Tags: crypto xor
Rating:
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.dat
The 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*n
Most possible char is needed to guess the key!
$ xortool -l 25 -c 'E' otp1.dat
4 possible key(s) of length 25:
UDCTFjw?}r!mtNe0N
