Decrypting in Python an string encrypted using .NET

Question

I am trying to encrypt a string using C# and decrypt it using Python. The encryption/decryption part works as expected (i.e. I am able to decrypt the string I originally encrypted). However the string returned by Python has 2 extra bytes at the beginning and each character is separated by a space.

**Original string** (before encryption -- encrypted using C#) = "Something you want to keep private with AES"

**Decrypted string** (using Python) = "��S o m e t h i n g  y o u   w a n t   t o   k e e p   p r i v a t e   w i t h  A E S"

Why am I getting these two extra bytes at the beginning of the string? Why all those spaces in the decrypted string? Any idea why?

Thanks!

Encryption with C#

public static string Encrypt<T>(string value, string password, string salt)
         where T : SymmetricAlgorithm, new()
{
    DeriveBytes rgb = new Rfc2898DeriveBytes(password, Encoding.Unicode.GetBytes(salt));

    SymmetricAlgorithm algorithm = new T();

    byte[] rgbKey = rgb.GetBytes(algorithm.KeySize >> 3);
    byte[] rgbIV = rgb.GetBytes(algorithm.BlockSize >> 3);

    ICryptoTransform transform = algorithm.CreateEncryptor(rgbKey, rgbIV);

    using (MemoryStream buffer = new MemoryStream())
    {
        using (CryptoStream stream = new CryptoStream(buffer, transform, CryptoStreamMode.Write))
        {
            using (StreamWriter writer = new StreamWriter(stream, Encoding.Unicode))
            {
                writer.Write(value);
            }
        }

        return Convert.ToBase64String(buffer.ToArray());
    }
}


string plain = "Something you want to keep private with AES";
string encrypted = CipherUtility.Encrypt<AesManaged>(plain, "password", "salt");

Decryption with Python + pycrypto

import base64, sys
import Crypto.Cipher.AES

password = base64.b64decode('PSCIQGfoZidjEuWtJAdn1JGYzKDonk9YblI0uv96O8s=') # See rgbKey
salt = base64.b64decode('ehjtnMiGhNhoxRuUzfBOXw==') # See rgbIV
aes = Crypto.Cipher.AES.new(password, Crypto.Cipher.AES.MODE_CBC, salt)
text = base64.b64decode('QpHn/fnraLswwI2Znt1xTaBzRtDqO4V5QI78jLOlVsbvaIs0yXMUlqJhQtK+su2hYn28G2vNyLkj0zLOs+RIjElCSqJv1aK/Yu8uY07oAeStqRt4u/DVUzoWlxdrlF0u')

print aes.decrypt(text)

How can I decrypt using Python for .NET RSA algorithm encryption ? — Neo
– Neo, Commented Oct 18, 2017 at 10:05

Mark Byers · Accepted Answer · 2012-05-12 17:25:40Z

12

The string is encoded to bytes using the UTF-16 encoding. The first two bytes are a BOM. Then each character is encoded to two bytes.

From the documentation for Encoding.Unicode:

Gets an encoding for the UTF-16 format using the little endian byte order.

To get the original string you need to decode it back from UTF-16 bytes to a Unicode string.

print aes.decrypt(text).decode('utf-16')

answered May 12, 2012 at 17:25

Mark Byers

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3 Comments

Martin Over a year ago

Wow! Thank you Mark! Now that I am decoding the string using utf-16 (as you proposed), the resulting string is: "Something you want to keep private with AESࠈࠈࠈࠈ". Any idea how to get rid of the last 4 characters?

Luka Rahne Over a year ago

Try to set padding for SymmetricAlgorithm to zeros msdn.microsoft.com/en-us/library/…. By default it uses PKCS7

john johnson Over a year ago

i was looking for a method of encrypting in .net/C# and decrypting in python and came across this post.. i too am getting the padding. Mine shows up as "test2ЄЄ" with the extra E looking chars . i tried changing PaddingMode to None on the .net side from PKCS7 and it made no difference??

Little Boy · Accepted Answer · 2019-05-28 15:19:23Z

0

 def decrypted(self) -> str:
    _pwd = base64.b64decode(self._key)
    _salt = base64.b64decode(self._salt)
    _aes = Crypto.Cipher.AES.new(_pwd, Crypto.Cipher.AES.MODE_CBC, _salt)
    _text = base64.b64decode(self._encrypted_str)
    _decode = _aes.decrypt(_text).decode('utf-16')
    _regex = '[a-zA-Z0-9 +-,\/ ]+'
    return re.findall(_regex, _decode)

is using regex

answered May 28, 2019 at 15:19

Little Boy

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