Re: Cryptographically Generated Cookies

["Andy Steingruebl" <steingra () gmail com>]

On Dec 9, 2007 7:58 AM, Paul Johnston <paj () pajhome org uk> wrote:
> 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.

Barring other types of attacks the work-factor to attack these two is
roughly identical.

 1. Brute force the key used to encrypt the cookie
 vs.
 2. Brute force the session-id

Now, admittedly #1 can be done offline while #2 can only be done
online.  That said, its roughly the same number of operations.  On a
site that has a large number of active sessions,  you might get lucky
and come up with a valid session.  Or, you might get lucky and brute
force the key used to encrypt the cookie.

An ideal scheme to protect the cookie then might rely on a per-user
key, or at least a key that rotates relatively frequently so that it
isn't valid for a lot more time than the cookie itself.  If you also
do an HMAC for the cookie and use a different key (good practice) then
even a successful brute force on the encryption key doesn't
necessarily buy you a lot since you can't tamper with the data.

For large sites that are geographically distributed, a shared-nothing
approach with the client maintaining most/all of the state can be
quite preferable and can be achieved reasonably securely.

-- 
Andy Steingruebl
steingra () gmail com

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