30.11.2013 02:28, Nick Mathewson:
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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
- 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?]
- 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 RANDOMIt 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)