5 ms·

Tor TLS traffic can be distinguished by random domain in SNI during handshake (https://news.ycombinator.com/item?id=5505056 https://news.ycombinator.com/item?id

by monort 12y ago

Tor TLS traffic can be distinguished by random domain in SNI during handshake (https://news.ycombinator.com/item?id=5505056 https://news.ycombinator.com/item?id=5505056). If FBI can sniff all traffic originating from hosting provider, then it seems to be trivial to detect all Tor servers and go through them manually?

userbinator 12y ago
SNI is an extension, it's not required and makes for the traffic to be easily distinguishable, so what's the reason for Tor to send an SNI?
- monort 12y ago
  Probably it will be even easier to detect Tor nodes - just look for IPs which never receive SNI in TLS exchange on 443 port.
  - userbinator 12y ago
    Thinking over the protocol again I think TLS wasn't a great choice for Tor - not only because of SNI but other things like server certificates are also sent in cleartext during the handshake so it's easy to distinguish and just block all TLS traffic that uses self-signed or otherwise certificates not issued by the big CAs. Something more like SSH, not relying on central CAs, would be less distinguishable - is it Tor traffic, or is it just someone accessing his/her server remotelly.
    - conductor 12y ago
      I would suggest impersonating other encrypted protocols with heavy traffic like encrypted BitTorrent or P2P Skype (is it still P2P or MS centralized it already?). On the other hand, systems administrators are frequently blocking BitTorrent and Skype traffic so the "TLS" traffic on port 443 has more chances to pass.
- cesarb 12y ago
  SNI is optional for clients only in theory. In practice, since there is no way of knowing whether a server needs the SNI hostname to select the correct server certificate, current browsers always send the SNI extension. Therefore, since Tor tries to mimick common browsers and servers, it must always send a SNI extension.
  - hokeypokey 12y ago
    >Therefore, since Tor tries to mimick common browsers and servers, it must always send a SNI extension. Right, and since all browsers have no problem with plaintext http, TOR should be okay with it too. You know, to mimic traffic and stuff. That's the most BS excuse I've ever heard. Sending the domain in the clear on TOR is completely inexcusable. But then, so is having JS enabled by default. Run, do not walk, RUN away from TOR.
    - tedks 12y ago
      Actually, Tor has pluggable transports that mimic HTTP as well, though obviously the traffic is encrypted and then stenographed into HTTPish shape. Take your FUD elsewhere.
    - cesarb 12y ago
      The data Tor exchanges are fixed-size encrypted packets. If Tor used plaintext http, these encrypted packets would be visible, making it trivially distinguishable from a common browser or server. The TLS layer hides that difference; due to the TLS encryption, there's no difference between an HTML document, encrypted Tor packets, or a picture of a pony. Therefore, to mimic browser traffic, Tor has to use TLS. I believe you might be a bit confused with respect to what's in the SNI extension; the domain sent in the clear on Tor connections is not the domain requested by the browser, it is a completely unrelated randomly-generated domain name. Also, even without SNI the same can be found on the opposite direction: the certificate sent in the response also has a randomly-generated domain name.
      - slopoque 12y ago
        My understanding of an SSL handshake goes a bit like this... Everything after the https:// https:// is encrypted. Client does a DNS lookup on the host, but the host name in the request is encrypted. The path is encrypted. The get parameters are encrypted. Everything is encrypted to the server in the request. Everything coming back from the server is encrypted as well. Team SNI comes along and says, "But we don't really care about security! We want to host multiple domains on the same IP address! How will the server know where to direct the SSL request without the domains?" And so, in the name of reusing an IP address, security is now compromised for the domain. It is sent outside the SSL request, in the clear. Is that about right? SNI on TOR? Nice one.

by monort 12y ago

userbinator 12y ago

SNI is an extension, it's not required and makes for the traffic to be easily distinguishable, so what's the reason for Tor to send an SNI?

monort 12y ago
Probably it will be even easier to detect Tor nodes - just look for IPs which never receive SNI in TLS exchange on 443 port.
- userbinator 12y ago
  Thinking over the protocol again I think TLS wasn't a great choice for Tor - not only because of SNI but other things like server certificates are also sent in cleartext during the handshake so it's easy to distinguish and just block all TLS traffic that uses self-signed or otherwise certificates not issued by the big CAs. Something more like SSH, not relying on central CAs, would be less distinguishable - is it Tor traffic, or is it just someone accessing his/her server remotelly.
  - conductor 12y ago
    I would suggest impersonating other encrypted protocols with heavy traffic like encrypted BitTorrent or P2P Skype (is it still P2P or MS centralized it already?). On the other hand, systems administrators are frequently blocking BitTorrent and Skype traffic so the "TLS" traffic on port 443 has more chances to pass.
cesarb 12y ago
SNI is optional for clients only in theory. In practice, since there is no way of knowing whether a server needs the SNI hostname to select the correct server certificate, current browsers always send the SNI extension. Therefore, since Tor tries to mimick common browsers and servers, it must always send a SNI extension.
- hokeypokey 12y ago
  >Therefore, since Tor tries to mimick common browsers and servers, it must always send a SNI extension. Right, and since all browsers have no problem with plaintext http, TOR should be okay with it too. You know, to mimic traffic and stuff. That's the most BS excuse I've ever heard. Sending the domain in the clear on TOR is completely inexcusable. But then, so is having JS enabled by default. Run, do not walk, RUN away from TOR.
  - tedks 12y ago
    Actually, Tor has pluggable transports that mimic HTTP as well, though obviously the traffic is encrypted and then stenographed into HTTPish shape. Take your FUD elsewhere.
  - cesarb 12y ago
    The data Tor exchanges are fixed-size encrypted packets. If Tor used plaintext http, these encrypted packets would be visible, making it trivially distinguishable from a common browser or server. The TLS layer hides that difference; due to the TLS encryption, there's no difference between an HTML document, encrypted Tor packets, or a picture of a pony. Therefore, to mimic browser traffic, Tor has to use TLS. I believe you might be a bit confused with respect to what's in the SNI extension; the domain sent in the clear on Tor connections is not the domain requested by the browser, it is a completely unrelated randomly-generated domain name. Also, even without SNI the same can be found on the opposite direction: the certificate sent in the response also has a randomly-generated domain name.
    - slopoque 12y ago
      My understanding of an SSL handshake goes a bit like this... Everything after the https:// https:// is encrypted. Client does a DNS lookup on the host, but the host name in the request is encrypted. The path is encrypted. The get parameters are encrypted. Everything is encrypted to the server in the request. Everything coming back from the server is encrypted as well. Team SNI comes along and says, "But we don't really care about security! We want to host multiple domains on the same IP address! How will the server know where to direct the SSL request without the domains?" And so, in the name of reusing an IP address, security is now compromised for the domain. It is sent outside the SSL request, in the clear. Is that about right? SNI on TOR? Nice one.