<?php
// Checking Garbo's launch data on your server, in PHP 7.2 or newer (sodium is built in).
// https://developers.garbo.ph/#check
//
// Your page sends Garbo.WebApp.initData to your server; never trust initDataUnsafe there.
// - garbo_check_hash(): with your app's secret (from Garbo; keep it on your server only).
// - garbo_check_signature(): with Garbo's public key below; no secret needed.
// Both throw an Exception when the data isn't right, and return what's in it when it is.

// Garbo's public key for the launch data's signature (Ed25519, hex).
const GARBO_PUBLIC_KEY = 'a6b7e2bcbd35fa46f8040e430316409d1d6e24f98f5232e5d89b9c361f7525a0';

/** The launch data's fields, URL-decoded. */
function garbo_fields(string $initData): array {
    $fields = [];
    foreach (explode('&', $initData) as $part) {
        if ($part === '') continue;
        $pair = explode('=', $part, 2);
        $fields[urldecode($pair[0])] = urldecode($pair[1] ?? '');
    }
    return $fields;
}

/** Every field but the ones left out, sorted by name, as "name=value" lines. */
function garbo_check_string(array $fields, array $leaveOut): string {
    $kept = array_diff_key($fields, array_flip($leaveOut));
    ksort($kept, SORT_STRING);
    $lines = [];
    foreach ($kept as $name => $value) $lines[] = $name . '=' . $value;
    return implode("\n", $lines);
}

function garbo_read(array $fields, int $maxAgeSeconds): array {
    $fields['auth_date'] = (int) ($fields['auth_date'] ?? 0);
    $fields['user'] = isset($fields['user']) ? json_decode($fields['user'], true) : null;
    if ($maxAgeSeconds && time() - $fields['auth_date'] > $maxAgeSeconds) {
        throw new Exception('The launch data is too old.');
    }
    return $fields;
}

function garbo_check_hash(string $initData, string $appSecret, int $maxAgeSeconds = 86400): array {
    $fields = garbo_fields($initData);
    $secretKey = hash_hmac('sha256', $appSecret, 'WebAppData', true);
    $expected = hash_hmac('sha256', garbo_check_string($fields, ['hash']), $secretKey);
    if (!hash_equals($expected, $fields['hash'] ?? '')) {
        throw new Exception("The launch data was not signed with this app's secret.");
    }
    return garbo_read($fields, $maxAgeSeconds);
}

function garbo_check_signature(string $initData, string $appId, int $maxAgeSeconds = 86400, string $publicKey = GARBO_PUBLIC_KEY): array {
    $fields = garbo_fields($initData);
    if (($fields['app_id'] ?? '') !== $appId) throw new Exception('The launch data is for another app.');
    $signature = base64_decode(strtr($fields['signature'] ?? '', '-_', '+/'));
    $signed = $appId . ":WebAppData\n" . garbo_check_string($fields, ['hash', 'signature']);
    if (strlen($signature) !== SODIUM_CRYPTO_SIGN_BYTES ||
        !sodium_crypto_sign_verify_detached($signature, $signed, sodium_hex2bin($publicKey))) {
        throw new Exception('The launch data was not signed by Garbo.');
    }
    return garbo_read($fields, $maxAgeSeconds);
}