SCSC2026 Final - Deskripsi Palsu - Cryptography Writeup

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.

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

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/home/LIGHT/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
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793 regular file 2026-05-16 10:00:30.768230996 +0700

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

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

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output.txt: 4e486f7466547375466a4137665431344a58676b466a31354a7859366542593765544579663373714f696f36
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README.md:
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Sebuah grup hacker mengklaim mereka membuat sistem enkripsi “quantum military grade” yang mustahil dipecahkan.
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Mereka memakai:
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- XOR
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- Base64
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- Reverse string
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- Hex encoding
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- “Quantum randomness” ?
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…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.

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import base64
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hexs = '4e486f7466547375466a4137665431344a58676b466a31354a7859366542593765544579663373714f696f36'
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raw = base64.b64decode(bytes.fromhex(hexs) + b'==')
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for key in range(256):
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    pt = bytes(c ^ key for c in raw[::-1])
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    if b'scsc26' in pt:
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        print(pt.decode())

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scsc26{x0r_1s_n0t_m1l1t4ry_gr4d3}