import java.io.PrintWriter;
import java.math.BigInteger;
import java.util.Scanner;

/**
 * RSA算法
 */
public class RSA {

    private static final BigInteger a = BigInteger.valueOf('a');
    private static final Scanner sc = new Scanner(System.in);
    private static final PrintWriter print = new PrintWriter(System.out,true);
    // 公钥
    public static BigInteger n;
    public static BigInteger e;
    // 私钥
    public static BigInteger d;

    // 帮助信息
    private static final String help = "1. 生成密钥  2. RSA加密  3. RSA解密 4.重置数据 5.输入密钥  -1. 退出";

    public static void main(String[] args) {
        while (true) {
            print.println("当前数据" + (n == null ? "" : " n = "+n) + (e == null ? "" : " e = "+e) + (d == null ? "" : " d = "+d));
            print.println(help);
            int i = sc.nextInt();
            switch (i) {
                case 1 -> createN();
                case 2 -> getCipher();
                case 3 -> getPlain();
                case 4 -> init();
                case 5 -> {
                    print.println("请输入密钥 n,e,d");
                    n = sc.nextBigInteger();
                    e = sc.nextBigInteger();
                    d = sc.nextBigInteger();
                }
                case -1 -> System.exit(0);
            }
        }

    }

    private static void init() {
        n = null;
        e = null;
        d = null;
    }

    private static void getPlain() {
        if (d == null) {
            print.println("请输入密钥");
            d = sc.nextBigInteger();
        }
        print.println("请输入密文");
        getModPow(d);
    }

    private static void getModPow(BigInteger d) {
        sc.next().chars().mapToObj(c -> BigInteger.valueOf(c - 'a')).forEach(i -> {
            BigInteger j = i.modPow(d, n);
            j = j.add(a);
            print.print((char) j.intValue());
        });
        print.println();
    }

    private static void createN() {
        print.println("请输入两个质数p,q");
        BigInteger p = sc.nextBigInteger();
        BigInteger q = sc.nextBigInteger();
        n = p.multiply(q);
        BigInteger f = p.subtract(BigInteger.ONE).multiply(q.subtract(BigInteger.ONE));
        e = BigInteger.TWO;
        while (!f.gcd(e).equals(BigInteger.ONE)) {
            e = e.add(BigInteger.ONE);
        }
        d = e.modInverse(f);
        print.println("公钥: (" + n + "," + e + ")");
        print.println("私钥: (" + n + "," + d + ")");
    }

    private static void getCipher() {
        if (n == null || e == null) {
            print.println("请输入公钥n，e");
            n = sc.nextBigInteger();
            e = sc.nextBigInteger();
        }
        print.println("请输入明文");
        getModPow(e);
    }
}