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Re: [Cfrg] Safecurves draft

Bodo Moeller <bmoeller@acm.org> Thu, 09 January 2014 15:26 UTC

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References: <20140109031144.6111382.52184.8264@certicom.com> <20140109094731.GA12327@netbook.cypherspace.org> <CADMpkc+giuSZgrYmusRJmj5SyN9Dcu_Mdaqx5KQPyXGMmosFUw@mail.gmail.com> <CABqy+soXxjY+fEzpHP+_yn9Y1Xtapm_9OWbgDcA_J_Lukz_YLw@mail.gmail.com> <CADMpkcJFk2C5DPQX9RVWphUH25atsUX2vPA7RwNf8zbmR6dXJQ@mail.gmail.com> <CABqy+soX0xVWG0+vJs-_7O1Ur_hkDW0u0acCGZYrrtEci5QRXw@mail.gmail.com>
Date: Thu, 09 Jan 2014 16:26:42 +0100
Message-ID: <CADMpkcKptQrtXyaarkXiMpRyGmobEcywbTeTkkcb6uWB-yttwg@mail.gmail.com>
From: Bodo Moeller <bmoeller@acm.org>
To: Robert Ransom <rransom.8774@gmail.com>
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Cc: "cfrg@irtf.org" <cfrg@irtf.org>
Subject: Re: [Cfrg] Safecurves draft
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Robert Ransom <rransom.8774@gmail.com>:

> So while the Montgomery-form Curve25519 certainly has its use, allowing
> > applications to negotiate a different form for ECDH would be beneficial.
>
>

> Even if the party which generates a public key uses Edwards-form
> points internally for that operation, whoever generates the key can
> put it into Montgomery form for free before scaling, whereas whoever
> receives it would need to perform an extra coordinate inversion in
> order to convert from Edwards form to affine Montgomery form.


That's a good point.  As I've pointed out (or tried to point out, anyway),
the receiver might want to do the computations in Edwards form too, but
there's not that much to be gained from that, so it may not be worth the
extra complexity.

Bodo

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