* The original character, and the 3 characters that follow, have a slightly higher chance of ending up in the ciphertext* Collect a sufficiently large sample size, and bruteforce the flag based on the highest-frequency characters
[Read the writeup here](https://ctf.zeyu2001.com/2021/cybersecurityrumble-ctf/personal-encryptor-with-nonbreakable-inforation-theoretic-security)