Rating:
# Plaint..image (crypto, 620p, 27 solved)
```
Hey!
I encrypted this photo using simple XOR, but im sure its safe,
i used 40 byte key after all, no one would ever be able to bruteforce that ;)
```
Pretty standard challenge, we have a [jpeg image](https://raw.githubusercontent.com/TFNS/writeups/master/2020-04-25-IJCTF/plaintimage/flag.jpg.enc) encrypted with 40-bytes long repeating key XOR.
The idea is rather simple:
- Use some known plaintext bytes (eg. file header) to recover part of the keystream
- XOR known keystream bytes with ciphertext
- Hope that this uncovers some new plaintext parts of the data, which we can "extend"
We run:
```python
from crypto_commons.generic import xor_string, chunk_with_remainder
data = open("flag.jpg.enc", 'rb').read()
jpg_header = 'FF D8 FF E0 00 10 4A 46 49 46 00 01'.replace(" ", "").decode("hex")
key = xor_string(data, jpg_header)
extended_key = key + ("\0" * (40 - len(key)))
chunks = chunk_with_remainder(data, 40)
for c in chunks:
print(xor_string(extended_key, c))
```
And we can see two interesting chunks:
```
STUVWXYZcdef...
()*56789:CDE...
```
If we look inside some example JPG files with hexeditor we can see that such data in fact appear there, and that they are much longer!
We can use one of them -> `()*56789:CDEFGHIJSTUVWXYZcdefghijstuvwxy` to recover entire keystream and decrypt the flag:
```python
extended_key = xor_string(chunks[15], "()*56789:CDEFGHIJSTUVWXYZcdefghijstuvwxy")
open('out.jpg', 'wb').write(xor_string(data, extended_key * 1000))
```
And we get:
