I would like to share a tool I've found in a certain forum.
This is Nintendo 3DS ROM Decryptor Python based tool, which even works offline. Mainly made for Citra.
Please use legal way to get game files before use this tool.
Features & Effects: One key batch decrypt 3DS files.
Usage: Put your encrypted .3ds rom in Encrypted folder and run decrypt-all.cmd , you will find decrypted game in Decrypted folder.
Download: Click here
Authors: Tom Muller - decrypt scripts and xorer.exe (3dsiso forum) BrianTokyo - NDSTokyoTrim3.exe
FAQs?
Q: Does it work on CIA? A: No, you need to convert .cia to .3ds using this tool here
Q: How long it takes? A: Not much, depends on your ROM size.
Q: I can't find Decrypted rom in the folder! A: Your ROM is already decrypted or new eShop format CIA. You need to visit this thread here.
Q: There is a video tutorial? A: Yes, here it is on YouTube .