draft-ietf-tls-oob-pubkey-06.xml

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<rfc category="std" docName="draft-ietf-tls-oob-pubkey-06.txt" ipr="trust200902">

  <front>
    <!-- The abbreviated title is used in the page header - it is only necessary if the 
         full title is longer than 39 characters -->

    <title abbrev="TLS OOB Public Key Validation">Out-of-Band Public Key Validation for Transport Layer Security (TLS)</title>

    <!-- add 'role="editor"' below for the editors if appropriate -->

    <author role="editor" fullname="Paul Wouters" initials="P." surname="Wouters">
      <organization>Red Hat</organization>
      <address>
        <postal>
          <street/>
          <city/>
          <region/>
          <code/>
          <country/>
        </postal>
        <email>paul@nohats.ca</email>
      </address>
    </author>


        <author role="editor"  initials="H." surname="Tschofenig" fullname="Hannes Tschofenig">
            <organization>Nokia Siemens Networks</organization>
            <address>
                <postal>
                    <street>Linnoitustie 6</street>
                    <city>Espoo</city>
                    <code>02600</code>
                    <country>Finland</country>
                </postal>
                <phone>+358 (50) 4871445</phone>
                <email>Hannes.Tschofenig@gmx.net</email>
                <uri>http://www.tschofenig.priv.at</uri>
            </address>
        </author>


    <author fullname="John Gilmore" initials="J." surname="Gilmore">
      <organization />
      <address>
        <postal>
          <street>PO Box 170608</street>
          <city>San Francisco</city>
          <region>California</region>
          <code>94117</code>
          <country>USA</country>
        </postal>
        <phone>+1 415 221 6524</phone>
        <email>gnu@toad.com</email>
        <uri>https://www.toad.com/</uri>
      </address>
    </author>

    <author fullname="Samuel Weiler" initials="S." surname="Weiler">
      <organization>SPARTA, Inc.</organization>
      <address>
        <postal>
          <street>7110 Samuel Morse Drive</street>
          <city>Columbia, Maryland</city>
          <code>21046</code>
          <country>US</country>
        </postal>
        <email>weiler@tislabs.com</email>
      </address>
    </author>

 <author initials="T." surname="Kivinen" fullname="Tero Kivinen">
            <organization>AuthenTec</organization>
            <address>
                <postal>
                    <street>Eerikinkatu 28</street>
                    <city>HELSINKI</city>
                    <code>FI-00180</code>
                    <country>FI</country>
                </postal>
                <email>kivinen@iki.fi</email>
            </address>
        </author>
    <date year="2012" />


    <!-- If the month and year are both specified and are the current ones, xml2rfc will fill 
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         to specify at least a month (xml2rfc assumes day="1" if not specified for the 
         purpose of calculating the expiry date).  With drafts it is normally sufficient to 
         specify just the year. -->

    <!-- Meta-data Declarations -->

    <area>Security</area>

    <workgroup>TLS</workgroup>

    <!-- WG name at the upperleft corner of the doc,
         IETF is fine for individual submissions.  
         If this element is not present, the default is "Network Working Group",
         which is used by the RFC Editor as a nod to the history of the IETF. -->

    <keyword>TLS</keyword>
    <keyword>DNSSEC</keyword>
    <keyword>DANE</keyword>
    <keyword>Raw Public Key</keyword>

    <!-- Keywords will be incorporated into HTML output
         files in a meta tag but they have no effect on text or nroff
         output. If you submit your draft to the RFC Editor, the
         keywords will be used for the search engine. -->

    <abstract>
      <t>
         This document specifies a new certificate type for exchanging
         raw public keys in Transport Layer Security (TLS)
         and Datagram Transport Layer Security (DTLS) for use with out-of-band
         public key validation. Currently, TLS authentication can only occur via X.509-based Public
Key Infrastructure (PKI) or OpenPGP certificates. By specifying a minimum resource for raw
         public key exchange, implementations can use alternative public key validation
         methods.
      </t>
      <t>
         One such alternative public key valiation method is offered by the DNS-Based Authentication of Named Entities (DANE) together with DNS Security. Another alternative is to utilize pre-configured keys, as is the case with sensors and other
         embedded devices. The usage of raw public keys, instead of X.509-based certificates, leads to a smaller 
         code footprint. 
      </t>

      <t>This document introduces the support for raw public keys in TLS.</t>
    </abstract>
  </front>

   <middle>
    <section anchor="into" title="Introduction">
       <t>Traditionally, TLS server public keys are obtained in PKIX containers
          in-band using the TLS handshake and validated using trust anchors
          based on a  <xref target='PKIX'/> certification authority (CA). This
          method can add a complicated trust relationship that is difficult
          to validate. Examples of such complexity can be seen in
          <xref target='Defeating-SSL'/>.</t>

       <t>Alternative methods are available that allow a TLS client to obtain
          the TLS server public key:</t>

       <t><list style="symbols">
          <t>The TLS server public key is obtained from a DNSSEC secured resource records              
             using DANE <xref target="RFC6698"/>.</t>

          <t>The TLS server public key is obtained from a <xref target='PKIX'/>
             certificate chain from an Lightweight Directory Access Protocol (LDAP) <xref target="LDAP"/> server.</t>

          <t>The TLS client and server public key is provisioned into the operating system firmware image,
             and updated via software updates.</t>

          </list>
       </t>

       <t>Some smart objects use the UDP-based Constrained Application Protocol (CoAP) <xref target="I-D.ietf-core-coap"/> to
	      interact with a Web server to upload sensor data at a regular intervals, such as
          temperature readings. CoAP <xref target="I-D.ietf-core-coap"/> can utilize
          DTLS for securing the client-to-server communication. As part of the manufacturing process,
          the embeded device may be configured with the address and the public key of a
          dedicated CoAP server, as well as a public key for the client itself. 
		  The usage of X.509-based PKIX certificates <xref target='PKIX'/> may not suit all 
		  smart object deployments and would therefore be an unneccesarry burden.
       </t>
	   
	   <t>The Transport Layer Security (TLS) Protocol Version 1.2 <xref target="RFC5246"/> provides
         a framework for extensions to TLS as well as guidelines for
         designing such extensions.  This document defines an extension to indicate the support for raw public keys. </t>
      </section>
	  
      <section title="Terminology" anchor="terminology">
        <t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
        "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
        document are to be interpreted as described in <xref target="RFC2119">RFC&nbsp;2119</xref>.</t>

      </section>


    <section title="New TLS Extension">
  <t>
   This section describes the changes to the TLS handshake message
   contents when raw public key certificates are to be used.
   <xref target="flow"/> illustrates the exchange of messages as
   described in the sub-sections below. The client and the server exchange 
   the newly defined certificate_type extension to indicate their ability and desire to
   exchange raw public keys. These raw public keys, in the form of a SubjectPublicKeyInfo structure, are then carried inside the
   certificate payload. The SubjectPublicKeyInfo structure is defined in Section 4.1 of RFC 5280. Note that the SubjectPublicKeyInfo block does not only contain the raw keys, such as the public exponent and the modulus of an RSA public key, but also an algorithm identifier. The structure, as shown in <xref target="SubjectPublicKeyInfo"/>, is encoded in an ASN.1 format and therefore contains length information as well.</t>
   <t>
<figure anchor="SubjectPublicKeyInfo" title="SubjectPublicKeyInfo ASN.1 Structure.">
<artwork>
<![CDATA[    
   SubjectPublicKeyInfo  ::=  SEQUENCE  {
        algorithm            AlgorithmIdentifier,
        subjectPublicKey     BIT STRING  }
]]>
</artwork>
</figure>
</t>

  <t>The algorithm identifiers are Object Identifiers (OIDs). RFC 3279 <xref target="RFC3279"/>, for example, defines the following OIDs shown in <xref target="alg-ids"/>.</t>
  <t>
<figure anchor="alg-ids" title="Example Algorithm Identifiers.">
<artwork>
<![CDATA[    
Key Type               | Document                   | OID
-----------------------+----------------------------+-------------------
RSA                    | Section 2.3.1 of RFC 3279  | 1.2.840.113549.1.1
.......................|............................|...................
Digital Signature      |                            |
Algorithm (DSS)        | Section 2.3.2 of RFC 3279  | 1.2.840.10040.4.1
.......................|............................|...................
Elliptic Curve         |                            |
Digital Signature      |                            |
Algorithm (ECDSA)      | Section 2.3.5 of RFC 3279  | 1.2.840.10045.2.1
-----------------------+----------------------------+-------------------
]]>
</artwork>
</figure>
</t>

  <t>
<figure anchor="flow" title="Basic Raw Public Key TLS Exchange.">
<artwork>
<![CDATA[ 
 client_hello, 
 certificate_type          ->

                           <-  server_hello,
                               certificate_type,
                               certificate,
                               server_key_exchange,
                               certificate_request,
                               server_hello_done
 certificate,
 client_key_exchange,
 certificate_verify,
 change_cipher_spec,
 finished                  ->

                           <- change_cipher_spec,
                              finished

Application Data        <------->     Application Data
]]>
</artwork>
</figure>
</t>

   <t>
   The "certificate_type" TLS extension carries a list of supported certificate types the
   client can send and receive, sorted by client preference.  Two values are defined for each certificate types to differentiate whether a client or a server is able to process a certificate of a specific type or can also send it. This extension MUST be
   omitted if the client only supports X.509 certificates.  The
   "extension_data" field of this extension contains a
   CertTypeExtension structure.</t>
   
   <t>Note that the
   CertTypeExtension structure is being used both by the client
   and the server, even though the structure is only specified once in
   this document.
   </t>

<t>The structure of the CertTypeExtension is defined as follows:<figure anchor="types" title="CertTypeExtension Structure.">
<artwork>
<![CDATA[    
enum { client, server } ClientOrServerExtension;

enum { X.509-Accept (0), 
       X.509-Offer (1), 
       RawPublicKey-Accept (2),
       RawPublicKey-Offer (3), 
       (255) 
      } CertificateType;

struct {
   select(ClientOrServerExtension) 
       case client:
         CertificateType certificate_types<1..2^8-1>;
       case server:
         CertificateType certificate_type;
   }
} CertTypeExtension;
]]>
</artwork>
</figure>
</t>
<t>The '-Offer' postfix indicates that a TLS entity is able to send the indicated certificate type to the other communication partner. The '-Accept' postfix indicates that a TLS entity is able to receive the indicated certificate type.</t>

<t>No new cipher suites are required to use raw public keys.  All
   existing cipher suites that support a key exchange method compatible
   with the defined extension can be used.</t>

</section>

   <section title="TLS Handshake Extension">
   
  <section title="Client Hello">

   <t>
   In order to indicate the support of out-of-band raw public keys,
   clients MUST include an extension of type "certificate_type" to the extended
   client hello message.  The "certificate_type" TLS extension is assigned the
   value of [TBD] from the TLS ExtensionType registry.  This value is used
   as the extension number for the extensions in both the client hello
   message and the server hello message.  The hello extension mechanism
   is described in TLS 1.2 <xref target="RFC5246"/>.
   </t>


</section>

<section title="Server Hello">

<t>If the server receives a client hello that contains the "certificate_type"
   extension and chooses a cipher suite then two outcomes are possible.
   The server MUST either select a certificate type from the CertificateType
   field in the extended client hello or terminate the session with a
   fatal alert of type "unsupported_certificate".</t>

<t>The certificate type selected by the server is encoded in a
   CertTypeExtension structure, which is included in the extended
   server hello message using an extension of type "certificate_type".  Servers
   that only support X.509 certificates MAY omit including the
   "certificate_type" extension in the extended server hello.
</t>

<t>If the client supports the receiption of raw public keys and the server is able to provide such a raw public key then the TLS server MUST
   place the SubjectPublicKeyInfo structure into the Certificate payload.
   The public key MUST match the selected key exchange algorithm.</t>

</section>

<section title="Certificate Request">

<t>
   The semantics of this message remain the same as in the TLS
   specification.
</t>

</section>

<section title="Other Handshake Messages">

<t>All the other handshake messages are identical to the TLS
   specification.</t>

</section>

<section title="Client authentication">

<t>Client authentication by the TLS server is supported only through
   authentication of the received client SubjectPublicKeyInfo via an
   out-of-band method</t>
</section>

 </section>


	  <!-- ******************************************************************************************** -->


<section title="Examples">

<t><xref target="flow1"/>, <xref target="flow2"/>, and <xref target="flow3"/> illustrate example exchanges. </t> 
   
<!--  <t> 
    The "RawPublicKey" value
   in the 'raw-public-key' extension allows the client to provide an 
   indication to the server that it supports the raw public key extension 
   in this document. The server responds with a certificate payload that 
   contains the raw public key as defined in this document. 
   Note that the certificate payloads only 
   contain the SubjectPublicKeyInfo structure instead of the entire
   certificate. 
  </t>
  
  --> 
  
  <t>The first example shows an exchange where the TLS client indicates its ability to 
  receive raw public keys. This client is quite restricted since it is unable to process other certificate types sent by the server. It also does not have credentials it could send. The 'certificate_type' extension indicates this in [1]. When the TLS server receives the client hello it 
  processes the certificate_type extension. Since it also has a raw public key it indicates 
  in [2] that it had choosen to place the SubjectPublicKeyInfo structure into the Certificate 
  payload [3]. The client uses this raw public key in the TLS handshake and an out-of-band technique, 
  such as DANE, to verify its validity.</t>
  
  <t>
<figure anchor="flow1" title="Example with Raw Public Key provided by the TLS Server">
<artwork>
<![CDATA[ 
client_hello,
certificate_type=(RawPublicKey-Accept) -> // [1]

                         <-  server_hello,
                             certificate_type=(RawPublicKey-Offer), // [2]
                             certificate, // [3]
                             server_key_exchange,
                             server_hello_done

client_key_exchange,
change_cipher_spec,
finished                  ->

                         <- change_cipher_spec,
                            finished

Application Data        <------->     Application Data
]]>
</artwork>
</figure>
</t>

 <t>In our second example the TLS client as well as the TLS server use raw public keys. This is a use case envisioned for smart object networking. The TLS client in this case is an embedded device that is configured with a raw public key for use with TLS and is also able to process raw public keys sent by the server. Therefore, it indicates these capabilities in the 'certificate_type' extension in [1]. As in the previously shown example the server fulfills the client's request, indicates this via the 'RawPublicKey-Offer'in the certificate_type payload, and provides a raw public key into the Certificate payload back to the client (see [3]). The TLS server, however, demands client authentication and therefore a certificate_request is added [4]. The certificate_type payload in [2] indicates that the TLS server accepts raw public keys. The TLS client, who has a raw public key pre-provisioned, 
returns it in the Certificate payload [5] to the server.</t> 
<t>
<figure anchor="flow2" title="Example with Raw Public Key provided by the TLS Server and the Client">
<artwork>
<![CDATA[ 
client_hello,
certificate_type=(RawPublicKey-Offer, RawPublicKey-Accept) -> // [1]

                         <-  server_hello,
                             certificate_type=(RawPublicKey-Offer, 
                                         RawPublicKey-Accept) // [2]
                             certificate, // [3]
                             certificate_request, // [4]
                             server_key_exchange,
                             server_hello_done

certificate, // [5]
client_key_exchange,
change_cipher_spec,
finished                  ->

                         <- change_cipher_spec,
                            finished

Application Data        <------->     Application Data
]]>
</artwork>
</figure>
</t>

<t>In our last example we illustrate a combination of raw public key and X.509 usage. The client uses a raw public key 
for client authentication but the server provides an X.509 certificate. This exchange starts with the client indicating its ability to process X.509 certificates provided by the server, and the ability to send raw public keys. The server provides the X.509 certificate in [3] with the indication present in [2]. For client authentication, however, the server indicates in [2] that it is able to support raw public keys and requests a certificate from the client in [4]. The TLS client provides a raw public key in [5] after receiving and processing the TLS server hello message.</t>

 <t>
<figure anchor="flow3" title="Hybrid Certificate Example">
<artwork>
<![CDATA[ 
client_hello,
certificate_type=(X.509-Accept, RawPublicKey-Offer) -> // [1]

                         <-  server_hello,
                             certificate_type=(X.509-Offer, 
                                  RawPublicKey-Accept), // [2]
                             certificate, // [3]
                             certificate_request, // [4]
                             server_key_exchange,
                             server_hello_done
certificate, // [5]
client_key_exchange,
change_cipher_spec,
finished                  ->

                         <- change_cipher_spec,
                            finished

Application Data        <------->     Application Data
]]>
</artwork>
</figure>
</t>


</section> 



    <section title="Security Considerations" anchor="security">

	<t>The transmission of raw public keys, as described in this document, 
	provides benefits by lowering the over-the-air transmission overhead since 
	raw public keys are quite naturally smaller than an entire certificate. 
	There are also advantages from a codesize point of view for parsing and
    processing these keys. The crytographic procedures for assocating the 
	public key with the possession of a private key also follows standard 
	procedures.</t>

    <t>The main security challenge is, however, how to associate the public 
    key with a specific entity. This information will be needed to make 
    authorization decisions. Without a secure binding, man-in-the-middle 
    attacks may be the consequence. This document assumes that such 
	binding can be made out-of-band and we list a few examples in <xref target="into"/>. 
	DANE <xref target="RFC6698"/> offers one such approach. 
	If public keys are obtained using DANE, these public keys are authenticated via DNSSEC.
	Pre-configured keys is another out of band method for authenticating raw public keys.
	 While pre-configured keys are not suitable for 
	a generic Web-based e-commerce environment such keys are a reasonable approach
	for many smart object deployments where there is a close relationship between 
	the software running on the device and the server-side communication endpoint. 
	Regardless of the chosen mechanism for out-of-band public key validation an 
	assessment of the most suitable approach has to be made prior to the start of a 
	deployment to ensure the security of the system.</t>
	
    </section>

    <section anchor="IANA" title="IANA Considerations">
      <t>
   This document defines a new TLS extension, "certificate_type", assigned a
   value of [TBD] from the TLS ExtensionType registry defined in <xref target="RFC5246"/>.
   This value is used as the extension number for the extensions in both
   the client hello message and the server hello message.  The new
   extension type is used for certificate type negotiation.</t>

   <t>The "certificate_type" extension contains an 8-bit CertificateType field,
   for which a new registry, named "TLS Certificate Types", is
   established in this document, to be maintained by IANA.  The registry
   is segmented in the following way:
   <list style="numbers">
   <t>The values 0 - 3 are defined in <xref target="types"/>.</t>

   <t>Values from 3 through 223 decimal inclusive are assigned via IETF
       Consensus <xref target="RFC5226"/>.</t>

   <t>Values from 224 decimal through 255 decimal inclusive are
       reserved for Private Use <xref target="RFC5226"/>.</t>
   </list> 
   </t>
    </section>

  <section title="Acknowledgements" anchor="acknowledgements">
      <t>The feedback from the TLS working group meeting at IETF#81 has 
	     substantially shaped the document and we would like to thank the 
	     meeting participants for their input.  The support for hashes of 
	     public keys has been moved to <xref target="I-D.ietf-tls-cached-info"/> after the discussions at the IETF#82 
	     meeting and the feedback from Eric Rescorla.</t>
      <t>We would like to thank the following persons for their review comments: Martin Rex, Bill Frantz, Zach Shelby, 
	     Carsten Bormann, Cullen Jennings, Rene Struik, Alper Yegin, Jim Schaad, Paul Hoffman, Robert Cragie, Nikos Mavrogiannopoulos, Phil Hunt, John Bradley, Klaus Hartke, Stefan Jucker, and James Manger.</t>
      </section>

  </middle>

  <!--  *****BACK MATTER ***** -->

  <back>
    <!-- References split into informative and normative -->

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<references title="Normative References">
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.2119.xml"?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.5246.xml"?>

   <reference anchor='PKIX'>
      <front>
      <title>Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile</title>
      <author initials='D.' surname='Cooper' fullname='D. Cooper'>
      <organization /></author>
      <author initials='S.' surname='Santesson' fullname='S. Santesson'>
      <organization /></author>
      <author initials='S.' surname='Farrell' fullname='S. Farrell'>
      <organization /></author>
      <author initials='S.' surname='Boeyen' fullname='S. Boeyen'>
      <organization /></author>
      <author initials='R.' surname='Housley' fullname='R. Housley'>
      <organization /></author>
      <author initials='W.' surname='Polk' fullname='W. Polk'>
      <organization /></author>
      <date year='2008' month='May' />
      <abstract>
         <t>This memo profiles the X.509 v3 certificate and X.509 v2 certificate revocation list (CRL) for use in the Internet.  An overview of this approach and model is provided as an introduction.  The X.509 v3 certificate format is described in detail, with additional information regarding the format and semantics of Internet name forms.  Standard certificate extensions are described and two Internet-specific extensions are defined.  A set of required certificate extensions is specified.  The X.509 v2 CRL format is described in detail along with standard and Internet-specific extensions.  An algorithm for X.509 certification path validation is described.  An ASN.1 module and examples are provided in the appendices. [STANDARDS TRACK]</t></abstract></front>
      <seriesInfo name='RFC' value='5280' />
      <format type='TXT' octets='352580' target='ftp://ftp.isi.edu/in-notes/rfc5280.txt' />
   </reference>

</references>

<references title="Informative References">
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.5226.xml"?>  
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.3279.xml"?> 
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.6698.xml"?>  
<?rfc include="http://xml.resource.org/public/rfc/bibxml3/reference.I-D.ietf-core-coap.xml"?>  
<?rfc include="http://xml.resource.org/public/rfc/bibxml3/reference.I-D.ietf-tls-cached-info.xml"?>  


   <reference anchor='LDAP'>
      <front>
      <title>Lightweight Directory Access Protocol (LDAP): The Protocol</title>
      <author initials='J.' surname='Sermersheim' fullname='J. Sermersheim'>
      <organization /></author>
      <date year='2006' month='June' />
      <abstract>
         <t>This document describes the protocol elements, along with
         their semantics and encodings, of the Lightweight Directory
         Access Protocol (LDAP).  LDAP provides access to distributed
         directory services that act in accordance with X.500 data
         and service models.  These protocol elements are based
         on those described in the X.500 Directory Access Protocol
         (DAP). [STANDARDS TRACK]</t>
       </abstract>
      </front>
      <seriesInfo name='RFC' value='4511' />
      <format type='TXT' octets='150116' target='ftp://ftp.isi.edu/in-notes/rfc4511.txt' />
   </reference>
   
    <reference anchor='Defeating-SSL' target='http://www.blackhat.com/presentations/bh-dc-09/Marlinspike/BlackHat-DC-09-Marlinspike-Defeating-SSL.pdf'>
       <front>
       <title>New Tricks for Defeating SSL in Practice</title>
       <author initials='M.' surname='Marlinspike' fullname='Moxie Marlinspike'>
       <organization /></author>
       <date year='2009' month='February' />
       </front>
       <format type='PDF' target='http://www.blackhat.com/presentations/bh-dc-09/Marlinspike/BlackHat-DC-09-Marlinspike-Defeating-SSL.pdf' />
    </reference>
    
</references>

  </back>
</rfc>