Decrypt String in Python That Was Encrypted in Java

Question

I'm working on a server application in Python and an app to connect to it in Java. I need to encrypt the string in Java and then decrypt it in Python.

Everything works well with the Java code, but there is a problem with the Python code.

from Crypto.Cipher import AES
obj        = AES.new('0123456789abcdef', AES.MODE_CBC,IV="fedcba9876543210")
BLOCK_SIZE = 16
PADDING    = " "
pad        = lambda s: s + (BLOCK_SIZE - len(s) % BLOCK_SIZE) * PADDING
plain      = "password"
plain      = pad (plain)
ciph       = obj.encrypt(plain)
obj        = AES.new('0123456789abcdef', AES.MODE_CBC,IV="fedcba9876543210")
decrypted  = obj.decrypt(ciph)
result     = ""
for char in ciph:
    result += str((hex(ord(char))))[2:]
print result
print decrypted

The results of this are:

2af41fc02b33765b56433f59edb67dd3
password

Which is what I would expect and matches the Java output, however when I plug in the first of the 2 lines to decrypt it in the Python code, the output is totally off and displays random characters. I think that is due to the for loop at the end which makes it the same as the Java output. Is there a way to undo that for loop? Also using code at least similar to this is there a decent way to decrypt the first of the two output strings (in Python)?

I appreciate all responses, thank you ahead of time!

You probably need to convert from hex back to a binary byte string before doing the decrypt. — Hot Licks
– Hot Licks, Commented Nov 4, 2012 at 23:05
You should really use a random IV. Using a constant IV pretty much nullifies the purpose of the IV. If you don't use a random IV, two users with the same password will get the same encrypted password. — kichik
– kichik, Commented Nov 4, 2012 at 23:10

alexis · Accepted Answer · 2012-11-04 23:12:37Z

1

Every pair of digits in result is the character code of a char in hexadecimal. Here's a snippet that reproduces the transformation and undoes it (there are more elegant ways to do this, but it's simple and it works).

import re

ciph = "hello there"
result = ""
for c in ciph:
    result += str((hex(ord(c))))[2:]

# reverse it
orig = ""
for code in re.findall('..', result):
    orig += chr(int(code, 16))
print orig  # "hello there"

answered Nov 4, 2012 at 23:12

alexis

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