Cookies and sessions sohamsengupta@yahoo.com, soham.sengupta.java@gmail.com Saturday, February 23, 2019Saturday, February 23, 2019 http://www.flickr.com/photos/torkildr/3462607995/

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.) sohamsengupta@yahoo.com, soham.sengupta.java@gmail.com Saturday, February 23, 2019Saturday, February 23, 2019

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. sohamsengupta@yahoo.com, soham.sengupta.java@gmail.com Saturday, February 23, 2019Saturday, February 23, 2019

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. sohamsengupta@yahoo.com, soham.sengupta.java@gmail.com Saturday, February 23, 2019Saturday, February 23, 2019

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. sohamsengupta@yahoo.com, soham.sengupta.java@gmail.com Saturday, February 23, 2019Saturday, February 23, 2019

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 sohamsengupta@yahoo.com, soham.sengupta.java@gmail.com Saturday, February 23, 2019Saturday, February 23, 2019

Using just cookies for login Browser Server Type username & password Send username & password Authenticate Cookie = usernm&pwd Click a link or whatever Warning This design contains a serious security hole. Request page (send cookie) sohamsengupta@yahoo.com, soham.sengupta.java@gmail.com Send back page Saturday, February 23, 2019Saturday, February 23, 2019

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. sohamsengupta@yahoo.com, soham.sengupta.java@gmail.com Saturday, February 23, 2019Saturday, February 23, 2019

Using cookies for login Browser Server Filesystem or Database Type username & password Send username & password Authenticate Store a random number valid only for next 10 minutes Cookie = the random # Click a link or whatever Request page (send cookie) Check if the number is right; if so, give another 10 minutes sohamsengupta@yahoo.com, soham.sengupta.java@gmail.com Send back page Saturday, February 23, 2019Saturday, February 23, 2019

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, … sohamsengupta@yahoo.com, soham.sengupta.java@gmail.com Saturday, February 23, 2019Saturday, February 23, 2019

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 sohamsengupta@yahoo.com, soham.sengupta.java@gmail.com Saturday, February 23, 2019Saturday, February 23, 2019

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. sohamsengupta@yahoo.com, soham.sengupta.java@gmail.com Saturday, February 23, 2019Saturday, February 23, 2019

Simple example of using session <?php session_start(); if (isset($_SESSION['numhits'])) $_SESSION['numhits'] = $_SESSION['numhits']+ 1; else $_SESSION['numhits'] = 1; echo "You hit my server ".$_SESSION['numhits']." times."; ?> sohamsengupta@yahoo.com, soham.sengupta.java@gmail.com Saturday, February 23, 2019Saturday, February 23, 2019

Authentication (Using hardcoded username&pwd for now) <?php session_start(); /* login.php */ if (array_key_exists("username", $_REQUEST) && array_key_exists("password", $_REQUEST)) { /* here is where we would check the username and password */ $_SESSION['uid'] = 1; echo '<a href="inventory.php">View Inventory</a>'; } else { ?> <form action="login.php" method="POST"> <div>Username: <input type="text" name="username"></div> <div>Password: <input type="password" name="password"></div> <div><input type="submit" value="OK"></div> </form> <?php } <?php session_start(); /* inventory.php */ if (isset($_SESSION['uid'])) echo "This is where we would show inventory."; else echo "You need to <a href='login.php'>Log in</a>"; ?> sohamsengupta@yahoo.com, soham.sengupta.java@gmail.com Saturday, February 23, 2019Saturday, February 23, 2019

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."; ?> sohamsengupta@yahoo.com, soham.sengupta.java@gmail.com Saturday, February 23, 2019Saturday, February 23, 2019

Summarizing cookies vs sessions 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 sohamsengupta@yahoo.com, soham.sengupta.java@gmail.com Saturday, February 23, 2019Saturday, February 23, 2019

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 sohamsengupta@yahoo.com, soham.sengupta.java@gmail.com Saturday, February 23, 2019Saturday, February 23, 2019

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 sohamsengupta@yahoo.com, soham.sengupta.java@gmail.com Saturday, February 23, 2019Saturday, February 23, 2019

Yet another caveat After all of those warnings, you still can save secret data in cookies, IF IT IS ENCRYPTED You will see how to do this later in the term But we don't really use encrypted cookies much because it can cause usability problems. sohamsengupta@yahoo.com, soham.sengupta.java@gmail.com Saturday, February 23, 2019Saturday, February 23, 2019