Sample Code in Java

Generation of appKey

 public static byte[] createAESKey() {
   appKey = null;
      try {
          KeyGenerator gen = KeyGenerator.getInstance("AES");
          gen.init(256);
          SecretKey secret = gen.generateKey();
          appKey = secret.getEncoded();
          } catch (NoSuchAlgorithmException ex) {
          Logger.getLogger(Utilities.class.getName()).log(Level.SEVERE, null, ex);
        }
    return appKey;
 }          

Asymmetric Encryption (RSA)

The following Java code snippet can be used for encrypting the password and the appkey using the public key given by the e-Invoice System. The encryption method used here is RSA.

public static String encryptAsymmetricKey(String pubkey, String password) throws Exception{

    PublicKey publicKeys = convertPubStringToKey(pubkey);
    Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1PADDING");
    cipher.init(Cipher.ENCRYPT_MODE, publicKeys);
   byte[] encryptedText = cipher.doFinal(password.getBytes());
   String encryptedPassword = Base64.encodeBase64String(encryptedText);
   return encryptedPassword;
 }

 private static PublicKey convertPubStringToKey(String publikkey)
{
   PublicKey pubKey = null;
   byte[] publicBytes = Base64.decodeBase64(publikkey);
   X509EncodedKeySpec keySpec = new X509EncodedKeySpec(publicBytes);
   KeyFactory keyFactory;
  try {
   keyFactory = KeyFactory.getInstance("RSA");
    pubKey = keyFactory.generatePublic(keySpec);
  } catch (Exception e)
   {
  e.printStackTrace();
  }
   return pubKey;
}
public static String encryptAsymmetricKey(String pubkey, byte[] appKey) throws Exception
  {
   PublicKey publicKeys = covertPubStringToKey(pubkey);
   Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1PADDING");
   cipher.init(Cipher.ENCRYPT_MODE, publicKeys);
   byte[] encryptedText = cipher.doFinal(appKey);
   String encryptedAppKey = Base64.encodeBase64String(encryptedText);
   return encryptedAppKey;
}

Symmetric Key Encryption using Java.

private static String encryptBySymmetricKey(String json, String decryptedSek)
  {
   byte[] sekByte = Base64.decodeBase64(decryptedSek);
    Key aesKey = new SecretKeySpec(sekByte, "AES");
   try {

    Cipher cipher = Cipher.getInstance("AES/ECB/PKCS5Padding");
    cipher.init(Cipher.ENCRYPT_MODE, aesKey);
     byte[] encryptedjsonbytes = cipher.doFinal(json.getBytes());
    String encryptedJson = Base64.encodeBase64String(encryptedjsonbytes);
    return encryptedJson;
 }
  catch(Exception e) {
  return "Exception "+e;
   }
}

Symmetric Key Decryption using Java

public static String decrptyBySyymetricKey(String encryptedSek, byte[] appKey)
 {
  Key aesKey = new SecretKeySpec(appKey, "AES"); // converts bytes(32 byte random generated) to key
  try {
  Cipher cipher = Cipher.getInstance("AES/ECB/PKCS5Padding"); // encryption type = AES with padding PKCS5
   cipher.init(Cipher.DECRYPT_MODE, aesKey); // initiate decryption type with the key
  byte[] encryptedSekBytes = Base64.decodeBase64(encryptedSek); // decode the base64 encryptedSek to bytes
   byte[] decryptedSekBytes = cipher.doFinal(encryptedSekBytes); // decrypt the encryptedSek with the initialized cipher containing the key(Results in bytes)
  String decryptedSek = Base64.encodeBase64String(decryptedSekBytes); // convert the decryptedSek(bytes) to Base64 StriNG
   return decryptedSek; // return results in base64 string
  }catch(Exception e)
  {
   return "Exception; "+e;
  }
}

Decoding the Signed eInvoice

 private void decodeSignedJWT(String signedText)
 {
   try {
     BASE64Decoder decoder = new BASE64Decoder();
     String[]  splitSignedText = signedText.split("\\.");
     String decodedSigned =new String(decoder.decodeBuffer(splitSignedText[0]));
     decodedSigned = decodedSigned +"\n Content:"+(new String(decoder.decodeBuffer(splitSignedText[1])));
     decodedSigned.replaceAll("\\\"", "\"");
     System.out.println("\nDecoded Text:" + decodedSigned);
   } catch (IOException ex) {
     Logger.getLogger(Main.class.getName()).log(Level.SEVERE, null, ex);
    }
  }