Thursday, April 20, 2006

Stateless web pages with hashes

Recently I've been working on a web application that requires some state to be passed between pages. I really didn't want to keep server side state and then give the user a cookie or some other token that I'd have to track in the server side application, age out if discarded etc.

I hit upon the idea of keeping everything in hidden fields passed between page transitions by form POSTs. Of course, the problem with hidden fields is that someone could fake the information and submit a form with their notion of state. For example, if this were a commerce application, someone could alter the contents of their own shopping cart and perhaps even the prices they have to pay.

To get around this problem I include two extra pieces of information in the form: the Unix epoch time when the form was delivered to the user and a hash that covers all the contents of the form. For example, a typical form might look like:
<form action=http://www.mywebsite.com/form.cgi method=POST>
<input type=hidden name=hash value=ff9f5c4a0d10d7ab384ad0f95ff3727f>
<input type=hidden name=now value=1145516605>
<input type=hidden name=cart value=agwiji8973cnwiei938943>
<input type=submit value="Checkout" name=checkout>
</form>
Here the form contains a cart value that is just an encoded version of the contents of the user's cart (note I say encoded and not encrypted; there's no protection inherent in the string encoding the cart contents: it's just safe to be passed in a form).

The time the form was sent to the user is in the now field and is just the Unix epoch time when the page was generated on the server side.

The hash is an MD5 hash of the now, the cart, the IP address of the person who requested the page and a salt value known only to the web server. The salt prevents an attacker from generating their own hashes and hence faking the form values, but it means that the web server can verify the validity of the form.

The now value means that old forms can be timed out just be checking the epoch time against the value in the form. The hashing of the IP address means that only the person for whom the form was generated can submit it.

I'm sure this isn't new to anyone who's written web applications. And it appears that Steve Gibson over at GRC is doing something similar with his e-commerce system and there's apparently something called View State in ASP.

Anyone who is a web expert care to comment?

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