Read the first post maybe?
"rxMode is probably the first free custom firmware for the Nintendo 3DS.
Since it's part of rxTools, it is bootable from the main menu.
How it works :
It supports all Gateway emuNAND up to the latest version.
It will boot in emuNAND in automatic, if it is present in your sdcard.
To make it work, you need to put the firmware.bin file in the root of your sdcard, just like rxTools.dat; the problem is that it contains material protected by copyright, so it's illegal to share this file, though we can legally download it from the Nintendo CDN. I included a simple tool that does it for you (with sources) : click on cdn_firm.exe and it will download the file for you."
To decrypt stuff you only need rxtools while the firmware.bin is used for rxmode (emunand).