Private Information Retrieval: Share Conversions vs Decoding Polynomials

A private information retrieval (PIR) protocol enables a client to retrieve a bit from an N bit database replicated among k servers in such a way that each server learns no information about the retrieved bit. Modern PIR protocols with information-theoretic privacy (Efremenko, SICOMP, 2012; Dvir and Gopi, STOC, 2015; Ghasemi et al., STOC, 25) are based on matching vectors over a composite number m. To construct a PIR protocol from the matching vectors, these protocols use a decoding polynomial, a sparse polynomial that returns a non-zero value on 1 and returns zero on a certain set implied by the matching vectors. Beimel et al. (CCC, 2012) abstracted the properties required by the transformation computed by the decoding polynomial, defining the notion of share conversion. In such a conversion, a set of parties is given shares of a secret in one secret-sharing scheme, and each party locally computes a new share (without any communication) such that the new shares are shares in a second secret-sharing scheme of a related secret. Beimel et al. showed that share conversion can replace the decoding polynomial in the PIR protocol of Efremenko and constructed a share conversion from the ring Z₆ to the field F2₂. This share conversion cannot be computed by a decoding polynomial, as decoding polynomials convert shares from a ring Z_m to a finite field of characteristic p such that p does not divide m. Alon et al. (TCC, 2025) simplified and generalized the PIR protocols of Dvir and Gopi and Ghasemi et al., using share conversion; however, in this protocol, the share conversion is from a ring Z_m to a finite field of characteristic p such that p does not divide m. In this paper, we study the power of share conversions. Our main result proves that if there is a k-party share conversion from a ring Z_m to a finite field of characteristic p such that p does not divide m, then there is a k sparse decoding polynomial from a ring Z_m to a finite field of characteristic p. This result implies that using share conversion in the protocol of Alon et al. can only improve the communication complexity by a constant factor. In addition, we show that if there is a k-party share conversion from a ring Z_m to a finite field, where m is a product of r distinct primes, then k≥r+1, i.e., the number of servers in the resulting PIR protocols using the appropriate matching vectors is at least r+1. A similar result was recently proved by Ghasemi and Kopparty (ITCS 26); our lower bound also applies to the case in which the characteristic of the field divides m.