Re: [TLS] draft-ietf-tls-tls13-26 is vulnerable to externally set PSK identity enumeration

[Benjamin Kaduk <[e-mail]>]

On Fri, Mar 16, 2018 at 09:11:32AM -0400, Christian Huitema wrote:
> 
> 
> On 3/15/2018 5:51 PM, Benjamin Kaduk wrote:
> > On Thu, Mar 15, 2018 at 12:25:38PM +0100, Hubert Kario wrote:
> > ...
> >> we do not have a reliable mechanism of differentiating between external and 
> >> resumption PSKs while parsing Client Hello
> > Well, a valid external PSK (identity) the server will of course
> > recognize, and we have a SHOULD-level requirement that the
> > obfuscated_ticket_age is zero for external PSKs.  I haven't gotten
> > to think through whether there is still potential for information
> > leakage about external PSK identities, but it seems like there would
> > not be, provided that the server prefers resumption to external-PSK
> > full handshakes.
> >
> 
> I am concerned with the privacy issues linked to these "external PSK
> identities". If a system is set so that clients use static PSK
> identities, then the identity is an identifier and the client's
> movements and connections can be tracked. I don't think privacy is
> improved if we make it easy to differentiate external identities from
> resumption tickets.

Oh, of course, the privacy considerations of the current external
PSK scheme are terrible!  This follows naturally from external PSKs
having not really been a first-class citizen while we were designing
things; we stuffed resumption PSKs together with external PSKs for
the convenience of having them use the same binder construct and
only needing to have one extension at the end of the ClientHello.
Resumption flows get single-use tickets for privacy preservation,
and external PSKs get infinite use and a gigantic correlation
channel.


> If you want to use PSK with some level of privacy, you might adopt a
> different setup. For example, servers could provision the clients with a
> set of single-use external PSK identities. But then, that looks a lot
> like resumption. Or, clients could generate single-use external PSK
> identities by encrypting their long term identity and a nonce with the
> public key of the server, a design which of course has its own set of
> issues.
> 

But, as you note, this is something of an open problem for how to do
well, and this document is already approved by the IESG.  (It's also
not entirely clear that the TLS WG would be the best place to design
this sort of scheme, though of course it could choose to do so.)

So to me the open question is whether we consider enumeration of
external PSK identifiers to be something we can address reasonably
at this stage of the document's lifecycle at all.  (I also note that
the presence of a CVE number for a similar issue does not
necessarily mean anything -- CVE assignments can occur for
situations with no actual security impact and/or against the wishes
of the software authors.)  I don't think anyone has proposed a
minimal change that would close the enumeration channel, and given
that external PSKs already have bad privacy properties, it seems
like we may just have to accept this state of affairs for this
document.


Hubert also says:

% so there's no reliable way to say that, yes, this identity is or is not a
% resumption ticket, if I can't decrypt it

Mostly.  External PSKs are established out of band, and that
provisioning process *could* include knowledge that the
obfuscated_ticket_age would always be zero when those PSKs are in
use, and that would be reliable for those specific parties.

It's probably also worth considering the two cases for server
behavior when presented with a PSK id that is neither a known
external PSK nor a known resumption ticket -- the server could
either treat it as an unknown external PSK id or as a resumption
ticket that fails to decrypt.  The latter case fails because the
attacker can try candidate external identities and the server falls
back to a full handshake unless the PSK ID is good.  (Well, maybe
the server rejects PSK IDs that are shorter than a ticket would be.)
The first case is not really usable since it would lead to spurious
triggering of the proposed "at most one external PSK" check.

So, in addition to the "we provision external PSKs only when we know
that obfuscated_ticket_age will be zero", deployments could also
agree that external PSK ids are shorter than a given length and
resumption PSKs are larger, which would again provide a reliable
differentiator between resumption and external.

The privacy issues remain terrible, of course.

% I'd really prefer we exhaust other possibilities before sacrificing support
% for multiple external PSK. With TLS 1.2 we had ticket_hint to guide PSK
% selection, now we're left with just server IP or hostname.

I think that "do nothing and accept external PSK enumeration as a
risk" is more likely than sacrificing support for multiple external
PSKs, personally.

-Benjamin