[tbb-dev] A proposal for verifying http downloads

[Tom Ritter]

We also discussed this proposal at the monthly Tor/Mozilla meeting.

Dennis pointed out that an attacker who is modifying a page served
over an insecure connection can change the link to point to the
malicious file, so it seems more valuable to just block such a
download outright.

If the link is served over HTTPS however, that might be a situation to
allow (and verify) it.

Dennis also pointed out that sometimes users are served customized
binaries or only allowed to download a file once. Both would break
verification.

We discussed using an exit node to measure how common it is for a Tor
user to download an executable file or non-opaque over HTTP. Such an
experiment shouldn't be done without talking to the Tor Research
Safety Group. But the idea would only be to record the number of such
files downloaded and the total amount of traffic pushed by Tor, to
give a statistic such as "For every 5 terabit of trafic, users
download ~4 executable files insecurely." No record of type of file,
file itself, location, time, or anything else should be kept.

Finally, we talked about checking an authenticode-style signature
block on an executable to make such a verification faster. A short
proposal would be:
1) Check if the file has an authenticode signature. If not, verify by
some other means
2) Download the PE Header and the signature block at the end of the
file over a new circuit
3) Compare the Header,, Signature Block, and Signature Public Key and
confirm they match
4) Confirm that the signature isn't a weak signature that would verify
any file (Dennis pointed this out too)

At this point we should be assured that a) The OS will check the
authenticode block b) if the file hash doesn't match the signature
block the OS won't run it c) the file hash was the same on both
circuit downloads.

-tom


On Thu, 7 Mar 2019 at 06:06, Tom Ritter <tom at ritter.vg> wrote:
>
> Third of three.
>
> This one is definitely the least fleshed out. The short version is
> verify most types of files downloaded over a potentially insecure
> connection.
>
> -tom