Shredder - Misc

In this challenge, we get floppy.img which is a FAT12 image. We can extract an ELF executable shredder from this image. Then I use testdisk tool to try to recover deleted files and I find flag.txt but it is gibberish or encrypted stuff. If you run the shredder file, you get

./shredder passes files

Next step is to reverse the shredder binary to see what it does, and it XORs every byte with the contents of flag.txt file. Therefore, we create this script:

import sys

with open('flag.txt', 'rb') as f:
    x =
    for a in range(int(sys.argv[1])):
        tmp = ""
        for i in range(len(x)):
            tmp += chr(ord(x[i]) ^ (0xa1 + a))
        x = tmp

        if "SECT" in x:

Then we get the flag: SECT{1f_U_574y_r1gh7_wh3r3_U_R,_7h3n_p30pl3_w1ll_3v3n7u4lly_c0m3_70_U}