TestRsa.java

import javax.crypto.Cipher;
import java.nio.charset.Charset;
import java.security.*;

import static javax.crypto.Cipher.DECRYPT_MODE;
import static javax.crypto.Cipher.ENCRYPT_MODE;

public class TestRsa {

    private static final String KEY_ALGORITHM = "RSA";
    private static final String SIGNATURE_ALGORITHM = "SHA256WithRSA";

    private static final Charset CHARSET = Charset.forName("UTF-8");
    private static final String TEST_CONTENT = "This is a test content.";

    public static void main(String[] args) throws Exception {
        final KeyPair key = generateKeyPair();

        final byte[] encrypted1 = encrypt(key.getPrivate(), TEST_CONTENT);
        final String decrypted1 = decrypt(key.getPublic(), encrypted1);
        if (!TEST_CONTENT.equals(decrypted1)) {
            throw new IllegalStateException("There was something going wrong while encrypting/decrypting.");
        }

        final byte[] encrypted2 = encrypt(key.getPublic(), TEST_CONTENT);
        final String decrypted2 = decrypt(key.getPrivate(), encrypted2);
        if (!TEST_CONTENT.equals(decrypted2)) {
            throw new IllegalStateException("There was something going wrong while encrypting/decrypting.");
        }

        final byte[] signature = sign(key.getPrivate(), TEST_CONTENT);
        if (!verify(key.getPublic(), TEST_CONTENT, signature)) {
            throw new IllegalStateException("There was something going wrong while signing/verifying.");
        }
    }

    private static KeyPair generateKeyPair() throws Exception {
        final KeyPairGenerator keyGen = KeyPairGenerator.getInstance(KEY_ALGORITHM);
        keyGen.initialize(4096);
        return keyGen.generateKeyPair();
    }

    private static byte[] encrypt(Key key, String what) throws Exception {
        final Cipher cipher = Cipher.getInstance(KEY_ALGORITHM);
        cipher.init(ENCRYPT_MODE, key);
        return cipher.doFinal(what.getBytes(CHARSET));
    }

    private static String decrypt(Key key, byte[] what) throws Exception {
        final Cipher encryptor = Cipher.getInstance(KEY_ALGORITHM);
        encryptor.init(DECRYPT_MODE, key);
        return new String(encryptor.doFinal(what), CHARSET);
    }

    private static byte[] sign(PrivateKey key, String what) throws Exception {
        final Signature instance = Signature.getInstance(SIGNATURE_ALGORITHM);
        instance.initSign(key);
        instance.update(what.getBytes(CHARSET));
        return instance.sign();
    }

    private static boolean verify(PublicKey key, String what, byte[] signature) throws Exception {
        final Signature instance = Signature.getInstance(SIGNATURE_ALGORITHM);
        instance.initVerify(key);
        instance.update(what.getBytes(CHARSET));
        return instance.verify(signature);
    }

}