It’s hard to imagine a single blockchain would suffice for all our society’s use cases. A multi-chain future seems inevitable. However, a multi-chain future without interoperability between the blockchains could be paralleled to the Internet before TCP/IP. Today’s blockchains are too fragmented and are by-nature not interoperable, hindering mass adoption of the technologies. For example, a decentralized application (dApp) must be married to a specific blockchain. If a user onboards into the crypto ecosystem through a given dApp, this fragmentation makes immense barriers for the user to fluidly adopt or try a dApp on another chain. To address the issues of interoperability, there have been a few proposals and projects that specifically emphasize the ability to inter-operate. However, the majority of interoperability systems only apply to specific blockchains, standardize their protocols within their own systems requiring other 1 blockchains to adopt or through complicated, restricted, and/or less secure bridges to join (see Figure 1). In this whitepaper we propose a novel, public L1 blockchain that actively and agnostically connects blockchains and facilitates interoperability. Furthermore, we propose a generic smart contract on blockchain that can hold and manipulate assets on external blockchains directly, thereby enabling generic smart contracts that can custody assets on external chains. This opens the door to boundless cross-chain dApps. Blockchains are naturally closed systems. The goal of this whitepaper is to design and specify a practical system that is generic in its inter-blockchain capability, without forcing existing blockchains to adopt new standards or a new blockchain that every assets needs to move to, and do so in a decentralized, byzantine fault tolerant way. In other words, we aim to create a public blockchain that supports real cross blockchain transactions, message delivery, and general cross-chain smart contracts. According to our extensive survey, to satisfy this goal, the best pragmatic approach is the decentralized notary scheme on top of an incentivized Proof-of-Stake replicated state machine (aka blockchain) which we call ZetaChain. ZetaChain is first of all a public blockchain with Proof-of-Stake validators. It’s trusted that a super majority (>66% nodes) of the validator nodes are honest and act according to protocol, and collectively serve as notaries. In addition to being a blockchain, interoperability requires observing other blockchains. Thus each ZetaChain validator node is attached with an observer that scans other blockchains for relevant events (event log, transaction, or state at a certain time). The observers report the relevant events to ZetaChain and reach consensus. ZetaChain uses custom logic to update its state in response to the reported events. On the other hand, in order to change state on other blockchains, each validator is also attached with a signer holding a key share. Collectively all the validators hold a single public/private key pair which can initiate transactions on other blockchains to change state directly. The signature scheme can be some kind of threshold signature scheme such as GG18/GG20 ECDSA/EdDSA, or BLS threshold/aggregate signatures, depending on the cryptography on different chains and their smart contract capability/cost. The presence of a single public key and address in the ZetaChain system allows ZetaChain to custody assets on external blockchains which might not have adequate smart contract capability such as Bitcoin. Such ability allows powerful cross-chain (or omnichain) dApps to be built on top of native ZetaChain cross-chain smart contracts. This capability looks much like on Ethereum where a smart contract can be trusted to manage assets according to predetermined rules, except on ZetaChain, a smart contract can leverage and manage assets on any connected blockchain. In summary, ZetaChain is designed to be a decentralized cross-blockchain smart contract platform. The vision of ZetaChain is to be a public computer on all important blockchains, on top of which cross-blockchain decentralized applications can be easily built as public, trustless, and persistent smart contracts.