Re: [Cfrg] Static-DH oracles, strong-DH security

Taylor R Campbell <campbell+cfrg@mumble.net> Fri, 22 November 2019 00:02 UTC

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From: Taylor R Campbell <campbell+cfrg@mumble.net>
To: Alex Davidson <alex.davidson92@gmail.com>
CC: cfrg@ietf.org, Dan Brown <danibrown@blackberry.com>
In-reply-to: <CAD5V+fMP9F5oYfoEGmDHUVNQd0NoHDJ95-zSdLR0hDktwaT-Tw@mail.gmail.com> (alex.davidson92@gmail.com)
Date: Fri, 22 Nov 2019 00:01:54 +0000
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Subject: Re: [Cfrg] Static-DH oracles, strong-DH security
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> Date: Thu, 21 Nov 2019 11:38:00 +0000
> From: Alex Davidson <alex.davidson92@gmail.com>;
> 
> > Even a global network like Cloudflare can't answer more than 2^64
> > sequential queries: at one nanosecond per query that would be nearly
> > five hundred years. So we really only need to worry about divisors d
> > of p +/- 1 where p is the order of the group in which the oracle
> > operates and where d is below 2^64.
> 
> So, my only comment on your analysis is that I think restricting the
> number of queries to 2^64 is something that I would personally like to
> avoid.

To be clear: I don't mean to suppose limiting adversaries to 2^64
queries overall; rather, I mean to suppose limiting them to 2^64
queries made _one after the other_ -- not in parallel.

What matters is the _latency_ of a query, not the _bandwidth_ of the
global Cloudflare network.  And I think it is quite reasonable to
suppose limiting the latency of a query to at least a nanosecond:

- Even if scalar multiplication were instantaneous, Cloudflare would
  have to be physically located less than 15cm away from me in order
  for the information to propagate within one nanosecond -- unless I
  figure out something about physics that would earn me a lot more
  than a few captcha-skipping tokens are worth!

- Even if all the networking latency and parsing overhead were
  dispensed with (perhaps the oracle is on a shared VPS host), it's
  hard to imagine that computing a scalar multiplication could take
  less than a nanosecond to begin with.