Re: [TLS] Don't Split HelloRetryRequest

Christopher Patton <cpatton@cloudflare.com> Thu, 01 April 2021 18:20 UTC

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From: Christopher Patton <cpatton@cloudflare.com>
Date: Thu, 1 Apr 2021 11:20:15 -0700
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To: Stephen Farrell <stephen.farrell@cs.tcd.ie>
Cc: Martin Thomson <mt@lowentropy.net>, "<tls@ietf.org>" <tls@ietf.org>
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Subject: Re: [TLS] Don't Split HelloRetryRequest
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One way HRR is used is in case the client's and server's cipher suite
> preferences don't intersect. This feature is an essential part of TLS, as
> there's no a priori reason why the client and server will initially
> advertise overlapping preferences. (They usually do, hence the claim that
> HRR is rare.) I don't think aborting the handshake instead of HRR is an
> acceptable solution, as this would mean there are deployments with which
> TLS couldn't be used.
>

Slight refinement: David B. pointed out to me that "cipher suite
preference" isn't quite the right term here. The client provides key shares
in its CH that it guesses the server can use; if it's wrong, then the
server replies with HRR. A more accurate statement would be that "HRR is
essential for ensuring the client sends a key share the server supports."