Re: [CFRG] I-D Action: draft-irtf-cfrg-rsa-blind-signatures-02.txt

Ian Goldberg <iang@uwaterloo.ca> Wed, 18 August 2021 21:49 UTC

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Date: Wed, 18 Aug 2021 17:49:21 -0400
From: Ian Goldberg <iang@uwaterloo.ca>
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Subject: Re: [CFRG] I-D Action: draft-irtf-cfrg-rsa-blind-signatures-02.txt
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On Mon, Aug 02, 2021 at 01:29:12PM -0400, Ian Goldberg wrote:
> On Mon, Aug 02, 2021 at 08:48:49AM -0700, Christopher Wood wrote:
> > As of now, there are no more outstanding issues against this draft. The editors think this version is feature complete and would welcome additional review. Please send any and all feedback either here on the list or as an issue:
> > 
> >    https://github.com/cfrg/draft-irtf-cfrg-blind-signatures/issues
> 
> In 8.6, should "Signers can enforce concurrent sessions" be "Signers can
> enforce a limit on concurrent sessions"?

Here are more notes.  I did not check the test vectors.

1:

"secret keys entities" -> "secret keys to entities"
"are doubled" -> "is doubled"

8.1:

There's a MUST here, but the thing that MUST be included just says "such
as RSA blinding".  First, "RSA blinding" should have a reference so that
implementers know exactly what they MUST implement (especially since
this document itself might at first glance appear to be that), but the
larger question is what does "MUST implement an X such as Y" actually
mean?  It's OK to implement X if you do it in a manner other than Y?
What if you do it in a really bad manner?

8.2:

"is that signer" -> "is that the signer"

8.6:

"Signers can enforce concurrent sessions, though the limit
(approximately 256) for reasonably secure elliptic curve groups is small
enough to make large-scale signature generation prohibitive.": this
sentence makes it sound as though, if you keep concurrency to less than
256, you're safe.  That's not (as I understand it) the case.  The paper
shows a very fast attack for 256 using its new [PolytimeROS] attack, but
the original subexponential [Wagner02] attack works for much smaller
levels of parallelism: for 15 parallel sessions, the attack runs in
~2^55 time (says [FPS20]), which is much lower than the desired security
parameter.  Even Clause Blind Schnorr just multiplies the complexity by
2^15 (for 15 parallel sessions), so ~2^70, which is still a far cry from
the desired 2^128 security level.

10.2:

The [WM99] entry is missing authors, date, URL.

[BLS-Proposal], [PolytimeROS], [RemoteTiming] are missing dates


-- 
Ian Goldberg
Canada Research Chair in Privacy Enhancing Technologies
Professor, Cheriton School of Computer Science
University of Waterloo