Вижинер

import java.util.Scanner;

public class Vigenere {

    private static String characters = "АБВГДЕЖЗИЙКЛМНОПРСТУФХЦЧШЩЬЪЫЭЮЯ";
    private static final int N = characters.length();

    private static String encode(String input, String keyword)
    {
        input = input.toUpperCase();
        keyword = keyword.toUpperCase();

        String result = "";

        int keyword_index = 0;

        for (char symbol : input.toCharArray())
        {
            if (symbol == ' ')
            {
                result += " ";
                continue;
            }

            int c = (characters.indexOf(symbol)
                  + characters.indexOf(keyword.charAt(keyword_index))) % N;

            result += characters.charAt(c);

            keyword_index++;

            if ((keyword_index) == keyword.length())
                keyword_index = 0;
        }

        return result;
    }

    private static String decode(String input, String keyword)
    {
        input = input.toUpperCase();
        keyword = keyword.toUpperCase();

        String result = "";

        int keyword_index = 0;

        for (char symbol : input.toCharArray())
        {
            if (symbol == ' ')
            {
                result += " ";
                continue;
            }

            int p = (characters.indexOf(symbol) + N -
                     characters.indexOf(keyword.charAt(keyword_index))) % N;

            result += characters.charAt(p);

            keyword_index++;

            if ((keyword_index) == keyword.length())
                keyword_index = 0;
        }

        return result;
    }

    public static void main(String []args)
    {
        Scanner in = new Scanner(System.in);
        String input, key;

        while (true)
        {
            System.out.println("Введите текст для шифрования: ");
            input = in.nextLine();

            System.out.println("Введите ключ: ");
            key = in.nextLine();

            String cipher = encode(input, key);
            System.out.println("Зашифрованный текст: " + cipher);
            System.out.println("Расшифровка: " + decode(cipher, key));

            System.out.println("Продолжить [д/н]? ");

            if(in.nextLine().equalsIgnoreCase("н"))
                break;
        }
    }
}