cv25519 scalar byte order

Daniel Kahn Gillmor dkg at
Mon Apr 9 20:51:03 CEST 2018

Over in,
a discussion was started about scalar byte order for OpenPGP curve 25519

On Mon 2018-04-09 18:53:53 +0200, Werner Koch wrote:
> On Mon, 19 Feb 2018 17:24, dkg at said:
> [ gniibe wrote: ]
>>> That would be incorrect.  The prefix (e.g. 0x40) indicates a _point_
>>> format and not the format of a scalar.  Thus skey[3] MAY not have this
>>> prefix.
>> what does this "MAY NOT" mean?  if this is an attempt at RFC 2119
>> language, i don't understand it.  Do you mean "MUST NOT" ?
> I was thinking SHOULD NOT but indeed it MUST be MUST NOT.
>> What steps are needed to clarify the documentation here so that we can
>> have interoperable implementations?
> I can't remember an open issue regaring this in the WG.  Should be
> handled there anyway,

I'm moving this discussion to the WG :)


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