Validates authentication cookie.

The checks include making sure that the authentication cookie is set and
pulling in the contents (if $cookie is not used).

Makes sure the cookie is not expired. Verifies the hash in cookie is what is
should be and compares the two.

function wp_validate_auth_cookie( $cookie = '', $scheme = '' ) {
		$cookie_elements = wp_parse_auth_cookie( $cookie, $scheme );
		if ( ! $cookie_elements ) {
			/**
			 * Fires if an authentication cookie is malformed.
			 *
			 * @since 2.7.0
			 *
			 * @param string $cookie Malformed auth cookie.
			 * @param string $scheme Authentication scheme. Values include 'auth', 'secure_auth',
			 *                       or 'logged_in'.
			 */
			do_action( 'auth_cookie_malformed', $cookie, $scheme );
			return false;
		}

		$scheme     = $cookie_elements['scheme'];
		$username   = $cookie_elements['username'];
		$hmac       = $cookie_elements['hmac'];
		$token      = $cookie_elements['token'];
		$expired    = $cookie_elements['expiration'];
		$expiration = $cookie_elements['expiration'];

		// Allow a grace period for POST and Ajax requests.
		if ( wp_doing_ajax() || 'POST' === $_SERVER['REQUEST_METHOD'] ) {
			$expired += HOUR_IN_SECONDS;
		}

		// Quick check to see if an honest cookie has expired.
		if ( $expired user_pass, 8, 4 );

		$key = wp_hash( $username . '|' . $pass_frag . '|' . $expiration . '|' . $token, $scheme );

		// If ext/hash is not present, compat.php's hash_hmac() does not support sha256.
		$algo = function_exists( 'hash' ) ? 'sha256' : 'sha1';
		$hash = hash_hmac( $algo, $username . '|' . $expiration . '|' . $token, $key );

		if ( ! hash_equals( $hash, $hmac ) ) {
			/**
			 * Fires if a bad authentication cookie hash is encountered.
			 *
			 * @since 2.7.0
			 *
			 * @param string[] $cookie_elements {
			 *     Authentication cookie components. None of the components should be assumed
			 *     to be valid as they come directly from a client-provided cookie value.
			 *
			 *     @type string $username   User's username.
			 *     @type string $expiration The time the cookie expires as a UNIX timestamp.
			 *     @type string $token      User's session token used.
			 *     @type string $hmac       The security hash for the cookie.
			 *     @type string $scheme     The cookie scheme to use.
			 * }
			 */
			do_action( 'auth_cookie_bad_hash', $cookie_elements );
			return false;
		}

		$manager = WP_Session_Tokens::get_instance( $user->ID );
		if ( ! $manager->verify( $token ) ) {
			/**
			 * Fires if a bad session token is encountered.
			 *
			 * @since 4.0.0
			 *
			 * @param string[] $cookie_elements {
			 *     Authentication cookie components. None of the components should be assumed
			 *     to be valid as they come directly from a client-provided cookie value.
			 *
			 *     @type string $username   User's username.
			 *     @type string $expiration The time the cookie expires as a UNIX timestamp.
			 *     @type string $token      User's session token used.
			 *     @type string $hmac       The security hash for the cookie.
			 *     @type string $scheme     The cookie scheme to use.
			 * }
			 */
			do_action( 'auth_cookie_bad_session_token', $cookie_elements );
			return false;
		}

		// Ajax/POST grace period set above.
		if ( $expiration ID;
	}
endif;

if ( ! function_exists( 'wp_generate_auth_cookie' ) ) :
	/**
	 * Generates authentication cookie contents.
	 *
	 * @since 2.5.0
	 * @since 4.0.0 The `$token` parameter was added.
	 *
	 * @param int    $user_id    User ID.
	 * @param int    $expiration The time the cookie expires as a UNIX timestamp.
	 * @param string $scheme     Optional. The cookie scheme to use: 'auth', 'secure_auth', or 'logged_in'.
	 *                           Default 'auth'.
	 * @param string $token      User's session token to use for this cookie.
	 * @return string Authentication cookie contents. Empty string if user does not exist.
	 */