[TLS] Re: Implicit ECH Config for TLS 1.3 – addressing public_name fingerprinting

[Kazuho Oku <[e-mail]>]

Hello Nick, folks,

I only had the chance to look at the draft and the presentation during
IETF, but I actually like this idea.

To address the concern of outer SNI becoming an observation vector, the
presentation suggested that clients should use "any choice of valid DNS
names."

But as some pointed out, the problem of choosing something valid remains.
Some clients might pick only within a certain suffix. Different clients
might choose them differently.

So, instead, I wonder if we could just say that all ECH clients should use
one value. Using one is simpler, and provides the guarantee that nothing
leaks other than the fact that ECH is used.

And to take another step forward, we can even choose the most popular
hostname around the world to be that one outer SNI. By doing so, we can
hide the ECH traffic being small during the adoption period.

I do understand that the content network operators do not want to abuse the
hostnames that their customers own for protecting other customers.

But what I am saying here is that we can shift the responsibility of
choosing the outer SNI from the content network operators to the clients.
If clients start sending whatever outer SNI that they like, content network
operators have no choice but to ignore that field.


2025年2月26日(水) 14:16 Nick Sullivan <[e-mail]>:

> Hi everyone,
>
> I’ve put together a draft, “Implicit ECH Configuration for TLS 1.3” (
> https://www.ietf.org/archive/id/draft-sullivan-tls-implicit-ech-00.html)
> as a potential starting point for improving ECH’s “do not stick out”
> compliance. Global deployments of ECH have become biased because a single
> public_name dominates most ECH connections, making it a prime target for
> fingerprinting (see https://github.com/net4people/bbs/issues/417) As
> discussed on the TLS WG mailing list (see
> https://mailarchive.ietf.org/arch/msg/tls/4rq4sZzpI9rjYgDLJ2IO-vG9DRw/)
> the outer SNI remains the primary identifier that enables on-path
> adversaries to identify ECH traffic.
>
> To mitigate these linkability risks, various past proposals were
> considered. One idea was to randomize or override the outer SNI rather than
> always using the provided public_name. For example, Stephen Farrell
> suggested allowing clients to use an arbitrary or blank outer SNI (for
> certain use cases like censorship circumvention). This would, in theory,
> make the outer handshake less predictable, increasing traffic diversity
> across ECH connections. However, others in the WG (e.g. Chris Wood)
> cautioned that relaxing this requirement essentially reintroduces domain
> fronting, a side-effect the group was wary of.
>
> The consensus was that fallback reliability and simplicity favored
> sticking with the public_name in SNI. See Github discussions:
> https://github.com/tlswg/draft-ietf-tls-esni/issues/396
> <https://github.com/tlswg/draft-ietf-tls-esni/issues/396#:~:text=For%20at%20least%20command%20line,benefit%20from%20that%20option%20too>
> .
>
> Relatedly, early drafts used an 8-byte config_id, but as documented in
> discussions around 2020-2021, it was shortened to one byte to reduce its
> uniqueness and tracking potential—a change that was well received by
> privacy advocates yet noted by implementers as complicating the deployment
> complexity for multi-key scenarios, though not enough to hinder deployment.
>
> Implicit ECH Configuration, introduced in
> draft-sullivan-tls-implicit-ech-00, builds on this prior work to propose a
> mode of ECH that minimizes explicit signaling of the server’s identity.
> This draft introduces an optional “implicit” mode via a new extension in
> ECHConfigContents. When this extension is present, clients MAY choose any
> valid outer SNI and a randomized config_id instead of relying on a
> potentially globally dominant public_name. Client-facing servers, in turn,
> MUST perform uniform trial decryption to ensure that every handshake is
> processed identically, regardless of whether a valid or a phony config_id
> or outer SNI is provided.
>
> This approach enables clients to adopt custom strategies for maintaining
> broad reachability, ensuring that a single public_name does not become a
> reliable way for external observers to distinguish ECH from ECH GREASE at
> scale. It is also useful for improving privacy when client-facing servers
> support only one or a small number of domains, as it enables clients to
> choose the outer SNI such that it is not merely a direct stand-in for the
> inner name.
>
> Importantly, I don’t believe this approach reintroduces domain fronting.
> It’s not possible to use implicit configuration ECH to connect to one site
> on a server and then trick that server into serving HTTP responses for a
> second, different site when the TLS certificate used to establish the
> connection is not authoritative for that second site – the essential thing
> that distinguishes domain fronting from other techniques. Implicit mode
> effectively relegates the outer SNI to a mostly symbolic role for these
> connections, used solely for ensuring network reachability—similar to how
> certain legacy TLS 1.2 messages were retained in TLS 1.3 to address network
> ossification issues.
>
> This change may have fit into the main ECH draft if it had been proposed
> earlier. However, ECH has already been submitted to IESG for publication,
> so I put this together as a standalone extension. I welcome your feedback
> on this proposal as we work to reduce fingerprinting risks without
> sacrificing deployability.
>
>
> Nick
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-- 
Kazuho Oku