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Versions: (draft-miller-xmpp-posh-prooftype) 00 01 02 03 draft-ietf-xmpp-posh

Network Working Group                                          M. Miller
Internet-Draft                                            P. Saint-Andre
Intended status: Standards Track                     Cisco Systems, Inc.
Expires: March 31, 2014                               September 27, 2013


                      PKIX over Secure HTTP (POSH)
                          draft-miller-posh-02

Abstract

   Experience has shown that it is extremely difficult to deploy proper
   PKIX certificates for TLS in multi-tenanted environments, since
   certification authorities will not issue certificates for hosted
   domains to hosting services, hosted domains do not want hosting
   services to hold their private keys, and hosting services wish to
   avoid liability for holding those keys.  As a result, domains hosted
   in multi-tenanted environments often deploy non-HTTP applications
   such as email and instant messaging using certificates that identify
   the hosting service, not the hosted domain.  Such deployments force
   end users and peer services to accept a certificate with an improper
   identifier, resulting in obvious security implications.  This
   document defines two methods that make it easier to deploy
   certificates for proper server identity checking in non-HTTP
   application protocols.  The first method enables the TLS client
   associated with a user agent or peer application server to obtain the
   end-entity certificate of a hosted domain over secure HTTP as an
   alternative to standard PKIX techniques.  The second method enables a
   hosted domain to securely delegate a non-HTTP application to a
   hosting service using redirects provided by HTTPS itself or by a
   pointer in a file served over HTTPS at the hosted domain.

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 March 31, 2014.



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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
   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
   2.  Discussion Venue . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Obtaining Verification Materials . . . . . . . . . . . . . . .  4
     4.1.  Source Domain Possesses PKIX Certificate . . . . . . . . .  6
     4.2.  Source Domain References PKIX Certificate  . . . . . . . .  7
     4.3.  Performing Verification  . . . . . . . . . . . . . . . . .  8
   5.  Secure Delegation  . . . . . . . . . . . . . . . . . . . . . .  9
   6.  Order of Operations  . . . . . . . . . . . . . . . . . . . . .  9
   7.  Caching Results  . . . . . . . . . . . . . . . . . . . . . . . 10
   8.  Alternates and Roll-over . . . . . . . . . . . . . . . . . . . 10
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 13
     11.2. Informative References . . . . . . . . . . . . . . . . . . 14
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 15
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15















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1.  Introduction

   We start with a thought experiment.

   Imagine that you work on the operations team of a hosting company
   that provides the "foo" service (or email or instant messaging or
   social networking service) for ten thousand different customer
   organizations.  Each customer wants their service to be identified by
   the customer's domain name (e.g., foo.example.com), not the hosting
   company's domain name (e.g., hosting.example.net).

   In order to properly secure each customer's "foo" service via
   Transport Layer Security (TLS) [RFC5246], you need to obtain PKIX
   certificates [RFC5280] containing identifiers such as
   foo.example.com, as explained in the "CertID" specification
   [RFC6125].  Unfortunately, you can't obtain such certificates
   because:

   o  Certification authorities won't issue such certificates to you
      because you work for the hosting company, not the customer
      organization.

   o  Customers won't obtain such certificates and then give them (plus
      the associated private keys) to you because their legal department
      is worried about liability.

   o  You don't want to install such certificates (plus the associated
      private keys) on your servers anyway because your legal department
      is worried about liability, too.

   Given your inability to deploy public keys / certificates containing
   the right identifiers, your back-up approach was always to use a
   certificate containing hosting.example.net as the identifier.
   However, more and more customers and end users are complaining about
   warning messages in user agents and the inherent security issues
   involved with taking a "leap of faith" to accept the identity
   mismatch between the source domain (foo.example.com) and the
   delegated domain (hosting.example.net).

   This situation is both insecure and unsustainable.  You have
   investigated the possibility of using DNS Security [RFC4033] and DNS-
   Based Authentication of Named Entities (DANE) [RFC6698] to solve the
   problem.  However, your customers and your operations team have told
   you that they will not be able to deploy DNSSEC and DANE for several
   years at least.  The product managers in your company are pushing you
   to find a method that can be deployed more quickly to overcome the
   lack of proper server identity checking for your hosted customers.




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   One possible approach is to ask each customer to provide the public
   key / certificate for the "foo" service at a special HTTPS URI on
   their website ("https://foo.example.com/.well-known/posh.foo.json" is
   one possibility).  This could be a public key that you generate for
   the customer, but because the customer hosts it via HTTPS, any user
   agent can find that public key and check it against the public key
   you provide during TLS negotiation for the "foo" service (as one
   added benefit, the customer never needs to hand you a private key).
   Alternatively, the customer can redirect requests for that special
   HTTPS URI to an HTTPS URI at your own website, thus making it
   explicit that they have delegated the "foo" service to you.

   The approach sketched out above, called POSH ("PKIX Over Secure
   HTTP"), is explained in the remainder of this document.


2.  Discussion Venue

   The discussion venue for this document is the posh@ietf.org mailing
   list; visit https://www.ietf.org/mailman/listinfo/posh for
   subscription information and discussion archives.


3.  Terminology

   This document inherits security terminology from [RFC5280].  The
   terms "source domain", "derived domain", "reference identifier", and
   "presented identifier" are used as defined in the "CertID"
   specification [RFC6125].

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


4.  Obtaining Verification Materials

   Server identity checking (see [RFC6125]) involves three different
   aspects:

   1.  A proof of the TLS server's identity (in PKIX, this takes the
       form of a PKIX certificate [RFC5280]).

   2.  Rules for checking the certificate (which vary by application
       protocol, although [RFC6125] attempts to harmonize those rules).





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   3.  The materials that a TLS client uses to verify the TLS server's
       identity or check the TLS server's proof (in PKIX, this takes the
       form of chaining the end-entity certificate back to a trusted
       root and performing all validity checks as described in
       [RFC5280], [RFC6125], and the relevant application protocol
       specification).

   When POSH is used, the first two aspects remain the same: the TLS
   server proves it identity by presenting a PKIX certificate [RFC5280]
   and the certificate is checked according to the rules defined in the
   appropriate application protocol specification (such as [RFC6120] for
   XMPP).  However, the TLS client obtains the materials it will use to
   verify the server's proof by retrieving a JSON Web Key (JWK) set
   [JOSE-JWK] over HTTPS ([RFC2616] and [RFC2818]) from a well-known URI
   [RFC5785].

   The process for retrieving a PKIX certificate over secure HTTP is as
   follows.

   1.  The TLS client performs an HTTPS GET at the source domain to the
       path "/.well-known/posh.{servicedesc}.json".  The value of
       "{servicedesc}" is application-specific; see Section 9 of this
       document for more details.  For example, if the application
       protocol is some hypothetical "Foo" service, then "{servicedesc}"
       could be "foo"; thus if a Foo client were to use POSH to verify a
       Foo server for the domain "foo.example.com", the HTTPS GET
       request would be as follows:


   GET /.well-known/posh.foo.json HTTP/1.1
   Host: foo.example.com


   2.  The source domain HTTPS server responds in one of three ways:

       *  If it possesses a PKIX certificate for the requested path, it
          responds as detailed in Section 4.1.

       *  If it has a reference to where the PKIX certificate can be
          obtained, it responds as detailed in Section 4.2.

       *  If it does not have any PKIX certificate for the requested
          path, it responds with a client error status code (e.g., 404).








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4.1.  Source Domain Possesses PKIX Certificate

   If the source domain HTTPS server possesses the certificate
   information, it responds to the HTTPS GET with a success status code
   and the message body set to a JSON Web Key (JWK) set [JOSE-JWK].  The
   JWK set MUST contain at least one JWK object, and MUST contain an
   "expires" field whose value is the number of seconds after which the
   TLS client ought to consider the key information to be stale (further
   explained under Section 7).

   Each included JWK object MUST possess the following information:

   o  The "kty" field set to the appropriate key type used for TLS
      connections (e.g., "RSA" for a certificate using an RSA key).

   o  The required public parameters for the key type (e.g., "n" and "e"
      for a certificate using an RSA key).

   o  The "x5t" field set to the certificate thumbprint, as described in
      section 3.6 of [JOSE-JWK].

   Each JWK object MUST NOT possess the private parameters for the key
   type (e.g., "d", "p", "q" for a certificate using an RSA key).

   Each JWK object MAY possess other parameters as desired by
   application servers (e.g., the "x5c" field containing the entire
   X.509 certificate chain, as per section 3.7 of [JOSE-JWK]).

   The following example illustrates the usage described above.






















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   Example Content Response

   HTTP/1.1 200 OK
   Content-Type: application/jwk-set+json
   Content-Length: 2785

   {
     "keys": [
       {
         "kty":"RSA",
         "kid":"c8fb8b80-1193-11e3-b2b1-835742119fe8",
         "n":"ANxwssdcU3LbODErec3owrwUhlzjtuskAn8rAcBMRPImn5xA
              JRX-1T5g2D7MTozWWFk4TlpgzAR5slvM0tc35qAI9I0Cqk4Z
              LChQrYsWuY7alTrnNXdusHUYc6Eq89DZaH2knTcp57wAXzJP
              IG_tpBi5F7ck9LVRvRjybix0HJ7i4YrL-GeLuSgrjO4-GDcX
              Ip8oV0FMKZH-NoMfUITlWYl_JcX1D0WUAiuAnvWtD4Kh_qMJ
              U6FZuupZGHqPdc3vrXtp27LWgxzxjFa9qnOU6y53vCCJXLLI
              5sy2fCwEDzLJqh2T6UItIzjrSUZMIsK8r2pXkroI0uYuNn3W
              y-jAzK8",
         "e":"AQAB",
         "x5t":"UpjRI_A3afKE8_AIeTZ5o1dECTY"
       }
     ],
     "expires": 604800
   }


   The "expires" value is a hint regarding the expiration of the keying
   materials.  If no "expires" field is included, a TLS client SHOULD
   consider these verification materials invalid.  See Section 7 for how
   to reconcile this "expires" field with the reference's "expires"
   field.

4.2.  Source Domain References PKIX Certificate

   If the source domain HTTPS server has a reference to the certificate
   information, it responds to the HTTPS GET with a JSON document.  The
   document MUST contain a "url" field whose value is the HTTPS URL
   where TLS clients can obtain the actual JWK set, and MUST contain an
   "expires" field whose value is the number of seconds after which the
   TLS client ought to consider the delegation to be stale (further
   explained under Section 7).









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   Example Reference Response

   HTTP/1.1 200 Ok
   Content-Type: application/json
   Content-Length: 78

   {
     "url":"https://hosting.example.net/.well-known/posh.foo.json",
     "expires":86400
   }

   The client performs an HTTPS GET for the URL specified in the "url"
   field value.  The HTTPS server for the URI to which the client has
   been redirected responds to the request with a JWK set.  The content
   retrieved from the "url" location MUST NOT itself be a reference
   (i.e., containing a "url" fields instead of a "keys" field), in order
   to prevent circular delegations.

      Note: The JSON document returned by the source domain HTTPS server
      MUST contain either a reference or a JWK-set, but MUST NOT contain
      both.

      Note: See Section 10 for discussion about HTTPS redirects.

   The "expires" value is a hint regarding the expiration of the source
   domain's delegation of service to the delegated domain.  If no
   "expires" field is included, a TLS client SHOULD consider the
   delegation invalid.  See Section 7 for guidelines about reconciling
   this "expires" field with the JWK-set's "expires" field.

4.3.  Performing Verification

   The TLS client compares the PKIX information obtained from the TLS
   server against each JWK object in the POSH results, until a match is
   found or the collection of POSH verification materials is exhausted.
   If none of the JWK objects match the TLS server PKIX information, the
   TLS client SHOULD reject the connection (the TLS client might still
   accept the connection if other verification schemes are successful).

   The TLS client SHOULD compare the fingerprint of the PKIX certificate
   from the TLS server against the "x5t" field of the JWK object (note
   the "x5t" field is the base64url encoding of the fingerprint).

   The TLS client MAY verify the certificate chain provided in the "x5c"
   field of the JWK object (if present), but it MUST NOT implicitly
   consider the final certificate in the "x5c" field to be a trust
   anchor itself; the TLS client only uses the end entity certificate
   information for verification.



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5.  Secure Delegation

   The delegation from the source domain to the delegated domain can be
   considered secure if the certificate offered by the TLS server
   matches the POSH certificate, regardless of how the POSH certificates
   are obtained.


6.  Order of Operations

   In order for the TLS client to perform verification of reference
   identifiers without potentially compromising data, POSH processes
   MUST be complete before any application-level data is exchanged for
   the source domain.  The TLS client SHOULD perform all POSH retrievals
   before opening any socket connections to the application protocol
   server.  For application protocols that use DNS SRV, the POSH
   processes ideally ought to be done in parallel with resolving the SRV
   records and the addresses of any targets, similar to the "happy
   eyeballs" approach for IPv4 and IPv6 [RFC6555].

   The following diagram illustrates the possession flow:



   Client                 Domain                 Server
   ------                 ------                 ------
     |                      |                      |
     |    Request POSH      |                      |
     |--------------------->|                      |
     |                      |                      |
     |   Return POSH keys   |                      |
     |<---------------------|                      |
     |                      |                      |
     |              Service TLS Handshake          |
     |<===========================================>|
     |                      |                      |
     |                 Service Data                |
     |<===========================================>|
     |                      |                      |

   While the following diagram illustrates the reference flow:










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   Client                 Domain                 Server
   ------                 ------                 ------
     |                      |                      |
     |    Request POSH      |                      |
     |--------------------->|                      |
     |                      |                      |
     |   Return POSH url    |                      |
     |<---------------------|                      |
     |                      |                      |
     |                  Request POSH               |
     |-------------------------------------------->|
     |                      |                      |
     |                Return POSH keys             |
     |<--------------------------------------------|
     |                      |                      |
     |             Service TLS Handshake           |
     |<===========================================>|
     |                      |                      |
     |                 Service Data                |
     |<===========================================>|
     |                      |                      |


7.  Caching Results

   The TLS client MUST NOT cache results (reference or JWK-set)
   indefinitely.  If the source domain returns a reference, the TLS
   client MUST use the lower of the two "expires" values when
   determining how long to cache results (i.e., if the reference
   "expires" value is lower than the JWK-set "expires" value, honor the
   reference "expires" value).  Once the TLS client considers the
   results stale, it SHOULD perform the entire POSH process again
   starting with the HTTPS GET to the source domain.  The TLS client MAY
   use a lower value than any provided in the "expires" field(s), or not
   cache results at all.

   The TLS client SHOULD NOT rely on HTTP caching mechanisms, instead
   using the expiration hints provided in the POSH reference or JWK-set
   documents.  To that end, the HTTPS servers for source and derived
   domains SHOULD specify a 'Cache-Control' header indicating a very
   short duration (e.g., max-age=60) or "no-cache" to indicate that the
   response (redirect, reference, or content) is not appropriate to
   cache at the HTTP level.


8.  Alternates and Roll-over

   To indicate alternate PKIX certificates (such as when an existing



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   certificate will soon expire), the returned JWK set MAY contain
   multiple JWK objects.  The JWK set SHOULD be ordered with the most
   relevant certificate first as determined by the application service
   operator (e.g., the renewed certificate), followed by the next most
   relevant certificate (e.g., the certificate soonest to expire).  Here
   is an example:

   {
     "keys":[
       {
         "kty": "RSA",
         "kid": "cfc0ca70-1193-11e3-b2b1-835742119fe8",
         "n":   "AM-ktWkQ8btj_HEdAA6kOpzJGgoHNZsJmxjh_PifpgAUfQeq
                 MO_YBR100IdJZRzJfULyhRwn9bikCq87WToxgPWOnd3sH3qT
                 YiAcIR5S6tBbsyp6WYmwM1yuC0vLCo6SoDzdK1SvkQKM3QWk
                 0GFNU4l4qXYAMxaSw83i6yv5DBVbST7E92vS6Gq_4pgI26l1
                 0JhybZuTEVPRUCG6pTKAXQpLxmjJ5oG9M91RP17nsuQeE7Ng
                 0Ap4BBn5hocojkfthwgbX4lqBMecpBAnky5jn6slmzS_rL-L
                 w-_8hUldaTPD9MHlHPrvcsRV5uw8wK5MB6QyfS6wF4b0Kj2T
                 vYceNlE",
         "e":   "AQAB",
         "x5t": "Ae0sLVtm78VT-mQXJQop-ENOM6o"
       },
       {
         "kty": "RSA",
         "kid": "dbc28570-1193-11e3-b2b1-835742119fe8",
         "n":   "AM-ktWkQ8btj_HEdAA6kOpzJGgoHNZsJmxjh_PifpgAUfQeq
                 MO_YBR100IdJZRzJfULyhRwn9bikCq87WToxgPWOnd3sH3qT
                 YiAcIR5S6tBbsyp6WYmwM1yuC0vLCo6SoDzdK1SvkQKM3QWk
                 0GFNU4l4qXYAMxaSw83i6yv5DBVbST7E92vS6Gq_4pgI26l1
                 0JhybZuTEVPRUCG6pTKAXQpLxmjJ5oG9M91RP17nsuQeE7Ng
                 0Ap4BBn5hocojkfthwgbX4lqBMecpBAnky5jn6slmzS_rL-L
                 w-_8hUldaTPD9MHlHPrvcsRV5uw8wK5MB6QyfS6wF4b0Kj2T
                 vYceNlE",
         "e":   "AQAB",
         "x5t": "lYZC2n9TBpOaUsBclEIacQTKToA"
       }
     ]
   }


9.  IANA Considerations

   This document registers a well-known URI [RFC5785] for protocols that
   use POSH.  The completed template follows.






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      URI suffix:  posh.
      Change controller:  IETF
      Specification document:  [[ this document ]]
      Related information:  Because the "posh." string is merely a
         prefix, protocols that use POSH need to register particular
         URIs that are prefixed with the "posh." string.

   Note that the registered URI is "posh." (with a trailing dot).  This
   is merely a prefix to be placed at the front of well-known URIs
   [RFC5785] registered by protocols that use POSH, which themselves are
   responsible for the relevant registrations with the IANA.  The URIs
   registered by such protocols SHOULD match the URI template [RFC6570]
   path "/.well-known/posh.{servicedesc}.json"; that is, begin with
   "posh." and end with ".json" (indicating a media type of application/
   json [RFC4627] or application/jwk-set+json [JOSE-JWK]).

   For POSH-using protocols that rely on DNS SRV records [RFC2782], the
   "{servicedesc}" part of the well-known URI SHOULD be
   "{service}.{proto}", where the "{service}" is the DNS SRV "Service"
   prepended by the underscore character "_" and the "{proto}" is the
   DNS SRV "Proto" also prepended by the underscore character "_".  As
   an example, the well-known URI for XMPP server-to-server connections
   would be "posh._xmpp-server._tcp.json" since XMPP [RFC6120] registers
   a service name of "xmpp-server" and uses TCP as the underlying
   transport protocol.

   For other POSH-using protocols, the "{servicedesc}" part of the well-
   known URI can be any unique string or identifier for the protocol,
   which might be a service name registered with the IANA in accordance
   with [RFC6335] or which might be an unregistered name.  As an
   example, the well-known URI for the mythical "Foo" service could be
   "posh.foo.json".

   Note: As explained in [RFC5785], the IANA registration policy
   [RFC5226] for well-known URIs is Specification Required.


10.  Security Considerations

   This document supplements but does not supersede the security
   considerations provided in specifications for application protocols
   that decide to use POSH (e.g., [RFC6120] and [RFC6125] for XMPP).
   Specifically, the security of requests and responses sent via HTTPS
   depends on checking the identity of the HTTP server in accordance
   with [RFC2818].  Additionally, the security of POSH can benefit from
   other HTTP hardening protocols, such as HSTS [RFC6797] and key
   pinning [KEYPIN], especially if the TLS client shares some



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   information with a common HTTPS implementation (e.g., platform-
   default web browser).

   Note well that POSH is used by a TLS client to obtain the public key
   of a TLS server to which it might connect for a particular
   application protocol such as IMAP or XMPP.  POSH does not enable a
   hosted domain to transfer private keys to a hosting service via
   HTTPS.  POSH also does not enable a TLS server to engage in
   certificate enrollment with a certification authority via HTTPS, as
   is done in Enrollment over Secure Transport [EST].

   A web server at the source domain might redirect an HTTPS request to
   another URL.  The location provided in the redirect response MUST
   specify an HTTPS URL.  Source domains SHOULD use only temporary
   redirect mechanisms, such as HTTP status codes 302 (Found) and 307
   (Temporary Redirect).  Clients MAY treat any redirect as temporary,
   ignoring the specific semantics for 301 (Moved Permanently) and 308
   (Permanent Redirect) [HTTP-STATUS-308].  To protect against circular
   references, clients MUST NOT follow an infinite number of redirects.
   It is RECOMMENDED that clients follow no more than 10 redirects,
   although applications or implementations can require that fewer
   redirects be followed.


11.  References

11.1.  Normative References

   [JOSE-JWK]
              Jones, M., "JSON Web Key (JWK)",
              draft-ietf-jose-json-web-key-16 (work in progress),
              September 2013.

   [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.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

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

   [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.

   [RFC5785]  Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
              Uniform Resource Identifiers (URIs)", RFC 5785,
              April 2010.

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, March 2011.

11.2.  Informative References

   [EST]      Pritikin, M., Yee, P., and D. Harkins, "Enrollment over
              Secure Transport", draft-ietf-pkix-est-09 (work in
              progress), August 2013.

   [HTTP-STATUS-308]
              Reschke, J., "The Hypertext Transfer Protocol (HTTP)
              Status Code 308 (Permanent Redirect)",
              draft-reschke-http-status-308-07 (work in progress),
              March 2012.

   [KEYPIN]   Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
              Extension for HTTP", draft-ietf-websec-key-pinning-08
              (work in progress), July 2013.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, May 2005.

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

   [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 6120, March 2011.

   [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
              Cheshire, "Internet Assigned Numbers Authority (IANA)



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              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", BCP 165,
              RFC 6335, August 2011.

   [RFC6555]  Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
              Dual-Stack Hosts", RFC 6555, April 2012.

   [RFC6570]  Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
              and D. Orchard, "URI Template", RFC 6570, March 2012.

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, August 2012.

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


Appendix A.  Acknowledgements

   Many thanks to Philipp Hancke, Joe Hildebrand, and Tobias Markmann
   for their implementation feedback.  Thanks also to Dave Cridland, Max
   Pritikin, and Joe Salowey for their input on the specification.


Authors' Addresses

   Matthew Miller
   Cisco Systems, Inc.
   1899 Wynkoop Street, Suite 600
   Denver, CO  80202
   USA

   Email: mamille2@cisco.com


   Peter Saint-Andre
   Cisco Systems, Inc.
   1899 Wynkoop Street, Suite 600
   Denver, CO  80202
   USA

   Email: psaintan@cisco.com








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