L40-Unconditional Sec - Perfect Secrecy Vernam cipher...

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Perfect Secrecy Perfect Secrecy
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Vernam Vernam cipher (1917) cipher (1917) Message: M 1 , M 2 , . .., M n Key: K 1 , K 2 , . .., K n Ciphertext: C 1 , C 2 , . .., C n , where C i = M i K i
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Weaknesses of Weaknesses of Vernam Vernam Cipher Cipher • If key is reused: C 1 = M 1 K and C 2 = M 2 K C 1 C 2 = M 1 K M 2 K = M 1 M 2 • If attacker knows M 1 , s/he can obtain M 2 (and vice versa) – Known plaintext attacks
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Security of Ciphers Security of Ciphers - - Revisited Revisited • Cryptographic schemes that are provably 100% secure ( Unconditional Cryptography Unconditional Cryptography ) ) – Perfect Secrecy Perfect Secrecy • An opponent cannot break the scheme: – even if unlimited text is available – even if unlimited time is available – even if unlimited computing power is available • Strength of scheme relies on a mathematical proof that the opponent does not have enough information to break the cryptographic scheme
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Perfect Cipher Perfect Cipher • A cipher is perfect if for any i and j we have: Pr(M i |C j ) = p(M i ) This means that the ciphertext does not reveal any information about the plaintext
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This note was uploaded on 09/15/2010 for the course CSE IT325 taught by Professor Maniklaldas during the Spring '09 term at Punjab Engineering College.

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L40-Unconditional Sec - Perfect Secrecy Vernam cipher...

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