Any site claiming to provide a "mega decryption key decoder" is almost certainly a scam designed to: Deliver Malware:

Where did you (e.g., a public forum, an email, a blog)?

When a user decides to share a file or folder via a public link, MEGA offers two primary ways to generate that URL. Understanding these two formats explains why you might be prompted for a key. 1. Links with the Embedded Key (Standard)

, meaning the platform itself never sees your files or your keys. MEGA Help Centre