MarlinFirmware/buildroot/share/PlatformIO/scripts/chitu_crypt.py

import os
import struct
Import("env")

# Relocate firmware from 0x08000000 to 0x08008800
for define in env['CPPDEFINES']:
    if define[0] == "VECT_TAB_ADDR":
        env['CPPDEFINES'].remove(define)
env['CPPDEFINES'].append(("VECT_TAB_ADDR", "0x8008800"))

custom_ld_script = os.path.abspath("buildroot/share/PlatformIO/ldscripts/chitu_f103.ld")
for i, flag in enumerate(env["LINKFLAGS"]):
    if "-Wl,-T" in flag:
        env["LINKFLAGS"][i] = "-Wl,-T" + custom_ld_script
    elif flag == "-T":
        env["LINKFLAGS"][i + 1] = custom_ld_script


def calculate_crc(contents, seed):
    accumulating_xor_value = seed;

    for i in range(0, len(contents), 4):
        value = struct.unpack('<I', contents[ i : i + 4])[0]
        accumulating_xor_value = accumulating_xor_value ^ value
    return accumulating_xor_value

def xor_block(r0, r1, block_number, block_size, file_key):
    # This is the loop counter
    loop_counter = 0x0

    # This is the key length
    key_length = 0x18

    # This is an initial seed
    xor_seed = 0x4bad

    # This is the block counter
    block_number = xor_seed * block_number

    #load the xor key from the file
    r7 =  file_key

    for loop_counter in range(0, block_size):
        # meant to make sure different bits of the key are used.
        xor_seed = int(loop_counter/key_length)

        # IP is a scratch register / R12
        ip = loop_counter - (key_length * xor_seed)

        # xor_seed = (loop_counter * loop_counter) + block_number
        xor_seed = (loop_counter * loop_counter) + block_number

        # shift the xor_seed left by the bits in IP.
        xor_seed = xor_seed >> ip

        # load a byte into IP
        ip = r0[loop_counter]

        # XOR the seed with r7
        xor_seed = xor_seed ^ r7

        # and then with IP
        xor_seed = xor_seed ^ ip

        #Now store the byte back
        r1[loop_counter] = xor_seed & 0xFF

        #increment the loop_counter
        loop_counter = loop_counter + 1


def encrypt_file(input, output_file, file_length):
    input_file = bytearray(input.read())
    block_size = 0x800
    key_length = 0x18
    file_key = 0xDAB27F94

    xor_crc = 0xef3d4323;

    # the input file is exepcted to be in chunks of 0x800
    # so round the size
    while len(input_file) % block_size != 0:
        input_file.extend(b'0x0')

    # write the file header
    output_file.write(struct.pack(">I", 0x443D2D3F))
    # encrypt the contents using a known file header key

    # write the file_key
    output_file.write(struct.pack(">I", 0x947FB2DA))

    #TODO - how to enforce that the firmware aligns to block boundaries?
    block_count = int(len(input_file) / block_size)
    print "Block Count is ", block_count
    for block_number in range(0, block_count):
        block_offset = (block_number * block_size)
        block_end = block_offset + block_size
        block_array = bytearray(input_file[block_offset: block_end])
        xor_block(block_array, block_array, block_number, block_size, file_key)
        for n in range (0, block_size):
            input_file[block_offset + n] = block_array[n]

        # update the expected CRC value.
        xor_crc = calculate_crc(block_array, xor_crc)

    # write CRC
    output_file.write(struct.pack("<I", xor_crc))

    # finally, append the encrypted results.
    output_file.write(input_file)
    return


# Encrypt ${PROGNAME}.bin and save it as 'update.cbd'
def encrypt(source, target, env):
    firmware = open(target[0].path, "rb")
    update = open(target[0].dir.path +'/update.cbd', "wb")
    length = os.path.getsize(target[0].path)

    encrypt_file(firmware, update, length)

    firmware.close()
    update.close()

env.AddPostAction("$BUILD_DIR/${PROGNAME}.bin", encrypt);