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Document security considerations around OMEMO and webcrypto

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JC Brand 2018-10-19 12:26:03 +02:00
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docs/source/features.rst
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@ -39,6 +39,49 @@ an untrusted device. You would in any case not be able to decrypt previously
received OMEMO messages, due to the Signal Protocol's forward secrecy and the
fact that you don't have a pre-existing session.
Security considerations for browser-based crypto
------------------------------------------------
Crypto apps deployed via regular web hosting can be described as relying on
"host-based" security.
Host-based security services require you to trust the host every time you access
it, whereas with installable desktop software you trust the host when you
download/install the software (and whenever it gets updated).
The dynamic nature of "host-based" systems makes it impractical for security
researchers to do security audits because the hosted code can change at any
time.
In such a setup you need to fully trust the host that serves you the JavaScript code.
The host that serves the JavaScript code is not necessarily the same host that
stores and procesess your chat messages. So using OMEMO can still protect your
messages from snooping on the XMPP server where they're stored encrypted.
In other words, you do have to trust the webserver that hosts Converse for you,
but you don't necessarily have to trust the XMPP server (if it's on a different host),
because it never gets hold of your private key.
One way to improve this situation is to host Converse yourself, especially if
you host it locally on your own machine. If you're not able to do that, then
at least make sure you use a reputable host that serves files over HTTPS and
that set `CSP <https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy>`_
headers.
Due to these reasons, it's NOT a good idea to use encrypted messaging with a
browser-based solution in life-threatening situations.
Security can be increased by using an installable app (like one based on `Electron <https://electronjs.org/>`_)
with a strict Content Security Policy.
Look out for an Electron based version of Converse coming in the following months.
For further reading on the challenges of web-based crypto, take a look at these
articles:
* `What's wrong with webcrypto? <https://tonyarcieri.com/whats-wrong-with-webcrypto>`_
* `Heartbleed and JavaScript crypto <https://tankredhase.com/2014/04/13/heartbleed-and-javascript-crypto/>`_
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