diff --git a/acme/version_go112.go b/acme/version_go112.go
index b9efdb59e5..cc5fab604b 100644
--- a/acme/version_go112.go
+++ b/acme/version_go112.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build go1.12
-// +build go1.12
 
 package acme
 
diff --git a/argon2/blamka_amd64.go b/argon2/blamka_amd64.go
index a014ac92aa..063e7784f8 100644
--- a/argon2/blamka_amd64.go
+++ b/argon2/blamka_amd64.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build amd64 && gc && !purego
-// +build amd64,gc,!purego
 
 package argon2
 
diff --git a/argon2/blamka_amd64.s b/argon2/blamka_amd64.s
index b2cc051504..6713accac0 100644
--- a/argon2/blamka_amd64.s
+++ b/argon2/blamka_amd64.s
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build amd64 && gc && !purego
-// +build amd64,gc,!purego
 
 #include "textflag.h"
 
@@ -200,8 +199,8 @@ TEXT ·mixBlocksSSE2(SB), 4, $0-32
 	MOVQ out+0(FP), DX
 	MOVQ a+8(FP), AX
 	MOVQ b+16(FP), BX
-	MOVQ a+24(FP), CX
-	MOVQ $128, BP
+	MOVQ c+24(FP), CX
+	MOVQ $128, DI
 
 loop:
 	MOVOU 0(AX), X0
@@ -214,7 +213,7 @@ loop:
 	ADDQ  $16, BX
 	ADDQ  $16, CX
 	ADDQ  $16, DX
-	SUBQ  $2, BP
+	SUBQ  $2, DI
 	JA    loop
 	RET
 
@@ -223,8 +222,8 @@ TEXT ·xorBlocksSSE2(SB), 4, $0-32
 	MOVQ out+0(FP), DX
 	MOVQ a+8(FP), AX
 	MOVQ b+16(FP), BX
-	MOVQ a+24(FP), CX
-	MOVQ $128, BP
+	MOVQ c+24(FP), CX
+	MOVQ $128, DI
 
 loop:
 	MOVOU 0(AX), X0
@@ -239,6 +238,6 @@ loop:
 	ADDQ  $16, BX
 	ADDQ  $16, CX
 	ADDQ  $16, DX
-	SUBQ  $2, BP
+	SUBQ  $2, DI
 	JA    loop
 	RET
diff --git a/argon2/blamka_ref.go b/argon2/blamka_ref.go
index 167c59d2d5..16d58c650e 100644
--- a/argon2/blamka_ref.go
+++ b/argon2/blamka_ref.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build !amd64 || purego || !gc
-// +build !amd64 purego !gc
 
 package argon2
 
diff --git a/blake2b/blake2bAVX2_amd64.go b/blake2b/blake2bAVX2_amd64.go
index 56bfaaa17d..199c21d27a 100644
--- a/blake2b/blake2bAVX2_amd64.go
+++ b/blake2b/blake2bAVX2_amd64.go
@@ -2,8 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.7 && amd64 && gc && !purego
-// +build go1.7,amd64,gc,!purego
+//go:build amd64 && gc && !purego
 
 package blake2b
 
diff --git a/blake2b/blake2bAVX2_amd64.s b/blake2b/blake2bAVX2_amd64.s
index 4b9daa18d9..9ae8206c20 100644
--- a/blake2b/blake2bAVX2_amd64.s
+++ b/blake2b/blake2bAVX2_amd64.s
@@ -2,8 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.7 && amd64 && gc && !purego
-// +build go1.7,amd64,gc,!purego
+//go:build amd64 && gc && !purego
 
 #include "textflag.h"
 
diff --git a/blake2b/blake2b_amd64.go b/blake2b/blake2b_amd64.go
deleted file mode 100644
index 5fa1b32841..0000000000
--- a/blake2b/blake2b_amd64.go
+++ /dev/null
@@ -1,25 +0,0 @@
-// Copyright 2016 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-//go:build !go1.7 && amd64 && gc && !purego
-// +build !go1.7,amd64,gc,!purego
-
-package blake2b
-
-import "golang.org/x/sys/cpu"
-
-func init() {
-	useSSE4 = cpu.X86.HasSSE41
-}
-
-//go:noescape
-func hashBlocksSSE4(h *[8]uint64, c *[2]uint64, flag uint64, blocks []byte)
-
-func hashBlocks(h *[8]uint64, c *[2]uint64, flag uint64, blocks []byte) {
-	if useSSE4 {
-		hashBlocksSSE4(h, c, flag, blocks)
-	} else {
-		hashBlocksGeneric(h, c, flag, blocks)
-	}
-}
diff --git a/blake2b/blake2b_amd64.s b/blake2b/blake2b_amd64.s
index ae75eb9afc..adfac00c15 100644
--- a/blake2b/blake2b_amd64.s
+++ b/blake2b/blake2b_amd64.s
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build amd64 && gc && !purego
-// +build amd64,gc,!purego
 
 #include "textflag.h"
 
diff --git a/blake2b/blake2b_ref.go b/blake2b/blake2b_ref.go
index b0137cdf02..6e28668cd1 100644
--- a/blake2b/blake2b_ref.go
+++ b/blake2b/blake2b_ref.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build !amd64 || purego || !gc
-// +build !amd64 purego !gc
 
 package blake2b
 
diff --git a/blake2b/register.go b/blake2b/register.go
index 9d8633963c..54e446e1d2 100644
--- a/blake2b/register.go
+++ b/blake2b/register.go
@@ -2,9 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.9
-// +build go1.9
-
 package blake2b
 
 import (
diff --git a/blake2s/blake2s_386.go b/blake2s/blake2s_386.go
index b4463fb4dc..97f629617e 100644
--- a/blake2s/blake2s_386.go
+++ b/blake2s/blake2s_386.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build 386 && gc && !purego
-// +build 386,gc,!purego
 
 package blake2s
 
diff --git a/blake2s/blake2s_386.s b/blake2s/blake2s_386.s
index 603d00ca32..919c026541 100644
--- a/blake2s/blake2s_386.s
+++ b/blake2s/blake2s_386.s
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build 386 && gc && !purego
-// +build 386,gc,!purego
 
 #include "textflag.h"
 
diff --git a/blake2s/blake2s_amd64.go b/blake2s/blake2s_amd64.go
index becdaa120f..8a7310254e 100644
--- a/blake2s/blake2s_amd64.go
+++ b/blake2s/blake2s_amd64.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build amd64 && gc && !purego
-// +build amd64,gc,!purego
 
 package blake2s
 
diff --git a/blake2s/blake2s_amd64.s b/blake2s/blake2s_amd64.s
index e9df7a7c21..fe4b818a33 100644
--- a/blake2s/blake2s_amd64.s
+++ b/blake2s/blake2s_amd64.s
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build amd64 && gc && !purego
-// +build amd64,gc,!purego
 
 #include "textflag.h"
 
diff --git a/blake2s/blake2s_ref.go b/blake2s/blake2s_ref.go
index 799dba0c41..38ce8e283f 100644
--- a/blake2s/blake2s_ref.go
+++ b/blake2s/blake2s_ref.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build (!amd64 && !386) || !gc || purego
-// +build !amd64,!386 !gc purego
 
 package blake2s
 
diff --git a/blake2s/register.go b/blake2s/register.go
index ef79ff3c67..3156148a42 100644
--- a/blake2s/register.go
+++ b/blake2s/register.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build go1.9
-// +build go1.9
 
 package blake2s
 
diff --git a/chacha20/chacha_arm64.go b/chacha20/chacha_arm64.go
index 94c71ac1ac..661ea132e0 100644
--- a/chacha20/chacha_arm64.go
+++ b/chacha20/chacha_arm64.go
@@ -2,8 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.11 && gc && !purego
-// +build go1.11,gc,!purego
+//go:build gc && !purego
 
 package chacha20
 
diff --git a/chacha20/chacha_arm64.s b/chacha20/chacha_arm64.s
index 63cae9e6f0..7dd2638e88 100644
--- a/chacha20/chacha_arm64.s
+++ b/chacha20/chacha_arm64.s
@@ -2,8 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.11 && gc && !purego
-// +build go1.11,gc,!purego
+//go:build gc && !purego
 
 #include "textflag.h"
 
diff --git a/chacha20/chacha_noasm.go b/chacha20/chacha_noasm.go
index 025b49897e..db42e6676a 100644
--- a/chacha20/chacha_noasm.go
+++ b/chacha20/chacha_noasm.go
@@ -2,8 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build (!arm64 && !s390x && !ppc64le) || (arm64 && !go1.11) || !gc || purego
-// +build !arm64,!s390x,!ppc64le arm64,!go1.11 !gc purego
+//go:build (!arm64 && !s390x && !ppc64le) || !gc || purego
 
 package chacha20
 
diff --git a/chacha20/chacha_ppc64le.go b/chacha20/chacha_ppc64le.go
index da420b2e97..3a4287f990 100644
--- a/chacha20/chacha_ppc64le.go
+++ b/chacha20/chacha_ppc64le.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build gc && !purego
-// +build gc,!purego
 
 package chacha20
 
diff --git a/chacha20/chacha_ppc64le.s b/chacha20/chacha_ppc64le.s
index 5c0fed26f8..66aebae258 100644
--- a/chacha20/chacha_ppc64le.s
+++ b/chacha20/chacha_ppc64le.s
@@ -20,7 +20,6 @@
 // due to the calling conventions and initialization of constants.
 
 //go:build gc && !purego
-// +build gc,!purego
 
 #include "textflag.h"
 
diff --git a/chacha20/chacha_s390x.go b/chacha20/chacha_s390x.go
index 4652247b8a..683ccfd1c3 100644
--- a/chacha20/chacha_s390x.go
+++ b/chacha20/chacha_s390x.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build gc && !purego
-// +build gc,!purego
 
 package chacha20
 
diff --git a/chacha20/chacha_s390x.s b/chacha20/chacha_s390x.s
index f3ef5a019d..1eda91a3d4 100644
--- a/chacha20/chacha_s390x.s
+++ b/chacha20/chacha_s390x.s
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build gc && !purego
-// +build gc,!purego
 
 #include "go_asm.h"
 #include "textflag.h"
diff --git a/chacha20poly1305/chacha20poly1305_amd64.go b/chacha20poly1305/chacha20poly1305_amd64.go
index 78b8ce2911..c384b407ac 100644
--- a/chacha20poly1305/chacha20poly1305_amd64.go
+++ b/chacha20poly1305/chacha20poly1305_amd64.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build gc && !purego
-// +build gc,!purego
 
 package chacha20poly1305
 
diff --git a/chacha20poly1305/chacha20poly1305_amd64.s b/chacha20poly1305/chacha20poly1305_amd64.s
index 867c181a14..731d2ac6db 100644
--- a/chacha20poly1305/chacha20poly1305_amd64.s
+++ b/chacha20poly1305/chacha20poly1305_amd64.s
@@ -5,7 +5,6 @@
 // This file was originally from https://golang.org/cl/24717 by Vlad Krasnov of CloudFlare.
 
 //go:build gc && !purego
-// +build gc,!purego
 
 #include "textflag.h"
 // General register allocation
@@ -184,11 +183,31 @@ GLOBL ·andMask<>(SB), (NOPTR+RODATA), $240
 #define shiftD1Right BYTE $0x66; BYTE $0x45; BYTE $0x0f; BYTE $0x3a; BYTE $0x0f; BYTE $0xd2; BYTE $0x04 // PALIGNR $4, X10, X10
 #define shiftD2Right BYTE $0x66; BYTE $0x45; BYTE $0x0f; BYTE $0x3a; BYTE $0x0f; BYTE $0xdb; BYTE $0x04 // PALIGNR $4, X11, X11
 #define shiftD3Right BYTE $0x66; BYTE $0x45; BYTE $0x0f; BYTE $0x3a; BYTE $0x0f; BYTE $0xff; BYTE $0x04 // PALIGNR $4, X15, X15
+
 // Some macros
+
+// ROL rotates the uint32s in register R left by N bits, using temporary T.
+#define ROL(N, R, T) \
+	MOVO R, T; PSLLL $(N), T; PSRLL $(32-(N)), R; PXOR T, R
+
+// ROL16 rotates the uint32s in register R left by 16, using temporary T if needed.
+#ifdef GOAMD64_v2
+#define ROL16(R, T) PSHUFB ·rol16<>(SB), R
+#else
+#define ROL16(R, T) ROL(16, R, T)
+#endif
+
+// ROL8 rotates the uint32s in register R left by 8, using temporary T if needed.
+#ifdef GOAMD64_v2
+#define ROL8(R, T) PSHUFB ·rol8<>(SB), R
+#else
+#define ROL8(R, T) ROL(8, R, T)
+#endif
+
 #define chachaQR(A, B, C, D, T) \
-	PADDD B, A; PXOR A, D; PSHUFB ·rol16<>(SB), D                            \
+	PADDD B, A; PXOR A, D; ROL16(D, T) \
 	PADDD D, C; PXOR C, B; MOVO B, T; PSLLL $12, T; PSRLL $20, B; PXOR T, B \
-	PADDD B, A; PXOR A, D; PSHUFB ·rol8<>(SB), D                             \
+	PADDD B, A; PXOR A, D; ROL8(D, T) \
 	PADDD D, C; PXOR C, B; MOVO B, T; PSLLL $7, T; PSRLL $25, B; PXOR T, B
 
 #define chachaQR_AVX2(A, B, C, D, T) \
diff --git a/chacha20poly1305/chacha20poly1305_noasm.go b/chacha20poly1305/chacha20poly1305_noasm.go
index f832b33d45..34e6ab1df8 100644
--- a/chacha20poly1305/chacha20poly1305_noasm.go
+++ b/chacha20poly1305/chacha20poly1305_noasm.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build !amd64 || !gc || purego
-// +build !amd64 !gc purego
 
 package chacha20poly1305
 
diff --git a/cryptobyte/asn1.go b/cryptobyte/asn1.go
index 6fc2838a3f..2492f796af 100644
--- a/cryptobyte/asn1.go
+++ b/cryptobyte/asn1.go
@@ -733,13 +733,14 @@ func (s *String) ReadOptionalASN1OctetString(out *[]byte, outPresent *bool, tag
 	return true
 }
 
-// ReadOptionalASN1Boolean sets *out to the value of the next ASN.1 BOOLEAN or,
-// if the next bytes are not an ASN.1 BOOLEAN, to the value of defaultValue.
-// It reports whether the operation was successful.
-func (s *String) ReadOptionalASN1Boolean(out *bool, defaultValue bool) bool {
+// ReadOptionalASN1Boolean attempts to read an optional ASN.1 BOOLEAN
+// explicitly tagged with tag into out and advances. If no element with a
+// matching tag is present, it sets "out" to defaultValue instead. It reports
+// whether the read was successful.
+func (s *String) ReadOptionalASN1Boolean(out *bool, tag asn1.Tag, defaultValue bool) bool {
 	var present bool
 	var child String
-	if !s.ReadOptionalASN1(&child, &present, asn1.BOOLEAN) {
+	if !s.ReadOptionalASN1(&child, &present, tag) {
 		return false
 	}
 
@@ -748,7 +749,7 @@ func (s *String) ReadOptionalASN1Boolean(out *bool, defaultValue bool) bool {
 		return true
 	}
 
-	return s.ReadASN1Boolean(out)
+	return child.ReadASN1Boolean(out)
 }
 
 func (s *String) readASN1(out *String, outTag *asn1.Tag, skipHeader bool) bool {
diff --git a/cryptobyte/asn1_test.go b/cryptobyte/asn1_test.go
index e3f53a932e..93760b06e9 100644
--- a/cryptobyte/asn1_test.go
+++ b/cryptobyte/asn1_test.go
@@ -115,6 +115,28 @@ func TestReadASN1OptionalInteger(t *testing.T) {
 	}
 }
 
+const defaultBool = false
+
+var optionalBoolTestData = []readASN1Test{
+	{"empty", []byte{}, 0xa0, true, false},
+	{"invalid", []byte{0xa1, 0x3, 0x1, 0x2, 0x7f}, 0xa1, false, defaultBool},
+	{"missing", []byte{0xa1, 0x3, 0x1, 0x1, 0x7f}, 0xa0, true, defaultBool},
+	{"present", []byte{0xa1, 0x3, 0x1, 0x1, 0xff}, 0xa1, true, true},
+}
+
+func TestReadASN1OptionalBoolean(t *testing.T) {
+	for _, test := range optionalBoolTestData {
+		t.Run(test.name, func(t *testing.T) {
+			in := String(test.in)
+			var out bool
+			ok := in.ReadOptionalASN1Boolean(&out, test.tag, defaultBool)
+			if ok != test.ok || ok && out != test.out.(bool) {
+				t.Errorf("in.ReadOptionalASN1Boolean() = %v, want %v; out = %v, want %v", ok, test.ok, out, test.out)
+			}
+		})
+	}
+}
+
 func TestReadASN1IntegerSigned(t *testing.T) {
 	testData64 := []struct {
 		in  []byte
diff --git a/cryptobyte/builder.go b/cryptobyte/builder.go
index c05ac7d16d..cf254f5f1e 100644
--- a/cryptobyte/builder.go
+++ b/cryptobyte/builder.go
@@ -95,6 +95,11 @@ func (b *Builder) AddUint32(v uint32) {
 	b.add(byte(v>>24), byte(v>>16), byte(v>>8), byte(v))
 }
 
+// AddUint48 appends a big-endian, 48-bit value to the byte string.
+func (b *Builder) AddUint48(v uint64) {
+	b.add(byte(v>>40), byte(v>>32), byte(v>>24), byte(v>>16), byte(v>>8), byte(v))
+}
+
 // AddUint64 appends a big-endian, 64-bit value to the byte string.
 func (b *Builder) AddUint64(v uint64) {
 	b.add(byte(v>>56), byte(v>>48), byte(v>>40), byte(v>>32), byte(v>>24), byte(v>>16), byte(v>>8), byte(v))
diff --git a/cryptobyte/cryptobyte_test.go b/cryptobyte/cryptobyte_test.go
index dc90e81fa9..9b55d52e32 100644
--- a/cryptobyte/cryptobyte_test.go
+++ b/cryptobyte/cryptobyte_test.go
@@ -179,6 +179,27 @@ func TestUint32(t *testing.T) {
 	}
 }
 
+func TestUint48(t *testing.T) {
+	var b Builder
+	var u uint64 = 0xfefcff3cfdfc
+	b.AddUint48(u)
+	if err := builderBytesEq(&b, 254, 252, 255, 60, 253, 252); err != nil {
+		t.Error(err)
+	}
+
+	var s String = b.BytesOrPanic()
+	var v uint64
+	if !s.ReadUint48(&v) {
+		t.Error("ReadUint48() = false, want true")
+	}
+	if v != u {
+		t.Errorf("v = %x, want %x", v, u)
+	}
+	if len(s) != 0 {
+		t.Errorf("len(s) = %d, want 0", len(s))
+	}
+}
+
 func TestUint64(t *testing.T) {
 	var b Builder
 	b.AddUint64(0xf2fefefcff3cfdfc)
diff --git a/cryptobyte/string.go b/cryptobyte/string.go
index 0531a3d6f1..10692a8a31 100644
--- a/cryptobyte/string.go
+++ b/cryptobyte/string.go
@@ -81,6 +81,17 @@ func (s *String) ReadUint32(out *uint32) bool {
 	return true
 }
 
+// ReadUint48 decodes a big-endian, 48-bit value into out and advances over it.
+// It reports whether the read was successful.
+func (s *String) ReadUint48(out *uint64) bool {
+	v := s.read(6)
+	if v == nil {
+		return false
+	}
+	*out = uint64(v[0])<<40 | uint64(v[1])<<32 | uint64(v[2])<<24 | uint64(v[3])<<16 | uint64(v[4])<<8 | uint64(v[5])
+	return true
+}
+
 // ReadUint64 decodes a big-endian, 64-bit value into out and advances over it.
 // It reports whether the read was successful.
 func (s *String) ReadUint64(out *uint64) bool {
diff --git a/curve25519/internal/field/_asm/go.mod b/curve25519/internal/field/_asm/go.mod
index 5b3dba315e..f6902f4a64 100644
--- a/curve25519/internal/field/_asm/go.mod
+++ b/curve25519/internal/field/_asm/go.mod
@@ -1,10 +1,16 @@
 module asm
 
-go 1.16
+go 1.18
 
 require (
-	github.com/mmcloughlin/avo v0.4.0
-	golang.org/x/crypto v0.0.0
+	github.com/mmcloughlin/avo v0.5.0
+	golang.org/x/crypto v0.1.0
 )
 
-replace golang.org/x/crypto v0.0.0 => ../../../..
+require (
+	golang.org/x/mod v0.8.0 // indirect
+	golang.org/x/sys v0.14.0 // indirect
+	golang.org/x/tools v0.6.0 // indirect
+)
+
+replace golang.org/x/crypto v0.1.0 => ../../../..
diff --git a/curve25519/internal/field/_asm/go.sum b/curve25519/internal/field/_asm/go.sum
index 4c9bbf675b..96b7915d4f 100644
--- a/curve25519/internal/field/_asm/go.sum
+++ b/curve25519/internal/field/_asm/go.sum
@@ -1,34 +1,51 @@
-github.com/mmcloughlin/avo v0.4.0 h1:jeHDRktVD+578ULxWpQHkilor6pkdLF7u7EiTzDbfcU=
-github.com/mmcloughlin/avo v0.4.0/go.mod h1:RW9BfYA3TgO9uCdNrKU2h6J8cPD8ZLznvfgHAeszb1s=
-github.com/yuin/goldmark v1.4.0/go.mod h1:mwnBkeHKe2W/ZEtQ+71ViKU8L12m81fl3OWwC1Zlc8k=
-golang.org/x/arch v0.0.0-20210923205945-b76863e36670/go.mod h1:5om86z9Hs0C8fWVUuoMHwpExlXzs5Tkyp9hOrfG7pp8=
+github.com/mmcloughlin/avo v0.5.0 h1:nAco9/aI9Lg2kiuROBY6BhCI/z0t5jEvJfjWbL8qXLU=
+github.com/mmcloughlin/avo v0.5.0/go.mod h1:ChHFdoV7ql95Wi7vuq2YT1bwCJqiWdZrQ1im3VujLYM=
+github.com/yuin/goldmark v1.4.13/go.mod h1:6yULJ656Px+3vBD8DxQVa3kxgyrAnzto9xy5taEt/CY=
+golang.org/x/arch v0.1.0/go.mod h1:5om86z9Hs0C8fWVUuoMHwpExlXzs5Tkyp9hOrfG7pp8=
 golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
-golang.org/x/crypto v0.0.0-20191011191535-87dc89f01550/go.mod h1:yigFU9vqHzYiE8UmvKecakEJjdnWj3jj499lnFckfCI=
-golang.org/x/mod v0.4.2 h1:Gz96sIWK3OalVv/I/qNygP42zyoKp3xptRVCWRFEBvo=
-golang.org/x/mod v0.4.2/go.mod h1:s0Qsj1ACt9ePp/hMypM3fl4fZqREWJwdYDEqhRiZZUA=
-golang.org/x/net v0.0.0-20190404232315-eb5bcb51f2a3/go.mod h1:t9HGtf8HONx5eT2rtn7q6eTqICYqUVnKs3thJo3Qplg=
+golang.org/x/crypto v0.0.0-20210921155107-089bfa567519/go.mod h1:GvvjBRRGRdwPK5ydBHafDWAxML/pGHZbMvKqRZ5+Abc=
+golang.org/x/mod v0.6.0-dev.0.20220419223038-86c51ed26bb4/go.mod h1:jJ57K6gSWd91VN4djpZkiMVwK6gcyfeH4XE8wZrZaV4=
+golang.org/x/mod v0.6.0/go.mod h1:4mET923SAdbXp2ki8ey+zGs1SLqsuM2Y0uvdZR/fUNI=
+golang.org/x/mod v0.8.0 h1:LUYupSeNrTNCGzR/hVBk2NHZO4hXcVaW1k4Qx7rjPx8=
+golang.org/x/mod v0.8.0/go.mod h1:iBbtSCu2XBx23ZKBPSOrRkjjQPZFPuis4dIYUhu/chs=
 golang.org/x/net v0.0.0-20190620200207-3b0461eec859/go.mod h1:z5CRVTTTmAJ677TzLLGU+0bjPO0LkuOLi4/5GtJWs/s=
-golang.org/x/net v0.0.0-20210805182204-aaa1db679c0d/go.mod h1:9nx3DQGgdP8bBQD5qxJ1jj9UTztislL4KSBs9R2vV5Y=
-golang.org/x/net v0.0.0-20211112202133-69e39bad7dc2/go.mod h1:9nx3DQGgdP8bBQD5qxJ1jj9UTztislL4KSBs9R2vV5Y=
+golang.org/x/net v0.0.0-20210226172049-e18ecbb05110/go.mod h1:m0MpNAwzfU5UDzcl9v0D8zg8gWTRqZa9RBIspLL5mdg=
+golang.org/x/net v0.0.0-20220722155237-a158d28d115b/go.mod h1:XRhObCWvk6IyKnWLug+ECip1KBveYUHfp+8e9klMJ9c=
+golang.org/x/net v0.1.0/go.mod h1:Cx3nUiGt4eDBEyega/BKRp+/AlGL8hYe7U9odMt2Cco=
+golang.org/x/net v0.6.0/go.mod h1:2Tu9+aMcznHK/AK1HMvgo6xiTLG5rD5rZLDS+rp2Bjs=
+golang.org/x/net v0.10.0/go.mod h1:0qNGK6F8kojg2nk9dLZ2mShWaEBan6FAoqfSigmmuDg=
 golang.org/x/sync v0.0.0-20190423024810-112230192c58/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
-golang.org/x/sync v0.0.0-20210220032951-036812b2e83c/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
+golang.org/x/sync v0.0.0-20220722155255-886fb9371eb4/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
+golang.org/x/sync v0.1.0 h1:wsuoTGHzEhffawBOhz5CYhcrV4IdKZbEyZjBMuTp12o=
+golang.org/x/sync v0.1.0/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
 golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
-golang.org/x/sys v0.0.0-20190412213103-97732733099d/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20201119102817-f84b799fce68/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
-golang.org/x/sys v0.0.0-20210423082822-04245dca01da/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20210615035016-665e8c7367d1/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
-golang.org/x/sys v0.0.0-20210809222454-d867a43fc93e/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
-golang.org/x/sys v0.0.0-20211030160813-b3129d9d1021 h1:giLT+HuUP/gXYrG2Plg9WTjj4qhfgaW424ZIFog3rlk=
-golang.org/x/sys v0.0.0-20211030160813-b3129d9d1021/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.0.0-20220520151302-bc2c85ada10a/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.0.0-20220722155257-8c9f86f7a55f/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.1.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.5.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.8.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.14.0 h1:Vz7Qs629MkJkGyHxUlRHizWJRG2j8fbQKjELVSNhy7Q=
+golang.org/x/sys v0.14.0/go.mod h1:/VUhepiaJMQUp4+oa/7Zr1D23ma6VTLIYjOOTFZPUcA=
 golang.org/x/term v0.0.0-20201126162022-7de9c90e9dd1/go.mod h1:bj7SfCRtBDWHUb9snDiAeCFNEtKQo2Wmx5Cou7ajbmo=
+golang.org/x/term v0.0.0-20210927222741-03fcf44c2211/go.mod h1:jbD1KX2456YbFQfuXm/mYQcufACuNUgVhRMnK/tPxf8=
+golang.org/x/term v0.1.0/go.mod h1:jbD1KX2456YbFQfuXm/mYQcufACuNUgVhRMnK/tPxf8=
+golang.org/x/term v0.5.0/go.mod h1:jMB1sMXY+tzblOD4FWmEbocvup2/aLOaQEp7JmGp78k=
+golang.org/x/term v0.8.0/go.mod h1:xPskH00ivmX89bAKVGSKKtLOWNx2+17Eiy94tnKShWo=
+golang.org/x/term v0.14.0/go.mod h1:TySc+nGkYR6qt8km8wUhuFRTVSMIX3XPR58y2lC8vww=
 golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
-golang.org/x/text v0.3.6/go.mod h1:5Zoc/QRtKVWzQhOtBMvqHzDpF6irO9z98xDceosuGiQ=
+golang.org/x/text v0.3.3/go.mod h1:5Zoc/QRtKVWzQhOtBMvqHzDpF6irO9z98xDceosuGiQ=
+golang.org/x/text v0.3.7/go.mod h1:u+2+/6zg+i71rQMx5EYifcz6MCKuco9NR6JIITiCfzQ=
+golang.org/x/text v0.4.0/go.mod h1:mrYo+phRRbMaCq/xk9113O4dZlRixOauAjOtrjsXDZ8=
+golang.org/x/text v0.7.0/go.mod h1:mrYo+phRRbMaCq/xk9113O4dZlRixOauAjOtrjsXDZ8=
+golang.org/x/text v0.9.0/go.mod h1:e1OnstbJyHTd6l/uOt8jFFHp6TRDWZR/bV3emEE/zU8=
+golang.org/x/text v0.14.0/go.mod h1:18ZOQIKpY8NJVqYksKHtTdi31H5itFRjB5/qKTNYzSU=
 golang.org/x/tools v0.0.0-20180917221912-90fa682c2a6e/go.mod h1:n7NCudcB/nEzxVGmLbDWY5pfWTLqBcC2KZ6jyYvM4mQ=
 golang.org/x/tools v0.0.0-20191119224855-298f0cb1881e/go.mod h1:b+2E5dAYhXwXZwtnZ6UAqBI28+e2cm9otk0dWdXHAEo=
-golang.org/x/tools v0.1.7 h1:6j8CgantCy3yc8JGBqkDLMKWqZ0RDU2g1HVgacojGWQ=
-golang.org/x/tools v0.1.7/go.mod h1:LGqMHiF4EqQNHR1JncWGqT5BVaXmza+X+BDGol+dOxo=
+golang.org/x/tools v0.1.12/go.mod h1:hNGJHUnrk76NpqgfD5Aqm5Crs+Hm0VOH/i9J2+nxYbc=
+golang.org/x/tools v0.2.0/go.mod h1:y4OqIKeOV/fWJetJ8bXPU1sEVniLMIyDAZWeHdV+NTA=
+golang.org/x/tools v0.6.0 h1:BOw41kyTf3PuCW1pVQf8+Cyg8pMlkYB1oo9iJ6D/lKM=
+golang.org/x/tools v0.6.0/go.mod h1:Xwgl3UAJ/d3gWutnCtw505GrjyAbvKui8lOU390QaIU=
 golang.org/x/xerrors v0.0.0-20190717185122-a985d3407aa7/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
-golang.org/x/xerrors v0.0.0-20191011141410-1b5146add898/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
-golang.org/x/xerrors v0.0.0-20200804184101-5ec99f83aff1 h1:go1bK/D/BFZV2I8cIQd1NKEZ+0owSTG1fDTci4IqFcE=
-golang.org/x/xerrors v0.0.0-20200804184101-5ec99f83aff1/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
 rsc.io/pdf v0.1.1/go.mod h1:n8OzWcQ6Sp37PL01nO98y4iUCRdTGarVfzxY20ICaU4=
diff --git a/curve25519/internal/field/fe_amd64.go b/curve25519/internal/field/fe_amd64.go
index edcf163c4e..70c541692c 100644
--- a/curve25519/internal/field/fe_amd64.go
+++ b/curve25519/internal/field/fe_amd64.go
@@ -1,7 +1,6 @@
 // Code generated by command: go run fe_amd64_asm.go -out ../fe_amd64.s -stubs ../fe_amd64.go -pkg field. DO NOT EDIT.
 
 //go:build amd64 && gc && !purego
-// +build amd64,gc,!purego
 
 package field
 
diff --git a/curve25519/internal/field/fe_amd64.s b/curve25519/internal/field/fe_amd64.s
index 293f013c94..60817acc41 100644
--- a/curve25519/internal/field/fe_amd64.s
+++ b/curve25519/internal/field/fe_amd64.s
@@ -1,7 +1,6 @@
 // Code generated by command: go run fe_amd64_asm.go -out ../fe_amd64.s -stubs ../fe_amd64.go -pkg field. DO NOT EDIT.
 
 //go:build amd64 && gc && !purego
-// +build amd64,gc,!purego
 
 #include "textflag.h"
 
diff --git a/curve25519/internal/field/fe_amd64_noasm.go b/curve25519/internal/field/fe_amd64_noasm.go
index ddb6c9b8f7..9da280d1d8 100644
--- a/curve25519/internal/field/fe_amd64_noasm.go
+++ b/curve25519/internal/field/fe_amd64_noasm.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build !amd64 || !gc || purego
-// +build !amd64 !gc purego
 
 package field
 
diff --git a/curve25519/internal/field/fe_arm64.go b/curve25519/internal/field/fe_arm64.go
index af459ef515..075fe9b925 100644
--- a/curve25519/internal/field/fe_arm64.go
+++ b/curve25519/internal/field/fe_arm64.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build arm64 && gc && !purego
-// +build arm64,gc,!purego
 
 package field
 
diff --git a/curve25519/internal/field/fe_arm64.s b/curve25519/internal/field/fe_arm64.s
index 5c91e45892..3126a43419 100644
--- a/curve25519/internal/field/fe_arm64.s
+++ b/curve25519/internal/field/fe_arm64.s
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build arm64 && gc && !purego
-// +build arm64,gc,!purego
 
 #include "textflag.h"
 
diff --git a/curve25519/internal/field/fe_arm64_noasm.go b/curve25519/internal/field/fe_arm64_noasm.go
index 234a5b2e5d..fc029ac12d 100644
--- a/curve25519/internal/field/fe_arm64_noasm.go
+++ b/curve25519/internal/field/fe_arm64_noasm.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build !arm64 || !gc || purego
-// +build !arm64 !gc purego
 
 package field
 
diff --git a/go.mod b/go.mod
index 09b6f9b51d..037a097c75 100644
--- a/go.mod
+++ b/go.mod
@@ -1,12 +1,13 @@
 module github.com/tailscale/golang-x-crypto
 
-go 1.17
+go 1.18
 
 require (
-	golang.org/x/crypto v0.11.0
 	golang.org/x/net v0.10.0 // tagx:ignore
-	golang.org/x/sys v0.10.0
-	golang.org/x/term v0.10.0
+	golang.org/x/sys v0.15.0
+	golang.org/x/term v0.15.0
 )
 
-require golang.org/x/text v0.11.0 // indirect
+require golang.org/x/crypto v0.17.1-0.20240102205709-08396bb92b82
+
+require golang.org/x/text v0.14.0 // indirect
diff --git a/go.sum b/go.sum
index c0b399b859..a1462e15d5 100644
--- a/go.sum
+++ b/go.sum
@@ -1,43 +1,10 @@
-github.com/yuin/goldmark v1.4.13/go.mod h1:6yULJ656Px+3vBD8DxQVa3kxgyrAnzto9xy5taEt/CY=
-golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
-golang.org/x/crypto v0.0.0-20210921155107-089bfa567519/go.mod h1:GvvjBRRGRdwPK5ydBHafDWAxML/pGHZbMvKqRZ5+Abc=
-golang.org/x/crypto v0.11.0 h1:6Ewdq3tDic1mg5xRO4milcWCfMVQhI4NkqWWvqejpuA=
-golang.org/x/crypto v0.11.0/go.mod h1:xgJhtzW8F9jGdVFWZESrid1U1bjeNy4zgy5cRr/CIio=
-golang.org/x/mod v0.6.0-dev.0.20220419223038-86c51ed26bb4/go.mod h1:jJ57K6gSWd91VN4djpZkiMVwK6gcyfeH4XE8wZrZaV4=
-golang.org/x/mod v0.8.0/go.mod h1:iBbtSCu2XBx23ZKBPSOrRkjjQPZFPuis4dIYUhu/chs=
-golang.org/x/net v0.0.0-20190620200207-3b0461eec859/go.mod h1:z5CRVTTTmAJ677TzLLGU+0bjPO0LkuOLi4/5GtJWs/s=
-golang.org/x/net v0.0.0-20210226172049-e18ecbb05110/go.mod h1:m0MpNAwzfU5UDzcl9v0D8zg8gWTRqZa9RBIspLL5mdg=
-golang.org/x/net v0.0.0-20220722155237-a158d28d115b/go.mod h1:XRhObCWvk6IyKnWLug+ECip1KBveYUHfp+8e9klMJ9c=
-golang.org/x/net v0.6.0/go.mod h1:2Tu9+aMcznHK/AK1HMvgo6xiTLG5rD5rZLDS+rp2Bjs=
+golang.org/x/crypto v0.17.1-0.20240102205709-08396bb92b82 h1:Im4GabMwJDxh7eJBIF8XGVAyhmlqdBQmZV49AzWdKEk=
+golang.org/x/crypto v0.17.1-0.20240102205709-08396bb92b82/go.mod h1:gCAAfMLgwOJRpTjQ2zCCt2OcSfYMTeZVSRtQlPC7Nq4=
 golang.org/x/net v0.10.0 h1:X2//UzNDwYmtCLn7To6G58Wr6f5ahEAQgKNzv9Y951M=
 golang.org/x/net v0.10.0/go.mod h1:0qNGK6F8kojg2nk9dLZ2mShWaEBan6FAoqfSigmmuDg=
-golang.org/x/sync v0.0.0-20190423024810-112230192c58/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
-golang.org/x/sync v0.0.0-20220722155255-886fb9371eb4/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
-golang.org/x/sync v0.1.0/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
-golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
-golang.org/x/sys v0.0.0-20201119102817-f84b799fce68/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
-golang.org/x/sys v0.0.0-20210615035016-665e8c7367d1/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
-golang.org/x/sys v0.0.0-20220520151302-bc2c85ada10a/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
-golang.org/x/sys v0.0.0-20220722155257-8c9f86f7a55f/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
-golang.org/x/sys v0.5.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
-golang.org/x/sys v0.8.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
-golang.org/x/sys v0.10.0 h1:SqMFp9UcQJZa+pmYuAKjd9xq1f0j5rLcDIk0mj4qAsA=
-golang.org/x/sys v0.10.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
-golang.org/x/term v0.0.0-20201126162022-7de9c90e9dd1/go.mod h1:bj7SfCRtBDWHUb9snDiAeCFNEtKQo2Wmx5Cou7ajbmo=
-golang.org/x/term v0.0.0-20210927222741-03fcf44c2211/go.mod h1:jbD1KX2456YbFQfuXm/mYQcufACuNUgVhRMnK/tPxf8=
-golang.org/x/term v0.5.0/go.mod h1:jMB1sMXY+tzblOD4FWmEbocvup2/aLOaQEp7JmGp78k=
-golang.org/x/term v0.8.0/go.mod h1:xPskH00ivmX89bAKVGSKKtLOWNx2+17Eiy94tnKShWo=
-golang.org/x/term v0.10.0 h1:3R7pNqamzBraeqj/Tj8qt1aQ2HpmlC+Cx/qL/7hn4/c=
-golang.org/x/term v0.10.0/go.mod h1:lpqdcUyK/oCiQxvxVrppt5ggO2KCZ5QblwqPnfZ6d5o=
-golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
-golang.org/x/text v0.3.3/go.mod h1:5Zoc/QRtKVWzQhOtBMvqHzDpF6irO9z98xDceosuGiQ=
-golang.org/x/text v0.3.7/go.mod h1:u+2+/6zg+i71rQMx5EYifcz6MCKuco9NR6JIITiCfzQ=
-golang.org/x/text v0.7.0/go.mod h1:mrYo+phRRbMaCq/xk9113O4dZlRixOauAjOtrjsXDZ8=
-golang.org/x/text v0.9.0/go.mod h1:e1OnstbJyHTd6l/uOt8jFFHp6TRDWZR/bV3emEE/zU8=
-golang.org/x/text v0.11.0 h1:LAntKIrcmeSKERyiOh0XMV39LXS8IE9UL2yP7+f5ij4=
-golang.org/x/text v0.11.0/go.mod h1:TvPlkZtksWOMsz7fbANvkp4WM8x/WCo/om8BMLbz+aE=
-golang.org/x/tools v0.0.0-20180917221912-90fa682c2a6e/go.mod h1:n7NCudcB/nEzxVGmLbDWY5pfWTLqBcC2KZ6jyYvM4mQ=
-golang.org/x/tools v0.0.0-20191119224855-298f0cb1881e/go.mod h1:b+2E5dAYhXwXZwtnZ6UAqBI28+e2cm9otk0dWdXHAEo=
-golang.org/x/tools v0.1.12/go.mod h1:hNGJHUnrk76NpqgfD5Aqm5Crs+Hm0VOH/i9J2+nxYbc=
-golang.org/x/tools v0.6.0/go.mod h1:Xwgl3UAJ/d3gWutnCtw505GrjyAbvKui8lOU390QaIU=
-golang.org/x/xerrors v0.0.0-20190717185122-a985d3407aa7/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
+golang.org/x/sys v0.15.0 h1:h48lPFYpsTvQJZF4EKyI4aLHaev3CxivZmv7yZig9pc=
+golang.org/x/sys v0.15.0/go.mod h1:/VUhepiaJMQUp4+oa/7Zr1D23ma6VTLIYjOOTFZPUcA=
+golang.org/x/term v0.15.0 h1:y/Oo/a/q3IXu26lQgl04j/gjuBDOBlx7X6Om1j2CPW4=
+golang.org/x/term v0.15.0/go.mod h1:BDl952bC7+uMoWR75FIrCDx79TPU9oHkTZ9yRbYOrX0=
+golang.org/x/text v0.14.0 h1:ScX5w1eTa3QqT8oi6+ziP7dTV1S2+ALU0bI+0zXKWiQ=
+golang.org/x/text v0.14.0/go.mod h1:18ZOQIKpY8NJVqYksKHtTdi31H5itFRjB5/qKTNYzSU=
diff --git a/hkdf/hkdf.go b/hkdf/hkdf.go
index dda3f143be..f4ded5fee2 100644
--- a/hkdf/hkdf.go
+++ b/hkdf/hkdf.go
@@ -56,7 +56,9 @@ func (f *hkdf) Read(p []byte) (int, error) {
 
 	// Fill the rest of the buffer
 	for len(p) > 0 {
-		f.expander.Reset()
+		if f.counter > 1 {
+			f.expander.Reset()
+		}
 		f.expander.Write(f.prev)
 		f.expander.Write(f.info)
 		f.expander.Write([]byte{f.counter})
diff --git a/internal/alias/alias.go b/internal/alias/alias.go
index 69c17f822b..551ff0c353 100644
--- a/internal/alias/alias.go
+++ b/internal/alias/alias.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build !purego
-// +build !purego
 
 // Package alias implements memory aliasing tests.
 package alias
diff --git a/internal/alias/alias_purego.go b/internal/alias/alias_purego.go
index 4775b0a438..6fe61b5c6e 100644
--- a/internal/alias/alias_purego.go
+++ b/internal/alias/alias_purego.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build purego
-// +build purego
 
 // Package alias implements memory aliasing tests.
 package alias
diff --git a/internal/poly1305/bits_compat.go b/internal/poly1305/bits_compat.go
deleted file mode 100644
index 45b5c966b2..0000000000
--- a/internal/poly1305/bits_compat.go
+++ /dev/null
@@ -1,40 +0,0 @@
-// Copyright 2019 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-//go:build !go1.13
-// +build !go1.13
-
-package poly1305
-
-// Generic fallbacks for the math/bits intrinsics, copied from
-// src/math/bits/bits.go. They were added in Go 1.12, but Add64 and Sum64 had
-// variable time fallbacks until Go 1.13.
-
-func bitsAdd64(x, y, carry uint64) (sum, carryOut uint64) {
-	sum = x + y + carry
-	carryOut = ((x & y) | ((x | y) &^ sum)) >> 63
-	return
-}
-
-func bitsSub64(x, y, borrow uint64) (diff, borrowOut uint64) {
-	diff = x - y - borrow
-	borrowOut = ((^x & y) | (^(x ^ y) & diff)) >> 63
-	return
-}
-
-func bitsMul64(x, y uint64) (hi, lo uint64) {
-	const mask32 = 1<<32 - 1
-	x0 := x & mask32
-	x1 := x >> 32
-	y0 := y & mask32
-	y1 := y >> 32
-	w0 := x0 * y0
-	t := x1*y0 + w0>>32
-	w1 := t & mask32
-	w2 := t >> 32
-	w1 += x0 * y1
-	hi = x1*y1 + w2 + w1>>32
-	lo = x * y
-	return
-}
diff --git a/internal/poly1305/bits_go1.13.go b/internal/poly1305/bits_go1.13.go
deleted file mode 100644
index ed52b3418a..0000000000
--- a/internal/poly1305/bits_go1.13.go
+++ /dev/null
@@ -1,22 +0,0 @@
-// Copyright 2019 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-//go:build go1.13
-// +build go1.13
-
-package poly1305
-
-import "math/bits"
-
-func bitsAdd64(x, y, carry uint64) (sum, carryOut uint64) {
-	return bits.Add64(x, y, carry)
-}
-
-func bitsSub64(x, y, borrow uint64) (diff, borrowOut uint64) {
-	return bits.Sub64(x, y, borrow)
-}
-
-func bitsMul64(x, y uint64) (hi, lo uint64) {
-	return bits.Mul64(x, y)
-}
diff --git a/internal/poly1305/mac_noasm.go b/internal/poly1305/mac_noasm.go
index f184b67d98..333da285b3 100644
--- a/internal/poly1305/mac_noasm.go
+++ b/internal/poly1305/mac_noasm.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build (!amd64 && !ppc64le && !s390x) || !gc || purego
-// +build !amd64,!ppc64le,!s390x !gc purego
 
 package poly1305
 
diff --git a/internal/poly1305/sum_amd64.go b/internal/poly1305/sum_amd64.go
index 6d522333f2..164cd47d32 100644
--- a/internal/poly1305/sum_amd64.go
+++ b/internal/poly1305/sum_amd64.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build gc && !purego
-// +build gc,!purego
 
 package poly1305
 
diff --git a/internal/poly1305/sum_amd64.s b/internal/poly1305/sum_amd64.s
index 1d74f0f881..e0d3c64756 100644
--- a/internal/poly1305/sum_amd64.s
+++ b/internal/poly1305/sum_amd64.s
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build gc && !purego
-// +build gc,!purego
 
 #include "textflag.h"
 
diff --git a/internal/poly1305/sum_generic.go b/internal/poly1305/sum_generic.go
index e041da5ea3..ec2202bd7d 100644
--- a/internal/poly1305/sum_generic.go
+++ b/internal/poly1305/sum_generic.go
@@ -7,7 +7,10 @@
 
 package poly1305
 
-import "encoding/binary"
+import (
+	"encoding/binary"
+	"math/bits"
+)
 
 // Poly1305 [RFC 7539] is a relatively simple algorithm: the authentication tag
 // for a 64 bytes message is approximately
@@ -114,13 +117,13 @@ type uint128 struct {
 }
 
 func mul64(a, b uint64) uint128 {
-	hi, lo := bitsMul64(a, b)
+	hi, lo := bits.Mul64(a, b)
 	return uint128{lo, hi}
 }
 
 func add128(a, b uint128) uint128 {
-	lo, c := bitsAdd64(a.lo, b.lo, 0)
-	hi, c := bitsAdd64(a.hi, b.hi, c)
+	lo, c := bits.Add64(a.lo, b.lo, 0)
+	hi, c := bits.Add64(a.hi, b.hi, c)
 	if c != 0 {
 		panic("poly1305: unexpected overflow")
 	}
@@ -155,8 +158,8 @@ func updateGeneric(state *macState, msg []byte) {
 		// hide leading zeroes. For full chunks, that's 1 << 128, so we can just
 		// add 1 to the most significant (2¹²⁸) limb, h2.
 		if len(msg) >= TagSize {
-			h0, c = bitsAdd64(h0, binary.LittleEndian.Uint64(msg[0:8]), 0)
-			h1, c = bitsAdd64(h1, binary.LittleEndian.Uint64(msg[8:16]), c)
+			h0, c = bits.Add64(h0, binary.LittleEndian.Uint64(msg[0:8]), 0)
+			h1, c = bits.Add64(h1, binary.LittleEndian.Uint64(msg[8:16]), c)
 			h2 += c + 1
 
 			msg = msg[TagSize:]
@@ -165,8 +168,8 @@ func updateGeneric(state *macState, msg []byte) {
 			copy(buf[:], msg)
 			buf[len(msg)] = 1
 
-			h0, c = bitsAdd64(h0, binary.LittleEndian.Uint64(buf[0:8]), 0)
-			h1, c = bitsAdd64(h1, binary.LittleEndian.Uint64(buf[8:16]), c)
+			h0, c = bits.Add64(h0, binary.LittleEndian.Uint64(buf[0:8]), 0)
+			h1, c = bits.Add64(h1, binary.LittleEndian.Uint64(buf[8:16]), c)
 			h2 += c
 
 			msg = nil
@@ -219,9 +222,9 @@ func updateGeneric(state *macState, msg []byte) {
 		m3 := h2r1
 
 		t0 := m0.lo
-		t1, c := bitsAdd64(m1.lo, m0.hi, 0)
-		t2, c := bitsAdd64(m2.lo, m1.hi, c)
-		t3, _ := bitsAdd64(m3.lo, m2.hi, c)
+		t1, c := bits.Add64(m1.lo, m0.hi, 0)
+		t2, c := bits.Add64(m2.lo, m1.hi, c)
+		t3, _ := bits.Add64(m3.lo, m2.hi, c)
 
 		// Now we have the result as 4 64-bit limbs, and we need to reduce it
 		// modulo 2¹³⁰ - 5. The special shape of this Crandall prime lets us do
@@ -243,14 +246,14 @@ func updateGeneric(state *macState, msg []byte) {
 
 		// To add c * 5 to h, we first add cc = c * 4, and then add (cc >> 2) = c.
 
-		h0, c = bitsAdd64(h0, cc.lo, 0)
-		h1, c = bitsAdd64(h1, cc.hi, c)
+		h0, c = bits.Add64(h0, cc.lo, 0)
+		h1, c = bits.Add64(h1, cc.hi, c)
 		h2 += c
 
 		cc = shiftRightBy2(cc)
 
-		h0, c = bitsAdd64(h0, cc.lo, 0)
-		h1, c = bitsAdd64(h1, cc.hi, c)
+		h0, c = bits.Add64(h0, cc.lo, 0)
+		h1, c = bits.Add64(h1, cc.hi, c)
 		h2 += c
 
 		// h2 is at most 3 + 1 + 1 = 5, making the whole of h at most
@@ -287,9 +290,9 @@ func finalize(out *[TagSize]byte, h *[3]uint64, s *[2]uint64) {
 	// in constant time, we compute t = h - (2¹³⁰ - 5), and select h as the
 	// result if the subtraction underflows, and t otherwise.
 
-	hMinusP0, b := bitsSub64(h0, p0, 0)
-	hMinusP1, b := bitsSub64(h1, p1, b)
-	_, b = bitsSub64(h2, p2, b)
+	hMinusP0, b := bits.Sub64(h0, p0, 0)
+	hMinusP1, b := bits.Sub64(h1, p1, b)
+	_, b = bits.Sub64(h2, p2, b)
 
 	// h = h if h < p else h - p
 	h0 = select64(b, h0, hMinusP0)
@@ -301,8 +304,8 @@ func finalize(out *[TagSize]byte, h *[3]uint64, s *[2]uint64) {
 	//
 	// by just doing a wide addition with the 128 low bits of h and discarding
 	// the overflow.
-	h0, c := bitsAdd64(h0, s[0], 0)
-	h1, _ = bitsAdd64(h1, s[1], c)
+	h0, c := bits.Add64(h0, s[0], 0)
+	h1, _ = bits.Add64(h1, s[1], c)
 
 	binary.LittleEndian.PutUint64(out[0:8], h0)
 	binary.LittleEndian.PutUint64(out[8:16], h1)
diff --git a/internal/poly1305/sum_ppc64le.go b/internal/poly1305/sum_ppc64le.go
index 4a069941a6..4aec4874b5 100644
--- a/internal/poly1305/sum_ppc64le.go
+++ b/internal/poly1305/sum_ppc64le.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build gc && !purego
-// +build gc,!purego
 
 package poly1305
 
diff --git a/internal/poly1305/sum_ppc64le.s b/internal/poly1305/sum_ppc64le.s
index 58422aad23..d2ca5deeb9 100644
--- a/internal/poly1305/sum_ppc64le.s
+++ b/internal/poly1305/sum_ppc64le.s
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build gc && !purego
-// +build gc,!purego
 
 #include "textflag.h"
 
diff --git a/internal/poly1305/sum_s390x.go b/internal/poly1305/sum_s390x.go
index ec95966889..e1d033a491 100644
--- a/internal/poly1305/sum_s390x.go
+++ b/internal/poly1305/sum_s390x.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build gc && !purego
-// +build gc,!purego
 
 package poly1305
 
diff --git a/internal/poly1305/sum_s390x.s b/internal/poly1305/sum_s390x.s
index aa9e0494c9..0fe3a7c217 100644
--- a/internal/poly1305/sum_s390x.s
+++ b/internal/poly1305/sum_s390x.s
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build gc && !purego
-// +build gc,!purego
 
 #include "textflag.h"
 
diff --git a/internal/wycheproof/eddsa_test.go b/internal/wycheproof/eddsa_test.go
index 0a7fbb7e0e..9b97490955 100644
--- a/internal/wycheproof/eddsa_test.go
+++ b/internal/wycheproof/eddsa_test.go
@@ -3,14 +3,12 @@
 // license that can be found in the LICENSE file.
 
 //go:build go1.13
-// +build go1.13
 
 package wycheproof
 
 import (
+	"crypto/ed25519"
 	"testing"
-
-	"golang.org/x/crypto/ed25519"
 )
 
 func TestEddsa(t *testing.T) {
diff --git a/internal/wycheproof/wycheproof_test.go b/internal/wycheproof/wycheproof_test.go
index fcae97ce33..bbaae3b338 100644
--- a/internal/wycheproof/wycheproof_test.go
+++ b/internal/wycheproof/wycheproof_test.go
@@ -11,6 +11,7 @@ import (
 	"crypto/x509"
 	"encoding/hex"
 	"encoding/json"
+	"flag"
 	"fmt"
 	"log"
 	"os"
@@ -28,6 +29,11 @@ const wycheproofModVer = "v0.0.0-20191219022705-2196000605e4"
 var wycheproofTestVectorsDir string
 
 func TestMain(m *testing.M) {
+	flag.Parse()
+	if flag.Lookup("test.short").Value.(flag.Getter).Get().(bool) {
+		log.Println("skipping test that downloads testdata via 'go mod download' in short mode")
+		os.Exit(0)
+	}
 	if _, err := exec.LookPath("go"); err != nil {
 		log.Printf("skipping test because 'go' command is unavailable: %v", err)
 		os.Exit(0)
@@ -42,8 +48,6 @@ func TestMain(m *testing.M) {
 	// can be used in the following tests.
 	path := "github.com/google/wycheproof@" + wycheproofModVer
 	cmd := exec.Command("go", "mod", "download", "-json", path)
-	// TODO: enable the sumdb once the Trybots proxy supports it.
-	cmd.Env = append(os.Environ(), "GONOSUMDB=*")
 	output, err := cmd.Output()
 	if err != nil {
 		log.Fatalf("failed to run `go mod download -json %s`, output: %s", path, output)
diff --git a/nacl/sign/sign.go b/nacl/sign/sign.go
index 3f7e40967f..03323726d0 100644
--- a/nacl/sign/sign.go
+++ b/nacl/sign/sign.go
@@ -21,10 +21,10 @@
 package sign
 
 import (
+	"crypto/ed25519"
 	"io"
 
 	"github.com/tailscale/golang-x-crypto/internal/alias"
-	"golang.org/x/crypto/ed25519"
 )
 
 // Overhead is the number of bytes of overhead when signing a message.
diff --git a/ocsp/ocsp_test.go b/ocsp/ocsp_test.go
index 0bc194b2f4..a23d29bd1f 100644
--- a/ocsp/ocsp_test.go
+++ b/ocsp/ocsp_test.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build go1.7
-// +build go1.7
 
 package ocsp
 
diff --git a/otr/libotr_test_helper.c b/otr/libotr_test_helper.c
index b3ca072d48..aae03a3df7 100644
--- a/otr/libotr_test_helper.c
+++ b/otr/libotr_test_helper.c
@@ -5,7 +5,7 @@
 // This code can be compiled and used to test the otr package against libotr.
 // See otr_test.go.
 
-// +build ignore
+//go:build ignore
 
 #include <stdio.h>
 #include <stdlib.h>
diff --git a/salsa20/salsa/salsa20_amd64.go b/salsa20/salsa/salsa20_amd64.go
index c400dfcf7b..e76b44fe59 100644
--- a/salsa20/salsa/salsa20_amd64.go
+++ b/salsa20/salsa/salsa20_amd64.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build amd64 && !purego && gc
-// +build amd64,!purego,gc
 
 package salsa
 
diff --git a/salsa20/salsa/salsa20_amd64.s b/salsa20/salsa/salsa20_amd64.s
index c089277204..fcce0234b6 100644
--- a/salsa20/salsa/salsa20_amd64.s
+++ b/salsa20/salsa/salsa20_amd64.s
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build amd64 && !purego && gc
-// +build amd64,!purego,gc
 
 // This code was translated into a form compatible with 6a from the public
 // domain sources in SUPERCOP: https://bench.cr.yp.to/supercop.html
diff --git a/salsa20/salsa/salsa20_amd64_test.go b/salsa20/salsa/salsa20_amd64_test.go
index fc781f76f9..fe14604fc9 100644
--- a/salsa20/salsa/salsa20_amd64_test.go
+++ b/salsa20/salsa/salsa20_amd64_test.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build amd64 && !purego && gc
-// +build amd64,!purego,gc
 
 package salsa
 
diff --git a/salsa20/salsa/salsa20_noasm.go b/salsa20/salsa/salsa20_noasm.go
index 4392cc1ac7..9448760f26 100644
--- a/salsa20/salsa/salsa20_noasm.go
+++ b/salsa20/salsa/salsa20_noasm.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build !amd64 || purego || !gc
-// +build !amd64 purego !gc
 
 package salsa
 
diff --git a/sha3/hashes_generic.go b/sha3/hashes_generic.go
index c74fc20fcb..fe8c84793c 100644
--- a/sha3/hashes_generic.go
+++ b/sha3/hashes_generic.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build !gc || purego || !s390x
-// +build !gc purego !s390x
 
 package sha3
 
diff --git a/sha3/keccakf.go b/sha3/keccakf.go
index e5faa375c0..ce48b1dd3e 100644
--- a/sha3/keccakf.go
+++ b/sha3/keccakf.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build !amd64 || purego || !gc
-// +build !amd64 purego !gc
 
 package sha3
 
diff --git a/sha3/keccakf_amd64.go b/sha3/keccakf_amd64.go
index 248a38241f..b908696be5 100644
--- a/sha3/keccakf_amd64.go
+++ b/sha3/keccakf_amd64.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build amd64 && !purego && gc
-// +build amd64,!purego,gc
 
 package sha3
 
diff --git a/sha3/keccakf_amd64.s b/sha3/keccakf_amd64.s
index 4cfa54383b..1f53938861 100644
--- a/sha3/keccakf_amd64.s
+++ b/sha3/keccakf_amd64.s
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build amd64 && !purego && gc
-// +build amd64,!purego,gc
 
 // This code was translated into a form compatible with 6a from the public
 // domain sources at https://github.com/gvanas/KeccakCodePackage
@@ -320,9 +319,9 @@
 	MOVQ rDi, _si(oState); \
 	MOVQ rDo, _so(oState)  \
 
-// func keccakF1600(state *[25]uint64)
+// func keccakF1600(a *[25]uint64)
 TEXT ·keccakF1600(SB), 0, $200-8
-	MOVQ state+0(FP), rpState
+	MOVQ a+0(FP), rpState
 
 	// Convert the user state into an internal state
 	NOTQ _be(rpState)
diff --git a/sha3/register.go b/sha3/register.go
index 8b4453aac3..addfd5049b 100644
--- a/sha3/register.go
+++ b/sha3/register.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build go1.4
-// +build go1.4
 
 package sha3
 
diff --git a/sha3/sha3.go b/sha3/sha3.go
index fa182beb40..4884d172a4 100644
--- a/sha3/sha3.go
+++ b/sha3/sha3.go
@@ -121,11 +121,11 @@ func (d *state) padAndPermute(dsbyte byte) {
 	copyOut(d, d.buf)
 }
 
-// Write absorbs more data into the hash's state. It produces an error
-// if more data is written to the ShakeHash after writing
+// Write absorbs more data into the hash's state. It panics if any
+// output has already been read.
 func (d *state) Write(p []byte) (written int, err error) {
 	if d.state != spongeAbsorbing {
-		panic("sha3: write to sponge after read")
+		panic("sha3: Write after Read")
 	}
 	if d.buf == nil {
 		d.buf = d.storage.asBytes()[:0]
@@ -182,12 +182,16 @@ func (d *state) Read(out []byte) (n int, err error) {
 }
 
 // Sum applies padding to the hash state and then squeezes out the desired
-// number of output bytes.
+// number of output bytes. It panics if any output has already been read.
 func (d *state) Sum(in []byte) []byte {
+	if d.state != spongeAbsorbing {
+		panic("sha3: Sum after Read")
+	}
+
 	// Make a copy of the original hash so that caller can keep writing
 	// and summing.
 	dup := d.clone()
-	hash := make([]byte, dup.outputLen)
+	hash := make([]byte, dup.outputLen, 64) // explicit cap to allow stack allocation
 	dup.Read(hash)
 	return append(in, hash...)
 }
diff --git a/sha3/sha3_s390x.go b/sha3/sha3_s390x.go
index 63a3edb4ce..d861bca528 100644
--- a/sha3/sha3_s390x.go
+++ b/sha3/sha3_s390x.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build gc && !purego
-// +build gc,!purego
 
 package sha3
 
@@ -49,7 +48,7 @@ type asmState struct {
 	buf       []byte          // care must be taken to ensure cap(buf) is a multiple of rate
 	rate      int             // equivalent to block size
 	storage   [3072]byte      // underlying storage for buf
-	outputLen int             // output length if fixed, 0 if not
+	outputLen int             // output length for full security
 	function  code            // KIMD/KLMD function code
 	state     spongeDirection // whether the sponge is absorbing or squeezing
 }
@@ -72,8 +71,10 @@ func newAsmState(function code) *asmState {
 		s.outputLen = 64
 	case shake_128:
 		s.rate = 168
+		s.outputLen = 32
 	case shake_256:
 		s.rate = 136
+		s.outputLen = 64
 	default:
 		panic("sha3: unrecognized function code")
 	}
@@ -108,7 +109,7 @@ func (s *asmState) resetBuf() {
 // It never returns an error.
 func (s *asmState) Write(b []byte) (int, error) {
 	if s.state != spongeAbsorbing {
-		panic("sha3: write to sponge after read")
+		panic("sha3: Write after Read")
 	}
 	length := len(b)
 	for len(b) > 0 {
@@ -192,8 +193,8 @@ func (s *asmState) Read(out []byte) (n int, err error) {
 // Sum appends the current hash to b and returns the resulting slice.
 // It does not change the underlying hash state.
 func (s *asmState) Sum(b []byte) []byte {
-	if s.outputLen == 0 {
-		panic("sha3: cannot call Sum on SHAKE functions")
+	if s.state != spongeAbsorbing {
+		panic("sha3: Sum after Read")
 	}
 
 	// Copy the state to preserve the original.
diff --git a/sha3/sha3_s390x.s b/sha3/sha3_s390x.s
index a0e051b045..826b862c77 100644
--- a/sha3/sha3_s390x.s
+++ b/sha3/sha3_s390x.s
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build gc && !purego
-// +build gc,!purego
 
 #include "textflag.h"
 
diff --git a/sha3/shake.go b/sha3/shake.go
index d7be2954ab..bb69984027 100644
--- a/sha3/shake.go
+++ b/sha3/shake.go
@@ -17,26 +17,25 @@ package sha3
 
 import (
 	"encoding/binary"
+	"hash"
 	"io"
 )
 
-// ShakeHash defines the interface to hash functions that
-// support arbitrary-length output.
+// ShakeHash defines the interface to hash functions that support
+// arbitrary-length output. When used as a plain [hash.Hash], it
+// produces minimum-length outputs that provide full-strength generic
+// security.
 type ShakeHash interface {
-	// Write absorbs more data into the hash's state. It panics if input is
-	// written to it after output has been read from it.
-	io.Writer
+	hash.Hash
 
 	// Read reads more output from the hash; reading affects the hash's
 	// state. (ShakeHash.Read is thus very different from Hash.Sum)
-	// It never returns an error.
+	// It never returns an error, but subsequent calls to Write or Sum
+	// will panic.
 	io.Reader
 
 	// Clone returns a copy of the ShakeHash in its current state.
 	Clone() ShakeHash
-
-	// Reset resets the ShakeHash to its initial state.
-	Reset()
 }
 
 // cSHAKE specific context
@@ -81,8 +80,8 @@ func leftEncode(value uint64) []byte {
 	return b[i-1:]
 }
 
-func newCShake(N, S []byte, rate int, dsbyte byte) ShakeHash {
-	c := cshakeState{state: &state{rate: rate, dsbyte: dsbyte}}
+func newCShake(N, S []byte, rate, outputLen int, dsbyte byte) ShakeHash {
+	c := cshakeState{state: &state{rate: rate, outputLen: outputLen, dsbyte: dsbyte}}
 
 	// leftEncode returns max 9 bytes
 	c.initBlock = make([]byte, 0, 9*2+len(N)+len(S))
@@ -119,7 +118,7 @@ func NewShake128() ShakeHash {
 	if h := newShake128Asm(); h != nil {
 		return h
 	}
-	return &state{rate: rate128, dsbyte: dsbyteShake}
+	return &state{rate: rate128, outputLen: 32, dsbyte: dsbyteShake}
 }
 
 // NewShake256 creates a new SHAKE256 variable-output-length ShakeHash.
@@ -129,7 +128,7 @@ func NewShake256() ShakeHash {
 	if h := newShake256Asm(); h != nil {
 		return h
 	}
-	return &state{rate: rate256, dsbyte: dsbyteShake}
+	return &state{rate: rate256, outputLen: 64, dsbyte: dsbyteShake}
 }
 
 // NewCShake128 creates a new instance of cSHAKE128 variable-output-length ShakeHash,
@@ -142,7 +141,7 @@ func NewCShake128(N, S []byte) ShakeHash {
 	if len(N) == 0 && len(S) == 0 {
 		return NewShake128()
 	}
-	return newCShake(N, S, rate128, dsbyteCShake)
+	return newCShake(N, S, rate128, 32, dsbyteCShake)
 }
 
 // NewCShake256 creates a new instance of cSHAKE256 variable-output-length ShakeHash,
@@ -155,7 +154,7 @@ func NewCShake256(N, S []byte) ShakeHash {
 	if len(N) == 0 && len(S) == 0 {
 		return NewShake256()
 	}
-	return newCShake(N, S, rate256, dsbyteCShake)
+	return newCShake(N, S, rate256, 64, dsbyteCShake)
 }
 
 // ShakeSum128 writes an arbitrary-length digest of data into hash.
diff --git a/sha3/shake_generic.go b/sha3/shake_generic.go
index 5c0710ef98..8d31cf5be2 100644
--- a/sha3/shake_generic.go
+++ b/sha3/shake_generic.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build !gc || purego || !s390x
-// +build !gc purego !s390x
 
 package sha3
 
diff --git a/sha3/xor.go b/sha3/xor.go
index 59c8eb9418..7337cca88e 100644
--- a/sha3/xor.go
+++ b/sha3/xor.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build (!amd64 && !386 && !ppc64le) || purego
-// +build !amd64,!386,!ppc64le purego
 
 package sha3
 
diff --git a/sha3/xor_unaligned.go b/sha3/xor_unaligned.go
index 1ce606246d..870e2d16e0 100644
--- a/sha3/xor_unaligned.go
+++ b/sha3/xor_unaligned.go
@@ -3,8 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build (amd64 || 386 || ppc64le) && !purego
-// +build amd64 386 ppc64le
-// +build !purego
 
 package sha3
 
diff --git a/ssh/agent/client.go b/ssh/agent/client.go
index c3e112a939..fecba8eb38 100644
--- a/ssh/agent/client.go
+++ b/ssh/agent/client.go
@@ -16,6 +16,7 @@ import (
 	"bytes"
 	"crypto/dsa"
 	"crypto/ecdsa"
+	"crypto/ed25519"
 	"crypto/elliptic"
 	"crypto/rsa"
 	"encoding/base64"
@@ -26,7 +27,6 @@ import (
 	"math/big"
 	"sync"
 
-	"golang.org/x/crypto/ed25519"
 	"golang.org/x/crypto/ssh"
 )
 
@@ -141,9 +141,14 @@ const (
 	agentAddSmartcardKeyConstrained = 26
 
 	// 3.7 Key constraint identifiers
-	agentConstrainLifetime  = 1
-	agentConstrainConfirm   = 2
-	agentConstrainExtension = 3
+	agentConstrainLifetime = 1
+	agentConstrainConfirm  = 2
+	// Constraint extension identifier up to version 2 of the protocol. A
+	// backward incompatible change will be required if we want to add support
+	// for SSH_AGENT_CONSTRAIN_MAXSIGN which uses the same ID.
+	agentConstrainExtensionV00 = 3
+	// Constraint extension identifier in version 3 and later of the protocol.
+	agentConstrainExtension = 255
 )
 
 // maxAgentResponseBytes is the maximum agent reply size that is accepted. This
@@ -205,7 +210,7 @@ type constrainLifetimeAgentMsg struct {
 }
 
 type constrainExtensionAgentMsg struct {
-	ExtensionName    string `sshtype:"3"`
+	ExtensionName    string `sshtype:"255|3"`
 	ExtensionDetails []byte
 
 	// Rest is a field used for parsing, not part of message
diff --git a/ssh/agent/client_test.go b/ssh/agent/client_test.go
index 8ffaca7491..fdc8000654 100644
--- a/ssh/agent/client_test.go
+++ b/ssh/agent/client_test.go
@@ -29,6 +29,9 @@ func startOpenSSHAgent(t *testing.T) (client ExtendedAgent, socket string, clean
 		// types supported vary by platform.
 		t.Skip("skipping test due to -short")
 	}
+	if runtime.GOOS == "windows" {
+		t.Skip("skipping on windows, we don't support connecting to the ssh-agent via a named pipe")
+	}
 
 	bin, err := exec.LookPath("ssh-agent")
 	if err != nil {
@@ -366,7 +369,8 @@ func TestAuth(t *testing.T) {
 	go func() {
 		conn, _, _, err := ssh.NewServerConn(a, &serverConf)
 		if err != nil {
-			t.Fatalf("Server: %v", err)
+			t.Errorf("NewServerConn error: %v", err)
+			return
 		}
 		conn.Close()
 	}()
diff --git a/ssh/agent/server.go b/ssh/agent/server.go
index 9a769de03d..e35ca7ce31 100644
--- a/ssh/agent/server.go
+++ b/ssh/agent/server.go
@@ -7,6 +7,7 @@ package agent
 import (
 	"crypto/dsa"
 	"crypto/ecdsa"
+	"crypto/ed25519"
 	"crypto/elliptic"
 	"crypto/rsa"
 	"encoding/binary"
@@ -16,7 +17,6 @@ import (
 	"log"
 	"math/big"
 
-	"golang.org/x/crypto/ed25519"
 	"golang.org/x/crypto/ssh"
 )
 
@@ -208,7 +208,7 @@ func parseConstraints(constraints []byte) (lifetimeSecs uint32, confirmBeforeUse
 		case agentConstrainConfirm:
 			confirmBeforeUse = true
 			constraints = constraints[1:]
-		case agentConstrainExtension:
+		case agentConstrainExtension, agentConstrainExtensionV00:
 			var msg constrainExtensionAgentMsg
 			if err = ssh.Unmarshal(constraints, &msg); err != nil {
 				return 0, false, nil, err
diff --git a/ssh/agent/server_test.go b/ssh/agent/server_test.go
index 038018ebb1..7700d18f1a 100644
--- a/ssh/agent/server_test.go
+++ b/ssh/agent/server_test.go
@@ -53,10 +53,11 @@ func TestSetupForwardAgent(t *testing.T) {
 	incoming := make(chan *ssh.ServerConn, 1)
 	go func() {
 		conn, _, _, err := ssh.NewServerConn(a, &serverConf)
+		incoming <- conn
 		if err != nil {
-			t.Fatalf("Server: %v", err)
+			t.Errorf("NewServerConn error: %v", err)
+			return
 		}
-		incoming <- conn
 	}()
 
 	conf := ssh.ClientConfig{
@@ -71,8 +72,10 @@ func TestSetupForwardAgent(t *testing.T) {
 	if err := ForwardToRemote(client, socket); err != nil {
 		t.Fatalf("SetupForwardAgent: %v", err)
 	}
-
 	server := <-incoming
+	if server == nil {
+		t.Fatal("Unable to get server")
+	}
 	ch, reqs, err := server.OpenChannel(channelType, nil)
 	if err != nil {
 		t.Fatalf("OpenChannel(%q): %v", channelType, err)
@@ -240,7 +243,11 @@ func TestParseConstraints(t *testing.T) {
 			ExtensionDetails: []byte(fmt.Sprintf("details: %d", i)),
 		}
 		expect = append(expect, ext)
-		data = append(data, agentConstrainExtension)
+		if i%2 == 0 {
+			data = append(data, agentConstrainExtension)
+		} else {
+			data = append(data, agentConstrainExtensionV00)
+		}
 		data = append(data, ssh.Marshal(ext)...)
 	}
 	_, _, extensions, err := parseConstraints(data)
diff --git a/ssh/benchmark_test.go b/ssh/benchmark_test.go
index a13235d743..b356330b46 100644
--- a/ssh/benchmark_test.go
+++ b/ssh/benchmark_test.go
@@ -6,6 +6,7 @@ package ssh
 
 import (
 	"errors"
+	"fmt"
 	"io"
 	"net"
 	"testing"
@@ -90,16 +91,16 @@ func BenchmarkEndToEnd(b *testing.B) {
 	go func() {
 		newCh, err := server.Accept()
 		if err != nil {
-			b.Fatalf("Client: %v", err)
+			panic(fmt.Sprintf("Client: %v", err))
 		}
 		ch, incoming, err := newCh.Accept()
 		if err != nil {
-			b.Fatalf("Accept: %v", err)
+			panic(fmt.Sprintf("Accept: %v", err))
 		}
 		go DiscardRequests(incoming)
 		for i := 0; i < b.N; i++ {
 			if _, err := io.ReadFull(ch, output); err != nil {
-				b.Fatalf("ReadFull: %v", err)
+				panic(fmt.Sprintf("ReadFull: %v", err))
 			}
 		}
 		ch.Close()
diff --git a/ssh/certs.go b/ssh/certs.go
index fc04d03e19..27d0e14aa9 100644
--- a/ssh/certs.go
+++ b/ssh/certs.go
@@ -16,8 +16,9 @@ import (
 
 // Certificate algorithm names from [PROTOCOL.certkeys]. These values can appear
 // in Certificate.Type, PublicKey.Type, and ClientConfig.HostKeyAlgorithms.
-// Unlike key algorithm names, these are not passed to AlgorithmSigner and don't
-// appear in the Signature.Format field.
+// Unlike key algorithm names, these are not passed to AlgorithmSigner nor
+// returned by MultiAlgorithmSigner and don't appear in the Signature.Format
+// field.
 const (
 	CertAlgoRSAv01        = "ssh-rsa-cert-v01@openssh.com"
 	CertAlgoDSAv01        = "ssh-dss-cert-v01@openssh.com"
@@ -255,10 +256,17 @@ func NewCertSigner(cert *Certificate, signer Signer) (Signer, error) {
 		return nil, errors.New("ssh: signer and cert have different public key")
 	}
 
-	if algorithmSigner, ok := signer.(AlgorithmSigner); ok {
+	switch s := signer.(type) {
+	case MultiAlgorithmSigner:
+		return &multiAlgorithmSigner{
+			AlgorithmSigner: &algorithmOpenSSHCertSigner{
+				&openSSHCertSigner{cert, signer}, s},
+			supportedAlgorithms: s.Algorithms(),
+		}, nil
+	case AlgorithmSigner:
 		return &algorithmOpenSSHCertSigner{
-			&openSSHCertSigner{cert, signer}, algorithmSigner}, nil
-	} else {
+			&openSSHCertSigner{cert, signer}, s}, nil
+	default:
 		return &openSSHCertSigner{cert, signer}, nil
 	}
 }
@@ -432,7 +440,9 @@ func (c *CertChecker) CheckCert(principal string, cert *Certificate) error {
 }
 
 // SignCert signs the certificate with an authority, setting the Nonce,
-// SignatureKey, and Signature fields.
+// SignatureKey, and Signature fields. If the authority implements the
+// MultiAlgorithmSigner interface the first algorithm in the list is used. This
+// is useful if you want to sign with a specific algorithm.
 func (c *Certificate) SignCert(rand io.Reader, authority Signer) error {
 	c.Nonce = make([]byte, 32)
 	if _, err := io.ReadFull(rand, c.Nonce); err != nil {
@@ -440,8 +450,20 @@ func (c *Certificate) SignCert(rand io.Reader, authority Signer) error {
 	}
 	c.SignatureKey = authority.PublicKey()
 
-	// Default to KeyAlgoRSASHA512 for ssh-rsa signers.
-	if v, ok := authority.(AlgorithmSigner); ok && v.PublicKey().Type() == KeyAlgoRSA {
+	if v, ok := authority.(MultiAlgorithmSigner); ok {
+		if len(v.Algorithms()) == 0 {
+			return errors.New("the provided authority has no signature algorithm")
+		}
+		// Use the first algorithm in the list.
+		sig, err := v.SignWithAlgorithm(rand, c.bytesForSigning(), v.Algorithms()[0])
+		if err != nil {
+			return err
+		}
+		c.Signature = sig
+		return nil
+	} else if v, ok := authority.(AlgorithmSigner); ok && v.PublicKey().Type() == KeyAlgoRSA {
+		// Default to KeyAlgoRSASHA512 for ssh-rsa signers.
+		// TODO: consider using KeyAlgoRSASHA256 as default.
 		sig, err := v.SignWithAlgorithm(rand, c.bytesForSigning(), KeyAlgoRSASHA512)
 		if err != nil {
 			return err
diff --git a/ssh/certs_test.go b/ssh/certs_test.go
index e6004835ea..97b7486d0c 100644
--- a/ssh/certs_test.go
+++ b/ssh/certs_test.go
@@ -187,10 +187,30 @@ func TestHostKeyCert(t *testing.T) {
 	}
 
 	for _, test := range []struct {
-		addr    string
-		succeed bool
+		addr                    string
+		succeed                 bool
+		certSignerAlgorithms    []string // Empty means no algorithm restrictions.
+		clientHostKeyAlgorithms []string
 	}{
 		{addr: "hostname:22", succeed: true},
+		{
+			addr:                    "hostname:22",
+			succeed:                 true,
+			certSignerAlgorithms:    []string{KeyAlgoRSASHA256, KeyAlgoRSASHA512},
+			clientHostKeyAlgorithms: []string{CertAlgoRSASHA512v01},
+		},
+		{
+			addr:                    "hostname:22",
+			succeed:                 false,
+			certSignerAlgorithms:    []string{KeyAlgoRSASHA256, KeyAlgoRSASHA512},
+			clientHostKeyAlgorithms: []string{CertAlgoRSAv01},
+		},
+		{
+			addr:                    "hostname:22",
+			succeed:                 false,
+			certSignerAlgorithms:    []string{KeyAlgoRSASHA256, KeyAlgoRSASHA512},
+			clientHostKeyAlgorithms: []string{KeyAlgoRSASHA512}, // Not a certificate algorithm.
+		},
 		{addr: "otherhost:22", succeed: false}, // The certificate is valid for 'otherhost' as hostname, but we only recognize the authority of the signer for the address 'hostname:22'
 		{addr: "lasthost:22", succeed: false},
 	} {
@@ -207,14 +227,24 @@ func TestHostKeyCert(t *testing.T) {
 			conf := ServerConfig{
 				NoClientAuth: true,
 			}
-			conf.AddHostKey(certSigner)
+			if len(test.certSignerAlgorithms) > 0 {
+				mas, err := NewSignerWithAlgorithms(certSigner.(AlgorithmSigner), test.certSignerAlgorithms)
+				if err != nil {
+					errc <- err
+					return
+				}
+				conf.AddHostKey(mas)
+			} else {
+				conf.AddHostKey(certSigner)
+			}
 			_, _, _, err := NewServerConn(c1, &conf)
 			errc <- err
 		}()
 
 		config := &ClientConfig{
-			User:            "user",
-			HostKeyCallback: checker.CheckHostKey,
+			User:              "user",
+			HostKeyCallback:   checker.CheckHostKey,
+			HostKeyAlgorithms: test.clientHostKeyAlgorithms,
 		}
 		_, _, _, err = NewClientConn(c2, test.addr, config)
 
@@ -242,6 +272,20 @@ func (s *legacyRSASigner) Sign(rand io.Reader, data []byte) (*Signature, error)
 }
 
 func TestCertTypes(t *testing.T) {
+	algorithmSigner, ok := testSigners["rsa"].(AlgorithmSigner)
+	if !ok {
+		t.Fatal("rsa test signer does not implement the AlgorithmSigner interface")
+	}
+	multiAlgoSignerSHA256, err := NewSignerWithAlgorithms(algorithmSigner, []string{KeyAlgoRSASHA256})
+	if err != nil {
+		t.Fatalf("unable to create multi algorithm signer SHA256: %v", err)
+	}
+	// Algorithms are in order of preference, we expect rsa-sha2-512 to be used.
+	multiAlgoSignerSHA512, err := NewSignerWithAlgorithms(algorithmSigner, []string{KeyAlgoRSASHA512, KeyAlgoRSASHA256})
+	if err != nil {
+		t.Fatalf("unable to create multi algorithm signer SHA512: %v", err)
+	}
+
 	var testVars = []struct {
 		name   string
 		signer Signer
@@ -251,8 +295,10 @@ func TestCertTypes(t *testing.T) {
 		{CertAlgoECDSA384v01, testSigners["ecdsap384"], ""},
 		{CertAlgoECDSA521v01, testSigners["ecdsap521"], ""},
 		{CertAlgoED25519v01, testSigners["ed25519"], ""},
-		{CertAlgoRSAv01, testSigners["rsa"], KeyAlgoRSASHA512},
+		{CertAlgoRSAv01, testSigners["rsa"], KeyAlgoRSASHA256},
 		{"legacyRSASigner", &legacyRSASigner{testSigners["rsa"]}, KeyAlgoRSA},
+		{"multiAlgoRSASignerSHA256", multiAlgoSignerSHA256, KeyAlgoRSASHA256},
+		{"multiAlgoRSASignerSHA512", multiAlgoSignerSHA512, KeyAlgoRSASHA512},
 		{CertAlgoDSAv01, testSigners["dsa"], ""},
 	}
 
@@ -318,3 +364,45 @@ func TestCertTypes(t *testing.T) {
 		})
 	}
 }
+
+func TestCertSignWithMultiAlgorithmSigner(t *testing.T) {
+	type testcase struct {
+		sigAlgo   string
+		algoritms []string
+	}
+	cases := []testcase{
+		{
+			sigAlgo:   KeyAlgoRSA,
+			algoritms: []string{KeyAlgoRSA, KeyAlgoRSASHA512},
+		},
+		{
+			sigAlgo:   KeyAlgoRSASHA256,
+			algoritms: []string{KeyAlgoRSASHA256, KeyAlgoRSA, KeyAlgoRSASHA512},
+		},
+		{
+			sigAlgo:   KeyAlgoRSASHA512,
+			algoritms: []string{KeyAlgoRSASHA512, KeyAlgoRSASHA256},
+		},
+	}
+
+	cert := &Certificate{
+		Key:         testPublicKeys["rsa"],
+		ValidBefore: CertTimeInfinity,
+		CertType:    UserCert,
+	}
+
+	for _, c := range cases {
+		t.Run(c.sigAlgo, func(t *testing.T) {
+			signer, err := NewSignerWithAlgorithms(testSigners["rsa"].(AlgorithmSigner), c.algoritms)
+			if err != nil {
+				t.Fatalf("NewSignerWithAlgorithms error: %v", err)
+			}
+			if err := cert.SignCert(rand.Reader, signer); err != nil {
+				t.Fatalf("SignCert error: %v", err)
+			}
+			if cert.Signature.Format != c.sigAlgo {
+				t.Fatalf("got signature format %q, want %q", cert.Signature.Format, c.sigAlgo)
+			}
+		})
+	}
+}
diff --git a/ssh/channel.go b/ssh/channel.go
index c0834c00df..cc0bb7ab64 100644
--- a/ssh/channel.go
+++ b/ssh/channel.go
@@ -187,9 +187,11 @@ type channel struct {
 	pending    *buffer
 	extPending *buffer
 
-	// windowMu protects myWindow, the flow-control window.
-	windowMu sync.Mutex
-	myWindow uint32
+	// windowMu protects myWindow, the flow-control window, and myConsumed,
+	// the number of bytes consumed since we last increased myWindow
+	windowMu   sync.Mutex
+	myWindow   uint32
+	myConsumed uint32
 
 	// writeMu serializes calls to mux.conn.writePacket() and
 	// protects sentClose and packetPool. This mutex must be
@@ -332,14 +334,24 @@ func (ch *channel) handleData(packet []byte) error {
 	return nil
 }
 
-func (c *channel) adjustWindow(n uint32) error {
+func (c *channel) adjustWindow(adj uint32) error {
 	c.windowMu.Lock()
-	// Since myWindow is managed on our side, and can never exceed
-	// the initial window setting, we don't worry about overflow.
-	c.myWindow += uint32(n)
+	// Since myConsumed and myWindow are managed on our side, and can never
+	// exceed the initial window setting, we don't worry about overflow.
+	c.myConsumed += adj
+	var sendAdj uint32
+	if (channelWindowSize-c.myWindow > 3*c.maxIncomingPayload) ||
+		(c.myWindow < channelWindowSize/2) {
+		sendAdj = c.myConsumed
+		c.myConsumed = 0
+		c.myWindow += sendAdj
+	}
 	c.windowMu.Unlock()
+	if sendAdj == 0 {
+		return nil
+	}
 	return c.sendMessage(windowAdjustMsg{
-		AdditionalBytes: uint32(n),
+		AdditionalBytes: sendAdj,
 	})
 }
 
diff --git a/ssh/cipher.go b/ssh/cipher.go
index 951812ff9b..0da67d595e 100644
--- a/ssh/cipher.go
+++ b/ssh/cipher.go
@@ -16,8 +16,8 @@ import (
 	"hash"
 	"io"
 
-	"github.com/tailscale/golang-x-crypto/chacha20"
-	"golang.org/x/crypto/poly1305"
+	"github.com/tailscale/golang-x-crypto/internal/poly1305"
+	"golang.org/x/crypto/chacha20"
 )
 
 const (
diff --git a/ssh/client.go b/ssh/client.go
index bdc356cbdf..fd8c49749e 100644
--- a/ssh/client.go
+++ b/ssh/client.go
@@ -82,7 +82,7 @@ func NewClientConn(c net.Conn, addr string, config *ClientConfig) (Conn, <-chan
 
 	if err := conn.clientHandshake(addr, &fullConf); err != nil {
 		c.Close()
-		return nil, nil, nil, fmt.Errorf("ssh: handshake failed: %v", err)
+		return nil, nil, nil, fmt.Errorf("ssh: handshake failed: %w", err)
 	}
 	conn.mux = newMux(conn.transport)
 	return conn, conn.mux.incomingChannels, conn.mux.incomingRequests, nil
diff --git a/ssh/client_auth.go b/ssh/client_auth.go
index 409b5ea1d4..34bf089d0b 100644
--- a/ssh/client_auth.go
+++ b/ssh/client_auth.go
@@ -71,7 +71,9 @@ func (c *connection) clientAuthenticate(config *ClientConfig) error {
 	for auth := AuthMethod(new(noneAuth)); auth != nil; {
 		ok, methods, err := auth.auth(sessionID, config.User, c.transport, config.Rand, extensions)
 		if err != nil {
-			return err
+			// We return the error later if there is no other method left to
+			// try.
+			ok = authFailure
 		}
 		if ok == authSuccess {
 			// success
@@ -101,6 +103,12 @@ func (c *connection) clientAuthenticate(config *ClientConfig) error {
 				}
 			}
 		}
+
+		if auth == nil && err != nil {
+			// We have an error and there are no other authentication methods to
+			// try, so we return it.
+			return err
+		}
 	}
 	return fmt.Errorf("ssh: unable to authenticate, attempted methods %v, no supported methods remain", tried)
 }
@@ -217,21 +225,45 @@ func (cb publicKeyCallback) method() string {
 	return "publickey"
 }
 
-func pickSignatureAlgorithm(signer Signer, extensions map[string][]byte) (as AlgorithmSigner, algo string) {
+func pickSignatureAlgorithm(signer Signer, extensions map[string][]byte) (MultiAlgorithmSigner, string, error) {
+	var as MultiAlgorithmSigner
 	keyFormat := signer.PublicKey().Type()
 
-	// Like in sendKexInit, if the public key implements AlgorithmSigner we
-	// assume it supports all algorithms, otherwise only the key format one.
-	as, ok := signer.(AlgorithmSigner)
-	if !ok {
-		return algorithmSignerWrapper{signer}, keyFormat
+	// If the signer implements MultiAlgorithmSigner we use the algorithms it
+	// support, if it implements AlgorithmSigner we assume it supports all
+	// algorithms, otherwise only the key format one.
+	switch s := signer.(type) {
+	case MultiAlgorithmSigner:
+		as = s
+	case AlgorithmSigner:
+		as = &multiAlgorithmSigner{
+			AlgorithmSigner:     s,
+			supportedAlgorithms: algorithmsForKeyFormat(underlyingAlgo(keyFormat)),
+		}
+	default:
+		as = &multiAlgorithmSigner{
+			AlgorithmSigner:     algorithmSignerWrapper{signer},
+			supportedAlgorithms: []string{underlyingAlgo(keyFormat)},
+		}
+	}
+
+	getFallbackAlgo := func() (string, error) {
+		// Fallback to use if there is no "server-sig-algs" extension or a
+		// common algorithm cannot be found. We use the public key format if the
+		// MultiAlgorithmSigner supports it, otherwise we return an error.
+		if !contains(as.Algorithms(), underlyingAlgo(keyFormat)) {
+			return "", fmt.Errorf("ssh: no common public key signature algorithm, server only supports %q for key type %q, signer only supports %v",
+				underlyingAlgo(keyFormat), keyFormat, as.Algorithms())
+		}
+		return keyFormat, nil
 	}
 
 	extPayload, ok := extensions["server-sig-algs"]
 	if !ok {
-		// If there is no "server-sig-algs" extension, fall back to the key
-		// format algorithm.
-		return as, keyFormat
+		// If there is no "server-sig-algs" extension use the fallback
+		// algorithm.
+		algo, err := getFallbackAlgo()
+		return as, algo, err
 	}
 
 	// The server-sig-algs extension only carries underlying signature
@@ -245,15 +277,22 @@ func pickSignatureAlgorithm(signer Signer, extensions map[string][]byte) (as Alg
 		}
 	}
 
-	keyAlgos := algorithmsForKeyFormat(keyFormat)
+	// Filter algorithms based on those supported by MultiAlgorithmSigner.
+	var keyAlgos []string
+	for _, algo := range algorithmsForKeyFormat(keyFormat) {
+		if contains(as.Algorithms(), underlyingAlgo(algo)) {
+			keyAlgos = append(keyAlgos, algo)
+		}
+	}
+
 	algo, err := findCommon("public key signature algorithm", keyAlgos, serverAlgos)
 	if err != nil {
-		// If there is no overlap, try the key anyway with the key format
-		// algorithm, to support servers that fail to list all supported
-		// algorithms.
-		return as, keyFormat
+		// If there is no overlap, return the fallback algorithm to support
+		// servers that fail to list all supported algorithms.
+		algo, err := getFallbackAlgo()
+		return as, algo, err
 	}
-	return as, algo
+	return as, algo, nil
 }
 
 func (cb publicKeyCallback) auth(session []byte, user string, c packetConn, rand io.Reader, extensions map[string][]byte) (authResult, []string, error) {
@@ -267,14 +306,39 @@ func (cb publicKeyCallback) auth(session []byte, user string, c packetConn, rand
 		return authFailure, nil, err
 	}
 	var methods []string
-	for _, signer := range signers {
-		pub := signer.PublicKey()
-		as, algo := pickSignatureAlgorithm(signer, extensions)
+	var errSigAlgo error
 
+	origSignersLen := len(signers)
+	for idx := 0; idx < len(signers); idx++ {
+		signer := signers[idx]
+		pub := signer.PublicKey()
+		as, algo, err := pickSignatureAlgorithm(signer, extensions)
+		if err != nil && errSigAlgo == nil {
+			// If we cannot negotiate a signature algorithm store the first
+			// error so we can return it to provide a more meaningful message if
+			// no other signers work.
+			errSigAlgo = err
+			continue
+		}
 		ok, err := validateKey(pub, algo, user, c)
 		if err != nil {
 			return authFailure, nil, err
 		}
+		// OpenSSH 7.2-7.7 advertises support for rsa-sha2-256 and rsa-sha2-512
+		// in the "server-sig-algs" extension but doesn't support these
+		// algorithms for certificate authentication, so if the server rejects
+		// the key try to use the obtained algorithm as if "server-sig-algs" had
+		// not been implemented if supported from the algorithm signer.
+		if !ok && idx < origSignersLen && isRSACert(algo) && algo != CertAlgoRSAv01 {
+			if contains(as.Algorithms(), KeyAlgoRSA) {
+				// We retry using the compat algorithm after all signers have
+				// been tried normally.
+				signers = append(signers, &multiAlgorithmSigner{
+					AlgorithmSigner:     as,
+					supportedAlgorithms: []string{KeyAlgoRSA},
+				})
+			}
+		}
 		if !ok {
 			continue
 		}
@@ -317,22 +381,12 @@ func (cb publicKeyCallback) auth(session []byte, user string, c packetConn, rand
 		// contain the "publickey" method, do not attempt to authenticate with any
 		// other keys.  According to RFC 4252 Section 7, the latter can occur when
 		// additional authentication methods are required.
-		if success == authSuccess || !containsMethod(methods, cb.method()) {
+		if success == authSuccess || !contains(methods, cb.method()) {
 			return success, methods, err
 		}
 	}
 
-	return authFailure, methods, nil
-}
-
-func containsMethod(methods []string, method string) bool {
-	for _, m := range methods {
-		if m == method {
-			return true
-		}
-	}
-
-	return false
+	return authFailure, methods, errSigAlgo
 }
 
 // validateKey validates the key provided is acceptable to the server.
diff --git a/ssh/client_auth_test.go b/ssh/client_auth_test.go
index 70558a9a77..bf0aa1fe23 100644
--- a/ssh/client_auth_test.go
+++ b/ssh/client_auth_test.go
@@ -1076,3 +1076,209 @@ func TestCompatibleAlgoAndSignatures(t *testing.T) {
 		}
 	}
 }
+
+func TestPickSignatureAlgorithm(t *testing.T) {
+	type testcase struct {
+		name       string
+		extensions map[string][]byte
+	}
+	cases := []testcase{
+		{
+			name: "server with empty server-sig-algs",
+			extensions: map[string][]byte{
+				"server-sig-algs": []byte(``),
+			},
+		},
+		{
+			name:       "server with no server-sig-algs",
+			extensions: nil,
+		},
+	}
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			signer, ok := testSigners["rsa"].(MultiAlgorithmSigner)
+			if !ok {
+				t.Fatalf("rsa test signer does not implement the MultiAlgorithmSigner interface")
+			}
+			// The signer supports the public key algorithm which is then returned.
+			_, algo, err := pickSignatureAlgorithm(signer, c.extensions)
+			if err != nil {
+				t.Fatalf("got %v, want no error", err)
+			}
+			if algo != signer.PublicKey().Type() {
+				t.Fatalf("got algo %q, want %q", algo, signer.PublicKey().Type())
+			}
+			// Test a signer that uses a certificate algorithm as the public key
+			// type.
+			cert := &Certificate{
+				CertType: UserCert,
+				Key:      signer.PublicKey(),
+			}
+			cert.SignCert(rand.Reader, signer)
+
+			certSigner, err := NewCertSigner(cert, signer)
+			if err != nil {
+				t.Fatalf("error generating cert signer: %v", err)
+			}
+			// The signer supports the public key algorithm and the
+			// public key format is a certificate type so the cerificate
+			// algorithm matching the key format must be returned
+			_, algo, err = pickSignatureAlgorithm(certSigner, c.extensions)
+			if err != nil {
+				t.Fatalf("got %v, want no error", err)
+			}
+			if algo != certSigner.PublicKey().Type() {
+				t.Fatalf("got algo %q, want %q", algo, certSigner.PublicKey().Type())
+			}
+			signer, err = NewSignerWithAlgorithms(signer.(AlgorithmSigner), []string{KeyAlgoRSASHA512, KeyAlgoRSASHA256})
+			if err != nil {
+				t.Fatalf("unable to create signer with algorithms: %v", err)
+			}
+			// The signer does not support the public key algorithm so an error
+			// is returned.
+			_, _, err = pickSignatureAlgorithm(signer, c.extensions)
+			if err == nil {
+				t.Fatal("got no error, no common public key signature algorithm error expected")
+			}
+		})
+	}
+}
+
+// configurablePublicKeyCallback is a public key callback that allows to
+// configure the signature algorithm and format. This way we can emulate the
+// behavior of buggy clients.
+type configurablePublicKeyCallback struct {
+	signer          AlgorithmSigner
+	signatureAlgo   string
+	signatureFormat string
+}
+
+func (cb configurablePublicKeyCallback) method() string {
+	return "publickey"
+}
+
+func (cb configurablePublicKeyCallback) auth(session []byte, user string, c packetConn, rand io.Reader, extensions map[string][]byte) (authResult, []string, error) {
+	pub := cb.signer.PublicKey()
+
+	ok, err := validateKey(pub, cb.signatureAlgo, user, c)
+	if err != nil {
+		return authFailure, nil, err
+	}
+	if !ok {
+		return authFailure, nil, fmt.Errorf("invalid public key")
+	}
+
+	pubKey := pub.Marshal()
+	data := buildDataSignedForAuth(session, userAuthRequestMsg{
+		User:    user,
+		Service: serviceSSH,
+		Method:  cb.method(),
+	}, cb.signatureAlgo, pubKey)
+	sign, err := cb.signer.SignWithAlgorithm(rand, data, underlyingAlgo(cb.signatureFormat))
+	if err != nil {
+		return authFailure, nil, err
+	}
+
+	s := Marshal(sign)
+	sig := make([]byte, stringLength(len(s)))
+	marshalString(sig, s)
+	msg := publickeyAuthMsg{
+		User:     user,
+		Service:  serviceSSH,
+		Method:   cb.method(),
+		HasSig:   true,
+		Algoname: cb.signatureAlgo,
+		PubKey:   pubKey,
+		Sig:      sig,
+	}
+	p := Marshal(&msg)
+	if err := c.writePacket(p); err != nil {
+		return authFailure, nil, err
+	}
+	var success authResult
+	success, methods, err := handleAuthResponse(c)
+	if err != nil {
+		return authFailure, nil, err
+	}
+	if success == authSuccess || !contains(methods, cb.method()) {
+		return success, methods, err
+	}
+
+	return authFailure, methods, nil
+}
+
+func TestPublicKeyAndAlgoCompatibility(t *testing.T) {
+	cert := &Certificate{
+		Key:         testPublicKeys["rsa"],
+		ValidBefore: CertTimeInfinity,
+		CertType:    UserCert,
+	}
+	cert.SignCert(rand.Reader, testSigners["ecdsa"])
+	certSigner, err := NewCertSigner(cert, testSigners["rsa"])
+	if err != nil {
+		t.Fatalf("NewCertSigner: %v", err)
+	}
+
+	clientConfig := &ClientConfig{
+		User:            "user",
+		HostKeyCallback: InsecureIgnoreHostKey(),
+		Auth: []AuthMethod{
+			configurablePublicKeyCallback{
+				signer:          certSigner.(AlgorithmSigner),
+				signatureAlgo:   KeyAlgoRSASHA256,
+				signatureFormat: KeyAlgoRSASHA256,
+			},
+		},
+	}
+	if err := tryAuth(t, clientConfig); err == nil {
+		t.Error("cert login passed with incompatible public key type and algorithm")
+	}
+}
+
+func TestClientAuthGPGAgentCompat(t *testing.T) {
+	clientConfig := &ClientConfig{
+		User:            "testuser",
+		HostKeyCallback: InsecureIgnoreHostKey(),
+		Auth: []AuthMethod{
+			// algorithm rsa-sha2-512 and signature format ssh-rsa.
+			configurablePublicKeyCallback{
+				signer:          testSigners["rsa"].(AlgorithmSigner),
+				signatureAlgo:   KeyAlgoRSASHA512,
+				signatureFormat: KeyAlgoRSA,
+			},
+		},
+	}
+	if err := tryAuth(t, clientConfig); err != nil {
+		t.Fatalf("unable to dial remote side: %s", err)
+	}
+}
+
+func TestCertAuthOpenSSHCompat(t *testing.T) {
+	cert := &Certificate{
+		Key:         testPublicKeys["rsa"],
+		ValidBefore: CertTimeInfinity,
+		CertType:    UserCert,
+	}
+	cert.SignCert(rand.Reader, testSigners["ecdsa"])
+	certSigner, err := NewCertSigner(cert, testSigners["rsa"])
+	if err != nil {
+		t.Fatalf("NewCertSigner: %v", err)
+	}
+
+	clientConfig := &ClientConfig{
+		User:            "user",
+		HostKeyCallback: InsecureIgnoreHostKey(),
+		Auth: []AuthMethod{
+			// algorithm ssh-rsa-cert-v01@openssh.com and signature format
+			// rsa-sha2-256.
+			configurablePublicKeyCallback{
+				signer:          certSigner.(AlgorithmSigner),
+				signatureAlgo:   CertAlgoRSAv01,
+				signatureFormat: KeyAlgoRSASHA256,
+			},
+		},
+	}
+	if err := tryAuth(t, clientConfig); err != nil {
+		t.Fatalf("unable to dial remote side: %s", err)
+	}
+}
diff --git a/ssh/client_test.go b/ssh/client_test.go
index 281475596c..2621f0ea52 100644
--- a/ssh/client_test.go
+++ b/ssh/client_test.go
@@ -7,6 +7,9 @@ package ssh
 import (
 	"bytes"
 	"crypto/rand"
+	"errors"
+	"fmt"
+	"net"
 	"strings"
 	"testing"
 )
@@ -207,9 +210,12 @@ func TestBannerCallback(t *testing.T) {
 }
 
 func TestNewClientConn(t *testing.T) {
+	errHostKeyMismatch := errors.New("host key mismatch")
+
 	for _, tt := range []struct {
-		name string
-		user string
+		name                    string
+		user                    string
+		simulateHostKeyMismatch HostKeyCallback
 	}{
 		{
 			name: "good user field for ConnMetadata",
@@ -219,6 +225,13 @@ func TestNewClientConn(t *testing.T) {
 			name: "empty user field for ConnMetadata",
 			user: "",
 		},
+		{
+			name: "host key mismatch",
+			user: "testuser",
+			simulateHostKeyMismatch: func(hostname string, remote net.Addr, key PublicKey) error {
+				return fmt.Errorf("%w: %s", errHostKeyMismatch, bytes.TrimSpace(MarshalAuthorizedKey(key)))
+			},
+		},
 	} {
 		t.Run(tt.name, func(t *testing.T) {
 			c1, c2, err := netPipe()
@@ -243,8 +256,16 @@ func TestNewClientConn(t *testing.T) {
 				},
 				HostKeyCallback: InsecureIgnoreHostKey(),
 			}
+
+			if tt.simulateHostKeyMismatch != nil {
+				clientConf.HostKeyCallback = tt.simulateHostKeyMismatch
+			}
+
 			clientConn, _, _, err := NewClientConn(c2, "", clientConf)
 			if err != nil {
+				if tt.simulateHostKeyMismatch != nil && errors.Is(err, errHostKeyMismatch) {
+					return
+				}
 				t.Fatal(err)
 			}
 
@@ -254,3 +275,93 @@ func TestNewClientConn(t *testing.T) {
 		})
 	}
 }
+
+func TestUnsupportedAlgorithm(t *testing.T) {
+	for _, tt := range []struct {
+		name      string
+		config    Config
+		wantError string
+	}{
+		{
+			"unsupported KEX",
+			Config{
+				KeyExchanges: []string{"unsupported"},
+			},
+			"no common algorithm",
+		},
+		{
+			"unsupported and supported KEXs",
+			Config{
+				KeyExchanges: []string{"unsupported", kexAlgoCurve25519SHA256},
+			},
+			"",
+		},
+		{
+			"unsupported cipher",
+			Config{
+				Ciphers: []string{"unsupported"},
+			},
+			"no common algorithm",
+		},
+		{
+			"unsupported and supported ciphers",
+			Config{
+				Ciphers: []string{"unsupported", chacha20Poly1305ID},
+			},
+			"",
+		},
+		{
+			"unsupported MAC",
+			Config{
+				MACs: []string{"unsupported"},
+				// MAC is used for non AAED ciphers.
+				Ciphers: []string{"aes256-ctr"},
+			},
+			"no common algorithm",
+		},
+		{
+			"unsupported and supported MACs",
+			Config{
+				MACs: []string{"unsupported", "hmac-sha2-256-etm@openssh.com"},
+				// MAC is used for non AAED ciphers.
+				Ciphers: []string{"aes256-ctr"},
+			},
+			"",
+		},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			c1, c2, err := netPipe()
+			if err != nil {
+				t.Fatalf("netPipe: %v", err)
+			}
+			defer c1.Close()
+			defer c2.Close()
+
+			serverConf := &ServerConfig{
+				Config: tt.config,
+				PasswordCallback: func(conn ConnMetadata, password []byte) (*Permissions, error) {
+					return &Permissions{}, nil
+				},
+			}
+			serverConf.AddHostKey(testSigners["rsa"])
+			go NewServerConn(c1, serverConf)
+
+			clientConf := &ClientConfig{
+				User:   "testuser",
+				Config: tt.config,
+				Auth: []AuthMethod{
+					Password("testpw"),
+				},
+				HostKeyCallback: InsecureIgnoreHostKey(),
+			}
+			_, _, _, err = NewClientConn(c2, "", clientConf)
+			if err != nil {
+				if tt.wantError == "" || !strings.Contains(err.Error(), tt.wantError) {
+					t.Errorf("%s: got error %q, missing %q", tt.name, err.Error(), tt.wantError)
+				}
+			} else if tt.wantError != "" {
+				t.Errorf("%s: succeeded, but want error string %q", tt.name, tt.wantError)
+			}
+		})
+	}
+}
diff --git a/ssh/common.go b/ssh/common.go
index 5ce452bae8..7e9c2cbc64 100644
--- a/ssh/common.go
+++ b/ssh/common.go
@@ -10,7 +10,6 @@ import (
 	"fmt"
 	"io"
 	"math"
-	"strings"
 	"sync"
 
 	_ "crypto/sha1"
@@ -49,7 +48,8 @@ var supportedKexAlgos = []string{
 	// P384 and P521 are not constant-time yet, but since we don't
 	// reuse ephemeral keys, using them for ECDH should be OK.
 	kexAlgoECDH256, kexAlgoECDH384, kexAlgoECDH521,
-	kexAlgoDH14SHA256, kexAlgoDH14SHA1, kexAlgoDH1SHA1,
+	kexAlgoDH14SHA256, kexAlgoDH16SHA512, kexAlgoDH14SHA1,
+	kexAlgoDH1SHA1,
 }
 
 // serverForbiddenKexAlgos contains key exchange algorithms, that are forbidden
@@ -59,8 +59,9 @@ var serverForbiddenKexAlgos = map[string]struct{}{
 	kexAlgoDHGEXSHA256: {}, // server half implementation is only minimal to satisfy the automated tests
 }
 
-// preferredKexAlgos specifies the default preference for key-exchange algorithms
-// in preference order.
+// preferredKexAlgos specifies the default preference for key-exchange
+// algorithms in preference order. The diffie-hellman-group16-sha512 algorithm
+// is disabled by default because it is a bit slower than the others.
 var preferredKexAlgos = []string{
 	kexAlgoCurve25519SHA256, kexAlgoCurve25519SHA256LibSSH,
 	kexAlgoECDH256, kexAlgoECDH384, kexAlgoECDH521,
@@ -70,12 +71,12 @@ var preferredKexAlgos = []string{
 // supportedHostKeyAlgos specifies the supported host-key algorithms (i.e. methods
 // of authenticating servers) in preference order.
 var supportedHostKeyAlgos = []string{
-	CertAlgoRSASHA512v01, CertAlgoRSASHA256v01,
+	CertAlgoRSASHA256v01, CertAlgoRSASHA512v01,
 	CertAlgoRSAv01, CertAlgoDSAv01, CertAlgoECDSA256v01,
 	CertAlgoECDSA384v01, CertAlgoECDSA521v01, CertAlgoED25519v01,
 
 	KeyAlgoECDSA256, KeyAlgoECDSA384, KeyAlgoECDSA521,
-	KeyAlgoRSASHA512, KeyAlgoRSASHA256,
+	KeyAlgoRSASHA256, KeyAlgoRSASHA512,
 	KeyAlgoRSA, KeyAlgoDSA,
 
 	KeyAlgoED25519,
@@ -126,6 +127,14 @@ func isRSA(algo string) bool {
 	return contains(algos, underlyingAlgo(algo))
 }
 
+func isRSACert(algo string) bool {
+	_, ok := certKeyAlgoNames[algo]
+	if !ok {
+		return false
+	}
+	return isRSA(algo)
+}
+
 // supportedPubKeyAuthAlgos specifies the supported client public key
 // authentication algorithms. Note that this doesn't include certificate types
 // since those use the underlying algorithm. This list is sent to the client if
@@ -138,8 +147,6 @@ var supportedPubKeyAuthAlgos = []string{
 	KeyAlgoDSA,
 }
 
-var supportedPubKeyAuthAlgosList = strings.Join(supportedPubKeyAuthAlgos, ",")
-
 // unexpectedMessageError results when the SSH message that we received didn't
 // match what we wanted.
 func unexpectedMessageError(expected, got uint8) error {
@@ -269,16 +276,16 @@ type Config struct {
 	// unspecified, a size suitable for the chosen cipher is used.
 	RekeyThreshold uint64
 
-	// The allowed key exchanges algorithms. If unspecified then a
-	// default set of algorithms is used.
+	// The allowed key exchanges algorithms. If unspecified then a default set
+	// of algorithms is used. Unsupported values are silently ignored.
 	KeyExchanges []string
 
-	// The allowed cipher algorithms. If unspecified then a sensible
-	// default is used.
+	// The allowed cipher algorithms. If unspecified then a sensible default is
+	// used. Unsupported values are silently ignored.
 	Ciphers []string
 
-	// The allowed MAC algorithms. If unspecified then a sensible default
-	// is used.
+	// The allowed MAC algorithms. If unspecified then a sensible default is
+	// used. Unsupported values are silently ignored.
 	MACs []string
 }
 
@@ -295,7 +302,7 @@ func (c *Config) SetDefaults() {
 	var ciphers []string
 	for _, c := range c.Ciphers {
 		if cipherModes[c] != nil {
-			// reject the cipher if we have no cipherModes definition
+			// Ignore the cipher if we have no cipherModes definition.
 			ciphers = append(ciphers, c)
 		}
 	}
@@ -304,10 +311,26 @@ func (c *Config) SetDefaults() {
 	if c.KeyExchanges == nil {
 		c.KeyExchanges = preferredKexAlgos
 	}
+	var kexs []string
+	for _, k := range c.KeyExchanges {
+		if kexAlgoMap[k] != nil {
+			// Ignore the KEX if we have no kexAlgoMap definition.
+			kexs = append(kexs, k)
+		}
+	}
+	c.KeyExchanges = kexs
 
 	if c.MACs == nil {
 		c.MACs = supportedMACs
 	}
+	var macs []string
+	for _, m := range c.MACs {
+		if macModes[m] != nil {
+			// Ignore the MAC if we have no macModes definition.
+			macs = append(macs, m)
+		}
+	}
+	c.MACs = macs
 
 	if c.RekeyThreshold == 0 {
 		// cipher specific default
diff --git a/ssh/common_test.go b/ssh/common_test.go
index 96744dcf0f..a7beee8e88 100644
--- a/ssh/common_test.go
+++ b/ssh/common_test.go
@@ -82,11 +82,11 @@ func TestFindAgreedAlgorithms(t *testing.T) {
 	}
 
 	cases := []testcase{
-		testcase{
+		{
 			name: "standard",
 		},
 
-		testcase{
+		{
 			name: "no common hostkey",
 			serverIn: kexInitMsg{
 				ServerHostKeyAlgos: []string{"hostkey2"},
@@ -94,7 +94,7 @@ func TestFindAgreedAlgorithms(t *testing.T) {
 			wantErr: true,
 		},
 
-		testcase{
+		{
 			name: "no common kex",
 			serverIn: kexInitMsg{
 				KexAlgos: []string{"kex2"},
@@ -102,7 +102,7 @@ func TestFindAgreedAlgorithms(t *testing.T) {
 			wantErr: true,
 		},
 
-		testcase{
+		{
 			name: "no common cipher",
 			serverIn: kexInitMsg{
 				CiphersClientServer: []string{"cipher2"},
@@ -110,7 +110,7 @@ func TestFindAgreedAlgorithms(t *testing.T) {
 			wantErr: true,
 		},
 
-		testcase{
+		{
 			name: "client decides cipher",
 			serverIn: kexInitMsg{
 				CiphersClientServer: []string{"cipher1", "cipher2"},
diff --git a/ssh/doc.go b/ssh/doc.go
index f6bff60dc7..edbe63340d 100644
--- a/ssh/doc.go
+++ b/ssh/doc.go
@@ -13,6 +13,7 @@ others.
 
 References:
 
+	[PROTOCOL]: https://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/PROTOCOL?rev=HEAD
 	[PROTOCOL.certkeys]: http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/PROTOCOL.certkeys?rev=HEAD
 	[SSH-PARAMETERS]:    http://www.iana.org/assignments/ssh-parameters/ssh-parameters.xml#ssh-parameters-1
 
diff --git a/ssh/example_test.go b/ssh/example_test.go
index bee679603b..3920832c1a 100644
--- a/ssh/example_test.go
+++ b/ssh/example_test.go
@@ -7,6 +7,8 @@ package ssh_test
 import (
 	"bufio"
 	"bytes"
+	"crypto/rand"
+	"crypto/rsa"
 	"fmt"
 	"log"
 	"net"
@@ -14,6 +16,7 @@ import (
 	"os"
 	"path/filepath"
 	"strings"
+	"sync"
 
 	"golang.org/x/crypto/ssh"
 	"golang.org/x/crypto/ssh/terminal"
@@ -75,7 +78,6 @@ func ExampleNewServerConn() {
 	if err != nil {
 		log.Fatal("Failed to parse private key: ", err)
 	}
-
 	config.AddHostKey(private)
 
 	// Once a ServerConfig has been configured, connections can be
@@ -97,8 +99,15 @@ func ExampleNewServerConn() {
 	}
 	log.Printf("logged in with key %s", conn.Permissions.Extensions["pubkey-fp"])
 
+	var wg sync.WaitGroup
+	defer wg.Wait()
+
 	// The incoming Request channel must be serviced.
-	go ssh.DiscardRequests(reqs)
+	wg.Add(1)
+	go func() {
+		ssh.DiscardRequests(reqs)
+		wg.Done()
+	}()
 
 	// Service the incoming Channel channel.
 	for newChannel := range chans {
@@ -118,16 +127,22 @@ func ExampleNewServerConn() {
 		// Sessions have out-of-band requests such as "shell",
 		// "pty-req" and "env".  Here we handle only the
 		// "shell" request.
+		wg.Add(1)
 		go func(in <-chan *ssh.Request) {
 			for req := range in {
 				req.Reply(req.Type == "shell", nil)
 			}
+			wg.Done()
 		}(requests)
 
 		term := terminal.NewTerminal(channel, "> ")
 
+		wg.Add(1)
 		go func() {
-			defer channel.Close()
+			defer func() {
+				channel.Close()
+				wg.Done()
+			}()
 			for {
 				line, err := term.ReadLine()
 				if err != nil {
@@ -139,6 +154,36 @@ func ExampleNewServerConn() {
 	}
 }
 
+func ExampleServerConfig_AddHostKey() {
+	// Minimal ServerConfig supporting only password authentication.
+	config := &ssh.ServerConfig{
+		PasswordCallback: func(c ssh.ConnMetadata, pass []byte) (*ssh.Permissions, error) {
+			// Should use constant-time compare (or better, salt+hash) in
+			// a production setting.
+			if c.User() == "testuser" && string(pass) == "tiger" {
+				return nil, nil
+			}
+			return nil, fmt.Errorf("password rejected for %q", c.User())
+		},
+	}
+
+	privateBytes, err := os.ReadFile("id_rsa")
+	if err != nil {
+		log.Fatal("Failed to load private key: ", err)
+	}
+
+	private, err := ssh.ParsePrivateKey(privateBytes)
+	if err != nil {
+		log.Fatal("Failed to parse private key: ", err)
+	}
+	// Restrict host key algorithms to disable ssh-rsa.
+	signer, err := ssh.NewSignerWithAlgorithms(private.(ssh.AlgorithmSigner), []string{ssh.KeyAlgoRSASHA256, ssh.KeyAlgoRSASHA512})
+	if err != nil {
+		log.Fatal("Failed to create private key with restricted algorithms: ", err)
+	}
+	config.AddHostKey(signer)
+}
+
 func ExampleClientConfig_HostKeyCallback() {
 	// Every client must provide a host key check.  Here is a
 	// simple-minded parse of OpenSSH's known_hosts file
@@ -318,3 +363,38 @@ func ExampleSession_RequestPty() {
 		log.Fatal("failed to start shell: ", err)
 	}
 }
+
+func ExampleCertificate_SignCert() {
+	// Sign a certificate with a specific algorithm.
+	privateKey, err := rsa.GenerateKey(rand.Reader, 3072)
+	if err != nil {
+		log.Fatal("unable to generate RSA key: ", err)
+	}
+	publicKey, err := ssh.NewPublicKey(&privateKey.PublicKey)
+	if err != nil {
+		log.Fatal("unable to get RSA public key: ", err)
+	}
+	caKey, err := rsa.GenerateKey(rand.Reader, 3072)
+	if err != nil {
+		log.Fatal("unable to generate CA key: ", err)
+	}
+	signer, err := ssh.NewSignerFromKey(caKey)
+	if err != nil {
+		log.Fatal("unable to generate signer from key: ", err)
+	}
+	mas, err := ssh.NewSignerWithAlgorithms(signer.(ssh.AlgorithmSigner), []string{ssh.KeyAlgoRSASHA256})
+	if err != nil {
+		log.Fatal("unable to create signer with algoritms: ", err)
+	}
+	certificate := ssh.Certificate{
+		Key:      publicKey,
+		CertType: ssh.UserCert,
+	}
+	if err := certificate.SignCert(rand.Reader, mas); err != nil {
+		log.Fatal("unable to sign certificate: ", err)
+	}
+	// Save the public key to a file and check that rsa-sha-256 is used for
+	// signing:
+	// ssh-keygen -L -f <path to the file>
+	fmt.Println(string(ssh.MarshalAuthorizedKey(&certificate)))
+}
diff --git a/ssh/handshake.go b/ssh/handshake.go
index 07a1843e0a..56cdc7c21c 100644
--- a/ssh/handshake.go
+++ b/ssh/handshake.go
@@ -11,6 +11,7 @@ import (
 	"io"
 	"log"
 	"net"
+	"strings"
 	"sync"
 )
 
@@ -34,6 +35,16 @@ type keyingTransport interface {
 	// direction will be effected if a msgNewKeys message is sent
 	// or received.
 	prepareKeyChange(*algorithms, *kexResult) error
+
+	// setStrictMode sets the strict KEX mode, notably triggering
+	// sequence number resets on sending or receiving msgNewKeys.
+	// If the sequence number is already > 1 when setStrictMode
+	// is called, an error is returned.
+	setStrictMode() error
+
+	// setInitialKEXDone indicates to the transport that the initial key exchange
+	// was completed
+	setInitialKEXDone()
 }
 
 // handshakeTransport implements rekeying on top of a keyingTransport
@@ -50,6 +61,10 @@ type handshakeTransport struct {
 	// connection.
 	hostKeys []Signer
 
+	// publicKeyAuthAlgorithms is non-empty if we are the server. In that case,
+	// it contains the supported client public key authentication algorithms.
+	publicKeyAuthAlgorithms []string
+
 	// hostKeyAlgorithms is non-empty if we are the client. In that case,
 	// we accept these key types from the server as host key.
 	hostKeyAlgorithms []string
@@ -95,6 +110,10 @@ type handshakeTransport struct {
 
 	// The session ID or nil if first kex did not complete yet.
 	sessionID []byte
+
+	// strictMode indicates if the other side of the handshake indicated
+	// that we should be following the strict KEX protocol restrictions.
+	strictMode bool
 }
 
 type pendingKex struct {
@@ -141,6 +160,7 @@ func newClientTransport(conn keyingTransport, clientVersion, serverVersion []byt
 func newServerTransport(conn keyingTransport, clientVersion, serverVersion []byte, config *ServerConfig) *handshakeTransport {
 	t := newHandshakeTransport(conn, &config.Config, clientVersion, serverVersion)
 	t.hostKeys = config.hostKeys
+	t.publicKeyAuthAlgorithms = config.PublicKeyAuthAlgorithms
 	go t.readLoop()
 	go t.kexLoop()
 	return t
@@ -203,7 +223,10 @@ func (t *handshakeTransport) readLoop() {
 			close(t.incoming)
 			break
 		}
-		if p[0] == msgIgnore || p[0] == msgDebug {
+		// If this is the first kex, and strict KEX mode is enabled,
+		// we don't ignore any messages, as they may be used to manipulate
+		// the packet sequence numbers.
+		if !(t.sessionID == nil && t.strictMode) && (p[0] == msgIgnore || p[0] == msgDebug) {
 			continue
 		}
 		t.incoming <- p
@@ -435,6 +458,11 @@ func (t *handshakeTransport) readOnePacket(first bool) ([]byte, error) {
 	return successPacket, nil
 }
 
+const (
+	kexStrictClient = "kex-strict-c-v00@openssh.com"
+	kexStrictServer = "kex-strict-s-v00@openssh.com"
+)
+
 // sendKexInit sends a key change message.
 func (t *handshakeTransport) sendKexInit() error {
 	t.mu.Lock()
@@ -448,7 +476,6 @@ func (t *handshakeTransport) sendKexInit() error {
 	}
 
 	msg := &kexInitMsg{
-		KexAlgos:                t.config.KeyExchanges,
 		CiphersClientServer:     t.config.Ciphers,
 		CiphersServerClient:     t.config.Ciphers,
 		MACsClientServer:        t.config.MACs,
@@ -458,36 +485,55 @@ func (t *handshakeTransport) sendKexInit() error {
 	}
 	io.ReadFull(rand.Reader, msg.Cookie[:])
 
+	// We mutate the KexAlgos slice, in order to add the kex-strict extension algorithm,
+	// and possibly to add the ext-info extension algorithm. Since the slice may be the
+	// user owned KeyExchanges, we create our own slice in order to avoid using user
+	// owned memory by mistake.
+	msg.KexAlgos = make([]string, 0, len(t.config.KeyExchanges)+2) // room for kex-strict and ext-info
+	msg.KexAlgos = append(msg.KexAlgos, t.config.KeyExchanges...)
+
 	isServer := len(t.hostKeys) > 0
 	if isServer {
 		for _, k := range t.hostKeys {
-			// If k is an AlgorithmSigner, presume it supports all signature algorithms
-			// associated with the key format. (Ideally AlgorithmSigner would have a
-			// method to advertise supported algorithms, but it doesn't. This means that
-			// adding support for a new algorithm is a breaking change, as we will
-			// immediately negotiate it even if existing implementations don't support
-			// it. If that ever happens, we'll have to figure something out.)
-			// If k is not an AlgorithmSigner, we can only assume it only supports the
-			// algorithms that matches the key format. (This means that Sign can't pick
-			// a different default.)
+			// If k is a MultiAlgorithmSigner, we restrict the signature
+			// algorithms. If k is a AlgorithmSigner, presume it supports all
+			// signature algorithms associated with the key format. If k is not
+			// an AlgorithmSigner, we can only assume it only supports the
+			// algorithms that matches the key format. (This means that Sign
+			// can't pick a different default).
 			keyFormat := k.PublicKey().Type()
-			if _, ok := k.(AlgorithmSigner); ok {
+
+			switch s := k.(type) {
+			case MultiAlgorithmSigner:
+				for _, algo := range algorithmsForKeyFormat(keyFormat) {
+					if contains(s.Algorithms(), underlyingAlgo(algo)) {
+						msg.ServerHostKeyAlgos = append(msg.ServerHostKeyAlgos, algo)
+					}
+				}
+			case AlgorithmSigner:
 				msg.ServerHostKeyAlgos = append(msg.ServerHostKeyAlgos, algorithmsForKeyFormat(keyFormat)...)
-			} else {
+			default:
 				msg.ServerHostKeyAlgos = append(msg.ServerHostKeyAlgos, keyFormat)
 			}
 		}
+
+		if t.sessionID == nil {
+			msg.KexAlgos = append(msg.KexAlgos, kexStrictServer)
+		}
 	} else {
 		msg.ServerHostKeyAlgos = t.hostKeyAlgorithms
 
 		// As a client we opt in to receiving SSH_MSG_EXT_INFO so we know what
 		// algorithms the server supports for public key authentication. See RFC
 		// 8308, Section 2.1.
+		//
+		// We also send the strict KEX mode extension algorithm, in order to opt
+		// into the strict KEX mode.
 		if firstKeyExchange := t.sessionID == nil; firstKeyExchange {
-			msg.KexAlgos = make([]string, 0, len(t.config.KeyExchanges)+1)
-			msg.KexAlgos = append(msg.KexAlgos, t.config.KeyExchanges...)
 			msg.KexAlgos = append(msg.KexAlgos, "ext-info-c")
+			msg.KexAlgos = append(msg.KexAlgos, kexStrictClient)
 		}
+
 	}
 
 	packet := Marshal(msg)
@@ -593,6 +639,13 @@ func (t *handshakeTransport) enterKeyExchange(otherInitPacket []byte) error {
 		return err
 	}
 
+	if t.sessionID == nil && ((isClient && contains(serverInit.KexAlgos, kexStrictServer)) || (!isClient && contains(clientInit.KexAlgos, kexStrictClient))) {
+		t.strictMode = true
+		if err := t.conn.setStrictMode(); err != nil {
+			return err
+		}
+	}
+
 	// We don't send FirstKexFollows, but we handle receiving it.
 	//
 	// RFC 4253 section 7 defines the kex and the agreement method for
@@ -642,16 +695,21 @@ func (t *handshakeTransport) enterKeyExchange(otherInitPacket []byte) error {
 
 	// On the server side, after the first SSH_MSG_NEWKEYS, send a SSH_MSG_EXT_INFO
 	// message with the server-sig-algs extension if the client supports it. See
-	// RFC 8308, Sections 2.4 and 3.1.
+	// RFC 8308, Sections 2.4 and 3.1, and [PROTOCOL], Section 1.9.
 	if !isClient && firstKeyExchange && contains(clientInit.KexAlgos, "ext-info-c") {
+		supportedPubKeyAuthAlgosList := strings.Join(t.publicKeyAuthAlgorithms, ",")
 		extInfo := &extInfoMsg{
-			NumExtensions: 1,
-			Payload:       make([]byte, 0, 4+15+4+len(supportedPubKeyAuthAlgosList)),
+			NumExtensions: 2,
+			Payload:       make([]byte, 0, 4+15+4+len(supportedPubKeyAuthAlgosList)+4+16+4+1),
 		}
 		extInfo.Payload = appendInt(extInfo.Payload, len("server-sig-algs"))
 		extInfo.Payload = append(extInfo.Payload, "server-sig-algs"...)
 		extInfo.Payload = appendInt(extInfo.Payload, len(supportedPubKeyAuthAlgosList))
 		extInfo.Payload = append(extInfo.Payload, supportedPubKeyAuthAlgosList...)
+		extInfo.Payload = appendInt(extInfo.Payload, len("ping@openssh.com"))
+		extInfo.Payload = append(extInfo.Payload, "ping@openssh.com"...)
+		extInfo.Payload = appendInt(extInfo.Payload, 1)
+		extInfo.Payload = append(extInfo.Payload, "0"...)
 		if err := t.conn.writePacket(Marshal(extInfo)); err != nil {
 			return err
 		}
@@ -663,6 +721,12 @@ func (t *handshakeTransport) enterKeyExchange(otherInitPacket []byte) error {
 		return unexpectedMessageError(msgNewKeys, packet[0])
 	}
 
+	if firstKeyExchange {
+		// Indicates to the transport that the first key exchange is completed
+		// after receiving SSH_MSG_NEWKEYS.
+		t.conn.setInitialKEXDone()
+	}
+
 	return nil
 }
 
@@ -685,9 +749,16 @@ func (a algorithmSignerWrapper) SignWithAlgorithm(rand io.Reader, data []byte, a
 
 func pickHostKey(hostKeys []Signer, algo string) AlgorithmSigner {
 	for _, k := range hostKeys {
+		if s, ok := k.(MultiAlgorithmSigner); ok {
+			if !contains(s.Algorithms(), underlyingAlgo(algo)) {
+				continue
+			}
+		}
+
 		if algo == k.PublicKey().Type() {
 			return algorithmSignerWrapper{k}
 		}
+
 		k, ok := k.(AlgorithmSigner)
 		if !ok {
 			continue
diff --git a/ssh/handshake_test.go b/ssh/handshake_test.go
index f190cbfa91..2bc607b649 100644
--- a/ssh/handshake_test.go
+++ b/ssh/handshake_test.go
@@ -148,6 +148,7 @@ func TestHandshakeBasic(t *testing.T) {
 	clientDone := make(chan int, 0)
 	gotHalf := make(chan int, 0)
 	const N = 20
+	errorCh := make(chan error, 1)
 
 	go func() {
 		defer close(clientDone)
@@ -158,7 +159,9 @@ func TestHandshakeBasic(t *testing.T) {
 		for i := 0; i < N; i++ {
 			p := []byte{msgRequestSuccess, byte(i)}
 			if err := trC.writePacket(p); err != nil {
-				t.Fatalf("sendPacket: %v", err)
+				errorCh <- err
+				trC.Close()
+				return
 			}
 			if (i % 10) == 5 {
 				<-gotHalf
@@ -177,16 +180,15 @@ func TestHandshakeBasic(t *testing.T) {
 				checker.waitCall <- 1
 			}
 		}
+		errorCh <- nil
 	}()
 
 	// Server checks that client messages come in cleanly
 	i := 0
-	err = nil
 	for ; i < N; i++ {
-		var p []byte
-		p, err = trS.readPacket()
-		if err != nil {
-			break
+		p, err := trS.readPacket()
+		if err != nil && err != io.EOF {
+			t.Fatalf("server error: %v", err)
 		}
 		if (i % 10) == 5 {
 			gotHalf <- 1
@@ -198,8 +200,8 @@ func TestHandshakeBasic(t *testing.T) {
 		}
 	}
 	<-clientDone
-	if err != nil && err != io.EOF {
-		t.Fatalf("server error: %v", err)
+	if err := <-errorCh; err != nil {
+		t.Fatalf("sendPacket: %v", err)
 	}
 	if i != N {
 		t.Errorf("received %d messages, want 10.", i)
@@ -345,16 +347,16 @@ func TestHandshakeAutoRekeyRead(t *testing.T) {
 
 	// While we read out the packet, a key change will be
 	// initiated.
-	done := make(chan int, 1)
+	errorCh := make(chan error, 1)
 	go func() {
-		defer close(done)
-		if _, err := trC.readPacket(); err != nil {
-			t.Fatalf("readPacket(client): %v", err)
-		}
-
+		_, err := trC.readPacket()
+		errorCh <- err
 	}()
 
-	<-done
+	if err := <-errorCh; err != nil {
+		t.Fatalf("readPacket(client): %v", err)
+	}
+
 	<-sync.called
 }
 
@@ -393,6 +395,10 @@ func (n *errorKeyingTransport) readPacket() ([]byte, error) {
 	return n.packetConn.readPacket()
 }
 
+func (n *errorKeyingTransport) setStrictMode() error { return nil }
+
+func (n *errorKeyingTransport) setInitialKEXDone() {}
+
 func TestHandshakeErrorHandlingRead(t *testing.T) {
 	for i := 0; i < 20; i++ {
 		testHandshakeErrorHandlingN(t, i, -1, false)
@@ -618,3 +624,398 @@ func TestNoSHA2Support(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+func TestMultiAlgoSignerHandshake(t *testing.T) {
+	algorithmSigner, ok := testSigners["rsa"].(AlgorithmSigner)
+	if !ok {
+		t.Fatal("rsa test signer does not implement the AlgorithmSigner interface")
+	}
+	multiAlgoSigner, err := NewSignerWithAlgorithms(algorithmSigner, []string{KeyAlgoRSASHA256, KeyAlgoRSASHA512})
+	if err != nil {
+		t.Fatalf("unable to create multi algorithm signer: %v", err)
+	}
+	c1, c2, err := netPipe()
+	if err != nil {
+		t.Fatalf("netPipe: %v", err)
+	}
+	defer c1.Close()
+	defer c2.Close()
+
+	serverConf := &ServerConfig{
+		PasswordCallback: func(conn ConnMetadata, password []byte) (*Permissions, error) {
+			return &Permissions{}, nil
+		},
+	}
+	serverConf.AddHostKey(multiAlgoSigner)
+	go NewServerConn(c1, serverConf)
+
+	clientConf := &ClientConfig{
+		User:              "test",
+		Auth:              []AuthMethod{Password("testpw")},
+		HostKeyCallback:   FixedHostKey(testSigners["rsa"].PublicKey()),
+		HostKeyAlgorithms: []string{KeyAlgoRSASHA512},
+	}
+
+	if _, _, _, err := NewClientConn(c2, "", clientConf); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestMultiAlgoSignerNoCommonHostKeyAlgo(t *testing.T) {
+	algorithmSigner, ok := testSigners["rsa"].(AlgorithmSigner)
+	if !ok {
+		t.Fatal("rsa test signer does not implement the AlgorithmSigner interface")
+	}
+	multiAlgoSigner, err := NewSignerWithAlgorithms(algorithmSigner, []string{KeyAlgoRSASHA256, KeyAlgoRSASHA512})
+	if err != nil {
+		t.Fatalf("unable to create multi algorithm signer: %v", err)
+	}
+	c1, c2, err := netPipe()
+	if err != nil {
+		t.Fatalf("netPipe: %v", err)
+	}
+	defer c1.Close()
+	defer c2.Close()
+
+	// ssh-rsa is disabled server side
+	serverConf := &ServerConfig{
+		PasswordCallback: func(conn ConnMetadata, password []byte) (*Permissions, error) {
+			return &Permissions{}, nil
+		},
+	}
+	serverConf.AddHostKey(multiAlgoSigner)
+	go NewServerConn(c1, serverConf)
+
+	// the client only supports ssh-rsa
+	clientConf := &ClientConfig{
+		User:              "test",
+		Auth:              []AuthMethod{Password("testpw")},
+		HostKeyCallback:   FixedHostKey(testSigners["rsa"].PublicKey()),
+		HostKeyAlgorithms: []string{KeyAlgoRSA},
+	}
+
+	_, _, _, err = NewClientConn(c2, "", clientConf)
+	if err == nil {
+		t.Fatal("succeeded connecting with no common hostkey algorithm")
+	}
+}
+
+func TestPickIncompatibleHostKeyAlgo(t *testing.T) {
+	algorithmSigner, ok := testSigners["rsa"].(AlgorithmSigner)
+	if !ok {
+		t.Fatal("rsa test signer does not implement the AlgorithmSigner interface")
+	}
+	multiAlgoSigner, err := NewSignerWithAlgorithms(algorithmSigner, []string{KeyAlgoRSASHA256, KeyAlgoRSASHA512})
+	if err != nil {
+		t.Fatalf("unable to create multi algorithm signer: %v", err)
+	}
+	signer := pickHostKey([]Signer{multiAlgoSigner}, KeyAlgoRSA)
+	if signer != nil {
+		t.Fatal("incompatible signer returned")
+	}
+}
+
+func TestStrictKEXResetSeqFirstKEX(t *testing.T) {
+	if runtime.GOOS == "plan9" {
+		t.Skip("see golang.org/issue/7237")
+	}
+
+	checker := &syncChecker{
+		waitCall: make(chan int, 10),
+		called:   make(chan int, 10),
+	}
+
+	checker.waitCall <- 1
+	trC, trS, err := handshakePair(&ClientConfig{HostKeyCallback: checker.Check}, "addr", false)
+	if err != nil {
+		t.Fatalf("handshakePair: %v", err)
+	}
+	<-checker.called
+
+	t.Cleanup(func() {
+		trC.Close()
+		trS.Close()
+	})
+
+	// Throw away the msgExtInfo packet sent during the handshake by the server
+	_, err = trC.readPacket()
+	if err != nil {
+		t.Fatalf("readPacket failed: %s", err)
+	}
+
+	// close the handshake transports before checking the sequence number to
+	// avoid races.
+	trC.Close()
+	trS.Close()
+
+	// check that the sequence number counters. We reset after msgNewKeys, but
+	// then the server immediately writes msgExtInfo, and we close the
+	// transports so we expect read 2, write 0 on the client and read 1, write 1
+	// on the server.
+	if trC.conn.(*transport).reader.seqNum != 2 || trC.conn.(*transport).writer.seqNum != 0 ||
+		trS.conn.(*transport).reader.seqNum != 1 || trS.conn.(*transport).writer.seqNum != 1 {
+		t.Errorf(
+			"unexpected sequence counters:\nclient: reader %d (expected 2), writer %d (expected 0)\nserver: reader %d (expected 1), writer %d (expected 1)",
+			trC.conn.(*transport).reader.seqNum,
+			trC.conn.(*transport).writer.seqNum,
+			trS.conn.(*transport).reader.seqNum,
+			trS.conn.(*transport).writer.seqNum,
+		)
+	}
+}
+
+func TestStrictKEXResetSeqSuccessiveKEX(t *testing.T) {
+	if runtime.GOOS == "plan9" {
+		t.Skip("see golang.org/issue/7237")
+	}
+
+	checker := &syncChecker{
+		waitCall: make(chan int, 10),
+		called:   make(chan int, 10),
+	}
+
+	checker.waitCall <- 1
+	trC, trS, err := handshakePair(&ClientConfig{HostKeyCallback: checker.Check}, "addr", false)
+	if err != nil {
+		t.Fatalf("handshakePair: %v", err)
+	}
+	<-checker.called
+
+	t.Cleanup(func() {
+		trC.Close()
+		trS.Close()
+	})
+
+	// Throw away the msgExtInfo packet sent during the handshake by the server
+	_, err = trC.readPacket()
+	if err != nil {
+		t.Fatalf("readPacket failed: %s", err)
+	}
+
+	// write and read five packets on either side to bump the sequence numbers
+	for i := 0; i < 5; i++ {
+		if err := trC.writePacket([]byte{msgRequestSuccess}); err != nil {
+			t.Fatalf("writePacket failed: %s", err)
+		}
+		if _, err := trS.readPacket(); err != nil {
+			t.Fatalf("readPacket failed: %s", err)
+		}
+		if err := trS.writePacket([]byte{msgRequestSuccess}); err != nil {
+			t.Fatalf("writePacket failed: %s", err)
+		}
+		if _, err := trC.readPacket(); err != nil {
+			t.Fatalf("readPacket failed: %s", err)
+		}
+	}
+
+	// Request a key exchange, which should cause the sequence numbers to reset
+	checker.waitCall <- 1
+	trC.requestKeyExchange()
+	<-checker.called
+
+	// write a packet on the client, and then read it, to verify the key change has actually happened, since
+	// the HostKeyCallback is called _during_ the handshake, so isn't actually indicative of the handshake
+	// finishing.
+	dummyPacket := []byte{99}
+	if err := trS.writePacket(dummyPacket); err != nil {
+		t.Fatalf("writePacket failed: %s", err)
+	}
+	if p, err := trC.readPacket(); err != nil {
+		t.Fatalf("readPacket failed: %s", err)
+	} else if !bytes.Equal(p, dummyPacket) {
+		t.Fatalf("unexpected packet: got %x, want %x", p, dummyPacket)
+	}
+
+	// close the handshake transports before checking the sequence number to
+	// avoid races.
+	trC.Close()
+	trS.Close()
+
+	if trC.conn.(*transport).reader.seqNum != 2 || trC.conn.(*transport).writer.seqNum != 0 ||
+		trS.conn.(*transport).reader.seqNum != 1 || trS.conn.(*transport).writer.seqNum != 1 {
+		t.Errorf(
+			"unexpected sequence counters:\nclient: reader %d (expected 2), writer %d (expected 0)\nserver: reader %d (expected 1), writer %d (expected 1)",
+			trC.conn.(*transport).reader.seqNum,
+			trC.conn.(*transport).writer.seqNum,
+			trS.conn.(*transport).reader.seqNum,
+			trS.conn.(*transport).writer.seqNum,
+		)
+	}
+}
+
+func TestSeqNumIncrease(t *testing.T) {
+	if runtime.GOOS == "plan9" {
+		t.Skip("see golang.org/issue/7237")
+	}
+
+	checker := &syncChecker{
+		waitCall: make(chan int, 10),
+		called:   make(chan int, 10),
+	}
+
+	checker.waitCall <- 1
+	trC, trS, err := handshakePair(&ClientConfig{HostKeyCallback: checker.Check}, "addr", false)
+	if err != nil {
+		t.Fatalf("handshakePair: %v", err)
+	}
+	<-checker.called
+
+	t.Cleanup(func() {
+		trC.Close()
+		trS.Close()
+	})
+
+	// Throw away the msgExtInfo packet sent during the handshake by the server
+	_, err = trC.readPacket()
+	if err != nil {
+		t.Fatalf("readPacket failed: %s", err)
+	}
+
+	// write and read five packets on either side to bump the sequence numbers
+	for i := 0; i < 5; i++ {
+		if err := trC.writePacket([]byte{msgRequestSuccess}); err != nil {
+			t.Fatalf("writePacket failed: %s", err)
+		}
+		if _, err := trS.readPacket(); err != nil {
+			t.Fatalf("readPacket failed: %s", err)
+		}
+		if err := trS.writePacket([]byte{msgRequestSuccess}); err != nil {
+			t.Fatalf("writePacket failed: %s", err)
+		}
+		if _, err := trC.readPacket(); err != nil {
+			t.Fatalf("readPacket failed: %s", err)
+		}
+	}
+
+	// close the handshake transports before checking the sequence number to
+	// avoid races.
+	trC.Close()
+	trS.Close()
+
+	if trC.conn.(*transport).reader.seqNum != 7 || trC.conn.(*transport).writer.seqNum != 5 ||
+		trS.conn.(*transport).reader.seqNum != 6 || trS.conn.(*transport).writer.seqNum != 6 {
+		t.Errorf(
+			"unexpected sequence counters:\nclient: reader %d (expected 7), writer %d (expected 5)\nserver: reader %d (expected 6), writer %d (expected 6)",
+			trC.conn.(*transport).reader.seqNum,
+			trC.conn.(*transport).writer.seqNum,
+			trS.conn.(*transport).reader.seqNum,
+			trS.conn.(*transport).writer.seqNum,
+		)
+	}
+}
+
+func TestStrictKEXUnexpectedMsg(t *testing.T) {
+	if runtime.GOOS == "plan9" {
+		t.Skip("see golang.org/issue/7237")
+	}
+
+	// Check that unexpected messages during the handshake cause failure
+	_, _, err := handshakePair(&ClientConfig{HostKeyCallback: func(hostname string, remote net.Addr, key PublicKey) error { return nil }}, "addr", true)
+	if err == nil {
+		t.Fatal("handshake should fail when there are unexpected messages during the handshake")
+	}
+
+	trC, trS, err := handshakePair(&ClientConfig{HostKeyCallback: func(hostname string, remote net.Addr, key PublicKey) error { return nil }}, "addr", false)
+	if err != nil {
+		t.Fatalf("handshake failed: %s", err)
+	}
+
+	// Check that ignore/debug pacekts are still ignored outside of the handshake
+	if err := trC.writePacket([]byte{msgIgnore}); err != nil {
+		t.Fatalf("writePacket failed: %s", err)
+	}
+	if err := trC.writePacket([]byte{msgDebug}); err != nil {
+		t.Fatalf("writePacket failed: %s", err)
+	}
+	dummyPacket := []byte{99}
+	if err := trC.writePacket(dummyPacket); err != nil {
+		t.Fatalf("writePacket failed: %s", err)
+	}
+
+	if p, err := trS.readPacket(); err != nil {
+		t.Fatalf("readPacket failed: %s", err)
+	} else if !bytes.Equal(p, dummyPacket) {
+		t.Fatalf("unexpected packet: got %x, want %x", p, dummyPacket)
+	}
+}
+
+func TestStrictKEXMixed(t *testing.T) {
+	// Test that we still support a mixed connection, where one side sends kex-strict but the other
+	// side doesn't.
+
+	a, b, err := netPipe()
+	if err != nil {
+		t.Fatalf("netPipe failed: %s", err)
+	}
+
+	var trC, trS keyingTransport
+
+	trC = newTransport(a, rand.Reader, true)
+	trS = newTransport(b, rand.Reader, false)
+	trS = addNoiseTransport(trS)
+
+	clientConf := &ClientConfig{HostKeyCallback: func(hostname string, remote net.Addr, key PublicKey) error { return nil }}
+	clientConf.SetDefaults()
+
+	v := []byte("version")
+	client := newClientTransport(trC, v, v, clientConf, "addr", a.RemoteAddr())
+
+	serverConf := &ServerConfig{}
+	serverConf.AddHostKey(testSigners["ecdsa"])
+	serverConf.AddHostKey(testSigners["rsa"])
+	serverConf.SetDefaults()
+
+	transport := newHandshakeTransport(trS, &serverConf.Config, []byte("version"), []byte("version"))
+	transport.hostKeys = serverConf.hostKeys
+	transport.publicKeyAuthAlgorithms = serverConf.PublicKeyAuthAlgorithms
+
+	readOneFailure := make(chan error, 1)
+	go func() {
+		if _, err := transport.readOnePacket(true); err != nil {
+			readOneFailure <- err
+		}
+	}()
+
+	// Basically sendKexInit, but without the kex-strict extension algorithm
+	msg := &kexInitMsg{
+		KexAlgos:                transport.config.KeyExchanges,
+		CiphersClientServer:     transport.config.Ciphers,
+		CiphersServerClient:     transport.config.Ciphers,
+		MACsClientServer:        transport.config.MACs,
+		MACsServerClient:        transport.config.MACs,
+		CompressionClientServer: supportedCompressions,
+		CompressionServerClient: supportedCompressions,
+		ServerHostKeyAlgos:      []string{KeyAlgoRSASHA256, KeyAlgoRSASHA512, KeyAlgoRSA},
+	}
+	packet := Marshal(msg)
+	// writePacket destroys the contents, so save a copy.
+	packetCopy := make([]byte, len(packet))
+	copy(packetCopy, packet)
+	if err := transport.pushPacket(packetCopy); err != nil {
+		t.Fatalf("pushPacket: %s", err)
+	}
+	transport.sentInitMsg = msg
+	transport.sentInitPacket = packet
+
+	if err := transport.getWriteError(); err != nil {
+		t.Fatalf("getWriteError failed: %s", err)
+	}
+	var request *pendingKex
+	select {
+	case err = <-readOneFailure:
+		t.Fatalf("server readOnePacket failed: %s", err)
+	case request = <-transport.startKex:
+		break
+	}
+
+	// We expect the following calls to fail if the side which does not support
+	// kex-strict sends unexpected/ignored packets during the handshake, even if
+	// the other side does support kex-strict.
+
+	if err := transport.enterKeyExchange(request.otherInit); err != nil {
+		t.Fatalf("enterKeyExchange failed: %s", err)
+	}
+	if err := client.waitSession(); err != nil {
+		t.Fatalf("client.waitSession: %v", err)
+	}
+}
diff --git a/ssh/kex.go b/ssh/kex.go
index 927a90cd46..8a05f79902 100644
--- a/ssh/kex.go
+++ b/ssh/kex.go
@@ -23,6 +23,7 @@ const (
 	kexAlgoDH1SHA1                = "diffie-hellman-group1-sha1"
 	kexAlgoDH14SHA1               = "diffie-hellman-group14-sha1"
 	kexAlgoDH14SHA256             = "diffie-hellman-group14-sha256"
+	kexAlgoDH16SHA512             = "diffie-hellman-group16-sha512"
 	kexAlgoECDH256                = "ecdh-sha2-nistp256"
 	kexAlgoECDH384                = "ecdh-sha2-nistp384"
 	kexAlgoECDH521                = "ecdh-sha2-nistp521"
@@ -430,6 +431,17 @@ func init() {
 		hashFunc: crypto.SHA256,
 	}
 
+	// This is the group called diffie-hellman-group16-sha512 in RFC
+	// 8268 and Oakley Group 16 in RFC 3526.
+	p, _ = new(big.Int).SetString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
+
+	kexAlgoMap[kexAlgoDH16SHA512] = &dhGroup{
+		g:        new(big.Int).SetInt64(2),
+		p:        p,
+		pMinus1:  new(big.Int).Sub(p, bigOne),
+		hashFunc: crypto.SHA512,
+	}
+
 	kexAlgoMap[kexAlgoECDH521] = &ecdh{elliptic.P521()}
 	kexAlgoMap[kexAlgoECDH384] = &ecdh{elliptic.P384()}
 	kexAlgoMap[kexAlgoECDH256] = &ecdh{elliptic.P256()}
diff --git a/ssh/kex_test.go b/ssh/kex_test.go
index 327013b7d4..cb7f66a509 100644
--- a/ssh/kex_test.go
+++ b/ssh/kex_test.go
@@ -8,6 +8,7 @@ package ssh
 
 import (
 	"crypto/rand"
+	"fmt"
 	"reflect"
 	"sync"
 	"testing"
@@ -63,3 +64,43 @@ func TestKexes(t *testing.T) {
 		})
 	}
 }
+
+func BenchmarkKexes(b *testing.B) {
+	type kexResultErr struct {
+		result *kexResult
+		err    error
+	}
+
+	for name, kex := range kexAlgoMap {
+		b.Run(name, func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				t1, t2 := memPipe()
+
+				s := make(chan kexResultErr, 1)
+				c := make(chan kexResultErr, 1)
+				var magics handshakeMagics
+
+				go func() {
+					r, e := kex.Client(t1, rand.Reader, &magics)
+					t1.Close()
+					c <- kexResultErr{r, e}
+				}()
+				go func() {
+					r, e := kex.Server(t2, rand.Reader, &magics, testSigners["ecdsa"].(AlgorithmSigner), testSigners["ecdsa"].PublicKey().Type())
+					t2.Close()
+					s <- kexResultErr{r, e}
+				}()
+
+				clientRes := <-c
+				serverRes := <-s
+
+				if clientRes.err != nil {
+					panic(fmt.Sprintf("client: %v", clientRes.err))
+				}
+				if serverRes.err != nil {
+					panic(fmt.Sprintf("server: %v", serverRes.err))
+				}
+			}
+		})
+	}
+}
diff --git a/ssh/keys.go b/ssh/keys.go
index 28f3446e4c..9dc19d591a 100644
--- a/ssh/keys.go
+++ b/ssh/keys.go
@@ -11,13 +11,16 @@ import (
 	"crypto/cipher"
 	"crypto/dsa"
 	"crypto/ecdsa"
+	"crypto/ed25519"
 	"crypto/elliptic"
 	"crypto/md5"
+	"crypto/rand"
 	"crypto/rsa"
 	"crypto/sha256"
 	"crypto/x509"
 	"encoding/asn1"
 	"encoding/base64"
+	"encoding/binary"
 	"encoding/hex"
 	"encoding/pem"
 	"errors"
@@ -27,7 +30,6 @@ import (
 	"strings"
 
 	"github.com/tailscale/golang-x-crypto/ssh/internal/bcrypt_pbkdf"
-	"golang.org/x/crypto/ed25519"
 )
 
 // Public key algorithms names. These values can appear in PublicKey.Type,
@@ -295,6 +297,18 @@ func MarshalAuthorizedKey(key PublicKey) []byte {
 	return b.Bytes()
 }
 
+// MarshalPrivateKey returns a PEM block with the private key serialized in the
+// OpenSSH format.
+func MarshalPrivateKey(key crypto.PrivateKey, comment string) (*pem.Block, error) {
+	return marshalOpenSSHPrivateKey(key, comment, unencryptedOpenSSHMarshaler)
+}
+
+// MarshalPrivateKeyWithPassphrase returns a PEM block holding the encrypted
+// private key serialized in the OpenSSH format.
+func MarshalPrivateKeyWithPassphrase(key crypto.PrivateKey, comment string, passphrase []byte) (*pem.Block, error) {
+	return marshalOpenSSHPrivateKey(key, comment, passphraseProtectedOpenSSHMarshaler(passphrase))
+}
+
 // PublicKey represents a public key using an unspecified algorithm.
 //
 // Some PublicKeys provided by this package also implement CryptoPublicKey.
@@ -321,7 +335,7 @@ type CryptoPublicKey interface {
 
 // A Signer can create signatures that verify against a public key.
 //
-// Some Signers provided by this package also implement AlgorithmSigner.
+// Some Signers provided by this package also implement MultiAlgorithmSigner.
 type Signer interface {
 	// PublicKey returns the associated PublicKey.
 	PublicKey() PublicKey
@@ -336,9 +350,9 @@ type Signer interface {
 // An AlgorithmSigner is a Signer that also supports specifying an algorithm to
 // use for signing.
 //
-// An AlgorithmSigner can't advertise the algorithms it supports, so it should
-// be prepared to be invoked with every algorithm supported by the public key
-// format.
+// An AlgorithmSigner can't advertise the algorithms it supports, unless it also
+// implements MultiAlgorithmSigner, so it should be prepared to be invoked with
+// every algorithm supported by the public key format.
 type AlgorithmSigner interface {
 	Signer
 
@@ -349,6 +363,75 @@ type AlgorithmSigner interface {
 	SignWithAlgorithm(rand io.Reader, data []byte, algorithm string) (*Signature, error)
 }
 
+// MultiAlgorithmSigner is an AlgorithmSigner that also reports the algorithms
+// supported by that signer.
+type MultiAlgorithmSigner interface {
+	AlgorithmSigner
+
+	// Algorithms returns the available algorithms in preference order. The list
+	// must not be empty, and it must not include certificate types.
+	Algorithms() []string
+}
+
+// NewSignerWithAlgorithms returns a signer restricted to the specified
+// algorithms. The algorithms must be set in preference order. The list must not
+// be empty, and it must not include certificate types. An error is returned if
+// the specified algorithms are incompatible with the public key type.
+func NewSignerWithAlgorithms(signer AlgorithmSigner, algorithms []string) (MultiAlgorithmSigner, error) {
+	if len(algorithms) == 0 {
+		return nil, errors.New("ssh: please specify at least one valid signing algorithm")
+	}
+	var signerAlgos []string
+	supportedAlgos := algorithmsForKeyFormat(underlyingAlgo(signer.PublicKey().Type()))
+	if s, ok := signer.(*multiAlgorithmSigner); ok {
+		signerAlgos = s.Algorithms()
+	} else {
+		signerAlgos = supportedAlgos
+	}
+
+	for _, algo := range algorithms {
+		if !contains(supportedAlgos, algo) {
+			return nil, fmt.Errorf("ssh: algorithm %q is not supported for key type %q",
+				algo, signer.PublicKey().Type())
+		}
+		if !contains(signerAlgos, algo) {
+			return nil, fmt.Errorf("ssh: algorithm %q is restricted for the provided signer", algo)
+		}
+	}
+	return &multiAlgorithmSigner{
+		AlgorithmSigner:     signer,
+		supportedAlgorithms: algorithms,
+	}, nil
+}
+
+type multiAlgorithmSigner struct {
+	AlgorithmSigner
+	supportedAlgorithms []string
+}
+
+func (s *multiAlgorithmSigner) Algorithms() []string {
+	return s.supportedAlgorithms
+}
+
+func (s *multiAlgorithmSigner) isAlgorithmSupported(algorithm string) bool {
+	if algorithm == "" {
+		algorithm = underlyingAlgo(s.PublicKey().Type())
+	}
+	for _, algo := range s.supportedAlgorithms {
+		if algorithm == algo {
+			return true
+		}
+	}
+	return false
+}
+
+func (s *multiAlgorithmSigner) SignWithAlgorithm(rand io.Reader, data []byte, algorithm string) (*Signature, error) {
+	if !s.isAlgorithmSupported(algorithm) {
+		return nil, fmt.Errorf("ssh: algorithm %q is not supported: %v", algorithm, s.supportedAlgorithms)
+	}
+	return s.AlgorithmSigner.SignWithAlgorithm(rand, data, algorithm)
+}
+
 type rsaPublicKey rsa.PublicKey
 
 func (r *rsaPublicKey) Type() string {
@@ -512,6 +595,10 @@ func (k *dsaPrivateKey) Sign(rand io.Reader, data []byte) (*Signature, error) {
 	return k.SignWithAlgorithm(rand, data, k.PublicKey().Type())
 }
 
+func (k *dsaPrivateKey) Algorithms() []string {
+	return []string{k.PublicKey().Type()}
+}
+
 func (k *dsaPrivateKey) SignWithAlgorithm(rand io.Reader, data []byte, algorithm string) (*Signature, error) {
 	if algorithm != "" && algorithm != k.PublicKey().Type() {
 		return nil, fmt.Errorf("ssh: unsupported signature algorithm %s", algorithm)
@@ -961,13 +1048,16 @@ func (s *wrappedSigner) Sign(rand io.Reader, data []byte) (*Signature, error) {
 	return s.SignWithAlgorithm(rand, data, s.pubKey.Type())
 }
 
+func (s *wrappedSigner) Algorithms() []string {
+	return algorithmsForKeyFormat(s.pubKey.Type())
+}
+
 func (s *wrappedSigner) SignWithAlgorithm(rand io.Reader, data []byte, algorithm string) (*Signature, error) {
 	if algorithm == "" {
 		algorithm = s.pubKey.Type()
 	}
 
-	supportedAlgos := algorithmsForKeyFormat(s.pubKey.Type())
-	if !contains(supportedAlgos, algorithm) {
+	if !contains(s.Algorithms(), algorithm) {
 		return nil, fmt.Errorf("ssh: unsupported signature algorithm %q for key format %q", algorithm, s.pubKey.Type())
 	}
 
@@ -1142,16 +1232,27 @@ func ParseRawPrivateKeyWithPassphrase(pemBytes, passphrase []byte) (interface{},
 		return nil, fmt.Errorf("ssh: cannot decode encrypted private keys: %v", err)
 	}
 
+	var result interface{}
+
 	switch block.Type {
 	case "RSA PRIVATE KEY":
-		return x509.ParsePKCS1PrivateKey(buf)
+		result, err = x509.ParsePKCS1PrivateKey(buf)
 	case "EC PRIVATE KEY":
-		return x509.ParseECPrivateKey(buf)
+		result, err = x509.ParseECPrivateKey(buf)
 	case "DSA PRIVATE KEY":
-		return ParseDSAPrivateKey(buf)
+		result, err = ParseDSAPrivateKey(buf)
 	default:
-		return nil, fmt.Errorf("ssh: unsupported key type %q", block.Type)
+		err = fmt.Errorf("ssh: unsupported key type %q", block.Type)
+	}
+	// Because of deficiencies in the format, DecryptPEMBlock does not always
+	// detect an incorrect password. In these cases decrypted DER bytes is
+	// random noise. If the parsing of the key returns an asn1.StructuralError
+	// we return x509.IncorrectPasswordError.
+	if _, ok := err.(asn1.StructuralError); ok {
+		return nil, x509.IncorrectPasswordError
 	}
+
+	return result, err
 }
 
 // ParseDSAPrivateKey returns a DSA private key from its ASN.1 DER encoding, as
@@ -1241,28 +1342,106 @@ func passphraseProtectedOpenSSHKey(passphrase []byte) openSSHDecryptFunc {
 	}
 }
 
+func unencryptedOpenSSHMarshaler(privKeyBlock []byte) ([]byte, string, string, string, error) {
+	key := generateOpenSSHPadding(privKeyBlock, 8)
+	return key, "none", "none", "", nil
+}
+
+func passphraseProtectedOpenSSHMarshaler(passphrase []byte) openSSHEncryptFunc {
+	return func(privKeyBlock []byte) ([]byte, string, string, string, error) {
+		salt := make([]byte, 16)
+		if _, err := rand.Read(salt); err != nil {
+			return nil, "", "", "", err
+		}
+
+		opts := struct {
+			Salt   []byte
+			Rounds uint32
+		}{salt, 16}
+
+		// Derive key to encrypt the private key block.
+		k, err := bcrypt_pbkdf.Key(passphrase, salt, int(opts.Rounds), 32+aes.BlockSize)
+		if err != nil {
+			return nil, "", "", "", err
+		}
+
+		// Add padding matching the block size of AES.
+		keyBlock := generateOpenSSHPadding(privKeyBlock, aes.BlockSize)
+
+		// Encrypt the private key using the derived secret.
+
+		dst := make([]byte, len(keyBlock))
+		key, iv := k[:32], k[32:]
+		block, err := aes.NewCipher(key)
+		if err != nil {
+			return nil, "", "", "", err
+		}
+
+		stream := cipher.NewCTR(block, iv)
+		stream.XORKeyStream(dst, keyBlock)
+
+		return dst, "aes256-ctr", "bcrypt", string(Marshal(opts)), nil
+	}
+}
+
+const privateKeyAuthMagic = "openssh-key-v1\x00"
+
 type openSSHDecryptFunc func(CipherName, KdfName, KdfOpts string, PrivKeyBlock []byte) ([]byte, error)
+type openSSHEncryptFunc func(PrivKeyBlock []byte) (ProtectedKeyBlock []byte, cipherName, kdfName, kdfOptions string, err error)
+
+type openSSHEncryptedPrivateKey struct {
+	CipherName   string
+	KdfName      string
+	KdfOpts      string
+	NumKeys      uint32
+	PubKey       []byte
+	PrivKeyBlock []byte
+}
+
+type openSSHPrivateKey struct {
+	Check1  uint32
+	Check2  uint32
+	Keytype string
+	Rest    []byte `ssh:"rest"`
+}
+
+type openSSHRSAPrivateKey struct {
+	N       *big.Int
+	E       *big.Int
+	D       *big.Int
+	Iqmp    *big.Int
+	P       *big.Int
+	Q       *big.Int
+	Comment string
+	Pad     []byte `ssh:"rest"`
+}
+
+type openSSHEd25519PrivateKey struct {
+	Pub     []byte
+	Priv    []byte
+	Comment string
+	Pad     []byte `ssh:"rest"`
+}
+
+type openSSHECDSAPrivateKey struct {
+	Curve   string
+	Pub     []byte
+	D       *big.Int
+	Comment string
+	Pad     []byte `ssh:"rest"`
+}
 
 // parseOpenSSHPrivateKey parses an OpenSSH private key, using the decrypt
 // function to unwrap the encrypted portion. unencryptedOpenSSHKey can be used
 // as the decrypt function to parse an unencrypted private key. See
 // https://github.com/openssh/openssh-portable/blob/master/PROTOCOL.key.
 func parseOpenSSHPrivateKey(key []byte, decrypt openSSHDecryptFunc) (crypto.PrivateKey, error) {
-	const magic = "openssh-key-v1\x00"
-	if len(key) < len(magic) || string(key[:len(magic)]) != magic {
+	if len(key) < len(privateKeyAuthMagic) || string(key[:len(privateKeyAuthMagic)]) != privateKeyAuthMagic {
 		return nil, errors.New("ssh: invalid openssh private key format")
 	}
-	remaining := key[len(magic):]
-
-	var w struct {
-		CipherName   string
-		KdfName      string
-		KdfOpts      string
-		NumKeys      uint32
-		PubKey       []byte
-		PrivKeyBlock []byte
-	}
+	remaining := key[len(privateKeyAuthMagic):]
 
+	var w openSSHEncryptedPrivateKey
 	if err := Unmarshal(remaining, &w); err != nil {
 		return nil, err
 	}
@@ -1284,13 +1463,7 @@ func parseOpenSSHPrivateKey(key []byte, decrypt openSSHDecryptFunc) (crypto.Priv
 		return nil, err
 	}
 
-	pk1 := struct {
-		Check1  uint32
-		Check2  uint32
-		Keytype string
-		Rest    []byte `ssh:"rest"`
-	}{}
-
+	var pk1 openSSHPrivateKey
 	if err := Unmarshal(privKeyBlock, &pk1); err != nil || pk1.Check1 != pk1.Check2 {
 		if w.CipherName != "none" {
 			return nil, x509.IncorrectPasswordError
@@ -1300,18 +1473,7 @@ func parseOpenSSHPrivateKey(key []byte, decrypt openSSHDecryptFunc) (crypto.Priv
 
 	switch pk1.Keytype {
 	case KeyAlgoRSA:
-		// https://github.com/openssh/openssh-portable/blob/master/sshkey.c#L2760-L2773
-		key := struct {
-			N       *big.Int
-			E       *big.Int
-			D       *big.Int
-			Iqmp    *big.Int
-			P       *big.Int
-			Q       *big.Int
-			Comment string
-			Pad     []byte `ssh:"rest"`
-		}{}
-
+		var key openSSHRSAPrivateKey
 		if err := Unmarshal(pk1.Rest, &key); err != nil {
 			return nil, err
 		}
@@ -1337,13 +1499,7 @@ func parseOpenSSHPrivateKey(key []byte, decrypt openSSHDecryptFunc) (crypto.Priv
 
 		return pk, nil
 	case KeyAlgoED25519:
-		key := struct {
-			Pub     []byte
-			Priv    []byte
-			Comment string
-			Pad     []byte `ssh:"rest"`
-		}{}
-
+		var key openSSHEd25519PrivateKey
 		if err := Unmarshal(pk1.Rest, &key); err != nil {
 			return nil, err
 		}
@@ -1360,14 +1516,7 @@ func parseOpenSSHPrivateKey(key []byte, decrypt openSSHDecryptFunc) (crypto.Priv
 		copy(pk, key.Priv)
 		return &pk, nil
 	case KeyAlgoECDSA256, KeyAlgoECDSA384, KeyAlgoECDSA521:
-		key := struct {
-			Curve   string
-			Pub     []byte
-			D       *big.Int
-			Comment string
-			Pad     []byte `ssh:"rest"`
-		}{}
-
+		var key openSSHECDSAPrivateKey
 		if err := Unmarshal(pk1.Rest, &key); err != nil {
 			return nil, err
 		}
@@ -1415,6 +1564,131 @@ func parseOpenSSHPrivateKey(key []byte, decrypt openSSHDecryptFunc) (crypto.Priv
 	}
 }
 
+func marshalOpenSSHPrivateKey(key crypto.PrivateKey, comment string, encrypt openSSHEncryptFunc) (*pem.Block, error) {
+	var w openSSHEncryptedPrivateKey
+	var pk1 openSSHPrivateKey
+
+	// Random check bytes.
+	var check uint32
+	if err := binary.Read(rand.Reader, binary.BigEndian, &check); err != nil {
+		return nil, err
+	}
+
+	pk1.Check1 = check
+	pk1.Check2 = check
+	w.NumKeys = 1
+
+	// Use a []byte directly on ed25519 keys.
+	if k, ok := key.(*ed25519.PrivateKey); ok {
+		key = *k
+	}
+
+	switch k := key.(type) {
+	case *rsa.PrivateKey:
+		E := new(big.Int).SetInt64(int64(k.PublicKey.E))
+		// Marshal public key:
+		// E and N are in reversed order in the public and private key.
+		pubKey := struct {
+			KeyType string
+			E       *big.Int
+			N       *big.Int
+		}{
+			KeyAlgoRSA,
+			E, k.PublicKey.N,
+		}
+		w.PubKey = Marshal(pubKey)
+
+		// Marshal private key.
+		key := openSSHRSAPrivateKey{
+			N:       k.PublicKey.N,
+			E:       E,
+			D:       k.D,
+			Iqmp:    k.Precomputed.Qinv,
+			P:       k.Primes[0],
+			Q:       k.Primes[1],
+			Comment: comment,
+		}
+		pk1.Keytype = KeyAlgoRSA
+		pk1.Rest = Marshal(key)
+	case ed25519.PrivateKey:
+		pub := make([]byte, ed25519.PublicKeySize)
+		priv := make([]byte, ed25519.PrivateKeySize)
+		copy(pub, k[32:])
+		copy(priv, k)
+
+		// Marshal public key.
+		pubKey := struct {
+			KeyType string
+			Pub     []byte
+		}{
+			KeyAlgoED25519, pub,
+		}
+		w.PubKey = Marshal(pubKey)
+
+		// Marshal private key.
+		key := openSSHEd25519PrivateKey{
+			Pub:     pub,
+			Priv:    priv,
+			Comment: comment,
+		}
+		pk1.Keytype = KeyAlgoED25519
+		pk1.Rest = Marshal(key)
+	case *ecdsa.PrivateKey:
+		var curve, keyType string
+		switch name := k.Curve.Params().Name; name {
+		case "P-256":
+			curve = "nistp256"
+			keyType = KeyAlgoECDSA256
+		case "P-384":
+			curve = "nistp384"
+			keyType = KeyAlgoECDSA384
+		case "P-521":
+			curve = "nistp521"
+			keyType = KeyAlgoECDSA521
+		default:
+			return nil, errors.New("ssh: unhandled elliptic curve " + name)
+		}
+
+		pub := elliptic.Marshal(k.Curve, k.PublicKey.X, k.PublicKey.Y)
+
+		// Marshal public key.
+		pubKey := struct {
+			KeyType string
+			Curve   string
+			Pub     []byte
+		}{
+			keyType, curve, pub,
+		}
+		w.PubKey = Marshal(pubKey)
+
+		// Marshal private key.
+		key := openSSHECDSAPrivateKey{
+			Curve:   curve,
+			Pub:     pub,
+			D:       k.D,
+			Comment: comment,
+		}
+		pk1.Keytype = keyType
+		pk1.Rest = Marshal(key)
+	default:
+		return nil, fmt.Errorf("ssh: unsupported key type %T", k)
+	}
+
+	var err error
+	// Add padding and encrypt the key if necessary.
+	w.PrivKeyBlock, w.CipherName, w.KdfName, w.KdfOpts, err = encrypt(Marshal(pk1))
+	if err != nil {
+		return nil, err
+	}
+
+	b := Marshal(w)
+	block := &pem.Block{
+		Type:  "OPENSSH PRIVATE KEY",
+		Bytes: append([]byte(privateKeyAuthMagic), b...),
+	}
+	return block, nil
+}
+
 func checkOpenSSHKeyPadding(pad []byte) error {
 	for i, b := range pad {
 		if int(b) != i+1 {
@@ -1424,6 +1698,13 @@ func checkOpenSSHKeyPadding(pad []byte) error {
 	return nil
 }
 
+func generateOpenSSHPadding(block []byte, blockSize int) []byte {
+	for i, l := 0, len(block); (l+i)%blockSize != 0; i++ {
+		block = append(block, byte(i+1))
+	}
+	return block
+}
+
 // FingerprintLegacyMD5 returns the user presentation of the key's
 // fingerprint as described by RFC 4716 section 4.
 func FingerprintLegacyMD5(pubKey PublicKey) string {
diff --git a/ssh/keys_test.go b/ssh/keys_test.go
index 334ef74532..3e18488f34 100644
--- a/ssh/keys_test.go
+++ b/ssh/keys_test.go
@@ -8,6 +8,7 @@ import (
 	"bytes"
 	"crypto/dsa"
 	"crypto/ecdsa"
+	"crypto/ed25519"
 	"crypto/elliptic"
 	"crypto/rand"
 	"crypto/rsa"
@@ -15,13 +16,13 @@ import (
 	"encoding/base64"
 	"encoding/hex"
 	"encoding/pem"
+	"errors"
 	"fmt"
 	"io"
 	"reflect"
 	"strings"
 	"testing"
 
-	"golang.org/x/crypto/ed25519"
 	"golang.org/x/crypto/ssh/testdata"
 )
 
@@ -111,9 +112,9 @@ func TestKeySignVerify(t *testing.T) {
 }
 
 func TestKeySignWithAlgorithmVerify(t *testing.T) {
-	for _, priv := range testSigners {
-		if algorithmSigner, ok := priv.(AlgorithmSigner); !ok {
-			t.Errorf("Signers constructed by ssh package should always implement the AlgorithmSigner interface: %T", priv)
+	for k, priv := range testSigners {
+		if algorithmSigner, ok := priv.(MultiAlgorithmSigner); !ok {
+			t.Errorf("Signers %q constructed by ssh package should always implement the MultiAlgorithmSigner interface: %T", k, priv)
 		} else {
 			pub := priv.PublicKey()
 			data := []byte("sign me")
@@ -221,6 +222,16 @@ func TestParseEncryptedPrivateKeysWithPassphrase(t *testing.T) {
 	}
 }
 
+func TestParseEncryptedPrivateKeysWithIncorrectPassphrase(t *testing.T) {
+	pem := testdata.PEMEncryptedKeys[0].PEMBytes
+	for i := 0; i < 4096; i++ {
+		_, err := ParseRawPrivateKeyWithPassphrase(pem, []byte(fmt.Sprintf("%d", i)))
+		if !errors.Is(err, x509.IncorrectPasswordError) {
+			t.Fatalf("expected error: %v, got: %v", x509.IncorrectPasswordError, err)
+		}
+	}
+}
+
 func TestParseDSA(t *testing.T) {
 	// We actually exercise the ParsePrivateKey codepath here, as opposed to
 	// using the ParseRawPrivateKey+NewSignerFromKey path that testdata_test.go
@@ -281,6 +292,74 @@ func TestMarshalParsePublicKey(t *testing.T) {
 	}
 }
 
+func TestMarshalPrivateKey(t *testing.T) {
+	tests := []struct {
+		name string
+	}{
+		{"rsa-openssh-format"},
+		{"ed25519"},
+		{"p256-openssh-format"},
+		{"p384-openssh-format"},
+		{"p521-openssh-format"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			expected, ok := testPrivateKeys[tt.name]
+			if !ok {
+				t.Fatalf("cannot find key %s", tt.name)
+			}
+
+			block, err := MarshalPrivateKey(expected, "test@golang.org")
+			if err != nil {
+				t.Fatalf("cannot marshal %s: %v", tt.name, err)
+			}
+
+			key, err := ParseRawPrivateKey(pem.EncodeToMemory(block))
+			if err != nil {
+				t.Fatalf("cannot parse %s: %v", tt.name, err)
+			}
+
+			if !reflect.DeepEqual(expected, key) {
+				t.Errorf("unexpected marshaled key %s", tt.name)
+			}
+		})
+	}
+}
+
+func TestMarshalPrivateKeyWithPassphrase(t *testing.T) {
+	tests := []struct {
+		name string
+	}{
+		{"rsa-openssh-format"},
+		{"ed25519"},
+		{"p256-openssh-format"},
+		{"p384-openssh-format"},
+		{"p521-openssh-format"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			expected, ok := testPrivateKeys[tt.name]
+			if !ok {
+				t.Fatalf("cannot find key %s", tt.name)
+			}
+
+			block, err := MarshalPrivateKeyWithPassphrase(expected, "test@golang.org", []byte("test-passphrase"))
+			if err != nil {
+				t.Fatalf("cannot marshal %s: %v", tt.name, err)
+			}
+
+			key, err := ParseRawPrivateKeyWithPassphrase(pem.EncodeToMemory(block), []byte("test-passphrase"))
+			if err != nil {
+				t.Fatalf("cannot parse %s: %v", tt.name, err)
+			}
+
+			if !reflect.DeepEqual(expected, key) {
+				t.Errorf("unexpected marshaled key %s", tt.name)
+			}
+		})
+	}
+}
+
 type testAuthResult struct {
 	pubKey   PublicKey
 	options  []string
@@ -616,3 +695,34 @@ func TestSKKeys(t *testing.T) {
 		}
 	}
 }
+
+func TestNewSignerWithAlgos(t *testing.T) {
+	algorithSigner, ok := testSigners["rsa"].(AlgorithmSigner)
+	if !ok {
+		t.Fatal("rsa test signer does not implement the AlgorithmSigner interface")
+	}
+	_, err := NewSignerWithAlgorithms(algorithSigner, nil)
+	if err == nil {
+		t.Error("signer with algos created with no algorithms")
+	}
+
+	_, err = NewSignerWithAlgorithms(algorithSigner, []string{KeyAlgoED25519})
+	if err == nil {
+		t.Error("signer with algos created with invalid algorithms")
+	}
+
+	_, err = NewSignerWithAlgorithms(algorithSigner, []string{CertAlgoRSASHA256v01})
+	if err == nil {
+		t.Error("signer with algos created with certificate algorithms")
+	}
+
+	mas, err := NewSignerWithAlgorithms(algorithSigner, []string{KeyAlgoRSASHA256, KeyAlgoRSASHA512})
+	if err != nil {
+		t.Errorf("unable to create signer with valid algorithms: %v", err)
+	}
+
+	_, err = NewSignerWithAlgorithms(mas, []string{KeyAlgoRSA})
+	if err == nil {
+		t.Error("signer with algos created with restricted algorithms")
+	}
+}
diff --git a/ssh/mempipe_test.go b/ssh/mempipe_test.go
index 8697cd6140..f27339c51a 100644
--- a/ssh/mempipe_test.go
+++ b/ssh/mempipe_test.go
@@ -13,9 +13,10 @@ import (
 // An in-memory packetConn. It is safe to call Close and writePacket
 // from different goroutines.
 type memTransport struct {
-	eof     bool
-	pending [][]byte
-	write   *memTransport
+	eof        bool
+	pending    [][]byte
+	write      *memTransport
+	writeCount uint64
 	sync.Mutex
 	*sync.Cond
 }
@@ -63,9 +64,16 @@ func (t *memTransport) writePacket(p []byte) error {
 	copy(c, p)
 	t.write.pending = append(t.write.pending, c)
 	t.write.Cond.Signal()
+	t.writeCount++
 	return nil
 }
 
+func (t *memTransport) getWriteCount() uint64 {
+	t.write.Lock()
+	defer t.write.Unlock()
+	return t.writeCount
+}
+
 func memPipe() (a, b packetConn) {
 	t1 := memTransport{}
 	t2 := memTransport{}
@@ -81,6 +89,9 @@ func TestMemPipe(t *testing.T) {
 	if err := a.writePacket([]byte{42}); err != nil {
 		t.Fatalf("writePacket: %v", err)
 	}
+	if wc := a.(*memTransport).getWriteCount(); wc != 1 {
+		t.Fatalf("got %v, want 1", wc)
+	}
 	if err := a.Close(); err != nil {
 		t.Fatal("Close: ", err)
 	}
@@ -95,6 +106,9 @@ func TestMemPipe(t *testing.T) {
 	if err != io.EOF {
 		t.Fatalf("got %v, %v, want EOF", p, err)
 	}
+	if wc := b.(*memTransport).getWriteCount(); wc != 0 {
+		t.Fatalf("got %v, want 0", wc)
+	}
 }
 
 func TestDoubleClose(t *testing.T) {
diff --git a/ssh/messages.go b/ssh/messages.go
index 922032d952..b55f860564 100644
--- a/ssh/messages.go
+++ b/ssh/messages.go
@@ -349,6 +349,20 @@ type userAuthGSSAPIError struct {
 	LanguageTag string
 }
 
+// Transport layer OpenSSH extension. See [PROTOCOL], section 1.9
+const msgPing = 192
+
+type pingMsg struct {
+	Data string `sshtype:"192"`
+}
+
+// Transport layer OpenSSH extension. See [PROTOCOL], section 1.9
+const msgPong = 193
+
+type pongMsg struct {
+	Data string `sshtype:"193"`
+}
+
 // typeTags returns the possible type bytes for the given reflect.Type, which
 // should be a struct. The possible values are separated by a '|' character.
 func typeTags(structType reflect.Type) (tags []byte) {
diff --git a/ssh/mux.go b/ssh/mux.go
index 9654c01869..d2d24c635d 100644
--- a/ssh/mux.go
+++ b/ssh/mux.go
@@ -231,6 +231,12 @@ func (m *mux) onePacket() error {
 		return m.handleChannelOpen(packet)
 	case msgGlobalRequest, msgRequestSuccess, msgRequestFailure:
 		return m.handleGlobalPacket(packet)
+	case msgPing:
+		var msg pingMsg
+		if err := Unmarshal(packet, &msg); err != nil {
+			return fmt.Errorf("failed to unmarshal ping@openssh.com message: %w", err)
+		}
+		return m.sendMessage(pongMsg(msg))
 	}
 
 	// assume a channel packet.
diff --git a/ssh/mux_test.go b/ssh/mux_test.go
index 393017c08c..21f0ac3e32 100644
--- a/ssh/mux_test.go
+++ b/ssh/mux_test.go
@@ -5,10 +5,11 @@
 package ssh
 
 import (
+	"errors"
+	"fmt"
 	"io"
 	"sync"
 	"testing"
-	"time"
 )
 
 func muxPair() (*mux, *mux) {
@@ -29,14 +30,21 @@ func channelPair(t *testing.T) (*channel, *channel, *mux) {
 	go func() {
 		newCh, ok := <-s.incomingChannels
 		if !ok {
-			t.Fatalf("No incoming channel")
+			t.Error("no incoming channel")
+			close(res)
+			return
 		}
 		if newCh.ChannelType() != "chan" {
-			t.Fatalf("got type %q want chan", newCh.ChannelType())
+			t.Errorf("got type %q want chan", newCh.ChannelType())
+			newCh.Reject(Prohibited, fmt.Sprintf("got type %q want chan", newCh.ChannelType()))
+			close(res)
+			return
 		}
 		ch, _, err := newCh.Accept()
 		if err != nil {
-			t.Fatalf("Accept %v", err)
+			t.Errorf("accept: %v", err)
+			close(res)
+			return
 		}
 		res <- ch.(*channel)
 	}()
@@ -45,8 +53,12 @@ func channelPair(t *testing.T) (*channel, *channel, *mux) {
 	if err != nil {
 		t.Fatalf("OpenChannel: %v", err)
 	}
+	w := <-res
+	if w == nil {
+		t.Fatal("unable to get write channel")
+	}
 
-	return <-res, ch, c
+	return w, ch, c
 }
 
 // Test that stderr and stdout can be addressed from different
@@ -74,14 +86,14 @@ func TestMuxChannelExtendedThreadSafety(t *testing.T) {
 	go func() {
 		c, err := io.ReadAll(reader)
 		if string(c) != magic {
-			t.Fatalf("stdout read got %q, want %q (error %s)", c, magic, err)
+			t.Errorf("stdout read got %q, want %q (error %s)", c, magic, err)
 		}
 		rd.Done()
 	}()
 	go func() {
 		c, err := io.ReadAll(reader.Stderr())
 		if string(c) != magic {
-			t.Fatalf("stderr read got %q, want %q (error %s)", c, magic, err)
+			t.Errorf("stderr read got %q, want %q (error %s)", c, magic, err)
 		}
 		rd.Done()
 	}()
@@ -99,14 +111,20 @@ func TestMuxReadWrite(t *testing.T) {
 
 	magic := "hello world"
 	magicExt := "hello stderr"
+	var wg sync.WaitGroup
+	t.Cleanup(wg.Wait)
+	wg.Add(1)
 	go func() {
+		defer wg.Done()
 		_, err := s.Write([]byte(magic))
 		if err != nil {
-			t.Fatalf("Write: %v", err)
+			t.Errorf("Write: %v", err)
+			return
 		}
 		_, err = s.Extended(1).Write([]byte(magicExt))
 		if err != nil {
-			t.Fatalf("Write: %v", err)
+			t.Errorf("Write: %v", err)
+			return
 		}
 	}()
 
@@ -137,13 +155,15 @@ func TestMuxChannelOverflow(t *testing.T) {
 	defer writer.Close()
 	defer mux.Close()
 
-	wDone := make(chan int, 1)
+	var wg sync.WaitGroup
+	t.Cleanup(wg.Wait)
+	wg.Add(1)
 	go func() {
+		defer wg.Done()
 		if _, err := writer.Write(make([]byte, channelWindowSize)); err != nil {
 			t.Errorf("could not fill window: %v", err)
 		}
 		writer.Write(make([]byte, 1))
-		wDone <- 1
 	}()
 	writer.remoteWin.waitWriterBlocked()
 
@@ -160,7 +180,40 @@ func TestMuxChannelOverflow(t *testing.T) {
 	if _, err := reader.SendRequest("hello", true, nil); err == nil {
 		t.Errorf("SendRequest succeeded.")
 	}
-	<-wDone
+}
+
+func TestMuxChannelReadUnblock(t *testing.T) {
+	reader, writer, mux := channelPair(t)
+	defer reader.Close()
+	defer writer.Close()
+	defer mux.Close()
+
+	var wg sync.WaitGroup
+	t.Cleanup(wg.Wait)
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		if _, err := writer.Write(make([]byte, channelWindowSize)); err != nil {
+			t.Errorf("could not fill window: %v", err)
+		}
+		if _, err := writer.Write(make([]byte, 1)); err != nil {
+			t.Errorf("Write: %v", err)
+		}
+		writer.Close()
+	}()
+
+	writer.remoteWin.waitWriterBlocked()
+
+	buf := make([]byte, 32768)
+	for {
+		_, err := reader.Read(buf)
+		if err == io.EOF {
+			break
+		}
+		if err != nil {
+			t.Fatalf("Read: %v", err)
+		}
+	}
 }
 
 func TestMuxChannelCloseWriteUnblock(t *testing.T) {
@@ -169,20 +222,21 @@ func TestMuxChannelCloseWriteUnblock(t *testing.T) {
 	defer writer.Close()
 	defer mux.Close()
 
-	wDone := make(chan int, 1)
+	var wg sync.WaitGroup
+	t.Cleanup(wg.Wait)
+	wg.Add(1)
 	go func() {
+		defer wg.Done()
 		if _, err := writer.Write(make([]byte, channelWindowSize)); err != nil {
 			t.Errorf("could not fill window: %v", err)
 		}
 		if _, err := writer.Write(make([]byte, 1)); err != io.EOF {
 			t.Errorf("got %v, want EOF for unblock write", err)
 		}
-		wDone <- 1
 	}()
 
 	writer.remoteWin.waitWriterBlocked()
 	reader.Close()
-	<-wDone
 }
 
 func TestMuxConnectionCloseWriteUnblock(t *testing.T) {
@@ -191,20 +245,21 @@ func TestMuxConnectionCloseWriteUnblock(t *testing.T) {
 	defer writer.Close()
 	defer mux.Close()
 
-	wDone := make(chan int, 1)
+	var wg sync.WaitGroup
+	t.Cleanup(wg.Wait)
+	wg.Add(1)
 	go func() {
+		defer wg.Done()
 		if _, err := writer.Write(make([]byte, channelWindowSize)); err != nil {
 			t.Errorf("could not fill window: %v", err)
 		}
 		if _, err := writer.Write(make([]byte, 1)); err != io.EOF {
 			t.Errorf("got %v, want EOF for unblock write", err)
 		}
-		wDone <- 1
 	}()
 
 	writer.remoteWin.waitWriterBlocked()
 	mux.Close()
-	<-wDone
 }
 
 func TestMuxReject(t *testing.T) {
@@ -212,13 +267,21 @@ func TestMuxReject(t *testing.T) {
 	defer server.Close()
 	defer client.Close()
 
+	var wg sync.WaitGroup
+	t.Cleanup(wg.Wait)
+	wg.Add(1)
 	go func() {
+		defer wg.Done()
+
 		ch, ok := <-server.incomingChannels
 		if !ok {
-			t.Fatalf("Accept")
+			t.Error("cannot accept channel")
+			return
 		}
 		if ch.ChannelType() != "ch" || string(ch.ExtraData()) != "extra" {
-			t.Fatalf("unexpected channel: %q, %q", ch.ChannelType(), ch.ExtraData())
+			t.Errorf("unexpected channel: %q, %q", ch.ChannelType(), ch.ExtraData())
+			ch.Reject(RejectionReason(UnknownChannelType), UnknownChannelType.String())
+			return
 		}
 		ch.Reject(RejectionReason(42), "message")
 	}()
@@ -249,6 +312,7 @@ func TestMuxChannelRequest(t *testing.T) {
 
 	var received int
 	var wg sync.WaitGroup
+	t.Cleanup(wg.Wait)
 	wg.Add(1)
 	go func() {
 		for r := range server.incomingRequests {
@@ -277,7 +341,6 @@ func TestMuxChannelRequest(t *testing.T) {
 	}
 	if ok {
 		t.Errorf("SendRequest(no): %v", ok)
-
 	}
 
 	client.Close()
@@ -294,7 +357,7 @@ func TestMuxUnknownChannelRequests(t *testing.T) {
 	defer serverPipe.Close()
 	defer client.Close()
 
-	kDone := make(chan struct{})
+	kDone := make(chan error, 1)
 	go func() {
 		// Ignore unknown channel messages that don't want a reply.
 		err := serverPipe.writePacket(Marshal(channelRequestMsg{
@@ -304,7 +367,8 @@ func TestMuxUnknownChannelRequests(t *testing.T) {
 			RequestSpecificData: []byte{},
 		}))
 		if err != nil {
-			t.Fatalf("send: %v", err)
+			kDone <- fmt.Errorf("send: %w", err)
+			return
 		}
 
 		// Send a keepalive, which should get a channel failure message
@@ -316,44 +380,53 @@ func TestMuxUnknownChannelRequests(t *testing.T) {
 			RequestSpecificData: []byte{},
 		}))
 		if err != nil {
-			t.Fatalf("send: %v", err)
+			kDone <- fmt.Errorf("send: %w", err)
+			return
 		}
 
 		packet, err := serverPipe.readPacket()
 		if err != nil {
-			t.Fatalf("read packet: %v", err)
+			kDone <- fmt.Errorf("read packet: %w", err)
+			return
 		}
 		decoded, err := decode(packet)
 		if err != nil {
-			t.Fatalf("decode failed: %v", err)
+			kDone <- fmt.Errorf("decode failed: %w", err)
+			return
 		}
 
 		switch msg := decoded.(type) {
 		case *channelRequestFailureMsg:
 			if msg.PeersID != 2 {
-				t.Fatalf("received response to wrong message: %v", msg)
+				kDone <- fmt.Errorf("received response to wrong message: %v", msg)
+				return
+
 			}
 		default:
-			t.Fatalf("unexpected channel message: %v", msg)
+			kDone <- fmt.Errorf("unexpected channel message: %v", msg)
+			return
 		}
 
-		kDone <- struct{}{}
+		kDone <- nil
 
 		// Receive and respond to the keepalive to confirm the mux is
 		// still processing requests.
 		packet, err = serverPipe.readPacket()
 		if err != nil {
-			t.Fatalf("read packet: %v", err)
+			kDone <- fmt.Errorf("read packet: %w", err)
+			return
 		}
 		if packet[0] != msgGlobalRequest {
-			t.Fatalf("expected global request")
+			kDone <- errors.New("expected global request")
+			return
 		}
 
 		err = serverPipe.writePacket(Marshal(globalRequestFailureMsg{
 			Data: []byte{},
 		}))
 		if err != nil {
-			t.Fatalf("failed to send failure msg: %v", err)
+			kDone <- fmt.Errorf("failed to send failure msg: %w", err)
+			return
 		}
 
 		close(kDone)
@@ -361,10 +434,8 @@ func TestMuxUnknownChannelRequests(t *testing.T) {
 
 	// Wait for the server to send the keepalive message and receive back a
 	// response.
-	select {
-	case <-kDone:
-	case <-time.After(10 * time.Second):
-		t.Fatalf("server never received ack")
+	if err := <-kDone; err != nil {
+		t.Fatal(err)
 	}
 
 	// Confirm client hasn't closed.
@@ -372,10 +443,9 @@ func TestMuxUnknownChannelRequests(t *testing.T) {
 		t.Fatalf("failed to send keepalive: %v", err)
 	}
 
-	select {
-	case <-kDone:
-	case <-time.After(10 * time.Second):
-		t.Fatalf("server never shut down")
+	// Wait for the server to shut down.
+	if err := <-kDone; err != nil {
+		t.Fatal(err)
 	}
 }
 
@@ -385,20 +455,23 @@ func TestMuxClosedChannel(t *testing.T) {
 	defer serverPipe.Close()
 	defer client.Close()
 
-	kDone := make(chan struct{})
+	kDone := make(chan error, 1)
 	go func() {
 		// Open the channel.
 		packet, err := serverPipe.readPacket()
 		if err != nil {
-			t.Fatalf("read packet: %v", err)
+			kDone <- fmt.Errorf("read packet: %w", err)
+			return
 		}
 		if packet[0] != msgChannelOpen {
-			t.Fatalf("expected chan open")
+			kDone <- errors.New("expected chan open")
+			return
 		}
 
 		var openMsg channelOpenMsg
 		if err := Unmarshal(packet, &openMsg); err != nil {
-			t.Fatalf("unmarshal: %v", err)
+			kDone <- fmt.Errorf("unmarshal: %w", err)
+			return
 		}
 
 		// Send back the opened channel confirmation.
@@ -409,7 +482,8 @@ func TestMuxClosedChannel(t *testing.T) {
 			MaxPacketSize: channelMaxPacket,
 		}))
 		if err != nil {
-			t.Fatalf("send: %v", err)
+			kDone <- fmt.Errorf("send: %w", err)
+			return
 		}
 
 		// Close the channel.
@@ -417,7 +491,8 @@ func TestMuxClosedChannel(t *testing.T) {
 			PeersID: openMsg.PeersID,
 		}))
 		if err != nil {
-			t.Fatalf("send: %v", err)
+			kDone <- fmt.Errorf("send: %w", err)
+			return
 		}
 
 		// Send a keepalive message on the channel we just closed.
@@ -428,43 +503,51 @@ func TestMuxClosedChannel(t *testing.T) {
 			RequestSpecificData: []byte{},
 		}))
 		if err != nil {
-			t.Fatalf("send: %v", err)
+			kDone <- fmt.Errorf("send: %w", err)
+			return
 		}
 
 		// Receive the channel closed response.
 		packet, err = serverPipe.readPacket()
 		if err != nil {
-			t.Fatalf("read packet: %v", err)
+			kDone <- fmt.Errorf("read packet: %w", err)
+			return
 		}
 		if packet[0] != msgChannelClose {
-			t.Fatalf("expected channel close")
+			kDone <- errors.New("expected channel close")
+			return
 		}
 
 		// Receive the keepalive response failure.
 		packet, err = serverPipe.readPacket()
 		if err != nil {
-			t.Fatalf("read packet: %v", err)
+			kDone <- fmt.Errorf("read packet: %w", err)
+			return
 		}
 		if packet[0] != msgChannelFailure {
-			t.Fatalf("expected channel close")
+			kDone <- errors.New("expected channel failure")
+			return
 		}
-		kDone <- struct{}{}
+		kDone <- nil
 
 		// Receive and respond to the keepalive to confirm the mux is
 		// still processing requests.
 		packet, err = serverPipe.readPacket()
 		if err != nil {
-			t.Fatalf("read packet: %v", err)
+			kDone <- fmt.Errorf("read packet: %w", err)
+			return
 		}
 		if packet[0] != msgGlobalRequest {
-			t.Fatalf("expected global request")
+			kDone <- errors.New("expected global request")
+			return
 		}
 
 		err = serverPipe.writePacket(Marshal(globalRequestFailureMsg{
 			Data: []byte{},
 		}))
 		if err != nil {
-			t.Fatalf("failed to send failure msg: %v", err)
+			kDone <- fmt.Errorf("failed to send failure msg: %w", err)
+			return
 		}
 
 		close(kDone)
@@ -478,11 +561,7 @@ func TestMuxClosedChannel(t *testing.T) {
 	defer ch.Close()
 
 	// Wait for the server to close the channel and send the keepalive.
-	select {
-	case <-kDone:
-	case <-time.After(10 * time.Second):
-		t.Fatalf("server never received ack")
-	}
+	<-kDone
 
 	// Make sure the channel closed.
 	if _, ok := <-ch.incomingRequests; ok {
@@ -494,22 +573,29 @@ func TestMuxClosedChannel(t *testing.T) {
 		t.Fatalf("failed to send keepalive: %v", err)
 	}
 
-	select {
-	case <-kDone:
-	case <-time.After(10 * time.Second):
-		t.Fatalf("server never shut down")
-	}
+	// Wait for the server to shut down.
+	<-kDone
 }
 
 func TestMuxGlobalRequest(t *testing.T) {
+	var sawPeek bool
+	var wg sync.WaitGroup
+	defer func() {
+		wg.Wait()
+		if !sawPeek {
+			t.Errorf("never saw 'peek' request")
+		}
+	}()
+
 	clientMux, serverMux := muxPair()
 	defer serverMux.Close()
 	defer clientMux.Close()
 
-	var seen bool
+	wg.Add(1)
 	go func() {
+		defer wg.Done()
 		for r := range serverMux.incomingRequests {
-			seen = seen || r.Type == "peek"
+			sawPeek = sawPeek || r.Type == "peek"
 			if r.WantReply {
 				err := r.Reply(r.Type == "yes",
 					append([]byte(r.Type), r.Payload...))
@@ -539,10 +625,6 @@ func TestMuxGlobalRequest(t *testing.T) {
 		t.Errorf("SendRequest(\"no\", true, \"a\"): %v %v %v",
 			ok, data, err)
 	}
-
-	if !seen {
-		t.Errorf("never saw 'peek' request")
-	}
 }
 
 func TestMuxGlobalRequestUnblock(t *testing.T) {
@@ -692,7 +774,13 @@ func TestMuxMaxPacketSize(t *testing.T) {
 		t.Errorf("could not send packet")
 	}
 
-	go a.SendRequest("hello", false, nil)
+	var wg sync.WaitGroup
+	t.Cleanup(wg.Wait)
+	wg.Add(1)
+	go func() {
+		a.SendRequest("hello", false, nil)
+		wg.Done()
+	}()
 
 	_, ok := <-b.incomingRequests
 	if ok {
@@ -700,6 +788,43 @@ func TestMuxMaxPacketSize(t *testing.T) {
 	}
 }
 
+func TestMuxChannelWindowDeferredUpdates(t *testing.T) {
+	s, c, mux := channelPair(t)
+	cTransport := mux.conn.(*memTransport)
+	defer s.Close()
+	defer c.Close()
+	defer mux.Close()
+
+	var wg sync.WaitGroup
+	t.Cleanup(wg.Wait)
+
+	data := make([]byte, 1024)
+
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		_, err := s.Write(data)
+		if err != nil {
+			t.Errorf("Write: %v", err)
+			return
+		}
+	}()
+	cWritesInit := cTransport.getWriteCount()
+	buf := make([]byte, 1)
+	for i := 0; i < len(data); i++ {
+		n, err := c.Read(buf)
+		if n != len(buf) || err != nil {
+			t.Fatalf("Read: %v, %v", n, err)
+		}
+	}
+	cWrites := cTransport.getWriteCount() - cWritesInit
+	// reading 1 KiB should not cause any window updates to be sent, but allow
+	// for some unexpected writes
+	if cWrites > 30 {
+		t.Fatalf("reading 1 KiB from channel caused %v writes", cWrites)
+	}
+}
+
 // Don't ship code with debug=true.
 func TestDebug(t *testing.T) {
 	if debugMux {
diff --git a/ssh/server.go b/ssh/server.go
index d86617440e..d7cfeae719 100644
--- a/ssh/server.go
+++ b/ssh/server.go
@@ -64,6 +64,13 @@ type ServerConfig struct {
 	// Config contains configuration shared between client and server.
 	Config
 
+	// PublicKeyAuthAlgorithms specifies the supported client public key
+	// authentication algorithms. Note that this should not include certificate
+	// types since those use the underlying algorithm. This list is sent to the
+	// client if it supports the server-sig-algs extension. Order is irrelevant.
+	// If unspecified then a default set of algorithms is used.
+	PublicKeyAuthAlgorithms []string
+
 	hostKeys []Signer
 
 	// NoClientAuth is true if clients are allowed to connect without
@@ -213,9 +220,20 @@ func NewServerConn(c net.Conn, config *ServerConfig) (*ServerConn, <-chan NewCha
 	if fullConf.MaxAuthTries == 0 {
 		fullConf.MaxAuthTries = 6
 	}
+	if len(fullConf.PublicKeyAuthAlgorithms) == 0 {
+		fullConf.PublicKeyAuthAlgorithms = supportedPubKeyAuthAlgos
+	} else {
+		for _, algo := range fullConf.PublicKeyAuthAlgorithms {
+			if !contains(supportedPubKeyAuthAlgos, algo) {
+				c.Close()
+				return nil, nil, nil, fmt.Errorf("ssh: unsupported public key authentication algorithm %s", algo)
+			}
+		}
+	}
 	// Check if the config contains any unsupported key exchanges
 	for _, kex := range fullConf.KeyExchanges {
 		if _, ok := serverForbiddenKexAlgos[kex]; ok {
+			c.Close()
 			return nil, nil, nil, fmt.Errorf("ssh: unsupported key exchange %s for server", kex)
 		}
 	}
@@ -333,7 +351,7 @@ func checkSourceAddress(addr net.Addr, sourceAddrs string) error {
 	return fmt.Errorf("ssh: remote address %v is not allowed because of source-address restriction", addr)
 }
 
-func gssExchangeToken(gssapiConfig *GSSAPIWithMICConfig, firstToken []byte, s *connection,
+func gssExchangeToken(gssapiConfig *GSSAPIWithMICConfig, token []byte, s *connection,
 	sessionID []byte, userAuthReq userAuthRequestMsg) (authErr error, perms *Permissions, err error) {
 	gssAPIServer := gssapiConfig.Server
 	defer gssAPIServer.DeleteSecContext()
@@ -343,7 +361,7 @@ func gssExchangeToken(gssapiConfig *GSSAPIWithMICConfig, firstToken []byte, s *c
 			outToken     []byte
 			needContinue bool
 		)
-		outToken, srcName, needContinue, err = gssAPIServer.AcceptSecContext(firstToken)
+		outToken, srcName, needContinue, err = gssAPIServer.AcceptSecContext(token)
 		if err != nil {
 			return err, nil, nil
 		}
@@ -365,6 +383,7 @@ func gssExchangeToken(gssapiConfig *GSSAPIWithMICConfig, firstToken []byte, s *c
 		if err := Unmarshal(packet, userAuthGSSAPITokenReq); err != nil {
 			return nil, nil, err
 		}
+		token = userAuthGSSAPITokenReq.Token
 	}
 	packet, err := s.transport.readPacket()
 	if err != nil {
@@ -504,6 +523,11 @@ userAuthLoop:
 					authErr = nil
 				}
 			}
+
+			// allow initial attempt of 'none' without penalty
+			if authFailures == 0 {
+				authFailures--
+			}
 		case "password":
 			if config.PasswordCallback == nil {
 				authErr = errors.New("ssh: password auth not configured")
@@ -544,7 +568,7 @@ userAuthLoop:
 				return nil, parseError(msgUserAuthRequest)
 			}
 			algo := string(algoBytes)
-			if !contains(supportedPubKeyAuthAlgos, underlyingAlgo(algo)) {
+			if !contains(config.PublicKeyAuthAlgorithms, underlyingAlgo(algo)) {
 				authErr = fmt.Errorf("ssh: algorithm %q not accepted", algo)
 				break
 			}
@@ -596,13 +620,22 @@ userAuthLoop:
 				if !ok || len(payload) > 0 {
 					return nil, parseError(msgUserAuthRequest)
 				}
-
+				// Ensure the declared public key algo is compatible with the
+				// decoded one. This check will ensure we don't accept e.g.
+				// ssh-rsa-cert-v01@openssh.com algorithm with ssh-rsa public
+				// key type. The algorithm and public key type must be
+				// consistent: both must be certificate algorithms, or neither.
+				if !contains(algorithmsForKeyFormat(pubKey.Type()), algo) {
+					authErr = fmt.Errorf("ssh: public key type %q not compatible with selected algorithm %q",
+						pubKey.Type(), algo)
+					break
+				}
 				// Ensure the public key algo and signature algo
 				// are supported.  Compare the private key
 				// algorithm name that corresponds to algo with
 				// sig.Format.  This is usually the same, but
 				// for certs, the names differ.
-				if !contains(supportedPubKeyAuthAlgos, sig.Format) {
+				if !contains(config.PublicKeyAuthAlgorithms, sig.Format) {
 					authErr = fmt.Errorf("ssh: algorithm %q not accepted", sig.Format)
 					break
 				}
@@ -696,9 +729,7 @@ userAuthLoop:
 			return nil, authErr
 		}
 
-		if userAuthReq.Method != "none" {
-			authFailures++
-		}
+		authFailures++
 		if config.MaxAuthTries > 0 && authFailures >= config.MaxAuthTries {
 			// If we have hit the max attempts, don't bother sending the
 			// final SSH_MSG_USERAUTH_FAILURE message, since there are
diff --git a/ssh/server_test.go b/ssh/server_test.go
new file mode 100644
index 0000000000..a9b2bce0c1
--- /dev/null
+++ b/ssh/server_test.go
@@ -0,0 +1,140 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ssh
+
+import (
+	"io"
+	"net"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+func TestClientAuthRestrictedPublicKeyAlgos(t *testing.T) {
+	for _, tt := range []struct {
+		name      string
+		key       Signer
+		wantError bool
+	}{
+		{"rsa", testSigners["rsa"], false},
+		{"dsa", testSigners["dsa"], true},
+		{"ed25519", testSigners["ed25519"], true},
+	} {
+		c1, c2, err := netPipe()
+		if err != nil {
+			t.Fatalf("netPipe: %v", err)
+		}
+		defer c1.Close()
+		defer c2.Close()
+		serverConf := &ServerConfig{
+			PublicKeyAuthAlgorithms: []string{KeyAlgoRSASHA256, KeyAlgoRSASHA512},
+			PublicKeyCallback: func(conn ConnMetadata, key PublicKey) (*Permissions, error) {
+				return nil, nil
+			},
+		}
+		serverConf.AddHostKey(testSigners["ecdsap256"])
+
+		done := make(chan struct{})
+		go func() {
+			defer close(done)
+			NewServerConn(c1, serverConf)
+		}()
+
+		clientConf := ClientConfig{
+			User: "user",
+			Auth: []AuthMethod{
+				PublicKeys(tt.key),
+			},
+			HostKeyCallback: InsecureIgnoreHostKey(),
+		}
+
+		_, _, _, err = NewClientConn(c2, "", &clientConf)
+		if err != nil {
+			if !tt.wantError {
+				t.Errorf("%s: got unexpected error %q", tt.name, err.Error())
+			}
+		} else if tt.wantError {
+			t.Errorf("%s: succeeded, but want error", tt.name)
+		}
+		<-done
+	}
+}
+
+func TestNewServerConnValidationErrors(t *testing.T) {
+	serverConf := &ServerConfig{
+		PublicKeyAuthAlgorithms: []string{CertAlgoRSAv01},
+	}
+	c := &markerConn{}
+	_, _, _, err := NewServerConn(c, serverConf)
+	if err == nil {
+		t.Fatal("NewServerConn with invalid public key auth algorithms succeeded")
+	}
+	if !c.isClosed() {
+		t.Fatal("NewServerConn with invalid public key auth algorithms left connection open")
+	}
+	if c.isUsed() {
+		t.Fatal("NewServerConn with invalid public key auth algorithms used connection")
+	}
+
+	serverConf = &ServerConfig{
+		Config: Config{
+			KeyExchanges: []string{kexAlgoDHGEXSHA256},
+		},
+	}
+	c = &markerConn{}
+	_, _, _, err = NewServerConn(c, serverConf)
+	if err == nil {
+		t.Fatal("NewServerConn with unsupported key exchange succeeded")
+	}
+	if !c.isClosed() {
+		t.Fatal("NewServerConn with unsupported key exchange left connection open")
+	}
+	if c.isUsed() {
+		t.Fatal("NewServerConn with unsupported key exchange used connection")
+	}
+}
+
+type markerConn struct {
+	closed uint32
+	used   uint32
+}
+
+func (c *markerConn) isClosed() bool {
+	return atomic.LoadUint32(&c.closed) != 0
+}
+
+func (c *markerConn) isUsed() bool {
+	return atomic.LoadUint32(&c.used) != 0
+}
+
+func (c *markerConn) Close() error {
+	atomic.StoreUint32(&c.closed, 1)
+	return nil
+}
+
+func (c *markerConn) Read(b []byte) (n int, err error) {
+	atomic.StoreUint32(&c.used, 1)
+	if atomic.LoadUint32(&c.closed) != 0 {
+		return 0, net.ErrClosed
+	} else {
+		return 0, io.EOF
+	}
+}
+
+func (c *markerConn) Write(b []byte) (n int, err error) {
+	atomic.StoreUint32(&c.used, 1)
+	if atomic.LoadUint32(&c.closed) != 0 {
+		return 0, net.ErrClosed
+	} else {
+		return 0, io.ErrClosedPipe
+	}
+}
+
+func (*markerConn) LocalAddr() net.Addr  { return nil }
+func (*markerConn) RemoteAddr() net.Addr { return nil }
+
+func (*markerConn) SetDeadline(t time.Time) error      { return nil }
+func (*markerConn) SetReadDeadline(t time.Time) error  { return nil }
+func (*markerConn) SetWriteDeadline(t time.Time) error { return nil }
diff --git a/ssh/session_test.go b/ssh/session_test.go
index c4b9f0ea5b..807a913e5a 100644
--- a/ssh/session_test.go
+++ b/ssh/session_test.go
@@ -13,6 +13,7 @@ import (
 	"io"
 	"math/rand"
 	"net"
+	"sync"
 	"testing"
 
 	"golang.org/x/crypto/ssh/terminal"
@@ -27,8 +28,14 @@ func dial(handler serverType, t *testing.T) *Client {
 		t.Fatalf("netPipe: %v", err)
 	}
 
+	var wg sync.WaitGroup
+	t.Cleanup(wg.Wait)
+	wg.Add(1)
 	go func() {
-		defer c1.Close()
+		defer func() {
+			c1.Close()
+			wg.Done()
+		}()
 		conf := ServerConfig{
 			NoClientAuth: true,
 		}
@@ -36,9 +43,14 @@ func dial(handler serverType, t *testing.T) *Client {
 
 		conn, chans, reqs, err := NewServerConn(c1, &conf)
 		if err != nil {
-			t.Fatalf("Unable to handshake: %v", err)
+			t.Errorf("Unable to handshake: %v", err)
+			return
 		}
-		go DiscardRequests(reqs)
+		wg.Add(1)
+		go func() {
+			DiscardRequests(reqs)
+			wg.Done()
+		}()
 
 		for newCh := range chans {
 			if newCh.ChannelType() != "session" {
@@ -51,8 +63,10 @@ func dial(handler serverType, t *testing.T) *Client {
 				t.Errorf("Accept: %v", err)
 				continue
 			}
+			wg.Add(1)
 			go func() {
 				handler(ch, inReqs, t)
+				wg.Done()
 			}()
 		}
 		if err := conn.Wait(); err != io.EOF {
@@ -337,8 +351,13 @@ func TestServerWindow(t *testing.T) {
 		t.Fatal(err)
 	}
 	defer session.Close()
-	result := make(chan []byte)
 
+	serverStdin, err := session.StdinPipe()
+	if err != nil {
+		t.Fatalf("StdinPipe failed: %v", err)
+	}
+
+	result := make(chan []byte)
 	go func() {
 		defer close(result)
 		echoedBuf := bytes.NewBuffer(make([]byte, 0, windowTestBytes))
@@ -354,10 +373,6 @@ func TestServerWindow(t *testing.T) {
 		result <- echoedBuf.Bytes()
 	}()
 
-	serverStdin, err := session.StdinPipe()
-	if err != nil {
-		t.Fatalf("StdinPipe failed: %v", err)
-	}
 	written, err := copyNRandomly("stdin", serverStdin, origBuf, windowTestBytes)
 	if err != nil {
 		t.Errorf("failed to copy origBuf to serverStdin: %v", err)
@@ -647,30 +662,57 @@ func TestSessionID(t *testing.T) {
 		User:            "user",
 	}
 
+	var wg sync.WaitGroup
+	t.Cleanup(wg.Wait)
+
+	srvErrCh := make(chan error, 1)
+	wg.Add(1)
 	go func() {
+		defer wg.Done()
 		conn, chans, reqs, err := NewServerConn(c1, serverConf)
+		srvErrCh <- err
 		if err != nil {
-			t.Fatalf("server handshake: %v", err)
+			return
 		}
 		serverID <- conn.SessionID()
-		go DiscardRequests(reqs)
+		wg.Add(1)
+		go func() {
+			DiscardRequests(reqs)
+			wg.Done()
+		}()
 		for ch := range chans {
 			ch.Reject(Prohibited, "")
 		}
 	}()
 
+	cliErrCh := make(chan error, 1)
+	wg.Add(1)
 	go func() {
+		defer wg.Done()
 		conn, chans, reqs, err := NewClientConn(c2, "", clientConf)
+		cliErrCh <- err
 		if err != nil {
-			t.Fatalf("client handshake: %v", err)
+			return
 		}
 		clientID <- conn.SessionID()
-		go DiscardRequests(reqs)
+		wg.Add(1)
+		go func() {
+			DiscardRequests(reqs)
+			wg.Done()
+		}()
 		for ch := range chans {
 			ch.Reject(Prohibited, "")
 		}
 	}()
 
+	if err := <-srvErrCh; err != nil {
+		t.Fatalf("server handshake: %v", err)
+	}
+
+	if err := <-cliErrCh; err != nil {
+		t.Fatalf("client handshake: %v", err)
+	}
+
 	s := <-serverID
 	c := <-clientID
 	if bytes.Compare(s, c) != 0 {
@@ -725,6 +767,8 @@ func TestHostKeyAlgorithms(t *testing.T) {
 	serverConf.AddHostKey(testSigners["rsa"])
 	serverConf.AddHostKey(testSigners["ecdsa"])
 
+	var wg sync.WaitGroup
+	t.Cleanup(wg.Wait)
 	connect := func(clientConf *ClientConfig, want string) {
 		var alg string
 		clientConf.HostKeyCallback = func(h string, a net.Addr, key PublicKey) error {
@@ -738,7 +782,11 @@ func TestHostKeyAlgorithms(t *testing.T) {
 		defer c1.Close()
 		defer c2.Close()
 
-		go NewServerConn(c1, serverConf)
+		wg.Add(1)
+		go func() {
+			NewServerConn(c1, serverConf)
+			wg.Done()
+		}()
 		_, _, _, err = NewClientConn(c2, "", clientConf)
 		if err != nil {
 			t.Fatalf("NewClientConn: %v", err)
@@ -772,7 +820,11 @@ func TestHostKeyAlgorithms(t *testing.T) {
 	defer c1.Close()
 	defer c2.Close()
 
-	go NewServerConn(c1, serverConf)
+	wg.Add(1)
+	go func() {
+		NewServerConn(c1, serverConf)
+		wg.Done()
+	}()
 	clientConf.HostKeyAlgorithms = []string{"nonexistent-hostkey-algo"}
 	_, _, _, err = NewClientConn(c2, "", clientConf)
 	if err == nil {
@@ -805,14 +857,22 @@ func TestServerClientAuthCallback(t *testing.T) {
 		User:            someUsername,
 	}
 
+	var wg sync.WaitGroup
+	t.Cleanup(wg.Wait)
+	wg.Add(1)
 	go func() {
+		defer wg.Done()
 		_, chans, reqs, err := NewServerConn(c1, serverConf)
 		if err != nil {
 			t.Errorf("server handshake: %v", err)
 			userCh <- "error"
 			return
 		}
-		go DiscardRequests(reqs)
+		wg.Add(1)
+		go func() {
+			DiscardRequests(reqs)
+			wg.Done()
+		}()
 		for ch := range chans {
 			ch.Reject(Prohibited, "")
 		}
diff --git a/ssh/tcpip.go b/ssh/tcpip.go
index 80d35f5ec1..ef5059a11d 100644
--- a/ssh/tcpip.go
+++ b/ssh/tcpip.go
@@ -5,6 +5,7 @@
 package ssh
 
 import (
+	"context"
 	"errors"
 	"fmt"
 	"io"
@@ -332,6 +333,40 @@ func (l *tcpListener) Addr() net.Addr {
 	return l.laddr
 }
 
+// DialContext initiates a connection to the addr from the remote host.
+//
+// The provided Context must be non-nil. If the context expires before the
+// connection is complete, an error is returned. Once successfully connected,
+// any expiration of the context will not affect the connection.
+//
+// See func Dial for additional information.
+func (c *Client) DialContext(ctx context.Context, n, addr string) (net.Conn, error) {
+	if err := ctx.Err(); err != nil {
+		return nil, err
+	}
+	type connErr struct {
+		conn net.Conn
+		err  error
+	}
+	ch := make(chan connErr)
+	go func() {
+		conn, err := c.Dial(n, addr)
+		select {
+		case ch <- connErr{conn, err}:
+		case <-ctx.Done():
+			if conn != nil {
+				conn.Close()
+			}
+		}
+	}()
+	select {
+	case res := <-ch:
+		return res.conn, res.err
+	case <-ctx.Done():
+		return nil, ctx.Err()
+	}
+}
+
 // Dial initiates a connection to the addr from the remote host.
 // The resulting connection has a zero LocalAddr() and RemoteAddr().
 func (c *Client) Dial(n, addr string) (net.Conn, error) {
diff --git a/ssh/tcpip_test.go b/ssh/tcpip_test.go
index f1265cb496..4d85114727 100644
--- a/ssh/tcpip_test.go
+++ b/ssh/tcpip_test.go
@@ -5,7 +5,10 @@
 package ssh
 
 import (
+	"context"
+	"net"
 	"testing"
+	"time"
 )
 
 func TestAutoPortListenBroken(t *testing.T) {
@@ -18,3 +21,33 @@ func TestAutoPortListenBroken(t *testing.T) {
 		t.Errorf("version %q marked as broken", works)
 	}
 }
+
+func TestClientImplementsDialContext(t *testing.T) {
+	type ContextDialer interface {
+		DialContext(context.Context, string, string) (net.Conn, error)
+	}
+	// Belt and suspenders assertion, since package net does not
+	// declare a ContextDialer type.
+	var _ ContextDialer = &net.Dialer{}
+	var _ ContextDialer = &Client{}
+}
+
+func TestClientDialContextWithCancel(t *testing.T) {
+	c := &Client{}
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+	_, err := c.DialContext(ctx, "tcp", "localhost:1000")
+	if err != context.Canceled {
+		t.Errorf("DialContext: got nil error, expected %v", context.Canceled)
+	}
+}
+
+func TestClientDialContextWithDeadline(t *testing.T) {
+	c := &Client{}
+	ctx, cancel := context.WithDeadline(context.Background(), time.Now())
+	defer cancel()
+	_, err := c.DialContext(ctx, "tcp", "localhost:1000")
+	if err != context.DeadlineExceeded {
+		t.Errorf("DialContext: got nil error, expected %v", context.DeadlineExceeded)
+	}
+}
diff --git a/ssh/test/agent_unix_test.go b/ssh/test/agent_unix_test.go
index 43fbdb22eb..aeecaebb1d 100644
--- a/ssh/test/agent_unix_test.go
+++ b/ssh/test/agent_unix_test.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build aix || darwin || dragonfly || freebsd || linux || netbsd || openbsd || solaris
-// +build aix darwin dragonfly freebsd linux netbsd openbsd solaris
 
 package test
 
diff --git a/ssh/test/banner_test.go b/ssh/test/banner_test.go
index 3bfdd4b059..dd17db5a87 100644
--- a/ssh/test/banner_test.go
+++ b/ssh/test/banner_test.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build aix || darwin || dragonfly || freebsd || linux || netbsd || openbsd || solaris
-// +build aix darwin dragonfly freebsd linux netbsd openbsd solaris
 
 package test
 
diff --git a/ssh/test/cert_test.go b/ssh/test/cert_test.go
index 83dd534c5c..15c44fb58e 100644
--- a/ssh/test/cert_test.go
+++ b/ssh/test/cert_test.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build aix || darwin || dragonfly || freebsd || linux || netbsd || openbsd || solaris
-// +build aix darwin dragonfly freebsd linux netbsd openbsd solaris
 
 package test
 
diff --git a/ssh/test/dial_unix_test.go b/ssh/test/dial_unix_test.go
index 4a7ec31737..8ec8d50a50 100644
--- a/ssh/test/dial_unix_test.go
+++ b/ssh/test/dial_unix_test.go
@@ -3,13 +3,13 @@
 // license that can be found in the LICENSE file.
 
 //go:build !windows && !js && !wasip1
-// +build !windows,!js,!wasip1
 
 package test
 
 // direct-tcpip and direct-streamlocal functional tests
 
 import (
+	"context"
 	"fmt"
 	"io"
 	"net"
@@ -47,7 +47,11 @@ func testDial(t *testing.T, n, listenAddr string, x dialTester) {
 		}
 	}()
 
-	conn, err := sshConn.Dial(n, l.Addr().String())
+	ctx, cancel := context.WithCancel(context.Background())
+	conn, err := sshConn.DialContext(ctx, n, l.Addr().String())
+	// Canceling the context after dial should have no effect
+	// on the opened connection.
+	cancel()
 	if err != nil {
 		t.Fatalf("Dial: %v", err)
 	}
diff --git a/ssh/test/forward_unix_test.go b/ssh/test/forward_unix_test.go
index 1171bc3a14..611122b9e8 100644
--- a/ssh/test/forward_unix_test.go
+++ b/ssh/test/forward_unix_test.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build aix || darwin || dragonfly || freebsd || linux || netbsd || openbsd || solaris
-// +build aix darwin dragonfly freebsd linux netbsd openbsd solaris
 
 package test
 
diff --git a/ssh/test/multi_auth_test.go b/ssh/test/multi_auth_test.go
index 6c253a7547..14cf1cce12 100644
--- a/ssh/test/multi_auth_test.go
+++ b/ssh/test/multi_auth_test.go
@@ -15,7 +15,6 @@
 // (for linux) in file ./sshd_test_pw.c.
 
 //go:build linux
-// +build linux
 
 package test
 
@@ -77,27 +76,27 @@ func (ctx *multiAuthTestCtx) kbdIntCb(user, instruction string, questions []stri
 func TestMultiAuth(t *testing.T) {
 	testCases := []multiAuthTestCase{
 		// Test password,publickey authentication, assert that password callback is called 1 time
-		multiAuthTestCase{
+		{
 			authMethods:         []string{"password", "publickey"},
 			expectedPasswordCbs: 1,
 		},
 		// Test keyboard-interactive,publickey authentication, assert that keyboard-interactive callback is called 1 time
-		multiAuthTestCase{
+		{
 			authMethods:       []string{"keyboard-interactive", "publickey"},
 			expectedKbdIntCbs: 1,
 		},
 		// Test publickey,password authentication, assert that password callback is called 1 time
-		multiAuthTestCase{
+		{
 			authMethods:         []string{"publickey", "password"},
 			expectedPasswordCbs: 1,
 		},
 		// Test publickey,keyboard-interactive authentication, assert that keyboard-interactive callback is called 1 time
-		multiAuthTestCase{
+		{
 			authMethods:       []string{"publickey", "keyboard-interactive"},
 			expectedKbdIntCbs: 1,
 		},
 		// Test password,password authentication, assert that password callback is called 2 times
-		multiAuthTestCase{
+		{
 			authMethods:         []string{"password", "password"},
 			expectedPasswordCbs: 2,
 		},
diff --git a/ssh/test/server_test.go b/ssh/test/server_test.go
new file mode 100644
index 0000000000..e70241bc6e
--- /dev/null
+++ b/ssh/test/server_test.go
@@ -0,0 +1,98 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package test
+
+import (
+	"net"
+
+	"golang.org/x/crypto/ssh"
+)
+
+type exitStatusMsg struct {
+	Status uint32
+}
+
+// goServer is a test Go SSH server that accepts public key and certificate
+// authentication and replies with a 0 exit status to any exec request without
+// running any commands.
+type goTestServer struct {
+	listener net.Listener
+	config   *ssh.ServerConfig
+	done     <-chan struct{}
+}
+
+func newTestServer(config *ssh.ServerConfig) (*goTestServer, error) {
+	server := &goTestServer{
+		config: config,
+	}
+	listener, err := net.Listen("tcp", "127.0.0.1:")
+	if err != nil {
+		return nil, err
+	}
+	server.listener = listener
+	done := make(chan struct{}, 1)
+	server.done = done
+	go server.acceptConnections(done)
+
+	return server, nil
+}
+
+func (s *goTestServer) port() (string, error) {
+	_, port, err := net.SplitHostPort(s.listener.Addr().String())
+	return port, err
+}
+
+func (s *goTestServer) acceptConnections(done chan<- struct{}) {
+	defer close(done)
+
+	for {
+		c, err := s.listener.Accept()
+		if err != nil {
+			return
+		}
+		_, chans, reqs, err := ssh.NewServerConn(c, s.config)
+		if err != nil {
+			return
+		}
+		go ssh.DiscardRequests(reqs)
+		defer c.Close()
+
+		for newChannel := range chans {
+			if newChannel.ChannelType() != "session" {
+				newChannel.Reject(ssh.UnknownChannelType, "unknown channel type")
+				continue
+			}
+
+			channel, requests, err := newChannel.Accept()
+			if err != nil {
+				continue
+			}
+
+			go func(in <-chan *ssh.Request) {
+				for req := range in {
+					ok := false
+					switch req.Type {
+					case "exec":
+						ok = true
+						go func() {
+							channel.SendRequest("exit-status", false, ssh.Marshal(&exitStatusMsg{Status: 0}))
+							channel.Close()
+						}()
+					}
+					if req.WantReply {
+						req.Reply(ok, nil)
+					}
+				}
+			}(requests)
+		}
+	}
+}
+
+func (s *goTestServer) Close() error {
+	err := s.listener.Close()
+	// wait for the accept loop to exit
+	<-s.done
+	return err
+}
diff --git a/ssh/test/session_test.go b/ssh/test/session_test.go
index b44877ebef..69c8b86dd5 100644
--- a/ssh/test/session_test.go
+++ b/ssh/test/session_test.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build !windows && !js && !wasip1
-// +build !windows,!js,!wasip1
 
 package test
 
@@ -410,6 +409,9 @@ func TestKeyExchanges(t *testing.T) {
 	// are not included in the default list of supported kex so we have to add them
 	// here manually.
 	kexOrder = append(kexOrder, "diffie-hellman-group-exchange-sha1", "diffie-hellman-group-exchange-sha256")
+	// The key exchange algorithms diffie-hellman-group16-sha512 is disabled by
+	// default so we add it here manually.
+	kexOrder = append(kexOrder, "diffie-hellman-group16-sha512")
 	for _, kex := range kexOrder {
 		t.Run(kex, func(t *testing.T) {
 			server := newServer(t)
diff --git a/ssh/test/sshcli_test.go b/ssh/test/sshcli_test.go
new file mode 100644
index 0000000000..07d53d5956
--- /dev/null
+++ b/ssh/test/sshcli_test.go
@@ -0,0 +1,100 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package test
+
+import (
+	"bytes"
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"runtime"
+	"testing"
+
+	"github.com/tailscale/golang-x-crypto/internal/testenv"
+	"golang.org/x/crypto/ssh"
+	"golang.org/x/crypto/ssh/testdata"
+)
+
+func sshClient(t *testing.T) string {
+	if testing.Short() {
+		t.Skip("Skipping test that executes OpenSSH in -short mode")
+	}
+	sshCLI := os.Getenv("SSH_CLI_PATH")
+	if sshCLI == "" {
+		sshCLI = "ssh"
+	}
+	var err error
+	sshCLI, err = exec.LookPath(sshCLI)
+	if err != nil {
+		t.Skipf("Can't find an ssh(1) client to test against: %v", err)
+	}
+	return sshCLI
+}
+
+func TestSSHCLIAuth(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skipf("always fails on Windows, see #64403")
+	}
+	sshCLI := sshClient(t)
+	dir := t.TempDir()
+	keyPrivPath := filepath.Join(dir, "rsa")
+
+	for fn, content := range map[string][]byte{
+		keyPrivPath:                        testdata.PEMBytes["rsa"],
+		keyPrivPath + ".pub":               ssh.MarshalAuthorizedKey(testPublicKeys["rsa"]),
+		filepath.Join(dir, "rsa-cert.pub"): testdata.SSHCertificates["rsa-user-testcertificate"],
+	} {
+		if err := os.WriteFile(fn, content, 0600); err != nil {
+			t.Fatalf("WriteFile(%q): %v", fn, err)
+		}
+	}
+
+	certChecker := ssh.CertChecker{
+		IsUserAuthority: func(k ssh.PublicKey) bool {
+			return bytes.Equal(k.Marshal(), testPublicKeys["ca"].Marshal())
+		},
+		UserKeyFallback: func(conn ssh.ConnMetadata, key ssh.PublicKey) (*ssh.Permissions, error) {
+			if conn.User() == "testpubkey" && bytes.Equal(key.Marshal(), testPublicKeys["rsa"].Marshal()) {
+				return nil, nil
+			}
+
+			return nil, fmt.Errorf("pubkey for %q not acceptable", conn.User())
+		},
+	}
+
+	config := &ssh.ServerConfig{
+		PublicKeyCallback: certChecker.Authenticate,
+	}
+	config.AddHostKey(testSigners["rsa"])
+
+	server, err := newTestServer(config)
+	if err != nil {
+		t.Fatalf("unable to start test server: %v", err)
+	}
+	defer server.Close()
+
+	port, err := server.port()
+	if err != nil {
+		t.Fatalf("unable to get server port: %v", err)
+	}
+
+	// test public key authentication.
+	cmd := testenv.Command(t, sshCLI, "-vvv", "-i", keyPrivPath, "-o", "StrictHostKeyChecking=no",
+		"-p", port, "testpubkey@127.0.0.1", "true")
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("public key authentication failed, error: %v, command output %q", err, string(out))
+	}
+	// Test SSH user certificate authentication.
+	// The username must match one of the principals included in the certificate.
+	// The certificate "rsa-user-testcertificate" has "testcertificate" as principal.
+	cmd = testenv.Command(t, sshCLI, "-vvv", "-i", keyPrivPath, "-o", "StrictHostKeyChecking=no",
+		"-p", port, "testcertificate@127.0.0.1", "true")
+	out, err = cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("user certificate authentication failed, error: %v, command output %q", err, string(out))
+	}
+}
diff --git a/ssh/test/sshd_test_pw.c b/ssh/test/sshd_test_pw.c
index 2794a563a4..1e48619994 100644
--- a/ssh/test/sshd_test_pw.c
+++ b/ssh/test/sshd_test_pw.c
@@ -20,7 +20,7 @@
 // Run sshd:
 // LD_PRELOAD="sshd_test_pw.so" TEST_USER="..." TEST_PASSWD="..." sshd ...
 
-// +build ignore
+//go:build ignore
 
 #define _GNU_SOURCE
 #include <string.h>
diff --git a/ssh/test/test_unix_test.go b/ssh/test/test_unix_test.go
index 0d301c53ad..45882e66f5 100644
--- a/ssh/test/test_unix_test.go
+++ b/ssh/test/test_unix_test.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build aix || darwin || dragonfly || freebsd || linux || netbsd || openbsd || plan9 || solaris
-// +build aix darwin dragonfly freebsd linux netbsd openbsd plan9 solaris
 
 package test
 
diff --git a/ssh/testdata/keys.go b/ssh/testdata/keys.go
index ad95a81929..69f9eef4de 100644
--- a/ssh/testdata/keys.go
+++ b/ssh/testdata/keys.go
@@ -195,6 +195,12 @@ var SSHCertificates = map[string][]byte{
 	"rsa-sha2-256": []byte(`ssh-rsa-cert-v01@openssh.com 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 host.example.com
 `),
 	"rsa-sha2-512": []byte(`ssh-rsa-cert-v01@openssh.com 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 host.example.com
+`),
+	// Assumes "ca" key above in file named "ca", sign a user cert for "rsa.pub"
+	// using "testcertificate" as principal:
+	//
+	// ssh-keygen -s ca -I username -n testcertificate rsa.pub
+	"rsa-user-testcertificate": []byte(`ssh-rsa-cert-v01@openssh.com AAAAHHNzaC1yc2EtY2VydC12MDFAb3BlbnNzaC5jb20AAAAgr0MnhSf+KkQWEQmweJOGsJfOrUt80pQZDaI9YiwSfDUAAAADAQABAAAAgQC8A6FGHDiWCSREAXCq6yBfNVr0xCVG2CzvktFNRpue+RXrGs/2a6ySEJQb3IYquw7HlJgu6fg3WIWhOmHCjfpG0PrL4CRwbqQ2LaPPXhJErWYejcD8Di00cF3677+G10KMZk9RXbmHtuBFZT98wxg8j+ZsBMqGM1+7yrWUvynswQAAAAAAAAAAAAAAAQAAAAh1c2VybmFtZQAAABMAAAAPdGVzdGNlcnRpZmljYXRlAAAAAAAAAAD//////////wAAAAAAAACCAAAAFXBlcm1pdC1YMTEtZm9yd2FyZGluZwAAAAAAAAAXcGVybWl0LWFnZW50LWZvcndhcmRpbmcAAAAAAAAAFnBlcm1pdC1wb3J0LWZvcndhcmRpbmcAAAAAAAAACnBlcm1pdC1wdHkAAAAAAAAADnBlcm1pdC11c2VyLXJjAAAAAAAAAAAAAAEXAAAAB3NzaC1yc2EAAAADAQABAAABAQC+D11D0hEbn2Vglv4YRJ8pZNyHjIGmvth3DWOQrq++2vH2MujmGQDxfr4SVE9GpMBlKU3lwGbpgIBxAg6yZcNSfo6PWVU9ACg6NMFO+yMzc2MaG+/naQdNjSewywF5j2rkNO2XOaViRVSrZroe2B/aY2LTV0jDl8nu5NOjwRs1/s7SLe5z1rw/X0dpmXk0qJY3gQhmR8HZZ1dhEkJUGwaBCPd0T8asSYf1Ag2rUD4aQ28r3q69mbwfWOOa6rMemVZruUV5dzHwVNVNtVv+ImtnYtz8m8g+K0plaGptHn3KsaOnASkh3tujhaE7kvc4HR9Igli9+76jhZie3h/dTN5zAAABFAAAAAxyc2Etc2hhMi01MTIAAAEAFuA+67KvnlmcodIp0Lv4mR9UW/CHghAaN1csBJTkI8mx3wXKyIPTsS2uXboEhWD0a7S9gps2SEwC5m6E3kV2Rzg7aH1S03GZqMvVlK2VHe7fzuoW2yOKk6yEPjeTF0pKCFbUQ6mce8pRpD/zdvjG0Z287XM3c3Axlrn7qq7TS0MDTjEZ/dsUNFHxep3co/HuAsWVWPVDItr/FPnvZ6WVH1yc8N/AJn0gLHobkGgug22vI9rNIge1wrnXxU9BUSouzkau/PQsrCQapnn+I1H7HaQt0wdk45nxMP+ags+HRI9qpX/p8WDn6+zpqYqN/nfw2aoytyaJqhsV32Teuqtrgg== rsa.pub
 `),
 }
 
diff --git a/ssh/transport.go b/ssh/transport.go
index da015801ea..0424d2d37c 100644
--- a/ssh/transport.go
+++ b/ssh/transport.go
@@ -49,6 +49,9 @@ type transport struct {
 	rand      io.Reader
 	isClient  bool
 	io.Closer
+
+	strictMode     bool
+	initialKEXDone bool
 }
 
 // packetCipher represents a combination of SSH encryption/MAC
@@ -74,6 +77,18 @@ type connectionState struct {
 	pendingKeyChange chan packetCipher
 }
 
+func (t *transport) setStrictMode() error {
+	if t.reader.seqNum != 1 {
+		return errors.New("ssh: sequence number != 1 when strict KEX mode requested")
+	}
+	t.strictMode = true
+	return nil
+}
+
+func (t *transport) setInitialKEXDone() {
+	t.initialKEXDone = true
+}
+
 // prepareKeyChange sets up key material for a keychange. The key changes in
 // both directions are triggered by reading and writing a msgNewKey packet
 // respectively.
@@ -112,11 +127,12 @@ func (t *transport) printPacket(p []byte, write bool) {
 // Read and decrypt next packet.
 func (t *transport) readPacket() (p []byte, err error) {
 	for {
-		p, err = t.reader.readPacket(t.bufReader)
+		p, err = t.reader.readPacket(t.bufReader, t.strictMode)
 		if err != nil {
 			break
 		}
-		if len(p) == 0 || (p[0] != msgIgnore && p[0] != msgDebug) {
+		// in strict mode we pass through DEBUG and IGNORE packets only during the initial KEX
+		if len(p) == 0 || (t.strictMode && !t.initialKEXDone) || (p[0] != msgIgnore && p[0] != msgDebug) {
 			break
 		}
 	}
@@ -127,7 +143,7 @@ func (t *transport) readPacket() (p []byte, err error) {
 	return p, err
 }
 
-func (s *connectionState) readPacket(r *bufio.Reader) ([]byte, error) {
+func (s *connectionState) readPacket(r *bufio.Reader, strictMode bool) ([]byte, error) {
 	packet, err := s.packetCipher.readCipherPacket(s.seqNum, r)
 	s.seqNum++
 	if err == nil && len(packet) == 0 {
@@ -140,6 +156,9 @@ func (s *connectionState) readPacket(r *bufio.Reader) ([]byte, error) {
 			select {
 			case cipher := <-s.pendingKeyChange:
 				s.packetCipher = cipher
+				if strictMode {
+					s.seqNum = 0
+				}
 			default:
 				return nil, errors.New("ssh: got bogus newkeys message")
 			}
@@ -170,10 +189,10 @@ func (t *transport) writePacket(packet []byte) error {
 	if debugTransport {
 		t.printPacket(packet, true)
 	}
-	return t.writer.writePacket(t.bufWriter, t.rand, packet)
+	return t.writer.writePacket(t.bufWriter, t.rand, packet, t.strictMode)
 }
 
-func (s *connectionState) writePacket(w *bufio.Writer, rand io.Reader, packet []byte) error {
+func (s *connectionState) writePacket(w *bufio.Writer, rand io.Reader, packet []byte, strictMode bool) error {
 	changeKeys := len(packet) > 0 && packet[0] == msgNewKeys
 
 	err := s.packetCipher.writeCipherPacket(s.seqNum, w, rand, packet)
@@ -188,6 +207,9 @@ func (s *connectionState) writePacket(w *bufio.Writer, rand io.Reader, packet []
 		select {
 		case cipher := <-s.pendingKeyChange:
 			s.packetCipher = cipher
+			if strictMode {
+				s.seqNum = 0
+			}
 		default:
 			panic("ssh: no key material for msgNewKeys")
 		}
diff --git a/x509roots/fallback/bundle.go b/x509roots/fallback/bundle.go
index bcde653c7f..ad9ca4db1e 100644
--- a/x509roots/fallback/bundle.go
+++ b/x509roots/fallback/bundle.go
@@ -360,6 +360,55 @@ maHFCN795trV1lpFDMS3wrUU77QR/w4VtfX128a961qn8FYiqTxlVMYVqL2Gns2D
 lmh6cYGJ4Qvh6hEbaAjMaZ7snkGeRDImeuKHCnE96+RapNLbxc3G3mB/ufNPRJLv
 KrcYPqcZ2Qt9sTdBQrC6YB3y/gkRsPCHe6ed
 -----END CERTIFICATE-----
+# CN=Atos TrustedRoot Root CA ECC TLS 2021,O=Atos,C=DE
+# b2fae53e14ccd7ab9212064701ae279c1d8988facb775fa8a008914e663988a8
+-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----
+# CN=Atos TrustedRoot Root CA RSA TLS 2021,O=Atos,C=DE
+# 81a9088ea59fb364c548a6f85559099b6f0405efbf18e5324ec9f457ba00112f
+-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----
 # CN=Autoridad de Certificacion Firmaprofesional CIF A62634068,C=ES
 # 57de0583efd2b26e0361da99da9df4648def7ee8441c3b728afa9bcde0f9b26a
 -----BEGIN CERTIFICATE-----
@@ -397,43 +446,6 @@ f9DEMnDAGf/JOC0ULGb0QkTmVXYbgBVX/8Cnp6o5qtjTcNAuuuuUavpfNIbnYrX9
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-# CN=Autoridad de Certificacion Firmaprofesional CIF A62634068,C=ES
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------BEGIN CERTIFICATE-----
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 # CN=BJCA Global Root CA1,O=BEIJING CERTIFICATE AUTHORITY,C=CN
 # f3896f88fe7c0a882766a7fa6ad2749fb57a7f3e98fb769c1fa7b09c2c44d5ae
 -----BEGIN CERTIFICATE-----
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 -----END CERTIFICATE-----
+# CN=CommScope Public Trust ECC Root-01,O=CommScope,C=US
+# 11437cda7bb45e41365f45b39a38986b0de00def348e0c7bb0873633800bc38b
+-----BEGIN CERTIFICATE-----
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+-----END CERTIFICATE-----
+# CN=CommScope Public Trust ECC Root-02,O=CommScope,C=US
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+-----BEGIN CERTIFICATE-----
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+# CN=CommScope Public Trust RSA Root-01,O=CommScope,C=US
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+-----BEGIN CERTIFICATE-----
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+-----END CERTIFICATE-----
+# CN=CommScope Public Trust RSA Root-02,O=CommScope,C=US
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+-----BEGIN CERTIFICATE-----
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 # CN=D-TRUST BR Root CA 1 2020,O=D-Trust GmbH,C=DE
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------END CERTIFICATE-----
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------BEGIN CERTIFICATE-----
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------END CERTIFICATE-----
-# CN=E-Tugra Global Root CA RSA v3,OU=E-Tugra Trust Center,O=E-Tugra EBG A.S.,L=Ankara,C=TR
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------BEGIN CERTIFICATE-----
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------END CERTIFICATE-----
 # CN=Entrust Root Certification Authority - EC1,OU=See www.entrust.net/legal-terms+OU=(c) 2012 Entrust\, Inc. - for authorized use only,O=Entrust\, Inc.,C=US
 # 02ed0eb28c14da45165c566791700d6451d7fb56f0b2ab1d3b8eb070e56edff5
 -----BEGIN CERTIFICATE-----
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-# CN=Hongkong Post Root CA 1,O=Hongkong Post,C=HK
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------BEGIN CERTIFICATE-----
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------END CERTIFICATE-----
 # CN=Hongkong Post Root CA 3,O=Hongkong Post,L=Hong Kong,ST=Hong Kong,C=HK
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 -----BEGIN CERTIFICATE-----
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 -----END CERTIFICATE-----
+# CN=SSL.com TLS ECC Root CA 2022,O=SSL Corporation,C=US
+# c32ffd9f46f936d16c3673990959434b9ad60aafbb9e7cf33654f144cc1ba143
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+-----END CERTIFICATE-----
+# CN=SSL.com TLS RSA Root CA 2022,O=SSL Corporation,C=US
+# 8faf7d2e2cb4709bb8e0b33666bf75a5dd45b5de480f8ea8d4bfe6bebc17f2ed
+-----BEGIN CERTIFICATE-----
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+-----END CERTIFICATE-----
 # CN=SZAFIR ROOT CA2,O=Krajowa Izba Rozliczeniowa S.A.,C=PL
 # a1339d33281a0b56e557d3d32b1ce7f9367eb094bd5fa72a7e5004c8ded7cafe
 -----BEGIN CERTIFICATE-----
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 -----END CERTIFICATE-----
+# CN=Sectigo Public Server Authentication Root E46,O=Sectigo Limited,C=GB
+# c90f26f0fb1b4018b22227519b5ca2b53e2ca5b3be5cf18efe1bef47380c5383
+-----BEGIN CERTIFICATE-----
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+-----END CERTIFICATE-----
+# CN=Sectigo Public Server Authentication Root R46,O=Sectigo Limited,C=GB
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+-----BEGIN CERTIFICATE-----
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+QqszKbrAKbkTidOIijlBO8n9pu0f9GBj39ItVQGL
+-----END CERTIFICATE-----
 # CN=Secure Global CA,O=SecureTrust Corporation,C=US
 # 4200f5043ac8590ebb527d209ed1503029fbcbd41ca1b506ec27f15ade7dac69
 -----BEGIN CERTIFICATE-----
@@ -3133,6 +3228,58 @@ t88NkvuOGKmYSdGe/mBEciG5Ge3C9THxOUiIkCR1VBatzvT4aRRkOfujuLpwQMcn
 HL/EVlP6Y2XQ8xwOFvVrhlhNGNTkDY6lnVuR3HYkUD/GKvvZt5y11ubQ2egZixVx
 SK236thZiNSQvxaz2emsWWFUyBy6ysHK4bkgTI86k4mloMy/0/Z1pHWWbVY=
 -----END CERTIFICATE-----
+# CN=TrustAsia Global Root CA G3,O=TrustAsia Technologies\, Inc.,C=CN
+# e0d3226aeb1163c2e48ff9be3b50b4c6431be7bb1eacc5c36b5d5ec509039a08
+-----BEGIN CERTIFICATE-----
+MIIFpTCCA42gAwIBAgIUZPYOZXdhaqs7tOqFhLuxibhxkw8wDQYJKoZIhvcNAQEM
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+FGWsJwt0ivKH
+-----END CERTIFICATE-----
+# CN=TrustAsia Global Root CA G4,O=TrustAsia Technologies\, Inc.,C=CN
+# be4b56cb5056c0136a526df444508daa36a0b54f42e4ac38f72af470e479654c
+-----BEGIN CERTIFICATE-----
+MIICVTCCAdygAwIBAgIUTyNkuI6XY57GU4HBdk7LKnQV1tcwCgYIKoZIzj0EAwMw
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+/bpV6wfEU6s3qe4hsiFbYI89MvHVI5TWWA==
+-----END CERTIFICATE-----
 # CN=Trustwave Global Certification Authority,O=Trustwave Holdings\, Inc.,L=Chicago,ST=Illinois,C=US
 # 97552015f5ddfc3c8788c006944555408894450084f100867086bc1a2bb58dc8
 -----BEGIN CERTIFICATE-----
diff --git a/x509roots/gen_fallback_bundle.go b/x509roots/gen_fallback_bundle.go
index c22d1b0c38..ffea49b1e8 100644
--- a/x509roots/gen_fallback_bundle.go
+++ b/x509roots/gen_fallback_bundle.go
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build generate
-// +build generate
 
 //go:generate go run gen_fallback_bundle.go
 
@@ -18,6 +17,7 @@ import (
 	"go/format"
 	"io"
 	"log"
+	"mime"
 	"net/http"
 	"os"
 	"sort"
@@ -87,6 +87,16 @@ func main() {
 			log.Fatalf("failed to request %q: %s", *certDataURL, err)
 		}
 		defer resp.Body.Close()
+		if resp.StatusCode != http.StatusOK {
+			body, _ := io.ReadAll(io.LimitReader(resp.Body, 4<<10))
+			log.Fatalf("got non-200 OK status code: %v body: %q", resp.Status, body)
+		} else if ct, want := resp.Header.Get("Content-Type"), `text/plain; charset="UTF-8"`; ct != want {
+			if mediaType, _, err := mime.ParseMediaType(ct); err != nil {
+				log.Fatalf("bad Content-Type header %q: %v", ct, err)
+			} else if mediaType != "text/plain" {
+				log.Fatalf("got media type %q, want %q", mediaType, "text/plain")
+			}
+		}
 		certdata = resp.Body
 	}
 
@@ -95,6 +105,10 @@ func main() {
 		log.Fatalf("failed to parse %q: %s", *certDataPath, err)
 	}
 
+	if len(certs) == 0 {
+		log.Fatal("certdata.txt appears to contain zero roots")
+	}
+
 	sort.Slice(certs, func(i, j int) bool {
 		// Sort based on the stringified subject (which may not be unique), and
 		// break any ties by just sorting on the raw DER (which will be unique,