Implementation of AES as used by https://aesencryption.net

aes.py

#!/usr/bin/env python3
# -*- coding: utf-8 -*-
#
# (c) 2020 Bernd Busse
#
"""Implementation of AES as used by https://aesencryption.net."""

import base64
import sys

from cryptography.hazmat.backends import default_backend
from cryptography.hazmat.primitives.ciphers import Cipher
from cryptography.hazmat.primitives.ciphers.algorithms import AES
from cryptography.hazmat.primitives.ciphers.modes import CBC


AES_IV = b'\x31\x32\x33\x34\x35\x36\x37\x38\x62\x30\x7a\x32\x33\x34\x35\x6e'


class EncryptionError(RuntimeError):
    pass


class DecryptionError(RuntimeError):
    pass


def usage(executable):
    sys.stderr.write("usage: {} -e KEY KEYSIZE TEXT\n".format(executable))
    sys.stderr.write("       {} -d KEY KEYSIZE CIPHER\n".format(executable))
    sys.stderr.flush()


def encrypt(password, message, keysize=128):
    if keysize not in (128, 192, 256):
        raise EncryptionError(
                "Invalid key size: {} must be one of (128, 192, 256)"
                .format(keysize))

    aes_key = (password + '\0' * ((keysize // 8) - len(password))).encode("utf-8")
    cipher = Cipher(algorithm=AES(aes_key[:(keysize // 8)]),
                    mode=CBC(AES_IV),
                    backend=default_backend())
    cipher = cipher.encryptor()

    count = 16 - (len(message) % 16)
    c_text = cipher.update(message.encode("utf-8")) + \
        cipher.update(count * bytes([count])) + \
        cipher.finalize()

    try:
        return base64.b64encode(c_text).decode("utf-8")
    except UnicodeDecodeError:
        return base64.b64encode(c_text)


def decrypt(password, ciphertext, keysize=128):
    if keysize not in (128, 192, 256):
        raise DecryptionError(
                "Invalid key size: {} must be one of (128, 192, 256)"
                .format(keysize))

    aes_key = (password + '\0' * ((keysize // 8) - len(password))).encode("utf-8")
    cipher = Cipher(algorithm=AES(aes_key[:(keysize // 8)]),
                    mode=CBC(AES_IV),
                    backend=default_backend())
    cipher = cipher.decryptor()

    message = cipher.update(base64.b64decode(ciphertext)) + cipher.finalize()
    message = message[:-message[-1]]

    try:
        return message.decode("utf-8")
    except UnicodeDecodeError:
        return message


if __name__ == "__main__":
    if len(sys.argv) != 5:
        usage(sys.argv[0])
        exit(1)

    if sys.argv[1] == "-e":
        # Run encryption
        cipher = encrypt(sys.argv[2], sys.argv[4], keysize=int(sys.argv[3]))
        print("Ciphertext: {:s}".format(cipher))
    elif sys.argv[1] == "-d":
        # Run decryption
        plain = decrypt(sys.argv[2], sys.argv[4], keysize=int(sys.argv[3]))
        print("Plaintext: {:s}".format(plain))
    else:
        usage(sys.argv[0])
        exit(1)

Can you port this version for ios ?

How on earth did you derive the IV? The code on the site says it's '1234567890123456' but yours is clearly correct.

How on earth did you derive the IV? The code on the site says it's '1234567890123456' but yours is clearly correct.

Through the magic of the CBC mode:

• When decrypting the first ciphertext block, the IV is XOR-ed onto the output of the block cipher, yielding the plaintext.
• Given we can use the site as an oracle to generate the ciphertext for a known plaintext and key, we can calculate the output of the block cipher in ECB mode with the same ciphertext and key.
• Since we now know two inputs to the XOR function (output of block cipher and plaintext) we can calculate the third value which is the IV in this case.