<?php
# session_protect.php - HMAC-SHA1 authentication for session data.
# Version 0.0
#
# WARNING: This is experimental software!
#
# ---------------------------------------- 
# Copyright (c) 2006 Dwayne C. Litzenberger <dlitz@dlitz.net>
#
# Permission is hereby granted, free of charge, to any person obtaining
# a copy of this software and associated documentation files (the
# "Software"), to deal in the Software without restriction, including
# without limitation the rights to use, copy, modify, merge, publish,
# distribute, sublicense, and/or sell copies of the Software, and to
# permit persons to whom the Software is furnished to do so, subject to
# the following conditions:
# 
# The above copyright notice and this permission notice shall be
# included in all copies or substantial portions of the Software.
# 
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
# IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
# CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
# TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
# SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
# ---------------------------------------- 
#
# Usage: Call session_start_protected at the beginning of your script,
#        and call session_write_close_protected at the end.
#
# Security notes:
#   1. This might not protect you if you use register_globals.
#   2. This does not protect against PHP vulnerabilities.
#   3. This does not protect against programs that have access to your key.
#   4. This does not protect you if you do something stupid.

require_once "hmac_sha1.php";

function session_start_protected($key)
{
    if (!isset($key)) { die("key must be set"); }
    if (strlen($key) < 20) { die("key must be at least 20 bytes, preferrably much longer"); }
    
    @session_start();

    $s_cookie_params = session_get_cookie_params();
    $s_id = session_id();
    $s_name = session_name();
    $s_host = $_SERVER['HTTP_HOST'];
    
    $stored_hash = $_SESSION["session_protect__hash_$s_id"];
    $stored_data = $_SESSION["session_protect__data_$s_id"];
    unset($_SESSION["session_protect__hash_$s_id"]);
    unset($_SESSION["session_protect__data_$s_id"]);
    
    if (isset($stored_hash) and isset($stored_data)) {
        $real_hash = hmac_sha1($stored_data, $key);
    } else {
        $real_hash = null;
    }

    if (!is_null($real_hash) and ($real_hash === $stored_hash)) {
        # The hash is valid.  However, we must check that the other data is correct.
        $package = unserialize($stored_data);
        unset($stored_data);
        if (($package['proto'] === 'PHPSessionProtect/0.0') and
            ($package['id'] === $s_id) and
            ($package['name'] === $s_name) and
            ($package['cookie_params'] === $s_cookie_params) and
            ($package['host'] === $s_host) and
            ($package['expire'] < time()))
        {
            # The metadata is valid.  Load the authenticated session data.
            foreach(array_keys($_SESSION) as $k) {
                unset($_SESSION[$k]);
            }
            session_decode($package['session_data']);
            return;
        }
    }
    
    # Session data is invalid or unprotected.  Start a new session.
    session_destroy();
    foreach(array_keys($_SESSION) as $k) {
        unset($_SESSION[$k]);
    }
    session_regenerate_id();
    session_start();
    return;
}


function session_write_close_protected($key)
{
    if (!isset($key)) { die("key must be set"); }
    if (strlen($key) < 20) { die("key must be at least 20 bytes, preferrably much longer"); }
    
    $s_cookie_params = session_get_cookie_params();
    $s_id = session_id();
    $s_name = session_name();
    $s_expire = time() + $s_cookie_params['lifetime'];
    $s_host = $_SERVER['HTTP_HOST'];
    
    unset($_SESSION["session_protect__hash_$s_id"]);
    unset($_SESSION["session_protect__data_$s_id"]);
    ksort($_SESSION);
    
    $package = array(
        'proto' => 'PHPSessionProtect/0.0',
        'id' => $s_id,
        'name' => $s_name,
        'expire' => $s_expire,
        'cookie_params' => $s_cookie_params,
        'host' => $s_host,
        'session_data' => session_encode());
    ksort($package);
    
    $package_data = serialize($package);
    $hash = hmac_sha1($package_data, $key);
 
    $_SESSION["session_protect__hash_$s_id"] = $hash;
    $_SESSION["session_protect__data_$s_id"] = $package_data;

    session_write_close();
}