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Versions: (draft-stark-expect-ct) 00 01 02 03 04 05 06 07

HTTP                                                            E. Stark
Internet-Draft                                                    Google
Intended status: Experimental                          February 26, 2018
Expires: August 30, 2018


                      Expect-CT Extension for HTTP
                    draft-ietf-httpbis-expect-ct-03

Abstract

   This document defines a new HTTP header, named Expect-CT, that allows
   web host operators to instruct user agents to expect valid Signed
   Certificate Timestamps (SCTs) to be served on connections to these
   hosts.  When configured in enforcement mode, user agents (UAs) will
   remember that hosts expect SCTs and will refuse connections that do
   not conform to the UA's Certificate Transparency policy.  When
   configured in report-only mode, UAs will report the lack of valid
   SCTs to a URI configured by the host, but will allow the connection.
   By turning on Expect-CT, web host operators can discover
   misconfigurations in their Certificate Transparency deployments and
   ensure that misissued certificates accepted by UAs are discoverable
   in Certificate Transparency logs.

Note to Readers

   Discussion of this draft takes place on the HTTP working group
   mailing list (ietf-http-wg@w3.org), which is archived at
   https://lists.w3.org/Archives/Public/ietf-http-wg/ [1].

   Working Group information can be found at http://httpwg.github.io/
   [2]; source code and issues list for this draft can be found at
   https://github.com/httpwg/http-extensions/labels/expect-ct [3].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."



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   This Internet-Draft will expire on August 30, 2018.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Server and Client Behavior  . . . . . . . . . . . . . . . . .   5
     2.1.  Response Header Field Syntax  . . . . . . . . . . . . . .   5
       2.1.1.  The report-uri Directive  . . . . . . . . . . . . . .   6
       2.1.2.  The enforce Directive . . . . . . . . . . . . . . . .   6
       2.1.3.  The max-age Directive . . . . . . . . . . . . . . . .   7
       2.1.4.  Examples  . . . . . . . . . . . . . . . . . . . . . .   7
     2.2.  Server Processing Model . . . . . . . . . . . . . . . . .   7
       2.2.1.  HTTP-over-Secure-Transport Request Type . . . . . . .   8
       2.2.2.  HTTP Request Type . . . . . . . . . . . . . . . . . .   8
     2.3.  User Agent Processing Model . . . . . . . . . . . . . . .   8
       2.3.1.  Expect-CT Header Field Processing . . . . . . . . . .   8
       2.3.2.  HTTP-Equiv <meta> Element Attribute . . . . . . . . .   9
       2.3.3.  Noting Expect-CT  . . . . . . . . . . . . . . . . . .   9
       2.3.4.  Storage Model . . . . . . . . . . . . . . . . . . . .   9
     2.4.  Evaluating Expect-CT Connections for CT Compliance  . . .  10
   3.  Reporting Expect-CT Failure . . . . . . . . . . . . . . . . .  11
     3.1.  Generating a violation report . . . . . . . . . . . . . .  11
     3.2.  Sending a violation report  . . . . . . . . . . . . . . .  13
     3.3.  Receiving a violation report  . . . . . . . . . . . . . .  14
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
     4.1.  Maximum max-age . . . . . . . . . . . . . . . . . . . . .  15
     4.2.  Avoiding amplification attacks  . . . . . . . . . . . . .  15
   5.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  15
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   7.  Usability Considerations  . . . . . . . . . . . . . . . . . .  16
   8.  Authoring Considerations  . . . . . . . . . . . . . . . . . .  16



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     8.1.  HTTP Header . . . . . . . . . . . . . . . . . . . . . . .  16
   9.  Changes . . . . . . . . . . . . . . . . . . . . . . . . . . .  16
     9.1.  Since -02 . . . . . . . . . . . . . . . . . . . . . . . .  16
     9.2.  Since -01 . . . . . . . . . . . . . . . . . . . . . . . .  17
     9.3.  Since -00 . . . . . . . . . . . . . . . . . . . . . . . .  17
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     10.2.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  18
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  19

1.  Introduction

   This document defines a new HTTP header that enables UAs to identify
   web hosts that expect the presence of Signed Certificate Timestamps
   (SCTs) [I-D.ietf-trans-rfc6962-bis] in future Transport Layer
   Security (TLS) [RFC5246] connections.

   Web hosts that serve the Expect-CT HTTP header are noted by the UA as
   Known Expect-CT Hosts.  The UA evaluates each connection to a Known
   Expect-CT Host for compliance with the UA's Certificate Transparency
   (CT) Policy.  If the connection violates the CT Policy, the UA sends
   a report to a URI configured by the Expect-CT Host and/or fails the
   connection, depending on the configuration that the Expect-CT Host
   has chosen.

   If misconfigured, Expect-CT can cause unwanted connection failures
   (for example, if a host deploys Expect-CT but then switches to a
   legitimate certificate that is not logged in Certificate Transparency
   logs, or if a web host operator believes their certificate to conform
   to all UAs' CT policies but is mistaken).  Web host operators are
   advised to deploy Expect-CT with caution, by using the reporting
   feature and gradually increasing the interval where the UA remembers
   the host as a Known Expect-CT Host.  These precautions can help web
   host operators gain confidence that their Expect-CT deployment is not
   causing unwanted connection failures.

   Expect-CT is a trust-on-first-use (TOFU) mechanism.  The first time a
   UA connects to a host, it lacks the information necessary to require
   SCTs for the connection.  Thus, the UA will not be able to detect and
   thwart an attack on the UA's first connection to the host.  Still,
   Expect-CT provides value by 1) allowing UAs to detect the use of
   unlogged certificates after the initial communication, and 2)
   allowing web hosts to be confident that UAs are only trusting
   publicly-auditable certificates.







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1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in RFC
   2119 [RFC2119].

1.2.  Terminology

   Terminology is defined in this section.

   Certificate Transparency Policy  is a policy defined by the UA
      concerning the number, sources, and delivery mechanisms of Signed
      Certificate Timestamps that are served on TLS connections.  The
      policy defines the properties of a connection that must be met in
      order for the UA to consider it CT-qualified.

   Certificate Transparency Qualified  describes a TLS connection for
      which the UA has determined that a sufficient quantity and quality
      of Signed Certificate Timestamps have been provided.

   CT-qualified  See Certificate Transparency Qualified.

   CT Policy  See Certificate Transparency Policy.

   Effective Expect-CT Date  is the time at which a UA observed a valid
      Expect-CT header for a given host.

   Expect-CT Host  See HTTP Expect-CT Host.

   HTTP Expect-CT  is the overall name for the combined UA- and server-
      side security policy defined by this specification.

   HTTP Expect-CT Host  is a conformant host implementing the HTTP
      server aspects of HTTP Expect-CT.  This means that an Expect-CT
      Host returns the "Expect-CT" HTTP response header field in its
      HTTP response messages sent over secure transport.

   Known Expect-CT Host  is an Expect-CT Host that the UA has noted as
      such.  See Section 2.3.3 for particulars.

   UA is an acronym for "user agent".  For the purposes of this
      specification, a UA is an HTTP client application typically
      actively manipulated by a user [RFC7230].

   Unknown Expect-CT Host  is an Expect-CT Host that the UA has not
      noted.




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2.  Server and Client Behavior

2.1.  Response Header Field Syntax

   The "Expect-CT" header field is a new response header defined in this
   specification.  It is used by a server to indicate that UAs should
   evaluate connections to the host emitting the header for CT
   compliance (Section 2.4).

   Figure 1 describes the syntax (Augmented Backus-Naur Form) of the
   header field, using the grammar defined in [RFC5234] and the rules
   defined in Section 3.2 of [RFC7230].

   Expect-CT           = #expect-ct-directive
   expect-ct-directive = directive-name [ "=" directive-value ]
   directive-name      = token
   directive-value     = token / quoted-string

              Figure 1: Syntax of the Expect-CT header field

   Optional white space ("OWS") is used as defined in Section 3.2.3 of
   [RFC7230]. "token" and "quoted-string" are used as defined in
   Section 3.2.6 of [RFC7230].

   The directives defined in this specification are described below.
   The overall requirements for directives are:

   1.  The order of appearance of directives is not significant.

   2.  A given directive MUST NOT appear more than once in a given
       header field.  Directives are either optional or required, as
       stipulated in their definitions.

   3.  Directive names are case insensitive.

   4.  UAs MUST ignore any header fields containing directives, or other
       header field value data, that do not conform to the syntax
       defined in this specification.  In particular, UAs must not
       attempt to fix malformed header fields.

   5.  If a header field contains any directive(s) the UA does not
       recognize, the UA MUST ignore those directives.

   6.  If the Expect-CT header field otherwise satisfies the above
       requirements (1 through 5), the UA MUST process the directives it
       recognizes.





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2.1.1.  The report-uri Directive

   The OPTIONAL "report-uri" directive indicates the URI to which the UA
   SHOULD report Expect-CT failures (Section 2.4).  The UA POSTs the
   reports to the given URI as described in Section 3.

   The "report-uri" directive is REQUIRED to have a directive value, for
   which the syntax is defined in Figure 2.

   report-uri-value = absolute-URI

            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
   the "report-uri" is one that uses TLS (e.g., HTTPS), UAs MUST check
   Expect-CT compliance when the host in the "report-uri" is a Known
   Expect-CT Host; similarly, UAs MUST apply HSTS if the host in the
   "report-uri" is a Known HSTS Host.

   Note that the report-uri need not necessarily be in the same Internet
   domain or web origin as the host being reported about.

   UAs SHOULD make their best effort to report Expect-CT failures to the
   "report-uri", but they may fail to report in exceptional conditions.
   For example, if connecting to the "report-uri" itself incurs an
   Expect-CT failure or other certificate validation failure, the UA
   MUST cancel the connection.  Similarly, if Expect-CT Host A sets a
   "report-uri" referring to Expect-CT Host B, and if B sets a "report-
   uri" referring to A, and if both hosts fail to comply to the UA's CT
   Policy, the UA SHOULD detect and break the loop by failing to send
   reports to and about those hosts.

   UAs SHOULD limit the rate at which they send reports.  For example,
   it is unnecessary to send the same report to the same "report-uri"
   more than once.

2.1.2.  The enforce Directive

   The OPTIONAL "enforce" directive is a valueless directive that, if
   present (i.e., it is "asserted"), signals to the UA that compliance
   to the CT Policy should be enforced (rather than report-only) and
   that the UA should refuse future connections that violate its CT
   Policy.  When both the "enforce" directive and "report-uri" directive
   (as defined in Figure 2) are present, the configuration is referred
   to as an "enforce-and-report" configuration, signalling to the UA




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   both that compliance to the CT Policy should be enforced and that
   violations should be reported.

2.1.3.  The max-age Directive

   The "max-age" directive specifies the number of seconds after the
   reception of the Expect-CT header field during which the UA SHOULD
   regard the host from whom the message was received as a Known Expect-
   CT Host.

   The "max-age" directive is REQUIRED to be present within an "Expect-
   CT" header field.  The "max-age" directive is REQUIRED to have a
   directive value, for which the syntax (after quoted-string
   unescaping, if necessary) is defined in Figure 3.

   max-age-value = delta-seconds
   delta-seconds = 1*DIGIT

              Figure 3: Syntax of the max-age directive value

   "delta-seconds" is used as defined in Section 1.2.1 of [RFC7234].

2.1.4.  Examples

   The following examples demonstrate valid Expect-CT response header
   fields:

Expect-CT: max-age=86400,enforce

Expect-CT: max-age=86400, enforce, report-uri="https://foo.example/report"

Expect-CT: max-age=86400,report-uri="https://foo.example/report"

       Figure 4: Examples of valid Expect-CT response header fields

2.2.  Server Processing Model

   This section describes the processing model that Expect-CT Hosts
   implement.  The model has 2 parts: (1) the processing rules for HTTP
   request messages received over a secure transport (e.g.,
   authenticated, non-anonymous TLS); and (2) the processing rules for
   HTTP request messages received over non-secure transports, such as
   TCP.








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2.2.1.  HTTP-over-Secure-Transport Request Type

   When replying to an HTTP request that was conveyed over a secure
   transport, an Expect-CT Host SHOULD include in its response exactly
   one Expect-CT header field.  The header field MUST satisfy the
   grammar specified in Section 2.1.

   Establishing a given host as an Expect-CT Host, in the context of a
   given UA, is accomplished as follows:

   1.  Over the HTTP protocol running over secure transport, by
       correctly returning (per this specification) at least one valid
       Expect-CT header field to the UA.

   2.  Through other mechanisms, such as a client-side preloaded Expect-
       CT Host list.

2.2.2.  HTTP Request Type

   Expect-CT Hosts SHOULD NOT include the Expect-CT header field in HTTP
   responses conveyed over non-secure transport.  UAs MUST ignore any
   Expect-CT header received in an HTTP response conveyed over non-
   secure transport.

2.3.  User Agent Processing Model

   The UA processing model relies on parsing domain names.  Note that
   internationalized domain names SHALL be canonicalized according to
   the scheme in Section 10 of [RFC6797].

2.3.1.  Expect-CT Header Field Processing

   If the UA receives, over a secure transport, an HTTP response that
   includes an Expect-CT header field conforming to the grammar
   specified in Section 2.1, the UA MUST evaluate the connection on
   which the header was received for compliance with the UA's CT Policy,
   and then process the Expect-CT header field as follows.

   If the connection complies with the UA's CT Policy (i.e. the
   connection is CT-qualified), then the UA MUST either:

   o  Note the host as a Known Expect-CT Host if it is not already so
      noted (see Section 2.3.3), or

   o  Update the UA's cached information for the Known Expect-CT Host if
      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



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      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.

   If the connection does not comply with the UA's CT Policy (i.e. is
   not CT-qualified), then the UA MUST NOT note this host as a Known
   Expect-CT Host.

   If the header field includes a "report-uri" directive, and the
   connection does not comply with the UA's CT Policy (i.e. the
   connection is not CT-qualified), and the UA has not already sent an
   Expect-CT report for this connection, then the UA SHOULD send a
   report to the specified "report-uri" as specified in Section 3.

   The UA MUST ignore any Expect-CT header field not conforming to the
   grammar specified in Section 2.1.

2.3.2.  HTTP-Equiv <meta> Element Attribute

   UAs MUST NOT heed "http-equiv="Expect-CT"" attribute settings on
   "<meta>" elements [W3C.REC-html51-20161101] in received content.

2.3.3.  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),
   the UA MUST note the host as a Known Expect-CT Host, storing the
   host's domain name and its associated Expect-CT directives in non-
   volatile storage.  The domain name and associated Expect-CT
   directives are collectively known as "Expect-CT metadata".

   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, as specified in Section 2.3.4.

   For forward compatibility, the UA MUST ignore any unrecognized
   Expect-CT header directives, while still processing those directives
   it does recognize.  Section 2.1 specifies the directives "enforce",
   "max-age", and "report-uri", but future specifications and
   implementations might use additional directives.

2.3.4.  Storage Model

   Known Expect-CT Hosts are identified only by domain names, and never
   IP addresses.  If the substring matching the host production from the
   Request-URI (of the message to which the host responded)
   syntactically matches the IP-literal or IPv4address productions from




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   Section 3.2.2 of [RFC3986], then the UA MUST NOT note this host as a
   Known Expect-CT Host.

   Otherwise, if the substring does not congruently match an existing
   Known Expect-CT Host's domain name, per the matching procedure
   specified in Section 8.2 of [RFC6797], then the UA MUST add this host
   to the Known Expect-CT Host cache.  The UA caches:

   o  the Expect-CT Host's domain name,

   o  whether the "enforce" directive is present

   o  the Effective Expiration Date, which is the Effective Expect-CT
      Date plus the value of the "max-age" directive.  Alternatively,
      the UA MAY cache enough information to calculate the Effective
      Expiration Date.

   o  the value of the "report-uri" directive, if present.

   If any other metadata from optional or future Expect-CT header
   directives are present in the Expect-CT header, and the UA
   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
   the server set the max-age to the UA's upper limit.  For example, if
   the UA caps max-age at 5,184,000 seconds (60 days), and an Expect-CT
   Host sets a max- age directive of 90 days in its Expect-CT header,
   the UA MAY behave as if the max-age were effectively 60 days.  (One
   way to achieve this behavior is for the UA to simply store a value of
   60 days instead of the 90-day value provided by the Expect-CT host.)

2.4.  Evaluating Expect-CT Connections for CT Compliance

   When a UA connects to a Known Expect-CT Host using a TLS connection,
   if the TLS connection has errors, the UA MUST terminate the
   connection without allowing the user to proceed anyway.  (This
   behavior is the same as that required by [RFC6797].)

   If the 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.  It is acceptable to skip this CT
   compliance check for some hosts according to local policy.  For
   example, a UA may disable CT compliance checks for hosts whose
   validated certificate chain terminates at a user-defined trust



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   anchor, rather than a trust anchor built-in to the UA (or underlying
   platform).

   An Expect-CT Host is "expired" if the effective expiration date
   refers to a date in the past.  The UA MUST ignore any expired Expect-
   CT Hosts in its cache and not treat such hosts as Known Expect-CT
   hosts.

   If a connection to a Known 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 a non-recoverable error.

   If a connection to a Known CT Host violates the UA's CT policy, and
   if the Known Expect-CT Host's Expect-CT metadata includes a "report-
   uri", the UA SHOULD send an Expect-CT report to that "report-uri"
   (Section 3).

   A UA that has previously noted a host as a Known Expect-CT Host MUST
   evaluate CT compliance when setting up the TLS session, before
   beginning an HTTP conversation over the TLS channel.

   If the UA does not evaluate CT compliance, e.g. because the user has
   elected to disable it, or because a presented certificate chain
   chains up to a user-defined trust anchor, UAs SHOULD NOT send Expect-
   CT reports.

3.  Reporting Expect-CT Failure

   When the UA attempts to connect to a Known Expect-CT Host and the
   connection is not CT-qualified, the UA SHOULD report Expect-CT
   failures to the "report-uri", if any, in the Known Expect-CT Host's
   Expect-CT metadata.

   When the UA receives an Expect-CT response header field over a
   connection that is not CT-qualified, if the UA has not already sent
   an Expect-CT report for this connection, then the UA SHOULD report
   Expect-CT failures to the configured "report-uri", if any.

3.1.  Generating a violation report

   To generate a violation report object, the UA constructs a JSON
   object with the following keys and values:

   o  "date-time": the value for this key indicates the time the UA
      observed the CT compliance failure.  The value is a string
      formatted according to Section 5.6, "Internet Date/Time Format",
      of [RFC3339].



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   o  "hostname": the value is the hostname to which the UA made the
      original request that failed the CT compliance check.  The value
      is provided as a string.

   o  "port": the value is the port to which the UA made the original
      request that failed the CT compliance check.  The value is
      provided as an integer.

   o  "effective-expiration-date": the value indicates the Effective
      Expiration Date (see Section 2.3.4) for the Expect-CT Host that
      failed the CT compliance check.  The value is provided as a string
      formatted according to Section 5.6 of [RFC3339] ("Internet Date/
      Time Format").

   o  "served-certificate-chain": the value is the certificate chain as
      served by the Expect-CT Host during TLS session setup.  The value
      is provided as an array of strings, which MUST appear in the order
      that the certificates were served; each string in the array is the
      Privacy-Enhanced Mail (PEM) representation of each X.509
      certificate as described in [RFC7468].

   o  "validated-certificate-chain": the value is the certificate chain
      as constructed by the UA during certificate chain verification.
      (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].

   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:

      *  A "version" key, with an integer value.  The UA MUST set this
         value to "1" if the SCT is in the format defined in Section 3.2
         of [RFC6962] and "2" if it is in the format defined in
         Section 4.6 of [I-D.ietf-trans-rfc6962-bis].

      *  The "status" key, with a string value that the UA MUST set to
         one of the following values: "unknown" (indicating that the UA
         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




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         "invalid" (indicating that the SCT validation failed because
         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 or [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".

      *  The "serialized_sct" key, with a string value.  If the value of
         the "version" key is "1", the UA MUST set this value to the
         base64 encoded [RFC4648] serialized
         "SignedCertificateTimestamp" structure from Section 3.2 of
         [RFC6962].  If the value of the "version" key is "2", the UA
         MUST set this value to the base64 encoded [RFC4648] serialized
         "TransItem" structure representing the SCT, as defined in
         Section 4.6 of [I-D.ietf-trans-rfc6962-bis].

   o  "failure-mode": the value indicates whether the Expect-CT report
      was triggered by an Expect-CT policy in enforce or report-only
      mode.  The value is provided as a string.  The UA MUST set this
      value to "enforce" if the Expect-CT metadata indicates an
      "enforce" configuration, and "report-only" otherwise.

   o  "test-report": the value is set to true if the report is being
      sent by a testing client to verify that the reporting server
      behaves correctly.  The value is provided as a boolean, and MUST
      be set to true if the report serves to test the server's behavior
      and can be discarded.

3.2.  Sending a violation report

   The UA SHOULD report an Expect-CT failure when a connection to a
   Known Expect-CT Host does not comply with the UA's CT Policy and the
   host's Expect-CT metadata contains a "report-uri".  Additionally, the
   UA SHOULD report an Expect-CT failure when it receives an Expect-CT
   header field which contains the "report-uri" directive over a
   connection that does not comply with the UA's CT Policy.

   The steps to report an Expect-CT failure are as follows.

   1.  Prepare a JSON object "report object" with the single key
       "expect-ct-report", whose value is the result of generating a
       violation report object as described in Section 3.1.

   2.  Let "report body" be the JSON stringification of "report object".

   3.  Let "report-uri" be the value of the "report-uri" directive in
       the Expect-CT header field.



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   4.  Send an HTTP POST request to "report-uri" with a "Content-Type"
       header field of "application/expect-ct-report+json", and an
       entity body consisting of "report body".

   The UA MAY perform other operations as part of sending the HTTP POST
   request, for example sending a CORS preflight as part of [FETCH].

3.3.  Receiving a violation report

   Upon receiving an Expect-CT violation report, the report server MUST
   respond with a 2xx (Successful) status code if it can parse the
   request body as valid JSON and recognizes the hostname in the
   "hostname" field of the report.  If the report body cannot be parsed
   or the report server does not expect to receive reports for the
   hostname in the "hostname" field, the report server MUST respond with
   a 4xx (Client Error) status code.

   If the report's "test-report" key is set to true, the server MAY
   discard the report without further processing but MUST still return a
   2xx (Successful) status code.

4.  Security Considerations

   When UAs support the Expect-CT header, it becomes a potential vector
   for hostile header attacks against site owners.  If a site owner uses
   a certificate issued by a certificate authority which does not embed
   SCTs nor serve SCTs via OCSP or TLS extension, a malicious server
   operator or attacker could temporarily reconfigure the host to comply
   with the UA's CT policy, and add the Expect-CT header in enforcing
   mode with a long "max-age".  Implementing user agents would note this
   as an Expect-CT Host (see Section 2.3.3).  After having done this,
   the configuration could then be reverted to not comply with the CT
   policy, prompting failures.  Note this scenario would require the
   attacker to have substantial control over the infrastructure in
   question, being able to obtain different certificates, change server
   software, or act as a man-in-the-middle in connections.

   Site operators could themselves only cure this situation by one of:
   reconfiguring their web server to transmit SCTs using the TLS
   extension defined in Section 6.5 of [I-D.ietf-trans-rfc6962-bis],
   obtaining a certificate from an alternative certificate authority
   which provides SCTs by one of the other methods, or by waiting for
   the user agents' persisted notation of this as an Expect-CT host to
   reach its "max-age".  User agents may choose to implement mechanisms
   for users to cure this situation, as noted in Section 7.






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4.1.  Maximum max-age

   There is a security trade-off in that low maximum values provide a
   narrow window of protection for users that visit the Known Expect-CT
   Host only infrequently, while high maximum values might result in a
   denial of service to a UA in the event of a hostile header attack, or
   simply an error on the part of the site-owner.

   There is probably no ideal maximum for the "max-age" directive.
   Since Expect-CT is primarily a policy-expansion and investigation
   technology rather than an end-user protection, a value on the order
   of 30 days (2,592,000 seconds) may be considered a balance between
   these competing security concerns.

4.2.  Avoiding amplification attacks

   Another kind of hostile header attack uses the "report-uri" mechanism
   on many hosts not currently exposing SCTs as a method to cause a
   denial-of-service to the host receiving the reports.  If some highly-
   trafficked websites emitted a non-enforcing Expect-CT header with a
   "report-uri", implementing UAs' reports could flood the reporting
   host.  It is noted in Section 2.1.1 that UAs should limit the rate at
   which they emit reports, but an attacker may alter the Expect-CT
   header's fields to induce UAs to submit different reports to
   different URIs to still cause the same effect.

5.  Privacy Considerations

   Expect-CT can be used to infer what Certificate Transparency policy
   is in use, by attempting to retrieve specially-configured websites
   which pass one user agents' policies but not another's.  Note that
   this consideration is true of UAs which enforce CT policies without
   Expect-CT as well.

   Additionally, reports submitted to the "report-uri" could reveal
   information to a third party about which webpage is being accessed
   and by which IP address, by using individual "report-uri" values for
   individually-tracked pages.  This information could be leaked even if
   client-side scripting were disabled.

   Implementations must store state about Known Expect-CT Hosts, and
   hence which domains the UA has contacted.

   Violation reports, as noted in Section 3, contain information about
   the certificate chain that has violated the CT policy.  In some
   cases, such as organization-wide compromise of the end-to-end
   security of TLS, this may include information about the interception




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   tools and design used by the organization that the organization would
   otherwise prefer not be disclosed.

   Because Expect-CT causes remotely-detectable behavior, it's advisable
   that UAs offer a way for privacy-sensitive users to clear currently
   noted Expect-CT hosts, and allow users to query the current state of
   Known Expect-CT Hosts.

6.  IANA Considerations

   TBD

7.  Usability Considerations

   When the UA detects a Known Expect-CT Host in violation of the UA's
   CT Policy, users will experience denials of service.  It is advisable
   for UAs to explain the reason why.

8.  Authoring Considerations

8.1.  HTTP Header

   Expect-CT could be specified as a TLS extension or X.509 certificate
   extension instead of an HTTP response header.  Using an HTTP header
   as the mechanism for Expect-CT introduces a layering mismatch: for
   example, the software that terminates TLS and validates Certificate
   Transparency information might know nothing about HTTP.
   Nevertheless, an HTTP header was chosen primarily for ease of
   deployment.  In practice, deploying new certificate extensions
   requires certificate authorities to support them, and new TLS
   extensions require server software updates, including possibly to
   servers outside of the site owner's direct control (such as in the
   case of a third-party CDN).  Ease of deployment is a high priority
   for Expect-CT because it is intended as a temporary transition
   mechanism for user agents that are transitioning to universal
   Certificate Transparency requirements.

9.  Changes

9.1.  Since -02

   o  Add concept of test reports and specify that servers must respond
      with 2xx status codes to valid reports.

   o  Add "failure-mode" key to reports to allow report servers to
      distinguish report-only from enforced failures.





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9.2.  Since -01

   o  Change SCT reporting format to support both RFC 6962 and 6962-bis
      SCTs.

9.3.  Since -00

   o  Editorial changes

   o  Change Content-Type header of reports to 'application/expect-ct-
      report+json'

   o  Update header field syntax to match convention (issue #327)

   o  Reference RFC 6962-bis instead of RFC 6962

10.  References

10.1.  Normative References

   [FETCH]    van Kesteren, A., "Fetch", n.d.,
              <https://fetch.spec.whatwg.org/>.

   [HTML]     Hickson, I., Pieters, S., van Kesteren, A., Jaegenstedt,
              P., and D. Denicola, "HTML", n.d.,
              <https://html.spec.whatwg.org/>.

   [I-D.ietf-trans-rfc6962-bis]
              Laurie, B., Langley, A., Kasper, E., Messeri, E., and R.
              Stradling, "Certificate Transparency Version 2.0", draft-
              ietf-trans-rfc6962-bis-27 (work in progress), October
              2017.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3339]  Klyne, G. and C. Newman, "Date and Time on the Internet:
              Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
              <https://www.rfc-editor.org/info/rfc3339>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.





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   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/info/rfc5246>.

   [RFC6797]  Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
              Transport Security (HSTS)", RFC 6797,
              DOI 10.17487/RFC6797, November 2012,
              <https://www.rfc-editor.org/info/rfc6797>.

   [RFC6962]  Laurie, B., Langley, A., and E. Kasper, "Certificate
              Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
              <https://www.rfc-editor.org/info/rfc6962>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <https://www.rfc-editor.org/info/rfc7230>.

   [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
              RFC 7234, DOI 10.17487/RFC7234, June 2014,
              <https://www.rfc-editor.org/info/rfc7234>.

   [RFC7468]  Josefsson, S. and S. Leonard, "Textual Encodings of PKIX,
              PKCS, and CMS Structures", RFC 7468, DOI 10.17487/RFC7468,
              April 2015, <https://www.rfc-editor.org/info/rfc7468>.

   [W3C.REC-html51-20161101]
              Faulkner, S., Eicholz, A., Leithead, T., and A. Danilo,
              "HTML 5.1", World Wide Web Consortium Recommendation REC-
              html51-20161101, November 2016,
              <https://www.w3.org/TR/2016/REC-html51-20161101>.

10.2.  URIs

   [1] https://lists.w3.org/Archives/Public/ietf-http-wg/

   [2] http://httpwg.github.io/



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   [3] https://github.com/httpwg/http-extensions/labels/expect-ct

Author's Address

   Emily Stark
   Google

   Email: estark@google.com











































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