From 9a4e7578d5eb502e1d8eb7b83a910ea3114912c2 Mon Sep 17 00:00:00 2001
From: Moreno Ambrosin <ambrosin@google.com>
Date: Wed, 20 Nov 2024 05:18:44 -0800
Subject: [PATCH] Use `ecdsa.{SignASN1,VerifyASN1}` in
 `subtle.{ECDSASigner,ECDSAVerifier}`

This avoids using the legacy `ecdsa.Sign` [1] for DER encoded signatures -- this is still used for the IEEE_P1363 encoding, and `ecdsa.Verify` [2]. The `crypto/ecdsa` package is moving away from using `math/big` [3].

[1] https://github.com/golang/go/blob/fe7d97d0322c283462b38c0f53bc340b642fe1cb/src/crypto/ecdsa/ecdsa_legacy.go#L59
[2] https://github.com/golang/go/blob/fe7d97d0322c283462b38c0f53bc340b642fe1cb/src/crypto/ecdsa/ecdsa_legacy.go#L135
[3] https://github.com/golang/go/commit/08f2091ce0817346458d2ae984ccea77817cd516

PiperOrigin-RevId: 698354627
Change-Id: I3b83575d614a65127f480a41cf90d76288444f4c
---
 signature/subtle/ecdsa_signer.go   | 30 ++++++++++++++++++++----------
 signature/subtle/ecdsa_verifier.go | 28 +++++++++++++++++++---------
 2 files changed, 39 insertions(+), 19 deletions(-)

diff --git a/signature/subtle/ecdsa_signer.go b/signature/subtle/ecdsa_signer.go
index ecc20a6..64b9e13 100644
--- a/signature/subtle/ecdsa_signer.go
+++ b/signature/subtle/ecdsa_signer.go
@@ -69,15 +69,25 @@ func (e *ECDSASigner) Sign(data []byte) ([]byte, error) {
 	if err != nil {
 		return nil, err
 	}
-	r, s, err := ecdsa.Sign(rand.Reader, e.privateKey, hashed)
-	if err != nil {
-		return nil, fmt.Errorf("ecdsa_signer: signing failed: %s", err)
-	}
-	// format the signature
-	sig := NewECDSASignature(r, s)
-	ret, err := sig.EncodeECDSASignature(e.encoding, e.privateKey.PublicKey.Curve.Params().Name)
-	if err != nil {
-		return nil, fmt.Errorf("ecdsa_signer: signing failed: %s", err)
+	var signatureBytes []byte
+	switch e.encoding {
+	case "IEEE_P1363":
+		r, s, err := ecdsa.Sign(rand.Reader, e.privateKey, hashed)
+		if err != nil {
+			return nil, err
+		}
+		sig := NewECDSASignature(r, s)
+		signatureBytes, err = sig.EncodeECDSASignature(e.encoding, e.privateKey.PublicKey.Curve.Params().Name)
+		if err != nil {
+			return nil, fmt.Errorf("ecdsa_signer: signing failed: %s", err)
+		}
+	case "DER":
+		signatureBytes, err = ecdsa.SignASN1(rand.Reader, e.privateKey, hashed)
+		if err != nil {
+			return nil, fmt.Errorf("ecdsa_signer: signing failed: %s", err)
+		}
+	default:
+		return nil, fmt.Errorf("ecdsa_signer: unsupported encoding: %s", e.encoding)
 	}
-	return ret, nil
+	return signatureBytes, nil
 }
diff --git a/signature/subtle/ecdsa_verifier.go b/signature/subtle/ecdsa_verifier.go
index 70784bc..87a90bb 100644
--- a/signature/subtle/ecdsa_verifier.go
+++ b/signature/subtle/ecdsa_verifier.go
@@ -21,11 +21,10 @@ import (
 	"hash"
 	"math/big"
 
+	internalecdsa "github.com/tink-crypto/tink-go/v2/internal/signature/ecdsa"
 	"github.com/tink-crypto/tink-go/v2/subtle"
 )
 
-var errInvalidECDSASignature = errors.New("ecdsa_verifier: invalid signature")
-
 // ECDSAVerifier is an implementation of Verifier for ECDSA.
 // At the moment, the implementation only accepts signatures with strict DER encoding.
 type ECDSAVerifier struct {
@@ -67,17 +66,28 @@ func NewECDSAVerifierFromPublicKey(hashAlg string, encoding string, publicKey *e
 // Verify verifies whether the given signature is valid for the given data.
 // It returns an error if the signature is not valid; nil otherwise.
 func (e *ECDSAVerifier) Verify(signatureBytes, data []byte) error {
-	signature, err := DecodeECDSASignature(signatureBytes, e.encoding)
-	if err != nil {
-		return fmt.Errorf("ecdsa_verifier: %s", err)
-	}
 	hashed, err := subtle.ComputeHash(e.hashFunc, data)
 	if err != nil {
 		return err
 	}
-	valid := ecdsa.Verify(e.publicKey, hashed, signature.R, signature.S)
-	if !valid {
-		return errInvalidECDSASignature
+	var asn1Signature []byte
+	switch e.encoding {
+	case "DER":
+		asn1Signature = signatureBytes
+	case "IEEE_P1363":
+		decodedSig, err := internalecdsa.IEEEP1363Decode(signatureBytes)
+		if err != nil {
+			return err
+		}
+		asn1Signature, err = internalecdsa.ASN1Encode(decodedSig)
+		if err != nil {
+			return err
+		}
+	default:
+		return fmt.Errorf("ecdsa: unsupported encoding: %s", e.encoding)
+	}
+	if ok := ecdsa.VerifyASN1(e.publicKey, hashed, asn1Signature); !ok {
+		return fmt.Errorf("ecdsa_verifier: invalid signature")
 	}
 	return nil
 }