Martin Martin - 26 days ago 8
Python Question

Decrypting in Python an string encrypted using .NET

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)

Answer

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')
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