qgustavor/index.html

## index.html
<!DOCTYPE html><html lang="en"><meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>MEGA URL checksum</title><style>body{margin:40px auto;max-width:650px;
font:1.1em/1.5 -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
color:#444;padding:0 10px}h1,h2,h3{line-height:1.2}
label{display:flex;flex-direction:row}input{width:100%;margin-left:1em}
@media print{body{max-width:none}}</style>

<h1>MEGA URL checksum</h1>

<p>Enter a MEGA file URL and a file and this page will check if the file corresponds to the URL.
Dead URLs still work. Folder URLs (those with "!F" in the beginning) don't work because they
don't contain the file checksum.</p>

<form>
<label>URL: <input type="url"></label>
<label>File: <input type="file"></label>
<br>
<button>Check</button> <output></output>
</form>

<script src="https://cdnjs.cloudflare.com/ajax/libs/sjcl/1.0.7/sjcl.min.js" integrity="sha384-X0mQP9Ee57CRKxDlf/55LrrCIwWvXucDZvxkZ/rO5WF2By9Lwva4rEFd2LZLogOs" crossorigin="anonymous"></script>
<script src="https://cdn.rawgit.com/bitwiseshiftleft/sjcl/cc887b518c026407e4ad11ecef37ad9e40d4bd40/core/codecArrayBuffer.js" integrity="sha384-lVE6SeS3AsYL2Aa+6Gebj7kJAELvdqbItqycfASLo8x0cX0loPWIFSBdnMrcJExh" crossorigin="anonymous"></script>

<script>
var outputEl = document.querySelector('output')
document.forms[0].addEventListener('submit', function (e) {
  e.preventDefault()
  var url = this.elements[0].value
  var file = this.elements[1].files[0]

  if (!url) {
    outputEl.textContent = 'Missing URL'
    return
  }

  if (!file) {
    outputEl.textContent = 'Missing file'
    return
  }

  var parsedURL = url.match(/https:\/\/mega\.(?:co\.)?nz\/#![^!]{3,}!([A-Za-z0-9-_]{10,})/)
  if (!parsedURL) {
    outputEl.textContent = 'Invalid URL'
    return
  }

  var keySum = sjcl.codec.base64url.toBits(parsedURL[1])
  for (var i = 0; i < 4; i++) keySum[i] = keySum[i] ^ keySum[i + 4]

  var aes = new sjcl.cipher.aes(keySum.slice(0, 4))
  var nonce = keySum.slice(4, 6)
  var checksum = keySum.slice(6, 8)
  nonce = nonce.concat(nonce)

  processFile({
    aes: aes,
    checksum: checksum,
    file: file,
    finalMac: [0, 0, 0, 0],
    increment: 131072,
    mac: nonce.slice(),
    nonce: nonce,
    pos: 0,
    posNext: 131072
  })
})

var blockSize = 1024 * 1024
function processFile (state) {
  outputEl.textContent = 'Processing: ' +
    (state.pos * 100 / state.file.size).toFixed(2) +
    '% - ' + state.pos + ' / ' + state.file.size

  var reader = new FileReader()
  var slice = state.file.slice(state.pos, state.pos + blockSize)

  reader.addEventListener('load', function () {
    handleBlock(sjcl.codec.arrayBuffer.toBits(reader.result), state)

    if (state.pos >= state.file.size) {
      finishChecksum(state)
    } else {
      processFile(state)
    }
  })

  reader.readAsArrayBuffer(slice)
}

function handleBlock (result, state) {
  for (var pos = 0; pos < result.length; pos += 4) {
    for (var i = 0; i < 4; i++) state.mac[i] ^= result[pos + i]
    state.mac = state.aes.encrypt(state.mac)
    checkBounding(state)
  }
}

function checkBounding (state) {
  state.pos += 16

  if (state.pos >= state.posNext) {
    for (var i = 0; i < 4; i++) state.finalMac[i] ^= state.mac[i]

    state.finalMac = state.aes.encrypt(state.finalMac)
    state.mac = state.nonce.slice()

    if (state.increment < 1048576) state.increment += 131072
    state.posNext += state.increment
  }
}

function finishChecksum (state) {
  for (var i = 0; i < 4; i++) state.finalMac[i] ^= state.mac[i]
  var mac = state.aes.encrypt(state.finalMac)
  var sum = state.checksum

  outputEl.textContent = mac[0] ^ mac[1] === sum[0] && mac[2] ^ mac[3] === sum[1]
    ? 'Checksum matches'
    : "Checksum don't matches"
}
</script>
	<!DOCTYPE html><html lang="en"><meta charset="utf-8">
	<meta name="viewport" content="width=device-width, initial-scale=1">
	<title>MEGA URL checksum</title><style>body{margin:40px auto;max-width:650px;
	font:1.1em/1.5 -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
	color:#444;padding:0 10px}h1,h2,h3{line-height:1.2}
	label{display:flex;flex-direction:row}input{width:100%;margin-left:1em}
	@media print{body{max-width:none}}</style>

	<h1>MEGA URL checksum</h1>

	<p>Enter a MEGA file URL and a file and this page will check if the file corresponds to the URL.
	Dead URLs still work. Folder URLs (those with "!F" in the beginning) don't work because they
	don't contain the file checksum.</p>

	<form>
	<label>URL: <input type="url"></label>
	<label>File: <input type="file"></label>
	<br>
	<button>Check</button> <output></output>
	</form>

	<script src="https://cdnjs.cloudflare.com/ajax/libs/sjcl/1.0.7/sjcl.min.js" integrity="sha384-X0mQP9Ee57CRKxDlf/55LrrCIwWvXucDZvxkZ/rO5WF2By9Lwva4rEFd2LZLogOs" crossorigin="anonymous"></script>
	<script src="https://cdn.rawgit.com/bitwiseshiftleft/sjcl/cc887b518c026407e4ad11ecef37ad9e40d4bd40/core/codecArrayBuffer.js" integrity="sha384-lVE6SeS3AsYL2Aa+6Gebj7kJAELvdqbItqycfASLo8x0cX0loPWIFSBdnMrcJExh" crossorigin="anonymous"></script>

	<script>
	var outputEl = document.querySelector('output')
	document.forms[0].addEventListener('submit', function (e) {
	e.preventDefault()
	var url = this.elements[0].value
	var file = this.elements[1].files[0]

	if (!url) {
	outputEl.textContent = 'Missing URL'
	return
	}

	if (!file) {
	outputEl.textContent = 'Missing file'
	return
	}

	var parsedURL = url.match(/https:\/\/mega\.(?:co\.)?nz\/#![^!]{3,}!([A-Za-z0-9-_]{10,})/)
	if (!parsedURL) {
	outputEl.textContent = 'Invalid URL'
	return
	}

	var keySum = sjcl.codec.base64url.toBits(parsedURL[1])
	for (var i = 0; i < 4; i++) keySum[i] = keySum[i] ^ keySum[i + 4]

	var aes = new sjcl.cipher.aes(keySum.slice(0, 4))
	var nonce = keySum.slice(4, 6)
	var checksum = keySum.slice(6, 8)
	nonce = nonce.concat(nonce)

	processFile({
	aes: aes,
	checksum: checksum,
	file: file,
	finalMac: [0, 0, 0, 0],
	increment: 131072,
	mac: nonce.slice(),
	nonce: nonce,
	pos: 0,
	posNext: 131072
	})
	})

	var blockSize = 1024 * 1024
	function processFile (state) {
	outputEl.textContent = 'Processing: ' +
	(state.pos * 100 / state.file.size).toFixed(2) +
	'% - ' + state.pos + ' / ' + state.file.size

	var reader = new FileReader()
	var slice = state.file.slice(state.pos, state.pos + blockSize)

	reader.addEventListener('load', function () {
	handleBlock(sjcl.codec.arrayBuffer.toBits(reader.result), state)

	if (state.pos >= state.file.size) {
	finishChecksum(state)
	} else {
	processFile(state)
	}
	})

	reader.readAsArrayBuffer(slice)
	}

	function handleBlock (result, state) {
	for (var pos = 0; pos < result.length; pos += 4) {
	for (var i = 0; i < 4; i++) state.mac[i] ^= result[pos + i]
	state.mac = state.aes.encrypt(state.mac)
	checkBounding(state)
	}
	}

	function checkBounding (state) {
	state.pos += 16

	if (state.pos >= state.posNext) {
	for (var i = 0; i < 4; i++) state.finalMac[i] ^= state.mac[i]

	state.finalMac = state.aes.encrypt(state.finalMac)
	state.mac = state.nonce.slice()

	if (state.increment < 1048576) state.increment += 131072
	state.posNext += state.increment
	}
	}

	function finishChecksum (state) {
	for (var i = 0; i < 4; i++) state.finalMac[i] ^= state.mac[i]
	var mac = state.aes.encrypt(state.finalMac)
	var sum = state.checksum

	outputEl.textContent = mac[0] ^ mac[1] === sum[0] && mac[2] ^ mac[3] === sum[1]
	? 'Checksum matches'
	: "Checksum don't matches"
	}
	</script>