ecdsaVerify function

bool ecdsaVerify(EcPublicKey key, List<int> digest, Uint8List signatureDer)

Verifies an ECDSA signature (DER SEQUENCE { r, s }) over digest.

Implementation

bool ecdsaVerify(EcPublicKey key, List<int> digest, Uint8List signatureDer) {
  final BigInt r, s;
  try {
    final seq = DerObject.parse(signatureDer).children;
    r = seq[0].asInteger;
    s = seq[1].asInteger;
  } on Object {
    return false;
  }
  final curve = key.curve;
  final n = curve.n;
  if (r <= BigInt.zero || r >= n || s <= BigInt.zero || s >= n) return false;

  // leftmost bits of the digest, per SEC1 §4.1.4
  var e = BigInt.zero;
  for (final byte in digest) {
    e = (e << 8) | BigInt.from(byte);
  }
  final excess = digest.length * 8 - n.bitLength;
  if (excess > 0) e >>= excess;

  final w = s.modInverse(n);
  final u1 = (e * w) % n;
  final u2 = (r * w) % n;
  final point = _add(
      curve, _multiply(curve, curve.g, u1), _multiply(curve, (key.x, key.y), u2));
  if (point == null) return false;
  return point.$1 % n == r;
}