Overdrive: Making SPDZ Great Again

SPDZ denotes a multiparty computation scheme in the preprocessing model based on somewhat homomorphic encryption (SHE) in the form of BGV. At CCS '16, Keller et al. presented MASCOT, a replacement of the preprocessing phase using oblivious transfer instead of SHE, improving by two orders of magnitude on the SPDZ implementation by Damgard et al. (ESORICS '13). In this work, we show that using SHE is faster than MASCOT in many aspects: 1. We present a protocol that uses semi-homomorphic (addition-only) encryption. For two parties, our BGV-based implementation is six times faster than MASCOT on a LAN and 20 times faster in a WAN setting. The latter is roughly the reduction in communication. 2. We show that using the proof of knowledge in the original work by Damgard et al. (Crypto '12) is more efficient in practice than the one used in the implementation mentioned above by about one order of magnitude. 3. We present an improvement to the verification of the aforementioned proof of knowledge that increases the performance with a growing number of parties, doubling it for 16 parties.