TLS Termination Delegation in Browsers

What is this?
A call for a mechanism for a browser to delegate TLS termination to a proxy, without resorting to MITM, making it possible for the browser to properly warn users when it’s active and support this feature in environments where it’s useful.?

Originally posted with ActivityPub

I’d like a mechanism for a browser to delegate TLS termination to a proxy. This is the true analogue for Cloudflare, or Cloudfront, or Akamai, or Fastly: as a service owner, I delegate TLS termination to my CDN, so the CDN can process the traffic it receives. It’s not an interception, or a MITM, as far as the client is concerned the CDN is the service.

Behind the CDN, we use TLS to the origin too, to make sure the right content gets served securely.

As a browser operator, I want something similar. As it happens, Cloudflare provides that service too, but not in the way I want it to, so more on that later.

Browsers have a mechanism to use proxies for HTTP, and even to use HTTPS to connect to proxies. But they (quite reasonably in many ways) decline to delegate TLS termination to the proxy. If I ask for an HTTPS website, the browser will ask the proxy for a tunnelled connection. I want the browser to properly warn users when it’s active, but also properly support this in environments where it’s useful. Probably a privacy-preserving pony, too, while I’m at it? I’m quite sure it’s not even possible to implement in a way that satisfies everyone.

What happens instead is actual MITM that needs to try to hide from the browser. This is classic middleware box, and also what Cloudflare offer in their “zero trust” product – replace your OS certificate store with their root certificate and they’ll intercept every request and make sure it’s legit. Which is fine if that’s what you want but it’s too similar to black-hat MITM for my liking.

I’m the volunteer administrator of a small school network. Mostly I use custom DNS for content moderation, which isn’t very precise and itself only works because I can delegate secure DNS lookups – and if a client makes a plain text DNS lookup via the router, it’ll be upgraded to DoH before being sent to the service provider too :).

Neil replies


Does something like do what you have in mind (even if not the UX you might want)?

i.e. the https connection is between the client-on-the-server and the remote server, and there is a separate session between user terminal and the client-on-the-server, showing the content to the user?

I respond

Interesting project :). Not quite, I think - that moves the rendering to the server too.

It shares with my wish that it isn't compromising the secure connection, which is really important for me - as an individual (and a member of society) I want to know who I'm talking to and that no one is listening.

Where it differs is in composability. In my day job, I want to be able to redirect requests for development without compromising the rest of my browsing. In school, I want to expand the trust boundary of the user agent to encompass a filtering proxy. At home, I want to be sure I'm talking directly to the origin. So I want to use a regular browser, and plain HTTP proxies already support all these use cases for plain HTTP.

Where it goes wrong is that a binary filter (IP, DNS, or TLS) doesn't give as much signal or control to a malicious administrator as the ability to see which content is actually loaded and to manipulate it. And even if it's entirely clear to the user what's happening, the capability to force use of a proxy is bad.

We see from real-world reports that in practice, the alternative to a content filter isn't an outright block, but less granular blocking. So unless we can work out a way to avoid it being abused, I'm quite happy to retain privacy and security against malicious administrators even if it makes things more challenging for benign administrators.