[Standards-JIG] JEP-0170: dialback + TLS + SASL

Peter Saint-Andre stpeter at jabber.org
Wed Jan 18 17:35:57 UTC 2006


Philipp Hancke wrote:
> Peter Saint-Andre typeth:
>> JEP-0170 is, so far, silent on the order of TLS, SASL, and dialback 
>> for server-to-server communications. RFC 3920 basically says "dialback 
>> is a legacy protocol, use TLS then SASL" but I wonder if the following 
>> order makes sense:
>>
>> 1. Dialback
>> 2. TLS
> *nitpick*
> Section 5.1 rules 9&10 say you must discard any knowledge obtained
> before TLS.
> Hence there is no information gain from dialback.

I never claimed there was information gained from dialback that needed 
to be discarded in TLS. I'm saying only that you could use it to 
pre-screen connections (if their DNS setup is wrong, don't even bother 
with TLS+SASL).

>> 3. SASL
> Using what mechanism?
> 
> If you want to use dialback+EXTERNAL, the order would have to be
> 1. Dialback
> 2. SASL (EXTERNAL)
> 3. TLS

Why would it "have to be"?

RFC 3920 says you MUST do TLS before SASL, so your order violates that 
and I doubt the IESG would look kindly on a change (and no, don't ask me 
about the politics between the TLS folks and the SASL folks, I've never 
claimed to understand it).

> Yet another approach might be to use dialback once to negotiate a
> shared secret which can subsequently be used for SASL DIGEST-MD5...
> Of course this would not result in a authentication any stronger than
> dialback and only increases complexity.

-1 :-)

> If you really want to follow the order defined in the RFC,
> you could in theory do something like
> 1. TLS
> 2. SASL EXTERNAL modified to carry dialback keys in <auth/>
>                 (interpreting <auth/> as dialback step 4)
> but that requires a ambiguous usage of EXTERNAL,
> some tweaks to dialback (using <success/> in step 10)
> and is quite weird.

It would probably require a new SASL mechanism.

>  > For now I'm speaking practically -- I'm not talking about what will be
>  > acceptable in rfc3920bis, just what works on the network. We'll deal
>  > with the IETF implications later. :-)
> starttls+dialback is widely on the network for months now...
> You're probably to late to stop the juggernaut :-)

Are people using self-signed certs for this? If so, how hard is it to 
just do TLS + SASL EXTERNAL and be done with it? I see no reason for 
dialback if you've already done TLS, but maybe I'm missing something.

Peter


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