[tor-dev] Proposal 225: Strawman proposal: commit-and-reveal shared rng

[Sebastian G. <bastik.tor>]

30.11.2013 02:28, Nick Mathewson:
> [...]
Trimming to shorten the length.

> Filename: 225-strawman-shared-rand.txt
> Title: Strawman proposal: commit-and-reveal shared rng
> Author: Nick Mathewson
> Created: 2013-11-29
> Status: Draft
> 
> 1. Introduction
> 
>   This is a strawman proposal: I don't think we should actually build
>   it.  It's just a simple writeup of the more trivial commit-then-reveal
>   protocol for generating a shared random value.  It's insecure to the
>   extent that an adversary who controls b of the authorities gets to
>   choose among 2^b outcomes for the result of the protocol.
> 
>   See proposal 224, section HASHRING for some motivation of why we want
>   one of these in Tor.
> 
>   Let's do better!
> 
>   [TODO: Are we really stuck with Tor's nasty metaformat here?]
> 
> 2. The protocol
> 
>   Here's a protocol for producing a shared random value. It should run
>   less frequently than the directory consensus algorithm. It runs in
>   these phases.
> 
>     1. COMMITMENT
>     2. REVEAL
>     3. COMPUTE SHARED RANDOM
> 
>   It should be implemented by software other than Tor, which should be
>   okay for authorities.
> 
>   Note: This is not a great protocol. It has a number of failure
>   modes. Better protocols seem hard to implement, though, and it ought
>   to be possible to drop in a replacement here, if we do it right.
> 
>   At the start of phase 1, each participating authority publishes a
>   statement of the form:
> 
>       shared-random 1
>       shared-random-type commit
>       signing-key-certification (certification here; see proposal 220)
>       commitment-key-certification (certification here; see proposal 220)
>       published YYYY-MM-DD HH:MM:SS
>       period-start YYYY-MM-DD HH:MM:SS
>       attempt INT
>       commitment sha512 C
>       signature (made with commitment key; see proposal 220)
> 
>   The signing key is the one used for consensus votes, signed by the
>   directory authority identity key. The commitment key is used for this
>   protocol only. The signature is made with the commitment key. The
>   period-start value is the start of the period for which the shared
>   random value should be in use. The attempt value starts at 1, and
>   increments by 1 for each time that the protocol fails.
> 
>   The other fields should be self-explanatory.
> 
>   The commitment value C is a base64-encoded SHA-512 hash of a 256-bit
>   random value R.
> 
>   During the rest of phase 1, every authority collects the commitments
>   from other authorities, and publishes them to other authorities, as
>   they do today with directory votes.
> 
>   At the start of phase 2, each participating authority publishes:
> 
>       shared-random 1
>       shared-random-type reveal
>       signing-key-certification (certification here; see proposal 220)
>       commitment-key-certification (certification here; see proposal 220)
>       received-commitment ID sig
>       received-commitment ID sig
>       published YYYY-MM-DD HH:MM:SS
>       period-start YYYY-MM-DD HH:MM:SS
>       attempt INT
>       commitment sha512 C
>       reveal R
>       signature (made with commitment key; see proposal 220)
> 
>   The R value is the one used to generate C. The received-commitment
>   lines are the signatures on the documents from other authorities in
>   phase 1. All other fields are as in the commitments.
> 
>   During the rest of phase 2, every authority collects the
>   reveals from other authorities, as above with commitments.

(Still haven't read any paper linked from the "bitcoin hash thread".)

By definition each subverted authority will try to be the last one that
reveals. It would try to calculate whenever or not it is better to
reveal or fake an error, based on the result it knows.

How about specifying some sort of order for revealing? 8 Authorities
will be split into 7 for A and 1 for B. Authority B can only reveal its
commit after all A's revealed theirs, with some timeout, where its vote
would count if others are missing. Each time they get together for
voting a different Authority will be B.

With one Authority under my control I could try to game it once instead
of trying to do this all the time. This works with a fixed pattern,
where Authority 1, 2, 3, 4, 5, 6, 7 and 8 rotate each time it is to be
decided about the string. I'm not sure if there would be an improvement
if that would be more random to avoid being able to predict what
authority will be allowed to be the last one too far ahead.


Regards,
Sebastian G. (bastik)