C# AES CBC PKCS7 to Python

Question

I'm currently trying to translate a simple AES code from C# to Python. I know both of these languages pretty well, but I have no idea about the encryption field (especially AES). I wrote this AES code before in C#, but now I have no idea how to make it work in Python (I'm using PyCrypto since Python2.7 does not have built-in AES). Below is my C# code:

using System.Collections;
using System.Text;
using System.Security.Cryptography;

namespace DefaultClasses
{
    public class SimpleAES
    {
        private const string KEY = "someKey";
        private const string IV = "someIV";
        private AesCryptoServiceProvider _aes;
        private ICryptoTransform _crypto;

        public SimpleAES()
        {
            _aes = new AesCryptoServiceProvider();
            _aes.BlockSize = 128;
            _aes.KeySize = 256;
            _aes.Key = ASCIIEncoding.ASCII.GetBytes(KEY);
            _aes.IV = ASCIIEncoding.ASCII.GetBytes(IV);
            _aes.Padding = PaddingMode.PKCS7;
            _aes.Mode = CipherMode.CBC;
        }

        public string encrypt(string message)
        {
            _crypto = _aes.CreateEncryptor(_aes.Key, _aes.IV);
            byte[] encrypted = _crypto.TransformFinalBlock(
                ASCIIEncoding.ASCII.GetBytes(message), 0, ASCIIEncoding.ASCII.GetBytes(message).Length);
            _crypto.Dispose();
            return System.Convert.ToBase64String(encrypted);
        }

        public string decrypt(string message)
        {
            _crypto = _aes.CreateDecryptor(_aes.Key, _aes.IV);
            byte[] decrypted = _crypto.TransformFinalBlock(
                System.Convert.FromBase64String(message), 0, System.Convert.FromBase64String(message).Length);
            _crypto.Dispose();
            return ASCIIEncoding.ASCII.GetString(decrypted);
        }
    }
}

Please note that I want also to have BlockSize = 128, KeySize = 256, Padding = PKCS7, and Cipher CBC for Python. Thanks in advances!

Answer 1

I realize this question is almost a year old but I found it when looking for some solutions to communicate with a legacy windows process that uses the PKCS7 padding in its implementation of AES. Here is a quick example that works great for me. Hopefully it proves useful to someone else down the line. I had the same block size, key size, and padding as the question author specified.

from Crypto.Cipher import AES
from pkcs7 import PKCS7Encoder
import base64

shared_key = "abc123ty9TW1abc123ty9TW1" #some random key for a working example
IV = "rTF25nTrrTF25nTr" 

clear_text = "Testing 123"
aes = AES.new(shared_key, AES.MODE_CBC, IV)
aes.block_size = 128
cipher_text = base64.b64encode(aes.encrypt(PKCS7Encoder().encode(clear_text)))
print(cipher_text)

aes_decrypter = AES.new(shared_key, AES.MODE_CBC, IV)
aes_decrypter.block_size = 128
clear_text = PKCS7Encoder().decode(aes_decrypter.decrypt(base64.b64decode(cipher_text)))
print(clear_text)

The pkcs7 padding utility I'm using was copied from a Github project:

import binascii
import StringIO

class PKCS7Encoder(object):
    def __init__(self, k=16):
       self.k = k

    ## @param text The padded text for which the padding is to be removed.
    # @exception ValueError Raised when the input padding is missing or corrupt.
    def decode(self, text):
        '''
        Remove the PKCS#7 padding from a text string
        '''
        nl = len(text)
        val = int(binascii.hexlify(text[-1]), 16)
        if val > self.k:
            raise ValueError('Input is not padded or padding is corrupt')

        l = nl - val
        return text[:l]

    ## @param text The text to encode.
    def encode(self, text):
        '''
        Pad an input string according to PKCS#7
        '''
        l = len(text)
        output = StringIO.StringIO()
        val = self.k - (l % self.k)
        for _ in xrange(val):
            output.write('%02x' % val)
        return text + binascii.unhexlify(output.getvalue())