Sample Efficient Search to Decision for kLIN
摘要
The kLIN problem concerns solving noisy systems of random sparse linear equations mod 2. It gives rise to natural candidate hard CSP distributions and is a cornerstone of local cryptography. Recently, it was used in cryptographic constructions, under the name “sparse LPN”. For constant sparsity k and inverse polynomial noise rate, both search and decision versions of kLIN are statistically possible and conjectured to be computationally hard for \(n\ll m\ll n^{k/2}\) , where m is the number of k-sparse linear equations, and n is the number of variables. We show an algorithm that given access to a distinguisher for \((k-1)\) LIN with m samples, solves search kLIN with roughly O(nm) samples. Previously, it was only known how to reduce from search kLIN with \(O(m^3)\) samples, yielding guarantees for decision kLIN only when \(m \ll n^{k/6}\) . The reduction succeeds even if the distinguisher has sub-constant advantage at a small additive cost in sample complexity. Our technique applies with some restrictions to Goldreich’s function and kLIN with random coefficients over other finite fields.