crypto: use btcec/v2 for no-cgo (#24533)

This updates the no-cgo implementations in the crypto package to use
the github.com/btcsuite/btcd/btcec/v2 module instead of the older btcec
package that was part of the main github.com/btcsuite/btcd module.

name                   old time/op  new time/op  delta
EcrecoverSignature-32   198µs ± 0%   144µs ± 0%  -27.11%
VerifySignature-32      177µs ± 0%   128µs ± 0%  -27.44%
DecompressPubkey-32    20.9µs ± 0%  10.1µs ± 0%  -51.51%

Use (*ModNScalar).IsOverHalfOrder instead of math/big.Int when checking
for malleable signatures.

crypto/signature_nocgo.go

@@ -24,37 +24,48 @@ import (
 	"crypto/elliptic"
 	"errors"
 	"fmt"
-	"math/big"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
+	btc_ecdsa "github.com/btcsuite/btcd/btcec/v2/ecdsa"
 )
 
 // Ecrecover returns the uncompressed public key that created the given signature.
 func Ecrecover(hash, sig []byte) ([]byte, error) {
-	pub, err := SigToPub(hash, sig)
+	pub, err := sigToPub(hash, sig)
 	if err != nil {
 		return nil, err
 	}
-	bytes := (*btcec.PublicKey)(pub).SerializeUncompressed()
+	bytes := pub.SerializeUncompressed()
 	return bytes, err
 }
 
+func sigToPub(hash, sig []byte) (*btcec.PublicKey, error) {
+	if len(sig) != SignatureLength {
+		return nil, errors.New("invalid signature")
+	}
+	// Convert to btcec input format with 'recovery id' v at the beginning.
+	btcsig := make([]byte, SignatureLength)
+	btcsig[0] = sig[RecoveryIDOffset] + 27
+	copy(btcsig[1:], sig)
+
+	pub, _, err := btc_ecdsa.RecoverCompact(btcsig, hash)
+	return pub, err
+}
+
 // SigToPub returns the public key that created the given signature.
 func SigToPub(hash, sig []byte) (*ecdsa.PublicKey, error) {
-	// Convert to btcec input format with 'recovery id' v at the beginning.
-	btcsig := make([]byte, SignatureLength)
-	btcsig[0] = sig[64] + 27
-	copy(btcsig[1:], sig)
-
-	pub, _, err := btcec.RecoverCompact(btcec.S256(), btcsig, hash)
-	return (*ecdsa.PublicKey)(pub), err
+	pub, err := sigToPub(hash, sig)
+	if err != nil {
+		return nil, err
+	}
+	return pub.ToECDSA(), nil
 }
 
 // Sign calculates an ECDSA signature.
 //
 // This function is susceptible to chosen plaintext attacks that can leak
 // information about the private key that is used for signing. Callers must
-// be aware that the given hash cannot be chosen by an adversery. Common
+// be aware that the given hash cannot be chosen by an adversary. Common
 // solution is to hash any input before calculating the signature.
 //
 // The produced signature is in the [R || S || V] format where V is 0 or 1.
@@ -65,14 +76,20 @@ func Sign(hash []byte, prv *ecdsa.PrivateKey) ([]byte, error) {
 	if prv.Curve != btcec.S256() {
 		return nil, fmt.Errorf("private key curve is not secp256k1")
 	}
-	sig, err := btcec.SignCompact(btcec.S256(), (*btcec.PrivateKey)(prv), hash, false)
+	// ecdsa.PrivateKey -> btcec.PrivateKey
+	var priv btcec.PrivateKey
+	if overflow := priv.Key.SetByteSlice(prv.D.Bytes()); overflow || priv.Key.IsZero() {
+		return nil, fmt.Errorf("invalid private key")
+	}
+	defer priv.Zero()
+	sig, err := btc_ecdsa.SignCompact(&priv, hash, false) // ref uncompressed pubkey
 	if err != nil {
 		return nil, err
 	}
 	// Convert to Ethereum signature format with 'recovery id' v at the end.
 	v := sig[0] - 27
 	copy(sig, sig[1:])
-	sig[64] = v
+	sig[RecoveryIDOffset] = v
 	return sig, nil
 }
 
@@ -83,13 +100,20 @@ func VerifySignature(pubkey, hash, signature []byte) bool {
 	if len(signature) != 64 {
 		return false
 	}
-	sig := &btcec.Signature{R: new(big.Int).SetBytes(signature[:32]), S: new(big.Int).SetBytes(signature[32:])}
-	key, err := btcec.ParsePubKey(pubkey, btcec.S256())
+	var r, s btcec.ModNScalar
+	if r.SetByteSlice(signature[:32]) {
+		return false // overflow
+	}
+	if s.SetByteSlice(signature[32:]) {
+		return false
+	}
+	sig := btc_ecdsa.NewSignature(&r, &s)
+	key, err := btcec.ParsePubKey(pubkey)
 	if err != nil {
 		return false
 	}
 	// Reject malleable signatures. libsecp256k1 does this check but btcec doesn't.
-	if sig.S.Cmp(secp256k1halfN) > 0 {
+	if s.IsOverHalfOrder() {
 		return false
 	}
 	return sig.Verify(hash, key)
@@ -100,16 +124,26 @@ func DecompressPubkey(pubkey []byte) (*ecdsa.PublicKey, error) {
 	if len(pubkey) != 33 {
 		return nil, errors.New("invalid compressed public key length")
 	}
-	key, err := btcec.ParsePubKey(pubkey, btcec.S256())
+	key, err := btcec.ParsePubKey(pubkey)
 	if err != nil {
 		return nil, err
 	}
 	return key.ToECDSA(), nil
 }
 
-// CompressPubkey encodes a public key to the 33-byte compressed format.
+// CompressPubkey encodes a public key to the 33-byte compressed format. The
+// provided PublicKey must be valid. Namely, the coordinates must not be larger
+// than 32 bytes each, they must be less than the field prime, and it must be a
+// point on the secp256k1 curve. This is the case for a PublicKey constructed by
+// elliptic.Unmarshal (see UnmarshalPubkey), or by ToECDSA and ecdsa.GenerateKey
+// when constructing a PrivateKey.
 func CompressPubkey(pubkey *ecdsa.PublicKey) []byte {
-	return (*btcec.PublicKey)(pubkey).SerializeCompressed()
+	// NOTE: the coordinates may be validated with
+	// btcec.ParsePubKey(FromECDSAPub(pubkey))
+	var x, y btcec.FieldVal
+	x.SetByteSlice(pubkey.X.Bytes())
+	y.SetByteSlice(pubkey.Y.Bytes())
+	return btcec.NewPublicKey(&x, &y).SerializeCompressed()
 }
 
 // S256 returns an instance of the secp256k1 curve.