elite-checksum.py

#!/usr/bin/env python
#
# ******************************************************************************
#
# BBC MASTER ELITE CHECKSUM SCRIPT
#
# Written by Mark Moxon, and inspired by Kieran Connell's version for the
# cassette version of Elite
#
# This script applies encryption and checksums to the compiled binary for the
# main parasite game code. It reads the unencrypted "BCODE.unprot.bin" and
# "BDATA.unprot.bin" binaries and generates encrypted versions as "BCODE.bin"
# and "BDATA.bin"
#
# ******************************************************************************

from __future__ import print_function
import sys

argv = sys.argv
encrypt = True
release = 1

for arg in argv[1:]:
    if arg == "-u":
        encrypt = False
    if arg == "-rel1":
        release = 1
    if arg == "-rel2":
        release = 2

print("Master Elite Checksum")
print("Encryption = ", encrypt)

# Configuration variables for scrambling code and calculating checksums
#
# Values must match those in 3-assembled-output/compile.txt
#
# If you alter the source code, then you should extract the correct values for
# the following variables and plug them into the following, otherwise the game
# will fail the checksum process and will hang on loading
#
# You can find the correct values for these variables by building your updated
# source, and then searching compile.txt for "elite-checksum.py", where the new
# values will be listed

if release == 1:
    # SNG47
    f = 0x7F48                  # F%
    scramble_from = 0x2CC1      # G%
    na2_per_cent = 0x34CD       # NA2%
elif release == 2:
    # Compact
    f = 0x7FED                  # F%
    scramble_from = 0x2CC1      # G%
    na2_per_cent = 0x34CD       # NA2%

# Configuration variables for BCODE

load_address = 0x1300
seed = 0x19

if release == 1:
    # SNG47
    scramble_to = f - 1
elif release == 2:
    # Compact
    scramble_to = f - 1

# Load assembled code file for BCODE

data_block = bytearray()

elite_file = open("3-assembled-output/BCODE.unprot.bin", "rb")
data_block.extend(elite_file.read())
elite_file.close()

# Commander data checksum

commander_start = na2_per_cent - load_address
commander_offset = 0x52
CH = 0x4B - 2
CY = 0
for i in range(CH, 0, -1):
    CH = CH + CY + data_block[commander_start + i + 7]
    CY = (CH > 255) & 1
    CH = CH % 256
    CH = CH ^ data_block[commander_start + i + 8]

print("Commander checksum = ", hex(CH))

data_block[commander_start + commander_offset] = CH ^ 0xA9
data_block[commander_start + commander_offset + 1] = CH

# Encrypt game code

if encrypt:
    for n in range(scramble_from, scramble_to):
        data_block[n - load_address] = (data_block[n - load_address] + data_block[n + 1 - load_address]) % 256

    data_block[scramble_to - load_address] = (data_block[scramble_to - load_address] + seed) % 256

# Write output file for BCODE

output_file = open("3-assembled-output/BCODE.bin", "wb")
output_file.write(data_block)
output_file.close()

print("3-assembled-output/BCODE.bin file saved")

# Configuration variables for BDATA

load_address = 0x1300 + 0x5D00
seed = 0x62
scramble_from = 0x8000
scramble_to = 0xB1FF

data_block = bytearray()

# Load assembled code file for BDATA

elite_file = open("3-assembled-output/BDATA.unprot.bin", "rb")
data_block.extend(elite_file.read())
elite_file.close()

if encrypt:
    for n in range(scramble_from, scramble_to):
        data_block[n - load_address] = (data_block[n - load_address] + data_block[n + 1 - load_address]) % 256

    data_block[scramble_to - load_address] = (data_block[scramble_to - load_address] + seed) % 256

# Write output file for BDATA

output_file = open("3-assembled-output/BDATA.bin", "wb")
output_file.write(data_block)
output_file.close()

print("3-assembled-output/BDATA.bin file saved")