wormhole/lib/Parser.js

const constants = require("./constants");
const { createLog } = require('./logger');

const parserLog = createLog([ 'Wormhole', 'Parser' ]);

class WebsocketFrame {
    constructor() {
        // Just defining the structure of the WebsocketFrame,
        // I could actually just use a normal object but I
        // might need to add some methods to this in the future
        // so it's good enough

        // 1 byte - first byte
        this.FIN = undefined;
        this.RSVx = undefined;
        this.Opcode = undefined;
        
        // 1 byte - second byte
        this.MASK = undefined;
        this.PayloadLen = undefined;

        this.PayloadLenEx = undefined; // This is for when PayloadLen needs to be extended

        // 4 bytes - masking key
        this.MaskingKey = undefined;

        // (Extension Data + Application Data[PayloadLen]) (kind of) - payload data
        // TODO: Separate extension data from application data
        this.PayloadData = undefined;
    }
}

WebsocketFrame.prototype.toBuffer = function() {
    const packet = [];

    const firstByte = this.FIN << 0x07 | this.RSVx << 0x06 | this.Opcode;
    const secondByte = this.MASK << 0x07 | this.PayloadLen;
    const headerBuffer = Buffer.from([firstByte, secondByte]);
    packet.push(headerBuffer);

    let lenBuffer;
    if (this.PayloadLen === 126) {
        lenBuffer = Buffer.alloc(2);
        lenBuffer.writeUint16BE(this.PayloadLenEx);
    } else if (this.PayloadLen === 127) {
        lenBuffer = Buffer.alloc(8);
        lenBuffer.writeBigUint64BE(BigInt(this.PayloadLenEx));
    }
    lenBuffer && packet.push(lenBuffer);

    this.MASK && packet.push(this.MaskingKey);
    packet.push(this.PayloadData);

    return Buffer.concat(packet);
};

const parseWebsocketFrame = (data) => {
    const firstByte = data.getUint8(0);
    const secondByte = data.getUint8(1);

    const frame = new WebsocketFrame();

    // 1 byte - first byte
    frame.FIN = (firstByte >> 7); // >> 7 - Get most significant bit (FIN)
    frame.RSVx = (firstByte & 0x70) >> 4; // 0x70[0b01110000] - Get the 3 bits after the most significant bit (RSVx)
    frame.Opcode = (firstByte & 0x0F); // 0xF[0b00001111] - Get the last 4 bits (Opcode)

    // 1 byte - second byte
    frame.MASK = (secondByte >> 7); // >> 7 - Get most significant bit (MASK)
    frame.PayloadLen = (secondByte & 0x7F); // 0x7F[0b01111111] - Get last 7 bits (Payload len)

    let maskingKeyOffset = 2; // By default, theres a 2 byte offset. We will modify this in the cases below.

    // Handle Payload len cases and set the masking key offset
    if (frame.PayloadLen === 126) {
        frame.PayloadLenEx = data.getUint16(2);
        maskingKeyOffset = 4; // 4 byte offset, because we also read 2 bytes with getUint16 above (2 bytes (normal size) + 2 bytes)
    } else if (frame.PayloadLen === 127) {
        frame.PayloadLenEx = data.getBigUint64(2);
        maskingKeyOffset = 10; // 10 byte offset, because we also read 8 bytes with getBigUint64 above (2 bytes (normal size) + 8 bytes)
    } else {
        frame.PayloadLenEx = frame.PayloadLen;
    }
    
    let maskingKeyEnd = frame.MASK ? maskingKeyOffset + 4 : maskingKeyOffset; // (4 bytes because it is a 32 bit value)    
    if (frame.MASK) frame.MaskingKey = data.buffer.slice(maskingKeyOffset, maskingKeyEnd); // Create a new buffer starting at the masking key offset and ending at the masking key end (duh).

    // TODO: Separate extension data from application data
    frame.PayloadData = data.buffer.slice(maskingKeyEnd, Number(BigInt(maskingKeyEnd) + BigInt(frame.PayloadLenEx))); // Create a new buffer that starts at the end of the masking key and ends at (the end of the masking key + payload length)

    // TODO: implement unmasked string decoding
    if (frame.MASK) {
        switch (frame.Opcode) {
            case constants.opcodes.TEXT_FRAME: { // Denotes a text frame   
                const payloadDataView = new DataView(frame.PayloadData);
                const maskingKeyView = new DataView(frame.MaskingKey);
    
                let decoded = '';
    
                for (let i = 0; i < frame.PayloadData.byteLength; i++) {
                    decoded += (String.fromCharCode(payloadDataView.getUint8(i) ^ maskingKeyView.getUint8(i % 4)));
                }
                
                frame.DecodedPayloadData = decoded;

                break;
            }
    
            case constants.opcodes.BINARY_FRAME: { // Denotes a binary frame
                const payloadDataView = new DataView(frame.PayloadData);
                const maskingKeyView = new DataView(frame.MaskingKey);
    
                const decoded = [];
    
                for (let i = 0; i < frame.PayloadData.byteLength; i++) {
                    decoded.push(payloadDataView.getUint8(i) ^ maskingKeyView.getUint8(i % 4));
                }
                
                frame.DecodedPayloadData = Uint8Array.from(decoded);

                break;
            }
        }
    }

    return frame;
}

module.exports = {
    WebsocketFrame,

    parseWebsocketFrame
};