import java.io.File;
import java.io.IOException;
import java.nio.file.Files;
import java.security.*;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.PKCS8EncodedKeySpec;
import java.security.spec.X509EncodedKeySpec;
import java.util.Base64;

public class RsaUtils {
    private static final int DEFAULT_KEY_SIZE = 2048;
    private static final String ALGORITHM_RSA = "RSA";
    /**
     * 从文件中读取公钥
     *
     * @param fileName 公钥保存路径,相对于classpath
     * @return 公钥对象
     * @throws Exception
     */
    public static PublicKey getPublicKey(String fileName) throws Exception {
        byte[] bytes = readFile(fileName);
        return getPublicKey(bytes);
    }

    /**
     * 从文件中读取密钥
     *
     * @param fileName 私钥保存路径,相对于classpath
     * @return 私钥对象
     * @throws Exception
     */
    public static PrivateKey getPrivateKey(String fileName) throws Exception {
        byte[] bytes = readFile(fileName);
        return getPrivateKey(bytes);
    }

    /**
     * 获取公钥
     *
     * @param bytes 公钥的字节数组
     * @return
     * @throws Exception
     */
    private static PublicKey getPublicKey(byte[] bytes) throws Exception {
        bytes = Base64.getDecoder().decode(bytes);
        X509EncodedKeySpec spec = new X509EncodedKeySpec(bytes);
        KeyFactory factory = KeyFactory.getInstance(ALGORITHM_RSA);
        return factory.generatePublic(spec);
    }

    /**
     * 获取密钥
     *
     * @param bytes 私钥的字节数组
     * @return
     * @throws Exception
     */
    private static PrivateKey getPrivateKey(byte[] bytes) throws NoSuchAlgorithmException, InvalidKeySpecException {
        bytes = Base64.getDecoder().decode(bytes);
        PKCS8EncodedKeySpec spec = new PKCS8EncodedKeySpec(bytes);
        KeyFactory factory = KeyFactory.getInstance(ALGORITHM_RSA);
        return factory.generatePrivate(spec);
    }

    /**
     * 根据密文,生成rsa公钥和私钥,并写入文件
     *
     * @param publicKeyFileName  公钥文件路径
     * @param privateKeyFileName 私钥文件路径
     * @param secret             生成密钥的密文
     */
    public static void generateKey(String publicKeyFileName, String privateKeyFileName, String secret, int keySize) throws Exception {
        KeyPairGenerator keyPairGenerator = KeyPairGenerator.getInstance(ALGORITHM_RSA);
        SecureRandom secureRandom = new SecureRandom(secret.getBytes());
        keyPairGenerator.initialize(Math.max(keySize, DEFAULT_KEY_SIZE), secureRandom);
        KeyPair keyPair = keyPairGenerator.genKeyPair();
        // 获取公钥并写文件
        byte[] publicKeyBytes = keyPair.getPublic().getEncoded();
        publicKeyBytes = Base64.getEncoder().encode(publicKeyBytes);
        writeFile(publicKeyFileName, publicKeyBytes);
        // 获取私钥并写文件
        byte[] privateKeyBytes = keyPair.getPrivate().getEncoded();
        privateKeyBytes = Base64.getEncoder().encode(privateKeyBytes);
        writeFile(privateKeyFileName, privateKeyBytes);
    }

    private static byte[] readFile(String fileName) throws Exception {
        return Files.readAllBytes(new File(fileName).toPath());
    }

    private static void writeFile(String destPath, byte[] bytes) throws IOException {
        File dest = new File(destPath);
        if (!dest.exists()) {
            dest.createNewFile();
        }
        Files.write(dest.toPath(), bytes);
    }

    public static void main(String[] args) throws Exception {
        String privateKeyFile = "D:\\var\\temp\\id_key_rsa";
        String publicKeyFile = "D:\\var\\temp\\id_key_rsa.pub";

        RsaUtils.generateKey(publicKeyFile, privateKeyFile, "test123", 2048);
    }
}