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/**
  _____        _____                  _             
 |  __ \      / ____|                | |            
 | |  | | ___| |     _ __ _   _ _ __ | |_ ___  _ __ 
 | |  | |/ _ \ |    | '__| | | | '_ \| __/ _ \| '__|
 | |__| |  __/ |____| |  | |_| | |_) | || (_) | |   
 |_____/ \___|\_____|_|   \__, | .__/ \__\___/|_|   
                           __/ | |                  
                          |___/|_|                  
 */
class DeCryptor {

    private static final String TRANSFORMATION = "AES/GCM/NoPadding";
    private static final String ANDROID_KEY_STORE = "AndroidKeyStore";

    private KeyStore keyStore;

    DeCryptor() throws CertificateException, NoSuchAlgorithmException, KeyStoreException,
            IOException {
        initKeyStore();
    }

    private void initKeyStore() throws KeyStoreException, CertificateException,
            NoSuchAlgorithmException, IOException {
        keyStore = KeyStore.getInstance(ANDROID_KEY_STORE);
        keyStore.load(null);
    }

    String decryptData(final String alias, final byte[] encryptedData, final byte[] encryptionIv)
            throws UnrecoverableEntryException, NoSuchAlgorithmException, KeyStoreException,
            NoSuchProviderException, NoSuchPaddingException, InvalidKeyException, IOException,
            BadPaddingException, IllegalBlockSizeException, InvalidAlgorithmParameterException {

        final Cipher cipher = Cipher.getInstance(TRANSFORMATION);
        final GCMParameterSpec spec = new GCMParameterSpec(128, encryptionIv);
        cipher.init(Cipher.DECRYPT_MODE, getSecretKey(alias), spec);

        return new String(cipher.doFinal(encryptedData), "UTF-8");
    }

    private SecretKey getSecretKey(final String alias) throws NoSuchAlgorithmException,
            UnrecoverableEntryException, KeyStoreException {
        return ((KeyStore.SecretKeyEntry) keyStore.getEntry(alias, null)).getSecretKey();
    }
}

/**
  ______        _____                  _             
 |  ____|      / ____|                | |            
 | |__   _ __ | |     _ __ _   _ _ __ | |_ ___  _ __ 
 |  __| | '_ \| |    | '__| | | | '_ \| __/ _ \| '__|
 | |____| | | | |____| |  | |_| | |_) | || (_) | |   
 |______|_| |_|\_____|_|   \__, | .__/ \__\___/|_|   
                            __/ | |                  
                           |___/|_|                  
 */
class EnCryptor {

    private static final String TRANSFORMATION = "AES/GCM/NoPadding";
    private static final String ANDROID_KEY_STORE = "AndroidKeyStore";

    private byte[] encryption;
    private byte[] iv;

    EnCryptor() {
    }

    byte[] encryptText(final String alias, final String textToEncrypt)
            throws UnrecoverableEntryException, NoSuchAlgorithmException, KeyStoreException,
            NoSuchProviderException, NoSuchPaddingException, InvalidKeyException, IOException,
            InvalidAlgorithmParameterException, SignatureException, BadPaddingException,
            IllegalBlockSizeException {

        final Cipher cipher = Cipher.getInstance(TRANSFORMATION);
        cipher.init(Cipher.ENCRYPT_MODE, getSecretKey(alias));

        iv = cipher.getIV();

        return (encryption = cipher.doFinal(textToEncrypt.getBytes("UTF-8")));
    }

    @NonNull
    private SecretKey getSecretKey(final String alias) throws NoSuchAlgorithmException,
            NoSuchProviderException, InvalidAlgorithmParameterException {

        final KeyGenerator keyGenerator = KeyGenerator
                .getInstance(KeyProperties.KEY_ALGORITHM_AES, ANDROID_KEY_STORE);

        keyGenerator.init(new KeyGenParameterSpec.Builder(alias,
                KeyProperties.PURPOSE_ENCRYPT | KeyProperties.PURPOSE_DECRYPT)
                .setBlockModes(KeyProperties.BLOCK_MODE_GCM)
                .setEncryptionPaddings(KeyProperties.ENCRYPTION_PADDING_NONE)
                .build());

        return keyGenerator.generateKey();
    }

    byte[] getEncryption() {
        return encryption;
    }

    byte[] getIv() {
        return iv;
    }
}

/**
   _____                       _        _    _                      
  / ____|                     | |      | |  | |                     
 | (___   __ _ _ __ ___  _ __ | | ___  | |  | |___  __ _  __ _  ___ 
  \___ \ / _` | '_ ` _ \| '_ \| |/ _ \ | |  | / __|/ _` |/ _` |/ _ \
  ____) | (_| | | | | | | |_) | |  __/ | |__| \__ \ (_| | (_| |  __/
 |_____/ \__,_|_| |_| |_| .__/|_|\___|  \____/|___/\__,_|\__, |\___|
                        | |                               __/ |     
                        |_|                              |___/      
 */
public class MainActivity extends AppCompatActivity {

    private static final String TAG = MainActivity.class.getSimpleName();
    private static final String SAMPLE_ALIAS = "MYALIAS";

    @BindView (R.id.toolbar)
    Toolbar toolbar;

    @BindView (R.id.ed_text_to_encrypt)
    EditText edTextToEncrypt;

    @BindView (R.id.tv_encrypted_text)
    TextView tvEncryptedText;

    @BindView (R.id.tv_decrypted_text)
    TextView tvDecryptedText;

    private EnCryptor encryptor;
    private DeCryptor decryptor;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        ButterKnife.bind(this);
        setSupportActionBar(toolbar);

        encryptor = new EnCryptor();

        try {
            decryptor = new DeCryptor();
        } catch (CertificateException | NoSuchAlgorithmException | KeyStoreException |
                IOException e) {
            e.printStackTrace();
        }
    }

    @OnClick ({R.id.btn_encrypt, R.id.btn_decrypt})
    public void onClick(final View view) {

        final int id = view.getId();

        switch (id) {
            case R.id.btn_encrypt:
                encryptText();
                break;
            case R.id.btn_decrypt:
                decryptText();
                break;
        }
    }

    private void decryptText() {
        try {
            tvDecryptedText.setText(decryptor
                    .decryptData(SAMPLE_ALIAS, encryptor.getEncryption(), encryptor.getIv()));
        } catch (UnrecoverableEntryException | NoSuchAlgorithmException |
                KeyStoreException | NoSuchPaddingException | NoSuchProviderException |
                IOException | InvalidKeyException e) {
            Log.e(TAG, "decryptData() called with: " + e.getMessage(), e);
        } catch (IllegalBlockSizeException | BadPaddingException | InvalidAlgorithmParameterException e) {
            e.printStackTrace();
        }
    }

    private void encryptText() {

        try {
            final byte[] encryptedText = encryptor
                    .encryptText(SAMPLE_ALIAS, edTextToEncrypt.getText().toString());
            tvEncryptedText.setText(Base64.encodeToString(encryptedText, Base64.DEFAULT));
        } catch (UnrecoverableEntryException | NoSuchAlgorithmException | NoSuchProviderException |
                KeyStoreException | IOException | NoSuchPaddingException | InvalidKeyException e) {
            Log.e(TAG, "onClick() called with: " + e.getMessage(), e);
        } catch (InvalidAlgorithmParameterException | SignatureException |
                IllegalBlockSizeException | BadPaddingException e) {
            e.printStackTrace();
        }
    }
}