Checksums in savegames

Hello,

After exctracting an unencrypted savegame of Super Mario Galaxy, I spotted that there might be some kind of checksum in the file because I can't modify it without the game seeing it as a corrupted savegame.
I thought there could be those checksums in other savegames ans why not in Zelda TP?
So, is there a way to bypass this checksum? Has it already been with the Twilight Hack?
In Super Mario Galaxy savefile ("GameData.bin"), the checksum should be from 0x000 to 0x003 (Wiibrew). I checked this by extracting near-identical saves and 0x000 to 0x003 is always different.
Are Nintendo checksums calculated with the same method? If so, does anyone know how to bypass them?
Thanks in advance!

Nobody knows :\ ?

Don't know if there is a general method used by Nintendo...
but you can try these things.

1. Systematically change a single variable in the game (dunno if it's doable with SMG though), save the game... note the change and the savegame file.... gathering enough values to analyze the checksum function used... If the checksum function is complex you won't get anywhere with this method though

2. Disassemble the relevant game dol, find the function that does the actual checking, now you can either
a. Patch the return value of the function so that it always return the expected result.
b. If you have usbgecko, dump the memory and you can get the calculated checksum by just observing relevant address or registers.

3. Lurk at the segher's git, to see how that checksum function might look like... There is the zelda&lego batman checksum fix tool there...
[git.infradead.org]

Thanks for the answer,

I have already tried the first "method" and I think I'm gonna look to segher's git first ;)