1// Aim: Program to perform Rail fence cipher (Rail fence Cipher).
2public class railfence {
3    public static String encrypt(String text, int key) {
4        if (key <= 1) return text;
5        StringBuilder[] rail = new StringBuilder[key];
6        for (int i = 0; i < key; i++) rail[i] = new StringBuilder();
7
8        int dir = 1, row = 0;
9        for (char c : text.toCharArray()) {
10            rail[row].append(c);
11            row += dir;
12            if (row == 0 || row == key - 1) dir *= -1;
13        }
14
15        StringBuilder result = new StringBuilder();
16        for (StringBuilder sb : rail) result.append(sb);
17        return result.toString();
18    }
19
20    public static String decrypt(String cipher, int key) {
21        if (key <= 1) return cipher;
22        int len = cipher.length();
23        boolean[][] mark = new boolean[key][len];
24
25        int dir = 1, row = 0;
26        for (int i = 0; i < len; i++) {
27            mark[row][i] = true;
28            row += dir;
29            if (row == 0 || row == key - 1) dir *= -1;
30        }
31
32        char[] result = new char[len];
33        int idx = 0;
34        for (int i = 0; i < key; i++) {
35            for (int j = 0; j < len; j++) {
36                if (mark[i][j] && idx < len) {
37                    result[j] = cipher.charAt(idx++);
38                }
39            }
40        }
41
42        StringBuilder plain = new StringBuilder();
43        row = 0; dir = 1;
44        for (int i = 0; i < len; i++) {
45            plain.append(result[i]);
46            row += dir;
47            if (row == 0 || row == key - 1) dir *= -1;
48        }
49        return plain.toString();
50    }
51
52    public static void main(String[] args) {
53        String text = "WEAREDISCOVEREDFLEEATONCE";
54        int key = 3;
55        String encrypted = encrypt(text, key);
56        System.out.println("Encrypted: " + encrypted);
57        String decrypted = decrypt(encrypted, key);
58        System.out.println("Decrypted: " + decrypted);
59    }
60}
61