I uploaded a "SaveTool" type web service here: http://files.anibb.info/n3ds/xor.php
Instructions in the same place.
3DSaveTool worked well for me on Windows 7 but then crashed no matter what I did with it on a WinXP-SP3/netbook. The web service is an easily accessible alternative that works whatever your OS, as long as you can upload/download binaries to it.
It puts the 512 byte key at the end of the file and removes it when you re-encrypt, instead of separating it into a second file. Jl12