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Re: Referer (was Patches for wwwboard.pl)
From: adam () TWOFISH NETECT COM (Adam Shostack)
Date: Sat, 10 Oct 1998 15:31:35 -0400


        Licoln's suggestion is very good, but can be improved by using
the hmac construction rather than md5(key, data).  The hmac function
is documented in http://www-cse.ucsd.edu/users/mihir/papers/hmac.html,
or rfc2014.

        An elegant proof of this is too large to fit in this note.

Adam


On Fri, Oct 09, 1998 at 07:45:38AM -0800, Michael Blythe wrote:
Since the referer information comes from the remote client, that doesn't
prevent malicious destruction.  Anyone with netcat or telnet could forge the
referer field. It requires a little more expertise, but not much.

In September's 'Web Techniques', Lincoln Stein dicscusses the problem of
using the referer header as an authentication method for CGI scripts. He
suggests using MD5 to check whether a form's fields have been tampered
with. I'm not sure if this would work with the wwwboard, because of the way
the script is passing info in hidden fields, but it will work in other
applications:

Here's an abbreviated version of his suggestions:

1. Choose a secret key.
2. Decide which form fields should be untamperable.
3. Optional: put in consistency-checking hidden fields.
4. Concatenate the key, the form fields, and the consitency-checking fields.
   Compute the MD5 hash* and convert it to a hexadecimal string.
5. Include within the form a new hidden field containing the hash.
6. When the form is returned to your script, make sure all expected fields
are present, and that
   expected values are present in the consistency-checking fields.
Recompute the MD5 hash and
   compare it to the one in the form field. If they match, "you can be
pretty sure that the form
   hasn't been tampered with."


* in perl, the MD5 hash can be computed as follows:
$hash = MD5 -> hexhash(MD5->hexhash ($secret) "@untamperable @consistency");

/mb

--
"It is seldom that liberty of any kind is lost all at once."
                                                       -Hume



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