RSA encryption/Decryption with a specific public/private Key.

RSA.java

import javax.crypto.Cipher;
import java.security.*;
import java.security.spec.PKCS8EncodedKeySpec;
import java.security.spec.X509EncodedKeySpec;
import java.util.Base64;

/**
 * @author Anass AIT BEN EL ARBI
 * <ul>
 *     <li>AES/CBC/NoPadding (128)</li>
 *     <li>AES/CBC/PKCS5Padding (128)</li>
 *     <li>AES/ECB/NoPadding (128)</li>
 *     <li>AES/ECB/PKCS5Padding (128)</li>
 *     <li>RSA/ECB/PKCS1Padding (1024, 2048)</li>
 *     <li>RSA/ECB/OAEPWithSHA-1AndMGF1Padding (1024, 2048)</li>
 *     <li>RSA/ECB/OAEPWithSHA-256AndMGF1Padding (1024, 2048)</li>
 * </ul>
 * <p>
 * for more details @see <a href="https://docs.oracle.com/javase/7/docs/api/javax/crypto/Cipher.html">Java Ciphers</a>
 */

public class RSA {

    private PrivateKey privateKey;
    private PublicKey publicKey;

    private static final String PRIVATE_KEY_STRING = "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";
    private static final String PUBLIC_KEY_STRING =  "MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQCYQYM3FwZuQNLK7xRbuBbAcviy1vl9LB//Ubz+NFSBjgrrGNPwqgOuWF1qskBOY0AnoAZpwHlWEryvtz1OGNX5q9boqrhOrGQebJfek9JGvjysz3+KCqAIup8C1Enp4+cUxYy7BRorjF6wqTAjyJn/SQVZPPUyVeN17Nw0jSqI9QIDAQAB";


    public void init(){
        try {
            KeyPairGenerator generator = KeyPairGenerator.getInstance("RSA");
            generator.initialize(1024);
            KeyPair pair = generator.generateKeyPair();
            privateKey = pair.getPrivate();
            publicKey = pair.getPublic();
        } catch (Exception ignored) {
        }
    }

    public void initFromStrings(){
        try{
            X509EncodedKeySpec keySpecPublic = new X509EncodedKeySpec(decode(PUBLIC_KEY_STRING));
            PKCS8EncodedKeySpec keySpecPrivate = new PKCS8EncodedKeySpec(decode(PRIVATE_KEY_STRING));

            KeyFactory keyFactory = KeyFactory.getInstance("RSA");

            publicKey = keyFactory.generatePublic(keySpecPublic);
            privateKey = keyFactory.generatePrivate(keySpecPrivate);
        }catch (Exception ignored){}
    }


    public void printKeys(){
        System.err.println("Public key\n"+ encode(publicKey.getEncoded()));
        System.err.println("Private key\n"+ encode(privateKey.getEncoded()));
    }

    public String encrypt(String message) throws Exception {
        byte[] messageToBytes = message.getBytes();
        Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
        cipher.init(Cipher.ENCRYPT_MODE, publicKey);
        byte[] encryptedBytes = cipher.doFinal(messageToBytes);
        return encode(encryptedBytes);
    }

    private static String encode(byte[] data) {
        return Base64.getEncoder().encodeToString(data);
    }
    private static byte[] decode(String data) {
        return Base64.getDecoder().decode(data);
    }

    public String decrypt(String encryptedMessage) throws Exception {
        byte[] encryptedBytes = decode(encryptedMessage);
        Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
        cipher.init(Cipher.DECRYPT_MODE, privateKey);
        byte[] decryptedMessage = cipher.doFinal(encryptedBytes);
        return new String(decryptedMessage, "UTF8");
    }

    public static void main(String[] args) {
        RSA rsa = new RSA();
        rsa.initFromStrings();
        
        try{
            String encryptedMessage = rsa.encrypt("Hello World");
            String decryptedMessage = rsa.decrypt(encryptedMessage);

            System.err.println("Encrypted:\n"+encryptedMessage);
            System.err.println("Decrypted:\n"+decryptedMessage);
          
         }catch (Exception ingored){}

        

    }
}

thanks i resolved my issue

Does the encrypt always return same value with same input?

@VietDiepAnh Short answer is no.

if you try to encrypt the same text, you will get different cyphertexts each time due to the RSA padding scheme used (PKCS1Padding). It's done for security reasons to prevent building patterns that could reveal the original text or the key.

Edit: You might consider generating your own keys instead of using the keys I shared.

Thanks for your response, but When I use the public key I have, the generated ciphertext always different each time!

My target is to get same encrypted text every time for the same input and Public_key(base64) like you share!