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Re: [Cfrg] On the differences of Ed25519/448 and how it affects a vote on twoshakes-d

Brian Smith <brian@briansmith.org> Sun, 13 December 2015 22:08 UTC

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References: <CAA4PzX18bcS_awPg-YDAoo90537Ot=s_nf7k_Vt75OVSdvtDrQ@mail.gmail.com> <87fuzcng51.fsf@latte.josefsson.org> <20151209125944.GA26766@LK-Perkele-V2.elisa-laajakaista.fi> <566AEB08.9070302@st.com> <CAHOTMV+1am7eyn_H8JLdR_GCU9twonduEpxRnQTJEVOb+Gq6jg@mail.gmail.com> <566BDF12.9060501@gmail.com> <87r3iqj99g.fsf@latte.josefsson.org>
Date: Sun, 13 Dec 2015 12:08:50 -1000
Message-ID: <CAFewVt7ytzuHcKZ_285_ZMeqLD-3XtV1zy-=Ce4FfPsVDsi2Mg@mail.gmail.com>
From: Brian Smith <brian@briansmith.org>
To: Simon Josefsson <simon@josefsson.org>
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Subject: Re: [Cfrg] On the differences of Ed25519/448 and how it affects a vote on twoshakes-d
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Simon Josefsson <simon@josefsson.org> wrote:

> This is incorrect -- Ed25519 and Ed25519ph are domain separated since
> they are different algorithms.  You should never use the same key with
> both algorithms, just as you shouldn't use the wrong
> domain-separation-bits in Ed448 twoshakes-d with the same key.
>

You could say something similar about RSA w.r.t. RSA encryption vs RSA
PKCS#1 vs RSA PSS. However, in practice, people are and will use the same
RSA key for multiple algorithms unless there's something that makes it
difficult to do so. I suspect the same will happen with Ed25519 and
Ed25519ph. Consequently, I don't think we should consider Ed25519 and
Ed25519ph to be domain-separated in practice.

Cheers,
Brian
-- 
https://briansmith.org/

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