DRM Technologies – CPRM

Posted by Abhishek | DRM | Monday 12 May 2008 9:38 am

Maintaining the DRM thread here is my primer on CPRM;

In this era of computers, Digital Media (data, audio, video and other digital contents) storage and safety had never been so important be it for the users of personal computer or for other digital consumer devices such as Pen drives, USB disc, Personal Media devices.

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As for the Personal Media Devices (PMD) such as Personal Media Players (PMP) the requirement is the ability to store and move the legitimate content across systems.

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And with more popular techniques this move-ability is linked to the media and the content. From the users perspective he needs this unlimited and unrestricted access to his digital media content. However, many a times implementing a perfect scheme which will provide a seamless play-anywhere and play-unlimited capability is infeasible. Besides, while there is an expected legitimate use of this content there are various new ways devised everyday to break these content protection schemes to enable free use of protected content.

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The economics behind digital content protection/playback is huge and easily falls in the figure of multi-billion dollars every year.

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The copyright holders and publishers own the large portion of this multi-billion dollar pie and look up to the Crypto/Security enthusiast community to help them countering the problems of piracy. Various standardized and proprietary content protection (Digital Rights Management – DRM) schemes have been proposed on this topic which have been successful a large extent. One of them is “CPRM – Content Protection for Recordable Media”.

CPRM is simply a hardware based technology which controls copying, moving and deletion of digital media on computers and digital players. CPRM imposes restrictions through built in mechanisms in storage media. CPRM was developed by The 4C Entity consisting of IBM, Intel, Matsushita and Toshiba. The 4C Entity, LLC, Licensor of Content Protection for Recordable Media has defined the CPRM specifications. These specifications contain all kamagra gel cryptographic methods to protect digital media or entertainment content when recorded on physical media. The current cryptographic methods presently used are Cryptomeria cipher (C2) algorithm for symmetric encryption. CPRM specification is based on key management for interchangeable media, content encryption and how digital media is being renewed. C2 is a block cipher. The 4C Entity has defined it and has the license. C2 is successor to CSS (Content Scramble System) and mainly designed for CPRM for DRM restricted digital media and devices. The C2 symmetric key algorithm is a 10-round Feistel cipher. It has a key size of 56 bits and a block size of 64 bits.

The 4C Entity provides development or facsimile keys for the product which uses CPRM technology. CPRM is mainly implemented in Secure Digital Card and used to incorporate digital tags into storage media viz. recordable CDs (CD-R, CD-RW) and flash memory cards for MP3 players, etc. CPRM specifications have been mainly defined for DVD drives, portable ATA storage and Secure Digital (SD) memory cards. CPRM requires a table of secret device keys which should be embedded into every licensed device and media key block (MKB) which should be stored on every recordable media. CPRM complaint devices can also generate a media key by performing operations on MKB. In the case of a DVD/CD the MKB is permanently burnt into the control data area hidden within the disc’s lead-in area. Lead-in area is normally near the center of the disc and usually not accessible by users. This will prevent users from deleting or modifying MKB. Generally, disc manufacturers embed a media identifier or media ID into each piece of recordable media, which can not be deleted. Media ID specifies the type of media and it’s manufacturer and also includes a 40-bit serial number that uniquely identifies the disc. This helps to uniquely identify the disc which helps CPRM to bind the data to the media on which it is recorded. Each volume of content is also identified by a secret 64-bit title key that is stored on the disc in encrypted form. This key can only be decrypted by a 56-bit disc specific media unique key that in turn is calculated from the media key and the media ID. The title key which is needed in order to read or write content is thus bound to both content and media. Media ID doesn’t need to be kept secret as it can not be physically altered.

In nutshell, CPRM mainly binds copyrighted materials to the physical media. It allows discs to be recorded and played back on different devices but doesn’t let protected content to be copied to another piece of media. As for how successful CPRM has been could make a good debate, however, it still widely adopted by hardware manufacturers. For now I shall leave you with this much.

- Abhishek Anurag

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