Expect-CT Extension for HTTPGoogleestark@google.com
Applications and Real-Time
HTTPThis document defines a new HTTP header field, 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.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.This document defines a new HTTP header field that enables UAs to identify web hosts
that expect the presence of Signed Certificate Timestamps (SCTs)
in future Transport Layer Security (TLS)
connections.Web hosts that serve the Expect-CT HTTP 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 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.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 .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” is an abbreviation for “Certificate Transparency Qualified”.“CT Policy” is an abbreviation for “Certificate Transparency Policy”.“Effective Expect-CT Date” is the time at which a UA observed a valid
Expect-CT header field for a given host.“Expect-CT Host” is a conformant host implementing the HTTP server aspects of
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
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
.“Unknown Expect-CT Host” is an Expect-CT Host that the UA has not noted.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
(). describes the syntax (Augmented Backus-Naur Form) of the header field,
using the grammar defined in and the rules defined in
Section 3.2 of .Optional white space (OWS) is used as defined in Section 3.2.3 of
. token and quoted-string are used as defined in Section 3.2.6
of .The directives defined in this specification are described below. The overall
requirements for directives are:The order of appearance of directives is not significant.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.Directive names are case insensitive.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.If a header field contains any directive(s) the UA does not recognize, the
UA MUST ignore those directives.If the Expect-CT header field otherwise satisfies the above requirements (1
through 5), the UA MUST process the directives it recognizes.The OPTIONAL report-uri directive indicates the URI to which the UA SHOULD
report Expect-CT failures (). The UA POSTs the reports
to the given URI as described in .The report-uri directive is REQUIRED to have a directive value, for which the
syntax is defined in .absolute-URI is defined in Section 4.3 of .Hosts may set report-uris 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.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 ) 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.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
.delta-seconds is used as defined in Section 1.2.1 of .The following examples demonstrate valid Expect-CT response header fields: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.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
.Establishing a given host as an Expect-CT Host, in the context of a given UA,
is accomplished as follows: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.Through other mechanisms, such as a client-side preloaded Expect-CT Host
list.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
field received in an HTTP response conveyed over non-secure transport.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 .If the UA receives, over a secure transport, an HTTP response that includes an
Expect-CT header field conforming to the grammar specified in
, 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.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
), orUpdate 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
.The UA MUST ignore any Expect-CT header field not conforming to the grammar
specified in .UAs MUST NOT heed http-equiv="Expect-CT" attribute settings on <meta>
elements in received content.Upon receipt of the Expect-CT response header field over an error-free TLS
connection (including the validation adding in ), 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 field, as specified in
.For forward compatibility, the UA MUST ignore any unrecognized Expect-CT header
field directives, while still processing those directives it does
recognize. specifies the directives enforce,
max-age, and report-uri, but future specifications and implementations might
use additional directives.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 , 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 , 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 presentthe 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 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.)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
.)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).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 ().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.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.To generate a violation report object, the UA constructs a JSON object with the
following keys and values:“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 .“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.“effective-expiration-date”: the value indicates the Effective Expiration Date
(see ) 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
(“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 .“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 .“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 and 2 if
it is in the format defined in Section 4.6 of
.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 or Section 8.2.3 of
), or “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 and Section 6 of
. 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
serialized SignedCertificateTimestamp structure from Section 3.2 of
. If the value of the “version” key is 2, the UA MUST set this
value to the base64 encoded serialized TransItem structure
representing the SCT, as defined in Section 4.6 of
.“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”: 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.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.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 .Let report body be the JSON stringification of report object.Let report-uri be the value of the report-uri directive in the Expect-CT
header field.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 .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.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 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 field in
enforcing mode with a long max-age. Implementing user agents would note this
as an Expect-CT Host (see ). 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 , 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
.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.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 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.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 , 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.TBDWhen 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.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 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.Certificate Transparency Version 2.0This document describes version 2.0 of the Certificate Transparency (CT) protocol for publicly logging the existence of Transport Layer Security (TLS) server certificates as they are issued or observed, in a manner that allows anyone to audit certification authority (CA) activity and notice the issuance of suspect certificates as well as to audit the certificate logs themselves. The intent is that eventually clients would refuse to honor certificates that do not appear in a log, effectively forcing CAs to add all issued certificates to the logs. Logs are network services that implement the protocol operations for submissions and queries that are defined in this document.Key words for use in RFCs to Indicate Requirement LevelsIn many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and RoutingThe Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document provides an overview of HTTP architecture and its associated terminology, defines the "http" and "https" Uniform Resource Identifier (URI) schemes, defines the HTTP/1.1 message syntax and parsing requirements, and describes related security concerns for implementations.Augmented BNF for Syntax Specifications: ABNFInternet technical specifications often need to define a formal syntax. Over the years, a modified version of Backus-Naur Form (BNF), called Augmented BNF (ABNF), has been popular among many Internet specifications. The current specification documents ABNF. It balances compactness and simplicity with reasonable representational power. The differences between standard BNF and ABNF involve naming rules, repetition, alternatives, order-independence, and value ranges. This specification also supplies additional rule definitions and encoding for a core lexical analyzer of the type common to several Internet specifications. [STANDARDS-TRACK]Uniform Resource Identifier (URI): Generic SyntaxA Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource. This specification defines the generic URI syntax and a process for resolving URI references that might be in relative form, along with guidelines and security considerations for the use of URIs on the Internet. The URI syntax defines a grammar that is a superset of all valid URIs, allowing an implementation to parse the common components of a URI reference without knowing the scheme-specific requirements of every possible identifier. This specification does not define a generative grammar for URIs; that task is performed by the individual specifications of each URI scheme. [STANDARDS-TRACK]Hypertext Transfer Protocol (HTTP/1.1): CachingThe Hypertext Transfer Protocol (HTTP) is a stateless \%application- level protocol for distributed, collaborative, hypertext information systems. This document defines HTTP caches and the associated header fields that control cache behavior or indicate cacheable response messages.HTTP Strict Transport Security (HSTS)This specification defines a mechanism enabling web sites to declare themselves accessible only via secure connections and/or for users to be able to direct their user agent(s) to interact with given sites only over secure connections. This overall policy is referred to as HTTP Strict Transport Security (HSTS). The policy is declared by web sites via the Strict-Transport-Security HTTP response header field and/or by other means, such as user agent configuration, for example. [STANDARDS-TRACK]Date and Time on the Internet: TimestampsTextual Encodings of PKIX, PKCS, and CMS StructuresThis document describes and discusses the textual encodings of the Public-Key Infrastructure X.509 (PKIX), Public-Key Cryptography Standards (PKCS), and Cryptographic Message Syntax (CMS). The textual encodings are well-known, are implemented by several applications and libraries, and are widely deployed. This document articulates the de facto rules by which existing implementations operate and defines them so that future implementations can interoperate.Certificate TransparencyThis document describes an experimental protocol for publicly logging the existence of Transport Layer Security (TLS) certificates as they are issued or observed, in a manner that allows anyone to audit certificate authority (CA) activity and notice the issuance of suspect certificates as well as to audit the certificate logs themselves. The intent is that eventually clients would refuse to honor certificates that do not appear in a log, effectively forcing CAs to add all issued certificates to the logs.Logs are network services that implement the protocol operations for submissions and queries that are defined in this document.The Base16, Base32, and Base64 Data EncodingsThis document describes the commonly used base 64, base 32, and base 16 encoding schemes. It also discusses the use of line-feeds in encoded data, use of padding in encoded data, use of non-alphabet characters in encoded data, use of different encoding alphabets, and canonical encodings. [STANDARDS-TRACK]HTML - Living StandardWHATWGFetch - Living StandardWHATWGHTML 5.2The Transport Layer Security (TLS) Protocol Version 1.2This document specifies Version 1.2 of the Transport Layer Security (TLS) protocol. The TLS protocol provides communications security over the Internet. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. [STANDARDS-TRACK]Editorial changesAdd concept of test reports and specify that servers must respond with 2xx
status codes to valid reports.Add “failure-mode” key to reports to allow report servers to distinguish
report-only from enforced failures.Change SCT reporting format to support both RFC 6962 and 6962-bis SCTs.Editorial changesChange Content-Type header of reports to ‘application/expect-ct-report+json’Update header field syntax to match convention (issue #327)Reference RFC 6962-bis instead of RFC 6962