Re: [TLS] three ECHO issues

[Stephen Farrell <[e-mail]>]

Hiya,

On 08/03/2020 16:07, Christopher Wood wrote:
> Thanks for raising these issues! Please see inline below.
> 
> On 8 Mar 2020, at 8:18, Stephen Farrell wrote:
> 
>> Hiya,
>> 
>> Thanks for the new ECHO PR. [1] I think this is the right direction
>> but I have three issues with how it's done in the PR right now that
>> I think would benefit from list discussion before a new I-D is
>> produced or the PR is merged.
>> 
>> 1) Padding. This should be easy but somehow seems to be hard;-(
>> ISTM the current text is broken as it'd expose length information
>> about ALPN in the CH and also in the EncryptedExtensions. I think
>> it'd be good if the list reached some level of agreement on the
>> goals of padding here rather than keep making different tweaks that
>> don't work. I think the goal for padding ought be that we don't
>> expose information about the content of the inner CH via the length
>> of any h/s message. If we agree that (or some similar thing) then
>> hopefully it should be just a matter of tweaking the algorithm so
>> it works. (I've raised this before but seemingly not convincingly
>> enough;-)
> 
> We kept maximum_name_length so as to not change “too much” in the PR.
> As this is orthogonal to the overall change, it can be addressed 
> separately. Can you please file an issue and propose text?

Sure. But ECHO vs. ESNI does make a difference here e.g.
due to the ALPN in the inner. If the outcome of (2) below
is to keep the flexibility they may be other extensions
that create length issues, either just in the inner CH
or else there and possibly also in other bits of the h/s.

That said, if the goal I stated above is something on
which everyone's agreed, I'd be happy to craft a PR. But
if that goal is not agreed, then probably any PR I make
won't be useful, hence raising this on the list.

>> 2) Variance between inner and outer CH. The current scheme is that
>> almost any variation is allowed. In the work I've done so far
>> looking at how to code that up, the trial decryption required
>> becomes a major pain in the client code as soon as the TLS
>> key-share differs between inner and outer. I don't know if that
>> affects other code bases or if that's mostly an OpenSSL thing but
>> think we ought establish on the list if that level of flexibility
>> is needed now, or maybe later, or even never. The cost there is not
>> just working out how to code it up, but this also creates new
>> complex code paths that will be rarely executed which is usually a 
>> recipe for sadness later. So, I'd hope other code bases are checked
>> for this before we merge the PR. (The problem here could be
>> OpenSSL's internal, eh... "intricacy" is a polite term I guess, or
>> it could be me being dim, but it seems real;-)
> 
> Yes, this will be a pain to implement. But variance between the
> inner and outer CH is permitted, and indeed a goal.

I'm questioning whether that's a good goal or not. In my
analysis of the various extensions, only SNI and ALPN seem
to offer immediate value. Until one gets to new algos,
which may mean PQC, there doesn't seem much more that's
worth supporting really. And when one looks at the level
of complexity to implement generic variance, I'm really
not sure it's worthwhile. That said, I'd hack away at
it as needed if others have taken a look at their code
and found it ok to support. I've just not heard that so
far and am concerned we're trading v, simple text in
the I-D vs. really horribly complex and maybe flakey
code, which doesn't seem like a good plan to me.

Just to be clear: I do like the idea that the analysis
be done on the flexible/full-variance scheme and I also
like the idea of reconstructing a full CH from the
recovered plaintext of the inner CH. Even with all
that though, the ECHO spec can still impose interop
restrictions on inner/outer CH variance to simplify
implementation and make interop easier and more likely.
My guess is that that'd be a worthwhile simplification.

If more variation is needed later (e.g. to help
incremental deployment of PQC algs), then that could
just use a new extension instead of ECHO.

> 
>> 3) I think we might be better off leaving out the compression stuff
>> for now, and only figuring that out later. The current
>> OuterExtensions thing is pretty ugly, and if the result of (2)
>> above were that we constrain the variance between inner and outer
>> some more then a generic compression scheme may not be needed. I
>> do think we will want some compression thing in the RFC we end up
>> with, but we might be better off to get interop without that first
>> and then add compression later. (That's what I plan to do fwiw, so
>> this is a less pressing issue for me given I'll be ignoring it for
>> now:-)
> 
> Can you elaborate on why it’s ugly? 

Allowing more than 1 OuterExtensions - that'd break some
code in OpenSSL's generic extension handling. Not sure
how bad that'll be to handle.

> What could we do differently?

If e.g. we required the same TLS key-share in inner and
outer, then you could just omit most of the octets from
the inner and deterministically reconstruct. With a small
number of such MUSTs, there might be no real need for a
generic compression scheme at all. That's linked to (2)
above though.

If a generic compression is needed, then yes something
ugly will likely be needed, but it could be simple and
ugly, e.g. extension-specific, for those extensions that
would make a difference. (And maybe we could punt on it
too - e.g. require the definition of the PQC stuff to
spec this compression if that's when it actually gets
useful.)

> Text suggestions are welcome!

Sure. And happy to offer some, but would like to see
guidance from the WG on the mailing list first. (Sorry
for being less github-cooperative about this, but being
logged in there is not my normal state of being:-)

Cheers,
S.


> 
> Thanks, Chris