Message Recovery Attack in NTRU via Knapsack

In the present paper, we introduce a message-recovery attack based on the Modular Knapsack Problem, applicable to all variants of the NTRU-HPS cryptosystem. Assuming that a fraction $\epsilon$ of the coefficients of the message ${\bf{m}}\in\{-1,0,1\}^N$ and of the nonce vector ${\bf r}\in\{-1,0,1\}^N$ are known in advance at random positions, we reduce message decryption to finding a short vector in a lattice that encodes an instance of a modular knapsack system. This allows us to address a key question: how much information about ${\bf m}$, or about the pair $({\bf m},{\bf r})$, is required before recovery becomes feasible? A FLATTER reduction successfully recovers the message, in practice when $\epsilon\approx 0.45$. Our implementation finds ${\bf m}$ within a few minutes on a commodity desktop.