Cause: You are trying to use a retail console’s keys on a developer unit game, or vice versa. There are separate keys for dev units (PANDA) and retail. Fix: Verify you dumped from the correct type of console.
Before we proceed to usage, a serious disclaimer is required.
Obtaining 3ds aes-keys.txt exists in a legal gray area. 3ds aes-keys.txt
The Safe & Legal Path: Own a 3DS. Use open-source tools (like GodMode9) to dump the keys from your device. Do not download a pre-made file from the internet. This article exists for educational purposes about the structure of the file, not to facilitate illegal downloading.
Analysis and Implications of the Nintendo 3DS AES Key File (3ds aes-keys.txt) in Console Security and Digital Preservation Cause: You are trying to use a retail
The most common reason people search for 3ds aes-keys.txt is to get Citra (or its successor, Lime3DS/PabloMK7's fork) to run encrypted ROMs.
Step-by-step:
Without this file, you will see a black screen or an error: "Failed to load ROM: Crypto missing."