import ecdsa
from hashlib import sha256


class Signer:
    def sign_rfc6979(self, private_key: bytes, message: bytes) -> bytes:
        if len(private_key) == 0 or private_key is None:
            raise ValueError(f"Incorrect private_key: {private_key}")

        # SECP256k1 is the Bitcoin elliptic curve
        sk = ecdsa.SigningKey.from_string(private_key, curve=ecdsa.SECP256k1, hashfunc=sha256)

        signature = sk.sign(message, sigencode=ecdsa.util.sigencode_string)

        return signature