Re: Digest mess
John Franks (john@math.nwu.edu)
Tue, 6 Jan 1998 09:44:01 -0600 (CST)
On Tue, 6 Jan 1998, Scott Lawrence wrote:
>
>
> I would like to
> be able to implement it in a single parsing pass over the message,
> which is certainly impossible if arbitrary headers may be included
> by an attribute in a field that may be sent in a trailer.
>
I see no problem with a single pass. Remember the actual message
headers are irrelevant for authentication purposes. Conversely, the
"origin-headers" play no role as HTTP headers, but are used only for
authentication. There is no problem if the origin-headers come at the
end. The single pass is (1) read entity-body and calculate
H(entity-body), (2) read Auth-info and calculate entity-digest using
origin-headers field, and (3) compare calculated entity digest with
sender supplied entity-digest. A point worth emphasizing is that
the actual HTTP headers never get used in any way in the authentication
process.
> Getting back to the original problem - the prevention of the replay
> of a valid message in response to a different request. The server
> can already prevent replay of a past client message by changing the
> nonce value included in the challenge.
I don't understand this. Why wouldn't a man-in-the-middle replay
an earlier challenge with a valid, but old, nonce? In fact the
MIM would have to do this for the replay attack to work.
John Franks
john@math.nwu.edu