Re: [TLS] (no subject)

Ilari Liusvaara <ilariliusvaara@welho.com> Thu, 09 March 2017 12:50 UTC

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Date: Thu, 9 Mar 2017 14:41:08 +0200
From: Ilari Liusvaara <ilariliusvaara@welho.com>
To: Hubert Kario <hkario@redhat.com>
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Subject: Re: [TLS] (no subject)
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On Thu, Mar 09, 2017 at 12:50:03PM +0100, Hubert Kario wrote:
> On Thursday, 9 February 2017 22:17:33 CET Eric Rescorla wrote:
> > Hi folks,
> > 
> > We need to close on an issue about the size of the
> > state in the HelloRetryRequest. Because we continue the transcript
> > after HRR, if you want a stateless HRR the server needs to incorporate
> > the hash state into the cookie. However, this has two issues:
> 
> Isn't the whole CH2 supposed to be deterministically created from CH1 and HRR?

No.

Neither CH1 nor CH2 are deterministic w.r.t. one another.

> So you should be able (as the server) to recreate the CH1 given the hash (or 
> better yet, keyed HMAC) of the CH1 fairly easily? Bonus point: you 
> automatically reject technically malformed CH2 messages (ones with more 
> changes than prescribed) as you won't be able to create a CH1 that creates the 
> matching HMAC.

Actually, you can't create CH1 from just CH2. Back some versions it was
merely very annoying to do so, now (as in -18), you can't do it at all.


-Ilari