1. Exercise 1- Caesar Cipher:
2. public class CaesarCipher {
3. // Encryption method
4. public static String encrypt(String plaintext, int shift) {
5. String encryptedText = "";
6. for (char ch : plaintext.toCharArray()) {
7. if (Character.isLetter(ch)) {
8. char base = Character.isUpperCase(ch) ? 'A' : 'a';
9. encryptedText=encryptedText +((char) ((ch - base + shift) % 26
10. + base));
11. } else {
12. encryptedText+=ch;
13. }
14. }
15. return encryptedText;
16. }
17. // Decryption method
18. public static String decrypt(String ciphertext, int shift) {
19. // Decryption is the same as encryption with a negative shift
20. return encrypt(ciphertext, -shift);
21. }
22.  public static void main(String[] args) {
23. // Example Usage
24. String plaintext = "Hello, World!";
25. int shift = 3;
26. // Encryption
27. String encryptedText = encrypt(plaintext, shift);
28. System.out.println("Encrypted: " + encryptedText);
29. // Decryption
30. String decryptedText = decrypt(encryptedText, shift);
31. System.out.println("Decrypted: " + decryptedText);
32. }
33. }
34. Exercise 2- Affine Cipher:
35. import java.util.Random;
36. public class AffineCipherExample {
37. public static void main(String[] args) {
38. // Example Usage
39. String plaintext = "Hello, World!"; // Replace with your desired
40. plaintext
41. // Generate random values for a and b (ensuring a is coprime with
42. 26)
43.  int a = generateCoprimeWith26();
44. int b = new Random().nextInt(26); // b can be any random integer
45. // Encryption
46. String ciphertext = encrypt(plaintext, a, b);
47. System.out.println("Plaintext: " + plaintext);
48. System.out.println("Encrypted: " + ciphertext);
49. // Decryption
50. String decryptedText = decrypt(ciphertext, a, b);
51. System.out.println("Decrypted: " + decryptedText);
52. }
53. // Method to check if two numbers are coprime
54. private static boolean areCoprime(int a, int b) {
55. while (b != 0) {
56. int temp = b;
57. b = a % b;
58. a = temp;
59. }
60. return a == 1;
61. }
62. // Method to generate a random integer coprime with 26
63. private static int generateCoprimeWith26() {
64.  Random random = new Random();
65. int candidate;
66. do {
67. candidate = random.nextInt(25) + 1; // Generate a random
68. integer between 1 and 25
69. } while (!areCoprime(candidate, 26));
70. return candidate;
71. }
72. // Affine Cipher Encryption
73. private static String encrypt(String plaintext, int a, int b) {
74. String encryptedText = "";
75. for (char ch : plaintext.toCharArray()) {
76. if (Character.isLetter(ch)) {
77. char base = Character.isUpperCase(ch) ? 'A' : 'a';
78. encryptedText+=((char) ((a * (ch - base) + b) % 26 + base));
79. } else {
80. encryptedText+=ch;
81. }
82. }
83. return encryptedText;
84. }
85. // Affine Cipher Decryption
86. private static String decrypt(String ciphertext, int a, int b) {
87. // Calculate modular multiplicative inverse of a modulo 26
88. int aInverse = 0;
89. for (int i = 1; i < 26; i++) {
90. if ((a * i) % 26 == 1) {
91. System.out.println("a= "+ a);
92. System.out.println("aInverse= "+ i);
93. aInverse = i;
94. break;
95. }
96. }
97. String decryptedText = "";
98. for (char ch : ciphertext.toCharArray()) {
99. if (Character.isLetter(ch)) {
100. char base = Character.isUpperCase(ch) ? 'A' : 'a';
101. decryptedText+=((char) ((aInverse * (ch - base - b + 26) % 26) +
102. base));
103. } else {
104. decryptedText+=ch;
105. }
106. }
107.  return decryptedText;
108. }
109. }