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Re: [TLS] Point Compression

John Mattsson <john.mattsson@ericsson.com> Tue, 26 October 2021 07:58 UTC

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From: John Mattsson <john.mattsson@ericsson.com>
To: Andrey Jivsov <crypto@brainhub.org>, Carl Mehner <c@cem.me>
CC: IETF TLS <tls@ietf.org>
Thread-Topic: [TLS] Point Compression
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Date: Tue, 26 Oct 2021 07:58:08 +0000
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Subject: Re: [TLS] Point Compression
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There are several active documents also defining compact representation.

After the discussion regarding HPKE there was a suggestion EDHOC should define a compract format that could be reused by other protocols. That was done in an Appendix.
https://datatracker.ietf.org/doc/html/draft-ietf-lake-edhoc-12#page-67

Also as an outcome of the HPKE discussions Dan Harkins has written on compact representation for HPKE submitted to CFRG. This also defines a a general format than can be reused by other protocols.
https://datatracker.ietf.org/doc/draft-harkins-cfrg-dnhpke/

> the support for 'regular' (SEC1) compressed curves is more widespread.

What is support in this case? An implementation can just use one of the values "02" and "03" or flip a coin. If you have support of 'regular' (SEC1) compressed curves, then compact representation is trivial.

Cheers,
John

From: TLS <tls-bounces@ietf.org> on behalf of Andrey Jivsov <crypto@brainhub.org>
Date: Tuesday, 26 October 2021 at 07:15
To: Carl Mehner <c@cem.me>
Cc: IETF TLS <tls@ietf.org>
Subject: Re: [TLS] Point Compression
Do we have evidence that "02 <x>" or "03 <x>" is more widespread than <x> for NIST curves? I haven't seen "02 <x>" or "03 <x>" in deployed products in TLS / X.509 at all. So, I feel that for TLS space the slate is clean regarding compression. X25519 uses one coordinate, which is simiiar to doing <x> for NIST curves...

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