Electronic Colloquium on Computational Complexity

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TR01-015 | 9th February 2001 00:00

Information-Theoretic Private Information Retrieval: A Unified Construction



A Private Information Retrieval (PIR) protocol enables a user to
retrieve a data item from a database while hiding the identity of the
item being retrieved. In a $t$-private, $k$-server PIR protocol the
database is replicated among $k$ servers, and the user's privacy is
protected from any collusion of up to $t$ servers. The main
cost-measure of such protocols is the communication complexity of
retrieving a single bit of data.

This work addresses the information-theoretic setting for PIR, in
which the user's privacy should be unconditionally protected from
collusions of servers. We present a unified general construction,
whose abstract components can be instantiated to yield both old and
new families of PIR protocols. A main ingredient in the new protocols
is a generalization of a solution by Babai, Kimmel, and Lokam to a
communication complexity problem in the so-called simultaneous
messages model.

Our construction strictly improves upon previous constructions and
resolves some previous anomalies. In particular, we obtain:
(1) $t$-private $k$-server PIR protocols with communication
$O(n^{1/\lfloor (2k-1)/t\rfloor})$, where $n$ is the database size.
For $t>1$, this is a substantial asymptotic improvement over the
previous state of the art;
(2) a constant-factor improvement in the communication complexity of
1-private PIR, providing the first improvement to the $2$-server case
since PIR protocols were introduced;
(3) efficient PIR protocols with logarithmic query length.
The latter protocols have applications to the construction of
efficient families of locally decodable codes over large alphabets and
to PIR protocols with reduced work by the servers.

ISSN 1433-8092 | Imprint