I agree with hardburn.
What I use is taken from Lincoln Stein's Apache Modules book. I create a string which is passed to the cookie and is also saved in a database for comparison.
It might be overkill but the cookie string is updated on each page as one of the fields used to create it is a timestamp. As Lincoln points out this is extremely sensitive to the smallest change in passed parameters, therefore is very hard to spoof and (almost) insures randomness.
use MD5;
my $MAC = MD5->hexhash( $secret.
MD5->hexhash(join '', $secret, @fields)
);

[download]
The $secret variable holds a 128 character string, which should be as random as possible.
The @fields array holds whatever data you want to use, as stated before preferably not relevant user data, which combination should of course be unique for each session.
Changing the $secret string on a regular basis will also provide peace of mind ;-)

Update 

use MD5;
my $MAC = MD5->hexhash( $secret.
MD5->hexhash(join ':', $secret, @fields)
);

[download]
It looks like the book had a typo as this correct version of the code appears somewhere else in the book.
Sorry Lincoln, my bad!

jayrom

In reply to Re: Is this a secure way to prevent cookie tampering by jayrom
in thread Is this a secure way to prevent cookie tampering by EvdB

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