Cashu is a new ecash protocol for custodial Bitcoin applications that is tightly integrated into the Lightning network. An Ecash system consists of two parts, the mint and the ecash wallet. Ecash transactions are untraceable, instant, and require no fees. Cashu is built for Bitcoin. Wallets use the mint's Lightning node for making or receiving Bitcoin payments in exchange for ecash. A Cashu mint does not know who you are, what your balance is, or who you're transacting with. Users of a mint can exchange ecash privately without anyone being able to know who the involved parties are. Bitcoin payments are executed without anyone able to censor specific users.