Question

I am working on some PHP code to perform AES string encryption and decryption. The encryption is working fine but I can’t seem to decrypt it.

Below is the code that does the encryption and adds the required padding.

function encrypt($data) 
    { 
        $iv = "PRIVATE";
        $key = CIPHERKEY;

        return base64_encode(mcrypt_encrypt(MCRYPT_RIJNDAEL_128, $key, addpadding($data), MCRYPT_MODE_CBC, $iv));
    }

    function addpadding($string, $blocksize = 16)
    {
        $len = strlen($string);
        $pad = $blocksize - ($len % $blocksize);
        $string .= str_repeat(chr($pad), $pad);

        return $string;
    }

The code above is working fine the code below where it does the decryption keeps on failing. I try and do the decryption and then strip the padding but false is always returned from the padding function.

Below is the code that does the decryption and the stripping.

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($data)
    {
        $iv = "PRIVATE";
        $key = CIPHERKEY;

        //$decoded = mcrypt_decrypt(MCRYPT_RIJNDAEL_128, $key, $data, MCRYPT_MODE_CBC, $iv);
        $decrytped = mcrypt_decrypt(MCRYPT_RIJNDAEL_128, $key, $data, MCRYPT_MODE_CBC, $iv);
        $base64Decoded = base64_decode($decrytped);
        return strippadding($base64Decoded);
    }

Thanks for any help you can provide.

Answer 1

In your decryption method, the decrypt and base64 steps are backwards. It’s important that in whatever order you do your operations for encrypting, you do the reverse to decrypt.

In your encrypt method, you’re base64 encoding the ciphertext:

base64_encode(mcrypt_encrypt(MCRYPT_RIJNDAEL_128, $key, addpadding($data), MCRYPT_MODE_CBC, $iv));

When you decrypt, you need to undo the base64 encoding, and then decrypt that result.

$base64Decoded = base64_decode($data);
$decrytped = mcrypt_decrypt(MCRYPT_RIJNDAEL_128, $key, $base64Decoded, MCRYPT_MODE_CBC, $iv);

Also, it looks like mcrypt will handle padding uneven blocks, so your addpadding() method might be superfluous.