TLS                                                      P. Wouters, Ed.
Internet-Draft                                                   Red Hat
Intended status: Standards Track                      H. Tschofenig, Ed.
Expires: January 31, April 22, 2014                     Nokia Siemens Solutions and Networks
                                                              J. Gilmore

                                                               S. Weiler
                                                            SPARTA, Inc.
                                                              T. Kivinen
                                                               AuthenTec
                                                           July 30,
                                                        October 19, 2013

  Out-of-Band

  Using Raw Public Key Validation for Keys in Transport Layer Security (TLS)
                    draft-ietf-tls-oob-pubkey-09.txt and Datagram
                    Transport Layer Security (DTLS)
                    draft-ietf-tls-oob-pubkey-10.txt

Abstract

   This document specifies a new certificate type and two TLS
   extensions, one for the client and one for the server, extensions
   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.

Status of This Memo

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   This Internet-Draft will expire on January 31, April 22, 2014.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  New TLS  Structure of the Raw Public Key Extension . . . . . . . . . . . . . . . . . . . . . .   3   4
   4.  TLS Client and Server Handshake Extension . . . . . . . Behavior  . . . . . . . . . . . .   7   6
     4.1.  Client Hello  . . . . . . . . . . . . . . . . . . . . . .   7
     4.2.  Server Hello  . . . . . . . . . . . . . . . . . . . . . .   7
     4.3.  Certificate Request . . . . . . . . .  Client Authentication . . . . . . . . . .   7
     4.4.  Other Handshake Messages . . . . . . . .   8
   5.  Examples  . . . . . . . .   8
     4.5.  Client authentication . . . . . . . . . . . . . . . . . .   8
   5.  Examples  . . . .
     5.1.  TLS Server uses Raw Public Key  . . . . . . . . . . . . .   8
     5.2.  TLS Client and Server use Raw Public Keys . . . . . . . .   9
     5.3.  Combined Usage of Raw Public Keys and X.509 Certificate .   8  10
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  10  11
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11  12
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12  13
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  12  13
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  13  14
   Appendix A.  Example Encoding . . . . . . . . . . . . . . . . . .  13  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14  15

1.  Introduction

   Traditionally, TLS client and server public keys are obtained in PKIX
   containers in-band using as part of the TLS handshake procedure and are
   validated using trust anchors based on a [PKIX] PKIX certification authority (CA).
   (CA) [RFC5280].  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 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 via a Lightweight Directory Access Protocol (LDAP) [LDAP] [RFC4511]
      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
      /private key pair for the client itself.

   This document introduces the use of raw public keys in TLS/DTLS.  Raw
   public key thereby means that only a sub-set of the information found
   in typical certificates is utilized, namely the SubjectPublicKeyInfo
   structure of a PKIX certificates that carries the parameters
   necessary to describe the public key.  Other parameters also found in
   a PKIX certificate are omitted.  A consequence of omitting various
   certificate related structures is that the resulting raw public key
   is fairly small (compared to the original certificate) and does not
   require codepaths for the ASN.1 parser, for certificate path
   validation and other PKIX related processing tasks.  To further
   reduce the size of the exchanged information this specification can
   be combined with the TLS Cached Info extension
   [I-D.ietf-tls-cached-info], which enables TLS endpoints to just
   exchange fingerprints of their public keys (rather than the full
   public keys).

   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 is structured as follows: Section 3 defines the support
   structure of raw public keys.  It also defines the two new TLS extensions, extensions "client_certificate_type" and
   "server_certificate_type".
   "server_certificate_type", which can be used as part of an extended
   TLS handshake when raw public keys are to be used.  Section 4 defines
   the behavior of the TLS client and the TLS server.  Example exchanges
   are described in Section 5.  Finally, in Section 7 this document also
   registers a new value to the IANA certificate types registry for the
   support of raw public keys.

2.  Terminology

   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 RFC 2119 [RFC2119].

   We use the terms 'TLS server' and 'server' as well as 'TLS client'
   and 'client' interchangable.

3.  New TLS  Structure of the Raw Public Key Extension

   This section describes the changes to defines the two TLS extensions 'client_certificate_type'
   and 'server_certificate_type', which can be used as part of an
   extended TLS handshake message
   contents when raw public keys are to be used.  Figure  Section 4 illustrates
   defines the exchange behavior of messages as described in the sub-sections below.  The TLS 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 server using this
   extension.

   This specification reuses the SubjectPublicKeyInfo structure to exchange raw public keys.  These
   encode the raw public keys, in key and to convey that information within the form of a
   SubjectPublicKeyInfo structure, are then carried inside
   TLS handshake the Certificate payload.  The Certificate and the SubjectPublicKeyInfo
   structure payload is utilized as a container, as
   shown in Figure 1.  The shown Certificate structure is an adaptation
   of its original form [RFC5246].

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

   struct {
       select(certificate_type){

           // 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. Payload as a Container for the Raw Public Key.

   The SubjectPublicKeyInfo structure is defined in Section 4.1 of RFC
   5280 [PKIX] [RFC5280] 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 represented in an a
   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], 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) (DSA)        | Section 2.3.2 of RFC 3279  | 1.2.840.10040.4.1
   .......................|............................|...................
   Elliptic Curve         |                            |
   Digital Signature      |                            |
   Algorithm (ECDSA)      | Section 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' extension format for extended client hellos and 'server_certificate_type' extensions added extended server,
   via the "extension_data" field, is used to carry the client
   ClientCertTypeExtension and
   server hello messages.

    client_hello,
    client_certificate_type
    server_certificate_type   ->

                              <-  server_hello,
                                  client_certificate_type,
                                  server_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

               Figure 4: Basic Raw Public Key TLS Exchange.

   The semantic of the ServerCertTypeExtension structures.
   These two extensions structures are shown in Figure 4.  The CertificateType
   structure is an enum with values taken from the 'TLS Certificate
   Type' registry [TLS-Certificate-Types-Registry].

   struct {
           select(ClientOrServerExtension)
               case client:
                 CertificateType client_certificate_types<1..2^8-1>;
               case server:
                 CertificateType client_certificate_type;
           }
   } ClientCertTypeExtension;
   struct {
           select(ClientOrServerExtension)
               case client:
                 CertificateType server_certificate_types<1..2^8-1>;
               case server:
                 CertificateType server_certificate_type;
           }
   } ServerCertTypeExtension;

                  Figure 4: CertTypeExtension Structure.

4.  TLS Client and Server Handshake Behavior

   This specification extends the ClientHello and the ServerHello
   messages, according to the extension procedures defined as follows: in [RFC5246].
   It does not extend or modify any other TLS message.

   Note: 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.

   The high-level message exchange in Figure 5 shows the
   'client_certificate_type' and 'server_certificate_type' sent
      in extensions
   added to the client hello, may carry a list and server hello messages.

    client_hello,
    client_certificate_type,
    server_certificate_type   ->

                              <-  server_hello,
                                  client_certificate_type,
                                  server_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
               Figure 5: Basic Raw Public Key TLS Exchange.

4.1.  Client Hello

   In order to indicate the support of supported certificate
      types, sorted by client preference.  It is a list in raw public keys, clients include
   the case
      where 'client_certificate_type' and/or the 'server_certificate_type'
   extensions in an extended client supports multiple certificate types.  These hello message.  The hello extension MUST be omitted if the client only supports X.509
      certificates.
   mechanism is described in Section 7.4.1.4 of TLS 1.2 [RFC5246].

   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 types
   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 and 'server_certificate_type' sent in
   the client
      hello.  If the server does not send hello may carry a certificate_request payload
      or none list of the certificates supported certificate types,
   sorted by the client (as indicated preference.  It is a list in the 'server_certificate_type' in case where the
   client hello) match the
      server-supported supports multiple certificate types types.

   The TLS client MUST omit the 'server_certificate_type'
      payload sent 'client_certificate_type' extension in
   the server client 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 {
           select(ClientOrServerExtension)
               case client:
                 CertificateType client_certificate_types<1..2^8-1>;
               case server:
                 CertificateType client_certificate_type;
           }
   } ClientCertTypeExtension;

   struct {
           select(ClientOrServerExtension)
               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 if it does not possess a client certificate or is
   not configured to use raw public keys.  All
   existing cipher suites that support a key exchange method compatible one with the defined extension can be used.

4. given TLS Handshake Extension

4.1.  Client Hello

   In order to indicate the support of out-of-band raw public keys,
   clients server.  The TLS client
   MUST include omit the 'client_certificate_type' and 'server_certificate_type' extensions extension in an extended the client hello
   message.  The hello extension mechanism
   if it is described unable to process any certificate types from the server
   (which is a situation that should not occur in TLS 1.2
   [RFC5246]. normal circumstances).

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 but it
       does not have a certificate type in common with the client.  In
       this case it returns  Then
       the 'server_certificate_type' and indicates server terminates the selected certificate session with a fatal alert of type value.
       "unsupported_certificate".

   3.  The server supports the extension extensions defined in this document but
       does not have a and
       has at least one certificate type in common with the client.  In
       this case the server terminate processing rules described below are followed.

   If the session with a fatal alert client hello indicates support of
       type "unsupported_certificate". 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.

   If the TLS server also requests a certificate from the client (via
   the certificate_request) certificate_request message) 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 server does not send a certificate_request payload (for
   example, because client authentication happens at the application
   layer or no client hello indicates support authentication is required) or none of raw public keys in the
   'client_certificate_type' extension and
   certificates supported by the server chooses to use raw
   public keys then client (as indicated in the TLS server MUST place
   'server_certificate_type' in the SubjectPublicKeyInfo
   structure into client hello) match the Certificate payload.

4.3.  Certificate Request
   The semantics of this message remain server-
   supported certificate types then the same as 'server_certificate_type'
   payload in the TLS
   specification.

4.4.  Other Handshake Messages

   All the other handshake messages are identical to server hello is omitted.

4.3.  Client Authentication

   Authentication of the TLS
   specification.

4.5.  Client authentication

   Client authentication by client to 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

   This section illustrates a number of possible usage scenarios.

5.1.  TLS Server uses Raw Public Key

   This section shows an exchange example 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 send to the server and
   therefore omits the 'client_certificate_type' extension.  Hence, the
   client only populates the 'server_certificate_type' extension indicates this with
   the raw public key type, as shown 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 together
   with an out-of-
   band out-of-band validation technique, such as DANE, to verify its validity. it.

   client_hello,
   server_certificate_type=(RawPublicKey) -> // [1]
                            ->
                            <-  server_hello,
                                server_certificate_type=(RawPublicKey), // [2]
                                certificate, // [3]
                                server_key_exchange,
                                server_hello_done

   client_key_exchange,
   change_cipher_spec,
   finished                  ->

                            <- change_cipher_spec,
                               finished

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

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

5.2.  TLS Client and Server

   In our second use Raw Public Keys

   This section shows an example where 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_hello,
   client_certificate_type=(RawPublicKey) // [1]
   server_certificate_type=(RawPublicKey) // [1]
                            ->
                            <-  server_hello,
                                server_certificate_type=(RawPublicKey)//[2]
                                certificate, // [3]
                                client_certificate_type=(RawPublicKey)//[4]
                                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

   Figure 7: Example with Raw Public Key provided by the TLS Server and
                                the Client

   In our last example we illustrate a combination Client.

5.3.  Combined Usage of Raw Public Keys and X.509 Certificate

   This section shows an example combining raw public key keys and X.509 usage.
   certificates.  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_hello,
   server_certificate_type=(X.509)
   client_certificate_type=(RawPublicKey) // [1]
                            ->
                            <-  server_hello,
                                server_certificate_type=(X.509)//[2]
                                certificate, // [3]
                                client_certificate_type=(RawPublicKey)//[4]
                                certificate_request, // [5]
                                server_key_exchange,
                                server_hello_done
   certificate, // [6]
   client_key_exchange,
   change_cipher_spec,
   finished                  ->
                             <- change_cipher_spec,
                                finished

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

                   Figure 8: Hybrid Certificate Example 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 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 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.

   A downgrading attack is another possibility for an adversary to gain
   advantages.  Thereby, an attacker might try to influence the
   handshake exchange to make the parties select different certificate
   types than they would normally choose.

   For this attack, an attacker must actively change one or more
   handshake messages.  If this occurs, the client and server will
   compute different values for the handshake message hashes.  As a
   result, the parties will not accept each others' Finished messages.
   Without the master_secret, the attacker cannot repair the Finished
   messages, so the attack will be discovered.

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, Alexey Melnikov, 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.

   Christer Holmberg provided the General Area (Gen-Art) review, Yaron
   Sheffer provided the Security Directorate (SecDir) review, Bert
   Greevenbosch provided the Applications Area Directorate review, and
   Linda Dunbar provided the Operations Directorate review.

   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.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, May 2008.

   [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]
              , "TLS Certificate Types Registry", February 2013, <http:/
              /www.iana.org/assignments/tls-extensiontype-values#tls-
              extensiontype-values-2>.

   [X.690]    , "Information    ITU, "ITU-T Recommendation X.690 (2002) | ISO/IEC
              8825-1:2002, Information technology - ASN.1 encoding
              rules: > Specification of Basic Encoding Rules (BER),
              Canonical > Encoding Rules (CER) and Distinguished Encoding
              Rules >
              (DER).", RFC 5280, (DER)", 2002.

9.2.  Informative References

   [ASN.1-Dump]
              Gutmann, P., "ASN.1 Object Dump Program", February 2013,
              <http://www.cs.auckland.ac.nz/~pgut001/>.

   [Defeating-SSL]
              Marlinspike, M., "New Tricks for Defeating SSL in
              Practice", February 2009, <http://www.blackhat.com/
              presentations/bh-dc-09/Marlinspike/BlackHat-DC-09
              -Marlinspike-Defeating-SSL.pdf>.

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

   [I-D.ietf-tls-cached-info]
              Santesson, S. and H. Tschofenig, "Transport Layer Security
              (TLS) Cached Information Extension", draft-ietf-tls-
              cached-info-14
              cached-info-15 (work in progress), March October 2013.

   [LDAP]

   [RFC4511]  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

   The following example 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

   We used Peter Gutmann's ASN.1 decoder [ASN.1-Dump]) illustrates [ASN.1-Dump] to turn the above-
   shown byte-sequence into an ASN.1 structure, as shown in of the
   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

   Email: paul@nohats.ca
   Hannes Tschofenig (editor)
   Nokia Siemens Solutions and Networks
   Linnoitustie 6
   Espoo  02600
   Finland

   Phone: +358 (50) 4871445
   Email: Hannes.Tschofenig@gmx.net
   URI:   http://www.tschofenig.priv.at

   John Gilmore
   PO Box 170608
   San Francisco, California  94117
   USA

   Phone: +1 415 221 6524
   Email: gnu@toad.com
   URI:   https://www.toad.com/

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

   Email: weiler@tislabs.com

   Tero Kivinen
   AuthenTec
   Eerikinkatu 28
   HELSINKI  FI-00180
   FI

   Email: kivinen@iki.fi