I just sent an email to a journalist in reply to their questions about EthereumZcash integration, and I thought part of my email should be more widely posted:
There are three different but related R&D efforts going on in parallel here. (I know that's confusing.)
Zcash-on-Ethereum (“ZoE”) — a project to upgrade Ethereum to support zk-SNARKs, and the Zcash protocol on top of the Ethereum blockchain. Note that zk-SNARKs might be useful for other things in addition to Zcash, such as for example privacy-protecting digital identity systems.
ZRelay — a project to upgrade Ethereum to support the Zcash Proof-of-Work (which includes algorithms named “Equihash” and “BLAKE2”) so that smart contracts running on the Ethereum blockchain can process the Zcash blockchain.
Cross-Chain Atomic Transactions (“XCAT”) — a way to make a transaction which executes across multiple blockchains. This could be used, for example, to create decentralized exchange between Ethereum, Zcash, and Bitcoin blockchains.
We are supporting all three of these efforts, but I want to draw your attention to the last one — XCAT — because:
It works with Bitcoin in addition to Ethereum and Zcash.
It doesn't require any upgrade to the Ethereum, Zcash, or Bitcoin blockchains. It can run on the current blockchains. That means that we can deploy it and users can start using it and building on top of it sooner.