shredder.md

Shredder (misc)

Solution

floppy is a FAT12 image, we can extract an ELF executable shredder and a deleted file flag.txt from it using the tool fatcat

Try to analyze the behaviour of shredder, I found

• usage ./shredder passes files...
• Behaviour：Byte by byte XOR the contents of files with a random number generated by getrandom() with passes times, and unlink(delete) the file.

Have a look on the deleted flag.txt, it's encrypted, thus we can infer that flag.txt is deleted by shredder. What we have to do is simply try to XOR flag.txt with all possible values of a byte (0 ~ 255).

Exploit

#!/usr/bin/env python3

f = open("flag.txt")
s = f.read()
for i in range(255):
    print("".join([chr(ord(c) ^ i) for c in s]))

Flag

SECT{1f_U_574y_r1gh7_wh3r3_U_R,_7h3n_p30pl3_w1ll_3v3n7u4lly_c0m3_70_U}