What are the stated advantages and disadvantages of Zerocash with respect to Zerocoin?
Zerocash was trying to provide full privacy. They introduced efficiency improvements, by smaller proof size and faster verification. They also tried to enhance privacy by adding encryption to the amount and both the sender and receiver addresses.
In 2019, four of the eight major Zerocoin implementations switched from Zerocoin protocol to what?
Four of the Zerocoin implementations switched to Sigma.
What was the technical cause behind the 2017 “fake spend” incident?
There was a typo in the source code which was exploited to mint 370,000 additional Zcoins. The Zcoin team announced that 18,171 coins were generated through the exploit the fake spends caused inflation of the Zcoin supply.
Explain the different responses by Zcoin, PIVX and Veil to the 2019 attack on the Zerocoin core protocol
Zcoin disabled zerocoin mints and froze the funds in the accumulator until the release of Sigma.
PIVX deactivated the privacy feature of Zerocoin through a spork. They disabled Zerocoin minting but left Zerocoin spending enabled.
Veil team decided to deactivate the anonymizing feature from the Zerocoin protocol. The initial fix removed the zero-knowledge proof required to prevent double spend and replaced it with a single signature. This did not prevent attackers from stealing funds from the accumulator.