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# !/usr/bin/env python3
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from pathlib import Path
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BIN_PATH = "ghost_compiler"
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TARGET_PREFIX = b"BITSCTF{"
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FNV_OFFSET = 0xCBF29CE484222325
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FNV_PRIME = 0x100000001B3
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MASK64 = (1 << 64) - 1
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def ror64(x: int, n: int = 1) -> int:
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    return ((x >> n) | ((x << (64 - n)) & MASK64)) & MASK64
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def derive_key(blob: bytes, skip_off: int, skip_len: int = 0x40) -> int:
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    h = FNV_OFFSET
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    for i, bt in enumerate(blob):
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        if skip_off <= i < skip_off + skip_len:
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            continue
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        h ^= bt
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        h = (h * FNV_PRIME) & MASK64
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    return (0xCAFEBABE00000000 ^ h) & MASK64
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def decrypt_window(blob: bytes, off: int, key: int, n: int = 0x40) -> bytes:
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    out = bytearray()
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    k = key
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    for i in range(n):
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        out.append(blob[off + i] ^ (k & 0xFF))
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        k = ror64(k, 1)
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    return bytes(out)
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def main() -> None:
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    blob = Path(BIN_PATH).read_bytes()
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    for off in range(0, len(blob) - 0x40 + 1):
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        key = derive_key(blob, off, 0x40)
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        dec = decrypt_window(blob, off, key, 0x40)
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        if dec.startswith(TARGET_PREFIX) and b"}" in dec:
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            flag = dec.split(b"\x00", 1)[0].decode("utf-8", errors="ignore")
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            print(f"[+] offset = {off}")
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            print(f"[+] key    = {hex(key)}")
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            print(f"[+] flag   = {flag}")
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            return
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    print("[-] Flag window not found")
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if __name__ == "__main__":
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    main()

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python3 solve_ghost_compiler.py

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[+] offset = 12320
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[+] key    = 0x5145dd89c16375d8
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[+] flag   = BITSCTF{n4n0m1t3s_4nd_s3lf_d3struct_0ur0b0r0s}