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[TLS] 0RTT and HelloRetryRequest (Re: Narrowing the replay window)

Martin Thomson <martin.thomson@gmail.com> Wed, 30 March 2016 22:57 UTC

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Date: Thu, 31 Mar 2016 09:57:51 +1100
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From: Martin Thomson <martin.thomson@gmail.com>
To: Ilari Liusvaara <ilariliusvaara@welho.com>
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Cc: tls@ietf.org
Subject: [TLS] 0RTT and HelloRetryRequest (Re: Narrowing the replay window)
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On 31 Mar 2016 5:56 AM, "Ilari Liusvaara" <ilariliusvaara@welho.com> wrote:
> Then on topic of 0-RTT, how does 0-RTT key hashes behave if
> handshake is restarted (main handshake hash continues, but
> 0-RTT hash context currently needs to be separate from the
> main context)?

Good question. I don't recall that being discussed. I see three options :

1. Continue the hash, just like in 1-RTT

2. Treat HelloRetryRequest as a denial of the entire first flight.

3. Signal the choice.

Option 2 suits best if we consider HelloRetryRequest to be a DoS feature
exclusively or at least primarily. But we have other reasons for it and I
don't think that DoS mitigation is a big factor for TCP.

I think that option 1 is easy enough, since both sides have to extend the
hash in any case. 3 is just complexity.

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