Actors in the ACE Architecture
draft-gerdes-ace-actors-04
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| Authors | Stefanie Gerdes , Ludwig Seitz , Göran Selander , Carsten Bormann | ||
| Last updated | 2015-03-25 | ||
| Replaced by | draft-ietf-ace-actors | ||
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draft-gerdes-ace-actors-04
ACE Working Group S. Gerdes
Internet-Draft Universitaet Bremen TZI
Intended status: Informational L. Seitz
Expires: September 26, 2015 SICS Swedish ICT AB
G. Selander
Ericsson
C. Bormann, Ed.
Universitaet Bremen TZI
March 25, 2015
Actors in the ACE Architecture
draft-gerdes-ace-actors-04
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.
This document gives an overview of the necessary terminology and
introduces the actors in an architecture as guidance for the
development of authentication and authorization solutions for
constrained environments. The actors represent the relationships
between the logical functional entities involved.
We also present a problem description for authentication and
authorization in constrained-node networks, i.e. networks where some
devices have severe constraints on memory, processing, power and
communication bandwidth.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 26, 2015.
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 . . . . . . . . . . . . . . . . . . . . . . . 4
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 4
3. Security Objectives . . . . . . . . . . . . . . . . . . . . . 5
4. Authentication and Authorization . . . . . . . . . . . . . . 6
5. Autonomous Communication . . . . . . . . . . . . . . . . . . 7
6. Actors . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.1. Constrained Level Actors . . . . . . . . . . . . . . . . 8
6.2. Principal Level Actors . . . . . . . . . . . . . . . . . 9
6.3. Less-Constrained Level Actors . . . . . . . . . . . . . . 10
7. Architecture Variants . . . . . . . . . . . . . . . . . . . . 11
8. Kinds of Protocols . . . . . . . . . . . . . . . . . . . . . 14
8.1. Constrained Level Protocols . . . . . . . . . . . . . . . 14
8.1.1. Cross Level Support Protocols . . . . . . . . . . . . 14
8.2. Less-Constrained Level Protocols . . . . . . . . . . . . 15
9. Introduction to Problem Description . . . . . . . . . . . . . 15
9.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 15
10. Background . . . . . . . . . . . . . . . . . . . . . . . . . 16
11. Problem Description . . . . . . . . . . . . . . . . . . . . . 18
11.1. Authorization . . . . . . . . . . . . . . . . . . . . . 19
11.2. Authentication . . . . . . . . . . . . . . . . . . . . . 19
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11.3. Communication Security . . . . . . . . . . . . . . . . . 20
11.4. Cryptographic Keys . . . . . . . . . . . . . . . . . . . 20
12. Assumptions and Requirements . . . . . . . . . . . . . . . . 21
12.1. Architecture . . . . . . . . . . . . . . . . . . . . . . 21
12.2. Constrained Devices . . . . . . . . . . . . . . . . . . 22
12.3. Authentication . . . . . . . . . . . . . . . . . . . . . 23
12.4. Authorization . . . . . . . . . . . . . . . . . . . . . 23
12.5. Authorization Information . . . . . . . . . . . . . . . 23
12.6. Resource Access . . . . . . . . . . . . . . . . . . . . 24
12.7. Keys and Cipher Suites . . . . . . . . . . . . . . . . . 24
12.8. Network Considerations . . . . . . . . . . . . . . . . . 25
12.9. Legacy Considerations . . . . . . . . . . . . . . . . . 25
13. Security Considerations . . . . . . . . . . . . . . . . . . . 25
13.1. Physical Attacks on Sensor and Actuator Networks . . . . 26
13.2. Time Measurements . . . . . . . . . . . . . . . . . . . 27
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 27
16. Informative References . . . . . . . . . . . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29
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. Moreover, these tasks need to be assigned to logical
functional entities which perform the tasks: the actors in the
architecture. Thus, relations between the actors and requirements
for protocols can be identified.
In this document, an architecture is developed to represent the
relationships between the logical functional entities involved.
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1.1. Terminology
Readers are required to be familiar with the terms and concepts
defined in [RFC4949].
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
information.
Constrained node: a constrained device in the sense of [RFC7228].
Actor: A logical functional entity that performs one or more tasks.
Depending on the tasks an actor must perform, the device that
contains the actor may need to have certain system resources
available. Multiple actors may share, i.e. be present within, a
device or even a piece of software.
Resource Server (RS): An entity which hosts and represents a
Resource.
Client (C): An entity which attempts to access a resource on a
Server.
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 permissions concerning authorized
representations of a Resource.
Principal: An individual that is either RqP or RO or both.
Authorization Server (AS): An entity that prepares and endorses
authentication and authorization data for a Server.
Client Authorization Server (CAS): An entity that prepares and
endorses authentication and authorization data for a Client.
Attribute Binding Authority: An entity that is authorized to
validate claims about an entity.
2. Problem Statement
The scenario this document addresses can be summarized as follows:
o C wants to access R on a RS.
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o A priori, C and RS do not necessarily know each other and have no
security relationship.
o C and / or RS are constrained.
------- --------
| C | -- requests resource ---> | RS |
------- <-- 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. Security Objectives
The security objectives that can be addressed by an authorization
solution are 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 non-repudiation and accountability. They are not related to
authorization and thus are not in scope of this draft, but still
should be considered by Authenticated Authorization solutions. Non-
repudiation and accountability may require authentication on 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.
The importance of a security objective depends on the application the
authorization mechanism is used for. [I-D.ietf-ace-usecases]
indicates that security objectives differ for the various constrained
environment use cases.
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In many cases, one participating party might have different security
objectives than the other. However, to achieve a security objective,
both parties must participate in providing a solution. E.g., if RqP
requires the integrity of sensor value representations RS is hosting,
Both C and RS need to integrity-protect the transmitted data.
Moreover, RS needs to protect the access to the sensor representation
to prevent unauthorized users to manipulate the sensor values.
4. Authentication and Authorization
Authorization solutions aim at protecting the access to items of
interest, e.g. hardware or software resources or data: They enable
the principal of such a resource to control who can access it and
how.
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 use the term _authenticated
authorization_ to refer to a synthesis of mechanism for
authentication and authorization.
If used for authorization, the authenticated attributes must be
meaningful for the purpose of the authorization, i.e. the
authorization policy grants access permissions based on these
attributes. If the authorization policy assigns permissions to an
individual entity, the authenticated attributes must be suitable to
uniquely identify this entity.
In scenarios where devices are communicating autonomously there is
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 (e.g. light bulb) is likely more
important than that it has a unique identifier.
Principals (RqP and RO) need to decide about the required level of
granularity for the authorization, ranging from _device
authorization_ over _owner authorization_ to _binary authorization_
and _unrestricted authorization_. In the first case different access
permissions are granted to individual devices while in the second
case individual owners are authorized. If binary authorization is
used, all authenticated entities have the same access permissions.
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Unrestricted authorization for an item of interest means that no
authorization mechanism is used (not even by authentication) and all
entities are able to access the item as they see fit. More fine-
grained authorization does not necessarily provide more security.
Principals need to consider that an entity should only be granted the
permissions it really needs to ensure the confidentiality and
integrity of resources.
For all cases where an authorization solution is needed (all but
Unrestricted Authorization), the authorizing party needs to be able
to authenticate the party that is to be authorized. Authentication
is therefore required for messages that contain representations of an
accessed item. More precisely, the authorizing party needs to make
sure that the receiver of a message containing a representation, and
the sender of a message containing a representation are authorized to
receive and send this message, respectively. To achieve this, the
integrity 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 (only the client side or only the server
side) requires the integrity and / or confidentiality of a resource
value. In these cases, principals may decide to use binary
authorization which can be achieved by an authentication mechanism or
even unrestricted authorization where no authentication mechanism is
required. However, as indicated in Section 3, the security
objectives of both sides must be considered. The security objectives
of one side can often only be achieved with the help of the other
side. E.g., if the server requires the confidentiality of a resource
representation, the client must make sure that it does not send
resource updates to parties other than the server. Therefore, the
client must at least use binary authorization.
5. Autonomous Communication
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.
Moreover, these devices often do not have any user interfaces or
displays. Even if the principals are present at the time of access,
they may not be able to communicate directly with the device. The
devices therefore need to be able to communicate autonomously. In
some scenarios there is an active user at one endpoint of the
communication. Other scenarios ask for true machine to machine (M2M)
communication.
To achieve the principals' security objectives, the devices must be
enabled to enforce the security policies of their principals.
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6. Actors
This section describes the various actors in the architecture. 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.
6.1. 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).
o Validate that an entity is an authorized source for R.
o Securely transmit an access request.
RS performs the following tasks:
o Communicate in a secure way (provide for confidentiality and
integrity of messages).
o Validate the authorization of the requester to access the
requested resource as requested.
o Securely transmit a response to an access request.
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.
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As C and RS do not necessarily know each other they might belong to
different security domains.
------- --------
| C | -- requests resource ---> | RS | Constrained Level
------- <-- provides resource--- --------
Figure 2: Constrained Level Actors
6.2. 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,
e.g. 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.
------- -------
| RqP | | RO | Principal Level
------- -------
| |
in charge of in charge of
| |
V V
------- -------
| C | -- requests resource ---> | RS | Constrained Level
------- <-- provides resource--- -------
Figure 3: Constrained Level Actors and Principal Level Actors
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6.3. 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 source 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.
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------- -------
| RqP | | RO | Principal Level
------- -------
| |
in charge of in charge of
| |
V V
---------- ----------
| CAS | <- AuthN and AuthZ -> | AS | Less-Constrained Level
---------- ----------
| |
authentication authentication
and authorization and authorization
support support
| |
V V
------- -------
| C | -- requests resource ---> | RS | Constrained Level
------- <-- provides resource -- -------
Figure 4: Overview of all Complexity Levels
For more detailed graphics please consult the PDF version.
7. Architecture Variants
As mentioned in section Section 6, actors 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:
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------- --------
| 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 5: Combined C and CAS
If RS is located on a more powerful device, it can be combined with
AS:
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------- -------
| 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 6: 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
|
V
--------------
| CAS + AS | Less-Constrained Level
--------------
/ \
/ \
authentication authentication
and authorization and authorization
support support
/ \
V V
------- -------
| C | -- requests resource --> | RS | Constrained Level
------- <-- provides resource -- -------
Figure 7: CAS combined with AS
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8. 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.
8.1. 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 6.1). C and RS might not belong to the same security
domain. Therefore, constrained level protocols need to work between
different security domains.
FIXME
Figure 8: Constrained Level Tasks
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
mind.
8.1.1. 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.
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8.2. 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 Web Authorization Protocol (OAuth)
[RFC6749] and Kerberos [RFC4120] can likely be used without
modifications and there are no limitations for the use of a Public
Key Infrastructure (PKI).
FIXME
Figure 9: Less-constrained Level Tasks
9. Introduction to Problem Description
Authorization is the process of deciding what an entity ought to be
allowed to do. This memo is about properties of security protocols
to enable explicit and dynamic authorization of clients to access a
resource at a server, in particular in constrained environments when
the client and/or server are constrained nodes.
Relevant use cases are provided in [I-D.ietf-ace-usecases], which
also lists some authorization problems derived from the use cases.
In this memo we present a more specific problem description for
authentication and authorization in constrained RESTful environments
together with a detailed set of assumptions and requirements (cf.
Section 12).
9.1. Terminology
Certain security-related terms are to be understood in the sense
defined in [RFC4949]. These terms include, but are not limited to,
"authentication", "authorization", "confidentiality", "(data)
integrity", "message authentication code", and "verify".
RESTful terms including "resource", "representation", etc. are to be
understood as used in HTTP [RFC7231] and CoAP [RFC7252].
Terminology for constrained environments including "constrained
device", "constrained-node network", "class 1", etc. are defined in
[RFC7228].
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"Explicit" authorization is used here to describe the ability to
specify in some detail which entity has access to what and under what
conditions, as opposed to "implicit" authorization where an entity is
either allowed to access everything or nothing.
"Dynamic" authorization means that the access control polices and the
parameters on which they are evaluated may change during normal
operations, as opposed to "static" authorization meaning that access
control policies cannot be changed during normal operations and may
require some special procedure such as out-of-band provision.
10. Background
We assume a client-server setting, where a client wishes to access
some resource hosted by a server. Such resources may e.g. be sensor
data, configuration data, or actuator settings. Thus access to a
resource could be by different methods, some of which change the
state of the resource. In this memo, we consider the REST setting
i.e. GET, POST, PUT and DELETE, and application protocols in scope
are HTTP [RFC7231] and CoAP [RFC7252].
We assume that the roles of client and server are not fixed, i.e. a
node which is client could very well be server in some other context
and vice-versa. Further we assume that in some cases, clients are
not previously known to servers, thus we cannot assume that the
server has access control policies specific to that client when the
client initiates communication.
Finally we also assume that in a significant number of cases, the
server and/or the client are too constrained to handle the evaluation
of complex access control policies and related configuration on their
own. Many authorization solutions involve a centralized, trusted
third party, supporting the client and/or resource server. A trusted
third party provides a more scalable way to centrally manage
authorization policies, in order to ensure consistent authorization
decisions. The physical separation of policy decision and policy
enforcement is an established principle in policy based management,
e.g. [RFC2748].
Borrowing from OAuth 2.0 [RFC6749] terminology we name the entities:
client (C), resource server (RS), authorization server (AS - the
third party), and resource owner (RO). RO is in charge of the access
control policies implemented in the AS governing the actions of RS.
However, the RO need not be active in a constrained device access
control setting, so we cannot rely on timely interactions with the
RO. In the target setting RS is typically constrained, C may be
constrained, whereas AS is not assumed to be constrained.
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Since RS is constrained, we assume that it needs to offload
authorization policy management and/or authorization decision making
to AS. This means that some authorization information needs to be
transferred from AS to RS.
Protecting information carried between AS and RS, requires some a
priori established cryptographic keys. How those keys are
established is out of scope for this problem description.
AS may for example be implemented as a cloud service, in a home
server, or in a smartphone. C and RS may or may not have
connectivity to AS at the time of the access request, e.g. because
they cannot handle multiple, simultaneous connections. Another
reason for intermittent connectivity may be that constant
connectivity is not affordable (e.g. due to limited battery power, or
a sensor mobility business case for which cellular connectivity cost
too much or is not available). Obviously, in order for a client
request to reach RS there must be connectivity between C and RS, but
that could be a short range technology such as Bluetooth, ZigBee, or
NFC. Furthermore, if there is not sufficient authorization
information about C in RS, and neither C nor RS can access AS, access
requests will be denied. Therefore we assume that either C or RS can
access AS at some point in time, prior to the client's request.
As a summary, there are potentially three information flows that
needs to be protected (see Figure 10):
1. The transfer of authorization information from AS to RS
2. The transfer of cryptographic keys or credentials from AS to RS
and C, respectively
3. The access request/response procedure between C and RS
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+---------------+
| Authorization |
| Server |
| |
+---------------+
/ \ Authorization
Credentials, / \ Information
Keys / \
/ \ Credentials,
/ \ Keys
V V
+--------+ +-----------+
| Client | | Resource |
| |<---- Access procedure --->| Server |
| | | |
+--------+ +-----------+
Figure. Information flows that needs to be protected.
Only showing origin and destination, actual
flow may pass intermediary nodes.
Figure 10
NOTE:
The information flow in Figure 10 above enables RO to control the
interactions of a constrained RS by means of access control policies.
There is an ongoing discussion about an analogous information flow
enabling the stakeholder associated to C ("Requesting Party" in UMA
terminology [I-D.hardjono-oauth-umacore]) to control the interactions
of a constrained C by means of policies. While this would not be
policies for access control to resources, it could be useful in
certain settings which require dynamically changing interaction
patterns with a constrained client without updating firmware. Such a
solution could potentially reuse all security components required to
protect the information flow in 1., so no additional specifications
would be needed. This aspect is not discussed further in this draft.
11. Problem Description
A number of problems needs to be solved in order to achieve explicit
and dynamic authorization, as is described in this section.
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11.1. 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.
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 The RS needs to be able to verify the authenticity of the
authorization information. There is a trade-off here between
processing complexity and deployment complexity.
o The RS needs to enforce the authorization decisions of the AS.
The authorization information it obtained from AS might require
additional policy evaluation (e.g. matching against local access
control lists, evaluating local conditions). The required "policy
evaluation" at the RS needs to be adapted to the capabilities of
the constrained device.
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 RS at different times and in different ways
prior to or during the client request.
11.2. Authentication
The following problems need to be addressed, when considering
authentication:
o RS need to authenticate AS to ensure that the authorization
information and related data comes from the correct source.
o C may need to to authenticate AS to ensure that it 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 bind it to the relevant authorization information. In
other use cases, authentication and authorization of C may be
implicit, e.g. by encrypting the resource representation the RS
only providing access to those who possess the key to decrypt.
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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 AS may need to authenticate its communication partner (either C or
RS), in order to ensure it serves the correct device.
11.3. 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, e.g.
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.
11.4. 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.
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On the other hand asymmetric cryptography has benefits e.g. in
terms of deployment.
Key Establishment
How are the corresponding cryptographic keys established?
Considering Section 11.1 there must be a binding 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 - e.g. 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?
12. 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.
12.1. 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 also provide other services such as authenticating C on
behalf of RS, or providing cryptographic keys or credentials to
C and/or RS to secure the request/response procedure.
o The architecture may contain intermediary nodes between any pair
of C, RS and AS, such as e.g. forward/reverse proxies in the CoRE
architecture. The solution shall not unduly restrict the use of
intermediaries.
* The architecture shall support session based security and data
object security.
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12.2. 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
consumption
o AS is not a constrained device.
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 (e.g. 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 The solution shall support a simple scheme for expiring
authentication and authorization information on devices which are
unable to measure time (cf. section Section 13.2).
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12.3. Authentication
o RS need to authenticate AS to ensure that the authorization
information and related data comes from the correct source.
o Depending on use case, C, RS or AS may need to authenticate each
other.
12.4. Authorization
o The authorization decision is based on credentials presented by C,
the requested resource, the RESTful method, and local context in
RS at the time of the request, or on any subset of this
information.
o The authorization decision is taken either by AS or RS.
o The authorization decision is enforced by RS.
* RS needs to have access to 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
(e.g. resource descriptions).
o The solution may need to be able to support the delegation of
access rights.
12.5. Authorization Information
o Authorization information is transferred from AS to RS using
Agent, Push or Pull mechanisms [RFC2904].
o RS shall authenticate that the authorization information is coming
from AS.
o The authorization information may also be encrypted end-to-end
between AS and RS.
o 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.
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o Authorization information may be pre-configured in RS.
o Authorization information stored or cached in RS shall be possible
to change. The change of such information shall 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.
12.6. Resource Access
o Resources are accessed in a RESTful manner using GET, PUT, POST,
DELETE.
o By default, the resource request shall be integrity protected and
may be encrypted end-to-end from C to RS. It shall be possible
for RS to detect a replayed request.
o By default, the response to a request shall be integrity protected
and encrypted end-to-end from RS to C. It shall be possible for C
to detect a replayed response.
o RS shall be able to verify that the request comes from an
authorized client
o C shall 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).
12.7. Keys and Cipher Suites
o AS and RS have established cryptographic keys. Either AS and RS
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.
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o There must be a mechanism for RS to establish the necessary key(s)
to verify and decrypt the request.
o There must be a mechanism for C to establish the necessary key(s)
to verify and decrypt the response.
o There must be a mechanism for C to look up the supported cipher
suites of a RS.
12.8. Network Considerations
o The solution shall prevent network overload due to avoidable
communication with AS.
o The solution shall prevent network overload by compact
authorization information representation.
o The solution shall optimize the case where authorization
information does not change often.
o The solution where possible shall support an efficient mechanism
for providing authorization information to multiple RSs, for
example when multiple entities need to be configured or change
state.
12.9. Legacy Considerations
o The solution shall work with existing infrastructure.
o The solution shall support authorization of access to legacy
devices.
13. 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.
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13.1. 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 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.
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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.
13.2. 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.
14. IANA Considerations
This document has no actions for IANA.
15. 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
acknowledge input provided by Hummen et al. [HUM14delegation].
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16. Informative References
[HUM14delegation]
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.
[I-D.hardjono-oauth-umacore]
Hardjono, T., Maler, E., Machulak, M., and D. Catalano,
"User-Managed Access (UMA) Profile of OAuth 2.0", draft-
hardjono-oauth-umacore-12 (work in progress), February
2015.
[I-D.ietf-ace-usecases]
Seitz, L., Gerdes, S., Selander, G., Mani, M., and S.
Kumar, "ACE use cases", draft-ietf-ace-usecases-03 (work
in progress), March 2015.
[I-D.koster-core-coap-pubsub]
Koster, M., Keranen, A., and J. Jimenez, "Publish-
Subscribe Broker for the Constrained Application Protocol
(CoAP)", draft-koster-core-coap-pubsub-01 (work in
progress), March 2015.
[I-D.selander-ace-object-security]
Selander, G., Mattsson, J., and L. Seitz, "March 9, 2015",
draft-selander-ace-object-security-01 (work in progress),
March 2015.
[RFC2748] Durham, D., Boyle, J., Cohen, R., Herzog, S., Rajan, R.,
and A. Sastry, "The COPS (Common Open Policy Service)
Protocol", RFC 2748, January 2000.
[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, August
2000.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
July 2005.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2", RFC
4949, August 2007.
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[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.
[RFC6749] Hardt, D., "The OAuth 2.0 Authorization Framework", RFC
6749, October 2012.
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228, May 2014.
[RFC7230] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
(HTTP/1.1): Message Syntax and Routing", RFC 7230, June
2014.
[RFC7231] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
(HTTP/1.1): Semantics and Content", RFC 7231, June 2014.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252, June 2014.
Authors' Addresses
Stefanie Gerdes
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63906
Email: gerdes@tzi.org
Ludwig Seitz
SICS Swedish ICT AB
Scheelevaegen 17
Lund 223 70
Sweden
Email: ludwig@sics.se
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Goeran Selander
Ericsson
Faroegatan 6
Kista 164 80
Sweden
Email: goran.selander@ericsson.com
Carsten Bormann (editor)
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
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