ACE Working Group M. Tiloca
Internet-Draft RISE AB
Intended status: Standards Track L. Seitz
Expires: May 6, 2021 Combitech
F. Palombini
Ericsson AB
S. Echeverria
G. Lewis
CMU SEI
November 02, 2020
Notification of Revoked Access Tokens in the Authentication and
Authorization for Constrained Environments (ACE) Framework
draft-tiloca-ace-revoked-token-notification-03
Abstract
This document specifies a method of the Authentication and
Authorization for Constrained Environments (ACE) framework, which
allows an Authorization Server to notify Clients and Resource Servers
(i.e., registered devices) about revoked Access Tokens. The method
relies on resource observation for the Constrained Application
Protocol (CoAP), with Clients and Resource Servers observing a Token
Revocation List on the Authorization Server. Resulting unsolicited
notifications of revoked Access Tokens complement alternative
approaches such as token introspection, while not requiring
additional endpoints on Clients and Resource Servers.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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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 May 6, 2021.
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Copyright Notice
Copyright (c) 2020 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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 5
3. Token Hash . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. The TRL Resource . . . . . . . . . . . . . . . . . . . . . . 8
4.1. Update of the TRL Resource . . . . . . . . . . . . . . . 9
5. The TRL Endpoint . . . . . . . . . . . . . . . . . . . . . . 9
5.1. Full Query of the TRL . . . . . . . . . . . . . . . . . . 10
5.2. Diff Query of the TRL . . . . . . . . . . . . . . . . . . 10
6. Upon Registration . . . . . . . . . . . . . . . . . . . . . . 12
7. Notification of Revoked Tokens . . . . . . . . . . . . . . . 13
8. Interaction Examples . . . . . . . . . . . . . . . . . . . . 13
8.1. Full Query with Observation . . . . . . . . . . . . . . . 14
8.2. Diff Query with Observation . . . . . . . . . . . . . . . 15
8.3. Full Query with Observation and Additional Diff Query . . 17
9. Security Considerations . . . . . . . . . . . . . . . . . . . 20
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
11.1. Normative References . . . . . . . . . . . . . . . . . . 21
11.2. Informative References . . . . . . . . . . . . . . . . . 22
Appendix A. Usage of the Series Transfer Pattern . . . . . . . . 22
Appendix B. Usage of the "Cursor" Pattern . . . . . . . . . . . 23
B.1. Full Query Request . . . . . . . . . . . . . . . . . . . 24
B.2. Full Query Response . . . . . . . . . . . . . . . . . . . 24
B.3. Diff Query Request . . . . . . . . . . . . . . . . . . . 24
B.4. Diff Query Response . . . . . . . . . . . . . . . . . . . 25
B.4.1. Empty Collection . . . . . . . . . . . . . . . . . . 25
B.4.2. Cursor Not Specified in the Diff Query Request . . . 25
B.4.3. Cursor Specified in the Diff Query Request . . . . . 26
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28
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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 and stores it.
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 may
not 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 observe a Token Revocation List (TRL) resource on the
Authorization Server, in order to get an updated list of revoked, but
yet not expired, pertaining Access Tokens. In particular, registered
devices rely on resource observation [RFC7641] for the Constrained
Application Protocol (CoAP) [RFC7252]. The benefits of this method
are that it complements token introspection and does not require any
additional endpoints on the registered devices.
1.1. 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.
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Readers are expected to be familiar with the terms and concepts
described in the ACE framework for Authentication and Authorization
[I-D.ietf-ace-oauth-authz], as well as with terms and concepts
related to CBOR Web Tokens (CWTs) [RFC8392], and JSON Web Tokens
(JWTs) [RFC7519]. The terminology for entities in the considered
architecture is defined in OAuth 2.0 [RFC6749]. In particular, this
includes Client, Resource Server, and Authorization Server.
Readers are also expected to be familiar with the terms and concepts
related to CBOR [I-D.ietf-cbor-7049bis], JSON [RFC8259], the CoAP
protocol [RFC7252], CoAP Observe [RFC7641], and the use of hash
functions to name objects as defined in [RFC6920].
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 Authorization Server, and /authz-
info at the Resource Server. This document does not use the CoAP
definition of "endpoint", which is "An entity participating in the
CoAP protocol."
This specification also refers to the following terminology.
o Token hash: identifier of an Access Token, in binary format
encoding. The token hash has no relation to other possibly used
token identifiers, such as the "cti" (CWT ID) claim of CBOR Web
Tokens (CWTs) [RFC8392].
o Token Revocation List (TRL): a collection of token hashes, in
which the corresponding Access Tokens have been revoked but are
not expired yet.
o TRL resource: a resource on the Authorization Server, with a TRL
as its representation.
o TRL endpoint: an endpoint at the Authorization Server associated
to the TRL resource. The default name of the TRL endpoint in a
url-path is '/revoke/trl'. Implementations are not required to
use this name, and can define their own instead.
o Registered device: a device registered at the Authorization
Server, i.e. as a Client, or a Resource Server, or both. A
registered device acts as caller of the TRL endpoint.
o Administrator: entity authorized to get full access to the TRL at
the Authorization Server, and acting as caller of the TRL
endpoint. An administrator is not necessarily a registered device
as defined above, i.e. a Client requesting Access Tokens or a
Resource Server consuming Access Tokens. How the administrator
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authorization is established and verified is out of the scope of
this specification.
o Pertaining Access Token:
* With reference to an administrator, an Access Token issued by
the Authorization Server.
* With reference to a registered device, an Access Token intended
to be owned by that device. An Access Token pertains to a
Client if the Authorization Server has issued the Access Token
and provided it to that Client. An Access Token pertains to a
Resource Server if the Authorization Server has issued the
Access Token to be consumed by that Resource Server.
2. Protocol Overview
This protocol defines how a CoAP-based Authorization Server informs
Clients and Resource Servers, i.e. registered devices, about revoked
Access Tokens. How the relationship between the 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.
o Upon startup, the Authorization Server creates a 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
yet not expired.
o When a device registers at the Authorization Server, it receives
the url-path to the TRL resource. After the registration
procedure is finished, the registered device sends an Observation
Request to that TRL resource as described in [RFC7641], i.e. a GET
request with an Observe option set to 0 (register). 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, in order to get the current list of pertaining
revoked Access Tokens.
o 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 described in [RFC7641]. That is, one Observe
Notification is sent to each registered device the Access Token
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pertains to, and specifies the current updated list of token
hashes in the portion of the TRL pertaining to that device.
o An administrator can observe and access the TRL like a registered
device, while getting the full updated representation of the TRL.
Figure 1 provides a high-level overview of the service provided by
this protocol. Each dotted line associated to a pair of registered
devices indicates the Access Token that they both own. In
particular, Figure 1 shows the Observe Notifications sent by the
Authorization Server to four registered devices and one
administrator, upon revocation of the issued Access Tokens t1, t2 and
t3, with token hash th1, th2 and th3, respectively.
+---------------+
| |
| 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 :
:...........................................:
Figure 1: Protocol Overview
More detailed examples describing the protocol flow and message
exchange between the Authorization Server and a registered device are
provided in Section 8.
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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 5.6.2 of [I-D.ietf-ace-oauth-authz]), where the
Access Token was included and returned 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 6).
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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)
}
Figure 2: Example of Authorization Server response using CBOR
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)
}
Figure 3: Example of Authorization Server response using JSON
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 token hash, encoded as a CBOR byte
string. 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.
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4.1. Update of the TRL Resource
The Authorization Server updates the TRL in the following two cases.
o When a non-expired Access Token is revoked, the token hash of the
Access Token is added to the TRL resource representation. That
is, a CBOR byte string encoding the token hash is added to the
CBOR array used as TRL resource representation.
o When a revoked Access Token expires, the token hash of the Access
Token is removed from the TRL resource representation. That is,
the CBOR byte string encoding the token hash is removed from the
CBOR array used as TRL resource representation.
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 provides two types
of query of the TRL.
o Full query: the Authorization Server returns the token hashes of
the revoked Access Tokens currently in the TRL and pertaining to
the requester. The processing of a full query and the related
response format are defined in Section 5.1.
o Diff query: the Authorization Server returns a list of diff
entries. Each entry is related to one of the most recent updates,
in the portion of the TRL pertaining to the requester. In
particular, the entry associated to 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 5.2.
The TRL endpoint allows the following query parameter in a GET
request.
o 'diff': if included, it indicates to perform a diff query of the
TRL. Its value MUST be either: i) 0, indicating that a
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(notification) response should include as many diff entries as the
Authorization Server can provide; or ii) a positive integer
greater than 0, indicating the maximum number of diff entries that
a (notification) response should include.
5.1. 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 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.
5.2. 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' of 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
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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 encoding the token hash of an
Access Token, that pertained to the requester and that was
removed from the TRL during the update associated to the diff
entry.
* The second element is a CBOR array 'added'. Each element of
the array is a CBOR byte string encoding the token hash of an
Access Token, that pertains to the requester and that was
added to the TRL during the update associated to 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' of U elements as payload.
Each element of the array specifies one of the CBOR arrays 'diff-
entry' prepared at point 2 as diff entries.
Within the CBOR array 'diff', 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' relates to the most recent update to the
portion of the TRL pertaining to the requester.
The CDDL definition [RFC8610] of the CBOR array 'diff' formatted as
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 = [* diff-entry]
Figure 4: CDDL definition of the response payload following a Diff
Query request to the TRL endpoint
If the Authorization Server supports diff queries:
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o 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.
o 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.
o The Authorization Server SHOULD provide registered devices and
administrators with the value of N_MAX, upon their registration
(see Section 6).
If the Authorization Server does not support diff queries, it
proceeds as when processing a full query (see Section 5.1).
Appendix A discusses how the 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 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].
6. 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.
o The url-path to the TRL endpoint at the Authorization Server.
o 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.
o Optionally, a positive integer N_MAX, if the Authorization Server
supports diff queries of the TRL resource (see Section 5.2).
After the registration procedure is finished, the administrator or
registered device performs 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
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wish for a full query (see Section 5.1) or a diff query (see
Section 5.2) of the TRL.
The Authorization Server MUST reply using the CoAP response code 2.05
(Content) and including the CoAP Observe option in the response. The
payload of the response is formatted as defined in Section 5.1 or in
Section 5.2, in case the GET request was for a full query or a diff
query of the TRL, respectively.
7. Notification of Revoked Tokens
In the case the TRL is updated (see Section 4.1), the Authorization
Server sends Observe Notifications to every observer of the TRL
resource. Observe Notifications are sent as per Section 4.2 of
[RFC7641].
The payload of each Observe Notification is formatted as defined in
Section 5.1 or in Section 5.2, in case the original Observation
Request was for 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 5.1) or a diff query (see Section 5.2).
8. Interaction Examples
This section provides examples of interactions between a Resource
Server RS as registered device and an Authorization Server AS. The
Authorization Server supports both full query and diff query of the
TRL, as defined in Section 5.1 and Section 5.2, 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:
o a "trl" parameter, specifying the path of the TRL resource;
o a "trl-hash" parameter, specifying the hash function used to
computed token hashes as defined in Section 3;
o an "n-max" parameter, specifying the value of N_MAX, i.e. the
maximum number of TRL updates pertaining to each registered device
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that the Authorization Server retains for that device (see
Section 5.2);
o 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.
8.1. Full Query with Observation
Figure 5 shows an example interaction between the Resource Server RS
and the Authorization Server AS, considering a CoAP observation and a
full query of the TRL.
RS AS
| |
| Registration: POST |
+-------------------------------------->|
| |
|<--------------------------------------+
| 2.01 CREATED |
| Payload: { |
| ... |
| "trl" = "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) |
| . |
| . |
| . |
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| |
| (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: [] |
| |
Figure 5: Interaction for Full Query with Observation
8.2. Diff Query with Observation
Figure 6 shows an example interaction between the Resource Server RS
and the Authorization Server AS, considering a CoAP observation and a
diff query of the TRL.
The Resource Server indicates N=3 as value of the query parameter
"diff", i.e. as the maximum number of diff entries to be specified in
a response from the Authorization Server.
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RS AS
| |
| Registration: POST |
+--------------------------------------------->|
| |
|<---------------------------------------------+
| 2.01 CREATED |
| Payload: { |
| ... |
| "trl" = "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) |
| |
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| |
|<---------------------------------------------+
| 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)] ] |
| ] |
| |
Figure 6: Interaction for Diff Query with Observation
8.3. Full Query with Observation and Additional Diff Query
Figure 7 shows an example interaction between the Resource Server RS
and the Authorization Server AS, considering a CoAP observation and a
full query of the TRL.
The example also considers one of the notifications from the
Authorization Server to get lost in transmission, and thus not
reaching the Resource Server.
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When this happens, and after a waiting time defined by the
application has elapsed, the Resource Server sends a GET request with
no observation to the Authorization Server, to perform a diff query
of the TRL.
In particular, the Resource Server indicates N=8 as value of the
query parameter "diff", i.e. as the maximum number of diff entries to
be specified in a response from the Authorization Server.
RS AS
| |
| Registration: POST |
+--------------------------------------------->|
| |
|<---------------------------------------------+
| 2.01 CREATED |
| Payload: { |
| ... |
| "trl" = "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)] |
| . |
| . |
| . |
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| |
| (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)] ] |
| ] |
| |
Figure 7: Interaction for Full Query with Observation and Diff Query
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9. 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 regards 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
to 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 5.1). This could be exploited by attackers
to negatively affect the behavior of a registered device. Short
expiration times 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 an application policy.
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10. IANA Considerations
This document has no actions for IANA.
11. References
11.1. Normative References
[I-D.ietf-ace-oauth-authz]
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)", draft-ietf-ace-oauth-authz-35
(work in progress), June 2020.
[I-D.ietf-cbor-7049bis]
Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", draft-ietf-cbor-7049bis-16 (work
in progress), September 2020.
[Named.Information.Hash.Algorithm]
IANA, "Named Information Hash Algorithm",
<https://www.iana.org/assignments/named-information/named-
information.xhtml>.
[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>.
[RFC6920] Farrell, S., Kutscher, D., Dannewitz, C., Ohlman, B.,
Keranen, A., and P. Hallam-Baker, "Naming Things with
Hashes", RFC 6920, DOI 10.17487/RFC6920, April 2013,
<https://www.rfc-editor.org/info/rfc6920>.
[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>.
[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<https://www.rfc-editor.org/info/rfc7519>.
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[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015,
<https://www.rfc-editor.org/info/rfc7641>.
[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>.
[RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
"CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
May 2018, <https://www.rfc-editor.org/info/rfc8392>.
11.2. Informative References
[I-D.bormann-t2trg-stp]
Bormann, C. and K. Hartke, "The Series Transfer Pattern
(STP)", draft-bormann-t2trg-stp-03 (work in progress),
April 2020.
[RFC7009] Lodderstedt, T., Ed., Dronia, S., and M. Scurtescu, "OAuth
2.0 Token Revocation", RFC 7009, DOI 10.17487/RFC7009,
August 2013, <https://www.rfc-editor.org/info/rfc7009>.
[RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
Definition Language (CDDL): A Notational Convention to
Express Concise Binary Object Representation (CBOR) and
JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
June 2019, <https://www.rfc-editor.org/info/rfc8610>.
Appendix A. Usage of the Series Transfer Pattern
This section discusses how the diff query of the TRL defined in
Section 5.2 is a usage example of the Series Transfer Pattern defined
in [I-D.bormann-t2trg-stp].
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,
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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 to the registered device.
When responding to a diff query request from a registered device (see
Section 5.2), 'diff' is a subset of the collection associated to the
requester, where each 'diff_entry' record is a series item from that
collection. Note that 'diff' 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 'diff' query parameter in the diff query 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 to 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
Building on Appendix A, this section describes how the diff query of
the TRL defined in Section 5.2 can be further improved by using the
"Cursor" pattern of the Series Transfer Pattern (see Section 3.3 of
[I-D.bormann-t2trg-stp]).
This has two benefits. First, the Authorization Server can avoid
excessively big latencies when several diff entries have to be
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transferred, by delivering one adjacent subset at the time, in
different diff query responses. Second, a requester can retrieve
diff entries associated to TRL updates that, even if not the most
recent ones, occurred after a TRL update indicated as checkpoint.
To this end, each series item in an update collection is also
associated with an unsigned integer 'index', with value the absolute
counter of series items added to that collection 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.
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.
Finally, the full query and diff query exchanges defined in
Section 5.1 and Section 5.2 are extended as follows.
B.1. Full Query Request
No changes apply to what defined in Section 5.1.
B.2. Full Query Response
The response to a full query request (see Section 5.1) includes the
CBOR array of token hashes as well as a parameter 'cursor', encoded
either as a CBOR unsigned integer or the CBOR simple value Null.
The 'cursor' parameter specifies the value Null if there are
currently no updates pertinent to the requester, i.e. the update
collection for that requester is empty. This is the case from when
the requester registers at the Authorization Server until a first
update pertaining that requester occurs to the TRL.
Otherwise, the 'cursor' parameter takes the value of 'index' for the
last series item in the collection, as corresponding to the most
recent update pertaining to the requester occurred to the TRL.
B.3. Diff Query Request
In addition to the query parameter 'diff' (see Section 5.2), the
requester can specify a query parameter 'cursor', with value an
unsigned integer.
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B.4. Diff Query Response
When receiving the diff query request, the Authorization Server
proceeds as follows.
B.4.1. Empty Collection
If the collection associated to the requester has no elements, the
Authorization Server returns a diff query response that contains:
o The 'diff' parameter, encoding an empty CBOR array.
o A 'cursor' parameter, encoding the CBOR simple value Null.
o A 'more' parameter, encoding the CBOR simple value False.
B.4.2. Cursor Not Specified in the Diff Query Request
If the update collection associated to the requester is not empty and
the diff query request does not include the query parameter 'cursor',
the Authorization Server returns a diff query response that contains:
o The 'diff' CBOR array, including L = min(U, MAX_DIFF_BATCH) diff
entries. In particular:
* If U <= MAX_DIFF_BATCH, these diff entries are the last series
items in the collection associated to the requester,
corresponding to the L most recent TRL updates pertaining to
the requester.
* If U > MAX_DIFF_BATCH, these diff entries are the eldest of the
last L series items in the collection associated to the
requester, as corresponding to the first L of the U most recent
TRL updates pertaining to the requester.
o A 'cursor' parameter, encoded as a CBOR unsigned integer. This
takes the 'index' value of the series element of the collection
included as first diff entry in the 'diff' CBOR array. That is,
it takes the 'index' value of the series item in the collection
corresponding to the most recent update pertaining to the
requester and returned in this diff query response.
Note that 'cursor' takes the same 'index' value of the last series
item in the collection when U <= MAX_DIFF_BATCH.
o A 'more' parameter, encoded as the CBOR simple value False if U <=
MAX_DIFF_BATCH, or as the CBOR simple value True otherwise.
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If 'more' has value True, the requester can send a follow-up diff
query request including the query parameter 'cursor', with the
same value of the 'cursor' parameter included in this response.
This would result in the Authorization Server transfering the
following subset of series items as diff entries, i.e. resuming
from where interrupted in the previous transfer.
B.4.3. Cursor Specified in the Diff Query Request
If the update collection associated to the requester is not empty and
the diff query request includes the query parameter 'cursor' with
value P, the Authorization Server proceeds as follows.
o If no series item X with 'index' having value P is found in the
collection associated to the requester, then that item has been
previously removed from the history of updates for that requester
(see Appendix A). In this case, the Authorization Server returns
a diff query response that contains:
* The 'diff' parameter, encoding an empty CBOR array.
* A 'cursor' parameter, encoding the CBOR simple value Null.
* A 'more' parameter, encoding the CBOR simple value True.
With the combination ('cursor', 'more') = (Null, True), the
Authorization Server is signaling that the update collection is in
fact not empty, but that some series items have been lost due to
their removal, including the item with 'index' value P that the
requester wished to use as checkpoint.
When receiving this diff query response, the requester should send
a new full query request to the Authorization Server, in order to
fully retrieve the current pertaining portion of the TRL.
o If the series item X with 'index' having value P is found in the
collection associated to the requester, the Authorization Server
returns a diff query response that contains:
* The 'diff' CBOR array, including L = min(SUB_U, MAX_DIFF_BATCH)
diff entries, where SUB_U = min(NUM, SUB_SIZE), and SUB_SIZE is
the number of series items in the collection following the
series item X.
That is, these are the L updates pertaining to the requester
that immediately follow the series item X indicated as
checkpoint. In particular:
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+ If SUB_U <= MAX_DIFF_BATCH, these diff entries are the last
series items in the collection associated to the requester,
corresponding to the L most recent TRL updates pertaining to
the requester.
+ If SUB_U > MAX_DIFF_BATCH, these diff entries are the eldest
of the last L series items in the collection associated to
the requester, corresponding to the first L of the SUB_U
most recent TRL updates pertaining to the requester.
* A 'cursor' parameter, encoded as a CBOR unsigned integer. If L
is equal to 0, i.e. the series item X is the last one in the
collection, 'cursor' takes the same 'index' value of the last
series item in the collection. Otherwise, 'cursor' takes the
'index' value of the series element of the collection included
as first diff entry in the 'diff' CBOR array. That is, it
takes the 'index' value of the series item in the collection
corresponding to the most recent update pertaining to the
requester and returned in this diff query response.
Note that 'cursor' takes the same 'index' value of the last
series item in the collection when SUB_U <= MAX_DIFF_BATCH.
* A 'more' parameter, encoded as the CBOR simple value False if
SUB_U <= MAX_DIFF_BATCH, or as the CBOR simple value True
otherwise.
If 'more' has value True, the requester can send a follow-up
diff query request including the query parameter 'cursor', with
the same value of the 'cursor' parameter specified in this diff
query response. This would result in the Authorization Server
transfering the following subset of series items as diff
entries, i.e. resuming from where interrupted in the previous
transfer.
Acknowledgments
The authors sincerely thank Carsten Bormann, Benjamin Kaduk, 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).
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Authors' Addresses
Marco Tiloca
RISE AB
Isafjordsgatan 22
Kista SE-16440 Stockholm
Sweden
Email: marco.tiloca@ri.se
Ludwig Seitz
Combitech
Djaeknegatan 31
Malmoe SE-21135 Malmoe
Sweden
Email: ludwig.seitz@combitech.se
Francesca Palombini
Ericsson AB
Torshamnsgatan 23
Kista SE-16440 Stockholm
Sweden
Email: francesca.palombini@ericsson.com
Sebastian Echeverria
CMU SEI
4500 Fifth Avenue
Pittsburgh, PA 15213-2612
United States of America
Email: secheverria@sei.cmu.edu
Grace Lewis
CMU SEI
4500 Fifth Avenue
Pittsburgh, PA 15213-2612
United States of America
Email: glewis@sei.cmu.edu
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