Re: Eric Rescorla's Discuss on draft-ietf-httpbis-expect-ct-07: (with DISCUSS and COMMENT)

Eric Rescorla <ekr@rtfm.com> Wed, 21 November 2018 22:20 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Wed, 21 Nov 2018 14:16:51 -0800
Message-ID: <CABcZeBMhoB7MjE0Tpww66e8CD0nqMdswyjzSng19sdtQ6QjOfw@mail.gmail.com>
To: Emily Stark <estark@google.com>
Cc: IESG <iesg@ietf.org>, httpbis-chairs@ietf.org, Mark Nottingham <mnot@mnot.net>, draft-ietf-httpbis-expect-ct@ietf.org, HTTP Working Group <ietf-http-wg@w3.org>
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Subject: Re: Eric Rescorla's Discuss on draft-ietf-httpbis-expect-ct-07: (with DISCUSS and COMMENT)
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No problem. Just wanted to make sure I wasn't the hold-up.

-Ekr


On Wed, Nov 21, 2018 at 2:04 PM Emily Stark <estark@google.com> wrote:

> Sorry for the delay. I'm on maternity leave but hope to have these
> addressed in the next couple weeks.
>
> On Wed, Nov 21, 2018 at 1:52 PM Eric Rescorla <ekr@rtfm.com> wrote:
>
>> Following up, I don't see any response to these comments.
>>
>> -Ekr
>>
>>
>> On Thu, Sep 13, 2018 at 7:22 AM Eric Rescorla <ekr@rtfm.com> wrote:
>>
>>> I have one other non-blocking comment: Why is this document
>>> Experimental? People are already deploying CT without this. It seems like
>>> PS would make more sense or Informational.
>>>
>>> Alexey, I leave it to you.
>>> -Ekr
>>>
>>>
>>> On Wed, Sep 12, 2018 at 4:12 PM, Eric Rescorla <ekr@rtfm.com> wrote:
>>>
>>>> Eric Rescorla has entered the following ballot position for
>>>> draft-ietf-httpbis-expect-ct-07: Discuss
>>>>
>>>> When responding, please keep the subject line intact and reply to all
>>>> email addresses included in the To and CC lines. (Feel free to cut this
>>>> introductory paragraph, however.)
>>>>
>>>>
>>>> Please refer to
>>>> https://www.ietf.org/iesg/statement/discuss-criteria.html
>>>> for more information about IESG DISCUSS and COMMENT positions.
>>>>
>>>>
>>>> The document, along with other ballot positions, can be found here:
>>>> https://datatracker.ietf.org/doc/draft-ietf-httpbis-expect-ct/
>>>>
>>>>
>>>>
>>>> ----------------------------------------------------------------------
>>>> DISCUSS:
>>>> ----------------------------------------------------------------------
>>>>
>>>> Rich version of this review at:
>>>> https://mozphab-ietf.devsvcdev.mozaws.net/D4579
>>>>
>>>>
>>>> This generally seems like a sound mechanism, but I believe there are
>>>> some points here that are sufficiently unclear they might create
>>>> interop problems,s o I am balloting DISCUSS.
>>>>
>>>> Most importantly, this document just says you support CT, but that
>>>> creates a potential interop problem if say 6962-tris had a different
>>>> way of delivering CT information or a different syntax. I'm not saying
>>>> you need a version here, but you need to indicate that it's not
>>>> forward-looking.
>>>>
>>>> Also, see below.
>>>>
>>>> DETAIL
>>>> S 2.4.
>>>> >      beginning an HTTP conversation over the TLS channel.
>>>> >
>>>> >      If a connection to a Known Expect-CT Host violates the UA's CT
>>>> policy
>>>> >      (i.e., the connection is not CT-qualified), and if the Known
>>>> Expect-
>>>> >      CT Host's Expect-CT metadata indicates an "enforce"
>>>> configuration,
>>>> >      the UA MUST treat the CT compliance failure as an error.
>>>>
>>>> Is this supposed to be a hard failure, as with HSTS. If not, how does
>>>> it interact with HSTS's hard failure reqs.
>>>>
>>>>
>>>> S 3.1.
>>>> >         (This may differ from the value of the
>>>> "served-certificate-chain"
>>>> >         key.)  The value is provided as an array of strings, which
>>>> MUST
>>>> >         appear in the order matching the chain that the UA validated;
>>>> each
>>>> >         string in the array is the Privacy-Enhanced Mail (PEM)
>>>> >         representation of each X.509 certificate as described in
>>>> >         [RFC7468].
>>>>
>>>> What happens if you try to construct multiple paths?
>>>>
>>>>
>>>> S 3.1.
>>>> >            does not have or does not trust the public key of the log
>>>> from
>>>> >            which the SCT was issued), "valid" (indicating that the UA
>>>> >            successfully validated the SCT as described in Section 5.2
>>>> of
>>>> >            [RFC6962] or Section 8.2.3 of
>>>> [I-D.ietf-trans-rfc6962-bis]), or
>>>> >            "invalid" (indicating that the SCT validation failed
>>>> because
>>>> >            of, e.g., a bad signature).
>>>>
>>>> Is "invalid" anything other than the specific cases listed above?
>>>>
>>>>
>>>> ----------------------------------------------------------------------
>>>> COMMENT:
>>>> ----------------------------------------------------------------------
>>>>
>>>>
>>>> >      allows web host operators to instruct user agents to expect valid
>>>> >      Signed Certificate Timestamps (SCTs) to be served on connections
>>>> to
>>>> >      these hosts.  Expect-CT allows web host operators to discover
>>>> >      misconfigurations in their Certificate Transparency deployments
>>>> and
>>>> >      ensure that misissued certificates accepted by UAs are
>>>> discoverable
>>>> >      in Certificate Transparency logs.
>>>>
>>>> I don't believe that it does this. Consider a client which simply did
>>>> not support CT, then it would (a) accept a misissued certificate that
>>>> (b) was not discoverable
>>>>
>>>>
>>>> S 2.1.1.
>>>> >
>>>> >               Figure 2: Syntax of the report-uri directive value
>>>> >
>>>> >      "absolute-URI" is defined in Section 4.3 of [RFC3986].
>>>> >
>>>> >      Hosts may set "report-uri"s that use HTTP or HTTPS.  If the
>>>> scheme in
>>>>
>>>> Why are you allowing HTTP?
>>>>
>>>>
>>>> S 2.3.2.
>>>> >         the "enforce", "max-age", or "report-uri" header field value
>>>> >         directives convey information different from that already
>>>> >         maintained by the UA.  If the "max-age" directive has a value
>>>> of
>>>> >         0, the UA MUST remove its cached Expect-CT information if the
>>>> host
>>>> >         was previously noted as a Known Expect-CT Host, and MUST NOT
>>>> note
>>>> >         this host as a Known Expect-CT Host if it is not already
>>>> noted.
>>>>
>>>> As noted above, I think you need to clear the cache when you upgrade
>>>> to a potentially incompatible CT version, or otherwise reconfigure the
>>>> client.
>>>>
>>>>
>>>> S 2.3.2.1.
>>>> >         this host as a Known Expect-CT Host if it is not already
>>>> noted.
>>>> >
>>>> >   2.3.2.1.  Noting Expect-CT
>>>> >
>>>> >      Upon receipt of the Expect-CT response header field over an
>>>> error-
>>>> >      free TLS connection (including the validation adding in Section
>>>> 2.4),
>>>>
>>>> s/adding/added/?
>>>>
>>>>
>>>> S 2.3.2.1.
>>>> >      host's domain name and its associated Expect-CT directives in
>>>> non-
>>>> >      volatile storage.
>>>> >
>>>> >      To note a host as a Known Expect-CT Host, the UA MUST set its
>>>> Expect-
>>>> >      CT metadata given in the most recently received valid Expect-CT
>>>> >      header field, as specified in Section 2.3.2.2.
>>>>
>>>> This seems ungrammatical. Set it where?
>>>>
>>>>
>>>> S 2.3.2.2.
>>>> >
>>>> >   2.3.2.2.  Storage Model
>>>> >
>>>> >      If the substring matching the host production from the
>>>> Request-URI
>>>> >      (of the message to which the host responded) does not congruently
>>>> >      match an existing Known Expect-CT Host's domain name, per the
>>>>
>>>> I would say "exactly match" rather than "congruently match" unless
>>>> this ia term of art somewhere.
>>>>
>>>>
>>>> S 2.3.2.2.
>>>> >      understands them, the UA MAY note them as well.
>>>> >
>>>> >      UAs MAY set an upper limit on the value of max-age, so that UAs
>>>> that
>>>> >      have noted erroneous Expect-CT hosts (whether by accident or due
>>>> to
>>>> >      attack) have some chance of recovering over time.  If the server
>>>> sets
>>>> >      a max-age greater than the UA's upper limit, the UA MAY behave
>>>> as if
>>>>
>>>> This MAY seems out of place, given that you already said MAY.
>>>>
>>>>
>>>> S 2.4.
>>>> >
>>>> >      When a UA connects to a Known Expect-CT Host using a TLS
>>>> connection,
>>>> >      if the TLS connection has no errors, then the UA will apply an
>>>> >      additional correctness check: compliance with a CT Policy.  A UA
>>>> >      should evaluate compliance with its CT Policy whenever
>>>> connecting to
>>>> >      a Known Expect-CT Host, as soon as possible.  However, the check
>>>> can
>>>>
>>>> What does "as soon as possible" mean?
>>>>
>>>>
>>>> S 2.4.
>>>> >      terminates the connection due to an Expect-CT failure, this could
>>>> >      cause the UA to skip subsequent correctness checks.  When the CT
>>>> >      compliance check is skipped or bypassed, Expect-CT reports
>>>> >      (Section 3) will not be sent.
>>>> >
>>>> >      When CT compliance is evaluted for a Known Expect-CT Host, the UA
>>>>
>>>> Nit: evaluated
>>>>
>>>>
>>>> S 2.4.1.
>>>> >      "report-uri" (Section 3).
>>>> >
>>>> >   2.4.1.  Skipping CT compliance checks
>>>> >
>>>> >      It is acceptable for a UA to skip CT compliance checks for some
>>>> hosts
>>>> >      according to local policy.  For example, a UA may disable CT
>>>>
>>>> Should this be MAY?
>>>>
>>>>
>>>> S 3.1.
>>>> >
>>>> >      o  "scts": the value represents the SCTs (if any) that the UA
>>>> >         received for the Expect-CT host and their validation statuses.
>>>> >         The value is provided as an array of JSON objects.  The SCTs
>>>> may
>>>> >         appear in any order.  Each JSON object in the array has the
>>>> >         following keys:
>>>>
>>>> So these just apply to the EE cert? What about CT for the non-EE
>>>> certs?
>>>>
>>>>
>>>>
>>>>
>>>> S 3.1.
>>>> >            of, e.g., a bad signature).
>>>> >
>>>> >         *  The "source" key, with a string value that indicates from
>>>> where
>>>> >            the UA obtained the SCT, as defined in Section 3 of
>>>> [RFC6962]
>>>> >            and Section 6 of [I-D.ietf-trans-rfc6962-bis].  The UA
>>>> MUST set
>>>> >            the value to one of "tls-extension", "ocsp", or "embedded".
>>>>
>>>> What do these mean? They seem obvious, but you don't say.
>>>>
>>>>
>>>>
>>>