# rebuild CRC python3 - <<PY import binascii, sys data = open("$FILE", "rb").read() crc = binascii.crc32(data[0x10:]) & 0xffffffff new = data[:0x08] + crc.to_bytes(4, 'little') + data[0x0c:] open("$FIXED", "wb").write(new) print(f"[*] Fixed CRC = 0xcrc:08x") PY
The checksum is calculated over the , i.e. bytes starting at 0x10 . 4. Re‑building the Bride (Checksum) 4.1 Compute the correct CRC‑32 Python makes this trivial: Debrideur fileice.net
if __name__ == "__main__": if len(sys.argv) != 2: print(f"Usage: sys.argv[0] <debrideur_file>") sys.exit(1) fix(sys.argv[1]) #!/usr/bin/env bash # run_and_get_flag.sh – Build the bride, run debrideur, extract the flag. # rebuild CRC python3 - <<PY import binascii,
# 2️⃣ Execute and filter the flag ./debrideur "$FIXED" 2>/dev/null | grep -i -E 'flag\[^]+\}' Make them executable ( chmod +x rebuild.py run_and_get_flag.sh ) and you’re ready to solve the challenge in one command: Re‑building the Bride (Checksum) 4
def fix(fname): data = open(fname, "rb").read() payload = data[0x10:] # skip header + checksum field crc = binascii.crc32(payload) & 0xffffffff fixed = data[:0x08] + crc.to_bytes(4, "little") + data[0x0c:] out = fname + ".fixed" open(out, "wb").write(fixed) print(f"[+] Fixed file: out CRC=0xcrc:08x")