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<?php
/*
 * Copyright 2019 Google LLC
 *
 * 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\Auth;

use DateTime;
use Exception;
use Firebase\JWT\ExpiredException;
use Firebase\JWT\JWT;
use Firebase\JWT\SignatureInvalidException;
use Google\Auth\Cache\MemoryCacheItemPool;
use Google\Auth\HttpHandler\HttpClientCache;
use Google\Auth\HttpHandler\HttpHandlerFactory;
use GuzzleHttp\Psr7\Request;
use GuzzleHttp\Psr7\Utils;
use InvalidArgumentException;
use phpseclib\Crypt\RSA;
use phpseclib\Math\BigInteger;
use Psr\Cache\CacheItemPoolInterface;
use RuntimeException;
use SimpleJWT\InvalidTokenException;
use SimpleJWT\JWT as SimpleJWT;
use SimpleJWT\Keys\KeyFactory;
use SimpleJWT\Keys\KeySet;
use UnexpectedValueException;

/**
 * Wrapper around Google Access Tokens which provides convenience functions.
 *
 * @experimental
 */
class AccessToken
{
    const FEDERATED_SIGNON_CERT_URL = 'https://www.googleapis.com/oauth2/v3/certs';
    const IAP_CERT_URL = 'https://www.gstatic.com/iap/verify/public_key-jwk';
    const IAP_ISSUER = 'https://cloud.google.com/iap';
    const OAUTH2_ISSUER = 'accounts.google.com';
    const OAUTH2_ISSUER_HTTPS = 'https://accounts.google.com';
    const OAUTH2_REVOKE_URI = 'https://oauth2.googleapis.com/revoke';

    /**
     * @var callable
     */
    private $httpHandler;

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

    /**
     * @param callable $httpHandler [optional] An HTTP Handler to deliver PSR-7 requests.
     * @param CacheItemPoolInterface $cache [optional] A PSR-6 compatible cache implementation.
     */
    public function __construct(
        callable $httpHandler = null,
        CacheItemPoolInterface $cache = null
    ) {
        $this->httpHandler = $httpHandler
            ?: HttpHandlerFactory::build(HttpClientCache::getHttpClient());
        $this->cache = $cache ?: new MemoryCacheItemPool();
    }

    /**
     * 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 $token The JSON Web Token to be verified.
     * @param array $options [optional] Configuration options.
     * @param string $options.audience The indended recipient of the token.
     * @param string $options.issuer The intended issuer of the token.
     * @param string $options.cacheKey The cache key of the cached certs. Defaults to
     *        the sha1 of $certsLocation if provided, otherwise is set to
     *        "federated_signon_certs_v3".
     * @param string $options.certsLocation The location (remote or local) from which
     *        to retrieve certificates, if not cached. This value should only be
     *        provided in limited circumstances in which you are sure of the
     *        behavior.
     * @param bool $options.throwException Whether the function should throw an
     *        exception if the verification fails. This is useful for
     *        determining the reason verification failed.
     * @return array|bool the token payload, if successful, or false if not.
     * @throws InvalidArgumentException If certs could not be retrieved from a local file.
     * @throws InvalidArgumentException If received certs are in an invalid format.
     * @throws InvalidArgumentException If the cert alg is not supported.
     * @throws RuntimeException If certs could not be retrieved from a remote location.
     * @throws UnexpectedValueException If the token issuer does not match.
     * @throws UnexpectedValueException If the token audience does not match.
     */
    public function verify($token, array $options = [])
    {
        $audience = isset($options['audience'])
            ? $options['audience']
            : null;
        $issuer = isset($options['issuer'])
            ? $options['issuer']
            : null;
        $certsLocation = isset($options['certsLocation'])
            ? $options['certsLocation']
            : self::FEDERATED_SIGNON_CERT_URL;
        $cacheKey = isset($options['cacheKey'])
            ? $options['cacheKey']
            : $this->getCacheKeyFromCertLocation($certsLocation);
        $throwException = isset($options['throwException'])
            ? $options['throwException']
            : false; // for backwards compatibility

        // Check signature against each available cert.
        $certs = $this->getCerts($certsLocation, $cacheKey, $options);
        $alg = $this->determineAlg($certs);
        if (!in_array($alg, ['RS256', 'ES256'])) {
            throw new InvalidArgumentException(
                'unrecognized "alg" in certs, expected ES256 or RS256'
            );
        }
        try {
            if ($alg == 'RS256') {
                return $this->verifyRs256($token, $certs, $audience, $issuer);
            }
            return $this->verifyEs256($token, $certs, $audience, $issuer);
        } catch (ExpiredException $e) {  // firebase/php-jwt 3+
        } catch (\ExpiredException $e) { // firebase/php-jwt 2
        } catch (SignatureInvalidException $e) {  // firebase/php-jwt 3+
        } catch (\SignatureInvalidException $e) { // firebase/php-jwt 2
        } catch (InvalidTokenException $e) { // simplejwt
        } catch (DomainException $e) {
        } catch (InvalidArgumentException $e) {
        } catch (UnexpectedValueException $e) {
        }

        if ($throwException) {
            throw $e;
        }

        return false;
    }

    /**
     * Identifies the expected algorithm to verify by looking at the "alg" key
     * of the provided certs.
     *
     * @param array $certs Certificate array according to the JWK spec (see
     *                     https://tools.ietf.org/html/rfc7517).
     * @return string The expected algorithm, such as "ES256" or "RS256".
     */
    private function determineAlg(array $certs)
    {
        $alg = null;
        foreach ($certs as $cert) {
            if (empty($cert['alg'])) {
                throw new InvalidArgumentException(
                    'certs expects "alg" to be set'
                );
            }
            $alg = $alg ?: $cert['alg'];

            if ($alg != $cert['alg']) {
                throw new InvalidArgumentException(
                    'More than one alg detected in certs'
                );
            }
        }
        return $alg;
    }

    /**
     * Verifies an ES256-signed JWT.
     *
     * @param string $token The JSON Web Token to be verified.
     * @param array $certs Certificate array according to the JWK spec (see
     *        https://tools.ietf.org/html/rfc7517).
     * @param string|null $audience If set, returns false if the provided
     *        audience does not match the "aud" claim on the JWT.
     * @param string|null $issuer If set, returns false if the provided
     *        issuer does not match the "iss" claim on the JWT.
     * @return array|bool the token payload, if successful, or false if not.
     */
    private function verifyEs256($token, array $certs, $audience = null, $issuer = null)
    {
        $this->checkSimpleJwt();

        $jwkset = new KeySet();
        foreach ($certs as $cert) {
            $jwkset->add(KeyFactory::create($cert, 'php'));
        }

        // Validate the signature using the key set and ES256 algorithm.
        $jwt = $this->callSimpleJwtDecode([$token, $jwkset, 'ES256']);
        $payload = $jwt->getClaims();

        if (isset($payload['aud'])) {
            if ($audience && $payload['aud'] != $audience) {
                throw new UnexpectedValueException('Audience does not match');
            }
        }

        // @see https://cloud.google.com/iap/docs/signed-headers-howto#verifying_the_jwt_payload
        $issuer = $issuer ?: self::IAP_ISSUER;
        if (!isset($payload['iss']) || $payload['iss'] !== $issuer) {
            throw new UnexpectedValueException('Issuer does not match');
        }

        return $payload;
    }

    /**
     * Verifies an RS256-signed JWT.
     *
     * @param string $token The JSON Web Token to be verified.
     * @param array $certs Certificate array according to the JWK spec (see
     *        https://tools.ietf.org/html/rfc7517).
     * @param string|null $audience If set, returns false if the provided
     *        audience does not match the "aud" claim on the JWT.
     * @param string|null $issuer If set, returns false if the provided
     *        issuer does not match the "iss" claim on the JWT.
     * @return array|bool the token payload, if successful, or false if not.
     */
    private function verifyRs256($token, array $certs, $audience = null, $issuer = null)
    {
        $this->checkAndInitializePhpsec();
        $keys = [];
        foreach ($certs as $cert) {
            if (empty($cert['kid'])) {
                throw new InvalidArgumentException(
                    'certs expects "kid" to be set'
                );
            }
            if (empty($cert['n']) || empty($cert['e'])) {
                throw new InvalidArgumentException(
                    'RSA certs expects "n" and "e" to be set'
                );
            }
            $rsa = new RSA();
            $rsa->loadKey([
                'n' => new BigInteger($this->callJwtStatic('urlsafeB64Decode', [
                    $cert['n'],
                ]), 256),
                'e' => new BigInteger($this->callJwtStatic('urlsafeB64Decode', [
                    $cert['e']
                ]), 256),
            ]);

            // create an array of key IDs to certs for the JWT library
            $keys[$cert['kid']] =  $rsa->getPublicKey();
        }

        $payload = $this->callJwtStatic('decode', [
            $token,
            $keys,
            ['RS256']
        ]);

        if (property_exists($payload, 'aud')) {
            if ($audience && $payload->aud != $audience) {
                throw new UnexpectedValueException('Audience does not match');
            }
        }

        // support HTTP and HTTPS issuers
        // @see https://developers.google.com/identity/sign-in/web/backend-auth
        $issuers = $issuer ? [$issuer] : [self::OAUTH2_ISSUER, self::OAUTH2_ISSUER_HTTPS];
        if (!isset($payload->iss) || !in_array($payload->iss, $issuers)) {
            throw new UnexpectedValueException('Issuer does not match');
        }

        return (array) $payload;
    }

    /**
     * Revoke an OAuth2 access token or refresh token. This method will revoke the current access
     * token, if a token isn't provided.
     *
     * @param string|array $token The token (access token or a refresh token) that should be revoked.
     * @param array $options [optional] Configuration options.
     * @return bool Returns True if the revocation was successful, otherwise False.
     */
    public function revoke($token, array $options = [])
    {
        if (is_array($token)) {
            if (isset($token['refresh_token'])) {
                $token = $token['refresh_token'];
            } else {
                $token = $token['access_token'];
            }
        }

        $body = Utils::streamFor(http_build_query(['token' => $token]));
        $request = new Request('POST', self::OAUTH2_REVOKE_URI, [
            'Cache-Control' => 'no-store',
            'Content-Type'  => 'application/x-www-form-urlencoded',
        ], $body);

        $httpHandler = $this->httpHandler;

        $response = $httpHandler($request, $options);

        return $response->getStatusCode() == 200;
    }

    /**
     * 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.
     *
     * @param string $location The location from which to retrieve certs.
     * @param string $cacheKey The key under which to cache the retrieved certs.
     * @param array $options [optional] Configuration options.
     * @return array
     * @throws InvalidArgumentException If received certs are in an invalid format.
     */
    private function getCerts($location, $cacheKey, array $options = [])
    {
        $cacheItem = $this->cache->getItem($cacheKey);
        $certs = $cacheItem ? $cacheItem->get() : null;

        $gotNewCerts = false;
        if (!$certs) {
            $certs = $this->retrieveCertsFromLocation($location, $options);

            $gotNewCerts = true;
        }

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

        // Push caching off until after verifying certs are in a valid format.
        // Don't want to cache bad data.
        if ($gotNewCerts) {
            $cacheItem->expiresAt(new DateTime('+1 hour'));
            $cacheItem->set($certs);
            $this->cache->save($cacheItem);
        }

        return $certs['keys'];
    }

    /**
     * Retrieve and cache a certificates file.
     *
     * @param $url string location
     * @param array $options [optional] Configuration options.
     * @return array certificates
     * @throws InvalidArgumentException If certs could not be retrieved from a local file.
     * @throws RuntimeException If certs could not be retrieved from a remote location.
     */
    private function retrieveCertsFromLocation($url, array $options = [])
    {
        // If we're retrieving a local file, just grab it.
        if (strpos($url, 'http') !== 0) {
            if (!file_exists($url)) {
                throw new InvalidArgumentException(sprintf(
                    'Failed to retrieve verification certificates from path: %s.',
                    $url
                ));
            }

            return json_decode(file_get_contents($url), true);
        }

        $httpHandler = $this->httpHandler;
        $response = $httpHandler(new Request('GET', $url), $options);

        if ($response->getStatusCode() == 200) {
            return json_decode((string) $response->getBody(), true);
        }

        throw new RuntimeException(sprintf(
            'Failed to retrieve verification certificates: "%s".',
            $response->getBody()->getContents()
        ), $response->getStatusCode());
    }

    private function checkAndInitializePhpsec()
    {
        // @codeCoverageIgnoreStart
        if (!class_exists('phpseclib\Crypt\RSA')) {
            throw new RuntimeException('Please require phpseclib/phpseclib v2 to use this utility.');
        }
        // @codeCoverageIgnoreEnd

        $this->setPhpsecConstants();
    }

    private function checkSimpleJwt()
    {
        // @codeCoverageIgnoreStart
        if (!class_exists('SimpleJWT\JWT')) {
            throw new RuntimeException('Please require kelvinmo/simplejwt ^0.2 to use this utility.');
        }
        // @codeCoverageIgnoreEnd
    }

    /**
     * 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
     * @codeCoverageIgnore
     */
    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', RSA::MODE_OPENSSL);
            }
        }
    }

    /**
     * Provide a hook to mock calls to the JWT static methods.
     *
     * @param string $method
     * @param array $args
     * @return mixed
     */
    protected function callJwtStatic($method, array $args = [])
    {
        $class = 'Firebase\JWT\JWT';
        return call_user_func_array([$class, $method], $args);
    }

    /**
     * Provide a hook to mock calls to the JWT static methods.
     *
     * @param array $args
     * @return mixed
     */
    protected function callSimpleJwtDecode(array $args = [])
    {
        return call_user_func_array(['SimpleJWT\JWT', 'decode'], $args);
    }

    /**
     * Generate a cache key based on the cert location using sha1 with the
     * exception of using "federated_signon_certs_v3" to preserve BC.
     *
     * @param string $certsLocation
     * @return string
     */
    private function getCacheKeyFromCertLocation($certsLocation)
    {
        $key = $certsLocation === self::FEDERATED_SIGNON_CERT_URL
            ? 'federated_signon_certs_v3'
            : sha1($certsLocation);

        return 'google_auth_certs_cache|' . $key;
    }
}