A toy decryption algorithm using AES in CBC mode, with PKCS5 padding scheme.

decrypt_aes_cbc.py

from Crypto.Cipher import AES, XOR  # from pycrypto library (https://www.dlitz.net/software/pycrypto/)


block_size = AES.block_size


def decrypt(key, ciphertext):
    # assume: len(key) == 16 bytes; PKCS5 padding scheme; len(IV) == 16 bytes
    iv = ciphertext[:block_size]
    ciph = ciphertext[block_size:]

    assert len(ciph) % block_size == 0

    cipher = AES.new(key)
    plain_blocks = []
    previous_ciph_block = iv
    for i in range(len(ciph) / block_size):
        xor = XOR.new(previous_ciph_block)

        ciph_block = ciph[(i*block_size):((i+1)*block_size)]
        plain_block = xor.decrypt(cipher.decrypt(ciph_block))

        previous_ciph_block = ciph_block
        plain_blocks.append(plain_block)

    # remove plaintext padding
    padded_plaintext = ''.join(plain_blocks)
    num_padded_bytes = ord(padded_plaintext[-1])

    return padded_plaintext[:(0-num_padded_bytes)]


if __name__ == '__main__':
    keys_ciphertexts_hex = [
            ('140b41b22a29beb4061bda66b6747e14', '4ca00ff4c898d61e1edbf1800618fb2828a226d160dad07883d04e008a7897ee2e4b7465d5290d0c0e6c6822236e1daafb94ffe0c5da05d9476be028ad7c1d81'),
            ('140b41b22a29beb4061bda66b6747e14', '5b68629feb8606f9a6667670b75b38a5b4832d0f26e1ab7da33249de7d4afc48e713ac646ace36e872ad5fb8a512428a6e21364b0c374df45503473c5242a253'),
            ]

    for key_hex, ciphertext_hex in keys_ciphertexts_hex:
        key = key_hex.decode('hex')
        ciphertext = ciphertext_hex.decode('hex')

        print repr(decrypt(key, ciphertext))