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Re: [Cfrg] On why point and APIs formats matter (Re: Submission of curve25519 to NIST from CFRG -> was RE: On "non-NIST")

Nico Williams <nico@cryptonector.com> Thu, 12 March 2015 16:26 UTC

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Date: Thu, 12 Mar 2015 11:26:23 -0500
From: Nico Williams <nico@cryptonector.com>
To: James Cloos <cloos@jhcloos.com>
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Subject: Re: [Cfrg] On why point and APIs formats matter (Re: Submission of curve25519 to NIST from CFRG -> was RE: On "non-NIST")
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On Thu, Mar 12, 2015 at 09:13:59AM -0400, James Cloos wrote:
> >>>>> "NW" == Nico Williams <nico@cryptonector.com> writes:
> 
> NW> In addition to API simplicity, for which Curve25519 sets a high bar!,
> NW> there are other reasons to stick with its use of little-endian encoding:
> 
> Those reasons are valid for keeping 25519 as it is, but do not imply
> that new curves also need to be little-endian.

Indeed it doesn't.

> Each curve should define all of its particulars, with points et alia
> always -- of course -- stored and transmitted as fixed lenght octet
> strings.  But that allows any curve to choose big-endian.

+1.

Though Montgomery x-only point representations naturally lend themselves
to fixed-length x representations with whatever endianness is most
convenient in the most common case (x85_64), which happens to be
little-endian.  I'd expect little-endian to be the common choice, but
I'd certainly not require it!

Nico
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