I thought Apple was supposed to be trying to get rid of DRM?
Maybe they only wanted to do that with other people's music.
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For digital rights management (DRM) of e.g. digitally delivered music or video, a technique to make the decryption keys more secure is needed, according to Apple.
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Therefore in accordance with this disclosure each piece of encrypted content when distributed is accompanied by an initial key value, which is itself not necessarily a decryption key. For purposes of decrypting the content, a first key is generated from the initial key value using a known (predetermined) function. The content for purposes of encryption (and later decryption) is fragmented into a plurality of portions or blocks. These need not be of equal length. The first key generated from the initial key value is used to decrypt the first portion or block of the message. However that key is not useful for the remaining blocks. Instead an additional key is derived (again typically by some predetermined mathematical function or other type of algorithm) for each successive key, each key being associated with one block. Thereby instead of only one key being used for decrypting the entire piece of content (message), a typical piece of content (song or video, for instance) has multiple keys.
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In the alternative, the initial key value is used together with a portion of the encrypted content or some other changing parameter to obtain the value of each content key associated with each block or portion. In other words, the initial key is a master key and the content information provides the derivation parameter so that each key value is a function of the initial key value and some seed value (the initial key value) derived from the particular content of each block.
http://www.macsimumnews.com/index.php/arch...ecryption_keys/
