Efficient Transciphering with Chacha21

Transciphering is a powerful technique to reduce communication overhead of fully homomorphic encryption. We propose to use the standardized cipher Chacha20 to construct transciphering. We implement a full bitwise homomorphic version of Chacha20 and analyze different strategies and techniques to implement it with non-binary messages. We also propose a non-blackbox way of computing homomorphic XOR gates for free within Chacha20 round evaluation. This yields a significant reduction on the computation cost, with an estimate of 238.08 s to run the cipher in our best serial implementation, with our throughput being up to 2 times higher than the state of the art.