정보보호입문 Decryption Algorithm

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본문내용

Task 1 ) Decryption Algorithm

Initial State : Ciphertext

Input : Ciphertext of English Alphabet = (C1, C2, …, Cn) and key k
(We have XMZVH) k = 3

Step 1-1) Ci → Yi
X -> 23 M -> 12 Z -> 25 V -> 21 H -> 7

Step 1-2) Xi = Yi – 3(k value) and 만약, Xi 값이 (-) 값이 나오면 +26
즉, ) Xi <- (Yi - k) mod 26

교수님해답 : Yi <- (Xi – k) mod 26
If (Xi – k) >= 0 Then Yi <- (Xi – k) mod 26
If (Xi – k) < 0 Then Yi <- (26 + Xi - k) mod 26
X -> (23 - 3) mod 26 -> 20
M -> (12 - 3) mod 26 -> 9
Z -> (25 -3) mod 26 -> 22
V -> (21 -3) mod 26 -> 18
H -> (7 -3) mod 26 -> 4

Step 1-3) Xi → Pi
X -> 20 -> U
M -> 9 -> J
Z -> 22 -> W
V -> 18 -> S
H -> 4 -> E
◆ UJWSE

Output : Plaintext of English Alphabet = (P1, P2, …, Pn)

Task 2 ) 자바로 프로그래밍하기
import java.io.*;
public class Report1 {
public static void main(String[] args) throws IOException {
BufferedReader in = new BufferedReader(new InputStreamReader(System.in));

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참고 자료

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