decrypt_upk.py

decrypt_upk.py

#!/usr/bin/env python3
import struct, hashlib, sys

TEA_DELTA   = 0x9e3779b9
TEA_ROUNDS  = 16
TEA_MAGIC   = b'TEA\x00'
KEY_CONST_1 = 0x6e35ba0c
KEY_CONST_2 = 0x9a8b7c6e

def derive_key(seed):
    t = seed & 0xFFFFFFFF
    out = []
    for i in range(16):
        t = (i + (t ^ KEY_CONST_2)) & 0xFFFFFFFF
        t = (t + KEY_CONST_1) & 0xFFFFFFFF
        out.append(t & 0xFF)
    return bytes(out)

def tea_decrypt(v0, v1, key):
    k0, k1, k2, k3 = struct.unpack('<4I', key)
    s = (TEA_DELTA * TEA_ROUNDS) & 0xFFFFFFFF
    for _ in range(TEA_ROUNDS):
        v1 = (v1 - (((v0 << 4) + k2) ^ (v0 + s) ^ ((v0 >> 5) + k3))) & 0xFFFFFFFF
        v0 = (v0 - (((v1 << 4) + k0) ^ (v1 + s) ^ ((v1 >> 5) + k1))) & 0xFFFFFFFF
        s = (s - TEA_DELTA) & 0xFFFFFFFF
    return v0, v1

def main():
    if len(sys.argv) < 2:
        print(f"usage: {sys.argv[0]} <file.upk> [out.tar]")
        sys.exit(1)

    data = open(sys.argv[1], 'rb').read()

    # offsets from bin4ry uniteabag.
    assert data[:4] == b'UTPK'
    name   = data[48:112].split(b'\x00')[0].decode()
    seed   = data[28:32]
    sign1  = data[32:40]
    sign2  = data[40:48]
    pl_sz  = struct.unpack('<Q', data[16:24])[0]
    ftype  = data[24]

    key = derive_key(int.from_bytes(seed, 'little'))

    print(f"package : {name}")
    print(f"seed    : {seed.hex()}")
    print(f"key     : {key.hex()}")
    print(f"size    : {pl_sz} bytes (type={ftype})")

    payload = data[112:112 + pl_sz]
    md5 = hashlib.md5(payload).digest()
    if md5 == sign1 + sign2:
        print("md5     : ok")
    else:
        print(f"md5     : FAIL (expected {sign1.hex()}{sign2.hex()}, got {md5.hex()})")

    assert payload[:4] == TEA_MAGIC
    encrypted = payload[4:]

    out = bytearray()
    for i in range(0, len(encrypted), 8):
        v0, v1 = struct.unpack_from('<II', encrypted, i)
        d0, d1 = tea_decrypt(v0, v1, key)
        out.extend(struct.pack('<II', d0, d1))

    # lol
    if out[257:262] == b'ustar':
        print("format  : tar (ustar)")
    else:
        print(f"format  : unknown (starts with {bytes(out[:32]).hex()})")

    output = sys.argv[2] if len(sys.argv) > 2 else sys.argv[1].replace('.upk', '.tar')
    open(output, 'wb').write(bytes(out))
    print(f"saved   : {output} ({len(out)} bytes)")

if __name__ == '__main__':
    main()