AES 256 bit key encryption with PBKDF2 in Win RT and WP8 - windows-phone-8

Question

I am trying to port a WP8 application to Win 8.1 store App. The app uses password based AES 256 for encryption. The confusion I am having is that I am able to decrypt the text from WP8, encrypted using 256 bit key, but only using a 32 bit key in Win RT.
Below is the Window Phone 8 code. Here note that Rfc2898DeriveBytes wants it in bytes, so the key used is 256 bits.
private static string Decrypt(string dataToDecrypt, string password, string salt)
{
AesManaged aes = null;
MemoryStream memoryStream = null;
try
{
//Generate a Key based on a Password and HMACSHA1 pseudo-random number generator
Rfc2898DeriveBytes rfc2898 = new Rfc2898DeriveBytes(password, Encoding.UTF8.GetBytes(salt), 10000);
//Create AES algorithm
aes = new AesManaged();
aes.KeySize = aes.LegalKeySizes[0].MaxSize; // returns 256
aes.BlockSize = aes.LegalBlockSizes[0].MaxSize; // return 128
aes.Key = rfc2898.GetBytes(aes.KeySize);
aes.IV = rfc2898.GetBytes(aes.BlockSize);
//Create Memory and Crypto Streams
memoryStream = new MemoryStream();
CryptoStream cryptoStream = new CryptoStream(memoryStream, aes.CreateDecryptor(), CryptoStreamMode.Write);
//Decrypt Data
byte[] data = Convert.FromBase64String(dataToDecrypt);
cryptoStream.Write(data, 0, data.Length);
cryptoStream.FlushFinalBlock();
//Return Decrypted String
byte[] decryptBytes = memoryStream.ToArray();
//Dispose
if (cryptoStream != null)
cryptoStream.Dispose();
//Retval
return Encoding.UTF8.GetString(decryptBytes, 0, decryptBytes.Length);
}
catch (Exception ex)
{
MasterData.CryptographyExceptionOccured = true;
Debug.WriteLine(ex.Message);
Debug.WriteLine(ex.StackTrace);
return "";
}
finally
{
if (memoryStream != null)
memoryStream.Dispose();
if (aes != null)
aes.Clear();
}
}
Below is the Win RT code (Win 8.1) Here note that the CryptographicEngine.DeriveKeyMaterial method requires the key size in bits. But I am able to decrypt the 256 bit encryption in WP8 using the below Win RT code, which seems to use 32 bits only. ( The password and salt used are same in WP8 / Win RT)
private static void GenerateKeyMaterial(string password, string salt, uint iterationCount, out IBuffer keyMaterial, out IBuffer iv)
{
// Setup KDF parameters for the desired salt and iteration count
IBuffer saltBuffer = CryptographicBuffer.ConvertStringToBinary(salt, BinaryStringEncoding.Utf8);
KeyDerivationParameters kdfParameters = KeyDerivationParameters.BuildForPbkdf2(saltBuffer, iterationCount);
// Get a KDF provider for PBKDF2, and store the source password in a Cryptographic Key
KeyDerivationAlgorithmProvider kdf = KeyDerivationAlgorithmProvider.OpenAlgorithm(KeyDerivationAlgorithmNames.Pbkdf2Sha256);
// KeyDerivationAlgorithmProvider kdf = KeyDerivationAlgorithmProvider.OpenAlgorithm(KeyDerivationAlgorithmNames.Pbkdf2Sha1); IBuffer passwordBuffer = CryptographicBuffer.ConvertStringToBinary(password, BinaryStringEncoding.Utf8);
CryptographicKey passwordSourceKey = kdf.CreateKey(passwordBuffer);
// Generate key material from the source password, salt, and iteration count. Only call DeriveKeyMaterial once,
// since calling it twice will generate the same data for the key and IV.
int keySize = 256 / 8;
int ivSize = 128 / 8;
uint totalDataNeeded = (uint)(keySize + ivSize);
IBuffer keyAndIv = CryptographicEngine.DeriveKeyMaterial(passwordSourceKey, kdfParameters, totalDataNeeded);
// Split the derived bytes into a seperate key and IV
byte[] keyMaterialBytes = keyAndIv.ToArray();
keyMaterial = WindowsRuntimeBuffer.Create(keyMaterialBytes, 0, keySize, keySize);
iv = WindowsRuntimeBuffer.Create(keyMaterialBytes, keySize, ivSize, ivSize);
}
So to use a 256 bit encryption in Win RT, what should be the key size in the above code? How is it possible to decrypt a 256 bit encrypted string using a 32 bit key. See the above WP8 and Win RT code.