Subject: RE: [Sip-security] A minimalist security framework for SIP

From: "Sanjoy Sen" <[email-masked]>
Date: Mon, 26 Nov 2001 13:49:58 -0600

All,

Thanks for bringing this up, Jari! As Henning has rightly pointed out in his
draft, several types of security solutions are likely to exist and will be
applicable to different markets/terminals. Currently, Digest is the most
widespread authentication framework available and I strongly favor an
approach which will minimally enhance the existing framework to satisfy the
first two of the three key requirements - authentication, integrity and
confidentiality. The benefits are code reuse and a cheap, lightweight
solution.

There has been three drafts which discusses SIP/Digest enhancements:
1) Jonathan's "pnonce" draft - needs no standard's change
2) Undery's "Access Authentication" draft  - needs standard's enhancements
3) Sen/Valerius/Torvinens' "one-hop Digest" draft - need very minimal
enhancement and perhaps some behavior enhancements on the part of the
Proxy/UA
The third  draft has been recently published at 
http://search.ietf.org/internet-drafts/draft-sen-sipping-onehop-digest-00.tx
t

This draft attempts to address the need for integrity protect entire SIP
messages over a single hop between the UA and a Proxy as required by certain
network operators such as the third-generation mobile network operators. The
draft also addresses other issues not addressed by the other two drafts,
such as authentication of messages forwarded by the Proxy etc. Some of the
issues discussed in this draft fits well with the discussion in Section 8
(In-session authentication for SIP) of Henning's SIP security draft.
The draft discusses some potential solutions (without bending the rules of
SIP/HTTP) and is intended to be a "request for comments". 

The issues identified are:
1) UAC-Proxy message authentication: It is not clear from RFC2617/bis05 how
the response forwarded upstream by the Proxy towards the UAC will be
authenticated. The authors propose using the "Authentication-Info" header
for this purpose. 
2) UAS-Proxy message authentication: Bis05 does not allow the Proxy to add
the Proxy-Authorization header in a request unless it is resubmitted.
Jonathan had suggested in the list that "resubmission" can be performed by
changing the topmost Via branch id. This would need some behavior
enhancements in the Proxy/UAS. Also, Bis05 does not mention the use of
Proxy-Authentication-Info which is used by HTTP/Digest for authenticating
responses from the UAS to the Proxy.
3) In order to avoid excess round-trip, a Proxy should be able to piggyback
its challenge in a 401 or 407 response that it forwards upstream to the UAC.
Proxies today are allowed to merge multiple challenges in case of forking. 
4) Integrity protect complete (or complete to the extent possible) messages:
The undery draft makes this a dynamic list of headers. Jonathan proposes
predictive nonce computed on these headers. The sen draft proposes a new
qop-options parameter - auth-extd-int - to indicate to the UA/Proxy that a
static, pre-negototiated protection level is needed. 

For some more details on these issues and example call flows, please refer
to the draft.

The intent of this mail is to stimulate some discussions on these issues
and, if possible, resolve them before the IETF meeting.

Regards,
Sanjoy

> -----Original Message-----
> From: Jari Arkko [mailto:[email-masked]]
> Sent: Sunday, November 25, 2001 1:18 AM
> To: Henning Schulzrinne; Charles Kalmanek
> Cc: sip-security
> Subject: Re: [Sip-security] A minimalist security framework for SIP
> 
> 
> > Re: lack of proxy authentication.  At a minimum, I think
> > the draft should state the assumptions under which this
> > provides acceptable security.  Pre-configured proxy address
> > in clients?  Only for use behind firewalls?  Spoofing a
> > proxy is pretty easy on a LAN.
> 
> The current Digest proxy mechanisms are pretty useless,
> imho. It only works one way, it doesn't protect all of the messages
> even where it could, it is hard to protect responses, etc. So
> we need some work here. My personal view of what should
> get is a two-level solution where the first level is mandatory
> and the second one is optional for those who have the CPU.
> The first level should provide at least integrity and replay
> protection, and operate based on symmetric crypto (e.g.
> enhanced digest). The second level should provide in
> addition operation in a N x N networks using asymmetric
> crypto, and confidentiality (e.g. S/MIME). You could perhaps
> compare these levels to having manually keyed IPsec, and
> IPsec with certs. (Of course, IPsec wouldn't cut it because we
> need to go through proxies.)
> 
> Question: does anyone know if S/MIME gives replay
> protection? Does everyone agree that we need replay
> protection? I would assume so, unless we want to allow
> attackers to replay a dangerous or expensive INVITE.
> 
> As for making Digest better, the following may help (though I
> think it should be extended to cover end-to-end as well, and
> perhaps merged with the undery work):
> 
> http://search.ietf.org/internet-drafts/draft-sen-sipping-oneho
p-digest-00.txt

Jari

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---

From: "James Undery" <[email-masked]>
Date: Tue, 27 Nov 2001 12:25:48 -0000

Hi,

Firstly I do intend to address Proxy-Authenticate (although this harder than
it appears at first due to Proxy-Authentication-Info), secondly I'll
probably be making it an extension of digest and thirdly I'll include
"step-down attack protection" (without the additional messages of the Sen
draft).

The gist of these changes will be the draft will require no extenstions to
SIP (although Proxy-Authentication-Info type behaviour will require some
modification for the server to client authentication). Implementations aware
of the draft will be able to spot step-down attack in the challange (via
pnonces that will still be unguessable and potentially statelessly generated
and validated by the server).

Currently I don't have the time to publish this draft, although expect to
release it early January.

James

-----Original Message-----
From: [email-masked] [mailto:[email-masked]]On
Behalf Of Sanjoy Sen
Sent: 26 November 2001 19:50
To: 'Jari Arkko'; Henning Schulzrinne; Charles Kalmanek
Cc: sip-security; Sanjoy Sen
Subject: RE: [Sip-security] A minimalist security framework for SIP

All,

p-digest-00.txt
Jari