Masked comparison circumventing compression in post-quantum schemes

Abstract

Various embodiments relate to a method for securely comparing a first polynomial represented by a plurality of arithmetic shares and a second compressed polynomial represented by a bitstring where the bits in the bitstring correspond to coefficients of the second polynomial, including: performing a first masked shift of the shares of the coefficients of the first polynomial based upon the start of the interval corresponding to the compressed coefficient of the second polynomial and a modulus value; performing a second masked shift of the shares of the coefficients of the first polynomial based upon the end of the interval corresponding to the compressed coefficient of the second polynomial; bitslicing the most significant bit of the first masked shift of the shares coefficients of the first polynomial; bitslicing the most significant bit of the second masked shift of the shares coefficients of the first polynomial; and combining the first bitsliced bits and the second bitsliced bits using an AND function to produce an output including a plurality of shares indicating that the first polynomial would compress to a bitstream matching the bitstream representing the second compressed polynomial.