Re: [Cfrg] On h2c implementation

"Riad S. Wahby" <rsw@jfet.org> Sat, 07 December 2019 16:54 UTC

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Date: Sat, 07 Dec 2019 17:54:45 +0100
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To: cfrg@irtf.org,Watson Ladd <watsonbladd@gmail.com>
From: "Riad S. Wahby" <rsw@jfet.org>
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Subject: Re: [Cfrg] On h2c implementation
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On December 7, 2019 6:55:26 AM GMT+01:00, Watson Ladd <watsonbladd@gmail.com> wrote:
>This is a common way to avoid the cost of a full carry chain, but has the
>disadvantage that the arithmetic functions need to be wrapped to properly
>carry out comparisons to constants.

Hello Watson,

Thanks for the feedback! We'll definitely give more thought to this issue. If you have any other details that you'd like to discuss off-list, happy to chat more.

Speaking only for myself (I haven't yet discussed with the other authors), my inclination is that details of field arithmetic implementations are quite a bit below the level of abstraction the hash-to-curve document is aiming for. For example, Section 4 currently says:

"Guidance on implementing these low-level operations in constant time is beyond the scope of this document."

(If I understand your email correctly, this isn't exactly what you're discussing, but it's in the ballpark and to me suggests a similar conclusion.)

Of course, I could be wrong and/or things could change! Again, happy to chat more.

Best,

-=rsw