Re: [CHANNEL-BINDING] [sasl] Updates to draft-altman-tls-channel-bindings (PLEASE REVIEW)

Dave Cridland <dave@cridland.net> Thu, 18 March 2010 13:45 UTC

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Date: Thu, 18 Mar 2010 13:45:13 +0000
From: Dave Cridland <dave@cridland.net>
To: "Pasi.Eronen@nokia.com" <Pasi.Eronen@nokia.com>, Nicolas Williams <Nicolas.Williams@sun.com>, "channel-binding@ietf.org" <channel-binding@ietf.org>, SASL Working Group <sasl@ietf.org>, "tls@ietf.org" <tls@ietf.org>
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Subject: Re: [CHANNEL-BINDING] [sasl] Updates to draft-altman-tls-channel-bindings (PLEASE REVIEW)
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On Thu Mar 18 11:49:11 2010, Pasi.Eronen@nokia.com wrote:
> 
> A question for SASL: both GS2 and SCRAM are currently in
> RFC Editor AUTH48, and they have a normative reference
> to the IANA registration page for 'tls-unique'.
> 
> Since it appears we're about to change that registration
> quite radically, what's the impact on GS2/SCRAM? Have GS2
> and/or SCRAM implementors implemented the 'tls-unique'
> binding?
> 
> 
This SCRAM implementor did, yes. And I followed the specification.  
Aren't I naïve?


> And should we publish the RFCs at this time, if we know one
> of the normative references is, well, unstable (likely to
> change in non-backward-compatible way soon)?
> 
> Best regards,
> Pasi
> 
> > -----Original Message-----
> > From: sasl-bounces@ietf.org [mailto:sasl-bounces@ietf.org] On  
> Behalf Of
> > ext Nicolas Williams
> > Sent: 18 March, 2010 01:15
> > To: channel-binding@ietf.org; sasl@ietf.org; tls@ietf.org
> > Subject: [sasl] Updates to draft-altman-tls-channel-bindings  
> (PLEASE
> > REVIEW)
> >
> > Now that TLS re-negotiation security (RFC5746) is done we need to
> > finish
> > draft-altman-tls-channel-bindings.
> >
> > It's become clear that MSFT implemented and deployed something  
> slightly
> > different than the 'tls-unique' binding description that had been
> > registered.  They seem to be the only implementors to date; their
> > implementation is secure, therefore I propose that we simply  
> modify the
> > description of 'tls-unique' and be done.
> >
> > The differences are:
> >
> >  - it's not always the client's Finished message, but the _first_
> >    Finished message in the relevant handshake (in a session  
> resumption
> >    handshake the server sends its Finished message first);
> >

This is quite annoying. It's harder for me (personally) to implement,  
given the APIs I'm working with. Moreover, since it changes with a  
(relatively) unusual feature of TLS, I suspect people will get this  
wrong - most uses of TLS in, for example, XMPP or IMAP do not appear  
to use session resumption much.


> >  - the Finished message is taken from the latest/inner-most TLS
> >    handshake, rather than the outer-most one
> >

This unnerves me. I'd prefer to use only the first negotiation from  
each side, since that effectively means that both ends know which one  
that was. Moreover, in principle a negotiation could occur during the  
SASL exchange, which would then invalidate the channel binding.

Dave.
-- 
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