Walking Onions: week 3 update On our current grant from the zcash foundation, I'm working on a full specification for the Walking Onions design. I'm going to try to send out these updates once a week.
My previous updates are linked below: Week 1: formats, preliminaries, git repositories, binary diffs, metaformat decisions, and Merkle Tree trickery. https://lists.torproject.org/pipermail/tor-dev/2020-March/014178.html Week 2: Specifying details of SNIP and ENDIVE formats, in CDDL. https://lists.torproject.org/pipermail/tor-dev/2020-March/014181.html You might like to check out those updates, and their references, if this update doesn't make sense to you. === As you might recall, the SNIPs need to be nice and small, but they need to be completely self-contained. The ENDIVE, on the other hand, needs to be diff-friendly, and compressible. Relays need to derive all the SNIPs from the ENDIVE. The ENDIVE and SNIP formats that I worked on during last week [SNIPFMT] was designed to meet these criteria. This week, I wrote up the algorithms to extract signed SNIPs from an ENDIVE. This is in section 4 [RECONST] of the specs in my work-in-progress repository [GITREPO] This was mainly straightforward consequence from the work I did last week, but there were a few things that turned up. One trickier aspect of this work is that we want to move most of the decision-making to the authorities, and have the relays simply do an expansion algorithm. Updating all the relays would be slow, so relays need to be able to handle new index types and data types without fully understanding them. That means that we need to specify an algorithm that can work flexibly with many future kinds of data. Another tricky aspect is that the data which the relays need to put into SNIPs all needs to be signed, so all of that data needs to come out the same when relays calculate it as the when the authorities calculate it. Thus, all the algorithms for building Merkle trees and indices and SNIPRouterData bodies need to be deterministic. I think that I got all of this relatively right in [RECONST], but there are probably some mistakes hiding in there. I'm hoping that there is a cleaner alternative to the weighted-index reconstruction algorithm -- the one that's there right now puts a lot of requirements on the input data, in order to avoid floating-point operations. === After working on ENDIVE expansion, I moved on to the problem of authorities voting. There's a lot of uncertainty here and decisions I'll need to make. The biggest decision is how the new voting process for ENDIVEs should relate to the current voting process for consensus documents. The choices seem to be: * Have two processes with different kinds of votes. * Expand the information that goes into current votes, and generate ENDIVEs as a new kind of consensus from the current voting process. * Design the ENDIVE voting process so that it can also be used to generate current-style consensuses. Using separate processes might be simplest, but it does carry risk for duplicated code, and for the two voting processes reaching divergent results. It would also be potentially bad if an authority could vote differently in each process. The second approach -- where ENDIVEs are another kind of consensus generated from current-style voting documents -- has some charm to it, but it perpetuates our current consensus-voting system, and makes us specify a second encoding of CBOR objects as text, if we want to vote on them meaningfully. This is a bit nasty, since the standard-specified text encoding of cbor is not a bijection, and doesn't mesh too well with our old directory format. Therefore the third approach seems a little better to me now. In it, we'd specify our votes as a CBOR-based format loosely based on the current vote format, and describe a way to extract from them the same data that is currently used for our voting algorithm. This would let us encode CBOR within the document naturally, while not having to throw away too much of our existing voting specification and voting code. I've got a few more ideas here, though I don't have any specs yet. I've been collecting the ideas in a notes file [VNOTES] so I don't forget them. === While working on voting notes, I found some areas where the ENDIVE and SNIP formats might be better. First, I hadn't thought about authority certificates. Those should go in, so that we can have the ENDIVE-signing keys be signed themselves with the authorities' long-lived identities. Second, there seem to be some opportunities for transmitting indices in a simpler way. Instead of assigning a weight to each router separately, for example, we could derive one weighted index from another weighted index by re-weighting it, as we do with the "bandwidth-weights" values in the current consensus. Teor also made some good suggestions for improving the diff format; I incorporated one and made a note to check the other. === I also spent a little while this week reading about BLS signatures, since they are the leading candidate for how we might do threshold signatures here. === Next week I'm going to try to get voting all specified. This will be a pretty deep dive, since I will need to maintain some backward compatibility with existing voting algorithms. There may be opportunities for simplification, however. === [GITREPO] https://github.com/nmathewson/walking-onions-wip [RECONST] https://github.com/nmathewson/walking-onions-wip/blob/master/specs/04-reconstructing-endives.m d [SNIPFMT] https://github.com/nmathewson/walking-onions-wip/blob/master/specs/02-endives-and-snips.md [VNOTES] https://github.com/nmathewson/walking-onions-wip/blob/master/notes/voting.txt _______________________________________________ tor-dev mailing list tor-dev@lists.torproject.org https://lists.torproject.org/cgi-bin/mailman/listinfo/tor-dev
