Web Security                                                    C. Evans
Internet-Draft                                                 C. Palmer
Intended status: Standards Track                               R. Sleevi
Expires: December 09, 2013                                  Google, Inc.
                                                           June 07, 2013

                 Public Key Pinning Extension for HTTP


   This memo describes an extension to the HTTP protocol allowing web
   host operators to instruct user agents (UAs) 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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 09, 2013.

Copyright Notice

   Copyright (c) 2013 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

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   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  . . . . . . . . . . . . . . . . . . . . . . . .   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.  The strict Directive  . . . . . . . . . . . . . . . .   6
       2.1.5.  Examples  . . . . . . . . . . . . . . . . . . . . . .   6
     2.2.  Server Processing Model . . . . . . . . . . . . . . . . .   7
       2.2.1.  HTTP-over-Secure-Transport Request Type . . . . . . .   7
       2.2.2.  HTTP Request Type . . . . . . . . . . . . . . . . . .   7
     2.3.  User Agent Processing Model . . . . . . . . . . . . . . .   8
       2.3.1.  Public-Key-Pins Response Header Field Processing  . .   8
       2.3.2.  Noting a Pinned Host - Storage Model  . . . . . . . .   9
       2.3.3.  HTTP-Equiv <Meta> Element Attribute . . . . . . . . .   9
       2.3.4.  UA Processing Examples  . . . . . . . . . . . . . . .   9
     2.4.  Semantics of Pins . . . . . . . . . . . . . . . . . . . .  10
     2.5.  Noting Pins . . . . . . . . . . . . . . . . . . . . . . .  11
     2.6.  Validating Pinned Connections . . . . . . . . . . . . . .  11
     2.7.  Interactions With Preloaded Pin Lists . . . . . . . . . .  12
     2.8.  Pinning Self-Signed End Entities  . . . . . . . . . . . .  13
   3.  Reporting Pin Validation Failure  . . . . . . . . . . . . . .  13
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
     4.1.  Maximum max-age . . . . . . . . . . . . . . . . . . . . .  15
     4.2.  Using includeSubdomains Safely  . . . . . . . . . . . . .  16
     4.3.  Backup Pins . . . . . . . . . . . . . . . . . . . . . . .  16
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   6.  Usability Considerations  . . . . . . . . . . . . . . . . . .  17
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  17
   8.  What's Changed  . . . . . . . . . . . . . . . . . . . . . . .  17
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  18
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  19
   Appendix A.  Fingerprint Generation . . . . . . . . . . . . . . .  19
   Appendix B.  Deployment Guidance  . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

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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".  At least one UA (Google Chrome) has experimented with
   shipping with a user-extensible embedded set of pins.  Although
   effective, this does not scale.  This proposal addresses the scale

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

   We intend for hosts to use public key pinning 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)
   indicates to a UA that it SHOULD perform Pin Validation (Section 2.6)
   in regards to the host emitting the response message containing this
   header field, and provides the necessary information for the UA to do

   Figure 1 describes the ABNF (Augmented Backus-Naur Form) syntax 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 =

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       "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 the 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 either "sha1" or "sha256".  The quoted-string

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   is a sequence of base 64 digits: the base 64-encoded SPKI
   Fingerprint.  See Section 2.4.

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.

   NOTE: A max-age value of zero (i.e., "max-age=0") signals the UA to
   cease regarding the host as a Known Pinned Host, including the
   includeSubDomains directive (if asserted for that Known Pinned Host).
   See Section 2.3.1.

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 Public-Key-Pins-Report-Only header, 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 Public-Key-
   Pins-Report-Only header, the UA performs Pin Validation only to check
   whether or not it should POST a report, but not for causing
   connection failure.

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   If a Host sets both the Public-Key-Pins header and the Public-Key-
   Pins-Report-Only header, the UA MUST NOT enforce Pin Validation, and
   MUST note only the pins and directives given in the Public-Key-Pins-
   Report-Only header.

   When used in the Public-Key-Pins header, the presence of a report-uri
   directive indicates to the UA that the UA MUST enforce Pin
   Validation, and the UA SHOULD also, in the event of Pin Validation
   failure, POST a report to the report-uri.

   Note that the report-uri need not necessarily be in the same Internet
   domain or web origin as the Known Pinned Host.

   Hosts may set report-uris that use HTTP, HTTPS, or other schemes.  If
   the scheme in the report-uri is 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.

   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 (and MAY attempt to re-send the report
   later).  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.

   UAs SHOULD limit the rate at which they send reports.  For example,
   it is unnecessary to send the same report to the same report-uri more
   than once.

2.1.4.  The strict Directive

   The OPTIONAL "strict" directive is a valueless directive which, if
   present (i.e., it is "asserted"), signals to the UA that the Pinning
   Policy contained should be applied to the Pinned Host exactly as
   specified, ignoring local client policy.

2.1.5.  Examples

   Figure 3 shows some example response header fields using the pins
   extension (folded for clarity).

   Public-Key-Pins: max-age=500;

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   Public-Key-Pins: max-age=31536000;

   Public-Key-Pins: pin-sha1="4n972HfV354KP560yw4uqe/baXc=";

   Public-Key-Pins: pin-sha1="4n972HfV354KP560yw4uqe/baXc=";
       max-age=2592000; includeSubDomains

                      Figure 3: HPKP Header Examples

2.2.  Server Processing Model

   This section describes the processing model that Pinned Hosts
   implement.  The model comprises two facets: the processing rules for
   HTTP request messages received over a secure transport (e.g. TLS
   [RFC5246]); and 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 that MUST satisfy the grammar specified above in
   Section 2.1.  If the Pinned Host does not include the PKP header
   field, and if the connection passed Pin Validation, UAs MUST treat
   the host as if it had set its max-age to 0 (see Section 2.3.1).

   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

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2.3.  User Agent Processing Model

   This section describes the HTTP Public Key Pinning processing model
   for UAs.

   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 HSTS Host if it is not already so noted
      (see Section 2.3.2),


   o  Update the UA's cached information for the Known Pinned Host if
      any of of the max-age, includeSubDomains, strict, 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 and strict directives, 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 in an HTTP
      response message over secure transport, then the UA MUST process
      only the first such header field.


   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  The UA MUST ignore any PKP header fields not conforming to the
      grammar specified in Section 2.1.

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2.3.2.  Noting a Pinned Host - Storage Model

   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 time of the observation (also known as the Effective Pin
   Date), the value of the max-age directive, whether or not the
   includeSubDomains or strict directives are asserted, the value of the
   report-uri directive (if present), and any other metadata from
   optional or future PKP header directives.

   UAs SHOULD 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 SHOULD behave as if the
   server set the max-age to the UA's upper limit.  For example, if the
   UA caps max-age at 2592000 seconds (30 days), and a Pinned Host sets
   a max-age directive of 60 days in its Valid Pinning Header, the UA
   SHOULD behave as if the max-age were effectively 30 days.  (One way
   to achieve this behavior is for the UA to simply store a value of 30
   days instead of the 60 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.

   A Known Pinned Host is "expired" if the Effective Pin Date plus the
   max-age 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.

2.3.3.  HTTP-Equiv <Meta> Element Attribute

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

2.3.4.  UA Processing Examples


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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
   algorithms are SHA-1, identified as "sha1", and SHA-256, identified
   as "sha256".  (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 "sha1" and "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 SubjectPublicKeyInfo of a certificate 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 SubjectPublicKeyInfo fields in certificates.

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2.5.  Noting Pins

   Upon receipt of the Public-Key-Pins 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
   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 Public-
      Key-Pins 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 Public-Key-Pins response header field does not meet all three
   of these criteria, the UA MUST NOT note the host as a Pinned Host.  A
   Public-Key-Pins response header field that meets all these critera is
   known as a Valid Pinning Header.

   The UA MUST ignore Public-Key-Pins response header fields received on
   connections that do not meet the first criterion.

   Whenever a UA receives a Valid Pinning Header, it MUST set its
   Pinning Metadata to the exact Pins, max-age, and (if any) report-uri
   and strict mode given in the most recently received Valid Pinning

   For forward compatibility, the UA MUST ignore any unrecognized
   Public-Key-Pins header directives, while still processing those
   directives it does recognize.  Section 2.1 specifies the directives
   max-age, pins, includeSubDomains, report-uri, and strict, but future
   specifications and implementations might use additional directives.

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

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   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.
   However, if the Pinned Host Metadata indicates that the Pinned Host
   is operating in "strict mode" (see Section 2.1.4), then the UA MUST
   perform Pin Validation.

   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.
   (For the purposes of Pin Validation, the UA MUST ignore certificates
   whose SPKI cannot be taken in isolation and 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 Failure as a non-
   recoverable error.  Any procedure that matches the results of this
   Pin Validation procedure is considered equivalent.

   Note that, 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

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.

   UAs that support additional sources of pinning information MUST use
   the most recently observed pinning information when performing Pin
   Validation for a host.  The most recently observed pinning
   information is determined based upon the most recent Effective Pin
   Date, as described in Section 2.3.2.

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   If the result of noting a Valid Pinning Header is to disable pinning
   for the host, such as through supplying a max-age directive with a
   value of 0, UAs MUST allow this new information to override any other
   pinning data.  That is, a host must be able to un-pin itself, even in
   the presence of built-in pins.

   Example: A UA may ship with a pre-configured list of pins that are
   collected from past observations of Valid Pinning Headers supplied by
   hosts.  In such a solution, the pre-configured list should track when
   the Valid Pinning Header was last observed, in order to permit site
   operators to later update the value by supplying a new Valid Pinning
   Header.  Updates to such a pre-configured list should not update the
   Effective Pin Dates for each host unless the list vendor has actually
   observed a more recent header.  This is to prevent situations where
   updating the Effective Pin Date on a pre-configured list of pins may
   effectively extend the max-age beyond the site operator's stated

   Example: An alternative example would be a UA that ships with a pre-
   configured list of pins that are collected through out-of-band means,
   such as direct contact with the site operator.  In such a solution,
   the site operator accepts responsibility for keeping the configured
   Valid Pinning Header in sync with the vendor's list, allowing the UA
   vendor to have each update to the list be treated as as an update of
   the Effective Pin Date.

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 message to the URI; the JSON message
   takes this form:

     "date-time": date-time,
     "hostname": hostname,
     "port": port,
     "certificate-chain": [
       pem1, ... pemN
     "known-pins": [

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       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 SHOULD appear only

   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 either as a string or as an

   The certificate-chain is the certificate chain, as constructed by the
   UA during certificate chain verification.  (This may differ from the
   certificate chain as served by the Known Pinned Host, of course.)  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 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.

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

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   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 2.3.2, UAs SHOULD 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 30
   days (2592000 seconds) may be considered a balance between the two
   competing security concerns.

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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 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: pin-sha1="ABC..."; pin-sha1="DEF..."; includeSubdomains

                Figure 7: example.com Valid Pinning Header

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

   Public-Key-Pins: pin-sha1="GHI..."; pin-sha1="JKL..."

           Figure 8: 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 first
   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 take care that they 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 inadvertant Pin Failure is
   to keep a Backup Pin. A Backup Pin is a fingerprint for the public

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   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 Public-Key-
   Pins 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.)

5.  IANA Considerations

   This document has no actions for IANA.

6.  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

7.  Acknowledgements

   Thanks to Tobias Gondrom, Jeff Hodges, Adam Langley, Nicolas
   Lidzborski, SM, James Manger, Eric Rescorla, Paul Hoffman, and Yoav
   Nir for suggestions and edits that clarified the text.  Thanks to
   Trevor Perrin for suggesting a mechanism to affirmatively break pins
   ([pin-break-codes]).  Adam Langley provided the SPKI fingerprint
   generation code.

8.  What's Changed

   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).

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   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.

9.  References

9.1.  Normative References

              Josefsson, S. and S. Leonard, "Text Encodings of PKIX and
              CMS Structures", draft-josefsson-pkix-textual-01 (work in
              progress), July 2012.

   [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.

   [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.

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   [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.

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

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

9.2.  Informative References

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

              Perrin, T., "Self-Asserted Key Pinning", September 2011,

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

Appendix A.  Fingerprint Generation

   This Go program generates SPKI Fingerprints, suitable for use in
   pinning, from PEM-encoded certificates.  It is non-normative.

   package main

   import (

   func main() {
          if len(os.Args) < 2 {
                  fmt.Printf("Usage: %s PEM-filename\n", os.Args[0])

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          pemBytes, err := ioutil.ReadFile(os.Args[1])
          if err != nil {
          block, _ := pem.Decode(pemBytes)
          if block == nil {
                  panic("No PEM structure found")
          derBytes := block.Bytes
          certs, err := x509.ParseCertificates(derBytes)
          if err != nil {
          cert := certs[0]
          h := sha1.New()
          digest := h.Sum()

          fmt.Printf("Hex: %x\nBase64: %s\n", digest,

            Figure 9: 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.

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   o  Operators SHOULD periodically exercise their Backup Pin plan
      -\u002D 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

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