[asterisk-users] is encrypted iax safe and secure?

[Tilghman Lesher]

On Tuesday 05 February 2008 09:22:29 Cavalera Claudio Luigi wrote:
> Hello,
> I'm doing some research concerning iax encryption, I haven't find any
> clients (softphones or hardphones) which implement so I have not tested
> it yet.
>
> There was also this message on asterisk-security mailing list
> http://archives.free.net.ph/message/20070507.101933.222987b2.en.html
> which got no answers and this makes me think that this iax encryption is
> not much interesting for the community.
>
> Anyway, in iax specification there is this statement:
> "Only the data portion of the messages are encoded."
>
> Which are the consequences of this, is it true as stated on
> http://www.voip-info.org/wiki/view/IAX+encryption
> that
> "The calling/called numbers are still passed in the clear over encrypted
> IAX, so you are still vulnerable to traffic analysis."
> ?
>
> If it's true how to deal with this?
> Would you consider media payload encryption enough?
> Maybe it's better to just forget about iax encryption and consider some
> more general approach like using openvpn
> http://www.voip-info.org/wiki/view/IAX_OpenVPN ?
>
> This half-encrypted iax encryption doesn't make much sense to me,
> therefore I think there's probably something I'm
> missing/misunderstanding.

Is it important for you to conceal that a call was made from abc to xyz on
thus-and-such a date?  Or do you merely need to conceal the content of a
call?  You can already do traffic analysis and figure out that a call
occurred, just not what the endpoints are (even if you encrypted the entire
link).  The only way to get around that is to continuously send random garbage 
through the pipe at the same rate and consistency as would occur with a real
IAX2 call.  And the endpoints are only as specific as the systems on either
end choose to make them.  If you used some system of src/dst obfuscation, you
could conceal even that information, though repeated calls to various
destinations could still be paired and correlated.

IAX2 encryption is designed to obscure the same information as is obscured
when you encrypt a call over the PSTN -- the content is protected, but the
existence of such a call is not.  Remember that a potential attacker will
always choose the weakest link, and will probably attack the audio stream
at a different location, if she cannot listen to the IP stream directly (such
as a true wiretap on an analog endpoint or breaking into one of the two
machines involved in the encryption).  The idea is to make the IAX2 link
unattractive as a potential target of wiretapping (whereas before it would
have been the most obvious choice), thus forcing the attacker to choose a
different attack scenario.

-- 
Tilghman