C# Encryption to PHP Decryption

Question

I'm trying to encrypt some (cookie) data in C# and then decrypt it in PHP. I have chosen to use Rijndael encryption. I've almost got it working, except only part of the text is decrypted! I started working from this example: Decrypt PHP encrypted string in C#

Here's the text (JSON) that I am encrypting (sensitive information removed):

{"DisplayName":"xxx", "Username": "yyy", "EmailAddress":"zzz"}

So I login to the C# app which creates/encodes the cookie from stored Key and IV and then redirects to the PHP app which is supposed to decrypt/read the cookie. When I decrypt the cookie, it comes out like this:

{"DisplayName":"xxx","F�A ;��HP=D�������4��z����ť���k�#E���R�j�5�\�t. t�D��" 

UPDATE: i've gotten a little bit further and this is now the result

string(96) "{"DisplayName":"xxx","Username":"yyy","EmailAddress"�)ق��-�J��k/VV-v� �9�B`7^" 

As you can see, it starts decrypting it, but then gets messed up...

When Decrypt the string it comes out correct (with padding, which I have a function to remove padding), but if I change the test string by one character I get garbage again:

B�nHL�Ek    �¿?�UΣlO����OЏ�M��NO/�f.M���Lƾ�CC�Y>F��~�qd�+

Here's the c# code I use to generate the random Key and IV:

UPDATE: I'm just using static key/IV for now, here they are:

Key: lkirwf897+22#bbtrm8814z5qq=498j5
IV: 741952hheeyy66#cs!9hjv887mxx7@8y

RijndaelManaged symmetricKey = new RijndaelManaged();
symmetricKey.BlockSize = 256;
symmetricKey.KeySize = 256;
symmetricKey.Padding = PaddingMode.Zeros;
symmetricKey.Mode = CipherMode.CBC;
string key = Convert.ToBase64String(symmetricKey.Key);
string IV = Convert.ToBase64String(symmetricKey.IV);

I then save the key and IV to a database to be retrieved later for encoding/decoding.

This is the full encryption class:

public static class Encryption
    {
        public static string Encrypt(string prm_text_to_encrypt, string prm_key, string prm_iv)
        {
            var sToEncrypt = prm_text_to_encrypt;

            var rj = new RijndaelManaged()
            {
                Padding = PaddingMode.PKCS7,
                Mode = CipherMode.CBC,
                KeySize = 256,
                BlockSize = 256,
                //FeedbackSize = 256
            };

            var key = Encoding.ASCII.GetBytes(prm_key);
            var IV = Encoding.ASCII.GetBytes(prm_iv);
            //var key = Convert.FromBase64String(prm_key);
            //var IV = Convert.FromBase64String(prm_iv);

            var encryptor = rj.CreateEncryptor(key, IV);

            var msEncrypt = new MemoryStream();
            var csEncrypt = new CryptoStream(msEncrypt, encryptor, CryptoStreamMode.Write);

            var toEncrypt = Encoding.ASCII.GetBytes(sToEncrypt);

            csEncrypt.Write(toEncrypt, 0, toEncrypt.Length);
            csEncrypt.FlushFinalBlock();

            var encrypted = msEncrypt.ToArray();

            return (Convert.ToBase64String(encrypted));
        }

        public static string Decrypt(string prm_text_to_decrypt, string prm_key, string prm_iv)
        {

            var sEncryptedString = prm_text_to_decrypt;

            var rj = new RijndaelManaged()
            {
                Padding = PaddingMode.PKCS7,
                Mode = CipherMode.CBC,
                KeySize = 256,
                BlockSize = 256,
                //FeedbackSize = 256
            };

            var key = Encoding.ASCII.GetBytes(prm_key);
            var IV = Encoding.ASCII.GetBytes(prm_iv);
            //var key = Convert.FromBase64String(prm_key);
            //var IV = Convert.FromBase64String(prm_iv);

            var decryptor = rj.CreateDecryptor(key, IV);

            var sEncrypted = Convert.FromBase64String(sEncryptedString);

            var fromEncrypt = new byte[sEncrypted.Length];

            var msDecrypt = new MemoryStream(sEncrypted);
            var csDecrypt = new CryptoStream(msDecrypt, decryptor, CryptoStreamMode.Read);

            csDecrypt.Read(fromEncrypt, 0, fromEncrypt.Length);

            return (Encoding.ASCII.GetString(fromEncrypt));
        }

        public static void GenerateKeyIV(out string key, out string IV)
        {
            var rj = new RijndaelManaged()
            {
                Padding = PaddingMode.PKCS7,
                Mode = CipherMode.CBC,
                KeySize = 256,
                BlockSize = 256,
                //FeedbackSize = 256
            };
            rj.GenerateKey();
            rj.GenerateIV();

            key = Convert.ToBase64String(rj.Key);
            IV = Convert.ToBase64String(rj.IV);
        }
    }

Here's the PHP code I am using to decrypt the data:

    function decryptRJ256($key,$iv,$string_to_decrypt)
{
    $string_to_decrypt = base64_decode($string_to_decrypt);
    $rtn = mcrypt_decrypt(MCRYPT_RIJNDAEL_256, $key, $string_to_decrypt, MCRYPT_MODE_CBC, $iv);
    //$rtn = rtrim($rtn, "\0\4");
    $rtn = unpad($rtn);
    return($rtn);
}

function unpad($value)
{
    $blockSize = mcrypt_get_block_size(MCRYPT_RIJNDAEL_256, MCRYPT_MODE_CBC);
    //apply pkcs7 padding removal
    $packing = ord($value[strlen($value) - 1]);
    if($packing && $packing < $blockSize){
        for($P = strlen($value) - 1; $P >= strlen($value) - $packing; $P--){
            if(ord($value{$P}) != $packing){
                $packing = 0;
            }//end if
        }//end for
    }//end if 

    return substr($value, 0, strlen($value) - $packing); 
}

$ky = 'lkirwf897+22#bbtrm8814z5qq=498j5'; // 32 * 8 = 256 bit key
$iv = '741952hheeyy66#cs!9hjv887mxx7@8y'; // 32 * 8 = 256 bit iv

$enc = $_COOKIE["MyCookie"];

$dtext = decryptRJ256($ky, $iv, $enc);
var_dump($dtext);

I am a little unsure about this part, because all of the example code I've seen simply passes in the base64 encoded string directly to the decryptor, but in my example, I have to base64_decode it before I pass it otherwise I get the error that the key and IV are not the correct length.

UPDATE: I'm using ASCII keys in the format needed by PHP. If I generate keys from the RijndaelManaged class they dont work on the PHP side, but I can use keys that are known to work on PHP side and use them in the RijndaelManaged C# side.

Please let me know if I left out any pertinent information. TIA!

Answer 1

For posterity I'm placing the fully completed solution here.

C# Encryption Class

public static class Encryption
{
    public static string Encrypt(string prm_text_to_encrypt, string prm_key, string prm_iv)
    {
        var sToEncrypt = prm_text_to_encrypt;

        var rj = new RijndaelManaged()
        {
            Padding = PaddingMode.PKCS7,
            Mode = CipherMode.CBC,
            KeySize = 256,
            BlockSize = 256,
        };

        var key = Convert.FromBase64String(prm_key);
        var IV = Convert.FromBase64String(prm_iv);

        var encryptor = rj.CreateEncryptor(key, IV);

        var msEncrypt = new MemoryStream();
        var csEncrypt = new CryptoStream(msEncrypt, encryptor, CryptoStreamMode.Write);

        var toEncrypt = Encoding.ASCII.GetBytes(sToEncrypt);

        csEncrypt.Write(toEncrypt, 0, toEncrypt.Length);
        csEncrypt.FlushFinalBlock();

        var encrypted = msEncrypt.ToArray();

        return (Convert.ToBase64String(encrypted));
        }

    public static string Decrypt(string prm_text_to_decrypt, string prm_key, string prm_iv)
    {

        var sEncryptedString = prm_text_to_decrypt;

        var rj = new RijndaelManaged()
        {
            Padding = PaddingMode.PKCS7,
            Mode = CipherMode.CBC,
            KeySize = 256,
            BlockSize = 256,
        };

        var key = Convert.FromBase64String(prm_key);
        var IV = Convert.FromBase64String(prm_iv);

        var decryptor = rj.CreateDecryptor(key, IV);

        var sEncrypted = Convert.FromBase64String(sEncryptedString);

        var fromEncrypt = new byte[sEncrypted.Length];

        var msDecrypt = new MemoryStream(sEncrypted);
        var csDecrypt = new CryptoStream(msDecrypt, decryptor, CryptoStreamMode.Read);

        csDecrypt.Read(fromEncrypt, 0, fromEncrypt.Length);

        return (Encoding.ASCII.GetString(fromEncrypt));
        }

    public static void GenerateKeyIV(out string key, out string IV)
    {
        var rj = new RijndaelManaged()
        {
            Padding = PaddingMode.PKCS7,
            Mode = CipherMode.CBC,
            KeySize = 256,
            BlockSize = 256,
        };
        rj.GenerateKey();
        rj.GenerateIV();

        key = Convert.ToBase64String(rj.Key);
        IV = Convert.ToBase64String(rj.IV);
    }
}

PHP Decryption Snippet

<?php
function decryptRJ256($key,$iv,$encrypted)
{
    //PHP strips "+" and replaces with " ", but we need "+" so add it back in...
    $encrypted = str_replace(' ', '+', $encrypted);

    //get all the bits
    $key = base64_decode($key);
    $iv = base64_decode($iv);
    $encrypted = base64_decode($encrypted);

    $rtn = mcrypt_decrypt(MCRYPT_RIJNDAEL_256, $key, $encrypted, MCRYPT_MODE_CBC, $iv);
    $rtn = unpad($rtn);
    return($rtn);
}

//removes PKCS7 padding
function unpad($value)
{
    $blockSize = mcrypt_get_block_size(MCRYPT_RIJNDAEL_256, MCRYPT_MODE_CBC);
    $packing = ord($value[strlen($value) - 1]);
    if($packing && $packing < $blockSize)
    {
        for($P = strlen($value) - 1; $P >= strlen($value) - $packing; $P--)
        {
            if(ord($value{$P}) != $packing)
            {
                $packing = 0;
            }
        }
    }

    return substr($value, 0, strlen($value) - $packing); 
}
?>
<pre>
<?php

$enc = $_COOKIE["MyCookie"];

$ky = ""; //INSERT THE KEY GENERATED BY THE C# CLASS HERE
$iv = ""; //INSERT THE IV GENERATED BY THE C# CLASS HERE

$json_user = json_decode(decryptRJ256($ky, $iv, $enc), true);

var_dump($json_user);

?>

Answer 2

Since the string is partially OK, but there is gibberish at the end it would suggest a padding problem within the encryption which expects exact blocks of 256 bytes. I suggest setting the padding as PKCS7 (PaddingMode.PKCS7) instead of Zeros on the C# side which PHP will understand without issues (as it's the default mode on that parser).

Edit: Oops, I did not notice that you had the following in your PHP:

$enc = $_COOKIE["MyCookie"];

This is the caveat. PHP is likely not getting the encrypted data as-is and is running some urldecode sanitizing. You should print this variable to see that it really matches what is being sent from the C# code.

Edit2:

Convert the whitespaces to missing + characters from the cookie by adding this:

str_replace(' ', '+', $enc);