Add optional encryption

Cryptography
- Use AES-GCM-256 for encryption
- Use PBKDF2 with SHA-256 and 100000 iterations for key derivation

Behavior
- A human-readable password will be automatically be generated when not set
- The password will be appended to the URI as fragment #password
- The IV (12 bytes) is prepended to the ciphertext
- Everything is base64 encoded (33% overhead)
- Update is disabled when encryption fails to avoid overwriting

Fixes petercunha#12

public/javascripts/crypt.js

@@ -0,0 +1,25 @@
+const unambiguousChars = '23456789abcdefghjkmnpqrstwxyz';  // easily human-readable
+async function generatePassword(length) {
+  let password = window.crypto.getRandomValues(new Uint32Array(length));
+  return password.reduce((str,chr)=>str+unambiguousChars[chr%unambiguousChars.length],'');  // chars from unambiguousChars list
+}
+async function deriveKey(password, salt) {
+  let keyMaterial = await window.crypto.subtle.importKey("raw", new TextEncoder().encode(password), "PBKDF2", false, ["deriveBits", "deriveKey"]);
+  return window.crypto.subtle.deriveKey(
+    { name: "PBKDF2", salt: salt, iterations: 100000, hash: "SHA-256" },
+    keyMaterial,
+    { name: "AES-GCM", length: 256},
+    true,
+    [ "encrypt", "decrypt" ]
+  );
+}
+const IV_LENGTH = 12;  /// 96 bits
+async function encrypt(plaintext, key) {
+  let iv = window.crypto.getRandomValues(new Uint8Array(IV_LENGTH));
+  let ciphertext = await window.crypto.subtle.encrypt({ name: "AES-GCM", iv: iv }, key, new TextEncoder().encode(plaintext));
+  return btoa(String.fromCharCode(...iv,...new Uint8Array(ciphertext)));  // base64
+}
+async function decrypt(bufferEncoded, key) {
+  let buffer = Uint8Array.from(atob(bufferEncoded), c => c.charCodeAt(0))  //base64
+  return new TextDecoder().decode(await window.crypto.subtle.decrypt({ name: "AES-GCM", iv: buffer.subarray(0, IV_LENGTH) }, key, buffer.subarray(IV_LENGTH)));
+}

public/javascripts/worker.js

@@ -12,24 +12,47 @@ if (f.substr(f.length - 1) == '/') {
   pathname = temp[temp.length - 1]
 }
 
-// Ask server latest data
-socket.emit('sync', { path: pathname })
-
-// Set TextArea as server response
-socket.on('notify', function (data) {
-  if (data.path == pathname) {
-    document.getElementById('text').value = data.content
-  } else {
-    console.log('Recieved: ' + data.path + ' Have: ' + pathname)
+// Run when DOM ready
+window.onload = async function() {
+  var textElement = document.getElementById('text')
+
+  // Initialize encryption
+  const salt = new TextEncoder().encode('hcvDtp51dTiaLGFXKBWD6r7OhQMZ87ph')
+  if (!window.location.hash) {
+    window.location.hash = await generatePassword(16)
   }
-})
+  const key = await deriveKey(window.location.hash.substr(1), salt)
+
+  // Ask server latest data
+  socket.emit('sync', { path: pathname })
+
+  // Set TextArea as server response
+  socket.on('notify', async function (data) {
+    if (data.path == pathname) {
+      if (data.content) {
+        try {
+          data.content = await decrypt(data.content, key)
+        } catch(e) {
+          console.log('Unable to decrypt note. Disable update.')
+          window.processText = function() {}  // avoid overwrite
+          textElement.disabled = true
+          textElement.value = 'Unable to decrypt note. Please append the correct password to the URL "#<password>" and reload.'
+          return
+        }
+      }
+      textElement.value = data.content
+    } else {
+      console.log('Recieved: ' + data.path + ' Have: ' + pathname)
+    }
+  })
 
-// Send latest data to server
-function processText () {
-  var x = document.getElementById('text').value
+  // Send latest data to server
+  window.processText = async function () {
+    var x = textElement.value
 
-  if (cache != x) {
-    socket.emit('data', { text: x, path: pathname })
-    cache = x
+    if (cache != x) {
+      socket.emit('data', { text: await encrypt(x, key), path: pathname })
+      cache = x
+    }
   }
 }

views/pad.pug

@@ -9,6 +9,7 @@ html
 
     script(src="/socket.io/socket.io.js" type="text/javascript")
     script(src="/javascripts/worker.js", type="text/javascript")
+    script(src="/javascripts/crypt.js", type="text/javascript")
 
   body
     textarea(id="text", autocomplete="off", onkeyup="processText()", placeholder=temp)