Worst-Case to Average-Case Reductions for Basic Linear Algebra

Abstract

We study worst-case to average-case reductions for basic linear-algebraic problems over finite fields. A seminal work of Blum, Luby, and Rubinfeld established that any algorithm that successfully multiplies more than $75\%$ of matrix pairs over any finite field, implies an algorithm with essentially the same running time multiplying any pair of matrices. A sequence of follow-ups reduced the needed fraction of successful input pairs to any positive constant. Despite most of the other basic linear algebraic operations being computationally equivalent to matrix multiplication in the worst case, no such worst-case to average-case reductions were known for them. Recently, Vaikuntanathan and Zamir introduced a cryptographic primitive called trapdoor matrices and used them to prove such worst-case to average-case reductions for matrix inversion, solving a linear system, and computing the determinant, all conditioned on common cryptographic assumptions. Our main result is the first unconditional time-preserving worst-case to average-case reduction for matrix inversion. Any algorithm that correctly inverts any constant positive fraction of uniformly random invertible matrices implies a worst-case such algorithm. We also significantly strengthen the conditional reductions based on trapdoored matrices: We establish reductions for deciding invertibility and computing rank at the optimal success thresholds, and qualitatively improve the success threshold needed in the reduction for determinant computation---while previous reductions all required the success probability to be at least $1/2$, we show that any fixed positive average-case success probability suffices for exact worst-case determinant computation over infinitely many finite fields.