add support for verifying IEEE P1363 encoded ECDSA sigs (#1686)

* add support for verifying IEEE P1363 encoded ECDSA sigs

Signed-off-by: Bob Callaway <bcallaway@google.com>

* improve unmarshal to struct for ASN.1 check

Signed-off-by: Bob Callaway <bcallaway@google.com>

---------

Signed-off-by: Bob Callaway <bcallaway@google.com>

pkg/signature/ecdsa.go

@@ -20,9 +20,11 @@ import (
 	"crypto/ecdsa"
 	"crypto/elliptic"
 	"crypto/rand"
+	"encoding/asn1"
 	"errors"
 	"fmt"
 	"io"
+	"math/big"
 
 	"github.com/sigstore/sigstore/pkg/signature/options"
 )
@@ -190,8 +192,23 @@ func (e ECDSAVerifier) VerifySignature(signature, message io.Reader, opts ...Ver
 		return fmt.Errorf("invalid ECDSA public key for %s", e.publicKey.Params().Name)
 	}
 
-	if !ecdsa.VerifyASN1(e.publicKey, digest, sigBytes) {
-		return errors.New("invalid signature when validating ASN.1 encoded signature")
+	asnParseTest := struct {
+		R, S *big.Int
+	}{}
+	if _, err := asn1.Unmarshal(sigBytes, &asnParseTest); err == nil {
+		if !ecdsa.VerifyASN1(e.publicKey, digest, sigBytes) {
+			return errors.New("invalid signature when validating ASN.1 encoded signature")
+		}
+	} else {
+		// deal with IEEE P1363 encoding of signatures
+		if len(sigBytes) == 0 || len(sigBytes) > 132 || len(sigBytes)%2 != 0 {
+			return errors.New("ecdsa: Invalid IEEE_P1363 encoded bytes")
+		}
+		r := new(big.Int).SetBytes(sigBytes[:len(sigBytes)/2])
+		s := new(big.Int).SetBytes(sigBytes[len(sigBytes)/2:])
+		if !ecdsa.Verify(e.publicKey, digest, r, s) {
+			return errors.New("invalid signature when validating IEEE_P1363 encoded signature")
+		}
 	}
 
 	return nil

pkg/signature/ecdsa_test.go

@@ -72,6 +72,13 @@ func TestECDSASignerVerifier(t *testing.T) {
 		t.Errorf("unexpected error creating verifier: %v", err)
 	}
 	testingVerifier(t, v, "ecdsa", crypto.SHA256, sig, message)
+
+	// test IEEE P1363 encoded signature
+	// created via
+	// import "github.com/tink-crypto/tink-go/signature/subtle"
+	// subtle.NewECDSASignerFromPrivateKey("SHA256", "IEEE_P1363", privKey.(*ecdsa.PrivateKey))
+	ieeeSig, _ := base64.StdEncoding.DecodeString("bmvQGbNZEJyS3HAVPiuXY0/BGFcUq5cl22v3+sttMxBls5LO0W52qR2yk6q0E59wDNs15gvaODeTZ2d6MwH9OQ==")
+	testingVerifier(t, v, "ecdsa", crypto.SHA256, ieeeSig, message)
 }
 
 func TestECDSASignerVerifierUnsupportedHash(t *testing.T) {