Internet DRAFT - draft-campbell-oauth-sts
draft-campbell-oauth-sts
OAuth Working Group B. Campbell
Internet-Draft J. Bradley
Intended status: Standards Track Ping Identity
Expires: January 24, 2016 July 23, 2015
OAuth 2.0 Token Exchange: an STS for the REST of us
draft-campbell-oauth-sts-03
Abstract
An OAuth 2.0 framework for exchanging security tokens enabling
authorization servers to act as lightweight HTTP and JSON based
security token services.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Notation and Conventions . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.3. Delegation vs. Impersonation Semantics . . . . . . . . . 3
2. Security Token Request . . . . . . . . . . . . . . . . . . . 4
3. Security Token Response . . . . . . . . . . . . . . . . . . . 6
3.1. Successful Security Token Response . . . . . . . . . . . 6
3.2. Error Response . . . . . . . . . . . . . . . . . . . . . 7
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . 8
7.2. Informative References . . . . . . . . . . . . . . . . . 8
Appendix A. Open Issues . . . . . . . . . . . . . . . . . . . . 9
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 10
Appendix C. Document History . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
A security token service (STS) is a service capable of validating and
issuing security tokens, which enables web service clients to obtain
appropriate temporary access credentials for resources in
heterogeneous environments or across security domains. Clients have
historically used WS-Trust [WS-Trust] as the protocol to interact
with an STS for token exchange. However WS-Trust is a fairly
heavyweight framework which uses XML, SOAP, WS-Security, XML-
Signatures, etc. while the trend in modern web development has been
towards more lightweight services utilizing RESTful patterns and
JSON. The OAuth 2.0 Authorization Framework [RFC6749] and OAuth 2.0
Bearer Tokens [RFC6750] have emerged as popular standards for
authorizing and securing access to HTTP and RESTful resources but do
not provide all that is needed to support generic STS interactions.
This specification defines a lightweight protocol extending OAuth 2.0
that enables clients to request and obtain security tokens from
authorization servers acting in the role of an STS. There is support
for enabling one party to act on behalf of another as well as
enabling one party to delegate constrained authority to another.
Similar to OAuth 2.0, this specification focuses on client developer
simplicity and requires only an HTTP client and JSON parser, which
are nearly universally available in modern development environments.
The STS protocol defined in this specification is not itself RESTful
(an STS doesn't lend itself particularly well to a REST approach) but
does utilize communication patterns and data formats that should be
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more palatable to developers accustom to working with RESTful
systems.
A new security token request grant type and the associated specific
parameters for a security token request to the token endpoint are
defined by this specification. A security token response is a normal
OAuth 2.0 response from the token endpoint with some additional
parameters defined herein to provide information to the client.
The security tokens obtained from an STS could be used in a variety
of contexts, the specifics of which are beyond the scope of this
document.
The scope of this specification is limited to the definition of a
framework and basic wire protocol for an STS style token exchange
utilizing OAuth 2.0. The syntax, semantics and security
characteristics of the tokens themselves (both those presented to the
AS and those obtained by the client) are explicitly out of scope and
no requirements are placed on the trust model in which an
implementation might be deployed. Additional profiles may provide
more detailed requirements around the specific nature of the parties
and trust involved, whether signatures and/or encryption of tokens is
required, etc., however, such details will often be policy decisions
made with respect to the specific needs of individual deployments and
will be configured or implemented accordingly.
1.1. Requirements Notation and Conventions
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 RFC
2119 [RFC2119].
1.2. Terminology
This specification uses the terms "authorization server" "token
endpoint", "access token request", "access token response", and
"client" defined by OAuth 2.0 [RFC6749].
1.3. Delegation vs. Impersonation Semantics
When principal A impersonates principal B, A is given all the rights
that B has within some defined rights context. Whereas, with
delegation semantics, principal A still has its own identity separate
from B and it is explicitly understood that while B may have
delegated its rights to A, any actions taken are being taken by A and
not B. In a sense, A is an agent for B.
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Delegation semantics are therefore different than impersonation
semantics, with which it is sometimes confused. When principal A
impersonates principal B, then in so far as any entity receiving such
a token is concerned, they are actually dealing with B. It is true
that some members of the identity system might have awareness that
impersonation is going on but it is not a requirement. For all
intents and purposes, when A is impersonating B, A is B.
A security token with delegation semantics is requested using this
framework by including both an on_behalf_of token and an act_as token
in the request. The on_behalf_of token represents the identity of
the party on behalf of whom the token is being requested while the
act_as token represents the identity of the party to whom the access
rights of the returned token are being delegated. In this case, the
token returned to the client will contain claims about both parties.
A security token with impersonation semantics is requested using this
framework by including an on_behalf_of token in the request and
omitting the act_as token. The on_behalf_of token represents the
identity of the party on behalf of whom the token is being requested
the token returned to the client will contain claims about that
party.
2. Security Token Request
A client requests a security token by making a token request to the
authorization server's token endpoint using the extension grant type
mechanism defined in Section 4.5 of OAuth 2.0 [RFC6749].
Client authentication to the authorization server is done using the
normal mechanisms provided by OAuth 2.0. Section 2.3.1 of The OAuth
2.0 Authorization Framework [RFC6749] defines password-based
authentication of the client, however, client authentication is
extensible and other mechanisms are allowed. For example, [RFC7522]
and [RFC7523] define client authentication using SAML Assertions and
JSON Web Tokens respectively. Other mechanisms, such as TLS client
authentication, are also possible. The supported methods of client
authentication and whether or not to allow unauthenticated or
unidentified clients are deployment decisions that are at the
discretion of the authorization server.
The client makes a general security token request to the token
endpoint with an extension grant type by including the following
parameters using the "application/x-www-form-urlencoded" format with
a character encoding of UTF-8 in the HTTP request entity-body:
grant_type
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REQUIRED. The value "urn:ietf:params:oauth:grant-type:security-
token-request" indicates that it is a security token request.
aud
OPTIONAL. Indicates the location of the service or resource where
the client intends to use the requested security token. The value
MUST be an absolute URI as defined by Section 4.3 of [RFC3986].
The URI MAY include a query component but MUST NOT include a
fragment component. When applicable, the value of this parameter
also typically informs the audience restrictions on the returned
security token.
scope
OPTIONAL. A list of space-delimited, case-sensitive strings that
allow the client to specify the desired scope of requested
security token in the context of the service or resource where the
token will be used (possibly indicated by the "aud" parameter).
requested_security_token_type
OPTIONAL. Identifier for the type of the requested security
token. For example, a JWT can be requested with the identifier
"urn:ietf:params:oauth:token-type:jwt", which is defined in JSON
Web Token [RFC7519]. If the requested type is unspecified, the
returned token type is at the discretion of the authorization
server and may be dictated by knowledge of the requirements of the
service or resource whose location is indicated by the "aud"
parameter.
on_behalf_of
REQUIRED. The value of this request parameter is a security token
which represents the identity of the party on behalf of whom the
request is being made. Typically the subject of this token will
be the primary subject of the security token returned in response
to this request.
on_behalf_of_token_type
REQUIRED. An identifier that indicates the type of the security
token sent with the "on_behalf_of" parameter.
act_as
OPTIONAL. The value of this request parameter is a security token
which represents the identity of the party that is authorized to
use the requested security token. When this parameter is present,
it indicates that the client wants a token that contains claims
about two distinct entities: 1) the entity represented by the
token in the "on_behalf_of" parameter as the primary subject and
2) the entity represented by this token as a party who is
authorized to act on behalf of that subject.
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act_as_token_type
REQUIRED when the "act_as" parameter is present in the request but
MUST NOT be included otherwise. The value of this parameter is an
identifier that indicates the type of the security token sent with
the "act_as" parameter.
3. Security Token Response
The authorization server responds to a security token request with a
normal OAuth 2.0 response from the token endpoint as defined in
Section 5 of RFC 6749 [RFC6749]. Additional details and explanation
are provided in the following subsections.
3.1. Successful Security Token Response
If the request is valid and meets all policy and other criteria of
the authorization server, a successful token response is constructed
by adding the following parameters to the entity-body of the HTTP
response using the "application/json" media type as defined by
[RFC4627] and an HTTP 200 status code. The parameters are serialized
into a JavaScript Object Notation (JSON) structure by adding each
parameter at the top level. Parameter names and string values are
included as JSON strings. Numerical values are included as JSON
numbers. The order of parameters does not matter and can vary.
access_token
REQUIRED. The security token issued by the authorization server
in response to the security token request. The "access_token"
parameter from Section 5.1 of RFC 6749 [RFC6749] is used here to
carry the requested security token, which allows this token
exchange framework to use the existing OAuth 2.0 request and
response constructs defined for the token endpoint.
security_token_type
REQUIRED. An identifier for the general type of the returned
security token. For example, if the security token is a JWT, this
value of the "security_token_type" is
"urn:ietf:params:oauth:token-type:jwt".
token_type
REQUIRED. A case insensitive value describing the type of the
token issued as discussed in Section 7.1 of RFC 6749 [RFC6749].
Note that this value is different from the value of the
"security_token_type" and provides the client with information
about how to utilize the token to access protected resources. For
example, a value of "Bearer" as defined in [RFC6750] indicates
that the security token is a bearer token and the client can
simply present it as is without any additional proof of
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eligibility beyond the contents of the token itself. A value of
"PoP", on the other hand, indicates that use of the token will
require demonstrating possession of a cryptographic key associated
with the security token ([I-D.ietf-oauth-pop-key-distribution]
describes the "PoP" token type).
expires_in
RECOMMENDED. The validity lifetime, in seconds, of the issued
security token. For example, the value 3600 denotes that the
token will expire in one hour from the time the response was
generated.
scope
OPTIONAL, if the scope of the security token is identical to the
scope requested by the client; otherwise, REQUIRED.
refresh_token
NOT RECOMMENDED. Refresh tokens will typically not be issued in
response to a "urn:ietf:params:oauth:grant-type:security-token-
request" grant type requests.
3.2. Error Response
If either the "on_behalf_of" or "act_as" tokens are invalid for any
reason, or are unacceptable based on policy, the authorization server
MUST construct an error response as defined in Section 5.2 of OAuth
2.0 [RFC6749] The value of the "error" parameter MUST be the
"invalid_grant" error code. The authorization server MAY include
additional information regarding the reasons for the error using the
"error_description" or "error_uri" parameters.
4. Examples
[[ TODO: at least two examples, with and without act_as, showing a
request/response exchange and including some relevant internal
details of the tokens involved ]]
5. IANA Considerations
[[ TODO ]] The "urn:ietf:params:oauth:grant-type:security-token-
request" Grant Type is to be registered in the IANA
urn:ietf:params:oauth registry established in [RFC6755].
[[ TODO ]] Other parameters like "requested_security_token_type",
"on_behalf_of", "on_behalf_of_token_type", "act_as", etc. need to be
registered in the appropriate registries. The "aud" parameter needs
to be registered too but that may well get done in
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[I-D.ietf-oauth-pop-key-distribution] and aud may/does have wider
applicability so perhaps deserves it's own little spec?
6. Security Considerations
[[ TODO ]]
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
RFC2119, March 1997,
<http://www.rfc-editor.org/info/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,
<http://www.rfc-editor.org/info/rfc3986>.
[RFC4627] Crockford, D., "The application/json Media Type for
JavaScript Object Notation (JSON)", RFC 4627, DOI
10.17487/RFC4627, July 2006,
<http://www.rfc-editor.org/info/rfc4627>.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<http://www.rfc-editor.org/info/rfc6749>.
7.2. Informative References
[I-D.ietf-oauth-pop-key-distribution]
Bradley, J., Hunt, P., Jones, M., and H. Tschofenig,
"OAuth 2.0 Proof-of-Possession: Authorization Server to
Client Key Distribution", draft-ietf-oauth-pop-key-
distribution-01 (work in progress), March 2015.
[I-D.jones-oauth-token-exchange]
Jones, M., Nadalin, A., and C. Baker, "OAuth 2.0 Token
Exchange", draft-jones-oauth-token-exchange-01 (work in
progress), July 2014.
[OpenID.Core]
Sakimura, N., Bradley, J., Jones, M., Medeiros, B., and C.
Mortimore, "OpenID Connect Core 1.0", February 2014.
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[RFC6750] Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
Framework: Bearer Token Usage", RFC 6750, DOI 10.17487/
RFC6750, October 2012,
<http://www.rfc-editor.org/info/rfc6750>.
[RFC6755] Campbell, B. and H. Tschofenig, "An IETF URN Sub-Namespace
for OAuth", RFC 6755, DOI 10.17487/RFC6755, October 2012,
<http://www.rfc-editor.org/info/rfc6755>.
[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<http://www.rfc-editor.org/info/rfc7519>.
[RFC7522] Campbell, B., Mortimore, C., and M. Jones, "Security
Assertion Markup Language (SAML) 2.0 Profile for OAuth 2.0
Client Authentication and Authorization Grants", RFC 7522,
DOI 10.17487/RFC7522, May 2015,
<http://www.rfc-editor.org/info/rfc7522>.
[RFC7523] Jones, M., Campbell, B., and C. Mortimore, "JSON Web Token
(JWT) Profile for OAuth 2.0 Client Authentication and
Authorization Grants", RFC 7523, DOI 10.17487/RFC7523, May
2015, <http://www.rfc-editor.org/info/rfc7523>.
[WS-Trust]
Nadalin, A., Goodner, M., Gudgin, M., Barbir, A., and H.
Granqvist, "WS-Trust 1.4 (incorporating Approved Errata
01)", February 2012, <http://docs.oasis-open.org/ws-sx/ws-
trust/v1.4/ws-trust.html>.
Appendix A. Open Issues
Some, but surely not all, decisions to be made with potential
associated draft updates:
o Are the constructs for expressing delegation and impersonation the
'right' ones? Do they provide sufficient flexibility while being
reasonably understandable and implementable?
o Does it really make sense to use the act_as and on_behalf_of
terms? They come with some baggage.
o More guidance on what delegation should look like in the returned
token? I.e. refer to "azp" in [OpenID.Core]? Or something else?
o Do we need to codify if/how the identity of the client end up in
returned token? Should it be an AS decision? A special case of
delegation/act_as? Something else?
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o Does the error response need a way to convey additional
information beyond what OAuth 2.0 provides?
o Exactly how the presentation of PoP or other non-bearer tokens
works. Should a challenge-response mechanism be considered rather
than trying to stuff the whole PoP into a single request?
Appendix B. Acknowledgements
The author wishes to thank Michael Jones for bringing forth the
concept of OAuth 2.0 Token Exchange with
[I-D.jones-oauth-token-exchange]. This draft borrows heavily from
Jones' work while striving to provide a syntax that is more
consistent with OAuth 2.0 [RFC6749], which will hopefully be more
familiar to developers and easier to understand and implement.
The author also wishes to thank John Bradley for his endless patience
and willingness to share his expertise.
The following individuals also contributed ideas, feedback, and
wording that shaped and formed the final specification:
Chuck Mortimore, Justin Richter, Phil Hunt, and Scott Tomilson.
Appendix C. Document History
[[ to be removed by the RFC Editor before publication as an RFC ]]
-03
o The aud parameter is now OPTIONAL rather than REQUIRED.
o Update references for JWT and SAML/JWT assertion frameworks to the
new RFCs.
-02
o Refreshing draft before -01 expires.
-01
o Add Bradley as an author.
-00
o Gotta start somewhere...
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Authors' Addresses
Brian Campbell
Ping Identity
Email: brian.d.campbell@gmail.com
John Bradley
Ping Identity
Email: ve7jtb@ve7jtb.com
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