Demo

Author: shivasubrahmanya

ciphers/lab2.cpp

@@ -0,0 +1,90 @@
+#include<iostream>
+#include<vector>
+#include<string>
+
+using namespace std;
+
+int modInverse(int n)
+{
+    n %= 26;
+    for(int i = 0; i < 26; i++)
+    {
+        if((n * i) % 26 == 1)
+            return i;
+    }
+    return -1;
+}
+
+string encrypt(string text, int key[2][2])
+{
+    string cipher = "";
+    for(int i = 0; i < text.length(); i += 2)
+    {
+        int p1 = text[i] - 'A';
+        int p2 = text[i+1] - 'A';
+
+        cipher += char((key[0][0]*p1 + key[0][1]*p2) % 26 + 'A');
+        cipher += char((key[1][0]*p1 + key[1][1]*p2) % 26 + 'A');
+    }
+    return cipher;
+}
+
+string decrypt(string cipher, int key[2][2])
+{
+    int det = (key[0][0]*key[1][1] - key[0][1]*key[1][0]) % 26;
+    if(det < 0) det += 26;
+
+    int invdet = modInverse(det);
+    if(invdet == -1) return "Key is not invertible";
+
+    int invKey[2][2];
+
+    invKey[0][0] = ( key[1][1] * invdet ) % 26;
+    invKey[0][1] = (-key[0][1] * invdet ) % 26;
+    invKey[1][0] = (-key[1][0] * invdet ) % 26;
+    invKey[1][1] = ( key[0][0] * invdet ) % 26;
+
+    for(int i = 0; i < 2; i++)
+        for(int j = 0; j < 2; j++)
+            if(invKey[i][j] < 0)
+                invKey[i][j] += 26;
+
+    string plain = "";
+
+    for(int i = 0; i < cipher.length(); i += 2)
+    {
+        int c1 = cipher[i] - 'A';
+        int c2 = cipher[i+1] - 'A';
+
+        plain += char((invKey[0][0]*c1 + invKey[0][1]*c2) % 26 + 'A');
+        plain += char((invKey[1][0]*c1 + invKey[1][1]*c2) % 26 + 'A');
+    }
+
+    return plain;
+}
+
+int main()
+{
+    int key[2][2];
+    string message;
+
+    cout << "Enter 4 integers for the key matrix (2x2): ";
+    for(int i = 0; i < 2; i++)
+        for(int j = 0; j < 2; j++)
+            cin >> key[i][j];
+
+    cout << "Enter message (uppercase letters only): ";
+    cin >> message;
+
+    // Ensure even length
+    if(message.length() % 2 != 0)
+        message += 'X';
+
+    string cipher = encrypt(message, key);
+    cout << "Encrypted message: " << cipher << endl;
+
+    string decrypted = decrypt(cipher, key);
+    cout << "Decrypted message: " << decrypted << endl;
+
+    return 0;
+}