Simple MEGA downloader

mega.js

export async function getMegaFile (megaUrl) {
  const { handler, wrappedKey } = parseMegaUrl(megaUrl)
  const url = await getDownloadUrl(handler)
  const response = await fetch(url)
  if (!response.ok) throw Error('MEGA request failed with ' + response.statusText)
  const data = await response.arrayBuffer()
  
  return decryptAesCtr(data, unwrapMegaKey(wrappedKey))
}

function unwrapMegaKey (originalKey) {
  const key = new Uint8Array(originalKey)
  for (let i = 0; i < 16; i++) key[i] = key[i] ^ key[16 + i] 
  return key
}

async function decryptAesCtr (data, key) {
  const keyWrapper = await window.crypto.subtle.importKey('raw', key.slice(0, 16), {
    name: 'AES-CTR'
  }, false, ['decrypt'])

  const counter = new Uint8Array(16)
  counter.set(key.slice(16, 24), 0)
  const plainText = await crypto.subtle.decrypt({
    name: 'AES-CTR',
    counter,
    length: 64
  }, keyWrapper, data)

  return plainText
}

export function parseMegaUrl (megaUrl) {
  const parsedUrl = new URL(megaUrl)
  const handler = parsedUrl.pathname.split('/').pop()
  const wrappedKey = decodeBase64(parsedUrl.hash.substr(1))
  return { handler, wrappedKey }
}

export async function getDownloadUrl (handler) {
  const response = await fetch('https://g.api.mega.co.nz/cs', {
    method: 'POST',
    body: JSON.stringify([{
      a: 'g',
      g: 1,
      ssl: 2,
      p: handler
    }])
  })
  if (!response.ok) throw Error(`MEGA returned with ${response.statusText}`)
  const data = await response.json()
  return data[0].g
}

function decodeBase64 (s) {
  let i
  // atob also works without = padding
  // https://infra.spec.whatwg.org/#forgiving-base64-decode
  const d = atob(s.replace(/-/g, '+').replace(/_/g, '/'))
  const b = new Uint8Array(d.length)
  for (i = 0; i < d.length; i++) b[i] = d.charCodeAt(i)
  return b
}