Re: [cryptography] Preventing Time Correlation Attacks on Leaks: Help! :-)
----- Forwarded message from Ben Laurie
Hey Peter,
thanks for your analysis!
I think we need to provide some additional input!
In the context of GlobaLeaks where, stating from our Threat Model at https://docs.google.com/document/d/1niYFyEar1FUmStC03OidYAIfVJf18ErUFwSWCmWB..., the Whistleblower can also be NON anonymous but approach a submission with "Confidential" level (over HTTPS over the internet) .
No anonymity, but forced disclaimer ( https://github.com/globaleaks/GlobaLeaks/issues/260) and acceptance to take the risk.
So, let's say that whistleblower is already in a bad position, but he accepted this condition.
We are not considering in any way to add actions/protection on Whistleblower-Side but only on Receiver-Side that is where the "bad guy" would be able to read Notification information sent and apply Time Correlation on the Whistleblower-Action.
Today if a Whistleblower make a submission, the system immediatelly send a notification to the Receiver.
That's bad, because it leave a trace that allow time correlation.
Who can read Receiver's email and traffic, can make a correlation on other data source where the whistleblower may leave traffic-traces (like a proxy, but also internet traffic dump, phisical badge/access logs, surveillance camera, etc) .
Which kind of logic / algorithm to apply on the Receiver's notification timing in order to prevent / reduce the likelihood that a time correlation pattern is possible?
A random delay between a lower bounday and an upper boundary seems like the most simple and effective approach to defeat this kind of correlation.
However this does not work on very low-traffic globaleaks node.
What do you think?
I think that if you want to send messages that are hard to trace, there's an existing technology: mixmaster, with an existing server network. Or, better yet, finish off mixminion, Even better: implement Minx (the fixed version).
-- Fabio Pietrosanti (naif) HERMES - Center for Transparency and Digital Human Rightshttp://logioshermes.org - http://globaleaks.org - http://tor2web.org
Il 8/21/13 4:17 AM, Peter Maxwell ha scritto:
Hi Fabio,
While I don't mean to be dismissive, I suspect your threat model is flawed for the following reasons:
i. Most mid to large companies would not permit the use of Tor within their infrastructure and even if the hypothetical company did, it doesn't take a whole lot of effort to track down the handful of users within a company using Tor/stunnel/ssh/VPN. For that matter, I understand some companies even install private CA certificates into the browsers on company computers and decrypt outgoing SSL/TLS traffic at their web-proxy/firewall... in that situation, you're WB is going to stand out like a sore thumb as they'll be the only TLS connection that isn't being decrypted (because it's Tor). So unless you want your whistle-blowers to literally advertise their presence as worthy of attention, they aren't going to do the leak from an company system directly.
ii. So, presuming i. is valid - and I suspect anyone who has worked within a competent security operations team will tell you the same - then you must assume the whistle-blower will do the leak from either their personal systems, a burn computer or a public system. If we make the assumption that the WB has taken the data out of the company/organisation on removable media or otherwise made it available to themselves outside the company infrastructure in a secure manner (while sometimes difficult, that is still far easier than i.) then your attacker can only see the WB's traffic if they are actively monitoring the WB's use of computers outside the company, in which case said WB has far bigger problems to worry about. If the attacker cannot monitor the timing of the leak, your problem is not framed in the manner you've presented.
iii. Even if your model was realistic, you cannot adequately defend against traffic analysis for such a low-traffic network: you need other traffic to hide in, lots of it, from other users within the same company - it's not realistic for this type of service.
iv. There are more subtle problems you are going to come across, not least of which are issues such as document tagging/water-marking/document versioning and the ability for the attacker - your hypothetical manager - to correlate leaked documents against the access rights and access patterns of potential whistle-blowers. For that matter, simple forensic analysis of staff computers is usually more than sufficient (and yes, organisations do this).
It's also Isle of Man that people like hiding their ill-gotten-gains in, not "Island of Mann" ;-) Interestingly, I think anyone who has used Isle of Man accounts for tax avoidance are scuppered as the HMRC has signed an agreement with the authorities there for automatic disclosure.
Anyway, as far as I can see it, you have two different scenarios to consider with one being significantly more difficult to solve than the other:
A. The scenario where the whistle-blower is able to take the data out the company on removable media or paper-copy. This is the easy one to solve. Personally I would favour a combination of asymmetric encryption with single-use keypairs and USB sticks in the post, but I'm old fashioned that way.
B. The scenario where the whistle-blower has to leak from the company/organisation's network. This is extremely difficult indeed. If I were approaching this problem myself, my first considerations would be: how to make the traffic look like normal web-traffic; how to ensure no forensic traces are left; and how to do that without installation of third-party software as that is a dead give-away. If the quantity of data is larger than a few hundred Mb, the problem is probably not solvable.
That's my tuppence-worth, hope that helps,
Peter
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