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Re: Digest and pipelining

From: David W. Morris <dwm@xpasc.com>
Date: Tue, 20 Jan 1998 17:55:27 -0800 (PST)
To: Paul Leach <paulle@microsoft.com>
Cc: http-wg@cuckoo.hpl.hp.com
Message-Id: <Pine.GSO.3.96.980120173302.1221E-100000@shell1.aimnet.com>
X-Mailing-List: <http-wg@cuckoo.hpl.hp.com> archive/latest/5236

On Tue, 20 Jan 1998, Paul Leach wrote:

> If we can rely on requests being delivered in order, then I have a simple
> algorithm to do replay prevention with pipelining.
> 1. The server sends a nonce which is a random number cocatenated with a
> timestamp. It sends an "opaque" value (part of) which is a "client ID".
> 1. The client uses a nonce which is the concatentation of the nonce the
> server sent it and a 3 digit count (mod 1000) of the number of requests it
> has sent to the server, plus a client chosen random string (this last
> prevents dictionary attacks). It sends back the 3 digits plus the client
> chosen random string.
> 2. The server uses (part of) the opaque field to identify the client, and
> concatenates the nonce it keeps associated with the opaque field with the
> nonce sent by the client to get the complete nonce that it uses to check the
> client's request. It keeps a counter of the requests associated with the
> client ID; this count and that received from the client must match; after
> matching it is incremented by one. The timestamp is checked to make sure it
> isn't too old. When it expires, the association between the client ID and
> the nonce and count can be discarded.
> Can we depend on the requests being in order?

Order can only be depended upon if the client ONLY uses a single
connection at a time to connect with a server. Irrespective of whether
that connection is persistent or uses pipelining.

> If we can't, then the server would need to keep a (say) 64 bit mask plus a
> base count, and check that the count in the request hasn't been seen before;
> when the count gets to be 0 mod 32, then the base count gets bumped by 32
> and the mask gets shifted right by 32 bits. This would handle requests as
> long as they weren't too out of order.

This algorithm will break as soon as request#32 arrives while #29 
is pending. The algorithm I think would work if the increment rule is
that slots 0-31 have been used, then increment the base and shift...

But given a web page as complex as some from a representative site
such as Microsoft where I've counted more than 30 requests as a
result of a single user URL click, and given 4 persistent connections,
I'd have a hard time accepting a very small limit like 32 on the
order of randomness. Perhaps 128 (with 256 bits in the mask)  would work
for the near term but even that seems like a poor protical choice.

Instead limit the scope of counter derrived nonce usage to insure
that requests are seen in order:

Require re-authentication by the client for each parallel connection. Each
persistent connection would thus get a unique client ID and hence nonce
In the case of multiple connections for single requests,
the client may use a counter nonce IFF all requests have completed and
the counter-nonce is derrived from the parallel connection with the
most recent Date: header. Or because of the complexity of this approach,
don't support sequencial nonces outside of a single persistent connection.

BUT ... I'm not sure this is an improvement on the suggestion from
someone (JohnF or BenL?) that each sequencial nonce be computed from
the previous nonce by rehashing the prior nonce and shared secret, perhaps
with a counter.

Of course, this all becomes a delightful challenge when a proxy gets in
the middle and doesn't maintain a one-one relationship between client and
server connections. This would raise havoc with order expectations as

Dave Morris
Received on Tuesday, 20 January 1998 18:00:17 UTC

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