Internet DRAFT - draft-ietf-gnap-resource-servers
draft-ietf-gnap-resource-servers
GNAP J. Richer, Ed.
Internet-Draft Bespoke Engineering
Intended status: Standards Track F. Imbault
Expires: 22 August 2024 acert.io
19 February 2024
Grant Negotiation and Authorization Protocol Resource Server Connections
draft-ietf-gnap-resource-servers-05
Abstract
GNAP defines a mechanism for delegating authorization to a piece of
software, and conveying that delegation to the software. This
extension defines methods for resource servers (RS) to connect with
authorization servers (AS) in an interoperable fashion.
Status of This Memo
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This Internet-Draft will expire on 22 August 2024.
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Copyright (c) 2024 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Access Tokens . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. General-purpose Access Token Model . . . . . . . . . . . 4
2.1.1. Value . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.2. Issuer . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.3. Audience . . . . . . . . . . . . . . . . . . . . . . 5
2.1.4. Key Binding . . . . . . . . . . . . . . . . . . . . . 6
2.1.5. Flags . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1.6. Access Rights . . . . . . . . . . . . . . . . . . . . 7
2.1.7. Time Validity Window . . . . . . . . . . . . . . . . 8
2.1.8. Token Identifier . . . . . . . . . . . . . . . . . . 8
2.1.9. Authorizing Resource Owner . . . . . . . . . . . . . 9
2.1.10. End User . . . . . . . . . . . . . . . . . . . . . . 9
2.1.11. Client Instance . . . . . . . . . . . . . . . . . . . 10
2.1.12. Label . . . . . . . . . . . . . . . . . . . . . . . . 10
2.1.13. Parent Grant Request . . . . . . . . . . . . . . . . 10
2.1.14. AS-Specific Access Tokens . . . . . . . . . . . . . . 11
2.2. Access Token Formats . . . . . . . . . . . . . . . . . . 12
3. Resource-Server-Facing API . . . . . . . . . . . . . . . . . 13
3.1. RS-facing AS Discovery . . . . . . . . . . . . . . . . . 13
3.2. Protecting RS requests to the AS . . . . . . . . . . . . 14
3.3. Token Introspection . . . . . . . . . . . . . . . . . . . 16
3.4. Registering a Resource Set . . . . . . . . . . . . . . . 20
3.5. Error Responses . . . . . . . . . . . . . . . . . . . . . 23
4. Deriving a downstream token . . . . . . . . . . . . . . . . . 24
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 26
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
6.1. Well-Known URI . . . . . . . . . . . . . . . . . . . . . 26
6.2. GNAP Grant Request Parameters . . . . . . . . . . . . . . 26
6.3. GNAP Token Formats Registry . . . . . . . . . . . . . . . 27
6.3.1. Registry Template . . . . . . . . . . . . . . . . . . 27
6.3.2. Initial Registry Contents . . . . . . . . . . . . . . 27
6.4. GNAP Token Introspection Request Registry . . . . . . . . 28
6.4.1. Registry Template . . . . . . . . . . . . . . . . . . 28
6.4.2. Initial Registry Contents . . . . . . . . . . . . . . 28
6.5. GNAP Token Introspection Response Registry . . . . . . . 29
6.5.1. Registry Template . . . . . . . . . . . . . . . . . . 29
6.5.2. Initial Registry Contents . . . . . . . . . . . . . . 29
6.6. GNAP Resource Set Registration Request Parameters . . . . 30
6.6.1. Registry Template . . . . . . . . . . . . . . . . . . 31
6.6.2. Initial Registry Contents . . . . . . . . . . . . . . 31
6.7. GNAP Resource Set Registration Response Parameters . . . 31
6.7.1. Registry Template . . . . . . . . . . . . . . . . . . 32
6.7.2. Initial Registry Contents . . . . . . . . . . . . . . 32
6.8. GNAP RS-Facing Discovery Document Fields . . . . . . . . 32
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6.8.1. Registry Template . . . . . . . . . . . . . . . . . . 32
6.8.2. Initial Registry Contents . . . . . . . . . . . . . . 33
6.9. GNAP RS-Facing Error Codes . . . . . . . . . . . . . . . 33
6.9.1. Registration Template . . . . . . . . . . . . . . . . 33
6.9.2. Initial Contents . . . . . . . . . . . . . . . . . . 34
7. Security Considerations . . . . . . . . . . . . . . . . . . . 34
7.1. TLS Protection in Transit . . . . . . . . . . . . . . . . 34
7.2. Token Validation . . . . . . . . . . . . . . . . . . . . 34
7.3. Cacheing Token Validation Result . . . . . . . . . . . . 35
7.4. Key Proof Validation . . . . . . . . . . . . . . . . . . 35
7.5. Token Exfiltration . . . . . . . . . . . . . . . . . . . 36
7.6. Token Re-Use by an RS . . . . . . . . . . . . . . . . . . 36
7.7. Token Format Considerations . . . . . . . . . . . . . . . 36
7.8. Over-sharing Token Contents . . . . . . . . . . . . . . . 37
7.9. Resource References . . . . . . . . . . . . . . . . . . . 37
7.10. Token Re-Issuance From an Untrusted AS . . . . . . . . . 37
7.11. Introspection of Token Keys . . . . . . . . . . . . . . . 38
7.12. RS Registration and Management . . . . . . . . . . . . . 38
8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 39
8.1. Token Contents . . . . . . . . . . . . . . . . . . . . . 39
8.2. Token Use Disclosure through Introspection . . . . . . . 39
8.3. Mapping a User to an AS . . . . . . . . . . . . . . . . . 40
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 40
9.1. Normative References . . . . . . . . . . . . . . . . . . 40
9.2. Informative References . . . . . . . . . . . . . . . . . 41
Appendix A. Document History . . . . . . . . . . . . . . . . . . 41
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 42
1. Introduction
The core GNAP specification ([GNAP]) defines distinct roles for the
authorization server (AS) and the resource server (RS). However, the
core specification does not define how the RS answers important
questions, such as whether a given access token is still valid or
what set of access rights the access token is approved for.
While it's possible for the AS and RS to be tightly coupled, such as
a single deployed server with a shared storage system, GNAP does not
presume or require such a tight coupling. It is increasingly common
for the AS and RS to be run and managed separately, particularly in
cases where a single AS protects multiple RS's simultaneously.
This specification defines a set of RS-facing APIs that an AS can
make available for advanced loosely-coupled deployments.
Additionally, this document defines a general-purpose model for
access tokens, which can be used in structured, formatted access
tokens or in the API. This specification also defines a method for
an RS to derive a downstream token for calling another chained RS.
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The means of the authorization server issuing the access token to the
client instance and the means of the client instance presenting the
access token to the resource server are the subject of the core GNAP
specification [GNAP].
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.
This document contains non-normative examples of partial and complete
HTTP messages, JSON structures, URLs, query components, keys, and
other elements. Some examples use a single trailing backslash \ to
indicate line wrapping for long values, as per [RFC8792]. The \
character and leading spaces on wrapped lines are not part of the
value.
Terminology specific to GNAP is defined in the terminology section of
the core specification [GNAP], and provides definitions for the
protocol roles: authorization server (AS), client, resource server
(RS), resource owner (RO), end user; as well as the protocol
elements: attribute, access token, grant, privilege, protected
resource, right, subject, subject information. The same definitions
are used in this document.
2. Access Tokens
Access tokens are a mechanism for an AS to provide a client instance
limited access to an RS. These access tokens are artifacts
representing a particular set of access rights granted to the client
instance to act on behalf of the RO. While the format of access
tokens varies in different systems (see discussion in Section 2.2),
the concept of an access token is consistent across all GNAP systems.
2.1. General-purpose Access Token Model
Access tokens represent a common set of aspects across different GNAP
deployments. This is not intended to be a universal or comprehensive
list, but instead to provide guidance to implementors when developing
data structures and associated systems across a GNAP deployment.
These data structures are communicated between the AS and RS either
by using a structured token or an API-like mechanism like token
introspection. This general-purpose data model does not assume
either approach, and in fact both can be used together to convey
different pieces of information. Where possible, mappings to
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concrete data fields in common standards understood by the RS are
provided for each item in the model.
2.1.1. Value
All access tokens have a unique value. This is the string that is
passed on the wire between parties. The AS chooses the value, which
can be structured as in Section 2.2 or unstructured. When the token
is structured, the token value also has a _format_ known to the AS
and RS, and the other items in this token model are contained within
the token's value in some fashion. When the token is unstructured,
the values are usually retrieved by the RS using a service like token
introspection described in Section 3.3.
The access token value is conveyed the value field of an access_token
response from Section 3.2 of [GNAP].
The format and content of the access token value is opaque to the
client software. While the client software needs to be able to carry
and present the access token value, the client software is never
expected nor intended to be able to understand the token value
itself.
2.1.2. Issuer
The access token is issued by the AS in a standard GNAP transaction.
The AS will often need to identify itself in order to recognize
tokens that it has issued, particularly in cases where tokens from
multiple different AS's could be presented.
This information is not usually conveyed directly to the client
instance, since the client instance should know this information
based on where it receives the token from.
In a [JWT] formatted token or a token introspection response, this
corresponds to the iss claim.
2.1.3. Audience
The access token is intended for use at one or more RS's. The AS can
identify those RS's to allow each RS to ensure that the token is not
receiving a token intended for someone else. The AS and RS have to
agree on the nature of any audience identifiers represented by the
token, but the URIs of the RS are a common pattern.
In a [JWT] formatted token or token introspection response, this
corresponds to the aud claim.
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In cases where more complex access is required, the location field of
objects in the access array can also convey audience information. In
such cases, the client instance might need to know the audience
information in order to differentiate between possible RS's to
present the token to.
2.1.4. Key Binding
Access tokens in GNAP are bound to the client instance's registered
or presented key, except in cases where the access token is a bearer
token. For all tokens bound to a key, the AS and RS need to be able
to identify which key the token is bound to, otherwise an attacker
could substitute their own key during presentation of the token. In
the case of an asymmetric algorithm, the model for the AS and RS need
only contain the public key, while the client instance will also need
to know the private key in order to present the token appropriately.
In the case of a symmetric algorithm, all parties will need to either
know or be able to derive the shared key.
The source of this key information can vary depending on circumstance
and deployment. For example, an AS could decide that all tokens
issued to a client instance are always bound to that client
instance's current key. When the key needs to be dereferenced, the
AS looks up the client instance to which the token was issued and
finds the key information there. The AS could alternatively bind
each token to a specific key that is managed separately from client
instance information. In such a case, the AS determines the key
information directly. This approach allows the client instance to
use a different key for each request, or allows the AS to issue a key
for the client instance to use with the particular token.
In all cases, the key binding also includes a proofing mechanism,
along with any parameters needed for that mechanism such as a signing
or digest algorithm. If such information is not stored, an attacker
could present a token with a seemingly-valid key using an insecure
and incorrect proofing mechanism.
This value is conveyed to the client instance in the key field of the
access_token response in Section 3.2 of [GNAP]. Since the common
case is that the token is bound to the client instance's registered
key, this field can be omitted in this case since the client will be
aware of its own key.
In a [JWT] formatted token, this corresponds to the cnf
(confirmation) claim. In a token introspection response, this
corresponds to the key claim.
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In the case of a bearer token, all parties need to know that a token
has no key bound to it, and will therefore reject any attempts to use
the bearer token with a key in an undefined way.
2.1.5. Flags
GNAP access tokens can have multiple data flags associated with them
that indicate special processing or considerations for the token.
For example, whether the token is a bearer token, or should be
expected to be durable across grant updates.
The client can request a set of flags in the access_token request in
[GNAP].
These flags are conveyed from the AS to the client in the flags field
of the access_token response in Section 3.2 of [GNAP].
For token introspection, flags are returned in the flags field of the
response.
2.1.6. Access Rights
Access tokens are tied to a limited set of access rights. These
rights specify in some detail what the token can be used for, how,
and where. The internal structure of access rights are detailed in
Section 8 of [GNAP].
The access rights associated with an access token are calculated from
the rights available to the client instance making the request, the
rights available to be approved by the RO, the rights actually
approved by the RO, and the rights corresponding to the RS in
question. The rights for a specific access token are a subset of the
overall rights in a grant request.
These rights are requested by the client instance in the access field
of the access_token request in Section 2.1 of [GNAP].
The rights associated with an issued access token are conveyed to the
client instance in the access field of the access_token response in
Section 3.2 of [GNAP].
In token introspection responses, this corresponds to the access
claim.
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2.1.7. Time Validity Window
The access token can be limited to a certain time window outside of
which it is no longer valid for use at an RS. This window can be
explicitly bounded by an expiration time and a not-before time, or it
could be calculated based on the issuance time of the token. For
example, an RS could decide that it will accept tokens for most calls
within an hour of a token's issuance, but only within five minutes of
the token's issuance for certain high-value calls.
Since access tokens could be revoked at any time for any reason
outside of a client instance's control, the client instance often
does not know or concern itself with the validity time window of an
access token. However, this information can be made available to it
using the expires_in field of an access token response in Section 3.2
of [GNAP].
The issuance time of the token is conveyed in the iat claim of a
[JWT] formatted token or a token introspection response.
The expiration time of a token, after which it is to be rejected, is
conveyed in the exp claim of a [JWT] formatted token or a token
introspection response.
The starting time of a token's validity window, before which it is to
be rejected, is conveyed in the nbf claim of a [JWT] formatted token
or a token introspection response.
2.1.8. Token Identifier
Individual access tokens often need a unique internal identifier to
allow the AS to differentiate between multiple separate tokens. This
value of the token can often be used as the identifier, but in some
cases a separate identifier is used.
This separate identifier can be conveyed in the jti claim of a [JWT]
formatted token or a token introspection response.
This identifier is not usually exposed to the client instance using
the token, since the client instance only needs to use the token by
value.
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2.1.9. Authorizing Resource Owner
Access tokens are approved on behalf of a resource owner (RO). The
identity of this RO can be used by the RS to determine exactly which
resource to access, or which kinds of access to allow. For example,
an access token used to access identity information can hold a user
identifier to allow the RS to determine which profile information to
return. The nature of this information is subject to agreement by
the AS and RS.
This corresponds to the sub claim of a [JWT] formatted token or a
token introspection response.
Detailed RO information is not returned to the client instance when
an access token is requested alone, and in many cases returning this
information to the client instance would be a privacy violation on
the part of the AS. Since the access token represents a specific
delegated access, the client instance needs only to use the token at
its target RS. Following the profile example, the client instance
does not need to know the account identifier to get specific
attributes about the account represented by the token.
GNAP does allow for the return of subject information separately from
the access token, in the form of identifiers and assertions. These
values are returned directly to the client separately from any access
tokens that are requested, though it's common that they represent the
same party.
2.1.10. End User
The end user is the party operating the client software. The client
instance can facilitate the end user interacting with the AS in order
to determine the end user's identity, gather authorization, and
provide the results of that information back to the client instance.
In many instances, the end user is the same party as the resource
owner. However, in some cases, the two roles can be fulfilled by
different people, where the RO is consulted asynchronously. The
token model should be able to reflect these kinds of situations by
representing the end user and RO separately. For example, an end
user can request a financial payment, but the RO is the holder of the
account that the payment would be withdrawn from. The RO would be
consulted for approval by the AS outside of the flow of the GNAP
request. A token in such circumstances would need to show both the
RO and end user as separate entities.
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2.1.11. Client Instance
Access tokens are issued to a specific client instance by the AS.
The identity of this instance can be used by the RS to allow specific
kinds of access, or other attributes about the access token. For
example, an AS that binds all access tokens issued to a particular
client instance to that client instance's most recent key rotation
would need to be able to look up the client instance in order to find
the key binding detail.
This corresponds to the client_id claim of a [JWT] formatted token or
the instance_id field of a token introspection response.
The client is not normally informed of this information separately,
since a client instance can usually correctly assume that it is the
client instance to which a token that it receives was issued.
2.1.12. Label
When multiple access tokens are requested or a client instance uses
token labels, the parties will need to keep track of which labels
were applied to each individual token. Since labels can be re-used
across different grant requests, the token label alone is not
sufficient to uniquely identify a given access token in a system.
However, within the context of a grant request, these labels are
required to be unique.
A client instance can request a specific label using the label field
of an access_token request in Section 2.1 of [GNAP].
The AS can inform the client instance of a token's label using the
label field of an access_token response in Section 3.2 of [GNAP].
This corresponds to the label field of a token introspection
response.
2.1.13. Parent Grant Request
All access tokens are issued in the context of a specific grant
request from a client instance. The grant request itself represents
a unique tuple of:
* The AS processing the grant request
* The client instance making the grant request
* The RO (or set of RO's) approving the grant request (or needing to
approve it)
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* The access rights granted by the RO
* The current state of the grant request, as defined in Section 1.5
of [GNAP]
The AS can use this information to tie common information to a
specific token. For instance, instead of specifying a client
instance for every issued access token for a grant, the AS can store
the client information in the grant itself and look it up by
reference from the access token.
The AS can also use this information when a grant request is updated.
For example, if the client instance asks for a new access token from
an existing grant, the AS can use this link to revoke older non-
durable access tokens that had been previously issued under the
grant.
A client instance will have its own model of an ongoing grant
request, especially if that grant request can be continued using the
API defined in Section 5 of [GNAP] where several pieces of
statefulness need to be kept in hand. The client instance might need
to keep an association with the grant request that issued the token
in case the access token expires or does not have sufficient access
rights, so that the client instance can get a new access token
without having to restart the grant request process from scratch.
Since the grant itself does not need to be identified in any of the
protocol messages, GNAP does not define a specific grant identifier
to be conveyed between any parties in the protocol. Only the AS
needs to keep an explicit connection between an issued access token
and the parent grant that issued it.
2.1.14. AS-Specific Access Tokens
When an access token is used for the grant continuation API defined
in Section 5 of [GNAP] (the continuation access token) the token
management API defined in Section 6 of [GNAP] (the token management
access token), or the RS-facing API defined in Section 3 (the
resource server management access token), the AS MUST separate these
access tokens from others usable at RS's. The AS can do this through
the use of a flag on the access token data structure, by using a
special internal access right, or any other means at its disposal.
Just like other access tokens in GNAP, the contents of these AS-
specific access tokens are opaque to the software presenting the
token. Unlike other access tokens, the contents of these AS-specific
access tokens are also opaque to the RS.
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The client instance is given continuation access tokens only as part
of the continue field of the grant response in Section 3.1 of [GNAP].
The client instance is given token management access tokens only as
part of the manage field of the grant response in Section 3.1.2 of
[GNAP]. The means by which the RS is given resource server
management access tokens is out of scope of this specification, but
methods could include pre-configuration of the token value with the
RS software or granting the access token through a standard GNAP
process.
For continuation access tokens and token management access tokens, a
client instance MUST take steps to differentiate these special-
purpose access tokens from access tokens used at RS's. To facilitate
this, a client instance can store AS-specific access tokens
separately from other access tokens in order to keep them from being
confused with each other and used at the wrong endpoints.
An RS should never see an AS-specific access token presented, so any
attempts to process one MUST fail. When introspection is used, the
AS MUST NOT return an active value of true for AS-specific access
tokens to the RS. If an AS implements its protected endpoints in
such a way as it uses token introspection internally, the AS MUST
differentiate these AS-specific access tokens from those issued for
use at an external RS.
2.2. Access Token Formats
When the AS issues an access token for use at an RS, the RS needs to
have some means of understanding what the access token is for in
order to determine how to respond to the request. The core GNAP
protocol makes neither assumptions nor demands on the format or
contents of the access token, and in fact, the token format and
contents are opaque to the client instance. However, such token
formats can be the topic of agreements between the AS and RS.
Self-contained structured token formats allow for the conveyance of
information between the AS and RS without requiring the RS to call
the AS at runtime as described in Section 3.3. Structured tokens can
also be used in combination with introspection, allowing the token
itself to carry one class of information and the introspection
response to carry another.
Some token formats, such as Macaroons [MACAROON] and Biscuits
[BISCUIT], allow for the RS to derive sub-tokens without having to
call the AS as described in Section 4.
The supported token formats can be communicated dynamically at
runtime between the AS and RS in several places.
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* The AS can declare its supported token formats as part of RS-
facing discovery Section 3.1
* The RS can require a specific token format be used to access a
registered resource set Section 3.4
* The AS can return the token's format in an introspection response
Section 3.3
In all places where the token format is listed explicitly, it MUST be
one of the registered values in the GNAP Token Formats Registry
Section 6.3.
3. Resource-Server-Facing API
To facilitate runtime and dynamic connections, the AS can offer an
RS-Facing API consisting of one or more of the following optional
pieces.
* Discovery
* Introspection
* Token chaining
* Resource reference registration
3.1. RS-facing AS Discovery
A GNAP AS offering RS-facing services can publish its features on a
well-known discovery document using the URL .well-known/gnap-as-rs
appended to the grant request endpoint URL.
The discovery response is a JSON document [RFC8259] consisting of a
single JSON object with the following fields:
grant_request_endpoint (string): The location of the AS's grant
request endpoint defined by Section 9 of [GNAP]. This URL MUST be
the same URL used by client instances in support of GNAP requests.
The RS can use this to derive downstream access tokens, if
supported by the AS. The location MUST be a URL [RFC3986] with a
scheme component that MUST be https, a host component, and
optionally, port, path and query components and no fragment
components. REQUIRED. See Section 4.
introspection_endpoint (string): The URL of the endpoint offering
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introspection. The location MUST be a URL [RFC3986] with a scheme
component that MUST be https, a host component, and optionally,
port, path and query components and no fragment components.
REQUIRED if the AS supports introspection. An absent value
indicates that the AS does not support introspection. See
Section 3.3.
token_formats_supported (array of strings): A list of token formats
supported by this AS. The values in this list MUST be registered
in the GNAP Token Format Registry in Section 6.3. OPTIONAL.
resource_registration_endpoint (string): The URL of the endpoint
offering resource registration. The location MUST be a URL
[RFC3986] with a scheme component that MUST be https, a host
component, and optionally, port, path and query components and no
fragment components. REQUIRED if the AS supports dynamic resource
registration. An absent value indicates that the AS does not
support this feature. See Section 3.4.
key_proofs_supported (array of strings) A list of the AS's supported
key proofing mechanisms. The values of this list correspond to
possible values of the proof field of the key section of the
request. Values MUST be in the GNAP Key Proofing Methods
registry. OPTIONAL.
Additional fields are defined in the GNAP RS-Facing Discovery
Document Fields registry Section 6.8.
3.2. Protecting RS requests to the AS
Unless otherwise specified, the RS MUST protect its calls to the AS
using any of the signature methods defined by GNAP. This signing
method MUST cover all of the appropriate portions of the HTTP request
message, including any body elements, tokens, or headers required for
functionality.
The RS MAY present its keys by reference or by value in a similar
fashion to a client instance calling the AS in the core protocol of
GNAP, described in [GNAP]. In the protocols defined here, this takes
the form of the resource server identifying itself using a key field
or by passing an instance identifier directly.
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POST /continue HTTP/1.1
Host: server.example.com
Authorization: GNAP 80UPRY5NM33OMUKMKSKU
Signature-Input: sig1=...
Signature: sig1=...
Content-Type: application/json
"resource_server": {
"key": {
"proof": "httpsig",
"jwk": {
"kty": "EC",
"crv": "secp256k1",
"kid": "2021-07-06T20:22:03Z",
"x": "-J9OJIZj4nmopZbQN7T8xv3sbeS5-f_vBNSy_EHnBZc",
"y": "sjrS51pLtu3P4LUTVvyAIxRfDV_be2RYpI5_f-Yjivw"
}
}
}
or by reference:
POST /continue HTTP/1.1
Host: server.example.com
Signature-Input: sig1=...
Signature: sig1=...
Content-Type: application/json
{
"resource_server": "7C7C4AZ9KHRS6X63AJAO"
}
The means by which an RS's keys are made known to the AS are out of
scope of this specification. The AS MAY require an RS to pre-
register its keys or could allow calls from arbitrary keys in a
trust-on-first-use model.
The AS MAY issue access tokens to the RS for protecting the RS-facing
API endpoints, called a resource server management access token. If
such tokens are issued, the RS MUST present them to the RS-facing API
endpoints along with the RS authentication.
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POST /continue HTTP/1.1
Host: server.example.com
Authorization: GNAP 80UPRY5NM33OMUKMKSKU
Signature-Input: sig1=...
Signature: sig1=...
Content-Type: application/json
{
"resource_server": "7C7C4AZ9KHRS6X63AJAO"
}
3.3. Token Introspection
The AS issues access tokens representing a set of delegated access
rights to be used at one or more RSs. The AS can offer an
introspection service to allow an RS to validate that a given access
token:
* has been issued by the AS
* has not expired
* has not been revoked
* is appropriate for the RS identified in the call
When the RS receives an access token, it can call the introspection
endpoint at the AS to get token information.
+--------+ +------+ +------+
| Client +--(1)->| RS | | AS |
|Instance| | +--(2)->| |
| | | | | |
| | | |<-(3)--+ |
| | | | +------+
| |<-(4)--+ |
+--------+ +------+
1. The client instance calls the RS with its access token.
2. The RS introspects the access token value at the AS. The RS
signs the request with its own key (not the client instance's key
or the token's key).
3. The AS validates the access token value and the Resource Server's
request and returns the introspection response for the token.
4. The RS fulfills the request from the client instance.
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The RS signs the request with its own key and sends the value of the
access token as the body of the request as a JSON object with the
following members:
access_token (string): REQUIRED. The access token value presented
to the RS by the client instance.
proof (string): RECOMMENDED. The proofing method used by the client
instance to bind the token to the RS request. The value MUST be
in the GNAP Key Proofing Methods registry.
resource_server (string or object): REQUIRED. The identification
used to authenticate the resource server making this call, either
by value or by reference as described in Section 3.2.
access (array of strings/objects): OPTIONAL. The minimum access
rights required to fulfill the request. This MUST be in the
format described in Section 8 of [GNAP].
Additional fields are defined in the GNAP Token Introspection Request
registry Section 6.4.
POST /introspect HTTP/1.1
Host: server.example.com
Content-Type: application/json
Signature-Input: sig1=...
Signature: sig1=...
Digest: sha256=...
{
"access_token": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
"proof": "httpsig",
"resource_server": "7C7C4AZ9KHRS6X63AJAO"
}
The AS MUST validate the access token value and determine if the
token is active. The parameters of the request provide a context for
the AS to evaluate the access token, and the AS MUST take all
provided parameters into account when evaluating if the token is
active. If the AS is unable to process part of the request, such as
not understanding part of the access field presented, the AS MUST NOT
indicate the token as active.
An active access token is defined as a token that
* was issued by the processing AS,
* has not been revoked,
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* has not expired,
* is bound using the proof method indicated,
* is appropriate for presentation at the identified RS, and
* is appropriate for the access indicated (if present),
The AS responds with a data structure describing the token's current
state and any information the RS would need to validate the token's
presentation, such as its intended proofing mechanism and key
material.
active (boolean): REQUIRED. If true, the access token presented is
active, as defined above. If any of the criteria for an active
token are not true, or if the AS is unable to make a determination
(such as the token is not found), the value is set to false and
other fields are omitted.
If the access token is active, additional fields from the single
access token response structure defined in Section 3.2.1 of [GNAP]
are included. In particular, these include the following:
access (array of strings/objects): REQUIRED. The access rights
associated with this access token. This MUST be in the format
described in the Section 8 of [GNAP]. This array MAY be filtered
or otherwise limited for consumption by the identified RS,
including being an empty array, indicating that the token has no
explicit access rights that can be disclosed to the RS.
key (object/string): REQUIRED if the token is bound. The key bound
to the access token, to allow the RS to validate the signature of
the request from the client instance. If the access token is a
bearer token, this MUST NOT be included.
flags (array of strings): OPTIONAL. The set of flags associated
with the access token.
exp (integer): OPTIONAL. The timestamp after which this token is no
longer valid. Expressed as a integer seconds from UNIX Epoch.
iat (integer): OPTIONAL. The timestamp at which this token was
issued by the AS. Expressed as a integer seconds from UNIX Epoch.
nbf (integer): OPTIONAL. The timestamp before which this token is
not valid. Expressed as a integer seconds from UNIX Epoch.
aud (string or array of strings): OPTIONAL. Identifiers for the
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resource servers this token can be accepted at.
sub (string): OPTIONAL. Identifier of the resource owner who
authorized this token.
iss (string): REQUIRED. Grant endpoint URL of the AS that issued
this token.
instance_id (string): OPTIONAL. The instance identifier of the
client instance that the token was issued to.
Additional fields are defined in the GNAP Token Introspection
Response registry Section 6.5.
The response MAY include any additional fields defined in an access
token response and MUST NOT include the access token value itself.
HTTP/1.1 200 OK
Content-Type: application/json
Cache-Control: no-store
{
"active": true,
"access": [
"dolphin-metadata", "some other thing"
],
"key": {
"proof": "httpsig",
"jwk": {
"kty": "RSA",
"e": "AQAB",
"kid": "xyz-1",
"alg": "RS256",
"n": "kOB5rR4Jv0GMeL...."
}
}
}
When processing the results of the introspection response, the RS
MUST determine the appropriate course of action. For instance, if
the RS determines that the access token's access rights are not
sufficient for the request to which the token was attached, the RS
can return an error or a public resource, as appropriate for the RS.
In all cases, the final determination of the response is at the
discretion of the RS.
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3.4. Registering a Resource Set
If the RS needs to, it can post a set of resources as described in
the Resource Access Rights section of [GNAP] to the AS's resource
registration endpoint along with information about what the RS will
need to validate the request.
access (array of objects/strings): REQUIRED. The list of access
rights associated with the request in the format described in the
"Resource Access Rights" section of [GNAP].
resource_server (string or object): REQUIRED. The identification
used to authenticate the resource server making this call, either
by value or by reference as described in Section 3.2.
token_formats_supported (array of strings): OPTIONAL. The token
formats the RS is able to process for accessing the resource. The
values in this array MUST be registered in the GNAP Token Formats
Registry in Section 6.3. If the field is omitted, the token
format is at the discretion of the AS. If the AS does not support
any of the requested token formats, the AS MUST return an error to
the RS.
token_introspection_required (boolean): OPTIONAL. If present and
set to true, the RS expects to make a token introspection request
as described in Section 3.3. If absent or set to false, the RS
does not anticipate needing to make an introspection request for
tokens relating to this resource set. If the AS does not support
token introspection for this RS, the AS MUST return an error to
the RS.
Additional fields are defined in the GNAP Resource Set Registration
Request registry Section 6.6.
The RS MUST identify itself with its own key and sign the request.
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POST /resource HTTP/1.1
Host: server.example.com
Content-Type: application/json
Signature-Input: sig1=...
Signature: sig1=...
Digest: ...
{
"access": [
{
"actions": [
"read",
"write",
"dolphin"
],
"locations": [
"https://server.example.net/",
"https://resource.local/other"
],
"datatypes": [
"metadata",
"images"
]
},
"dolphin-metadata"
],
"resource_server": "7C7C4AZ9KHRS6X63AJAO"
}
The AS responds with a reference appropriate to represent the
resources list that the RS presented in its request as well as any
additional information the RS might need in future requests.
resource_reference (string): REQUIRED. A single string representing
the list of resources registered in the request. The RS MAY make
this handle available to a client instance as part of a discovery
response as described in Section 9.1 of [GNAP] or as documentation
to client software developers.
instance_id (string): OPTIONAL. An instance identifier that the RS
can use to refer to itself in future calls to the AS, in lieu of
sending its key by value. See Section 3.2.
introspection_endpoint (string): OPTIONAL. The introspection
endpoint of this AS, used to allow the RS to perform token
introspection. See Section 3.3.
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Additional fields are defined in the GNAP Resource Set Registration
Response Registry Section 6.7.
HTTP/1.1 200 OK
Content-Type: application/json
Cache-Control: no-store
{
"resource_reference": "FWWIKYBQ6U56NL1"
}
If a resource was previously registered, the AS MAY return the same
resource reference value as in previous responses.
If the registration fails, the AS returns an HTTP 400 Bad Request
error to the RS indicating that the registration was not successful.
The client instance can then use the resource_reference value as a
string-type access reference as defined in Section 8.1 of [GNAP].
This value MAY be combined with any other additional access rights
requested by the client instance.
{
"access_token": {
"access": [
"FWWIKYBQ6U56NL1",
{
"type": "photo-api",
"actions": [
"read",
"write",
"dolphin"
],
"locations": [
"https://server.example.net/",
"https://resource.local/other"
],
"datatypes": [
"metadata",
"images"
]
},
"dolphin-metadata"
]
},
"client": "client-12351.bdxqf"
}
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3.5. Error Responses
In the case of an error from the RS-facing API, the AS responds to
the RS with an HTTP 400 (Bad Request) status code and a JSON object
consisting of a single error field, which is either an object or a
string.
When returned as a string, the error value is the error code:
{
error: "invalid_access"
}
When returned as an object, the error object contains the following
fields:
code (string): A single ASCII error code defining the error.
REQUIRED.
description (string): A human-readable string description of the
error intended for the developer of the client. OPTIONAL.
{
"error": {
"code": "invalid_access",
"description": "Access to 'foo' is not permitted for this RS."
}
}
This specification defines the following error code values:
"invalid_request": The request is missing a required parameter,
includes an invalid parameter value or is otherwise malformed.
"invalid_resource_server": The request was made from an RS that was
not recognized or allowed by the AS, or the RS's signature
validation failed.
"invalid_access" The RS is not permitted to register or introspect
for the requested "access" value.
Additional error codes can be defined in the GNAP RS-Facing Error
Codes Registry (Section 6.9).
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4. Deriving a downstream token
Some architectures require an RS to act as a client instance and use
a derived access token for a secondary RS. Since the RS is not the
same entity that made the initial grant request, the RS is not
capable of referencing or modifying the existing grant. As such, the
RS needs to request or generate a new token access token for its use
at the secondary RS. This internal secondary token is issued in the
context of the incoming access token.
While it is possible to use a token format (Section 2) that allows
for the RS to generate its own secondary token, the AS can allow the
RS to request this secondary access token using the same process used
by the original client instance to request the primary access token.
Since the RS is acting as its own client instance from the
perspective of GNAP, this process uses the same grant endpoint,
request structure, and response structure as a client instance's
request.
+--------+ +-------+ +------+ +-------+
| Client +--(1)->| RS1 | | AS | | RS2 |
|Instance| | +--(2)->| | | |
| | | |<-(3)--+ | | |
| | | | +------+ | |
| | | | | |
| | | +-----------(4)------->| |
| | | |<----------(5)--------+ |
| |<-(6)--+ | | |
+--------+ +-------+ +-------+
1. The client instance calls RS1 with an access token.
2. RS1 presents that token to the AS to get a derived token for use
at RS2. RS1 indicates that it has no ability to interact with
the RO. Note that RS1 signs its request with its own key, not
the token's key or the client instance's key.
3. The AS returns a derived token to RS1 for use at RS2.
4. RS1 calls RS2 with the token from (3).
5. RS2 fulfills the call from RS1.
6. RS1 fulfills the call from the original client instance.
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If the RS needs to derive a token from one presented to it, it can
request one from the AS by making a token request as described in
[GNAP] and presenting the existing access token's value in the
"existing_access_token" field.
Since the RS is acting as a client instance, the RS MUST identify
itself with its own key in the client field and sign the request just
as any client instance would, as described in Section 3.2. The AS
MUST determine that the token being presented is appropriate for use
at the RS making the token chaining request.
POST /tx HTTP/1.1
Host: server.example.com
Content-Type: application/json
Detached-JWS: ejy0...
{
"access_token": {
"access": [
{
"actions": [
"read",
"write",
"dolphin"
],
"locations": [
"https://server.example.net/",
"https://resource.local/other"
],
"datatypes": [
"metadata",
"images"
]
},
"dolphin-metadata"
]
},
"client": "7C7C4AZ9KHRS6X63AJAO",
"existing_access_token": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0"
}
The AS responds with a token for the downstream RS2 as described in
[GNAP]. The downstream RS2 could repeat this process as necessary
for calling further RS's.
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5. Acknowledgements
The editors would like to thank the feedback of the following
individuals for their reviews, implementations, and contributions:
Aaron Parecki, Adrian Gropper, Andrii Deinega, Annabelle Backman,
Dmitry Barinov, Fabien Imbault, Florian Helmschmidt, George Fletcher,
Justin Richer, Kathleen Moriarty, Leif Johansson, Mike Varley, Nat
Sakimura, Takahiko Kawasaki, Yaron Sheffer.
Finally, the editors want to acknowledge the immense contributions of
Aaron Parecki to the content of this document. We thank him for his
insight, input, and hard work, without which GNAP would not have
grown to what it is.
6. IANA Considerations
IANA is requested to add values to existing registries and to create
5 registries in the Grant Negotiation and Authorization Protocol
registry.
6.1. Well-Known URI
The "gnap-as-rs" URI suffix is registered into the Well-Known URIs
Registry to support RS-facing discovery of the AS.
URI Suffix: gnap-as-rs
Change Controller: IETF
Specification Document: Section 3.1 of RFC xxxx
Status: Permanent
6.2. GNAP Grant Request Parameters
The following parameter is registered into the GNAP Grant Request
Parameters registry:
Name: existing_access_token
Type: string
Specification document(s): Section 4 of RFC xxxx
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6.3. GNAP Token Formats Registry
This document defines a GNAP token format, for which IANA is asked to
create and maintain a new registry titled "GNAP Token Formats".
Initial values for this registry are given in Section 6.3.2. Future
assignments and modifications to existing assignment are to be made
through the Specification Required registration policy [RFC8126].
The Designated Expert (DE) is expected to ensure that all
registrations follow the template presented in Section 6.3.1. The DE
is expected to ensure that the format's definition is sufficiently
unique from other formats provided by existing parameters. The DE is
expected to ensure that the format's definition specifies the format
of the access token in sufficient detail to allow for the AS and RS
to be able to communicate the token information.
6.3.1. Registry Template
Name The name of the format.
Status Whether or not the format is in active use. Possible values
are Active and Deprecated.
Description Human-readable description of the access token format.
Reference The specification that defines the token format.
6.3.2. Initial Registry Contents
+===============+========+====================+============+
| Name | Status | Description | Reference |
+===============+========+====================+============+
| jwt-signed | Active | JSON Web Token, | [JWT] |
| | | signed with JWS | |
+---------------+--------+--------------------+------------+
| jwt-encrypted | Active | JSON Web Token, | [JWT] |
| | | encrypted with JWE | |
+---------------+--------+--------------------+------------+
| macaroon | Active | Macaroon | [MACAROON] |
+---------------+--------+--------------------+------------+
| biscuit | Active | Biscuit | [BISCUIT] |
+---------------+--------+--------------------+------------+
| zcap | Active | ZCAP | [ZCAPLD] |
+---------------+--------+--------------------+------------+
Table 1
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6.4. GNAP Token Introspection Request Registry
This document defines GNAP token introspection, for which IANA is
asked to create and maintain a new registry titled "GNAP Token
Introspection Request". Initial values for this registry are given
in Section 6.4.2. Future assignments and modifications to existing
assignment are to be made through the Specification Required
registration policy [RFC8126].
The Designated Expert (DE) is expected to ensure that all
registrations follow the template presented in Section 6.4.1. The DE
is expected to ensure that the claim's definition is sufficiently
orthogonal to other claims defined in the registry so as avoid
overlapping functionality. The DE is expected to ensure that the
claim's definition specifies the syntax and semantics of the claim in
sufficient detail to allow for the AS and RS to be able to
communicate the token values.
6.4.1. Registry Template
Name The name of the claim.
Type The JSON data type of the claim value.
Reference The specification that defines the token.
6.4.2. Initial Registry Contents
The table below contains the initial contents of the GNAP Token
Introspection Registry.
+=================+=================+=========================+
| Name | Type | Reference |
+=================+=================+=========================+
| access_token | string | Section 3.3 of RFC xxxx |
+-----------------+-----------------+-------------------------+
| proof | string | Section 3.3 of RFC xxxx |
+-----------------+-----------------+-------------------------+
| resource_server | object/string | Section 3.3 of RFC xxxx |
+-----------------+-----------------+-------------------------+
| access | array of | Section 3.3 of RFC xxxx |
| | strings/objects | |
+-----------------+-----------------+-------------------------+
Table 2
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6.5. GNAP Token Introspection Response Registry
This document defines GNAP token introspection, for which IANA is
asked to create and maintain a new registry titled "GNAP Token
Introspection Response". Initial values for this registry are given
in Section 6.5.2. Future assignments and modifications to existing
assignment are to be made through the Specification Required
registration policy [RFC8126].
The Designated Expert (DE) is expected to ensure that all
registrations follow the template presented in Section 6.5.1. The DE
is expected to ensure that the claim's definition is sufficiently
orthogonal to other claims defined in the registry so as avoid
overlapping functionality. The DE is expected to ensure that the
claim's definition specifies the syntax and semantics of the claim in
sufficient detail to allow for the AS and RS to be able to
communicate the token values.
6.5.1. Registry Template
Name The name of the claim.
Type The JSON data type of the claim value.
Reference The specification that defines the token.
6.5.2. Initial Registry Contents
The table below contains the initial contents of the GNAP Token
Introspection Registry.
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+=============+==========================+=========================+
| Name | Type | Reference |
+=============+==========================+=========================+
| active | boolean | Section 3.3 of RFC xxxx |
+-------------+--------------------------+-------------------------+
| access | array of strings/objects | Section 3.3 of RFC xxxx |
+-------------+--------------------------+-------------------------+
| key | object/string | Section 3.3 of RFC xxxx |
+-------------+--------------------------+-------------------------+
| flags | array of strings | Section 3.3 of RFC xxxx |
+-------------+--------------------------+-------------------------+
| exp | integer | Section 3.3 of RFC xxxx |
+-------------+--------------------------+-------------------------+
| iat | integer | Section 3.3 of RFC xxxx |
+-------------+--------------------------+-------------------------+
| nbf | integer | Section 3.3 of RFC xxxx |
+-------------+--------------------------+-------------------------+
| aud | string or array of | Section 3.3 of RFC xxxx |
| | strings | |
+-------------+--------------------------+-------------------------+
| sub | string | Section 3.3 of RFC xxxx |
+-------------+--------------------------+-------------------------+
| iss | string | Section 3.3 of RFC xxxx |
+-------------+--------------------------+-------------------------+
| instance_id | string | Section 3.3 of RFC xxxx |
+-------------+--------------------------+-------------------------+
Table 3
6.6. GNAP Resource Set Registration Request Parameters
This document defines a means to register a resource set for a GNAP
AS, for which IANA is asked to create and maintain a new registry
titled "GNAP Resource Set Registration Request Parameters". Initial
values for this registry are given in Section 6.6.2. Future
assignments and modifications to existing assignment are to be made
through the Expert Review registration policy [RFC8126].
The Designated Expert (DE) is expected to ensure that all
registrations follow the template presented in Section 6.6.1. The DE
is expected to ensure that the parameter's definition is sufficiently
orthogonal to other claims defined in the registry so as avoid
overlapping functionality. The DE is expected to ensure that the
parameter's definition specifies the syntax and semantics of the
claim in sufficient detail to allow for the AS and RS to be able to
communicate the resource set.
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6.6.1. Registry Template
Name The name of the parameter.
Type The JSON data type of the parameter value.
Reference The specification that defines the token.
6.6.2. Initial Registry Contents
The table below contains the initial contents of the GNAP Resource
Set Registration Request Parameters Registry.
+==============================+=================+=============+
| Name | Type | Reference |
+==============================+=================+=============+
| access | array of | Section 3.4 |
| | strings/objects | of RFC xxxx |
+------------------------------+-----------------+-------------+
| resource_server | string or | Section 3.4 |
| | object | of RFC xxxx |
+------------------------------+-----------------+-------------+
| token_formats_supported | string | Section 3.4 |
| | | of RFC xxxx |
+------------------------------+-----------------+-------------+
| token_introspection_required | boolean | Section 3.4 |
| | | of RFC xxxx |
+------------------------------+-----------------+-------------+
Table 4
6.7. GNAP Resource Set Registration Response Parameters
This document defines a means to register a resource set for a GNAP
AS, for which IANA is asked to create and maintain a new registry
titled "GNAP Resource Set Registration Response Parameters". Initial
values for this registry are given in Section 6.7.2. Future
assignments and modifications to existing assignment are to be made
through the Expert Review registration policy [RFC8126].
The Designated Expert (DE) is expected to ensure that all
registrations follow the template presented in Section 6.7.1. The DE
is expected to ensure that the parameter's definition is sufficiently
orthogonal to other claims defined in the registry so as avoid
overlapping functionality. The DE is expected to ensure that the
parameter's definition specifies the syntax and semantics of the
claim in sufficient detail to allow for the AS and RS to be able to
communicate the resource set.
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6.7.1. Registry Template
Name The name of the parameter.
Type The JSON data type of the parameter value.
Reference The specification that defines the token.
6.7.2. Initial Registry Contents
The table below contains the initial contents of the GNAP Resource
Set Registration Response Parameters Registry.
+========================+========+=========================+
| Name | Type | Reference |
+========================+========+=========================+
| resource_reference | string | Section 3.4 of RFC xxxx |
+------------------------+--------+-------------------------+
| instance_id | string | Section 3.4 of RFC xxxx |
+------------------------+--------+-------------------------+
| introspection_endpoint | string | Section 3.4 of RFC xxxx |
+------------------------+--------+-------------------------+
Table 5
6.8. GNAP RS-Facing Discovery Document Fields
This document defines a means to for a GNAP AS to be discovered by a
GNAP RS, for which IANA is asked to create and maintain a new
registry titled "GNAP RS-Facing Discovery Document Fields". Initial
values for this registry are given in Section 6.8.2. Future
assignments and modifications to existing assignment are to be made
through the Expert Review registration policy [RFC8126].
The Designated Expert (DE) is expected to ensure that all
registrations follow the template presented in Section 6.8.1. The DE
is expected to ensure that the claim's definition is sufficiently
orthogonal to other claims defined in the registry so as avoid
overlapping functionality. The DE is expected to ensure that the
claim's definition specifies the syntax and semantics of the claim in
sufficient detail to allow for RS to be able to communicate with the
AS.
6.8.1. Registry Template
Name The name of the parameter.
Type The JSON data type of the parameter value.
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Reference The specification that defines the token.
6.8.2. Initial Registry Contents
The table below contains the initial contents of the GNAP RS-Facing
Discovery Registry.
+================================+==========+=============+
| Name | Type | Reference |
+================================+==========+=============+
| introspection_endpoint | string | Section 3.1 |
| | | of RFC xxxx |
+--------------------------------+----------+-------------+
| token_formats_supported | array of | Section 3.1 |
| | strings | of RFC xxxx |
+--------------------------------+----------+-------------+
| resource_registration_endpoint | string | Section 3.1 |
| | | of RFC xxxx |
+--------------------------------+----------+-------------+
| grant_request_endpoint | string | Section 3.1 |
| | | of RFC xxxx |
+--------------------------------+----------+-------------+
| key_proofs_supported | array of | Section 3.1 |
| | strings | of RFC xxxx |
+--------------------------------+----------+-------------+
Table 6
6.9. GNAP RS-Facing Error Codes
This document defines a set of errors that the AS can return to the
RS, for which IANA is asked to create and maintain a new registry
titled "GNAP RS-Facing Error Codes". Initial values for this
registry are given in Section 6.9.2. Future assignments and
modifications to existing assignment are to be made through the
Specification Required registration policy [RFC8126].
The DE is expected to ensure that all registrations follow the
template presented in Section 6.9.1. The DE is expected to ensure
that the error response is sufficiently unique from other errors to
provide actionable information to the client instance. The DE is
expected to ensure that the definition of the error response
specifies all conditions in which the error response is returned, and
what the client instance's expected action is.
6.9.1. Registration Template
Error:
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A unique string code for the error.
Specification document(s):
Reference to the document(s) that specify the value, preferably
including a URI that can be used to retrieve a copy of the
document(s). An indication of the relevant sections may also be
included but is not required.
6.9.2. Initial Contents
+=========================+===========================+
| Error | Specification document(s) |
+=========================+===========================+
| invalid_request | Section 3.5 of RFC xxxx |
+-------------------------+---------------------------+
| invalid_resource_server | Section 3.5 of RFC xxxx |
+-------------------------+---------------------------+
| invalid_access | Section 3.5 of RFC xxxx |
+-------------------------+---------------------------+
Table 7
7. Security Considerations
In addition to the normative requirements in this document and in
[GNAP], implementors are strongly encouraged to consider these
additional security considerations in implementations and deployments
of GNAP.
7.1. TLS Protection in Transit
All requests in GNAP made over untrusted network connections have to
be made over TLS as outlined in [BCP195] to protect the contents of
the request and response from manipulation and interception by an
attacker. This includes all requests from a client instance to the
RS and all requests from the RS to an AS.
7.2. Token Validation
The RS has a responsibility to validate the incoming access token in
a manner consistent with its deployment. For self-contained
stateless tokens such as those described in Section 2.2, this
consists of actions such as validating the token's signature and
ensuring the relevant fields and results are appropriate for the
request being made. For reference-style tokens or tokens that are
otherwise opaque to the RS, the token introspection RS-facing API can
be used to provide updated information about the state of the token,
as described in Section 3.3.
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The RS needs to validate that a token:
* Is intended for this RS (audience restriction)
* Is presented using the appropriate key for the token (see also
Section 7.4) Subject identification (the RS knows who authorized
the token) Issuer restriction (the RS knows who created the token,
including signing a structure or providing introspection to prove
this)
Even though key proofing mechanisms have to cover the value of the
token, validating the key proofing alone is not sufficient to protect
a request to an RS. If an RS validates only the presentation method
as described in Section 7.4 without validating the token itself, an
attacker could use a compromised key or a confused deputy to make
arbitrary calls to the RS beyond what has been authorized by the RO.
7.3. Cacheing Token Validation Result
Since token validation can be an expensive process, requiring either
cryptographic operations or network calls to an introspection service
as described in Section 3.3, an RS could cache the results of token
validation for a particular token. The trade offs of using a cached
validation for a token present an important decision space for
implementors: relying on a cached validation result increases
performance and lowers processing overhead, but it comes at the
expense of the liveness and accuracy of the information in the cache.
While a cached value is in use at the RS, an attacker could present a
revoked token and have it accepted by the RS.
As with any cache, the consistency of this cache can be managed in a
variety of ways. One of the most simple methods is managing the
lifetime of the cache in order to balance the performance and
security properties. Too long of a cache, and an attacker has a
larger window in which to use a revoked token. Too short of a window
and the benefits of using the cache are diminished. It is also
possible that an AS could send a proactive signal to the RS to
invalidate revoked access tokens, though such a mechanism is outside
the scope of this specification.
7.4. Key Proof Validation
For key-bound access tokens, the proofing method needs to be
validated alongside the value of the token itself as described in
Section 7.2. The process of validation is defined by the key
proofing method, as described in Section 7.3 of [GNAP].
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If the proofing method is not validated, an attacker could use a
compromised token without access to the token's bound key.
The RS also needs to ensure that the proofing method is appropriate
for the key associated with the token, including any choice of
algorithm or identifiers.
The proofing should be validated independently on each request to the
RS, particularly as aspects of the call could vary. As such, the RS
should never cache the results of a proof validation from one message
and apply it to a subsequent message.
7.5. Token Exfiltration
Since the RS sees the token value, it is possible for a compromised
RS to leak that value to an attacker. As such, the RS needs to
protect token values as sensitive information and protect them from
exfiltration.
This is especially problematic with bearer tokens and tokens bound to
a shared key, since an RS has access to all information necessary to
create a new, valid request using the token in question.
7.6. Token Re-Use by an RS
If the access token is a bearer token, or the RS has access to the
key material needed to present the token, the RS could be tricked
into re-using an access token presented to it by a client. While it
is possible to build a system that makes use of this artifact as a
feature, it is safer to exchange the incoming access token for
another contextual token for use by the RS, as described in
Section 4. This access token can be bound to the RS's own keys and
limited to access needed by the RS, instead of the full set of rights
associated with the token issued to the client instance.
7.7. Token Format Considerations
With formatted tokens, the format of the token is likely to have its
own considerations, and the RS needs to follow any such
considerations during the token validation process. The application
and scope of these considerations is specific to the format and
outside the scope of this specification.
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7.8. Over-sharing Token Contents
The contents of the access token model divulge to the RS information
about the access token's context and rights. This is true whether
the contents are parsed from the token itself or sent in an
introspection response.
It's likely that every RS does not need to know all details of the
token model, especially in systems where a single access token is
usable across multiple RS's. An attacker could use this to gain
information about the larger system by compromising only one RS. By
limiting the information available to only that which is relevant to
a specific RS, such as using a limited introspection reply as defined
in Section 3.3, a system can follow a principle of least disclosure
to each RS.
7.9. Resource References
Resource references, as returned by the protocol in Section 3.4, are
intended to be opaque to both the RS and the client. However, since
they are under the control of the AS, the AS can put whatever content
it wants into the reference value. This value could unintentionally
disclose system structure or other internal details if it processed
by an unintended party. Furthermore, such patterns could lead to the
client software and RS depending on certain structures being present
in the reference value, which diminishes the separation of concerns
of the different roles in a GNAP system.
To mitigate this, the AS should only use fully random or encrypted
values for resource references.
7.10. Token Re-Issuance From an Untrusted AS
It is possible for an attacker's client instance to issue its own
tokens to another client instance, acting as an AS that the second
client instance has chosen to trust. If the token is a bearer token
or the re-issuance is bound using an AS-provided key, the target
client instance will not be able to tell that the token was
originally issued by the valid AS. This process allows an attacker
to insert their own session and rights into an unsuspecting client
instance, in the guise of a token valid for the attacker that appears
to have been issued to the target client instance on behalf of its
own RO.
This attack is predicated on a misconfiguration with the targeted
client, as it has been configured to get tokens from the attacker's
AS and use those tokens with the target RS, which has no association
with the attacker's AS. However, since the token is ultimately
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coming from the trusted AS, and is being presented with a valid key,
the RS has no way of telling that the token was passed through an
intermediary.
To mitigate this, the RS can publish its association with the trusted
AS through either discovery or documentation. Therefore, a client
properly following this association would only go directly to the
trusted RS directly for access tokens for the RS.
Furthermore, limiting the use of bearer tokens and AS-provided keys
to only highly trusted AS's and limited circumstances prevents the
attacker from being able to willingly exfiltrate their token to an
unsuspecting client instance.
7.11. Introspection of Token Keys
The introspection response defined in Section 3.3 provides a means
for the AS to tell the RS the key material needed to validate the key
proof of the request. Capture of the introspection response can
expose these security keys to an attacker. In the case of asymmetric
cryptography, only the public key is exposed, and the token cannot be
re-used by the attacker based on this result alone. This could
potentially divulge information about the client instance that was
unknown otherwise.
If an access token is bound to a symmetric key, the RS will need
access to the full key value in order to validate the key proof of
the request, as described in Section 7.4. However, divulging the key
material to the RS also gives the RS the ability to create a new
request with the token. In this circumstance, the RS is under
similar risk of token exfiltration and re-use as a bearer token, as
described in Section 7.6. Consequently, symmetric keys should only
be used in systems where the RS can be fully trusted to not create a
new request with tokens presented to it.
7.12. RS Registration and Management
Most functions of the RS-facing API in Section 3 are protected by
requiring the RS to present proof of a signing key along with the
request, in order to identify the RS making the call, potentially
coupled with an AS-specific access token. This practice allows the
AS to differentially respond to API calls to different RS's, such as
answering introspection calls with only the access rights relevant to
a given RS instead of all access rights an access token could be good
for.
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While the means by which an RS and its keys become known to the AS is
out of scope for this specification, it is anticipated that common
practice will be to statically register an RS, allowing it to protect
specific resources or certain classes of resources. Fundamentally,
the RS can only offer the resources that it serves. However, a rogue
AS could attempt to register a set of resources that mimics a
different RS in order to solicit an access token usable at the target
RS. If the access token is a bearer token or is bound to a symmetric
key that is known to the RS, then the attacker's RS gains the ability
and knowledge needed to use that token elsewhere.
In some ecosystems, dynamic registration of an RS and its associated
resources is feasible. In such systems, the identity of the RS could
be conveyed by a URI passed in the location field of an access rights
request, thereby allowing the AS to limit the view the RS has into
the larger system.
8. Privacy Considerations
8.1. Token Contents
The contents of the access token could potentially contain personal
information about the end-user, RO, or other parties. This is true
whether the contents are parsed from the token itself or sent in an
introspection response.
While an RS will sometimes need this information for processing, it's
often the case that an RS is exposed to these details only in
passing, and not intentionally. For example, disclosure of a medical
record number in the contents of an access token usable for both
medial and non-medical APIs.
To mitigate this, the a limited token introspection response can be
used, as defined in Section 3.3.
8.2. Token Use Disclosure through Introspection
When introspection is used by an RS, the AS is made aware of a
particular token being used at a particular RS. When the RS is a
separate system, the AS would not otherwise have insight into this
action. This can potentially lead to the AS learning about patterns
and actions of particular end users by watching which RS's are
accessed and when.
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8.3. Mapping a User to an AS
When the client instance receives information about the protecting AS
from an RS, this can be used to derive information about the
resources being protected without releasing the resources themselves.
For example, if a medical record is protected by a personal AS, an
untrusted client could call an RS to discover the location of the AS
protecting the record. Since the AS is tied strongly to a single RO,
the untrusted and unauthorized client software can gain information
about the resource being protected without accessing the record
itself.
9. References
9.1. Normative References
[BCP195] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", May 2015,
<https://www.rfc-editor.org/info/bcp195>.
[GNAP] Richer, J. and F. Imbault, "Grant Negotiation and
Authorization Protocol", Work in Progress, Internet-Draft,
draft-ietf-gnap-core-protocol-18, 10 February 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-gnap-
core-protocol-18>.
[JWT] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<https://www.rfc-editor.org/rfc/rfc7519>.
[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/rfc/rfc2119>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/rfc/rfc3986>.
[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/rfc/rfc8174>.
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[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/rfc/rfc8259>.
[RFC8792] Watsen, K., Auerswald, E., Farrel, A., and Q. Wu,
"Handling Long Lines in Content of Internet-Drafts and
RFCs", RFC 8792, DOI 10.17487/RFC8792, June 2020,
<https://www.rfc-editor.org/rfc/rfc8792>.
9.2. Informative References
[BISCUIT] "Biscuit Authorization", n.d.,
<https://www.biscuitsec.org/>.
[MACAROON] "Macaroons: Cookies with Contextual Caveats for
Decentralized Authorization in the Cloud", 2014,
<https://research.google/pubs/pub41892/>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/rfc/rfc8126>.
[ZCAPLD] "Authorization Capabilities for Linked Data", 2023,
<https://w3c-ccg.github.io/zcap-spec/>.
Appendix A. Document History
* -05
- Added discussion of access tokens used to call the RS-facing AS
APIs.
- Updated IANA sections to align with core (and each other).
- Added IANA section on introspection requests.
- Added error responses.
- Added extended discussion on resource server registration
practices.
* -04
- Editorial cleanup.
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- Updated IANA requirements, including "specification required"
registration.
- Added privacy and security considerations.
- Clarified and expanded token introspection request and
response.
- Clarified and expanded resource set registration request and
response, include example of use of resource reference.
- Updated discovery.
- Allow optional tokens on RS-facing API requests.
- Tighter controls over derived tokens.
* -03
- Added token model.
- Added IANA sections.
* -02
- Editorial and formatting fixes.
* -01
- Better described RS authentication.
- Added access token format registry.
- Filled out introspection protocol.
- Filled out resource registration protocol.
- Expanded RS-facing discovery mechanisms.
- Moved client-facing RS response back to GNAP core document.
* -00
- Extracted resource server section.
Authors' Addresses
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Justin Richer (editor)
Bespoke Engineering
Email: ietf@justin.richer.org
URI: https://bspk.io/
Fabien Imbault
acert.io
Email: fabien.imbault@acert.io
URI: https://acert.io/
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