facebook messenger encryption

encrypt.js

// configure with jQuery 3

async function aesGcmEncrypt(plaintext, password) {
    const pwUtf8 = new TextEncoder().encode(password);                                 // encode password as UTF-8
    const pwHash = await crypto.subtle.digest('SHA-256', pwUtf8);                      // hash the password

    const iv = crypto.getRandomValues(new Uint8Array(12));                             // get 96-bit random iv

    const alg = { name: 'AES-GCM', iv: iv };                                           // specify algorithm to use

    const key = await crypto.subtle.importKey('raw', pwHash, alg, false, ['encrypt']); // generate key from pw

    const ptUint8 = new TextEncoder().encode(plaintext);                               // encode plaintext as UTF-8
    const ctBuffer = await crypto.subtle.encrypt(alg, key, ptUint8);                   // encrypt plaintext using key
 
    const ctArray = Array.from(new Uint8Array(ctBuffer));                              // ciphertext as byte array
    const ctStr = ctArray.map(byte => String.fromCharCode(byte)).join('');             // ciphertext as string
    const ctBase64 = btoa(ctStr);                                                      // encode ciphertext as base64

    const ivHex = Array.from(iv).map(b => ('00' + b.toString(16)).slice(-2)).join(''); // iv as hex string

    return ivHex+ctBase64;                                                             // return iv+ciphertext
}

async function aesGcmDecrypt(ciphertext, password) {
    const pwUtf8 = new TextEncoder().encode(password);                                 // encode password as UTF-8
    const pwHash = await crypto.subtle.digest('SHA-256', pwUtf8);                      // hash the password

    const iv = ciphertext.slice(0,24).match(/.{2}/g).map(byte => parseInt(byte, 16));  // get iv from ciphertext

    const alg = { name: 'AES-GCM', iv: new Uint8Array(iv) };                           // specify algorithm to use

    const key = await crypto.subtle.importKey('raw', pwHash, alg, false, ['decrypt']); // use pw to generate key

    const ctStr = atob(ciphertext.slice(24));                                          // decode base64 ciphertext
    const ctUint8 = new Uint8Array(ctStr.match(/[\s\S]/g).map(ch => ch.charCodeAt(0)));// ciphertext as Uint8Array
    // note: why doesn't ctUint8 = new TextEncoder().encode(ctStr) work?

    const plainBuffer = await crypto.subtle.decrypt(alg, key, ctUint8);                // decrypt ciphertext using key
    const plaintext = new TextDecoder().decode(plainBuffer);                           // decode password from UTF-8

    return plaintext;                                                                  // return the plaintext
}

var password = '';
var persist = false;
customjsReady("._fl2", e => {
  var pass = document.createElement('input');
  pass.type = 'password';
  pass.addEventListener('input', x => {
    password = pass.value;
    if (!persist) {
      return;
    }
    decryptAll();
  });
  $(e).prepend(pass);

  var persistcb = document.createElement('input');
  persistcb.type = 'checkbox';
  persistcb.addEventListener('change', () => {
    persist = persistcb.checked;
    if (persist) {
      decryptAll();
    } else {
      uncryptAll();
    }
  });
  $(e).prepend(persistcb);
});

function decrypt(x) {
  var r = $(x).data('received');
  if (r === undefined) {
    r = x.innerText;
    $(x).data('received', x.innerText);
  }
  x.innerText = r;
  if (password === '') {
    return;
  }
  try {
    aesGcmDecrypt(r, password).then((str) => x.innerText = str);
  } catch (e) {
    x.innerText = r;
  }
}

function decryptAll() { $('._58nk').each(function (i) { decrypt(this); }); }
function uncryptAll() {
  $('._58nk').each(function (i)  {
      this.innerText = $(this).data('received');
  });
}

customjsReady('._58nk', x => {
  if (persist) {
    decrypt(x);
  }
});

function hook() {
  var oldsend = require("MercuryMessageActions").prototype.send;
  require("MercuryMessageActions").prototype.send = function(i, j, k) {
    if (password === '') {
      return oldsend.call(this, i, j, k);
    }
    aesGcmEncrypt(i.body, password).then(str => {
      i.body = str;
      return oldsend.call(this, i, j, k);
    })
  };
}

function hooker() {
  try { hook(); } catch (e) { setTimeout(hooker, 10); }
}
hooker();

$(document).keydown(e => {
  if (e.keyCode === 18 && !persist) {
    decryptAll();
  }
});

$(document).keyup(e => {
  if (e.keyCode === 18 && !persist) {
    uncryptAll();
  }
});