HostKeyAlgorithms: add rsa-sha2-256 and rsa-sha2-512 for ssh-rsa

knownhosts.go

@@ -76,13 +76,23 @@ func (hkcb HostKeyCallback) HostKeyAlgorithms(hostWithPort string) (algos []stri
 	// example by https://github.com/golang/crypto/pull/254.
 	hostKeys := hkcb.HostKeys(hostWithPort)
 	seen := make(map[string]struct{}, len(hostKeys))
-	for _, key := range hostKeys {
-		typ := key.Type()
+	addAlgo := func(typ string) {
 		if _, already := seen[typ]; !already {
 			algos = append(algos, typ)
 			seen[typ] = struct{}{}
 		}
 	}
+	for _, key := range hostKeys {
+		typ := key.Type()
+		if typ == ssh.KeyAlgoRSA {
+			// KeyAlgoRSASHA256 and KeyAlgoRSASHA512 are only public key algorithms,
+			// not public key formats, so they can't appear as a PublicKey.Type.
+			// The corresponding PublicKey.Type is KeyAlgoRSA. See RFC 8332, Section 2.
+			addAlgo(ssh.KeyAlgoRSASHA512)
+			addAlgo(ssh.KeyAlgoRSASHA256)
+		}
+		addAlgo(typ)
+	}
 	return algos
 }
 

knownhosts_test.go

@@ -42,10 +42,10 @@ func TestHostKeyAlgorithms(t *testing.T) {
 	}
 
 	expectedAlgorithms := map[string][]string{
-		"only-rsa.example.test:22":     {"ssh-rsa"},
+		"only-rsa.example.test:22":     {"rsa-sha2-512", "rsa-sha2-256", "ssh-rsa"},
 		"only-ecdsa.example.test:22":   {"ecdsa-sha2-nistp256"},
 		"only-ed25519.example.test:22": {"ssh-ed25519"},
-		"multi.example.test:2233":      {"ssh-rsa", "ecdsa-sha2-nistp256", "ssh-ed25519"},
+		"multi.example.test:2233":      {"rsa-sha2-512", "rsa-sha2-256", "ssh-rsa", "ecdsa-sha2-nistp256", "ssh-ed25519"},
 		"192.168.1.102:2222":           {"ecdsa-sha2-nistp256", "ssh-ed25519"},
 		"unknown-host.example.test":    {}, // host not in file
 		"multi.example.test:22":        {}, // different port than entry in file