RNCryptor IOS + Javascript encryption / decryption AES 256

Question

I am new to encryption and i am trying to get some symmetric encryption usign AES256 going from a mobile app to a webpage through websockets.

I Encrypt the data using RNCryptor default settings

IOS CODE

   NSString* message = @"testmessage";
   NSData* pubData = [message dataUsingEncoding:NSUTF8StringEncoding];
   NSData *encryptedData = [RNEncryptor encryptData:pubData
                                        withSettings:kRNCryptorAES256Settings
                                            password:@"test"
                                               error:&error];
    if(error) {
        NSLog(@"Error encrypting %@", [error localizedDescription]);
    }

    NSString* encryptedString = [encryptedData base64Encoding];
    NSLog(@"Sending message %@", encryptedString);
    [self.session publishData:[encryptedString dataUsingEncoding:NSUTF8StringEncoding] onTopic:@"test12345"];

Here is the output going out over websockets AgEBnXPPvAkJb7YVapwCVNd5SQw4JwqU7BfLsEXNZyKy9SazfJT8w16Y/hYY7aKxuz3Kuy2tAXXX/cHCc3PMhvG+fzSfrslRVMKvD6L+oWvXLg==

JAVASCRIPT CODE - I receive the message and i try to parse it and display it

function onMessageArrived(message) {

  var rawData = base64.decode(message.payloadString);
  var encryptionSalt = rawData.substr(2,8);
  var hmacSalt = rawData.substr(10,8);
  var iv = rawData.substr(18, 16);
  var ciphertext = rawData.substr(34, rawData.length-34-32);
  var hmac = rawData.substr(rawData.length-32, 32);
  var password = "test";

  var key = CryptoJS.PBKDF2(password, encryptionSalt, { keySize: 256 / 32, iterations: 10000});

  var plaintextArray = CryptoJS.AES.decrypt(
    { ciphertext: CryptoJS.enc.Utf8.parse(ciphertext) },
    CryptoJS.enc.Hex.parse(key),
    { iv: CryptoJS.enc.Latin1.parse(iv) }
  );

   showScreen('<span style="color: blue;">User: ' + CryptoJS.enc.Latin1.stringify(plaintextArray) + '</span>');
};

For some reason the code gets stuck on generating the key (maybe 10k iterations are too much for CryptoJS??? thats the iterations used on IOS )

I have tried a bunch of different things with this and the output is garbage im not actually getting the message decrypted. Any help would be greatly appreciated. If you are going to recommend SJCL please provide some code . RNCryptor uses its own message format. I use it because of the randomized iv it provides . Please recommend a different library if you know of any as well.

THanks for reading.

Answer 1

Please follow the documentation for the RNCryptor file format: https://github.com/RNCryptor/RNCryptor-Spec/blob/master/RNCryptor-Spec-v3.md

You should be able to derive all the necessary data from file and add your shared secret...

Specification for RNCryptor data format version 3

Byte:     |    0    |    1    |      2-9       |  10-17   | 18-33 | <-      ...     -> | n-32 - n |
Contents: | version | options | encryptionSalt | HMACSalt |  IV   | ... ciphertext ... |   HMAC   |

• version (1 byte): Data format version. Currently 3.
• options (1 byte): bit 0 - uses password
• encryptionSalt (8 bytes): iff option includes "uses password"
• HMACSalt (8 bytes): iff options includes "uses password"
• IV (16 bytes) ciphertext (variable) -- Encrypted in CBC mode HMAC (32 bytes)

All data is in network order (big-endian).