Web Security                                                    C. Evans
Internet-Draft                                                 C. Palmer
Intended status: Standards Track                               R. Sleevi
Expires: December 14, 18, 2014                                  Google, Inc.
                                                           June 12, 16, 2014

                 Public Key Pinning Extension for HTTP


   This memo describes an extension to the HTTP protocol allowing web
   host operators to instruct user agents to remember ("pin") the hosts'
   cryptographic identities for a given period of time.  During that
   time, UAs will require that the host present 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 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 Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on December 14, 18, 2014.

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Server and Client Behavior  . . . . . . . . . . . . . . . . .   3
     2.1.  Response Header Field Syntax  . . . . . . . . . . . . . .   3
       2.1.1.  The max-age Directive . . . . . . . . . . . . . . . .   5
       2.1.2.  The includeSubDomains Directive . . . . . . . . . . .   5
       2.1.3.  The report-uri Directive  . . . . . . . . . . . . . .   5
       2.1.4.  Examples  . . . . . . . . . . . . . . . . . . . . . .   6
     2.2.  Server Processing Model . . . . . . . . . . . . . . . . .   7
       2.2.1.  HTTP-over-Secure-Transport Request Type . . . . . . .   7
       2.2.2.  HTTP Request Type . . . . . . . . . . . . . . . . . .   8
     2.3.  User Agent Processing Model . . . . . . . . . . . . . . .   8
       2.3.1.  Public-Key-Pins Response Header Field Processing  . .   8
       2.3.2.  Interaction of Public-Key-Pins and Public-Key-Pins-
               Report-Only . . . . . . . . . . . . . . . . . . . . .   9
       2.3.3.  Noting a Pinned Host - Storage Model  . . . . . . . .  10
       2.3.4.  HTTP-Equiv <Meta> Element Attribute . . . . . . . . .  11
     2.4.  Semantics of Pins . . . . . . . . . . . . . . . . . . . .  11
     2.5.  Noting Pins . . . . . . . . . . . . . . . . . . . . . . .  12
     2.6.  Validating Pinned Connections . . . . . . . . . . . . . .  13
     2.7.  Interactions With Preloaded Pin Lists . . . . . . . . . .  14
     2.8.  Pinning Self-Signed End Entities  . . . . . . . . . . . .  14
   3.  Reporting Pin Validation Failure  . . . . . . . . . . . . . .  14
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  16  17
     4.1.  Maximum max-age . . . . . . . . . . . . . . . . . . . . .  17  18
     4.2.  Using includeSubDomains Safely  . . . . . . . . . . . . .  17  18
     4.3.  Backup Pins . . . . . . . . . . . . . . . . . . . . . . .  18  19
     4.4.  Interactions With Cookie Scoping  . . . . . . . . . . . .  19  20
   5.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  19  20
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20  21
   7.  Usability Considerations  . . . . . . . . . . . . . . . . . .  21  22
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  21  22
   9.  What's Changed  . . . . . . . . . . . . . . . . . . . . . . .  21  22
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  22  23
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  22  23
     10.2.  Informative References . . . . . . . . . . . . . . . . .  23  24
   Appendix A.  Fingerprint Generation . . . . . . . . . . . . . . .  23  24
   Appendix B.  Deployment Guidance  . . . . . . . . . . . . . . . .  23  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24  25

1.  Introduction

   We propose a new HTTP header to enable a web host to express to user
   agents (UAs) which Subject Public Key Info (SPKI) structure(s) UAs
   SHOULD expect to be present in the host's certificate chain in future
   connections using TLS (see [RFC5246]).  We call this "public key
   pinning" (PKP).  At least one UA (Google Chrome) has experimented
   with the idea by shipping with a user-extensible embedded set of
   Pins.  Although effective, this does not scale.  This proposal
   addresses the scale problem.

   Deploying PKP safely will require operational and organizational
   maturity due to the risk that hosts may make themselves unavailable
   by pinning to a (set of) SPKI(s) that becomes invalid.  (See
   Section 4.)  We believe that, with care, host operators can greatly
   reduce the risk of main-in-the-middle (MITM) attacks and other false-
   authentication problems for their users without incurring undue risk.

   We intend for hosts to use PKP together with HSTS ([RFC6797]), but is
   possible to pin keys without requiring HSTS.

   This draft is being discussed on the WebSec Working Group mailing
   list, websec@ietf.org.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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 HTTP response header field (PKP header field) and
   Public-Key-Pins-Report-Only response header field (PKP-RO header
   field) indicate to a UA that it should perform Pin Validation
   (Section 2.6) in regards to the host emitting the response message
   containing these header fields, and provide the necessary information
   for the UA to do so.

   Figure 1 describes the syntax (Augmented Backus-Naur Form) of the
   header field.  It is based on the Generic Grammar defined in
   Section 2 of [RFC2616] (which includes a notion of "implied linear
   whitespace", also known as "implied *LWS").

Public-Key-Pins =
    "Public-Key-Pins" ":" [ directive ] *( ";" [ directive ] )
Public-Key-Pins-Report-Only =
    "Public-Key-Pins-Report-Only" ":" [ directive ] *( ";" [ directive ] )

directive        = simple-directive
                   / pin-directive

simple-directive = directive-name [ "=" directive-value ]
directive-name   = token
directive-value  = token
                   / quoted-string

pin-directive    = "pin-" token "=" quoted-string

                       Figure 1: HPKP Header Syntax

   token and quoted-string are used as defined in [RFC2616],
   Section 2.2.

   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.  All simple-directives MUST appear only once in a PKP header
       field.  Directives are either optional or required, as stipulated
       in their definitions.

   3.  Directive names are case-insensitive.

   4.  UAs MUST ignore any PKP header fields containing directives, or
       other header field value data, that do not conform to the syntax
       defined in this specification.

   5.  If a PKP header field contains any directive(s) the UA does not
       recognize, the UA MUST ignore those directives.

   6.  If the PKP header field otherwise satisfies the above
       requirements (1 through 5), the UA MUST process the directives it

   Additional directives extending the semantic functionality of the PKP
   header field can be defined in other specifications, with a registry
   (having an IANA policy definition of IETF Review [RFC2616]) defined
   for them at such time.  Such future directives will be ignored by UAs
   implementing only this specification, as well as by generally non-
   conforming UAs.

   In the pin-directive, the token is the name of a cryptographic hash
   algorithm, and MUST be "sha256".  (In the future, additional hash
   algorithms MAY be registered and used.)  The quoted-string is a
   sequence of base 64 digits: the base 64-encoded SPKI Fingerprint
   ([RFC4648]).  See Section 2.4.

   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 connection passed Pin Validation with
   the UA's existing noted pins for the Host (i.e. the Host is a Known
   Pinned Host), the UA MUST cease to consider the Host as a Known
   Pinned Host.  (I.e. the UA should fail open.)  The UA SHOULD indicate
   to users that the Host is no longer a Known Pinned Host.

2.1.1.  The max-age Directive

   The REQUIRED "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 delta-seconds production is specified in

   The syntax of the max-age directive's REQUIRED value (after quoted-
   string unescaping, if necessary) is defined as:

   max-age-value = delta-seconds
   delta-seconds = 1*DIGIT

                      Figure 2: max-age Value Syntax

   delta-seconds is used as defined in [RFC2616], Section 3.3.2.

2.1.2.  The includeSubDomains Directive

   The OPTIONAL includeSubDomains directive is a valueless directive
   which, 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.

2.1.3.  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, HTTPS, or other schemes.  If
   the scheme in the report-uri is one that uses TLS (e.g.  HTTPS or
   WSS), 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 Known Pinned Host.

   UAs SHOULD make their best effort to report Pin Validation failures
   to the report-uri, but 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.

2.1.4.  Examples

   Figure 3 shows some example response header fields using the Pins
   extension.  (Lines are folded to fit.)
   Public-Key-Pins: max-age=3000;

   Public-Key-Pins: max-age=2592000;

   Public-Key-Pins: max-age=2592000;

   Public-Key-Pins-Report-Only: max-age=2592000;


       max-age=10000; includeSubDomains

                      Figure 3: 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.  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, MAY be accomplished over the HTTP protocol, which is in
   turn running over secure transport, by correctly returning (per this
   specification) at least one valid PKP header field to the UA.  Other
   mechanisms, such as a client-side pre-loaded Known Pinned Host list
   MAY also be used.

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

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),


   o  Update the UA's cached information for the Known Pinned Host if
      any of of the max-age, includeSubDomains, or report-uri header
      field value directives convey information different than that
      already maintained by the UA.

   o  The max-age value is essentially a "time to live" value relative
      to the time of the most recent observation of the PKP header

   o  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.

   o  If a UA receives more than one PKP header field or more than one
      PKP-RO header fieled 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


   o  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).

   o  Similarly, if the UA receives the HTTP response over insecure
      transport, the UA MUST ignore any present PKP-RO header field(s).

   o  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

   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 only to check whether or not
   it should POST a report, but not for causing 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 enforcing Pin Validation.

   If a Host sets the PKP-RO header, the UA SHOULD note the Pins and
   directives given in the PKP-RO header as specified by the 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 note process the Pins and directives given in the PKP-RO header.
   If the UA does note process the Pins and directives in the PKP-RO header
   it SHOULD evaluate the specified policy and SHOULD report any would-be 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 all
   expired Known Pinned Hosts from its cache if, at any time, an expired
   Known Pinned Host exists in the cache.

   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

   Otherwise, if the substring does not congruently match a Known Pinned
   Host's domain name, per the matching procedure specified in
   Section 8.2 of [RFC6797], then the UA MUST note this host as a Known
   Pinned Host, caching the Pinned Host's domain name and noting along
   with it the Effective Expiration Date (or enough information to
   calculate it, i.e. the Effective Pin Date and the value of the max-
   age directive), whether or not the includeSubDomains directive is
   asserted, the value of the report-uri directive (if present).  If any
   other metadata from optional or future PKP header directives is
   present in the Valid Pinning Header, the UA MAY note them if it
   understands them, and need not note them if it does not understand

   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 5184000 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.

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 subjectPublicKeyInfo (SPKI) field 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.
   (See [RFC4648].)

   In this version of the specification, the known cryptographic hash
   algorithm is SHA-256, identified as "sha256" ([RFC4634]).  (Future
   versions of this specification 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;
   UAs MUST recognize "sha256".

   Figure 4 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 4: SPKI Definition

   If the certificate's subjectPublicKeyInfo is incomplete when taken in
   isolation, such as when holding a DSA key without domain parameters,
   a public key pin cannot be formed.  Hence, pins using these keys
   cannot be pinned.

   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 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 observe these conditions when noting a Host:

   o  The UA MUST note the Pins if and only if 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 UA MUST note the Pins if and only if 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 UA MUST note the Pins if and only if 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 critera 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 SHOULD NOT need not note any pins or other policy expressed in the PKP-
   response header field. 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, 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, but MAY allow Pin Validation to be disabled for 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.

   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.

   Although the UA has previously received Pins at the HTTP layer, it
   can and MUST perform Pin Validation at the TLS layer, before
   beginning an HTTP conversation over the TLS channel.  The TLS layer
   thus evaluates TLS connections with pinning information the UA
   received previously, regardless of mechanism: statically preloaded,
   via HTTP header, or some other means (possibly in the TLS layer

   If Pin Validation is not in effect (e.g. because the user has elected
   to disable it, or because a presented certificate chain chains up to
   a locally-installed anchor), and if the server has set a report-uri
   in a PKP or PKP-RO header, the UA SHOULD NOT send any reports to the
   report-uri for the given certificate chain.

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

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 ([RFC4627]) 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,
     "served-certificate-chain": [
       pem1, ... pemN
     "validated-certificate-chain": [
       pem1, ... pemN
     "known-pins": [
       known-pin1, ... known-pinN

                       Figure 5: 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.

   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 PEM
   representation of each X.509 certificate as described in

   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
   [I-D.josefsson-pkix-textual].  For UAs that build certificate chains
   in more than one way during the validation process, they 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

   known-pin = token "=" quoted-string

                        Figure 6: Known Pin Syntax

   As in Section 2.4, the token refers to the algorithm name, and the
   quoted-string refers to the base 64 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 \-escape
   the embedded double quotes in the quoted-string part of the known-

   Figure 7 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
      -----END CERTIFICATE-----",
    "validated-certificate-chain": [
      "-----BEGIN CERTIFICATE-----\n
      -----END CERTIFICATE-----",
    "known-pins": [

              Figure 7: 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 could 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
   potentially 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...";

                Figure 8: example.com Valid Pinning Header

   Assume subdomain.example.com sets a Valid Pinning Header such as

   Public-Key-Pins: pin-sha256="GHI..."; pin-sha256="JKL..."

           Figure 9: 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 conections.  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.)

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 omit 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 originating host (not its subdomains), and should
   only be sent over HTTPS (not HTTP).

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
   if 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

      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 (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

      4.  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

   o  example.com can use SNI 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, giving
          some UAs Valid Pinning Headers for some, but not all
          subdomains (causing subsequent requests for
          n.fingerprint.example.com to fail), and giving some UAs no
          Valid Pinning Header for other subdomains (causing subsequent
          requests for m.fingerprint.example.com to succeed).

   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.  A forensic attacker might
   find this information useful, even if the user has cleared other
   parts of the UA's state.

6.  IANA Considerations

   IANA is requested to register the header described in this document
   in the "Message Headers" registry, with the following parameters:

   o  Header Field Names should be "Public-Key-Pins" and "Public-Key-

   o  Protocol should be "http"

   o  Status should be "standard"

   o  Reference should be this document

7.  Usability Considerations

   When pinning works to detect impostor Pinned Hosts, users will
   experience denial of service.  UAs MUST explain the reason why, i.e.
   that it was impossible to verify the confirmed cryptographic identity
   of the host.

   UAs MUST have a way for users to clear current Pins for Pinned Hosts.
   UAs SHOULD have a way for users to query the current state of Pinned

8.  Acknowledgements

   Thanks to Tobias Gondrom, Jeff Hodges, Paul Hoffman, Ivan Krstic,
   Adam Langley, Nicolas Lidzborski, SM, James Manger, Yoav Nir, Trevor
   Perrin, Eric Rescorla, Tom Ritter, and Yan Zhu for suggestions and
   edits that clarified the text.

9.  What's Changed


   Removed the strict directive.

   Removed the requirement that the server set the Valid Pinning Header
   on every response.

   Added normative references for SHA, JSON, and base-64.

   Added the Privacy Considerations section.

   Changed non-normative pin generation code from Go to POSIX shell
   script using openssl.

   Changed max-max-age from SHOULD to MAY, and used the example of 60
   days instead of 30.

   Removed the section "Pin Validity Times", which was intended to be in
   harmony with [I-D.perrin-tls-tack].  Now using max-age purely as
   specified in [RFC6797].

   Added new directives: includeSubDomains, report-uri and strict.

   Added a new variant of the PKP Header: Public-Key-Pins-Report-Only.

   Removed the section on pin break codes and verifiers, in favor the of
   most-recently-received policy (Section 2.5).

   Now using a new header field, Public-Key-Pins, separate from HSTS.
   This allows hosts to use pinning separately from Strict Transport

   Explicitly requiring that UAs perform Pin Validation before the HTTP
   conversation begins.

   Backup Pins are now required.

   Separated normative from non-normative material.  Removed tangential
   and out-of-scope non-normative discussion.

10.  References

10.1.  Normative References

              Josefsson, S. and S. Leonard, "Text Encodings of PKIX and
              CMS Structures", draft-josefsson-pkix-textual-03 (work in
              progress), April 2014.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC3339]  Klyne, G., Ed. and C. Newman, "Date and Time on the
              Internet: Timestamps", RFC 3339, July 2002.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66, RFC
              3986, January 2005.

   [RFC4627]  Crockford, D., "The application/json Media Type for
              JavaScript Object Notation (JSON)", RFC 4627, July 2006.

   [RFC4634]  Eastlake, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and HMAC-SHA)", RFC 4634, July 2006.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, October 2006.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [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.

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              April 2011.

   [RFC6797]  Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
              Transport Security (HSTS)", RFC 6797, November 2012.

              Raggett, D., Hors, A., and I. Jacobs, "HTML 4.01
              Specification", World Wide Web Consortium Recommendation
              REC-html401-19991224, December 1999,

10.2.  Informative References

              Marlinspike, M., "Trust Assertions for Certificate Keys",
              draft-perrin-tls-tack-02 (work in progress), January 2013.

              Langley, A., "Public Key Pinning", May 2011,

Appendix A.  Fingerprint Generation

   This 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 | base64

            Figure 10: Example SPKI Fingerprint Generation Code

Appendix B.  Deployment Guidance

   This section is non-normative guidance which 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 either root or
      intermediary signer compromise.

   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.

Authors' Addresses

   Chris Evans
   Google, Inc.
   1600 Amphitheatre Pkwy
   Mountain View, CA  94043

   Email: cevans@google.com

   Chris Palmer
   Google, Inc.
   1600 Amphitheatre Pkwy
   Mountain View, CA  94043

   Email: palmer@google.com
   Ryan Sleevi
   Google, Inc.
   1600 Amphitheatre Pkwy
   Mountain View, CA  94043

   Email: sleevi@google.com