Notification of Revoked Access Tokens in the Authentication and Authorization for Constrained Environments (ACE) Framework

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Table of Contents

1. Introduction

Authentication and Authorization for Constrained Environments (ACE) [I-D.ietf-ace-oauth-authz] is a framework that enforces access control on IoT devices acting as Resource Servers. In order to use ACE, both Clients and Resource Servers have to register with an Authorization Server and become a registered device. Once registered, a Client can send a request to the Authorization Server, to obtain an Access Token for a Resource Server. For a Client to access the Resource Server, the Client must present the issued Access Token at the Resource Server, which then validates it before storing it (see Section 5.10.1.1 of [I-D.ietf-ace-oauth-authz]).¶

Even though Access Tokens have expiration times, there are circumstances by which an Access Token may need to be revoked before its expiration time, such as: (1) a registered device has been compromised, or is suspected of being compromised; (2) a registered device is decommissioned; (3) there has been a change in the ACE profile for a registered device; (4) there has been a change in access policies for a registered device; and (5) there has been a change in the outcome of policy evaluation for a registered device (e.g., if policy assessment depends on dynamic conditions in the execution environment, the user context, or the resource utilization).¶

As discussed in Section 6.1 of [I-D.ietf-ace-oauth-authz], only client-initiated revocation is currently specified [RFC7009] for OAuth 2.0 [RFC6749], based on the assumption that Access Tokens in OAuth are issued with a relatively short lifetime. However, this is not expected to be the case for constrained, intermittently connected devices, that need Access Tokens with relatively long lifetimes.¶

This document specifies a method for allowing registered devices to access and possibly subscribe to a Token Revocation List (TRL) resource on the Authorization Server, in order to obtain an updated list of revoked, but yet not expired, pertaining Access Tokens. In particular, registered devices can subscribe to the TRL at the Authorization Server by using resource observation [RFC7641] for the Constrained Application Protocol (CoAP) [RFC7252].¶

Unlike in the case of token introspection (see Section 5.9 of [I-D.ietf-ace-oauth-authz]), a registered device does not provide an owned Access Token to the Authorization Server for inquiring about its current state. Instead, registered devices simply obtain an updated list of revoked, but yet not expired, pertaining Access Tokens, as efficiently identified by corresponding hash values.¶

The benefits of this method are that it complements token introspection, and it does not require any additional endpoints on the registered devices. The only additional requirements for registered devices are a request/response interaction with the Authorization Server to access and possibly subscribe to the TRL (see Section 2), and the lightweight computation of hash values to use as Token identifiers (see Section 3).¶

2. Protocol Overview

This protocol defines how a CoAP-based Authorization Server informs Clients and Resource Servers, i.e., registered devices, about pertaining revoked Access Tokens. How the relationship between a registered device and the Authorization Server is established is out of the scope of this specification.¶

At a high level, the steps of this protocol are as follows.¶

• Upon startup, the Authorization Server creates a single TRL resource. At any point in time, the TRL resource represents the list of all revoked Access Tokens issued by the Authorization Server that are not expired yet.¶

• When a device registers at the Authorization Server, it also receives the url-path to the TRL resource.¶

  After the registration procedure is finished, the registered device can send an Observation Request to the TRL resource as described in [RFC7641], i.e., a GET request with an Observe option set to 0 (register). By doing so, the registered device effectively subscribes to the TRL resource, as interested to receive notifications about its update. Upon receiving the request, the Authorization Server adds the registered device to the list of observers of the TRL resource.¶

  At any time, the registered device can send a GET request to the TRL endpoint. When doing so, it can request for: the current list of pertaining revoked Access Tokens (see Section 6); or the most recent TRL updates occurred over the list of pertaining revoked Access Tokens (see Section 7). In either case, the registered device may especially rely on an Observation Request for subscribing to the TRL resource as discussed above.¶

• When an Access Token is revoked, the Authorization Server adds the corresponding token hash to the TRL. Also, when a revoked Access Token eventually expires, the Authorization Server removes the corresponding token hash from the TRL.¶

  In either case, after updating the TRL, the Authorization Server sends Observe Notifications as per [RFC7641]. That is, an Observe Notification is sent to each registered device subscribed to the TRL resource and to which the Access Token pertains.¶

  Depending on the specific subscription established through the observation request, the notification provides the current updated list of revoked Access Tokens in the portion of the TRL pertaining to that device (see Section 6), or rather the most recent TRL updates occurred over that list of pertaining revoked Access Tokens (see Section 7).¶

  Further Observe Notifications may be sent, consistently with ongoing additional observations of the TRL resource.¶

• An administrator can access and subscribe to the TRL like a registered device, while getting the full updated representation of the TRL.¶

Figure 1 shows a high-level overview of the service provided by this protocol. In particular, it shows the Observe Notifications sent by the Authorization Server to one administrator and four registered devices, upon revocation of the issued Access Tokens t1, t2 and t3, with token hash th1, th2 and th3, respectively. Each dotted line associated with a pair of registered devices indicates the Access Token that they both own.¶

                    +----------------------+
                    | Authorization Server |
                    +-----------o----------+
                    revoke/trl  |  TRL: {th1,th2,th3}
                                |
 +-----------------+------------+------------+------------+
 |                 |            |            |            |
 | th1,th2,th3     | th1,th2    | th1        | th3        | th2,th3
 v                 v            v            v            v
+---------------+ +----------+ +----------+ +----------+ +----------+
| Administrator | | Client 1 | | Resource | | Client 2 | | Resource |
|               | |          | | Server 1 | |          | | Server 2 |
+---------------+ +----------+ +----------+ +----------+ +----------+
                     :    :        :           :            :    :
                     :    :   t1   :           :     t3     :    :
                     :    :........:           :............:    :
                     :                   t2                      :
                     :...........................................:

Section 10 provides examples of the protocol flow and message exchange between the Authorization Server and a registered device.¶

3. Token Hash

The token hash of an Access Token is computed as follows.¶

1. The Authorization Server defines ENCODED_TOKEN, as the content of the 'access_token' parameter in the Authorization Server response (see Section 5.8.2 of [I-D.ietf-ace-oauth-authz]), where the Access Token was included and provided to the requesting Client.¶

   Note that the content of the 'access_token' parameter is either:¶

   • A CBOR byte string, if the Access Token was transported using CBOR. With reference to the example in Figure 2, and assuming the string's length in bytes to be 119 (i.e., 0x77 in hexadecimal), then ENCODED_TOKEN takes the bytes {0x58 0x77 0xd0 0x83 0x44 0xa1 ...}, i.e., the raw content of the parameter 'access_token'.¶
   • A text string, if the Access Token was transported using JSON. With reference to the example in Figure 3, ENCODED_TOKEN takes "2YotnFZFEjr1zCsicMWpAA", i.e., the raw content of the parameter 'access_token'.¶

2. The Authorization Server defines HASH_INPUT as follows.¶

   • If CBOR was used to transport the Access Token (as a CWT or JWT), HASH_INPUT takes the same value of ENCODED_TOKEN.¶

   • If JSON was used to transport the Access Token (as a CWT or JWT), HASH_INPUT takes the serialization of ENCODED_TOKEN.¶

     In either case, HASH_INPUT results in the binary representation of the content of the 'access_token' parameter from the Authorization Server response.¶

3. The Authorization Server generates a hash value of HASH_INPUT as per Section 6 of [RFC6920]. The resulting output in binary format is used as the token hash. Note that the used binary format embeds the identifier of the used hash function, in the first byte of the computed token hash.¶

   The specifically used hash function MUST be collision-resistant on byte-strings, and MUST be selected from the "Named Information Hash Algorithm" Registry [Named.Information.Hash.Algorithm].¶

   The Authorization Server specifies the used hash function to registered devices during their registration procedure (see Section 8).¶

2.01 Created
Content-Format: application/ace+cbor
Max-Age: 85800
Payload:
{
   access_token : h'd08344a1...'
   (remainder of the Access Token omitted for brevity)
   token_type : pop,
   expires_in : 86400,
   profile    : coap_dtls,
   (remainder of the response omitted for brevity)
}

HTTP/1.1 200 OK
Content-Type: application/json
Cache-Control: no-store
Pragma: no-cache
Payload:
{
   "access_token" : "2YotnFZFEjr1zCsicMWpAA"
   (remainder of the Access Token omitted for brevity)
   "token_type" : "pop",
   "expires_in" : 86400,
   "profile"    : "coap_dtls",
   (remainder of the response omitted for brevity)
}

4. The TRL Resource

Upon startup, the Authorization Server creates a single TRL resource, encoded as a CBOR array.¶

Each element of the array is a CBOR byte string, with value the token hash of an Access Token. The order of the token hashes in the CBOR array is irrelevant, and the CBOR array MUST be treated as a set in which the order has no significant meaning.¶

The TRL is initialized as empty, i.e., the initial content of the TRL resource representation MUST be an empty CBOR array.¶

5. The TRL Endpoint

Consistent with Section 6.5 of [I-D.ietf-ace-oauth-authz], all communications between a caller of the TRL endpoint and the Authorization Server MUST be encrypted, as well as integrity and replay protected. Furthermore, responses from the Authorization Server to the caller MUST be bound to the caller's request.¶

The Authorization Server MUST implement measures to prevent access to the TRL endpoint by entities other than registered devices and authorized administrators.¶

The TRL endpoint supports only the GET method, and allows two types of query of the TRL.¶

• Full query: the Authorization Server returns the token hashes of the revoked Access Tokens currently in the TRL and pertaining to the requester. The Authorization Server MUST support this type of query. The processing of a full query and the related response format are defined in Section 6.¶

• Diff query: the Authorization Server returns a list of diff entries. Each diff entry is related to one of the most recent updates, in the portion of the TRL pertaining to the requester. The Authorization Server MAY support this type of query.¶

  The entry associated with one of such updates contains a list of token hashes, such that: i) the corresponding revoked Access Tokens pertain to the requester; and ii) they were added to or removed from the TRL at that update. The processing of a diff query and the related response format are defined in Section 7.¶

The TRL endpoint allows the following query parameters in a GET request. The Authorization Server MUST silently ignore unknown query parameters.¶

• 'pmax': if included, it follows the semantics defined in Section 3.2.2 of [I-D.ietf-core-conditional-attributes]. This query parameter is relevant only in case the GET request is specifically an Observation Request, i.e., if it includes the CoAP Observe option set to 0 (register). In such a case, this parameter indicates the maximum time, in seconds, between two consecutive notifications for the observation in question, regardless whether the TRL resource has changed or not.¶

  If the Observation Request does not include the 'pmax' parameter, the maximum time to consider is up to the Authorization Server. If the Observation Request includes the 'pmax' parameter, its value MUST be greater than zero, otherwise the Authorization Server MUST return a 4.00 (Bad Request) response.¶

  If the GET request is not an Observation Request, the Authorization Server MUST ignore the 'pmax' parameter, in case this is included.¶

• 'diff': if included, it indicates to perform a diff query of the TRL (see Section 7). Its value MUST be either:¶

  • the integer 0, indicating that a (notification) response should include as many diff entries as the Authorization Server can provide in the response; or¶
  • a positive integer greater than 0, indicating the maximum number of diff entries that a (notification) response should include.¶

  If the Authorization Server does not support diff queries, it ignores the query parameter 'diff' when present in the GET request and proceeds like when processing a full query of the TRL (see Section 6).¶

6. Full Query of the TRL

In order to produce a (notification) response to a GET request asking for a full query of the TRL, the Authorization Server performs the following actions.¶

1. From the current TRL resource representation, the Authorization Server builds a set HASHES, such that:¶

   • If the requester is a registered device, HASHES specifies the token hashes of the Access Tokens pertaining to that registered device. The Authorization Server can use the authenticated identity of the registered device to perform the necessary filtering on the TRL resource representation.¶
   • If the requester is an administrator, HASHES specifies all the token hashes in the current TRL resource representation.¶

2. The Authorization Server sends a 2.05 (Content) Response to the requester, with a CBOR array 'full-set' as payload. Each element of the array specifies one of the token hashes from the set HASHES, encoded as a CBOR byte string.¶

   The order of the token hashes in the CBOR array is irrelevant, i.e., the CBOR array MUST be treated as a set in which the order has no significant meaning.¶

The CDDL definition [RFC8610] of the CBOR array 'full-set' specified as payload in the response from the Authorization Server is provided below.¶

   token-hash = bytes
   full-set = [* token-hash]

7. Diff Query of the TRL

In order to produce a (notification) response to a GET request asking for a diff query of the TRL, the Authorization Server performs the following actions.¶

1. The Authorization Server defines the positive integer NUM. If the value N specified in the query parameter 'diff' in the GET request is equal to 0 or greater than a pre-defined positive integer N_MAX, then NUM takes the value of N_MAX. Otherwise, NUM takes N.¶

2. The Authorization Server prepares U = min(NUM, SIZE) diff entries, where SIZE <= N_MAX is the number of TRL updates pertaining to the requester and currently stored at the Authorization Server. That is, the diff entries are related to the U most recent TRL updates pertaining to the requester. In particular, the first entry refers to the most recent of such updates, the second entry refers to the second from last of such updates, and so on.¶

   Each diff entry is a CBOR array 'diff-entry', which includes the following two elements.¶

   • The first element is a CBOR array 'removed'. Each element of the array is a CBOR byte string, with value the token hash of an Access Token such that: it pertained to the requester; and it was removed from the TRL during the update associated with the diff entry.¶
   • The second element is a CBOR array 'added'. Each element of the array is a CBOR byte string, with value the token hash of an Access Token such that: it pertains to the requester; and it was added to the TRL during the update associated with the diff entry.¶

   The order of the token hashes in the CBOR arrays 'removed' and 'added' is irrelevant. That is, the CBOR arrays 'removed' and 'added' MUST be treated as a set in which the order of elements has no significant meaning.¶

3. The Authorization Server prepares a 2.05 (Content) Response for the requester, with a CBOR array 'diff-set' of U elements as payload. Each element of the CBOR array 'diff-set' specifies one of the CBOR arrays 'diff-entry' prepared at step 2 as diff entries.¶

   Within the CBOR array 'diff-set', the CBOR arrays 'diff-entry' MUST be sorted to reflect the corresponding updates to the TRL in reverse chronological order. That is, the first 'diff-entry' element of 'diff-set' relates to the most recent update to the portion of the TRL pertaining to the requester. The second 'diff-entry' element of 'diff-set' relates to the second from last most recent update to that portion, and so on.¶

The CDDL definition [RFC8610] of the CBOR array 'diff-set' specified as payload in the response from the Authorization Server is provided below.¶

   token-hash = bytes
   trl-patch = [* token-hash]
   diff-entry = [removed: trl-patch, added: trl-patch]
   diff-set = [* diff-entry]

If the Authorization Server supports diff queries:¶

• The Authorization Server MUST return a 4.00 (Bad Request) response in case the query parameter 'diff' of the GET request specifies a value other than 0 or than a positive integer.¶

  The response MUST have Content-Format "application/ace-trl+cbor". The payload of the response is a CBOR map, which MUST include the 'error' field with value 0 ("Invalid parameter value") and MAY include the 'error_description' field to provide additional context.¶

• The Authorization Server MUST keep track of N_MAX most recent updates to the portion of the TRL that pertains to each caller of the TRL endpoint. The particular method to achieve this is implementation-specific.¶
• When SIZE is equal to N_MAX, and a new TRL update occurs as pertaining to a registered device, the Authorization Server MUST first delete the oldest stored update for that device, before storing this latest update as the most recent one for that device.¶
• The Authorization Server SHOULD provide registered devices and administrators with the value of N_MAX, upon their registration (see Section 8).¶

Appendix A discusses how performing a diff query of the TRL is in fact a usage example of the Series Transfer Pattern defined in [I-D.bormann-t2trg-stp].¶

Appendix B discusses how the execution of a diff query of the TRL can be further improved by using the "Cursor" pattern defined in Section 3.3 of [I-D.bormann-t2trg-stp].¶

8. Upon Registration

During the registration process at the Authorization Server, an administrator or a registered device receives the following information as part of the registration response.¶

• The url-path to the TRL endpoint at the Authorization Server.¶
• The hash function used to compute token hashes. This is specified as an integer or a text string, taking value from the "ID" or "Hash Name String" column of the "Named Information Hash Algorithm" Registry [Named.Information.Hash.Algorithm], respectively.¶
• Optionally, a positive integer N_MAX, if the Authorization Server supports diff queries of the TRL resource (see Section 7).¶

After the registration procedure is finished, the administrator or registered device can send a GET request to the TRL resource, including the CoAP Observe option set to 0 (register), in order to start an observation of the TRL resource at the Authorization Server as per Section 3.1 of [RFC7641]. The GET request can express the wish for a full query (see Section 6) or a diff query (see Section 7) of the TRL.¶

In case the request is successfully processed, the Authorization Server replies with a response specifying the CoAP response code 2.05 (Content) and including the CoAP Observe option. The payload of the response is formatted as defined in Section 6 or in Section 7, in case the GET yielded the execution of a full query or a diff query of the TRL, respectively.¶

Further details about the registration process at the Authorization Server are out of scope for this specification. Note that the registration process is also out of the scope of the ACE framework for Authentication and Authorization (see Section 5.5 of [I-D.ietf-ace-oauth-authz]).¶

9. Notification of Revoked Tokens

When the TRL is updated (see Section 4.1), the Authorization Server sends Observe Notifications to the observers of the TRL resource. Observe Notifications are sent as per Section 4.2 of [RFC7641].¶

If the 'pmax' query parameter was specified in the Observation Request starting an observation (see Section 5), the Authorization Server might accordingly send additional Observe Notifications to the associated observer. That is, the Authorization Server ensures that no more than pmax seconds elapse between two consecutive notifications sent to that observer, regardless whether the TRL resource has changed or not. If the 'pmax' query parameter was not specified in the Observation Request, a possible maximum time to consider is up to the Authorization Server.¶

The payload of each Observe Notification is formatted as defined in Section 6 or in Section 7, in case the original Observation Request yielded the execution of a full query or a diff query of the TRL, respectively.¶

Furthermore, an administrator or a registered device can send additional GET requests to the TRL endpoint at any time, in order to retrieve the token hashes of the pertaining revoked Access Tokens. When doing so, the caller of the TRL endpoint can perform a full query (see Section 6) or a diff query (see Section 7).¶

When receiving a response from the TRL endpoint, a registered device MUST expunge every stored Access Token associated with a token hash specified in the response.¶

When a Resource Server RS receives a response from the TRL endpoint specifying the token hash H associated with a certain revoked Access Token, the RS might not have received and stored that Access Token yet. This occurs if the Access Token is revoked before it is successfully posted to the Authorization Information Endpoint at the RS (see Section 5.10.1 of [I-D.ietf-ace-oauth-authz]). Such a delay can be due, for example, to messages that get lost in transmission, or rather to the Client experiencing failures in sending or deliberately holding the Access Token back.¶

In such a case, the RS performs the following actions.¶

• The RS MUST store the token hash H, until gaining knowledge that the associated revoked Access Token is also expired.¶

  This can happen when receiving a subsequent response from the TRL endpoint (i.e., indicating that the token hash is not in the TRL portion pertaining to the RS anymore), or when the Access Token is posted to the Authorization Information Endpoint and is found to be expired based on its 'exp' claim [RFC7519], if included.¶

• The RS MUST NOT accept as valid and store an Access Token posted to the Authorization Information Endpoint, if the corresponding token hash H is among the stored ones.¶

10. Interaction Examples

This section provides examples of interactions between a Resource Server RS as a registered device and an Authorization Server AS. The Authorization Server supports both full query and diff query of the TRL, as defined in Section 6 and Section 7, respectively.¶

The details of the registration process are omitted, but it is assumed that the Resource Server sends an unspecified payload to the Authorization Server, which replies with a 2.01 (Created) response.¶

The payload of the registration response is a CBOR map, which includes the following entries:¶

• a "trl_path" parameter, specifying the path of the TRL resource;¶
• a "trl_hash" parameter, specifying the hash function used to computed token hashes as defined in Section 3;¶
• an "n_max" parameter, specifying the value of N_MAX, i.e., the maximum number of TRL updates pertaining to each registered device that the Authorization Server retains for that device (see Section 7);¶
• possible further parameters related to the registration process.¶

Furthermore, 'h(x)' refers to the hash function used to compute the token hashes, as defined in Section 3 of this specification and according to [RFC6920]. Assuming the usage of CWTs transported in CBOR, 'bstr.h(t1)' and 'bstr.h(t2)' denote the byte-string representations of the token hashes for the Access Tokens t1 and t2, respectively.¶

RS                                     AS
|                                       |
| Registration: POST                    |
+-------------------------------------->|
|                                       |
|<--------------------------------------+
|        2.01 CREATED                   |
|         Payload: {                    |
|          ...                          |
|            "trl_path" = "revoke/trl", |
|            "trl_hash" = "sha-256",    |
|            "n_max" = 10               |
|         }                             |
|                                       |
| GET Observe: 0                        |
|  coap://example.as.com/revoke/trl/    |
+-------------------------------------->|
|                                       |
|<--------------------------------------+
|              2.05 CONTENT Observe: 42 |
|               Payload: []             |
|                   .                   |
|                   .                   |
|                   .                   |
|                                       |
|    (Access Tokens t1 and t2 issued    |
|   and successfully submitted to RS)   |
|                   .                   |
|                   .                   |
|                   .                   |
|                                       |
|                                       |
|     (Access Token t1 is revoked)      |
|                                       |
|<--------------------------------------+
|              2.05 CONTENT Observe: 53 |
|               Payload: [bstr.h(t1)]   |
|                   .                   |
|                   .                   |
|                   .                   |
|                                       |
|     (Access Token t2 is revoked)      |
|                                       |
|<--------------------------------------+
|              2.05 CONTENT Observe: 64 |
|               Payload: [bstr.h(t1),   |
|                         bstr.h(t2)]   |
|                   .                   |
|                   .                   |
|                   .                   |
|                                       |
|       (Access Token t1 expires)       |
|                                       |
|<--------------------------------------+
|              2.05 CONTENT Observe: 75 |
|               Payload: [bstr.h(t2)]   |
|                   .                   |
|                   .                   |
|                   .                   |
|                                       |
|       (Access Token t2 expires)       |
|                                       |
|<--------------------------------------+
|              2.05 CONTENT Observe: 86 |
|               Payload: []             |
|                                       |

RS                                            AS
|                                              |
| Registration: POST                           |
+--------------------------------------------->|
|                                              |
|<---------------------------------------------+
|               2.01 CREATED                   |
|                Payload: {                    |
|                   ...                        |
|                   "trl_path" = "revoke/trl", |
|                   "trl_hash" = "sha-256",    |
|                   "n_max" = 10               |
|                }                             |
|                                              |
| GET Observe: 0                               |
|  coap://example.as.com/revoke/trl?diff=3     |
+--------------------------------------------->|
|                                              |
|<---------------------------------------------+
|                     2.05 CONTENT Observe: 42 |
|                      Payload: []             |
|                        .                     |
|                        .                     |
|                        .                     |
|                                              |
|         (Access Tokens t1 and t2 issued      |
|         and successfully submitted to RS)    |
|                        .                     |
|                        .                     |
|                        .                     |
|                                              |
|          (Access Token t1 is revoked)        |
|                                              |
|<---------------------------------------------+
|            2.05 CONTENT Observe: 53          |
|             Payload: [                       |
|                        [ [], [bstr.h(t1)] ]  |
|                      ]                       |
|                                              |
|                                              |
|                        .                     |
|                        .                     |
|                        .                     |
|                                              |
|          (Access Token t2 is revoked)        |
|                                              |
|<---------------------------------------------+
|            2.05 CONTENT Observe: 64          |
|             Payload: [                       |
|                        [ [], [bstr.h(t2)] ], |
|                        [ [], [bstr.h(t1)] ]  |
|                      ]                       |
|                        .                     |
|                        .                     |
|                        .                     |
|                                              |
|          (Access Token t1 expires)           |
|                                              |
|<---------------------------------------------+
|            2.05 CONTENT Observe: 75          |
|             Payload: [                       |
|                        [ [bstr.h(t1)], [] ], |
|                        [ [], [bstr.h(t2)] ], |
|                        [ [], [bstr.h(t1)] ]  |
|                      ]                       |
|                        .                     |
|                        .                     |
|                        .                     |
|                                              |
|          (Access Token t2 expires)           |
|                                              |
|<---------------------------------------------+
|            2.05 CONTENT Observe: 86          |
|             Payload: [                       |
|                        [ [bstr.h(t2)], [] ], |
|                        [ [bstr.h(t1)], [] ], |
|                        [ [], [bstr.h(t2)] ]  |
|                      ]                       |
|                                              |

RS                                            AS
|                                              |
| Registration: POST                           |
+--------------------------------------------->|
|                                              |
|<---------------------------------------------+
|               2.01 CREATED                   |
|                Payload: {                    |
|                   ...                        |
|                   "trl_path" = "revoke/trl", |
|                   "trl_hash" = "sha-256",    |
|                   "n_max" = 10               |
|                }                             |
|                                              |
| GET Observe: 0                               |
|  coap://example.as.com/revoke/trl/           |
+--------------------------------------------->|
|                                              |
|<---------------------------------------------+
|                     2.05 CONTENT Observe: 42 |
|                      Payload: []             |
|                       .                      |
|                       .                      |
|                       .                      |
|                                              |
|      (Access Tokens t1 and t2 issued         |
|      and successfully submitted to RS)       |
|                       .                      |
|                       .                      |
|                       .                      |
|                                              |
|         (Access Token t1 is revoked)         |
|                                              |
|<---------------------------------------------+
|                     2.05 CONTENT Observe: 53 |
|                      Payload: [bstr.h(t1)]   |
|                                              |
|                       .                      |
|                       .                      |
|                       .                      |
|                                              |
|         (Access Token t2 is revoked)         |
|                                              |
|<---------------------------------------------+
|                     2.05 CONTENT Observe: 64 |
|                      Payload: [bstr.h(t1),   |
|                                bstr.h(t2)]   |
|                       .                      |
|                       .                      |
|                       .                      |
|                                              |
|         (Access Token t1 expires)            |
|                                              |
|<---------------------------------------------+
|                     2.05 CONTENT Observe: 75 |
|                      Payload: [bstr.h(t2)]   |
|                        .                     |
|                        .                     |
|                        .                     |
|                                              |
|         (Access Token t2 expires)            |
|                                              |
|       X<-------------------------------------+
|                     2.05 CONTENT Observe: 86 |
|                      Payload: []             |
|                        .                     |
|                        .                     |
|                        .                     |
|                                              |
|       (Enough time has passed since          |
|       the latest received notification)      |
|                                              |
| GET                                          |
|  coap://example.as.com/revoke/trl?diff=8     |
+--------------------------------------------->|
|                                              |
|<---------------------------------------------+
|            2.05 CONTENT                      |
|             Payload: [                       |
|                        [ [bstr.h(t2)], [] ], |
|                        [ [bstr.h(t1)], [] ], |
|                        [ [], [bstr.h(t2)] ], |
|                        [ [], [bstr.h(t1)] ]  |
|                      ]                       |
|                                              |

11. ACE Token Revocation List Parameters

This specification defines a number of parameters that can be transported in the response from the TRL endpoint, when the response payload is encoded as a CBOR map. Note that such a response MUST use the Content-Format "application/ace-trl+cbor" defined in Section 14.2 of this specification.¶

The table below summarizes them, and specifies the CBOR value to use as abbreviation instead of the full descriptive name.¶

+-------------------+------------+-----------------------+
| Name              | CBOR Value | CBOR Type             |
+-------------------+------------+-----------------------+
| full-set          | TBD        | array                 |
|                   |            |                       |
+-------------------+------------+-----------------------+
| cursor            | TBD        | simple value "null" / |
|                   |            | unsigned integer      |
+-------------------+------------+-----------------------+
| diff-set          | TBD        | array                 |
|                   |            |                       |
+-------------------+------------+-----------------------+
| more              | TBD        | simple value "true"   |
|                   |            | or "false"            |
+-------------------+------------+-----------------------+
| error             | TBD        | int                   |
|                   |            |                       |
+-------------------+------------+-----------------------+
| error_description | TBD        | tstr                  |
|                   |            |                       |
+-------------------+------------+-----------------------+

12. ACE Token Revocation List Error Identifiers

This specification defines a number of values that the Authorization Server can include as error identifiers, in the 'error' field of an error response from the TRL endpoint. This applies to error responses whose payload is encoded as a CBOR map and whose Content-Format is "application/ace-trl+cbor".¶

+-------+---------------------------+
| Value | Description               |
+-------+---------------------------+
|   0   | Invalid parameter value   |
+-------+---------------------------+
|   1   | Invalid set of parameters |
+-------+---------------------------+
|   2   | Out of bound cursor value |
+-------+---------------------------+

13. Security Considerations

Security considerations are inherited from the ACE framework for Authentication and Authorization [I-D.ietf-ace-oauth-authz], from [RFC8392] as to the usage of CWTs, from [RFC7519] as to the usage of JWTs, from [RFC7641] as to the usage of CoAP Observe, and from [RFC6920] with regard to resource naming through hashes. The following considerations also apply.¶

The Authorization Server MUST ensure that each registered device can access and retrieve only its pertaining portion of the TRL. To this end, the Authorization Server can perform the required filtering based on the authenticated identity of the registered device, i.e., a (non-public) identifier that the Authorization Server can securely relate to the registered device and the secure association they use to communicate.¶

Disclosing any information about revoked Access Tokens to entities other than the intended registered devices may result in privacy concerns. Therefore, the Authorization Server MUST ensure that, other than registered devices accessing their own pertaining portion of the TRL, only authorized and authenticated administrators can retrieve the full TRL. To this end, the Authorization Server may rely on an access control list or similar.¶

If a registered device has many non-expired Access Tokens associated with itself that are revoked, the pertaining portion of the TRL could grow to a size bigger than what the registered device is prepared to handle upon reception, especially if relying on a full query of the TRL resource (see Section 6). This could be exploited by attackers to negatively affect the behavior of a registered device. Issuing Access Tokens with not too long expiration time could help reduce the size of a TRL, but an Authorization Server SHOULD take measures to limit this size.¶

Most of the communication about revoked Access Tokens presented in this specification relies on CoAP Observe Notifications sent from the Authorization Server to a registered device. The suppression of those notifications by an external attacker that has access to the network would prevent registered devices from ever knowing that their pertaining Access Tokens have been revoked. To avoid this, a registered device SHOULD NOT rely solely on the CoAP Observe notifications. In particular, a registered device SHOULD also regularly poll the Authorization Server for the most current information about revoked Access Tokens, by sending GET requests to the TRL endpoint according to a related application policy.¶

14. IANA Considerations

RFC EDITOR: Throughout this section, please replace [this document] with the RFC number of this specification and remove this note.¶

This document has the following actions for IANA.¶

Appendix A. On using the Series Transfer Pattern

Performing a diff query of the TRL as specified in Section 7 is a usage example of the Series Transfer Pattern defined in [I-D.bormann-t2trg-stp].¶

That is, a diff query enables the transfer of a series of TRL updates, with the Authorization Server specifying U <= N_MAX diff entries as the U most recent updates to the portion of the TRL pertaining to a registered device.¶

For each registered device, the Authorization Server maintains an update collection of maximum N_MAX items. Each time the TRL changes, the Authorization Server performs the following operations for each registered device.¶

1. The Authorization Server considers the portion of the TRL pertaining to that registered device. If the TRL portion is not affected by this TRL update, the Authorization Server stops the processing for that registered device.¶
2. Otherwise, the Authorization Server creates two sets 'trl_patch' of token hashes, i.e., one "removed" set and one "added" set, as related to this TRL update.¶
3. The Authorization Server fills the two sets with the token hashes of the removed and added Access Tokens, respectively, from/to the TRL portion from step 1.¶
4. The Authorization Server creates a new series item including the two sets from step 3, and adds the series item to the update collection associated with the registered device.¶

When responding to a diff query request from a registered device (see Section 7), 'diff-set' is a subset of the collection associated with the requester, where each 'diff_entry' record is a series item from that collection. Note that 'diff-set' specifies the whole current collection when the value of U is equal to SIZE, i.e., the current number of series items in the collection.¶

The value N of the query parameter 'diff' in the GET request allows the requester and the Authorization Server to trade the amount of provided information with the latency of the information transfer.¶

Since the collection associated with each registered device includes up to N_MAX series item, the Authorization Server deletes the oldest series item when a new one is generated and added to the end of the collection, due to a new TRL update pertaining to that registered device. This addresses the question "When can the server decide to no longer retain older items?" in Section 3.2 of [I-D.bormann-t2trg-stp].¶

Appendix B. Usage of the "Cursor" Pattern

This section defines how the execution of a diff query of the TRL specified in Section 7 can be extended, by using the "Cursor" pattern of the Series Transfer Pattern (see Section 3.3 of [I-D.bormann-t2trg-stp]).¶

[ TODO¶

Merge what is defined below into the document body.¶

• What is defined below for "Full Query Response" becomes part of the full query processing in the document body. The successful response payload is a CBOR map if the AS supports both the "Diff Query" mode and the "Cursor" pattern, or just the CBOR array full-set otherwise.¶
• The diff-query processing in the document body becomes as defined below for "Diff Query Request" and "Diff Query Response". The successful response payload is a CBOR map if the AS supports both the "Diff Query" mode and the "Cursor" pattern, or just the CBOR array diff-set otherwise.¶
• An example using also the "Cursor" pattern can be added in "Interaction Examples".¶

]¶

This has two benefits. First, the Authorization Server can avoid excessively big latencies when several diff entries have to be transferred, by delivering one adjacent subset at the time, in different diff query responses. Second, a requester can retrieve diff entries associated with TRL updates that, even if not the most recent ones, occurred after a TRL update indicated as reference point.¶

To this end, each series item X in an update collection is also associated with an unsigned integer 'index', with value the absolute counter of series items added to that collection until and including X, minus 1. That is, the first series item added to a collection has 'index' with value 0. Then, the values of 'index' are used as cursor information.¶

Within an update collection, the unsigned integer LAST_INDEX denotes the value of 'index' associated with the latest added series item, i.e., the total number of series items added to the collection minus 1.¶

Furthermore, the Authorization Server defines an unsigned integer MAX_DIFF_BATCH <= N_MAX, specifying the maximum number of diff entries to be included in a single diff query response. If supporting diff queries, the Authorization Server SHOULD provide registered devices and administrators with the value of MAX_DIFF_BATCH, upon their registration (see Section 8).¶

Finally, the full query and diff query exchanges defined in Section 6 and Section 7 are extended as follows.¶

In particular, successful responses from the TRL endpoint MUST use the Content-Format "application/ace-trl+cbor" defined in Section 14.2 of this specification.¶

Appendix C. Document Updates

RFC EDITOR: Please remove this section.¶

Acknowledgments

The authors sincerely thank Carsten Bormann, Benjamin Kaduk, Marco Rasori, Michael Richardson, Jim Schaad, Goeran Selander and Travis Spencer for their comments and feedback.¶

The work on this document has been partly supported by VINNOVA and the Celtic-Next project CRITISEC; and by the H2020 project SIFIS-Home (Grant agreement 952652).¶

Authors' Addresses