简单RSA算法

simple_rsa.java

package com.dcl;

import java.math.BigInteger;
import java.util.HashMap;
import java.util.Map;

public class Test {

    public static Map int2Char = new HashMap<Integer, Character>() {{
        put(0, 'a');
        put(1, 'b');
        put(2, 'c');
        put(3, 'd');
        put(4, 'e');
        put(5, 'f');
        put(6, 'g');
        put(7, 'h');
        put(8, 'i');
        put(9, 'j');
        put(10, 'k');
        put(11, 'l');
        put(12, 'm');
        put(13, 'n');
        put(14, 'o');
        put(15, 'p');
        put(16, 'q');
        put(17, 'r');
        put(18, 's');
        put(19, 't');
        put(20, 'u');
        put(21, 'v');
        put(22, 'w');
        put(23, 'x');
        put(24, 'y');
        put(25, 'z');
    }};
    public static Map char2Int = new HashMap<Character, Integer>() {{
        put('a', 0);
        put('b', 1);
        put('c', 2);
        put('d', 3);
        put('e', 4);
        put('f', 5);
        put('g', 6);
        put('h', 7);
        put('i', 8);
        put('j', 9);
        put('k', 10);
        put('l', 11);
        put('m', 12);
        put('n', 13);
        put('o', 14);
        put('p', 15);
        put('q', 16);
        put('r', 17);
        put('s', 18);
        put('t', 19);
        put('u', 20);
        put('v', 21);
        put('w', 22);
        put('x', 23);
        put('y', 24);
        put('z', 25);
    }};

    public static int getSecretKey(int e, int oula) {
        int secretKey = 1, n = 1;

        while (true) {
            if ((oula * n) + 1 == e * secretKey) {
                return secretKey;
            }
            secretKey++;
            if ((oula * n) + 1 < e * secretKey) {
                n++;
            }
        }
    }

    public static void main(String[] args) {

        int p, q, N;
        p = 7;
        q = 11;
        N = p * q;
        int oula = (p - 1) * (q - 1);
        int e = 13;

        int d;

        d = getSecretKey(e, oula);

        System.out.println("p: " + p);
        System.out.println("q: " + q);
        System.out.println("N: " + N);
        System.out.println("oula: " + oula);
        System.out.println("e: " + e);
        System.out.println("d: " + d);

        System.out.println("encoded key is: {" + e + ", " + N + "}");
        System.out.println("public key is: {" + d + ", " + N + "}");

        System.out.println(encodeMessage("hello", e, N));
        System.out.println(decodeMessage(encodeMessage("hello", e, N), d, N));
    }

    private static String decodeMessage(String encodedMessage, int e, int N) {
        StringBuilder sb = new StringBuilder();
        int step = getBitNum(N);
        int start = 0;
        int length = encodedMessage.length();
        while (start < length) {
            String word = encodedMessage.substring(start, start + step);
            BigInteger messageInNum = new BigInteger(word).pow(e).mod(new BigInteger(String.valueOf(N)));
            sb.append(int2Char.get(messageInNum.intValue()));
            start += step;
        }

        return sb.toString();
    }

    private static int getBitNum(int N) {
        int num = 1;
        while (N / 10 > 0) {
            N /= 10;
            num++;
        }
        return num;
    }

    private static String encodeMessage(String message, int sec, int N) {
        StringBuilder sb = new StringBuilder();
        int num = getBitNum(N);
        for (char ch : message.toCharArray()) {
            BigInteger temp = new BigInteger(char2Int.get(ch).toString()).pow(sec).mod(new BigInteger(String.valueOf(N)));

            sb.append(String.format("%0" + num + "d", temp.intValue()));
        }
        return sb.toString();
    }

}