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Re: HTTP Bar BoF

From: Adam Roach <adam@estacado.net>
Date: Wed, 12 Jul 2006 17:47:43 +0000
Message-ID: <44B5359C.9090903@estacado.net>
To: Mark Nottingham <mnot@mnot.net>
CC: HTTP Working Group <ietf-http-wg@w3.org>
Three years ago (and periodically since then), I have raised an issue 
around ambiguity relating to the handling of md5-sess in RFC2617.
This is complicated. The explanation of what is wrong takes a lot of 
reading and a *lot* of thought. I'm sorry about that.

I *did* get Scott to understand the problem at one point in time, circa 
2004. There *is* a problem here. The fix is minor. If we're revising the 
document, can we *please* make this unambiguous so that implementors 
have some chance of interoperating?

Unfortunately, I have prior commitments that prevent me from attending 
the BOF.

/a

Mark Nottingham wrote:
>
> It seems like a fair number of people are interested in this, so:
>
> 2) Errata Candidates <http://skrb.org/ietf/http_errata.html>


attached mail follows:



In implementing MD5-sess, we've encountered some
ambiguities with the interaction between MD5 sessions
and nextnonce.

The only description of signaling the end of an
authentication session is in section 3.3. In particular,
it says that an authentication session lasts until the
client receives another WWW-Authenticate header.

Presumably, the session ends at this point because
the nonce has changed. Using this logic, the authentication
session should also terminate if the client receives a
response with an Authentication-Info header containing a
nextnonce parameter.

Termination of a session with nextnonce isn't mentioned
anywhere, though -- so it's not clear whether this was
the intention when the RFC was written. In particular, it's
not clear to me what value changing a nonce within the
same session would provide.

To be specific, when using MD5-sess, the proper client
behavior upon receiving nextnonce is not particularly clear.
To illustrate the ambiguity, I'll demonstrate the exact
issue with a use case.

1) Client sends a request.

2) Server challenges request, with an algorithm of "MD5-Sess".
   We will call the nonce sent in this challenge "nonce-value-1".

3) Client re-sends request, with response to challenge. This response
   includes a cnonce, which we will call "cnonce-value-1". The
   response is calculated as follows:

   KD(H(unq(username-value) ":" unq(realm-value) ":" passwd)
      ":" unq(nonce-value-1) ":" unq(cnonce-value-1),
      unq(nonce-value-1) ":" nc-value ":" unq(cnonce-value-1) ":"
      unq(qop-value) ":" Method ":" digest-uri-value)

4) Server responds to request with a 200 response. This response
   contains a new nonce in a nextnonce parameter. We will call
   this value "nonce-value-2".

5) Later, the client sends a second request. It preemptively includes
   an "Authentication" header with a cnonce that we will call
   "cnonce-value-2".

Here's where the confusion lies. For the highest chance of success,
should the client calculate the response-digest as:

A) KD(H(unq(username-value) ":" unq(realm-value) ":" passwd)
      ":" unq(nonce-value-1) ":" unq(cnonce-value-1),
      unq(nonce-value-2) ":" nc-value ":" unq(cnonce-value-2) ":"
      unq(qop-value) ":" Method ":" digest-uri-value)

or as:

B) KD(H(unq(username-value) ":" unq(realm-value) ":" passwd)
      ":" unq(nonce-value-2) ":" unq(cnonce-value-2),
      unq(nonce-value-2) ":" nc-value ":" unq(cnonce-value-2) ":"
      unq(qop-value) ":" Method ":" digest-uri-value)

The text in the document would support A (since only WWW-Authenticate
ends an authentication session and H(A1) "is calculated only once" per
authentication session), while B seems far more intuitive.

So, what have people here actually implemented?

/a
Received on Wednesday, 12 July 2006 18:34:41 GMT

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