crypto: add DecompressPubkey, VerifySignature (#15615)

We need those operations for p2p/enr.

Also upgrade github.com/btcsuite/btcd/btcec to the latest version
and improve BenchmarkSha3. The benchmark printed extra output 
that confused tools like benchstat and ignored N.

crypto/signature_cgo.go

@@ -27,10 +27,12 @@ import (
 	"github.com/ethereum/go-ethereum/crypto/secp256k1"
 )
 
+// Ecrecover returns the uncompressed public key that created the given signature.
 func Ecrecover(hash, sig []byte) ([]byte, error) {
 	return secp256k1.RecoverPubkey(hash, sig)
 }
 
+// SigToPub returns the public key that created the given signature.
 func SigToPub(hash, sig []byte) (*ecdsa.PublicKey, error) {
 	s, err := Ecrecover(hash, sig)
 	if err != nil {
@@ -58,6 +60,22 @@ func Sign(hash []byte, prv *ecdsa.PrivateKey) (sig []byte, err error) {
 	return secp256k1.Sign(hash, seckey)
 }
 
+// VerifySignature checks that the given public key created signature over hash.
+// The public key should be in compressed (33 bytes) or uncompressed (65 bytes) format.
+// The signature should have the 64 byte [R || S] format.
+func VerifySignature(pubkey, hash, signature []byte) bool {
+	return secp256k1.VerifySignature(pubkey, hash, signature)
+}
+
+// DecompressPubkey parses a public key in the 33-byte compressed format.
+func DecompressPubkey(pubkey []byte) (*ecdsa.PublicKey, error) {
+	x, y := secp256k1.DecompressPubkey(pubkey)
+	if x == nil {
+		return nil, fmt.Errorf("invalid public key")
+	}
+	return &ecdsa.PublicKey{X: x, Y: y, Curve: S256()}, nil
+}
+
 // S256 returns an instance of the secp256k1 curve.
 func S256() elliptic.Curve {
 	return secp256k1.S256()