| OAuth Working Group | J. Richer |
| Internet-Draft | The MITRE Corporation |
| Intended status: Standards Track | M. Jones |
| Expires: February 6, 2015 | Microsoft |
| J. Bradley | |
| Ping Identity | |
| M. Machulak | |
| Newcastle University | |
| P. Hunt | |
| Oracle Corporation | |
| August 5, 2014 |
OAuth 2.0 Dynamic Client Registration Protocol
draft-ietf-oauth-dyn-reg-19
This specification defines mechanisms for dynamically registering OAuth 2.0 clients with authorization servers. Registration requests send a set of desired client metadata values to the authorization server and the resulting registration responses return a client identifier to use at the authorization server and the client metadata values registered for the client. The client can then use this registration information to communicate with the authorization server using the OAuth 2.0 protocol. This specification also defines a set of common client metadata fields and values for clients to use during registration.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 6, 2015.
Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved.
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In order for an OAuth 2.0 client to utilize an OAuth 2.0 authorization server, the client needs specific information to interact with the server, including an OAuth 2.0 client identifier to use at that server. This specification describes how an OAuth 2.0 client can be dynamically registered with an authorization server to obtain this information.
As part of the registration process, this specification also defines a mechanism for the client to present the authorization server with a set of metadata, such as a set of valid redirection URIs. This metadata can either be communicated in a self-asserted fashion or as a set of metadata called a software statement, which is digitally signed or MACed; in the case of a software statement, the issuer is vouching for the validity of the data about the client.
Traditionally, registration of a client with an authorization server is performed manually. The mechanisms defined in this specification can be used either for a client to dynamically register itself with authorization servers or for a client developer to programmatically register the client with authorization servers. Multiple applications using OAuth 2.0 have previously developed mechanisms for accomplishing such registrations. This specficiation generalizes the registration mechanisms defined by the OpenID Connect Dynamic Client Registration 1.0 [OpenID.Registration] specification and used by the User Managed Access (UMA) Profile of OAuth 2.0 [I-D.hardjono-oauth-umacore] specification in a way that is compatible with both, while being applicable to a wider set of OAuth 2.0 use cases.
The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'MAY', and 'OPTIONAL' in this document are to be interpreted as described in [RFC2119].
Unless otherwise noted, all the protocol parameter names and values are case sensitive.
This specification uses the terms "access token", "authorization code", "authorization endpoint", "authorization grant", "authorization server", "client", "client identifier", "client secret", "grant type", "protected resource", "redirection URI", "refresh token", "resource owner", "resource server", "response type", and "token endpoint" defined by OAuth 2.0 [RFC6749] and uses the term "Claim" defined by JSON Web Token (JWT) [JWT].
This specification defines the following terms:
+--------(A)- Initial Access Token (OPTIONAL)
|
| +----(B)- Software Statement (OPTIONAL)
| |
v v
+-----------+ +---------------+
| |--(C)- Client Registration Request -->| Client |
| Client or | | Registration |
| Developer |<-(D)- Client Information Response ---| Endpoint |
| | +---------------+
+-----------+
Figure 1: Abstract Dynamic Client Registration Flow
The abstract OAuth 2.0 client dynamic registration flow illustrated in Figure 1 describes the interaction between the client or developer and the endpoint defined in this specification. This figure does not demonstrate error conditions. This flow includes the following steps:
Clients have a set of metadata values associated with their client identifier at an authorization server, such as the list of valid redirection URIs or a display name.
The client metadata values are used in two ways:
The following client metadata fields are defined by this specification. The implementation and use of all client metadata fields is OPTIONAL, unless stated otherwise.
Additional values can be defined via the IANA OAuth Token Endpoint Authentication Methods Registry established in
Section 5.2. Absolute URIs can also be used as values for this parameter without being registered. If unspecified or omitted, the default is client_secret_basic, denoting HTTP Basic Authentication Scheme as specified in Section 2.3.1 of OAuth 2.0.Authorization Servers MAY allow for other values as defined in grant type extensions to OAuth 2.0. The extension process is described in OAuth 2.0 Section 2.5. If the token endpoint is used in the grant type, the value of this parameter MUST be the same as the value of the
grant_type parameter passed to the token endpoint defined in the extension. If omitted, the default is that the client will use only the authorization_code Grant Type.Authorization servers MAY allow for other values as defined in response type extensions to OAuth 2.0. The extension process is described in OAuth 2.0 Section 2.5. If the authorization endpoint is used by the grant type, the value of this parameter MUST be the same as the value of the
response_type parameter passed to the authorization endpoint defined in the extension. If omitted, the default is that the client will use only the code response type.Extensions and profiles of this specification MAY expand this list. The authorization server MUST ignore any client metadata values sent by the client that it does not understand.
Client metadata values can either be communicated directly in the body of a registration request, as described in Section 3.1, or included as claims in a software statement, as described in Section 2.3, or a mixture of both. If the same client metadata name is present in both locations and the software statement is trusted by the authorization server, the value of a claim in the software statement MUST take precedence.
The grant_types and response_types values described above are partially orthogonal, as they refer to arguments passed to different endpoints in the OAuth protocol. However, they are related in that the grant_types available to a client influence the response_types that the client is allowed to use, and vice versa. For instance, a grant_types value that includes authorization_code implies a response_types value that includes code, as both values are defined as part of the OAuth 2.0 authorization code grant. As such, a server supporting these fields SHOULD take steps to ensure that a client cannot register itself into an inconsistent state, for example by returning an invalid_client_metadata error response to an inconsistent registration request.
The correlation between the two fields is listed in the table below.
| grant_types value includes: | response_types value includes: |
|---|---|
| authorization_code | code |
| implicit | token |
| password | (none) |
| client_credentials | (none) |
| refresh_token | (none) |
| urn:ietf:params:oauth:grant-type:jwt-bearer | (none) |
| urn:ietf:params:oauth:grant-type:saml2-bearer | (none) |
Extensions and profiles of this document that introduce new values to either the grant_types or response_types parameter MUST document all correspondences between these two parameter types.
Human-readable client metadata values and client metadata values that reference human-readable values MAY be represented in multiple languages and scripts. For example, the values of fields such as client_name, tos_uri, policy_uri, logo_uri, and client_uri might have multiple locale-specific values in some client registrations to facilitate use in different locations.
To specify the languages and scripts, BCP47 [RFC5646] language tags are added to client metadata member names, delimited by a # character. Since JSON [RFC7159] member names are case sensitive, it is RECOMMENDED that language tag values used in Claim Names be spelled using the character case with which they are registered in the IANA Language Subtag Registry [IANA.Language]. In particular, normally language names are spelled with lowercase characters, region names are spelled with uppercase characters, and languages are spelled with mixed case characters. However, since BCP47 language tag values are case insensitive, implementations SHOULD interpret the language tag values supplied in a case insensitive manner. Per the recommendations in BCP47, language tag values used in metadata member names should only be as specific as necessary. For instance, using fr might be sufficient in many contexts, rather than fr-CA or fr-FR.
For example, a client could represent its name in English as "client_name#en": "My Client" and its name in Japanese as "client_name#ja-Jpan-JP": "\u30AF\u30E9\u30A4\u30A2\u30F3\u30C8\u540D" within the same registration request. The authorization server MAY display any or all of these names to the resource owner during the authorization step, choosing which name to display based on system configuration, user preferences or other factors.
If any human-readable field is sent without a language tag, parties using it MUST NOT make any assumptions about the language, character set, or script of the string value, and the string value MUST be used as-is wherever it is presented in a user interface. To facilitate interoperability, it is RECOMMENDED that clients and servers use a human-readable field without any language tags in addition to any language-specific fields, and it is RECOMMENDED that any human-readable fields sent without language tags contain values suitable for display on a wide variety of systems.
Implementer's Note: Many JSON libraries make it possible to reference members of a JSON object as members of an object construct in the native programming environment of the library. However, while the # character is a valid character inside of a JSON object's member names, it is not a valid character for use in an object member name in many programming environments. Therefore, implementations will need to use alternative access forms for these claims. For instance, in JavaScript, if one parses the JSON as follows, var j = JSON.parse(json);, then the member client_name#en-us can be accessed using the JavaScript syntax j["client_name#en-us"].
A software statement is a JSON Web Token (JWT) [JWT] that asserts metadata values about the client software as a bundle. A set of claims that can be used in a software statement are defined in Section 2. When presented to the authorization server as part of a client registration request, the software statement MUST be digitally signed or MACed using JWS [JWS] and MUST contain an iss (issuer) claim denoting the party attesting to the claims in the software statement. It is RECOMMENDED that software statements be digitally signed using the RS256 signature algorithm, although particular applications MAY specify the use of different algorithms.
The means by which a client or developer obtains a software statement are outside the scope of this specification. Some common methods could include a client developer generating a client-specific JWT registering with a software API publisher to obtain a software statement for a class of clients. The software statement is typically distributed with all instances of a client application.
The criteria by which authorization servers determine whether to trust and utilize the information in a software statement are beyond the scope of this specification.
In some cases, authorization servers MAY choose to accept a software statement value directly as a client identifier in an authorization request, without a prior dynamic client registration having been performed. The circumstances under which an authorization server would do so, and the specific software statement characteristics required in this case, are beyond the scope of this specification.
The client registration endpoint is an OAuth 2.0 endpoint defined in this document that is designed to allow a client to be registered with the authorization server. The client registration endpoint MUST accept HTTP POST messages with request parameters encoded in the entity body using the application/json format. The client registration endpoint MUST be protected by a transport-layer security mechanism, and the server MUST support TLS 1.2 RFC 5246 [RFC5246] and/or TLS 1.0 [RFC2246] and MAY support additional transport-layer mechanisms meeting its security requirements. When using TLS, the client MUST perform a TLS/SSL server certificate check, per RFC 6125 [RFC6125].
The client registration endpoint MAY be an OAuth 2.0 protected resource and accept an initial access token in the form of an OAuth 2.0 [RFC6749] access token to limit registration to only previously authorized parties. The method by which the initial access token is obtained by the client or developer is generally out-of-band and is out of scope for this specification. The method by which the initial access token is verified and validated by the client registration endpoint is out of scope for this specification.
To support open registration and facilitate wider interoperability, the client registration endpoint SHOULD allow registration requests with no authorization (which is to say, with no initial access token in the request). These requests MAY be rate-limited or otherwise limited to prevent a denial-of-service attack on the client registration endpoint.
This operation registers a client with the authorization server. The authorization server assigns this client a unique client identifier, optionally assigns a client secret, and associates the metadata provided in the request with the issued client identifier. The request includes any client metadata parameters being specified for the client during the registration. The authorization server MAY provision default values for any items omitted in the client metadata.
To register, the client or developer sends an HTTP POST to the client registration endpoint with a content type of application/json. The HTTP Entity Payload is a JSON [RFC7159] document consisting of a JSON object and all requested client metadata values as top-level members of that JSON object.
Client metadata values may also be provided in a software statement, as described in Section 2.3. Software statements are included in the requesting JSON object using this member:
For example, if the server supports open registration (with no initial access token), the client could send the following registration request to the client registration endpoint:
The following is a non-normative example request not using an initial access token (with line wraps within values for display purposes only):
POST /register HTTP/1.1
Content-Type: application/json
Accept: application/json
Host: server.example.com
{
"redirect_uris":[
"https://client.example.org/callback",
"https://client.example.org/callback2"],
"client_name":"My Example Client",
"client_name#ja-Jpan-JP":
"\u30AF\u30E9\u30A4\u30A2\u30F3\u30C8\u540D",
"token_endpoint_auth_method":"client_secret_basic",
"logo_uri":"https://client.example.org/logo.png",
"jwks_uri":"https://client.example.org/my_public_keys.jwks",
"example_extension_parameter": "example_value"
}
Alternatively, if the server supports authorized registration, the developer or the client will be provisioned with an initial access token. (The method by which the initial access token is obtained is out of scope for this specification.) The developer or client sends the following authorized registration request to the client registration endpoint. Note that the initial access token sent in this example as an OAuth 2.0 Bearer Token [RFC6750], but any OAuth 2.0 token type could be used by an authorization server.
The following is a non-normative example request using an initial access token (with line wraps within values for display purposes only):
POST /register HTTP/1.1
Content-Type: application/json
Accept: application/json
Authorization: Bearer ey23f2.adfj230.af32-developer321
Host: server.example.com
{
"redirect_uris":["https://client.example.org/callback",
"https://client.example.org/callback2"],
"client_name":"My Example Client",
"client_name#ja-Jpan-JP":
"\u30AF\u30E9\u30A4\u30A2\u30F3\u30C8\u540D",
"token_endpoint_auth_method":"client_secret_basic",
"policy_uri":"https://client.example.org/policy.html",
"jwks":{"keys":[{...omitted for brevity...}]},
"example_extension_parameter": "example_value"
}
In the following example, some registration parameters are conveyed as claims in a software statement, while some values specific to the client instance are conveyed as regular parameters (with line wraps within values for display purposes only):
POST /register HTTP/1.1
Content-Type: application/json
Accept: application/json
Host: server.example.com
{
"redirect_uris":[
"https://client.example.org/callback",
"https://client.example.org/callback2"
],
"software_statement":"eyJhbGciOiJFUzI1NiJ9.
eyJpc3Mi[...omitted for brevity...].
J9l-ZhwP[...omitted for brevity...]",
"scope":"read write",
"example_extension_parameter":"example_value"
}
Upon successful registration, the authorization server returns a client identifier for the client. The server responds with an HTTP 201 Created code and a body of type application/json with content as described in Section 4.1.
Upon an unsuccessful registration, the authorization server responds with an error, as described in Section 4.2.
The following responses are sent in response to registration requests.
The response contains the client identifier as well as the client secret, if the client is a confidential client. The response MAY contain additional fields as specified by extensions to this specification.
Additionally, the authorization server MUST return all registered metadata about this client, including any fields provisioned by the authorization server itself. The authorization server MAY reject or replace any of the client's requested metadata values submitted during the registration or update requests and substitute them with suitable values.
The response is an application/json document with all parameters as top-level members of a JSON object [RFC7159].
If a software statement was used as part of the registration, its value MUST be returned in the response along with other metadata. Client metadata elements used from the software statement MUST also be returned directly as top-level client metadata values in the registration response (possibly with different values, since the values requested and the values used may differ).
Following is a non-normative example response:
HTTP/1.1 201 Created
Content-Type: application/json
Cache-Control: no-store
Pragma: no-cache
{
"client_id":"s6BhdRkqt3",
"client_secret": "cf136dc3c1fc93f31185e5885805d",
"client_id_issued_at":2893256800,
"client_secret_expires_at":2893276800,
"redirect_uris":[
"https://client.example.org/callback",
"https://client.example.org/callback2"],
"grant_types": ["authorization_code", "refresh_token"],
"client_name":"My Example Client",
"client_name#ja-Jpan-JP":
"\u30AF\u30E9\u30A4\u30A2\u30F3\u30C8\u540D",
"token_endpoint_auth_method":"client_secret_basic",
"logo_uri":"https://client.example.org/logo.png",
"jwks_uri":"https://client.example.org/my_public_keys.jwks",
"example_extension_parameter": "example_value"
}
When an OAuth 2.0 error condition occurs, such as the client presenting an invalid initial access token, the authorization server returns an error response appropriate to the OAuth 2.0 token type.
When a registration error condition occurs, the authorization server returns an HTTP 400 status code (unless otherwise specified) with content type application/json consisting of a JSON object [RFC7159] describing the error in the response body.
Two members are defined for inclusion in the JSON object:
Other members MAY also be included, and if not understood, MUST be ignored.
This specification defines the following error codes:
Following is a non-normative example of an error response resulting from a redirection URI that has been blacklisted by the authorization server (with line wraps within values for display purposes only):
HTTP/1.1 400 Bad Request
Content-Type: application/json
Cache-Control: no-store
Pragma: no-cache
{
"error": "invalid_redirect_uri",
"error_description": "The redirection URI
http://sketchy.example.com is not allowed by this server."
}
Following is a non-normative example of an error response resulting from an inconsistent combination of response_types and grant_types values (with line wraps within values for display purposes only):
HTTP/1.1 400 Bad Request
Content-Type: application/json
Cache-Control: no-store
Pragma: no-cache
{
"error": "invalid_client_metadata",
"error_description": "The grant type 'authorization_code' must be
registered along with the response type 'code' but found only
'implicit' instead."
}
This specification establishes the OAuth Dynamic Registration Client Metadata registry.
OAuth registration client metadata values are registered with a Specification Required ([RFC5226]) after a two-week review period on the oauth-ext-review@ietf.org mailing list, on the advice of one or more Designated Experts. However, to allow for the allocation of values prior to publication, the Designated Expert(s) may approve registration once they are satisfied that such a specification will be published.
Registration requests must be sent to the oauth-ext-review@ietf.org mailing list for review and comment, with an appropriate subject (e.g., "Request to register OAuth Dynamic Registration Client Metadata name: example").
Within the review period, the Designated Expert(s) will either approve or deny the registration request, communicating this decision to the review list and IANA. Denials should include an explanation and, if applicable, suggestions as to how to make the request successful.
IANA must only accept registry updates from the Designated Expert(s) and should direct all requests for registration to the review mailing list.
The initial contents of the OAuth Dynamic Registration Client Metadata registry are:
This specification establishes the OAuth Token Endpoint Authentication Methods registry.
Additional values for use as token_endpoint_auth_method metadata values are registered with a Specification Required ([RFC5226]) after a two-week review period on the oauth-ext-review@ietf.org mailing list, on the advice of one or more Designated Experts. However, to allow for the allocation of values prior to publication, the Designated Expert(s) may approve registration once they are satisfied that such a specification will be published.
Registration requests must be sent to the oauth-ext-review@ietf.org mailing list for review and comment, with an appropriate subject (e.g., "Request to register token_endpoint_auth_method value: example").
Within the review period, the Designated Expert(s) will either approve or deny the registration request, communicating this decision to the review list and IANA. Denials should include an explanation and, if applicable, suggestions as to how to make the request successful.
IANA must only accept registry updates from the Designated Expert(s) and should direct all requests for registration to the review mailing list.
The initial contents of the OAuth Token Endpoint Authentication Methods registry are:
Since requests to the client registration endpoint result in the transmission of clear-text credentials (in the HTTP request and response), the authorization server MUST require the use of a transport-layer security mechanism when sending requests to the registration endpoint. The server MUST support TLS 1.2 RFC 5246 [RFC5246] and/or TLS 1.0 [RFC2246] and MAY support additional transport-layer mechanisms meeting its security requirements. When using TLS, the client MUST perform a TLS/SSL server certificate check, per RFC 6125 [RFC6125].
For clients that use redirect-based grant types such as authorization_code and implicit, authorization servers MUST require clients to register their redirection URI values. This can help mitigate attacks where rogue actors inject and impersonate a validly registered client and intercept its authorization code or tokens through an invalid redirection URI or open redirector. Additionally, in order to prevent hijacking of the return values of the redirection, registered redirection URI values MUST be one of:
Public clients MAY register with an authorization server using this protocol, if the authorization server's policy allows them. Public clients use a none value for the token_endpoint_auth_method metadata field and are generally used with the implicit grant type. Often these clients will be short-lived in-browser applications requesting access to a user's resources and access is tied to a user's active session at the authorization server. Since such clients often do not have long-term storage, it's possible that such clients would need to re-register every time the browser application is loaded. Additionally, such clients may not have ample opportunity to unregister themselves using the delete action before the browser closes. To avoid the resulting proliferation of dead client identifiers, an authorization server MAY decide to expire registrations for existing clients meeting certain criteria after a period of time has elapsed.
Since different OAuth 2.0 grant types have different security and usage parameters, an authorization server MAY require separate registrations for a piece of software to support multiple grant types. For instance, an authorization server might require that all clients using the authorization_code grant type make use of a client secret for the token_endpoint_auth_method, but any clients using the implicit grant type do not use any authentication at the token endpoint. In such a situation, a server MAY disallow clients from registering for both the authorization_code and implicit grant types simultaneously. Similarly, the authorization_code grant type is used to represent access on behalf of an end user, but the client_credentials grant type represents access on behalf of the client itself. For security reasons, an authorization server could require that different scopes be used for these different use cases, and as a consequence it MAY disallow these two grant types from being registered together by the same client. In all of these cases, the authorization server would respond with an invalid_client_metadata error response.
Unless used as a claim in a software statement, the authorization server MUST treat all client metadata as self-asserted. For instance, a rogue client might use the name and logo of a legitimate client that it is trying to impersonate. Additionally, a rogue client might try to use the software identifier or software version of a legitimate client to attempt to associate itself on the authorization server with instances of the legitimate client. To counteract this, an authorization server needs to take steps to mitigate this risk by looking at the entire registration request and client configuration. For instance, an authorization server could issue a warning if the domain/site of the logo doesn't match the domain/site of redirection URIs. An authorization server could also refuse registration requests from a known software identifier that is requesting different redirection URIs or a different client homepage URI. An authorization server can also present warning messages to end users about dynamically registered clients in all cases, especially if such clients have been recently registered or have not been trusted by any users at the authorization server before.
In a situation where the authorization server is supporting open client registration, it must be extremely careful with any URL provided by the client that will be displayed to the user (e.g. logo_uri, tos_uri, client_uri, and policy_uri). For instance, a rogue client could specify a registration request with a reference to a drive-by download in the policy_uri. The authorization server SHOULD check to see if the logo_uri, tos_uri, client_uri, and policy_uri have the same host and scheme as the those defined in the array of redirect_uris and that all of these URIs resolve to valid web pages.
Clients MAY use both the direct JSON object and the JWT-encoded software statement to present client metadata to the authorization server as part of the registration request. A software statement is cryptographically protected and represents claims made by the issuer of the statement, while the JSON object represents the self-asserted claims made by the client or developer directly. If the software statement is valid and trusted, the values of client metadata within the software statement MUST take precedence over those metadata values presented in the plain JSON object, which could have been modified en route.
The software statement is an item that is self-asserted by the client, even though its contents have been digitally signed or MACed by the issuer of the software statement. As such, presentation of the software statement is not sufficient in most cases to fully identity a piece of client software. An initial access token, in contrast, does not necessarily contain information about a particular piece of client software but instead represents authorization to use the registration endpoint. An authorization server MUST consider the full registration request, including the software statement, initial access token, and JSON client metadata values, when deciding whether to honor a given registration request.
If an authorization server receives a registration request for a client that uses the same software_id and software_version values as another client, the server should treat the new registration as being suspect. It is possible that the new client is trying to impersonate the existing client.
Since a client identifier is a public value that can be used to impersonate a client at the authorization endpoint, an authorization server that decides to issue the same client identifier to multiple instances of a registered client MUST be very particular about the circumstances under which this occurs. For instance, the authorization server can limit a given client identifier to clients using the same redirect-based flow and the same redirection URIs. An authorization server SHOULD NOT issue the same client secret to multiple instances of a registered client, even if they are issued the same client identifier, or else the client secret could be leaked, allowing malicious imposters to impersonate a confidential client.
| [I-D.hardjono-oauth-umacore] | Hardjono, T., "User-Managed Access (UMA) Profile of OAuth 2.0", Internet-Draft draft-hardjono-oauth-umacore-10, July 2014. |
| [OAuth.Registration.Management] | Richer, J., Jones, M., Bradley, J., Machulak, M. and P. Hunt, "OAuth 2.0 Dynamic Client Registration Management Protocol", Internet-Draft draft-ietf-oauth-dyn-reg-management, August 2014. |
| [OpenID.Registration] | Sakimura, N., Bradley, J. and M. Jones, "OpenID Connect Dynamic Client Registration 1.0", February 2014. |
This appendix describes different ways that this specification can be utilized, including describing some of the choices that may need to be made. Some of the choices are independent and can be used in combination, whereas some of the choices are interrelated.
Authorization servers that support open registration allow registrations to be made with no initial access token. This allows all client software to register with the authorization server.
Authorization servers that support protected registration require that an initial access token be used when making registration requests. While the method by which a client or developer receives this initial access token and the method by which the authorization server validates this initial access token are out of scope for this specification, a common approach is for the developer to use a manual pre-registration portal at the authorization server that issues an initial access token to the developer.
When a software statement is not used in the registration request, the authorization server must be willing to use client metadata values without them being digitally signed or MACed (and thereby attested to) by any authority. (Note that this choice is independent of the Open versus Protected choice, and that an initial access token is another possible form of attestation.)
A software statement can be used in a registration request to provide attestation by an authority for a set of client metadata values. This can be useful when the authorization server wants to restrict registration to client software attested to by a set of authorities or when it wants to know that multiple registration requests refer to the same piece of client software.
In some use cases, client software will dynamically register itself with an authorization server to obtain a client identifier and other information needed to interact with the authorization server. In this case, no client identifier for the authorization server is packaged with the client software.
In some cases, the developer (or development software being used by the developer) will pre-register the client software with the authorization server or a set of authorization servers. In this case, the client identifier value(s) for the authorization server(s) can be packaged with the client software.
In some cases, each deployed instance of a piece of client software will dynamically register and obtain distinct client identifier values. This can be advantageous, for instance, if the code flow is being used, as it also enables each client instance to have its own client secret. This can be useful for native clients, which cannot maintain the secrecy of a client secret value packaged with the software, but which may be able to maintain the secrecy of a per-instance client secret.
In some cases, each deployed instance of a piece of client software will share a common client identifier value. For instance, this is often the case for in-browser clients using the implicit flow, when no client secret is involved. Particular authorization servers might choose, for instance, to maintain a mapping between software statement values and client identifier values, and return the same client identifier value for all registration requests for a particular piece of software. The circumstances under which an authorization server would do so, and the specific software statement characteristics required in this case, are beyond the scope of this specification.
In some cases, authorization servers will maintain state about registered clients, typically indexing this state using the client identifier value. This state would typically include the client metadata values associated with the client registration, and possibly other state specific to the authorization server's implementation. When stateful registration is used, operations to support retrieving and/or updating this state may be supported. One possible set of operations upon stateful registrations is described in the [OAuth.Registration.Management] specification.
In some cases, authorization servers will be implemented in a manner the enables them to not maintain any local state about registered clients. One means of doing this is to encode all the registration state in the returned client identifier value, and possibly encrypting the state to the authorization server to maintain the confidentiality and integrity of the state.
The authors thank the OAuth Working Group, the User-Managed Access Working Group, and the OpenID Connect Working Group participants for their input to this document. In particular, the following individuals have been instrumental in their review and contribution to various versions of this document: Amanda Anganes, Derek Atkins, Tim Bray, Domenico Catalano, Donald Coffin, Vladimir Dzhuvinov, George Fletcher, Thomas Hardjono, Phil Hunt, William Kim, Torsten Lodderstedt, Eve Maler, Josh Mandel, Nov Matake, Tony Nadalin, Nat Sakimura, Christian Scholz, and Hannes Tschofenig.
[[ to be removed by the RFC editor before publication as an RFC ]]
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