rcx_correct

#!/usr/bin/env python

# Correct the checksums in EPSON IPL blocks in an .rcx firmware file.
# James Wah 2020
# This file is in the public domain.

import sys
import hashlib
import struct
import rcx

if len(sys.argv) != 3:
    print("usage: %s infile.rcx outfile.rcx" % sys.argv[0])

with open(sys.argv[1], 'rb') as fp:
    data = fp.read()

fp = open(sys.argv[2], 'w+b')
fp.write(data)


def correct_ipl(offset):
    fp.seek(offset, 0)
    ipl_header = fp.read(0x100)

    assert ipl_header[:9] == b'EPSON IPL'

    header_hash = hashlib.sha1(ipl_header[:0xe])

    hitem_bytes, = struct.unpack('<H', ipl_header[0xe:0x10])
    hitems = hitem_bytes // 0x24
    assert hitems == 3

    for idx, body_base in enumerate([0x1000, 0x3000]):
        hdr_base = 0x10 + 0x24*idx
        hi_data = ipl_header[hdr_base:hdr_base+0x10]
        f0, f2, f4, _, f8, _, length = struct.unpack('<HHHHHHL', hi_data)

        fp.seek(offset + body_base, 0)
        checksum = sum(bytearray(fp.read(length)))
        fp.seek(offset + hdr_base + 2)
        fp.write(struct.pack('<H', checksum & 0xffff))

        if idx > 0:
            fp.seek(offset + body_base, 0)
            body_hash = hashlib.sha1(fp.read(length))
            body_hash.update(header_hash.digest())

            fp.seek(offset + hdr_base + 0x10)
            fp.write(body_hash.digest())


header_end, cfg = rcx.parse_rcx(data)

correct_ipl(header_end)
pointer = header_end

for name, section in cfg.items():
    if name.startswith('Z_'):
        length = int(section['5'])
        pointer += length
        if pointer >= len(data):
            break
        if data[pointer:pointer+9] == b'EPSON IPL':
            correct_ipl(pointer)