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Re: Some comments on Digest Auth

From: Dave Kristol <dmk@bell-labs.com>
Date: Mon, 19 Jan 1998 09:57:45 -0500
Message-Id: <34C369E8.6F5992E1@bell-labs.com>
To: Yaron Goland <yarong@microsoft.com>
Cc: http-wg@cuckoo.hpl.hp.com
Yaron Goland wrote:
> 
> I wish someone would explain to me how having multiple outstanding attacks
> could lead to degraded security assuming that the multiple nonces do NOT
> have lifetimes greater than that of the previous single nonce. That is, the
> server sends you a nonce and starts a hypothetical count down. Once the
> count down is expired that nonce will not be accepted. My proposal is to
> allow the server, using a separate header, to return multiple nonces.
> However I suspect that the server should use the same count down for ALL the
> previously returned nonces. Use 'em or lose 'em, as it were.
> 
> As for ordering of requests, I'm still not sure how big an issue this is. It
> would be great if some server side implementers could weigh in on the issue.

Here's my two grams' worth. :-)

I'm ill-qualified to address the issue of degraded security, but let me
try to address the server implementation issues.

A nonce can be self-describing.  That is, the server can choose a form
for a nonce that encodes its lifetime.  That's attractive, because it
means the server can avoid having a database of nonces.  The lifetime
can be made arbitrarily long or short, as the server's needs require.

If, instead, the server provides a *list* of nonces in a
WWW-Authenticate header, then it would be obliged to remember what they
were.  Otherwise a rogue client could return a digest for a nonce of its
own choosing, along with the nonce chosen, disregarding the list.  More
particularly, it could replay a previous nonce.  If the nonces from the
list are meant to be single-use, then the server must at least remember
the set of nonces already used, or not yet used, so it can detect
responses for nonces that have been used.

Now consider the proposal where a set of nonces must be received by a
server in the order it sent them.  A browser may open multiple
connections.  On its first connection it gets a base page, a
WWW-Authenticate, and a list of nonces.  It responds with an
Authorization header and opens multiple new connections to retrieve
images for the page.  Which nonce should the browser use on which
connection?  How can it be sure that the server will process the
connections and accompanying requests in the order that it sends them? 
If the server's receive-order is different from the browser's
send-order, the server must reject the out-of-order requests, even
though the nonces are otherwise good.  If "the server" is really a farm
of servers, the connections may be to multiple machines, and the
inter-server communication may affect the apparent sequence of nonce
use.

Conclusions:  requiring a client and server to honor a sequence of
nonces is hard.  Allowing them to pick from a list of nonces, as long as
there's no reuse is easier, but still hard.  I don't see any particular
added value in sending a list of nonces, all of which have independent
(or simultaneous) time-out properties, instead of a single such nonce.

Dave Kristol
Received on Monday, 19 January 1998 07:00:40 EST

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