Tags: misc xor
Rating:
# Hidden Flag (Misc 296 points)
We are given an [image](chall.png).
Lets see what type of file it is.
```
vagrant@ubuntu-xenial:~$ file chall.png
chall.png: data
vagrant@ubuntu-xenial:~$
```
It is named with .png but it's headers don't match png format. Maybe the file is encrypted.
Lets see for any embedded strings in it.
```
vagrant@ubuntu-xenial:~$ strings chall.png
..
..
key is invisible
vagrant@ubuntu-xenial:~$
```
It says "key is invisible". Maybe the file is encrypted with the key "invisible".
Lets try XOR first.
```
infile = "chall.png"
outfile = "dec.png"
key = "invisible"
s = open(infile, "r").read()
p = ""
for i in range(len(s)):
p += chr(ord(s[i]) ^ ord(key[i%len(key)]))
open(outfile, "w").write(p)
```
```
vagrant@ubuntu-xenial:~$ file dec.png
dec.png: PNG image data, 665 x 268, 8-bit/color RGB, non-interlaced
vagrant@ubuntu-xenial:~$
```
After decrypting the original file, we got a png. Lets open it and check.
There is our flag.
