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RE: Some comments on Digest Auth

From: Yaron Goland <yarong@microsoft.com>
Date: Mon, 19 Jan 1998 10:40:22 -0800
Message-Id: <3FF8121C9B6DD111812100805F31FC0D0113C654@red-msg-59.dns.microsoft.com>
To: 'Dave Kristol' <dmk@bell-labs.com>
Cc: http-wg@cuckoo.hpl.hp.com
X-Mailing-List: <http-wg@cuckoo.hpl.hp.com> archive/latest/5226
"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." - So Spoke Dave Kristol

ASSUMPTION: Avoiding replay attacks is important enough to most implementers
that either the standard will require or implementers will voluntarily
refuse to accept the same nonce twice.

GOAL OF THIS MESSAGE: To demonstrates that the current digest auth
mechanism, from the point of view of performance in situations where we wish
to prevent replay attacks, is unacceptably sub-optimal.

Let us say, for the sake of argument, that we were willing to rewrite our
entire browser implementation to maximize its performance with the current
digest auth proposal. In that case, because we know a server will not accept
the same nonce twice, we would never pipeline a request that needed auth and
never make simultaneous requests that needed auth. Thus using auth
essentially removes all the performance benefits of using HTTP/1.1.

Now, back in the real world, we are not going to change our entire browser
architecture just to make digest happy. When we download an HTML page with
links in it we open two connections and start making both simultaneous and
pipelined requests, regardless of the need for authentication. This means
that all the requests will have the same nonce. Assuming the server wishes
to protect itself from replay attacks it will only accept one of the
requests and reject all the others. This means that if N requests are made
one is accepted and the other N-1 are rejected. Assuming the server is
itself optimal it will give each of the rejected requests a unique nonce and
the next N-1 requests will be accepted. Thus ONLY adding somewhere between 1
to N-1 additional round trips depending upon how you measure round trip
costs in dual connection pipelined situations. 

I have sent out a proposal which demonstrates how to implement a server such
that it will never accept the same nonce twice and doesn't require any
ordering. Hopefully we can use that post as the basis for further


> -----Original Message-----
> From:	Dave Kristol [SMTP:dmk@bell-labs.com]
> Sent:	Monday, January 19, 1998 6:58 AM
> To:	Yaron Goland
> Cc:	http-wg@cuckoo.hpl.hp.com
> Subject:	Re: Some comments on Digest Auth
> 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 17:21:50 UTC

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