Re: [TLS] 0-RTT & resumption

Eric Rescorla <ekr@rtfm.com> Tue, 04 August 2015 12:41 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Tue, 4 Aug 2015 05:40:48 -0700
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To: Ilari Liusvaara <ilari.liusvaara@elisanet.fi>
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Subject: Re: [TLS] 0-RTT & resumption
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On Mon, Aug 3, 2015 at 11:51 PM, Ilari Liusvaara <
ilari.liusvaara@elisanet.fi> wrote:

> On Sat, Jul 25, 2015 at 09:07:49PM +0200, Eric Rescorla wrote:
> >
> >
> > We agreed on how to do this in Prague. The sticking point was
> establishing
> > the cipher suite. I have WIP text on my machine for both of these which I
> > will be
> > sending early next week, once I get enough sleep to be able to clean it
> up,
> > so I'd ask people to sit tight till then.
> >
>
> Okay, now the PR (#211) seems to be up, let's review


Oops. It wasn't supposed to be done. I meant to make it a PR against my
own branch so that I could get early comments from MT, but obviously that
didn't work out. Regardless, thanks for your comments.


- Lacks client-driven client authentication[1]. All client auth is server
>   driven, which I think isn't very useful in real world (there are all
>   sorts of bad hacks[2] trying to work around lack of client-driven auth).
>

This is just extending existing practice for 1-RTT handshakes.

I tend to agree with you that it would be good to change that, but I didn't
want
to do that in this PR, because it would be a big semantic change in TLS.
I suggest you start a new thread on this topic, or perhaps add it to
Andrei's
client auth thread?


- EncryptedExtensions looks to be mandatory in some exchanges, optional
>   in others. I agree it should be mandatory in all (issue #213).
>

Me too. That's just editing error.



> - "The client's cryptographic determining parameters match the parameters
>   that the server has negotiated based on the rest of the ClientHello."
>   ... Does that mean the client has to guess what ciphersuite the server
>   will choose (more than pure-PSK vs. GDHE, which is unvaoidable with
>   just one encrypted block)?
>

The client knows what the server selected based on his previous offer and
the server's configuration (which by hypothesis is still extant) so the
server
should choose the same value again.


> - Am I reading the syntax wrong, or does the extensions field in server
>   configuration only allow exactly one extension (shouldn't it be zero
>   or more)?
>

Yes. good catch.


Also, regarding issue #212, unless the Certificate is handled specially,
> it would mean that the signature does not cover certificate. And not
> signing the certificate (esp. the public key within) causes problems
> in some exotic cases (I don't know if any of those cases pop up in TLS
> 1.3).
>

This seems like a good argument.



> I think it would simplify the security analysis a bit if CertificateVerify
> was always immediately before Finished and covered everything before that
> point.
>

Isn't this always the case presently? Are you just thinking we should say
it's
a rule?

-Ekr