Decrypt Access token using PHP for C# Encryption

Question

I have an API URL with specific access token which was encrypted with C#(Below Code) and I want to Decrypt it using PHP post request by passing access token to parameters. Can anyone help me out to solve this problem.

Thanks in Advance!!

C# Code for Encryption:

private String AES_encrypt(String Input)
   {
   var aes = new RijndaelManaged();
   aes.KeySize = 256;
   aes.BlockSize = 256;
   aes.Padding = PaddingMode.PKCS7;
   aes.Key =Convert.FromBase64String("QdZx1B0ZIcLK7DPNRK09wc/rjP4WnxtE");
   aes.IV = Convert.FromBase64String("hBSE4tn6e/5c3YVKFZ54Iisi4MiDyCO0HJO+WZBeXoY=");
   var encrypt = aes.CreateEncryptor(aes.Key, aes.IV);
   byte[] xBuff = null;
   using (var ms = new MemoryStream())
   {
   using (var cs = new CryptoStream(ms, encrypt, CryptoStreamMode.Write))
   {
     byte[] xXml = Encoding.UTF8.GetBytes(Input);
     cs.Write(xXml, 0, xXml.Length);
   }
     xBuff = ms.ToArray();
   }
     String Output = Convert.ToBase64String(xBuff);
     return Output;
   }

So far I tried to decrypt it with the below code

function strippadding($string)
    {
        $slast = ord(substr($string, -1));
        $slastc = chr($slast);
        $pcheck = substr($string, -$slast);
        if(preg_match("/$slastc{".$slast."}/", $string)){
        $string = substr($string, 0, strlen($string)-$slast);
            return $string;
        } else {
            return false;
        }
   }
function decrypt($string)
    {
        $key = base64_decode("DZR");
        $iv = base64_decode("Shravan");
        $string = base64_decode($string);
        return strippadding(mcrypt_decrypt(MCRYPT_RIJNDAEL_256, $key, $string, MCRYPT_MODE_CBC, $iv));
    }

Fill out the items below:

Use this key and iv that are below.

key = QdZx1B0ZIcLK7DPNRK09wc/rjP4WnxtE

iv= hBSE4tn6e/5c3YVKFZ54Iisi4MiDyCO0HJO+WZBeXoY=

Run some text through your AES_encrypt() function and whatever comes out paste on the next line.

encrypted text = put your encrypted text here.

Answer 1

$xXml = openssl_decrypt(
   $Output, #openssl_decrypt works with base64 encoded data
   'AES-256-CBC',
   base64_decode("QdZx1B0ZIcLK7DPNRK09wc/rjP4WnxtE"), #key
   OPENSSL_RAW_DATA,
   base64_decode("hBSE4tn6e/5c3YVKFZ54Iisi4MiDyCO0HJO+WZBeXoY=") #IV
);

Now $xXml is the binary form of the input string in UTF-8 encoded.

And make sure openssl is included in your PHP build.

Answer 2

You have not provided me with any encrypted text to be able to test this with.

Here is what I think you need to do:

In your C# code you need to change the block size to 128 bits:

aes.BlockSize = 128;

In your C# code your IV needs to be 128 bits or 16 bytes long. It needs to equal your selected block size.

So for now this needs to be your IV:

IV = HWeR102dxMjRHZlxTqL2aA==

Your key is set for 256 bits: So here is a 256 bit key:

Key = aZUEBKSsYRKA6CGQbwFwvIS8rUnW7YA2hVMNHnnf844=

C# has functions that will automatically generate a cryptographically strong string for you of a certain length. I suggest you find these functions and learn how to use them so you can generate your own keys and IVs.

Now for the PHP portion.

You should use the OpenSSL library instead of the Mcrypt library. Mcrypt is deprecated and is no longer supported. So here is an OpenSSL solution.

Since the block size is now 128 bits and the key size is 256 bits it will now be compatible with the openssl library's AES-256-CBC function.

$key = 'aZUEBKSsYRKA6CGQbwFwvIS8rUnW7YA2hVMNHnnf844='; //256 bit key.  
$iv = 'HWeR102dxMjRHZlxTqL2aA=='; //128 bit IV length.  The same as the block size that is set in the C#.

function decrypt($string, $key, $iv){

  $cipherText = base64_decode($string);  //We are going to use raw data.
  return openssl_decrypt($cipherText, 'AES-256-CBC', base64_decode($key), OPENSSL_RAW_DATA, base64_decode($iv));
  //Note that I did not specify no padding in the function.  By default it is PKCS#7 which is what is set in the C# code.
}

The best I can tell this should work for you. This assumption is predicated on the fact that your AES_encrypt() is working correctly and that you have OpenSSL on your machine. Which you probably do.

Hope it helps!