Category: Cryptography

Flag: scsc26{x0r_1s_n0t_m1l1t4ry_gr4d3}

Description: flag format: scsc26{…}

The artifact was a ZIP archive named Crypto_4.zip. File triage showed a small archive, and the listing showed two useful files: Crypto_4/output.txt and Crypto_4/README.md.

file '/home/rei/Downloads/SCSC2026Final/Crypto_4.zip' && stat -c '%s %F %y' '/home/rei/Downloads/SCSC2026Final/Crypto_4.zip'

/home/rei/Downloads/SCSC2026Final/Crypto_4.zip: Zip archive data, made by v2.0 UNIX, extract using at least v2.0, last modified, last modified Sun, May 16 2026 09:21:52, uncompressed size 0, method=store
793 regular file 2026-05-16 10:00:30.768230996 +0700

unzip -l '/home/rei/Downloads/SCSC2026Final/Crypto_4.zip'; xxd '/home/rei/Downloads/SCSC2026Final/Crypto_4.zip' | head -n 8

Archive:  /home/rei/Downloads/SCSC2026Final/Crypto_4.zip
  Length      Date    Time    Name
---------  ---------- -----   ----
        0  05-16-2026 09:21   Crypto_4/
       88  05-16-2026 09:21   Crypto_4/output.txt
      260  05-16-2026 09:19   Crypto_4/README.md
---------                     -------
      348                     3 files
00000000: 504b 0304 1400 0000 0000 ba4a b05c 0000  PK.........J.\..
00000010: 0000 0000 0000 0000 0000 0900 2000 4372  ............ .Cr
00000020: 7970 746f 5f34 2f75 780b 0001 0400 0000  ypto_4/ux.......
00000030: 0004 0000 0000 5554 0d00 07c0 d407 6afc  ......UT......j.
00000040: d407 6ab0 d207 6a50 4b03 0414 0008 0008  ..j...jPK.......
00000050: 00bc 4ab0 5c00 0000 0000 0000 0000 0000  ..J.\...........
00000060: 0013 0020 0043 7279 7074 6f5f 342f 6f75  ... .Crypto_4/ou
00000070: 7470 7574 2e74 7874 7578 0b00 0104 0000  tput.txtux......

The archive contents gave the route. output.txt held one hex-looking string, and README.md named the claimed layers: XOR, Base64, Reverse string, Hex encoding, plus a joke about “Quantum randomness”.

output.txt: 4e486f7466547375466a4137665431344a58676b466a31354a7859366542593765544579663373714f696f36
README.md:
Sebuah grup hacker mengklaim mereka membuat sistem enkripsi “quantum military grade” yang mustahil dipecahkan.

Mereka memakai:

- XOR
- Base64
- Reverse string
- Hex encoding
- “Quantum randomness” ?

…atau setidaknya begitu kata mereka.

The first layer was hex. Decoding it produced printable Base64-looking bytes, and Base64 decoding produced non-printable bytes. XOR brute force on those bytes produced a reversed flag at key 73. Reversing first, then XORing every byte with 0x49, produced the flag.

import base64

hexs = '4e486f7466547375466a4137665431344a58676b466a31354a7859366542593765544579663373714f696f36'
raw = base64.b64decode(bytes.fromhex(hexs) + b'==')

for key in range(256):
    pt = bytes(c ^ key for c in raw[::-1])
    if b'scsc26' in pt:
        print(pt.decode())

scsc26{x0r_1s_n0t_m1l1t4ry_gr4d3}