Given a digest, public key (as hex), and a base64 signature,
    verify that the public key signed the digest.
    Return True if so
    Return False if not

def verify_digest(hash_hex, pubkey_hex, sigb64, hashfunc=hashlib.sha256):
    """
    Given a digest, public key (as hex), and a base64 signature,
    verify that the public key signed the digest.
    Return True if so
    Return False if not
    """
    if not isinstance(hash_hex, (str, unicode)):
        raise ValueError("hash hex is not a string")

    hash_hex = str(hash_hex)
    pubk_uncompressed_hex = keylib.key_formatting.decompress(pubkey_hex)
    sig_r, sig_s = decode_signature(sigb64)

    pubk = ec.EllipticCurvePublicNumbers.from_encoded_point(ec.SECP256K1(), pubk_uncompressed_hex.decode('hex')).public_key(default_backend())
    signature = encode_dss_signature(sig_r, sig_s)

    try:
        pubk.verify(signature, hash_hex.decode('hex'), ec.ECDSA(utils.Prehashed(hashes.SHA256())))
        return True
    except InvalidSignature:
        return False