<?php

/**
 * OpenSSL Modular Exponentiation Engine
 *
 * PHP version 5 and 7
 *
 * @author    Jim Wigginton <terrafrost@php.net>
 * @copyright 2017 Jim Wigginton
 * @license   http://www.opensource.org/licenses/mit-license.html  MIT License
 * @link      http://pear.php.net/package/Math_BigInteger
 */

declare(strict_types=1);

namespace phpseclib3\Math\BigInteger\Engines;

use phpseclib3\Crypt\RSA\Formats\Keys\PKCS8;
use phpseclib3\Exception\OutOfRangeException;
use phpseclib3\Exception\UnexpectedValueException;
use phpseclib3\Math\BigInteger;

/**
 * OpenSSL Modular Exponentiation Engine
 *
 * @author  Jim Wigginton <terrafrost@php.net>
 */
abstract class OpenSSL
{
    /**
     * Test for engine validity
     */
    public static function isValidEngine(): bool
    {
        return extension_loaded('openssl') && static::class != __CLASS__;
    }

    /**
     * Performs modular exponentiation.
     */
    public static function powModHelper(Engine $x, Engine $e, Engine $n): Engine
    {
        if ($n->getLengthInBytes() < 31 || $n->getLengthInBytes() > 16384) {
            throw new OutOfRangeException('Only modulo between 31 and 16384 bits are accepted');
        }

        $key = PKCS8::savePublicKey(
            new BigInteger($n),
            new BigInteger($e)
        );

        $plaintext = str_pad($x->toBytes(), $n->getLengthInBytes(), "\0", STR_PAD_LEFT);

        // this is easily prone to failure. if the modulo is a multiple of 2 or 3 or whatever it
        // won't work and you'll get a "failure: error:0906D06C:PEM routines:PEM_read_bio:no start line"
        // error. i suppose, for even numbers, we could do what PHP\Montgomery.php does, but then what
        // about odd numbers divisible by 3, by 5, etc?
        if (!openssl_public_encrypt($plaintext, $result, $key, OPENSSL_NO_PADDING)) {
            throw new UnexpectedValueException(openssl_error_string());
        }

        $class = $x::class;
        return new $class($result, 256);
    }
}