PHP 7.4 direct replacement for mcrypt decryption

Question

I have a legacy database with content that was encrypted with mcrypt using DES (yes, I know, it was a long time ago) The encryption method is like this:

/**
 * General encryption routine for generating a reversible ciphertext
 * @param String $string the plain text to encrypt
 * @param String $key the encryption key to use
 * @return String the cypher text result
 */
function encrypt($string, $key)
{
    srand((double) microtime() * 1000000);
    /* Open module, and create IV */
    $td = mcrypt_module_open('des', '', 'cfb', '');
    $ksub = substr(md5($key), 0, mcrypt_enc_get_key_size($td));
    $iv_size = mcrypt_enc_get_iv_size($td);
    $iv = mcrypt_create_iv($iv_size, MCRYPT_RAND);
    /* Initialize encryption handle */
    if (mcrypt_generic_init($td, $ksub, $iv) != -1)
    {
        /* Encrypt data */
        $ctxt = mcrypt_generic($td, $string);
        mcrypt_generic_deinit($td);
        mcrypt_module_close($td);
        $ctxt = $iv . $ctxt;
        return base64_encode($ctxt);
    } //end if
}

and the decryption method is like this:

/**
 * General decryption routine for recovering a plaintext
 * @param String $string the cypher text to decrypt
 * @param String $key the encryption key to use
 * @return String the plain text result
 */
function decrypt($string, $key)
{
    $ptxt = base64_decode($string);
    /* Open module, and create IV */
    $td = mcrypt_module_open('des', '', 'cfb', '');
    $ksub = substr(md5($key), 0, mcrypt_enc_get_key_size($td));
    $iv_size = mcrypt_enc_get_iv_size($td);
    $iv = substr($ptxt, 0, $iv_size);
    $ptxtsub = substr($ptxt, $iv_size);
    /* Initialize encryption handle */
    if (mcrypt_generic_init($td, $ksub, $iv) != -1)
    {
        /* Encrypt data */
        $ctxt = mdecrypt_generic($td, $ptxtsub);
        mcrypt_generic_deinit($td);
        mcrypt_module_close($td);
        return $ctxt;
    } //end if
}

I need to extract this data in a PHP7.4 environment, even if only to re-encrypt it with something better, but I'm not sure how to reproduce the mcrypt operations with stuff that exists in PHP7.4 like sodium. I suppose one method would be to spin up some sort of legacy PHP installation that still has mcrypt and do it offline, but is there a more direct way of coding a decryption method?

Answer 1

While mcrypt is not part of PHP anymore (for good reasons), it still exists as module you can install for PHP 7.4

https://pecl.php.net/package/mcrypt

Install it, make sure to re-encrypt all data, once all old data is updated, change your code to not use it anymore and remove the extension.