Re: [Cfrg] I-D Action: draft-irtf-cfrg-hash-to-curve-07.txt

Björn Haase <bjoern.m.haase@web.de> Tue, 28 April 2020 15:14 UTC

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Date: Tue, 28 Apr 2020 17:13:52 +0200
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Subject: Re: [Cfrg] I-D Action: draft-irtf-cfrg-hash-to-curve-07.txt
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Hi H2C team,

I just had a short look at the draft. The first thing I have looked at
more closely is the expand_message_xmd function. There is something
which I don't understand in this construction regarding the Z_pad string
which is  prepended in the hash invocation for b_0.

Iwould understand a zero-padding that would first insert some
information that should be maintained confidential and a Z_pad string
that fills up the remaining rest of the input block of the hash function
with zeros. However, I did not get the point, why filling the first
block with zeros should provide any advantage.

Could somebody explain the reasoning?

Yours,

Björn