signature method
sign the hash of message with the private key
Implementation
Signature signature(String hexHash) {
final rs = base.sign(base.secp256k1.n, base.secp256k1.p, base.secp256k1.a,
D, base.secp256k1.G, BigInt.parse(hexHash, radix: 16));
return Signature(rs[0], rs[1]);
}