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

[Eric Rescorla <[e-mail]>]

On Thu, Dec 28, 2017 at 9:51 AM, Matt Caswell <[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]>> 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.

-Ekr



> Matt
>
>