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Re: [TLS] PR #699: Simplify traffic key expansion

Kazuho Oku <kazuhooku@gmail.com> Thu, 20 October 2016 19:52 UTC

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From: Kazuho Oku <kazuhooku@gmail.com>
Date: Fri, 21 Oct 2016 04:52:37 +0900
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To: Eric Rescorla <ekr@rtfm.com>
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Subject: Re: [TLS] PR #699: Simplify traffic key expansion
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Hi,

While trying to implement NewSessionTicket, I have noticed that
resumption_psk is derived from resumption_secret.

Is there any need to expand resumption_psk from resumption_secret?

To me, it is unclear why resumption_secret cannot be used directly as
a psk, since the two values have the same lengths and since the secret
is only used for deriving the psk.

Maybe is this something we could also simplify?


2016-10-18 5:08 GMT+09:00 Eric Rescorla <ekr@rtfm.com>:
> Hi folks,
>
> https://github.com/tlswg/tls13-spec/pull/699/files
>
> A while back Steven Valdez pointed out that now that we have the PSK binder
> change and dual key ladders, each set of traffic keys is generated from a
> different
> base secret which has a label folded in [0], so we don't need to have the
> "phase" parameter in the traffic key calculation in Section 7.3, which
> simplifies things
> a bit. Due to an oversight, this didn't make it into the PR, but it seems
> straightforward.
>
> Please let me know ASAP if I have missed something here or you otherwise
> object.
>
> -Ekr
>
>
> [0] client_early_traffic_secret, [sender]_handshake_traffic_secret,
> [sender]_traffic_secret_N respectively
>
> _______________________________________________
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>



-- 
Kazuho Oku

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