PIR with Low Storage Overhead: Coding instead of Replication

Private information retrieval (PIR) protocols allow a user to retrieve a data item from a database without revealing any information about the identity of the item being retrieved. Specifically, in information-theoretic $k$-server PIR, the database is replicated among $k$ non-communicating servers, and each server learns nothing about the item retrieved by the user. The cost of PIR protocols is usually measured in terms of their communication complexity, which is the total number of bits exchanged between the user and the servers, and storage overhead, which is the ratio between the total number of bits stored on all the servers and the number of bits in the database. Since single-server information-theoretic PIR is impossible, the storage overhead of all existing PIR protocols is at least $2$. In this work, we show that information-theoretic PIR can be achieved with storage overhead arbitrarily close to the optimal value of $1$, without sacrificing the communication complexity. Specifically, we prove that all known $k$-server PIR protocols can be efficiently emulated, while preserving both privacy and communication complexity but significantly reducing the storage overhead. To this end, we distribute the $n$ bits of the database among $s+r$ servers, each storing $n/s$ coded bits (rather than replicas). For every fixed $k$, the resulting storage overhead $(s+r)/s$ approaches $1$ as $s$ grows; explicitly we have $r\le k\sqrt{s}(1+o(1))$. Moreover, in the special case $k = 2$, the storage overhead is only $1 + \frac{1}{s}$. In order to achieve these results, we introduce and study a new kind of binary linear codes, called here $k$-server PIR codes. We then show how such codes can be constructed, and we establish several bounds on the parameters of $k$-server PIR codes. Finally, we briefly discuss extensions of our results to nonbinary alphabets, to robust PIR, and to $t$-private PIR.