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That's because you are glossing over a lot of packet inspection deeper than the TCP level with a glib "Read the HTTP request".

You might want to think about existing surveillance systems. Analysis of telephone traffic is often done purely on the CDR (the caller, callee, and length of call, in simple terms) without the equivalent of deep packet inspection to read the HTTP request, which would be analysis of the actual audio data themselves. The HTTPS case would likewise need just the total octets transferred over the TCP connection for fingerprinting.

There's a lot of glib handwaving in this discussion about identical sizes, not based upon actual measurements of the Debian archive. I quickly looked at the package cache in one of my Debian machines:

    jdebp% ls -l|awk 'x[$5]++'
    -rw-r--r-- 1 root root      3314 Feb 16  2018 nosh-run-freedesktop-system-bus_1.37_amd64.deb
    -rw-r--r-- 1 root root     35190 Dec 14  2016 redo_1.3_amd64.deb
    -rw-r--r-- 1 root root   1114546 Feb 25  2018 udev_232-25+deb9u2_amd64.deb
    jdebp %

It turns out that in practice size alone almost does uniquely identify package in this sample. The other file that is 35190 bytes is version 1.2 of the same package, leaving just 2 possible ambiguities out of 847 packages. It seems likely that this holds after encryption as well.

So the remaining question is how much HTTP pipelining ameliorates this, which no-one here has yet actually analysed.

I'm not sure I understand your reply. Are you replying to me as if I said that determined attackers cannot trace back HTTPS traffic to individual APT packages? Because I said no such thing.

I just made the distinction between casual eavesdroppers and determined attackers. Those determined attackers exist and are quite capable, I'm sure. I said as much in my post.

You might also want to look into your use of the word 'glib' here. I find it an uncharitable interpretation of my post to call it 'glib' or 'glib handwaving', to be honest. Makes it seem to me as if I should be defending something I said, but I'm not sure what.

There's really no difference between a "casual" eavesdropper and a "serious" one. In what world do you live in where the former camp even exists? No one is casually spying on your apt updates, and anyone who is "seriously" spying on your apt updates can trivially manage to identify them by size. HTTPs really doesn't add anything here.

Of course no-one is spying casually on HTTP APT traffic specifically. Nobody is arguing that strawman - nobody here is "living in that world", give me some credit please.

But people spying casually on HTTP traffic in general do exist. People able to spy on HTTP traffic in general casually is one of the main reasons we care about HTTPS in the first place. Even though people can do a targeted content length analysis for nearly all other the stuff we read/watch/download online, too. We still care about HTTPS for all of that. And we should probably care for that with APT too, if only a little bit.

TL;DR HTTPS gives you potentially more confidentiality but not guaranteed as known vulnerability exists which an advanced attacker can exploit. You should not assume confidentiality when using APT over HTTPS. The severity of this issue in a CVSS is going to be very low because it is only an information leak.

What?

ISPs, for example, eavesdrop us all the time, and they do it quite casually. They will modify your unprotected HTTP requests, inject ads, log everything they are able to, and sell the data if they can.

Wouldn't help. If almost all of the files' sizes are unique (I'm pretty sure compressed package sizes aren't even block-aligned), and you know the sizes ahead of time, and you can make test requests for samples of what headers are being sent/received, it's trivial to calculate which combination of packages would result in a given stream length using pipelining. You'd have to add countermeasures like padding or fake data.

You could quantize these with up to 10% padding and cause a very large number of collisions, but that wouldn't be useful without HTTPS. Is the core argument that privacy is not attainable, or that it is not valuable?

I think the core argument first of all is that it is not attainable trivially by "just using HTTPS". So it's a question of costs vs. benefits, where the costs are pretty big (change the whole infrastructure).

This is wrong. A great deal more privacy is attainable by trivially using HTTPS. Privacy in the presence of stream inspection is more difficult, but attainable by padding files to have quantized lengths.

You are entirely incorrect in this instance, for all the reasons laid out in this thread.

Looking at stream sizes is not "advanced deep packet inspection", it's in fact the opposite of that.

Up to 10% padding likely isn't sufficient to provide privacy. Think of the long tail of large packages.

Quantizing up to 10% collapses 43,000 values into 120. It's not perfect but it removes a huge amount of information.

How do you arrive at 120? I guess that makes sense if the biggest packages is within 92709x (1.1^120) the size of the smallest. But that doesn't seem like enough range, just eyeballing it. If you have a 1kB package at the low end, I'd be surprised if Debian didn't have a package bigger than 92 MB.

Presenting it as 120 from 43,000 is a bit of an oversimplification, because the average isn't meaningful. The long tail is going to have the worst privacy and the small packages will (probably) have the most.

A scheme like this might be workable but requires being really careful about the security properties you're claiming (i.e., of those 120, probably half are unique, large packages). And obviously, this scheme requires up to 10% additional bandwidth, in the case of the chosen 10% threshold. If buckets change over time, packages moving between buckets may leak a lot of information.

"Step 1: ... etc."

This sounds less like some sort of massively impossible barrier to overcome and more like a Project Euler problem, and one not all that far into the sequence, either.

One of the things you have to overcome if you want to think like a security person is that, yes, there are attackers that will put some effort into attacking you if you are a target of any consequence, certainly effort far exceeding what you just described. I've watched some people at the company I work for have to overcome that handicap myself. Yes, there are attackers that are not just script kiddies and actually, like, have skills and such.

Attackers won't jump through infinite hoops, but getting a foothold on a network somewhere where they'd like more access, seeing that they can watch a new system in your network getting provisioned, and cross-checking that against a list of known vulnerabilities by looking at package sizes would be boringly mundane for them, not something wildly exotic.

There are two reasons you want to compromise a host:

1) You're building a botnet (or, these days, are crypto mining). In that case you're not targeting a specific machine, you just want many of them.

2) You want to exfiltrate information from a host, or sabotage it. In that case you're targeting a specific machine.

I'd argue that in both cases, the proposed attack vector of inferring installed software versions through apt downloads is inferior, or at least more involved. In case 1) you're better off scanning for known vulnerabilities or make use of shodan and the likes. In case 2) you're probably going to probe the server anyways. It might take a little more time than if you just had a complete list of installed packages and their version (given you were somehow able to eavesdrop on the host in the first place), but you'll most likely determine at least what OS is running and what technology stack their internet-facing services are running on after some nmapping.

Or, looking at it from the other perspective, I wouldn't really feel much safer if apt were using https. I'm not against it, but I don't think it's a priority, especially if it needs a lot of coordination between different people, which always turns out to be very time consuming. Just being fast with updating packages seems a better investment of that time.

> Or, looking at it from the other perspective, I wouldn't really feel much safer if apt were using https. I'm not against it, but I don't think it's a priority, especially if it needs a lot of coordination between different people, which always turns out to be very time consuming. Just being fast with updating packages seems a better investment of that time.

This is exactly my position, to be fair. We're all bike-shedding here as far as I'm concerned - including this very website. I think the position that HTTPS doesn't help you is a little bit disingenuous, and the only fair position is that "coordinating this stuff takes time and effort we don't feel is worth the negligible advantages" (as you say, and as this website says) is a more acceptable argument than "the negligible advantages don't exist" (as this website seems to also want to say).

A: Assuming package privacy could somehow be protected (a question I leave to part B), then I agree that there is no improvement in security against capable attackers: they would have to focus on hacking the APT servers, on which the attack surface can be minimized, and monitoring and logging any deviations can be tracked and published for all to inspect.

B: if the packages are concatenated to each other and a random length noise string, we can substantially frustrate nation state / ISP level attackers to the point of forcing them to get this information from the endpoints themselves: Either end user, or the APT server must already have been compromised.

1) End user not yet compromised: in order to capture these, they must attack the APT server.

2) End user already compromised: on each update of all compromised users, information is sent to C&C, so this would produce lots of opportunity for attentive users to discover the implant.

When focussing on the APT, which would give the cleanest record of attack surfaces, the community can put man power on designing minimalist APT servers, and inspecting published deviations in communications can lead to uncovering 0days.

EDIT: changed disagree to agree, as I (incorrectly) thought you were arguing it would not make attacks more expensive, woops!

Any self signed certificate? Then the attacker who is already capable of intercepting packets can just serve whatever and proxy your request. Your argument to the main argument falls short just the same..

Agreed, went a bit overboard there. Removed the part about self-signed certificates.

Steps 1-4 are very easy; they just require some dev work. Thinking an attacker won't bother because it sounds annoying to implement is security by obscurity.

No, it's security against a specific class of attacker that your threat model is considering.

We know there are nation states that build profiles of each user based on their HTTP requests, but we don't know of any that have written custom software specifically to target Debian users.

It would take them half an hour to write that custom software. Under what threat model is "nation-states that can't spend half an hour writing code" a meaningful adversary? Note in particular that they can identify users by previous traffic statistics (and just normal traffic flow statistics that might be kept as a routine log by any network administrator, even); they don't need to write the code in advance. Given the number of bytes transferred, the times, and an archive of the state of the Debian archive (which is publicly available), they can always identify past downloads.

(Would it help if I wrote that code right now and put it on GitHub?)

> It would take them half an hour to write that custom software.

It would take a software engineer half an hour to write that custom software. It would take a government years to amass the political will to target such a small section of the population, and then potentially hundreds of thousands of dollars for a government contractor to offer a solution and implement it.

There's always going to be a big difference in threat level between a piece of software which already exists and a piece of software that could exist. For example, when you're snatched off the streets by the secret police, and they go to investigate what you've been doing in the country, they might be able to request from HQ a list of HTTP addresses fetched from the IP address associated with your apartment, but they're unlikely to be able to request that HQ write some software to go back retrospectively and count bytes of individual connections you made.

> (Would it help if I wrote that code right now and put it on GitHub?)

No, but it would help if you wrote a patch for APT which made it use HTTP range requests to hide the size of the files it downloads. That should only take half an hour, right?

I continue to be confused by this threat model where all government agencies you're worried about are plagued by massive levels of US-style governmental bureaucracy and can't get anything done, yet they're capable of being meaningful threats. (Also where the only entities you're worried about are government entities.)

The entities I'm worried about have bought off-the-shelf surveillance tools which record the HTTP requests associated with each IP/MAC address. This is a minimum viable product for governments and ISPs (not to mention businesses, like hotels and coffee shops), and it is reasonable to think that such a system is deployed on orders of magnitude more networks than a system that tries to infer Debian package downloads from counting bytes of HTTPS traffic.

> Read the HTTP request payload

The thing to note here is that the only reason this seems easy is that there is tooling readily available for such a task. If you didn't have such tooling, you'd find it more difficult to implement then your HTTPS case even _with_ tooling.

The same principle applies to your HTTPS case. Your argument disappears as soon as there is tooling. That tooling only needs to be written once. Perhaps it already has been done and exists in the circles where people want to surveil apt users. One possible reason such tooling isn't widely available is that apt doesn't use HTTPS by default, and one outcome may be that if apt switched to HTTPS the tooling would appear.

I have half a mind to write the tooling and publish it just to eliminate this argument. It really isn't very difficult.