Re: [TLS] Interaction between cookies and middlebox compat mode

[Eric Rescorla <[e-mail]>]

On Thu, Dec 28, 2017 at 10:02 AM, Matt Caswell <[e-mail]> wrote:

>
>
> On 28/12/17 17:55, Eric Rescorla wrote:
> >
> > On Thu, Dec 28, 2017 at 9:51 AM, Matt Caswell <[e-mail]
> > <mailto:[e-mail]>> wrote:
> >
> >
> >
> >     On 28/12/17 17:42, Eric Rescorla wrote:
> >     >
> >     >
> >     > On Thu, Dec 28, 2017 at 8:12 AM, Matt Caswell <[e-mail]
> <mailto:[e-mail]>
> >     > <mailto:[e-mail] <mailto:[e-mail]>>> wrote:
> >     >
> >     >
> >     >
> >     >     On 28/12/17 12:28, Eric Rescorla wrote:
> >     >     >     I think it would be helpful
> >     >     >     to be more explicit in the text if that is the case,
> >     i.e. identify the
> >     >     >     first point in the handshake and the last point in the
> >     handshake where
> >     >     >     CCS is valid. There probably should also be some words
> >     about how servers
> >     >     >     implementing older TLS versions should handle a CCS that
> >     comes first.
> >     >     >
> >     >     >
> >     >     > I could add those.
> >     >     >
> >     >     >
> >     >     >     However, I'm concerned about the added complexity of
> >     interpreting things
> >     >     >     that way. Suddenly a CCS arriving is no longer handled
> >     by just dropping
> >     >     >     it and forgetting it - you now have to store state about
> >     that and
> >     >     >     remember it later on in the process in other TLS
> >     versions. The CCS
> >     >     >     workaround was supposed to be a simple no-op to
> >     implement and it no
> >     >     >     longer appears that way in this interpretation.
> >     >     >
> >     >     >
> >     >     > Well, it seems like the issue here is you want the client to
> >     send CH1,
> >     >     > CCS, CH2
> >     >     > so we need the server to accept that. Am I missing something?
> >     >
> >     >     The point is a stateless server will not know about CH1 at the
> >     point
> >     >     that it receives CCS.
> >     >
> >     >
> >     > Well, sort of.
> >     >
> >     > Specifically, there are three valid things that a server (whether
> >     stateless
> >     > or stateful) can receive:
> >     >
> >     > - CH1 [I.e. a CH without a cookie]
> >     > - CH2 [i.e., a CH with a cookie]
> >     > - CCS
> >     >
> >     > It should respond to any other message with an alert and abort the
> >     > handshake.
> >     > A stateful server should also tear down the transport connection,
> so
> >     > that subsequent
> >     > messages are considered an error. This obviously isn't an option
> for a
> >     > stateless server,
> >     > so, yes, a stateless server might in principle receive arbitrary
> >     amounts
> >     > of junk
> >     > before CH1 or between CH1 and CH2, and it would still survive,
> >     albeit by
> >     > sending alerts.
> >     >
> >     >
> >     >
> >     >     Actually, as Ilari points out, there could be any
> >     >     junk (including partial records) arriving between CH1 and CH2.
> >     So this
> >     >     feels more like a special case for stateless servers.
> >     >
> >     >     In other words I would prefer to say that a CCS that arrives
> >     first is
> >     >     not allowed. That simplifies the general case and requires no
> >     special
> >     >     coding for servers implementing older versions of TLS.
> >     >
> >     >
> >     > This issue only seems to arise for people who are both doing TLS
> >     1.3 and
> >     > TLS 1.2 *and* doing stateless implementations, which is kind of an
> odd
> >     > configuration because a number of the conditions in TLS 1.3 that
> >     involve
> >     > HRR (and thus can be stateless). It doesn't arise for QUIC
> (because no
> >     > TLS 1.2) and mostly doesn't arise for DTLS (if you reject all
> kinds of
> >     > junk).  Or am I wrong?
> >
> >     Correct, although technically the wording of draft-22 (in your
> >     interpretation) *requires* that a server receiving a CCS first MUST
> >     ignore it - even though that should never happen except in the weird
> >     scenario above. That is why I prefer to say that a CCS arriving
> first is
> >     always an error for the general case.
> >
> >
> > Well, you can receive a CCS first any time you're stateless. What's
> unusual
> > is having to subsequently reject it if you are stateless and *then*
> > negotiate
> > 1.2. My point is that this doesn't seem like a very big hardship for the
> > reasons
> > above.
>
> I must be missing your point. According to the spec as it stands even
> with a stateful server I MUST ignore a CCS that comes first. Since this
> is a stateful server it may end up negotiating TLSv1.2 - which requires
> us to abort the handshake if the CCS comes first. No sensible
> implementation will ever send a CCS first in this scenario, so why am I
> required by the spec to ignore it and implement the extra complexity in
> TLSv1.2 handling?
>
> In reality I wouldn't bother to implement this which would make me
> technically non-compliant. I would prefer it if the wording were fixed
> to not require this.
>

OK, I understand your point now, I think it's fine to reject this case as
long as
you properly handle things in the stateless case. If you want to submit a
PR,
I will take a look.

-Ekr


> Matt
>
>
>