safer-crypto

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A non observable Web Crypto API.

Background

The Web Crypto API is configurable by default, and even if frozen in user-land, it's still vulnerable to Promise.prototype.then pollution.

While it is true that having unknown 3rd parts scripts interfering with your code is the root of unreliable JS, it is too damn simple to hook into the crypto API and sniff user passwords, server salts, encrypted data in clear, and so on and so fort.

Attacks Examples

// A crypto.subtle based attack
(() => {
  const {encrypt, decrypt, importKey} = crypto.subtle;
  Object.defineProperties(crypto.subtle, {
    importKey: {
      value(_, password) {
        console.log('user password', password);
        return importKey.apply(this, arguments);
      }
    },
    encrypt: {
      value(algo, key, data) {
        console.log('encrypting', new TextDecoder().decode(data));
        return encrypt.apply(this, arguments);
      }
    },
    decrypt: {
      value() {
        return decrypt.apply(this, arguments).then(data => {
          console.log('decrypted', new TextDecoder().decode(data));
          return data;
        });
      }
    }
  });
})();

// A promise based attack
(() => {
  const {then} = Promise.prototype;
  Object.defineProperties(Promise.prototype, {
    then: {
      value(fn) {
        return typeof fn === 'function' ?
          then.call(this, result => {
            console.log('busted', result);
            return fn(result);
          }) :
          then.call(this, fn, result => {
            console.log('busted', result);
            return result;
          });
      }
    }
  });
})();

The Solution

Based on safer-promise, hence on safer-function, this module should be included as top-most dependency, to ensure no third parts or polyfill could've interfered with the native version of its dependencies: the Function.prototype and the Promise.

Bear in mind this module does not change in any way globals variables, classes, or prototypes, it simply secures a custom crypto and its subtle reference in a not observable way.

import crypto from 'safer-crypto';
import {encode, decode} from 'safer-text';

const {log} = console;

crypto.subtle
  .digest(
    'SHA-256',
    encode('safer encryption')
  )
  .then(sha256Buffer => {
    log(sha256Buffer); // ArrayBuffer
  });