Exercise 1- Caesar Cipher:
public class CaesarCipher {
// Encryption method
public static String encrypt(String plaintext, int shift) {
String encryptedText = "";
for (char ch : plaintext.toCharArray()) {
if (Character.isLetter(ch)) {
char base = Character.isUpperCase(ch) ? 'A' : 'a';
encryptedText=encryptedText +((char) ((ch - base + shift) % 26
+ base));
} else {
encryptedText+=ch;
}
}
return encryptedText;
}
// Decryption method
public static String decrypt(String ciphertext, int shift) {
// Decryption is the same as encryption with a negative shift
return encrypt(ciphertext, -shift);
}
 public static void main(String[] args) {
// Example Usage
String plaintext = "Hello, World!";
int shift = 3;
// Encryption
String encryptedText = encrypt(plaintext, shift);
System.out.println("Encrypted: " + encryptedText);
// Decryption
String decryptedText = decrypt(encryptedText, shift);
System.out.println("Decrypted: " + decryptedText);
}
}
Exercise 2- Affine Cipher:
import java.util.Random;
public class AffineCipherExample {
public static void main(String[] args) {
// Example Usage
String plaintext = "Hello, World!"; // Replace with your desired
plaintext
// Generate random values for a and b (ensuring a is coprime with
26)
 int a = generateCoprimeWith26();
int b = new Random().nextInt(26); // b can be any random integer
// Encryption
String ciphertext = encrypt(plaintext, a, b);
System.out.println("Plaintext: " + plaintext);
System.out.println("Encrypted: " + ciphertext);
// Decryption
String decryptedText = decrypt(ciphertext, a, b);
System.out.println("Decrypted: " + decryptedText);
}
// Method to check if two numbers are coprime
private static boolean areCoprime(int a, int b) {
while (b != 0) {
int temp = b;
b = a % b;
a = temp;
}
return a == 1;
}
// Method to generate a random integer coprime with 26
private static int generateCoprimeWith26() {
 Random random = new Random();
int candidate;
do {
candidate = random.nextInt(25) + 1; // Generate a random
integer between 1 and 25
} while (!areCoprime(candidate, 26));
return candidate;
}
// Affine Cipher Encryption
private static String encrypt(String plaintext, int a, int b) {
String encryptedText = "";
for (char ch : plaintext.toCharArray()) {
if (Character.isLetter(ch)) {
char base = Character.isUpperCase(ch) ? 'A' : 'a';
encryptedText+=((char) ((a * (ch - base) + b) % 26 + base));
} else {
encryptedText+=ch;
}
}
return encryptedText;
}
// Affine Cipher Decryption
private static String decrypt(String ciphertext, int a, int b) {
// Calculate modular multiplicative inverse of a modulo 26
int aInverse = 0;
for (int i = 1; i < 26; i++) {
if ((a * i) % 26 == 1) {
System.out.println("a= "+ a);
System.out.println("aInverse= "+ i);
aInverse = i;
break;
}
}
String decryptedText = "";
for (char ch : ciphertext.toCharArray()) {
if (Character.isLetter(ch)) {
char base = Character.isUpperCase(ch) ? 'A' : 'a';
decryptedText+=((char) ((aInverse * (ch - base - b + 26) % 26) +
base));
} else {
decryptedText+=ch;
}
}
 return decryptedText;
}
}