DeCus is a new decentralized custody scheme. eBTC Powered by DeCus, eBTC has crosschain interoperability and can be extended to other blochchains. It is an ERC20 token trustlessly backed by BTC, designed to bring liquidity of BTC to multiple DeFi protocols. BTC can be 1-to-1 converted into eBTC and vice-versa. eBTC is the first decentralized cross-chain BTC solution that is free from over-collateralization and settlement risk. 5x-15x collateral efficiency can be achieved because every BTC pledged in the system is capable of securing custody for more than 5 BTC (and 5 minted eBTC respectively). Based on the above analysis, DeCus implements the decentralized custody scheme with fewer groups in representative, and achieves impressive low collateral ratio.For example, with a polynomial design over prime fields, DeCus is able to achieve efficiency factor $\eta=A/C$ as much as 20 with $n=121$ custodians, which means the collateral is <5% of total assets under custody! It’s also possible to have the collateral kept together with other assets under custody thanks to the above results. From this feature we introduce eBTC, which is a decentralized cross-chain BTC token 1:1 backed by BTC, with low collateralization rate and truly BTC as collateral. In eBTC, the custodians are assigned into overlapping groups following DeCus, where every group is a multisign BTC address and every custodian belongs to multiple groups.