Subject: RE: [Sip] verifying identity of sender

From: "Peterson, Jon" <[email-masked]>
Date: Wed, 6 Oct 2004 14:27:04 -0400

Juha,

Fundamentally, in the only context in which certificates are widely used
today, a certificate is a contract that binds a set of keying material to a
domain name. What we need here for sip-identity is a way of binding a set of
keying material to a domain name. There are, of course, alternatives to
certificates, like putting keying material in the DNS, and, I guess, relying
on the DNS to "make a few SIP requests", as you suggest. However, the
insecurity of the DNS at the moment makes these sorts of approaches
problematic. Also, I think the security community is deeply suspicious of
attempts to reinvent the certificate construct in some other way. This is
what certificates are for: if we need this property, we should use
certificates.

Perhaps even more fundamentally, without TLS, sip-identity-03 would not work
at all. TLS assumes that your SIP proxies have a domain certificate. People
must be able to verify this certificate. TLS is mandatory-to-implement for
SIP proxy servers according to RFC3261. In other words, I'm not requiring
anything that isn't required of proxy servers by RFC3261 already. Finally, I
don't really think certificates are prohibitively expensive, and so on.

> by the way, i don't think that this requirement holds true in the
> proposed solution either, since the uas must acquire the certificate of
> the signing domain, which may mean network lookup.

Yes, it may mean network lookup, but doesn't necessarily. There are
certainly other ways for the UAS to acquire certificates. This system does
not require a network lookup to work because certificates are
self-verifying. Any system that relies on a network lookup is troublesome,
generally, because caching credentials because very problematic. That much
said, probably I should probably make the phrasing of that requirement more
precise.

Jon Peterson
NeuStar, Inc.

> -----Original Message-----
> From: Juha Heinanen [mailto:[email-masked]]
> Sent: Wednesday, October 06, 2004 1:30 AM
> To: [email-masked]
> Subject: [Sip] verifying identity of sender
> 
> 
> i read draft-ietf-sip-identity-03 and noticed that in order to use it,
> the domains must still have private/public certificates that are "valid
> back to a trusted CA".
> 
> my domain tutpro.com has not purchased such certificate and i would bet
> that the same holds true for a VERY large percent of all internet
> domains.  as long as this is true, the proposed identity verification
> solution may work in theory but not is practice.
> 
> when thinking about this, i started to wonder if it would be possible
> for the UAS to verify the identity of the UAC from the domain via a few
> sip requests, i.e., by breaking this requirement:
> 
>       User agents that receive identity assurances must be able to
>       validate these assurances without performing any network lookup.
> 
> by the way, i don't think that this requirement holds true in the
> proposed solution either, since the uas must acquire the certificate of
> the signing domain, which may mean network lookup.
> 
> so how about a certificate free system where the UAS authenticates the
> request with the domain by sending to the proxy of the domain a
> verification request?  when the proxy receives the verification request
> it challenges the UAC and then replies to the UAS.  comments?
> 
> -- juha
> 
> _______________________________________________
> Sip mailing list  https://www1.ietf.org/mailman/listinfo/sip
> This list is for NEW development of the core SIP Protocol
> Use [email-masked] for questions on current sip
> Use [email-masked] for new developments on the application of sip
> 

_______________________________________________
Sip mailing list  https://www1.ietf.org/mailman/listinfo/sip
This list is for NEW development of the core SIP Protocol
Use [email-masked] for questions on current sip
Use [email-masked] for new developments on the application of sip

---

From: Juha Heinanen <[email-masked]>
Date: Wed, 6 Oct 2004 21:49:23 +0300

Peterson, Jon writes:

 > However, the

i didn't try to propose any new certificate construct.  uas would just
send a verification request to the proxy of the domain and would get
back a reply.

 > don't really think certificates are prohibitively expensive, and so

implementation of tls in the proxy is one thing and getting a
certificate for it that is verifiable to a known CA is another thing.  i
can imagine that large operators can do it, but not small companies and
private people who use sip more or less in peer-to-peer mode.

-- juha

---

From: Dean Willis <[email-masked]>
Date: Wed, 06 Oct 2004 14:48:44 -0500

>said, probably I should probably make the phrasing of that requirement more
Checking certificate revocation lists, which is an important part of 
TLS, does seem to require network lookup.

--
Dean