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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 somethin

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.

tedunangst 10y ago
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.
- jerf 10y ago
  I don't see anything that makes this unique to CloudFlare, either. (You imply this in your point, but given the specificity of the accusation, I think it's worth clearly pointing out.)
  - SwellJoe 10y ago
    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.
    - jerf 10y ago
      But if we're talking about The Adversary, then they're already deeper in than CloudFlare will ever be, so... what's different?
      - yoo1I 10y ago
        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.
        deleted 10y ago
        [deleted]
      - SwellJoe 10y ago
        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.
- onecooldev24 10y ago
  Data between you and tor nodes are encrypted, no way your idea will work.
  - tedunangst 10y ago
    Encryption doesn't obscure the size or frequency of requests.
    - onecooldev24 10y ago
      It does obscure the size, maybe not the frequency. Best have JS disabled when you come across this.
      - garrettr_ 10y ago
        > It does obscure the size 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.
        onecooldev24 10y ago
        > since Tor always sends fixed-width cells you send 1px of data or 500px tor always send fixed width cells. There is no question of padding here.
        mynameisvlad 10y ago
        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.
        onecooldev24 10y ago
- sitkack 10y ago
  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.
akerro 10y ago
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.
- tedunangst 10y ago
  Random IP IDs won't do jack to prevent correlation. Probably even make it easier if actual fragments are involved.
drostie 10y ago
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.
yoo1I 10y ago
It has enough detail: 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.
- jgrahamc 10y ago
  But it's a Google-served reCAPTCHA so there's nothing to measure on CloudFlare. So I disagree that there's detail here. Need real technical detail to be able to take action. If this were a paper or PoC then would be different. If there's a way to do that then please report it to us.
  - yoo1I 10y ago
    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. Tor users on google fiber take note.
    - lucb1e 10y ago
      > users on google fiber take note. 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.

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.

tedunangst 10y ago

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.

jerf 10y ago
I don't see anything that makes this unique to CloudFlare, either. (You imply this in your point, but given the specificity of the accusation, I think it's worth clearly pointing out.)
- SwellJoe 10y ago
  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.
  - jerf 10y ago
    But if we're talking about The Adversary, then they're already deeper in than CloudFlare will ever be, so... what's different?
    - yoo1I 10y ago
      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.
      - deleted 10y ago
        [deleted]
    - SwellJoe 10y ago
      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.
onecooldev24 10y ago
Data between you and tor nodes are encrypted, no way your idea will work.
- tedunangst 10y ago
  Encryption doesn't obscure the size or frequency of requests.
  - onecooldev24 10y ago
    It does obscure the size, maybe not the frequency. Best have JS disabled when you come across this.
    - garrettr_ 10y ago
      > It does obscure the size 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.
      - onecooldev24 10y ago
        > since Tor always sends fixed-width cells you send 1px of data or 500px tor always send fixed width cells. There is no question of padding here.
        mynameisvlad 10y ago
        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.
        onecooldev24 10y ago
sitkack 10y ago
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.

akerro 10y ago

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.

tedunangst 10y ago
Random IP IDs won't do jack to prevent correlation. Probably even make it easier if actual fragments are involved.

drostie 10y ago

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.

yoo1I 10y ago

It has enough detail: 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.

jgrahamc 10y ago
But it's a Google-served reCAPTCHA so there's nothing to measure on CloudFlare. So I disagree that there's detail here. Need real technical detail to be able to take action. If this were a paper or PoC then would be different. If there's a way to do that then please report it to us.
- yoo1I 10y ago
  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. Tor users on google fiber take note.
  - lucb1e 10y ago
    > users on google fiber take note. 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.