Universität Bremen TZI

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Internet Internet-Draft 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. To transfer detailed authorization information from an authorization manager (such as an ACE-OAuth Authorization Server) to a device, a representation format is needed. This document provides a suggestion for such a format, the Authorization Information Format (AIF). AIF is defined both as a general structure that can be used for many different applications and as a specific refinement that describes REST resources and the permissions on them.

Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at . Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 30 January 2021.

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Table of Contents

• .  Introduction
  • .  Terminology
• .  Information Model
  • .  REST-specific model
  • .  Limitations
  • .  Extended REST-specific model
• .  Data Model
• .  Media Types
• .  IANA Considerations
  • .  Media Types
  • .  Registries
  • .  Content-Format
• .  Security Considerations
• .  References
  • .  Normative References
  • .  Informative References
• Acknowledgements
• Author's Address

Introduction (See Abstract.)

Terminology This memo uses terms from and . The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here. These words may also appear in this document in lower case as plain English words, absent their normative meanings. (Note that this document is itself informational, but it is discussing normative statements that MUST be put into concrete terms in each specification that makes use of this document.) The term "byte", abbreviated by "B", is used in its now customary sense as a synonym for "octet".

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). 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 an object. 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). As a consequence, AIF MUST be used in a way that the subject of the authorizations is unambiguously identified (e.g., as part of the armor around it). The generic model of a such a capability list is a list of pairs of object identifiers and the permissions the subject has on the object(s) identified.

Definition of Generic AIF = [* [Toid, Tperm]] ]]>

In a specific data model, the object identifier (Toid) will often be a text string, and the set of permissions (Tperm) will be represented by a bitset in turn represented as a number (see ).

Likely shape of a specific AIF ]]>

REST-specific model In the specific instantiation of the REST resources and the permissions on them, for the object identifiers (Toid), 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 ) are considered 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). As a consequence, AIF MUST be used in a way that it is unambiguous who is the target (enforcement point) of these authorizations. For the permissions (Tperm), we simplify the model permissions to giving the subset of the CoAP methods permitted. This model is summarized in . An authorization instance in the AIF Information Model

local-part	Permission Set
/s/light	GET
/a/led	PUT, GET
/dtls	POST

Limitations This simple information model only allows granting permissions for statically identifiable objects, e.g. URIs for the REST-specific instantiation. One might be tempted to extend the model towards URI templates , however, that requires some considerations of the ease and unambiguity of matching a given URI against a set of templates in an AIF object. This simple information model also doesn't allow further conditionalizing access based on state outside the identification of objects (e.g., "opening a door is allowed if that isn't locked"). Finally, the model does not provide any special access for a set of resources that are specific to a subject, e.g. that the subject created itself by previous operations (PUT, POST) or that were specifically created for the subject by others.

Extended REST-specific model The extended REST-specific model addresses the need to provide defined access to dynamic resources that were created by the subject itself, specifically, a resource that is made known to the subject by providing Location-* options in a CoAP result or using the Location header field in HTTP (the Location-indicating mechanisms). (The concept is somewhat comparable to "ACL inheritance" in NFSv4 , except that it does not use a containment relationship but the fact that the dynamic resource was created from a resource to which the subject had access.) An authorization instance in the AIF Information Model

local-part	Permission Set
/a/make-coffee	POST, Dynamic-GET, Dynamic-DELETE

For a method X, the presence of a Dynamic-X permission means that the subject holds permission to exercise the method X on resources that have been returned by a Location-indicating mechanism to a request that the subject made to the resource listed (/a/make-coffee in the example, which might return the location of a resource that allows GET to find out about the status and DELETE to cancel the coffee-making operation). Since the use of the extension defined in this section can be detected by the mentioning of the Dynamic-X permissions, there is no need for another explicit switch between the basic and the extended model; the extended model is always presumed once a Dynamic-X permission is present.

Data Model Different data model specializations can be defined for the generic information model given above. In this section, we will give the data model for basic REST authorization. As discussed, the object identifier is specialized as a text string giving a relative URI (local-part as absolute path on the server serving as enforcement point). The permission set is specialized to a single number by the following steps:

• 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.
• The (non-dynamic) methods in the permission sets are converted into their CoAP method numbers, minus 1.
• Dynamic-X permissions are converted into what the number would have been for X, plus a Dynamic-Offset chosen as 32 (e.g., 35 for Dynamic-DELETE).
• 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 power values.

This data model could be interchanged in the JSON representation given in .

An authorization instance encoded in JSON (46 bytes)

In CDDL , a straightforward specification of the data model (including both the methods from and the new ones from , identified by the method code minus 1) is:

AIF in CDDL path = tstr ; URI relative to enforcement point permissions = uint .bits methods methods = &( GET: 0 POST: 1 PUT: 2 DELETE: 3 FETCH: 4 PATCH: 5 iPATCH: 6 Dynamic-GET: 32; 0 .plus Dynamic-Offset Dynamic-POST: 33; 1 .plus Dynamic-Offset Dynamic-PUT: 34; 2 .plus Dynamic-Offset Dynamic-DELETE: 35; 3 .plus Dynamic-Offset Dynamic-FETCH: 36; 4 .plus Dynamic-Offset Dynamic-PATCH: 37; 5 .plus Dynamic-Offset Dynamic-iPATCH: 38; 6 .plus Dynamic-Offset ) ]]>

A representation of this information in CBOR is given in ; again, several optimizations/improvements are possible.

An authorization instance encoded in CBOR (29 bytes)

Note that choosing 32 as Dynamic-Offset means that all future CoAP methods that can be registered can be represented both as themselves and in the Dynamic-X variant, but only the dynamic forms of methods 1 to 21 are typically usable in a JSON form .

Media Types This specification defines media types for the generic information model, expressed in JSON (application/aif+json) or in CBOR (application/aif+cbor). These media types have parameters for specifying Toid and Tperm; default values are the values "local-uri" for Toid and "REST-method-set" for Tperm. [Insert lots of boilerplate here] A specification that wants to use Generic AIF with different Toid and/or Tperm is expected to request these as media type parameters () and register a corresponding Content-Format ().

IANA Considerations

Media Types See .

Registries IANA is requested to create a registry for AIF with two sub-registries for Toid and Tperm, populated with:

Subregistry	name	Description/Specification
Toid	local-part	local-part of URI as specified in [RFCthis]
Tperm	REST-method-set	set of REST methods represented as specified in [RFCthis]

The registration policy is Specification required . The designated expert will engage with the submitter to ascertain the requirements of this document are addressed.

Content-Format IANA is requested to register Content-Format numbers in the CoRE Parameters Registry , as follows:

Security Considerations (TBD. Some issues are already discussed in the security considerations of and in .)

References Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This Glossary provides definitions, abbreviations, and explanations of terminology for information system security. The 334 pages of entries offer recommendations to improve the comprehensibility of written material that is generated in the Internet Standards Process (RFC 2026). The recommendations follow the principles that such writing should (a) use the same term or definition whenever the same concept is mentioned; (b) use terms in their plainest, dictionary sense; (c) use terms that are already well-established in open publications; and (d) avoid terms that either favor a particular vendor or favor a particular technology or mechanism over other, competing techniques that already exist or could be developed. This memo provides information for the Internet community. The Constrained Application Protocol (CoAP) is a specialized web transfer protocol for use with constrained nodes and constrained (e.g., low-power, lossy) networks. The nodes often have 8-bit microcontrollers with small amounts of ROM and RAM, while constrained networks such as IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs) often have high packet error rates and a typical throughput of 10s of kbit/s. The protocol is designed for machine- to-machine (M2M) applications such as smart energy and building automation. CoAP provides a request/response interaction model between application endpoints, supports built-in discovery of services and resources, and includes key concepts of the Web such as URIs and Internet media types. CoAP is designed to easily interface with HTTP for integration with the Web while meeting specialized requirements such as multicast support, very low overhead, and simplicity for constrained environments. Many protocols make use of points of extensibility that use constants to identify various protocol parameters. To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations are often coordinated by a central record keeper. For IETF protocols, that role is filled by the Internet Assigned Numbers Authority (IANA). To make assignments in a given registry prudently, guidance describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed. This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry. This is the third edition of this document; it obsoletes RFC 5226. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. This document proposes a notational convention to express Concise Binary Object Representation (CBOR) data structures (RFC 7049). Its main goal is to provide an easy and unambiguous way to express structures for protocol messages and data formats that use CBOR or JSON. Informative References This specification defines a profile of the ACE framework that allows constrained servers to delegate client authentication and authorization. The protocol relies on DTLS version 1.2 for communication security between entities in a constrained network using either raw public keys or pre-shared keys. A resource- constrained server can use this protocol to delegate management of authorization information to a trusted host with less severe limitations regarding processing power and memory. Work in Progress This memo specifies a profile for the Authentication and Authorization for Constrained Environments (ACE) framework. It utilizes Object Security for Constrained RESTful Environments (OSCORE) to provide communication security, server authentication, and proof-of-possession for a key owned by the client and bound to an OAuth 2.0 access token. Work in Progress IANA A Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource. This specification defines the generic URI syntax and a process for resolving URI references that might be in relative form, along with guidelines and security considerations for the use of URIs on the Internet. The URI syntax defines a grammar that is a superset of all valid URIs, allowing an implementation to parse the common components of a URI reference without knowing the scheme-specific requirements of every possible identifier. This specification does not define a generative grammar for URIs; that task is performed by the individual specifications of each URI scheme. [STANDARDS-TRACK] This document describes the Network File System (NFS) version 4 minor version 1, including features retained from the base protocol (NFS version 4 minor version 0, which is specified in RFC 3530) and protocol extensions made subsequently. Major extensions introduced in NFS version 4 minor version 1 include Sessions, Directory Delegations, and parallel NFS (pNFS). NFS version 4 minor version 1 has no dependencies on NFS version 4 minor version 0, and it is considered a separate protocol. Thus, this document neither updates nor obsoletes RFC 3530. NFS minor version 1 is deemed superior to NFS minor version 0 with no loss of functionality, and its use is preferred over version 0. Both NFS minor versions 0 and 1 can be used simultaneously on the same network, between the same client and server. [STANDARDS-TRACK] A URI Template is a compact sequence of characters for describing a range of Uniform Resource Identifiers through variable expansion. This specification defines the URI Template syntax and the process for expanding a URI Template into a URI reference, along with guidelines for the use of URI Templates on the Internet. [STANDARDS-TRACK] The Concise Binary Object Representation (CBOR) is a data format whose design goals include the possibility of extremely small code size, fairly small message size, and extensibility without the need for version negotiation. These design goals make it different from earlier binary serializations such as ASN.1 and MessagePack. The Hypertext Transfer Protocol (HTTP) is a stateless \%application- level protocol for distributed, collaborative, hypertext information systems. This document defines the semantics of HTTP/1.1 messages, as expressed by request methods, request header fields, response status codes, and response header fields, along with the payload of messages (metadata and body content) and mechanisms for content negotiation. I-JSON (short for "Internet JSON") is a restricted profile of JSON designed to maximize interoperability and increase confidence that software can process it successfully with predictable results. The methods defined in RFC 7252 for the Constrained Application Protocol (CoAP) only allow access to a complete resource, not to parts of a resource. In case of resources with larger or complex data, or in situations where resource continuity is required, replacing or requesting the whole resource is undesirable. Several applications using CoAP need to access parts of the resources. This specification defines the new CoAP methods, FETCH, PATCH, and iPATCH, which are used to access and update parts of a resource. JavaScript Object Notation (JSON) is a lightweight, text-based, language-independent data interchange format. It was derived from the ECMAScript Programming Language Standard. JSON defines a small set of formatting rules for the portable representation of structured data. This document removes inconsistencies with other specifications of JSON, repairs specification errors, and offers experience-based interoperability guidance. The Internet of Things (IoT) concept refers to the usage of standard Internet protocols to allow for human-to-thing and thing-to-thing communication. The security needs for IoT systems are well recognized, and many standardization steps to provide security have been taken -- for example, the specification of the Constrained Application Protocol (CoAP) secured with Datagram Transport Layer Security (DTLS). However, security challenges still exist, not only because there are some use cases that lack a suitable solution, but also because many IoT devices and systems have been designed and deployed with very limited security capabilities. In this document, we first discuss the various stages in the lifecycle of a thing. Next, we document the security threats to a thing and the challenges that one might face to protect against these threats. Lastly, we discuss the next steps needed to facilitate the deployment of secure IoT systems. This document can be used by implementers and authors of IoT specifications as a reference for details about security considerations while documenting their specific security challenges, threat models, and mitigations. This document is a product of the IRTF Thing-to-Thing Research Group (T2TRG).

Acknowledgements Jim Schaad and Francesca Palombini provided comments that shaped the direction of this document.

Author's Address Universität Bremen TZI

Postfach 330440 Bremen D-28359 Germany +49-421-218-63921 cabo@tzi.org