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){}

        

    }
}

Try below example.

    private PrivateKey getBase64PrivateKey() throws DataEncryptionException {
    StringBuilder pkcs8Lines = new StringBuilder();

    try (BufferedReader rdr = new BufferedReader(new FileReader(base64PrivateKey))) {
        String line;
        while ((line = rdr.readLine()) != null) {
            pkcs8Lines.append(line);
        }

        String pkcs8Pem = pkcs8Lines.toString();
        pkcs8Pem = pkcs8Pem.replace("-----BEGIN PRIVATE KEY-----", "");
        pkcs8Pem = pkcs8Pem.replace("-----END PRIVATE KEY-----", "");
        pkcs8Pem = pkcs8Pem.replaceAll("\\s+","");

        byte [] pkcs8EncodedBytes = Base64.getDecoder().decode(pkcs8Pem);

        PKCS8EncodedKeySpec keySpec = new PKCS8EncodedKeySpec(pkcs8EncodedBytes);
        KeyFactory kf = KeyFactory.getInstance("RSA");
        return kf.generatePrivate(keySpec);
    }catch (IOException | InvalidKeySpecException | NoSuchAlgorithmException e) {
        throw new DataEncryptionException(e.getMessage());
    }

}

 private byte[] decryptByteArray(String encryptedMessage) throws NoSuchPaddingException, NoSuchAlgorithmException, InvalidKeyException, IllegalBlockSizeException, BadPaddingException {
        Cipher decryptCipher = Cipher.getInstance("RSA/ECB/OAEPWithSHA-256AndMGF1Padding");
        decryptCipher.init(Cipher.DECRYPT_MODE, getBase64PrivateKey());
        return decryptCipher.doFinal(Base64.getDecoder().decode(encryptedMessage));
    }

    public String decrypt(String encryptedMessage) throws NoSuchPaddingException, IllegalBlockSizeException, NoSuchAlgorithmException,  BadPaddingException, InvalidKeyException {
        LOGGER.info(() -> "Start decrypting data.");
        return new String(decryptByteArray(encryptedMessage), StandardCharsets.UTF_8);
    }

I keep running this code and keep getting the same error:
"Padding error in decryption"
Do you know why?

@manuelfdg , I have just tested it on an online debugger and it works check it out, or send me your code.

https://www.jdoodle.com/ia/1jUv