Google Research

Privacy-Preserving Secure Cardinality and Frequency Estimation

Google, LLC (2020)


In this paper we introduce a new family of methods for cardinality and frequency estimation. These methods combine aspects of HyperLogLog (HLL) and Bloom filters in order to build a sketch that, like HLL, is substantially more compact than a Bloom filter, but like a Bloom filter maintains the ability to union sketches with a bucket-wise sum. Together these properties enable the creation of a scalable secure multi-party computation protocol that takes advantage of homomorphic encryption to combine sketches across multiple untrusted parties. The protocol limits the amount of information that participants learn to differentially private estimates of the union of sketches and some partial information about the Venn diagram of the per-sketch cardinalities.

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