[encoding] How can I send and receive WebSocket messages on the server side?

• How can I send and receive messages on the server side using WebSocket, as per the protocol?

• Why do I get seemingly random bytes at the server when I send data from the browser to the server? It the data encoded somehow?

• How does the framing work in both the server → client and client → server directions?

pimvdb's answer implemented in python:

def DecodedCharArrayFromByteStreamIn(stringStreamIn):
    #turn string values into opererable numeric byte values
    byteArray = [ord(character) for character in stringStreamIn]
    datalength = byteArray[1] & 127
    indexFirstMask = 2 
    if datalength == 126:
        indexFirstMask = 4
    elif datalength == 127:
        indexFirstMask = 10
    masks = [m for m in byteArray[indexFirstMask : indexFirstMask+4]]
    indexFirstDataByte = indexFirstMask + 4
    decodedChars = []
    i = indexFirstDataByte
    j = 0
    while i < len(byteArray):
        decodedChars.append( chr(byteArray[i] ^ masks[j % 4]) )
        i += 1
        j += 1
    return decodedChars

An Example of usage:

fromclient = '\x81\x8c\xff\xb8\xbd\xbd\xb7\xdd\xd1\xd1\x90\x98\xea\xd2\x8d\xd4\xd9\x9c'
# this looks like "?ŒOÇ¿¢gÓ ç\Ð=«ož" in unicode, received by server
print DecodedCharArrayFromByteStreamIn(fromclient)
# ['H', 'e', 'l', 'l', 'o', ' ', 'W', 'o', 'r', 'l', 'd', '!']

Implementation in Go

Encode part (server -> browser)

func encode (message string) (result []byte) {
  rawBytes := []byte(message)
  var idxData int

  length := byte(len(rawBytes))
  if len(rawBytes) <= 125 { //one byte to store data length
    result = make([]byte, len(rawBytes) + 2)
    result[1] = length
    idxData = 2
  } else if len(rawBytes) >= 126 && len(rawBytes) <= 65535 { //two bytes to store data length
    result = make([]byte, len(rawBytes) + 4)
    result[1] = 126 //extra storage needed
    result[2] = ( length >> 8 ) & 255
    result[3] = ( length      ) & 255
    idxData = 4
  } else {
    result = make([]byte, len(rawBytes) + 10)
    result[1] = 127
    result[2] = ( length >> 56 ) & 255
    result[3] = ( length >> 48 ) & 255
    result[4] = ( length >> 40 ) & 255
    result[5] = ( length >> 32 ) & 255
    result[6] = ( length >> 24 ) & 255
    result[7] = ( length >> 16 ) & 255
    result[8] = ( length >>  8 ) & 255
    result[9] = ( length       ) & 255
    idxData = 10
  }

  result[0] = 129 //only text is supported

  // put raw data at the correct index
  for i, b := range rawBytes {
    result[idxData + i] = b
  }
  return
}

Decode part (browser -> server)

func decode (rawBytes []byte) string {
  var idxMask int
  if rawBytes[1] == 126 {
    idxMask = 4
  } else if rawBytes[1] == 127 {
    idxMask = 10
  } else {
    idxMask = 2
  }

  masks := rawBytes[idxMask:idxMask + 4]
  data := rawBytes[idxMask + 4:len(rawBytes)]
  decoded := make([]byte, len(rawBytes) - idxMask + 4)

  for i, b := range data {
    decoded[i] = b ^ masks[i % 4]
  }
  return string(decoded)
}

C++ Implementation (not by me) here. Note that when your bytes are over 65535, you need to shift with a long value as shown here.

In addition to the PHP frame encoding function, here follows a decode function:

function Decode($M){
    $M = array_map("ord", str_split($M));
    $L = $M[1] AND 127;

    if ($L == 126)
        $iFM = 4;
    else if ($L == 127)
        $iFM = 10;
    else
        $iFM = 2;

    $Masks = array_slice($M, $iFM, 4);

    $Out = "";
    for ($i = $iFM + 4, $j = 0; $i < count($M); $i++, $j++ ) {
        $Out .= chr($M[$i] ^ $Masks[$j % 4]);
    }
    return $Out;
}

I've implemented this and also other functions in an easy-to-use WebSocket PHP class here.

JavaScript implementation:

function encodeWebSocket(bytesRaw){
    var bytesFormatted = new Array();
    bytesFormatted[0] = 129;
    if (bytesRaw.length <= 125) {
        bytesFormatted[1] = bytesRaw.length;
    } else if (bytesRaw.length >= 126 && bytesRaw.length <= 65535) {
        bytesFormatted[1] = 126;
        bytesFormatted[2] = ( bytesRaw.length >> 8 ) & 255;
        bytesFormatted[3] = ( bytesRaw.length      ) & 255;
    } else {
        bytesFormatted[1] = 127;
        bytesFormatted[2] = ( bytesRaw.length >> 56 ) & 255;
        bytesFormatted[3] = ( bytesRaw.length >> 48 ) & 255;
        bytesFormatted[4] = ( bytesRaw.length >> 40 ) & 255;
        bytesFormatted[5] = ( bytesRaw.length >> 32 ) & 255;
        bytesFormatted[6] = ( bytesRaw.length >> 24 ) & 255;
        bytesFormatted[7] = ( bytesRaw.length >> 16 ) & 255;
        bytesFormatted[8] = ( bytesRaw.length >>  8 ) & 255;
        bytesFormatted[9] = ( bytesRaw.length       ) & 255;
    }
    for (var i = 0; i < bytesRaw.length; i++){
        bytesFormatted.push(bytesRaw.charCodeAt(i));
    }
    return bytesFormatted;
}

function decodeWebSocket (data){
    var datalength = data[1] & 127;
    var indexFirstMask = 2;
    if (datalength == 126) {
        indexFirstMask = 4;
    } else if (datalength == 127) {
        indexFirstMask = 10;
    }
    var masks = data.slice(indexFirstMask,indexFirstMask + 4);
    var i = indexFirstMask + 4;
    var index = 0;
    var output = "";
    while (i < data.length) {
        output += String.fromCharCode(data[i++] ^ masks[index++ % 4]);
    }
    return output;
}

C# Implementation

Browser -> Server

    private String DecodeMessage(Byte[] bytes)
    {
        String incomingData = String.Empty;
        Byte secondByte = bytes[1];
        Int32 dataLength = secondByte & 127;
        Int32 indexFirstMask = 2;
        if (dataLength == 126)
            indexFirstMask = 4;
        else if (dataLength == 127)
            indexFirstMask = 10;

        IEnumerable<Byte> keys = bytes.Skip(indexFirstMask).Take(4);
        Int32 indexFirstDataByte = indexFirstMask + 4;

        Byte[] decoded = new Byte[bytes.Length - indexFirstDataByte];
        for (Int32 i = indexFirstDataByte, j = 0; i < bytes.Length; i++, j++)
        {
            decoded[j] = (Byte)(bytes[i] ^ keys.ElementAt(j % 4));
        }

        return incomingData = Encoding.UTF8.GetString(decoded, 0, decoded.Length);
    }

Server -> Browser

    private static Byte[] EncodeMessageToSend(String message)
    {
        Byte[] response;
        Byte[] bytesRaw = Encoding.UTF8.GetBytes(message);
        Byte[] frame = new Byte[10];

        Int32 indexStartRawData = -1;
        Int32 length = bytesRaw.Length;

        frame[0] = (Byte)129;
        if (length <= 125)
        {
            frame[1] = (Byte)length;
            indexStartRawData = 2;
        }
        else if (length >= 126 && length <= 65535)
        {
            frame[1] = (Byte)126;
            frame[2] = (Byte)((length >> 8) & 255);
            frame[3] = (Byte)(length & 255);
            indexStartRawData = 4;
        }
        else
        {
            frame[1] = (Byte)127;
            frame[2] = (Byte)((length >> 56) & 255);
            frame[3] = (Byte)((length >> 48) & 255);
            frame[4] = (Byte)((length >> 40) & 255);
            frame[5] = (Byte)((length >> 32) & 255);
            frame[6] = (Byte)((length >> 24) & 255);
            frame[7] = (Byte)((length >> 16) & 255);
            frame[8] = (Byte)((length >> 8) & 255);
            frame[9] = (Byte)(length & 255);

            indexStartRawData = 10;
        }

        response = new Byte[indexStartRawData + length];

        Int32 i, reponseIdx = 0;

        //Add the frame bytes to the reponse
        for (i = 0; i < indexStartRawData; i++)
        {
            response[reponseIdx] = frame[i];
            reponseIdx++;
        }

        //Add the data bytes to the response
        for (i = 0; i < length; i++)
        {
            response[reponseIdx] = bytesRaw[i];
            reponseIdx++;
        }

        return response;
    }

PHP Implementation:

function encode($message)
{
    $length = strlen($message);

    $bytesHeader = [];
    $bytesHeader[0] = 129; // 0x1 text frame (FIN + opcode)

    if ($length <= 125) {
            $bytesHeader[1] = $length;
    } else if ($length >= 126 && $length <= 65535) {
            $bytesHeader[1] = 126;
            $bytesHeader[2] = ( $length >> 8 ) & 255;
            $bytesHeader[3] = ( $length      ) & 255;
    } else {
            $bytesHeader[1] = 127;
            $bytesHeader[2] = ( $length >> 56 ) & 255;
            $bytesHeader[3] = ( $length >> 48 ) & 255;
            $bytesHeader[4] = ( $length >> 40 ) & 255;
            $bytesHeader[5] = ( $length >> 32 ) & 255;
            $bytesHeader[6] = ( $length >> 24 ) & 255;
            $bytesHeader[7] = ( $length >> 16 ) & 255;
            $bytesHeader[8] = ( $length >>  8 ) & 255;
            $bytesHeader[9] = ( $length       ) & 255;
    }

    $str = implode(array_map("chr", $bytesHeader)) . $message;

    return $str;
}

Clojure, the decode function assumes frame is sent as map of {:data byte-array-buffer :size int-size-of-buffer}, because the actual size may not be the same size as the byte-array depending on chunk size of your inputstream.

Code posted here: https://gist.github.com/viperscape/8918565

(defn ws-decode [frame]
  "decodes websocket frame"
  (let [data (:data frame)
        dlen (bit-and (second data) 127)
        mstart (if (== dlen 127) 10 (if (== dlen 126) 4 2))
        mask (drop 2 (take (+ mstart 4) data))
        msg (make-array Byte/TYPE (- (:size frame) (+ mstart 4)))]
   (loop [i (+ mstart 4), j 0]
      (aset-byte msg j (byte (bit-xor (nth data i) (nth mask (mod j 4)))))
      (if (< i (dec(:size frame))) (recur (inc i) (inc j))))
    msg))

(defn ws-encode [data]
  "takes in bytes, return websocket frame"
  (let [len (count data)
        blen (if (> len 65535) 10 (if (> len 125) 4 2))
        buf (make-array Byte/TYPE (+ len blen))
        _ (aset-byte buf 0 -127) ;;(bit-or (unchecked-byte 0x80) 
                                           (unchecked-byte 0x1)
        _ (if (= 2 blen) 
            (aset-byte buf 1 len) ;;mask 0, len
            (do
              (dorun(map #(aset-byte buf %1 
                      (unchecked-byte (bit-and (bit-shift-right len (*(- %2 2) 8))
                                               255)))
                      (range 2 blen) (into ()(range 2 blen))))
              (aset-byte buf 1 (if (> blen 4) 127 126))))
        _ (System/arraycopy data 0 buf blen len)]
    buf))

Thank you for the answer, i would like to add onto hfern's(above) Python version to include the Sending function if any one is interested.

def DecodedWebsockRecieve(stringStreamIn):
    byteArray =  stringStreamIn 
    datalength = byteArray[1] & 127
    indexFirstMask = 2 
    if datalength == 126:
        indexFirstMask = 4
    elif datalength == 127:
        indexFirstMask = 10
    masks = [m for m in byteArray[indexFirstMask : indexFirstMask+4]]
    indexFirstDataByte = indexFirstMask + 4
    decodedChars = []
    i = indexFirstDataByte
    j = 0
    while i < len(byteArray):
        decodedChars.append( chr(byteArray[i] ^ masks[j % 4]) )
        i += 1
        j += 1
    return ''.join(decodedChars)

def EncodeWebSockSend(socket,data):
    bytesFormatted = []
    bytesFormatted.append(129)

    bytesRaw = data.encode()
    bytesLength = len(bytesRaw)
    if bytesLength <= 125 :
        bytesFormatted.append(bytesLength)
    elif bytesLength >= 126 and bytesLength <= 65535 :
        bytesFormatted.append(126)
        bytesFormatted.append( ( bytesLength >> 8 ) & 255 )
        bytesFormatted.append( bytesLength & 255 )
    else :
        bytesFormatted.append( 127 )
        bytesFormatted.append( ( bytesLength >> 56 ) & 255 )
        bytesFormatted.append( ( bytesLength >> 48 ) & 255 )
        bytesFormatted.append( ( bytesLength >> 40 ) & 255 )
        bytesFormatted.append( ( bytesLength >> 32 ) & 255 )
        bytesFormatted.append( ( bytesLength >> 24 ) & 255 )
        bytesFormatted.append( ( bytesLength >> 16 ) & 255 )
        bytesFormatted.append( ( bytesLength >>  8 ) & 255 )
        bytesFormatted.append( bytesLength & 255 )

    bytesFormatted = bytes(bytesFormatted)
    bytesFormatted = bytesFormatted + bytesRaw
    socket.send(bytesFormatted) 

Usage for reading:

bufSize = 1024     
read = DecodedWebsockRecieve(socket.recv(bufSize))

Usage for writing:

EncodeWebSockSend(sock,"hellooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo")

Java implementation (if any one requires)

Reading : Client to Server

        int len = 0;            
        byte[] b = new byte[buffLenth];
        //rawIn is a Socket.getInputStream();
        while(true){
            len = rawIn.read(b);
            if(len!=-1){

                byte rLength = 0;
                int rMaskIndex = 2;
                int rDataStart = 0;
                //b[0] is always text in my case so no need to check;
                byte data = b[1];
                byte op = (byte) 127;
                rLength = (byte) (data & op);

                if(rLength==(byte)126) rMaskIndex=4;
                if(rLength==(byte)127) rMaskIndex=10;

                byte[] masks = new byte[4];

                int j=0;
                int i=0;
                for(i=rMaskIndex;i<(rMaskIndex+4);i++){
                    masks[j] = b[i];
                    j++;
                }

                rDataStart = rMaskIndex + 4;

                int messLen = len - rDataStart;

                byte[] message = new byte[messLen];

                for(i=rDataStart, j=0; i<len; i++, j++){
                    message[j] = (byte) (b[i] ^ masks[j % 4]);
                }

                parseMessage(new String(message)); 
                //parseMessage(new String(b));

                b = new byte[buffLenth];

            }
        }

Writing : Server to Client

public void brodcast(String mess) throws IOException{
    byte[] rawData = mess.getBytes();

    int frameCount  = 0;
    byte[] frame = new byte[10];

    frame[0] = (byte) 129;

    if(rawData.length <= 125){
        frame[1] = (byte) rawData.length;
        frameCount = 2;
    }else if(rawData.length >= 126 && rawData.length <= 65535){
        frame[1] = (byte) 126;
        int len = rawData.length;
        frame[2] = (byte)((len >> 8 ) & (byte)255);
        frame[3] = (byte)(len & (byte)255); 
        frameCount = 4;
    }else{
        frame[1] = (byte) 127;
        int len = rawData.length;
        frame[2] = (byte)((len >> 56 ) & (byte)255);
        frame[3] = (byte)((len >> 48 ) & (byte)255);
        frame[4] = (byte)((len >> 40 ) & (byte)255);
        frame[5] = (byte)((len >> 32 ) & (byte)255);
        frame[6] = (byte)((len >> 24 ) & (byte)255);
        frame[7] = (byte)((len >> 16 ) & (byte)255);
        frame[8] = (byte)((len >> 8 ) & (byte)255);
        frame[9] = (byte)(len & (byte)255);
        frameCount = 10;
    }

    int bLength = frameCount + rawData.length;

    byte[] reply = new byte[bLength];

    int bLim = 0;
    for(int i=0; i<frameCount;i++){
        reply[bLim] = frame[i];
        bLim++;
    }
    for(int i=0; i<rawData.length;i++){
        reply[bLim] = rawData[i];
        bLim++;
    }

    out.write(reply);
    out.flush();

}