[rtcweb] Identity and PSTN gateways

[Harald Alvestrand <[e-mail]>]

One thing that has come up repeatedly in the discussion is the claim 
that "you can't have a verified identity when you talk to someone via a 
telephone gateway" (and therefore <insert your favourite security 
mechanism here> is not needed / not an added benefit / other claim).

I think this is a fallacy.

Sure, as people have commented numerous times, telephone numbers are 
identities; they're being used as such every time someone prints them on 
a business card or a billboard.

When you're connecting via a gateway to the PSTN, the gateway operator 
gives you a guarantee that you're being connected to the right person; 
that's what gateways are for.

This makes for a fairly simple mapping to the "identity / identity 
provider" model we've been bandying about for the "full-blown" IdP / 
endpoint case:

The identity is the telephone number.
The identity provider (one of many possible ones for the number) is the 
gateway operator.

Thus - if you call a telephone number via a gateway, you would perform a 
DTLS key exchange with the gateway, and an identity verification 
exchange with the gateway operator; you would then guarantee that the 
gateway operator vouches for this being a legitimate gateway function 
that you can reach for that number.

That's just about the best guarantee you can get when talking to the 
telephone system. But if we're using the IdP + DTLS-SRTP version, the 
exchange guarantees you that:
a) nobody is listening in between you and the gateway (even if they 
snooped your signalling)
b) the gateway operator vouches for the gateway being the right gateway 
to reach that number

Seems like a little bit better than what you get with SDES. Only a little.

                        Harald