Re: Post-quantum secure hierachical deterministic key derivation

[Jeff Burdges]

> On 21 Dec 2020, at 06:49, Martin Schanzenbach <mschanzenbach@posteo.de> wrote:
> this looks promising for PQ-secure GNS key blinding:
> https://eprint.iacr.org/2020/1149.pdf


It’s cool that lattice-based schemes can do so many nifty things like this, and 
tor might want the same functionality, but a priori one expects such tricks 
weaken lattice-based schemes.

There is a claim in the second paragraph in section 5.1 page 24 that this 
weakening does not occur here, assuming the randomization is honest.  I have 
not explored this claim under their honest randomness assumption, but it'd 
clearly requires proof even there.  In fact, I doubt this randomization can be 
considered honest in the blockchain or GNS threat models, so I think the 
authors do not really understand for what their protocol expects to be used.

In tor’s case, one might consider directory authorities honest.  There exist 
few directory authorities though regardless, so if tor made directory 
authorities propose randomness using a VRF implemented with a hash-based 
signature, then you could bound the adversaries influence over the randomness 
by the number of signing directory authorities since an honest directory 
authority signed.  This is almost surely useful.

Assuming section 5.1 fails then one might still use the protocol, but..  There 
is strictly more room for debate over the parameter choices here than for a 
lattice-based signature without this functionality.  Indeed, I seriously doubt 
you'd deploy this using exactly the same parameters a lattice-based signature 
without this functionality, given the similar threats, lifetime, etc.  And 
folks do not yet agree about even the regular parameter!

I also think the protocol requires further exploration of linkability between 
keys, which maybe matters under some blockchain usages of HDKD like zcash, 
definitely matters for Tor, and maybe matters for GNS.  It appears the paper 
does not explore this.

It’s kinda a problem with our brave new lattice based post-quantium future that 
niche crypto becomes more of a second class citizen than with elliptic curves, 
but if security levels become established then at least parameterizing the 
niche stuff becomes more realistic.

Jeff

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