RSA encrypt and decrypt in JAVA

java_rsa.java

package tests;

import org.apache.commons.codec.binary.Base64;

import javax.crypto.Cipher;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.security.KeyFactory;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.security.spec.PKCS8EncodedKeySpec;
import java.security.spec.X509EncodedKeySpec;

public class Test {
    /**
      Generate key file using openssl
      1、key pair
      openssl genrsa -out private_key.pem 2048
      2、public key
      openssl rsa -in private_key.pem -pubout -outform DER -out tst_public.der
      3、private key
      openssl pkcs8 -topk8 -inform PEM -outform DER -in private_key.pem -out private_key.der -nocrypt
    */
  
    public static void main(String[] args) throws Exception {
        String privateKeyFilePath = "/path/to/private/key/file";
        Path privateKeyPath = Paths.get(privateKeyFilePath);
        byte[] privateKeyBytes = Files.readAllBytes(privateKeyPath);
        PrivateKey privateKey = KeyFactory.getInstance("RSA").generatePrivate(new PKCS8EncodedKeySpec(privateKeyBytes));

        String publicKeyFilePath = "/path/to/public/key/file";
        Path publicKeyPath = Paths.get(publicKeyFilePath);
        byte[] publicKeyBytes = Files.readAllBytes(publicKeyPath);
        PublicKey publicKey = KeyFactory.getInstance("RSA").generatePublic(new X509EncodedKeySpec(publicKeyBytes));

        String str = "abcdefghijklmnopqrstuvwxyz1234567890";

        Cipher cipher = Cipher.getInstance("RSA");
        cipher.init(Cipher.ENCRYPT_MODE, privateKey);
        byte[] encStr = cipher.doFinal(str.getBytes());
        System.out.println(new String(Base64.encodeBase64(encStr)));

        Cipher cipher1 = Cipher.getInstance("RSA");
        cipher1.init(Cipher.DECRYPT_MODE, publicKey);
        byte[] decStr = cipher1.doFinal(encStr);
        System.out.println(new String(decStr));
    }
}

Reference http://stackoverflow.com/questions/17513791/how-to-load-public-ssh-key-from-a-file-in-java