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A controversy surrounding the AACS cryptographic key arose in April 2007 when the Motion Picture Association of America and the Advanced Access Content System Licensing Administrator, LLC (AACS LA) began issuing cease and desist letters to websites publishing a 128-bit (16-byte) number, represented in hexadecimal as 09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0 (commonly referred to as 09 F9), a cryptographic key for HD DVDs and Blu-ray Discs. The letters demanded the immediate removal of the key and any links to it, citing the anti-circumvention provisions of the United States Digital Millennium Copyright Act (DMCA). In response to widespread Internet postings of the key, the AACS LA issued various press statements, praising those websites that complied with their requests as acting in a "responsible manner", warning that "legal and technical tools" were adapting to the situation. The controversy was further escalated in early May 2007, when aggregate news site Digg received a DMCA cease and desist notice and then removed numerous articles on the matter and banned users reposting the information. This sparked what some describe as a digital revolt or "cyber-riot", in which users posted and spread the key on Digg, and throughout the Internet ''en masse'', thereby leading to a Streisand effect. The AACS LA described this situation as an "interesting new twist". == Background == Hexadecimal is a base-16 numeral system used in the fields of computer programming and mathematics. The key is an ordinary number most widely known by its hexadecimal representation; in decimal notation, it is 13,256,278,887,989,457,651,018,865,901,401,704,640.〔Read as: "thirteen undecillion, two hundred and fifty-six decillion, two hundred and seventy-eight nonillion, eight hundred and eighty-seven octillion, nine hundred and eighty-nine septillion, four hundred and fifty-seven sextillion, six hundred and fifty-one quintillion, eighteen quadrillion, eight hundred and sixty-five trillion, nine-hundred and one billion, four hundred and one million, seven hundred and four thousand, six hundred and forty"〕 Because the encryption key may be used as part of circumvention technology forbidden by the DMCA, its possession and distribution has been viewed as illegal by the AACS, as well as by some legal professionals.〔 Since it is a 128-bit numerical value, it was dubbed an illegal number.〔(Blogger News Network / ‘Illegal Number’ Triggers Flood of MPAA Cease-and Desist Letters )〕〔(ButtUgly: Main_blogentry_010507_1 )〕〔(Protected Blog Login « WordPress.com )〕 Opponents to the expansion of the scope of copyright criticize the idea of making a particular number illegal. Commercial HD DVDs and Blu-ray Discs integrate copy protection technology specified by the AACS LA. There are several interlocking encryption mechanisms, such that cracking one part of the system does not necessarily crack other parts. Therefore, the "09 F9" key is only one of many parts that are needed to play a disc on an unlicensed player. The AACS system can be used to revoke a key of a specific playback device, after it is known to have been compromised, as it has for WinDVD. The compromised players can still be used to view old discs, but not newer releases without encryption keys for the compromised players. If other players are then cracked, further revocation would lead to legitimate users of compromised players being forced to upgrade or replace their player software or firmware in order to view new discs. Each playback device comes with a binary tree of secret device and processing keys. The processing key in this tree, a requirement to play the AACS encrypted discs, is selected based on the device key and the information on the disc to be played. As such, a processing key such as the "09 F9" key is not revoked, but newly produced discs cause the playback devices to select a different valid processing key to decrypt the discs. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「AACS encryption key controversy」の詳細全文を読む スポンサード リンク
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