Send and receive binary data over web sockets in Javascript?

Question

It is possible to send and receive binary data over web sockets in Javascript? Could I, for example, implement an SSH client using web sockets?

Answer 1

The next draft (hybi-07) of the WebSockets specification is being implemented in most browsers and it will add built-in binary support to the protocol and API.

However, until then, WebSockets payload is encoded as UTF-8. In order to send binary data you must use some way of encoding the binary data as UTF-8.

There are many options but here are two that I have used:

UTF-8:

You can actually encode a byte stream directly to UTF-8.

The python to encode and decode would look something like this:

from codecs import (utf_8_encode, utf_8_decode,
                    latin_1_encode, latin_1_decode)

utf_8_encode(unicode(buf, 'latin-1'))[0]      # encode

latin_1_encode(utf_8_decode(utf8_buf)[0])[0]  # decode

In Javascript:

chr = data.charCodeAt(N)  // to 'decode' at position N of the message

// Enocde array of bytes (0-255) to UTF-8
data = array.map(function (num) {
    return String.fromCharCode(num); }).join('');

UTF-8 encode notes:

• For binary data that is evenly distributed across value 0-255, then size of the payload is 50% larger than the raw binary data.

• The Flash WebSockets emulator web-socket-js may have trouble with the encoding of 0 (zero).

Base 64:

In python:

from base64 import b64encode, b64decode

data = b64encode(buf)    # encode binary buffer to b64

buf = b64decode(data)    # decode b64 to binary buffer

To encode and decode the messages on the Javascript side:

data = window.btoa(msg)  // Encode to base64

msg = window.atob(data)  // Decode base64
msg.charCodeAt(N)        // Read decode byte at N

Base 64 notes:

• Evenly distributed binary data (0-255) will be 33% larger than the raw data.

• There is less python side overhead to base64 encoding than there is to UTF-8 encoding. However, there is a bit more Javascript side overhead to decoding base64 (UTF-8 doesn't need decoding in Javascript since the browser has already converted the UTF-8 to the Javascript native UTF-16).

• Update: This assumes the binary data is encoded to a UTF-8 string as shown above with character values that range from 0-255. Specifically, window.atob does not support character values above 255. See this mozilla bug. The same limitation applies to Chrome.

websockify:

WebSockify is a proxy/bridge that allows a WebSockets capable browser to communicate with any arbitrary binary service. It was created to allow noVNC to communicate with existing VNC servers. websockify uses base64 encode/decode of the binary data and also provides a websock.js library for use in Javascript. The websock.js has an API similar to regular WebSocket but it is handles binary data transparently and is designed to communicate with websockify. Disclaimer: I created websockify and noVNC.

ssh client:

Technically you could implement a browser ssh client over WebSockets (and I've considered it), however, this will require doing SSH encryption and decryption in the browser which will be slow. Given that WebSockets has an encrypted WSS (TLS) mode, it probably makes more sense to do plain telnet over WebSocket WSS.

In fact, websockify includes an example telnet client.

You would launch websockify on HOSTNAME like this (telnetd is from krb5-telnetd):

sudo ./websockify 2023 --web . --wrap-mode=respawn -- telnetd -debug 2023

Then navigate to http://HOSTNAME:2023/wstelnet.html?hostname=HOSTNAME&port=2023

See the websockify README for more information. To use WSS encryption you will need to create an SSL key as described on the noVNC advanced usage wiki page

Answer 2

One good and safe way to send and receive binary data is with base64 or base128 (where 128 has just 1/7 overhead instead of 1/3).

Yes an SSH Client is possible.

A proof for this is that there are already a lot of solutions out there that run in common browsers, but most of them still needs a custom server side implementation. You can look here for more information: http://en.wikipedia.org/wiki/Web-based_SSH

Answer 3

Now you can send and receive binary data easily, this article explain lot of thinks : http://blog.mgechev.com/2015/02/06/parsing-binary-protocol-data-javascript-typedarrays-blobs/

Here is how I receive binary numpy array sent with python (my_nparray.tobytes()) in my browser:

ws = new WebSocket("ws://localhost:51234");
ws.binaryType = 'blob';
var buffer;

ws.onmessage = function (evt) {
    var reader = new FileReader();
    reader.readAsArrayBuffer(evt.data);
    reader.addEventListener("loadend", function(e)
    {
        buffer = new Uint16Array(e.target.result);  // arraybuffer object
    });
};

You can convert typed array to javascript array with this:

Array.prototype.slice.call(buffer.slice());

Answer 4

Hmm, maybe WebSockets could somehow be combined with this: http://ie.microsoft.com/testdrive/HTML5/TypedArrays/

Answer 5

You cannot implement a SSH client in a browser using WebSockets without the help of a web server that will act as the SSH client or as sort of WebSocket-to-SSH proxy.

The WebSocket protocol allows to send arbitrary binary data (not even UTF-8 or Base-64 encoded) BUT that data are encapsulated in frames whose format is defined by WebSocket protocol (see RFC6455) and has nothing to do with SSH protocol. This encapsulation is hidden by the Javascript at web browser side but the server that receives the WebSocket connection receives it and must also implement it so that connection can be established.

So it could be possible to exchange the SSH protocol as payload of the WebSocket protocol but not to implement a standard SSH client.