# Exam1Sols - CIS 3362 Exam#1 Classical Encryption Schemes...

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CIS 3362 Exam #1 – Classical Encryption Schemes Solution Date: 9/22/08 1) (10 pts) Encrypt "KNIGHTS" using the affine cipher with the encryption function f(x) = (11x + 6)%26. f(10) = 11(10) + 6 = 116 % 26 = 12 (M) f(13) = 11(13) + 6 = 149 % 26 = 19 (T) f(8) = 11(8) + 6 = 94 % 26 = 16 (Q) f(6) = 11(6) + 6 = 72 % 26 = 20 (U) f(7) = 11(7) + 6 = 83 % 26 = 5 (F) f(19) = 11(19) + 6 = 215 % 26 = 7 (H) f(18) = 11(18) + 6 = 204 % 26 == 22 (W) Answer = MTQUFHW (1 pt for each letter, 3 pts bonus for getting 5 or more) 2) (10 pts) Determine the corresponding decryption function for the encryption function given in question #1. x = 11y + 6 mod 26 (2 pts) x – 6 = 11y mod 26 (2 pts) 19(x – 6) = 19(11y) mod 26 (2 pts) y = 19x – 114 (mod 26) (2 pts) y = 19x + 16 (mod 26) (2 pts)

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3) (15 pts) Determine the value of 46 -1 mod 87. 87 = 1 x 46 + 41 46 = 1 x 41 + 5 41 = 8 x 5 + 1 (3 pts) 41 – 8 x 5 = 1 41 – 8(46 – 41) = 1 41 – 8 x 46 + 8 x 41 = 1 9 x 41 – 8 x 46 = 1 (5 pts) 9(87 – 46) – 8 x 46 = 1
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Exam1Sols - CIS 3362 Exam#1 Classical Encryption Schemes...

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