Content Scrambling System (CSS)
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What it is
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a DVD content-protection system built around a proprietary 40-bit Stream Cipher; a historical example of a cryptographic construction that was broken
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Construction
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uses two Linear Feedback Shift Register (LFSR)s, of lengths 17 and 25 bits
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each register contains one fixed bit, leaving secret initialization bits—not 42 bits of key entropy
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combines output bytes using addition with carry; the actual construction also includes substitutions and mode-dependent operations
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Security
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the short key permits exhaustive search, but structural attacks can do substantially better
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in the simplified known-output attack, enumerate the possible secret states of the smaller register, infer the larger register’s state, and check additional output
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this attack requires sufficient known generator output; it is not a universal claim that any CSS ciphertext yields its key immediately
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Lesson
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combining predictable components does not automatically produce a cryptographically secure generator
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Distinguish from
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Cascading Style Sheets—the unrelated web styling language also abbreviated CSS
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Source
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https://www.cs.cmu.edu/~dst/DeCSS/Kesden/index.html
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