Function _decrypt

pgpy/symenc.py:38–56

(ct, key, alg, iv=None)

def _decrypt(ct, key, alg, iv=None):
    if iv is None:
        """
        Instead of using an IV, OpenPGP prefixes a string of length
        equal to the block size of the cipher plus two to the data before it
        is encrypted. The first block-size octets (for example, 8 octets for
        a 64-bit block length) are random, and the following two octets are
        copies of the last two octets of the IV.
        """
        iv = b'\x00' * (alg.block_size // 8)

    try:
        decryptor = Cipher(alg.cipher(key), modes.CFB(iv), default_backend()).decryptor()

    except UnsupportedAlgorithm as ex:  # pragma: no cover
        raise PGPDecryptionError from ex

    else:
        return bytearray(decryptor.update(ct) + decryptor.finalize())