MCrypt rijndael-128 to OpenSSL aes-128-ecb conversion

Question

Since Mcrypt is deprecated, I want to use OpenSSL instead in my code since we already using php 7.0.17 in our server and there's no tell when they upgrade it.

Some third party API (hosted on PHP 5.x probably and using mcrypt), is taking encrypted data. They've provided methods which they are using to encrypt/decrypt strings.

Here are they

$secret = 'a0a7e7997b6d5fcd55f4b5c32611b87c' ;

public function encrypt128($str)
    {
        $block = mcrypt_get_block_size("rijndael_128", "ecb");
        $pad   = $block - (strlen($str) % $block);
        $str .= str_repeat(chr($pad), $pad);

        return base64_encode(mcrypt_encrypt(MCRYPT_RIJNDAEL_128, $secret, $str, MCRYPT_MODE_ECB));
    }

public function decrypt128($str)
    {
        $str = base64_decode($str);
        $str = mcrypt_decrypt(MCRYPT_RIJNDAEL_128, $secret, $str, MCRYPT_MODE_ECB);

        $len = strlen($str);
        $pad = ord($str[$len - 1]);

        return substr($str, 0, strlen($str) - $pad);
    }

using these methods string small1 if encrypted becomes v7IXp5vVaFVXXlt/MN8BVw==

---

We want to use openssl_encrypt in our side such that if we encrypt same string with OpenSSL it must give same results as Mcrypt. I've researched that mcrypt using rijndael-128 Mode ecb should be compatible with OpenSSL aes-128-ecb.

For last few hours, I've been trying to make my own method to encrypt strings serving same result by using OpenSSL. So far I've come to this

public function sslEncrypt128($str)
{
    $secret = 'a0a7e7997b6d5fcd55f4b5c32611b87c';
    return base64_encode(openssl_encrypt($str, 'aes-128-ecb', $secret, OPENSSL_RAW_DATA));
}

But it produces different string SxJ3+EdaeItZx3/EwGTUbw== for same as above input. I don't know if it is flag's problem or padding's, any pointers will be welcome.

I've added the code here to test online https://3v4l.org/v2J2N

Thanks in advance.

Answer 1

In your specific example I've found that by changing aes-128-ecb to aes-256-ecb, it produces the same output as the legacy mcrypt_encrypt.

Answer 2

Here is what worked for me:

<?php

$str = 'Content';
if (strlen($str) % 16) {
    $str = str_pad($str, strlen($str) + 16 - strlen($str) % 16, "\0");
}

$key = 'KEY';
if (strlen($key) % 16) {
    $key = str_pad($key, strlen($key) + 16 - strlen($key) % 16, "\0");
}

$res1 = mcrypt_encrypt(MCRYPT_RIJNDAEL_128, $key, $str, MCRYPT_MODE_ECB);
echo strToHex($res1) . ' | mcrypt_encrypt';

echo "<hr>";
echo strToHex(openssl_decrypt($res1, "aes-128-ecb", $key, OPENSSL_RAW_DATA | OPENSSL_NO_PADDING)) . ' | openssl_decrypt';

echo "<hr>";

$res2 = openssl_encrypt($str, "aes-128-ecb", $key, OPENSSL_RAW_DATA | OPENSSL_NO_PADDING);
echo strToHex($res2) . ' | openssl_encrypt';

echo "<hr>";
echo strToHex(openssl_decrypt($res2, "aes-128-ecb", $key, OPENSSL_RAW_DATA | OPENSSL_NO_PADDING)) . ' | openssl_decrypt';


function strToHex($string) {
    $hex = '';
    for ($i = 0; $i < strlen($string); $i++) {
        $ord     = ord($string[$i]);
        $hexCode = dechex($ord);
        $hex     .= substr('0' . $hexCode, -2);
    }

    return strToUpper($hex);
}

Answer 3

Most likely the key was expected to be used as hex (it already is in hex format) not as a string to be converted to hex.

---

mcrypt:

mcrypt does not support standard PKCS#7 (née PKCS#5) padding, only non-standard null padding but the padding is being explicitly added prior to mcrypt.

The encryption v7IXp5vVaFVXXlt/MN8BVw== is the correct encryption based on PKCS#7 padding. ECB mode and the key as a string.

See: mcrypt - AES CALCULATOR.

In hex, notice the data padding is clearly visible:
key: 6130613765373939376236643566636435356634623563333236313162383763
data: 736D616C6C310A0A0A0A0A0A0A0A0A0A
encrypted: BFB217A79BD56855575E5B7F30DF0157

In Base64:
encrypted: v7IXp5vVaFVXXlt/MN8BVw==

---

OpenSSL:

Notice the key is 256-bits but the OpenSSL call with "aes-128-ecb" seems to imply a 128-but key. So the keys don't match.

See: OpenSSL - AES CALCULATOR

In hex, notice the data padding is clearly visible:
key: 61306137653739393762366435666364
data: 736D616C6C310A0A0A0A0A0A0A0A0A0A
encrypted: 4B1277F8475A788B59C77FC4C064D46F

In Base64:
encrypted: SxJ3+EdaeItZx3/EwGTUbw==