Make built-in types implement the new DigestSigner and DigestVerify interface

README.md

@@ -137,6 +137,16 @@ See [example_test.go](./example_test.go) for more examples.
 Untagged COSE_Sign1 messages can be signed and verified as above, using
 `cose.UntaggedSign1Message` instead of `cose.Sign1Message`.
 
+#### Signing and Verification of digested payloads
+
+When `cose.NewSigner` is used with PS{256,384,512} or ES{256,384,512}, the returned signer
+can be casted to the `cose.DigestSigner` interface, whose `SignDigest` method signs an
+already digested message.
+
+When `cose.NewVerifier` is used with PS{256,384,512} or ES{256,384,512}, the returned verifier
+can be casted to the `cose.DigestVerifier` interface, whose `VerifyDigest` method verifies an
+already digested message.
+
 ### About hashing
 
 `go-cose` does not import any hash package by its own to avoid linking unnecessary algorithms to the final binary.

ecdsa.go

@@ -57,6 +57,13 @@ func (es *ecdsaKeySigner) Sign(rand io.Reader, content []byte) ([]byte, error) {
 	if err != nil {
 		return nil, err
 	}
+	return es.SignDigest(rand, digest)
+}
+
+// SignDigest signs message digest with the private key, possibly using
+// entropy from rand.
+// The resulting signature should follow RFC 8152 section 8.
+func (es *ecdsaKeySigner) SignDigest(rand io.Reader, digest []byte) ([]byte, error) {
 	r, s, err := ecdsa.Sign(rand, es.key, digest)
 	if err != nil {
 		return nil, err
@@ -86,6 +93,13 @@ func (es *ecdsaCryptoSigner) Sign(rand io.Reader, content []byte) ([]byte, error
 	if err != nil {
 		return nil, err
 	}
+	return es.SignDigest(rand, digest)
+}
+
+// SignDigest signs message digest with the private key, possibly using
+// entropy from rand.
+// The resulting signature should follow RFC 8152 section 8.
+func (es *ecdsaCryptoSigner) SignDigest(rand io.Reader, digest []byte) ([]byte, error) {
 	sigASN1, err := es.signer.Sign(rand, digest, nil)
 	if err != nil {
 		return nil, err
@@ -153,7 +167,15 @@ func (ev *ecdsaVerifier) Verify(content []byte, signature []byte) error {
 	if err != nil {
 		return err
 	}
+	return ev.VerifyDigest(digest, signature)
+}
 
+// VerifyDigest verifies message digest with the public key, returning nil
+// for success.
+// Otherwise, it returns ErrVerification.
+//
+// Reference: https://datatracker.ietf.org/doc/html/rfc8152#section-8.1
+func (ev *ecdsaVerifier) VerifyDigest(digest, signature []byte) error {
 	// verify signature
 	r, s, err := decodeECDSASignature(ev.key.Curve, signature)
 	if err != nil {

rsa.go

@@ -24,14 +24,20 @@ func (rs *rsaSigner) Algorithm() Algorithm {
 //
 // Reference: https://datatracker.ietf.org/doc/html/rfc8152#section-8
 func (rs *rsaSigner) Sign(rand io.Reader, content []byte) ([]byte, error) {
-	hash := rs.alg.hashFunc()
-	digest, err := computeHash(hash, content)
+	digest, err := rs.alg.computeHash(content)
 	if err != nil {
 		return nil, err
 	}
+	return rs.SignDigest(rand, digest)
+}
+
+// SignDigest signs message digest with the private key, possibly using
+// entropy from rand.
+// The resulting signature should follow RFC 8152 section 8.
+func (rs *rsaSigner) SignDigest(rand io.Reader, digest []byte) ([]byte, error) {
 	return rs.key.Sign(rand, digest, &rsa.PSSOptions{
 		SaltLength: rsa.PSSSaltLengthEqualsHash, // defined in RFC 8230 sec 2
-		Hash:       hash,
+		Hash:       rs.alg.hashFunc(),
 	})
 }
 
@@ -54,12 +60,20 @@ func (rv *rsaVerifier) Algorithm() Algorithm {
 //
 // Reference: https://datatracker.ietf.org/doc/html/rfc8152#section-8
 func (rv *rsaVerifier) Verify(content []byte, signature []byte) error {
-	hash := rv.alg.hashFunc()
-	digest, err := computeHash(hash, content)
+	digest, err := rv.alg.computeHash(content)
 	if err != nil {
 		return err
 	}
-	if err := rsa.VerifyPSS(rv.key, hash, digest, signature, &rsa.PSSOptions{
+	return rv.VerifyDigest(digest, signature)
+}
+
+// VerifyDigest verifies message digest with the public key, returning nil
+// for success.
+// Otherwise, it returns ErrVerification.
+//
+// Reference: https://datatracker.ietf.org/doc/html/rfc8152#section-8.1
+func (rv *rsaVerifier) VerifyDigest(digest, signature []byte) error {
+	if err := rsa.VerifyPSS(rv.key, rv.alg.hashFunc(), digest, signature, &rsa.PSSOptions{
 		SaltLength: rsa.PSSSaltLengthEqualsHash, // defined in RFC 8230 sec 2
 	}); err != nil {
 		return ErrVerification

signer.go

@@ -23,6 +23,16 @@ type Signer interface {
 	Sign(rand io.Reader, content []byte) ([]byte, error)
 }
 
+// DigestSigner is an interface for private keys to sign digested COSE signatures.
+type DigestSigner interface {
+	Signer
+
+	// SignDigest signs message digest with the private key, possibly using
+	// entropy from rand.
+	// The resulting signature should follow RFC 8152 section 8.
+	SignDigest(rand io.Reader, digest []byte) ([]byte, error)
+}
+
 // NewSigner returns a signer with a given signing key.
 // The signing key can be a golang built-in crypto private key, a key in HSM, or
 // a remote KMS.
@@ -34,6 +44,8 @@ type Signer interface {
 // public key of type `*rsa.PublicKey`, `*ecdsa.PublicKey`, or
 // `ed25519.PublicKey` are accepted.
 //
+// The returned signer for rsa and ecdsa keys also implements `cose.DigestSigner`.
+//
 // Note: `*rsa.PrivateKey`, `*ecdsa.PrivateKey`, and `ed25519.PrivateKey`
 // implement `crypto.Signer`.
 func NewSigner(alg Algorithm, key crypto.Signer) (Signer, error) {

verifier.go

@@ -22,9 +22,21 @@ type Verifier interface {
 	Verify(content, signature []byte) error
 }
 
+// DigestVerifier is an interface for public keys to verify digested COSE signatures.
+type DigestVerifier interface {
+	Verifier
+
+	// VerifyDigest verifies message digest with the public key, returning nil
+	// for success.
+	// Otherwise, it returns ErrVerification.
+	VerifyDigest(digest, signature []byte) error
+}
+
 // NewVerifier returns a verifier with a given public key.
 // Only golang built-in crypto public keys of type `*rsa.PublicKey`,
 // `*ecdsa.PublicKey`, and `ed25519.PublicKey` are accepted.
+//
+// The returned signer for rsa and ecdsa keys also implements `cose.DigestSigner`.
 func NewVerifier(alg Algorithm, key crypto.PublicKey) (Verifier, error) {
 	switch alg {
 	case AlgorithmPS256, AlgorithmPS384, AlgorithmPS512: