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That's really interesting, if i'm understanding this correctly it's basically another form of compression leaking information about the underlying data. Would t

That's really interesting, if i'm understanding this correctly it's basically another form of compression leaking information about the underlying data. Would they need to pad everything to a deterministic size or would they just need to change some early parameters during the encoding process to throw off the rest of the chain? But in the end it still doesn't mean TLS isn't worth it for all the other benefits (not that you implied that).

saurik 6y ago
The way static "streaming" video works is that you divide the video into a number of small segments--maybe every 10 seconds or every 2 seconds or something--and then encode each of those segments at various quality levels and store all of those files on HTTP servers. There is then a "manifest" file that lets the client learn what qualities are available and what their URLs will be for various timestamps. The client then just starts downloading chunks. If it is finding itself with free time, it starts downloading higher quality (larger) chunks in the future. If it isn't downloading them fast enough, it downloads lower quality (smaller) chunks in the future. The issue then is that no matter what you do with the compression algorithms, the user is downloading these chunks and you can see the pattern of packets in one direction setting up requests and the packets in the other direction replying with the chunks and you can fingerprint what movie someone is watching. It isn't some kind of encoding issue, as the data is all encrypted: it is the entire concept of taking fixed length segments of a movie that will compress to some non-determinstic size. If you take the first two minutes of every Star Wars movie, divide each up into 10 second segments, and then compress those segments, the sequence of sizes of the segments will be pretty unique. What is so great about this particular paper is that you don't even really need to analyze the TLS layer and try to pay close attention to really figure out the request/responses: they just fingerprint the TCP flow and that's sufficient, which in retrospect doesn't surprise me as what you are really looking for is some kind of rate of requests to responses for the chunks over the course of those first few minutes of watching the video, and don't really need to know for sure where the boundaries are: you have a long enough sequence and a small enough catalog (there aren't tens of millions or billions of videos on Netflix) to get a really strong fingerprint using just the relative rates. To fix this you really need to either inject extra random traffic (such as extra packets to the server that break up the request rates) that adds so much noise that you can't figure out the signal "in time"--if it takes longer to fingerprint a movie than the length of a typical movie, that's "good enough"--or you need to destroy signal (which is a better description of what we do if the video segments are all the same size: at that point all movies are by definition the same sequence over and over again; if you then pad the length of every movie to the same 4 hour runtime and force the user to download all of the padded black video frames, you essentially 100% solve the problem ;P).
- SketchySeaBeast 6y ago
  Doesn't this inspection of the manifest have a fairly limited scope in how it can be exploited though? I know the attacker will be able to see what someone is watching, but only for non-novel information, right? You can't use that technique to guess anything new, and had to have yourself already seen the video?
  - saurik 6y ago
    What I am talking about hers is definitely focussed only on "fingerprint known content", as that is the goal of the "figure out which Netflix movie I am watching" use case (as well as the "what model am I watching on myFreeCams/Chaturbate" use case, which might feel worse ;P... it is also the "what video am I watching on PornHub" use case, but that's inherently harder as there are so many many videos and they are often super short: much harder than the moderate catalog of long movies and tv shows on Netflix or the comparatively scant number of available live feed on camsites). That said, as noted by someone else on this thread, there has been some work done figuring out what people are saying by analyzing encrypted speech packets; but I imagine that kind of technique would be almost impossible to pull off with these segments on the order of multiple seconds long, and including the video in the stream would seem to make that a total non-starter.
  - tialaramex 6y ago
    Well one of the things that paper does is they just go fetch the index. Every Netflix client that is watching say 1080p Rick & Morty s1e2 fetches an index of the bits that make up that video, and then fetches the relevant chunks of data. So if you just fetch that index, no need to "watch" the video, you know OK 1390450 bytes is the 1st chunk of data making up that video, and then 2046917 bytes is the 2nd chunk. When you see that somebody fetched 1390450 bytes of encrypted data from Netflix, they're probably watching s1e2 of Rick & Morty in 1080p. When they fetch 2046917 bytes of encrypted data that confirms it. So it's "non-novel" but they don't need to (relatively expensively) actually watch the videos they want to match. It's likely any sophisticated adversary has an index of all the common videos from sites like Netflix, the Play Store, Disney+ and so on. There is definitely more a client could do to make this harder, but there'd be a sharp trade-off where to make it impossible for passive eavesdroppers to know what you saw costs both you and the "broadcaster" a lot of bandwidth, and bandwidth ain't free. I'd like to see Netflix at least gently stroll in that direction, maybe use some TLS padding to make it a bit harder to do this trick, but I don't expect them to set off at a sprint and ruin their profitability in pursuit of a non-goal.

saurik 6y ago

The way static "streaming" video works is that you divide the video into a number of small segments--maybe every 10 seconds or every 2 seconds or something--and then encode each of those segments at various quality levels and store all of those files on HTTP servers. There is then a "manifest" file that lets the client learn what qualities are available and what their URLs will be for various timestamps. The client then just starts downloading chunks. If it is finding itself with free time, it starts downloading higher quality (larger) chunks in the future. If it isn't downloading them fast enough, it downloads lower quality (smaller) chunks in the future. The issue then is that no matter what you do with the compression algorithms, the user is downloading these chunks and you can see the pattern of packets in one direction setting up requests and the packets in the other direction replying with the chunks and you can fingerprint what movie someone is watching. It isn't some kind of encoding issue, as the data is all encrypted: it is the entire concept of taking fixed length segments of a movie that will compress to some non-determinstic size. If you take the first two minutes of every Star Wars movie, divide each up into 10 second segments, and then compress those segments, the sequence of sizes of the segments will be pretty unique. What is so great about this particular paper is that you don't even really need to analyze the TLS layer and try to pay close attention to really figure out the request/responses: they just fingerprint the TCP flow and that's sufficient, which in retrospect doesn't surprise me as what you are really looking for is some kind of rate of requests to responses for the chunks over the course of those first few minutes of watching the video, and don't really need to know for sure where the boundaries are: you have a long enough sequence and a small enough catalog (there aren't tens of millions or billions of videos on Netflix) to get a really strong fingerprint using just the relative rates. To fix this you really need to either inject extra random traffic (such as extra packets to the server that break up the request rates) that adds so much noise that you can't figure out the signal "in time"--if it takes longer to fingerprint a movie than the length of a typical movie, that's "good enough"--or you need to destroy signal (which is a better description of what we do if the video segments are all the same size: at that point all movies are by definition the same sequence over and over again; if you then pad the length of every movie to the same 4 hour runtime and force the user to download all of the padded black video frames, you essentially 100% solve the problem ;P).

SketchySeaBeast 6y ago
Doesn't this inspection of the manifest have a fairly limited scope in how it can be exploited though? I know the attacker will be able to see what someone is watching, but only for non-novel information, right? You can't use that technique to guess anything new, and had to have yourself already seen the video?
- saurik 6y ago
  What I am talking about hers is definitely focussed only on "fingerprint known content", as that is the goal of the "figure out which Netflix movie I am watching" use case (as well as the "what model am I watching on myFreeCams/Chaturbate" use case, which might feel worse ;P... it is also the "what video am I watching on PornHub" use case, but that's inherently harder as there are so many many videos and they are often super short: much harder than the moderate catalog of long movies and tv shows on Netflix or the comparatively scant number of available live feed on camsites). That said, as noted by someone else on this thread, there has been some work done figuring out what people are saying by analyzing encrypted speech packets; but I imagine that kind of technique would be almost impossible to pull off with these segments on the order of multiple seconds long, and including the video in the stream would seem to make that a total non-starter.
- tialaramex 6y ago
  Well one of the things that paper does is they just go fetch the index. Every Netflix client that is watching say 1080p Rick & Morty s1e2 fetches an index of the bits that make up that video, and then fetches the relevant chunks of data. So if you just fetch that index, no need to "watch" the video, you know OK 1390450 bytes is the 1st chunk of data making up that video, and then 2046917 bytes is the 2nd chunk. When you see that somebody fetched 1390450 bytes of encrypted data from Netflix, they're probably watching s1e2 of Rick & Morty in 1080p. When they fetch 2046917 bytes of encrypted data that confirms it. So it's "non-novel" but they don't need to (relatively expensively) actually watch the videos they want to match. It's likely any sophisticated adversary has an index of all the common videos from sites like Netflix, the Play Store, Disney+ and so on. There is definitely more a client could do to make this harder, but there'd be a sharp trade-off where to make it impossible for passive eavesdroppers to know what you saw costs both you and the "broadcaster" a lot of bandwidth, and bandwidth ain't free. I'd like to see Netflix at least gently stroll in that direction, maybe use some TLS padding to make it a bit harder to do this trick, but I don't expect them to set off at a sprint and ruin their profitability in pursuit of a non-goal.