// load pub-key from file
FileInputStream fis = new FileInputStream("public.pem");
BufferedInputStream bis = new BufferedInputStream(fis);
DataInputStream dis = new DataInputStream(bis);
byte[] keyBytes = new byte[dis.available()];
dis.readFully(keyBytes);
dis.close();

// transform bytes to PublicKey object
X509EncodedKeySpec spec = new X509EncodedKeySpec(keyBytes);
KeyFactory kf = KeyFactory.getInstance("RSA");
PublicKey publicKey = kf.generatePublic(spec);

    private static String encrypt(RSAPrivateKey key, String algorithm, String input) throws Exception {
        // https://docs.oracle.com/en/java/javase/21/docs/api/java.base/javax/crypto/Cipher.html
        // https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/security/spec/AlgorithmParameterSpec.html
        Cipher cipher = Cipher.getInstance(algorithm);

        // Initializes this {@code Cipher} object with a key.
        //     *
        //     * <p>The {@code Cipher} object is initialized for one of the following four
        //     * operations:
        //     * encryption, decryption, key wrapping or key unwrapping, depending
        //     * on the value of {@code opmode}.
        cipher.init(Cipher.ENCRYPT_MODE, key);
        byte[] cipherText = cipher.doFinal(input.getBytes());
        return encoder.encodeToString(cipherText);
    }

private static String sign(RSAPrivateKey key, String input) throws Exception {
    var privateSignature = Signature.getInstance("SHA256withRSA");
    privateSignature.initSign(key);
    privateSignature.update(input.getBytes());
    byte[] signature = privateSignature.sign();
    return encoder.encodeToString(signature);
}