Lecture Note
University
California State University, Los AngelesCourse
CS4780 | Cryptography and Information SecurityPages
3
Academic year
2023
Julie Elizabeth John
Views
0
Substituition Techniques 1. Caesar Cipher in It involves replaung each letter of the alphabet with a letter standing 3 places further down the alphabet C = (Pt 3) mod 2.6 = (0+3) mod 26 5 a bc = 3 mod 26 J3 def = 3 P CC - 3) mod 26 = (3-3) mod 26 O mod 26 = Eg. PT = W.xyz wxyz CT = ZABC Here K =3 I Let us to each assign a numerical equivalent a b C f j K d e g 0 I 2 3 2456789 l m n 0 P q S t u V 11 12 13 14 15 16 17 18 19 20 21 w a y 8 22 23 24 25 Then the algrithm can be expressed as C 1 E(3,P) (P+3) mod 26 where C Cipher text P- - Plain text
(Plain text expressed in lower case and Ciphes text expressed in upper case and E in italics 1 General Eneryphon scheme C E(K,P) = (pt K) mod 26 Then P = D.(K, c) = (c- K) mod 26 Q. Plain text jomon + 3) mod 26 (Encrypting) C = (9. = 12 mod 26 = 12 M C = (14 + 3) mod 26 = 17 mod 26 40M0I = 17 R. C = ( 12+ 3 mod 26 = 15 mod 26 15 3 P C = ( 14 73) f mod 26 = 17 R. C = C 18H3 mod 26 16 mod 26 C MR.RRQ Then Decrypting P : 1= (12-3) mod 26 = 9 mod 26 29 (J)
P = (17-3) mod 26 = 14 mod 26 0 P = ( 15-3) mod 26 12 mod 26 = M P = (17 -3) mod 26 = 14 mod 26 P= (16 - 3) mod 26 = 13 mod. 26 = N P= JOMON - Disadvantages of Gesar cipher 1. are Know R. The encryption and decryption algorithms 2. There are only 25 Keye to try 3. The Language of PT is Known. Mono - alphabetic ciphers the caesar cipheo is not safe; and With only 25 alphabetic Keys increasing key space can be achieved by allowing an arbitrary substitution . so there are 26 factorial substitution are possible
Substituition Techniques
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