It is sometimes convenient to reveal a small part of a transaction to a counter- party in a way that allows them to check the signatures and sign it themselves. A typical use case for this is an oracle, defined as a network service that is trusted to sign transactions containing statements about the world outside the ledger only if the statements are true. Here are some example statements an oracle might check: • The price of a stock at a particular moment was X. • An agreed upon interest rate at a particular moment was Y. • If a specific organisation has declared bankruptcy. • Weather conditions in a particular place at a particular time.