I'm currently researching user authentication protocols for a website I'm developing. I would like to create an authentication cookie so users can stay logged in between pages.
Here is my first bash:
cookie = user_id|expiry_date|HMAC(user_id|expiry_date, k)
Where k is
HMAC(user_id|expiry_date, sk) and sk is a 256 bit key only known to the server. HMAC is a SHA-256 hash. Note that '|' is a separator, not just concatenation.
This looks like this in PHP (Note, this question is language-agnostic, PHP is just an example):
$key = hash_hmac('sha256', $user_id . '|' . $expiry_time, SECRET_KEY); $digest = hash_hmac('sha256', $user_id . '|' . $expiry_time, $key); $cookie = $user_id . '|' . $expiry_time . '|' . $digest;
I can see that it's vulnerable to Replay Attacks as stated in A Secure Cookie Protocol, but should be resistant to Volume Attacks, and Cryptographic Splicing.
THE QUESTION: Am I on the right lines here, or is there a massive vulnerability that I've missed? Is there a way to defend against Replay Attacks that works with dynamically assigned IP addresses and doesn't use sessions? I don't want to be storing anything on the server-side to have this work.
Also, I'm not planning or rolling my own. I'm asking this to better judge what solution I should choose. So no "Just use X solution" answers without some sort of explanation.
The most recent material I have read:
Dos and Don'ts of Client Authentication on the Web aka Fu et al.
A Secure Cookie Protocol
aka Liu et al.
which expands on the previous method
Hardened Stateless Session Cookies
which also expands on the previous method.
As the subject is extremely complicated I'm am only looking for answers from security experts with real world experience in creating and breaking authentication schemes.