Re: [TLS] Possible blocking of Encrypted SNI extension in China

"Blumenthal, Uri - 0553 - MITLL" <> Sun, 09 August 2020 17:14 UTC

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From: "Blumenthal, Uri - 0553 - MITLL" <>
To: David Fifield <>
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Thread-Topic: [TLS] Possible blocking of Encrypted SNI extension in China
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Date: Sun, 09 Aug 2020 17:09:49 +0000
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Subject: Re: [TLS] Possible blocking of Encrypted SNI extension in China
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I’m pretty sure your reasoning is wrong. In the ideal world, if *everybody* enabled ESNI - then *maybe* the GFW would relent. 

The way things are - is not smart pretending reality is not what it is. 

Using your terminology - better blend with the crowd, because you aren’t likely to live long enough to see the crowd change to match you. 

There are a lot of technical details why the whole crowd won’t change regardless of your wishes - e.g., who controls TLS implementations in various devices - but I won’t go there. 


> On Aug 9, 2020, at 11:36, David Fifield <> wrote:
> On Thu, Jul 30, 2020 at 11:16:50AM -0700, Christian Huitema wrote:
>> Thanks for the report. I think this relates to our ambivalence about the
>> requirement for ESNI to not "stick out". That requirement is hard to
>> meet, and designs have drifted towards an acceptation that it is OK to
>> stick out as long as a sufficiently large share of the traffic does it.
>> If that share is large, goes the reasoning, it would be too costly for
>> censors to just "drop everything that looks like ESNI". Well, given
>> actors like the Great Firewall, one wonders.
> There's nothing wrong with that reasoning, in my opinion. To blend in
> with a crowd, you can change yourself to match the crowd; or you can
> change the crowd to match yourself. My feeling is that ESNI is currently
> easy to block (or to put it in terms I like, *inexpensive* to block)
> because very few TLS connections use it--nothing valuable depends on it
> yet. Whereas if encrypted SNI were somehow deployed suddenly and
> massively such that it became a normal feature of TLS connections both
> essential and inessential, it would be more difficult (read: expensive)
> to block.
> After all, even the GFW is not all-powerful. Surely it would prefer to
> abolish TLS altogether, but it's too late for that. At this point,
> blocking TLS would cost too much--not in terms of implementing firewall
> rules, but in how much essential communication it would damage. Put
> another way, the GFW itself, and the people who operate/manage it, would
> feel some of the pain of blocking.
> I don't mean to imply that coordinated deployment is the only path to
> success, just saying that if SNI encryption were already widespread,
> even an obvious tag like 0xffce would not be a useful distinguisher. And
> though I find it useful to think of these things in terms of the costs
> of overblocking, it's not an infallible guide. A large organization like
> the GFW is made up of many conflicting motivations, and it is as prone
> as any other to making bad decisions and enacting policies that are
> evidently against its own interests. For that reason I believe it's
> possible to reduce the risk surrounding the deployment of encrypted SNI,
> but not eliminate it completely.
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