Simple RSA demo for learning purpose.

RSA.java

import java.io.*;
import java.math.BigInteger;
import java.security.SecureRandom;

/**
 * Created by MainasuK on 2017-5-9.
 */
public class RSA {

    // public key
    private BigInteger n;
    private BigInteger e;

    // private key
    private BigInteger d;

    public RSA(int length) {
        SecureRandom secureRandom = new SecureRandom();

        // Choose prime p and q
        BigInteger p = BigInteger.probablePrime(length, secureRandom);
        BigInteger q = BigInteger.probablePrime(length, secureRandom);

        // n = q * p
        this.n  = p.multiply(q);

        // phi = (p - 1) * (q - 1)
        BigInteger phi  = p.subtract(BigInteger.ONE).multiply(q.subtract(BigInteger.ONE));

        // Choose e which gcd(e, phi) is 1 and great than 1 & less than phi
        do {
            this.e = new BigInteger(phi.bitLength(), secureRandom);
        } while (e.compareTo(BigInteger.ONE) <= 0 || e.compareTo(phi) >= 0 ||
                !e.gcd(phi).equals(BigInteger.ONE));

        // Choose d which de = 1 (mod phi), it's must exists due to gcd(e, phi) = 1
        this.d = e.modInverse(phi);
    }

    public void encode(File plaintextFile, File encryptFile) throws IOException {
        encode(plaintextFile, encryptFile, this.e, this.n);
    }

    public void encode(File plaintextFile, File encryptFile, BigInteger keyE, BigInteger keyN) throws IOException {
        System.out.println("对以下内容进行加密，公钥 (e, n) " + keyE + ", " + keyN + "): ");
        conver(plaintextFile, encryptFile, keyE, keyN);
        System.out.println("");
    }

    public void decode(File encryptFile, File decodeFile) throws IOException {
        decode(encryptFile, decodeFile, this.d, this.n);
    }

    public void decode(File encryptFile, File decodeFile, BigInteger keyD, BigInteger keyN) throws IOException {
        System.out.println("对以下内容进行解密，私钥 (d, n) " + keyD + ", " + keyN + "): ");
        conver(encryptFile, decodeFile, keyD, keyN);
        System.out.println("");
    }

    private void conver(File inputFile, File outputFile, BigInteger exponent, BigInteger mod) throws IOException {
        FileOutputStream fos = new FileOutputStream(outputFile);
        if (!outputFile.exists()) {
            outputFile.createNewFile();
        }
        FileInputStream inputStream = new FileInputStream(inputFile);

        byte[] fromBytes = new byte[(int) inputFile.length()];
        inputStream.read(fromBytes);

        System.out.println("--------- 源文件 ---------");
        System.out.println(new String(fromBytes, "UTF-8"));
        System.out.println("--------- 结束 ---------");

        BigInteger convertContent = new BigInteger(fromBytes).modPow(exponent, mod);
        String content = new String(convertContent.toByteArray(), "UTF-8");
        System.out.println("--------- 目标文件 ---------");
        System.out.println(content);
        System.out.println("--------- 结束 ---------");

        fos.write(convertContent.toByteArray());
        fos.close();
    }


    public BigInteger getKeyE() {
        return e;
    }

    public BigInteger getKeyN() {
        return n;
    }

    public static void main(String args[]) throws IOException {
        RSA rsa = new RSA(100);

        File plaintextFile = new File("plain.txt");
        if (!plaintextFile.exists()) {
            plaintextFile.createNewFile();

            FileWriter writer = new FileWriter(plaintextFile);
            writer.append("This is plaintext");
            writer.flush();
        }

        File encryptFile = new File("encrypt.txt");
        File decodeFile = new File("decode.txt");

        rsa.encode(plaintextFile, encryptFile);
        rsa.decode(encryptFile, decodeFile);
    }

}