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rfc:rfc7469

Internet Engineering Task Force (IETF) C. Evans Request for Comments: 7469 C. Palmer Category: Standards Track R. Sleevi ISSN: 2070-1721 Google, Inc.

                                                            April 2015
               Public Key Pinning Extension for HTTP

Abstract

 This document defines a new HTTP header that allows web host
 operators to instruct user agents to remember ("pin") the hosts'
 cryptographic identities over a period of time.  During that time,
 user agents (UAs) will require that the host presents a certificate
 chain including at least one Subject Public Key Info structure whose
 fingerprint matches one of the pinned fingerprints for that host.  By
 effectively reducing the number of trusted authorities who can
 authenticate the domain during the lifetime of the pin, pinning may
 reduce the incidence of man-in-the-middle attacks due to compromised
 Certification Authorities.

Status of This Memo

 This is an Internet Standards Track document.
 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).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7469.

Evans, et al. Standards Track [Page 1] RFC 7469 Public Key Pinning Extension for HTTP April 2015

Copyright Notice

 Copyright (c) 2015 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
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Evans, et al. Standards Track [Page 2] RFC 7469 Public Key Pinning Extension for HTTP April 2015

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   5
 2.  Server and Client Behavior  . . . . . . . . . . . . . . . . .   5
   2.1.  Response Header Field Syntax  . . . . . . . . . . . . . .   5
     2.1.1.  The Pin Directive . . . . . . . . . . . . . . . . . .   6
     2.1.2.  The max-age Directive . . . . . . . . . . . . . . . .   7
     2.1.3.  The includeSubDomains Directive . . . . . . . . . . .   7
     2.1.4.  The report-uri Directive  . . . . . . . . . . . . . .   7
     2.1.5.  Examples  . . . . . . . . . . . . . . . . . . . . . .   8
   2.2.  Server Processing Model . . . . . . . . . . . . . . . . .   9
     2.2.1.  HTTP-over-Secure-Transport Request Type . . . . . . .   9
     2.2.2.  HTTP Request Type . . . . . . . . . . . . . . . . . .   9
   2.3.  User Agent Processing Model . . . . . . . . . . . . . . .  10
     2.3.1.  Public-Key-Pins Response Header Field Processing  . .  10
     2.3.2.  Interaction of Public-Key-Pins and Public-Key-Pins-
             Report-Only . . . . . . . . . . . . . . . . . . . . .  11
     2.3.3.  Noting a Pinned Host - Storage Model  . . . . . . . .  11
     2.3.4.  HTTP-Equiv <Meta> Element Attribute . . . . . . . . .  13
   2.4.  Semantics of Pins . . . . . . . . . . . . . . . . . . . .  13
   2.5.  Noting Pins . . . . . . . . . . . . . . . . . . . . . . .  14
   2.6.  Validating Pinned Connections . . . . . . . . . . . . . .  15
   2.7.  Interactions with Preloaded Pin Lists . . . . . . . . . .  16
   2.8.  Pinning Self-Signed End Entities  . . . . . . . . . . . .  16
 3.  Reporting Pin Validation Failure  . . . . . . . . . . . . . .  16
 4.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
   4.1.  Maximum max-age . . . . . . . . . . . . . . . . . . . . .  19
   4.2.  Using includeSubDomains Safely  . . . . . . . . . . . . .  20
   4.3.  Backup Pins . . . . . . . . . . . . . . . . . . . . . . .  21
   4.4.  Interactions With Cookie Scoping  . . . . . . . . . . . .  21
   4.5.  Hostile Pinning . . . . . . . . . . . . . . . . . . . . .  21
 5.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  22
 6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  24
 7.  Usability Considerations  . . . . . . . . . . . . . . . . . .  24
 8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  24
   8.1.  Normative References  . . . . . . . . . . . . . . . . . .  24
   8.2.  Informative References  . . . . . . . . . . . . . . . . .  26
 Appendix A.  Fingerprint Generation . . . . . . . . . . . . . . .  27
 Appendix B.  Deployment Guidance  . . . . . . . . . . . . . . . .  27
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  28
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  28

Evans, et al. Standards Track [Page 3] RFC 7469 Public Key Pinning Extension for HTTP April 2015

1. Introduction

 This document defines a new HTTP header that enables UAs to determine
 which Subject Public Key Info (SPKI) structures will be present in a
 web host's certificate chain in future Transport Layer Security (TLS)
 [RFC5246] connections.
 Deploying Public Key Pinning (PKP) safely will require operational
 and organizational maturity due to the risk that hosts may make
 themselves unavailable by pinning to a set of SPKIs that becomes
 invalid (see Section 4).  With care, host operators can greatly
 reduce the risk of man-in-the-middle (MITM) attacks and other false-
 authentication problems for their users without incurring undue risk.
 PKP is meant to be used together with HTTP Strict Transport Security
 (HSTS) [RFC6797], but it is possible to pin keys without requiring
 HSTS.
 A Pin is a relationship between a hostname and a cryptographic
 identity (in this document, one or more of the public keys in a chain
 of X.509 certificates).  Pin Validation is the process a UA performs
 to ensure that a host is in fact authenticated with its previously
 established Pin.
 Key pinning is a trust-on-first-use (TOFU) mechanism.  The first time
 a UA connects to a host, it lacks the information necessary to
 perform Pin Validation; UAs can only apply their normal cryptographic
 identity validation.  (In this document, it is assumed that UAs apply
 X.509 certificate chain validation in accord with [RFC5280].)
 The UA will not be able to detect and thwart a MITM attacking the
 UA's first connection to the host.  (However, the requirement that
 the MITM provide an X.509 certificate chain that can pass the UA's
 validation requirements, without error, mitigates this risk
 somewhat.)  Worse, such a MITM can inject its own PKP header into the
 HTTP stream, and pin the UA to its own keys.  To avoid post facto
 detection, the attacker would have to be in a position to intercept
 all future requests to the host from that UA.
 Thus, key pinning as described in this document is not a perfect
 defense against MITM attackers capable of passing certificate chain
 validation procedures -- nothing short of pre-shared keys can be.
 However, it provides significant value by allowing host operators to
 limit the number of certification authorities that can vouch for the
 host's identity, and allows UAs to detect in-process MITM attacks
 after the initial communication.

Evans, et al. Standards Track [Page 4] RFC 7469 Public Key Pinning Extension for HTTP April 2015

1.1. Requirements Language

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

2. Server and Client Behavior

2.1. Response Header Field Syntax

 The "Public-Key-Pins" and "Public-Key-Pins-Report-Only" header
 fields, also referred to within this specification as the PKP and
 PKP-RO header fields, respectively, are new response headers defined
 in this specification.  They are used by a server to indicate that a
 UA should perform Pin Validation (Section 2.6) for the host emitting
 the response message, and to provide the necessary information for
 the UA to do so.
 Figure 1 describes the syntax (Augmented Backus-Naur Form) of the
 header fields, using the grammar defined in [RFC5234] and the rules
 defined in Section 3.2 of [RFC7230].  The field values of both header
 fields conform to the same rules.
 Public-Key-Directives = directive *( OWS ";" OWS directive )
 directive             = directive-name [ "=" directive-value ]
 directive-name        = token
 directive-value       = token
                       / quoted-string
                     Figure 1: HPKP Header Syntax
 Optional white space (OWS) is used as defined in Section 3.2.3 of
 [RFC7230]. token and quoted-string are used as defined in
 Section 3.2.6 of [RFC7230].
 The directives defined in this specification are described below.
 The overall requirements for directives are:
 1.  The order of appearance of directives is not significant.
 2.  With the exception of pin-directives with the same pin-directive-
     name (see below), 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.

Evans, et al. Standards Track [Page 5] RFC 7469 Public Key Pinning Extension for HTTP April 2015

 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 PKP or PKP-RO header field otherwise satisfies the above
     requirements (1 through 5), the UA MUST process the directives it
     recognizes.
 Additional directives extending the semantic functionality of the
 header fields can be defined in other specifications.  The first such
 specification will need to define a registry for such directives.
 Such future directives will be ignored by UAs implementing only this
 specification, as well as by generally non-conforming UAs.
 When a connection passes Pin Validation using the UA's noted Pins for
 the host at the time, the host becomes a Known Pinned Host.

2.1.1. The Pin Directive

 The pin directive specifies a way for web host operators to indicate
 a cryptographic identity that should be bound to a given web host.
 The syntax of a pin directive is as follows:
 pin-directive       = pin-directive-name "=" pin-directive-value
 pin-directive-name  = "pin-" token
 pin-directive-value = quoted-string
                    Figure 2: Pin Directive Syntax
 In the pin-directive, the token is the name of a cryptographic hash
 algorithm.  The only algorithm allowed at this time is "sha256",
 i.e., the hash algorithm SHA256 [RFC6234]; additional algorithms may
 be allowed for use in this context in the future.  The quoted-string
 is a sequence of base 64 digits: the base64-encoded SPKI Fingerprint
 [RFC4648] (see Section 2.4).
 According to the processing rules of Section 2.1, the UA MUST ignore
 pin-directives with tokens naming hash algorithms it does not
 recognize.  If the set of remaining effective pin-directives is
 empty, and if the host is a Known Pinned Host, the UA MUST cease to
 consider the host as a Known Pinned Host (the UA should fail open).
 The UA should indicate to users that the host is no longer a Known
 Pinned Host.

Evans, et al. Standards Track [Page 6] RFC 7469 Public Key Pinning Extension for HTTP April 2015

 Note, per the processing rules of Section 2.1, the pin-directive-name
 is case insensitive.

2.1.2. The max-age Directive

 The "max-age" directive specifies the number of seconds after the
 reception of the PKP header field during which the UA SHOULD regard
 the host (from whom the message was received) as a Known Pinned Host.
 The "max-age" directive is REQUIRED to be present within a "Public-
 Key-Pins" header field.  The "max-age" directive is meaningless
 within a "Public-Key-Pins-Report-Only" header field, and UAs MUST
 ignore it and not cache the header.  See Section 2.3.3.
 The max-age directive is REQUIRED to have a directive value, for
 which the syntax (after quoted-string unescaping, if necessary) is
 defined as:
 max-age-value = delta-seconds
 delta-seconds = 1*DIGIT
                    Figure 3: max-age Value Syntax
 delta-seconds is used as defined in [RFC7234], Section 1.2.1.
 See Section 2.3.3 for limitations on the range of values for max-age.

2.1.3. The includeSubDomains Directive

 The OPTIONAL includeSubDomains directive is a valueless directive
 that, if present (i.e., it is "asserted"), signals to the UA that the
 Pinning Policy applies to this Pinned Host as well as any subdomains
 of the host's domain name.

2.1.4. The report-uri Directive

 The OPTIONAL report-uri directive indicates the URI to which the UA
 SHOULD report Pin Validation failures (Section 2.6).  The UA POSTs
 the reports to the given URI as described in Section 3.
 When used in the PKP or PKP-RO headers, the presence of a report-uri
 directive indicates to the UA that in the event of Pin Validation
 failure it SHOULD POST a report to the report-uri.  If the header is
 Public-Key-Pins, the UA should do this in addition to terminating the
 connection (as described in Section 2.6).

Evans, et al. Standards Track [Page 7] RFC 7469 Public Key Pinning Extension for HTTP April 2015

 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 perform
 Pinning Validation when the host in the report-uri is a Known Pinned
 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 Pin Validation failures
 to the report-uri, but they may fail to report in exceptional
 conditions.  For example, if connecting the report-uri itself incurs
 a Pinning Validation failure or other certificate validation failure,
 the UA MUST cancel the connection.  Similarly, if Known Pinned Host A
 sets a report-uri referring to Known Pinned Host B, and if B sets a
 report-uri referring to A, and if both hosts fail Pin Validation, the
 UA SHOULD detect and break the loop by failing to send reports to and
 about those hosts.
 In any case of report failure, the UA MAY attempt to re-send the
 report later.
 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 per distinct set of declared Pins.

2.1.5. Examples

 Figure 4 shows some example PKP and PKP-RO response header fields.
 (Lines are folded to fit.)
 Public-Key-Pins: max-age=3000;
     pin-sha256="d6qzRu9zOECb90Uez27xWltNsj0e1Md7GkYYkVoZWmM=";
     pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g="
 Public-Key-Pins: max-age=2592000;
     pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g=";
     pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ="
 Public-Key-Pins: max-age=2592000;
     pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g=";
     pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ=";
     report-uri="http://example.com/pkp-report"
 Public-Key-Pins-Report-Only: max-age=2592000;
     pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g=";
     pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ=";
     report-uri="https://other.example.net/pkp-report"

Evans, et al. Standards Track [Page 8] RFC 7469 Public Key Pinning Extension for HTTP April 2015

 Public-Key-Pins:
     pin-sha256="d6qzRu9zOECb90Uez27xWltNsj0e1Md7GkYYkVoZWmM=";
     pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ=";
     max-age=259200
 Public-Key-Pins:
     pin-sha256="d6qzRu9zOECb90Uez27xWltNsj0e1Md7GkYYkVoZWmM=";
     pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g=";
     pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ=";
     max-age=10000; includeSubDomains
       Figure 4: HTTP Public Key Pinning (HPKP) Header Examples

2.2. Server Processing Model

 This section describes the processing model that Pinned 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, a Pinned Host SHOULD include in its response exactly one
 PKP header field, exactly one PKP-RO header field, or one of each.
 Each instance of either header field MUST satisfy the grammar
 specified in Section 2.1.
 Establishing a given host as a Known Pinned 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
     PKP header field to the UA.
 2.  Through other mechanisms, such as a client-side preloaded Known
     Pinned Host List.

2.2.2. HTTP Request Type

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

Evans, et al. Standards Track [Page 9] RFC 7469 Public Key Pinning Extension for HTTP April 2015

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. Public-Key-Pins Response Header Field Processing

 If the UA receives, over a secure transport, an HTTP response that
 includes a PKP header field conforming to the grammar specified in
 Section 2.1, and there are no underlying secure transport errors or
 warnings (see Section 2.5), the UA MUST either:
 o  Note the host as a Known Pinned Host if it is not already so noted
    (see Section 2.3.3),
 or,
 o  Update the UA's cached information for the Known Pinned Host if
    any of the max-age, includeSubDomains, or report-uri header field
    value directives convey information different from that already
    maintained by the UA.
 The max-age value is essentially a "time to live" value relative to
 the time of the most recent observation of the PKP header field.  If
 the max-age header field value token has a value of 0, the UA MUST
 remove its cached Pinning Policy information (including the
 includeSubDomains directive, if asserted) if the Pinned Host is
 Known, or, MUST NOT note this Pinned Host if it is not yet Known.
 If a UA receives more than one PKP header field or more than one PKP-
 RO header field in an HTTP response message over secure transport,
 then the UA MUST process only the first PKP header field (if present)
 and only the first PKP-RO header field (if present).
 If the UA receives the HTTP response over insecure transport, or if
 the PKP header is not a Valid Pinning Header (see Section 2.5), the
 UA MUST ignore any present PKP header field(s).  Similarly, if the UA
 receives the HTTP response over insecure transport, the UA MUST
 ignore any present PKP-RO header field(s).  The UA MUST ignore any
 PKP or PKP-RO header fields not conforming to the grammar specified
 in Section 2.1.

Evans, et al. Standards Track [Page 10] RFC 7469 Public Key Pinning Extension for HTTP April 2015

2.3.2. Interaction of Public-Key-Pins and Public-Key-Pins-Report-Only

 A server MAY set both the "Public-Key-Pins" and "Public-Key-Pins-
 Report-Only" headers simultaneously.  The headers do not interact
 with one another, but the UA MUST process the PKP header and SHOULD
 process both.
 The headers are processed according to Section 2.3.1.
 When the PKP-RO header is used with a report-uri, the UA SHOULD POST
 reports for Pin Validation failures to the indicated report-uri,
 although the UA MUST NOT enforce Pin Validation.  That is, in the
 event of Pin Validation failure when the host has set the PKP-RO
 header, the UA performs Pin Validation to check whether or not it
 should POST a report, but not whether it should cause a connection
 failure.
 Note: There is no purpose to using the PKP-RO header without the
 report-uri directive.  User Agents MAY discard such headers without
 interpreting them further.
 When the PKP header is used with a report-uri, the UA SHOULD POST
 reports for Pin Validation failures to the indicated report-uri, as
 well as enforce Pin Validation.
 If a host sets the PKP-RO header, the UA SHOULD note the Pins and
 directives given in the PKP-RO header, ignoring any max-age
 directive.  If the UA does note the Pins and directives in the PKP-RO
 header, it SHOULD evaluate the specified policy and SHOULD report any
 would-be Pin Validation failures that would occur if the report-only
 policy were enforced.
 If a host sets both the PKP header and the PKP-RO header, the UA MUST
 note and enforce Pin Validation as specified by the PKP header, and
 SHOULD process the Pins and directives given in the PKP-RO header.
 If the UA does process the Pins and directives in the PKP-RO header,
 it SHOULD evaluate the specified policy and SHOULD report any would-
 be Pin Validation failures that would occur if the report-only policy
 were enforced.

2.3.3. Noting a Pinned Host - Storage Model

 The Effective Pin Date of a Known Pinned Host is the time that the UA
 observed a Valid Pinning Header for the host.  The Effective
 Expiration Date of a Known Pinned Host is the Effective Pin Date plus
 the max-age.  A Known Pinned Host is "expired" if the Effective
 Expiration Date refers to a date in the past.  The UA MUST ignore any
 expired Known Pinned Hosts in its cache.

Evans, et al. Standards Track [Page 11] RFC 7469 Public Key Pinning Extension for HTTP April 2015

 For example, if a UA is beginning to perform Pin Validation for a
 Known Pinned Host and finds that the cached pinning information for
 the host indicates an Effective Expiration Date in the past, the UA
 MUST NOT continue with Pin Validation for the host, and MUST consider
 the host to no longer be a Known Pinned Host.
 Known Pinned 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 Pinned Host.
 Otherwise, if the substring does not congruently match an existing
 Known Pinned 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 Pinned Host cache.  The UA caches:
 o  the Pinned Host's domain name,
 o  the Effective Expiration Date, or enough information to calculate
    it (the Effective Pin Date and the value of the max-age
    directive),
 o  whether or not the includeSubDomains directive is asserted, and
 o  the value of the report-uri directive, if present.
 If any other metadata from optional or future PKP header directives
 are present in the Valid Pinning 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 Pins (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 Valid Pinning 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 Pinned Host.)  For
 UA implementation guidance on how to select a maximum max-age, see
 Section 4.1.
 The UA MUST NOT modify any pinning metadata of any superdomain
 matched Known Pinned Host.

Evans, et al. Standards Track [Page 12] RFC 7469 Public Key Pinning Extension for HTTP April 2015

 The UA MUST NOT cache information derived from a PKP-RO header.
 (PKP-RO headers are useful only at the time of receipt and
 processing.)

2.3.4. HTTP-Equiv <Meta> Element Attribute

 UAs MUST NOT heed http-equiv="Public-Key-Pins" or
 http-equiv="Public-Key-Pins-Report-Only" attribute settings on <meta>
 elements [W3C.REC-html401-19991224] in received content.

2.4. Semantics of Pins

 An SPKI Fingerprint is defined as the output of a known cryptographic
 hash algorithm whose input is the DER-encoded ASN.1 representation of
 the Subject Public Key Info (SPKI) of an X.509 certificate.  A Pin is
 defined as the combination of the known algorithm identifier and the
 SPKI Fingerprint computed using that algorithm.
 The SPKI Fingerprint is encoded in base 64 for use in an HTTP header
 [RFC4648].
 In this version of the specification, the known cryptographic hash
 algorithm is SHA-256, identified as "sha256" [RFC6234].  (Future
 specifications may add new algorithms and deprecate old ones.)  UAs
 MUST ignore Pins for which they do not recognize the algorithm
 identifier.  UAs MUST continue to process the rest of a PKP response
 header field and note Pins for algorithms they do recognize.
 Figure 5 reproduces the definition of the SubjectPublicKeyInfo
 structure in [RFC5280].
 SubjectPublicKeyInfo  ::=  SEQUENCE  {
     algorithm            AlgorithmIdentifier,
     subjectPublicKey     BIT STRING  }
 AlgorithmIdentifier  ::=  SEQUENCE  {
     algorithm            OBJECT IDENTIFIER,
     parameters           ANY DEFINED BY algorithm OPTIONAL  }
                       Figure 5: SPKI Definition
 If the certificate's Subject Public Key Info is incomplete when taken
 in isolation, such as when holding a DSA key without domain
 parameters, a public key pin cannot be formed.
 We pin public keys, rather than entire certificates, to enable
 operators to generate new certificates containing old public keys
 (see [why-pin-key]).

Evans, et al. Standards Track [Page 13] RFC 7469 Public Key Pinning Extension for HTTP April 2015

 See Appendix A for an example non-normative program that generates
 SPKI Fingerprints from certificates.

2.5. Noting Pins

 Upon receipt of the PKP response header field, the UA notes the host
 as a Known Pinned Host, storing the Pins and their associated
 directives in non-volatile storage (for example, along with the HSTS
 metadata).  The Pins and their associated directives are collectively
 known as Pinning Metadata.
 The UA MUST note the Pins for a Host if and only if all three of the
 following conditions hold:
 o  It received the PKP response header field over an error-free TLS
    connection.  If the host is a Pinned Host, this includes the
    validation added in Section 2.6.
 o  The TLS connection was authenticated with a certificate chain
    containing at least one of the SPKI structures indicated by at
    least one of the given SPKI Fingerprints (see Section 2.6).
 o  The given set of Pins contains at least one Pin that does NOT
    refer to an SPKI in the certificate chain.  (That is, the host
    must set a Backup Pin; see Section 4.3.)
 If the PKP response header field does not meet all three of these
 criteria, the UA MUST NOT note the host as a Pinned Host.  A PKP
 response header field that meets all these criteria is known as a
 Valid Pinning Header.
 Whenever a UA receives a Valid Pinning Header, it MUST set its
 Pinning Metadata to the exact Pins, Effective Expiration Date
 (computed from max-age), and (if any) report-uri given in the most
 recently received Valid Pinning Header.
 For forward compatibility, the UA MUST ignore any unrecognized PKP
 and PKP-RO header directives, while still processing those directives
 it does recognize.  Section 2.1 specifies the directives max-age,
 Pins, includeSubDomains, and report-uri, but future specifications
 and implementations might use additional directives.
 Upon receipt of a PKP-RO response header field, the UA SHOULD
 evaluate the policy expressed in the field, and SHOULD generate and
 send a report (see Section 3).  However, failure to validate the Pins
 in the field MUST have no effect on the validity or non-validity of
 the policy expressed in the PKP field or in previously noted Pins for
 the Known Pinned Host.

Evans, et al. Standards Track [Page 14] RFC 7469 Public Key Pinning Extension for HTTP April 2015

 The UA need not note any Pins or other policy expressed in the PKP-RO
 response header field, except for the purpose of determining that it
 has already sent a report for a given policy.  UAs SHOULD make a best
 effort not to inundate report-uris with redundant reports.

2.6. Validating Pinned Connections

 When a UA connects to a Pinned 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 determine whether
 to apply a new, additional correctness check: Pin Validation.  A UA
 SHOULD perform Pin Validation whenever connecting to a Known Pinned
 Host, as soon as possible (e.g., immediately after receiving the
 Server Certificate message).  It is acceptable to allow Pin
 Validation to be disabled for some Hosts according to local policy.
 For example, a UA may disable Pin Validation for Pinned 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).
 To perform Pin Validation, the UA will compute the SPKI Fingerprints
 for each certificate in the Pinned Host's validated certificate
 chain, using each supported hash algorithm for each certificate.  (As
 described in Section 2.4, certificates whose SPKI cannot be taken in
 isolation cannot be pinned.)  The UA MUST ignore superfluous
 certificates in the chain that do not form part of the validating
 chain.  The UA will then check that the set of these SPKI
 Fingerprints intersects the set of SPKI Fingerprints in that Pinned
 Host's Pinning Metadata.  If there is set intersection, the UA
 continues with the connection as normal.  Otherwise, the UA MUST
 treat this Pin Validation failure as a non-recoverable error.  Any
 procedure that matches the results of this Pin Validation procedure
 is considered equivalent.
 A UA that has previously noted a host as a Known Pinned Host MUST
 perform Pin Validation when setting up the TLS session, before
 beginning an HTTP conversation over the TLS channel.
 UAs send validation failure reports only when Pin Validation is
 actually in effect.  Pin Validation might not be in effect, e.g.,
 because the user has elected to disable it, or because a presented
 certificate chain chains up to a user-defined trust anchor.  In such
 cases, UAs SHOULD NOT send reports.

Evans, et al. Standards Track [Page 15] RFC 7469 Public Key Pinning Extension for HTTP April 2015

2.7. Interactions with Preloaded Pin Lists

 UAs MAY choose to implement additional sources of pinning
 information, such as through built-in lists of pinning information.
 Such UAs should allow users to override such additional sources,
 including disabling them from consideration.
 The effective policy for a Known Pinned Host that has both built-in
 Pins and Pins from previously observed PKP header response fields is
 implementation-defined.

2.8. Pinning Self-Signed End Entities

 If UAs accept hosts that authenticate themselves with self-signed end
 entity certificates, they MAY also allow hosts to pin the public keys
 in such certificates.  The usability and security implications of
 this practice are outside the scope of this specification.

3. Reporting Pin Validation Failure

 When a Known Pinned Host has set the report-uri directive, the UA
 SHOULD report Pin Validation failures to the indicated URI.  The UA
 does this by POSTing a JSON [RFC7159] message to the URI; the JSON
 message takes this form:
 {
   "date-time": date-time,
   "hostname": hostname,
   "port": port,
   "effective-expiration-date": expiration-date,
   "include-subdomains": include-subdomains,
   "noted-hostname": noted-hostname,
   "served-certificate-chain": [
     pem1, ... pemN
   ],
   "validated-certificate-chain": [
     pem1, ... pemN
   ],
   "known-pins": [
     known-pin1, ... known-pinN
   ]
 }
                     Figure 6: JSON Report Format
 Whitespace outside of quoted strings is not significant.  The key/
 value pairs may appear in any order, but each MUST appear only once.

Evans, et al. Standards Track [Page 16] RFC 7469 Public Key Pinning Extension for HTTP April 2015

 The date-time indicates the time the UA observed the Pin Validation
 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 Pin Validation.  It is provided as a string.
 The port is the port to which the UA made the original request that
 failed Pin Validation.  It is provided as an integer.
 The effective-expiration-date is the Effective Expiration Date for
 the noted Pins.  It is provided as a string formatted according to
 Section 5.6, "Internet Date/Time Format", of [RFC3339].
 include-subdomains indicates whether or not the UA has noted the
 includeSubDomains directive for the Known Pinned Host.  It is
 provided as one of the JSON identifiers "true" or "false".
 noted-hostname indicates the hostname that the UA noted when it noted
 the Known Pinned Host.  This field allows operators to understand why
 Pin Validation was performed for, e.g., foo.example.com when the
 noted Known Pinned Host was example.com with includeSubDomains set.
 The served-certificate-chain is the certificate chain, as served by
 the Known Pinned Host during TLS session setup.  It is provided as an
 array of strings; 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; each string pem1, ... pemN is the PEM
 representation of each X.509 certificate as described in [RFC7468].
 UAs that build certificate chains in more than one way during the
 validation process SHOULD send the last chain built.  In this way,
 they can avoid keeping too much state during the validation process.
 The known-pins are the Pins that the UA has noted for the Known
 Pinned Host.  They are provided as an array of strings with the
 syntax:
 known-pin = token "=" quoted-string
                      Figure 7: Known Pin Syntax

Evans, et al. Standards Track [Page 17] RFC 7469 Public Key Pinning Extension for HTTP April 2015

 As in Section 2.4, the token refers to the algorithm name, and the
 quoted-string refers to the base64 encoding of the SPKI Fingerprint.
 When formulating the JSON POST body, the UA MUST either use single-
 quoted JSON strings or use double-quoted JSON strings and backslash-
 escape the embedded double quotes in the quoted-string part of the
 known-pin.
 Figure 8 shows an example of a Pin Validation failure report.  (PEM
 strings are shown on multiple lines for readability.)
{
  "date-time": "2014-04-06T13:00:50Z",
  "hostname": "www.example.com",
  "port": 443,
  "effective-expiration-date": "2014-05-01T12:40:50Z"
  "include-subdomains": false,
  "served-certificate-chain": [
    "-----BEGIN CERTIFICATE-----\n
    MIIEBDCCAuygAwIBAgIDAjppMA0GCSqGSIb3DQEBBQUAMEIxCzAJBgNVBAYTAlVT\n
    ...
    HFa9llF7b1cq26KqltyMdMKVvvBulRP/F/A8rLIQjcxz++iPAsbw+zOzlTvjwsto\n
    WHPbqCRiOwY1nQ2pM714A5AuTHhdUDqB1O6gyHA43LL5Z/qHQF1hwFGPa4NrzQU6\n
    yuGnBXj8ytqU0CwIPX4WecigUCAkVDNx\n
    -----END CERTIFICATE-----",
    ...
  ],
  "validated-certificate-chain": [
    "-----BEGIN CERTIFICATE-----\n
    MIIEBDCCAuygAwIBAgIDAjppMA0GCSqGSIb3DQEBBQUAMEIxCzAJBgNVBAYTAlVT\n
    ...
    HFa9llF7b1cq26KqltyMdMKVvvBulRP/F/A8rLIQjcxz++iPAsbw+zOzlTvjwsto\n
    WHPbqCRiOwY1nQ2pM714A5AuTHhdUDqB1O6gyHA43LL5Z/qHQF1hwFGPa4NrzQU6\n
    yuGnBXj8ytqU0CwIPX4WecigUCAkVDNx\n
    -----END CERTIFICATE-----",
    ...
  ],
  "known-pins": [
    'pin-sha256="d6qzRu9zOECb90Uez27xWltNsj0e1Md7GkYYkVoZWmM="',
    "pin-sha256=\"E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g=\""
  ]
}
            Figure 8: Pin Validation Failure Report Example

Evans, et al. Standards Track [Page 18] RFC 7469 Public Key Pinning Extension for HTTP April 2015

4. Security Considerations

 Pinning public keys helps hosts strongly assert their cryptographic
 identity even in the face of issuer error, malfeasance, or
 compromise.  But, there is some risk that a host operator could lose
 (or lose control of) their host's private key (such as by operator
 error or host compromise).  If the operator had pinned only the key
 of the host's end-entity certificate, the operator would not be able
 to serve their web site or application in a way that UAs would trust
 for the duration of their pin's max-age.  (Recall that UAs MUST close
 the connection to a host upon Pin Failure.)
 Therefore, there is a necessary trade-off between two competing
 goods: pin specificity and maximal reduction of the scope of issuers
 on the one hand; and flexibility and resilience of the host's
 cryptographic identity on the other hand.  One way to resolve this
 trade-off is to compromise by pinning to the key(s) of the issuer(s)
 of the host's end-entity certificate(s).  Often, a valid certificate
 chain will have at least two certificates above the end-entity
 certificate: the intermediate issuer and the trust anchor.  Operators
 can pin any one or more of the public keys in this chain, and indeed
 MUST pin to issuers not in the chain (as, for example, a Backup Pin).
 Pinning to an intermediate issuer, or even to a trust anchor or root,
 still significantly reduces the number of issuers who can issue end-
 entity certificates for the Known Pinned Host, while still giving
 that host flexibility to change keys without a disruption of service.

4.1. Maximum max-age

 As mentioned in Section 2.3.3, UAs MAY cap the max-age value at some
 upper limit.  There is a security trade-off in that low maximum
 values provide a narrow window of protection for users who visit the
 Known Pinned Host only infrequently, while high maximum values might
 result in a UA's inability to successfully perform Pin Validation for
 a Known Pinned Host if the UA's noted Pins and the host's true Pins
 diverge.
 Such divergence could occur for several reasons, including: UA error;
 host operator error; network attack; or a Known Pinned Host that
 intentionally migrates all pinned keys, combined with a UA that has
 noted true Pins with a high max-age value and has not had a chance to
 observe the new true Pins for the host.  (This last example
 underscores the importance for host operators to phase in new keys
 gradually and to set the max-age value in accordance with their
 planned key migration schedule.)

Evans, et al. Standards Track [Page 19] RFC 7469 Public Key Pinning Extension for HTTP April 2015

 There is probably no ideal upper limit to the max-age directive that
 would satisfy all use cases.  However, a value on the order of 60
 days (5,184,000 seconds) may be considered a balance between the two
 competing security concerns.

4.2. Using includeSubDomains Safely

 It may happen that Pinned Hosts whose hostnames share a parent domain
 use different Valid Pinning Headers.  If a host whose hostname is a
 parent domain for another host sets the includeSubDomains directive,
 the two hosts' Pins may conflict with each other.  For example,
 consider two Known Pinned Hosts, example.com and
 subdomain.example.com.  Assume example.com sets a Valid Pinning
 Header such as this:
 Public-Key-Pins: max-age=12000; pin-sha256="ABC...";
     pin-sha256="DEF..."; includeSubDomains
              Figure 9: example.com Valid Pinning Header
 Assume subdomain.example.com sets a Valid Pinning Header such as
 this:
 Public-Key-Pins: pin-sha256="GHI..."; pin-sha256="JKL..."
         Figure 10: subdomain.example.com Valid Pinning Header
 Assume a UA that has not previously noted any Pins for either of
 these hosts.  If the UA first contacts subdomain.example.com, it will
 note the Pins in the Valid Pinning Header, and perform Pin Validation
 as normal on subsequent connections.  If the UA then contacts
 example.com, again it will note the Pins and perform Pin Validation
 on future connections.
 However, if the UA happened to visit example.com before
 subdomain.example.com, the UA would, due to example.com's use of the
 includeSubDomains directive, attempt to perform Pin Validation for
 subdomain.example.com using the SPKI hashes ABC... and DEF..., which
 are not valid for the certificate chains subdomain.example.com (which
 uses certificates with SPKIs GHI... and JLK...).  Thus, depending on
 the order in which the UA observes the Valid Pinning Headers for
 hosts example.com and subdomain.example.com, Pin Validation might or
 might not fail for subdomain.example.com, even if the certificate
 chain the UA receives for subdomain.example.com is perfectly valid.
 Thus, Pinned Host operators must use the includeSubDomains directive
 with care.  For example, they may choose to use overlapping pin sets
 for hosts under a parent domain that uses includeSubDomains, or to

Evans, et al. Standards Track [Page 20] RFC 7469 Public Key Pinning Extension for HTTP April 2015

 not use the includeSubDomains directive in their effective-second-
 level domains, or to simply use the same pin set for all hosts under
 a given parent domain.

4.3. Backup Pins

 The primary way to cope with the risk of inadvertent Pin Validation
 failure is to keep a Backup Pin.  A Backup Pin is a fingerprint for
 the public key of a secondary, not-yet-deployed key pair.  The
 operator keeps the backup key pair offline, and sets a pin for it in
 the PKP header.  Then, in case the operator loses control of their
 primary private key, they can deploy the backup key pair.  UAs, who
 have had the backup key pair pinned (when it was set in previous
 Valid Pinning Headers), can connect to the host without error.
 Because having a backup key pair is so important to recovery, UAs
 MUST require that hosts set a Backup Pin (see Section 2.5).  The down
 side of keeping a not-yet-deployed key pair is that, if an attacker
 gains control of the private key, she will be able to perform a MITM
 attack without being discovered.  Operators must take care to avoid
 leaking the key such as keeping it offline.

4.4. Interactions With Cookie Scoping

 HTTP cookies [RFC6265] set by a Known Pinned Host can be stolen by a
 network attacker who can forge web and DNS responses so as to cause a
 client to send the cookies to a phony subdomain of the host.  To
 prevent this, hosts SHOULD set the "secure" attribute and precisely
 scope the "domain" attribute on all security-sensitive cookies, such
 as session cookies.  These settings tell the browser that the cookie
 should only be sent back to the specific host(s) (and not, e.g., all
 subdomains of a given domain), and should only be sent over HTTPS
 (not HTTP).

4.5. Hostile Pinning

 An attacker who is able to obtain a valid certificate for a domain,
 either through misissuance by a Certification Authority or through
 other means, such as being the prior owner of a given domain, may
 attempt to perform 'hostile' pinning.  In this scenario, the attacker
 provides a Valid Pinning Header that pins to a set of SPKIs of the
 attacker's choice.  If a UA has not previously noted pins for that
 host, it may note the attacker's pins, preventing access to the
 legitimate site.
 This attack is mitigated through several means.  Most prominently,
 the attack can only persist for the maximum max-age (see
 Section 4.1).  Web host operators can reduce the opportunity for

Evans, et al. Standards Track [Page 21] RFC 7469 Public Key Pinning Extension for HTTP April 2015

 attack by working to preload the host's pins within the UA.
 Operators may further detect such misissuance through other means,
 such as certificate transparency ([RFC6962]).

5. Privacy Considerations

 Hosts can use HSTS or HPKP as a "super-cookie", by setting distinct
 policies for a number of subdomains.  For example, assume example.com
 wishes to track distinct UAs without explicitly setting a cookie, or
 that a previously set cookie is deleted from the UA's cookie store.
 Here are two attack scenarios.
 o  example.com can use report-uri and the ability to pin arbitrary
    identifiers to distinguish UAs.
    1.  example.com sets a Valid Pinning Header in its response to
        requests.  The header asserts the includeSubDomains directive
        and specifies a report-uri directive as well.  Pages served by
        the host also include references to subresource
        https://bad.example.com/foo.png.
    2.  The Valid Pinning Header includes a "pin" that is not really
        the hash of an SPKI but is instead an arbitrary distinguishing
        string sent only in response to a particular request.  For
        each request, the host creates a new, distinct distinguishing
        string and sets it as if it were a pin.
    3.  The certificate chain served by bad.example.com does not pass
        Pin Validation given the pin set the host asserted in step
        (1).  The HPKP-conforming UA attempts to report the Pin
        Validation failure to the specified report-uri, including the
        certificate chain it observed and the SPKI hashes it expected
        to see.  Among the SPKI hashes is the distinguishing string in
        step (2).
 o  Different site operators/origins can optionally collaborate by
    setting the report-uri to be in an origin they share
    administrative control of.  UAs MAY, therefore, refuse to send
    reports outside of the origin that set the PKP or PKP-RO header.
 o  example.com can use server name indication (SNI; [RFC3546]) and
    subdomains to distinguish UAs.
    1.  example.com sets a Valid Pinning Header in its response to
        requests.  The header asserts the includeSubDomains directive.

Evans, et al. Standards Track [Page 22] RFC 7469 Public Key Pinning Extension for HTTP April 2015

    2.  On a subsequent page view, the host responds with a page
        including the subresource https://0.fingerprint.example.com/
        foo.png, and the server responds using a certificate chain
        that does not pass Pin Validation for the pin-set defined in
        the Valid Pinning Header in step (1).  The HPKP-conforming UA
        will close the connection, never completing the request to
        0.fingerprint.example.com.  The host may thus note that this
        particular UA had noted the (good) Pins for that subdomain.
    3.  example.com can distinguish 2^N UAs by serving Valid Pinning
        Headers from an arbitrary number N distinct subdomains.  For
        any given subdomain n.fingerprint.example.com, the host may
        deliver a Valid Pinning Header to one UA, but not deliver it
        to a different UA.  The server may then change the
        configuration for n.fingerprint.example.com.  If the UA fails
        to connect, it was in the set of UAs that were pinned, which
        can be distinguished from the UAs that were not pinned, as
        they will succeed in connecting.  The host may repeat this for
        a sufficient number of subdomains necessary to distinguish
        individual UAs.
 o  Conforming implementations (as well as implementations conforming
    to [RFC6797]) must store state about which domains have set
    policies, hence which domains the UA has contacted.  Because these
    policies cause remotely detectable behaviors, it is advisable that
    UAs have a way for privacy-sensitive users to clear current Pins
    for Pinned Hosts and that UAs allow users to query the current
    state of Pinned Hosts.  In addition, note that because pinning a
    host implies a degree of persistent state, an attacker with
    physical access to a device may be able to recover information
    about hosts a user has visited, even if the user has cleared other
    parts of the UA's state.
 o  Pin reports, as noted in Section 3, contains information about the
    certificate chain that has failed pin validation.  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.

Evans, et al. Standards Track [Page 23] RFC 7469 Public Key Pinning Extension for HTTP April 2015

6. IANA Considerations

 IANA has registered the response headers described in this document
 under "Permanent Message Header Field Names" in the "Message Headers"
 registry [message-headers] with the following parameters:
 o  Header Field Names: Public-Key-Pins and Public-Key-Pins-Report-
    Only
 o  Protocol: http
 o  Status: standard
 o  Reference: RFC 7469

7. Usability Considerations

 When pinning works to detect impostor Pinned Hosts, users will
 experience denial of service.  It is advisable for UAs to explain the
 reason why, i.e., that it was impossible to verify the confirmed
 cryptographic identity of the host.
 It is advisable that UAs have a way for users to clear current Pins
 for Pinned Hosts and that UAs allow users to query the current state
 of Pinned Hosts.

8. References

8.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997,
            <http://www.rfc-editor.org/info/rfc2119>.
 [RFC3339]  Klyne, G. and C. Newman, "Date and Time on the Internet:
            Timestamps", RFC 3339, 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, January 2005,
            <http://www.rfc-editor.org/info/rfc3986>.
 [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
            Encodings", RFC 4648, October 2006,
            <http://www.rfc-editor.org/info/rfc4648>.

Evans, et al. Standards Track [Page 24] RFC 7469 Public Key Pinning Extension for HTTP April 2015

 [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
            Specifications: ABNF", STD 68, RFC 5234, 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, August 2008,
            <http://www.rfc-editor.org/info/rfc5246>.
 [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, May 2008,
            <http://www.rfc-editor.org/info/rfc5280>.
 [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
            (SHA and SHA-based HMAC and HKDF)", RFC 6234, May 2011,
            <http://www.rfc-editor.org/info/rfc6234>.
 [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
            April 2011, <http://www.rfc-editor.org/info/rfc6265>.
 [RFC6797]  Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
            Transport Security (HSTS)", RFC 6797, November 2012,
            <http://www.rfc-editor.org/info/rfc6797>.
 [RFC7159]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
            Interchange Format", RFC 7159, March 2014,
            <http://www.rfc-editor.org/info/rfc7159>.
 [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
            Protocol (HTTP/1.1): Message Syntax and Routing", RFC
            7230, 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, 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, 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>.

Evans, et al. Standards Track [Page 25] RFC 7469 Public Key Pinning Extension for HTTP April 2015

 [message-headers]
            IANA, "Message Headers",
            <http://www.iana.org/assignments/message-headers/>.

8.2. Informative References

 [RFC3546]  Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
            and T. Wright, "Transport Layer Security (TLS)
            Extensions", RFC 3546, June 2003,
            <http://www.rfc-editor.org/info/rfc3546>.
 [RFC6962]  Laurie, B., Langley, A., and E. Kasper, "Certificate
            Transparency", RFC 6962, June 2013,
            <http://www.rfc-editor.org/info/rfc6962>.
 [TACK]     Marlinspike, M., "Trust Assertions for Certificate Keys",
            Work in Progress, draft-perrin-tls-tack-02, January 2013.
 [why-pin-key]
            Langley, A., "Public Key Pinning", Imperial Violet: Adam
            Langley's Weblog, May 2011,
            <https://www.imperialviolet.org/2011/05/04/pinning.html>.

Evans, et al. Standards Track [Page 26] RFC 7469 Public Key Pinning Extension for HTTP April 2015

Appendix A. Fingerprint Generation

 This Portable Operating System Interface (POSIX) shell program
 generates SPKI Fingerprints, suitable for use in pinning, from PEM-
 encoded certificates.  It is non-normative.
 openssl x509 -noout -in certificate.pem -pubkey | \
     openssl asn1parse -noout -inform pem -out public.key
 openssl dgst -sha256 -binary public.key | openssl enc -base64
          Figure 11: Example SPKI Fingerprint Generation Code

Appendix B. Deployment Guidance

 This section is non-normative guidance that may smooth the adoption
 of public key pinning.
 o  Operators should get the backup public key signed by a different
    (root and/or intermediary) CA than their primary certificate, and
    store the backup key pair safely offline.  The semantics of an
    SPKI Fingerprint do not require the issuance of a certificate to
    construct a valid Pin. However, in many deployment scenarios, in
    order to make a Backup Pin operational, the server operator will
    need to have a certificate to deploy TLS on the host.  Failure to
    obtain a certificate through prior arrangement will leave clients
    that recognize the site as a Known Pinned Host unable to
    successfully perform Pin Validation until such a time as the
    operator can obtain a new certificate from their desired
    certificate issuer.
 o  It is most economical to have the backup certificate signed by a
    completely different signature chain than the live certificate, to
    maximize recoverability in the event of compromise of either the
    root or intermediary signer.
 o  Operators should periodically exercise their Backup Pin plan -- an
    untested backup is no backup at all.
 o  Operators should start small.  Operators should first deploy
    public key pinning by using the report-only mode together with a
    report-uri directive that points to a reliable report collection
    endpoint.  When moving out of report-only mode, operators should
    start by setting a max-age of minutes or a few hours and gradually
    increase max-age as they gain confidence in their operational
    capability.

Evans, et al. Standards Track [Page 27] RFC 7469 Public Key Pinning Extension for HTTP April 2015

Acknowledgements

 Thanks to Tobias Gondrom, Jeff Hodges, Paul Hoffman, Ivan Krstic,
 Adam Langley, Barry Leiba, Nicolas Lidzborski, SM, James Manger, Yoav
 Nir, Trevor Perrin, Eric Rescorla, Pete Resnick, Tom Ritter, and Yan
 Zhu for suggestions and edits that clarified the text.
 TACK [TACK] is a fruitful source of alternative design
 considerations.

Authors' Addresses

 Chris Evans
 Google, Inc.
 1600 Amphitheatre Pkwy
 Mountain View, CA  94043
 United States
 EMail: cevans@google.com
 Chris Palmer
 Google, Inc.
 1600 Amphitheatre Pkwy
 Mountain View, CA  94043
 United States
 EMail: palmer@google.com
 Ryan Sleevi
 Google, Inc.
 1600 Amphitheatre Pkwy
 Mountain View, CA  94043
 United States
 EMail: sleevi@google.com

Evans, et al. Standards Track [Page 28]

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