Re: [TLS] Pull request for session hash

Ilari Liusvaara <ilari.liusvaara@elisanet.fi> Fri, 07 November 2014 22:35 UTC

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Date: Sat, 8 Nov 2014 00:35:08 +0200
From: Ilari Liusvaara <ilari.liusvaara@elisanet.fi>
To: Eric Rescorla <ekr@rtfm.com>
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Subject: Re: [TLS] Pull request for session hash
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On Fri, Nov 07, 2014 at 02:01:27PM -0800, Eric Rescorla wrote:
> I've created a preliminary pull request to adapt the session hash fix for
> TLS 1.3
> for feedback from the WG.

Quick comment: I presume "{ServerKeyShare}" in figure 2 should be
"ServerKeyShare" (i.e. not encrypted).
 
> Points to note:
> 
> - IMPORTANT: Because the initial handshake messages are encrypted
>   under separate keys from the application data traffic, it seems like we
> need
>   to have 2 sets of CCS messages or none. In anticipation of removing
>   renegotiation (see PR https://github.com/tlswg/tls13-spec/pull/88).
>   Based on conversations with Alfredo and Martin, it seemed easier
>   to remove them. Warning: analysis needed here.

I would say that from implementation safety perspective (at least for
non-D TLS) it is better to do the state transitions implicitly.

Except this could cause fun issues if ContentType is encrypted.


I favor "do it wrong and blow up at runtime" over "do it wrong and
get a security hole". Even if the first would be more complex.

(This can also be used to argue that session hash should not be
restarted on retry, not that I think repeating data is a good idea
anyway).

> - I created a separate resumption master secret that is fed into the
>   key hierarchy. This makes the description of that simpler but will
>   need revisiting if we adopt either Rich's unification of session hash
>   and tickets or Karthik's PSK-as-tickets unification, but that seemed
>   like a separate issue.

Oh, on topic of multiple master secrets, which one does TLS-Extractor
get? :-)


-Ilari