@octalmage/node-appletv/src/lib/util/tlv.ts

A Node.js library for communicating with an Apple TV

/**
 * Type Length Value encoding/decoding, used by HAP as a wire format.
 * https://en.wikipedia.org/wiki/Type-length-value
 *
 * Originally based on code from github:KhaosT/HAP-NodeJS@0c8fd88 used
 * used per the terms of the Apache Software License v2.
 *
 * Original code copyright Khaos Tian <khaos.tian@gmail.com>
 *
 * Modifications copyright Zach Bean <zb@forty2.com>
 *  * Reformatted for ES6-style module
 *  * Rewrote encode() to be non-recursive; also simplified the logic
 *  * Rewrote decode()
 */

const Tag = {
    PairingMethod:  0x00,
    Username:       0x01,
    Salt:           0x02, // salt is 16 bytes long

    // could be either the SRP client public key (384 bytes) or the ED25519 public key (32 bytes), depending on context
    PublicKey:      0x03,
    Proof:          0x04, // 64 bytes
    EncryptedData:  0x05,
    Sequence:       0x06,
    ErrorCode:      0x07,
    BackOff:        0x08,
    Signature:      0x0A, // 64 bytes

    MFiCertificate: 0x09,
    MFiSignature:   0x0A
};

function encode(type, data, ...args: any[]): Buffer {

    var encodedTLVBuffer = Buffer.alloc(0);

    // coerce data to Buffer if needed
    if (typeof data === 'number')
      data = Buffer.from([data]);
    else if (typeof data === 'string')
      data = Buffer.from(data);

    if (data.length <= 255) {
        encodedTLVBuffer = Buffer.concat([Buffer.from([type,data.length]),data]);
    } else {
        var leftLength = data.length;
        var tempBuffer = Buffer.alloc(0);
        var currentStart = 0;

        for (; leftLength > 0;) {
            if (leftLength >= 255) {
                tempBuffer = Buffer.concat([tempBuffer,Buffer.from([type,0xFF]),data.slice(currentStart, currentStart + 255)]);
                leftLength -= 255;
                currentStart = currentStart + 255;
            } else {
                tempBuffer = Buffer.concat([tempBuffer,Buffer.from([type,leftLength]),data.slice(currentStart, currentStart + leftLength)]);
                leftLength -= leftLength;
            }
        }

        encodedTLVBuffer = tempBuffer;
    }

    // do we have more to encode?
    if (arguments.length > 2) {

      // chop off the first two arguments which we already processed, and process the rest recursively
      var remainingArguments = Array.prototype.slice.call(arguments, 2);
      var remainingTLVBuffer = encode.apply(this, remainingArguments);

      // append the remaining encoded arguments directly to the buffer
      encodedTLVBuffer = Buffer.concat([encodedTLVBuffer, remainingTLVBuffer]);
    }

    return encodedTLVBuffer;
}

function decode(data): {} {

    var objects = {};

    var leftLength = data.length;
    var currentIndex = 0;

    for (; leftLength > 0;) {
        var type = data[currentIndex];
        var length = data[currentIndex+1];
        currentIndex += 2;
        leftLength -= 2;

        var newData = data.slice(currentIndex, currentIndex+length);

        if (objects[type]) {
            objects[type] = Buffer.concat([objects[type],newData]);
        } else {
            objects[type] = newData;
        }

        currentIndex += length;
        leftLength -= length;
    }

    return objects;
}

export default {
    Tag,
    encode,
    decode
}