AES128 Encryption using PHP mcrypt is not outputting same as ASP.net

Question

I have a PHP application that needs to encrypt a challenge string that is consumed by an ASP.net web service however the output of my PHP implementation is not properly decrypted by .net. Exactly as in this question relating to iOS and .net: AES128 bit encryption string is not similar as on .net

Why are the outputs different and what can I do to my PHP to output the same as .net?

My PHP code looks like this:

$key = '128bit-16bytekey';
$value = '128bit-16byteval';
function fnEncrypt($value, $key)
{
    $ivsize = mcrypt_get_iv_size(MCRYPT_RIJNDAEL_128, MCRYPT_MODE_ECB);
    $iv     = mcrypt_create_iv($ivsize);
    return base64_encode(
            mcrypt_encrypt(
                MCRYPT_RIJNDAEL_128,
                $key, $value, 
                MCRYPT_MODE_ECB,$iv
            ));
}

.net like this

public static string EncryptData(string plainText)
    {
        string encryptionKey = AppConstants.AES_ENCRYPTDECRYPT_KEY;
        // Convert our plaintext into a byte array.
        // Let us assume that plaintext contains UTF8-encoded characters.
        byte[] plainTextBytes = Encoding.UTF8.GetBytes(plainText);

        // Use the password to generate pseudo-random bytes for the encryption
        // key. Specify the size of the key in bytes (instead of bits).
        ASCIIEncoding encoding = new ASCIIEncoding();
        byte[] keyBytes = new byte[16];

        byte[] tempKey = encoding.GetBytes(encryptionKey);

        for (int i = 0; i < keyBytes.Length; i++)
        {
            if (i < tempKey.Length)
            {
                keyBytes[i] = tempKey[i];
            }
            else
            {
                keyBytes[i] = 0;
            }
        }

        // Create uninitialized Rijndael encryption object.
        RijndaelManaged symmetricKey = new RijndaelManaged();

        //AesManaged symmetricKey = new AesManaged();


        //symmetricKey.Padding = PaddingMode.PKCS7;
        // It is reasonable to set encryption mode to Cipher Block Chaining
        // (CBC). Use default options for other symmetric key parameters.
        symmetricKey.Mode = CipherMode.ECB;

        // Generate encryptor from the existing key bytes and initialization 
        // vector. Key size will be defined based on the number of the key 
        // bytes.

        //ICryptoTransform encryptor = symmetricKey.CreateEncryptor(keyBytes,initVectorBytes);
        ICryptoTransform encryptor = symmetricKey.CreateEncryptor(keyBytes, null);

        // Define memory stream which will be used to hold encrypted data.
        MemoryStream memoryStream = new MemoryStream();

        // Define cryptographic stream (always use Write mode for encryption).
        CryptoStream cryptoStream = new CryptoStream(memoryStream,
                                                     encryptor,
                                                     CryptoStreamMode.Write);
        // Start encrypting.
        cryptoStream.Write(plainTextBytes, 0, plainTextBytes.Length);

        // Finish encrypting.
        cryptoStream.FlushFinalBlock();

        // Convert our encrypted data from a memory stream into a byte array.
        byte[] cipherTextBytes = memoryStream.ToArray();

        // Close both streams.
        memoryStream.Close();
        cryptoStream.Close();

        // Convert encrypted data into a base64-encoded string.
        string cipherText = Convert.ToBase64String(cipherTextBytes);

        // Return encrypted string.
        return cipherText;
    }

Sample outputs

PHP : Q54nP/tXq2rDTUwWw4ckkg==

.net : Q54nP/tXq2rDTUwWw4ckkpSt9CQiIzsg2xsQEndcqc8=

PHP : DQZdAB/lABXVOOoCdNM6HQ==

.net : DQZdAB/lABXVOOoCdNM6HZSt9CQiIzsg2xsQEndcqc8=

As in the question ref'd above the right hand side of the .net output are always the same and left side are consistent between the two implementations. I am pretty sure the IV is irrelevant and that it is something to do with how padding is handled in mcrypt.

If I decrypt either of the outputs in PHP it returns the same correct result.

Can anyone shed any light? I am unable to change the .net app. Thanks!

Answer 1

This happens due to the padding that is applied in .net. You can overcome this by adding the following line to your .net code:

symmetricKey.Padding = PaddingMode.None;

Or you can change the PHP code to get the same encrypted string:

// Add the lines below to your fnEncrypt function
$block = mcrypt_get_block_size(MCRYPT_RIJNDAEL_128, MCRYPT_MODE_ECB);
$len = strlen($value);
$padding = $block - ($len % $block);
$value .= str_repeat(chr($padding),$padding);

A similar issue is described in one of the comments from PHP manual pages: http://www.php.net//manual/en/function.mcrypt-encrypt.php#47973