crypto/tls: support crypto.MessageSigner private keys

Fixes #75656

Change-Id: I6bc71c80973765ef995d17b1450ea2026a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/724820
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Nicholas Husin <husin@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nicholas Husin <nsh@golang.org>

doc/next/6-stdlib/99-minor/crypto/tls/75656.md

@@ -0,0 +1,2 @@
+If [Certificate.PrivateKey] implements [crypto.MessageSigner], its SignMessage
+method is used instead of Sign in TLS 1.2 and later.

src/crypto/tls/auth.go

@@ -18,9 +18,13 @@ import (
 	"slices"
 )
 
-// verifyHandshakeSignature verifies a signature against pre-hashed
+// verifyHandshakeSignature verifies a signature against unhashed handshake contents.
-// (if required) handshake contents.
 func verifyHandshakeSignature(sigType uint8, pubkey crypto.PublicKey, hashFunc crypto.Hash, signed, sig []byte) error {
+	if hashFunc != directSigning {
+		h := hashFunc.New()
+		h.Write(signed)
+		signed = h.Sum(nil)
+	}
 	switch sigType {
 	case signatureECDSA:
 		pubKey, ok := pubkey.(*ecdsa.PublicKey)
@@ -61,6 +65,32 @@ func verifyHandshakeSignature(sigType uint8, pubkey crypto.PublicKey, hashFunc c
 	return nil
 }
 
+// verifyLegacyHandshakeSignature verifies a TLS 1.0 and 1.1 signature against
+// pre-hashed handshake contents.
+func verifyLegacyHandshakeSignature(sigType uint8, pubkey crypto.PublicKey, hashFunc crypto.Hash, hashed, sig []byte) error {
+	switch sigType {
+	case signatureECDSA:
+		pubKey, ok := pubkey.(*ecdsa.PublicKey)
+		if !ok {
+			return fmt.Errorf("expected an ECDSA public key, got %T", pubkey)
+		}
+		if !ecdsa.VerifyASN1(pubKey, hashed, sig) {
+			return errors.New("ECDSA verification failure")
+		}
+	case signaturePKCS1v15:
+		pubKey, ok := pubkey.(*rsa.PublicKey)
+		if !ok {
+			return fmt.Errorf("expected an RSA public key, got %T", pubkey)
+		}
+		if err := rsa.VerifyPKCS1v15(pubKey, hashFunc, hashed, sig); err != nil {
+			return err
+		}
+	default:
+		return errors.New("internal error: unknown signature type")
+	}
+	return nil
+}
+
 const (
 	serverSignatureContext = "TLS 1.3, server CertificateVerify\x00"
 	clientSignatureContext = "TLS 1.3, client CertificateVerify\x00"
@@ -77,21 +107,15 @@ var signaturePadding = []byte{
 	0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20,
 }
 
-// signedMessage returns the pre-hashed (if necessary) message to be signed by
+// signedMessage returns the (unhashed) message to be signed by certificate keys
-// certificate keys in TLS 1.3. See RFC 8446, Section 4.4.3.
+// in TLS 1.3. See RFC 8446, Section 4.4.3.
-func signedMessage(sigHash crypto.Hash, context string, transcript hash.Hash) []byte {
+func signedMessage(context string, transcript hash.Hash) []byte {
-	if sigHash == directSigning {
+	const maxSize = 64 /* signaturePadding */ + len(serverSignatureContext) + 512/8 /* SHA-512 */
-		b := &bytes.Buffer{}
+	b := bytes.NewBuffer(make([]byte, 0, maxSize))
-		b.Write(signaturePadding)
+	b.Write(signaturePadding)
-		io.WriteString(b, context)
+	io.WriteString(b, context)
-		b.Write(transcript.Sum(nil))
+	b.Write(transcript.Sum(nil))
-		return b.Bytes()
+	return b.Bytes()
-	}
-	h := sigHash.New()
-	h.Write(signaturePadding)
-	io.WriteString(h, context)
-	h.Write(transcript.Sum(nil))
-	return h.Sum(nil)
 }
 
 // typeAndHashFromSignatureScheme returns the corresponding signature type and

src/crypto/tls/common.go

@@ -1597,9 +1597,14 @@ var writerMutex sync.Mutex
 type Certificate struct {
 	Certificate [][]byte
 	// PrivateKey contains the private key corresponding to the public key in
-	// Leaf. This must implement crypto.Signer with an RSA, ECDSA or Ed25519 PublicKey.
+	// Leaf. This must implement [crypto.Signer] with an RSA, ECDSA or Ed25519
+	// PublicKey.
+	//
 	// For a server up to TLS 1.2, it can also implement crypto.Decrypter with
 	// an RSA PublicKey.
+	//
+	// If it implements [crypto.MessageSigner], SignMessage will be used instead
+	// of Sign for TLS 1.2 and later.
 	PrivateKey crypto.PrivateKey
 	// SupportedSignatureAlgorithms is an optional list restricting what
 	// signature algorithms the PrivateKey can be used for.

src/crypto/tls/handshake_client.go

@@ -781,15 +781,13 @@ func (hs *clientHandshakeState) doFullHandshake() error {
 			return fmt.Errorf("tls: client certificate private key of type %T does not implement crypto.Signer", chainToSend.PrivateKey)
 		}
 
-		var sigType uint8
-		var sigHash crypto.Hash
 		if c.vers >= VersionTLS12 {
 			signatureAlgorithm, err := selectSignatureScheme(c.vers, chainToSend, certReq.supportedSignatureAlgorithms)
 			if err != nil {
 				c.sendAlert(alertHandshakeFailure)
 				return err
 			}
-			sigType, sigHash, err = typeAndHashFromSignatureScheme(signatureAlgorithm)
+			sigType, sigHash, err := typeAndHashFromSignatureScheme(signatureAlgorithm)
 			if err != nil {
 				return c.sendAlert(alertInternalError)
 			}
@@ -799,23 +797,31 @@ func (hs *clientHandshakeState) doFullHandshake() error {
 				tlssha1.Value() // ensure godebug is initialized
 				tlssha1.IncNonDefault()
 			}
+			if hs.finishedHash.buffer == nil {
+				c.sendAlert(alertInternalError)
+				return errors.New("tls: internal error: did not keep handshake transcript for TLS 1.2")
+			}
+			signOpts := crypto.SignerOpts(sigHash)
+			if sigType == signatureRSAPSS {
+				signOpts = &rsa.PSSOptions{SaltLength: rsa.PSSSaltLengthEqualsHash, Hash: sigHash}
+			}
+			certVerify.signature, err = crypto.SignMessage(key, c.config.rand(), hs.finishedHash.buffer, signOpts)
+			if err != nil {
+				c.sendAlert(alertInternalError)
+				return err
+			}
 		} else {
-			sigType, sigHash, err = legacyTypeAndHashFromPublicKey(key.Public())
+			sigType, sigHash, err := legacyTypeAndHashFromPublicKey(key.Public())
 			if err != nil {
 				c.sendAlert(alertIllegalParameter)
 				return err
 			}
-		}
+			signed := hs.finishedHash.hashForClientCertificate(sigType)
-
+			certVerify.signature, err = key.Sign(c.config.rand(), signed, sigHash)
-		signed := hs.finishedHash.hashForClientCertificate(sigType, sigHash)
+			if err != nil {
-		signOpts := crypto.SignerOpts(sigHash)
+				c.sendAlert(alertInternalError)
-		if sigType == signatureRSAPSS {
+				return err
-			signOpts = &rsa.PSSOptions{SaltLength: rsa.PSSSaltLengthEqualsHash, Hash: sigHash}
+			}
-		}
-		certVerify.signature, err = key.Sign(c.config.rand(), signed, signOpts)
-		if err != nil {
-			c.sendAlert(alertInternalError)
-			return err
 		}
 
 		if _, err := hs.c.writeHandshakeRecord(certVerify, &hs.finishedHash); err != nil {

src/crypto/tls/handshake_client_tls13.go

@@ -664,7 +664,7 @@ func (hs *clientHandshakeStateTLS13) readServerCertificate() error {
 	if sigType == signaturePKCS1v15 || sigHash == crypto.SHA1 {
 		return c.sendAlert(alertInternalError)
 	}
-	signed := signedMessage(sigHash, serverSignatureContext, hs.transcript)
+	signed := signedMessage(serverSignatureContext, hs.transcript)
 	if err := verifyHandshakeSignature(sigType, c.peerCertificates[0].PublicKey,
 		sigHash, signed, certVerify.signature); err != nil {
 		c.sendAlert(alertDecryptError)
@@ -783,12 +783,12 @@ func (hs *clientHandshakeStateTLS13) sendClientCertificate() error {
 		return c.sendAlert(alertInternalError)
 	}
 
-	signed := signedMessage(sigHash, clientSignatureContext, hs.transcript)
+	signed := signedMessage(clientSignatureContext, hs.transcript)
 	signOpts := crypto.SignerOpts(sigHash)
 	if sigType == signatureRSAPSS {
 		signOpts = &rsa.PSSOptions{SaltLength: rsa.PSSSaltLengthEqualsHash, Hash: sigHash}
 	}
-	sig, err := cert.PrivateKey.(crypto.Signer).Sign(c.config.rand(), signed, signOpts)
+	sig, err := crypto.SignMessage(cert.PrivateKey.(crypto.Signer), c.config.rand(), signed, signOpts)
 	if err != nil {
 		c.sendAlert(alertInternalError)
 		return errors.New("tls: failed to sign handshake: " + err.Error())

src/crypto/tls/handshake_server.go

@@ -780,19 +780,27 @@ func (hs *serverHandshakeState) doFullHandshake() error {
 				tlssha1.Value() // ensure godebug is initialized
 				tlssha1.IncNonDefault()
 			}
+			if hs.finishedHash.buffer == nil {
+				c.sendAlert(alertInternalError)
+				return errors.New("tls: internal error: did not keep handshake transcript for TLS 1.2")
+			}
+			if err := verifyHandshakeSignature(sigType, pub, sigHash, hs.finishedHash.buffer, certVerify.signature); err != nil {
+				c.sendAlert(alertDecryptError)
+				return errors.New("tls: invalid signature by the client certificate: " + err.Error())
+			}
 		} else {
 			sigType, sigHash, err = legacyTypeAndHashFromPublicKey(pub)
 			if err != nil {
 				c.sendAlert(alertIllegalParameter)
 				return err
 			}
+			signed := hs.finishedHash.hashForClientCertificate(sigType)
+			if err := verifyLegacyHandshakeSignature(sigType, pub, sigHash, signed, certVerify.signature); err != nil {
+				c.sendAlert(alertDecryptError)
+				return errors.New("tls: invalid signature by the client certificate: " + err.Error())
+			}
 		}
 
-		signed := hs.finishedHash.hashForClientCertificate(sigType, sigHash)
-		if err := verifyHandshakeSignature(sigType, pub, sigHash, signed, certVerify.signature); err != nil {
-			c.sendAlert(alertDecryptError)
-			return errors.New("tls: invalid signature by the client certificate: " + err.Error())
-		}
 		c.peerSigAlg = certVerify.signatureAlgorithm
 
 		if err := transcriptMsg(certVerify, &hs.finishedHash); err != nil {

src/crypto/tls/handshake_server_tls13.go

@@ -845,12 +845,12 @@ func (hs *serverHandshakeStateTLS13) sendServerCertificate() error {
 		return c.sendAlert(alertInternalError)
 	}
 
-	signed := signedMessage(sigHash, serverSignatureContext, hs.transcript)
+	signed := signedMessage(serverSignatureContext, hs.transcript)
 	signOpts := crypto.SignerOpts(sigHash)
 	if sigType == signatureRSAPSS {
 		signOpts = &rsa.PSSOptions{SaltLength: rsa.PSSSaltLengthEqualsHash, Hash: sigHash}
 	}
-	sig, err := hs.cert.PrivateKey.(crypto.Signer).Sign(c.config.rand(), signed, signOpts)
+	sig, err := crypto.SignMessage(hs.cert.PrivateKey.(crypto.Signer), c.config.rand(), signed, signOpts)
 	if err != nil {
 		public := hs.cert.PrivateKey.(crypto.Signer).Public()
 		if rsaKey, ok := public.(*rsa.PublicKey); ok && sigType == signatureRSAPSS &&
@@ -1081,7 +1081,7 @@ func (hs *serverHandshakeStateTLS13) readClientCertificate() error {
 		if sigType == signaturePKCS1v15 || sigHash == crypto.SHA1 {
 			return c.sendAlert(alertInternalError)
 		}
-		signed := signedMessage(sigHash, clientSignatureContext, hs.transcript)
+		signed := signedMessage(clientSignatureContext, hs.transcript)
 		if err := verifyHandshakeSignature(sigType, c.peerCertificates[0].PublicKey,
 			sigHash, signed, certVerify.signature); err != nil {
 			c.sendAlert(alertDecryptError)

src/crypto/tls/key_agreement.go

@@ -127,25 +127,8 @@ func md5SHA1Hash(slices [][]byte) []byte {
 }
 
 // hashForServerKeyExchange hashes the given slices and returns their digest
-// using the given hash function (for TLS 1.2) or using a default based on
+// using a hash based on the sigType. It can only be used for TLS 1.0 and 1.1.
-// the sigType (for earlier TLS versions). For Ed25519 signatures, which don't
+func hashForServerKeyExchange(sigType uint8, slices ...[]byte) []byte {
-// do pre-hashing, it returns the concatenation of the slices.
-func hashForServerKeyExchange(sigType uint8, hashFunc crypto.Hash, version uint16, slices ...[]byte) []byte {
-	if sigType == signatureEd25519 {
-		var signed []byte
-		for _, slice := range slices {
-			signed = append(signed, slice...)
-		}
-		return signed
-	}
-	if version >= VersionTLS12 {
-		h := hashFunc.New()
-		for _, slice := range slices {
-			h.Write(slice)
-		}
-		digest := h.Sum(nil)
-		return digest
-	}
 	if sigType == signatureECDSA {
 		return sha1Hash(slices)
 	}
@@ -207,14 +190,13 @@ func (ka *ecdheKeyAgreement) generateServerKeyExchange(config *Config, cert *Cer
 		return nil, fmt.Errorf("tls: certificate private key of type %T does not implement crypto.Signer", cert.PrivateKey)
 	}
 
-	var sigType uint8
+	var sig []byte
-	var sigHash crypto.Hash
 	if ka.version >= VersionTLS12 {
 		ka.signatureAlgorithm, err = selectSignatureScheme(ka.version, cert, clientHello.supportedSignatureAlgorithms)
 		if err != nil {
 			return nil, err
 		}
-		sigType, sigHash, err = typeAndHashFromSignatureScheme(ka.signatureAlgorithm)
+		sigType, sigHash, err := typeAndHashFromSignatureScheme(ka.signatureAlgorithm)
 		if err != nil {
 			return nil, err
 		}
@@ -222,25 +204,31 @@ func (ka *ecdheKeyAgreement) generateServerKeyExchange(config *Config, cert *Cer
 			tlssha1.Value() // ensure godebug is initialized
 			tlssha1.IncNonDefault()
 		}
+		signed := slices.Concat(clientHello.random, hello.random, serverECDHEParams)
+		if (sigType == signaturePKCS1v15 || sigType == signatureRSAPSS) != ka.isRSA {
+			return nil, errors.New("tls: certificate cannot be used with the selected cipher suite")
+		}
+		signOpts := crypto.SignerOpts(sigHash)
+		if sigType == signatureRSAPSS {
+			signOpts = &rsa.PSSOptions{SaltLength: rsa.PSSSaltLengthEqualsHash, Hash: sigHash}
+		}
+		sig, err = crypto.SignMessage(priv, config.rand(), signed, signOpts)
+		if err != nil {
+			return nil, errors.New("tls: failed to sign ECDHE parameters: " + err.Error())
+		}
 	} else {
-		sigType, sigHash, err = legacyTypeAndHashFromPublicKey(priv.Public())
+		sigType, sigHash, err := legacyTypeAndHashFromPublicKey(priv.Public())
 		if err != nil {
 			return nil, err
 		}
-	}
+		signed := hashForServerKeyExchange(sigType, clientHello.random, hello.random, serverECDHEParams)
-	if (sigType == signaturePKCS1v15 || sigType == signatureRSAPSS) != ka.isRSA {
+		if (sigType == signaturePKCS1v15) != ka.isRSA {
-		return nil, errors.New("tls: certificate cannot be used with the selected cipher suite")
+			return nil, errors.New("tls: certificate cannot be used with the selected cipher suite")
-	}
+		}
-
+		sig, err = priv.Sign(config.rand(), signed, sigHash)
-	signed := hashForServerKeyExchange(sigType, sigHash, ka.version, clientHello.random, hello.random, serverECDHEParams)
+		if err != nil {
-
+			return nil, errors.New("tls: failed to sign ECDHE parameters: " + err.Error())
-	signOpts := crypto.SignerOpts(sigHash)
+		}
-	if sigType == signatureRSAPSS {
-		signOpts = &rsa.PSSOptions{SaltLength: rsa.PSSSaltLengthEqualsHash, Hash: sigHash}
-	}
-	sig, err := priv.Sign(config.rand(), signed, signOpts)
-	if err != nil {
-		return nil, errors.New("tls: failed to sign ECDHE parameters: " + err.Error())
 	}
 
 	skx := new(serverKeyExchangeMsg)
@@ -300,6 +288,18 @@ func (ka *ecdheKeyAgreement) processServerKeyExchange(config *Config, clientHell
 	if len(sig) < 2 {
 		return errServerKeyExchange
 	}
+	if ka.version >= VersionTLS12 {
+		ka.signatureAlgorithm = SignatureScheme(sig[0])<<8 | SignatureScheme(sig[1])
+		sig = sig[2:]
+		if len(sig) < 2 {
+			return errServerKeyExchange
+		}
+	}
+	sigLen := int(sig[0])<<8 | int(sig[1])
+	if sigLen+2 != len(sig) {
+		return errServerKeyExchange
+	}
+	sig = sig[2:]
 
 	if !slices.Contains(clientHello.supportedCurves, ka.curveID) {
 		return errors.New("tls: server selected unoffered curve")
@@ -333,12 +333,6 @@ func (ka *ecdheKeyAgreement) processServerKeyExchange(config *Config, clientHell
 	var sigType uint8
 	var sigHash crypto.Hash
 	if ka.version >= VersionTLS12 {
-		ka.signatureAlgorithm = SignatureScheme(sig[0])<<8 | SignatureScheme(sig[1])
-		sig = sig[2:]
-		if len(sig) < 2 {
-			return errServerKeyExchange
-		}
-
 		if !isSupportedSignatureAlgorithm(ka.signatureAlgorithm, clientHello.supportedSignatureAlgorithms) {
 			return errors.New("tls: certificate used with invalid signature algorithm")
 		}
@@ -350,26 +344,27 @@ func (ka *ecdheKeyAgreement) processServerKeyExchange(config *Config, clientHell
 			tlssha1.Value() // ensure godebug is initialized
 			tlssha1.IncNonDefault()
 		}
+		if (sigType == signaturePKCS1v15 || sigType == signatureRSAPSS) != ka.isRSA {
+			return errServerKeyExchange
+		}
+		signed := slices.Concat(clientHello.random, serverHello.random, serverECDHEParams)
+		if err := verifyHandshakeSignature(sigType, cert.PublicKey, sigHash, signed, sig); err != nil {
+			return errors.New("tls: invalid signature by the server certificate: " + err.Error())
+		}
 	} else {
 		sigType, sigHash, err = legacyTypeAndHashFromPublicKey(cert.PublicKey)
 		if err != nil {
 			return err
 		}
-	}
+		if (sigType == signaturePKCS1v15) != ka.isRSA {
-	if (sigType == signaturePKCS1v15 || sigType == signatureRSAPSS) != ka.isRSA {
+			return errServerKeyExchange
-		return errServerKeyExchange
+		}
+		signed := hashForServerKeyExchange(sigType, clientHello.random, serverHello.random, serverECDHEParams)
+		if err := verifyLegacyHandshakeSignature(sigType, cert.PublicKey, sigHash, signed, sig); err != nil {
+			return errors.New("tls: invalid signature by the server certificate: " + err.Error())
+		}
 	}
 
-	sigLen := int(sig[0])<<8 | int(sig[1])
-	if sigLen+2 != len(sig) {
-		return errServerKeyExchange
-	}
-	sig = sig[2:]
-
-	signed := hashForServerKeyExchange(sigType, sigHash, ka.version, clientHello.random, serverHello.random, serverECDHEParams)
-	if err := verifyHandshakeSignature(sigType, cert.PublicKey, sigHash, signed, sig); err != nil {
-		return errors.New("tls: invalid signature by the server certificate: " + err.Error())
-	}
 	return nil
 }
 

src/crypto/tls/prf.go

@@ -221,23 +221,9 @@ func (h finishedHash) serverSum(masterSecret []byte) []byte {
 	return h.prf(masterSecret, serverFinishedLabel, h.Sum(), finishedVerifyLength)
 }
 
-// hashForClientCertificate returns the handshake messages so far, pre-hashed if
+// hashForClientCertificate returns the handshake messages so far, pre-hashed,
-// necessary, suitable for signing by a TLS client certificate.
+// suitable for signing by a TLS 1.0 and 1.1 client certificate.
-func (h finishedHash) hashForClientCertificate(sigType uint8, hashAlg crypto.Hash) []byte {
+func (h finishedHash) hashForClientCertificate(sigType uint8) []byte {
-	if (h.version >= VersionTLS12 || sigType == signatureEd25519) && h.buffer == nil {
-		panic("tls: handshake hash for a client certificate requested after discarding the handshake buffer")
-	}
-
-	if sigType == signatureEd25519 {
-		return h.buffer
-	}
-
-	if h.version >= VersionTLS12 {
-		hash := hashAlg.New()
-		hash.Write(h.buffer)
-		return hash.Sum(nil)
-	}
-
 	if sigType == signatureECDSA {
 		return h.server.Sum(nil)
 	}

src/crypto/tls/tls_test.go

@@ -2390,3 +2390,74 @@ func TestECH(t *testing.T) {
 
 	check()
 }
+
+func TestMessageSigner(t *testing.T) {
+	t.Run("TLSv10", func(t *testing.T) { testMessageSigner(t, VersionTLS10) })
+	t.Run("TLSv12", func(t *testing.T) { testMessageSigner(t, VersionTLS12) })
+	t.Run("TLSv13", func(t *testing.T) { testMessageSigner(t, VersionTLS13) })
+}
+
+func testMessageSigner(t *testing.T, version uint16) {
+	clientConfig, serverConfig := testConfig.Clone(), testConfig.Clone()
+	serverConfig.ClientAuth = RequireAnyClientCert
+	clientConfig.MinVersion = version
+	clientConfig.MaxVersion = version
+	serverConfig.MinVersion = version
+	serverConfig.MaxVersion = version
+	clientConfig.Certificates = []Certificate{{
+		Certificate: [][]byte{testRSACertificate},
+		PrivateKey:  messageOnlySigner{testRSAPrivateKey},
+	}}
+	serverConfig.Certificates = []Certificate{{
+		Certificate: [][]byte{testRSACertificate},
+		PrivateKey:  messageOnlySigner{testRSAPrivateKey},
+	}}
+
+	_, _, err := testHandshake(t, clientConfig, serverConfig)
+	if version < VersionTLS12 {
+		if err == nil {
+			t.Fatal("expected failure for TLS 1.0/1.1")
+		}
+	} else {
+		if err != nil {
+			t.Fatalf("unexpected failure: %s", err)
+		}
+	}
+
+	clientConfig.Certificates = []Certificate{{
+		Certificate: [][]byte{testECDSACertificate},
+		PrivateKey:  messageOnlySigner{testECDSAPrivateKey},
+	}}
+	serverConfig.Certificates = []Certificate{{
+		Certificate: [][]byte{testECDSACertificate},
+		PrivateKey:  messageOnlySigner{testECDSAPrivateKey},
+	}}
+
+	_, _, err = testHandshake(t, clientConfig, serverConfig)
+	if version < VersionTLS12 {
+		if err == nil {
+			t.Fatal("expected failure for TLS 1.0/1.1")
+		}
+	} else {
+		if err != nil {
+			t.Fatalf("unexpected failure: %s", err)
+		}
+	}
+}
+
+type messageOnlySigner struct{ crypto.Signer }
+
+func (s messageOnlySigner) Public() crypto.PublicKey {
+	return s.Signer.Public()
+}
+
+func (s messageOnlySigner) Sign(rand io.Reader, msg []byte, opts crypto.SignerOpts) (signature []byte, err error) {
+	return nil, errors.New("messageOnlySigner: Sign called")
+}
+
+func (s messageOnlySigner) SignMessage(rand io.Reader, msg []byte, opts crypto.SignerOpts) (signature []byte, err error) {
+	h := opts.HashFunc().New()
+	h.Write(msg)
+	digest := h.Sum(nil)
+	return s.Signer.Sign(rand, digest, opts)
+}