Secure Multi-Party Computation (SMPC or MPC) is a cryptographic technique that enables multiple parties to jointly compute a function over their combined private inputs without revealing those inputs to each other. Each party learns only the final result, not any other party's data.

How MPC Works (Simplified)

• Secret Sharing: Each party's input is split into random "shares" distributed to other parties. No single party has enough shares to reconstruct any input.
• Computation on Shares: Parties perform computations on their shares, exchanging intermediate results according to a predefined protocol.
• Result Reconstruction: Only the final result can be reconstructed from the combined output shares — intermediate values and original inputs remain hidden.

MPC Protocols

• Garbled Circuits (Yao's Protocol): One party "garbles" the computation into an encrypted circuit; the other evaluates it without learning intermediate values. Efficient for two-party computation.
• Secret Sharing (Shamir, BGW): Distribute data as polynomial shares among multiple parties. Supports addition natively; multiplication requires communication rounds.
• Oblivious Transfer (OT): A protocol where a sender transfers one of multiple items to a receiver without learning which item was selected.

Applications in AI/ML

• Privacy-Preserving ML Training: Multiple hospitals train a model on their combined patient data without any hospital sharing raw records.
• Federated Analytics: Aggregate statistics across organizations without exposing individual data points.
• Private Inference: A user sends an encrypted query to a model, receives the result, and the model operator never sees the query.
• Data Marketplaces: Validate data quality or compute on purchased data without revealing it before payment.

Challenges

• Performance: MPC is orders of magnitude slower than plaintext computation due to communication and cryptographic overhead.
• Communication: Parties must exchange messages proportional to the computation size, requiring reliable, high-bandwidth networks.
• Complexity: Designing and implementing correct MPC protocols requires deep cryptographic expertise.

MPC is gaining traction in healthcare, finance, and cross-organizational AI where data sharing is legally or competitively impossible but joint computation is valuable.