[tor-dev] UX improvement proposal: Onion auto-redirects using Alt-Svc HTTP header

[George Kadianakis]

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



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UX improvement proposal: Onion redirects using Onion-Location HTTP header
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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:
      https://trac.torproject.org/projects/tor/ticket/21952

2. Proposal

   We introduce a new HTTP header called "Onion-Location" with the exact same
   restrictions and semantics as the Location HTTP header. 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.

   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
   techniques.

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).

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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