Re: [Cfrg] draft-fluhrer-lms-more-parm-sets-01

Russ Housley <housley@vigilsec.com> Thu, 23 April 2020 20:21 UTC

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From: Russ Housley <housley@vigilsec.com>
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Date: Thu, 23 Apr 2020 16:21:19 -0400
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Cc: Scott Fluhrer <sfluhrer@cisco.com>, IRTF CFRG <cfrg@irtf.org>
To: Uri Blumenthal <uri@ll.mit.edu>
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Subject: Re: [Cfrg] draft-fluhrer-lms-more-parm-sets-01
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> On Apr 23, 2020, at 3:42 PM, Blumenthal, Uri - 0553 - MITLL <uri@ll.mit.edu> wrote:
> 
> On 4/23/20, 15:03, "Cfrg on behalf of Russ Housley" <cfrg-bounces@irtf.org on behalf of housley@vigilsec.com> wrote:
> 
> 
>> 2) SHAKE256-256 and SHAKE256-192:  Why use an Extendable-Output
>> Function (XOF)?
> 
> Because it's pseudorandom by provable design rather than conjecture (like in case of SHA-1 and SHA-2)?

I think that is true for SHA3-256, which has the fixed size output.

>> Since the output in the application is always 256 bits or 192 bits, the normal reason
>> for picking an XOF does not seem relevant.
> 
> There are no reasons NOT to, either.

FIPS 202, Appendix A seems to offer some warning in this direction.

Russ