# unimod


in this chall he gave me encryption code
```python
import random

flag = open('flag.txt', 'r').read()
ct = ''
k = random.randrange(0,0xFFFD)
for c in flag:
ct += chr((ord(c) + k) % 0xFFFD)

open('out', 'w').write(ct)

```
he picks a random key between 0 and 65533
so key < 10e9 then we can do a bruteforce .

```python

f = open('out','r').read()
print(f)

for i in range(0 , 0xFFFD ):
h = ''
for j in f:
h+= chr(( ord(j) -i )% 0xFFFD)
if 'flag' in h :
print(h)
```

flag: flag{4e68d16a61bc2ea72d5f971344e84f11}

Original writeup (https://github.com/diko-ct/write-ups/tree/main/nahamcon_ctf_2022/unimod).