Several famous tools have claimed to possess a "master key" or have been colloquially labeled as such:

Modern decryption requires active, valid session tokens linked to a premium subscription. The server validates the user's account permissions before ever serving the content keys. If the session token is invalid or lacks the required subscription tier (e.g., trying to access HiFi audio on a free account), the license server refuses to deliver the necessary decryption components. The Current State of Third-Party Downloaders

Unlike many modern services that exclusively use Advanced Encryption Standard (AES), Deezer heavily utilized the cipher in Cipher Block Chaining (CBC) mode. Blowfish is a symmetric-key block cipher, meaning the same key is used for both encrypting and decrypting the data. 2. Derivation of Track Keys