So I followed this tutorial: https://gbatemp.net/threads/decrypt-re-encrypt-saves-for-most-games-sort-of.364522/ to make my MH4U .3dz legit dump from original EUR cartridge save in the old Repeating CTR Fail and that worked, I played all the tutorial for 20 minutes and the game is working fine. The savegame can now be opened with 3DSExplorer and I can extract key and image bins too. I also followed this: http://3dbrew.org/wiki/Savegames#Repeating_CTR_Fail and I found a file splitting tool simply called Free File Splitter to make the 512 bytes chunks. With Ultracompare I also found my keystream which is also present on the key.bin extracted with 3DSExplorer. Now what I'm not albe to find is a tool that can XOR that keystream from the savegame to decrypt it. How can I do that? Are there such tools around?