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crypto7_new - CIS CIS 5371 Cryptography 7 Symmetric...

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CIS 5371 C t h Cryptography 7. Symmetric encryption symmetric cryptography 1
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C t hi t Cryptographic systems C t t ( M C K K’ G E D ) Cryptosystem: ( M,C,K,K’,G,E,D M, plaintext message space C, ciphertext message space K, K’, encryption and decryption key spaces G N K K’ k i l i h G : K’, key generation algorithm E : M K C, encryption algorithm D : C K’ M decryption algorithm K M, decryption algorithm G,E,D must be efficient symmetric cryptography 2
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E l Examples C t t ( M C K K’ G E D ) Cryptosystem: ( M,C,K,K’,G,E,D Substitution Cipher : M = C = Z 26 , with K=K’ The encryption algorithm is a mapping E k :M C -- E k (x) = (x), where k K is the key. The dencryption algorithm is a mapping D k :M C -- D k (y) = -1 (y). Shift Ciphe M C K K’ Z ith Shift Cipher : M = C = K = K’ = 26 , with -- E k (x) = x + k mod26 -- D k (y) = y k mod26 symmetric cryptography 3 (y) y k mod26 where x,y Z 26
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