This short piece doesn't have much detail. But if reCAPTCHA is usable to deanonymize Tor users then I would like to know about it in detail so I can do something about it.
I didn't see anything that makes it unique to recaptcha. Any fingerprint able traffic pattern that can be observed coming and going will work.
I could make a website that adds random(1, 64) one pixel images to each page. As you browse the site, you'll be broadcasting 6 bits of identifier with every click.
I believe the "unique to CloudFlare" element is that CloudFlare effectively sees traffic for significant portions of the web...but is one entity. So, a powerful enough hostile actor (say, a state) would only need to compromise one entity (CloudFlare) to exploit users of thousands of websites, including many major ones. Er, well, two entities, because they also need entrance data. So, if a state were to compromise an ISP and CloudFlare it would give that state a lot of Tor users identities.
Very few small-ish entities have such a large reach and can interject themselves into so many connections on the web.
CloudFlare is an endpoint for predictable actions; and serves overseas websites. So, even a state that doesn't necessarily have power to infiltrate a foreign host might still be able to identify users of those foreign hosts.
So, say I use Tor to make political comments on a foreign website; one that I have reasonable trust is outside the reach of my government. But, say that website uses CloudFlare and CloudFlare has servers that are within reach of my government. That's the difference. It is a difference of degree, rather than kind, but a difference nonetheless.
Powerful actors have always had some ability to compromise Tor by compromising the requesting side (the ISP of the target of an investigation, for example), and then the receiving side (the website where your suspect does the thing you're investigating them for...possibly a honey pot setup specifically to catch people who do this thing, or possibly a website whose owner has already been arrested, prosecuted and made a deal that allowed access to the systems). CloudFlare just adds an additional element of uncertainty for Tor users: Will this CAPTCHA take place in a way and place that allows someone to narrow down my identity?
As with a lot of the security concerns about Tor, one has to take it as weights on a scale. Who are my attackers, and what level of attack can they bring against my traffic? If your privacy concerns don't include state level actors, then this is probably a theoretical attack. If your attackers do include state level actors, then it is a concern to be aware of. State level actors have other means of compromising your identity and traffic, of course, but this is one of them, and if I understand it correctly, it is a valid concern.
The difference is that reCAPTCHA provides a detectable traffic pattern and is already widely deployed. This provides plausible deniability. Other than that, I don't see a difference.
The problem is that traffic is passed all the way through the network. A solution would be to have a transformative proxy on the inside of the tor network running as a hidden service that made requests on your behalf. Then it could possibly respond with .har files or some other transformed asset that doesn't match the same traffic signature. VPN into Tor, terminate at a high level proxy and then exit Tor through this intermediary.
Encryption does not inherently obscure the size of plaintext. Protocols may choose to pad plaintext for various reasons, and both Tor (since Tor always sends fixed-width cells) and TLS (when it uses a block cipher mode) do so. However, the amount of padding is typically small and can hardly be said to "obscure" the size of a request - it is not a defense against traffic analysis.
I believe what he's saying is that it'll pad to fit those cells. If you're sending 500b of data in 16b chunks, you'll need some padding (12b) in there to fit into 32x16 = 512b.
If the data is just a few bytes, tor would pad it with null and then encrypt it. The final encrypted cell would have no revealing information except for the time it was sent out at.
As I get it, it's irrelevant to JS. "Select all X" image-based captcha can be done without it. For example, a set of checkboxes with CSS background-image for its :checked (or :hover/:active/whatever) states would also do the trick (unless you patch/configure your browser to not optimize/delay the load of resources until they're actually required)
The claim is that an adversary who can measure traffic on CloudFlare's side (i.e. you) and the users ISP (i.e. your hypothetical friend mallory) can collude by measuring and comparing the bursts of packets generated during puzzle solving on the ISP side and the receipt of said packets on CF's side.
This information is enough to figure out that Alice wanted to reach example.com via TOR.
This works because reCAPTCHA has a detectable data signature. But you are in the position to inject any javascript you like anyhow, so it's not really reCAPTCHA specific in a technical sense, it's just that that would be a good coverstory if Eve were to try to make you and mallory cooperate to de-anonymize Alice.
You're right of course wrt to Google serving reCAPTCHA, in this case you'd just be providing plausible deniability if Google and Mallory-ISP were to collude to exploit this.
This is why I got quite scared when I first heard of Google Fiber.
It's in Google's interest to provide good, fast and cheap service: they will gain more customers and more people will be able to use more Internet services (many of which are from Google or use Google -- adwords, analytics, etc.). Thus they provide speeds for prices that are very hard to compete with for normal ISPs, since normal ISPs don't have the luxury of being the world's most popular, well, so many things (search engine, mapping service, email service, ad service, etc.).
If one company knows everything about you and controls a big enough stake in your life, that sounds very scary to me. Not because Google is bad, but because it's one company able to control many basic services.
That's all well and good except for, to my knowledge, the reCAPTCHA widget being served by Google and communicating solely with Google as 'jgrahamc points out. The amount of Cloudflare blame in this article does not mesh well with plain logic (I'm kinda disappointed with Cryptome, TBH). I believe the only thing handed to Cloudflare during and after the reCAPTCHA is solved is a token of some kind indicating Google's confidence the user is real. If I'm wrong about this I'd be surprised, because it would then be possible for server-side operators to tamper with Google's machine learning that they're doing with reCAPTCHA users.
I'm almost positive these claims are completely false, for example:
> Cloudflare can conveniently serve few more images to specific users
> Each click on one of the images in the puzzle generates a total of about 50 packets between Tor user's computer and the Cloudflare's server (about half are requests and half are real-time responses from the server.)
> The packet group has predictable sizes and patterns, so all the adversary has to do is note the easily detectable signature of the "image click" event, and correlate it with the same on the Cloudflare side.
There is no API documentation in the reCAPTCHA widget about your server having to handle real-time requests from users solving the widget or serve images, so there is no Cloudflare side. It wouldn't make sense from an API perspective; why would I have to add a bunch of code to my server to handle this stuff? Google runs that. Look here:
Do you see a "handle real time image click events" API here for Cloudflare to deploy? You do not. Google would have to build backends for their machine learning and fraud detection algorithms in every language an API user would ever run, and then they also lose obscurity by shipping them. The image click events almost certainly go only to Google, never Cloudflare, so I think whoever sent this tip didn't understand what they were looking at in Wireshark.
The possible threat vector here is Google, not Cloudflare. Cloudflare just happens to have deployed Google's reCAPTCHA widely. The article is misleading and incredibly light on important detail; how about even a screenshot of a packet capture showing traffic to Cloudflare? If you want my honest take, I read this as a Tor user annoyed they have to solve reCAPTCHAs on Cloudflare sites (the "insistence" and quoted "protects" bits are the clue) and looking for something to hit them with, and a lack of diligence on Cryptome's part before posting it.
That's because this is a speculation; e.g. "this obvious opportunity is not the proof" is admitted in the text itself. As mentioned, the requirement is the ability to correlate two different traffic signals: from your computer to Tor, and from Tor to the exit. So, the agency trying to trace you needs to be listening at both of those points. Their approach is simply to have one of those be the ISPs (presumably this ranges into the hundreds of thousands if not millions of computers) of US citizens (foreign ISPs would seem much harder to monitor), and the other being the CloudFlare servers (only maybe hundreds or thousands of machines needed to log this?).
Actually, watching the entrance and exit nodes in this fashion is probably more expensive than simply hosting your own entrance and exit nodes. It would be within the NSA's power to, say, host or monitor 500 of the 1000ish exit nodes by now, collecting 50% of the exit traffic at almost no real cost. Entrance traffic is harder as the network is larger, but if you hosted (or, again, captured the traffic to) another 2k non-exit relays you might be able to capture 10-20% of the entrance traffic. The basic points I'm making here are: (1) that there are way fewer relay nodes to monitor than there are ISPs, if you would prefer surveillance; and (2) you are not restricted to surveillance or even to your own nation--there's literally nothing stopping the NSA from purchasing VPSes in the Netherlands and Germany and Sweden and running Tor on them, and it'll seem like a very geographically diverse set when you're looking at it with Vidalia.
Combined together the NSA can maybe deanonymize about 5-10% of the Tor traffic to the Internet right now with a much cheaper method, and this is where it gets interesting: the Tor default is to have 3 hops, which means that in addition to correlating traffic patterns you get to correlate on the IP address of the hop in the middle, even if that hop is not colluding with you. So even in the face of network jitter you have a 32-bit identifier which links together packets above and beyond simple network traffic into or out of Tor. And you only need to operate a few thousand computers to do it -- far fewer than you'd need to monitor the US ISPs in general.
You can also try to watch specific popular exits like Cloudflare, but doing this removes this awesome IP address that you get for the middle hop, and you still need either a relay node or else to be tapping a given user's IP, to try to deanonymize them.
If deanonymization would be based on size and patterns of TCP connections made to cloudfare server you should use BSD configuration flag to make them more random with net.inet.ip.random_id=1. But as long as we can't measure this... it's no proof and no defense.
