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> ZCash is pretty explicit about the difference between shielded and transparent addresses....

This is an important point. Having two very distinct addresses (different lengths, different prefixes, different RPC APIs) makes it very obvious to users when they have the benefits of shielded transactions, and when they don't. Thus users make an explicit choice to forgo privacy when they use transparent addresses.

The problem of only 28% of transactions currently being shielded (https://explorer.zcha.in/statistics/network https://explorer.zcha.in/statistics/timeseries?hashrate=fals...) is a separate ecosystem problem, where usage of shielded addresses with third parties (like wallets and exchanges) requires them to use new APIs, instead of just interacting with the new block chain via the Bitcoin API. IIUC Monero has also encountered these issues, and it is something we are both working on improving.

Sorry if I'm looking at the data wrong, but this chart suggests less than 5% are shielded https://explorer.zcha.in/statistics/value

Is the chart wrong?

Perhaps it could have something to do with termonology (%value vs %transactions).

For value I get: 42588 / (891015+222753) = 3.8% Shielded Value / (Cumulative Miner's Reward + Cumulative Founder's Reward)

Perhaps it is 28% of transactions are shielded worth only 4% of zec value?

Note that the number of prior shielded transactions (not the proportion, and not the value) is what is actually relevant to the privacy of new shielded transactions. Roughly speaking, the privacy you get with Zcash is comparable to what you would get with Monero if you could use all previous shielded transactions (over 120000 of them, currently) as mixins. That's why the criticisms of Zcash based on the percentage use of shielding (either by transactions or value) are totally missing the point.

-- Daira Hopwood (Zcash developer)

The number of note commitments can be found using 'zcash-cli getblockchaininfo' and is currently 301068 commitments, i.e. 150534 JoinSplits (so a bit more than the 120000 I said).

The chart isn't wrong. It's just measuring something different than the metric str4d is referring to.

Underneath that pie chart, there is a caption: "Transparent value (stored in t-addresses) vs shielded value (stored in z-addresses), in ZEC." In other words, that pie chart shows the number of ZEC that are currently residing in transparent vs shielded addresses at the specific point in time that you load that page.

The Advanced Network Stats page - https://explorer.zcha.in/statistics/network - has a box labelled "Shielded Transaction Percentage", which indicates what percentage of transactions involve a shielded value. You can see more details for different amounts of time on https://explorer.zcha.in/statistics/usage

> The point they are trying to make is that the anonymity set between shielded addresses is that of all transactions in the anonymous set.

This way of stating is somewhat questionable in light of the claims in the second half of the paper.

What is shown in the second half of the paper is that all possible sources are not equally likely and this most probably applies to Zcash (and every other coin) as well. In the Figure 1 illustration of Zcash, it is most likely that the rightmost (most recent) arc is the correct one. Of course this can't be stated with certainty in either coin.

Another way of interpreting the trend shown in Figure 8 is that Zcash gains little (though of course it still gains something) from including all transactions in the anonymity set (arbitrarily far to the right) because once one departs from focusing predominantly on the more recent transactions, the effective anonymity set does not grow much.

> Instead, it looks like clients weren't even doing basic checks:

>> We find that among Monero transaction inputs with one or more mixins, 62% of these are deducible, i.e. they can be incontrovertibly linked to the prior TXO they spend.

There are no basic checks that can solve that issue. It was fixed in a different way.

> even if Monero fixes everything in this upcoming release,

Most of the issues in the paper were already addressed in the past, and the paper says this. The remaining issue is the time bias which the paper states has already been improved, but can be improved further.

You're mistaken in saying that it is most likely that the actual note is the most recent one for Zcash. The figure gives a slightly misleading impression because it has to show few enough inputs to fit on the page. The number of possible inputs is the total number of previous shielded notes (before the JoinSplit anchor) that the adversary does not control or know to have been spent. There have been around 129000 JoinSplits so far, each creating two notes; I'll get back to this with a more precise number. In any case, the probability of the actual note being an output from the most recent prior JoinSplit is extremely small, even taking into account recency bias.

Another way of saying this is that in Zcash, the content of a fully shielded transaction does not give an adversary any more information about the possible input distribution than they could guess without seeing the content (i.e. only based on the timestamp and the number of JoinSplits in that transaction). In Monero, the adversary can refine their guess of the distribution based on the inputs that are actually mixed in, and that is what creates the privacy weakness.

Figure 8 does not apply to Zcash, it is specific to Monero, as the caption states.

-- Daira Hopwood (Zcash developer)

Yes you are likely correct that "most recent" being the "most likely" is not accurate. However, there is a distribution and it has a peak. It is certainly not flat, so it is incorrect to say that the entire set constitutes an "effective anonymity set" while at the same time claiming that Monero's ring signatures only have an "effective mixin size" that is smaller than the actual size due to the same non-uniform distribution.

> In Monero, the adversary can refine their guess of the distribution based on the inputs that are actually mixed in, and that is what creates the privacy weakness.

That is not what is claimed in Section 4 of the paper. Section 4 merely indicates that of potential outputs, the time distribution introduces a bias toward the most recent (actually in Monero this might be inaccurate in some cases too: very, very recent might be less likely than merely very recent; the paper does not examine this). In Zerocash the same time distribution bias exists, though across a larger set of potential coins (or notes or whatever it is you call it).

However, very old members of that set are essentially irrelevant as their probability in the distribution is almost certainly extremely low (this is the same reason that more older outputs in Monero are essentially irrelevant).

As far as I know we've never claimed that the distribution is flat or that the "effective anonymity" is equivalent to a uniform distribution over prior notes (I certainly didn't claim that). One of the advantages of Zcash's approach is that you don't need to know the distribution in order to have a strong privacy claim. As I said, this is because the content of a transaction is not revealed, and so the attacker's advantage is no better than guessing based on their prior knowledge (plus the little information that can be inferred from timestamps and number of JoinSplits in a transaction).

It's the same claim as for semantically secure encryption, for example: no competent cryptographer would claim that encrypting a message implies that the adversary's knowledge of the plaintext distribution is uniform; only that the ciphertext gives the attacker no further information (apart from length, typically) about the distribution.

The comment to which I replied (not by you) claimed that the anonymity set is all shielded transactions.

It is, in the same sense that the first order anonymity set of Monero transactions is all outputs included in the ring signature which can't be proven implausible (e.g. using the methods in Section 3 of the paper). However, Section 4 of the paper points out that a non-uniform distribution means this is reduced, in practice, to a smaller effective degree. The same method can be used with Zcash to estimate a smaller effective degree since many previous shielded transactions are probabilistically unlikely.

This is certainly not 'deanonymization' or 'tracing' or any such thing, but it isn't that in the Monero case either.

The number of note commitments can be found using 'zcash-cli getblockchaininfo' and is currently 301068 commitments, i.e. 150534 JoinSplits (so a bit more than the 129000 I said).

What do you mean "if Monero fixes everything"? They already fixed everything, which is why the paper shows the decline in their ability to deduce things, and the paper COMPLETELY STOPS showing data after RingCT went into action.

Perhaps you should familiarise yourself with the papers that Monero themselves published on this in September 2014, and the follow-up in January 2015? Here-

https://lab.getmonero.org/pubs/MRL-0001.pdf

https://lab.getmonero.org/pubs/MRL-0004.pdf

Now the recommendations made in that 2nd paper were only instituted in the v2 hard fork in March 2016, because hard forks are hard and it was their first one, but it doesn't change the fact that they published two papers on it to warn the community, made immediate changes so that updated clients used minimum ring sig sizes, and then hard forked to ban mixin 0. Publishing a paper on an already-discovered and already-solved issue two years later isn't particularly interesting or novel.