Re: [Sip] RE: Identity after reinvite

[Paul Kyzivat <[e-mail]>]

Jon,

One of your key points below seems to be to resist tying any of these 
remedies to the identity draft itself. I have no problem with that - 
despite the remaining problems the identity feature does add value and 
need not wait for solution of the other problems. I didn't intend to 
imply otherwise.

More comments below.

	Paul

Peterson, Jon wrote:
>>>The problem is that, due to retargeting, the URI in the To field of the 
>>>INVITE may not identify the user that was eventually reached. In RFC 
>>>3261, requests from the called party to the caller carry the value from 
>>>the original To field in the From. As a result, the requests will have a
> 
>>>From field value which does not actually correspond to the originator of
>>>the reuqest.
>>
>>Very good point!
> 
> This is a more significant problem with mid-dialog requests and the request
> identity system, agreed.
> 
>> > The identity service will then reject these requests.
>>
>>Hmm. Will the request be rejected, or will it just not get a 
>>authentication that it is valid? I suppose that is local policy. It may 
>>be necessary to permit this, without authentication, just to avoid 
>>breaking these common scenarios.
> 
> Yes, this is the crux of the matter. The text in Section 6 of
> sip-identity-03 says the following:
> 
>    If the identity field contains a SIP or SIPS URI,
>    the authentication service MUST extract the hostname portion of the
>    identity field and compare it to the domain(s) for which it is
>    responsible...
>    If the authentication service is not responsible for the identity in
>    question, it MAY handle the request normally, but it MUST NOT add an
>    Identity header; see below for more information on authentication
>    service handling of an existing Identity header.
> 
> So it isn't that the auth service will reject the request; it MAY just
> forward the request normally. What will happen in that instance? Well, if
> the auth service role is instantiated by a proxy server, it will forward the
> request in accordance with the URI without adding an Identity header; it
> won't challenge the request, or what have you.

Good. You confirm what I was assuming.

>>>The solution, which rfc3261 warns of, is that a UA has to put its actual
>>>identity in the From field URI, not what it saw in the To field of the 
>>>original request.
>>
>>Yes, this would be a good solution, if we can sort out the compatibility 
>>issues getting to it.
> 
> I am extremely wary of predicating request identity on solving the
> connected-party problem, in part because I believe doing so will dictate a
> direction for the response identity problem. If we go down this path here
> (changing the From header field value), we will be forced to do the same for
> response identity. I think this is the wrong approach, as sip-identity-02
> suggested; I think we are sacrificing certain valuable security properties
> for the long term if we go down this path.

I agree the basic request identity problem is separable from the others. 
But it does seem that connected-party, response-identity, and reverse 
direction requests in dialog are all entangled, and that changing 
to/from may be a single solution good for all of them.

> I have a number of arguments about this, but here are just three:
> 
> - Identity will succeed in the marketplace if it can be used
> opportunistically. Requiring UAs to change the From header field in this
> fashion forces them to become identity-aware, which is an impediment to
> adoption. Basically, we force UAs to be either backwards-compliant with
> RFC2543 or compliant with Identity. Tough choice, especially given that the
> UA might not be aware of its need to be either of those things.

Agree.

> - I also question whether or not the callee UA will even know which identity
> it needs to stick in the From header of requests in the backwards direction:
> after all, if the UA is registered under multiple AoRs, what exactly about
> the dialog-forming request will tell it definitively which identity it is
> supposed to use? While in some cases, this could be inferred from the
> Request-URI of the dialog-forming requests, it won't be sufficient in all
> cases, I think.  The previous hop (last proxy server to handle the request)
> might also be a hint. Now, it could be that there's a good answer to this
> question, but if so, we need to know that answer.

There are many reasons why a UA with multiple registrations needs to 
know which one was the source of an incoming call. And the UA can easily 
achieve this by using distinct contacts in the registrations. I don't 
think we need to lose sleep over those that choose not to do so.

> - Retargeting is inherently insecure, and no amount of wiggling with the
> sip-identity mechanism will repair this. If Alice sent a request to Bob, and
> a new request in the backwards direction comes from Edgar, I think there is
> -no- circumstance in which Alice should not view this as a potential
> security risk, regardless of the presence or absence of an Identity header,
> SIPS, or what have you. Trying to explain this aware flies in the face of
> common sense, and will be incomprehensible to actual users who will need to
> make security decisions based on the evidence collected from the protocol by
> their user agent.

Of course it is a potential security risk, which is why it is good to 
report that it happened. (A good UA will hopefully note that the 
connected party is not the called party and make a big fuss at its UI.) 
But it is still good to know who in fact you ended up talking to.

This is however not in any way a surprising event. It happens all the 
time for a variety of reasons. Usually it is detected because the voice 
you hear isn't the one you expected to hear. The fact that we can do 
better than that is itself of value. But I don't think we need to 
deprecate the call any further. While it would be good to know more 
about why the retargetting occured, in most cases people will do fine 
without that.

>>>This will break all compatibility with rfc2543, but
>>>will work with other rfc3261 compliant endpoints, since the URI is no 
>>>longer used for dialog identification.
>>
>>If we were going to switch to this, then it could begin with the first 
>>response to the invite. That would be the first step to really solving 
>>the connected-partiy-id problem.
> 
> Not just 'could begin' but 'should begin'. If the identity you see in the
> response to the INVITE does not equal the identity you see in requests in
> the backwards direction, I believe that is extremely problematic. Yes, this
> is what I mean by solving the response identity problem.

The 'could' was a response to Jonathan, who didn't seem to have that in 
mind.

Re having the identity in a subsequent request differ from the one in 
the response to the invite: I'm not sure it is so problematic. The 
identity can change in mid call, and that will then probably first 
manifest itself in the first message sent following the identity change. 
This could happen if one party is a B2BUA or gateway.

Perhaps it would be more problematic if the change was first seen in 
some random message (e.g. INFO or OPTIONS) rather than in a message 
intended to change dialog state (e.g. INVITE or UPDATE). But it is only 
problematic if we enact some rule that says changes in identity must be 
formally announced.

>>>Doesn't sips prevent that? With sips, you won't be able to identify the 
>>>called party from the 200 OK, but it will guarantee that no one except 
>>>the party you reach can terminate the call.
>>
>>Well, if there are multiple hops that isn't proof against every possible 
>>attack. But it certainly makes it harder. And the kinds of attacks that 
>>would work here could probably just as well compromise the 
>>authentication server itself.
> 
> Don't forget, for example, that SIPS does not apply after the domain
> indicated by the Request-URI of the request, and so on. SIPS is not a
> powerful mechanism. Until we all acknowledge that we were mistaken in our
> formulation in RFC3261, and that SIPS, when present, signifies unambiguously
> that TLS should be used end-to-end, I do not think SIPS can be said to solve
> any of our problems. Once we have the connect-reuse mechanism nailed down
> firmly, I think it will be possible to shift to that understanding of SIPS,
> if we want to.
> 
>>>Indeed, it occurs to me that one can "simulate" response identity by 
>>>having the called party send an UPDATE or nearly any other request in 
>>>the reverse direction immediately upon receipt of the ACK. If you 
>>>structure the From field of that request as I have proposed above, the 
>>>net result provides a form of response identity - it tells you who was 
>>>reached. This doesnt tell you *why* it was reached (as redirection 
>>>would), but it seems valuable nonetheless.
>>
>>As I mention above, if the To were changed in responses, then the caller 
>>would know right away, without the extra request. But it wouldn't be 
>>signed. However then the way forward would be clear - have a callee 
>>suthentiation server sign the To header of responses.
> 
> Yet more justification for the idea that this solution must share its core
> with our longer-term response identity mechanism.
> 
>>>>There is a very important distinction in sophistication between an
>>>>attacker who can merely formulate a plausible dialog-forming request
>>>>with an
>>>>inappropriate From header field versus an attacker who can capture
>>>>dialog-forming requests and improvise appropriate responses (with
>>>>correct
>>>>tags, correct Call-ID/CSeq, etc).
>>>
>>>sips, of course, prevents this fully.
> 
> SIPS would be sufficient if it were not crippled by restrictions about the
> final administrative domain, and the ambiguity about finality in the context
> of retargeting.
> 
> Jon Peterson
> NeuStar, Inc.


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