Decrypt in Python an encrypted message in Java

Question

I'm trying to decrypt in Python (with M2Crypto) an encrypted message generated in Java with this library

My code (which I actually found here) works decrypting messages encrypted by itself, but not from Java's library, I get the following error:

EVPError: 'wrong final block length'

I have tried both *aes_128_cbc* and *aes_128_ecb* and I get the same error.

I guess the failure is that Java's result is Ascii's encoded and Python's code is expecting some other encoding (as it works with base64) but I don't know where to make the change (in my Python's code). I'm open to use any other Python encryption library.

Thanks

import M2Crypto
from base64 import b64encode, b64decode

ENC=1
DEC=0

def AES_build_cipher(key, iv, op=ENC):
    """"""""
    return M2Crypto.EVP.Cipher(alg='aes_128_cbc', key=key, iv=iv, op=op)

def AES_encryptor(key,msg, iv=None):
    """"""
    #Decode the key and iv
    key = b64decode(key)
    if iv is None:
        iv = '\0' * 16
    else:
        iv = b64decode(iv)

   # Return the encryption function
    def encrypt(data):
        cipher = AES_build_cipher(key, iv, ENC)
        v = cipher.update(data)
        v = v + cipher.final()
        del cipher
        v = b64encode(v)
        return v
    print "AES encryption successful\n"
    return encrypt(msg)

def AES_decryptor(key,msg, iv=None):
    """"""
    #Decode the key and iv
    key = b64decode(key)
    print key
    print
    if iv is None:
        iv = '\0' * 16
    else:
        iv = b64decode(iv)

   # Return the decryption function
    def decrypt(data):
        data = b64decode(data)
        cipher = AES_build_cipher(key, iv, DEC)
        v = cipher.update(data)
        v = v + cipher.final()
        del cipher
        return v
    print "AES dencryption successful\n"
    return decrypt(msg)

if __name__ == "__main__":
    result = AES_decryptor(b64encode(SECRET_KEY), msg=encrypted_message)

user156548 · Accepted Answer · 2011-09-27 11:08:48Z

1

What does "ascii encoded" mean? As you know, my code expected base64 input and produced base64 output. Removing the calls to b64decode and b64encode in the encrypt and decrypt functions will let you pass in raw data, then it'll be up to you to decode the input from Java into raw bytes.

answered Sep 27, 2011 at 11:08

user156548

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1 Comment

Ima Over a year ago

You are right, I have deleted every b64encode/decode call, but I have made some other changes to match Java JCE encryption results in ECB mode (not in CBC as the code uses). I also have replaced key value for key.decode("hex") in both methods and v = b64encode(v) for v = v.encode("hex") in the encryption method.

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