Zcoin is the first full implementation of the Zerocoin Protocol, which allows users to have complete privacy via Zero-Knowledge cryptographic proofs. It is worth noting that Zcoin is unrelated to other cryptocurrencies utilizing the Zerocash Protocol. Although Zerocash is a development from Zerocoin, their respective implementations are not simple forks of each other, but rely on different cryptographic assumptions with various tradeoffs. Both approaches supplement each other quite nicely, and a good way to describe them would be sibling projects. The Zerocoin Protocol is being actively researched and improved, such as by a new extension that when implemented would hide transactions and address balances.
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