ACE Working Group                                              S. Gerdes
Internet-Draft                                   Universitaet Bremen TZI
Intended status: Informational                                  L. Seitz
Expires: March 31, April 21, 2016                              SICS Swedish ICT AB
                                                             G. Selander
                                                         C. Bormann, Ed.
                                                 Universitaet Bremen TZI
                                                      September 28,
                                                        October 19, 2015

     An architecture for authorization in constrained environments


   Constrained-node networks are networks where some nodes have severe
   constraints on code size, state memory, processing capabilities, user
   interface, power and communication bandwidth (RFC 7228).

   This document provides terminology, and identifies the elements that
   an architecture needs to address, providing a problem statement, for
   authentication and authorization in these networks.

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   This Internet-Draft will expire on March 31, April 21, 2016.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Architecture and High-level Problem Statement . . . . . . . .   5
     2.1.  Elements of an Architecture . . . . . . . . . . . . . . .   5
     2.2.  Architecture Variants . . . . . . . . . . . . . . . . . .   8
     2.3.  Information flows . . . . . . . . . . . . . . . . . . . .  11
     2.4.  Problem statement . . . . . . . . . . . . . . . . . . . .  12  11
   3.  Security Objectives . . . . . . . . . . . . . . . . . . . . .  12
     3.1.  End-to-End Security Objectives  . . . . . . . . . . . in Multi-Hop Scenarios . .  13
   4.  Authentication and Authorization  . . . . . . . . . . . . . .  13
   5.  Actors and their Tasks  . . . . . . . . . . . . . . . . . . .  15
     5.1.  Constrained Level Actors  . . . . . . . . . . . . . . . .  16
     5.2.  Principal Level Actors  . . . . . . . . . . . . . . . . .  17
     5.3.  Less-Constrained Level Actors . . . . . . . . . . . . . .  17
   6.  Kinds of Protocols  . . . . . . . . . . . . . . . . . . . . .  18
     6.1.  Constrained Level Protocols . . . . . . . . . . . . . . .  18
       6.1.1.  Cross Level Support Protocols . . . . . . . . . . . .  19
     6.2.  Less-Constrained Level Protocols  . . . . . . . . . . . .  19
   7.  Elements of a Solution  . . . . . . . . . . . . . . . . . . .  19
     7.1.  Authorization . . . . . . . . . . . . . . . . . . . . . .  19
     7.2.  Authentication  . . . . . . . . . . . . . . . . . . . . .  20
     7.3.  Communication Security  . . . . . . . . . . . . . . . . .  20  21
     7.4.  Cryptographic Keys  . . . . . . . . . . . . . . . . . . .  21
   8.  Assumptions and Requirements  . . . . . . . . . . . . . . . .  22
     8.1.  Architecture  . . . . . . . . . . . . . . . . . . . . . .  22
     8.2.  Constrained Devices . . . . . . . . . . . . . . . . . . .  22  23
     8.3.  Authentication  . . . . . . . . . . . . . . . . . . . . .  23  24
     8.4.  Server-side Authorization . . . . . . . . . . . . . . . .  24
     8.5.  Client-side Authorization Information . . . . . . . . . .  24
     8.6.  Server-side Authorization Information . . . . . . . . . .  24  25
     8.7.  Resource Access . . . . . . . . . . . . . . . . . . . . .  25
     8.8.  Keys and Cipher Suites  . . . . . . . . . . . . . . . . .  25  26
     8.9.  Network Considerations  . . . . . . . . . . . . . . . . .  26
     8.10. Legacy Considerations . . . . . . . . . . . . . . . . . .  26
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  26  27
     9.1.  Physical Attacks on Sensor and Actuator Networks  . . . .  27
     9.2.  Time Measurements . . . . . . . . . . . . . . . . . . . .  28
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  28  29
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  29
   12. Informative References  . . . . . . . . . . . . . . . . . . .  29
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  31

1.  Introduction

   Constrained nodes are small devices with limited abilities which in
   many cases are made to fulfill a specific simple task.  They have
   limited hardware resources such as processing power, memory, non-
   volatile storage and transmission capacity and additionally in most
   cases do not have user interfaces and displays.  Due to these
   constraints, commonly used security protocols are not always easily

   Constrained nodes are expected to be integrated in all aspects of
   everyday life and thus will be entrusted with vast amounts of data.
   Without appropriate security mechanisms attackers might gain control
   over things relevant to our lives.  Authentication and authorization
   mechanisms are therefore prerequisites for a secure Internet of

   Authorization is about who can do what to which objects.
   Authentication specifically addresses the who, but is often specific
   to the authorization that is required (for example, it may be
   sufficient to authenticate the age of an actor, so no identifier is
   needed or even desired).  Authentication often involves credentials,
   only some of which need to be long-lived and generic; others may be
   directed towards specific authorizations (but still possibly long-
   lived).  Authorization then makes use of these credentials, as well
   as other information (such as the time of day).  This means that the
   application-induced complexity of authenticated authorization can
   often be moved back and forth between these two aspects.

   In some cases authentication and authorization can be addressed by
   static configuration provisioned during manufacturing or deployment
   by means of fixed trust anchors and static access control lists.
   This is particularly applicable to siloed, fixed-purpose deployments.

   However, as the need for flexible access to assets already deployed
   increases, the legitimate set of authorized entities as well as their
   specific privileges cannot be conclusively defined during deployment,
   without any need for change during the lifetime of the device.
   Moreover, several use cases illustrate the need for fine-grained
   access control policies, for which for instance a basic access
   control list concept may not be sufficiently powerful.

   The limitations of the constrained nodes ask for security mechanisms
   which take the special characteristics of constrained environments
   into account; not all constituents may be able to perform all
   necessary tasks by themselves.  In order to meet the security
   requirements in constrained scenarios, the necessary tasks need to be
   assigned to logical functional entities.

   In order to be able to achieve complex security objectives between
   actors some of which are hosted on simple ("constrained") devices,
   some of the actors will make use of help from other, less constrained
   actors.  (This offloading is not specific to networks with
   constrained nodes, but their constrainedness as the main motivation

   We therefore group the logical functional entities by whether they
   can be assigned to a constrained device ("constrained level") or need
   higher function platforms ("less-constrained level"); the latter does
   not necessarily mean high-function, "server" or "cloud" platforms.
   Note that assigning a logical functional entity to the constrained
   level does not mean that the specific implementation needs to be
   constrained, only that it _can_ be.

   This document provides some terminology, and identifies the elements
   an architecture needs to address, representing the relationships
   between the logical functional entities involved; on this basis, a
   problem description for authentication and authorization in
   constrained-node networks is provided.

1.1.  Terminology

   Readers are required to be familiar with the terms and concepts
   defined in [RFC4949], including "authentication", "authorization",
   "confidentiality", "(data) integrity", "message authentication code",
   and "verify".

   REST terms including "resource", "representation", etc. are to be
   understood as used in HTTP [RFC7231] and CoAP [RFC7252]; the latter
   also defines additional terms such as "endpoint".

   Terminology for constrained environments including "constrained
   device", "constrained-node network", "class 1", etc. is defined in

   In addition, this document uses the following terminology:

   Resource (R):  an item of interest which is represented through an
      interface.  It might contain sensor or actuator values or other

   Constrained node:  a constrained device in the sense of [RFC7228].

   Actor:  A logical functional entity that performs one or more tasks.
      Multiple actors may be present within a single device or a single
      piece of software.

   Resource Server (RS):  An entity which hosts and represents a

   Client (C):  An entity which attempts to access a resource on an RS.

   Principal:  (Used in its English sense here, and specifically as:) An
      individual that is either RqP or RO or both.

   Resource Owner (RO):  The principal that is in charge of the resource
      and controls its access permissions.

   Requesting Party (RqP):  The principal that is in charge of the
      Client and controls the requests a Client makes and its acceptance
      of responses.

   Authorization Server (AS):  An entity that prepares and endorses
      authentication and authorization data for a Resource Server.

   Client Authorization Server (CAS):  An entity that prepares and
      endorses authentication and authorization data for a Client.

   Authenticated Authorization:  A synthesis of mechanisms for
      authentication and authorization.

   Note that other authorization architectures such as OAuth [RFC6749]
   or UMA [I-D.hardjono-oauth-umacore] focus on the authorization
   problems on the RS side, in particular what accesses to resources the
   RS is to allow.  In this document the term authorization includes
   this aspect, but is also used for the client-side aspect of
   authorization, i.e., more generally to describe allowed interactions
   with other endpoints.

2.  Architecture and High-level Problem Statement

2.1.  Elements of an Architecture

   This document deals with how to control and protect resource-based
   interaction between potentially constrained endpoints.  The following
   setting is assumed:

   o  An endpoint may host functionality of one or more actors.

   o  C in one endpoint requests to access R on a RS in another

   o  A priori, the endpoints do not necessarily have a pre-existing
      security relationship to each other.

   o  Either of the endpoints, or both, may be constrained.

   Without loss of generality, we focus on the C functionality in one
   endpoint, which we therefore also call C, accessing the RS
   functionality in another endpoint, which we therefore also call RS.

   The constrained level and its security objectives are detailed in
   Section 5.1.

           --------------                          --------------
           |  -------   |                          |  -------   |
           |  |  C  | ------ requests resource -----> | RS  |   |
           |  ------- <----- provides resource ------ -------   |
           |  Endpoint  |                          |  Endpoint  |
           --------------                          --------------

                        Figure 1: Constrained Level

   The authorization decisions at the endpoints are made on behalf of
   the principals that control the endpoints.  To reuse OAuth and UMA
   terminology, the present document calls C's controlling principal the
   Requesting Party (RqP), and calls RS's controlling principal the
   Resource Owner (RO).  Each principal makes authorization decisions
   (possibly encapsulating them into security policies) which the
   endpoint it controls then enforces.

   The specific security objectives will vary, but for any specific
   version of this scenario will include one or more of:

   o  Objectives of type 1: No entity not authorized by the RO has
      access to (or otherwise gains knowledge of) R.

   o  Objectives of type 2: C is exchanging information with (sending a
      request to, accepting a response from) a resource only where it
      can ascertain that RqP has authorized the exchange with R.

   Objectives of type 1 require performing authorization on the Resource
   Server side while objectives of type 2 require performing
   authorization on the Client side.

   More on the security objectives of the principal level in
   Section 5.2.

     -------                           -------
     | RqP |                           |  RO | Principal Level
     -------                           -------
        |                                 |
   in charge of                      in charge of
        |                                 |
        V                                 V
     -------                           -------
     |  C  |  -- requests resource --> | RS  | Constrained Level
     -------  <-- provides resource--  -------

              Figure 2: Constrained Level and Principal Level

   The use cases defined in [I-D.ietf-ace-usecases] demonstrate that
   constrained devices are often used for scenarios where their
   principals are not present at the time of the communication, are not
   able to communicate directly with the device because of a lack of
   user interfaces or displays, or may prefer the device to communicate

   Moreover, constrained endpoints may need support with tasks requiring
   heavy processing, large memory or storage, or interfacing to humans,
   such as management of security policies defined by a principal.  The
   principal, in turn, requires some agent maintaining the policies
   governing how its endpoints will interact.

   For these reasons, another level of nodes is introduced in the
   architecture, the less-constrained level.  Using OAuth terminology,
   AS acts on behalf of the RO to control and support the RS in handling
   access requests, employing a pre-existing security relationship with
   RS.  We complement this with CAS acting on behalf of RqP to control
   and support the C in making resource requests and acting on the
   responses received, employing a pre-existing security relationship
   with C.  To further relieve the constrained level, authorization (and
   related authentication) mechanisms may be employed between CAS and AS
   (Section 6.2).  (Again, both CAS and AS are conceptual entities
   controlled by their respective principals.  Many of these entities,
   often acting for different principals, can be combined into a single
   server implementation; this of course requires proper segregation of
   the control information provided by each principal.)
    -------                           -------
    | RqP |                           |  RO | Principal Level
    -------                           -------
       |                                 |
   controls                          controls
       |                                 |
       V                                 V
   --------                          -------
   |  CAS |  <- AuthN and AuthZ ->   |  AS |  Less-Constrained Level
   --------                          -------
       |                                 |
   controls and supports        controls and supports
   authentication               authentication
   and authorization            and authorization
       |                                 |
       V                                 V
    -------                           -------
    |  C  |  -- requests resource --> | RS  | Constrained Level
    -------  <-- provides resource--  -------

                      Figure 3: Overall architecture

   Figure 3 shows all three levels considered in this document.  Note
   that the vertical arrows point down to illustrate exerting control
   and providing support; this is complemented by information flows that
   often are bidirectional.  Note also that not all entities need to be
   ready to communicate at any point in time; for instance, RqP may have
   provided enough information to CAS that CAS can autonomously
   negotiate access to RS with AS for C based on this information.

2.2.  Architecture Variants

   The elements of the architecture described above are architectural.
   In a specific scenario, several elements can share a single device or
   even be combined in a single piece of software.  If C is located on a
   more powerful device, it can be combined with CAS:

      -------                          --------
      | RqP |                          |  RO  |   Principal Level
      -------                          --------
        |                                  |
   in charge of                       in charge of
        |                                  |
        V                                  V
   ------------                        --------
   |  CAS + C | <- AuthN and AuthZ ->  |  AS  |  Less-Constrained Level
   ------------                        --------
               ^                            |
                \__                         |
                   \___                 authentication
                       \___             and authorization
        requests resource/ \___          support
        provides resource      \___        |
                                   \___    |
                                       V   V
                                        | RS  | Constrained Level

                       Figure 4: Combined C and CAS

   If RS is located on a more powerful device, it can be combined with

      -------                            -------
      | RqP |                            |  RO |   Principal Level
      -------                            -------
        |                                  |
   in charge of                       in charge of
        |                                  |
        V                                  V
   ----------                        -----------
   |  CAS   | <- AuthN and AuthZ ->  | RS + AS |  Less-Constrained Level
   ----------                        -----------
        |                           ^
   authentication               ___/
   and authorization        ___/
   support              ___/ request resource / provides resource
        |           ___/
        V       ___/
     -------   /
     |  C  | <-

                       Figure 5: Combined AS and RS

   If C and RS have the same principal, CAS and AS can be combined.

                   | RqP = RO |   Principal Level
                     in charge of
                 |  CAS + AS  |  Less-Constrained Level
                /                \
               /                  \
   authentication               authentication
   and authorization            and authorization
   support                      support
            /                        \
           V                          V
     -------                          -------
     |  C  | -- requests resource --> | RS  | Constrained Level
     ------- <-- provides resource -- -------

                      Figure 6: CAS combined with AS

2.3.  Information flows

   In this subsection, we complement  Problem statement

   We now formulate the abstracted architecture
   described above with a discussion problem statement in terms of the information
   flows in scope,
   mentioning that each endpoint may assume both a client the architecture focuses on.

   The interaction with the nodes on the principal level, RO and a server
   role RqP, is
   not involving constrained nodes and that communication may be via intermediaries. therefore can employ an existing
   mechanism.  The less-constrained nodes, CAS and AS, control the interactions
   between the endpoints by supporting support the potentially
   constrained nodes nodes, C and RS, with control information, for example
   permissions of clients, conditions on resources, attributes of client
   and resource servers, keys and credentials.  The  This control information
   may be rather different for C and RS, reflecting the intrinsic
   asymmetry with C initiating the request for access to a resource, and
   RS acting on a received request, and C finally acting on the received

   The potential information flows are shown in Figure 7.  The arrows with control
   information only indicate origin and destination of information,
   actual message flow may pass intermediary nodes (both nodes that are
   identified in the architecture and other nodes).  The direction
   of the vertical arrows expresses the exertion of control; actual
   information flow is bidirectional.

   The message flow may pass unprotected paths and thus need to be
   protected, potentially beyond a single REST hop (Section 3.1):

     -------  ------                                     -------  ------
        | CAS |  | AS |
     | CAS |                                     | AS  |
     -------  ------                                     -------  ------
            |      |                           |      |
            |      |                           |      |
            |      | control                   |
     a ^ | b   a = requests for control info     a ^ |      | information b
       | |     b = control information             | |
       |      |
       -------------------                -------------------
       |    |      |     |                |    |      |     |
       |    v      |     |                |    |      v
       | -------  ---------- request  -----------> -------  |
       | | C1  | <---------- response ------------ | RS2 |  |
       | -------   |     |                |    |   -------  |
       | v                                         |                | v            |
     -------  |                |                                     -------
     | C   |        | RS1 | <---- ------ request ----- -------------------> | C2 RS  |
     |     |        ------- ----- <----- response ---> -------         |
       |                 |                |                 |
       |   Endpoint 1    | ------------------- |   Endpoint 2     |
       -------------------                -------------------
     -------                                     -------

           Figure 7: Information flows that need to be protected

   o  We assume that the necessary keys/credentials for protecting the
      control information between the potentially constrained nodes and
      their associated less-constrained nodes are pre-established, for
      example as part of the commissioning procedure.

   o  The messages between the endpoints also need to be protected,
      potentially end-to-end through intermediary nodes (Section 3.1).  Any necessary keys/credentials for protecting the interaction
      between the endpoints potentially constrained nodes will need to be
      established and maintained as part of a solution.

2.4.  Problem statement

   The problem statement for authorization in constrained environments
   can be summarized as follows:

   o  The interaction between potentially constrained endpoints is
      controlled by control information provided by less-constrained
      nodes on behalf of the principals of the endpoints.

   o  The interaction between the endpoints needs to be secured, as well
      as the establishment of the necessary keys for securing the
      interaction, potentially end-to-end through intermediary nodes.

   o  The mechanism for transferring control information needs to be
      secured, potentially end-to-end through intermediary nodes.  Pre-
      established keying material may need to be employed for
      establishing the keys used to protect these information flows.

3.  Security Objectives

   The security objectives that are addressed by an authorization
   solution include confidentiality and integrity.  Additionally,
   allowing only selected entities limits the burden on system
   resources, thus helping to achieve availability.  Misconfigured or
   wrongly designed authorization solutions can result in availability
   breaches: Users might no longer be able to use data and services as
   they are supposed to.

   Authentication mechanisms can achieve additional security objectives
   such as accountability and third-party verifiability.  These
   additional objectives are not directly related to authorization and
   thus are not in scope of this draft, but may nevertheless be
   relevant.  Accountability and third-party verifiability may require
   authentication on a device level, if it is necessary to determine
   which device performed an action.  In other cases it may be more
   important to find out who is responsible for the device's actions.
   See also Section 4 for more discussion about authentication and

   The security objectives and their relative importance differ for the
   various constrained environment applications and use cases

   In many cases, one participating party has different security
   objectives than another.  To achieve a security objective of one
   party, another party may be required to provide a service.  For
   example, if RqP requires the integrity of representations of a
   resource R that RS is hosting, both C and RS need to partake in
   integrity-protecting the transmitted data.  Moreover, RS needs to
   protect any write access to this resource as well as to relevant
   other resources (such as configuration information, firmware update
   resources) to prevent unauthorized users from manipulating R.

3.1.  End-to-End Security Objectives in Multi-Hop Scenarios

   In many cases, the information flows described in Section 2.3 cross
   multiple client-server pairings but still need to be protected end-to-end. end-
   to-end.  For example, AS may not be connected to RS (or may not want
   to exercise such a connection), relying on C for transferring
   authorization information.  As the authorization information is
   related to the permissions granted to C, C must not be in a position
   to manipulate this information, which therefore requires integrity
   protection on the way between AS and RS.

   As another example, resource representations sent between endpoints
   may be stored in intermediary nodes, such as caching proxies or pub-
   sub brokers.  Where these intermediaries cannot be relied on to
   fulfill the security objectives of the endpoints, these will need to
   protect the exchanges end-to-end. beyond a single client-server exchange.

   Note that there may also be cases of intermediary nodes that very
   much partake in the security objectives to be achieved.  What is  The question
   what are the
   endpoint to pairs of endpoints between which the communication needs
   end-to-end protection (and which aspect of protection) is defined by
   the use case.  Two examples of intermediary nodes executing security

   o  To enable a trustworthy publication service, a pub-sub broker may
      be untrusted with the plaintext content of a publication
      (confidentiality), but required to verify that the publication is
      performed by claimed publisher and is not a replay of an old
      publication (authenticity/integrity).

   o  To comply with requirements of transparency, a gateway may be
      allowed to read, verify (authenticity) but not modify (integrity)
      a resource representation which therefore also is end-to-end
      integrity protected from the server towards a client behind the

   In order to support the required communication and application
   security, keying material needs to be established between the
   relevant nodes in the architecture.

4.  Authentication and Authorization

   Server-side authorization solutions aim at protecting the access to
   items of interest, for instance hardware or software resources or
   data: They enable the resource owner to control who can access it and

   To determine if an entity is authorized to access a resource, an
   authentication mechanism is needed.  According to the Internet
   Security Glossary [RFC4949], authentication is "the process of
   verifying a claim that a system entity or system resource has a
   certain attribute value."  Examples for attribute values are the ID
   of a device, the type of the device or the name of its owner.

   The security objectives the authorization mechanism aims at can only
   be achieved if the authentication and the authorization mechanism
   work together correctly.  We speak of authenticated authorization to
   refer to the required synthesis of mechanism for authentication and

   Where used for authorization, the set of authenticated attributes
   must be meaningful for this purpose, i.e., authorization decisions
   must be possible based on these attributes.  If the authorization
   policy assigns permissions to an individual entity, the set of
   authenticated attributes must be suitable to uniquely identify this

   In scenarios where devices are communicating autonomously there is
   often less need to uniquely identify an individual device: For a
   principal, the fact that a device belongs to a certain company or
   that it has a specific type (such as a light bulb) or location may be
   more important than that it has a unique identifier.

   (As a special case for the authorization of read access to a
   resource, RS may simply make an encrypted representation available to
   anyone [OSCAR].  In this case, controlling read access to that
   resource can be reduced to controlling read access to the key;
   partially removing access also requires a timely update of the key
   for RS and all participants still authorized.)

   Principals (RqP and RO) need to decide about the required level of
   granularity for the authorization.  For example, we distinguish
   device authorization from owner authorization, and flat authorization
   from unrestricted authorization.  In the first case different access
   permissions are granted to individual devices while in the second
   case individual owners are authorized.  If flat authorization is
   used, all authenticated entities are implicitly authorized and have
   the same access permissions.  Unrestricted authorization for an item
   of interest means that no authorization mechanism is used for
   accessing this resource (not even by authentication) and all entities
   are able to access the item as they see fit (note that an
   authorization mechanism may still be used to arrive at the decision
   to employ unrestricted authorization).

   More fine-grained authorization does not necessarily provide more
   security but can be more flexible.  Principals need to consider that
   an entity should only be granted the permissions it really needs
   (principle of least privilege), to ensure the confidentiality and
   integrity of resources.

   For all cases where an authorization solution is needed (all but
   Unrestricted Authorization), the enforcing party needs to be able to
   authenticate the party that is to be authorized.  Authentication is
   therefore required for messages that contain (or otherwise update)
   representations of an accessed item.  More precisely: The enforcing
   party needs to make sure that the receiver of a message containing a
   representation is authorized to receive it, both in the case of a
   client sending a representation to a server and vice versa.  In
   addition, it needs to ensure that the actual sender of a message
   containing a representation is indeed the one authorized to send this
   message, again for both the client-to-server and server-to-client
   case.  To achieve this, integrity protection of these messages is
   required: Authenticity cannot be assured if it is possible for an
   attacker to modify the message during transmission.

   In some cases, only one side (client or server side) requires the
   integrity and / or confidentiality of a resource value.  Principals
   may decide to omit authentication (unrestricted authorization), or
   use flat authorization (just employing an authentication mechanism).
   However, as indicated in Section 3, the security objectives of both
   sides must be considered, which can often only be achieved when the
   the other side can be relied on to perform some security service.

5.  Actors and their Tasks

   This and the following section look at the resulting architecture
   from two different perspectives: This section provides a more
   detailed description of the various "actors" in the architecture, the
   logical functional entities performing the tasks required.  The
   following section then will focus on the protocols run between these
   functional entities.

   For the purposes of this document, an actor consists of a set of
   tasks and additionally has a security domain (client domain or server
   domain) and a level (constrained, principal, less-constrained).
   Tasks are assigned to actors according to their security domain and
   required level.

   Note that actors are a concept to understand the security
   requirements for constrained devices.  The architecture of an actual
   solution might differ as long as the security requirements that
   derive from the relationship between the identified actors are
   considered.  Several actors might share a single device or even be
   combined in a single piece of software.  Interfaces between actors
   may be realized as protocols or be internal to such a piece of

   A more detailed discussion of the tasks the actors have to perform in
   order to achieve specific security objectives is provided in

5.1.  Constrained Level Actors

   As described in the problem statement (see Section 2), either C or RS
   or both of them may be located on a constrained node.  We therefore
   define that C and RS must be able to perform their tasks even if they
   are located on a constrained node.  Thus, C and RS are considered to
   be Constrained Level Actors.

   C performs the following tasks:

   o  Communicate in a secure way (provide for confidentiality and
      integrity of messages), including access requests.

   o  Validate that an entity is an authorized server for R.

   RS performs the following tasks:

   o  Communicate in a secure way (provide for confidentiality and
      integrity of messages), including responses to access requests.

   o  Validate the authorization of the requester to access the
      requested resource as requested.

   R is an item of interest such as a sensor or actuator value.  R is
   considered to be part of RS and not a separate actor.  The device on
   which RS is located might contain several resources of different ROs.
   For simplicity of exposition, these resources are described as if
   they had separate RS.

   As C and RS do not necessarily know each other they might belong to
   different security domains.

   (See Figure 8.)
           -------                            --------
           |  C  |  -- requests resource ---> |  RS  | Constrained Level
           -------  <-- provides resource---  --------

                    Figure 8: Constrained Level Actors

5.2.  Principal Level Actors

   Our objective is that C and RS are under control of principals in the
   physical world, the Requesting Party (RqP) and the Resource Owner
   (RO) respectively.  The principals decide about the security policies
   of their respective endpoints and belong to the same security domain.

   RqP is in charge of C, i.e. RqP specifies security policies for C,
   such as with whom C is allowed to communicate.  By definition, C and
   RqP belong to the same security domain.

   RqP must fulfill the following task:

   o  Configure for C authorization information for sources for R.

   RO is in charge of R and RS.  RO specifies authorization policies for
   R and decides with whom RS is allowed to communicate.  By definition,
   R, RS and RO belong to the same security domain.

   RO must fulfill the following task:

   o  Configure for RS authorization information for accessing R.

   (See Figure 2.)

5.3.  Less-Constrained Level Actors

   Constrained level actors can only fulfill a limited number of tasks
   and may not have network connectivity all the time.  To relieve them
   from having to manage keys for numerous endpoints and conducting
   computationally intensive tasks, another complexity level for actors
   is introduced.  An actor on the less-constrained level belongs to the
   same security domain as its respective constrained level actor.  They
   also have the same principal.

   The Client Authorization Server (CAS) belongs to the same security
   domain as C and RqP.  CAS acts on behalf of RqP.  It assists C in
   authenticating RS and determining if RS is an authorized server for
   R.  CAS can do that because for C, CAS is the authority for claims
   about RS.

   CAS performs the following tasks:

   o  Validate on the client side that an entity has certain attributes.

   o  Obtain authorization information about an entity from C's
      principal (RqP) and provide it to C.

   o  Negotiate means for secure communication to communicate with C.

   The Authorization Server (AS) belongs to the same security domain as
   R, RS and RO.  AS acts on behalf of RO.  It supports RS by
   authenticating C and determining C's permissions on R.  AS can do
   that because for RS, AS is the authority for claims about C.

   AS performs the following tasks:

   o  Validate on the server side that an entity has certain attributes.

   o  Obtain authorization information about an entity from RS'
      principal (RO) and provide it to RS.

   o  Negotiate means for secure communication to communicate with RS.

6.  Kinds of Protocols

   Devices on the less-constrained level potentially are more powerful
   than constrained level devices in terms of processing power, memory,
   non-volatile storage.  This results in different characteristics for
   the protocols used on these levels.

6.1.  Constrained Level Protocols

   A protocol is considered to be on the constrained level if it is used
   between the actors C and RS which are considered to be constrained
   (see Section 5.1).  C and RS might not belong to the same security
   domain.  Therefore, constrained level protocols need to work between
   different security domains.

   Commonly used Internet protocols can not in every case be applied to
   constrained environments.  In some cases, tweaking and profiling is
   required.  In other cases it is beneficial to define new protocols
   which were designed with the special characteristics of constrained
   environments in mind.

   On the constrained level, protocols need to address the specific
   requirements of constrained environments.  Examples for protocols
   that consider these requirements is the transfer protocol CoAP
   (Constrained Application Protocol) [RFC7252] and the Datagram
   Transport Layer Security Protocol (DTLS) [RFC6347] which can be used
   for channel security.

   Constrained devices have only limited storage space and thus cannot
   store large numbers of keys.  This is especially important because
   constrained networks are expected to consist of thousands of nodes.
   Protocols on the constrained level should keep this limitation in

6.1.1.  Cross Level Support Protocols

   Protocols which operate between a constrained device on one side and
   the corresponding less-constrained device on the other are considered
   to be (cross level) support protocols.  Protocols used between C and
   CAS or RS and AS are therefore support protocols.

   Support protocols must consider the limitations of their constrained
   endpoint and therefore belong to the constrained level protocols.

6.2.  Less-Constrained Level Protocols

   A protocol is considered to be on the less-constrained level if it is
   used between the actors CAS and AS.  CAS and AS might belong to
   different security domains.

   On the less-constrained level, HTTP [RFC7230] and Transport Layer
   Security (TLS) [RFC5246] can be used alongside or instead of CoAP and
   DTLS.  Moreover, existing security solutions for authentication and
   authorization such as the OAuth web authorization framework [RFC6749]
   and Kerberos [RFC4120] can likely be used without modifications and
   there are no limitations for the use of a Public Key Infrastructure

7.  Elements of a Solution

   Without anticipating specific solutions, the following considerations
   may be helpful in discussing them.

7.1.  Authorization

   The core problem we are trying to solve is authorization.  The
   following problems related to authorization need to be addressed:

   o  AS needs to transfer authorization information to RS and CAS needs
      to transfer authorization information to C.

   o  The transferred authorization information needs to follow a
      defined format and encoding, which must be efficient for
      constrained devices, considering size of authorization information
      and parser complexity.

   o  C and RS need to be able to verify the authenticity of the
      authorization information they receive.  Here as well, there is a
      trade-off between processing complexity and deployment complexity.

   o  The RS needs to enforce the authorization decisions of the AS,
      while C needs to abide with the authorization decisions of the
      CAS.  The authorization information might require additional
      policy evaluation (such as matching against local access control
      lists, evaluating local conditions).  The required "policy
      evaluation" at the constrained actors needs to be adapted to the
      capabilities of the devices implementing them.

   o  Finally, as is indicated in the previous bullet, for a particular
      authorization decision there may be different kinds of
      authorization information needed, and these pieces of information
      may be transferred to C and RS at different times and in different
      ways prior to or during the client request.

7.2.  Authentication

   The following problems need to be addressed, when considering

   o  RS needs to authenticate AS, and C needs to authenticate CAS, to
      ensure that the authorization information and related data comes
      from the correct source.

   o  CAS and AS may need to to authenticate each other, both to perform
      the required business logic and to ensure that CAS gets security
      information related to the resources from the right source.

   o  In some use cases RS needs to authenticate some property of C, in
      order to map it to the relevant authorization information.  In
      other use cases, authentication and authorization of C may be
      implicit, for example by encrypting the resource representation
      the RS only providing access to those who possess the key to

   o  C may need to authenticate RS, in order to ensure that it is
      interacting with the right resources.  Alternatively C may just
      verify the integrity of a received resource representation.

   o  CAS and AS need to authenticate their communication partner (C or
      RS), in order to ensure it serves the correct device.

7.3.  Communication Security

   There are different alternatives to provide communication security,
   and the problem here is to choose the optimal one for each scenario.
   We list the available alternatives:

   o  Session-based security at transport layer such as DTLS [RFC6347]
      offers security, including integrity and confidentiality
      protection, for the whole application layer exchange.  However,
      DTLS may not provide end-to-end security over multiple hops.
      Another problem with DTLS is the cost of the handshake protocol,
      which may be too expensive for constrained devices especially in
      terms of memory and power consumption for message transmissions.

   o  An alternative is object security at application layer, for
      instance using [I-D.selander-ace-object-security].  Secure objects
      can be stored or cached in network nodes and provide security for
      a more flexible communication model such as publish/subscribe
      (compare e.g.  CoRE Mirror Server [I-D.koster-core-coap-pubsub]).
      A problem with object security is that it can not provide
      confidentiality for the message headers.

   o  Hybrid solutions using both session-based and object security are
      also possible.  An example of a hybrid is where authorization
      information and cryptographic keys are provided by AS in the
      format of secure data objects, but where the resource access is
      protected by session-based security.

7.4.  Cryptographic Keys

   With respect to cryptographic keys, we see the following problems
   that need to be addressed:

   Symmetric vs Asymmetric Keys
      We need keys both for protection of resource access and for
      protection of transport of authentication and authorization
      information.  Do we want to support solutions based on asymmetric
      keys or symmetric keys in both cases?  There are classes of
      devices that can easily perform symmetric cryptography, but
      consume considerably more time/battery for asymmetric operations.
      On the other hand asymmetric cryptography has benefits such as in
      terms of deployment.

   Key Establishment
      How are the corresponding cryptographic keys established?
      Considering Section 7.1 there must be a mapping between these keys
      and the authorization information, at least in the sense that AS
      must be able to specify a unique client identifier which RS can
      verify (using an associated key).  One of the use cases of
      [I-D.ietf-ace-usecases] describes spontaneous change of access
      policies - such as giving a hitherto unknown client the right to
      temporarily unlock your house door.  In this case C is not
      previously known to RS and a key must be provisioned by AS.

   Revocation and Expiration
      How are keys replaced and how is a key that has been compromised
      revoked in a manner that reaches all affected parties, also
      keeping in mind scenarios with intermittent connectivity?

8.  Assumptions and Requirements

   In this section we list a set of candidate assumptions and
   requirements to make the problem description in the previous sections
   more concise and precise.

8.1.  Architecture

   The architecture consists of at least the following types of nodes:

   o  RS hosting resources, and responding to access requests

   o  C requesting access to resources

   o  AS supporting the access request/response procedure by providing
      authorization information to RS

      *  AS may support this by aiding RS in authenticating C, or
         providing cryptographic keys or credentials to C and/or RS to
         secure the request/response procedure.

   o  CAS supporting the access request/response procedure by providing
      authorization information to C

      *  CAS may support this by aiding C in authenticating RS,
         forwarding information between AS and C (possibly ultimately
         for RS), or providing cryptographic keys or credentials to C
         and/or RS to secure the request/response procedure.

   o  The architecture allows for intermediary nodes between any pair of
      C, RS, AS, and CAS, such as forward or reverse proxies in the CoRE
      architecture.  (Solutions may or may not support all

      *  The architecture does not make a choice between session based
         security and data object security.

8.2.  Constrained Devices

   o  C and/or RS may be constrained in terms of power, processing,
      communication bandwidth, memory and storage space, and moreover:

      *  unable to manage complex authorization policies

      *  unable to manage a large number of secure connections

      *  without user interface

      *  without constant network connectivity

      *  unable to precisely measure time

      *  required to save on wireless communication due to high power

   o  CAS and AS are not assumed to be constrained devices.

   o  All devices under consideration can process symmetric cryptography
      without incurring an excessive performance penalty.

      *  We assume the use of a standardized symmetric key algorithm,
         such as AES.

      *  Except for the most constrained devices we assume the use of a
         standardized cryptographic hash function such as SHA-256.

   o  Public key cryptography requires additional resources (such as
      RAM, ROM, power, specialized hardware).

   o  A DTLS handshake involves significant computation, communication,
      and memory overheads in the context of constrained devices.

      *  The RAM requirements of DTLS handshakes with public key
         cryptography are prohibitive for certain constrained devices.

      *  Certificate-based DTLS handshakes require significant volumes
         of communication, RAM (message buffers) and computation.

   o  A solution will need to consider support for a simple scheme for
      expiring authentication and authorization information on devices
      which are unable to measure time (cf. section Section 9.2).

8.3.  Authentication

   o  RS needs to authenticate AS to ensure that the authorization
      information and related data comes from the correct source.

   o  Similary, C needs to authenticate CAS to ensure that the
      authorization information and related data comes from the correct

   o  Depending on use case and authorization requirements, C, RS, CAS,
      or AS may need to authenticate messages from each other.

8.4.  Server-side Authorization

   o  RS enforces authorization for access to a resource based on
      credentials presented by C, the requested resource, the REST
      method, and local context in RS at the time of the request, or on
      any subset of this information.

   o  The credentials presented by C may have been provided by CAS.

   o  The underlying authorization decision is taken either by AS or RS.

   o  The authorization decision is enforced by RS.

      *  RS needs to have authorization information in order to verify
         that C is allowed to access the resource as requested.

      *  RS needs to make sure that it provides resource access only to
         authorized clients.

   o  Apart from authorization for access to a resource, authorization
      may also be required for access to information about a resource
      (for instance, resource descriptions).

   o  The solution may need to be able to support the delegation of
      access rights.

8.5.  Client-side Authorization Information

   o  C enforces client-side authorization by protecting its requests to
      RS and by authenticating results from RS, making use of decisions
      and policies as well as keying material provided by CAS.

8.6.  Server-side Authorization Information

   o  Authorization information is transferred from AS to RS using
      Agent, Push or Pull mechanisms [RFC2904].

   o  RS needs to authenticate that the authorization information is
      coming from AS (integrity).

   o  The authorization information may also be encrypted end-to-end
      between AS and RS (confidentiality).

   o  The architecture supports the case where RS may not be able to
      communicate with AS at the time of the request from C.

   o  RS may store or cache authorization information.

   o  Authorization information may be pre-configured in RS.

   o  Authorization information stored or cached in RS needs to be
      possible to change.  The change of such information needs to be
      subject to authorization.

   o  Authorization policies stored on RS may be handled as a resource,
      i.e. information located at a particular URI, accessed with
      RESTful methods, and the access being subject to the same
      authorization mechanics.  AS may have special privileges when
      requesting access to the authorization policy resources on RS.

   o  There may be mechanisms for C to look up the AS which provides
      authorization information about a particular resource.

8.7.  Resource Access

   o  Resources are accessed in a RESTful manner using GET, PUT, POST,

   o  By default, the resource request needs to be integrity protected
      and may be encrypted end-to-end from C to RS.  It needs to be
      possible for RS to detect a replayed request.

   o  By default, the response to a request needs to be integrity
      protected and encrypted end-to-end from RS to C.  It needs to be
      possible for C to detect a replayed response.

   o  RS needs to be able to verify that the request comes from an
      authorized client

   o  C needs to be able to verify that the response to a request comes
      from the intended RS.

   o  There may be resources whose access need not be protected (e.g.
      for discovery of the responsible AS).

8.8.  Keys and Cipher Suites

   o  A constrained node and its authorization manager (i.e., RS and AS,
      and C and CAS) have established cryptographic keys.  For example,
      they share a secret key or each have the other's public key.

   o  The transfer of authorization information is protected with
      symmetric and/or asymmetric keys.

   o  The access request/response can be protected with symmetric and/or
      asymmetric keys.

   o  There must be a mechanism for RS to establish the necessary key(s)
      to verify and decrypt the request and to protect the response.

   o  There must be a mechanism for C to establish the necessary key(s)
      to protect the request and to verify and decrypt the response.

   o  There must be a mechanism for C to obtain the supported cipher
      suites of a RS.

8.9.  Network Considerations

   o  A solution will need to consider network overload due to avoidable
      communication of a constrained node with its authorization manager
      (C with CAS, RS with AS).

   o  A solution will need to consider network overload by compact
      authorization information representation.

   o  A solution may want to optimize the case where authorization
      information does not change often.

   o  A solution may consider support for an efficient mechanism for
      providing authorization information to multiple RSs, for example
      when multiple entities need to be configured or change state.

8.10.  Legacy Considerations

   o  A solution may consider interworking with existing infrastructure.

   o  A solution may consider supporting authorization of access to
      legacy devices.

9.  Security Considerations

   This document discusses authorization-related tasks for constrained
   environments and describes how these tasks can be mapped to actors in
   the architecture.

   The entire document is about security.  Security considerations
   applicable to authentication and authorization in RESTful
   environments are provided in e.g.  OAuth 2.0 [RFC6749].

   In this section we focus on specific security aspects related to
   authorization in constrained-node networks.  Section 11.6 of
   [RFC7252], "Constrained node considerations", discusses implications
   of specific constraints on the security mechanisms employed.  A wider
   view of security in constrained-node networks is provided in

9.1.  Physical Attacks on Sensor and Actuator Networks

   The focus of this work is on constrained-node networks consisting of
   connected sensors and actuators.  The main function of such devices
   is to interact with the physical world by gathering information or
   performing an action.  We now discuss attacks performed with physical
   access to such devices.

   The main threats to sensors and actuator networks are:

   o  Unauthorized access to data to and from sensors and actuators,
      including eavesdropping and manipulation of data.

   o  Denial-of-service making the sensor/actuator unable to perform its
      intended task correctly.

      A number of attacks can be made with physical access to a device
      including probing attacks, timing attacks, power attacks, etc.
      However, with physical access to a sensor or actuator device it is
      possible to directly perform attacks equivalent of eavesdropping,
      manipulating data or denial of service.  For example:

   o  Instead of eavesdropping the sensor data or attacking the
      authorization system to gain access to the data, the attacker
      could make its own measurements on the physical object.

   o  Instead of manipulating the sensor data the attacker could change
      the physical object which the sensor is measuring, thereby
      changing the payload data which is being sent.

   o  Instead of manipulating data for an actuator or attacking the
      authorization system, the attacker could perform an unauthorized
      action directly on the physical object.

   o  A denial-of-service attack could be performed physically on the
      object or device.

      All these attacks are possible by having physical access to the
      device, since the assets are related to the physical world.
      Moreover, this kind of attacks are in many cases straightforward
      (requires no special competence or tools, low cost given physical
      access, etc.)

      As a conclusion, if an attacker has full physical access to a
      sensor or actuator device, then much of the security functionality
      elaborated in this draft is not effective to protect the asset
      during the physical attack.

      Since it does not make sense to design a solution for a situation
      that cannot be protected against we assume there is no need to
      protect assets which are exposed during a physical attack.  In
      other words, either an attacker does not have physical access to
      the sensor or actuator device, or if it has, the attack shall only
      have effect during the period of physical attack, and shall be
      limited in extent to the physical control the attacker exerts
      (e.g., must not affect the security of other devices.)

9.2.  Time Measurements

   Measuring time with certain accuracy is important to achieve certain
   security properties, for example to determine whether a public key
   certificate, access token or some other assertion is valid.

   Dynamic authorization in itself requires the ability to handle expiry
   or revocation of authorization decisions or to distinguish new
   authorization decisions from old.

   For certain categories of devices we can assume that there is an
   internal clock which is sufficiently accurate to handle the time
   measurement requirements.  If RS can connect directly to AS it could
   get updated in terms of time as well as revocation information.

   If RS continuously measures time but can't connect to AS or other
   trusted source, time drift may have to be accepted and it may not be
   able to manage revocation.  However, it may still be able to handle
   short lived access rights within some margins, by measuring the time
   since arrival of authorization information or request.

   Some categories of devices in scope may be unable measure time with
   any accuracy (e.g. because of sleep cycles).  This category of
   devices is not suitable for the use cases which require measuring
   validity of assertions and authorizations in terms of absolute time.

10.  IANA Considerations

   This document has no actions for IANA.

11.  Acknowledgements

   The authors would like to thank Olaf Bergmann, Robert Cragie, Klaus
   Hartke, Sandeep Kumar, John Mattson, Corinna Schmitt, Mohit Sethi,
   Hannes Tschofenig, Vlasios Tsiatsis and Erik Wahlstroem for
   contributing to the discussion, giving helpful input and commenting
   on previous forms of this draft.  The authors would also like to
   specifically acknowledge input provided by Hummen and others

12.  Informative References

              Hummen, R., Shafagh, H., Raza, S., Voigt, T., and K.
              Wehrle, "Delegation-based Authentication and Authorization
              for the IP-based Internet of Things", 11th IEEE
              International Conference on Sensing, Communication, and
              Networking (SECON'14), June 30 - July 3, 2014.

              Garcia-Morchon, O., Kumar, S., Keoh, S., Hummen, R., and
              R. Struik, "Security Considerations in the IP-based
              Internet of Things", draft-garcia-core-security-06 (work
              in progress), September 2013.

              Gerdes, S., "Authorization-Related Tasks in Constrained
              Environments", draft-gerdes-ace-tasks-00 (work in
              progress), September 2015.

              Hardjono, T., Maler, E., Machulak, M., and D. Catalano,
              "User-Managed Access (UMA) Profile of OAuth 2.0", draft-
              hardjono-oauth-umacore-13 (work in progress), April 2015.

              Seitz, L., Gerdes, S., Selander, G., Mani, M., and S.
              Kumar, "ACE use cases", draft-ietf-ace-usecases-06 draft-ietf-ace-usecases-09 (work
              in progress), September October 2015.

              Koster, M., Keranen, A., and J. Jimenez, "Publish-
              Subscribe Broker for the Constrained Application Protocol
              (CoAP)", draft-koster-core-coap-pubsub-02 (work in
              progress), July 2015.

              Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "June 29, 2015", draft-selander-ace-object-security-02
              (work in progress), June 2015.

   [OSCAR]    Vucinic, M., Tourancheau, B., Rousseau, F., Duda, A.,
              Damon, L., and R. Guizzetti, "OSCAR: Object Security
              Architecture for the Internet of Things", CoRR vol.
              abs/1404.7799, 2014.

   [RFC2904]  Vollbrecht, J., Calhoun, P., Farrell, S., Gommans, L.,
              Gross, G., de Bruijn, B., de Laat, C., Holdrege, M., and
              D. Spence, "AAA Authorization Framework", RFC 2904, DOI
              10.17487/RFC2904, August 2000,

   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
              Kerberos Network Authentication Service (V5)", RFC 4120,
              DOI 10.17487/RFC4120, July 2005,

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2", FYI
              36, RFC 4949, DOI 10.17487/RFC4949, August 2007,

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/
              RFC5246, August 2008,

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <>.

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228, DOI 10.17487/
              RFC7228, May 2014,

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing", RFC
              7230, DOI 10.17487/RFC7230, June 2014,

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI
              10.17487/RFC7231, June 2014,

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252, DOI 10.17487/
              RFC7252, June 2014,

Authors' Addresses

   Stefanie Gerdes
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359

   Phone: +49-421-218-63906

   Ludwig Seitz
   SICS Swedish ICT AB
   Scheelevaegen 17
   Lund  223 70


   Goeran Selander
   Faroegatan 6
   Kista  164 80

   Carsten Bormann (editor)
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359

   Phone: +49-421-218-63921