AES Decryption Exception change

Question

I have an AES Cryptography wrapper and unit tests that has been working for over a year. Now after installing VS 2012 (or maybe an update to .net Framework 4) the unit tests do not pass. The streamreader block was throwing a CryptographicException when I passed in a bad pass but is not throwing a ArgumentNullException.

The code is up at. https://github.com/jnaus/Cryptography

Here is the unit test that now does not work. (BadSaltTest has the same problem)

[TestMethod]
[ExpectedException(typeof(CryptographicException),
  "Bad password was inappropriately allowed")]
public void BadPasswordTest()
{
    var cipherText = EncryptString();
    var decryptedText = AESCryptography.DecryptStringAES
        (cipherText,"A bad password", salt);
}

Test Result: Test method CryptographyTest.AESTest.BadPasswordTest threw exception System.ArgumentNullException, but exception System.Security.Cryptography.CryptographicException was expected. Exception message: System.ArgumentNullException: Value cannot be null. Parameter name: inputBuffer

Decrypt code.

public static string DecryptStringAES(string cipherText, 
    string password, byte[] salt)
{   
    RijndaelManaged aesAlg = null;
    string plaintext = null;

    try 
    {
        // generate the key from the shared secret and the salt
        Rfc2898DeriveBytes key = new Rfc2898DeriveBytes(password, salt);

        // Create a RijndaelManaged object
        // with the specified key and IV.
        aesAlg = new RijndaelManaged();
        aesAlg.Key = key.GetBytes(aesAlg.KeySize/8);
        aesAlg.IV = key.GetBytes(aesAlg.BlockSize/8);

        // Create a decrytor to perform the stream transform.
        ICryptoTransform decryptor = aesAlg.CreateDecryptor(aesAlg.Key, 
            aesAlg.IV);
        // Create the streams used for decryption.                
        byte[] bytes = Convert.FromBase64String(cipherText);

        using (MemoryStream msDecrypt = new MemoryStream(bytes)) 
        {
            using (CryptoStream csDecrypt = 
                new CryptoStream(msDecrypt, decryptor, CryptoStreamMode.Read)) 
            {
                //StreamReader now gives ArgumentNullException
                using (StreamReader srDecrypt = new StreamReader(csDecrypt)) 
                {
                    // Read the decrypted bytes from the decrypting stream
                    // and place them in a string.
                    plaintext = srDecrypt.ReadToEnd();
                }
            }
        }
    }
    finally 
    {
        // Clear the RijndaelManaged object.
        if (aesAlg != null)
        {
            aesAlg.Clear();
        }
    }
    return plaintext;
}

Answer 1

After searching around a bit more, this indeed does appear to be a bug as confimred by Microsoft (see Microsoft Connect). Personally not a big fan of the suggested workaround as it really shouldn't be needed but it will do in the mean time I suppose.

Answer 2

The code you posted above worked for me (I had to write some code to create an encrypted string to pass). Compiled and run in VS2012 using .Net Framework 4.

The encryption code I used was:

    private static string EncryptStringAES(string plainText, string password, byte[] salt)
    {
        RijndaelManaged aesAlg = null;
        string cypherText = null;

        try
        {
            // generate the key from the shared secret and the salt
            Rfc2898DeriveBytes key = new Rfc2898DeriveBytes(password, salt);

            // Create a RijndaelManaged object
            // with the specified key and IV.
            aesAlg = new RijndaelManaged();
            aesAlg.Key = key.GetBytes(aesAlg.KeySize / 8);
            aesAlg.IV = key.GetBytes(aesAlg.BlockSize / 8);

            // Create an encryptor to perform the stream transform.
            ICryptoTransform encryptor = aesAlg.CreateEncryptor(aesAlg.Key, aesAlg.IV);
            // Create the streams used for encryption.                
            byte[] bytes = new UTF8Encoding().GetBytes(plainText);

            using (MemoryStream msEncrypt = new MemoryStream())
            {
                using (CryptoStream csEncrypt = new CryptoStream(msEncrypt, encryptor, CryptoStreamMode.Write))
                {
                    csEncrypt.Write(bytes, 0, bytes.Length);
                    csEncrypt.FlushFinalBlock();
                    cypherText = Convert.ToBase64String(msEncrypt.ToArray());
                }
            }
        }
        finally
        {
            // Clear the RijndaelManaged object.
            if (aesAlg != null)
            {
                aesAlg.Clear();
            }
        }
        return cypherText;
    }

Used the following to make the method calls:

        byte[] salt =  new byte[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 };
        string test = DecryptStringAES(EncryptStringAES("This is a test", "Test", salt), "Test", salt);

Resulting string (Test) contained "This is a test".