Re: [Cfrg] I-D Action: draft-irtf-cfrg-hpke-02.txt

Michael Scott <mike.scott@miracl.com> Sat, 07 December 2019 10:20 UTC

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From: Michael Scott <mike.scott@miracl.com>
Date: Sat, 7 Dec 2019 10:21:59 +0000
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To: Ilari Liusvaara <ilariliusvaara@welho.com>
Cc: "cfrg@irtf.org" <cfrg@irtf.org>
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Subject: Re: [Cfrg] I-D Action: draft-irtf-cfrg-hpke-02.txt
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On Sat, Dec 7, 2019 at 9:50 AM Ilari Liusvaara <ilariliusvaara@welho.com>;
wrote:

> On Fri, Dec 06, 2019 at 07:11:01PM -0800, Benjamin Kaduk wrote:
> > On Wed, Dec 04, 2019 at 11:29:53AM +0000, Michael Scott wrote:
> > >
> > > Some feed-back: It would be a good idea to explicitly warn the reader
> that
> > > the X25519/X448 keys are actually in little-endian format, whereas for
> > > P256/521 they are big-endian.
> >
> > I think the general idea is that X25519 and X448 APIs act on fixed-length
> > byte strings and you are not supposed to care about the internal
> structure
> > or encoding.
>
> Correct, from RFC7748:
>
> "Although the functions work internally with integers, the inputs and
> outputs are 32-byte strings (for X25519) or 56-byte strings (for X448)
> and this specification defines their encoding."
>
>
> -Ilari
>
>

Interesting point: Are these drafts primarily blueprints for developers who
want to implement them, or user manuals for implementations (which may not
yet exist). In practise, for good or ill,  they are used as blueprints by
implementers (as highlighted just now by Watson Ladd in another thread),
and I would suggest that is part of their purpose. Why else provide test
vectors?

But I agree that over time as they become more established and standardised
they do become primarily user manuals for use with implementations which
can be assumed to be fully compliant. In which case friendly hints to
struggling developers would become redundant.

Mike



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