TLS                                                      P. Wouters Wouters, Ed.
Internet-Draft                                                   Red Hat
Intended status: Standards Track                              J. Gilmore                      H. Tschofenig, Ed.
Expires: January 17, April 25, 2013                           Nokia Siemens Networks
                                                              J. Gilmore

                                                               S. Weiler
                                                            SPARTA, Inc.
                                                              T. Kivinen
                                                           H. Tschofenig
                                                  Nokia Siemens Networks
                                                           July 16,
                                                        October 22, 2012

  Out-of-Band Public Key Validation for Transport Layer Security
                    draft-ietf-tls-oob-pubkey-04.txt (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 January 17, April 25, 2013.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  New TLS Extensions Extension  . . . . . . . . . . . . . . . . . . . . . .  4
   4.  TLS Handshake Extension  . . . . . . . . . . . . . . . . . . .  5  7
     4.1.  Client Hello . . . . . . . . . . . . . . . . . . . . . . .  5  7
     4.2.  Server Hello . . . . . . . . . . . . . . . . . . . . . . .  6  7
     4.3.  Certificate Request  . . . . . . . . . . . . . . . . . . .  6  8
     4.4.  Certificate Payload  . .  Other Handshake Messages . . . . . . . . . . . . . . . . .  6  8
     4.5.  Other TLS Messages . .  Client authentication  . . . . . . . . . . . . . . . . . .  6  8
   5.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . .  6  8
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9 11
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10 12
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10 12
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11 13
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 11 13
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 11 13
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12 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 [I-D.ietf-dane-protocol]. [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] does 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 Extensions

   In order to indicate the support for multiple certificate types two
   new extensions are defined by this specification with the following

   cert-send:  The certificate payload in this message contains a
      certificate of Extension

   This section describes the type indicated by this extension.

   cert-receive:  By including this extension an entity indicates that
      it is able changes to recieve and process the indicated certificate types.
      This list is sorted by preference.

     enum { X.509(0), RawPublicKey(1), (255) } CertType;

     CertType cert-receive <1..2^8-1>;

     CertType cert-send;

                  Figure 1: New TLS Extension Structures

   No new cipher suites are required for use with handshake message
   contents when raw public keys.  All
   existing cipher suites that support a key exchange method compatible
   with the key in the certificate can certificates are to be used in combination with raw
   public key certificate types.

4.  TLS Handshake Extension

   This section describes used.  Figure 3
   illustrates the semantic exchange of messages as described in the 'cert-send' sub-sections
   below.  The client and the 'cert-
   receive' extensions for server exchange the different handshake messages.

4.1.  Client Hello

   To allow a TLS client newly defined
   certificate_type extension to indicate that it is able their ability and desire to receive a
   exchange raw public keys.  These raw public keys, in the form of a specific type it MAY include
   SubjectPublicKeyInfo structure, are then carried inside the 'cert-receive'
   certificate payload.  The SubjectPublicKeyInfo structure is defined
   in Section 4.1 of RFC 5280.  Note that the client hello message.  To indicate SubjectPublicKeyInfo block
   does not only contain the ability to
   process a raw public key by the server the TLS client MUST include
   the 'cert-receive' with keys, such as the value one (1) (indicating "RawPublicKey")
   in public exponent and
   the list modulus of supported certificate types.  If a TLS client only
   supports X.509 certificates it MAY include this extension to indicate
   support 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 it.

   Future documents may define additional 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 that require
   addition 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 registered.

   Note: 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.

   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 'cert-
   "certificate_type" extension and chooses a cipher suite then two
   outcomes are possible.  The server MUST either select a certificate
   type from client-provided list the CertificateType field in the extended client hello or
   terminate the session with a fatal alert of type
   "unsupported_certificate".  In the former case the procedure in
   Section 4.4 MUST be followed.

4.3.  Certificate Request

   The Certificate Request payload sent by the TLS server to the TLS
   client MUST be accompanied by a 'cert-receive' extension, which
   indicates to the TLS client the certificate type selected by the server supports.

4.4.  Certificate Payload

   Certificate payloads MUST be accompanied by is encoded in a 'cert-send' extension,
   CertTypeExtension structure, which indicates the certificate format found is included in the Certificate
   payload itself.

   The list extended server
   hello message using an extension of supported certificate types to choose from MUST have been
   obtained via the 'cert-receive' extension.  This ensures that a
   Certificate payload only contains a certificate type "certificate_type".  Servers
   that is also
   supported by only support X.509 certificates MAY omit including the recipient.

   "certificate_type" extension in the 'RawPublicKey' certificate type extended server hello.

   If the client supports the receiption of raw public keys and the
   server is selected able to provide such a raw public key then the TLS server
   MUST place the SubjectPublicKeyInfo structure MUST be placed into the Certificate
   payload.  The type of the asymmetric public key MUST match the selected key exchange

4.5.  Other

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 2, 5, Figure 3, 6, and Figure 4 7 illustrate example message

   The first example shows an exchange where the TLS client indicates
   its ability to process two certificate types, namely raw public keys
   and X.509 certificates via the 'cert-receive' 'certificate_type' extension (see [1]). in [1].
   When the TLS server receives the client hello it processes the cert-
   certificate_type extension and 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]).  The client uses this raw public key in the TLS handshake and
   an out-of-band technique, such as DANE, to verify its validatity.

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

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

finished                  ->

                         <- change_cipher_spec,

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

     Figure 2: 5: Example with Raw Public Key provided by the TLS Server

   In our second example both the TLS client and 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 raw public keys and therefore it indicates this
   capability via the 'cert-receive' 'certificate_type' extension in [1].  As in the
   previously shown example the server fulfills the client's request and
   provides a raw public key into the Certificate payload back to the
   client (see [2] and [3]).  The TLS server, however, demands client
   authentication and for this reason therefore a Certificate_Request payload certificate_request is added [4], which comes with an indication of [4].  The
   certificate_type payload indicates the TLS server supported
   certificate types types, see [2], and particularly that the TLS server is
   also able to process raw public keys sent by the server, see [5]. client.  The TLS
   client, who has a raw public key pre-provisioned, returns it in the
   Certificate payload
   [7] [5] to the server with the indication about its content [6]. server.

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

                            <-  server_hello,
                                            RawPublicKey-Accept) // [2]
                                certificate, // [3]
                                certificate_request, // [4]
                               cert-receive=(RawPublicKey, X.509) // [5]

  cert-send=RawPublicKey, // [6]

   certificate, // [7] [5]
   finished                  ->

                            <- change_cipher_spec,

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

   Figure 3: 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. 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 [5] [2] that it is able
   to support raw public keys as well as X.509 certificates. keys.  The TLS client provides a raw public key
   in [7] [5] after receiving and processing the indication in [6]. TLS server hello message.

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

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

  cert-send=RawPublicKey, // [6]
   certificate, // [7] [5]
   finished                  ->

                            <- change_cipher_spec,

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

                   Figure 4: 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 [I-D.ietf-dane-protocol] [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 two a new TLS extension, 'cert-send' and 'cert-
   receive', and their values need to be added to "certificate_type",
   assigned a value of [TBD] from the TLS ExtensionType registry created by RFC 5246 defined
   in [RFC5246].

   The values  This value is used as the extension number for the
   extensions in these both the client hello message and the server hello
   message.  The new extensions extension type is used for certificate type

   The "certificate_type" extension contains an 8-bit CertificateType
   field, for which a new registry, named "Certificate "TLS Certificate Types", is
   established in this document, to be maintained by IANA.  The registry
   is segmented in the following way:

   1.  The value (0) is values 0 - 3 are defined in this document. Figure 4.

   2.  Values from 2 3 through 223 decimal inclusive are assigned using
       the 'Specification Required' policy defined in RFC 5226 via IETF
       Consensus [RFC5226].

   3.  Values from 224 decimal through 255 decimal inclusive are
       reserved for 'Private Use', see 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-10 (work in progress), June 2012.

              Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", draft-ietf-dane-protocol-23
              draft-ietf-core-coap-12 (work in progress), June October 2012.

              Santesson, S. and H. Tschofenig, "Transport Layer Security
              (TLS) Cached Information Extension",
              draft-ietf-tls-cached-info-13 (work in progress),
              December 2011.
              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.

   [RFC6091]  Mavrogiannopoulos, N.

   [RFC6698]  Hoffman, P. and D. Gillmor, "Using OpenPGP Keys
              for J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS) Authentication",
              Protocol: TLSA", RFC 6091, February 2011. 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


   Hannes Tschofenig
   Nokia Siemens Networks
   Linnoitustie 6
   Espoo  02600

   Phone: +358 (50) 4871445