Re: [CFRG] Call for adoption for draft-wood-cfrg-rsa-blind-signatures

"Riad S. Wahby" <rsw@jfet.org> Wed, 28 April 2021 12:33 UTC

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Date: Wed, 28 Apr 2021 08:33:03 -0400
From: "Riad S. Wahby" <rsw@jfet.org>
To: Jeff Burdges <burdges@gnunet.org>
Cc: "Stanislav V. Smyshlyaev" <smyshsv@gmail.com>, CFRG <cfrg@irtf.org>, Taler <taler@gnu.org>
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Subject: Re: [CFRG] Call for adoption for draft-wood-cfrg-rsa-blind-signatures
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Jeff Burdges <burdges@gnunet.org> wrote:
> We need a strong clarification that blinding factors should be
> rejection sampled from the RSA group, meaning same bit width
> and rejection if they exceed the modulus.  I’ve some GCD test
> in GNU Taler’s code but that’s unnecessary since n - phi(n) = pq
> - (p-1)(q-1) = p + q -1 << n.

As an alternative to rejection sampling, why not sample log2(n)+128
bits, interpret as an integer, and reduce mod n? That gives a result
statistically close to uniform mod n without a loop.

Or log2(n)+256 bits, if that feels better :)

Agreed that the GCD test is superfluous. The chance of getting a "bad"
value is morally equivalent to the chance of guessing a factor of n.

Cheers,

-=rsw