Surae's End of November (2017!) Update

Hello, everyone! Sarang posted his update a few days ago to give the community time to review his work before the end of the month. I was hoping to finish multisig off before the end of this month... so I held off on writing this update until then... but it looks like I'm somewhere between 2 days and a week behind on that estimate.

MRL Announcements

Meetings. We are holding weekly meetings on Mondays at 17:00 UTC. Logs are to be posted on my github soon(tm). Usually we alternate between "office hours" and "research meetings." At office hours, we want members of the community to come in and be able to ask questions, so we are considering opening up a relay to the freenode channel during office hours times, unless things get out of hand.

POW-Difficulty Replacement Contest. Some time in December, I am going to formalize an FFS "idea" to open up a multiple-round contest for possible replacements for our proof of work game. The first round would have a 3- or 6-month deadline. Personally, I would love it if this FFS could have an unbounded reward amount. If the community is extremely generous, we could easily whip up a large enough reward to spur lots and lots of interest across the world.

The Bitcoin POW game uses SHA256 to find nonces that produce hashes with sufficiently small digests according to the Bitcoin difficulty metric. Our current POW game uses CryptoNight to find nonces that produce hashes with sufficiently small digests according to the CryptoNote difficulty metric. The winner need not be proof of work. My current thoughts are roughly this:

All submissions will be public. Submissions that minimize incentives for centralized mining (or maximize disincentives) will be preferred over submissions that do not. Submissions that are elegant will be preferred over submissions that are not. Submissions that have provable claims about desirable properties will be preferred over submissions that do not (e.g. for either the Bitcoin or the Monero POW games, the necessary and sufficient network conditions for these games to produce blocks in a Poisson process have not been identified, to my understanding). Submissions that have a smaller environmental impact will be preferred over submissions that have a larger impact. And so on. I would like as many ideas as possible about a judging rubric for the first round. Especially if a large amount of money will be put up as a prize.

The details of the next round would be announced along with the winners of the first round. The reward funds should be released when a set of judges agree on a winner. MRL and Monero Core should each have representation on the panel of judges, and there ought to be at least one independent judge not directly associated with the Monero Project, like Peter Todd, Tim Ruffing, or someone along those lines. But, again, this is just an idea. If the community doesn't like it, we can drop it.

Here is a rundown for November

Multisig. Almost done. I know, I know, it's been forever. We, as a community, have recently come to see how important it is to carefully and formally ensure the correctness of our schemes before proceeding. Multisig is a delicate thing because a naively implemented multisig can reveal information about the participants.

I'm finishing vetting key creation today, finishing signatures tomorrow and the next day. Then I'm passing the result off to moneromooo and luigi to ensure that my description of their code is accurate up to their understanding. Then onto Sarang for final reviews before submission, hopefully by the end of the month. I have my life until Sunday evening blocked off to finish this. A copy of the document will be made available to the community ASAP (an older version is on my github), after more checking and writing is completed.

This whitepaper on multisig will be broken into two papers: one will be intended for peer review describing multi-ring signatures, and one will be a Monero Standard. More about that later...

RTRS RingCT column-linkability and amortization. You may say "what? I thought we were putting RTRS RingCT on the back burner?" Well, I'm still think ing about amortization of signatures. I'm thinking it will be possible (although perhaps not feasible) for miners to include amortized signatures upon finding new blocks. This would allow users to cite an amortized signature for fast verification, but has some possible drawbacks. But more exciting, I'm also chatting with Tim Ruffing, one of the authors on the RTRS RingCT papers: he thinks he has a solution to our "linkability by columns" problem with MLSAG and RingCT. Currently we try to avoid using more than one ring signature per recipient. This avoids linking distinct outputs based on bundling of these ring signatures. Ruffing believes RTRS RingCT can be tweaked to prove several commitments in a vector of commitments; this would allow a single RTRS RingCT to be computed and checked for each output being spent.

Once all the details are checked, I'll write up a document and make a copy of it available to the community. If it works, of course.

Consequences of bulletproofs. In my last end-of-month update I hinted at issues with an exponential space-time trade-off in RTRS RingCT. Due to the speed and space savings with bulletproofs, it may now be feasible to implement RTRS RingCT. With improved verification time savings with bulletproofs we can relax our requirements for verification times for signatures. This will allow the slightly longer verification times of RTRS RingCT to be counter-acted. Solving the problem "what ring sizes can we really get away with?" involves some modeling and solving some linear programming problems (linear programming, or linear optimization, is an anachronistically named area of applied mathematics involved with optimizing logistic problems... see here for more information).

Hence, we will be inserting bulletproofs into Monero with low friction, and then we will look into the logistics of moving to RTRS RingCT.

Monero Standards. Right now, we don't have a comprehensive list of how Monero works, all the various primitives and how they all fit together. Sarang and I have begun working on some Monero Standards that are similar to the original Cryptonote Standards (see here for more information). For each standard, from our hash function on upward, we will describe the standard, provide a justification for Monero's choices in those standards (complete with references), as well as a list of possible replacement standards. For example, our Monero RingCT Standard should describe the RingCT scheme described by shen, which is essentially a ring signature with linear combinations of signing keys + amount commitments. Under the "possible replacements" section, we would describe both the RTRS RingCT scheme and the doubly efficient zk-snark technology as two separate options.

These standards may take awhile to complete, and will be living documents as we change the protocol over the years. In the meantime, it will make it dramatically easier for future researchers to step into MRL and pick up where previous researchers have left off.

Hierarchical view keys. Exploiting the algebra we currently use for computing one-time keys, the sub-address scheme plays with view keys in a certain way, allowing a user to have one single view key for many wallets. Similarly, we may split a view key into several shares, where each subset of shares can be used to grant partial view access to the wallet. A receiver can request that a sender use a particular basepoint in their transaction key where different subsets of shares of the view key grant access to transactions with different basepoints in their transaction keys. None of these are protocol-level observations, they are wallet-level observations. Moreover, these require only that a receiver optionally specify a basepoint.

In other words: hierarchical view keys are a latent feature of our one-time address scheme that has not seen specific development yet. It's a rather low priority compared to the other projects under development; it grants users fine-grained control over their legal compliance, but Monero Standards will have great long-term impact on development and research at Monero.

Criticisms. Monero has suffered some recent criticisms about our hash function. I want to briefly address them.

First, I believe part of the criticism came from a confusion between Keccak3, SHA-3, and Keccak: we have never claimed to use SHA-3 as our hash function, we have only used the Keccak3 hash function, which is a legacy choice inherited from the original CryptoNote reference code. Many developers confuse the two, but Keccak3 was the hash function on which SHA-3 is based. In particular, the Keccak sponge construction can be used to fashion lots and lots of primitives, all of which could fairly be called "Keccak:" both Keccak3 and SHA-3 are Keccak constructions. This may be a subtle nomenclature issue, but it's important because a good portion of our criticisms say "Hey, they aren't using SHA-3!"

Second, I believe part of the criticism also comes from our choice of library, which in my opinion isn't a big deal as long as the library does what it says on the tin. In this case, our hash function is a valid implementation of Keccak3 according to the Keccak3 documentation. The most important criticism, from my point of view, is our choice of pre-SHA-3 Keccak3 as our hash function. Keccak3 underwent lots of analysis during the SHA contest, and Keccak3 is a well-vetted hash funtion. However, it has not been chosen as an international standard. There is a sentiment in the cryptocurrency community to distrust standards, which is probably a healthy sentiment. In this case, however, it means that our choice of hash function is not likely to be supported in common, well-vetted libraries in the future. Moreover, since SHA-3 is an international standard, it shall be undergoing heavy stress testing over the coming decades, a benefit Keccak3 shall not enjoy.

Last month, after some discussions, we made changes to our choice of PRNG in Monero to match the PRNG for Bitcoin. There has since been some discussions instantiated by anonimal about this choice of PRNG. We at MRL are doing our best to assist the core team in weighing the relative costs and benefits of switching to a library like crypto++, and so we believe these criticisms fall into the same category. We intend to address these issues and make formal recommendations in the aforementioned Monero Standards. Sorry for using the word aforementioned.

Things that didn't move much include a) educational outreach, b) SPECTRE, c) anti-ASIC roadmap, d) refund transactions. Most of which was on hold to complete multisig.

As far as educational outreach, I contacted a few members of a few math/cs depts at universities around me, but I haven't gotten anything hopeful yet. I wanted to go local (with respect to me) to make it easier to organize, but that's looking less likely. No matter how enthusiastic of a department we find, garnering participation from faculty members, beginning an application process for new students, squirelling up funding, working out logistics of getting teachers or lecturers/speakers from point A to point B, where to stash students, etc would be a challenge to finish before, say, July. And some schools start their fall semesters in mid-August. So I'm thinking that Summer 2019 is reasonable as the first Monero Summer School... and would be a real fun way to finish off a two-year post-doc!

December plan. I am going to finish multisig, and then finish the zk-lit review with Jeffrey Quesnelle, since these are both slam dunks. Any other time in December I have will be devoted to a) looking into the logistics of using the bulletproofs + RTRS RingCT set-up, b) reading the new zk-stark paper and assessing its importance for Monero, c) beginning work on Monero Standards, which includes addressing our hash function criticisms, our PRNG, etc.

Thank you again! This is an incredible opportunity, and this community is filled with some smart cookies. Every day is a challenge, and I couldn't ask for a more fun thing to be doing with my life right now. I'm hoping that my work ends up making Monero better for you.