Short answer: Tornado Cash is a set of non-custodial smart contracts that use zero-knowledge proofs to let a withdrawal prove membership in a deposit set without revealing which deposit it corresponds to.
Protocol, contracts, and interfaces
Immutable contracts execute on public blockchains, while web interfaces make those contracts easier to access. The primary Classic interface is available at tornadocash.eth.limo.
What zero-knowledge proofs contribute
A user proves knowledge of the secret associated with a valid deposit commitment. The proof can be verified without publishing which exact commitment belongs to that user. Public metadata and user behavior can still weaken privacy, so “private” should not be read as “impossible to analyze.”
Open-source and non-custodial
Tornado Cash contracts, cryptographic circuits, and interface source code are publicly available. Assets are controlled through smart-contract rules and the private note created for a deposit.