This used to be an appendix in my drafts for proposal 224, but I split it into its own document since it's orthogonal to the rest of 224.
See section 1 for appropriate provisos and disclaimers. I'm writing this as a proposal mainly so other researchers can write better proposals for this kind of thing.
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.
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?)