<?php
declare(strict_types=1);

/**
 * SRP6 salt/verifier generation compatible with AzerothCore's authserver
 * (same constants and algorithm as TrinityCore/MaNGOS, mandated by the WoW
 * client itself, so this is stable across expansions/versions).
 *
 * Algorithm (matches AzerothCore's SRP6::MakeRegistrationData /
 * ac-nodejs-srp6's computeVerifier):
 *   s = random 32 bytes                              (salt)
 *   h1 = SHA1(UPPER(username) + ":" + UPPER(password))
 *   x  = SHA1(s . h1)  interpreted as a LITTLE-endian integer
 *   v  = g^x mod N                                    (verifier)
 * Both s and v are stored little-endian, zero-padded to 32 bytes, in the
 * `account`.`salt` / `account`.`verifier` BINARY(32) columns.
 */
final class Srp6
{
    // Fixed 256-bit prime and generator used by the WoW client's SRP6 implementation.
    private const N_HEX = '894B645E89E1535BBDAD5B8B290650530801B18EBFBF5E8FAB3C82872A3E9BB7';
    private const G = 7;
    private const KEY_LENGTH = 32;

    /**
     * @return array{salt: string, verifier: string} raw 32-byte binary strings
     */
    public static function makeRegistrationData(string $username, string $password): array
    {
        if (!extension_loaded('gmp')) {
            throw new RuntimeException('Chybí PHP rozšíření GMP (php-gmp), nutné pro výpočet SRP6 verifieru.');
        }

        $username = strtoupper($username);
        $password = strtoupper($password);

        $salt = random_bytes(self::KEY_LENGTH);

        $h1 = sha1($username . ':' . $password, true);
        $xHash = sha1($salt . $h1, true);

        $x = gmp_import($xHash, 1, GMP_LSW_FIRST | GMP_LITTLE_ENDIAN);
        $n = gmp_init(self::N_HEX, 16);
        $g = gmp_init(self::G, 10);

        $v = gmp_powm($g, $x, $n);

        $verifier = self::gmpToLittleEndianBytes($v, self::KEY_LENGTH);

        return [
            'salt' => $salt,
            'verifier' => $verifier,
        ];
    }

    private static function gmpToLittleEndianBytes(\GMP $value, int $length): string
    {
        $bytes = gmp_export($value, 1, GMP_LSW_FIRST | GMP_LITTLE_ENDIAN);
        if (strlen($bytes) > $length) {
            throw new RuntimeException('SRP6 verifier přesáhl očekávanou délku.');
        }

        return str_pad($bytes, $length, "\0", STR_PAD_RIGHT);
    }
}