Knapsack-based cryptosystems used to be popular in the beginning of public key cryptography before all but the Chor-Rivest cryptosystem (B. Chor and R.L. Rivest, 1985) was broken. The article demonstrates how to break this one with its suggested parameters: GF(p²⁴) and GF(256²⁵). We also show the direction of possible extensions of our attack