Two Halves Make a Whole: Reducing Data Transfer in Garbled Circuits using Half Gates.
Samee Zahur, Mike Rosulek, David Evans · 2014
Abstract. The well-known classical constructions of garbled circuits use four ciphertexts per gate, although various methods have been proposed to reduce this cost. The best previously known methods for optimizing AND gates (two ci-phertexts; Pinkas et al., ASIACRYPT 2009) and XOR gates (zero ciphertexts; Kolesnikov and Schneider, ICALP 2008) were incompatible, so most implemen-tations used the best known method compatible with free-XOR gates (three ci-phertexts; Kolesnikov and Schneider, ICALP 2008). In this work we show how to simultaneously garble AND gates using two ciphertexts and XOR gates using zero ciphertexts, resulting in smaller garbled circuits than any prior scheme. The main idea behind our construction is to break an AND gate into two half-gates — AND gates for which one party knows one input. Each half-gate can be garbled with a single ciphertext, so our construction uses two ciphertexts for each AND gate while being compatible with free-XOR gates. The price for the reduction in size is that the evaluator must perform two cryptographic operations per AND gate, rather than one as in previous schemes. We experimentally demonstrate that our garbling scheme leads to an overall decrease in time (up to 25%), bandwidth (up to 33%), and energy use (up to 20%) over several benchmark applications. We show that our construction is optimal for a large class of garbling schemes encompassing all known practical garbling techniques. 1