On Tuesday, 20 March 2018 22:21:06 CET Eric Rescorla wrote: > On Tue, Mar 20, 2018 at 7:42 PM, Hubert Kario <hka...@redhat.com> wrote: > > On Monday, 19 March 2018 14:38:05 CET Eric Rescorla wrote: > > > On Mon, Mar 19, 2018 at 1:33 PM, Nikos Mavrogiannopoulos < > > > > n...@redhat.com> > > > > > wrote: > > > > On Fri, 2018-03-16 at 14:45 -0500, Benjamin Kaduk wrote: > > > > > 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. > > > > > > > > I agree. > > > > > > > > > > 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. > > > > > > > > That's a good general remark, but not really the case for the > > > > vulnerabilities that Hubert pointed out. > > > > > > > > > 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. > > > > > > > > I believe that this can happen in an interoperable way if the protocol > > > > mandates it (which is not the case now). These specific parties may > > > > use > > > > software from different vendors which may use different conventions if > > > > the protocol is not clear enough. > > > > > > > > > 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. > > > > > > > > That cannot easily happen. I can have multiple servers answering to > > > > the > > > > same hostname each using a different implementation. Any conventions > > > > used in one implementation would not apply to another. > > > > > > > > > % 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. > > > > > > > > The problem is that you personally are not affected by that risk and I > > > > guess that makes it easy for you to accept it. TLS1.2 with PSK did > > > > explicitly prevent enumeration (by asking implementations to proceed > > > > to > > > > handshake even with unknown usernames, and making up a key), meaning > > > > that this is a risk that the designers of PSK (external) intentionally > > > > ruled out. Going that path, it would be a step back in PSK security > > > > for > > > > TLS1.3. > > > > > > Nikos, > > > > > > Just as a clarification, I believe it's possible for a PSK-only > > > implementation to > > > avoid this attack by behaving uniformly for "unknown ID" and "invalid > > > binder" > > > > supporting both external PSK authentication and certificate based > > authentication for a single IoT gateway would be quite expected, I'd say > > > > so just because PSK-only implementations are not vulnerable, I don't think > > we > > can expect all servers that deploy external PSKs will also disable > > certificate > > based authentication (not to mention that it's rather unlikely that users > > will > > expect that adding certificate will suddenly make their server vulnerable) > > I'm not making an argument, I'm merely stating what I believe to be the > facts.
ok, then I agree with those facts :) -- Regards, Hubert Kario Senior Quality Engineer, QE BaseOS Security team Web: www.cz.redhat.com Red Hat Czech s.r.o., Purkyňova 115, 612 00 Brno, Czech Republic
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