TLS                                                      P. Wouters, Ed.
Internet-Draft                                                   Red Hat
Intended status: Standards Track                      H. Tschofenig, Ed.
Expires: January 17, 31, 2014                         Nokia Siemens Networks
                                                              J. Gilmore

                                                               S. Weiler
                                                            SPARTA, Inc.
                                                              T. Kivinen
                                                           July 16, 30, 2013

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


   This document specifies a new certificate type and two TLS
   extensions, one for the client and one for the server, for exchanging
   raw public keys in Transport Layer Security (TLS) and Datagram
   Transport Layer Security (DTLS) for use with out-of-band public key

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  New TLS Extension . . . . . . . . . . . . . . . . . . . . . .   3
   4.  TLS Handshake Extension . . . . . . . . . . . . . . . . . . .   7
     4.1.  Client Hello  . . . . . . . . . . . . . . . . . . . . . .   7
     4.2.  Server Hello  . . . . . . . . . . . . . . . . . . . . . .   7
     4.3.  Certificate Request . . . . . . . . . . . . . . . . . . .   7
     4.4.  Other Handshake Messages  . . . . . . . . . . . . . . . .   7   8
     4.5.  Client authentication . . . . . . . . . . . . . . . . . .   8
   5.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11  12
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Appendix A.  Example Encoding . . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   Traditionally, TLS client and 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

   Alternative methods are available that allow a TLS clients/servers to
   obtain the TLS servers/client public key:

   o  TLS clients can obtain the TLS server public key from a DNSSEC
      secured resource records using DANE [RFC6698].

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

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

      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 embedded 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 mechanism defined herein only provides authentication when an
   out-of-band mechanism is also used to bind the public key to the
   entity presenting the key.

   This document registers a new value to the IANA certificate types
   registry for the support of raw public keys.  It also defines two new
   TLS extensions, "client_certificate_type" and

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 keys are to be used.  Figure 4 illustrates
   the exchange of messages as described in the sub-sections below.  The
   client and the server exchange make use of two new TLS extensions,
   namely 'client_certificate_type' and 'server_certificate_type', and
   an already available IANA TLS Certificate Type registry
   [TLS-Certificate-Types-Registry] 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 Certificate and the SubjectPublicKeyInfo
   structure is shown in Figure 1.

   opaque ASN.1Cert<1..2^24-1>;

   struct {
           // certificate type defined in this document.
           case RawPublicKey:
             opaque ASN.1_subjectPublicKeyInfo<1..2^24-1>;

           // X.509 certificate defined in RFC 5246
           case X.509:
             ASN.1Cert certificate_list<0..2^24-1>;

           // Additional certificate type based on TLS
           // Certificate Type Registry
   } Certificate;

                   Figure 1: TLS Certificate Structure.

   The SubjectPublicKeyInfo structure is defined in Section 4.1 of RFC
   5280 [PKIX] and 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 algorithm identifier can also include
   parameters.  The structure, as shown in Figure 2, is encoded in an
   DER encoded ASN.1 format [X.690] and therefore contains length
   information as well.  An example is provided in Appendix A.

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

      AlgorithmIdentifier   ::=  SEQUENCE  {
           algorithm               OBJECT IDENTIFIER,
           parameters              ANY DEFINED BY algorithm OPTIONAL  }

              Figure 2: SubjectPublicKeyInfo ASN.1 Structure.

   The algorithm identifiers are Object Identifiers (OIDs).  RFC 3279
   [RFC3279] and [RFC5480] [RFC5480], for example, define the following OIDs shown
   in Figure 3.  Note that this list is not exhaustive and more OIDs may
   be defined in future RFCs.  RFC 5480 also defines a number of OIDs.

   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 2 of RFC 5480      | 1.2.840.10045.2.1

              Figure 3: Example Algorithm Object Identifiers.

   The message exchange in Figure 4 shows the 'client_certificate_type'
   and 'server_certificate_type' extensions added to the client and
   server hello messages.

    server_certificate_type   ->

                              <-  server_hello,
    finished                  ->

                              <- change_cipher_spec,

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

               Figure 4: Basic Raw Public Key TLS Exchange.

   The semantic of the two extensions is defined as follows:

      The 'client_certificate_type' and 'server_certificate_type' sent
      in the client hello, may carry a list of supported certificate
      types, sorted by client preference.  It is a list in the case
      where the client supports multiple certificate types.  These
      extension MUST be omitted if the client only supports X.509
      certificates.  The 'client_certificate_type' sent in the client
      hello indicates the certificate types the client is able to
      provide to the server, when requested using a certificate_request
      message.  The 'server_certificate_type' in the client hello
      indicates the type of certificates the client is able to process
      when provided by the server in a subsequent certificate payload.

      The 'client_certificate_type' returned in the server hello
      indicates the certificate type found in the attached certificate
      payload.  Only a single value is permitted.  The
      'server_certificate_type' in the server hello indicates the type
      of certificates the client is requested to provide in a subsequent
      certificate payload.  The value conveyed in the
      'server_certificate_type' MUST be selected from one of the values
      provided in the 'server_certificate_type' sent in the client
      hello.  If the server does not send a certificate_request payload
      or none of the certificates supported by the client (as indicated
      in the 'server_certificate_type' in the client hello) match the
      server-supported certificate types the 'server_certificate_type'
      payload sent in the server hello is omitted.

   The "extension_data" field of this extension contains the
   ClientCertTypeExtension or the ServerCertTypeExtension structure, as
   shown in Figure 5.  The CertificateType structure is an enum with
   with values from TLS Certificate Type Registry.

   struct {
               case client:
                 CertificateType client_certificate_types<1..2^8-1>;
               case server:
                 CertificateType client_certificate_type;
   } ClientCertTypeExtension;

   struct {
               case client:
                 CertificateType server_certificate_types<1..2^8-1>;
               case server:
                 CertificateType server_certificate_type;
   } ServerCertTypeExtension;

                  Figure 5: 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 the 'client_certificate_type' and
   'server_certificate_type' extensions in an extended client hello
   message.  The hello extension mechanism is described in TLS 1.2

4.2.  Server Hello

   If the server receives a client hello that contains the
   'client_certificate_type' and 'server_certificate_type' extensions
   and chooses a cipher suite then three outcomes are possible:

   1.  The server does not support the extension defined in this
       document.  In this case the server returns the server hello
       without the extensions defined in this document.

   2.  The server supports the extension defined in this document and
       has at least one certificate type in common with the client.  In
       this case it returns the 'server_certificate_type' and indicates
       the selected certificate type value.

   3.  The server supports the extension defined in this document but
       does not have a certificate type in common with the client.  In
       this case the server terminate the session with a fatal alert of
       type "unsupported_certificate".

   If the TLS server also requests a certificate from the client (via
   the certificate_request) it MUST include the
   'client_certificate_type' extension with a value chosen from the list
   of client-supported certificates types (as provided in the
   'client_certificate_type' of the client hello).

   If the client hello indicates support of raw public keys in the
   'client_certificate_type' extension and the server chooses to use raw
   public keys then the TLS server MUST place the SubjectPublicKeyInfo
   structure into the Certificate payload.

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 6, Figure 7, and Figure 8 illustrate example exchanges.

   The first example shows an exchange where the TLS client indicates
   its ability to receive and validate raw public keys from the server.
   In our example the client is quite restricted since it is unable to
   process other certificate types sent by the server.  It also does not
   have credentials (at the TLS layer) it could send.  The
   'client_certificate_type' extension indicates this in [1].  When the
   TLS server receives the client hello it processes the
   'client_certificate_type' extension.  Since it also has a raw public
   key it indicates in [2] that it had chosen 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.

   server_certificate_type=(RawPublicKey) -> // [1]

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

   finished                  ->

                            <- change_cipher_spec,

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

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

   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 [1].  As in the previously shown
   example the server fulfills the client's request, indicates this via
   the "RawPublicKey" value in the server_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.

   client_certificate_type=(RawPublicKey) // [1]
   server_certificate_type=(RawPublicKey) // [1]
                            <-  server_hello,
                                certificate, // [3]
                                certificate_request, // [4]

   certificate, // [5]
   finished                  ->

                            <- change_cipher_spec,

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

   Figure 7: 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 (see [1]).  The server provides the X.509 certificate
   in [3] with the indication present in [2].  For client authentication
   the server indicates in [4] that it selected the raw public key
   format and requests a certificate from the client in [5].  The TLS
   client provides a raw public key in [6] after receiving and
   processing the TLS server hello message.

   client_certificate_type=(RawPublicKey) // [1]
                            <-  server_hello,
                                certificate, // [3]
                                certificate_request, // [5]
   certificate, // [6]
   finished                  ->

                             <- change_cipher_spec,

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

                   Figure 8: 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 code size point of
   view for parsing and processing these keys.  The cryptographic
   procedures for associating 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.  Without a secure binding between
   identity and key the protocol will be vulnerable to masquerade and
   man-in-the-middle attacks.  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.  In order to address these
   vulnerabilities, specifications that make use of the extension MUST
   specify how the identity and public key are bound.  In addition to
   ensuring the binding is done out-of-band an implementation also needs
   to check the status of that binding.

   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

   IANA is asked to register a new value in the "TLS Certificate Types"
   registry of Transport Layer Security (TLS) Extensions
   [TLS-Certificate-Types-Registry], as follows:

   Value: 2
   Description: Raw Public Key
   Reference: [[THIS RFC]]

   This document asks IANA to allocate two new TLS extensions,
   "client_certificate_type" and "server_certificate_type", from the TLS
   ExtensionType registry defined in [RFC5246].  These extensions are
   used in both the client hello message and the server hello message.
   The new extension type is used for certificate type negotiation.  The
   values carried in these extensions are taken from the TLS Certificate
   Types registry [TLS-Certificate-Types-Registry].

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.

   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, Barry Leiba,
   Paul Hoffman, Robert Cragie, Nikos Mavrogiannopoulos, Phil Hunt, John
   Bradley, Klaus Hartke, Stefan Jucker, Kovatsch Matthias, Daniel Kahn
   Gillmor, Peter Sylvester, Hauke Mehrtens, and James Manger.  Nikos
   Mavrogiannopoulos contributed the design for re-using the certificate
   type registry.  Barry Leiba contributed guidance for the IANA
   consideration text.  Stefan Jucker, Kovatsch Matthias, and Klaus
   Hartke provided implementation feedback regarding the
   SubjectPublicKeyInfo structure.

   We would like to thank our TLS working group chairs, Eric Rescorla
   and Joe Salowey, for their guidance and support.  Finally, we would
   like to thank Sean Turner, who is the responsible security area
   director for this work for his review comments and suggestions.

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.

   [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.

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

   [RFC5480]  Turner, S., Brown, D., Yiu, K., Housley, R., and T. Polk,
              "Elliptic Curve Cryptography Subject Public Key
              Information", RFC 5480, March 2009.

              , "TLS Certificate Types Registry", February 2013, <http:/

   [X.690]    , "Information technology - ASN.1 encoding rules: >
              Specification of Basic Encoding Rules (BER), Canonical >
              Encoding Rules (CER) and Distinguished Encoding Rules >
              (DER).", RFC 5280, 2002.

9.2.  Informative References

              Gutmann, P., "ASN.1 Object Dump Program", February 2013,

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

              Shelby, Z., Hartke, K., and C. Bormann, "Constrained
              Application Protocol (CoAP)", draft-ietf-core-coap-18
              (work in progress), June 2013.

              Santesson, S. and H. Tschofenig, "Transport Layer Security
              (TLS) Cached Information Extension", draft-ietf-tls-
              cached-info-14 (work in progress), March 2013.

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

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

Appendix A.  Example Encoding

   For example, the following hex sequence describes a
   SubjectPublicKeyInfo structure inside the certificate payload:

          0     1     2     3     4     5     6     7     8     9
   1  | 0x30, 0x81, 0x9f, 0x30, 0x0d, 0x06, 0x09, 0x2a, 0x86, 0x48,
   2  | 0x86, 0xf7, 0x0d, 0x01, 0x01, 0x01, 0x05, 0x00, 0x03, 0x81,
   3  | 0x8d, 0x00, 0x30, 0x81, 0x89, 0x02, 0x81, 0x81, 0x00, 0xcd,
   4  | 0xfd, 0x89, 0x48, 0xbe, 0x36, 0xb9, 0x95, 0x76, 0xd4, 0x13,
   5  | 0x30, 0x0e, 0xbf, 0xb2, 0xed, 0x67, 0x0a, 0xc0, 0x16, 0x3f,
   6  | 0x51, 0x09, 0x9d, 0x29, 0x2f, 0xb2, 0x6d, 0x3f, 0x3e, 0x6c,
   7  | 0x2f, 0x90, 0x80, 0xa1, 0x71, 0xdf, 0xbe, 0x38, 0xc5, 0xcb,
   8  | 0xa9, 0x9a, 0x40, 0x14, 0x90, 0x0a, 0xf9, 0xb7, 0x07, 0x0b,
   9  | 0xe1, 0xda, 0xe7, 0x09, 0xbf, 0x0d, 0x57, 0x41, 0x86, 0x60,
   10 | 0xa1, 0xc1, 0x27, 0x91, 0x5b, 0x0a, 0x98, 0x46, 0x1b, 0xf6,
   11 | 0xa2, 0x84, 0xf8, 0x65, 0xc7, 0xce, 0x2d, 0x96, 0x17, 0xaa,
   12 | 0x91, 0xf8, 0x61, 0x04, 0x50, 0x70, 0xeb, 0xb4, 0x43, 0xb7,
   13 | 0xdc, 0x9a, 0xcc, 0x31, 0x01, 0x14, 0xd4, 0xcd, 0xcc, 0xc2,
   14 | 0x37, 0x6d, 0x69, 0x82, 0xd6, 0xc6, 0xc4, 0xbe, 0xf2, 0x34,
   15 | 0xa5, 0xc9, 0xa6, 0x19, 0x53, 0x32, 0x7a, 0x86, 0x0e, 0x91,
   16 | 0x82, 0x0f, 0xa1, 0x42, 0x54, 0xaa, 0x01, 0x02, 0x03, 0x01,
   17 | 0x00, 0x01

      Figure 9: Example SubjectPublicKeyInfo Structure Byte Sequence.

   The decoded byte-sequence shown in Figure 9 (for example using
   Peter's ASN.1 decoder [ASN.1-Dump]) illustrates the structure, as
   shown in Figure 10.

   Offset  Length   Description
      0     3+159:   SEQUENCE {
      3      2+13:     SEQUENCE {
      5       2+9:      OBJECT IDENTIFIER Value (1 2 840 113549 1 1 1)
                 :             PKCS #1, rsaEncryption
     16       2+0:      NULL
                 :      }
     18     3+141:    BIT STRING, encapsulates {
     22     3+137:      SEQUENCE {
     25     3+129:        INTEGER Value (1024 bit)
    157       2+3:        INTEGER Value (65537)
                 :        }
                 :      }
                 :    }

      Figure 10: Decoding of Example SubjectPublicKeyInfo Structure.

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