keywrap.py 文件源码

def _unwrap_core(wrapping_key, a, r, backend):
    # Implement RFC 3394 Key Unwrap - 2.2.2 (index method)
    decryptor = Cipher(AES(wrapping_key), ECB(), backend).decryptor()
    n = len(r)
    for j in reversed(range(6)):
        for i in reversed(range(n)):
            # pack/unpack are safe as these are always 64-bit chunks
            atr = struct.pack(
                ">Q", struct.unpack(">Q", a)[0] ^ ((n * j) + i + 1)
            ) + r[i]
            # every decryption operation is a discrete 16 byte chunk so
            # it is safe to reuse the decryptor for the entire operation
            b = decryptor.update(atr)
            a = b[:8]
            r[i] = b[-8:]

    assert decryptor.finalize() == b""
    return a, r