This is XOR cipher with the flag as repeated key.```php//For these problems I'll only change these 2 lines:$flag = trim(file_get_contents("../../flag1.txt"));$key = pad_string($flag, 0); ```
We use [CyberChef](https://gchq.github.io/CyberChef/) to dump ciphertext to file, then [xortool](https://github.com/hellman/xortool) to find the key length of 25 and try to reveal some part of the flag.```$ xortool otp1.datThe most probable key lengths: 2:   9.6% 5:  15.6%10:  13.0%13:   7.7%15:  10.3%20:   8.6%22:   6.1%25:  15.4%30:   6.1%50:   7.6%Key-length can be 5*nMost possible char is needed to guess the key!$ xortool -l 25 -c 'E' otp1.dat4 possible key(s) of length 25:UDCTFjw?}r!mtNe0N