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Versions: 00

Network Working Group                                      H. Tschofenig
Internet-Draft                                                  ARM Ltd.
Intended status: Best Current Practice                 February 14, 2014
Expires: August 18, 2014

  Authentication and Authorization for Constrained Environments (ACE):
                Overview of Existing Security Protocols


   This document surveys existing three party authentication and
   authorization protocols for use with Internet of Things use cases.
   The discussed protocol frameworks are Kerberos, OAuth, ABFAB, and the
   certificate model.  The aim is to understand whether any of the
   available standardized security protocols are re-usable for
   constrained environments.  A future version of this document will
   provide a more detailed analysis against the requirements.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 18, 2014.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must

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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

1.  Introduction

   [I-D.seitz-ace-usecases] introduces a number of use cases that
   require device-to-device authentication whereby both devices may be
   constrained.  [I-D.ietf-lwig-terminology] discusses the different
   types of constraints of these devices.

   This document aims to raise the high-level question about the
   possible re-use of existing three party authentication and key
   exchange protocols for use in IoT environments.  This version of the
   document does not aim to map requirements derived from the use cases
   against these protocols.  Such a detailed analysis is premature at
   this point when use case descriptions are still in flux.

   The starting assumption for the architectures in this document is
   that a device (a client) wants to access some resource (referred as
   service in this document).  It unfortunately does not have any
   relationship with the server offering that service.  Figure 1 shows
   the scenario graphically.

        +-----------+    no prior         +-----------+
        |  Client   |    relationship     |  Service  |
        |           |                     |           |
        +-----------+                     +-----------+

                       Figure 1: Two Party Scenario.

   Imagine that the client is a light-switch and the service is a light-

   Today, companies solve this case by using a pairing protocol (at the
   link layer typically) where the two devices execute a special
   imprinting/pairing protocol to establish an initial key by using out-
   of-band (OOB) channel.  This OOB channel can come in many forms:

   o  Using an alternative communication channel, such as a USB stick,
      Ethernet cable

   o  Human involvement by comparing hashed keys, entering passkeys,
      scanning QR codes

   o  Second wireless connectivity (e.g., infrared)

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   o  Proximity-based information

   The pairing is a suitable approach where wireless communication
   replaces a wired communication technology previously used.  For
   example, a headset connected to a music player using a wired
   connection is replaced with the wireless version.  Not all use cases
   do, however, allow users to pair their device with other devices
   upfront.  Consider an enterprise with electronic door locks.  It is
   hard to imagine an employee who has to pair their digital key with
   every door in the building first before they use the system.

   Requiring every device to pair with every other device upfront is
   often inconvenient or not feasible.  Hence, this document does not
   explore pairing solutions further.  To offer an improved user
   experience with better scalability properties a device might either
   share credentials with some trusted third party.  There are various
   ways how credentials can be shared with these trusted third parties.
   For example, credentials may be provisioned during the manufacturing
   process or devices may have been paired with the trusted third party
   (in case the trusted third party is local to the user).  In fact,
   today is it very common for IoT devices to have at least credentials
   pre-provisioned for use with the vendor / manufacturer of the device
   to allow software updates to be provided securely.

   Thus, we move to a model where the device (client) shares some
   credentials with a trusted third party.  This trusted third party
   does not need to be a server on the Internet; ideally it could also
   be operated locally within someones' home, within an enterprise, or
   within a factory.

   This three party architecture is shown in Figure 2.

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                                          |  Trusted  |
             +--------------------------->|Third Party|
             |        Key                 +-----------+
             |                                  ^
             |                                  |
             |                                  |
             |                                  | Key
             |                                  |
             |                                  |
             |                                  |
             v                                  v
        +-----------+  no prior           +-----+-----+
        |           |  relationship       |           |
        |   Client  | <..................>| Service   |
        +-----------+                     +-----------+

                      Figure 2: Three Party Scenario.

   This three party architecture and messaging pattern has been explored
   with prior IETF work and this document lists the most relevant
   efforts (on a high level).

   The goal of the communication exchange is that the client has been
   authorized to access the service, and is able to secure the exchange
   of information.  The client and the service may, optionally, possess
   keying material for future use of the service with the benefit of
   better performance for future interactions.

   Note: This document does not aim to cover the use cases in their
   entirety.  First, we assume that the security protocol interaction
   for link layer authentication is outside the scope.  The focus of
   this document is on the application layer interactions when accessing
   services.  Second, this document does not survey access control
   policy languages and mechanisms for managing these access control
   policies.  These policies are important since many of the systems
   described below only provide an answer to the question 'Who is the
   holder of this key?' and standards for answering the question 'Can
   this key be used for this purpose?' (authorization) are often
   realized in a proprietary way.

   While Figure 1 shows three parties the protocols described in
   Section 2 have been generalized to four or even multi-party scenario.
   The result is shown in Figure 2.

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   +-----------+                      +-----------+
   |  Trusted  |   Agreement w/key    | Trusted   |
   |  Third    |<-------------------->| Third     |
   |  Party A  |                      | Party B   |
   +-----------+                      +-----------+
         ^                                  ^
         |                                  |
         | Key                              | Key
         |                                  |
         |                                  |
         |                                  |
         |                                  |
         v                                  v
    +-----------+  no prior           +-----+-----+
    |           |  relationship       |           |
    |   Client  | <..................>| Service   |
    +-----------+                     +-----------+

             Figure 3: Generalization of Three Party Scenario.

2.  Three Party Security Frameworks

   This section introduces four authentication and authorization
   frameworks standardized in the IETF.  The description is
   intentionally kept at a high level and a reader is encouraged to
   consult the referenced documents for details and various options
   these protocols offer.  The terminology with each of these protocols
   is lightly different and appropriate mappings have been applied.

   To demonstrate the level of maturity of these frameworks availability
   of products, source code, and deployment experience is mentioned for
   each of these frameworks.  Note, however, that this experience does
   not imply suitability for use with the IoT environment.

2.1.  Application Bridging for Federated Access Beyond Web Architecture

   This section describes the Application Bridging for Federated Access
   Beyond Web architecture [I-D.ietf-abfab-arch], which builds on the
   Authentication, Authorization and Accounting (AAA) framework.  The
   AAA framework re-uses the Extensible Authentication Protocol (EAP)
   [RFC5247] and EAP methods for the authentication protocol
   capabilities.  A detailed description of the AAA keying framework can
   be found in [RFC5247].

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                                       |   Identity   |
                                       |   Provider   |
                                       |    (IdP)     |
                                         * EAP      o RADIUS
                                         *          o
                                         *          o
      +-------------+                  +-v----------v--+
      |             |                  |               |
      | Client      |  EAP/EAP Method  |  Relying      |
      |             |<****************>|  Party        |
      |             |  GSS-API         |               |
      |             |<---------------->|               |
      |             |  Application     |               |
      |             |  Data            |               |
      |             |<================>|               |
      +-------------+                  +---------------+

   Terminology Mapping:
    - The term 'Relying Party' corresponds to the 'service'.
    - The term 'Identity Provider' corresponds to the
      'trusted third party'.

                       Figure 4: ABFAB Architecture.

   With the message exchange shown in Figure 4 the client wants to
   obtain access to a service and starts interacting with that service.
   Since no prior relationship between the client and the service is
   assumed the EAP message exchanges is relayed by the service and the
   EAP server component of the IdP.  Between the client and the service
   these EAP payloads are encapsulated within the GSS-API.  After a
   successful authentication and authorization session keys are
   delivered from the IdP to the service and can then be used to secure
   the application layer data exchange between the client and the

   While the use of EAP and the AAA architecture has mostly found use
   for network access authentication the work on ABFAB applies this
   architecture to application layer services.


   o  Re-uses existing protocols: RADIUS, GSS-API, EAP, EAP methods.

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   o  Security properties of the AAA / EAP framework well studied and
      large deployments of the AAA framework exist.

   o  Products and open source code exists for EAP, EAP methods, RADIUS,
      and the GSS-API.  The extensions needed for ABFAB also have been
      implemented but they are less mature compared to the EAP/AAA

   o  Large range of EAP methods available offering all possible
      authentication and key exchange protocols for authentication
      between the client and the AAA server.  These mechanisms have been
      deployed and are in widespread use today.  While many EAP methods
      have been standardized only a few are in widespread use in non-IoT
      environments.  However, there are many (open source)
      implementations available such that further experience concerning
      IoT suitability can be gathered.

   o  IoT devices might use the AAA/EAP architecture for network access
      authentication (e.g., WLAN-based, IEEE 802.15.4-based ZigBee-IP

   o  The AAA framework also supports authentication in a federated

   o  Authorization information is conveyed within RADIUS (and
      potentially in SAML assertions, as envisioned by ABFAB).


   o  The initial authentication and authorization exchange requires
      real-time interaction between the AAA server and the service.

   o  Deployments have so far used this architecture mainly for network
      access and for specific applications (VoIP) only.  Experience with
      other applications, as ABFAB envisions, is rather limited.

   o  ABFAB architecture uses layering of EAP within the GSS-API, which
      adds additional overhead.  A binding for the transport of EAP
      payloads in CoAP, for example, does not exist.

   o  No unified authorization policy language has been defined for the
      AAA/EAP architecture.  Instead, RADIUS attributes carry
      information about access control decisions.

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2.2.  Kerberos

   Kerberos [RFC4120] is authentication system for distributed
   environments that has enjoyed deployment for more than three decades.
   The security properties have been extensively studied and various
   implementations exist.

                  | Authentication |
                  | Server (AS)    |
                  ^        /
       Request   /        /
       Ticket   /        /
               /        /Ticket
              /        /{SK}C-KDC
             /        /
            /        /
           /        /
          /        v
        +-----------+                             +-----------+
        |           |   Ticket + Authenticator    |           |
        |  Client   |---------------------------->|  Server   |
        |           |<===========================>|           |
        +-----------+    Application Data         +-----------+

   Terminology Mapping:
     - The term AS corresponds to the 'trusted third party.'
     - The term Server corresponds to the 'service'.

                            Figure 5: Kerberos.

   The Kerberos exchange shown in Figure 5 illustrates a client who
   wants to get access to a server.  It first has to interact with the
   Authentication Server (AS) to request a ticket.  In response, the AS
   provides a ticket, which is a data structure encrypted with a key
   known only between the server and the AS.  This ticket includes
   information about the client, a session key (SK) for later use, and
   various other security relevant information elements.  The client
   also obtains the session key encrypted with a key it shares with the

   When a service access is required then the client interacts with the
   server and presents the ticket along with an Authenticator.  The
   Authenticator demonstrates that the client was able to decrypt the
   session key with the key it shares with the AS and that it was able

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   to apply this key to compute a keyed message digest over several
   fields, including a time-stamp, when accessing the service.  The
   time-stamp avoids replay attacks.


   o  Re-uses existing protocol: Kerberos

   o  Security properties well studied and large deployments exist.

   o  Products and open source service exist.

   o  Most parts of Kerberos, particularly the ticket concept, are
      designed with symmetric key cryptography, which improves
      performance.  The Kerberos ticket is consequently fairly small and
      uses a binary encoding.

   o  Kerberos also supports cross-realm authentication for scalable

   o  Kerberos also specifies a UDP-based transport.

   o  The message exchanges between the client and the service can be
      tailored to the need of the application.


   o  Each ticket is only usable for a single service (intentionally).
      As such, new tickets have to be requested whenever the client
      wants to access a new service or when the ticket expired.

   o  For the authentication between the client and the KDC a limited
      number of authentication protocols have been specified.

   o  Kerberos uses ASN.1 for encoding of the ticket and various
      messages.  This may increase implementation complexity but the
      binary encoding is more efficient than other encodings, like XML
      or JSON.

   o  No standardized access control policy has been standardized for
      inclusion inside a ticket.  Proprietary policies are, however,
      used in real-world deployments.

   o  A CoAP binding for the KRB_PRIV and the KRB_SAFE message exchanges
      used to secure application data between the client and the service
      have not been defined.

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2.3.  OAuth

   The OAuth protocol is a recent development for the Web, which re-uses
   the Kerberos interaction pattern with influences from the Web /
   mobile app space.  It initially aimed to solve the problem of
   delegated access to protected resources where websites asked users to
   share their long-term password.  Over time OAuth has been used in
   other use cases that require delegated access.

                            |Server (AS)  |
                            ^        /
                 Request   /        /
                 Access   /        /
                 Token   /        /Access Token
                        /        /
                       /        /
                      /        /
                     /        /
      O             /        v
     /|\          +-----------+                  +-----------+
      |    -----> |           |   Access Token   | Resource  |
     / \   <----- |  Client   |----------------->|  Server   |
   Resource       |           |<================>|   (RS)    |
    Owner         +-----------+ Application Data +-----------+

   Terminology Mapping:
     - The term AS corresponds to the 'trusted third party'.
     - The term RS corresponds to the 'service'.

                 Figure 6: Simplified OAuth Architecture.

   Figure 6 shows the high-level OAuth message exchange.  The canonical
   OAuth example allows a web user (resource owner) to grant a printing
   service (client) access to her private photos stored at a photo
   sharing service (resource server), without sharing her username and
   password.  Instead, she authenticates directly with the authorization
   server which issues the printing service delegation-specific


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   o  Re-uses existing protocols: OAuth Core [RFC6749], OAuth Bearer
      Token [RFC6750] OAuth MAC Token [I-D.ietf-oauth-v2-http-mac]/ HOTK
      [I-D.tschofenig-oauth-hotk], JWT [I-D.ietf-oauth-json-web-token].

   o  Large deployments in the Web environment exist, which use the
      OAuth Bearer Token.

   o  Products and open source service exist.

   o  OAuth is flexible with regard to the used cryptography.  A
      standardized format for the access token has been described with
      the JSON Web Token (JWT).  For security protection of the JWT
      various specifications from the JOSE working group are available.

   o  The message exchanges between the client and the service can be
      tailored to the need of the application.  Bindings are available
      for HTTPS and SASL [I-D.ietf-kitten-sasl-oauth].

   o  With regard to the offered security mechanism the interaction
      between the client and the resource server gives several choices:
      The OAuth Bearer Token requires a TLS exchange between the client
      and the resource server.  The MAC Token specification is
      conceptually similar to Kerberos; a version based on asymmetric
      cryptography exists as well (see HOTK).


   o  For an environment with more than one authorization server or
      where the authorization server is located in a different domain
      than the resource server the standardization work is still in
      progress.  Efforts have mostly be done in Kantara with the User-
      Managed-Access (UMA) working group.

   o  A binding for CoAP does not exist for the client to authorization
      server nor for the client to resource server.

   o  The OAuth architecture does not standardize the authentication
      procedure of the resource owner to the authorization server
      itself.  This is a common approach for the Web environment where a
      number of different authentication protocols are in use in the
      browser.  As such, the protocol works with any authentication

2.4.  Certificate Model

   Prior work on the Public Key Infrastructure, certificate formats,
   certificate extensions, and various certificate management protocols
   can be re-used in the IoT context.  With respect to the use cases

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   described in [I-D.seitz-ace-usecases] certificates may be short-lived
   and might need to contain attributes (which may be used for making
   access control decisions) rather than purely relying on the identity
   of users and their devices.

   For the purpose of dynamic provisioning short-lived certificates,
   this document envisions to re-use a subset of the functionality
   offered by protocols like the Certificate Management over CMS (CMC)
   [RFC5272], the Certificate Management Protocol (CMP) [RFC4210], the
   Simple Certificate Enrollment Protocol [I-D.nourse-scep],
   Certification Request Syntax Standard - PKCS#10 [RFC2315] (with TLS
   or with PKCS#7 [RFC2986] ).  While these protocols offer slightly
   different features, on a high-level the all fulfill the same
   function.  Note that the management of trust anchors may be provided
   by a different protocol, such as Trust Anchor Management Protocol
   (TAMP) [RFC5934].

   Of course, certificates do not necessarily need to be short-lived and
   could even be provisioned during the manufacturing process and never
   changed during the lifetime of the device.  The drawback of such an
   approach is, however, that mechanisms for certificate revocation have
   to be provided.  Furthermore, privacy concerns might be arise since
   the same client certificate content will be shown to every service
   rather than information that is only relevant for a specific purpose.

                  |  Authority  |
                  ^        /
       Request   /        /
       (Short)  /        /
       Lived   /        /(Short) Lived
       Cert   /        / Certificate
             /        /
            /        /
           /        /
          /        v
        +-----------+                             +-----------+
        |           |   DTLS with certificate     |           |
        |           |   or app layer msg w/cert   |           |
        |  Client   |---------------------------->|  Service  |
        |           |<===========================>|           |
        +-----------+      Application Data       +-----------+

                       Figure 7: Certificate Model.

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   o  Re-uses existing protocols: DTLS (or application protocol), CMP/
      CMC/PKCS#10/SCEP, specifications (certificate format - RFC 6818
      [RFC6818]), and concepts (PKI).

   o  Large deployments on the Web and with enterprise system exist.

   o  Products and open source code exists.

   o  The certificate format offers flexibility in terms of content.
      New extensions have been defined over time.

   o  Certificates can be used with DTLS without any additional
      modifications.  Certificates can also used with application
      security mechanisms.

   o  Authorization information may be placed in an extension of a
      public key certificate or in a separate attribute certificate
      [RFC3281].  Earlier work on KeyNote [RFC2704]could be re-used as
      it provides a more flexible authorization policy language.

   o  A single certificate can be used with a number of different

   o  Various PKI management protocols have been defined and they offer
      some flexibility.  The properties vary on the specific use cases.


   o  The certificate format and the PKI management protocols use ASN.1.

   o  No UDP or CoAP transport is defined for CMC/CMP/SCEP.  For PKCS#10
      no transport is defined at all.

   o  The public key infrastructure only focuses on asymmetric
      cryptography.  A separate body of work is available for
      provisioning symmetric keys (like one-time-keys), such as the
      Portable Symmetric Key Container (PSKC) [RFC6030] and Dynamic
      Symmetric Key Provisioning Protocol (DSKPP) ([RFC6063]).

   o  Protocols for certificate enrollment are in use but many
      deployments use their own strategy for distributing certificates
      (typically long-lived) to their users.

   o  Asymmetric cryptography is computationally more expensive than
      symmetric cryptography but offers additional security benefits.

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3.  Conclusion

   Several existing protocols can be used to meet the use cases outlined
   in [I-D.seitz-ace-usecases].  Each technology presented here offers a
   number of possibilities for profiling to make them work on for
   constrained devices.  Despite the range of available security
   protocols, the use cases suggest that there is a need to profile and
   to extend those in order to make them fit for the constrained

   The right choice of authentication and authorization protocol will
   heavily depend on the envisioned usage environment.

   It is, however, also worth noting that several aspects that are not
   discussed in this document although they appear as requirements in
   the use case document, namely

   o  a language for describing access control policies,

   o  the encoding of these policies, and

   o  the container for associating these policies with keying material.

4.  Security Considerations

   This entire document is about security.

5.  IANA Considerations

   This document does not require any actions by IANA.

6.  Acknowledgements

   The author would like to thank Stefanie Gerdes for her review

7.  Informative References

              Howlett, J., Hartman, S., Tschofenig, H., Lear, E., and J.
              Schaad, "Application Bridging for Federated Access Beyond
              Web (ABFAB) Architecture", draft-ietf-abfab-arch-10 (work
              in progress), December 2013.

              Mills, W., Showalter, T., and H. Tschofenig, "A set of
              SASL Mechanisms for OAuth", draft-ietf-kitten-sasl-
              oauth-12 (work in progress), December 2013.

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              Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained Node Networks", draft-ietf-lwig-terminology-06
              (work in progress), December 2013.

              Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", draft-ietf-oauth-json-web-token-15 (work in
              progress), January 2014.

              Richer, J., Mills, W., Tschofenig, H., and P. Hunt, "OAuth
              2.0 Message Authentication Code (MAC) Tokens", draft-ietf-
              oauth-v2-http-mac-05 (work in progress), January 2014.

              Pritikin, M., Nourse, A., and J. Vilhuber, "Simple
              Certificate Enrollment Protocol", draft-nourse-scep-23
              (work in progress), September 2011.

              Seitz, L., Gerdes, S., and G. Selander, "ACE use cases",
              draft-seitz-ace-usecases-00 (work in progress), February

              Bradley, J., Hunt, P., Nadalin, A., and H. Tschofenig,
              "The OAuth 2.0 Authorization Framework: Holder-of-the-Key
              Token Usage", draft-tschofenig-oauth-hotk-03 (work in
              progress), January 2014.

   [RFC2315]  Kaliski, B., "PKCS #7: Cryptographic Message Syntax
              Version 1.5", RFC 2315, March 1998.

   [RFC2704]  Blaze, M., Feigenbaum, J., Ioannidis, J., and A.
              Keromytis, "The KeyNote Trust-Management System Version
              2", RFC 2704, September 1999.

   [RFC2986]  Nystrom, M. and B. Kaliski, "PKCS #10: Certification
              Request Syntax Specification Version 1.7", RFC 2986,
              November 2000.

   [RFC3281]  Farrell, S. and R. Housley, "An Internet Attribute
              Certificate Profile for Authorization", RFC 3281, April

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   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
              Kerberos Network Authentication Service (V5)", RFC 4120,
              July 2005.

   [RFC4210]  Adams, C., Farrell, S., Kause, T., and T. Mononen,
              "Internet X.509 Public Key Infrastructure Certificate
              Management Protocol (CMP)", RFC 4210, September 2005.

   [RFC5247]  Aboba, B., Simon, D., and P. Eronen, "Extensible
              Authentication Protocol (EAP) Key Management Framework",
              RFC 5247, August 2008.

   [RFC5272]  Schaad, J. and M. Myers, "Certificate Management over CMS
              (CMC)", RFC 5272, June 2008.

   [RFC5934]  Housley, R., Ashmore, S., and C. Wallace, "Trust Anchor
              Management Protocol (TAMP)", RFC 5934, August 2010.

   [RFC6030]  Hoyer, P., Pei, M., and S. Machani, "Portable Symmetric
              Key Container (PSKC)", RFC 6030, October 2010.

   [RFC6063]  Doherty, A., Pei, M., Machani, S., and M. Nystrom,
              "Dynamic Symmetric Key Provisioning Protocol (DSKPP)", RFC
              6063, December 2010.

   [RFC6749]  Hardt, D., "The OAuth 2.0 Authorization Framework", RFC
              6749, October 2012.

   [RFC6750]  Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
              Framework: Bearer Token Usage", RFC 6750, October 2012.

   [RFC6818]  Yee, P., "Updates to the Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 6818, January 2013.

Author's Address

   Hannes Tschofenig
   ARM Ltd.
   110 Fulbourn Rd
   Cambridge  CB1 9NJ
   Great Britain

   Email: Hannes.tschofenig@gmx.net
   URI:   http://www.tschofenig.priv.at

Tschofenig               Expires August 18, 2014               [Page 16]

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