code: func builtinJWTVerifyES256(a ast.Value, b ast.Value) (ast.Value, error) {
  	return builtinJWTVerify(a, b, func(publicKey interface{}, digest []byte, signature []byte) error {
  		publicKeyEcdsa, ok := publicKey.(*ecdsa.PublicKey)
  		if !ok {
  			return fmt.Errorf("incorrect public key type")
  		}
  		r, s := &big.Int{}, &big.Int{}
  		n := len(signature) / 2
  		r.SetBytes(signature[:n])
  		s.SetBytes(signature[n:])
  		if ecdsa.Verify(publicKeyEcdsa, digest, r, s) {
  			return nil
  		}
  		return fmt.Errorf("ECDSA signature verification error")
  	})
  }
docs: // Implements ES256 JWT signature verification.