There's a buffer overflow while appending `.txt` to filename. Overwrite the `e` and try [Common Modulus Attack](https://crypto.stackexchange.com/questions/1614/rsa-cracking-the-same-message-is-sent-to-two-different-people-problem) to crack the plaintext of `flag.txt`.

Original writeup (https://gist.github.com/cubarco/dc725bcaf737017c501188f2fce1680f#file-secret_fs-py).