Validate firebase requests using jwt.

jwt.js

/*
You can obtain a token, client-side like this:

const { currentUser } = firebase.auth()
if (!currentUser) throw new Error('User has to be logged in.')
const idToken = await currentUser.getIdToken()
*/


const http = require('http')
const https = require('https')
const crypto = require('crypto')
const publicKeysUrl = 'https://www.googleapis.com/robot/v1/metadata/x509/securetoken@system.gserviceaccount.com'
const PORT = process.env.PORT || '8000'
const HEADERS = {
  'Content-Type': 'text/json',
  'Access-Control-Allow-Methods': 'POST, GET',
  'Access-Control-Allow-Credentials': 'false',
  'Access-Control-Allow-Headers': 'Content-Type, Authorization, Accept'
}

const replaceAll = (input, searchValue, replaceValue) => {
  return input
    .toString()
    .split(searchValue)
    .join(replaceValue)
}

const isBadInput = value => {
  typeof value === 'string'
    && value.trim() !== ''
}

const requestPublicTokens = () => new Promise((resolve, reject) => {
  https.get(publicKeysUrl, res => {
    const { headers: { expires } } = res
    if (res.statusCode !== 200) return reject(new Error(res.statusMessage))
    let result = ''
    res.on('error', error => {
      reject(error)
    })
    res.on('data', data => result += data)
    res.on('end', () => {
      const publicKeys = JSON.parse(result)
      const expiresAt = Date.parse(expires)
      resolve({ publicKeys, expiresAt })
    })
  })
})

const getPublicTokensMemoized = () => {
  let value = { expiresAt: 0, publicKeys: null }
  return async () => {
    if (Date.now() > value.expiresAt) {
      value = await requestPublicTokens()
    }
    return value.publicKeys
  }
}

const fromBase64 = value => {
  try {
    return Buffer.from(
      jwtToValidBase64(value),
      'base64').toString('utf8')
  } catch (err) {
    throw new Error('JWT token has an invalid base64 encoding.')
  }
}

const parse = value => {
  try {
    return JSON.parse(
      fromBase64(
        value
      )
    )
  } catch (err) {
    logError(err)
    throw new Error('decoded JWT token contains invalid JSON syntax.')
  }
}

const jwtToValidBase64 = base64Url => {
  base64Url = base64Url.toString()

  const padding = 4 - base64Url.length % 4
  if (padding !== 4) {
    base64Url = base64Url + '==='.slice(0, padding)
  }

  return replaceAll(replaceAll(base64Url, '-', '+'), '_', '/')
}

const getPublicKeys = getPublicTokensMemoized()

const authenticate = async (authorization) => {
  if (isBadInput(authorization)) {
    throw new Error('No jwt token.')
  }
  const [rawHeader, rawBody, rawSignature] = authorization
    .replace('Bearer ', '')
    .replace('JWT ', '')
    .trim()
    .split('.')
  const { kid, alg, typ } = parse(rawHeader)

  if (!alg.includes('RS')) throw new Error(`Unsupported algorithm ${alg}. This example only allows RSxxx`)

  const bits = alg.replace('RS', '')

  const publicKeys = await getPublicKeys()

  const publicKey = typeof publicKeys === 'string' ? publicKeys : publicKeys[kid]

  const verifier = crypto.createVerify('RSA-SHA' + bits)
  const signature = jwtToValidBase64(rawSignature)
  verifier.update(rawHeader + '.' + rawBody)
  const isValid = verifier.verify(publicKey, signature, 'base64')

  if (!isValid) throw new Error('Invalid jwt token, verification was unsuccessful.')

  return parse(rawBody)
}

const server = http.createServer(async (req, res) => {
  const { headers: { authorization }, url, method, } = req

  const CORS = {
    'Access-Control-Allow-Origin': req.headers.origin,
  }

  if (method === 'OPTIONS') {
    res.writeHead(200, { ...HEADERS, ...CORS })
    res.end()
    return
  }

  let token;
  try {
    token = await authenticate(authorization)
  } catch (err) {
    const status = 401
    const title = err.message
    res.writeHead(status, title, { ...HEADERS, ...CORS })
    res.write(JSON.stringify({ errors: [{ status, title }] }))
    res.end()
    return
  }

  res.writeHead(200, { ...HEADERS, ...CORS })
  res.write(JSON.stringify({ data: token }))
  res.end()
})

server.listen(PORT)

console.log(`Listening to port: ${PORT}`)