Encrypted Title Key - Decryption Procedure

Decryption Procedure

So that the players with license can achieve reading the discs’ content, there are some decryption procedures before achieving the reading. The discs have a volume identifier called VID (Volume ID), the Encrypted Title Key and a decryption key (Media Key Block).

The players have some keys, according to each model, called Device Keys, which are granted by the AACS organization. In the reproduction moment, one of these keys decrypts the contained MKB in the disc and as a result of this process, the Media Key, is obtained. The Media Key is combined with the VID (Volume ID) and the Volume Unique Key (KVU) is originated so that the decryption of the Encrypted Title Key can finally be done and in consequence the necessary Title Key is obtained to decrypt and reproduce the discs’ content.

To codify the Encrypted Title Key, a codification is made following the next formula:

AES-128E (Kvu, Kt ⊕ Nonce ⊕ AES_H(Volume ID || title_id))

It is possible to demonstrate, with a simple analysis of the formula, that the result is obtained of a combination between the Volume ID and a Title identifier obtained from the Media Key, giving as a result the Kvu (Volume Unique Key).