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Expect-CT Extension for HTTP
RFC 9163

Document Type RFC - Experimental (June 2022)
Author estark@google.com
Last updated 2022-06-07
Replaces draft-stark-expect-ct
Stream Internet Engineering Task Force (IETF)
Formats
Reviews
Stream WG state Submitted to IESG for Publication
Document shepherd Mark Nottingham
Shepherd write-up Show Last changed 2018-07-31
IESG IESG state RFC 9163 (Experimental)
Action Holders
(None)
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Alexey Melnikov
Send notices to Mark Nottingham <mnot@mnot.net>
IANA IANA review state IANA OK - Actions Needed
IANA action state RFC-Ed-Ack
RFC 9163


Internet Engineering Task Force (IETF)                          E. Stark
Request for Comments: 9163                                        Google
Category: Experimental                                         June 2022
ISSN: 2070-1721

                      Expect-CT Extension for HTTP

Abstract

   This document defines a new HTTP header field named "Expect-CT",
   which allows web host operators to instruct user agents (UAs) 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 (CT)
   deployments.  Further, web host operators can use Expect-CT to ensure
   that if a UA that supports Expect-CT accepts a misissued certificate,
   that certificate will be discoverable in Certificate Transparency
   logs.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are candidates for any level of
   Internet Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9163.

Copyright Notice

   Copyright (c) 2022 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 Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Requirements Language
     1.2.  Terminology
   2.  Server and Client Behavior
     2.1.  Response Header Field Syntax
       2.1.1.  The report-uri Directive
       2.1.2.  The enforce Directive
       2.1.3.  The max-age Directive
       2.1.4.  Examples
     2.2.  Host Processing Model
       2.2.1.  HTTP-over-Secure-Transport Request Type
       2.2.2.  HTTP Request Type
     2.3.  User Agent Processing Model
       2.3.1.  Missing or Malformed Expect-CT Header Fields
       2.3.2.  Expect-CT Header Field Processing
       2.3.3.  Reporting
     2.4.  Evaluating Expect-CT Connections for CT Compliance
       2.4.1.  Skipping CT Compliance Checks
   3.  Reporting Expect-CT Failure
     3.1.  Generating a Violation Report
     3.2.  Sending a Violation Report
     3.3.  Receiving a Violation Report
   4.  Usability Considerations
   5.  Authoring Considerations
   6.  Privacy Considerations
   7.  Security Considerations
     7.1.  Hostile Header Attacks
     7.2.  Maximum max-age
     7.3.  Amplification Attacks
   8.  IANA Considerations
     8.1.  Header Field Registry
     8.2.  Media Types Registry
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Author's Address

1.  Introduction

   This document defines a new HTTP header field ([RFC9110],
   Section 6.3) that enables UAs to identify web hosts that expect the
   presence of Signed Certificate Timestamps (SCTs) [RFC9162] in
   subsequent Transport Layer Security (TLS) [RFC8446] connections.

   Web hosts that serve the Expect-CT header field 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 precautions by using the reporting
   feature and gradually increasing the time interval during which the
   UA regards 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.

   Expect-CT is similar to HTTP Strict Transport Security (HSTS)
   [RFC6797] and HTTP Public Key Pinning (HPKP) [RFC7469].  HSTS allows
   websites to declare themselves accessible only via secure
   connections, and HPKP allows websites to declare their cryptographic
   identifies.  Similarly, Expect-CT allows websites to declare
   themselves accessible only via connections that are compliant with CT
   Policy.

   This Expect-CT specification is compatible with [RFC6962] and
   [RFC9162], but not necessarily with future versions of Certificate
   Transparency.  UAs will ignore Expect-CT header fields from web hosts
   that use future versions of Certificate Transparency, unless a future
   version of this document specifies how they should be processed.

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
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

1.2.  Terminology

   Terminology is defined in this section.

   "Certificate Transparency Policy"
      A policy defined by the UA concerning the number, sources, and
      delivery mechanisms of Signed Certificate Timestamps that are
      associated with 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"
      An abbreviation for "Certificate Transparency Qualified".

   "CT Policy"
      An abbreviation for "Certificate Transparency Policy".

   "Effective Expect-CT Date"
      The time at which a UA observed a valid Expect-CT header field for
      a given host.

   "Expect-CT Host"
      A conformant host implementing the HTTP server aspects of Expect-
      CT.  This means that an Expect-CT Host returns the Expect-CT
      response header field in its HTTP response messages sent over
      secure transport.  The term "host" is equivalent to "server" in
      this specification.

   "Known Expect-CT Host"
      An Expect-CT Host that the UA has noted as such.  See
      Section 2.3.2.1 for particulars.

   "User Agent (UA)"
      For the purposes of this specification, a UA is an HTTP client
      application typically actively manipulated by a user [RFC9110].

   "Unknown Expect-CT Host"
      An Expect-CT Host that the UA has not noted.

2.  Server and Client Behavior

2.1.  Response Header Field Syntax

   The Expect-CT response header field is a new field defined in this
   specification.  It is used by a server to indicate that UAs should
   evaluate connections to the host emitting the header field 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 5 of [RFC9110].  The "#" ABNF extension is
   specified in Section 5.6.1 of [RFC9110].

   Expect-CT           = 1#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

   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 does 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), and Expect-CT is not disabled for
       local policy reasons (as discussed in Section 2.4.1), the UA MUST
       process the directives it recognizes.

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 = (DQUOTE absolute-URI DQUOTE) / absolute-URI

             Figure 2: Syntax of the report-uri Directive Value

   The 'report-uri-value' MUST be quoted if it contains any character
   not allowed in 'token'.

   absolute-URI is defined in Section 4.3 of [RFC3986].

   UAs MUST ignore any report-uri that does not use the HTTPS scheme.
   UAs MUST check Expect-CT compliance when the host in the report-uri
   is a Known Expect-CT Host; similarly, UAs MUST apply HSTS [RFC6797]
   if the host in the report-uri is a Known HSTS Host.

   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.

   Note that the report-uri need not necessarily be in the same Internet
   domain or web origin as the host being reported about.  Hosts are in
   fact encouraged to use a separate host as the report-uri so that CT
   failures on the Expect-CT Host do not prevent reports from being
   sent.

   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 in the same web-browsing session.

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, signaling to the UA that both
   compliance to the CT Policy should be enforced and 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.

   If a response contains an Expect-CT header field, then the response
   MUST contain an Expect-CT header field with a max-age directive.  (A
   max-age directive need not appear in every Expect-CT header field in
   the response.)  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 1.3 of [RFC9111].

2.1.4.  Examples

   The following three examples demonstrate valid Expect-CT response
   header fields (where the second splits the directives into two field
   instances):

   Expect-CT: max-age=86400, enforce

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

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

        Figure 4: Examples of Valid Expect-CT ResponseHeader Fields

2.2.  Host 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.

2.2.1.  HTTP-over-Secure-Transport Request Type

   An Expect-CT Host includes an Expect-CT header field in its response.
   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) a 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.

2.3.  User Agent Processing Model

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

   The UA stores Known Expect-CT Hosts and their associated Expect-CT
   directives.  This data is collectively known as a host's "Expect-CT
   metadata".

2.3.1.  Missing or Malformed Expect-CT Header Fields

   If an HTTP response does not include an Expect-CT header field that
   conforms to the grammar specified in Section 2.1, then the UA MUST
   NOT update any Expect-CT metadata.

2.3.2.  Expect-CT Header Field Processing

   If the UA receives an HTTP response over a secure transport 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 field was received for compliance with the UA's CT
   Policy, and then process the Expect-CT header field as follows.  UAs
   MUST ignore any Expect-CT header field received in an HTTP response
   conveyed over non-secure transport.

   If the connection does not comply with the UA's CT Policy (i.e., the
   connection is not CT qualified), then the UA MUST NOT update any
   Expect-CT metadata.  If the header field includes a report-uri
   directive, the UA SHOULD send a report to the specified report-uri
   (Section 2.3.3).

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

   *  Note the host as a Known Expect-CT Host if it is not already so
      noted (see Section 2.3.2.1) or

   *  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 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 a UA receives an Expect-CT header field over a CT-compliant
   connection that uses a version of Certificate Transparency other than
   [RFC6962] or [RFC9162], the UA MUST ignore the Expect-CT header field
   and clear any Expect-CT metadata associated with the host.

2.3.2.1.  Noting Expect-CT

   Upon receipt of the Expect-CT response header field over an error-
   free TLS connection (with X.509 certificate chain validation as
   described in [RFC5280], as well as the validation described in
   Section 2.4 of this document), 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.

   To note a host as a Known Expect-CT Host, the UA MUST set its Expect-
   CT metadata in its Known Expect-CT Host cache (as specified in
   Section 2.3.2.2), using the metadata given in the most recently
   received valid Expect-CT header field.

   For forward compatibility, the UA MUST ignore any unrecognized
   Expect-CT header field 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.

2.3.2.2.  Storage Model

   If the substring matching the host production from the Request-URI
   (of the message to which the host responded) does not exactly match
   an existing Known Expect-CT Host's domain name, per the matching
   procedure for a Congruent Match specified in Section 8.2 of
   [RFC6797], then the UA MUST add this host to the Known Expect-CT Host
   cache.  The UA caches:

   *  the Expect-CT Host's domain name.

   *  whether the enforce directive is present.

   *  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.  The Effective Expiration Date is calculated from
      when the UA observed the Expect-CT header field and is independent
      of when the response was generated.

   *  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 field, 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
   field, 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.)

2.3.3.  Reporting

   If the UA receives, over a secure transport, an HTTP response that
   includes an Expect-CT header field with 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.

2.4.  Evaluating Expect-CT Connections for CT Compliance

   When a UA sets up a TLS connection, the UA determines whether the
   host is a Known Expect-CT Host according to its Known Expect-CT Host
   cache.  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.

   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.  However, the check can be skipped for local
   policy reasons (as discussed in Section 2.4.1) or in the event that
   other checks cause the UA to terminate the connection before CT
   compliance is evaluated.  For example, a Public Key Pinning failure
   [RFC7469] could cause the UA to terminate the connection before CT
   compliance is checked.  Similarly, if the UA 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 evaluated for a Known Expect-CT Host, the UA
   MUST evaluate compliance when setting up the TLS session, before
   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.  The UA MAY
   allow the user to bypass the error unless connection errors should
   have no user recourse due to other policies in effect (such as HSTS,
   as described in Section 12.1 of [RFC6797]).

   If a connection to a Known Expect-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).

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
   compliance checks for hosts whose validated certificate chain
   terminates at a user-defined trust anchor rather than a trust anchor
   built in to the UA (or underlying platform).

   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
   [RFC8259] object with the following keys and values:

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

   "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.

   "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.

   "scheme"
      (optional) The value is the scheme with which the UA made the
      original request that failed the CT compliance check.  The value
      is provided as a string.  This key is optional and is assumed to
      be "https" if not present.

   "effective-expiration-date"
      The value indicates the Effective Expiration Date (see
      Section 2.3.2.2) for the Expect-CT Host that failed the CT
      compliance check, in UTC.  The value is provided as a string
      formatted according to Section 5.6 of [RFC3339], "Internet Date/
      Time Format".

   "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].

   "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 PEM
      representation of each X.509 certificate as described in
      [RFC7468].  The first certificate in the chain represents the end-
      entity certificate being verified.  UAs that build certificate
      chains in more than one way during the validation process SHOULD
      send the last chain built.

   "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] or 2 if it is in the format defined in Section 4.5
         of [RFC9162].

   *     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 8.1.3 of [RFC9162]); or "invalid"
         (indicating that the SCT validation failed because of a bad
         signature or an invalid timestamp).

   *     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 [RFC9162].  The UA MUST set the value to one
         of the following: "tls-extension", "ocsp", or "embedded".
         These correspond to the three methods of delivering SCTs in the
         TLS handshake that are described in Section 3.3 of [RFC6962].

   *     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].  The base64 encoding
         is defined in Section 4 of [RFC4648].  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 4.5 of [RFC9162].

   "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.

   "test-report"
      (optional) The value is set to true if the report is being sent by
      a testing client to verify that the report 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 Expect-CT failures for Known Expect-CT Hosts:
   that is, 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 Expect-CT failures for hosts for
   which it does not have any stored Expect-CT metadata; that is, when
   the UA connects to a host and receives an Expect-CT header field that
   contains the report-uri directive, the UA SHOULD report an Expect-CT
   failure if the connection 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.

   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, such as sending a Cross-Origin Resource Sharing (CORS)
   preflight as part of [FETCH].

   Future versions of this specification may need to modify or extend
   the Expect-CT report format.  They may do so by defining a new top-
   level key to contain the report, replacing the "expect-ct-report"
   key.  Section 3.3 defines how report servers should handle report
   formats that they do not support.

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, the report conforms to the format
   described in Section 3.1, and it recognizes the scheme, hostname, and
   port in the "scheme", "hostname", and "port" fields of the report.
   If the report body cannot be parsed or does not conform to the format
   described in Section 3.1, or the report server does not expect to
   receive reports for the scheme, hostname, or port in the report, then
   the report server MUST respond with a 400 Bad Request status code.

   As described in Section 3.2, future versions of this specification
   may define new report formats that are sent with a different top-
   level key.  If the report server does not recognize the report
   format, the report server MUST respond with a 501 Not Implemented
   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.  If the "test-report" key is absent or
   set to false, the server SHOULD store the report for processing and
   analysis by the owner of the Expect-CT Host.

4.  Usability Considerations

   When the UA detects a Known Expect-CT Host in violation of the UA's
   CT Policy, end users will experience denials of service.  It is
   advisable for UAs to explain to users why they cannot access the
   Expect-CT Host, e.g., in a user interface that explains that the
   host's certificate cannot be validated.

5.  Authoring Considerations

   Expect-CT could be specified as a TLS extension or X.509 certificate
   extension instead of an HTTP response header field.  Using an HTTP
   header field 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 field 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 Content Delivery Network (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.

6.  Privacy Considerations

   Expect-CT can be used to infer what Certificate Transparency Policy a
   UA is using by attempting to retrieve specially configured websites
   that pass one user agent's policies but not another's.  Note that
   this consideration is true of UAs that enforce CT policies without
   Expect-CT as well.

   Additionally, reports submitted to the report-uri could reveal
   information to a third party about which web page 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 store state about Known Expect-CT Hosts and, hence,
   which domains the UA has contacted.  Implementations may choose to
   not store this state subject to local policy (e.g., in the private
   browsing mode of a web browser).

   Violation reports, as noted in Section 3, contain information about
   the certificate chain that has violated the CT Policy.  In some
   cases, such as an organization-wide compromise of the end-to-end
   security of TLS, this may include information about the interception
   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 end users to clear
   currently noted Expect-CT Hosts and allow users to query the current
   state of Known Expect-CT Hosts.

7.  Security Considerations

7.1.  Hostile Header Attacks

   When UAs support the Expect-CT header field, it becomes a potential
   vector for hostile header attacks against site owners.  If a site
   owner uses a certificate issued by a certificate authority that does
   not embed SCTs nor serve SCTs via the Online Certificate Status
   Protocol (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 field in enforcing mode
   with a long max-age.  Implementing user agents would note this as an
   Expect-CT Host (see Section 2.3.2.1).  After having done this, the
   configuration could then be reverted to not comply with the CT
   Policy, prompting failures.  Note that 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 can mitigate this situation by one of the following:
   reconfiguring their web server to transmit SCTs using the TLS
   extension defined in Section 6.5 of [RFC9162]; obtaining a
   certificate from an alternative certificate authority that provides
   SCTs by one of the other methods; or by waiting for the user agent's
   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 4.

7.2.  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.

7.3.  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 field
   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 fields to induce UAs to submit different reports to
   different URIs to still cause the same effect.

8.  IANA Considerations

8.1.  Header Field Registry

   This document registers the "Expect-CT" header field in the
   "Hypertext Transfer Protocol (HTTP) Field Name Registry" registry
   located at <https://www.iana.org/assignments/http-fields>.

   Header field name:  Expect-CT

   Applicable protocol:  http

   Status:  permanent

   Author/Change controller:  IETF

   Specification document(s):  This document

   Related information:  (empty)

8.2.  Media Types Registry

   This document registers the application/expect-ct-report+json media
   type (which uses the suffix established in [RFC6839]) for Expect-CT
   violation reports in the "Media Types" registry as follows.

   Type name:  application

   Subtype name:  expect-ct-report+json

   Required parameters:  n/a

   Optional parameters:  n/a

   Encoding considerations:  binary

   Security considerations:  See Section 7

   Interoperability considerations:  n/a

   Published specification:  This document

   Applications that use this media type:  UAs that implement
      Certificate Transparency compliance checks and reporting

   Additional information:

      Deprecated alias names for this type: n/a

      Magic number(s): n/a

      File extension(s): n/a

      Macintosh file type code(s): n/a

   Person & email address to contact for further information:
      Emily Stark (estark@google.com)

   Intended usage:  COMMON

   Restrictions on usage:  none

   Author:  Emily Stark (estark@google.com)

   Change controller:  IETF

9.  References

9.1.  Normative References

   [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>.

   [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>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/info/rfc5280>.

   [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>.

   [RFC6839]  Hansen, T. and A. Melnikov, "Additional Media Type
              Structured Syntax Suffixes", RFC 6839,
              DOI 10.17487/RFC6839, January 2013,
              <https://www.rfc-editor.org/info/rfc6839>.

   [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>.

   [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>.

   [RFC7469]  Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
              Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April
              2015, <https://www.rfc-editor.org/info/rfc7469>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.

   [RFC9110]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,
              <https://www.rfc-editor.org/info/rfc9110>.

   [RFC9111]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Caching", STD 98, RFC 9111,
              DOI 10.17487/RFC9111, June 2022,
              <https://www.rfc-editor.org/info/rfc9111>.

   [RFC9162]  Laurie, B., Messeri, E., and R. Stradling, "Certificate
              Transparency Version 2.0", RFC 9162, DOI 10.17487/RFC9162,
              December 2021, <https://www.rfc-editor.org/info/rfc9162>.

9.2.  Informative References

   [FETCH]    WHATWG, "Fetch - Living Standard",
              <https://fetch.spec.whatwg.org>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

Author's Address

   Emily Stark
   Google
   Email: estark@google.com