<sarang> Let's go ahead and get started with GREETINGS <kinghat> o/ <sgp_> hello <sarang> That's long enough! <sarang> Let's move to ROUNDTABLE <sarang> suraeNoether: what up with you <suraeNoether> i'm terribly ill this morning, so my update will be very brief. my work in this past week has involved three incomplete tasks: <suraeNoether> 1) CLSAG linkable anonymity proof required some thought. sarang and i have thought about it and we have a strategy to finish writing the proof. sarang: do you want to make the changes to our LA definition or do you want i should? <sarang> suraeNoether: I have a writeup for LA in my notebook that I'm transcribing to TeX <suraeNoether> and proof* not just the definition <sarang> it works just fine <sarang> On that note <sarang> Do you have any thoughts on linkability (not LA) <sarang> I don't particularly like the Backes definition <suraeNoether> uh one sec <sarang> Triptych has a version of linkability+non-frameability that I like better <suraeNoether> is there soemthing wrong with the definition we proposed initially? <suraeNoether> iirc that one's from bender <sarang> It's not formalized quite enough, in the apparent opinion of the reviewer <sarang> I think it needs just minor work <sarang> Triptych formalizes it a tad more IMO <sarang> I can add that to the writeup if you like <suraeNoether> well <suraeNoether> for the sake of the audience, can you describe the 3 different definitions you want to consider? or 2, assuming you want to bail on backes' <sarang> Backes requires the following for an LRS: completeness, linkable anonymity, linkability, non-frameability <sarang> Right now we combine linkability and non-frameability with non-standard terminology <sarang> Backes uses a particular linkability definition: can the adversary use q keys to generate q+1 non-linking signatures? <sarang> Where q is scaled via the security parameter <sarang> I don't particularly like this definition over the "usual" one about producing two linking signatures, but I think it's important to frame the definition as a challenger-player interaction <sarang> Our current method does this very informally <sarang> I propose a combined linkability definition in my Triptych writeup that's a slight formalization of what CLSAG has now <sarang> (it could easily be split into linkability and non-frameability) <suraeNoether> hmmmm q scaling with the security parameter is the weird part to me: if the security parameter goes up, so does q... and so this means, for example, the adversary can't produce 3 signatures using 2 keys without some linking occurring. this feels *weaker* than the statement "can't produce two signatures using the same key without them being linked" <sarang> Yeah, which is why I don't really like it <sarang> didn't sit well with me <suraeNoether> and we want the property with q=1 anyway to prevent double-spending <sarang> So I am proposing not using the Backes definition, but simply formalizing what we have now, a la Triptych <sarang> then it's more clear what the linkability player has access to in terms of keys etc. <suraeNoether> okay, i'm going to read more deeply into that this afternoon <sarang> IMO it's a pretty straightforward formalization <sarang> doesn't affect much in practice <suraeNoether> backes' definition with q=1 seems to me to imply backes' definition with greater q, but it's possible that it doesn't technically reduce the way it seems. i'll think more about it <sarang> That definition doesn't make assumptions about linking tags being equal AFAICT <sarang> Whereas ours does <sarang> I think that's part of it <sarang> Anyway, you were talking about work you'd been doing, before I barged in =p <suraeNoether> moving along, my next incomplete task is reviewing triptych's security proofs more deeply, which dovetails with this :P <sarang> Yeah, a nice tie-in <suraeNoether> finally, i'm working on matching simulations today. i'm experiencing a data management and presentation issue, but i hope for the end of the day a nice graph displaying performance of Eve as a function of ring size and churn length <sarang> Nice! <suraeNoether> this will come along with a push to my repo with all the code used to generate that, and explanations so people can replicate it <sarang> word <suraeNoether> that's it, if i had presented in the other order then your "barging" would have been a great segue into *your* work for the week :P <sarang> We can pretend otherwise <sarang> I have completed a draft of the Triptych preprint, which is now in suraeNoether's hands <sgp_> suraeNoether: I'm really looking forward to that chart <sarang> it includes my proposed linkability+non-frameability definition <sarang> Figured out the CLSAG linkable anonymity definition, which is not as strong as Backes, but does the job IMO <sarang> I've also been working with Aram from Zcoin on some related Groth proving system stuff <sgp_> what's the shortfall on the linkable anonymity definition, even if there's no practical difference? <sarang> There will be a neat paper coming out from them on that shortly, which they graciously provided to me in advance <sarang> sgp_: Backes permits key corruption, which doesn't work with our DDH hardness assumption <sarang> Instead, we assume the adversary can obtain key images <sarang> And that the adversary can pack rings with their own malicious keys <sgp_> sarang: thanks <sarang> (which you can assume are trivially corrupted) <sarang> This is already stronger than the existing definition that was used <sarang> Otherwise, I also wish to update the DLSAG paper (which will appear next year in conference proceedings) with the CLSAG security model, since they are structurally extremely similar <sarang> So overall, a lot of tedious (but still interesting) stuff involving formal definitions and proofs <sarang> When suraeNoether finishes his review of the Triptych preprint, it'll go to the IACR archive <sarang> and presumably any CLSAG/DLSAG updates as well <suraeNoether> hmm Backes' linkability definition is a puzzle i have very little intuition about: should it be harder or easier to present 2 signatures from the same key without linking the signatures than it should be to present 201 signatures from 200 different keys without any of them linking? *taps chin* <sarang> The adversary picks which keys IIRC, right? <suraeNoether> yeah, adversary can use KeyGen or any other way of selecting the verification keys <suraeNoether> may not even know the secret key, so it's genuinely adversarial <sarang> ya <sarang> The adversarial generation isn't really a big deal, since soundness implies the adversary's choice of keys satisfy the verification equations <sarang> and then you rely on the one-way mapping <suraeNoether> actually, it's not clear; each verification key needs to be in \mathcal{VK}, and it's not specified where that comes from, i'm assuming from the challenger <suraeNoether> in which case the adversary has to pick challenge keys to break linkability, it's not enough for the adversary to pack all rings with fake pubkeys <sarang> Backes even notes that generating q such signatures is trivial, since you simply use separate keys <sarang> Fake pubkeys should be acceptable <sarang> since the adversary does all this offline, or otherwise generates the pubkeys in its own (seemingly) valid transactions <sarang> The q=1 case feels like some kind of targeted linking attack, where the general q case seems like a broader "hope for a collision somewhere" attack <sarang> suraeNoether: thoughts? <suraeNoether> nothing concrete. the way this definition is written feels very very counter-intuitive to the way you and i have discussed linkability in the past. <sarang> Yeah, and I haven't seen it anywhere else <sarang> Again, I don't feel any particular need to use it <sarang> But getting the existing definition more formalized in a challenger-player sense seems wise <suraeNoether> agreed <sarang> roger <sarang> OK, that's my update <sarang> Does anyone else have interesting (or uninteresting) research to share? <suraeNoether> ok, dude, i think i know the problem here <suraeNoether> with that definition <suraeNoether> or at least my problem with it <sarang> Ooh, go on <suraeNoether> linkability is a property that has a "correctness" component and a "soundness" component. to correctly link two things means to link them when they should be linked. to soundly link two things is to *only* link them when they should be linked <suraeNoether> you called this positive and negative linkability at some point <suraeNoether> i feel like this definition is mashing the two together <suraeNoether> or attempting to <suraeNoether> anyway, my thoughts don't go deeper than that yet <sarang> Backes uses non-frameability to show that you can't make signatures that _appear_ to link without knowing/using the same key <sarang> and linkability to mean that you can't make sigs with the same key(s) but different tag(s) <sarang> The reviewer didn't like the CLSAG paper's use of positive/negative/soundness in linkability <suraeNoether> hmm <suraeNoether> okay, that's going to require more thought <suraeNoether> anywya, now i'm done. :P <sarang> A lot of this is simply getting the right terminology for the definition(s) of choice <sarang> I happen to like using linkability to refer to both <sarang> since that's typically what you want <sarang> but it's two different concepts <sarang> OK, we can move on to any other research <sarang> or to the next topic, QUESTIONS <suraeNoether> i have a pretty general observation <suraeNoether> which may be relevant in terms of independent interest <suraeNoether> a property like linkability applies to all ZK proofs. for example, our ring signatures are ZK proofs of knowledge of a secret key. but they are *linkable* proofs of knowledge, so that if the same witness data (keys) are used for two different proofs (signatures), then an observer can link them. <suraeNoether> so just like ZK proofs have a property of correctness (if you know a witness, the proof is valid) and a property of soundness (if you don't know a witness, your proof is invalid), a linkable ZK proof is going to have a dual pair of notions for linkability <suraeNoether> i bring this up so that the next version of snarks has an L floating around <sarang> There's a related-ish property in sigma protocols, quasi-unique responses <sarang> But that relates to responses to the verifier challenge <suraeNoether> more reading to do :\ <sarang> There's probably a subtle relationship to (SHV)ZK <sarang> and therefore witness indistinguishability <sarang> (which follows from SHVZK) <suraeNoether> anyway <sarang> Normally, providing two proofs should not reveal distinguishing information about the witnesses <suraeNoether> right <sarang> Hopefully you will enjoy the Triptych paper, which builds a linkable construction on top of a sigma protocol :) <suraeNoether> i enjoyed it the last time i read it, and the tiem before that. it takes awhile to digest :P <suraeNoether> ok, i gotta bounce, i'm not feeling well; my list of 3 unfinished tasks is also my list of action items today <sarang> roger <sarang> My ACTION ITEMS are getting these new definitions and proofs typeset and finalized, determining their DLSAG applicability, a few other organizational issues on the CLSAG paper to prepare it for resubmission, and getting Triptych submitted on review <sarang> Any other final thoughts, comments, or questions before this meeting ends? <moneromooo> I have an unrelated question. <sarang> ? <moneromooo> I was wondering whether atomic swaps between two cryptonotes with hte same curve etc (ie, not the general case) is possible now. <moneromooo> Well, assuming the tooling was there of course, which it isn't. <moneromooo> In theory I mean. <sarang> I don't know of a good way that retains indistinguishability as well as DLSAG does, and that still has the tracing issue <sarang> If you were willing to accept and mitigate the tracing issue, then its method could do it <sarang> its = DLSAG's <moneromooo> What is the tracing issue already ? <sarang> The fixed basepoint used for dual-address key images allows determination of unwanted signature linking <sarang> It isn't clear how to do a DLSAG-type construction with the variable-basepoint key images used currently <sarang> I should more precisely say, the use of a fixed basepoint and having output private keys used as the corresponding key image discrete log (this doesn't exist in more recent constructions that use a fixed basepoint but in a different way) <sarang> Oh, suraeNoether: do you think it's useful in the LA definition to include the linking tag oracle separately from the signature oracle? <sarang> The player can get the linking tag oracle result simply by querying the signature oracle on a public key by using a random ring and message (and ignoring everything but the returned linking tag) <sarang> Having a separate oracle only really serves to make it clear that the player doesn't necessarily need to convince a user to sign messages, but can obtain linking tags otherwise <sarang> (although in this security model, it can do both)