[ProgClub list] Javascript Cryptography Considered Harmful

John Elliot jj5 at progclub.org
Mon Aug 29 09:36:28 AEST 2011

I bumped into this article today, Javascript Cryptography Considered 


It seems particularly pertinent, given that I've just released an 
encryption library, half of which is a Javascript crypto library:


I thought about a few of the issues raised by the author while I was 
developing my library, and given that I've thought about it, I have a 
few things to add regarding his comments.

Firstly, you can't really always rely on SSL/TLS. AIUI many Big 
Organisations setup their desktop SOE image with a trusted root 
certificate for their own CA. They then proxy all HTTPS activity though 
a proxy that issues its own certs (which are trusted) and the user is 
none the wiser that their bank balance is being reviewed by some creep 
in the Orwellian network security department. Using client-side 
encryption in addition to SSL/TLS frustrates such review by forcing a 
would-be peeping-tom to jump though a few extra hoops before he can get 
at your bank balance. It's not perfect, but it certainly raises the bar.

Secondly, using client-side encryption raises the bar for passive 
(rather than active) network interference, such as, for instance, a 
net-ops officer using tcpdump or wireshark to capture network traffic. 
By using the encryption library you can avoid unauthorised viewing by 
the sort of casual inspection that is common on networks (directly 
tampering with the scripting components, or reversing the crypto would 
be *less likely*).

Thirdly, you can protect users from doing silly things like saving the 
page as HTML with the sensitive content embedded in it. This is a bit of 
a double-edged sword, because sometimes you will want the user to be 
able to save a copy of their information in a HTML page on their 
computer, but in some scenarios this is not desirable. The way I use my 
javascript crypto library is like this:

1. In the HTML content for the page, I render the encrypted content in a 
<span> element with an 'encrypted' class, e.g.:

  <span class="encrypted">27632472673..encrypted..2389429837278</span>

2. On the page load event, I run over all encrypted content and decrypt 
it with the user's secret which is kept in a "secure" session cookie.

3. If the user saves the page, their content is encrypted in the HTML on 
disk. Optionally you can prompt the user for their secret (for 
decryption) when they reload the HTML page that they have saved (as the 
secret will not be in their session cookie any more).

Generally I think that using the Javascript crypto library is worth it 
for the added security you get for a few common use-case scenarios. It's 
not perfect, but it definitely raises the bar. As a "system" the code 
acts more like an "obfuscator" than an "encryptor", although it does 
employ (somewhat flawed) crypto processes.

I'd be happy to hear others' thoughts. What do you think? Am I wasting 
my time by encrypting user data with Javascript? Or do the benefits 
outweigh the minor inconvenience of having to implement it in the first 


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