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Shannon Recap Perfect & Practical Secrecy.

Shannon Recap Perfect & Practical Secrecy. Science basis for encryption OR how well we can keep secrets when we want to. Esoteric: UNCERTAINTY & ENTROPY, LANGUAGE RATE & REDUNDANCY, KEY EQUIVOCATION/UNICITY DISTANCE, AND WORK FUNCTIONS. Uncertainty & Entropy

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Shannon Recap Perfect & Practical Secrecy.

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  1. Shannon Recap Perfect & Practical Secrecy. Science basis for encryption OR how well we can keep secrets when we want to. Esoteric: UNCERTAINTY & ENTROPY, LANGUAGE RATE & REDUNDANCY, KEY EQUIVOCATION/UNICITY DISTANCE, AND WORK FUNCTIONS. Shannon Recap

  2. Uncertainty & Entropy Consider a crossword puzzle: Across: First man Down: Intermediate Knowing the first word has reduced the uncertainty (entropy) of the second word from 264 to 263 (actually more since not all letters are possible). This makes cracking the encoding easier [word is “mean”]. Shannon Recap

  3. Language Rate/Redundancy All languages are filled with redundancy and that helps break codes. Consider striking the vowels in a sentence: xt slxws yxx dxwn bxt thx sxntxncx xs stxll lxgxblx, xsn't xt? Can you decipher it - Yes It slows you down but the sentence is still legible isn’t it? The consonants seem to be giving us most of the information we need. There is a lot of redundancy. Shannon Recap

  4. Language Rate/Redundancy Redundancy is the boone of the analyst. He/she doesn’t have to recover all the text, only a few pieces that can be Used to fill in the blanks. Redundancy doesn’t do the secret keepers any favors. That is why some real messages you see are so cryptic – to limit redundancy. Shannon Recap

  5. Key Equivocation/Unicity Distance Unicity distance is generally a measure of security of a cipher (larger is better) and specifically indicates: How many ciphertext characters are necessary to break the encryption – the number of characters required to ensure only one key could have encoded the message. This tells the analyst the minimum number of ciphertext characters that must be used to break the cipher. Shannon Recap

  6. Work Function Theoretical work function – not useful or practical. Historical work function – very practical, but has limits. Limited because the historical is the best known attack and there are many folks that keep that information secret (NSA, KGB, etc.) Consequently, public historical work function may give a misleading picture of what is possible. Still, it is the best we can do in the open literature. Shannon Recap

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