| Internet-Draft | Alternative ACE Workflow and Parameters | March 2024 |
| Tiloca & Selander | Expires 5 September 2024 | [Page] |
This document updates the Authentication and Authorization for Constrained Environments Framework (ACE, RFC 9200) as follows. First, it defines a new, alternative workflow that the Authorization Server can use for uploading an access token to a Resource Server on behalf of the Client. Second, it defines new parameters and encodings for the OAuth 2.0 token endpoint at the Authorization Server. Third, it amends two of the requirements on profiles of the framework. Finally, it deprecates the original payload format of error responses that convey an error code, when CBOR is used to encode message payloads. For such error responses, it defines a new payload format aligned with RFC 9290, thus updating in this respect also the profiles of ACE defined in RFC 9202, RFC 9203, and RFC 9431.¶
This note is to be removed before publishing as an RFC.¶
Discussion of this document takes place on the Authentication and Authorization for Constrained Environments Working Group mailing list (ace@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/ace/.¶
Source for this draft and an issue tracker can be found at https://github.com/ace-wg/ace-workflow-and-params.¶
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 https://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 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 5 September 2024.¶
Copyright (c) 2024 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 (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶
The Authentication and Authorization for Constrained Environments (ACE) framework [RFC9200] defines an architecture to enforce access control for constrained devices. A Client (C) requests an assertion of granted permissions from an Authorization Server (AS) in the form of an access token, then uploads the access token to the target Resource Server (RS), and finally accesses protected resources at the RS according to the permissions specified in the access token.¶
The framework has as main building blocks the OAuth 2.0 framework [RFC6749], the Constrained Application Protocol (CoAP) [RFC7252] for message transfer, CBOR [RFC8949] for compact encoding, and COSE [RFC9052][RFC9053] for self-contained protection of access tokens. In addition, separate profile documents define in detail how the participants in the ACE architecture communicate, especially as to the security protocols that they use.¶
This document updates [RFC9200] as follows.¶
It defines a new, alternative protocol workflow for the ACE framework (see Section 2), according to which the AS uploads the access token to the RS on behalf of C, and then informs C about the outcome. The new workflow is especially convenient in deployments where the communication leg between C and the RS is constrained, but the communication leg between the AS and the RS is not.¶
The new workflow has no ambition to replace the original workflow. The AS can use one workflow or the other depending, for example, on the specific RS for which an access token has been issued and the nature of the communication leg with that RS.¶
It defines additional parameters and encodings for the OAuth 2.0 token endpoint at the AS (see Section 3). These include:¶
"token_upload", used by C to inform the AS that it opts in to use the new ACE workflow, and by the AS to inform C about the outcome of the token uploading to the RS per the new workflow.¶
"rs_cnf2", used by the AS to provide C with the public keys of the RSs in the group-audience for which the access token is issued (see Section 6.9 of [RFC9200]).¶
"aud2", used by the AS to provide C with the identifiers of the RSs in the group-audience for which the access token is issued.¶
"anchor_cnf", used by the AS to provide C with the public keys of trust anchors, which C can use to validate the public key of an RS (e.g., as provided in the parameter "rs_cnf" defined in [RFC9201] or in the parameter "rs_cnf2" defined in this document).¶
It amends two of the requirements on profiles of the ACE framework (see Section 4).¶
It deprecates the original payload format of error responses that convey an error code, when CBOR is used to encode message payloads in the ACE framework. For such error responses, it defines a new payload format according to the problem-details format specified in [RFC9290] (see Section 5).¶
In this respect, it also updates the profiles of the ACE framework defined in [RFC9202], [RFC9203], and [RFC9431].¶
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 expected to be familiar with the terms and concepts described in the ACE framework for Authentication and Authorization [RFC9200][RFC9201], as well as with terms and concepts related to CBOR Web Tokens (CWTs) [RFC8392] and CWT Confirmation Methods [RFC8747].¶
The terminology for entities in the considered architecture is defined in OAuth 2.0 [RFC6749]. In particular, this includes Client (C), Resource Server (RS), and Authorization Server (AS).¶
Readers are also expected to be familiar with the terms and concepts related to the CoAP protocol [RFC7252], CDDL [RFC8610], CBOR [RFC8949], and COSE [RFC9052][RFC9053].¶
Note that, unless otherwise indicated, the term "endpoint" is used here following its OAuth definition, aimed at denoting resources such as /token and /introspect at the AS, and /authz-info at the RS. This document does not use the CoAP definition of "endpoint", which is "An entity participating in the CoAP protocol."¶
Furthermore, this document uses the following term.¶
Token series: the set comprising all the access tokens issued by the same AS for the same pair (Client, Resource Server).¶
Profiles of ACE can provide their extended and specialized definition, e.g., by further taking into account the public authentication credentials of C and the RS.¶
Examples throughout this document are expressed in CBOR diagnostic notation without the tag and value abbreviations.¶
As defined in Section 4 of [RFC9200], the ACE framework considers what is shown in Figure 1 as its basic protocol workflow.¶
That is, the Client first sends an access token request to the token endpoint at the AS (step A), specifying permissions that it seeks to obtain for accessing protected resources at the RS, possibly together with information on its own public authentication credentials.¶
Then, if the request has been successfully verified, authenticated, and authorized, the AS replies to the Client (step B), providing an access token and possibly additional parameters as access information including the actually granted permissions.¶
Finally, the Client uploads the access token to the RS and, consistently with the permissions granted according to the access token, accesses a resource at the RS (step C), which replies with the result of the resource access (step F). Details about what protocol the Client and the RS use to establish a secure association, mutually authenticate, and secure their communications are defined in the specifically used profile of ACE, e.g., [RFC9202][RFC9203][RFC9431][I-D.ietf-ace-edhoc-oscore-profile][I-D.ietf-ace-group-oscore-profile][RFC9431].¶
Further interactions are possible between the AS and the RS, i.e., the exchange of an introspection request and response where the AS validates a previously issued access token for the RS (steps D and E).¶
+--------+ +---------------+ | |---(A)-- Token Request ------->| | | | | Authorization | | |<--(B)-- Access Token ---------| Server | | | + Access Information | | | | + Refresh Token (optional) +---------------+ | | ^ | | | Introspection Request (D)| | | Client | Response | |(E) | | (optional exchange) | | | | | v | | +--------------+ | |---(C)-- Token + Request ----->| | | | | Resource | | |<--(F)-- Protected Resource ---| Server | | | | | +--------+ +--------------+
This section defines a new, alternative protocol workflow shown in Figure 2, which MAY be supported by the AS. Unlike in the original protocol workflow, the AS uploads the access token to the RS on behalf of the Client, and then informs the Client about the outcome.¶
If the token uploading has been successfully completed, the AS does not provide the access token to the Client altogether. Instead, the Client simply establishes a secure association with the RS (if that has not happened already), and then accesses protected resources at the RS according to the permissions granted per the access token and specified by the AS as access information.¶
+--------+ +----------------------------+ | |---(A)-- Token Request ------->| | | | | Authorization | | |<--(B)-- Token Response -------| Server | | | + Access Information | | | | + Access Token (optional) +----------------------------+ | | + Refresh Token (optional) ^ | | ^ | | | | | | Token-Upload | | Introspection Request (D)| | (A1)| | Request | Client | Response | |(E) | |(A2) Response | | (optional exchange) | | | | | | | v v | | | +----------------------------+ | |---(C1)-- Token (Optional) --->| | | | | | | |---(C2)-- Protected Request -->| Resource | | | | Server | | |<--(F)--- Protected Resource --| | | | | | +--------+ +----------------------------+
More specifically, the new workflow consists of the following steps.¶
Step A - Like in the original workflow, the Client sends an Access Token Request to the token endpoint at the AS, with the additional indication that it opts in to use the alternative workflow.¶
As defined in Section 3.1, this information is conveyed to the AS by means of the "token_upload" parameter.¶
Step A1 - This new step consists of the AS uploading the access token to the RS, typically at the authz-info endpoint, just like the Client does in the original workflow.¶
Step A2 - This new step consists of the RS replying to the AS, following the uploading of the access token at step A1.¶
Step B - In the Access Token Response, the AS tells the Client that it has attempted to upload the access token to the RS, specifying the outcome of the token uploading based on the reply received from the RS at step A2.¶
As defined in Section 3.1, this information is conveyed to the Client by means of the "token_upload" parameter. If the token uploading has succeeded, the AS does not provide the Client with the access token. Otherwise, the AS provides the Client with the access token.¶
Step C1 - This step occurs only if the token uploading from the AS has failed, and the AS has provided the Client with the access token at step B. In such a case, the Client uploads the access token to the RS just like at step C of the original workflow.¶
Step C2 - The Client attempts to access a protected resource at the RS, according to the permissions granted per the access token and specified by the AS as access information at step B.¶
Steps D, E, and F are as in the original workflow.¶
The new workflow has no ambition to replace the original workflow defined in [RFC9200]. The AS can use one workflow or the other depending, for example, on the specific RS for which the access token has been issued and the nature of the communication leg with that RS.¶
The rest of this section defines a number of additional parameters and encodings for the OAuth 2.0 token endpoint at the AS.¶
This section defines the additional parameter "token_upload". The parameter can be used in an Access Token Request sent by C to the token endpoint at the AS, as well as in the successful Access Token Response sent as reply by the AS.¶
In an Access Token Request¶
The parameter "token_upload" is OPTIONAL in an Access Token Request. If present, this parameter MUST encode the CBOR simple value true (0xf5). The presence of the parameter indicates that C opts in to use the new, alternative ACE workflow defined in Section 2, whose actual use for uploading the issued access token to the RS is an exclusive prerogative of the AS.¶
If the AS supports the new ACE workflow and the Access Token Request includes the parameter "token_upload" with value the CBOR simple value true (0xf5), then the AS MAY use the new ACE workflow to upload the access token to the RS on behalf of C. Otherwise, the AS MUST NOT use the new ACE workflow.¶
In an Access Token Response¶
The parameter "token_upload" is REQUIRED in a successful Access Token Response with response code 2.01 (Created), if both the following conditions apply. Otherwise, the parameter "token_upload" MUST NOT be present.¶
The corresponding Access Token Request included the parameter "token_upload", with value the CBOR simple value true (0xf5).¶
The AS has attempted to upload the issued access token at the RS as per the new ACE workflow, irrespective of the result of the token upload.¶
When the parameter "token_upload" is present in the Access Token Response, the following applies.¶
If the token upload at the RS was successful, then the parameter "token_upload" MUST encode the CBOR simple value true (0xf5), and the access token MUST NOT be included in the Access Token Response.¶
If the token upload at the RS was not successful, then the parameter "token_upload" MUST encode the CBOR simple value false (0xf4), and the access token MUST be included in the Access Token Response.¶
Figure 3 shows an example with first an Access Token Request from C to the AS, and then an Access Token Response from the AS to C, following the issue of an access token bound to a symmetric PoP key.¶
The Access Token Response specifies the parameter "token_upload" with value the CBOR simple value true (0xf5), which indicates that the AS has successfully uploaded the access token to the RS on behalf of C.¶
Consistently, the Access Token Response does not include the access token, while it still includes the parameter "cnf" specifying the symmetric PoP key bound to the access token.¶
/ Access Token Request /
Header: POST (Code=0.02)
Uri-Host: "as.example.com"
Uri-Path: "token"
Content-Format: application/ace+cbor
Payload:
{
"audience" : "tempSensor4711",
"scope" : "read",
"token_upload" : true
}
/ Access Token Response /
Header: Created (Code=2.01)
Content-Format: application/ace+cbor
Max-Age: 3560
Payload:
{
"token_upload" : true,
"expires_in" : 3600,
"cnf" : {
"COSE_Key" : {
"kty" : 1,
"kid" : h'3d027833fc6267ce',
"k" : h'73657373696f6e6b6579'
}
}
}
Figure 4 shows another example with first an Access Token Request from C to the AS, and then an Access Token Response from the AS to C, also following the issue of an access token bound to a symmetric PoP key.¶
In this example, the Access Token Response includes the parameter "token_upload" with value the CBOR simple value false (0xf4), which indicates that the AS has failed to upload the access token to the RS on behalf of C. The Access Token Response also includes the access token and the parameter "cnf" specifying the symmetric PoP key bound to the access token.¶
Note that, even though the AS has failed to upload the access token to the RS, the response code 2.01 (Created) is used when replying to C, since the Access Token Request as such has been successfully processed at the AS, with the following issue of the access token.¶
/ Access Token Request /
Header: POST (Code=0.02)
Uri-Host: "as.example.com"
Uri-Path: "token"
Content-Format: application/ace+cbor
Payload:
{
"audience" : "tempSensor4711",
"scope" : "read",
"token_upload" : true
}
/ Access Token Response /
Header: Created (Code=2.01)
Content-Format: application/ace+cbor
Max-Age: 3560
Payload:
{
"access_token" : h'd08343a1'/...
(remainder of CWT omitted for brevity;
CWT contains the symmetric PoP key in the "cnf" claim)/,
"token_upload" : false,
"expires_in" : 3600,
"cnf" : {
"COSE_Key" : {
"kty" : 1,
"kid" : h'3d027833fc6267ce',
"k" : h'73657373696f6e6b6579'
}
}
}
This section defines the additional parameters "rs_cnf2" and "aud2" for an Access Token Response, sent by the AS in reply to a request to the token endpoint from C.¶
The parameter "rs_cnf2" is OPTIONAL if the token type is "pop", asymmetric keys are used, and the access token is issued for an audience that includes multiple RSs (i.e., a group-audience, see Section 6.9 of [RFC9200]). Otherwise, the parameter "rs_cnf2" MUST NOT be present.¶
This parameter specifies information about the public keys used by the RSs of a group-audience for authenticating themselves to C, and is used in case the binding between the public keys and the corresponding RS identities are not established through other means. If this parameter is absent, either the RSs in the group-audience do not use a public key, or the AS knows that the RSs can authenticate themselves to C without additional information.¶
If present, this parameter MUST encode a non-empty CBOR array of N elements, where N is the number of RSs in the group-audience for which the access token is issued. Each element of the CBOR array specifies the public key of one RS in the group-audience, and MUST 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. It is not required that all the elements of the CBOR array rely on the same confirmation method.¶
Each of the public keys may contain parameters specifying information such as the public key algorithm and use (e.g., by means of the parameters "alg" or "key_ops" in a COSE_Key structure). If such information is specified, a Client MUST NOT use a public key that is incompatible with the profile or PoP algorithm according to that information. 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).¶
The parameter "aud2" is OPTIONAL and specifies the identifiers of the RSs in the group-audience for which the access token is issued.¶
If present, this parameter MUST encode a non-empty CBOR array of N elements, where N is the number of RSs in the group-audience for which the access token is issued. Each element of the CBOR array in the "aud2" parameter MUST be a CBOR text string, with value the identifier of one RS in the group-audience.¶
The element of the CBOR array referring to an RS in the group-audience SHOULD have the same value that would be used to identify that RS through the parameter "aud" of an Access Token Request to the AS (see Section 5.8.2 of [RFC9200]) and of an Access Token Response from the AS (see Section 5.8.2 of [RFC9200]), when requesting and issuing an access token for that individual RS.¶
The parameter "aud2" is REQUIRED if the parameter "rs_cnf2" is present. In such a case, the i-th element of the CBOR array in the "aud2" parameter MUST be the identifier of the RS whose public key is specified as the i-th element of the CBOR array in the "rs_cnf2" parameter.¶
Figure 5 shows an example of Access Token Response from the AS to C, following the issue of an access token for a group-audience composed of two RSs "rs1" and "rs2", and bound to C's public key as asymmetric PoP key. The Access Token Response includes the access token, as well as the parameters "aud2" and "rs_cnf2". These specify the public key of the two RSs as intended recipients of the access token and the identifiers of those two RSs, respectively.¶
Header: Created (Code=2.01)
Content-Format: application/ace+cbor
Max-Age: 3600
Payload:
{
"access_token" : b64'SlAV32hk'/...
(remainder of CWT omitted for brevity;
CWT contains the client's RPK in the "cnf" claim)/,
"expires_in" : 3600,
"aud2" : ["rs1", "rs2"],
"rs_cnf2" : [
{
"COSE_Key" : {
"kty" : 2,
"crv" : 1,
"x" : h'bbc34960526ea4d32e940cad2a234148
ddc21791a12afbcbac93622046dd44f0',
"y" : h'4519e257236b2a0ce2023f0931f1f386
ca7afda64fcde0108c224c51eabf6072'
}
},
{
"COSE_Key" : {
"kty" : 2,
"crv" : 1,
"x" : h'ac75e9ece3e50bfc8ed6039988952240
5c47bf16df96660a41298cb4307f7eb6',
"y" : h'6e5de611388a4b8a8211334ac7d37ecb
52a387d257e6db3c2a93df21ff3affc8'
}
}
]
}
This section defines the additional parameter "anchor_cnf" for an Access Token Response, sent by the AS in reply to a request to the token endpoint from C.¶
The parameter "anchor_cnf" is OPTIONAL if the token type is "pop" and asymmetric keys are used. Otherwise, the parameter "anchor_cnf" MUST NOT be present.¶
This parameter specifies information about the public keys of trust anchors, which C can use to validate the public key of the RS/RSs included in the audience for which the access token is issued. This parameter can be used when the access token is issued for an audience including one RS or multiple RSs.¶
If this parameter is absent, either the RS/RSs in the audience do not use a public key, or the AS knows that C can validate the public key of such RS/RSs without additional information (e.g., C has already obtained the required public keys of the involved trust anchors from the AS or through other means).¶
If present, this parameter MUST encode a non-empty CBOR array that MUST be treated as a set, i.e., the order of its elements has no meaning. Each element of the CBOR array specifies the public key of one trust anchor, which can be used to validate the public key of at least one RS included in the audience for which the access token is issued. Each element of the CBOR array MUST 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. It is not required that all the elements of the CBOR array rely on the same confirmation method.¶
Each of the public keys specified in the parameter "anchor_cnf" may contain parameters specifying information such as the public key algorithm and use (e.g., by means of the parameters "alg" or "key_ops" in a COSE_Key structure). If such information is specified, a Client MUST NOT use a public key that is incompatible with the profile, or with the public keys to validate and the way to validate those.¶
The presence of this parameter does not require that the Access Token Response also includes the parameter "rs_cnf" defined in [RFC9201] or the parameter "rs_cnf2" defined in Section 3.2 of this document. That is, C may be able to obtain the public keys of the RS/RSs for which the access token is issued through other means.¶
When the Access Token Response includes both the parameter "anchor_cnf" and the parameter "aud2" defined in Section 3.2, then C MUST make sure that a public key PK_RS is associated with an RS identified by an element of "aud2", before using any of the public keys specified in "anchor_cnf" to validate PK_RS.¶
When the Access Token Response includes the parameter "anchor_cnf" but not the parameter "aud2", then C can use any of the public keys specified in "anchor_cnf" to validate the public key PK_RS of any RS in the targeted audience. This allows C to use the access token with an RS that is deployed later on as part of the same audience, which is particularly useful in the case of a group-audience.¶
Figure 6 shows an example of Access Token Response from the AS to C, following the issue of an access token for a group-audience, and bound to C's public key as asymmetric PoP key.¶
The identifier of the group-audience was specified by the "aud" parameter of the Access Token Request to the AS and is specified by the "aud" claim of the issued access token, and is not repeated in the Access Token Response from the AS.¶
The Access Token Response includes the parameter "anchor_cnf". This specifies the public key of a trust anchor that C can use to validate the public keys of any RS with which the access token is going to be used. The public key of the trust anchor is here conveyed within an X.509 certificate used as public authentication credential for that trust anchor, by means of the CWT confirmation method "x5chain" defined in [I-D.ietf-ace-edhoc-oscore-profile].¶
Header: Created (Code=2.01)
Content-Format: application/ace+cbor
Max-Age: 3600
Payload:
{
"access_token" : b64'SlAV32hk'/...
(remainder of CWT omitted for brevity;
CWT contains the client's RPK in the "cnf" claim)/,
"expires_in" : 3600,
"anchor_cnf" : [
{
"x5chain" : h'308201363081dea003020102020301f50d30
0a06082a8648ce3d04030230163114301206
035504030c0b524643207465737420434130
1e170d3230303130313030303030305a170d
3231303230323030303030305a3022312030
1e06035504030c1730312d32332d34352d46
462d46452d36372d38392d41423059301306
072a8648ce3d020106082a8648ce3d030107
03420004b1216ab96e5b3b3340f5bdf02e69
3f16213a04525ed44450b1019c2dfd3838ab
ac4e14d86c0983ed5e9eef2448c6861cc406
547177e6026030d051f7792ac206a30f300d
300b0603551d0f040403020780300a06082a
8648ce3d04030203470030440220445d798c
90e7f500dc747a654cec6cfa6f037276e14e
52ed07fc16294c84660d02205a33985dfbd4
bfdd6d4acf3804c3d46ebf3b7fa62640674f
c0354fa056dbaea6'
}
]
}
Appendix C of [RFC9200] compiles a list of requirements on the profiles of ACE. This document amends two of those requirements as follows.¶
The text of the fifth requirement¶
Specify the security protocol the client and RS must use to protect their communication (e.g., OSCORE or DTLS). This must provide encryption and integrity and replay protection (Section 5.8.4.3).¶
is replaced by the following text:¶
Specify the security protocol the client and RS must use to protect their communication (e.g., OSCORE or DTLS). In combination with the used communication protocol, this must provide encryption, integrity and replay protection, and a binding between requests and responses (Section 5.8.4.3 and Section 6.5).¶
The text of the tenth requirement¶
Specify the communication and security protocol for interactions between the client and AS. This must provide encryption, integrity protection, replay protection, and a binding between requests and responses (Sections 5 and 5.8).¶
is replaced by the following text:¶
Specify the communication and security protocol for interactions between the client and AS. The combined use of those protocols must provide encryption, integrity protection, replay protection, and a binding between requests and responses (Sections 5 and 5.8).¶
At the time of writing, all the profiles of ACE that are published as RFC (i.e., [RFC9202][RFC9203][RFC9431]) already comply with the two updated requirements as formulated above.¶
This section deprecates the original payload format of error responses conveying an error code, when CBOR is used to encode message payloads in the ACE framework. That format is referred to, e.g., when defining the error responses of Sections 5.8.3 and 5.9.3 of [RFC9200].¶
Also, this section defines a new payload format that allows such error responses to convey an error code together with further error-specific information, according to the problem-details format specified in [RFC9290].¶
Such error responses MUST have Content-Format set to application/concise-problem-details+cbor. The payload of these error responses MUST be a CBOR map specifying a Concise Problem Details data item (see Section 2 of [RFC9290]). The CBOR map is formatted as follows.¶
It MUST include the Custom Problem Detail entry "ace-error" registered in Section 7.3 of this document.¶
This entry is formatted as a CBOR map including only one field, namely "error-code". The map key for "error-code" is the CBOR unsigned integer with value 0. The value of "error-code" is a CBOR integer specifying the error code associated with the occurred error. This value is taken from the "CBOR Value" column of the "OAuth Error Code CBOR Mappings" registry [ACE.OAuth.Error.Code.CBOR.Mappings].¶
The new payload format MUST use the field "error-code" in order to convey the same information that the original payload format conveys through the "error" parameter (see, e.g., Sections 5.8.3 and 5.9.3 of [RFC9200]).¶
The CDDL notation [RFC8610] of the "ace-error" entry is given below.¶
ace-error = {
&(error-code: 0) => int
}
¶
It MAY include further Standard Problem Detail entries or Custom Problem Detail entries (see [RFC9290]). The following Standard Problem Detail entries are of particular relevance for the ACE framework.¶
"detail" (map key -2): its value is a CBOR text string that specifies a human-readable, diagnostic description of the occurred error (see Section 2 of [RFC9290]).¶
The diagnostic text is intended for software engineers as well as for device and network operators, in order to aid debugging and provide context for possible intervention. The diagnostic message SHOULD be logged by the sender of the error response. The entry "detail" is unlikely relevant in an unattended setup where human intervention is not expected.¶
The new payload format MUST use the Standard Problem Detail entry "detail" in order to convey the same information that the original payload format conveys through the "error_description" parameter (see, e.g., Sections 5.8.3 and 5.9.3 of [RFC9200]).¶
"instance" (map key -3): its value is a URI reference identifying the specific occurrence of the error (see Section 2 of [RFC9290]).¶
The new payload format MUST use the Standard Problem Detail entry "instance" in order to convey the same information that the original payload format conveys through the "error_uri" parameter (see, e.g., Sections 5.8.3 and 5.9.3 of [RFC9200]).¶
An example of error response using the problem-details format is shown in Figure 7.¶
Header: Bad Request (Code=4.00)
Content-Format: application/concise-problem-details+cbor
Payload:
{
/ title / -1: "Incompatible ACE profile",
/ detail / -2: "The RS supports only the OSCORE profile",
/ ace-error/ TBD: {
/ error_code / 0: 8 / incompatible_ace_profiles /
}
}
Note to RFC Editor: In the figure above, please replace "TBD" with the unsigned integer assigned as key value to the Custom Problem Detail entry "ace-error" (see Section 7.3). Then, please delete this paragraph.¶
When the ACE framework is used with CBOR for encoding message payloads, the following applies.¶
It is RECOMMENDED that Authorization Servers, Clients, and Resource Servers support the payload format defined in this section.¶
Authorization Servers, Clients, and Resource Servers that support the payload format defined in this section MUST use it when composing an outgoing error response that conveys an error code.¶
The same security considerations from the ACE framework for Authentication and Authorization [RFC9200] apply to this document, together with those from the specifically used transport profile of ACE, e.g., [RFC9202][RFC9203][RFC9431][I-D.ietf-ace-edhoc-oscore-profile][I-D.ietf-ace-group-oscore-profile][RFC9431].¶
When using the problem-details format defined in [RFC9290] for error responses, then the privacy and security considerations from Sections 4 and 5 of [RFC9290] also apply.¶
Editor's note: add more security considerations.¶
This document has the following actions for IANA.¶
Note to RFC Editor: Please replace all occurrences of "[RFC-XXXX]" with the RFC number of this specification and delete this paragraph.¶
IANA is asked to add the following entries to the "OAuth Parameters" registry.¶
Name: "token_upload"¶
Parameter Usage Location: token request and token response¶
Change Controller: IESG¶
Reference: [RFC-XXXX]¶
Name: "rs_cnf2"¶
Parameter Usage Location: token response¶
Change Controller: IESG¶
Reference: [RFC-XXXX]¶
IANA is asked to add the following entries to the "OAuth Parameters CBOR Mappings" following the procedure specified in [RFC9200].¶
IANA is asked to register the following entry in the "Custom Problem Detail Keys" registry within the "Constrained RESTful Environments (CoRE) Parameters" registry group.¶
For any transport profile of ACE, the following holds.¶
The new ACE workflow defined in Section 2 is effectively possible to use. This is beneficial for deployments where the communication leg between C and the RS is constrained, but the communication leg between the AS and RS is not.¶
When the new ACE workflow is used, the parameter "token_upload" defined in Section 3.1 is used:¶
When the RPK mode of the DTLS profile is used (see Section 3.2 of [RFC9202]), it becomes possible for the AS to effectively issue an access token intended to an audience that includes multiple RSs. This is enabled by the parameters "rs_cnf2" and "aud2" defined in Section 3.2, as well as by the parameter "anchor_cnf" defined in Section 3.3. This seamlessly applies also if the profile uses Transport Layer Security (TLS) [RFC8446], as defined in [RFC9430].¶
When the EDHOC and OSCORE profile is used [I-D.ietf-ace-edhoc-oscore-profile], it becomes possible for the AS to effectively issue an access token intended to an audience that includes multiple RSs. This is enabled by the parameters "rs_cnf2" and "aud2" defined in Section 3.2, as well as by the parameter "anchor_cnf" defined in Section 3.3.¶
The following discusses open points related to the use of the new ACE workflow defined in Section 2.¶
In some profiles of ACE, C can request a new access token to update its access rights, while preserving the same secure association with the RS. The new access token supersedes the current one stored at the RS, as they are both part of the same token series.¶
When using the original ACE workflow, C uploads the new access token to the RS by protecting the message exchange through the secure association with the RS. This allows the RS to determine that the upload of such access token is for updating the access rights of C.¶
When using the new ACE workflow, the AS uploads the new access token to the RS also when an update of access rights for C is to be performed. This message exchange would be protected through the secure association between the AS and the RS. However, this secure association does not help the RS retrieve the stored access token to supersede, as that is rather bound to the secure association with C.¶
In order for the new ACE workflow to also allow the dynamic update of access rights, it is required that the new access token updating the access rights of C includes an explicit indication for the RS. Such an indication can point the RS to the token series in question (hence to the current access token to supersede), irrespective of the secure association used to protect the token uploading.¶
In some profiles of ACE, such an indication is in fact already present in issued access tokens:¶
In the PSK mode of the DTLS profile [RFC9202], the token series is indicated by the parameter "kid" within the claim "cnf" of the new access token. This has the same value of the parameter "kid" in the COSE_Key within the claim "cnf" from the first access token of the token series.¶
In the OSCORE profile [RFC9203], the token series is indicated by the parameter "kid" within the claim "cnf" of the new access token. This has the same value of the parameter "id" in the OSCORE_Input_Material object within the claim "cnf" from the first access token of the token series.¶
In the EDHOC and OSCORE profile [I-D.ietf-ace-edhoc-oscore-profile], the token series is indicated by the parameter "kid" within the claim "cnf" of the new access token. This has the same value of the parameter "id" in the EDHOC_Information object within the claim "cnf" from the first access token of the token series.¶
In the three cases above, the update of access rights is possible because there is a value used as de facto "token series ID". This value does not change throughout the lifetime of a token series, and it is used to associate the new access token with the previous one in the same series to be superseded.¶
Such a token series ID is required to have a unique value from a namespace/pool that the AS exclusively controls. This is in fact what happens in the profiles of ACE above, where the AS is the entity creating the mentioned objects or COSE Key included in the first access token of a token series.¶
However, this may generally not hold and it is not what happens in other known cases, i.e., the DTLS profile in RPK mode [RFC9203] and the Group OSCORE profile [I-D.ietf-ace-group-oscore-profile]. At the moment, the dynamic update of access rights is not possible for those, neither in the original nor in the new ACE workflow.¶
In order to make the update of access rights possible also for such cases, as well as both in the original and in the new ACE workflow, those cases can rely on a new parameter and claim "token_series_id" (see Appendix B.2), which specifies a unique identifier of the token series which an access token belongs to.¶
As to existing profiles of ACE, the above has no intention to change the current behavior when the update of access rights occurs, irrespective of the used ACE workflow and especially when using the original workflow.¶
If future profiles rely on a construction where the AS creates the object or the key included in the claim "cnf" of the first access token in a token series, and a unique ID generated by the AS is included in such object or key, then that ID must be used as de facto "token series ID", rather than the new parameter "token_series_id".¶
After the AS has successfully uploaded the access token to the RS when using the new ACE workflow, C does not obtain the access token altogether. It follows that C cannot re-upload the Access Token to the RS by itself, e.g., in order to perform a key update like defined for the OSCORE profile [RFC9203].¶
Even in such a case, the token re-uploading can be allowed by relying on a new parameter "token_hash", which the AS provides to C and specifies the hash of the access token (see Appendix B.2).¶
Then, C can practically "re-upload" the access token to the RS, by sending a request to the authz-info endpoint that includes the parameter "token_hash" instead of the parameter "access_token". Such a request may include further parameters, depending on what is defined for the used transport profile.¶
If the RS still stores the access token in question, then the RS can identify it by means of the received token hash, and take the same actions that would have been taken in case the full access token was re-uploaded to the authz-info endpoint.¶
Some profiles of ACE require that C and the RS generate information to be exchanged when uploading the access token.¶
For example, in the OSCORE profile [RFC9203], C and the RS exchange the nonces N1 and N2 together with their OSCORE Recipient IDs ID1 and ID2, when uploading to the RS the first access token of a token series, as well as when re-uploading any access token (e.g., in order to perform a key update).¶
Evidently, using the new ACE workflow prevents C and the RS from directly performing the required exchanges above, since the uploading of the access token does not rely on a direct interaction between C and the RS like in the original ACE workflow. For some profiles of ACE, this may prevent the use of the new ACE workflow altogether.¶
This issue can be solved by having the AS acting as intermediary also for the exchange of C- and RS-generated information, by relying on two new parameters "to_rs" and "from_rs" (see Appendix B.2). In particular, C can use "to_rs" for providing the AS with C-generated information, to be relayed to the RS when uploading the access token. Also, the RS can use "from_rs" for providing the AS with RS-generated information when replying to the token uploading, and to be relayed to C.¶
With reference to the two cases mentioned above, "to_rs" can specify the nonce N1 generated by C, while "from_rs" can specify the nonce N2 generated by the RS.¶
The following discusses possible, further new parameters that can be defined for addressing the open points raised earlier in Appendix B.¶
"token_series_id" - This parameter specifies the unique identifier of a token series, thus ensuring that C can dynamically update its access rights, irrespective of the used ACE workflow (see Appendix B.1.1).¶
When issuing the first access token of a token series, the AS specifies this parameter in the Access Token Response to C, with value TS_ID. Also, the AS includes a claim "token_series_id" with the same value in the access token.¶
When C requests a new access token in the same tokes series for dynamically updating its access rights, C specifies TS_ID as value of the parameter "token_series_id" of the Access Token Request, which MUST omit the parameter "req_cnf" (see Section 3.1 of [RFC9201]). The AS specifies the same value within the claim "token_series_id" of the new access token.¶
When this parameter is used, the information about the token series in question has to be specified in that parameter and in the corresponding token claim. Instead, the "req_cnf" parameter and the "cnf" claim are used for their main purpose, i.e., for specifying the public authentication credential of the Client, by value or by reference.¶
If a profile of ACE can use or is already using a different parameter/claim as de-facto identifier of the token series, then that profile will continue to do so, and will not use this new parameter "token_series_id".¶
"token_hash" - This parameter specifies the hash of an access token that the AS has successfully issued and uploaded to the RS as per the new ACE workflow, and thus that the AS does not provide to C (see Appendix B.1.1).¶
The AS specifies this parameter in a successful Access Token Response, in case the parameter "token_upload" is also specified as encoding the CBOR simple value true (0xf5) (see Section 3.1). The parameter value is the hash computed over the value that the parameter "access_token" would have had in that same response message, if it was included therein specifying the access token.¶
C specifies this parameter in the request sent to the authz-info endpoint at the RS for "re-uploading" the same access token, e.g., in order to perform a key update (see Appendix B.1.2).¶
This parameter also allows C to seamlessly use the method defined in [I-D.ietf-ace-revoked-token-notification] for learning of revoked access tokens, even when the new ACE workflow is used and C does not obtain the access token, which makes it impossible for C to compute the token hash by itself.¶
"to_rs" - When using the new ACE workflow, this parameter specifies C-generated information that, according to the used profile of ACE, C has to provide to the RS together with the access token if using the original ACE workflow. This allows the AS to relay such information to the RS upon uploading the access token on behalf of C (see Appendix B.1.3).¶
First, C specifies this parameter in the Access Token Request sent to the AS. Then, the AS specifies this parameter in the request to the RS sent for uploading the access token on behalf of C, by simply relaying the value received from C. The used profile of ACE has to define the detailed content and semantics of the information specified in the parameter value.¶
"from_rs" - When using the new ACE workflow, this parameter specifies RS-generated information that, according to the used profile of ACE, the RS has to provide to C after the uploading of an access token if using the original ACE workflow. This allows the AS to relay such information to C after having uploaded the access token on behalf of C (see Appendix B.1.3).¶
First, the RS specifies this parameter in the response sent to the AS, after the upload of an access token through a request from the AS. Then, the AS specifies this parameter in the Access Token Response to C, by simply relaying the value received from the RS. The used profile of ACE has to define the detailed content and semantics of the information specified in the parameter value.¶
This section is to be removed before publishing as an RFC.¶
Definition of the "token series" moved to the "Terminology" section.¶
Clarifications and fixes on using parameters in messages.¶
Amendeded two of the requirements on profiles of the framework.¶
The Client has to opt-in for using the alternative workflow.¶
Parameter "token_uploaded" renamed to "token_upload".¶
Updated format of error response payload to use RFC 9290.¶
Security considerations inherited from other documents.¶
Editorial fixes and improvements.¶
The authors sincerely thank Christian Amsüss, Rikard Höglund, and Dave Robin for their comments and feedback. The work on this document has been partly supported by the H2020 project SIFIS-Home (Grant agreement 952652).¶