[tor-dev] UX improvement proposal: Onion auto-redirects using Alt-Svc HTTP header
tom at ritter.vg
Fri Dec 8 16:27:41 UTC 2017
On 8 December 2017 at 09:06, George Kadianakis <desnacked at riseup.net> wrote:
> As discussed in this mailing list and in IRC, I'm posting a subsequent
> version of this proposal. Basic improvements:
> - Uses a new custom HTTP header, instead of Alt-Svc or Location.
> - Does not do auto-redirect; it instead suggests the onion based on
> antonella's mockup: https://trac.torproject.org/projects/tor/attachment/ticket/21952/21952.png
> UX improvement proposal: Onion redirects using Onion-Location HTTP header
> 1. Motivation:
> Lots of high-profile websites have onion addresses these days (e.g. Tor ,
> NYT, blockchain, ProPublica). All those websites seem confused on what's
> the right way to inform their users about their onion addresses. Here are
> some confusion examples:
> a) torproject.org does not even advertise their onion address to Tor users (!!!)
> b) blockchain.info throws an ugly ASCII page to Tor users mentioning their onion
> address and completely wrecking the UX (loses URL params, etc.)
> c) ProPublica has a "Browse via Tor" section which redirects to the onion site.
> Ideally there would be a consistent way for websites to inform their users
> about their onion counterpart. This would provide the following positives:
> + Tor users would use onions more often. That's important for user
> education and user perception, and also to partially dispell the darkweb myth.
> + Website operators wouldn't have to come up with ad-hoc ways to advertise
> their onion services, which sometimes results in complete breakage of
> the user experience (particularly with blockchain)
> This proposal specifies a simple way forward here that's far from perfect,
> but can still provide benefits and also improve user-education around onions
> so that in the future we could employ more advanced techniques.
> Also see Tor ticket #21952 for more discussion on this:
> 2. Proposal
> We introduce a new HTTP header called "Onion-Location" with the exact same
> restrictions and semantics as the Location HTTP header.
For reference, this is https://tools.ietf.org/html/rfc7231#section-7.1.2
> Websites can use the
> Onion-Location HTTP header to specify their onion counterpart, in the same
> way that they would use the Location header.
> The Tor Browser intercepts the Onion-Location header (if any) and informs
> the user of the existense of the onion site, giving them the option to visit
> it. Tor Browser only does so if the header is served over HTTPS.
> Browsers that don't support Tor SHOULD ignore the Onion-Location header.
> 3. Improvements
> 4. Drawbacks
> 4.1. No security/performance benefits
> While we could come up with onion redirection proposals that provide
> security and performance benefits, this proposal does not actually provide
> any of those.
> As a matter of fact, the security remains the same as connecting to normal
> websites (since we trust its HTTP headers), and the performance gets worse
> since we first need to connect to the website, get its headers, and then
> also connect to the onion.
I would specifically call out that the user has provided any
identifying information (cookies) that may be present, as well as
opened themselves to any possible browser-based attack vector served
by the target domain.
> Still _all_ the website approaches mentioned in the "Motivation" section
> suffer from the above drawbacks, and sysadmins still come up with ad-hoc
> ways to inform users abou their onions. So this simple proposal will still
> help those websites and also pave the way forward for future auto-redirect
> 4.2. Defining new HTTP headers is not the best idea
> This proposal defines a new non-standard HTTP header. This is not great
> because it makes Tor into a "special" thing that needs to be supported with
> special headers. However, the fact that it's a new HTTP header that only
> works for Tor is a positive thing since it means that non-Tor browsers will
> just ignore it.
> Furthermore, another drawback is that this HTTP header will increase the
> bandwidth needlessly if it's also served to non-Tor clients. Hence websites
> with lots of client traffic are encouraged to use tools that detect Tor
> users and only serve the header to them (e.g. tordnsel).
I would talk about how users could experience false positives and
false negatives if this mechanism is used.
I think it is also worth addressing that this does not stop sysadmins
from (trying to) detect tor users, and send the onion address in the
Location header, thus triggering a non-prompting redirect. But that
they should consider the potential user confusion of being redirected
to an odd looking domain in such a scenario. And state that this
mechanism is designed to provide a browser-supported option to
consistently offer an onion service in a hopefully less-confusing way.
> 5. The future
> As previously discussed, this is just a simple proposal to introduce the
> redirection concept to people, and also to help some sysadmins who are
> currently coming up with weird ways to inform people about their
> onions. It's not the best way to do this, but it's definitely one of the
> simplest ways.
> In the future we could implement with more advanced auto-redirect proposals like:
> a) Have a "domains to onions" map into HTTPS-everywhere and have it do the
> autoredirects for us (performance benefits, and security benefits under many
> threat models).
> b) Bake onion addresses into SSL certificates and Let's Encrypt as suggested
> by comment:42 in #21952.
> But both of the designs above require non-trivial engineering/policy work
> and would still confuse people. So I think starting with a simple approach
> that will educate users and then moving to more advanced designs is a more
> normative way to go.
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