Subject: [Sip] S/MIME, Authentication and Integrity

From: "James Undery" <[email-masked]>
Date: Mon, 28 Jan 2002 14:51:50 -0000

Hi,

Looking over section 21.4.1 I am having trouble seeing how any of the
claims can be justified against a MiTM attack. Without a
challange/credentials framework S/MIME can't offer Authentication or
Integrity e.g. a MiTM just removes the S/MIME protection (and optionally
adds it's own).

James

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---

From: "Peterson, Jon" <[email-masked]>
Date: Mon, 28 Jan 2002 10:35:18 -0600

The attack is mentioned in 21.2, and discussed relatively extensively in
22.4.2.

The basic jist of the text is that while the use of the S/MIME mechanism
admits of the startup MiTM attack, it nevertheless improves the security of
SIP. We contend that this isn't entirely dissimilar to the degree that SSH
improves telnet security.

Jon Peterson
NeuStar, Inc.

> -----Original Message-----
> From: James Undery [mailto:[email-masked]]
> Sent: Monday, January 28, 2002 6:52 AM
> To: SIP (E-mail)
> Subject: [Sip] S/MIME, Authentication and Integrity
> 
> 
> Hi,
>

---

From: "James Undery" <[email-masked]>
Date: Mon, 28 Jan 2002 17:03:17 -0000

comments inline.

> From: Peterson, Jon [mailto:[email-masked]]

> The attack is mentioned in 21.2, and discussed relatively 

The replacement of a certificate is mentioned, the removal of
certificates isn't.

> improves telnet security.

I'd agree it improves security, I'd just disagree it was anywhere near
the amount SSH improves telnet. The reason for this is if I telnet to a
machine I am much more likely to have obtained a reliable
fingerprint/copy of the certificate for the machine. The other reason
and the one that definately isn't addressed is the lack of framework for
the key exchange, which SSH provides.

James

---

From: "Peterson, Jon" <[email-masked]>
Date: Mon, 28 Jan 2002 12:21:12 -0600

On removal of certificates, yes, I totally agree that's an omission. I
thought we had some text about how to handle this (and a couple of similar
cases, like what happens when S/MIME is unsupported by a recipient) but now
I can't find it anywhere in the text. Hmm, been doing too much cut and paste
lately to be sure. Well, it'll be added in the build next Monday. Removal is
I think pretty easy to detect (easier that replacement), and it's fairly
obvious what you should do in that case.

The relatively 'likeliness' of having obtained a reliable fingerprint is
debatable; there are certainly some people who use SSH that make sure to
acquire an original fingerprint from the same LAN as the host in question
before they go on a business trip or what have you. In some SIP
architectures this certainty won't be achievable - but then again, in some
it will be. Clearly there are all sorts of customary SIP intermediaries,
such as proxy servers, that potentially have carte blanche to act as MiTMs
which are not a concern in SSH. So I don't entirely disagree with you - but,
that much said, I think it's pretty clear why we draw the analogy. Perhaps
the most important similarity between the two is that the oversight of a
human is required in trust establishment.

Jon Peterson
NeuStar, Inc.

> Sent: Monday, January 28, 2002 9:03 AM
> To: Peterson, Jon; SIP (E-mail)
> Subject: RE: [Sip] S/MIME, Authentication and Integrity
> 
> 
> comments inline.
>