Improve security of transposition ciphers
First determine the permutation domain. If $L_2 &gt; L_1$, then the cipher is broken by $\lceil  log_2 (L_2) \rceil$ pairs of plaintext/ciphertect. The security improvement is small, as the attacker only needs to use $\lceil  log_2 (L_2) \rceil - \lceil  log_2 (L_1) \rceil$ additional pairs to find the key.