So is this something that can be added on top of Bitcoin or is it a separate coin with improvements? It isn't clear to me which it is from the abstract.
"Zerocash can be deployed atop any ledger (even one maintained by a central bank). Here, we briefly detail integration with the Bitcoin protocol."
"Zerocash breaks compatibility with the Bitcoin network. While Zerocash could be integrated into Bitcoin (the actual currency and its supporting software) via a “flag day” where a super-majority of Bitcoin miners simultaneously adopt the new software, we neither expect nor advise such integration in the near future and suggest using Zerocash in a separate altcoin."
"Integrating Zerocash into Bitcoin consists of adding a new transaction type, Zerocash transactions, and modifying the protocol and software to invoke Zerocash’s DAP interface to create and verify these transactions. There are at least two possible approaches to this integration. The first approach replaces all bitcoins with zerocoins, making all transactions anonymous at the cost of losing any additional Bitcoin functionality provided by, e.g., the Bitcoin scripting language (see Section 6.1).
The second approach maintains this functionality, adding a parallel Zerocash currency, zerocoin, which can be converted to and from bitcoin at a one-to-one rate "
The ZeroCoin stuff was just not really technically viable as it was. Also as evidence by the fact that no altcoins have picked it up. The trusted initialization requirement was also kinda lame.
ZeroCash improves the performance from the network's perspective greatly (at the expense of client performance), so it might be easier to make work. It also greatly improves the anonymity by hiding the values, at the same time it makes the trusted initialization worse. Mixed bag.
("My initial read of their paper was interesting, but it was two to three orders of magnitude more resource intensive than would be required to make it actually viable ... On the plus side— approaches can only get better")
I do hope that cryptographic privacy improvements are themselves not too controversial to deploy. Bitcoin as it is, is basically a privacy disaster which is only mitigated by the fact that Bitcoin is a niche thing that no one is forced to use. If you felt compelled to use Bitcoin in your daily personal and business life the current state of privacy there would cause a lot of harm. I cringe a bit at people trying to promote Bitcoin to 'authorities' with the "it's not private" argument, I think that stems from a misunderstanding of what authorities should want (e.g. they shouldn't want people to be made vulnerable through a loss of privacy), and a lack of knoweldge into how to discuss that privacy isn't incompatible with the public interest (quite the opposite), and that trying to deny privacy seldom reduces privacy for criminals (since they can buy their way to privacy) but mostly for innocent individuals... but I guess we'll see how things pan out.
Right now the proposed privacy technology has to little maturity and too many tradeoffs to consider it in Bitcoin, but when that isn't the case I hope we can adopt them, but even without doing so the privacy techniques that can already be used in Bitcoin and cannot be stopped (like CoinJoins and CoinSwaps) can help a lot if they become more widely used.
The major objections are to the inherent complexity of Zerocoin (it uses actual bleeding edge crypto); the performance/size issues (it used to be hours per transaction, I think it's down to minutes now); and the potential for regulatory backlash at a time where Bitcoin people were in appeasement mode, to some extent.
Compared to the crypto used in Zerocash, Zerocoin's internals are fairly elementary.
Zerocoin used one-way accumulators and discrete log based ZKPs, which are fairly approachable to anyone who has taken an undergraduate course in cryptography. Zerocash, however, uses Ben-Sasson's highly efficient zk-SNARK construct [0], the details of which are probably fully understood by a handful of people in the world. There's a reason the people who geek out over this kind of thing (#bitcoin-wizards) call it 'Moon math'.
It's easy to do so, considering that the new zerocash protocol's whitepaper references zerocoin and claims that its new coins will still be called zerocoins once implemented.
