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Versions: 00

ACE Working Group                                              S. Gerdes
Internet-Draft                                   Universitaet Bremen TZI
Intended status: Informational                        September 29, 2015
Expires: April 1, 2016


        Authorization-Related Tasks in Constrained Environments
                       draft-gerdes-ace-tasks-00

Abstract

   Constrained nodes are small devices which are limited in terms of
   processing power, memory, non-volatile storage and transmission
   capacity.  Due to these constraints, commonly used security protocols
   are not easily applicable.  Nevertheless, an authentication and
   authorization solution is needed to ensure the security of these
   devices.

   Due to the limitations of the constrained nodes it is especially
   important to develop a light-weight security solution which is
   adjusted to the relevant security objectives of each participating
   party in this environment.  Necessary security measures must be
   identified and applied where needed.

   In this document, the required security related tasks are identified
   as guidance for the development of authentication and authorization
   solutions for constrained environments.

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
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   This Internet-Draft will expire on April 1, 2016.







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Copyright Notice

   Copyright (c) 2015 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
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Tasks . . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Basic Scenario Tasks  . . . . . . . . . . . . . . . . . .   4
     3.2.  Security-Related Tasks  . . . . . . . . . . . . . . . . .   4
     3.3.  Authentication-Related Tasks  . . . . . . . . . . . . . .   5
     3.4.  Authorization-Related Tasks . . . . . . . . . . . . . . .   5
   4.  Actors  . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Constrained Level Actors  . . . . . . . . . . . . . . . .   7
     4.2.  Principal Level Actors  . . . . . . . . . . . . . . . . .   8
     4.3.  Less-Constrained Level Actors . . . . . . . . . . . . . .   9
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  11
   8.  Informative References  . . . . . . . . . . . . . . . . . . .  11
   Appendix A.  List of Tasks  . . . . . . . . . . . . . . . . . . .  11
     A.1.  Basic Scenario  . . . . . . . . . . . . . . . . . . . . .  12
       A.1.1.  Processing Information  . . . . . . . . . . . . . . .  12
       A.1.2.  Sending Information . . . . . . . . . . . . . . . . .  13
     A.2.  Security-Related Tasks  . . . . . . . . . . . . . . . . .  15
       A.2.1.  Information Authenticity  . . . . . . . . . . . . . .  15
       A.2.2.  Authorization Validation  . . . . . . . . . . . . . .  16
       A.2.3.  Transmission Security . . . . . . . . . . . . . . . .  17
       A.2.4.  Obtain Authorization information  . . . . . . . . . .  17
       A.2.5.  Attribute Binding . . . . . . . . . . . . . . . . . .  18
       A.2.6.  Configuration of Authorization Information  . . . . .  20
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  20






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1.  Introduction

   Constrained nodes are small devices with limited abilities which in
   many cases are made to fulfill a single 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
   applicable.

   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
   Things.

   The limitations of the constrained nodes ask for security mechanisms
   which take the special characteristics of constrained environments
   into account.  Therefore, it is crucial to identify the tasks which
   must be performed to meet the security requirements in constrained
   scenarios.

   In this document, the required authorization-related tasks are
   identified as guidance for the development of authentication and
   authorization solutions for constrained environments.

1.1.  Terminology

   Readers are required to be familiar with the terms and concepts
   defined in [RFC4949] and [I-D.ietf-ace-actors].

2.  Problem Statement

   The scenario this document addresses can be summarized as follows:

   o  C wants to access R on a S.

   o  A priori, C and S do not necessarily know each other and have no
      security relationship.

   o  C and / or S are constrained.









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              -------                            -------
              |  C  |  -- requests resource ---> |  S  |
              -------  <-- provides resource---  -------


                         Figure 1: Basic Scenario

   The security requirements of any specific version of this scenario
   will include one or more of:

   o  Rq0.1: No unauthorized entity has access to (or otherwise gains
      knowledge of) R.

   o  Rq0.2: C is exchanging status updates of a resource only with
      authorized resources.  (When C attempts to access R, that access
      reaches an authorized R).

   Rq0.1 requires authorization on the server side while Rq0.2 requires
   authorization on the client side.

3.  Tasks

   This section gives an overview of the tasks which must be performed
   in the given scenario (see Section 2) to meet any specific security
   requirements.

   Either C or S or both of them are constrained.  Therefore tasks which
   must be conducted by either C or S must be performable by constrained
   nodes.

3.1.  Basic Scenario Tasks

   This document does not assume a specific transfer protocol.  We
   assume however, that at least the following information is exchanged
   between the client and the server:

   o  C transmits to S which resource it requests to access, the kind of
      action it wants to perform on the resource, and the parameters
      needed for the action.

   o  S transmits to C the result of the attempted access.

3.2.  Security-Related Tasks

   The reason for the communication is that C wants S to process some
   information.  S' reaction to C's access request might be processed by
   C.  The reason for using an authorization solution is to validate




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   that the entity that sent the information used for processing is
   authorized to provide it.

   To validate if a sender is authorized to send a received piece of
   information, the receiver must determine the sender's authorization.
   Correspondingly, to validate if a receiver is allowed to receive a
   message, the sender must determine its authorization.  This can only
   be achieved with the help of an authentication mechanism.

3.3.  Authentication-Related Tasks

   Several steps must be conducted for authenticating certain attributes
   of an entity and validating the authenticity of an information:

   1.  Attribute binding: The attribute that shall be verifiable must be
       bound to a verifier, e.g. a key.  To achieve this, an entity that
       is authorized to conduct the attribute binding, the attribute
       binding authority, checks if an entity actually has the
       attributes it claims to have and then binds them to a verifier.
       The binding authority must provide some kind of endorsement
       information which enables other entities to validate this
       binding.

   Note: The attribute binding can be conducted using either symmetric
   or asymmetric cryptography.

   1.  Verifier validation: The entity that wants to authenticate the
       source of an information checks the attribute-verifier-binding
       using the endorsement provided by the attribute binding
       authority.

   2.  Authentication: The verifier is used for authenticating the
       source of a data item, i.e. it is checked whether the data item
       is bound to the verifier.  Thus the attributes of the source can
       be determined.

   Step 1 is addressed in Appendix A.2.5.  After the first step is
   conducted, step 2 and step 3 can be performed when needed.  They must
   be performed together and thus are examined together as well.  Tasks
   for step 2 and 3 are Information authenticity (see Appendix A.2.1)
   and secure communication (see Appendix A.2.3).

3.4.  Authorization-Related Tasks

   Several steps must be conducted for explicit authorization:

   1.  Configuration of authorization information: The respective
       principals (COP and ROP) must configure the authorization



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       information according to their authorization policy.  An
       authorization information must contain one or more permissions
       and the attribute an entity must have to apply to this
       authorization.

   2.  Obtaining authorization information: Authorization information
       must be made available to the entity which enforces the
       authorization.

   3.  Authorization validation: The authorization of an entity with
       certain attributes must be confirmed by applying the request in
       conjunction with authenticated attributes to the policy provided
       by the authorization information.

   4.  Authorization enforcement: According to the result of the
       authorization validation the access to a resource is granted or
       denied.

   Tasks for step 1 are defined in Appendix A.2.6.  Appendix A.2.4
   addresses step 2.  After step 1 and step 2 are conducted, step 3 and
   step 4 can be performed when needed.  Step 3 and step 4 must be
   performed together and thus are examined together.  Appendix A.2.2
   introduces tasks for these steps.

4.  Actors

   This section describes how the tasks defined in Appendix A are mapped
   to the various actors in the architecture (see also
   [I-D.ietf-ace-actors]).  An actor consists of a set of tasks and
   additionally has an 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: 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 software.

   The concept of actors is used to assign the tasks defined in
   Appendix A to logical functional entities.







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4.1.  Constrained Level Actors

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

   C performs the following tasks:

   o  Negotiate means for secure communication (Task TSecureComm, see
      Appendix A.2.3).

   o  Validate that an entity is an authorized source for R (Task
      TValSourceAuthz, see Appendix A.2.2).

   o  Securely transmit an access request (Task TSendReq, see
      Appendix A.1.2).

   o  Validate that the response to an access request is authentic (Task
      TAuthnResp, see Appendix A.2.1).

   o  Process the response to an access request (Task TProcResp, see
      Appendix A.1.1).

   S performs the following tasks:

   o  Negotiate means for secure communication (Task TSecureComm, see
      Appendix A.2.3).

   o  Validate the authenticity of an access request (Task TAuthnReq,
      see Appendix A.2.1).

   o  Validate the authorization of the requester to access the
      requested resource as requested (Task TValAccessAuthZ, see
      Appendix A.2.2).

   o  Process an access request (Task TProcReq, see Appendix A.1.1).

   o  Securely transmit a response to an access request (Task TSendResp,
      see Appendix A.1.2).

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




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

           -------                            -------
           |  C  |  -- requests resource ---> |  S  | Constrained Level
           -------  <-- provides resource---  -------


                    Figure 2: Constrained Level Actors

4.2.  Principal Level Actors

   Our objective is that C and S are under control of principals in the
   physical world, the Client Overseeing Principal (COP) and the
   Resource Overseeing Principal (ROP) respectively.  The principals
   decide about the security policies of their respective endpoints and
   belong to the same security domain.

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

   COP must fulfill the following task:

   o  Configure for C authorization information for sources for R (Task
      TConfigSourceAuthz, see Appendix A.2.6).

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

   ROP must fulfill the following task:

   o  Configure for S authorization information for accessing R (Task
      TConfigAccessAuthz, see Appendix A.2.6).
















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      -------                           -------
      | COP |                           | ROP | Principal Level
      -------                           -------
        |                                 |
   in charge of                      in charge of
        |                                 |
        V                                 V
     -------                            -------
     |  C  |  -- requests resource ---> |  S  | Constrained Level
     -------  <-- provides resource---  -------


       Figure 3: Constrained Level Actors and Principal Level Actors

4.3.  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 (CAM) belongs to the same security
   domain as C and COP.  CAM acts on behalf of COP.  It assists C in
   authenticating S and determining if S is an authorized source for R.
   CAM can do that because for C, CAM is the authority for claims about
   S.

   CAM performs the following tasks:

   o  Validate on the client side that an entity has certain attributes
      (Task TValSourceAttr, see Appendix A.2.5).

   o  Obtain authorization information about an entity from C's
      principal (COP) and provide it to C.  (Task TObtainSourceAuthz,
      see Appendix A.2.4).

   o  Negotiate means for secure communication to communicate with C
      (Task TSecureComm, see Appendix A.2.3).

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

   SAM performs the following tasks:



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   o  Validate on the server side that an entity has certain attributes
      (Task TValReqAttr, see Appendix A.2.5).

   o  Obtain authorization information about an entity from S' principal
      (ROP) and provide it to S (Task TObtainAccessAuthz, see
      Appendix A.2.4).

   o  Negotiate means for secure communication to communicate with S
      (Task TSecureComm, see Appendix A.2.3).

      -------                          -------
      | COP |                          | ROP |   Principal Level
      -------                          -------
        |                                  |
   in charge of                       in charge of
        |                                  |
        V                                  V
   ----------                        ---------
   |  CAM   | <- AuthN and AuthZ ->  |  SAM  |  Less-Constrained Level
   ----------                        ---------
        |                                  |
   authentication                     authentication
   and authorization                  and authorization
   support                            support
        |                                  |
        V                                  V
     -------                            -------
     |  C  |  -- requests resource ---> |  S  | Constrained Level
     -------  <-- provides resource --  -------


                Figure 4: Overview of all Complexity Levels

   For more detailed graphics please consult the PDF version.

5.  IANA Considerations

   None

6.  Security Considerations

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







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7.  Acknowledgments

   The author would like to thank Carsten Bormann, Olaf Bergmann, Robert
   Cragie and Klaus Hartke for their valuable input and feedback.

8.  Informative References

   [I-D.ietf-ace-actors]
              Gerdes, S., Seitz, L., Selander, G., and C. Bormann, "An
              architecture for authorization in constrained
              environments", draft-ietf-ace-actors-00 (work in
              progress), August 2015.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2", FYI
              36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
              <http://www.rfc-editor.org/info/rfc4949>.

Appendix A.  List of Tasks

   This section defines the tasks which must be performed in the given
   scenario (see Section 2) starting from communication related tasks
   and then deriving the required security-related tasks.  An overview
   of the tasks can be found in Section 3.

   A task has the following structure:

   o  The name of the task which has the form TXXX

   o  One or more Requirements (if applicable) of the form RqXXX

   o  One or more Preconditions (if applicable) of the form PreXXX

   o  One or more Postconditions (if applicable) of the form PostXXX

   Requirements have to be met _while_ performing the task.  They derive
   directly from the scenario (see Section 2) or from the security
   requirements defined for the scenario.  Preconditions have to be
   fulfilled _before_ conducting the task.  Postconditions are the
   _results_ of the completed task.

   We start our analysis with the processing tasks and define which
   preconditions need to be fulfilled before these tasks can be
   conducted.  We then determine which tasks therefore need to be
   performed first (have postconditions which match the respective
   preconditions).

   Note: Regarding the communication, C and S are defined as entities
   each having their set of attributes and a verifier which is bound to



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   these attributes.  Attributes are not necessarily usable to identify
   an individual C or S.  Several entities might have the same
   attributes.

A.1.  Basic Scenario

   The intended result of the interaction between C and S is that C has
   successfully accessed R.  C gets to know that its access request was
   successful by receiving the answer from S.

   The transmission of information from C to S comprises two parts:
   sending the information on one side and receiving and processing it
   on the other.  Security has to be considered at each of these steps.

A.1.1.  Processing Information

   The purpose of the communication between C and S is C's intent to
   access R.  To achieve this, S must process the information about the
   requested access and C must process the information in the response
   to a requested access.  The request and the response might both
   contain resource values.

   The confidentiality and integrity of R require that only authorized
   entities are able to access R (see Rq0.1).  Therefore, C and S must
   check that the information is authentic and that the source of the
   information is authorized to provide it, before the information can
   be processed.  C must validate that S is an authorized source for R.
   S must validate that C is authorized to access R as requested.

   If proxies are used, it depends on the type of proxy how they are
   integrated into the communication and what kind of security
   relationships need to be established.  A future version of this
   document will provide more details on this topic.  At this point we
   assume that C and S might receive the information either from S or C
   directly or from a proxy which is authorized to speak for the
   respective communication partner.

   o  Task TProcResp: Process the response to an access request.
      Description: C processes the response to an access request
      according to the reason for requesting the resource in the first
      place.  The response might include resource values or information
      about the results of a request.  Requirements: * RqProcResp.1: Is
      performed by C (derives from the problem statement).  *
      RqProcResp.2: Must be performable by a constrained device (derives
      from the problem statement: C and / or S are constrained).
      Preconditions: * PreProcResp.1: A response to an access request
      was sent (see Appendix A.1.2).  * PreProcResp.2 (required for
      Rq0.2): C validated that the response to an access request is



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      authentic, i.e. it stems from the entity requested in TSendReq
      (see Appendix A.1.2), i.e.  S or an entity which is authorized to
      speak for S (see Appendix A.2.1).  * PreProcResp.3 (required for
      Rq0.2): C validated that S or the entity which is authorized to
      speak for S is an authorized source for R (see Appendix A.2.2).
      Postcondition: * PostProcResp.1: C processed the response.

   o  Task TProcReq: Process an access request.  Description: S either
      performs an action on the resource according to the information in
      the request, or determines the reason for not performing an
      action.  Requirements: * RqProcReq.1: Is performed by S.  *
      RqProcReq.2: Must be performable by a constrained device (derives
      from the problem statement: C and / or S are constrained).
      Preconditions: * PreProcReq.1: An access request was sent (see
      Appendix A.1.2).  * PreProcReq.2 (needed for Rq0.1): S validated
      that the request is authentic, i.e. it stems from C or an entity
      which is authorized to speak for C and is fresh. (see
      Appendix A.2.1).  * PreProcReq.3 (needed for Rq0.1): S validated
      the authorization of C or the entity which is authorized to speak
      for C to access the resource as requested (see Appendix A.2.2).
      Postconditions: * PostProcReq.1: The access request was processed
      (fulfills PreSendResp.1, see Appendix A.1.2).

   Note: The preconditions PreProcReq.2 and PreProcReq.3 must be
   conducted together.  S must assure that the response is bound to a
   verifier, the verifier is bound to certain attributes and the
   authorization information refers to these attributes.

A.1.2.  Sending Information

   The information needed for processing has to be transmitted at some
   point.  C has to transmit to S which resource it wants to access with
   which actions and parameters.  S has to transmit to C the result of
   the request.  The request and the response might both contain
   resource values.  To fulfill Rq0.1, the confidentiality and integrity
   of the transmitted data has to be assured.

   If proxies are used, it depends on the type of proxy how they need to
   be handled.  A future version of this document will provide more
   details on this topic.  At this point we assume that C and S might
   transmit the message either to S and C directly or to a proxy which
   is authorized to speak for the respective communication partner.

   o  Task TSendReq: Securely transmit an access request.  Description:
      C wants to access a resource R hosted by the resource server S.
      To achieve this, it has to transmit some information to S such as
      the resource to be accessed, the action to be performed on the
      resource and, if a writing access is requested, the value to



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      write.  C might send the request directly to S or to an entity
      which is authorized to speak for S.  C assures that the request
      reaches the proper R.  C binds the request to C's verifier to
      ensure the integrity of the message.  C uses means to assure that
      no unauthorized entity is able to access the information in the
      request.  Requirements: * RqSendReq.1: Is performed by C (derives
      from problem statement).  * RqSendReq.2: Must be performable by a
      constrained device (derives from the problem statement: C and / or
      S are constrained).  * RqSendReq.3: As the request might contain
      resource values, the confidentiality and integrity of the request
      must be ensured during transmission.  Only authorized parties must
      be able to read or modify the request (derives from Rq0.1).
      Preconditions: * PreSendReq.1: Validate that the receiver is an
      authorized source for R (see Appendix A.2.2).  * PreSendReq.2: To
      assure that the request reaches the proper S, that no unauthorized
      party is able to access the request, and that the information in
      the request is bound to C's verifier it is necessary to negotiate
      means for secure communication with S (see Appendix A.2.3).
      Postconditions: * PostSendReq.1: The request was sent securely to
      S (necessary for Rq0.1) (fulfills PreProcReq.1, see
      Appendix A.1.1).

   Note: The preconditions PreSendReq.1 and PreSendReq.2 must be
   conducted together.  C must assure that the request reaches an entity
   with certain attributes and that the authorization information refers
   to these attributes.

   o  Task TSendResp: Securely transmit a response to an access request.
      Description: S sends a response to an access request to inform C
      about the result of the request.  S must assure that response
      reaches the requesting C.  S might send the response to C or to an
      entity which is authorized to speak for C.  The response might
      contain resource values.  S binds the request to S's verifier to
      ensure the integrity of the message.  S uses means to assure that
      no unauthorized entity is able to access the information in the
      response.  Requirements: * RqSendResp.1: Is performed by S
      (derives from the problem statement).  * RqSendResp.2: Must be
      performable by a constrained device (derives from the problem
      statement: C and / or S are constrained).  * RqSendResp.3: As the
      response might contain resource values, the confidentiality and
      integrity of the response must be ensured during transmission.
      Only authorized parties must be able to read or modify the
      response (derives from Rq0.1).  Preconditions: * PreSendResp.1: An
      access request was processed (see Appendix A.1.1).  *
      PreSendResp.2: If information about R is transmitted, validate
      that the receiver is authorized to access R (see Appendix A.2.2).
      * PreSendResp.3: S must assure that the response reaches the
      requesting C, no unauthorized party is able to access the response



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      and the information in the response is bound to S' verifier: Means
      for secure communication were negotiated (see Appendix A.2.3).
      Postconditions: * PostSendResp.1: A response to an access request
      was sent (fulfills PreProcResp.1, see Appendix A.1.1).

A.2.  Security-Related Tasks

A.2.1.  Information Authenticity

   This section addresses information authentication, i.e. using the
   verifier to validate the source of an information.  Information
   authentication must be conducted before processing received
   information.  C must validate that a response to an access request is
   fresh, really stems from the queried S (or an entity which is
   authorized to speak for S) and was not modified during transmission.
   S must validate that the information in the access request is fresh,
   really stems from C (or an entity which is authorized to speak for C)
   and was not modified during transmission.

   The entity which processes the information must be the entity which
   is validating the source of the information.

   C and S must assure that the authenticated source of the information
   is authorized to provide the information.

   o  Task TAuthnResp: Validate that the response to an access request
      is authentic.  Description: C checks if the response to an access
      request stems from an entity in possession of the respective
      verifier and is fresh.  Thus, C validates that the response stems
      from the queried S or an entity which is authorized to speak for
      S.  Requirements: * RqAuthnResp.1: Must be performed by C.  *
      RqAuthnResp.2: Must be performable by a constrained device
      (derives from the problem statement: C and / or S are
      constrained).  Preconditions: * PreAuthnResp.1: Means for secure
      communication were negotiated (see Appendix A.2.3).
      Postconditions: * PostAuthnResp.1: C knows that the response came
      from S (fulfills PreProcResp.2, see Appendix A.1.1).

   o  Task TAuthnReq: Validate the authenticity of a request.
      Description: S checks if the request stems from an entity in
      possession of the respective verifier and is fresh.  Thus, S
      validates that the request stems from C or an entity which is
      authorized to speak for C.  Requirements: * RqAuthnReq.1: Must be
      performed by S.  * RqAuthnReq.2: Must be performable by a
      constrained device (derives from the problem statement: C and / or
      S are constrained).  Preconditions: * PreAuthnReq.1: Means for
      secure communication were negotiated (see Appendix A.2.3).




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      Postconditions: * PostAuthnReq.1: S knows that the request is
      authentic (fulfills PreProcReq.2, see Appendix A.1.1).

A.2.2.  Authorization Validation

   This section addresses the validation of the authorization of an
   entity.  The entity which processes the information must validate
   that the source of the information is authorized to provide it.  The
   processing entity has to verify that the source of the information
   has certain attributes which authorize it to provide the information:
   C must validate that S (or the entity which speaks for S) is in
   possession of attributes which are necessary for being an authorized
   source for R.  S must validate that C (or the entity which speaks for
   C) has attributes which are necessary for a permission to access R as
   requested.

   o  Task TValSourceAuthz: Validate that an entity is an authorized
      source for R.  Description: C checks if according to COP's
      authorization policy and the authentication endorsement provided
      by the attribute binding authority, S (or an entity which speaks
      for S) is authorized to be a source for R.  S assures that the
      entity's verifier is bound to certain attributes and the
      authorization information refers to these attributes.
      Requirements: * RqValSourceAuthz.1: Is performed by C *
      RqValSourceAuthz.2: Must be performable by a constrained device
      (derives from the problem statement: C and / or S are
      constrained).  Preconditions: * PreValSourceAuthz.1: Authorization
      information about the entity is available.  Requires obtaining
      authorization information about the entity from COP (see
      Appendix A.2.4).  * PreValSourceAuthz.2: Means to validate that
      the entity has certain attributes which are relevant for the
      authorization: Requires validation of claims about S (see
      Appendix A.2.5).  Postconditions: * PostValSourceAuthz.1: The
      entity which performs the task knows that an entity is an
      authorized source for R (fulfills PreProcResp.3, see
      Appendix A.1.1 and PreSendReq.1, see Appendix A.1.2).

   o  Task TValAccessAuthZ: Validate the authorization of the requester
      to access the requested resource as requested.  Description: ROP
      configures which clients are authorized to perform which action on
      R.  S has to check if according to ROP's authorization policy and
      the authentication endorsement provided by the attribute binding
      authority, C (or an entity which speaks for C) is authorized to
      access R as requested.  S assures that requester's verifier is
      bound to certain attributes and the authorization information
      refers to these attributes.  Requirements: * RqValAccessAuthz.1:
      Is performed by S * RqValAccessAuthz.2: Must be performable by a
      constrained device (derives from the problem statement: C and / or



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      S are constrained).  Preconditions: * PreValAccessAuthz.1:
      Authorization information about the entity are available.
      Requires obtaining authorization information about the entity from
      ROP (see Appendix A.2.4).  * PreValAccessAuthz.2: Means to
      validate that the entity has certain attributes which are relevant
      for the authorization: Requires validation of claims about C or
      the entity which speaks for C (see Appendix A.2.5).
      Postconditions: * PostValAccessAuthz.1: The entity which performs
      the task knows that an entity is authorized to access R with the
      requested action (fulfills PreProcReq.3, see Appendix A.1.1).

A.2.3.  Transmission Security

   To ensure the confidentiality and integrity of information during
   transmission means for secure communication have to be negotiated
   between the communicating parties.

   o  Task TSecureComm: Negotiate means for secure communication.
      Description: To ensure the confidentiality and integrity of
      transmitted information, means for secure communication (e.g.
      session keys, used cipher suites, etc.) have to be negotiated.
      Channel security as well as object security solutions are
      possible.  Details depend on the used solution and are not in the
      scope of this document.  Requirements: * RqSecureComm.1: Must be
      performable by a constrained device (derives from the problem
      statement: C and / or S are constrained).  Preconditions: *
      PreSecureComm.1: Sender and receiver must be able to validate that
      the entity in possession of a certain verifier has the claimed
      attributes. (see Appendix A.2.5).  Postconditions: *
      PostSecureComm.1: C and S can communicate securely: The integrity
      and confidentiality of information is ensured during transmission.
      The sending entity can use means to assure that the information
      reaches the intended receiver so that no unauthorized party is
      able to access the information.  The sending entity can bind the
      information to the entity's verifier (fulfills PreSendResp.3 and
      PreSendReq.2, see Appendix A.1.2 as well as PreAuthnResp.1 and
      PreAuthnReq.1, see Appendix A.2.1).

A.2.4.  Obtain Authorization information

   As described in Section 3.4, the authorization of an entity requires
   several steps.  The authorization information must be configured by
   the principal and provided to the enforcing entity.

   o  Task TObtainSourceAuthz: Obtain authorization information about an
      entity from C's principal (COP).  Description: COP defines
      authorized sources for R.  The authorization information must be
      made available to C to enable it to enforce COP's authorization



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      information.  To facilitate the configuration for the principal
      this device should have a user interface.  The authorization
      information has to be made available to C in a secure way.
      Requirements: * RqObtainSourceAuthz.1: Must be performed by an
      entity which is authorized by COP.  * RqObtainSourceAuthz.2: Must
      be performed by an entity which is authorized to speak for COP
      concerning authorized sources for R.  * RqObtainSourceAuthz.3:
      Should be performed by a device which can provide some sort of
      user interface to facilitate the configuration of authorization
      information for COP.  Preconditions: * PreObtainSourceAuthz.1: COP
      configured authorized sources for R (see Appendix A.2.6).
      Postconditions: * PostObtainSourceAuthz.1: C obtained S'
      authorization to be a source for R (fulfills PreValSourceAuthz.1,
      see Appendix A.2.2).

   o  Task TObtainAccessAuthz: Obtain authorization information about an
      entity from S' principal (ROP).  Description: ROP defines if and
      how C is authorized to access R.  The authorization information
      must be made available to S to enable it to enforce ROP's
      authorization policies.  To facilitate the configuration for the
      principal this device should have a user interface.  The
      authorization information has to be made available to S in a
      secure way.  Requirements: * RqObtainAccessAuthz.1: Must be
      performed by an entity which is authorized by ROP.  *
      RqObtainAccessAuthz.2: Must be performed by an entity which is
      authorized to speak for ROP concerning authorization of access to
      R.  * RqObtainAccessAuthz.3: Should be performed by a device which
      can provide some sort of user interface to facilitate the
      configuration of authorization information for ROP.
      Preconditions: * PreObtainAccessAuthz.1: ROP configured
      authorization information for the access to R (see
      Appendix A.2.6).  Postconditions: * PostObtainAccessAuthz.1: S
      obtained C's authorization for accessing R (fulfills
      PreValAccessAuthz.1, see Appendix A.2.2).

A.2.5.  Attribute Binding

   As described in Section 3.3, several steps must be conducted for
   authentication.  This section addresses the binding of attributes to
   a verifier.

   For authentication it is necessary to validate if an entity has
   certain attributes.  An example for such an attribute in the physical
   world is the name of a person or her age.  In constrained
   environments, attributes might be the name of the owner or the type
   of device.  Authorizations are bound to such attributes.





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   The possession of attributes must be verifiable.  For that purpose,
   attributes must be bound to a verifier.  An example for a verifier in
   the physical world is a passport.  In constrained environments, a
   verifier will likely be the knowledge of a secret.

   At some point, an authority has to check if an entity in possession
   of the verifier really possesses the claimed attributes.  In the
   physical world, government agencies check your name and age before
   they give you a passport.

   The entity that validates the claims has to provide some kind of seal
   to make its endorsement verifiable for other entities and thus bind
   the attributes to the verifier.  In the physical world passports are
   stamped by the issuing government agencies (and must only be provided
   by government agencies anyway).

   o  Task TValSourceAttr: Validate on the client side that an entity
      has certain attributes.  Description: The claim that an entity has
      certain attributes has to be checked and made available for C in a
      secure way.  The validating party states that an entity in
      possession of a certain key has certain attributes and provides C
      with means to validate this endorsement.  Requirements: *
      RqValSourceAttr.1: Must be performed by an entity which is
      authorized by COP to validate claims about S.  *
      RqValSourceAttr.3: The executing entity must have the means to
      fulfill the task (e.g. enough storage space, computational power,
      a user interface to facilitate the configuration of authentication
      policies).  Postconditions: * PostValSourceAttr.1: Means for
      authenticating (validating the attribute-verifier-binding of)
      other entities were given to C in form of a verifiable endorsement
      (fulfills PreValSourceAuthz.2, see Appendix A.2.2 and
      PreSecureComm.1, see Appendix A.2.3).

   o  Task TValReqAttr: Validate on the server side that an entity has
      certain attributes.  Description: The claim that an entity has
      certain attributes has to be checked and made available for S in a
      secure way.  The validating party states that an entity in
      possession of a certain key has certain attributes and provides S
      with means to validate this endorsement.  Requirements: *
      RqValReqAttr.1: Must be performed by an entity which is authorized
      by ROP to validate claims about C.  * RqValReqAttr.2: The
      executing entity must have the means to fulfill the task (e.g.
      enough storage space, computational power, a user interface to
      facilitate the configuration of authentication policies).
      Postconditions: * PostValReqAttr.1: Means for authenticating
      (validating the attribute-verifier-binding of) other entities were
      given to S in form of a verifiable endorsement (fulfills




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      PreValSourceAuthz.2, see Appendix A.2.2 and PreSecureComm.1, see
      Appendix A.2.3).

A.2.6.  Configuration of Authorization Information

   As stated in Section 3.4, several steps have to be conducted for
   authorization.  This section is about the configuration of
   authorization information.

   The principal of an endpoint or resource wants to be in control of
   her device and her data.  For that purpose, she has to configure
   authorization information.  COP might want to configure which
   attributes an entity must have to be allowed to represent R.  ROP
   might want to configure which attributes are required for accessing R
   with a certain action.

   o  Task TConfigSourceAuthz: Configure for C authorization information
      for sources for R.  Description: COP has to define authorized
      sources for R.  Requirements: * RqConfigSourceAuthz.1: Must be
      provided by COP.  Postconditions: * PostConfigSourceAuthz.1: The
      authorization information are available to an endpoint which
      performs TObtainSourceAuthz (fulfills PreObtainSourceAuthz.1 see
      Appendix A.2.4).

   o  Task TConfigAccessAuthz: Configure for S authorization information
      for accessing R.  Description: ROP has to configure if and how an
      entity with certain attributes is allowed to access R.
      Requirements: * RqConfigAccessAuthz.1: Must be provided by ROP.
      Postconditions: * PostConfigAccessAuthz.1: The authorization
      information are available to the endpoint which performs
      TObtainAccessAuthz (fulfills PreObtainAccessAuthz.1, see
      Appendix A.2.4).

Author's Address

   Stefanie Gerdes
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359
   Germany

   Phone: +49-421-218-63906
   Email: gerdes@tzi.org








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