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crypto/ed25519: implement Ed25519ctx and Ed25519ph with context
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This is missing a test for Ed25519ph with context, since the RFC doesn't
provide one.

Fixes #31804

Change-Id: I20947374c51c6b22fb2835317d00edf816c9a2d2
Reviewed-on: https://go-review.googlesource.com/c/go/+/404274
Auto-Submit: Filippo Valsorda <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Filippo Valsorda <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
Reviewed-by: Roland Shoemaker <[email protected]>
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@FiloSottile @gopherbot
FiloSottile authored and gopherbot committed Nov 21, 2022
1 parent 8614c52 commit 831c650
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Showing 3 changed files with 124 additions and 25 deletions.
1 change: 1 addition & 0 deletions api/next/31804.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -2,3 +2,4 @@ pkg crypto/ed25519, func VerifyWithOptions(PublicKey, []uint8, []uint8, *Options
pkg crypto/ed25519, method (*Options) HashFunc() crypto.Hash #31804
pkg crypto/ed25519, type Options struct #31804
pkg crypto/ed25519, type Options struct, Hash crypto.Hash #31804
pkg crypto/ed25519, type Options struct, Context string #31804
90 changes: 65 additions & 25 deletions src/crypto/ed25519/ed25519.go
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Original file line number Diff line number Diff line change
Expand Up @@ -81,18 +81,30 @@ func (priv PrivateKey) Seed() []byte {
// crypto.Hash(0) and the message must not be hashed, as Ed25519 performs two
// passes over messages to be signed.
func (priv PrivateKey) Sign(rand io.Reader, message []byte, opts crypto.SignerOpts) (signature []byte, err error) {
switch opts.HashFunc() {
case crypto.SHA512:
hash := opts.HashFunc()
context := ""
if opts, ok := opts.(*Options); ok {
context = opts.Context
}
if l := len(context); l > 255 {
return nil, errors.New("ed25519: bad Ed25519ph context length: " + strconv.Itoa(l))
}
switch {
case hash == crypto.SHA512: // Ed25519ph
if l := len(message); l != sha512.Size {
return nil, errors.New("ed25519: bad Ed25519ph message hash length: " + strconv.Itoa(l))
}
signature := make([]byte, SignatureSize)
sign(signature, priv, message, domPrefixPh)
sign(signature, priv, message, domPrefixPh, context)
return signature, nil
case hash == crypto.Hash(0) && context != "": // Ed25519ctx
signature := make([]byte, SignatureSize)
sign(signature, priv, message, domPrefixCtx, context)
return signature, nil
case crypto.Hash(0):
case hash == crypto.Hash(0): // Ed25519
return Sign(priv, message), nil
default:
return nil, errors.New("ed25519: expected opts zero (unhashed message, for standard Ed25519) or SHA-512 (for Ed25519ph)")
return nil, errors.New("ed25519: expected opts.HashFunc() zero (unhashed message, for standard Ed25519) or SHA-512 (for Ed25519ph)")
}
}

Expand All @@ -102,8 +114,9 @@ type Options struct {
// Hash can be zero for regular Ed25519, or crypto.SHA512 for Ed25519ph.
Hash crypto.Hash

// TODO(filippo): add Context, a string of at most 255 bytes which when
// non-zero selects Ed25519ctx.
// Context, if not empty, selects Ed25519ctx or provides the context string
// for Ed25519ph. It can be at most 255 bytes in length.
Context string
}

func (o *Options) HashFunc() crypto.Hash { return o.Hash }
Expand Down Expand Up @@ -162,20 +175,24 @@ func Sign(privateKey PrivateKey, message []byte) []byte {
// Outline the function body so that the returned signature can be
// stack-allocated.
signature := make([]byte, SignatureSize)
sign(signature, privateKey, message, domPrefixPure)
sign(signature, privateKey, message, domPrefixPure, "")
return signature
}

// Domain separation prefixes used to disambiguate Ed25519/Ed25519ph.
// Domain separation prefixes used to disambiguate Ed25519/Ed25519ph/Ed25519ctx.
// See RFC 8032, Section 2 and Section 5.1.
const (
// domPrefixPure is empty for pure Ed25519.
domPrefixPure = ""
// domPrefixPh is dom2(phflag=1, context="") for Ed25519ph.
domPrefixPh = "SigEd25519 no Ed25519 collisions\x01\x00"
// domPrefixPh is dom2(phflag=1) for Ed25519ph. It must be followed by the
// uint8-length prefixed context.
domPrefixPh = "SigEd25519 no Ed25519 collisions\x01"
// domPrefixCtx is dom2(phflag=0) for Ed25519ctx. It must be followed by the
// uint8-length prefixed context.
domPrefixCtx = "SigEd25519 no Ed25519 collisions\x00"
)

func sign(signature, privateKey, message []byte, domPrefix string) {
func sign(signature, privateKey, message []byte, domPrefix, context string) {
if l := len(privateKey); l != PrivateKeySize {
panic("ed25519: bad private key length: " + strconv.Itoa(l))
}
Expand All @@ -189,7 +206,11 @@ func sign(signature, privateKey, message []byte, domPrefix string) {
prefix := h[32:]

mh := sha512.New()
mh.Write([]byte(domPrefix))
if domPrefix != domPrefixPure {
mh.Write([]byte(domPrefix))
mh.Write([]byte{byte(len(context))})
mh.Write([]byte(context))
}
mh.Write(prefix)
mh.Write(message)
messageDigest := make([]byte, 0, sha512.Size)
Expand All @@ -202,7 +223,11 @@ func sign(signature, privateKey, message []byte, domPrefix string) {
R := (&edwards25519.Point{}).ScalarBaseMult(r)

kh := sha512.New()
kh.Write([]byte(domPrefix))
if domPrefix != domPrefixPure {
kh.Write([]byte(domPrefix))
kh.Write([]byte{byte(len(context))})
kh.Write([]byte(context))
}
kh.Write(R.Bytes())
kh.Write(publicKey)
kh.Write(message)
Expand All @@ -222,36 +247,47 @@ func sign(signature, privateKey, message []byte, domPrefix string) {
// Verify reports whether sig is a valid signature of message by publicKey. It
// will panic if len(publicKey) is not PublicKeySize.
func Verify(publicKey PublicKey, message, sig []byte) bool {
return verify(publicKey, message, sig, domPrefixPure)
return verify(publicKey, message, sig, domPrefixPure, "")
}

// VerifyWithOptions reports whether sig is a valid signature of message by
// publicKey. A valid signature is indicated by returning a nil error.
// If opts.HashFunc() is crypto.SHA512, the pre-hashed variant Ed25519ph is used
// and message is expected to be a SHA-512 hash, otherwise opts.HashFunc() must
// If opts.Hash is crypto.SHA512, the pre-hashed variant Ed25519ph is used
// and message is expected to be a SHA-512 hash, otherwise opts.Hash must
// be crypto.Hash(0) and the message must not be hashed, as Ed25519 performs two
// passes over messages to be signed.
func VerifyWithOptions(publicKey PublicKey, message, sig []byte, opts *Options) error {
switch opts.HashFunc() {
case crypto.SHA512:
switch {
case opts.Hash == crypto.SHA512: // Ed25519ph
if l := len(message); l != sha512.Size {
return errors.New("ed25519: bad Ed25519ph message hash length: " + strconv.Itoa(l))
}
if !verify(publicKey, message, sig, domPrefixPh) {
if l := len(opts.Context); l > 255 {
return errors.New("ed25519: bad Ed25519ph context length: " + strconv.Itoa(l))
}
if !verify(publicKey, message, sig, domPrefixPh, opts.Context) {
return errors.New("ed25519: invalid signature")
}
return nil
case opts.Hash == crypto.Hash(0) && opts.Context != "": // Ed25519ctx
if l := len(opts.Context); l > 255 {
return errors.New("ed25519: bad Ed25519ctx context length: " + strconv.Itoa(l))
}
if !verify(publicKey, message, sig, domPrefixCtx, opts.Context) {
return errors.New("ed25519: invalid signature")
}
return nil
case crypto.Hash(0):
if !verify(publicKey, message, sig, domPrefixPure) {
case opts.Hash == crypto.Hash(0): // Ed25519
if !verify(publicKey, message, sig, domPrefixPure, "") {
return errors.New("ed25519: invalid signature")
}
return nil
default:
return errors.New("ed25519: expected opts zero (unhashed message, for standard Ed25519) or SHA-512 (for Ed25519ph)")
return errors.New("ed25519: expected opts.Hash zero (unhashed message, for standard Ed25519) or SHA-512 (for Ed25519ph)")
}
}

func verify(publicKey PublicKey, message, sig []byte, domPrefix string) bool {
func verify(publicKey PublicKey, message, sig []byte, domPrefix, context string) bool {
if l := len(publicKey); l != PublicKeySize {
panic("ed25519: bad public key length: " + strconv.Itoa(l))
}
Expand All @@ -266,7 +302,11 @@ func verify(publicKey PublicKey, message, sig []byte, domPrefix string) bool {
}

kh := sha512.New()
kh.Write([]byte(domPrefix))
if domPrefix != domPrefixPure {
kh.Write([]byte(domPrefix))
kh.Write([]byte{byte(len(context))})
kh.Write([]byte(context))
}
kh.Write(sig[:32])
kh.Write(publicKey)
kh.Write(message)
Expand Down
58 changes: 58 additions & 0 deletions src/crypto/ed25519/ed25519_test.go
View file
Original file line number Diff line number Diff line change
Expand Up @@ -71,6 +71,10 @@ func TestSignVerifyHashed(t *testing.T) {
t.Errorf("valid signature rejected: %v", err)
}

if err := VerifyWithOptions(public, hash[:], sig, &Options{Hash: crypto.SHA256}); err == nil {
t.Errorf("expected error for wrong hash")
}

wrongHash := sha512.Sum512([]byte("wrong message"))
if VerifyWithOptions(public, wrongHash[:], sig, &Options{Hash: crypto.SHA512}) == nil {
t.Errorf("signature of different message accepted")
Expand All @@ -85,6 +89,60 @@ func TestSignVerifyHashed(t *testing.T) {
if VerifyWithOptions(public, hash[:], sig, &Options{Hash: crypto.SHA512}) == nil {
t.Errorf("invalid signature accepted")
}

// The RFC provides no test vectors for Ed25519ph with context, so just sign
// and verify something.
sig, err = private.Sign(nil, hash[:], &Options{Hash: crypto.SHA512, Context: "123"})
if err != nil {
t.Fatal(err)
}
if err := VerifyWithOptions(public, hash[:], sig, &Options{Hash: crypto.SHA512, Context: "123"}); err != nil {
t.Errorf("valid signature rejected: %v", err)
}
if err := VerifyWithOptions(public, hash[:], sig, &Options{Hash: crypto.SHA512, Context: "321"}); err == nil {
t.Errorf("expected error for wrong context")
}
if err := VerifyWithOptions(public, hash[:], sig, &Options{Hash: crypto.SHA256, Context: "123"}); err == nil {
t.Errorf("expected error for wrong hash")
}
}

func TestSignVerifyContext(t *testing.T) {
// From RFC 8032, Section 7.2
key, _ := hex.DecodeString("0305334e381af78f141cb666f6199f57bc3495335a256a95bd2a55bf546663f6dfc9425e4f968f7f0c29f0259cf5f9aed6851c2bb4ad8bfb860cfee0ab248292")
expectedSig, _ := hex.DecodeString("55a4cc2f70a54e04288c5f4cd1e45a7bb520b36292911876cada7323198dd87a8b36950b95130022907a7fb7c4e9b2d5f6cca685a587b4b21f4b888e4e7edb0d")
message, _ := hex.DecodeString("f726936d19c800494e3fdaff20b276a8")
context := "foo"

private := PrivateKey(key)
public := private.Public().(PublicKey)
sig, err := private.Sign(nil, message, &Options{Context: context})
if err != nil {
t.Fatal(err)
}
if !bytes.Equal(sig, expectedSig) {
t.Error("signature doesn't match test vector")
}
if err := VerifyWithOptions(public, message, sig, &Options{Context: context}); err != nil {
t.Errorf("valid signature rejected: %v", err)
}

if VerifyWithOptions(public, []byte("bar"), sig, &Options{Context: context}) == nil {
t.Errorf("signature of different message accepted")
}
if VerifyWithOptions(public, message, sig, &Options{Context: "bar"}) == nil {
t.Errorf("signature with different context accepted")
}

sig[0] ^= 0xff
if VerifyWithOptions(public, message, sig, &Options{Context: context}) == nil {
t.Errorf("invalid signature accepted")
}
sig[0] ^= 0xff
sig[SignatureSize-1] ^= 0xff
if VerifyWithOptions(public, message, sig, &Options{Context: context}) == nil {
t.Errorf("invalid signature accepted")
}
}

func TestCryptoSigner(t *testing.T) {
Expand Down

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