Filename: 225-strawman-shared-rand.txt
Title: Strawman proposal: commit-and-reveal shared rng
Author: Nick Mathewson
Created: 2013-11-29
Status: Superseded
Superseded-by: 250
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.
At the start of phase 3, each participating authority either has a
reveal from every authority that it received a commitment from, or it
does not. Each participating authority then says
shared-random 1
shared-random-type finish
signing-key-certification (certification here; see proposal 220)
commitment-key-certification (certification here; see proposal 220)
received-commitment ID sig R
received-commitment ID sig R ...
published YYYY-MM-DD HH:MM:SS
period-start YYYY-MM-DD HH:MM:SS
attempt INT
consensus C
signature (made with commitment key; see proposal 220)
Where C = SHA256(ID | R | ID | R | ID | R | ...) where the ID
values appear in ascending order and the R values appear after
their corresponding ID values.
See [SHAREDRANDOM-REFS] for more discussion here.
(TODO: should this be its own spec? If so, does it have to use our
regular metaformat or can it use something less sucky?)