Re: [CFRG] RSA blind signatures

Hello Chris,

It’s critically important the blinding factor r be a uniformly random integer mod n, which I think deserves more emphasis than you give.  There is an easy deanonymization attack if r were say generated a random integer mod 2^{floor(log2 n)}.  You hould emphasize that random_integer should be instantiated with a CSPRNG and rejection sampling, maybe even specify the rejection sampling algorithm starting with shake or chacha.  

If I recall, RSA-PSS depends upon signer randomness for its security arguments.  As such, one should ideally not base an RSA blind signature off PSS but instead specify a full domain hash (FDH).  

At this point, one could specify the blinding factor be produced by applying the FDH to system randomness.  This is what I did for Taler’s blind RSA signatures: https://taler.net/en/

Initially I wanted to point you to the RSA-FDH-VRF in https://datatracker.ietf.org/doc/draft-irtf-cfrg-vrf/ except..  Actually the RSA-FDH-VRF draft does not properly specify the FDH either, but only points to https://tools.ietf.org/html/rfc8017 which does not specify the FDH.

An FDH is a pretty easy notion but people get this wrong.  Also, there might be interoperability advantages in specifying it more fully. 

Best,
Jeff

p.s.  I think one should not deploy RSA-FDH-VRF but instead work through all the tricks to make Rabin-Williams deterministic.  It’s not too hard but not as easy as RSA-FDH-VRF.  I’ve no looked at wether Rabin-Williams could be adopted to blind signatures, but I think some issues arose beyond what one alters for a Rabin-Williams VRF. 

> On 23 Feb 2021, at 18:37, Christopher Wood <caw@heapingbits.net> wrote:
> 
> There are a growing number of use cases where we need something like VOPRFs but with public verifiability [1,2]. Given the results in 2020/945 [3], it seems prudent to try and fill the gap with something we know is reasonably safe. To that end, here's a draft describing RSA-based blind signatures:
> 
>   https://chris-wood.github.io/draft-wood-cfrg-blind-signatures/draft-wood-cfrg-rsa-blind-signatures.html
> 
> (I missed the deadline yesterday, so apologies for not having an actual datatracker draft to point at.)
> 
> Obviously, something better than RSA (in terms of bandwidth and overall messages) would be great. But it's not clear what that is right now.
> 
> Time permitting, I'd like to request some time on the agenda to present this to the group at IETF 110.
> 
> Thanks,
> Chris
> 
> [1] https://github.com/ietf-wg-privacypass/base-drafts/issues/40
> [2] https://github.com/privacycg/private-click-measurement/issues/27
> [3] https://eprint.iacr.org/2020/945.pdf
> 
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