Re: [TLS] Don't Split HelloRetryRequest

Martin Thomson <mt@lowentropy.net> Tue, 06 April 2021 02:29 UTC

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Date: Tue, 06 Apr 2021 12:28:40 +1000
From: "Martin Thomson" <mt@lowentropy.net>
To: tls@ietf.org
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Subject: Re: [TLS] Don't Split HelloRetryRequest
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I've created a few pull requests that make the changes I propose.  I think that the whole suite of related issues are closed as a result.

The main one is here: https://github.com/tlswg/draft-ietf-tls-esni/pull/417
There's a bit of rewriting here, but the change is not that large.  I would expect most implementations to be compliant already (it's much more work not to be).

The whole set: https://github.com/tlswg/draft-ietf-tls-esni/pulls/martinthomson

On Thu, Apr 1, 2021, at 12:57, Martin Thomson wrote:
> I just reviewed the proposal to split HelloRetryRequest into outer and 
> (encrypted) inner.
> 
> https://github.com/tlswg/draft-ietf-tls-esni/pull/407/files
> 
> I'm strongly opposed to taking the change.  It complicates the design 
> greatly and unnecessarily.
> 
> The existing design has some flaws, but they can be fixed more 
> elegantly than this.
> 
> (Apologies if this seems a little long.  I'm writing down every 
> possible argument I can think of, because I can't guarantee that I will 
> be coherent at the meeting.)
> 
> # HelloRetryRequest Has a Narrow Purpose
> 
> Let's first address the key question of what HelloRetryRequest exists 
> to do.  It exists to ensure that the client and server are able to 
> jointly agree on keys to use for the remainder of the handshake.  This 
> is a very narrow scope.
> 
> Furthermore, the particulars of key agreement are public.  This is 
> important as we can also say that all hidden servers need to use the 
> same configuration as it relates to cipher suites, key exchange, and 
> related parameters, as the results of negotiation are sent in the clear 
> in the ServerHello.
> 
> My claim here is that there is no value in protecting any parameter 
> that might change in response to HelloRetryRequest.
> 
> # Don't Seek Complexity
> 
> It is entirely possible to imagine scenarios where an inner ClientHello 
> has different preferences from an outer ClientHello.  While in theory 
> we can construct a design that would support that (the pull request 
> does this well enough), to do so only serves to increase complexity.  
> It does not address any real use case or problem that I'm aware of.
> 
> If we allow for the client to provide different values in inner and 
> outer ClientHello messages, then the current design means that the 
> client is faced with some ambiguity about which of the two messages a 
> HelloRetryRequest applies to.  If we try to create an indicator, then 
> that leaks.
> 
> We could solve the problem by making the protocol more complex.  Or we 
> could avoid it.
> 
> This problem is entirely avoidable.
> 
> # Matching Inner and Outer Values
> 
> When we get right down to it, there are a very small number of things 
> that truly change in response to HelloRetryRequest.  And all of these 
> changes are to values that do not need confidentiality protection.
> 
> The draft lists three fields that change: ciphersuites, key_share, and 
> version.  From my perspective, changing cipher suites, supported 
> groups, or versions would be a big mistake.  So what changes is even 
> more limited.  Just the shares in key_share.
> 
> On this basis, a client that offers cipher suites, groups, versions, 
> and key shares that are identical in both inner and outer ClientHello 
> messages will always receive a HelloRetryRequest that applies equally 
> to both messages.
> 
> The only adjustment that is acceptable with respect to these fields 
> being identical is the addition of TLS 1.2-only options to the outer 
> ClientHello (or the removal of the same from the inner ClientHello if 
> you prefer it that way around).  This is a fine optimization on the 
> basis that accepting ECH represents a commitment to support TLS 1.3 (or 
> higher).  But it is really just an optimization (the draft makes this 
> mandatory, which is silly).  The client can therefore remove options 
> from the inner ClientHello only if it is impossible to select them with 
> TLS 1.3 or higher.
> 
> For new extensions, if they define a means of adjustment or correction 
> via HelloRetryRequest (there is currently just one: password_salt, 
> which I haven't examined), then they too need to follow this 
> restriction.   It's not an onerous one.
> 
> Follow this simple constraint and HelloRetryRequest will always apply 
> equally to both inner and outer ClientHello and everything works.  
> Conveniently, this is more or less exactly what the current draft says. 
>  Almost.
> 
> The draft currently allows inner and outer ClientHello to have 
> different types of key share.  The way it handles this is terrible: it 
> recommends breaking the transcript.  To me, that seems like it would 
> only serve to open the protocol up to downgrade attack.
> 
> Incidentally, I don't see a problem with having a different key share 
> *value* in inner and outer ClientHello.  There's no point in doing that 
> unless you believe that these values leak information (they really 
> shouldn't), but it wouldn't break this model if a client decided to do 
> that.
> 
> https://github.com/tlswg/draft-ietf-tls-esni/issues/333 appears to be 
> concerned about the cookie only applying to one or other ClientHello.  
> I don't see how is the case, so I'm just going to say that this is 
> fixed by having HelloRetryRequest apply to both inner and outer 
> ClientHello messages.  If the client receives HelloRetryRequest that 
> applies to just one of the two, then the problem is that the client is 
> faulty.  That would be treated as a programming error as normal (crash, 
> open a bug report, send an internal_error alert, etc...).
> 
> Then there are the things that more or less have to change in response 
> to HelloRetryRequest, but really only because the ClientHello changes: 
> padding, pre_shared_key, and ECH itself.  For those, we need to address 
> a minor inconsistency problem at the level of the core protocol itself.
> 
> # Addressing Minor HelloRetryRequest Problems
> 
> We do need to fix RFC 8446 rules regarding HelloRetryRequest.  David 
> already suggested some minute adjustments for that problem in 
> https://github.com/tlswg/draft-ietf-tls-esni/issues/358 .  The short 
> version is that extensions can define their own rules for how they 
> change after HelloRetryRequest.  This is a good amendment, especially 
> as it relates to extensions that are not known to the server.
> 
> That tweak does have deployment issues, because the original rules have 
> been interpreted too literally in some cases, but that should not 
> affect ECH specifically.  Servers that have this bug won't be able to 
> deploy ECH without fixing the bug and that's OK.  Other servers will 
> only see grease.
> 
> The draft currently mandates that greasing values not change after 
> HelloRetryRequest, which will avoid this compatibility bug, but also 
> reveal the fraud.  I can tolerate that small amount of leakage.
> 
> # Avoiding HelloRetryRequest
> 
> I think that Nick's suggestion for helping avoid HelloRetryRequest by 
> placing hints about key shares in DNS SVCB/HTTPS records is a fine one.
> 
> I see the arguments about this being about the configuration needing to 
> speak for backend servers when the record relates to frontend servers.  
> But my perspective here is that you already need to ensure that backend 
> servers have a consistent cryptographic support profile; adding a small 
> number of frontend servers to the set that need to be made consistent 
> isn't that difficult.  If this consistency is not possible in some 
> deployments, that's understandable, but then it is an optional 
> enhancement that won't be available to those deployments, that's all.
> 
> Of course, this is an extension that we can pursue separately.
> 
> # Conclusion
> 
> I'm firmly opposed to splitting HelloRetryRequest.  I would like to 
> deploy ECH and this doesn't really help with that.
> 
> I don't agree that there is a problem that needs to be fixed with the 
> current draft.
> 
> On the other hand, I can guarantee that this change will delay Firefox 
> deployment significantly (that is, for an indefinite period).  It would 
> require rearchitecting a piece of code that is rarely used already 
> (despite being a source of significant complexity) and replacing it 
> with code that is even more complex and would include paths that are 
> even more lightly used.