ClientEncrypt.java

public class ClientEncrypt {
  
private static String publicKeyString = "<your_public_key_here>";
  
  public static void main (String args[]) {
    try {
      
      // 1. generate secret key using AES
      KeyGenerator keyGenerator = KeyGenerator.getInstance("AES");
      keyGenerator.init(128); // AES is currently available in three key sizes: 128, 192 and 256 bits.The design and strength of all key lengths of the AES algorithm are sufficient to protect classified information up to the SECRET level
      SecretKey secretKey = keyGenerator.generateKey();
      
      // 2. get string which needs to be encrypted
      String text = "<your_string_which_needs_to_be_encrypted_here>";
      
      // 3. encrypt string using secret key
      byte[] raw = secretKey.getEncoded();
      SecretKeySpec skeySpec = new SecretKeySpec(raw, "AES");
      Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
      cipher.init(Cipher.ENCRYPT_MODE, skeySpec, new IvParameterSpec(new byte[16]));
      String cipherTextString = Base64.encodeToString(cipher.doFinal(text.getBytes(Charset.forName("UTF-8"))), Base64.DEFAULT);
      
      // 4. get public key
      X509EncodedKeySpec publicSpec = new X509EncodedKeySpec(Base64.decode(publicKeyString, Base64.DEFAULT));
      KeyFactory keyFactory = KeyFactory.getInstance("RSA");
      PublicKey publicKey = keyFactory.generatePublic(publicSpec);
      
      // 6. encrypt secret key using public key
      Cipher cipher2 = Cipher.getInstance("RSA/ECB/OAEPWithSHA1AndMGF1Padding");   
      cipher2.init(Cipher.ENCRYPT_MODE, publicKey);  
      String encryptedSecretKey = Base64.encodeToString(cipher2.doFinal(secretKey.getEncoded()), Base64.DEFAULT);
      
      // 7. pass cipherTextString (encypted sensitive data) and encryptedSecretKey to your server via your preferred way.
      // Tips:
      // You may use JSON to combine both the strings under 1 object.
      // You may use a volley call to send this data to your server.
      
    } catch (NoSuchAlgorithmException | InvalidKeyException | NoSuchPaddingException | IllegalBlockSizeException | BadPaddingException | InvalidKeySpecException | InvalidAlgorithmParameterException e) {
      e.printStackTrace();
    }
  }
}