public class Vigenere {
    private final int SIZE = 26;
    private final int BASE = 65;
    private char[][] cryptex = new char[SIZE][SIZE];

    // constructors ========================================================
    public Vigenere() {
        // generate Vigenere cryptex
        for (int row = 0; row < SIZE; row++) {
            int i = 0;
            for (int col = 0; col < SIZE; col++) {
                cryptex[row][col] = (char)(('A' + row + i++) % SIZE + BASE);
            }
        }
    }

    // getters/setters =====================================================

    // methods =============================================================
    public char encrypt(char letter, char key) {
        return cryptex[letter % SIZE][key % SIZE];
    }

    public String encrypt(String message, String key) {
        StringBuilder sb = new StringBuilder();
        message = message.toUpperCase();
        key = key.toUpperCase();

        for (int i = 0, j = 0; i < message.length(); i++) {
            if (message.charAt(i) >= 'A' && message.charAt(i) <= 'Z') {
                sb.append(encrypt(message.charAt(i), key.charAt(j++ % key.length())));
            }
            else
                sb.append(message.charAt(i));
        }

        return sb.toString();
    }

    public char decrypt(char letter, char key) {
        int index = findLetter(letter, key);
        return cryptex[index][0];
    }

    public String decrypt(String cipher, String key) {
        StringBuilder sb = new StringBuilder();
        cipher = cipher.toUpperCase();
        key = key.toUpperCase();

        for (int i = 0, j = 0; i < cipher.length(); i++) {
            if (cipher.charAt(i) >= 'A' && cipher.charAt(i) <= 'Z') {
                sb.append(decrypt(cipher.charAt(i), key.charAt(j++ % key.length())));
            }
            else
                sb.append(cipher.charAt(i));
        }

        return sb.toString();
    }

    // helper methods ======================================================
    private int findLetter(char letter, char key) {
        for (int i = 0; i < SIZE; i++) {
            if (cryptex[i][key % SIZE] == letter)
                return i;
        }
        return -1;
    }
}