[davidsarah]

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)

[polarized]

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).

[daira]

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.

[polarized]

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.

[davidsarah]

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).