Internet DRAFT - draft-identity-chaining
draft-identity-chaining
oauth A. Schwenkschuster
Internet-Draft P. Kasselmann
Intended status: Informational Microsoft
Expires: 11 January 2024 K. Burgin
M. Jenkins
NSA-CCSS
10 July 2023
Identity Chaining across Trust Domains
draft-identity-chaining-00
Abstract
This specification defines a mechanism to preserve identity and call
chain information across trust domains that use the OAuth 2.0
Framework.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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This Internet-Draft will expire on 11 January 2024.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Identity Chaining Across Trust Domains . . . . . . . . . . . 3
2.1. Use Case . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1.1. Preserve User Context across Multi-cloud, Multi-Hybrid
environments . . . . . . . . . . . . . . . . . . . . 4
2.1.2. API Security Use Case . . . . . . . . . . . . . . . . 4
2.2. Overview . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3. Authorization Server Discovery . . . . . . . . . . . . . 6
2.4. Token Exchange . . . . . . . . . . . . . . . . . . . . . 6
2.4.1. Request . . . . . . . . . . . . . . . . . . . . . . . 7
2.4.2. Processing rules . . . . . . . . . . . . . . . . . . 7
2.4.3. Authorization grant type . . . . . . . . . . . . . . 7
2.4.4. Response . . . . . . . . . . . . . . . . . . . . . . 8
2.4.5. Example . . . . . . . . . . . . . . . . . . . . . . . 8
2.5. Authorization Grant . . . . . . . . . . . . . . . . . . . 8
2.5.1. Request . . . . . . . . . . . . . . . . . . . . . . . 9
2.5.2. Processing rules . . . . . . . . . . . . . . . . . . 9
2.5.3. Response . . . . . . . . . . . . . . . . . . . . . . 9
2.5.4. Example . . . . . . . . . . . . . . . . . . . . . . . 10
2.6. Claims transcription . . . . . . . . . . . . . . . . . . 10
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
4. Security Considerations . . . . . . . . . . . . . . . . . . . 11
4.1. Client Authentication . . . . . . . . . . . . . . . . . . 11
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.1. Normative References . . . . . . . . . . . . . . . . . . 11
5.2. Informative References . . . . . . . . . . . . . . . . . 12
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 12
A.1. Resource server acting as client . . . . . . . . . . . . 12
A.2. Authorization server acting as client . . . . . . . . . . 14
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 16
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17
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1. Introduction
Applications often require access to resources that are distributed
across multiple trust domains where each trust domain has its own
OAuth 2.0 authorization server. As a result, developers are often
faced with the situation that a protected resource is located in a
different trust domain and thus protected by a different
authorization server. A request may transverse multiple resource
servers in multiple trust domains before completing. All protected
resources involved in such a request need to know on whose behalf the
request was originally initiated (i.e. the user), what authorization
was granted and optionally which other resource servers were called
prior to making an authorization decision. This information needs to
be preserved, even when a request crosses one or more trust domains.
Preserving this information is referred to as identity chaining.
This document defines a mechanism for preserving identity chaining
information across trust domains using a combination of OAuth 2.0
Token Exchange [RFC8693] and Assertion Framework for OAuth 2.0 Client
Authentication and Authorization Grants [RFC7521].
1.1. Requirements Language
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.
2. Identity Chaining Across Trust Domains
This specification describes a combination of OAuth 2.0 Token
Exchange [RFC8693] and Assertion Framework for OAuth 2.0 Client
Authentication and Authorization Grants [RFC7521] to achieve identity
chaining across trust domains.
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A client in trust domain A that needs to access a resource server in
trust domain B requests an authorization grant from the authorization
server for trust domain A via a token exchange. The client in trust
domain A presents the received grant as an assertion to the
authorization server in domain B in order to obtain an access token
for the protected resource in domain B. The client in domain A may
be a resource server, or it may be the authorization server itself.
A client in trust domain A that needs to access a resource server in
trust domain B requests an authorization grant from the authorization
server for trust domain A via a token exchange. The client in trust
domain A presents the received grant as an assertion to the
authorization server in domain B in order to obtain an access token
for the protected resource in domain B. The client in domain A may
be a resource server, or it may be the authorization server itself.
2.1. Use Case
This section describes two use cases addressed in this specification.
2.1.1. Preserve User Context across Multi-cloud, Multi-Hybrid
environments
A user attempts to access a service that is implemented as a number
of on-premise and cloud-based microservices. Both the on-premise and
cloud-based services are segmented by multiple trust boundaries that
span one or more on-premise or cloud service environments. Every
microservice can apply an authorization policy that takes the context
of the original user, as well as intermediary microservices into
account, irrespective of where the microservices are running and even
when a microservice in one trust domain calls another service in
another trust domain.
2.1.2. API Security Use Case
A home devices company provides a “Camera API” to enable access to
home cameras. Partner companies use this Camera API to integrate the
camera feeds into their security dashboards. Using OAuth between the
partner and the Camera API, a partner can request the feed from a
home camera to be displayed in their dashboard. The user has an
account with the camera provider. The user may be logged in to view
the partner provided dashboard, or they may authorize emergency
access to the camera. The home devices company must be able to
independently verify that the request originated and was authorized
by a user who is authorized to view the feed of the requested home
camera.
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2.2. Overview
The Identity Chaining flow outlined below describes how a combination
of OAuth 2.0 Token Exchange [RFC8693] and Assertion Framework for
OAuth 2.0 Client Authentication and Authorization Grants [RFC7521]
are used to address the use cases identified. The appendix include
two additional examples that describe how this flow is used. In one
example, the resource server acts as the client and in the other, the
authorization server acts as the client.
+-------------+ +-------------+ +---------+
|Authorization| +--------+ |Authorization| |Protected|
|Server | |Client | |Server | |Resource |
|Domain A | |Domain A| |Domain B | |Domain B |
+-------------+ +--------+ +-------------+ +---------+
| | | |
| |----+ | |
| | | (A) discover | |
| |<---+ Authorization | |
| | Server | |
| | Domain B | |
| | | |
| | | |
| (B) exchange token | | |
| [RFC 8693] | | |
|<-------------------| | |
| | | |
| (C) <authorization | | |
| grant> | | |
| - - - - - - - - - >| | |
| | | |
| | (D) present | |
| | authorization grant | |
| | [RFC 7521] | |
| | ------------------->| |
| | | |
| | (E) <access token> | |
| | <- - - - - - - - - -| |
| | | |
| | (F) access |
| | --------------------------------->|
| | | |
| | | |
Figure 1: Identity Chaining Flow
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The flow illustrated in Figure 1 shows the steps the client in trust
Domain A needs to perform to access a protected resource in trust
domain B. In this flow, the client has a way to discover the
authorization server in Domain B and a trust relationship exists
between Domain A and Domain B (e.g., through federation). It
includes the following:
* (A) The client of Domain A needs to discover the authorization
server of Domain B. See Authorization Server Discovery
(Section 2.3).
* (B) The client exchanges its token at the authorization server of
its own domain (Domain A) for an authorization grant that can be
used at the authorization server in Domain B. See Token Exchange
(Section 2.4).
* (C) The authorization server of Domain A processes the request and
returns an authorization grant that the client can use with the
authorization server of Domain B. This requires a trust
relationship between Domain A and Domain B (e.g., through
federation).
* (D) The client presents the authorization grant to the
authorization server of Domain B. See Authorization Grant
(Section 2.5).
* (E) Authorization server of Domain B validates the authorization
grant and returns an access token.
* (F) The client now possesses an access token to access the
protected resource in Domain B.
2.3. Authorization Server Discovery
This specification does not define authorization server discovery. A
client MAY contact the resource server and leverage the WWW-
Authentication response (see section 3 of [RFC6750]), maintain a
static mapping or use other means to identify the authorization
server.
2.4. Token Exchange
The client performs token exchange as defined in [RFC8693] with the
authorization server for its own domain (e.g., Domain A) in order to
obtain an authorization grant that can be used with the authorization
server of a different domain (e.g., Domain B) as specified in section
1.3 of [RFC6749].
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2.4.1. Request
The parameters described in section 2.1 of [RFC8693] apply here with
the following restrictions:
requested_token_type
OPTIONAL according to [RFC8693]. In the context of this
specification this parameter SHOULD NOT be used. See
Authorization grant type (Section 2.4.3).
scope
OPTIONAL. Additional scopes to indicate scopes included in
returned authorization grant. See Claims transcription
(Section 2.6).
resource
REQUIRED if audience is not set. URI of authorization server of
targeting domain (domain B).
audience
REQUIRED if resource is not set. Well known/logical name of
authorization server of targeting domain (domain B).
2.4.2. Processing rules
* If the request itself is not valid or if the given resource or
audience are unknown, or are unacceptable based on policy, the
authorization server MUST deny the request.
* The authorization server MAY add, remove or change claims. See
Claims transcription (Section 2.6).
2.4.3. Authorization grant type
The authorization grant format and content is part of a contract
between the authorization servers. To achieve a maintainable and
flexible systems clients SHOULD NOT request a specific
requested_token_type during the token exchange and SHOULD NOT require
a certain format or parse the authorization grant (e.g., if the token
is encoded as a JWT). The issued_token_type parameter in the
response indicates the type and SHOULD be passed into the assertion
request. This allows flexibility for authorization servers to change
format and content.
Authorization servers MAY use an existing grant type such us
urn:ietf:params:oauth:grant-type:jwt-bearer to indicate a JWT or
urn:ietf:params:oauth:grant-type:saml2-bearer to indicate SAML.
Other grant types MAY be used to indicate other formats.
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2.4.4. Response
All of section 2.2 of [RFC8693] applies. In addition, the following
applies to implementations that conform to this specification.
* The "aud" claim in the returned authorization grant MUST identify
the requested authorization server. This corresponds with RFC
7523 Section 3, Point 3 (https://datatracker.ietf.org/doc/html/
rfc7523#section-3) and is there to reduce missuse and to prevent
clients from presenting access tokens as an authorization grant to
an authorization server in a different domain.
* The "aud" claim included in the returned authorization grant MAY
identify multiple authorization servers, provided that trust
relationships exist with them (e.g. through federation). It is
RECOMMENDED that the "aud" claim is restricted to a single
authorization server to prevent an authorization server in one
domain from presenting the client's authorization grant to an
authorization server in a different trust domain. For example,
this will prevent the authorization server in Domain B from
presenting the authorization grant it received from the client in
Domain A to the authorization server for Domain C.
2.4.5. Example
The example belows shows the message invoked by the client in trust
domain A to perform token exchange with the authorization server in
domain A (https://a.org/auth) to receive an authorization grant for
the authorization server in trust domain B (https://b.org/auth).
curl --location 'https://a.org/auth/token' \
--form 'grant_type="urn:ietf:params:oauth:grant-type:token-exchange"' \
--form 'subject_token="ey.."' \
--form 'subject_token_type="urn:ietf:params:oauth:token-type:access_token"' \
--form 'resource="https://b.org/auth"'
2.5. Authorization Grant
The client presents the authorization grant it received from the
authorization server in its own domain and presents it to the
authorization server in the domain of the resources server it wants
to access as defined in the "Assertion Framework for OAuth 2.0 Client
Authentication and Authorization Grants" [RFC7521].
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2.5.1. Request
If the authorization grant is in the form of a JWT bearer token, the
client SHOULD use the "JSON Web Token (JWT) Profile for OAuth 2.0
Client Authentication and Authorization Grants" as defined in
[RFC7521]. Otherwise, the client SHOULD request an access token
using the "Assertion Framework for OAuth 2.0 Client Authentication
and Authorization Grants" as defined in [RFC7521] (Section 4.1). For
the purpose of this specification the following descriptions apply:
grant_type
REQUIRED. In context of this specification clients SHOULD use the
type identifier returned by the token exchange (issued_token_type
response). See authorization grant type (Section 2.4.3) for more
details.
assertion
REQUIRED. Authorization grant returned by the token exchange
(access_token response).
scope
OPTIONAL.
The client MAY indicate the audience it is trying to access through
the scope parameter or the resource parameter defined in [RFC8707].
2.5.2. Processing rules
All of [RFC7521] (Section 5.2 in specific) applies, along with the
following processing rules:
* The request MUST be denied if the presented authorization grant
does not include an "aud" claim identifying the authorization
server that processes the request.
* The authorization server SHOULD deny the request if it is not able
to identify the subject.
* Due to policy the request MAY be denied (for instance if the
federation from domain A is not allowed).
2.5.3. Response
The authorization server responds with an access token as described
in section 5.1 of [RFC6749].
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2.5.4. Example
The example belows shows how the client in trust domain A presents an
authorization grant to the authorization server in trust domain B
(https://b.org/auth) to receive an access token for a protected
resource in trust domain B.
curl --location --request GET 'https://b.org/auth/token' \
--form 'grant_type="urn:ietf:params:oauth:grant-type:jwt-bearer"' \
--form 'assertion="ey..."'
2.6. Claims transcription
Authorization servers MAY transcribe claims when either producing
authorization grants in the token exchange flow or access tokens in
the assertion flow.
* *Transcribing the subject identifier*: Subject identifier can
differ between the parties involved. For instance: A user is
known at domain A by "johndoe@a.org" but in domain B by
"doe.john@b.org". The mapping from one identifier to the other
MAY either happen in the token exchange step and the updated
identifer is reflected in returned authorization grant or in the
assertion step where the updated identifier would be reflected in
the access token. To support this both authorization servers MAY
add, change or remove claims as described above.
* *Selective disclosure*: Authorization servers MAY remove or hide
certain due to privacy requirements or reduced trust towards the
targeting trust domain. To hide and enclose claims [SD-JWT] MAY
be used.
* *Controlling scope*: Clients MAY use the scope parameter to
control transcribed claims (e.g. downscoping). Authorization
Servers SHOULD verify that requested scopes are not higher
priveleged than the scopes of presented subject_token.
* *Including authorization grant claims*: The authorization server
performing the assertion flow MAY leverage claims from the
presented authorization grant and include them in the returned
access token. The populated claims SHOULD be namespaced or
validated to prevent the injection of invalid claims.
The representation of transcribed claims and their format is not
defined in this specification.
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3. IANA Considerations
To be added.
4. Security Considerations
4.1. Client Authentication
Authorization Servers SHOULD follow the OAuth 2.0 Security Best
Current Practice [OAUTH2-BCP] for client authentication.
5. References
5.1. Normative References
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<https://www.rfc-editor.org/rfc/rfc6749>.
[RFC6750] Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
Framework: Bearer Token Usage", RFC 6750,
DOI 10.17487/RFC6750, October 2012,
<https://www.rfc-editor.org/rfc/rfc6750>.
[RFC8693] Jones, M., Nadalin, A., Campbell, B., Ed., Bradley, J.,
and C. Mortimore, "OAuth 2.0 Token Exchange", RFC 8693,
DOI 10.17487/RFC8693, January 2020,
<https://www.rfc-editor.org/rfc/rfc8693>.
[RFC7521] Campbell, B., Mortimore, C., Jones, M., and Y. Goland,
"Assertion Framework for OAuth 2.0 Client Authentication
and Authorization Grants", RFC 7521, DOI 10.17487/RFC7521,
May 2015, <https://www.rfc-editor.org/rfc/rfc7521>.
[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, <https://www.rfc-editor.org/rfc/rfc7523>.
[RFC8707] Campbell, B., Bradley, J., and H. Tschofenig, "Resource
Indicators for OAuth 2.0", RFC 8707, DOI 10.17487/RFC8707,
February 2020, <https://www.rfc-editor.org/rfc/rfc8707>.
[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>.
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[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>.
5.2. Informative References
[SD-JWT] Fett, D., Yasuda, K., and B. Campbell, "Selective
Disclosure for JWTs (SD-JWT)", n.d.,
<https://datatracker.ietf.org/doc/html/draft-ietf-oauth-
selective-disclosure-jwt-04>.
[OAUTH2-BCP]
Lodderstedt, T., Bradley, J., Labunets, A., and D. Fett,
"OAuth 2.0 Security Best Current Practice", n.d.,
<https://datatracker.ietf.org/doc/html/draft-ietf-oauth-
security-topics>.
Appendix A. Examples
This section contains two examples, demonstrating how this
specification may be used in different environments with specific
requirements. The first example shows the resource server acting as
the client and the second example shows the authorization server
acting as the client.
A.1. Resource server acting as client
Resources servers may act as clients if the following is true:
* Authorization Server B is reachable by the resource server by
network and is able to perform the appropiate client
authentication (if required).
* The resource server has the ability to determine the authorization
server of the protected resource outside its trust domain.
The flow would look like this:
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+-------------+ +--------+ +-------------+ +---------+
|Authorization| |Resource| |Authorization| |Protected|
|Server | |Server | |Server | |Resource |
|Domain A | |Domain A| |Domain B | |Domain B |
+-------------+ +--------+ +-------------+ +---------+
| | | |
| | (A) access (unauthenticated) |
| | --------------------------------->|
| | | |
| | (B) <WWW-Authenticate header> |
| | <- - - - - - - - - - - - - - - - -|
| | | |
| (C) exchange token | | |
| [RFC 8693] | | |
|<--------------------| | |
| | | |
| (D) <authorization | | |
| grant> | | |
| - - - - - - - - - ->| | |
| | | |
| | (E) present | |
| | authorization | |
| | grant [RFC 7521] | |
| | ------------------->| |
| | | |
| | (F) <access token> | |
| | <- - - - - - - - - -| |
| | | |
| | (G) access |
| | --------------------------------->|
| | | |
| | | |
Figure 2: Resource server acting as client
The flow contains the following steps:
(A) The resource server of domain A needs to access protected
resource in Domain B. It requires an access token to do so which it
does not possess. To receive information about the authorization
server which protecs the resource in domain B it calls the resource
unauthenticated.
(B) The protected resource returns the WWW-Authenticate header to
indicate its authorization server.
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(C) Now, after the resource server has identified the authorization
server for Domain B, the resource server requests an authorization
grant for the authorization server in Domain B from its own
authorization server (Domain A). This happens via the token exchange
protocol.
(D) If successful, the authorization server returns an authorization
grant to the resource server.
(E) The resource server presents the authorization grant to the
authorization server of Domain B.
(F) The authorization server of Domain B uses claims from the
authorization grant to identify the user and its access. If access
is granted an access token is returned.
(G) The resource server uses the access token to access the protected
resource at Domain B.
A.2. Authorization server acting as client
Authorization servers may act as clients too. This can be necessary
because of following reasons:
* Resource servers may not have knowledge of authorization servers.
* Resource servers may not have network access to other
authorization servers.
* A strict access control on resources outside the trust domain is
required and enforced by authorization servers.
* Authorization servers require client authentication. Managing
clients for resource servers outside of the trust domain is not
intended.
The flow when authorization servers act as client would look like
this:
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+--------+ +-------------+ +-------------+ +---------+
|Resource| |Authorization| |Authorization| |Protected|
|Server | |Server | |Server | |Resource |
|Domain A| |Domain A | |Domain B | |Domain B |
+--------+ +-------------+ +-------------+ +---------+
| | | |
| (A) request token for| | |
| protected resource | | |
| in domain B. | | |
| -------------------->| | |
| | | |
| |----+ | |
| | | (B) determine | |
| |<---+ authorization | |
| | server B | |
| | | |
| | | |
| |----+ | |
| | | (C) issue | |
| |<---+ authorization | |
| | grant ("internal | |
| | token exchange") | |
| | | |
| | | |
| | (D) present | |
| | authorization grant | |
| | [RFC 7521] | |
| | --------------------->| |
| | | |
| | (E) <access token> | |
| | <- - - - - - - - - - -| |
| | | |
| (F) <access token> | | |
| <- - - - - - - - - - | | |
| | | |
| | (G) access | |
| ---------------------------------------------------------->|
| | | |
| | | |
Figure 3: Authorization server acting as client
The flow contains the following steps:
(A) The resource server of Domain A requests a token for the
protected resource in Domain B from the authorization server in
Domain A. This specification does not cover this step. A profile of
Token Exchange [RFC8693] may be used.
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(B) The authorization server (of Domain A) determines the
authorization server (of Domain B). This could have been passed by
the client, is statically maintained or dynamically resolved.
(C) Once the authorization server is determined an authorization
grant is issued internally. This reflects to Token exchange
(Section 2.4) of this specification and can be seen as an "internal
token exchange".
(D) The issued authorization grant is presented to the authorization
server of Domain B. This presentation happens between the
authorization servers and authorization server A may be required to
perform client authentication while doing so.
(E) The authorization server of Domain B returns an access token for
access to the protected resource in Domain B to the authorization
server in Domain A.
(F) The authorization server of Domain A returns that access token to
the resource server in Domain A.
(G) The resource server in Domain A uses the received access token to
access the protected resource in Domain B.
Acknowledgements
Joe Jubinski, Justin Richer
Contributors
Atul Tulshibagwale
SGNL
Email: atuls@sgnl.ai
George Fletcher
Capital One
Email: george.fletcher@capitalone.com
Rifaat Shekh-Yusef
EY
Email: rifaat.shekh-yusef@ca.ey.com
Hannes Tschofenig
Email: hannes.tschofenig@gmx.net
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Internet-Draft Identity Chaining across Trust Domains July 2023
Authors' Addresses
Arndt Schwenkschuster
Microsoft
Email: arndts@microsoft.com
Pieter Kasselmann
Microsoft
Email: pieter.kasselman@microsoft.com
Kelley Burgin
Email: kelley.burgin@gmail.com
Mike Jenkins
NSA-CCSS
Email: mjjenki@cyber.nsa.gov
Schwenkschuster, et al. Expires 11 January 2024 [Page 17]
