HTTP Public Key Pinning: Public Key Pinning Extension for HTTP

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.

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.

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

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.

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.

   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.

   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).

   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"

   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.

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.

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.

   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.

   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]).

   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.

   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.

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.

   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

   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

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.)

   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
   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
   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.

      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.

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>.

   [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>.

   [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>.

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.

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