Cryptographically Generated Cookies

[Paul Johnston <paj () pajhome org uk>]

Hi,

People have talked a lot about storing data on the client and 
cryptographically generating cookies.

My recommendation for security is to not do either. All you store on the 
client is a session ID - a 128-bit random number (plus a CSRF token 
where needed). Any data is stored on the server side, keyed by the 
session ID. This is the most secure approach, but it needs a lot of 
database access on the server.

Any approach that involves a crypto-generated cookie and such is a 
design decision to improve efficiency at some cost to security. Whether 
these security issues justify the performance gains will depend on the 
application. As a rough guide, I'd say this is unsuitable for anything 
that handles money, but probably ok for most other systems.

In terms of session IDs, you can generate these cryptographically, using 
a fairly simple formula:
   (userid, timestamp, hash(userid + timestamp + server_secret))
Think of this as a token saying "user X logged in at time Y, and this 
number Z proves the server authorised it". And this can be done without 
any use of a database on the server.
This has two problems:
1) You can't have a proper logout function - the only way a token 
expires is when it times out.
2) If the server_secret is leaked, your website security vanishes.

In terms of storing data on the client, in most situations you don't 
mind the client seeing this data (it's about them anyway) but you would 
want to stop them tampering with it. As such, protecting it with a MAC 
(a keyed hash) with the secret kept on the server will work. This 
doesn't stop replay attacks, although you could potentially include a 
timestamp as well to partially address this. You could also use 
encryption to maintain confidentiality of that data.

Regards,

Paul

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