AES Example in Java

Encrypter.java

import javax.crypto.Cipher;
import javax.crypto.KeyGenerator;
import javax.crypto.SecretKey;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.SecretKeySpec;
import java.io.ByteArrayOutputStream;
import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;
import java.util.Base64;

public class Encrypter {

    private static final String ALGORITHM = "AES/CBC/PKCS5Padding";
    private static final String KEY_ALGORITHM = "AES";

    public String makeMeAKey() throws NoSuchAlgorithmException {
        SecretKey key = getAESKey();
        return Base64.getEncoder().encodeToString(key.getEncoded());
    }

    private SecretKey getAESKey() throws NoSuchAlgorithmException {
        KeyGenerator keyGen = KeyGenerator.getInstance(KEY_ALGORITHM);
        keyGen.init(256, SecureRandom.getInstanceStrong());
        return keyGen.generateKey();
    }

    public String encrypt(String input, String encodedKey) throws Exception {
        IvParameterSpec iv = gimmeGimmeGimmeAnIvAfterMidnight();
        Cipher cipher = Cipher.getInstance(ALGORITHM);
        cipher.init(Cipher.ENCRYPT_MODE, keyFromString(encodedKey), iv);
        byte[] cypherText = cipher.doFinal(input.getBytes());

        ByteArrayOutputStream out = new ByteArrayOutputStream();
        out.write(iv.getIV());
        out.write(cypherText);

        return Base64.getEncoder().encodeToString(out.toByteArray());
    }

    private SecretKey keyFromString(String base64Key) {
        byte[] decodedKey = Base64.getDecoder().decode(base64Key);
        return new SecretKeySpec(decodedKey, 0, decodedKey.length, KEY_ALGORITHM);
    }

    private IvParameterSpec gimmeGimmeGimmeAnIvAfterMidnight() {
        byte[] iv = new byte[16];
        new SecureRandom().nextBytes(iv);
        return new IvParameterSpec(iv);
    }

    public String decrypt(String cypherText, String encodedKey) throws Exception {
        byte[] cyperPlusIv = Base64.getDecoder().decode(cypherText);
        IvParameterSpec iv = new IvParameterSpec(cyperPlusIv, 0, 16);

        ByteArrayOutputStream cipherOnly = new ByteArrayOutputStream();
        cipherOnly.write(cyperPlusIv, 16, cyperPlusIv.length - 16);

        Cipher cipher = Cipher.getInstance(ALGORITHM);
        cipher.init(Cipher.DECRYPT_MODE, keyFromString(encodedKey), iv);
        byte[] plainText = cipher.doFinal(cipherOnly.toByteArray());
        return new String(plainText);
    }

    public static void main(String[] args) {
        try {
            Encrypter encrypter = new Encrypter();

            String secretKey = encrypter.makeMeAKey();
            System.err.println("My key is " + secretKey);

            String plaintext = "Now is the winter of your discontent";
            System.err.println("Plaintext is " + plaintext);

            String encrypted = encrypter.encrypt(plaintext, secretKey);
            System.err.println("Encrypted is " + encrypted);

            String decrypted = encrypter.decrypt(encrypted, secretKey);
            System.err.println("Descrypted is " + decrypted);

        } catch (Exception e) {
            e.printStackTrace();
        }
    }

}