<?php
// SPDX-License-Identifier: GPL-3.0-or-later
declare(strict_types=1);

namespace Aiwend\Secrets;

/**
 * The default encryption key of a site that has not configured one: derived (HKDF-SHA256) from the eight secret keys and salts that
 * WordPress keeps in wp-config.php (AUTH_KEY ... NONCE_SALT). Nothing is written anywhere, so this works on every host, including read-only
 * file systems, containers and any web server, and the key never sits in the database next to the ciphertext.
 *
 * It is offered only when ALL eight values are defined in wp-config.php and look genuine (long enough, not the sample placeholder, not
 * all the same). WordPress itself falls back to values stored in the database when they are missing; such a key would live beside the
 * ciphertext, so it is refused here and the administrator is asked to set AIWEND_SECRET_KEY instead.
 *
 * Limit, stated plainly: like a key in wp-config.php it does not protect against someone who can read the site's files. And if a host or a
 * security plugin changes the salts, the derived key changes: Plugin detects that with a stored check value, refuses to use a different
 * key silently, and tells the administrator to restore the salts or define AIWEND_SECRET_KEY (saved provider keys then have to be entered again).
 */
final class SaltKey
{
    public const NAMES = ['AUTH_KEY', 'SECURE_AUTH_KEY', 'LOGGED_IN_KEY', 'NONCE_KEY', 'AUTH_SALT', 'SECURE_AUTH_SALT', 'LOGGED_IN_SALT', 'NONCE_SALT'];
    public const INFO = 'aiwend/secret-key/v1';

    /**
     * @param (callable(string): ?string)|null $constant returns the value of a wp-config.php constant, or null (tests inject one)
     * @return string|null the raw 32-byte key, or null when the site's salts are missing or placeholders
     */
    public static function derive(?callable $constant = null): ?string
    {
        $constant ??= static fn(string $name): ?string => defined($name) && is_string(constant($name)) ? (string) constant($name) : null;
        $parts = [];
        foreach (self::NAMES as $name) {
            $v = $constant($name);
            if (!is_string($v) || !self::genuine($v)) {
                return null;
            }
            $parts[] = $v;
        }
        if (count(array_unique($parts)) !== count($parts)) {
            return null; // the same value used twice: not what WordPress generates
        }
        return hash_hkdf('sha256', implode("\0", $parts), SODIUM_CRYPTO_AEAD_XCHACHA20POLY1305_IETF_KEYBYTES, self::INFO);
    }

    public static function genuine(string $v): bool
    {
        return strlen($v) >= 32 && !str_contains(strtolower($v), 'put your unique phrase here') && count(array_unique(str_split($v))) >= 8;
    }

    /** A value that proves the derived key is the same one as before without revealing it (stored in an option). */
    public static function checkValue(string $rawKey): string
    {
        return hash_hmac('sha256', 'aiwend-key-check', $rawKey);
    }
}