Decrypt Meshtastic packets from MQTT

decrypt.js

import * as crypto from 'node:crypto'
import { loadProtobufs } from './pbs.js'

const pbroot = await loadProtobufs()
const Data = pbroot.lookupType('meshtastic.Data')

// Usage: decryptPacket(
//   Buffer.from('1PG7OiApB1nwvP+rz05pAQ==', 'base64'),
//   {from, id, encrypted})

export function decryptPacket(psk, packet) {
  const packetId = packet.id
  const fromNodeId = packet.from
  const ciphertext = packet.encrypted

  // guard against overflows
  if (typeof packetId !== 'number' ||
      typeof fromNodeId !== 'number')
    throw new TypeError('id/from should be in numbers')

  const iv = new ArrayBuffer(16)
  const ivView = new DataView(iv)
  ivView.setUint32(0, packetId, true)
  ivView.setUint32(8, fromNodeId, true)

  const algo = psk.length === 32 ? 'aes-256-ctr' : 'aes-128-ctr'

  const decipher = crypto.createDecipheriv(algo, psk, iv)

  let plaintext = decipher.update(ciphertext)
  plaintext = Buffer.concat([plaintext, decipher.final()])

  try {
    return Data.decode(plaintext)
  } catch (err) {  // i.e. input is incorrect
    return null
  }
}

pbs.js

import protobuf from 'protobufjs'

/** @type {protobuf.Root} */
let root

export async function loadProtobufs() {
  if (root == null) {
    let defaultResolve = protobuf.util.path.resolve
    protobuf.Root.prototype.resolvePath = function(p, ip, an) {
      return defaultResolve('protobufs/xxx', ip, an)
    }
    try {
      root = await protobuf.load('meshtastic/mqtt.proto')
    } finally {
      protobuf.Root.prototype.resolvePath = defaultResolve
    }
  }
  return root
}