Cookies and sessions

1. Cookies and sessions http://www.flickr.com/photos/torkildr/3462607995/

2. HTTP is “stateless” • By default, web servers are “forgetful” • As far as they can tell, every request comes from a totally new and different browser • (Not exactly true. We'll discuss persistent connections in the context of performance.)

3. Pros of stateless servers • Chief benefit: Potential for replication • Improved performance: A sysadmin can fire up N copies of a website (on N machines) and any machine can serve each request. • Improved reliability: If a machine crashes, then another can be started up in its place, and no data gets lost in the process.

4. Cons of stateless servers • Chief problem: Keeping track of which requests "go together" • Security challenge: If a user submits username & password in http request X, then tries to access resources in http request Y, how does the server know that request Y is from somebody who already logged in? • By the time that request Y comes in, the server will already have forgotten that request X ever occurred. • And on a replicated system, request Y might be served by a different machine than request X.

5. Cookies to the rescue! • Reminder: • Cookie = a piece of data that is automatically copied between the client and server • Cookies can be set by the client (as in the last unit) or by the server.

6. A simple way to use cookies for login… • When user sends a valid username & password in request X, the server replies with a cookie containing the username & password • When user subsequently makes request Y, the browser sends along the cookie. • Sounds appealing: user only needs to log in once • Serious security hole: anybody who gets his hands on the user's computer can see cookies

7. Using just cookies for login Browser Server Type username & password Send username & password Authenticate Cookie = usernm&pwd Click a linkor whatever WarningThis design contains a serious security hole. Request page(send cookie) Send back page

8. A more secure way of cookies+login • When user sends a valid username & password in request X, the server replies with a cookie containing a secret that the client couldn't possibly have guessed. • When user subsequently makes request Y, the browser sends along the cookie. • Since the client couldn't have guessed this value without logging in, the server knows that the user did in fact previously log in.

9. Using cookies for login Filesystem or Database Browser Server Type username & password Send username & password Authenticate Store a random numbervalid only for next 10 minutes Cookie = the random # Click a linkor whatever Request page(send cookie) Check if the number is right;if so, give another 10 minutes Send back page

10. Session = state stored across requests • This is what we call a "session" • Session is basically an add-on to the basic http functionality of a website • So that the website can remember information across requests. • You can store lots of stuff in session • Numbers, strings, stringified objects, …

11. Pros of sessions • Stores information between requests • Much more secure than the simple cookie-based approach I showed you • A bad person would need to steal the random number (cookie) within 10 minutes of its creation

12. Cons of sessions • Requires your web server to have write-access to some sort of storage medium • File system, database, …, if you want replication • Otherwise just use memory (lost on server crash) • Requires user to access site every few minutes • Though you can configure longer or shorter times • This is a tradeoff between usability & security. • EECS servers currently are set to 24 minutes.

13. Simple example of using session <?php session_start(); // MUST BE the 1st line of your php if (isset($_SESSION['numhits'])) $_SESSION['numhits'] = $_SESSION['numhits']+ 1; else $_SESSION['numhits'] = 1; echo "You hit my server ".$_SESSION['numhits']." times."; ?>

14. Authentication (Using hardcoded username&pwd for now) <?phpsession_start(); /* login.php */ if (array_key_exists("username", $_REQUEST) && array_key_exists("password", $_REQUEST)) { /* here is where we would check the username and password */ $_SESSION['uid'] = 1; echo '<a href="inventory.php">View Inventory</a>'; } else { ?> <form action="login.php" method="POST"> <div>Username: <input type="text" name="username"></div> <div>Password: <input type="password" name="password"></div> <div><input type="submit" value="OK"></div> </form> <?php } ?> <?phpsession_start(); /* inventory.php */ if (isset($_SESSION['uid'])) echo "This is where we would show inventory."; else echo "You need to <a href='login.php'>Log in</a>"; ?>

15. You can set cookies without session <?php $nhits = isset($_COOKIE['numhits']) ? $_COOKIE['numhits'] : 0; $nhits = $nhits + 1; setcookie('numhits', $nhits, time()+86400*365); /* expires in 365 days */ echo "You hit my server ".$nhits." times."; ?>

16. Summarizing cookies vs sessions • Cookies • Little bits of data that are stored on client but also copied automatically to the server • Useful for storing little bits of data on the client, but they are visible to everybody • So don't store sensitive data in cookies • Sessions • Data is stored on the server (e.g., filesystem), keyed by a random number • The random number is sent as a cookie to the browser • And the random number expires after a little while

17. When to use cookies vs sessions • Use cookies when • You need to save a small amount of data between requests, and it doesn't need to be kept secret • Use sessions when • You need to save a larger amount of data between requests, or when the data needs to be secret

18. Examples of information not to store in unencrypted cookies • Passwords • Credit card numbers • Social security numbers • Student ID numbers • Birthdates • List of diseases the user has contracted • Anything that must be kept secret

19. Yet another caveat • After all of those warnings, you still can save secret data in cookies, IF IT IS ENCRYPTED • But we don't really use encrypted cookies much because it can cause usability problems.