One area to explore would be the concept of smart contracts, currently best exampled by experiments with blockchain technology - Ethereum and transparency in prediction markets would have places to start. One possible solution to a real-world non-criminal application of that problem would be to hire a neutral third party with appropriate confidentiality trust - like a lawyer - to create an agreement, witness it, and hold it in trust.

This becomes less reliable when you start delving into the ethical problems outlined by other responders if you're trying to create something deniable, but you can probably work around that with an 'if-then' agreement with a lawyer and a pre-sealed envelope.