Free Flag Encrypter

ringzer0ctf

Task: a Merkle-Hellman knapsack cryptosystem encrypts a flag as a subset-sum integer; only the public key and ciphertext are given (private trapdoor lost). Solution: the knapsack density is ~0.74 (below the 0.9408 threshold), so the subset selection is recovered via LLL reduction of a CJLOSS lattice, then decoded as 8-bit ASCII.