Network Working Group C. Bormann
Internet-Draft Universitaet Bremen TZI
Intended status: Informational January 17, 2014
Expires: July 21, 2014
An Authorization Information Format (AIF) for ACE
draft-bormann-core-ace-aif-00
Abstract
Constrained Devices as they are used in the "Internet of Things" need
security. One important element of this security is that devices in
the Internet of Things need to be able to decide which operations
requested of them should be considered authorized, need to ascertain
that the authorization to request the operation does apply to the
actual requester, and need to ascertain that other devices they place
requests on are the ones they intended.
On the ACE mailing list, an activity to create specifications for
such authenticated authorization for constrained devices is
contemplated.
One potential work item is an Authorization Information Format (AIF).
This document provides a strawman for such a format that is intended
to enable further discussion of the objectives for its development.
Status of This Memo
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This Internet-Draft will expire on July 21, 2014.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 2
2. Information Model . . . . . . . . . . . . . . . . . . . . . . 2
2.1. Limitations . . . . . . . . . . . . . . . . . . . . . . . 3
3. Data Model . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
(See Abstract.)
1.1. Terminology
This memo uses terms from [I-D.ietf-core-coap] and [RFC4949].
The term "byte", abbreviated by "B", is used in its now customary
sense as a synonym for "octet".
2. Information Model
Authorizations are generally expressed through some data structures
that are cryptographically secured (or transmitted in a secure way).
This section discusses the information model underlying the payload
of that data (as opposed to the cryptographic armor around it).
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For the purposes of this strawman, the underlying access control
model will be that of an access matrix, which gives a set of
permissions for each possible combination of a subject and on object.
For the objects, we simply use the URI of a resource on a CoAP
server. More specifically, the parts of the URI that identify the
server ("authority" in [RFC3986]) are considered to be the realm of
the authentication mechanism (which are handled in the cryptographic
armor); we therefore focus on the "path-absolute" and "query" parts
of the URI (URI "local-part" in this specification, as expressed by
the Uri-Path and Uri-Query options in CoAP). Similarly, we do not
concern the AIF format with the subject for which the AIF object is
issued, focusing the AIF object on a single row in the access matrix
(such a row traditionally is also called a capability list).
At the information model level, this leaves a set of pairs of local
URIs and related permissions. We simplify the model for the
permissions to simply giving the subset of the CoAP methods
permitted. This model is summarized in Table 1 (what is a row in an
access matrix is now just a set of pairs, so it looks like a pair of
columns):
+------------+----------------+
| local-part | Permission Set |
+------------+----------------+
| /s/light | GET |
| | |
| /a/led | PUT, GET |
| | |
| /dtls | POST |
+------------+----------------+
Table 1: An authorization instance in the AIF Information Model
In this example an authenticated subject is authorized to access
three resources on the server to which this authorization information
applies. Different operations are allowed on the individual objects,
e. g. read access (CoAP method GET) to /s/light, or create access
(CoAP method POST) on /dtls.
2.1. Limitations
This simple information model only allows granting permissions for
static URIs. It is probably necessary to extend the model towards
URI templates [RFC6570], however, that requires some considerations
of the ease and unambiguity of matching a given URI against a set of
templates in an AIF object.
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This simple information model also doesn't allow conditionalizing
access (e.g., "opening a door is allowed if that isn't exhibiting the
state 'locked'").
Finally, the model does not provide any dynamic functions such as
enabling special access for a set of resources that are specific to a
subject, e.g. those that the subject created itself by previous
operations (PUT, POST) or that were specifically created for the
subject by others.
3. Data Model
For representing the AIF object discussion in Section 2, the
permission set is reduced to a single number by the following steps:
o The entries in the table that specify the same local-part are
merged into a single entry that specifies the union of the
permission sets
o The methods in the permission sets are converted into their CoAP
method numbers
o The set of numbers is converted into a single number by taking
each number to the power of two and computing the inclusive OR of
the binary representations of all the numbers.
This strawman data model could be interchanged in the JSON [RFC4627]
representation given in Figure 1 (more extensible/more compact
representations are possible).
[["/s/light", 1], ["/a/led", 5], ["/dtls", 2]]
Figure 1: An authorization instance encoded in JSON (46 bytes)
A straightforward representation of the same information in CBOR
[RFC7049] is given in Figure 2; again, several optimizations/
improvements are possible.
83 # array(3)
82 # array(2)
68 # text(8)
2f732f6c69676874 # "/s/light"
01 # unsigned(1)
82 # array(2)
66 # text(6)
2f612f6c6564 # "/a/led"
05 # unsigned(5)
82 # array(2)
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65 # text(5)
2f64746c73 # "/dtls"
02 # unsigned(2)
Figure 2: An authorization instance encoded in CBOR (29 bytes)
4. IANA Considerations
This document makes no requirements on IANA. (This section to be
removed by RFC editor.)
5. Security Considerations
(TBD. Some issues are already discussed in the security
considerations of [I-D.ietf-core-coap] and in
[I-D.garcia-core-security].)
6. Acknowledgements
TBD
7. References
7.1. Normative References
[I-D.ietf-core-coap]
Shelby, Z., Hartke, K., and C. Bormann, "Constrained
Application Protocol (CoAP)", draft-ietf-core-coap-18
(work in progress), June 2013.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2", RFC
4949, August 2007.
7.2. Informative References
[I-D.garcia-core-security]
Garcia-Morchon, O., Kumar, S., Keoh, S., Hummen, R., and
R. Struik, "Security Considerations in the IP-based
Internet of Things", draft-garcia-core-security-06 (work
in progress), September 2013.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC
3986, January 2005.
[RFC4627] Crockford, D., "The application/json Media Type for
JavaScript Object Notation (JSON)", RFC 4627, July 2006.
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[RFC6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
and D. Orchard, "URI Template", RFC 6570, March 2012.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, October 2013.
Author's Address
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
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