PHP equivalent of .net AES encryption

Question

I am working on a data exchange integration with my client and the data they send me is encrypted using their C# encrypt method (below).

My app is running PHP 5.3 and I need an equivalent code to decrypt the data they send. I have the PHP code but it'd not decrypt the client data correctly for me.

Clearly I am making some mistake in my encryption/decryption methods, IV key or something. Can anyone spot the mistake?

Thanks.

C# Code (From my client):

using System;
using System.Security.Cryptography;
using System.Text;
using System.IO;

public class Program
{
    public static void Main()
    {
        var text = "this is a plain string";
        var enc = Program.Encrypt(text);
        Console.WriteLine(enc);
        Console.WriteLine(Program.Decrypt(enc));
    }

    public static string Encrypt(string clearText)
    {
        var EncryptionKey = "1234567890123456";
        byte[] clearBytes = Encoding.Unicode.GetBytes(clearText);
        using (Aes encryptor = Aes.Create())
        {
            byte[] IV = new byte[15];
            var rand = new Random();
            rand.NextBytes(IV);
            Rfc2898DeriveBytes pdb = new Rfc2898DeriveBytes(EncryptionKey, IV);
            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(IV) + Convert.ToBase64String(ms.ToArray());
            }
        }

        return clearText;
    }

    public static string Decrypt(string cipherText)
    {
        var EncryptionKey = "1234567890123456";
        byte[] IV = Convert.FromBase64String(cipherText.Substring(0, 20));
        cipherText = cipherText.Substring(20).Replace(" ", "+");
        byte[] cipherBytes = Convert.FromBase64String(cipherText);
        using (Aes encryptor = Aes.Create())
        {
            Rfc2898DeriveBytes pdb = new Rfc2898DeriveBytes(EncryptionKey, IV);
            encryptor.Key = pdb.GetBytes(32);
            encryptor.IV = pdb.GetBytes(16);
            using (MemoryStream ms = new MemoryStream())
            {
                using (CryptoStream cs = new CryptoStream(ms, encryptor.CreateDecryptor(), CryptoStreamMode.Write))
                {
                    cs.Write(cipherBytes, 0, cipherBytes.Length);
                    cs.Close();
                }

                cipherText = Encoding.Unicode.GetString(ms.ToArray());
            }
        }

        return cipherText;
    }
}

PHP Code I have:

public function encrypt($plainText)
{
    $secretKey = '1234567890123456';

    return rtrim(
        base64_encode(
            mcrypt_encrypt(
                MCRYPT_RIJNDAEL_256,
                $secretKey, $plainText,
                MCRYPT_MODE_ECB,
                mcrypt_create_iv(
                    mcrypt_get_iv_size(
                        MCRYPT_RIJNDAEL_256,
                        MCRYPT_MODE_ECB
                    ),
                    MCRYPT_RAND)
            )
        ), "\0"
    );
}

public function decrypt($encodedData)
{
    $secretKey = '1234567890123456';

    return rtrim(
        mcrypt_decrypt(
            MCRYPT_RIJNDAEL_256,
            $secretKey,
            base64_decode($encodedData),
            MCRYPT_MODE_ECB,
            mcrypt_create_iv(
                mcrypt_get_iv_size(
                    MCRYPT_RIJNDAEL_256,
                    MCRYPT_MODE_ECB
                ),
                MCRYPT_RAND
            )
        ), "\0"
    );
}

Answer 1

Can anyone spot the mistake?

Yes, and the big one isn't really your fault: mcrypt's confusing API strikes again.

That said, there are actually multiple mistakes here.

return rtrim( // unnecessary
    base64_encode(
        mcrypt_encrypt(
            MCRYPT_RIJNDAEL_256, // Not AES
            $secretKey, $plainText,
            MCRYPT_MODE_ECB, // BAD, use MCRYPT_MODE_CBC or 'ctr' instead
            mcrypt_create_iv(
                mcrypt_get_iv_size(      // unless you're going make this
                    MCRYPT_RIJNDAEL_256, // configurable, you should just
                    MCRYPT_MODE_ECB      // hard-code this as an integer
                ),
                MCRYPT_RAND) // BAD, use MCRYPT_DEV_URANDOM
        )
    ), "\0"
); 

If you're going to generate an IV, it should be communicated so your recipient can decrypt the same first block successfully. The C# code does this, the PHP does not.

From a cryptography engineering perspective, you should consider, both in C# land and in PHP, deploying an Encrypt then Authenticate protocol. See this blog post on encryption and authentication. Also, all the crypto code you've ever written is probably broken.

Answer 2

It seems like the PHP Script is Using the wrong Mode:
https://msdn.microsoft.com/en-us/library/system.security.cryptography.symmetricalgorithm.mode%28v=vs.110%29.aspx
The C# Functions do not set any Mode so the default is CBC.
The PHP part uses ECB instead, which is not only wrong, but insecure.