Among the many problems faced by the blockchain, the interoperability between the blockchains greatly limits the application space of the blockchain. Regardless of whether it is a public chain or a private chain, cross-chain technology is the key to realizing the Internet of Value. It is a good medicine to save the blockchain from scattered islands and a bridge for the expansion and connection of the blockchain.
The current mainstream cross-chain technologies include:
1. Notary schemes
2. Sidechains/relays
3. Hash-locking
4. Distributed private key control
Lists are compared:
The above table lists the basic differences of various cross-chain technologies. Early cross-chain technologies include Ripple and BTC Relay, which are more concerned with asset transfer; existing cross-chain technologies are represented by Polkadot and Cosmos. It is a cross-chain infrastructure; the newly emerging FUSION implements multi-currency smart contracts, which is a public chain with great application value, on which rich cross-chain financial applications can be generated.
1. Notarization Technology: Ripple Interledger Protocol
As early as 2012, Ripple Labs proposed the Interledger protocol to connect different ledgers and achieve collaboration between them. The Interledger protocol is applicable to all accounting systems and can accommodate the differences of all accounting systems. The goal of the protocol is to create a global unified payment standard and create a unified network financial transmission protocol.
The Interledger protocol enables two different accounting systems to freely transfer money to each other through third-party "connectors" or "validators." The accounting system does not need to trust the "connector", because the protocol uses a cryptographic algorithm to create a fund escrow for the two accounting systems with the connector, and when all participants reach a consensus on the transaction, they can trade with each other. The protocol removes the trust required by transaction participants, and the connector cannot lose or steal funds, which means that such transactions do not need the protection of legal contracts and excessive review, greatly reducing the threshold. At the same time, only the accounting system involved can track the transaction, and the details of the transaction can be hidden. The "validator" operates through an encryption algorithm, so the details of the transaction cannot be directly seen. In theory, the protocol is compatible with any online bookkeeping system, and banks' existing bookkeeping systems can use the protocol with only minor changes. This enables direct transactions between banks without a central counterparty or correspondent bank.
2. Sidechain Technology: BTC Relay
The side chain is a new type of blockchain based on anchoring a token on the original chain, just as the dollar is anchored to gold. Sidechains connect various chains, and other blockchains can exist independently. However, it is difficult to build cross-chain smart contracts on side chains now, so it is difficult to realize various financial functions, which is why the existing blockchains have not made progress in the fields of stocks, bonds, derivatives, etc.
BTC Relay was born and grown with the support of the Ethereum Foundation, and it is considered to be the first sidechain on the blockchain. BTC Relay connects the Ethereum network with the Bitcoin network through the use of Ethereum smart contracts, allowing users to verify Bitcoin transactions on Ethereum. It creates a small version of the Bitcoin blockchain through the Ethereum smart contract, but the smart contract needs to obtain the Bitcoin network data, which is relatively difficult to achieve decentralization. BTC Relay has made a meaningful attempt to communicate across blockchains, opening a channel for communication between different blockchains.
3. Relay technology: Polkadot and COSMOS
Polkadot
Polkadot is a public chain launched by the main core developers of the original Ethereum. It aims to solve two major challenges preventing the spread and acceptance of blockchain technology today: instant scalability and extensibility. Polkadot plans to integrate the private chain/consortium chain into the consensus network of the public chain, while maintaining the original data privacy and permissioned features of the private chain/consortium chain. It can connect multiple blockchains to each other.
In Polkadot's view, other blockchains are all parallel chains. Polkadot can transfer the tokens on the original chain to the original chain address similar to multi-signature control through relay-chain technology, and carry out Temporarily locked, the transaction result on the relay chain will be voted by these signers to determine whether it will take effect. It also introduced the fisherman role to report and monitor transactions. Through Polkadot, Bitcoin, Ethereum, etc. can be linked to Polkadot to realize cross-chain communication.
At present, Polkadot is still mainly based on Ethereum, realizing its interconnection with private chains, and aiming at upgrading other public chain networks, and finally allowing Ethereum to communicate directly with any chain.
COSMOS
Cosmos是tendermint团队推出的一个支持跨链交互的异构网络。Cosmos采用的Tendermint共识算法，是一个类似实用拜占庭容错共识引擎，具有高性能、一致性等特点，而且在其严格的分叉责任制保证下，能够防止怀有恶意的参与者做出不当操作。
Cosmos上的第一个空间叫做"Cosmos Hub"。Cosmos Hub中心是一种多资产权益证明加密货币网络，它通过简单的管理机制来实现网络的改动与更新，还可以通过连接其他空间来实现扩展。
Cosmos 网络的中心及各个空间可以通过区块链间通信（IBC）协议进行沟通，这种协议是针对区块链网络的，类似UDP或TCP网络协议。 代币可以安全快速地从一个空间传递到另一个空间，两者之间无需体现汇兑流动性。相反，空间内部所有代币的转移都会通过Cosmos 中心，它会记录每个空间所持有的代币总量。这个中心会将每个空间与其他故障空间隔离开。因为每个人都可以将新空间连接到Cosmos中心，所以Cosmos也可以兼容未来新的区块链。
这一架构解决了当今区块链领域面临的许多问题，包括应用程序互操作性、可扩展性、以及无缝更新性。比如，从 Bitcoind、Go-Ethereum、ZCash或其他区块链系统中衍生出来的空间，都可以接入Cosmos中心。这些空间允许Cosmos实现无限扩展，从而满足全球交易的需求。
4.哈希锁定技术：Lighting network
Lightning network闪电网络提供了一个可扩展的bitcoin微支付通道网络，它极大提升了比特币网络链外的交易处理能力。交易双方若在区块链上预先设有支付通道，就可以多次、高频、双向地实现快速确认的微支付；双方若无直接的点对点支付通道，只要网络中存在一条连通双方的、由多个支付通道构成的支付路径，闪电网络也可以利用这条支付路径实现资金在双方之间的可靠转移。 闪电网络的关键技术是HTLC哈希锁定技术，基本原理如下：Alice和Bob可以达成这样一个协议：协议将锁定Alice的0.1 BTC，在时刻T到来之前（T以未来的某个区块链高度表述），如果Bob能够向Alice出示一个适当的R（称为秘密），使得R的哈希值等于事先约定的值H(R)，Bob就能获得这0.1 BTC；如果直到时刻T过去Bob仍然未能提供一个正确的R，这0.1 BTC将自动解冻并归还Alice。
闪电网络并不试图解决单次支付的问题，其假设是单次支付的金额足够小，即使一方违约另一方的损失也非常小，风险可以承受。因此使用时必须注意“微支付”这个前提。
5.分布式私钥控制技术WanChain
WanChain万维链也支持主流公有链间的跨链交易，但首先需要完成在万维链上的注册，确保万维链能够对该链进行唯一识别。对于跨链交易，万维链利用多方计算和门限密钥共享方案。当一种未注册资产由原有链转移到万维链上时,万维链节点会使用一个基于协议的内置资产模板,根据跨链交易信息部署新的智能合约创建新的资产。当一种已注册资产由原有链转移到万维链上时,万维链节点会为用户在已有合约中发放相应等值代币,确保了原有链资产在万维链上仍然可以相互交易流通。
万维链通过分布式的方式完成不同区块链账本的连接及价值交换。它采用通用的跨链协议以及记录跨链交易、链内交易的分布式账本，公有链、私有链还是联盟链，均能接入万维链，实现不同区块链账本的连接及资产的跨账本转移。
但是，实现各种链映射到一条链上只是完成了第一步，如果上面的智能合约还是像现在的仅为交易触发，智能合约没办法实现分布式运算和多触发机制，那多币种智能合约能实现仍然是相当有限的功能。
FUSION
各种加密资产可以通过分布式私钥生成与控制技术被映射到FUSION公有链上。多种被映射的加密资产可以在其公有链上进行自由交互。实现和解除分布式控制权管理的操作称为：锁入（Lock-in）和解锁（Lock-out）。锁入是对所有通过密钥控制的数字资产实现分布式控制权管理和资产映射的过程。解锁是锁入的逆向操作，将数字资产的控制权交还给所有者。
由于各种资产映射到FUISON上，其原链上的功能丧失了，其主要属性是金融价值。所以FUSION要做的是价值互联网时代的金融，即加密金融。之所以叫加密金融是因为，各种加密金融的资产都是由私钥控制的。
FUSION (fusion.org)协议支持各种代币之间的价值转移的同时，在并行运算和多触发机制方面进行了极大的革新，从而实现了高效率的、面向多币种和多种触发机制的增强型智能合约。
FUSION通过多层共识机制和记账节点分组，实现了一定的并行计算。多层机制使合约计算与计算结果的记账分步完成，记账节点分组使不同的智能合约由不同的记账节点分组完成。
FUSION想要做的加密金融合约还必须有多种触发机制才能完成完整的金融功能。针对这一问题，FUSION引入了“先知”的功能。虽然这一功能已经被多次提出，但一直没有很好的实现手段，这是由于智能合约每次加载相当浪费资源，也不现实。FUSION创造性的使用了“Calling list”机制将各种触发条件与智能合约分开储备，记账节点只要加载触发条件的列表即可，被触发时才加载智能合约，这使各种交易触发、时间触发和事件触发（包括链外数据触发）可以写到触发列表里。
How the Internet of Value can be called a "net" instead of a set of parallel lines requires various cross-chain technologies. However, cross-chain transactions are only the most superficial requirements for cross-chains, and multi-currency contracts are the essence of cross-chains. In addition, cross-chain enables tokens on various original chains to be converted into financial assets, resulting in encrypted financial applications. However, in order to achieve complete financial functions, smart contracts need to be enhanced, and parallel computing is used to improve the efficiency of smart contracts, so that encrypted finance can complete complete financial functions, which is the future direction of cross-chain technology.
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