Encryption Oracle 12

encryption_oracle_12.py

from Crypto.Cipher import AES
from random import randint

def pad_pkcs7(buffer, block_size):
    if len(buffer) % block_size:
        padding = (len(buffer) / block_size + 1) * block_size - len(buffer)
    else:
        padding = 0
    # Padding size must be less than a byte 
    assert 0 <= padding <= 255
    new_buffer = bytearray()
    new_buffer[:] = buffer
    new_buffer += bytearray([chr(padding)] * padding)
    return new_buffer

def random_key(length):
    key = bytearray(length)
    for i in range(length):
        key[i] = chr(randint(0, 255)) 
    return key

key = bytes(random_key(16))

def aes_128_ecb_enc(buffer, key):
    obj = AES.new(key, AES.MODE_ECB)
    return bytearray(obj.encrypt(bytes(buffer)))

def encryption_oracle(data):
    unknown_string = bytearray((
        "Um9sbGluJyBpbiBteSA1LjAKV2l0aCBteSByYWctdG9wIGRvd24gc28gbXkg\n" +
        "aGFpciBjYW4gYmxvdwpUaGUgZ2lybGllcyBvbiBzdGFuZGJ5IHdhdmluZyBq\n" +
        "dXN0IHRvIHNheSBoaQpEaWQgeW91IHN0b3A/IE5vLCBJIGp1c3QgZHJvdmUg\n" +
        "YnkK"
    ).decode("base64"))
    plaintext = pad_pkcs7(
        data + unknown_string,
        AES.block_size,
    )
    return aes_128_ecb_enc(plaintext, key)