Re: [TLS] Securely disabling ECH

[Safe Browsing <[e-mail]>]

Hi Nick,

Thanks for your input.

Yes, what I am suggesting is instead of having: "Depending on whether X,
this may Y or Z".
Rather state: "If X, then MUST Y, else MUST Z"

So, instead of this text in Section 8.2:

"Depending on whether the client is configured to accept the proxy's
certificate as authoritative for the public name, this may trigger the
retry logic described in Section 6.1.6
<https://www.ietf.org/archive/id/draft-ietf-tls-esni-15.html#rejected-ech> or
result in a connection failure. ..."

Have something like this:

"*If* the client is configured to accept the proxy's certificate as
authoritative for the public name, the retry logic described in Section
6.1.6
<https://www.ietf.org/archive/id/draft-ietf-tls-esni-15.html#rejected-ech>
*MUST* be exercised. Else, a connection failure *MUST* result. ..."

Note that the second 'MUST' (the one in the 'else' case) is not new. It is
already stated in the first paragraph in Section 6.1.6 as required behavior.

Also note that the retry in this case would not involve the "retry_configs"
method mentioned in Section 6.1.6. Instead, it would be retrying the
handshake with a new transport connection and ECH disabled. This, ECH
disabled retry, is mentioned in Section 6.1.6, but only in the context
of "retry_configs"
containing non-supported versions, or if an earlier TLS version was
negotiated.

Thus, as I also stated in one of my earlier emails, Section 6.1.6 is not
very clear about the retry mechanism to be followed in the case discussed
here.

A suggestion for changing the text of one of the paragraphs of Section
6.1.6, to be more clear/explicit, would be to change:

"If none of the values provided in "retry_configs" contains a supported
version, or an earlier TLS version was negotiated, the client can regard
ECH as securely disabled by the server, and it SHOULD retry the handshake
with a new transport connection and ECH disabled."

To:

"If none of the values provided in "retry_configs" contains a supported
version, or an earlier TLS version was negotiated, *or an
**"encrypted_client_hello"
extension was not supplied by the server,* the client can regard ECH as
securely disabled by the server, and it *MUST* retry the handshake with a
new transport connection and ECH disabled."

This type of retry would be required only once, and a precondition for this
"securely disabled" state is already stated in one of the earlier
paragraphs in the same section. Namely, that the server/certificate must be
authoritative for the public name.

Again the 'MUST' in this suggested text. Note that it does not pertain to
the "retry_configs", as stated already. But rather to the case where ECH
has been disabled securely, in which case it would be good if the
specification can provide the way a stack must operate in such a situation
which the specification prescribes. I believe having a MUST here, and in
the previous instance mentioned at the top of this email, will make stacks
that wish to comply with the specification much more likely to to implement
this retry mechanism, resulting in a more robust ECH deployment and
operating environment.

To the comment about Section 8.1. Yes, that section has this text that is
relevant here:

"If the server does not understand the "encrypted_client_hello" extension
at all, it will ignore it as required by Section 4.1.2
<https://rfc-editor.org/rfc/rfc8446#section-4.1.2> of [RFC8446
<https://www.ietf.org/archive/id/draft-ietf-tls-esni-15.html#RFC8446>].
Provided the server can present a certificate valid for the public name,
the client can safely retry with updated settings, as described in Section
6.1.6
<https://www.ietf.org/archive/id/draft-ietf-tls-esni-15.html#rejected-ech>."

So it simply refers back to Section 6.1.6 again, which then gets back to my
earlier statement about Section 6.1.6 not being as clear as it can be about
this situation. I.e. the situation were a retry is with ECH disabled, due
to the lack of an ECH extension from the server, as opposed to the
"retry_config" retry method with ECH still enabled - they are of course
very different retry mechanisms.


On Wed, Oct 19, 2022 at 1:44 AM Nick Harper <[e-mail]> wrote:

>
>
> On Tue, Oct 18, 2022 at 8:56 PM Safe Browsing <[e-mail]>
> wrote:
>
>> The draft does consider this by allowing ECH to be disabled - as
>> discussed in this thread. Albeit at the cost of an extra round trip.
>> However, the connection retry logic in the presence of ECH disabling is
>> currently optional.
>>
>> The draft states, in Section 8.2:
>> “ this may trigger the retry logic”
>>
>> It seems this text must change to:
>> “ this MUST trigger the retry logic”
>>
>
> This language change would not make sense. The context for "this may
> trigger the retry logic" in section 8.2 offers two options. The sentence
> structure is "Depending on whether X, this may Y or Z", i.e. if X is
> resolved one way, then the client does Y, otherwise it does Z. Changing the
> "may" to "MUST" would result in stating "this MUST trigger the retry logic
> described in Section 6.1.6 or result in a connection failure", which
> doesn't really make sense, and wouldn't have the goal you'd like, since a
> connection failure instead of retry logic would satisfy the MUST.
>
> If your server is authoritative for the public name, then the behavior you
> care about is described in section 8.1.
>
> I suspect most implementations of ECH will implement the retry logic, as
> the misconfigurations and deployment concerns described in section 8.1 are
> an inevitability, and implementing the retry logic avoids connection
> failures that would occur without it. I doubt that adding a MUST would make
> someone more likely to implement the retry logic.
>
>>
>> In order to ensure functional connections in a TLS client agnostic
>> manner, in the presence of protocol level ECH disabling.
>>
>> I would appreciate your thoughts/input.
>>
>> On Oct 8, 2022, at 7:41 PM, Eric Rescorla <[e-mail]> wrote:
>>
>> 
>> If you are able to install a new trust anchor, then you should be able to
>> use the enterprise configuration mechanisms in browsers to disable ECH.
>>
>> -Ekr
>>
>>
>> On Fri, Oct 7, 2022 at 8:40 PM Safe Browsing <[e-mail]>
>> wrote:
>>
>>> Hi Rich,
>>>
>>> When I say “authoritative”, I mean it in the true TLS sense, in the way
>>> that I believe the ECH draft also suggests and requires.
>>>
>>> In other words, the middlebox serves a cert to the client that is
>>> cryptographically valid for the said public name of the client facing
>>> server.
>>>
>>> How can that be when the client facing server guards its private key
>>> properly? By re-signing the server certificate on the middlebox with a
>>> private key, local to the middle box only, for which the corresponding
>>> certificate has been installed in the trust store of the client, before
>>> sending it on to the client. Only after the original server certificate
>>> has been validated properly on the middlebox, of course. Message
>>> digests being managed accordingly/appropriately.
>>>
>>> That is a very typical setup for most (all?) TLS inspection devices
>>> (next gen firewalls and such).
>>>
>>> Thus this part of ECH, requiring the middlebox to be authoritative for
>>> the server, is well understood and prolifically exercised in inspected TLS
>>> sessions today. What is new is that these connections can now fail/close,
>>> in the “securely disabling ECH” case, and the onus is on the TLS client,
>>> not the application, to retry the connection without ECH.
>>>
>>> I am after such a client, if one exists already.
>>>
>>> Thank you.
>>>
>>> Sent from my phone
>>>
>>> On Oct 7, 2022, at 11:41 AM, Salz, Rich <[e-mail]> wrote:
>>>
>>> 
>>>
>>>
>>>
>>>    - Client <-> *Middlebox* <-> Client-facing server <-> Backend server
>>>
>>>
>>>
>>>    - With "Middlebox" really meaning a middlebox like a firewall or
>>>    similar.
>>>
>>>
>>>
>>> The middlebox is not allowed to modify traffic between the client and
>>> the server. Doing so would mean that the packets the client sent are not
>>> the ones that the server received, and the two message digests would
>>> disagree.  (If you think about things, it **has** to be this way,
>>> otherwise TLS would not be able to provide integrity guarantees.)
>>>
>>>
>>>
>>>    - From the draft, ECH seems to be designed to still allow successful
>>>    TLS connection establishment if the encrypted_client_hello extension is
>>>    dropped from the ClientHello on a conforming middlebox. Provided that the
>>>    middlebox is authoritative for the client-facing server's public name, as
>>>    reported/delivered by DNS to the client. We can assume that this is the
>>>    case here.
>>>
>>>
>>>
>>> I do not understand what you mean by this.  The word “authoritative” is
>>> used to mean that it has a certificate and keypair and can do TLS
>>> termination. DNS giving the client a particular IP address is not
>>> authoritative. It can be confusing because DNS terminology uses
>>> authoritative to mean that a particular entity can prepare data used for
>>> DNS responses.  But it is not authoritative in the TLS sense.
>>>
>>>
>>>
>>> I think your questions can be answered with those two overall
>>> corrections above.  If not, please continue the thread.  (And feel free to
>>> repost from your note since I trimmed for brevity.)
>>>
>>>
>>>
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>>>
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