Low-Latency MPC for Online Digital Signatures

Multi-party computation (MPC) protocols are used to jointly construct digital signatures on behalf of private keys which are never reassembled by any single participant. Unlike non-MPC multi-signature schemes, the security of these signatures does not depend on policies or other context during verification: existence of the signature is itself proof of a successful preceding MPC protocol execution. While their elevated security justifies their use, practical MPC protocols' execution is based on essentially distributed systems, beneath a signature API which emulates centralized signing services. Even with such abstraction, MPC-based signing in those \"distributed\" systems advances through a protocol-specific number of rounds, and involves node-to-node communication sequences.

While data at rest scenarios with infrequent signing---typical for custody of digital assets---may tolerate the inherent high latencies, transaction-oriented use of many MPC schemes may be impractical for low-latency applications. We describe a subset of practical MPC protocols which may be used in latency-critical applications. We describe how implementations may be specialized for low per-signature latencies, possibly at the cost of more significant offline preparation. We show which security properties are relaxed, compared to the selected MPC schemes' latency-insensitive full implementations. We show why the security procedures of transaction processing may already compensate for the relative differences between original protocols and their latency-specialized variants.

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