You should always generate your own encTitleKeys.bin from your own console to ensure compatibility and legality: by holding (Start) during power on. Navigate to [1:] SYSNAND CTRNAND -> dbs . Select ticket.db and choose Ticketdump options .
If you are following a modern 3DS modding guide (such as 3DS Hacks Guide ), you will be asked to find or use an EncTitleKeys.bin file. The recommended method for playing backups is dumping your own cartridges using godmode9 to create .cia files directly.