Low Cost Constant Round MPC Combining BMR and Oblivious Transfer

In this work, we present two new universally composable, actively secure, constant round multi-party protocols for generating BMR garbled circuits with free-XOR and reduced costs. (1) Our first protocol takes a generic approach using any secret-sharing based MPC protocol for binary circuits, and a correlated oblivious transfer functionality. (2) Our specialized protocol uses secret-sharing based MPC with information-theoretic MACs. This approach is less general, but requires no additional correlated OTs to compute the garbled circuit. In both approaches, the underlying secret-sharing based protocol is only used for one secure $F_2$ multiplication per AND gate. An interesting consequence of this is that, with current techniques, constant round MPC for binary circuits is not much more expensive than practical, non-constant round protocols. We demonstrate the practicality of our second protocol with an implementation, and perform experiments with up to $9$ parties securely computing the AES and SHA-256 circuits. Our running times improve upon the best possible performance with previous BMR-based protocols by 60 times

Efficient Constant-Round Multi-party Computation Combining BMR and SPDZ

© 2019, International Association for Cryptologic Research. Recently, there has been huge progress in the field of concretely efficient secure computation, even while providing security in the presence of malicious adversaries. This is especially the case in the two-party setting, where constant-round protocols exist that remain fast even over slow networks. However, in the multi-party setting, all concretely efficient fully secure protocols, such as SPDZ, require many rounds of communication. In this paper, we present a constant-round multi-party secure computation protocol that is fully secure in the presence of malicious adversaries and for any number of corrupted parties. Our construction is based on the constant-round protocol of Beaver et al. (the BMR protocol) and is the first version of that protocol that is concretely efficient for the dishonest majority case. Our protocol includes an online phase that is extremely fast and mainly consists of each party locally evaluating a garbled circuit. For the offline phase, we present both a generic construction (using any underlying MPC protocol) and a highly efficient instantiation based on the SPDZ protocol. Our estimates show the protocol to be considerably more efficient than previous fully secure multi-party protocols.status: publishe

Raziel: Private and Verifiable Smart Contracts on Blockchains

Raziel combines secure multi-party computation and proof-carrying code to provide privacy, correctness and verifiability guarantees for smart contracts on blockchains. Effectively solving DAO and Gyges attacks, this paper describes an implementation and presents examples to demonstrate its practical viability (e.g., private and verifiable crowdfundings and investment funds). Additionally, we show how to use Zero-Knowledge Proofs of Proofs (i.e., Proof-Carrying Code certificates) to prove the validity of smart contracts to third parties before their execution without revealing anything else. Finally, we show how miners could get rewarded for generating pre-processing data for secure multi-party computation.Comment: Support: cothority/ByzCoin/OmniLedge