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Filename : Verify.php
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<?php

/*
 * Copyright 2008 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

namespace Google\AccessToken;

use DateTime;
use DomainException;
use Exception;
use ExpiredException;
use Firebase\JWT\ExpiredException as ExpiredExceptionV3;
use Firebase\JWT\JWT;
use Firebase\JWT\Key;
use Firebase\JWT\SignatureInvalidException;
use Google\Auth\Cache\MemoryCacheItemPool;
use Google\Exception as GoogleException;
use GuzzleHttp\Client;
use GuzzleHttp\ClientInterface;
use InvalidArgumentException;
use LogicException;
use phpseclib3\Crypt\AES;
use phpseclib3\Crypt\PublicKeyLoader;
use phpseclib3\Math\BigInteger;
use Psr\Cache\CacheItemPoolInterface;

/**
 * Wrapper around Google Access Tokens which provides convenience functions
 *
 */
class Verify
{
    const FEDERATED_SIGNON_CERT_URL = 'https://www.googleapis.com/oauth2/v3/certs';
    const OAUTH2_ISSUER = 'accounts.google.com';
    const OAUTH2_ISSUER_HTTPS = 'https://accounts.google.com';

    /**
     * @var ClientInterface The http client
     */
    private $http;

    /**
     * @var CacheItemPoolInterface cache class
     */
    private $cache;

    /**
     * @var \Firebase\JWT\JWT
     */
    public $jwt;

    /**
     * Instantiates the class, but does not initiate the login flow, leaving it
     * to the discretion of the caller.
     */
    public function __construct(
        ?ClientInterface $http = null,
        ?CacheItemPoolInterface $cache = null,
        ?string $jwt = null
    ) {
        if (null === $http) {
            $http = new Client();
        }

        if (null === $cache) {
            $cache = new MemoryCacheItemPool();
        }

        $this->http = $http;
        $this->cache = $cache;
        $this->jwt = $jwt ?: $this->getJwtService();
    }

    /**
     * Verifies an id token and returns the authenticated apiLoginTicket.
     * Throws an exception if the id token is not valid.
     * The audience parameter can be used to control which id tokens are
     * accepted.  By default, the id token must have been issued to this OAuth2 client.
     *
     * @param string $idToken the ID token in JWT format
     * @param string $audience Optional. The audience to verify against JWt "aud"
     * @return array|false the token payload, if successful
     */
    public function verifyIdToken($idToken, $audience = null)
    {
        if (empty($idToken)) {
            throw new LogicException('id_token cannot be null');
        }

        // set phpseclib constants if applicable
        $this->setPhpsecConstants();

        // Check signature
        $certs = $this->getFederatedSignOnCerts();
        foreach ($certs as $cert) {
            try {
                $args = [$idToken];
                $publicKey = $this->getPublicKey($cert);
                if (class_exists(Key::class)) {
                    $args[] = new Key($publicKey, 'RS256');
                } else {
                    $args[] = $publicKey;
                    $args[] = ['RS256'];
                }
                $payload = \call_user_func_array([$this->jwt, 'decode'], $args);

                if (property_exists($payload, 'aud')) {
                    if ($audience && $payload->aud != $audience) {
                        return false;
                    }
                }

                // support HTTP and HTTPS issuers
                // @see https://developers.google.com/identity/sign-in/web/backend-auth
                $issuers = [self::OAUTH2_ISSUER, self::OAUTH2_ISSUER_HTTPS];
                if (!isset($payload->iss) || !in_array($payload->iss, $issuers)) {
                    return false;
                }

                return (array)$payload;
            } catch (ExpiredException $e) { // @phpstan-ignore-line
                return false;
            } catch (ExpiredExceptionV3 $e) {
                return false;
            } catch (SignatureInvalidException $e) {
                // continue
            } catch (DomainException $e) {
                // continue
            }
        }

        return false;
    }

    private function getCache()
    {
        return $this->cache;
    }

    /**
     * Retrieve and cache a certificates file.
     *
     * @param string $url location
     * @return array certificates
     * @throws \Google\Exception
     */
    private function retrieveCertsFromLocation($url)
    {
        // If we're retrieving a local file, just grab it.
        if (0 !== strpos($url, 'http')) {
            if (!$file = file_get_contents($url)) {
                throw new GoogleException(
                    "Failed to retrieve verification certificates: '".
                    $url."'."
                );
            }

            return json_decode($file, true);
        }

        // @phpstan-ignore-next-line
        $response = $this->http->get($url);

        if ($response->getStatusCode() == 200) {
            return json_decode((string)$response->getBody(), true);
        }
        throw new GoogleException(
            sprintf(
                'Failed to retrieve verification certificates: "%s".',
                $response->getBody()->getContents()
            ),
            $response->getStatusCode()
        );
    }

    // Gets federated sign-on certificates to use for verifying identity tokens.
    // Returns certs as array structure, where keys are key ids, and values
    // are PEM encoded certificates.
    private function getFederatedSignOnCerts()
    {
        $certs = null;
        if ($cache = $this->getCache()) {
            $cacheItem = $cache->getItem('federated_signon_certs_v3');
            $certs = $cacheItem->get();
        }


        if (!$certs) {
            $certs = $this->retrieveCertsFromLocation(
                self::FEDERATED_SIGNON_CERT_URL
            );

            if ($cache) {
                $cacheItem->expiresAt(new DateTime('+1 hour'));
                $cacheItem->set($certs);
                $cache->save($cacheItem);
            }
        }

        if (!isset($certs['keys'])) {
            throw new InvalidArgumentException(
                'federated sign-on certs expects "keys" to be set'
            );
        }

        return $certs['keys'];
    }

    private function getJwtService()
    {
        $jwt = new JWT();
        if ($jwt::$leeway < 1) {
            // Ensures JWT leeway is at least 1
            // @see https://github.com/google/google-api-php-client/issues/827
            $jwt::$leeway = 1;
        }

        return $jwt;
    }

    private function getPublicKey($cert)
    {
        $modulus = new BigInteger($this->jwt->urlsafeB64Decode($cert['n']), 256);
        $exponent = new BigInteger($this->jwt->urlsafeB64Decode($cert['e']), 256);
        $component = ['n' => $modulus, 'e' => $exponent];

        $loader = PublicKeyLoader::load($component);

        return $loader->toString('PKCS8');
    }

    /**
     * phpseclib calls "phpinfo" by default, which requires special
     * whitelisting in the AppEngine VM environment. This function
     * sets constants to bypass the need for phpseclib to check phpinfo
     *
     * @see phpseclib/Math/BigInteger
     * @see https://github.com/GoogleCloudPlatform/getting-started-php/issues/85
     */
    private function setPhpsecConstants()
    {
        if (filter_var(getenv('GAE_VM'), FILTER_VALIDATE_BOOLEAN)) {
            if (!defined('MATH_BIGINTEGER_OPENSSL_ENABLED')) {
                define('MATH_BIGINTEGER_OPENSSL_ENABLED', true);
            }
            if (!defined('CRYPT_RSA_MODE')) {
                define('CRYPT_RSA_MODE', AES::ENGINE_OPENSSL);
            }
        }
    }
}