We found a strange zip file. Sadly the content is encrypted. As only criminals encrypt their stuff we need to decrypt this urgently. Flag format is FLAGB**************S.
In this task we got a zip file containing a couple of other small zip files. All of them were encrypted, but their size was only up to 5 bytes - just like in recent Backdoor CTF task. See that writeup for details.
After decrypting, this is what we got:
1 main(
2 ){int
3 n=24
4 ;whil
5 e(n--
6 ){put
7 char(
8
9
10 ]^n);
11 }}
Files 8 and 9 were missing. However, basing on flag format from task description, we could guess what should go in these empty places:
In [1]: s="FLAGB"
In [2]: for i, c in zip(range(23,0,-1), s):
...: print chr(ord(c)^i)
...:
Q
Z
T
S
Q
It might be a bit of guess, but we see that the last letter is the same as the first one, indicating period equal to 4.
If we put "STZQ in file 8, and "[n%4 in file 9, then both files have size equal to 5, just like the others, and
the output of compiled program matches flag format.
main(){int n=24;while(n--){putchar("STZQ"[n%4]^n);}}
// Prints FLAGBHEC^TY_ZP][V\QWRXUS