  Secure multi-party computation (MPC) allows users to offload machine learning
inference on untrusted servers without having to share their privacy-sensitive
data. Despite their strong security properties, MPC-based private inference has
not been widely adopted in the real world due to their high communication
overhead. When evaluating ReLU layers, MPC protocols incur a significant amount
of communication between the parties, making the end-to-end execution time
multiple orders slower than its non-private counterpart.
  This paper presents HummingBird, an MPC framework that reduces the ReLU
communication overhead significantly by using only a subset of the bits to
evaluate ReLU on a smaller ring. Based on theoretical analyses, HummingBird
identifies bits in the secret share that are not crucial for accuracy and
excludes them during ReLU evaluation to reduce communication. With its
efficient search engine, HummingBird discards 87--91% of the bits during ReLU
and still maintains high accuracy. On a real MPC setup involving multiple
servers, HummingBird achieves on average 2.03--2.67x end-to-end speedup without
introducing any errors, and up to 8.64x average speedup when some amount of
accuracy degradation can be tolerated, due to its up to 8.76x communication
reduction.
