AES encryption/decryption in python

Sometimes I just need some encryption, so I wrote a script that fits some cases. The functions use the python Crypto library.

The security of the used encryption is ok, I wrote a PBKDF2-like Key Derivation Function, that hashes the password before truncating and using it as the AES key. The encryption function does not add random padding. This means an attacker can guess how long the plaintext was. Additionally, CBC is a non-authenticated mode, therefore if somebody flips a bit in your ciphertext the decryption routine won’t notice. This usually means an attacker can flip one bit, but the remaining blocks will be corrupted. So flipping a bit in the last block is easy. Moreover 13’370 derivation rounds might be too much or not enough for you.

def AESencrypt(password, plaintext, base64=False):
    import hashlib, os
    from Crypto.Cipher import AES
    SALT_LENGTH = 32
    DERIVATION_ROUNDS=13370
    BLOCK_SIZE = 16
    KEY_SIZE = 32
    MODE = AES.MODE_CBC
    
    salt = os.urandom(SALT_LENGTH)
    iv = os.urandom(BLOCK_SIZE)
    
    paddingLength = 16 - (len(plaintext) % 16)
    paddedPlaintext = plaintext+chr(paddingLength)*paddingLength
    derivedKey = password
    for i in range(0,DERIVATION_ROUNDS):
        derivedKey = hashlib.sha256(derivedKey+salt).digest()
    derivedKey = derivedKey[:KEY_SIZE]
    cipherSpec = AES.new(derivedKey, MODE, iv)
    ciphertext = cipherSpec.encrypt(paddedPlaintext)
    ciphertext = ciphertext + iv + salt
    if base64:
        import base64
        return base64.b64encode(ciphertext)
    else:
        return ciphertext.encode("hex")

def AESdecrypt(password, ciphertext, base64=False):
    import hashlib
    from Crypto.Cipher import AES
    SALT_LENGTH = 32
    DERIVATION_ROUNDS=13370
    BLOCK_SIZE = 16
    KEY_SIZE = 32
    MODE = AES.MODE_CBC
    
    if base64:
        import base64
        decodedCiphertext = base64.b64decode(ciphertext)
    else:
        decodedCiphertext = ciphertext.decode("hex")
    startIv = len(decodedCiphertext)-BLOCK_SIZE-SALT_LENGTH
    startSalt = len(decodedCiphertext)-SALT_LENGTH
    data, iv, salt = decodedCiphertext[:startIv], decodedCiphertext[startIv:startSalt], decodedCiphertext[startSalt:]
    derivedKey = password
    for i in range(0, DERIVATION_ROUNDS):
        derivedKey = hashlib.sha256(derivedKey+salt).digest()
    derivedKey = derivedKey[:KEY_SIZE]
    cipherSpec = AES.new(derivedKey, MODE, iv)
    plaintextWithPadding = cipherSpec.decrypt(data)
    paddingLength = ord(plaintextWithPadding[-1])
    plaintext = plaintextWithPadding[:-paddingLength]
    return plaintext
    
a = AESencrypt("password", "ABC")
print AESdecrypt("password", a)

One thought on “AES encryption/decryption in python

  1. Can you help me to desription the meaning of comands in AES encryption/descryption code! It need me in masters degre in faculty. Please help …thank you so much-jeta

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