Encrypting a string in .net and decrypting it in python

Question

I'm trying to encrypt password using system.Security.Cryptography which is working properly This is the code (.Net)

if (clearText == null)
{
    clearText = "";
}
string EncryptionKey = "****";
byte[] clearBytes = Encoding.Unicode.GetBytes(clearText);
using (Aes encryptor = Aes.Create())
{
    Rfc2898DeriveBytes pdb = new Rfc2898DeriveBytes(EncryptionKey, new byte[] { 0x49, 0x76, 0x61, 0x6e, 0x20, 0x4d, 0x65, 0x64, 0x76, 0x65, 0x64, 0x65, 0x76 });
    encryptor.Key = pdb.GetBytes(32);
    encryptor.IV = pdb.GetBytes(16);
    using (MemoryStream ms = new MemoryStream())
    {
        using (CryptoStream cs = new CryptoStream(ms, encryptor.CreateEncryptor(), CryptoStreamMode.Write))
        {
            cs.Write(clearBytes, 0, clearBytes.Length);
            cs.Close();
        }
        clearText = Convert.ToBase64String(ms.ToArray());
    }
}
return clearText;

And this is decryption code in python which is not working properly

def Decryptstr(self, text):
    try:
        EncryptionKey = "****"
        if text is None:
            return
        else:
            cipherbytes = base64.b64decode(text)
            salt = '\0x49\0x76\0x61\0x6e\0x20\0x4d\0x65\0x64\0x76\0x65\0x64\0x65\0x76'
            key_bytes = KDF.PBKDF2(EncryptionKey, salt, dkLen=32)
            iv = KDF.PBKDF2(EncryptionKey, salt,dkLen=16)
            cipher = AES.new(key_bytes, AES.MODE_CBC, iv)
            password = cipher.decrypt(cipherbytes).decode('utf-16')
            print(password)
            return password
    except Exception as err:
        print(err)

following is the output of the above code for the encrypted string ('eet123') 䏺꧴퐄妯৞軸힡薟

Any help will be appreciated.

In the C# code, how are you telling the Aes class that the mode is CBC? Or it is CBC by default? — ikerbera
– ikerbera, Commented Jan 3, 2019 at 6:46
by default the block cipher mode to use for encryption is CBC in c# @ikerbera — Abhishek Jain
– Abhishek Jain, Commented Jan 3, 2019 at 7:40
I think .GetBytes() returns the next n bytes. In that case your C# IV is bytes [32:48] while your Python IV is bytes [0:16] (which is also the first half of your key!). Also, Im not sure what is the defult hash algorithm in KDF.PBKDF2, but I think it's the same as Rfc2898DeriveBytes (SHA1). — t.m.adam
– t.m.adam, Commented Jan 3, 2019 at 7:42
hash algorithm is same for both KDF.PBKDF2 and Rfc2898DeriveBytes @t.m.adam — Abhishek Jain
– Abhishek Jain, Commented Jan 3, 2019 at 7:50
Write a complete implementation in C# that round trips your unencrypted text. Then write another complete implementation in python that does the same thing. Try to decompose them into small chunks of functionality that do the same things in both. It's then a) a lot easier to compare outputs of stages and see where they disagree and b) easy to then introduce a point where one hands off the data to the other, satisfying your original requirement. — Damien_The_Unbeliever
– Damien_The_Unbeliever, Commented Jan 3, 2019 at 12:56

Scott Baker · Accepted Answer · 2022-07-20 17:08:28Z

It seems that your PBKDF2HMAC key extraction in python side is incorrect. You need to pass correct parameters and get a 48 bytes key. Then use first 32 bytes as Key and last 16 bytes as IV (in your design).

Here is a working C#/Python code pair. First C# part:

static string encrypt(string clearText = "")
{
    if (clearText == null)
    {
        clearText = "";
    }

    string EncryptionKey = "****";
    byte[] clearBytes = Encoding.UTF8.GetBytes(clearText);
    using (Aes encryptor = Aes.Create())
    {
       Rfc2898DeriveBytes pdb = new Rfc2898DeriveBytes(EncryptionKey, new byte[] { 0x49, 0x76, 0x61, 0x6e, 0x20, 0x4d, 0x65, 0x64, 0x76, 0x65, 0x64, 0x65, 0x76 }, 100000, HashAlgorithmName.SHA1);
       encryptor.Key = pdb.GetBytes(32);
       encryptor.IV = pdb.GetBytes(16);
       encryptor.Mode = CipherMode.CBC;
       using (MemoryStream ms = new MemoryStream())
       {
            using (CryptoStream cs = new CryptoStream(ms, encryptor.CreateEncryptor(), CryptoStreamMode.Write))
            {
                cs.Write(clearBytes, 0, clearBytes.Length);
                cs.Close();
            }
        clearText = Convert.ToBase64String(ms.ToArray());
        }
    }
    return clearText;
}

About this C# code:

By default Rfc2898DeriveBytes uses SHA1 and only 1000 rounds. My suggestion is that you should use at least 100,000. I have seen code apps that use 1,000,000 rounds. You can change hash too if you like, but number of rounds is more important.
Specify mode too. Even though it uses CBC by default, I think it is better to specify it.
Since C# uses key length to select AES algorithm, this code uses AES-256.

Now Python part:

from cryptography.hazmat.primitives import hashes
from cryptography.hazmat.primitives.kdf.pbkdf2 import PBKDF2HMAC
from cryptography.hazmat.backends import default_backend

def Decryptstr(self, text):
    try:
        if text is None:
            return
        else:
            backend = default_backend()
            EncryptionKey = "****"
            salt = bytes([ 0x49, 0x76, 0x61, 0x6e, 0x20, 0x4d, 0x65, 0x64, 0x76, 0x65, 0x64, 0x65, 0x76 ])
            kdf = PBKDF2HMAC(algorithm=hashes.SHA1(),length=48,salt=salt,iterations=100000,backend=backend)
            key_bytes = kdf.derive(bytes(EncryptionKey, 'utf-8'))
            key = key_bytes[0:32]
            iv = key_bytes[32:]

            cipherbytes = base64.b64decode(text)
            cipher = AES.new(key, AES.MODE_CBC, iv)
            password = cipher.decrypt(cipherbytes).decode('utf-8')
            print(password)
            return password
    except Exception as err:
        print(err)

As you see, I used another PBKDF2HMAC library. I used it to create 48 bytes and used first 32 as Key and last 16 bytes as IV.

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