I know these sorts of questions come up, but I couldn't find a simple answer, most of the answers are "don't do that"

For play, I built myself a webapp, it uses sjcl to encrypt and decrypt notes on the client, whilst sending the encrypted form back to the server.

I am storing the key in local storage.

From my research, I am at risk from:

  1. Cross site scripting attacks (my application uses AJAX to send and receive the data from the server, I'm not sure if that is an attack vector).
  2. Local machine being hacked. At the moment the app can only be used on machines I have full control over, the moment I log in from a machine I do not control and type my key, that machine essentially has my key.
  3. Any type of flaw in local storage that lets a site from a different domain read the storage of other websites stored in the browser.

I am sure I have missed countless other attack vectors.

At first glance, I feel like it's impossible to design a client side encrypted note taking application that never sends the key to the server. With programming, there is almost always a way.

Is there a way I can make the app actually usable?

  • 3
    Can you trust that a proxy isn't changing the javascript code as it goes through the wire? Related on Security.SE Is client side encryption really better than server side?. The question at the heart of every security problem is 'how secure do you want it to be?' realizing that the more secure, the more complex, and the more expensive.
    – user40980
    Commented Nov 8, 2014 at 3:17
  • @MichaelT, Would using SSL migrate that particular attack?
    – Joseph
    Commented Nov 8, 2014 at 3:25
  • 1
    No. You've still got the man in the middle available there if any of the preconditions of SSL are not valid.
    – user40980
    Commented Nov 8, 2014 at 3:28
  • You can sort of avoid the MITM by sending a bootstrap page with far future expire headers which contains only the decryption logic and code to fetch an encrypted javascript bootstrap. Then as long as the initial download was secure, successive ones are as well. If you use HTML5 appcache for that initial bootstrap page, you can even guarantee that it doesn't get evicted from the cache (you can paint yourself into a nasty corner with far-future expiration and HTML5 appcache, but here it may be a benefit). Commented Nov 10, 2014 at 13:17

3 Answers 3


Basically, if the key is safe, the data is safe too (provided that its a good encryption algorithm and the key long enough, which I take as granted).

If I understood right, you only send encrypted data over the wire, not the key itself. If it's not the case, please discard this whole answer.

Assuming the key is never sent over the wire, the only way to compromise the data is to discover the key. This could happen if:

  • How the key is generated is predictible
  • Somebody having access to the file
  • A virus on the PC
  • DNS spoofing attacks (claiming to be your host/domain) -> Use TLS and certificates (see http://dev.w3.org/html5/webstorage/#privacy)
  • Exploiting a bug in some exotic browser/version (dunno if this already happenned, and if there are some vulnerability alerts or such)
  • ...is there anything else?

...perfect security is hard to achieve. But in your case, it seems an attacker must go to great length in order to hack it, or the user have a compromised machine from the start. Usually, the bottleneck is the user itself. ;)


In basic terms you assume, that if you encrypt data on client side (creating a token) and send token to server over HTTP, it will be secure - this is not true.

Hacker can get the token via network monitoring software (he does not need password) - that's it. Estimated time to bypass this is like less then 15 min.

  • The token is the encrypted data, so getting that doesn't matter right? The important thing to keep away from the hacker is the key, and that doesn't leave the client as it lives in local storage. But is that secure?
    – Joseph
    Commented Nov 10, 2014 at 14:12
  • So if your application was internet bank your task would be to keep pin codes unknown? Because client information and money are not safe! I am told you that hacker does not need to know your pin in order to scrape CI and money.
    – Margus
    Commented Nov 10, 2014 at 14:47

First, from what I understand, you don't show to user any data from other users, so you don't really have to worry about XSS. If you would show it, then you would have to make some countermeasures, such as character escaping (that actually should be used anyway). Hacked machine and browser security flaws are things you rather can't predict.

Second, the encryption thing you created doesn't really work. Sorry. Unless you are a genious with years of experience in security, but then you wouldn't need to ask this question. Among things that should be considered when designing encryption are:

  • Man in the Middle - can someone catch user's data by decrypting data from server on own machine and reencrypt them when sending to client? Attacker may simply ask real server for token, and when pretending to client that he is the server, he has the luxury to generate own set of keys/tokens, which will look as legit as original.
  • Network monitoring - are you sure you are not susceptible for instance to replay attack? Assume you order some items in some shop. If attacker records it and replays to the server, the same, encrypted data, will server detect it, or commit transaction again?
  • Key generation - are the keys you use completely random, or just predictably random? If attacker can generate the same set of keys your server stores, he doesn't even need to directly attack you, he can just sit back and other implications are pretty obvious. And believe me, standard random with time seed is very predictable.
  • Lots and lots more...

There are libraries like SSL that make this for you, made by people with years of experience and, as you probably realize, even they happen to mistakes, but nobody seems to be particularly eager to implement any alternative for some reason.

In case of security, the best option is to use checked and popular solutions, as they are probably also the most safe ones. As there is quite a few open-source and free libraries available, that should not be a problem. Ready encryption supported by browser will probably take care of more problems than you will ever probably imagine there is.

Unless of course I misunderstood you and you were not playing with webapps for webapp part of it, but for security thing. Then keep trying, and remember you will also learn a lot by attacking the app you created.

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