Keywords: CoAP, OAuth 2.0, Access Control, Authorization, Internet of Things
Internet Engineering Task Force (IETF) L. Seitz
Request for Comments: 9201 Combitech
Category: Standards Track August 2022
ISSN: 2070-1721
Additional OAuth Parameters for Authentication and Authorization for
Constrained Environments (ACE)
Abstract
This specification defines new parameters and encodings for the OAuth
2.0 token and introspection endpoints when used with the framework
for Authentication and Authorization for Constrained Environments
(ACE). These are used to express the proof-of-possession (PoP) key
the client wishes to use, the PoP key that the authorization server
has selected, and the PoP key the resource server uses to
authenticate to the client.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9201.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction
2. Terminology
3. Parameters for the Token Endpoint
3.1. Client-to-AS Request
3.2. AS-to-Client Response
4. Parameters for the Introspection Endpoint
5. Confirmation Method Parameters
6. CBOR Mappings
7. Requirements When Using Asymmetric Keys
8. Security Considerations
9. Privacy Considerations
10. IANA Considerations
10.1. OAuth Parameter Registration
10.2. OAuth Parameters CBOR Mappings Registration
10.3. OAuth Token Introspection Response CBOR Mappings
Registration
11. References
11.1. Normative References
11.2. Informative References
Acknowledgments
Author's Address
1. Introduction
The Authentication and Authorization for Constrained Environments
(ACE) specification [RFC9200] requires some new parameters for
interactions with the OAuth 2.0 [RFC6749] token and introspection
endpoints, as well as some new claims to be used in access tokens.
These parameters and claims can also be used in other contexts and
have therefore been put into a dedicated document to facilitate their
use in a manner independent of [RFC9200].
Note that although all examples are shown in Concise Binary Object
Representation (CBOR) [RFC8949], JSON [RFC8259] MAY be used as an
alternative for HTTP-based communications, as specified in [RFC9200].
2. Terminology
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 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Readers are assumed to be familiar with the terminology from
[RFC9200], especially the terminology for entities in the
architecture such as client (C), resource server (RS), and
authorization server (AS).
Terminology from [RFC8152] is used in the examples, especially
COSE_Key, which is defined in Section 7 of [RFC8152].
Note that the term "endpoint" is used here following its OAuth 2.0
[RFC6749] definition, which is to denote resources such as token and
introspection at the AS and authz-info at the RS. The Constrained
Application Protocol (CoAP) [RFC7252] definition, which is "[a]n
entity participating in the CoAP protocol", is not used in this
specification.
3. Parameters for the Token Endpoint
This section defines additional parameters for the interactions with
the token endpoint in the ACE framework [RFC9200].
3.1. Client-to-AS Request
This section defines the req_cnf parameter allowing clients to
request a specific PoP key in an access token from a token endpoint
in the ACE framework [RFC9200]:
req_cnf
OPTIONAL. This field contains information about the key the
client would like to bind to the access token for proof of
possession. It is RECOMMENDED that an AS rejects a request
containing a symmetric key value in the req_cnf field
(kty=Symmetric), since the AS is expected to be able to generate
better symmetric keys than a constrained client. (Note: this does
not apply to key identifiers referencing a symmetric key.) The AS
MUST verify that the client really is in possession of the
corresponding key. Profiles of [RFC9200] using this specification
MUST define the PoP method used by the AS if they allow clients to
use this request parameter. Values of this parameter follow the
syntax and semantics of the cnf claim either from Section 3.1 of
[RFC8747] for CBOR-based interactions or from Section 3.1 of
[RFC7800] for JSON-based interactions.
Figure 1 shows a request for an access token using the req_cnf
parameter to request a specific public key as a PoP key. The content
is displayed in CBOR diagnostic notation with line breaks for better
readability.
Header: POST (Code=0.02)
Uri-Host: "as.example.com"
Uri-Path: "token"
Content-Format: application/ace+cbor
Payload:
{
/ req_cnf / 4 : {
/ COSE_Key / 1 : {
/ kty / 1 : 2 /EC2/,
/ kid / 2 : h'11',
/ crv / -1 : 1 /P-256/,
/ x / -2 : h'BAC5B11CAD8F99F9C72B05CF4B9E26D24
4DC189F745228255A219A86D6A09EFF',
/ y / -3 : h'20138BF82DC1B6D562BE0FA54AB7804A3
A64B6D72CCFED6B6FB6ED28BBFC117E'
}
}
}
Figure 1: Example Request for an Access Token Bound to an
Asymmetric Key
3.2. AS-to-Client Response
This section defines the following additional parameters for an AS
response to a request to the token endpoint:
cnf
REQUIRED if the token type is "pop" and a symmetric key is used.
MAY be present for asymmetric PoP keys. This field contains the
PoP key that the AS selected for the token. Values of this
parameter follow the syntax and semantics of the cnf claim either
from Section 3.1 of [RFC8747] for CBOR-based interactions or from
Section 3.1 of [RFC7800] for JSON-based interactions. See
Section 5 for additional discussion of the usage of this
parameter.
rs_cnf
OPTIONAL if the token type is "pop" and asymmetric keys are used.
MUST NOT be present otherwise. This field contains information
about the public key used by the RS to authenticate. If this
parameter is absent, either the RS does not use a public key or
the AS knows that the RS can authenticate itself to the client
without additional information. Values of this parameter follow
the syntax and semantics of the cnf claim either from Section 3.1
of [RFC8747] for CBOR-based interactions or from Section 3.1 of
[RFC7800] for JSON-based interactions. See Section 5 for
additional discussion of the usage of this parameter.
Figure 2 shows an AS response containing a token and a cnf parameter
with a symmetric PoP key.
Header: Created (Code=2.01)
Content-Format: application/ace+cbor
Payload:
{
/ access_token / 1 : h'4A5015DF686428/...
(remainder of CWT omitted for brevity;
CWT contains COSE_Key in the "cnf" claim)/',
/ cnf / 8 : {
/ COSE_Key / 1 : {
/ kty / 1 : 4 / Symmetric /,
/ kid / 2 : h'DFD1AA97',
/ k / -1 : h'849B5786457C1491BE3A76DCEA6C427108'
}
}
}
Figure 2: Example AS Response with an Access Token Bound to a
Symmetric Key
Figure 3 shows an AS response containing a token bound to a
previously requested asymmetric PoP key (not shown) and an rs_cnf
parameter containing the public key of the RS.
Header: Created (Code=2.01)
Content-Format: application/ace+cbor
Payload:
{
/ access_token / 1 : h'D08343A1010AA1054D2A45DF6FBC5A5A/...
(remainder of CWT omitted for brevity)/',
/ rs_cnf / 41 : {
/ COSE_Key / 1 : {
/ kty / 1 : 2 /EC2/,
/ kid / 2 : h'12',
/ crv / -1 : 1 /P-256/,
/ x / -2 : h'BCEE7EAAC162F91E6F330F5771211E220
B8B546C96589B0AC4AD0FD24C77E1F1',
/ y / -3 : h'C647B38C55EFBBC4E62E651720F002D5D
75B2E0C02CD1326E662BCA222B90416'
}
}
}
Figure 3: Example AS Response Including the RS's Public Key
4. Parameters for the Introspection Endpoint
This section defines the use of CBOR instead of JSON for the cnf
introspection response parameter specified in Section 9.4 of
[RFC8705].
If CBOR is used instead of JSON in an interaction with the
introspection endpoint, the AS MUST use the parameter mapping
specified in Table 1 and the value must follow the syntax of cnf
claim values from Section 3.1 of [RFC8747].
Figure 4 shows an AS response to an introspection request including
the cnf parameter to indicate the PoP key bound to the token.
Header: Created (Code=2.01)
Content-Format: application/ace+cbor
Payload:
{
/ active / 10 : true,
/ scope / 9 : "read",
/ aud / 3 : "tempSensor4711",
/ cnf / 8 : {
/ COSE_Key / 1 : {
/ kty / 1 : 2 /EC2/,
/ kid / 2 : h'11',
/ crv / -1 : 1 /P-256/,
/ x / -2 : h'BAC5B11CAD8F99F9C72B05CF4B9E26D24
4DC189F745228255A219A86D6A09EFF',
/ y / -3 : h'20138BF82DC1B6D562BE0FA54AB7804A3
A64B6D72CCFED6B6FB6ED28BBFC117E'
}
}
}
Figure 4: Example Introspection Response
5. Confirmation Method Parameters
The confirmation method parameters are used in [RFC9200] as follows:
* req_cnf in the access token request C -> AS, OPTIONAL to indicate
the client's raw public key or the key identifier of a previously
established key between the C and RS that the client wishes to use
for proof of possession of the access token.
* cnf in the token response AS -> C, OPTIONAL if using an asymmetric
key or a key that the client requested via a key identifier in the
request. REQUIRED if the client didn't specify a req_cnf and
symmetric keys are used. Used to indicate the symmetric key
generated by the AS for proof of possession of the access token.
* cnf in the introspection response AS -> RS, REQUIRED if the access
token that was subject to introspection is a PoP token, absent
otherwise. Indicates the PoP key bound to the access token.
* rs_cnf in the token response AS -> C, OPTIONAL to indicate the
public key of the RS if it uses one to authenticate itself to the
client and the binding between the key and RS identity is not
established through other means.
Note that the COSE_Key structure in a confirmation claim or parameter
may contain an alg or key_ops parameter. If such parameters are
present, a client MUST NOT use a key that is incompatible with the
profile or PoP algorithm according to those parameters. An RS MUST
reject a proof of possession using such a key with a response code
equivalent to the CoAP code 4.00 (Bad Request).
If an access token is issued for an audience that includes several
RSs, the rs_cnf parameter MUST NOT be used, since the client cannot
determine for which RS the key applies. This document recommends to
specify a different endpoint that the client can use to acquire RS
authentication keys in such cases. The specification of such an
endpoint is out of scope for this document.
6. CBOR Mappings
If CBOR is used, the new parameters and claims defined in this
document MUST be mapped to CBOR types, as specified in Table 1, using
the given integer abbreviation for the map key.
+=========+==========+============+========================+
| Name | CBOR Key | Value Type | Usage |
+=========+==========+============+========================+
| req_cnf | 4 | map | token request |
+---------+----------+------------+------------------------+
| cnf | 8 | map | token response |
+---------+----------+------------+------------------------+
| cnf | 8 | map | introspection response |
+---------+----------+------------+------------------------+
| rs_cnf | 41 | map | token response |
+---------+----------+------------+------------------------+
Table 1: CBOR Mappings for New Parameters and Claims
7. Requirements When Using Asymmetric Keys
An RS using asymmetric keys to authenticate to the client MUST NOT
hold several different asymmetric key pairs applicable to the same
authentication algorithm. For example, when using DTLS, the RS MUST
NOT hold several asymmetric key pairs applicable to the same cipher
suite. The reason for this restriction is that the RS has no way of
determining which key to use before the client's identity is
established. Therefore, authentication attempts by the RS could
randomly fail based on which key the RS selects, unless the algorithm
negotiation produces a unique choice of key pair for the RS.
8. Security Considerations
This document is an extension to [RFC9200]. All security
considerations from that document apply here as well.
9. Privacy Considerations
This document is an extension to [RFC9200]. All privacy
considerations from that document apply here as well.
10. IANA Considerations
10.1. OAuth Parameter Registration
This section registers the following parameters in the "OAuth
Parameters" registry [IANA.OAuthParameters]:
Name: req_cnf
Parameter Usage Location: token request
Change Controller: IETF
Reference: Section 5 of RFC 9201
Name: rs_cnf
Parameter Usage Location: token response
Change Controller: IETF
Reference: Section 5 of RFC 9201
Name: cnf
Parameter Usage Location: token response
Change Controller: IETF
Reference: Section 5 of RFC 9201
10.2. OAuth Parameters CBOR Mappings Registration
This section registers the following parameter mappings in the "OAuth
Parameters CBOR Mappings" registry established in Section 8.10 of
[RFC9200].
Name: req_cnf
CBOR Key: 4
Value Type: map
Reference: Section 3.1 of RFC 9201
Original Specification: RFC 9201
Name: cnf
CBOR Key: 8
Value Type: map
Reference: Section 3.2 of RFC 9201
Original Specification: RFC 9201
Name: rs_cnf
CBOR Key: 41
Value Type: map
Reference: Section 3.2 of RFC 9201
Original Specification: RFC 9201
10.3. OAuth Token Introspection Response CBOR Mappings Registration
This section registers the following parameter mapping in the "OAuth
Token Introspection Response CBOR Mappings" registry established in
Section 8.12 of [RFC9200].
Name: cnf
CBOR Key: 8
Value Type: map
Reference: Section 4 of RFC 9201
Original Specification: [RFC8705]
11. References
11.1. Normative References
[IANA.OAuthParameters]
IANA, "OAuth Parameters",
<https://www.iana.org/assignments/oauth-parameters>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<https://www.rfc-editor.org/info/rfc6749>.
[RFC7800] Jones, M., Bradley, J., and H. Tschofenig, "Proof-of-
Possession Key Semantics for JSON Web Tokens (JWTs)",
RFC 7800, DOI 10.17487/RFC7800, April 2016,
<https://www.rfc-editor.org/info/rfc7800>.
[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC8152, July 2017,
<https://www.rfc-editor.org/info/rfc8152>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/info/rfc8259>.
[RFC8705] Campbell, B., Bradley, J., Sakimura, N., and T.
Lodderstedt, "OAuth 2.0 Mutual-TLS Client Authentication
and Certificate-Bound Access Tokens", RFC 8705,
DOI 10.17487/RFC8705, February 2020,
<https://www.rfc-editor.org/info/rfc8705>.
[RFC8747] Jones, M., Seitz, L., Selander, G., Erdtman, S., and H.
Tschofenig, "Proof-of-Possession Key Semantics for CBOR
Web Tokens (CWTs)", RFC 8747, DOI 10.17487/RFC8747, March
2020, <https://www.rfc-editor.org/info/rfc8747>.
[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/info/rfc8949>.
[RFC9200] Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
H. Tschofenig, "Authentication and Authorization for
Constrained Environments (ACE) Using the OAuth 2.0
Framework (ACE-OAuth)", RFC 9200, DOI 10.17487/RFC9200,
August 2022, <https://www.rfc-editor.org/info/rfc9200>.
11.2. Informative References
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
Acknowledgments
This document is a product of the ACE Working Group of the IETF.
Special thanks to Brian Campbell for his thorough review of this
document.
Ludwig Seitz worked on this document as part of the CelticNext
projects CyberWI and CRITISEC with funding from Vinnova.
Author's Address
Ludwig Seitz
Combitech
Djäknegatan 31
SE-211 35 Malmö
Sweden
Email: ludwig.seitz@combitech.com