Skip to main content

Transaction Tokens
draft-tulshibagwale-oauth-transaction-tokens-05

Document Type Active Internet-Draft (individual)
Authors Atul Tulshibagwale , George Fletcher , Pieter Kasselman
Last updated 2023-10-20
RFC stream (None)
Intended RFC status (None)
Formats
Stream Stream state (No stream defined)
Consensus boilerplate Unknown
RFC Editor Note (None)
IESG IESG state I-D Exists
Telechat date (None)
Responsible AD (None)
Send notices to (None)
draft-tulshibagwale-oauth-transaction-tokens-05
oauth                                                   A. Tulshibagwale
Internet-Draft                                                      SGNL
Intended status: Informational                               G. Fletcher
Expires: 22 April 2024                                       Capital One
                                                            P. Kasselman
                                                               Microsoft
                                                         20 October 2023

                           Transaction Tokens
            draft-tulshibagwale-oauth-transaction-tokens-05

Abstract

   Transaction Tokens (Txn-Tokens) enable workloads in a trusted domain
   to ensure that user identity and authorization context of an external
   programmatic request, such as an API invocation, are preserved and
   available to all workloads that are invoked as part of processing
   such a request.  Txn-Tokens also enable workloads within the trusted
   domain to optionally immutably assert to downstream workloads that
   they were invoked in the call chain of the request.

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://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 22 April 2024.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights

Tulshibagwale, et al.     Expires 22 April 2024                 [Page 1]
Internet-Draft                 Txn-Tokens                   October 2023

   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  What are Transaction Tokens?  . . . . . . . . . . . . . .   4
     2.2.  Creating Txn-Tokens . . . . . . . . . . . . . . . . . . .   4
       2.2.1.  Initial Creation  . . . . . . . . . . . . . . . . . .   4
       2.2.2.  Replacement Txn-Tokens  . . . . . . . . . . . . . . .   4
     2.3.  Txn-Token Lifetime  . . . . . . . . . . . . . . . . . . .   5
     2.4.  Benefits of Txn-Tokens  . . . . . . . . . . . . . . . . .   5
     2.5.  Txn-Token Issuance and Usage Flows  . . . . . . . . . . .   5
       2.5.1.  Basic Flow  . . . . . . . . . . . . . . . . . . . . .   5
       2.5.2.  Replacement Txn-Token Flow  . . . . . . . . . . . . .   7
   3.  Notational Conventions  . . . . . . . . . . . . . . . . . . .   8
   4.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   8
   5.  Txn-Token Format  . . . . . . . . . . . . . . . . . . . . . .   9
     5.1.  JWT Header  . . . . . . . . . . . . . . . . . . . . . . .   9
     5.2.  JWT Body  . . . . . . . . . . . . . . . . . . . . . . . .   9
       5.2.1.  Required Claims . . . . . . . . . . . . . . . . . . .   9
       5.2.2.  Optional Claims . . . . . . . . . . . . . . . . . . .  10
       5.2.3.  Example . . . . . . . . . . . . . . . . . . . . . . .  11
   6.  Txn-Token Service . . . . . . . . . . . . . . . . . . . . . .  11
   7.  Requesting Txn-Tokens . . . . . . . . . . . . . . . . . . . .  12
     7.1.  Txn-Token Request . . . . . . . . . . . . . . . . . . . .  12
     7.2.  Txn-Token Response  . . . . . . . . . . . . . . . . . . .  13
     7.3.  Creating Replacement Txn-Tokens . . . . . . . . . . . . .  13
       7.3.1.  Txn-Token Service Responsibilities  . . . . . . . . .  14
       7.3.2.  Replacement Txn-Token Request . . . . . . . . . . . .  14
       7.3.3.  Replacement Txn-Token Response  . . . . . . . . . . .  14
     7.4.  Mutual Authentication of the Txn-Token Request  . . . . .  14
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
     8.1.  OAuth Registry Contents . . . . . . . . . . . . . . . . .  15
     8.2.  JWT Registry Contents . . . . . . . . . . . . . . . . . .  15
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
     9.1.  Txn-Token Lifetime  . . . . . . . . . . . . . . . . . . .  16
     9.2.  Sender Constrained Tokens . . . . . . . . . . . . . . . .  16
     9.3.  Access Tokens . . . . . . . . . . . . . . . . . . . . . .  17
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     10.2.  Informative References . . . . . . . . . . . . . . . . .  18
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  18
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

Tulshibagwale, et al.     Expires 22 April 2024                 [Page 2]
Internet-Draft                 Txn-Tokens                   October 2023

1.  Introduction

   Modern computing architectures often use multiple independently
   running components called workloads.  In many cases, external
   invocations through externally visible interfaces such as APIs result
   in a number of internal workloads being invoked in order to process
   the external invocation.  These workloads often run in virtually or
   physically isolated networks.  These networks and the workloads
   running within their perimeter may be compromised by attackers
   through software supply chain, privileged user compromise or other
   attacks.  Workloads compromised through external attacks, malicious
   insiders or software errors can cause any or all of the following
   unauthorized actions:

   *  Invocations of workloads in the network without any external
      invocation being present

   *  Arbitrary user impersonation

   *  Parameter modification or augmentation

   The results of these actions are unauthorised access to resources.

2.  Overview

   Transaction Tokens (Txn-Token) are a means to mitigate damage from
   such attacks or spurious invocations.  A valid Txn-Token indicates a
   valid external invocation.  They ensure that the identity of the user
   or a workload that made the external request is preserved throughout
   subsequent workload invocations.  They preserve any context such as:

   *  Parameters of the original call

   *  Environmental factors, such as IP address of the original caller

   *  Any computed context that needs to be preserved in the call chain.
      This includes information that was not in the original request to
      the external endpoint.

   Cryptographically protected Txn-Tokens ensure that downstream
   workloads cannot make unauthorized modifications to such information,
   and cannot make spurious calls without the presence of an external
   trigger.

Tulshibagwale, et al.     Expires 22 April 2024                 [Page 3]
Internet-Draft                 Txn-Tokens                   October 2023

2.1.  What are Transaction Tokens?

   Txn-Tokens are short-lived, signed JWTs [RFC7519] that assert the
   identity of a user or a workload and assert an authorization context.
   The authorization context provides information expected to remain
   constant during the execution of a call as it passes through multiple
   workloads.

2.2.  Creating Txn-Tokens

2.2.1.  Initial Creation

   Txn-Tokens are typically created when a workload is invoked using an
   endpoint that is externally visible, and is authorized using a
   separate mechanism, such as an OAuth [RFC6749] access token or an
   OpenID Connect [OpenIdConnect] ID token.  This workload then performs
   an OAuth 2.0 Token Exchange [RFC8693] to obtain a Txn-Token.  To do
   this, it invokes a special Token Service (the Txn-Token Service) and
   provides context that is sufficient for it to generate a Txn-Token.
   This context MAY include:

   *  The external authorization token (e.g., the OAuth access token)

   *  Parameters that are required to be bound for the duration of this
      call

   *  Additional context, such as the incoming IP address, User Agent
      information, or other context that can help the Txn-Token Service
      to issue the Txn-Token

   The Txn-Token Service responds to a successful invocation by
   generating a Txn-Token.  The calling workload then uses the Txn-Token
   to authorize its calls to subsequent workloads.  Subsequent workloads
   may obtain Txn-Tokens of their own.

2.2.2.  Replacement Txn-Tokens

   A service within a call chain may choose to replace the Txn-Token.
   This can typically happen if the service wants to add to the context
   of the current Txn-Token

   To get a replacement Txn-Token, a service will request a new Txn-
   Token from the Txn-Token Service and provide the current Txn-Token
   and other parameters in the request.  The Txn-Token service must
   exercise caution in what kinds of replacement requests it supports so
   as to not negate the entire value of Txn-Tokens.

Tulshibagwale, et al.     Expires 22 April 2024                 [Page 4]
Internet-Draft                 Txn-Tokens                   October 2023

2.3.  Txn-Token Lifetime

   Txn-Tokens are expected to be short-lived (order of minutes, e.g., 5
   minutes), and as a result MAY be used only for the expected duration
   of an external invocation.  If the token or other credential
   presented to the Txn-Token service when requesting a Txn-Token has an
   expiration time, then the Txn-Token MUST NOT exceed the lifetime of
   the originally presented token or credential.  If a long-running
   process such as an batch or offline task is involved, it can use a
   separate mechanism to perform the external invocation, but the
   resulting Txn-Token is still short-lived.

2.4.  Benefits of Txn-Tokens

   Txn-Tokens help prevent spurious invocations by ensuring that a
   workload receiving an invocation can independently verify the user or
   workload on whose behalf an external call was made and any context
   relevant to the processing of the call.  Through the presence of
   additional signatures on the Txn-Token, a workload receiving an
   invocation can also independently verify that specific workloads were
   within the path of the call before it was invoked.

2.5.  Txn-Token Issuance and Usage Flows

2.5.1.  Basic Flow

   Figure 1 shows the basic flow of how Txn-Tokens are used in an a
   multi-workload environment.

Tulshibagwale, et al.     Expires 22 April 2024                 [Page 5]
Internet-Draft                 Txn-Tokens                   October 2023

        1    ┌──────────────┐    2      ┌──────────────┐
   ─────────▶│              ├───────────▶              │
             │   External   │           │  Txn-Token   │
        7    │   Endpoint   │    3      │   Service    │
   ◀─────────┤              ◀───────────│              │
             └────┬───▲─────┘           └──────────────┘
                  │   │
                4 │   │ 6
             ┌────▼───┴─────┐
             │              │
             │   Internal   │
             │  µ-service   │
             │              │
             └────┬───▲─────┘
                  │   │
                  ▼   │
                    o
                5   o    6
                    o
                  │   ▲
                  │   │
             ┌────▼───┴─────┐
             │              │
             │   Internal   │
             │  µ-service   │
             │              │
             └──────────────┘

              Figure 1: Basic Transaction Tokens Architecture

   1.  External endpoint is invoked using conventional authorization
       mechanism such as an OAuth 2.0 Access token

   2.  External endpoint provides context and incoming authorization
       (e.g., access token) to the Txn-Token Service

   3.  Txn-Token Service mints a Txn-Token that provides immutable
       context for the transaction and returns it to the requester

   4.  The external endpoint initiates a call to an internal
       microservice and provides the Txn-Token as authorization

   5.  Subsequent calls to other internal microservices use the same
       Txn-Token to authorize calls

   6.  Responses are provided to callers based on successful
       authorization by the invoked microservices

Tulshibagwale, et al.     Expires 22 April 2024                 [Page 6]
Internet-Draft                 Txn-Tokens                   October 2023

   7.  External client is provided a response to the external invocation

2.5.2.  Replacement Txn-Token Flow

   An intermediate service may decide to obtain a replacement Txn-Token
   from the Txn-Token service.  That flow is described below in Figure 2

        1    ┌──────────────┐    2      ┌──────────────┐
   ─────────▶│              ├───────────▶              │
             │   External   │           │              │
        10   │   Endpoint   │    3      │              │
   ◀─────────┤              ◀───────────│              │
             └────┬───▲─────┘           │              │
                  │   │                 │              │
                4 │   │ 9               │              │
             ┌────▼───┴─────┐           │              │
             │              │           │              │
             │   Internal   │           │              │
             │  µ-service   │           │              │
             │              │           │              │
             └────┬───▲─────┘           │  Txn-Token   │
                  │   │                 │   Service    │
                  ▼   │                 │              │
                    o                   │              │
                5   o    9              │              │
                  │ o ▲                 │              │
                  │   │                 │              │
                  │   │                 │              │
             ┌────▼───┴─────┐    6      │              │
             │              ├───────────▶              │
             │   Internal   │           │              │
             │  µ-service   │    7      │              │
             │              ◀───────────│              │
             └────┬───▲─────┘           │              │
                  │   │                 │              │
                  ▼   │                 └──────────────┘
                    o
                8   o    9
                    o
                  │   ▲
                  │   │
             ┌────▼───┴─────┐
             │              │
             │   Internal   │
             │  µ-service   │
             │              │
             └──────────────┘

Tulshibagwale, et al.     Expires 22 April 2024                 [Page 7]
Internet-Draft                 Txn-Tokens                   October 2023

                    Figure 2: Replacement Txn-Token Flow

   In the diagram above, steps 1-5 are the same as in Section 2.5.1

   6.  An intermediate service determines that it needs to obtain a
       Replacement Txn-Token.  It requests a Replacement Txn-Token from
       the Txn-Token Service.  It passes the incoming Txn-Token in the
       request, along with any additional context it needs to send the
       Txn-Token Service.

   7.  The Txn-Token Service responds with a replacement Txn-Token

   8.  The service that requested the Replacement Txn-Token uses that
       Txn-Token for downstream call authorization

   9.  Responses are provided to callers based on successful
       authorization by the invoked microservices

   10. External client is provided a response to the external invocation

3.  Notational 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 BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

4.  Terminology

   Workload:  An independent computational unit that can autonomously
      receive and process invocations, and can generate invocations of
      other workloads.  Examples of workloads include containerized
      microservices, monolithic services and infrastructure services
      such as managed databases.

   Trust Domain:  A virtually or physically separated network, which
      contains two or more workloads.  The workloads within an Trust
      Domain may be invoked only through published interfaces.  A Trust
      Domain must have an identifier that is used as the aud (audience)
      value in Txn-Tokens.  The format of this identifier is a universal
      resource identifier.  Each Trust Domain has exactly one Txn-Token
      Service.

   External Endpoint:  A published interface to an Trust Domain that
      results in the invocation of a workload within the Trust Domain.

   Call Chain:  A sequence of invocations that results from the

Tulshibagwale, et al.     Expires 22 April 2024                 [Page 8]
Internet-Draft                 Txn-Tokens                   October 2023

      invocation of an external endpoint.

   Transaction Token (Txn-Token):  A signed JWT that has a short
      lifetime, which provides immutable information about the user or
      workload, certain parameters of the call and certain contextual
      attributes of the call.

   Authorization Context:  A JSON object containing a set of claims that
      represent the immutable context of a call chain.

   Transaction Token Service (Txn-Token Service):  A special service
      within the Trust Domain, which issues Txn-Tokens to requesting
      workloads.  Each Trust Domain has exactly one Txn-Token Service.

5.  Txn-Token Format

   A Txn-Token is a JSON Web Token [RFC7519] protected by a JSON Web
   Signature [RFC7515].  The following describes the required values in
   a Txn-Token:

5.1.  JWT Header

   In the JWT Header:

   *  The typ claim MUST be present and MUST have the value txn_token.

   *  Key rotation of the signing key SHOULD be supported through the
      use of a kid claim.

   Figure 3 is a non-normative example of the JWT Header of a Txn-Token

   {
       "typ": "txn_token",
       "alg": "RS256",
       "kid": "identifier-to-key"
   }

                    Figure 3: Example: Txn-Token Header

5.2.  JWT Body

5.2.1.  Required Claims

   The JWT body MUST have the following claims:

   *  An iss claim, whose value is a URN [RFC8141] that uniquely
      identifies the workload or the Txn-Token Service that created the
      Txn-Token.

Tulshibagwale, et al.     Expires 22 April 2024                 [Page 9]
Internet-Draft                 Txn-Tokens                   October 2023

   *  An iat claim, whose value is the time at which the Txn-Token was
      created.

   *  An aud claim, whose value is a URN [RFC8141] that uniquely
      identifies the audience of the Txn-Token.  This MUST identify the
      trust domain in which the Txn-Token is used.

   *  An exp claim, whose value is the time at which the Txn-Token
      expires.

   *  A txn claim, whose value is the unique transaction identifier as
      defined in Section 2.2 of [RFC8417].  When used in the transaction
      token, it identifies the entire call chain.

   *  A sub_id claim, whose value is the unique identifier of the user
      or workload on whose behalf the call chain is being executed.  The
      format of this claim MAY be a Subject Identifier as specified in
      [SubjectIdentifiers].

   *  An azd claim, whose value is a JSON object that contains values
      that remain constant in the call chain.

5.2.2.  Optional Claims

   The JWT body MAY have the following claims:

5.2.2.1.  Requester Context

   The Txn-Token MAY contain an req_ctx claim, whose value is a JSON
   object the describes the requester context of the transaction.  This
   MAY include the IP address information of the originating user, as
   well as information about the computational entity that requested the
   Txn-Token.

   The JSON value of the req_ctx claim MAY include any values the Txn-
   Token Service determines are interesting to downstream services that
   rely on the Txn-Token.  The following claims are defined so that if
   they are included, they have the following meaning:

   *  req_ip The IP address of the requester.  This MAY be the end-user
      or a robotic process that requested the Transaction

   *  authn The authentication method used to idenitfy the requester.
      Its value is a URN that uniquely identifies the method used.

   *  req_wl The requesting workload.  A URN that uniquely identifies
      the computational entity that requested the Txn-Token.  This
      entity MUST be within the Trust Domain of the Txn-Token.

Tulshibagwale, et al.     Expires 22 April 2024                [Page 10]
Internet-Draft                 Txn-Tokens                   October 2023

5.2.2.2.  Purpose

   The Txn-Token MAY contain a purp claim, whose value specifies the
   purpose of the transaction.  The format of this claim is a JSON
   string.

5.2.3.  Example

   The figure below Figure 4 shows a non-normative example of the JWT
   body of a Txn-Token:

   {
       "iss": "https://trust-domain.example/txn-token-service",
       "iat": "1686536226000",
       "aud": "trust-domain.example",
       "exp": "1686536526000",
       "txn": "97053963-771d-49cc-a4e3-20aad399c312",
       "sub_id": {
           "format": "email",
           "email": "user@trust-domain.example"
       },
       "req_ctx": {
         "req_ip": "69.151.72.123", // env context of external call
         "authn": "urn:ietf:rfc:6749", // env context of the external call
         "req_wl": "apigateway.trust-domain.example" // the internal entity that requested the Txn-Token
       },
       "purp" : "trade.stocks",
       "azd": {
           "action": "BUY", // parameter of external call
           "ticker": "MSFT", // parameter of external call
           "quantity": "100", // parameter of external call
           "user_level": "vip" // computed value not present in external call
       }
   }

                     Figure 4: Example: Txn-Token Body

6.  Txn-Token Service

   A Txn-Token Service provides a OAuth 2.0 Token Exchange [RFC8693]
   endpoint that can respond to Txn-Token issuance requests.  The token
   exchange requests it supports require extra parameters than those
   defined in the OAuth 2.0 Token Exchange [RFC8693] specification.  The
   unique properties of the Txn-Token requests and responses are
   described below.  The Txn-Token Service MAY optionally support other
   OAuth 2.0 endpoints and features, but that is not a requirement for
   it to be a Txn-Token Service.

Tulshibagwale, et al.     Expires 22 April 2024                [Page 11]
Internet-Draft                 Txn-Tokens                   October 2023

   Each Trust Domain MUST have exactly one Txn-Token Service.

7.  Requesting Txn-Tokens

   A workload requests a Txn-Token from a Transaction Token Service
   using OAuth 2.0 Token Exchange [RFC8693].  The request to obtain a
   Txn-Token using this method is called a Txn-Token Request, and a
   successful response is called a Txn-Token Response.  A Txn-Token
   Request is a Token Exchange Request, as described in Section 2.1 of
   [RFC8693] with additional parameters.  A Txn-Token Response is a
   OAuth 2.0 token endpoint response, as described in Section 5 of
   [RFC6749], where the token_type in the response has the value
   txn_token.

7.1.  Txn-Token Request

   A Txn-Token Request is an OAuth 2.0 Token Exchange Request, as
   described in Section 2.1 of [RFC8693], with an additional parameter
   in the request.  The following parameters are required in the Txn-
   Token Request by the OAuth 2.0 Token Exchange specification
   [RFC8693]:

   *  The audience value MUST be set to the Trust Domain name

   *  The requested_token_type value MUST be
      urn:ietf:params:oauth:token-type:txn_token

   *  The subject_token value MUST be the external token received by the
      workload that authorized the call

   *  The subject_token_type value MUST be present and indicate the type
      of the authorization token present in the subject_token parameter

   The following additional parameter MUST be present in a Txn-Token
   Request:

   *  A parameter named rctx , whose value is a JSON object.  This
      object contains the request context, i.e. any information the
      Transaction Token Service needs to understand the context of the
      incoming request

   Figure 5 shows a non-normative example of a Txn-Token Request.

Tulshibagwale, et al.     Expires 22 April 2024                [Page 12]
Internet-Draft                 Txn-Tokens                   October 2023

   POST /txn-token-service/token_endpoint HTTP 1.1
   Host: txn-token-service.trust-domain.example
   Content-Type: application/x-www-form-urlencoded

   requested_token_type=urn%3Aietf%3Aparams%3Aoauth%3Atoken-type%3Atxn_token
   &audience=http%3A%2F%2Ftrust-domain.example
   &subject_token=eyJhbGciOiJFUzI1NiIsImtpZC...kdXjwhw
   &subject_token_type=urn%3Aietf%3Aparams%3Aoauth%3Atoken-type%3Aaccess_token
   &rctx=%7B%22param1%22%3A%22value1%22%2C%22param2%22%3A%22value2%22%2C%22ip_address%22%3A%2269.151.72.123%22%7D

                    Figure 5: Example: Txn-Token Request

7.2.  Txn-Token Response

   A successful response to a Txn-Token Request by a Transaction Token
   Service is called a Txn-Token Response.  If the Transaction Token
   Service responds with an error, the error response is as described in
   Section 5.2 of [RFC6749].  The following describes required values of
   a Txn-Token Response:

   *  The token_type value MUST be set to txn_token

   *  The access_token value MUST be the Txn-Token

   *  The response MUST NOT include the values expires_in, refresh_token
      and scope

   Figure 6 shows a non-normative example of a Txn-Token Response.

   HTTP/1.1 200 OK
   Content-Type: application/json
   Cache-Control: no-cache, no-store

   {
     "issued_token_type": "urn:ietf:params:oauth:token-type:txn_token",
     "access_token": "eyJCI6IjllciJ9...Qedw6rx"
   }

                   Figure 6: Example: Txn-Token Response

7.3.  Creating Replacement Txn-Tokens

   A workload within a call chain may request the Transaction Token
   Server to replace a Txn-Token.

   Workloads MAY request replacement Txn-Tokens in order to change (add
   to, remove or modify) the asserted values within a Txn-Token.

Tulshibagwale, et al.     Expires 22 April 2024                [Page 13]
Internet-Draft                 Txn-Tokens                   October 2023

   The value of the aud claim MUST remain unchanged in a replacement
   Txn-Token.  If the claim req_ctx is present in the original Txn-
   Token, then it MUST be present unchanged in the replacement Txn-
   Token.

7.3.1.  Txn-Token Service Responsibilities

   A Txn-Token Service replacing a Txn-Token must consider that
   modifying previously asserted values from existing Txn-Tokens can
   completely negate the benefits of Txn-Tokens.  When issuing
   replacement Txn-Tokens, a Transaction Token Server therefore:

   *  MAY enable modifications to asserted values that reduce the scope
      of permitted actions

   *  MAY enable additional asserted values

   *  SHOULD NOT enable modification to asserted values that expand the
      scope of permitted actions

7.3.2.  Replacement Txn-Token Request

   To request a replacement Txn-Token, the requester makes a Txn-Token
   Request as described in Section 7.1 but includes the Txn-Token to be
   replaced as the value of the subject_token parameter.

7.3.3.  Replacement Txn-Token Response

   A successful response by the Transaction Token Server to a
   Replacement Txn-Token Request is a Txn-Token Response as described in
   Section 7.2

7.4.  Mutual Authentication of the Txn-Token Request

   A Txn-Token Service MUST ensure that it authenticates any workloads
   requesting Txn-Tokens.  In order to do so:

   *  It MUST name a limited, pre-configured set of workloads that MAY
      request Txn-Tokens

   *  It MUST individually authenticate the requester as being one of
      the named requesters

   *  It SHOULD rely on mechanisms, such as [Spiffe] or some other means
      of performing MTLS [RFC8446], to securely authenticate the
      requester

Tulshibagwale, et al.     Expires 22 April 2024                [Page 14]
Internet-Draft                 Txn-Tokens                   October 2023

   *  It SHOULD NOT rely on insecure mechanisms, such as long-lived
      shared secrets to authenticate the requesters

   The requesting workload MUST have a pre-configured location for the
   Transaction Token Service.  It SHOULD rely on mechanisms, such as
   [Spiffe], to securely authenticate the Transaction Token Service
   before making a Txn-Token Request.

8.  IANA Considerations

   This specification registers the following claims defined in
   Section Section 5.1 to the OAuth Access Token Types Registry defined
   in [RFC6749], and the following claims defined in
   Section Section 5.2.1 in the IANA JSON Web Token Claims Registry
   defined in [RFC7519]

8.1.  OAuth Registry Contents

   *  Name: txn_token

   *  Description: JWT of type Transaction Token

   *  Additional Token Endpoint Response Parameters: none

   *  HTTP Authentication Schemes: TLS [RFC8446]

   *  Change Controller: IESG

   *  Specification Document: Section Section 5.1 of this specificaiton

8.2.  JWT Registry Contents

   *  Claim Name: azd

      -  Claim Description: The authorization context details

      -  Change Controller: IESG

      -  Specification Document: Section Section 5.2.1 of this
         specification

   *  Claim Name: req_ctx

      -  Claim Description: The requester context

      -  Change Controller: IESG

Tulshibagwale, et al.     Expires 22 April 2024                [Page 15]
Internet-Draft                 Txn-Tokens                   October 2023

      -  Specification Document: Section Section 5.2.2.1 of this
         specification

   *  Claim Name: purp

      -  Claim Description: The purpose of the transaction

      -  Change Controller: IESG

      -  Specification Document: Section Section 5.2.2.2 of this
         specification

9.  Security Considerations

9.1.  Txn-Token Lifetime

   A Txn-Token is not resistant to replay attacks.  A long-lived Txn-
   Token therefore represents a risk if it is stored in a file,
   discovered by an attacker, and then replayed.  For this reason, a
   Txn-Token lifetime must be kept short, not exceeding the lifetime of
   a call-chain.  Even for long-running "batch" jobs, a longer lived
   access token should be used to initiate the request to the batch
   endpoint.  It then obtains short-lived Txn-Tokens that may be used to
   authorize the call to downstream services in the call-chain.

   Because Txn-Tokens are short-lived, the Txn-Token response from the
   Txn-Token service does not contain the refresh_token field.  A Txn-
   Token cannot be issued by presenting a refresh_token.

   The expires_in and scope fields of the OAuth 2.0 Token Exchange
   specification [RFC8693] are also not used in Txn-Token responses.
   The expires_in is not required since the issued token has an exp
   field, which indicates the token lifetime.  The scope field is
   omitted from the request and therefore omitted in the response.

9.2.  Sender Constrained Tokens

   Although Txn-Tokens are short-lived, they MAY be sender constrained
   as an additional layer of defence to prevent them from being re-used
   by a compromised or malicious workload under the control of a hostile
   actor.

Tulshibagwale, et al.     Expires 22 April 2024                [Page 16]
Internet-Draft                 Txn-Tokens                   October 2023

9.3.  Access Tokens

   When creating Txn-Tokens, the Txn-Token MUST NOT contain the Access
   Token presented to the external endpoint.  If an Access Token is
   included in a Txn-Token, an attacker may extract the Access Token
   from the Txn-Token, and replay it to any Resource Server that can
   accept that Access Token.  Txn-Token expiry does not protect against
   this attack since the Access Token may remain valid even after the
   Txn-Token has expired.

10.  References

10.1.  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,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/rfc/rfc8446>.

   [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>.

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <https://www.rfc-editor.org/rfc/rfc7519>.

   [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
              2015, <https://www.rfc-editor.org/rfc/rfc7515>.

   [RFC8141]  Saint-Andre, P. and J. Klensin, "Uniform Resource Names
              (URNs)", RFC 8141, DOI 10.17487/RFC8141, April 2017,
              <https://www.rfc-editor.org/rfc/rfc8141>.

   [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>.

   [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>.

Tulshibagwale, et al.     Expires 22 April 2024                [Page 17]
Internet-Draft                 Txn-Tokens                   October 2023

   [RFC8417]  Hunt, P., Ed., Jones, M., Denniss, W., and M. Ansari,
              "Security Event Token (SET)", RFC 8417,
              DOI 10.17487/RFC8417, July 2018,
              <https://www.rfc-editor.org/rfc/rfc8417>.

   [OpenIdConnect]
              Sakimura, N., Bradley, J., Jones, M., Medeiros, B. de.,
              and C. Mortimore, "OpenID Connect Core 1.0 incorporating
              errata set 1", November 2014,
              <https://openid.net/specs/openid-connect-core-1_0.html>.

   [SubjectIdentifiers]
              Backman, A., Scurtescu, M., and P. Jain, "Subject
              Identifiers for Security Event Tokens", n.d.,
              <https://datatracker.ietf.org/doc/html/draft-ietf-
              secevent-subject-identifiers>.

10.2.  Informative References

   [Spiffe]   Cloud Native Computing Foundation, "Secure Production
              Identity Framework for Everyone", n.d.,
              <https://spiffe.io/docs/latest/spiffe-about/overview/>.

Acknowledgements

Contributors

   Dr. Kelley W. Burgin, PhD.
   MITRE Corporation
   Email: kburgin@mitre.org

   Hannes Tschofenig
   Arm Ltd.
   Email: Hannes.Tschofenig@arm.com

   Evan Gilman
   SPIRL
   Email: evan@spirl.com

   Arndt Schwenkschuster
   Microsoft
   Email: arndts@microsoft.com

Tulshibagwale, et al.     Expires 22 April 2024                [Page 18]
Internet-Draft                 Txn-Tokens                   October 2023

Authors' Addresses

   Atul Tulshibagwale
   SGNL
   Email: atul@sgnl.ai

   George Fletcher
   Capital One
   Email: george.fletcher@capitalone.com

   Pieter Kasselman
   Microsoft
   Email: pieter.kasselman@microsoft.com

Tulshibagwale, et al.     Expires 22 April 2024                [Page 19]