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TR06-127 | 7th October 2006 00:00

New Locally Decodable Codes and Private Information Retrieval Schemes


Authors: Sergey Yekhanin
Publication: 7th October 2006 22:59
Downloads: 1748


A q-query Locally Decodable Code (LDC) encodes an n-bit message
x as an N-bit codeword C(x), such that one can
probabilistically recover any bit x_i of the message
by querying only q bits of the codeword C(x), even after
some constant fraction of codeword bits has been corrupted.

We give new constructions of three query LDCs of vastly shorter
length than that of previous constructions. Specifically, given
any Mersenne prime p=2^t-1, we design three query LDCs of
length N=EXP(n^{1/t}), for every $n$. Based on the
largest known Mersenne prime, this translates to a length of less
than EXP(n^{10^{-7}}), compared to
EXP(n^{1/2}) in the previous constructions. It has
often been conjectured that there are infinitely many Mersenne
primes. Under this conjecture, our constructions yield three query
locally decodable codes of length N=EXP(n^{1/log log n}) for infinitely many $n$.

We also obtain analogous improvements for Private Information
Retrieval (PIR) schemes. We give 3-server PIR schemes with
communication complexity of O(n^{10^{-7}}) to access
an n-bit database, compared to the previous best scheme with
complexity O(n^{1/5.25}). Assuming again that there are
infinitely many Mersenne primes, we get 3-server PIR schemes of
communication complexity n^{O(1/log log n)} for infinitely many $n$.

Previous families of LDCs and PIR schemes were based on the
properties of low-degree multivariate polynomials over finite
fields. Our constructions are completely different and are
obtained by constructing a large number of vectors in a small
dimensional vector space whose inner products are restricted to
lie in an algebraically nice set.

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