is there a Dart Equivalent of the following Aes Encryption function in java

Question

I have set up a server that is listening for an encrypted byte array through the java function mentioned below. Earlier i was using java (android) to build my app so it was easy using the same java function but i am unable to figure out what is the dart equivalent(flutter) of that function which takes a string as input and returns an AES encrypted byte array as output ,which i can write to the tcp socket .

Also i'd really appreciate help on knowing how to write the generated byte array to the server and then read a similar response and decrypting it through dart (flutter)

I have been successful in writing simple strings and receiving simple strings as input and output to the tcp server through dart but cant do the same for encrypted byte arrays . In java i used DataOutputStream to send the response to server like this

DataOutputStream dOut = newDataOutputStream(socket.getOutputStream());

           byte[] s2 = Encrypt3.encrypt2(myString);

           dOut.writeInt(s2.length); // write length of the message
            dOut.write(s2);

Here is the java function i used for aes encryption

import java.security.Key;

import javax.crypto.Cipher;

import javax.crypto.spec.SecretKeySpec;


public class Encrypt3 {
public static String key = "mykey";

public static byte[] encrypt2(String text ){
    String encrypt ="";

    try{
        // Create key and cipher
        Key aesKey = new SecretKeySpec(key.getBytes(), "AES");
        Cipher cipher = Cipher.getInstance("AES");
        // encrypt the text
        cipher.init(Cipher.ENCRYPT_MODE, aesKey);
        byte[] encrypted = cipher.doFinal(text.getBytes());
        return encrypted ;
    }catch(Exception e){
        System.out.println(e);
    }
    return null ;
}


public static String decrypt2(byte[] encrypted2){
    String decrypt ="";
    try{
        Key aesKey = new SecretKeySpec(key.getBytes(), "AES");
        Cipher cipher = Cipher.getInstance("AES");
        // decrypt the text
        cipher.init(Cipher.DECRYPT_MODE, aesKey);
        decrypt = new String(cipher.doFinal(encrypted2));
    }catch(Exception e){
        System.out.println(e);
    }

    return decrypt ;
}


}

I expect the equivalent bytearray to be produced in dart when provided with a string as input,and then writing the byte array to a tcp server.

Thank You in advance

Solution

The way you are configuring the Java cipher defaults to Electronic Code Book (ECB) mode which shouldn't be used in a real cipher system. You are also relying on the default padding - PKCS5.

Use the pointycastle package, with the following imports:

import 'package:pointycastle/api.dart';
import 'package:pointycastle/block/aes_fast.dart';
import 'package:pointycastle/paddings/pkcs7.dart';
import 'package:pointycastle/padded_block_cipher/padded_block_cipher_impl.dart';
import 'package:pointycastle/block/modes/ecb.dart';

An approximate equivalent in Dart is:

List<int> encrypt2(String text) {
  // key must be 16 or 32 bytes - not sure how "mykey" could work
  // key should really be binary, not a String! Better to use a KDF.
  Uint8List key = Uint8List.fromList(
    utf8.encode('0123456789abcdef'),
  );

  PaddedBlockCipher cipher = PaddedBlockCipherImpl(
    PKCS7Padding(), // Java defaults to PKCS5 which is equivalent
    ECBBlockCipher(AESFastEngine()), // Very weak mode - don't use this in the real world
  );

  cipher.init(
    true,
    PaddedBlockCipherParameters<CipherParameters, CipherParameters>(
      KeyParameter(key),
      null,
    ),
  );

  return cipher.process(utf8.encode(text)); // this isn't the same as .toBytes, except for ASCII
}

Comments

[Admin]

I am new to encryption , i implemented this function in flutter ,and this returned me a List<Int> , i would like to write this list to my server and then decrypt it over there , can i simply do sock.write(encryptedList) to write it to my server ? And can you give me the encrypt and decrypt function for dart and java both Please ? Also do you suggest i use a different padding than pkcs7 ?

[Richard Heap]

It's probably sock.add in dart to send it. That dart encrypt should match the existing java decrypt in the question. Pkcs5/7 padding is acceptable for your purposes at the moment. Your main problem comes from using ECB. Read Wikipedia to see why. Decrypt is identical, but change true to false :-)

[Admin]

Thanks a lot , i have successfully encrypted the string and decrypted on my server However i am unable to decrypt on the dart side , i received this string through encryption 'Öb9÷Ñ\^] vê~§Ó<…>' which is actually "Text data" . I tried putting false in decrypt but it gives me this error when i pass this string to the decrypt function in dart " Invalid argument(s): Input data length must be a multiple of cipher's block size"

[Richard Heap]

Ob9... isn't an array of bytes, so not sure how you would expect to decrypt that. Where are you getting the encrypted data from? Your Java function returns byte[] as you might expect.

[Admin]

I am stuck at 1.)the way i am receiving the Stream of bytes from my server . and 2.) the return type of decrypt function , return statement 1.) To receive on dart i am using sock.listen((data) { } I assume data is the byte stream i expect , or should i use stdin for that (please give sample snippet if possible) 2.)my return type of decrypt function is string , it takes a list as an argument(the one we received) , i've changed true to false in cipher.init , now what should be the return statement for this ?

[Admin]

I want to first receive the byte array on dart and then decrypt it , but i face the above problems , by mistake i tried to convert the array to string hence the other comment , please ignore that

[Richard Heap]

sock.listen should yield bytes, yes. After decrypting the bytes you should get another byte array. Turn this back to characters - closest equiv to java toBytes is utf8.decode(somebytes)

[Admin]

for decrypting the recieved byte array , i have to pass it to the decrypt function , what should its argument be , earlier it was string when i was encrypting,now should it be the byte array ?

[Richard Heap]

yes and the last line becomes return utf8.decode(cipher.process(cipherText))

[Bhavin Patel]

@VaibhavDikha I'm using same Encrypt and decrypt functionality .So,please share class which you created in flutter for encrypt decrypt and api dunction for converting data please