Expect-CT Extension for HTTP
draft-ietf-httpbis-expect-ct-00

HTTP Working Group                                              E. Stark
Internet-Draft                                                    Google
Intended status: Experimental                           February 8, 2017
Expires: August 12, 2017

                      Expect-CT Extension for HTTP
                    draft-ietf-httpbis-expect-ct-00

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/ .

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

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
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Copyright Notice

   Copyright (c) 2017 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
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   publication of this document.  Please review these documents
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   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 . . . . . . . . . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Server and Client Behavior  . . . . . . . . . . . . . . . . .   4
     2.1.  Response Header Field Syntax  . . . . . . . . . . . . . .   4
       2.1.1.  The report-uri Directive  . . . . . . . . . . . . . .   5
       2.1.2.  The enforce Directive . . . . . . . . . . . . . . . .   6
       2.1.3.  The max-age Directive . . . . . . . . . . . . . . . .   6
     2.2.  Server Processing Model . . . . . . . . . . . . . . . . .   7
       2.2.1.  HTTP-over-Secure-Transport Request Type . . . . . . .   7
       2.2.2.  HTTP Request Type . . . . . . . . . . . . . . . . . .   7
     2.3.  User Agent Processing Model . . . . . . . . . . . . . . .   8
       2.3.1.  Expect-CT Header Field Processing . . . . . . . . . .   8
       2.3.2.  Noting an Expect-CT Host - Storage Model  . . . . . .   9
       2.3.3.  HTTP-Equiv <meta> Element Attribute . . . . . . . . .  10
     2.4.  Noting Expect-CT  . . . . . . . . . . . . . . . . . . . .  10
     2.5.  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
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
     4.1.  Maximum max-age . . . . . . . . . . . . . . . . . . . . .  14
     4.2.  Avoiding amplification attacks  . . . . . . . . . . . . .  15
   5.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  15
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   7.  Usability Considerations  . . . . . . . . . . . . . . . . . .  16
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  16
     8.2.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  17

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   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  17

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) [RFC6962] 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.

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].

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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.

   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 2.4 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 [RFC2616].

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

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 2.5).

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   Figure 1 describes the syntax (Augmented Backus-Naur Form) of the
   header field, using the grammar defined in RFC 5234 [RFC5234] and the
   rules defined in Section 3.2 of RFC 7230 [RFC7230].

   Expect-CT-Directives = directive *( OWS ";" OWS directive )
   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
   RFC 7230 [RFC7230]. "token" and "quoted-string" are used as defined
   in Section 3.2.6 of RFC 7230 [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.

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 2.5).  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.

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   report-uri-value = absolute-URI

            Figure 2: Syntax of the report-uri directive value

   "absolute-URI" is defined in Section 4.3 of RFC 3986 [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 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
   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.

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   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 RFC 7234
   [RFC7234].

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.

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.

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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 2.4), 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
      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.

   If a UA receives more than one Expect-CT header field in an HTTP
   response message over secure transport, then the UA MUST process only
   the first Expect-CT header field.

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

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2.3.2.  Noting an Expect-CT Host - Storage Model

   The "effective Expect-CT date" of a Known Expect-CT Host is the time
   that the UA observed a valid Expect-CT header for the host.  The
   "effective expiration date" of a Known Expect-CT Host is the
   effective Expect-CT date plus the max-age.  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.

   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
   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, or enough information to calculate
      it (the effective Expect-CT date and the value of the "max-age"
      directive),

   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 a Pinned 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.)

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2.3.3.  HTTP-Equiv <meta> Element Attribute

   UAs MUST NOT heed "http-equiv="Expect-CT"" attribute settings on
   "<meta>" elements [W3C.REC-html401-19991224] in received content.

2.4.  Noting Expect-CT

   Upon receipt of the Expect-CT response header field, the UA notes 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".

   The UA MUST note a host as a Known Expect-CT Host if and only if it
   received the Expect-CT response header field over an error-free TLS
   connection, including the validation added in Section 2.5.

   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.

   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.

2.5.  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
   anchor, rather than a trust anchor built-in to the UA (or underlying
   platform).

   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.

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   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
   message of the following form:

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   {
     "date-time": date-time,
     "hostname": hostname,
     "port": port,
     "effective-expiration-date": expiration-date,
     "served-certificate-chain": [ (MUST be in the order served)
       pem1, ... pemN
     ],
     "validated-certificate-chain": [
       pem1, ... pemN
     ],
     "scts": [
       sct1, ... sctN
     ]
   }

            Figure 4: JSON format of a violation report object

   Whitespace outside of quoted strings is not significant.  The key/
   value pairs may appear in any order, but each MUST appear only once.

   The "date-time" indicates the time the UA observed the CT compliance
   failure.  It is provided as a string formatted according to
   Section 5.6, "Internet Date/Time Format", of [RFC3339].

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

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

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

   The "served-certificate-chain" is the certificate chain, as served by
   the Expect-CT Host during TLS session setup.  It is provided as an
   array of strings, which MUST appear in the order that the
   certificates were served; each string "pem1", ... "pemN" is the
   Privacy-Enhanced Mail (PEM) representation of each X.509 certificate
   as described in [RFC7468].

   The "validated-certificate-chain" is the certificate chain, as
   constructed by the UA during certificate chain verification.  (This
   may differ from the "served-certificate-chain".)  It is provided as
   an array of strings, which MUST appear in the order matching the

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   chain that the UA validated; each string "pem1", ... "pemN" is the
   Privacy-Enhanced Mail (PEM) representation of each X.509 certificate
   as described in [RFC7468].

   The "scts" are JSON messages representing the SCTs (if any) that the
   UA received for the Expect-CT host and their validation statuses.
   The format of "sct1", ... "sctN" is shown in Figure 5.  The SCTs may
   appear in any order.

   {
     "sct": sct,
     "status": status,
     "source": source
   }

                  Figure 5: JSON format of an SCT object

   The "sct" is as defined in Section 4.1 of RFC 6962 [RFC6962].

   The "status" is a string 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 "invalid" (indicating that
   the SCT validation failed because of, e.g., a bad signature).

   The "source" is a string that indicates from where the UA obtained
   the SCT, as defined in Section 3.3 of [RFC6962].  The UA MUST set
   "source" to one of the following values: "tls-extension", "ocsp", or
   "embedded".

3.2.  Sending a violation report

   When an Expect-CT header field contains the "report-uri" directive,
   and the connection does not comply with the UA's CT Policy, or when
   the UA connects to a Known Expect-CT Host with Expect-CT metadata
   that contains a "report-uri", the UA SHOULD report the failure 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 Figure 4.

   2.  Let "report body" by 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.  Queue a task [4] to fetch [5] "report-uri", with the synchronous
       flag not set, using HTTP method "POST", with a "Content-Type"
       header field of "application/expect-ct-report", and an entity
       body consisting of "report body".

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 2.4).  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 3.3 of [RFC6962], 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.

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.

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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
   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

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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.  References

8.1.  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,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616,
              DOI 10.17487/RFC2616, June 1999,
              <http://www.rfc-editor.org/info/rfc2616>.

   [RFC3339]  Klyne, G. and C. Newman, "Date and Time on the Internet:
              Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
              <http://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,
              <http://www.rfc-editor.org/info/rfc3986>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <http://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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc6797>.

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

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   [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,
              <http://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,
              <http://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, <http://www.rfc-editor.org/info/rfc7468>.

   [W3C.REC-html401-19991224]
              Raggett, D., Hors, A., and I. Jacobs, "HTML 4.01
              Specification", World Wide Web Consortium Recommendation
              REC-html401-19991224, December 1999,
              <http://www.w3.org/TR/1999/REC-html401-19991224>.

8.2.  URIs

   [1] https://html.spec.whatwg.org/#queue-a-task

   [2] https://fetch.spec.whatwg.org/#fetching

Author's Address

   Emily Stark
   Google

   Email: estark@google.com

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