Tandem is a cryptographic engine for executing programs using Secure Multi-Party Computation (MPC), which allows 2 parties to compute a function without revealing their private inputs. The classic example from cryptography is the millionaires' problem, where two millionaires compute who is richer without revealing their wealth to each other.

MPC has in the past been mostly confined to academia, but thanks to some recent papers has finally become fast enough for practical applications. Tandem uses Garbled Circuits, encrypted circuits consisting of boolean gates. While there have been other implementations based on these ideas, Tandem tries to provide a much better user experience than previous attempts. For example, the Tandem repo includes a server, a command line client and client libs for wasm and native architectures. To make it easier to run practical applications, programs can be written in Garble, a custom Rust-like programming language that compiles to Garbled Circuits.

(The most practical application to date using Tandem is "Encryptle", a Wordle clone that runs entirely over MPC. Depending on latency, a client/server exchange takes about 400ms, which makes the UX very similar to the real Wordle, with the twist that the server cannot see the guess in plain text and the browser cannot see the solution in plain text. Instead, guess and solution are kept private and compared using MPC.)