Boktai save file checksums

The save files of Boktai games contain checksums that are verified when the save is loaded. After modifying save files with a hex editor or another tool, these checksums must be updated so the save will load correctly again.

The save file is divided into blocks. Each block size is a multiple of 8 bytes. After the last 8-byte chunk in a block, there is another 8-byte chunk containing the checksum. The checksum are these 4 halfwords:

  1. Constant 0xffff
  2. Byte size of this block, excluding this checksum
  3. Sum of halfwords in this block, excluding this checksum
  4. XOR-sum of halfwords in this block, excluding this checksum

Hex dump of the first block in Boktai 1 saves, with the checksum in bold. Note that each halfword is stored in the save file in big-endian byte order:

00000000: 0000 00e4 0003 cc18 0000 0101 0000 0000 00000010: 0000 0000 0000 0000 ffff 0018 ce00 cdfe

Boktai 1 save files contain the following blocks. Here is a Python script that updates all checksums of a Boktai 1 save.

Byte offset Byte size
0x0000 0x18
0x00c0 0x28
0x0200 0x28
0x0340 0x400
0x0780 0x418
0x0dc0 0x400
0x1200 0x418
0x1840 0x168
0x1a40 0x168

Tags: boktai boktai1 boktai2 boktai3