TLS                                                      P. Wouters, Ed.
Internet-Draft                                                   Red Hat
Intended status: Standards Track                      H. Tschofenig, Ed.
Expires: April 25, 2013                           Nokia Siemens Networks
                                                              J. Gilmore

                                                               S. Weiler
                                                            SPARTA, Inc.
                                                              T. Kivinen
                                                        October 22, 2012

  Out-of-Band Public Key Validation for Transport Layer Security (TLS)


   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.

   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.

   This document introduces the support for raw public keys in TLS.

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   This Internet-Draft will expire on April 25, 2013.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  New TLS Extension  . . . . . . . . . . . . . . . . . . . . . .  4
   4.  TLS Handshake Extension  . . . . . . . . . . . . . . . . . . .  7
     4.1.  Client Hello . . . . . . . . . . . . . . . . . . . . . . .  7
     4.2.  Server Hello . . . . . . . . . . . . . . . . . . . . . . .  7
     4.3.  Certificate Request  . . . . . . . . . . . . . . . . . . .  8
     4.4.  Other Handshake Messages . . . . . . . . . . . . . . . . .  8
     4.5.  Client authentication  . . . . . . . . . . . . . . . . . .  8
   5.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 13
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 13
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14

1.  Introduction

   Traditionally, TLS server public keys are obtained in PKIX containers
   in-band using the TLS handshake and validated using trust anchors
   based on a [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 [Defeating-SSL].

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

   o  The TLS server public key is obtained from a DNSSEC secured
      resource records using DANE [RFC6698].

   o  The TLS server public key is obtained from a [PKIX] certificate
      chain from an Lightweight Directory Access Protocol (LDAP) [LDAP]

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

   Some smart objects use the UDP-based Constrained Application Protocol
   (CoAP) [I-D.ietf-core-coap] to interact with a Web server to upload
   sensor data at a regular intervals, such as temperature readings.
   CoAP [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 [PKIX] may not
   suit all smart object deployments and would therefore be an
   unneccesarry burden.

   The Transport Layer Security (TLS) Protocol Version 1.2 [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.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  New TLS Extension

   This section describes the changes to the TLS handshake message
   contents when raw public key certificates are to be used.  Figure 3
   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 Figure 1, is encoded in an ASN.1 format
   and therefore contains length information as well.

      SubjectPublicKeyInfo  ::=  SEQUENCE  {
           algorithm            AlgorithmIdentifier,
           subjectPublicKey     BIT STRING  }

              Figure 1: SubjectPublicKeyInfo ASN.1 Structure.

   The algorithm identifiers are Object Identifiers (OIDs).  RFC 3279
   [RFC3279], for example, defines the following OIDs shown in Figure 2.

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

                 Figure 2: Example Algorithm Identifiers.

    certificate_type          ->

                              <-  server_hello,
    finished                  ->

                              <- change_cipher_spec,

   Application Data        <------->     Application Data

               Figure 3: Basic Raw Public Key TLS Exchange.

   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

   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.

   The structure of the CertTypeExtension is defined as follows:

   enum { client, server } ClientOrServerExtension;

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

   struct {
          case client:
            CertificateType certificate_types<1..2^8-1>;
          case server:
            CertificateType certificate_type;
   } CertTypeExtension;

                  Figure 4: CertTypeExtension Structure.

   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.

   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.

4.  TLS Handshake Extension

4.1.  Client Hello

   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 [RFC5246].

4.2.  Server Hello

   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

   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.

   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

4.3.  Certificate Request

   The semantics of this message remain the same as in the TLS

4.4.  Other Handshake Messages

   All the other handshake messages are identical to the TLS

4.5.  Client authentication

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

5.  Examples

   Figure 5, Figure 6, and Figure 7 illustrate example message exchanges.

   The first example shows an exchange where the TLS client indicates
   its ability to process two certificate types, namely receive raw public keys
   and X.509 certificates via 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 and since
   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 (see
   [3]). [3].  The client uses this raw public key in
   the TLS handshake and an out-of-band technique, such as DANE, to
   verify its validatity. validity.

certificate_type=(RawPublicKey-Accept) -> // [1]

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

finished                  ->

                         <- change_cipher_spec,

Application Data        <------->     Application Data

     Figure 5: Example with Raw Public Key provided by the TLS Server

   In our second example both the TLS client and 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
   only supports
   is configured with a raw public keys key for use with TLS and therefore is also able
   to process raw public keys sent by the server.  Therefore, it
   indicates this
   capability via these capabilities in the 'certificate_type' extension in
   [1].  As in the previously shown example the server fulfills the
   client's request 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 the TLS server supported
   certificate types, see [2], and particularly that the TLS server is
   also able to process accepts raw public keys sent by the client. keys.  The TLS
   client, who has a raw public key pre-provisioned, returns it in the
   Certificate payload [5] to the server.

   certificate_type=(RawPublicKey-Offer, RawPublicKey-Accept) -> // [1]

                            <-  server_hello,
                                            RawPublicKey-Accept) // [2]
                                certificate, // [3]
                                certificate_request, // [4]

   certificate, // [5]
   finished                  ->

                            <- change_cipher_spec,

   Application Data        <------->     Application Data

   Figure 6: Example with Raw Public Key provided by the TLS Server and
                                the Client

   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 using
   that format 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. 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.

   certificate_type=(X.509 Receive,
   certificate_type=(X.509-Accept, RawPublicKey-Offer) -> // [1]

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

                            <- change_cipher_spec,

   Application Data        <------->     Application Data

                   Figure 7: Hybrid Certificate Example

6.  Security Considerations

   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.

   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
   Section 1.  DANE [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.

7.  IANA Considerations

   This document defines a new TLS extension, "certificate_type",
   assigned a value of [TBD] from the TLS ExtensionType registry defined
   in [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

   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:

   1.  The values 0 - 3 are defined in Figure 4.

   2.  Values from 3 through 223 decimal inclusive are assigned via IETF
       Consensus [RFC5226].

   3.  Values from 224 decimal through 255 decimal inclusive are
       reserved for Private Use [RFC5226].

8.  Acknowledgements

   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 [I-D.ietf-tls-cached-info] after the
   discussions at the IETF#82 meeting and the feedback from Eric

   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.

9.  References
9.1.  Normative References

   [PKIX]     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.

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

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

9.2.  Informative References

              Marlinspike, M., "New Tricks for Defeating SSL in
              Practice", February 2009, <

              Shelby, Z., Hartke, K., Bormann, C., and B. Frank,
              "Constrained Application Protocol (CoAP)",
              draft-ietf-core-coap-12 (work in progress), October 2012.

              Santesson, S. and H. Tschofenig, "Transport Layer Security
              (TLS) Cached Information Extension",
              draft-ietf-tls-cached-info-13 (work in progress),
              September 2012.

   [LDAP]     Sermersheim, J., "Lightweight Directory Access Protocol
              (LDAP): The Protocol", RFC 4511, June 2006.

   [RFC3279]  Bassham, L., Polk, W., and R. Housley, "Algorithms and
              Identifiers for the Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 3279, April 2002.

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

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

Authors' Addresses

   Paul Wouters (editor)
   Red Hat


   Hannes Tschofenig (editor)
   Nokia Siemens Networks
   Linnoitustie 6
   Espoo  02600

   Phone: +358 (50) 4871445

   John Gilmore
   PO Box 170608
   San Francisco, California  94117

   Phone: +1 415 221 6524

   Samuel Weiler
   SPARTA, Inc.
   7110 Samuel Morse Drive
   Columbia, Maryland  21046


   Tero Kivinen
   Eerikinkatu 28
   HELSINKI  FI-00180