99 lines
2.9 KiB
Java
99 lines
2.9 KiB
Java
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//The ‘key’ for the Affine cipher consists of 2 numbers, we’ll call them a and b.
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// The following discussion assumes the use of a 26 character alphabet (m = 26).
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// a should be chosen to be relatively prime to m (i.e. a should have no factors in common with m).
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package Ciphers;
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import java.util.Scanner;
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class AffineCipher
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{
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static Scanner in = new Scanner(System.in);
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static String encryptMessage(char[] msg)
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{
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System.out.println("Enter key value a for encryption : ");
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int a = in.nextInt();
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System.out.println("Enter key value b for encryption : ");
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int b = in.nextInt();
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/// Initially empty cipher String
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String cipher = "";
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for (int i = 0; i < msg.length; i++)
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{
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// Avoid space to be encrypted
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/* applying encryption formula ( a x + b ) mod m
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{here x is msg[i] and m is 26} and added 'A' to
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bring it in range of ascii alphabet[ 65-90 | A-Z ] */
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if (msg[i] != ' ')
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{
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cipher = cipher
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+ (char) ((((a * (msg[i] - 'A')) + b) % 26) + 'A');
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} else // append space character
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{
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cipher += msg[i];
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}
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}
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return cipher;
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}
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static String decryptCipher(String cipher)
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{
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System.out.println("Enter key value a for decryption : ");
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int a = in.nextInt();
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System.out.println("Enter key value b for decryption : ");
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int b = in.nextInt();
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String msg = "";
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int a_inv = 0;
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int flag = 0;
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//Find a^-1 (the multiplicative inverse of a
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//in the group of integers modulo m.)
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for (int i = 0; i < 26; i++)
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{
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flag = (a * i) % 26;
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// Check if (a*i)%26 == 1,
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// if so, then i will be the multiplicative inverse of a
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if (flag == 1)
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{
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a_inv = i;
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}
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}
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for (int i = 0; i < cipher.length(); i++)
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{
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/*Applying decryption formula a^-1 ( x - b ) mod m
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{here x is cipher[i] and m is 26} and added 'A'
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to bring it in range of ASCII alphabet[ 65-90 | A-Z ] */
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if (cipher.charAt(i) != ' ')
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{
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msg = msg + (char) (((a_inv *
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((cipher.charAt(i) + 'A' - b)) % 26)) + 'A');
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}
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else // append space character
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{
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msg += cipher.charAt(i);
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}
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}
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return msg;
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}
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// Main method
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public static void main(String[] args)
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{
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String msg = "AFFINE CIPHER";
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// Encrypting message
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String cipherText = encryptMessage(msg.toCharArray());
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System.out.println("Encrypted Message is : " + cipherText);
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// Decrypting message
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System.out.println("Decrypted Message is: " + decryptCipher(cipherText));
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}
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}
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