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Lecture 3 Classical Cipher System SUBSTITUTION CIPHERS

Lecture 3 Classical Cipher System SUBSTITUTION CIPHERS. By: NOOR DHIA AL- SHAKARCHY 2012-2013. Classical cipher system. substitution. transposition. simple. Homophonic. polyalphabetic. polygram. High-order. Beal. Vigenere. Beafort. Variant beaufort. Key word mix. affin. shifted.

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Lecture 3 Classical Cipher System SUBSTITUTION CIPHERS

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  1. Lecture 3Classical Cipher SystemSUBSTITUTION CIPHERS By: NOOR DHIA AL- SHAKARCHY 2012-2013

  2. Classical cipher system substitution transposition simple Homophonic polyalphabetic polygram High-order Beal Vigenere Beafort Variant beaufort Key word mix affin shifted decimation Hill Play Fair

  3. 5- POLYGRAM SUBSTITUTION CIPHERS: PolyGram substitution ciphers are ciphers in which group of letters are encrypted together, and includes enciphering large blocks of letters. • Play Fair: 1- if m1,m2 in same row, then c1, c2 are the two characters to the right of m1, m2 respectively. 2- If m1, m2 in the same column, then c1, c2 are below the m1,m2. 3- If m1, m2 are in different rows and columns then c1, c2 are the other corners of rectangle. 4- If m1=m2 a null character (e.g. x) is inserted in the plaintext between m1, m2 to eliminate the double. 5- If the plaintext has an odd number of characters a null character is appended to the end of the plaintext.

  4. 5- POLYGRAM SUBSTITUTION CIPHERS:- Play Fair: Example: ∑ =A…Z M = RENAISSANCE K = M = RE NA IS SA NC EX Ek(M)= C= HG WC BH HR WF GV

  5. 5- POLYGRAM SUBSTITUTION CIPHERS: Hill Ciphers: Let d=2 , M= m1 m2, C= c1, c2 where: C1 =( k11m1 +k12m2) mod n C2 =(k21m1 +k22m2) mod n Where K = That is : = * mod n Ek(M) = K*M Dk(C) = K-1 *C mod n = K-1 K M mod n = M Where K.K-1 mod n = I (Identical matrix)

  6. 5- POLYGRAM SUBSTITUTION CIPHERS: Example: ∑ =A…Z M = EG K K-1 I Mod 26 = M = E G = 4 6 = * mod 26 = = To decipher:- = * mod 26 = =

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