Your web application requires users to log in before they can perform meaningful transactions with the application. Once the user is logged in, you need to track the session until the user logs out.
The solution to this problem is actually straightforward. If the user presents the proper password, you generate a session ID and return it to the client in a cookie. While the session is active, the client sends the session ID back to the server; the server verifies it against an internal table of sessions that has the relevant user information associated with each session ID, allowing the server to proceed without requiring the client to continually send the user name and password to the server. For maximum security, all communications should be done over an SSL-enabled connection.
The only trick is that the ID should be large and cryptographically random, in order to prevent hijacking attempts.
Unfortunately, there is little that can be done to prevent session hijacking if an attacker can somehow gain access to the session ID that you need to generate for the user if the login attempt is successful. Normally, the cookie used to carry the session ID should not be permanent (i.e., it expires when the user shuts down the browser), so most browsers will never store the cookie on disk, keeping the cookie only in memory. While this does not make it impossible for an attacker to obtain the session ID, it makes it considerably more difficult.
This issue underscores the need to use SSL properly, which is usually not a problem between browsers and web servers. Take heed of this for other applications of SSL, however. If certificates are not properly verified, allowing an attacker to execute a man-in-the-middle attack, the session ID can be captured. At that point, it hardly matters, though. If such an attack can be mounted, the attacker can do far worse than simply capture session IDs.
The only real requirement for generating a session ID is that it should be unique and not predictable. A base64-encoded 128-bit cryptographically strong random number should generally suffice for the task, but there are many other ways to achieve the same end. For example, you could hash a random number or encrypt some identifying data using a symmetric cipher. However you want to do it is fine?just make sure it's unique and unpredictable! You'll always need some element of randomness in your session IDs, though, so we recommend that you always include at least a 64-bit, cryptographically strong, random number.
Depending on how you choose to generate your session ID, you may require a lookup table keyed by the session ID. In the table, you'll need at least to keep the username associated with the session ID so that you know which user you're dealing with. You can also attach timing information to perform session expiration. If you don't want to get that fancy, and all you need to keep track of is the user's name or some kind of internal user ID, a good solution is to encrypt that information along with some other information. If you choose to do this, be sure to include a nonce, and properly MAC and encrypt the data (e.g., with CWC mode from Recipe 5.10, or as described in Recipe 6.18); the result will be the session ID. In some instances, you may want to bind the IP address into the cookie as well.
Recipe 5.10, Recipe 6.18