Re: [Cfrg] Call for adoption: draft-hdevalence-cfrg-ristretto-01

Greg Hudson <ghudson@mit.edu> Mon, 30 September 2019 16:37 UTC

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To: Alex Davidson <alex.davidson92@gmail.com>, Filippo Valsorda <filippo@ml.filippo.io>
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From: Greg Hudson <ghudson@mit.edu>
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Date: Mon, 30 Sep 2019 12:37:10 -0400
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Subject: Re: [Cfrg] Call for adoption: draft-hdevalence-cfrg-ristretto-01
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On 9/30/19 7:36 AM, Alex Davidson wrote:
> For the second point, there is a possible solution in defining a
> FROM_BYTES API
> function as part of the Ristretto interface. This would take non-uniform
> bytes,
> transform them to uniform bytes, and then invoke FROM_UNIFORM_BYTES on the
> output of this transformation. For ristretto255 it would be fine to use
> SHA512, [...]

Hashing non-uniform inputs does not produce uniform outputs.  Some
obvious statistical biases may be eliminated, but certain outputs are
still more likely than others (unless hash collisions manage to
precisely cancel out the non-uniformity of the input).  Is Ristretto
asking for too much by requiring a "uniformly distributed 64-byte string b"?