Applying Private Information Retrieval to Lightweight Bitcoin Clients

Abstract

Lightweight Bitcoin clients execute a Simple Payment Verification (SPV) protocol to verify the validity of transactions related to a particular user. Currently, lightweight clients
\nuse Bloom filters to significantly reduce the amount of bandwidth
\nrequired to validate a particular transaction. This is despite the
\nfact that research has shown that Bloom filters are insufficient
\nat preserving the privacy of clients’ queries.
\nIn this paper we describe our design of an SPV protocol
\nthat leverages Private Information Retrieval (PIR) to create fully
\nprivate and performant queries. We show that our protocol has
\na low bandwidth and latency cost; properties that make our
\nprotocol a viable alternative for lightweight Bitcoin clients and
\nother cryptocurrencies with a similar SPV model. In contract
\nto Bloom filters, our PIR-based approach offers deterministic
\nprivacy to the user.
\nAmong our results, we show that in the worst case, clients who
\nwould like to verify 100 transactions occurring in the past week
\nincurs a bandwidth cost of 33.54 MB with an associated latency
\nof approximately 4.8 minutes, when using our protocol. The
\nsame query executed using the Bloom-filter-based SPV protocol
\nincurs a bandwidth cost of 12.85 MB; this is a modest overhead
\nconsidering the privacy guarantees it provides.