Question

I am learning about encryption and using crypto-js I have made a Js & c# version.
What I am trying to accomplish is that the JS or c# version will be able to decode each other messages.

For testing I have kept the IV and KEY , paddding and mode the same in both the JS and C# instance.

I have them both decrypting and encrypting data respectivly but what I have yet to accomplish is providing an encrypted from JS be able to decode using c#.

JS

var key = CryptoJS.enc.Base64.parse('7061737323313233'); 
var iv = CryptoJS.enc.Base64.parse('7061737323313233'); 
var encrypted = CryptoJS.AES.encrypt("It works", key, 
 { keySize: 128 / 8, iv: iv, mode: CryptoJS.mode.CBC,padding: CryptoJS.pad.Pkcs7 }); 

var decrypted = CryptoJS.AES.decrypt(encrypted, key, { 
keySize: 128 / 8, iv: iv, mode: CryptoJS.mode.CBC, padding: CryptoJS.pad.Pkcs7 }); 

document.write('Encrypted :' + encrypted + '<br>');
document.write('Key :' + encrypted.key + '<br>');
document.write('Salt :' + encrypted.salt + '<br>');
document.write('iv :' + encrypted.iv + '<br>');
document.write('Decrypted : ' + decrypted + '<br>');
document.write('utf8 = ' + decrypted.toString(CryptoJS.enc.Utf8) + '<br>');

C#

  public void startEncryption(string original )
        {

            using (RijndaelManaged myRijndael = new RijndaelManaged())
            {
                //Settings
                myRijndael.Mode = CipherMode.CBC;
                myRijndael.Padding = PaddingMode.PKCS7;
                myRijndael.FeedbackSize = 128;

                keybytes = Encoding.UTF8.GetBytes("7061737323313233");
                //Should be made unique for each message!. TODO
                iv = Encoding.UTF8.GetBytes("7061737323313233");

                // Encrypt the string to an array of bytes.
                encrypted = EncryptStringToBytes(original, keybytes, iv);

                //Show Encrypted data
                txt_Output.Text = Convert.ToBase64String(encrypted);

                // Decrypt the bytes to a string.
                string roundtrip = DecryptStringFromBytes(encrypted, keybytes, iv);

                //Display the original data and the decrypted data.
                Console.WriteLine("Original:   {0}", original);
                Console.WriteLine("Round Trip: {0}", roundtrip);
            }


        }

Where the problem arises in decryption.

  private void btn_Decrypt_Click(object sender, EventArgs e)
    {
        Console.WriteLine("Decrypting..");
        using (RijndaelManaged myRijndael = new RijndaelManaged())
        {
            //Settings
            myRijndael.Mode = CipherMode.CBC;
            myRijndael.Padding = PaddingMode.PKCS7;
            myRijndael.FeedbackSize = 128;

            keybytes = Encoding.UTF8.GetBytes("7061737323313233");
            //Should be made unique for each message!. TODO
            iv = Encoding.UTF8.GetBytes("7061737323313233");

            // Decrypt the bytes to a string.
            string roundtrip = DecryptToString(txt_Output.Text);

            txt_Output.Text = roundtrip;
            //Display the original data and the decrypted data.

        }
    }

  public string DecryptToString(string TextValue)
    {

        return DecryptStringFromBytes(Convert.FromBase64String(TextValue), keybytes, iv);
    }


         static string DecryptStringFromBytes(byte[] cipherText, byte[] Key, byte[] IV)
    {
        // Check arguments.
        if (cipherText == null || cipherText.Length <= 0)
            throw new ArgumentNullException("cipherText");
        if (Key == null || Key.Length <= 0)
            throw new ArgumentNullException("Key");
        if (IV == null || IV.Length <= 0)
            throw new ArgumentNullException("Key");

        // Declare the string used to hold
        // the decrypted text.
        string plaintext = null;

        // Create an RijndaelManaged object
        // with the specified key and IV.
        using (RijndaelManaged rijAlg = new RijndaelManaged())
        {
            rijAlg.Key = Key;
            rijAlg.IV = IV;
            rijAlg.Padding = PaddingMode.PKCS7;
            rijAlg.Mode = CipherMode.CBC;

            // Create a decrytor to perform the stream transform.
            ICryptoTransform decryptor = rijAlg.CreateDecryptor(rijAlg.Key, rijAlg.IV);

            // Create the streams used for decryption.
           using (MemoryStream msDecrypt = new MemoryStream(cipherText))
           {
         using (CryptoStream csDecrypt =
        new CryptoStream(msDecrypt,decryptor,CryptoStreamMode.Read))
                 {
                 using (StreamReader srDecrypt = new StreamReader(csDecrypt))
                 {

                  // Read the decrypted bytes from the decrypting stream
                   // and place them in a string.
                  plaintext = srDecrypt.ReadToEnd();
                 }
             }
         }

       }

      return plaintext;

    }

I am producing different encrypted size strings:

JS:MhAP11fHa+fUfRzSw2UHVQ==
C#:+Ijpt1GDVgM4MqMAQUwf0Q==

I get Padding is invalid and cannot be removed, when trying to decrypt the JS string in c#
Where have I got wrong?.

Answer 1

Basically you run into encoding issues. First of all, you parse your IV using Base64 decoding in one implementation and one using direct character encoding in the other. Your Base64 strings don’t look like Base64 strings either.

Furthermore, many libraries (incorrectly) allow that incorrect key and IV sizes are used. This is however confusing as there is no generic way for key or IV expansion. So you should make sure that the binary representations of the key and IV are correct for the specific algorithm.

For AES you should use a key size of 128, 192 or 256 bits and an IV size identical to the block size, 128 bits. The IV should be randomly generated and communicated to the other side, e.g. by prefixing the IV to the ciphertext.