Add Hill Cipher (Hacktoberfest) (#2463)

Ciphers/HillCipher.java

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+package Ciphers;
+
+import java.util.*;
+
+/*
+* Java Implementation of Hill Cipher
+* Hill cipher is a polyalphabetic substitution cipher. Each letter is represented by a number belonging to the set Z26 where A=0 , B=1, ..... Z=25.
+* To encrypt a message, each block of n letters (since matrix size is n x n) is multiplied by an invertible n × n matrix, against modulus 26.
+* To decrypt the message, each block is multiplied by the inverse of the matrix used for encryption.
+* The cipher key and plaintext/ciphertext are user inputs.
+* @author Ojasva Jain
+*/
+
+public class HillCipher{
+static Scanner in = new Scanner (System.in);
+
+/* Following function encrypts the message
+*/
+static void encrypt(String message)
+{
+    message = message.toUpperCase();
+    // Get key matrix
+    System.out.println("Enter key matrix size");
+    int n = in.nextInt();
+    System.out.println("Enter Key/encryptionKey matrix ");
+    int keyMatrix[][] = new int [n][n];
+    for(int i=0;i<n;i++){
+        for(int j=0;j<n;j++){
+            keyMatrix[i][j] = in.nextInt();
+        }
+    }
+    //check if det = 0
+    if(determinant(keyMatrix,n)%26 == 0)
+    {
+        System.out.println("Invalid key, as determinant = 0. Program Terminated");
+        return;
+    }
+
+    int [][]messageVector = new int[n][1];
+    String CipherText="";
+    int cipherMatrix [][] = new int [n][1];
+    int j = 0;
+    while(j<message.length()){
+    for (int i = 0; i < n; i++){
+        if(j>=message.length()){ messageVector[i][0] = 23;}
+        else
+        messageVector[i][0] = (message.charAt(j))%65;
+        System.out.println(messageVector[i][0]);
+        j++; 
+    }
+    int x, i;
+    for (i = 0; i < n; i++)
+    {
+        cipherMatrix[i][0] = 0;
+
+            for (x = 0; x < n; x++)
+            {
+                cipherMatrix[i][0] += keyMatrix[i][x] * messageVector[x][0];
+            }
+            System.out.println(cipherMatrix[i][0]);
+            cipherMatrix[i][0] = cipherMatrix[i][0] % 26;
+        }
+    for (i = 0; i < n; i++)
+    CipherText += (char)(cipherMatrix[i][0] + 65);
+}
+System.out.println("Ciphertext: "+ CipherText);
+}
+//Following function decrypts a message
+static void decrypt(String message)
+{
+    message = message.toUpperCase();
+    // Get key matrix
+    System.out.println("Enter key matrix size");
+    int n = in.nextInt();
+    System.out.println("Enter inverseKey/decryptionKey matrix ");
+    int keyMatrix[][] = new int [n][n];
+    for(int i=0;i<n;i++){
+        for(int j=0;j<n;j++){
+            keyMatrix[i][j] = in.nextInt();
+        }
+    }
+    //check if det = 0
+    if(determinant(keyMatrix,n)%26 == 0)
+    {
+        System.out.println("Invalid key, as determinant = 0. Program Terminated");
+        return;
+    }
+    //solving for the required plaintext message
+    int [][]messageVector = new int[n][1];
+    String PlainText="";
+    int plainMatrix [][] = new int [n][1];
+    int j = 0;
+    while(j<message.length()){
+        for (int i = 0; i < n; i++){
+            if(j>=message.length()){ messageVector[i][0] = 23;}
+            else
+            messageVector[i][0] = (message.charAt(j))%65;
+            System.out.println(messageVector[i][0]);
+            j++; 
+        }
+        int x, i;
+        for (i = 0; i < n; i++)
+        {
+            plainMatrix[i][0] = 0;
+    
+                for (x = 0; x < n; x++)
+                {
+                    plainMatrix[i][0] += keyMatrix[i][x] * messageVector[x][0];
+                }
+    
+                plainMatrix[i][0] = plainMatrix[i][0] % 26;
+            }
+        for (i = 0; i < n; i++)
+        PlainText += (char)(plainMatrix[i][0] + 65);
+    }
+    System.out.println("Plaintext: "+PlainText);
+}
+
+// Determinant calculator
+public static int determinant(int a[][], int n){
+    int det = 0, sign = 1, p = 0, q = 0;
+
+    if(n==1){
+        det = a[0][0];
+    }
+    else{
+        int b[][] = new int[n-1][n-1];
+        for(int x = 0 ; x < n ; x++){
+            p=0;q=0;
+            for(int i = 1;i < n; i++){
+                for(int j = 0; j < n;j++){
+                    if(j != x){
+                        b[p][q++] = a[i][j];
+                        if(q % (n-1) == 0){
+                            p++;
+                            q=0;
+                        }
+                    }
+                }
+            }
+            det = det + a[0][x] *determinant(b, n-1) * sign;
+            sign = -sign;
+        }
+    }
+    return det;
+}
+
+// Function to implement Hill Cipher
+static void hillcipher(String message)
+{
+    message.toUpperCase();
+    System.out.println("What do you want to process from the message?");
+    System.out.println("Press 1: To Encrypt");
+    System.out.println("Press 2: To Decrypt");
+    short sc = in.nextShort();
+    if(sc == 1)
+    encrypt(message);
+    else if(sc == 2)
+    decrypt(message);
+    else
+    System.out.println("Invalid input, program terminated.");
+}
+
+// Driver code
+public static void main(String[] args)
+   {
+    // Get the message to be encrypted
+    System.out.println("Enter message");
+    String message = in.nextLine();
+    hillcipher(message);
+    }
+}