DLink HDR4 Decryptor

dec.py

# awful code for decrypting DLink TCLinux firmware files
# requires pycryptodome

import struct
import binascii
from Crypto.Cipher import AES

KEY = bytes.fromhex("11223344556677889900aabbccddeeff")
IV = bytes.fromhex("a5a5a5a5a5a5a5a5a5a5a5a5a5a5a5a5")

f = open("enc.bin", "rb").read()

assert f[:4] == b'4RDH', "unsupported file"

magic, flen, crc32 = struct.unpack(">LxxxxLL", f[:16])

print(f"{magic=:X} {flen=:X} {crc32=:X}")

cipher = AES.new(KEY, AES.MODE_CBC, iv=IV)

dec = cipher.decrypt(f[0x100:flen - (flen % 16) + 16])
dec = dec[:flen-0x100]

calc_crc = binascii.crc32(dec)
calc_crc ^= 0xa5a5a5a5 ^ 0xFFFFFFFF
 
print(f"{calc_crc=:X} {crc32^calc_crc=:X}")

assert crc32 == calc_crc, "invalid crc"

open("dec.bin", "wb").write(f[:0x100] + dec)