Internet Engineering Task Force (IETF)                        Y. Sheffer
Request for Comments: 9115                                        Intuit
Category: Standards Track                                       D. López
ISSN: 2070-1721                                        A. Pastor Perales
                                                          Telefonica I+D
                                                              T. Fossati
                                                                     ARM
                                                          September 2021


   An Automatic Certificate Management Environment (ACME) Profile for
                   Generating Delegated Certificates

Abstract

   This document defines a profile of the Automatic Certificate
   Management Environment (ACME) protocol by which the holder of an
   identifier (e.g., a domain name) can allow a third party to obtain an
   X.509 certificate such that the certificate subject is the delegated
   identifier while the certified public key corresponds to a private
   key controlled by the third party.  A primary use case is that of a
   Content Delivery Network (CDN), the third party, terminating TLS
   sessions on behalf of a content provider (the holder of a domain
   name).  The presented mechanism allows the holder of the identifier
   to retain control over the delegation and revoke it at any time.
   Importantly, this mechanism does not require any modification to the
   deployed TLS clients and servers.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9115.

Copyright Notice

   Copyright (c) 2021 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 and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
     1.1.  Terminology
     1.2.  Conventions Used in This Document
   2.  Protocol Flow
     2.1.  Preconditions
     2.2.  Overview
     2.3.  Delegated Identity Profile
       2.3.1.  Delegation Configuration
       2.3.2.  Order Object Transmitted from NDC to IdO and to ACME
               Server (STAR)
       2.3.3.  Order Object Transmitted from NDC to IdO and to ACME
               Server (Non-STAR)
       2.3.4.  Capability Discovery
       2.3.5.  Negotiating an Unauthenticated GET
       2.3.6.  Terminating the Delegation
     2.4.  Proxy Behavior
   3.  CA Behavior
   4.  CSR Template
     4.1.  Template Syntax
     4.2.  Example
   5.  Further Use Cases
     5.1.  CDN Interconnection (CDNI)
       5.1.1.  Multiple Parallel Delegates
       5.1.2.  Chained Delegation
     5.2.  Secure Telephone Identity Revisited (STIR)
   6.  IANA Considerations
     6.1.  New Fields in the "meta" Object within a Directory Object
     6.2.  New Fields in the Order Object
     6.3.  New Fields in the Account Object
     6.4.  New Error Types
     6.5.  CSR Template Extensions
   7.  Security Considerations
     7.1.  Trust Model
     7.2.  Delegation Security Goal
     7.3.  New ACME Channels
     7.4.  Restricting CDNs to the Delegation Mechanism
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Appendix A.  CSR Template: CDDL
   Appendix B.  CSR Template: JSON Schema
   Acknowledgements
   Authors' Addresses

1.  Introduction

   This document is related to [RFC8739], in that some important use
   cases require both documents to be implemented.  To avoid
   duplication, we give here a bare-bones description of the motivation
   for this solution.  For more details, please refer to the
   introductory sections of [RFC8739].

   An Identifier Owner (IdO) has agreements in place with one or more
   Name Delegation Consumer (NDC) to use and attest its identity.

   In the primary use case, the IdO is a content provider, and we
   consider a Content Delivery Network (CDN) provider contracted to
   serve the content over HTTPS.  The CDN terminates the HTTPS
   connection at one of its edge cache servers and needs to present its
   clients (browsers, mobile apps, set-top boxes) a certificate whose
   name matches the domain name of the URL that is requested, i.e., that
   of the IdO.  Understandably, some IdOs may balk at sharing their
   long-term private keys with another organization; equally, delegates
   would rather not have to handle other parties' long-term secrets.
   Other relevant use cases are discussed in Section 5.

   This document describes a profile of the ACME protocol [RFC8555] that
   allows the NDC to request from the IdO, acting as a profiled ACME
   server, a certificate for a delegated identity -- i.e., one belonging
   to the IdO.  The IdO then uses the ACME protocol (with the extensions
   described in [RFC8739]) to request issuance of a Short-Term,
   Automatically Renewed (STAR) certificate for the same delegated
   identity.  The generated short-term certificate is automatically
   renewed by the ACME Certification Authority (CA), is periodically
   fetched by the NDC, and is used to terminate HTTPS connections in
   lieu of the IdO.  The IdO can end the delegation at any time by
   simply instructing the CA to stop the automatic renewal and letting
   the certificate expire shortly thereafter.

   While the primary use case we address is a delegation of STAR
   certificates, the mechanism proposed here also accommodates long-
   lived certificates managed with the ACME protocol.  The most
   noticeable difference between long-lived and STAR certificates is the
   way the termination of the delegation is managed.  In the case of
   long-lived certificates, the IdO uses the "revokeCert" URL exposed by
   the CA and waits for the explicit revocation based on the Certificate
   Revocation List (CRL) and Online Certificate Status Protocol (OCSP)
   to propagate to the relying parties.

   In case the delegated identity is a domain name, this document also
   provides a way for the NDC to inform the IdO about the CNAME mappings
   that need to be installed in the IdO's DNS zone to enable the
   aliasing of the delegated name, thus allowing the complete name
   delegation workflow to be handled using a single interface.

   We note that other standardization efforts address the problem of
   certificate delegation for TLS connections, specifically
   [TLS-SUBCERTS] and [MGLT-LURK-TLS13].  The former extends the TLS
   certificate chain with a customer-owned signing certificate; the
   latter separates the server's private key into a dedicated, more-
   secure component.  Compared to these other approaches, the current
   document does not require changes to the TLS network stack of the
   client or the server, nor does it introduce additional latency to the
   TLS connection.

1.1.  Terminology

   IdO     Identifier Owner, the holder (current owner) of an identifier
           (e.g., a domain name) that needs to be delegated.  Depending
           on the context, the term IdO may also be used to designate
           the (profiled) ACME server deployed by the Identifier Owner
           or the ACME client used by the Identifier Owner to interact
           with the CA.

   NDC     Name Delegation Consumer, the entity to which the domain name
           is delegated for a limited time.  This is a CDN in the
           primary use case (in fact, readers may note the similarity of
           the two abbreviations).  Depending on the context, the term
           NDC may also be used to designate the (profiled) ACME client
           used by the Name Delegation Consumer.

   CDN     Content Delivery Network, a widely distributed network that
           serves the domain's web content to a wide audience at high
           performance.

   STAR    Short-Term, Automatically Renewed, as applied to X.509
           certificates.

   ACME    Automated Certificate Management Environment, a certificate
           management protocol [RFC8555].

   CA      Certification Authority, specifically one that implements the
           ACME protocol.  In this document, the term is synonymous with
           "ACME server deployed by the Certification Authority".

   CSR     Certificate Signing Request, specifically a PKCS#10 [RFC2986]
           Certificate Signing Request, as supported by ACME.

   FQDN    Fully Qualified Domain Name.

1.2.  Conventions Used in This Document

   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.  Protocol Flow

   This section presents the protocol flow.  For completeness, we
   include the ACME profile proposed in this document as well as the
   ACME STAR protocol described in [RFC8739].

2.1.  Preconditions

   The protocol assumes the following preconditions are met:

   *  The IdO exposes an ACME server interface to the NDC(s) comprising
      the account management interface.

   *  The NDC has registered an ACME account with the IdO.

   *  The NDC and IdO have agreed on a "CSR template" to use, including
      at a minimum: subject name (e.g., "abc.ido.example"), requested
      algorithms and key length, key usage, and extensions.  The NDC
      will use this template for every CSR created under the same
      delegation.

   *  The IdO has registered an ACME account with the Certification
      Authority (CA).

   Note that even if the IdO implements the ACME server role, it is not
   acting as a CA; in fact, from the point of view of the certificate
   issuance process, the IdO only works as a "policing" forwarder of the
   NDC's key pair and is responsible for completing the identity
   verification process towards the CA.

2.2.  Overview

   For clarity, the protocol overview presented here covers the main use
   case of this protocol, namely delegation of STAR certificates.
   Protocol behavior for non-STAR certificates is similar, and the
   detailed differences are listed in the following sections.

   The interaction between the NDC and the IdO is governed by the
   profiled ACME workflow detailed in Section 2.3.  The interaction
   between the IdO and the CA is ruled by ACME [RFC8555], ACME STAR
   [RFC8739], and any other ACME extension that applies (e.g.,
   [TOKEN-TNAUTHLIST] for Secure Telephone Identity Revisited (STIR)).

   The outline of the combined protocol for STAR certificates is as
   follows (Figure 1):

   *  NDC sends an Order1 for the delegated identifier to IdO.

   *  IdO creates an Order1 resource in state "ready" with a "finalize"
      URL.

   *  NDC immediately sends a "finalize" request (which includes the
      CSR) to the IdO.

   *  IdO verifies the CSR according to the agreed upon CSR template.

   *  If the CSR verification fails, Order1 is moved to an "invalid"
      state and everything stops.

   *  If the CSR verification is successful, IdO moves Order1 to state
      "processing" and sends a new Order2 (using its own account) for
      the delegated identifier to the CA.

   *  If the ACME STAR protocol fails, Order2 moves to "invalid", and
      the same state is reflected in Order1 (i.e., the NDC Order).

   *  If the ACME STAR run is successful (i.e., Order2 is "valid"), IdO
      copies the "star-certificate" URL from Order2 to Order1 and
      updates the Order1 state to "valid".

   The NDC can now download, install, and use the short-term certificate
   bearing the name delegated by the IdO.  The STAR certificate can be
   used until it expires, at which time the NDC is guaranteed to find a
   new certificate it can download, install, and use.  This continues
   with subsequent certificates until either Order1 expires or the IdO
   decides to cancel the automatic renewal process with the CA.

   Note that the interactive identifier authorization phase described in
   Section 7.5 of [RFC8555] is suppressed on the NDC-IdO side because
   the delegated identity contained in the CSR presented to the IdO is
   validated against the configured CSR template (Section 4.1).
   Therefore, the NDC sends the "finalize" request, including the CSR,
   to the IdO immediately after Order1 has been acknowledged.  The IdO
   SHALL buffer a (valid) CSR until the Validation phase completes
   successfully.

   Also note that the successful negotiation of the unauthenticated GET
   (Section 3.4 of [RFC8739]) is required in order to allow the NDC to
   access the "star-certificate" URL on the CA.

    .------.            .---------------.            .------.
   |  NDC   |          |       IdO       |          |  ACME  |
   +--------+          +--------+--------+          +--------+
   | Client |          | Server | Client |          | Server |
   '---+----'          '----+---+---+----'          '----+---'
       |                    |       |                    |
       |   Order1           |       |                    |
       |   Signature        |       |                    |
       o------------------->|       |                    |
       |                    |       |                    |
       | [ No identity    ] |       |                    |
       | [ validation via ] |       |                    |
       | [ authorizations ] |       |                    |
       |                    |       |                    |
       |   CSR              |       |                    |
       |   Signature        |       |                    |
       o------------------->|       |                    |
       |   Acknowledgement  |       |   Order2           |
       |<-------------------o       |   Signature        |
       |                    |       o------------------->|
       |                    |       |         Required   |
       |                    |       |   Authorizations   |
       |                    |       |<-------------------o
       |                    |       |   Responses        |
       |                    |       |   Signature        |
       |                    |       o------------------->|
       |                    |       |                    |
       |                    |       |<~~~~Validation~~~~>|
       |                    |       |                    |
       |                    |       |   CSR              |
       |                    |       |   Signature        |
       |                    |       o------------------->|
       |                    |       |   Acknowledgement  |
       |                    |       |<-------------------o
       |                    |       |                    |
       |<~~Await issuance~->|       |<~~Await issuance~~>|
       |                                                 |
       |     (unauthenticated) GET STAR certificate      |
       o------------------------------------------------>|
       |                 Certificate #1                  |
       |<------------------------------------------------o
       |     (unauthenticated) GET STAR certificate      |
       o------------------------------------------------>|
       |                 Certificate #2                  |
       |<------------------------------------------------o
       |                     [...]                       |
       |     (unauthenticated) GET STAR certificate      |
       o------------------------------------------------>|
       |                 Certificate #n                  |
       |<------------------------------------------------o

                 Figure 1: End-to-End STAR Delegation Flow

2.3.  Delegated Identity Profile

   This section defines a profile of the ACME protocol to be used
   between the NDC and IdO.

2.3.1.  Delegation Configuration

   The IdO must be preconfigured to recognize one or more NDCs and
   present them with details about certificate delegations that apply to
   each one.

2.3.1.1.  Account Object Extensions

   An NDC identifies itself to the IdO as an ACME account.  The IdO can
   delegate multiple names to an NDC, and these configurations are
   described through "delegation" objects associated with the NDC's
   account object on the IdO.

   As shown in Figure 2, the ACME account resource on the IdO is
   extended with a new "delegations" attribute:

   delegations (required, string):  A URL from which a list of
      delegations configured for this account (Section 2.3.1.3) can be
      fetched via a POST-as-GET request.

   {
     "status": "valid",
     "contact": [
       "mailto:delegation-admin@ido.example"
     ],
     "termsOfServiceAgreed": true,
     "orders": "https://example.com/acme/orders/saHpfB",
     "delegations": "https://acme.ido.example/acme/delegations/adFqoz"
   }

             Figure 2: Example Account Object with Delegations

2.3.1.2.  Delegation Lists

   Each account object includes a "delegations" URL from which a list of
   delegation configurations created by the IdO can be fetched via a
   POST-as-GET request.  The result of the request MUST be a JSON object
   whose "delegations" field is an array of URLs, each identifying a
   delegation configuration made available to the NDC account
   (Section 2.3.1.3).  The server MAY return an incomplete list, along
   with a "Link" header field with a "next" link relation indicating
   where further entries can be acquired.

   HTTP/1.1 200 OK
   Content-Type: application/json
   Link: <https://acme.ido.example/acme/directory>;rel="index"
   Link: <https://acme.ido.example/acme/delegations/adFqoz?/
         cursor=2>;rel="next"

   {
     "delegations": [
       "https://acme.ido.example/acme/delegation/ogfr8EcolOT",
       "https://acme.ido.example/acme/delegation/wSi5Lbb61E4",
       /* more URLs not shown for example brevity */
       "https://acme.ido.example/acme/delegation/gm0wfLYHBen"
     ]
   }

   Note that in the figure above,
   https://acme.ido.example/acme/delegations/adFqoz?cursor=2 includes a
   line break for the sake of presentation.

2.3.1.3.  Delegation Objects

   This profile extends the ACME resource model with a new read-only
   "delegation" object that represents a delegation configuration that
   applies to a given NDC.

   A "delegation" object contains the CSR template (see Section 4) that
   applies to that delegation and, optionally, any related CNAME mapping
   for the delegated identifiers.  Its structure is as follows:

   csr-template (required, object):  CSR template, as defined in
      Section 4.

   cname-map (optional, object):  A map of FQDN pairs.  In each pair,
      the name is the delegated identifier; the value is the
      corresponding NDC name that is aliased in the IdO's zone file to
      redirect the resolvers to the delegated entity.  Both names and
      values MUST be FQDNs with a terminating '.'.  This field is only
      meaningful for identifiers of type "dns".

   An example "delegation" object in JSON format is shown in Figure 3.

   {
     "csr-template": {
       "keyTypes": [
         {
           "PublicKeyType": "id-ecPublicKey",
           "namedCurve": "secp256r1",
           "SignatureType": "ecdsa-with-SHA256"
         }
       ],
       "subject": {
         "country": "CA",
         "stateOrProvince": "**",
         "locality": "**"
       },
       "extensions": {
         "subjectAltName": {
           "DNS": [
             "abc.ido.example"
           ]
         },
         "keyUsage": [
           "digitalSignature"
         ],
         "extendedKeyUsage": [
           "serverAuth"
         ]
       }
     },
     "cname-map": {
       "abc.ido.example.": "abc.ndc.example."
     }
   }

             Figure 3: Example Delegation Configuration Object

   In order to indicate which specific delegation applies to the
   requested certificate, a new "delegation" attribute is added to the
   order object on the NDC-IdO side (see Figures 4 and 7).  The value of
   this attribute is the URL pointing to the delegation configuration
   object that is to be used for this certificate request.  If the
   "delegation" attribute in the order object contains a URL that does
   not correspond to a configuration available to the requesting ACME
   account, the IdO MUST return an error response with status code 403
   (Forbidden), providing a problem document [RFC7807] with type
   "urn:ietf:params:acme:error:unknownDelegation".

2.3.2.  Order Object Transmitted from NDC to IdO and to ACME Server
        (STAR)

   If the delegation is for a STAR certificate, the request object
   created by the NDC:

   *  MUST have a "delegation" attribute indicating the preconfigured
      delegation that applies to this Order;

   *  MUST have entries in the "identifiers" field for each delegated
      name present in the configuration;

   *  MUST NOT contain the "notBefore" and "notAfter" fields; and

   *  MUST contain an "auto-renewal" object and, inside it, the fields
      listed in Section 3.1.1 of [RFC8739].  In particular, the "allow-
      certificate-get" attribute MUST be present and set to true.

   POST /acme/new-order HTTP/1.1
   Host: acme.ido.example
   Content-Type: application/jose+json

   {
     "protected": base64url({
       "alg": "ES256",
       "kid": "https://acme.ido.example/acme/acct/evOfKhNU60wg",
       "nonce": "Alc00Ap6Rt7GMkEl3L1JX5",
       "url": "https://acme.ido.example/acme/new-order"
     }),
     "payload": base64url({
       "identifiers": [
         {
           "type": "dns",
           "value": "abc.ido.example"
         }
       ],
       "auto-renewal": {
         "end-date": "2021-04-20T00:00:00Z",
         "lifetime": 345600,          // 4 days
         "allow-certificate-get": true
       },
       "delegation":
         "https://acme.ido.example/acme/delegation/gm0wfLYHBen"
     }),
     "signature": "g454e3hdBlkT4AEw...nKePnUyZTjGtXZ6H"
   }

                     Figure 4: New STAR Order from NDC

   The order object that is created on the IdO:

   *  MUST start in the "ready" state;

   *  MUST contain an "authorizations" array with zero elements;

   *  MUST contain the indicated "delegation" configuration;

   *  MUST contain the indicated "auto-renewal" settings; and

   *  MUST NOT contain the "notBefore" and "notAfter" fields.

   {
     "status": "ready",
     "expires": "2021-05-01T00:00:00Z",

     "identifiers": [
      {
        "type": "dns",
        "value": "abc.ido.example"
      }
     ],

     "auto-renewal": {
       "end-date": "2021-04-20T00:00:00Z",
       "lifetime": 345600,
       "allow-certificate-get": true
     },

     "delegation":
       "https://acme.ido.example/acme/delegation/gm0wfLYHBen",

     "authorizations": [],

     "finalize": "https://acme.ido.example/acme/order/TO8rfgo/finalize"
   }

                Figure 5: STAR Order Resource Created on IdO

   The Order is then finalized by the NDC supplying the CSR containing
   the delegated identifiers.  The IdO checks the provided CSR against
   the template contained in the "delegation" object that applies to
   this request, as described in Section 4.1.  If the CSR fails
   validation for any of the identifiers, the IdO MUST return an error
   response with status code 403 (Forbidden) and an appropriate type,
   e.g., "rejectedIdentifier" or "badCSR".  The error response SHOULD
   contain subproblems (Section 6.7.1 of [RFC8555]) for each failed
   identifier.  If the CSR is successfully validated, the order object
   status moves to "processing" and the twin ACME protocol instance is
   initiated on the IdO-CA side.

   The request object created by the IdO:

   *  MUST copy the identifiers sent by the NDC;

   *  MUST strip the "delegation" attribute; and

   *  MUST carry a copy of the "auto-renewal" object sent by the NDC.

   When the identifiers' authorization has been successfully completed
   and the certificate has been issued by the CA, the IdO:

   *  MUST move its Order resource status to "valid" and

   *  MUST copy the "star-certificate" field from the STAR Order
      returned by the CA into its Order resource.  When dereferenced,
      the "star-certificate" URL includes (via the "Cert-Not-Before" and
      "Cert-Not-After" HTTP header fields) the renewal timers needed by
      the NDC to inform its certificate reload logic.

   {
     "status": "valid",
     "expires": "2021-05-01T00:00:00Z",

     "identifiers": [
      {
        "type": "dns",
        "value": "abc.ido.example"
      }
     ],

     "auto-renewal": {
       "end-date": "2021-04-20T00:00:00Z",
       "lifetime": 345600,
       "allow-certificate-get": true
     },

     "delegation":
       "https://acme.ido.example/acme/delegation/gm0wfLYHBen",

     "authorizations": [],

     "finalize": "https://acme.ido.example/acme/order/TO8rfgo/finalize",

     "star-certificate": "https://acme.ca.example/acme/order/yTr23sSDg9"
   }

                Figure 6: STAR Order Resource Updated on IdO

   This delegation protocol is predicated on the NDC being able to fetch
   certificates periodically using an unauthenticated HTTP GET, since,
   in general, the NDC does not possess an account on the CA; as a
   consequence, it cannot issue the standard POST-as-GET ACME request.
   Therefore, before forwarding the Order request to the CA, the IdO
   SHOULD ensure that the selected CA supports unauthenticated GET by
   inspecting the relevant settings in the CA's directory object, as per
   Section 3.4 of [RFC8739].  If the CA does not support unauthenticated
   GET of STAR certificates, the IdO MUST NOT forward the Order request.
   Instead, it MUST move the Order status to "invalid" and set the
   "allow-certificate-get" in the "auto-renewal" object to "false".  The
   same occurs in case the Order request is forwarded and the CA does
   not reflect the "allow-certificate-get" setting in its Order
   resource.  The combination of "invalid" status and denied "allow-
   certificate-get" in the Order resource at the IdO provides an
   unambiguous (asynchronous) signal to the NDC about the failure
   reason.

2.3.2.1.  CNAME Installation

   If one of the objects in the "identifiers" list is of type "dns", the
   IdO can add the CNAME records specified in the "delegation" object to
   its zone, for example:

      abc.ido.example. CNAME abc.ndc.example.

2.3.3.  Order Object Transmitted from NDC to IdO and to ACME Server
        (Non-STAR)

   If the delegation is for a non-STAR certificate, the request object
   created by the NDC:

   *  MUST have a "delegation" attribute indicating the preconfigured
      delegation that applies to this Order;

   *  MUST have entries in the "identifiers" field for each delegated
      name present in the configuration; and

   *  MUST have the "allow-certificate-get" attribute set to true.

   POST /acme/new-order HTTP/1.1
   Host: acme.ido.example
   Content-Type: application/jose+json

   {
     "protected": base64url({
       "alg": "ES256",
       "kid": "https://acme.ido.example/acme/acct/evOfKhNU60wg",
       "nonce": "IYBkoQfaCS80UcCn9qH8Gt",
       "url": "https://acme.ido.example/acme/new-order"
     }),
     "payload": base64url({
       "identifiers": [
         {
           "type": "dns",
           "value": "abc.ido.example"
         }
       ],
       "delegation":
         "https://acme.ido.example/acme/delegation/gm0wfLYHBen",
       "allow-certificate-get": true
     }),
     "signature": "j9JBUvMigi4zodud...acYkEKaa8gqWyZ6H"
   }

                   Figure 7: New Non-STAR Order from NDC

   The order object that is created on the IdO:

   *  MUST start in the "ready" state;

   *  MUST contain an "authorizations" array with zero elements;

   *  MUST contain the indicated "delegation" configuration; and

   *  MUST contain the indicated "allow-certificate-get" setting.

   {
     "status": "ready",
     "expires": "2021-05-01T00:00:00Z",

     "identifiers": [
      {
        "type": "dns",
        "value": "abc.ido.example"
      }
     ],

     "delegation":
       "https://acme.ido.example/acme/delegation/gm0wfLYHBen",

     "allow-certificate-get": true,

     "authorizations": [],

     "finalize": "https://acme.ido.example/acme/order/3ZDlhYy/finalize"
   }

              Figure 8: Non-STAR Order Resource Created on IdO

   The Order finalization by the NDC and the subsequent validation of
   the CSR by the IdO proceed in the same way as for the STAR case.  If
   the CSR is successfully validated, the order object status moves to
   "processing" and the twin ACME protocol instance is initiated on the
   IdO-CA side.

   The request object created by the IdO:

   *  MUST copy the identifiers sent by the NDC;

   *  MUST strip the "delegation" attribute; and

   *  MUST copy the "allow-certificate-get" attribute.

   When the identifiers' authorization has been successfully completed
   and the certificate has been issued by the CA, the IdO:

   *  MUST move its Order resource status to "valid" and

   *  MUST copy the "certificate" field from the Order returned by the
      CA into its Order resource, as well as "notBefore" and "notAfter"
      if these fields exist.

   {
     "status": "valid",
     "expires": "2021-05-01T00:00:00Z",

     "identifiers": [
      {
        "type": "dns",
        "value": "abc.ido.example"
      }
     ],

     "delegation":
       "https://acme.ido.example/acme/delegation/gm0wfLYHBen",

     "allow-certificate-get": true,

     "authorizations": [],

     "finalize": "https://acme.ido.example/acme/order/3ZDlhYy/finalize",

     "certificate": "https://acme.ca.example/acme/order/YtR23SsdG9"
   }

              Figure 9: Non-STAR Order Resource Updated on IdO

   At this point of the protocol flow, the same considerations as in
   Section 2.3.2.1 apply.

   Before forwarding the Order request to the CA, the IdO SHOULD ensure
   that the selected CA supports unauthenticated GET by inspecting the
   relevant settings in the CA's directory object, as per Section 2.3.5.
   If the CA does not support unauthenticated GET of certificate
   resources, the IdO MUST NOT forward the Order request.  Instead, it
   MUST move the Order status to "invalid" and set the "allow-
   certificate-get" attribute to "false".  The same occurs in case the
   Order request is forwarded and the CA does not reflect the "allow-
   certificate-get" setting in its Order resource.  The combination of
   "invalid" status and denied "allow-certificate-get" in the Order
   resource at the IdO provides an unambiguous (asynchronous) signal to
   the NDC about the failure reason.

2.3.4.  Capability Discovery

   In order to help a client discover support for this profile, the
   directory object of an ACME server (typically, one deployed by the
   IdO) contains the following attribute in the "meta" field:

   delegation-enabled (optional, boolean):  Boolean flag indicating
      support for the profile specified in this memo.  An ACME server
      that supports this delegation profile MUST include this key and
      MUST set it to true.

   The IdO MUST declare its support for delegation using "delegation-
   enabled" regardless of whether it supports delegation of STAR
   certificates, non-STAR certificates, or both.

   In order to help a client discover support for certificate fetching
   using unauthenticated HTTP GET, the directory object of an ACME
   server (typically, one deployed by the CA) contains the following
   attribute in the "meta" field:

   allow-certificate-get (optional, boolean):  See Section 2.3.5.

2.3.5.  Negotiating an Unauthenticated GET

   In order to enable the name delegation of non-STAR certificates, this
   document defines a mechanism that allows a server to advertise
   support for accessing certificate resources via unauthenticated GET
   (in addition to POST-as-GET) and a client to enable this service with
   per-Order granularity.

   It is worth pointing out that the protocol elements described in this
   section have the same names and semantics as those introduced in
   Section 3.4 of [RFC8739] for the STAR use case (except, of course,
   they apply to the certificate resource rather than the star-
   certificate resource).  However, they differ in terms of their
   position in the directory meta and order objects; rather than being
   wrapped in an "auto-renewal" subobject, they are located at the top
   level.

   A server states its availability to grant unauthenticated access to a
   client's Order certificate by setting the "allow-certificate-get"
   attribute to "true" in the "meta" field inside the directory object:

   allow-certificate-get (optional, boolean):  If this field is present
      and set to "true", the server allows GET (and HEAD) requests to
      certificate URLs.

   A client states its desire to access the issued certificate via
   unauthenticated GET by adding an "allow-certificate-get" attribute to
   the payload of its newOrder request and setting it to "true".

   allow-certificate-get (optional, boolean):  If this field is present
      and set to "true", the client requests the server to allow
      unauthenticated GET (and HEAD) to the certificate associated with
      this Order.

   If the server accepts the request, it MUST reflect the attribute
   setting in the resulting order object.

   Note that even when the use of unauthenticated GET has been agreed
   upon, the server MUST also allow POST-as-GET requests to the
   certificate resource.

2.3.6.  Terminating the Delegation

   Identity delegation is terminated differently depending on whether or
   not this is a STAR certificate.

2.3.6.1.  By Cancellation (STAR)

   The IdO can terminate the delegation of a STAR certificate by
   requesting its cancellation (see Section 3.1.2 of [RFC8739]).

   Cancellation of the ACME STAR certificate is a prerogative of the
   IdO.  The NDC does not own the relevant account key on the CA;
   therefore, it can't issue a cancellation request for the STAR
   certificate.  Potentially, since it holds the STAR certificate's
   private key, it could request the revocation of a single STAR
   certificate.  However, STAR explicitly disables the revokeCert
   interface.

   Shortly after the automatic renewal process is stopped by the IdO,
   the last issued STAR certificate expires and the delegation
   terminates.

2.3.6.2.  By Revocation (Non-STAR)

   The IdO can terminate the delegation of a non-STAR certificate by
   requesting it to be revoked using the "revokeCert" URL exposed by the
   CA.

   According to Section 7.6 of [RFC8555], the revocation endpoint can be
   used with either the account key pair or the certificate key pair.
   In other words, an NDC that learns the "revokeCert" URL of the CA
   (which is publicly available via the CA's directory object) would be
   able to revoke the certificate using the associated private key.
   However, given the trust relationship between the NDC and IdO
   expected by the delegation trust model (Section 7.1), as well as the
   lack of incentives for the NDC to prematurely terminate the
   delegation, this does not represent a significant security risk.

2.4.  Proxy Behavior

   There are cases where the ACME Delegation flow should be proxied,
   such as the use case described in Section 5.1.2.  This section
   describes the behavior of such proxies.

   An entity implementing the IdO server role -- an "ACME Delegation
   server" -- may behave, on a per-identity case, either as a proxy into
   another ACME Delegation server or as an IdO and obtain a certificate
   directly.  The determining factor is whether it can successfully be
   authorized by the next-hop ACME server for the identity associated
   with the certificate request.

   The identities supported by each server and the disposition for each
   of them are preconfigured.

   Following is the proxy's behavior for each of the messages exchanged
   in the ACME Delegation process:

   New-order request:
      *  The complete "identifiers" attribute MUST be copied as is.
      *  Similarly, the "auto-renewal" object MUST be copied as is.
   New-order response:
      *  The "status", "expires", "authorizations", "identifiers", and
         "auto-renewal" attributes/objects MUST be copied as is.
      *  The "finalize" URL is rewritten so that the "finalize" request
         will be made to the proxy.
      *  Similarly, the "Location" header MUST be rewritten to point to
         an order object on the proxy.
      *  Any "Link" relations MUST be rewritten to point to the proxy.
   Get Order response:
      *  The "status", "expires", "authorizations", "identifiers", and
         "auto-renewal" attributes/objects MUST be copied as is.
      *  Similarly, the "star-certificate" URL (or the "certificate" URL
         in case of non-STAR requests) MUST be copied as is.
      *  The "finalize" URL is rewritten so that the "finalize" request
         will be made to the proxy.
      *  The "Location" header MUST be rewritten to point to an order
         object on the proxy.
      *  Any "Link" relations MUST be rewritten to point to the proxy.
   "finalize" request:
      *  The CSR MUST be copied as is.
   "finalize" response:
      *  The "Location" header, "Link" relations, and the "finalize"
         URLs are rewritten as for Get Order.

   We note that all the above messages are authenticated; therefore,
   each proxy must be able to authenticate any subordinate server.

3.  CA Behavior

   Although most of this document, and in particular Section 2, is
   focused on the protocol between the NDC and IdO, the protocol does
   affect the ACME server running in the CA.  A CA that wishes to
   support certificate delegation MUST also support unauthenticated
   certificate fetching, which it declares using "allow-certificate-get"
   (Section 2.3.5, Paragraph 3).

4.  CSR Template

   The CSR template is used to express and constrain the shape of the
   CSR that the NDC uses to request the certificate.  The CSR is used
   for every certificate created under the same delegation.  Its
   validation by the IdO is a critical element in the security of the
   whole delegation mechanism.

   Instead of defining every possible CSR attribute, this document takes
   a minimalist approach by declaring only the minimum attribute set and
   deferring the registration of further, more-specific attributes to
   future documents.

4.1.  Template Syntax

   The template is a JSON document.  Each field (with the exception of
   "keyTypes", see below) denotes one of the following:

   *  A mandatory field where the template specifies the literal value
      of that field.  This is denoted by a literal string, such as
      "abc.ido.example".

   *  A mandatory field where the content of the field is defined by the
      client.  This is denoted by "**".

   *  An optional field where the client decides whether the field is
      included in the CSR and, if so, what its value is.  This is
      denoted by "*".

   The NDC MUST NOT include any fields in the CSR, including any
   extensions, unless they are specified in the template.

   The structure of the template object is defined by the Concise Data
   Definition Language (CDDL) [RFC8610] document in Appendix A.  An
   alternative, nonnormative JSON Schema syntax is given in Appendix B.
   While the CSR template must follow the syntax defined here, neither
   the IdO nor the NDC are expected to validate it at runtime.

   The "subject" field and its subfields are mapped into the "subject"
   field of the CSR, as per Section 4.1.2.6 of [RFC5280].  Other
   extension fields of the CSR template are mapped into the CSR
   according to the table in Section 6.5.

   The "subjectAltName" field is currently defined for the following
   identifiers: DNS names, email addresses, and URIs.  New identifier
   types may be added in the future by documents that extend this
   specification.  Each new identifier type SHALL have an associated
   identifier validation challenge that the CA can use to obtain proof
   of the requester's control over it.

   The "keyTypes" property is not copied into the CSR.  Instead, this
   property constrains the "SubjectPublicKeyInfo" field of the CSR,
   which MUST have the type/size defined by one of the array members of
   the "keyTypes" property.

   When the IdO receives the CSR, it MUST verify that the CSR is
   consistent with the template contained in the "delegation" object
   referenced in the Order.  The IdO MAY enforce additional constraints,
   e.g., by restricting field lengths.  In this regard, note that a
   "subjectAltName" of type "DNS" can be specified using the wildcard
   notation, meaning that the NDC can be required ("**") or offered the
   possibility ("*") to define the delegated domain name by itself.  If
   this is the case, the IdO MUST apply application-specific checks on
   top of the control rules already provided by the CSR template to
   ensure the requested domain name is legitimate according to its local
   policy.

4.2.  Example

   The CSR template in Figure 10 represents one possible CSR template
   governing the delegation exchanges provided in the rest of this
   document.

   {
     "keyTypes": [
       {
         "PublicKeyType": "rsaEncryption",
         "PublicKeyLength": 2048,
         "SignatureType": "sha256WithRSAEncryption"
       },
       {
         "PublicKeyType": "id-ecPublicKey",
         "namedCurve": "secp256r1",
         "SignatureType": "ecdsa-with-SHA256"
       }
     ],
     "subject": {
       "country": "CA",
       "stateOrProvince": "**",
       "locality": "**"
     },
     "extensions": {
       "subjectAltName": {
         "DNS": [
           "abc.ido.example"
         ]
       },
       "keyUsage": [
         "digitalSignature"
       ],
       "extendedKeyUsage": [
         "serverAuth",
         "clientAuth"
       ]
     }
   }

                      Figure 10: Example CSR Template

5.  Further Use Cases

   This nonnormative section describes additional use cases implementing
   the STAR certificate delegation in nontrivial ways.

5.1.  CDN Interconnection (CDNI)

   [HTTPS-DELEGATION] discusses several solutions addressing different
   delegation requirements for the CDN Interconnection (CDNI)
   environment.  This section discusses two of the stated requirements
   in the context of the STAR delegation workflow.

   This section uses specific CDNI terminology, e.g., Upstream CDN
   (uCDN) and Downstream (dCDN), as defined in [RFC7336].

5.1.1.  Multiple Parallel Delegates

   In some cases, the content owner (IdO) would like to delegate
   authority over a website to multiple NDCs (CDNs).  This could happen
   if the IdO has agreements in place with different regional CDNs for
   different geographical regions or if a "backup" CDN is used to handle
   overflow traffic by temporarily altering some of the CNAME mappings
   in place.  The STAR delegation flow enables this use case naturally,
   since each CDN can authenticate separately to the IdO (via its own
   separate account) specifying its CSR, and the IdO is free to allow or
   deny each certificate request according to its own policy.

5.1.2.  Chained Delegation

   In other cases, a content owner (IdO) delegates some domains to a
   large CDN (uCDN), which in turn delegates to a smaller regional CDN
   (dCDN).  The IdO has a contractual relationship with uCDN, and uCDN
   has a similar relationship with dCDN.  However, IdO may not even know
   about dCDN.

   If needed, the STAR protocol can be chained to support this use case:
   uCDN could forward requests from dCDN to IdO and forward responses
   back to dCDN.  Whether such proxying is allowed is governed by policy
   and contracts between the parties.

   A mechanism is necessary at the interface between uCDN and dCDN, by
   which the uCDN can advertise:

   *  the names that the dCDN is allowed to use and

   *  the policy for creating the key material (allowed algorithms,
      minimum key lengths, key usage, etc.) that the dCDN needs to
      satisfy.

   Note that such mechanism is provided by the CSR template.

5.1.2.1.  Two-Level Delegation in CDNI

   A User Agent (UA), e.g., a browser or set-top box, wants to fetch the
   video resource at the following URI: "https://video.cp.example/
   movie".  Redirection between the content provider (CP) and upstream
   and downstream CDNs is arranged as a CNAME-based aliasing chain, as
   illustrated in Figure 11.

                                                    .------------.
                            video.cp.example ?     | .-----.      |
                 .---------------------------------->|     |      |
                |                  (a)             | | DNS |  CP  |
                |    .-------------------------------+     |      |
                |   |   CNAME video.ucdn.example   | '-----'      |
                |   |                               '------------'
                |   |
                |   |
    .-----------|---v--.                            .------------.
   |    .-----.-+-----. |   video.ucdn.example ?   | .-----.      |
   |    |     |       +----------------------------->|     |      |
   | UA | TLS |  DNS  | |          (b)             | | DNS | uCDN |
   |    |     |       |<-----------------------------+     |      |
   |    '--+--'-----+-' | CNAME video.dcdn.example | '-----'      |
    '------|----^---|--'                            '------------'
           |    |   |
           |    |   |
           |    |   |                               .------------.
           |    |   |      video.dcdn.example ?    | .-----.      |
           |    |    '------------------------------>|     |      |
           |    |                  (c)             | | DNS |      |
           |     '-----------------------------------+     |      |
           |                   A 192.0.2.1         | +-----+ dCDN |
           |                                       | |     |      |
            '--------------------------------------->| TLS |      |
                        SNI: video.cp.example      | |     |      |
                                                   | '-----'      |
                                                    '------------'

                         Figure 11: DNS Redirection

   Unlike HTTP-based redirection, where the original URL is supplanted
   by the one found in the "Location" header of the 302 response, DNS
   redirection is completely transparent to the User Agent.  As a
   result, the TLS connection to the dCDN edge is done with a Server
   Name Indication (SNI) equal to the "host" in the original URI -- in
   the example, "video.cp.example".  So, in order to successfully
   complete the handshake, the landing dCDN node has to be configured
   with a certificate whose "subjectAltName" field matches
   "video.cp.example", i.e., a content provider's name.

   Figure 12 illustrates the cascaded delegation flow that allows dCDN
   to obtain a STAR certificate that bears a name belonging to the
   content provider with a private key that is only known to the dCDN.

              .--------------------.
             |      .------.------. |
             |      | STAR | ACME |<-------------.
             |  CP  | dele | STAR | |             |
             |      | srv  | cli  +-----.         |
             |      '---+--'------' |    |        6
              '---------|------^---'     5        |
                        |      |         |     .--|-------.
                        |      |         |    | .-+----.   |
                        7      |          '---->| ACME |   |
                        |      |              | | STAR | C |
                        |      4              | +------| A |
                        |      |              | | HTTP |   |
                        |      |              | '----+-'   |
                        |   .-'                '--^--|----'
         .--------------v--|--.                   |  |
        |      .------.----+-. |                  |  10
        |      |      | STAR | |                  |  |
        | uCDN | CDNI | dele | |                  |  |
        |      |      | fwd  | |                  |  |
        |      '----+-'-+----' |                  |  |
         '-------^--|---|--^--'                   |  |
                 |  |   |  |                      |  |
                 |  2   8  |                      |  |
                 1  |   |  3                      |  |
                 |  |   |  |                      9  |
         .-------|--v---v--|---------.            |  |
        |      .-+----.----+-.------. |           |  |
        |      |      | STAR |      +------------'   |
        | dCDN | CDNI | dele | HTTP | |              |
        |      |      | cli  |      |<--------------'
        |      '------'------'------' |
         '---------------------------'

                  Figure 12: Two-Level Delegation in CDNI

   uCDN is configured to delegate to dCDN, and CP is configured to
   delegate to uCDN, both as defined in Section 2.3.1.

   1.   dCDN requests CDNI path metadata to uCDN.

   2.   uCDN replies with, among other CDNI metadata, the STAR
        delegation configuration, which includes the delegated content
        provider's name.

   3.   dCDN creates a key pair and the CSR with the delegated name.  It
        then places an order for the delegated name to uCDN.

   4.   uCDN forwards the received order to the content provider (CP).

   5.   CP creates an order for a STAR certificate and sends it to the
        CA.  The order also requests unauthenticated access to the
        certificate resource.

   6.   After all authorizations complete successfully, the STAR
        certificate is issued.

   7.   CP notifies uCDN that the STAR certificate is available at the
        order's "star-certificate" URL.

   8.   uCDN forwards the information to dCDN.  At this point, the ACME
        signaling is complete.

   9.   dCDN requests the STAR certificate using unauthenticated GET
        from the CA.

   10.  The CA returns the certificate.  Now dCDN is fully configured to
        handle HTTPS traffic in lieu of the content provider.

   Note that 9 and 10 repeat until the delegation expires or is
   terminated.

5.2.  Secure Telephone Identity Revisited (STIR)

   As a second use case, we consider the delegation of credentials in
   the STIR ecosystem [RFC9060].

   This section uses STIR terminology.  The term Personal Assertion
   Token (PASSporT) is defined in [RFC8225], and "TNAuthList" is defined
   in [RFC8226].

   In the STIR delegated mode, a service provider SP2 -- the NDC --
   needs to sign PASSporTs [RFC8225] for telephone numbers (e.g.,
   TN=+123) belonging to another service provider, SP1 -- the IdO.  In
   order to do that, SP2 needs a STIR certificate and a private key that
   includes TN=+123 in the TNAuthList [RFC8226] certificate extension.

   In detail (Figure 13):

   1.  SP1 and SP2 agree on the configuration of the delegation -- in
       particular, the CSR template that applies.

   2.  SP2 generates a private/public key pair and sends a CSR to SP1,
       requesting creation of a certificate with an SP1 name, an SP2
       public key, and a TNAuthList extension with the list of TNs that
       SP1 delegates to SP2.  (Note that the CSR sent by SP2 to SP1
       needs to be validated against the CSR template agreed upon in
       step 1.).

   3.  SP1 sends an order for the CSR to the CA.  The order also
       requests unauthenticated access to the certificate resource.

   4.  Subsequently, after the required TNAuthList authorizations are
       successfully completed, the CA moves the order to a "valid"
       state; at the same time, the star-certificate endpoint is
       populated.

   5.  The contents of the order are forwarded from SP1 to SP2 by means
       of the paired "delegation" order.

   6.  SP2 dereferences the "star-certificate" URL in the order to fetch
       the rolling STAR certificate bearing the delegated identifiers.

   7.  The STAR certificate is returned to SP2.

         .-------------------.
        |     .------.------. |
        |     | STAR | STAR |<--------------.
    .-->| SP1 | dele | dele | |              |
   |    |     | srv  | cli  +-----.          |
   |    |     '----+-'------' |    |         4
   |     '------^--|---------'     3         |
   |            |  |               |    .----|-----.
   |            |  5               |   | .---+--.   |
   |            |  |                '--->| ACME |   |
   |            |  |                   | | STAR | C |
   1            |  |                   | +------| A |
   |            |  |                .--->| HTTP |   |
   |            2  |               |   | '---+--'   |
   |            |  |               |    '----|-----'
   |     .------|--v---------.     6         |
   |    |     .-+----.------. |    |         7
   |    |     | STAR |      +-----'          |
    '-->| SP2 | dele | HTTP | |              |
        |     | cli  |      |<--------------'
        |     '----+-'-+----' |
         '-------------------'

                       Figure 13: Delegation in STIR

   As shown, the STAR delegation profile described in this document
   applies straightforwardly; the only extra requirement being the
   ability to instruct the NDC about the allowed TNAuthList values.
   This can be achieved by a simple extension to the CSR template.

6.  IANA Considerations

6.1.  New Fields in the "meta" Object within a Directory Object

   This document adds the following entries to the "ACME Directory
   Metadata Fields" registry:

            +=======================+============+===========+
            | Field Name            | Field Type | Reference |
            +=======================+============+===========+
            | delegation-enabled    | boolean    | RFC 9115  |
            +-----------------------+------------+-----------+
            | allow-certificate-get | boolean    | RFC 9115  |
            +-----------------------+------------+-----------+

                                 Table 1

6.2.  New Fields in the Order Object

   This document adds the following entries to the "ACME Order Object
   Fields" registry:

     +=======================+============+==============+===========+
     | Field Name            | Field Type | Configurable | Reference |
     +=======================+============+==============+===========+
     | allow-certificate-get | boolean    | true         | RFC 9115  |
     +-----------------------+------------+--------------+-----------+
     | delegation            | string     | true         | RFC 9115  |
     +-----------------------+------------+--------------+-----------+

                                  Table 2

6.3.  New Fields in the Account Object

   This document adds the following entries to the "ACME Account Object
   Fields" registry:

            +=============+============+==========+===========+
            | Field Name  | Field Type | Requests | Reference |
            +=============+============+==========+===========+
            | delegations | string     | none     | RFC 9115  |
            +-------------+------------+----------+-----------+

                                  Table 3

   Note that the "delegations" field is only reported by ACME servers
   that have "delegation-enabled" set to true in their meta Object.

6.4.  New Error Types

   This document adds the following entries to the "ACME Error Types"
   registry:

    +===================+================================+===========+
    | Type              | Description                    | Reference |
    +===================+================================+===========+
    | unknownDelegation | An unknown configuration is    | RFC 9115  |
    |                   | listed in the "delegation"     |           |
    |                   | attribute of the order request |           |
    +-------------------+--------------------------------+-----------+

                                 Table 4

6.5.  CSR Template Extensions

   IANA has established the "STAR Delegation CSR Template Extensions"
   registry, with "Specification Required" as its registration
   procedure.

   Each extension registered must specify:

   *  an extension name,

   *  an extension syntax, as a reference to a CDDL document that
      defines this extension, and

   *  the extension's mapping into an X.509 certificate extension.

   The initial contents of this registry are the extensions defined by
   the CDDL in Appendix A.

     +==================+===========+===============================+
     | Extension Name   | Extension | Mapping to X.509 Certificate  |
     |                  | Syntax    | Extension                     |
     +==================+===========+===============================+
     | keyUsage         | See       | [RFC5280], Section 4.2.1.3    |
     |                  | Appendix  |                               |
     |                  | A         |                               |
     +------------------+-----------+-------------------------------+
     | extendedKeyUsage | See       | [RFC5280], Section 4.2.1.12   |
     |                  | Appendix  |                               |
     |                  | A         |                               |
     +------------------+-----------+-------------------------------+
     | subjectAltName   | See       | [RFC5280], Section 4.2.1.6    |
     |                  | Appendix  | (note that only specific name |
     |                  | A         | formats are allowed: URI, DNS |
     |                  |           | name, email address)          |
     +------------------+-----------+-------------------------------+

                                 Table 5

   When evaluating a request for an assignment in this registry, the
   designated expert should follow this guidance:

   *  The definition must include a full CDDL definition, which the
      expert will validate.

   *  The definition must include both positive and negative test cases.

   *  Additional requirements that are not captured by the CDDL
      definition are allowed but must be explicitly specified.

7.  Security Considerations

7.1.  Trust Model

   The ACME trust model needs to be extended to include the trust
   relationship between NDC and IdO.  Note that once this trust link is
   established, it potentially becomes recursive.  Therefore, there has
   to be a trust relationship between each of the nodes in the
   delegation chain; for example, in case of cascading CDNs, this is
   contractually defined.  Note that when using standard [RFC6125]
   identity verification, there are no mechanisms available to the IdO
   to restrict the use of the delegated name once the name has been
   handed over to the first NDC.  It is, therefore, expected that
   contractual measures are in place to get some assurance that
   redelegation is not being performed.

7.2.  Delegation Security Goal

   Delegation introduces a new security goal: only an NDC that has been
   authorized by the IdO, either directly or transitively, can obtain a
   certificate with an IdO identity.

   From a security point of view, the delegation process has five
   separate parts:

   1.  enabling a specific third party (the intended NDC) to submit
       requests for delegated certificates

   2.  making sure that any request for a delegated certificate matches
       the intended "shape" in terms of delegated identities as well as
       any other certificate metadata, e.g., key length, x.509
       extensions, etc.

   3.  serving the certificate back to the NDC

   4.  handling revocation of the delegation

   5.  handling revocation of the certificate itself

   The first part is covered by the NDC's ACME account that is
   administered by the IdO, whose security relies on the correct
   handling of the associated key pair.  When a compromise of the
   private key is detected, the delegate MUST use the account
   deactivation procedures defined in Section 7.3.6 of [RFC8555].

   The second part is covered by the act of checking an NDC's
   certificate request against the intended CSR template.  The steps of
   shaping the CSR template correctly, selecting the right CSR template
   to check against the presented CSR, and making sure that the
   presented CSR matches the selected CSR template are all security
   relevant.

   The third part builds on the trust relationship between NDC and IdO
   that is responsible for correctly forwarding the certificate URL from
   the Order returned by the CA.

   The fourth part is associated with the ability of the IdO to
   unilaterally remove the "delegation" object associated with the
   revoked identity, therefore, disabling any further NDC requests for
   such identity.  Note that, in more extreme circumstances, the IdO
   might decide to disable the NDC account, thus entirely blocking any
   further interaction.

   The fifth is covered by two different mechanisms, depending on the
   nature of the certificate.  For STAR, the IdO shall use the
   cancellation interface defined in Section 2.3 of [RFC8739].  For non-
   STAR, the certificate revocation interface defined in Section 7.6 of
   [RFC8555]) is used.

   The ACME account associated with the delegation plays a crucial role
   in the overall security of the presented protocol.  This, in turn,
   means that (in delegation scenarios) the security requirements and
   verification associated with an ACME account may be more stringent
   than in base ACME deployments, since the out-of-band configuration of
   delegations that an account is authorized to use (combined with
   account authentication) takes the place of the normal ACME
   authorization challenge procedures.  Therefore, the IdO MUST ensure
   that each account is associated with the exact policies (via their
   matching "delegation" objects) that define which domain names can be
   delegated to the account and how.  The IdO is expected to use out-of-
   band means to preregister each NDC to the corresponding account.

7.3.  New ACME Channels

   Using the model established in Section 10.1 of [RFC8555], we can
   decompose the interactions of the basic delegation workflow, as shown
   in Figure 14.

   .-----. ACME Channel .--------.
   | NDC +------------->| IdO    |
   '--+--'              | server |
      |                 '--o-----'
      |                    |
      |                    |         ACME Channel
      |                    |  .------------>-------------.
      |                    |  |                          |
      |                 .--o--+--.                    .--+---.
      |                 | IdO    |                    |  CA  |
      |                 | client |                    '--+-+-'
      |                 '-----+--'                       | |
      |                       '-----------<--------------' |
      |                            Validation Channel      |
      '-------------------->-------------------------------'
                (subset of) ACME Channel [1]

   [1] Unauthenticated certificate fetch and non-STAR certificate
       revocation.

                  Figure 14: Delegation Channels Topology

   The considerations regarding the security of the ACME Channel and
   Validation Channel discussed in [RFC8555] apply verbatim to the IdO-
   CA leg.  The same can be said for the ACME Channel on the NDC-IdO
   leg.  A slightly different set of considerations apply to the ACME
   Channel between the NDC and CA, which consists of a subset of the
   ACME interface comprising two API endpoints: the unauthenticated
   certificate retrieval and, potentially, non-STAR revocation via
   certificate private key.  No specific security considerations apply
   to the former, but the privacy considerations in Section 6.3 of
   [RFC8739] do.  With regard to the latter, it should be noted that
   there is currently no means for an IdO to disable authorizing
   revocation based on certificate private keys.  So, in theory, an NDC
   could use the revocation API directly with the CA, therefore,
   bypassing the IdO.  The NDC SHOULD NOT directly use the revocation
   interface exposed by the CA unless failing to do so would compromise
   the overall security, for example, if the certificate private key is
   compromised and the IdO is not currently reachable.

   All other security considerations from [RFC8555] and [RFC8739] apply
   as is to the delegation topology.

7.4.  Restricting CDNs to the Delegation Mechanism

   When a website is delegated to a CDN, the CDN can in principle modify
   the website at will, e.g., create and remove pages.  This means that
   a malicious or breached CDN can pass the ACME (as well as common non-
   ACME) HTTPS-based validation challenges and generate a certificate
   for the site.  This is true regardless of whether or not the CNAME
   mechanisms defined in the current document is used.

   In some cases, this is the desired behavior; the domain holder trusts
   the CDN to have full control of the cryptographic credentials for the
   site.  However, this document assumes a scenario where the domain
   holder only wants to delegate restricted control and wishes to retain
   the capability to cancel the CDN's credentials at a short notice.

   The following is a possible mitigation when the IdO wishes to ensure
   that a rogue CDN cannot issue unauthorized certificates:

   *  The domain holder makes sure that the CDN cannot modify the DNS
      records for the domain.  The domain holder should ensure it is the
      only entity authorized to modify the DNS zone.  Typically, it
      establishes a CNAME resource record from a subdomain into a CDN-
      managed domain.

   *  The domain holder uses a Certification Authority Authorization
      (CAA) record [RFC8659] to restrict certificate issuance for the
      domain to specific CAs that comply with ACME and are known to
      implement [RFC8657].

   *  The domain holder uses the ACME-specific CAA mechanism [RFC8657]
      to restrict issuance to a specific CA account that is controlled
      by it and MUST require "dns-01" as the sole validation method.

   We note that the above solution may need to be tweaked depending on
   the exact capabilities and authorization flows supported by the
   selected CA.  In addition, this mitigation may be bypassed if a
   malicious or misconfigured CA does not comply with CAA restrictions.

8.  References

8.1.  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/info/rfc2119>.

   [RFC2986]  Nystrom, M. and B. Kaliski, "PKCS #10: Certification
              Request Syntax Specification Version 1.7", RFC 2986,
              DOI 10.17487/RFC2986, November 2000,
              <https://www.rfc-editor.org/info/rfc2986>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/info/rfc5280>.

   [RFC7807]  Nottingham, M. and E. Wilde, "Problem Details for HTTP
              APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016,
              <https://www.rfc-editor.org/info/rfc7807>.

   [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/info/rfc8174>.

   [RFC8555]  Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
              Kasten, "Automatic Certificate Management Environment
              (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,
              <https://www.rfc-editor.org/info/rfc8555>.

   [RFC8610]  Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
              Definition Language (CDDL): A Notational Convention to
              Express Concise Binary Object Representation (CBOR) and
              JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
              June 2019, <https://www.rfc-editor.org/info/rfc8610>.

   [RFC8739]  Sheffer, Y., Lopez, D., Gonzalez de Dios, O., Pastor
              Perales, A., and T. Fossati, "Support for Short-Term,
              Automatically Renewed (STAR) Certificates in the Automated
              Certificate Management Environment (ACME)", RFC 8739,
              DOI 10.17487/RFC8739, March 2020,
              <https://www.rfc-editor.org/info/rfc8739>.

8.2.  Informative References

   [HTTPS-DELEGATION]
              Fieau, F., Stephan, E., and S. Mishra, "CDNI extensions
              for HTTPS delegation", Work in Progress, Internet-Draft,
              draft-ietf-cdni-interfaces-https-delegation-06, 10
              September 2021, <https://datatracker.ietf.org/doc/html/
              draft-ietf-cdni-interfaces-https-delegation-06>.

   [json-schema-07]
              Wright, A., Andrews, H., and B. Hutton, "JSON Schema
              Validation: A Vocabulary for Structural Validation of
              JSON", Work in Progress, Internet-Draft, draft-handrews-
              json-schema-validation-02, 17 September 2019,
              <https://datatracker.ietf.org/doc/html/draft-handrews-
              json-schema-validation-02>.

   [MGLT-LURK-TLS13]
              Migault, D., "LURK Extension version 1 for (D)TLS 1.3
              Authentication", Work in Progress, Internet-Draft, draft-
              mglt-lurk-tls13-05, 26 July 2021,
              <https://datatracker.ietf.org/doc/html/draft-mglt-lurk-
              tls13-05>.

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
              2011, <https://www.rfc-editor.org/info/rfc6125>.

   [RFC7336]  Peterson, L., Davie, B., and R. van Brandenburg, Ed.,
              "Framework for Content Distribution Network
              Interconnection (CDNI)", RFC 7336, DOI 10.17487/RFC7336,
              August 2014, <https://www.rfc-editor.org/info/rfc7336>.

   [RFC8225]  Wendt, C. and J. Peterson, "PASSporT: Personal Assertion
              Token", RFC 8225, DOI 10.17487/RFC8225, February 2018,
              <https://www.rfc-editor.org/info/rfc8225>.

   [RFC8226]  Peterson, J. and S. Turner, "Secure Telephone Identity
              Credentials: Certificates", RFC 8226,
              DOI 10.17487/RFC8226, February 2018,
              <https://www.rfc-editor.org/info/rfc8226>.

   [RFC8657]  Landau, H., "Certification Authority Authorization (CAA)
              Record Extensions for Account URI and Automatic
              Certificate Management Environment (ACME) Method Binding",
              RFC 8657, DOI 10.17487/RFC8657, November 2019,
              <https://www.rfc-editor.org/info/rfc8657>.

   [RFC8659]  Hallam-Baker, P., Stradling, R., and J. Hoffman-Andrews,
              "DNS Certification Authority Authorization (CAA) Resource
              Record", RFC 8659, DOI 10.17487/RFC8659, November 2019,
              <https://www.rfc-editor.org/info/rfc8659>.

   [RFC9060]  Peterson, J., "Secure Telephone Identity Revisited (STIR)
              Certificate Delegation", RFC 9060, DOI 10.17487/RFC9060,
              September 2021, <https://www.rfc-editor.org/info/rfc9060>.

   [TLS-SUBCERTS]
              Barnes, R., Iyengar, S., Sullivan, N., and E. Rescorla,
              "Delegated Credentials for TLS", Work in Progress,
              Internet-Draft, draft-ietf-tls-subcerts-10, 24 January
              2021, <https://datatracker.ietf.org/doc/html/draft-ietf-
              tls-subcerts-10>.

   [TOKEN-TNAUTHLIST]
              Wendt, C., Hancock, D., Barnes, M., and J. Peterson,
              "TNAuthList profile of ACME Authority Token", Work in
              Progress, Internet-Draft, draft-ietf-acme-authority-token-
              tnauthlist-08, 27 March 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-acme-
              authority-token-tnauthlist-08>.

Appendix A.  CSR Template: CDDL

   Following is the normative definition of the CSR template using CDDL
   [RFC8610].  The CSR template MUST be a valid JSON document that is
   compliant with the syntax defined here.

   There are additional constraints not expressed in CDDL that MUST be
   validated by the recipient, including:

   *  the value of each "subjectAltName" entry is compatible with its
      type and

   *  the parameters in each "keyTypes" entry form an acceptable
      combination.

   csr-template-schema = {
     keyTypes: [ + $keyType ]
     ? subject: non-empty<distinguishedName>
     extensions: extensions
   }

   non-empty<M> = (M) .and ({ + any => any })

   mandatory-wildcard = "**"
   optional-wildcard = "*"
   wildcard = mandatory-wildcard / optional-wildcard

   ; regtext matches all text strings but "*" and "**"
   regtext = text .regexp "([^\*].*)|([\*][^\*].*)|([\*][\*].+)"

   regtext-or-wildcard = regtext / wildcard

   distinguishedName = {
     ? country: regtext-or-wildcard
     ? stateOrProvince: regtext-or-wildcard
     ? locality: regtext-or-wildcard
     ? organization: regtext-or-wildcard
     ? organizationalUnit: regtext-or-wildcard
     ? emailAddress: regtext-or-wildcard
     ? commonName: regtext-or-wildcard
   }

   $keyType /= rsaKeyType
   $keyType /= ecdsaKeyType

   rsaKeyType = {
     PublicKeyType: "rsaEncryption" ; OID: 1.2.840.113549.1.1.1
     PublicKeyLength: rsaKeySize
     SignatureType: $rsaSignatureType
   }

   rsaKeySize = uint

   ; RSASSA-PKCS1-v1_5 with SHA-256
   $rsaSignatureType /= "sha256WithRSAEncryption"
   ; RSASSA-PCKS1-v1_5 with SHA-384
   $rsaSignatureType /= "sha384WithRSAEncryption"
   ; RSASSA-PCKS1-v1_5 with SHA-512
   $rsaSignatureType /= "sha512WithRSAEncryption"
   ; RSASSA-PSS with SHA-256, MGF-1 with SHA-256, and a 32 byte salt
   $rsaSignatureType /= "sha256WithRSAandMGF1"
   ; RSASSA-PSS with SHA-384, MGF-1 with SHA-384, and a 48 byte salt
   $rsaSignatureType /= "sha384WithRSAandMGF1"
   ; RSASSA-PSS with SHA-512, MGF-1 with SHA-512, and a 64 byte salt
   $rsaSignatureType /= "sha512WithRSAandMGF1"

   ecdsaKeyType = {
     PublicKeyType: "id-ecPublicKey" ; OID: 1.2.840.10045.2.1
     namedCurve: $ecdsaCurve
     SignatureType: $ecdsaSignatureType
   }

   $ecdsaCurve /= "secp256r1" ; OID: 1.2.840.10045.3.1.7
   $ecdsaCurve /= "secp384r1" ; OID: 1.3.132.0.34
   $ecdsaCurve /= "secp521r1" ; OID: 1.3.132.0.3

   $ecdsaSignatureType /= "ecdsa-with-SHA256" ; paired with secp256r1
   $ecdsaSignatureType /= "ecdsa-with-SHA384" ; paired with secp384r1
   $ecdsaSignatureType /= "ecdsa-with-SHA512" ; paired with secp521r1

   subjectaltname = {
     ? DNS: [ + regtext-or-wildcard ]
     ? Email: [ + regtext ]
     ? URI: [ + regtext ]
     * $$subjectaltname-extension
   }

   extensions = {
     ? keyUsage: [ + keyUsageType ]
     ? extendedKeyUsage: [ + extendedKeyUsageType ]
     subjectAltName: non-empty<subjectaltname>
   }

   keyUsageType /= "digitalSignature"
   keyUsageType /= "nonRepudiation"
   keyUsageType /= "keyEncipherment"
   keyUsageType /= "dataEncipherment"
   keyUsageType /= "keyAgreement"
   keyUsageType /= "keyCertSign"
   keyUsageType /= "cRLSign"
   keyUsageType /= "encipherOnly"
   keyUsageType /= "decipherOnly"

   extendedKeyUsageType /= "serverAuth"
   extendedKeyUsageType /= "clientAuth"
   extendedKeyUsageType /= "codeSigning"
   extendedKeyUsageType /= "emailProtection"
   extendedKeyUsageType /= "timeStamping"
   extendedKeyUsageType /= "OCSPSigning"
   extendedKeyUsageType /= oid

   oid = text .regexp "([0-2])((\.0)|(\.[1-9][0-9]*))*"

Appendix B.  CSR Template: JSON Schema

   This appendix includes an alternative, nonnormative JSON Schema
   definition of the CSR template.  The syntax used is that of draft 7
   of JSON Schema, which is documented in [json-schema-07].  Note that
   later versions of this (now-expired) draft describe later versions of
   the JSON Schema syntax.  At the time of writing, a stable reference
   for this syntax is not yet available, and we have chosen to use the
   draft version, which is currently best supported by tool
   implementations.

   The same considerations about additional constraints checking
   discussed in Appendix A apply here as well.

   {
     "title": "JSON Schema for the STAR Delegation CSR template",
     "$schema": "http://json-schema.org/draft-07/schema#",
     "$id": "http://ietf.org/acme/drafts/star-delegation/csr-template",
     "$defs": {
       "distinguished-name": {
         "$id": "#distinguished-name",
         "type": "object",
         "minProperties": 1,
         "properties": {
           "country": {
             "type": "string"
           },
           "stateOrProvince": {
             "type": "string"
           },
           "locality": {
             "type": "string"
           },
           "organization": {
             "type": "string"
           },
           "organizationalUnit": {
             "type": "string"
           },
           "emailAddress": {
             "type": "string"
           },
           "commonName": {
             "type": "string"
           }
         },
         "additionalProperties": false
       },
       "rsaKeyType": {
         "$id": "#rsaKeyType",
         "type": "object",
         "properties": {
           "PublicKeyType": {
             "type": "string",
             "const": "rsaEncryption"
           },
           "PublicKeyLength": {
             "type": "integer"
           },
           "SignatureType": {
             "type": "string",
             "enum": [
               "sha256WithRSAEncryption",
               "sha384WithRSAEncryption",
               "sha512WithRSAEncryption",
               "sha256WithRSAandMGF1",
               "sha384WithRSAandMGF1",
               "sha512WithRSAandMGF1"
             ]
           }
         },
         "required": [
           "PublicKeyType",
           "PublicKeyLength",
           "SignatureType"
         ],
         "additionalProperties": false
       },
       "ecdsaKeyType": {
         "$id": "#ecdsaKeyType",
         "type": "object",
         "properties": {
           "PublicKeyType": {
             "type": "string",
             "const": "id-ecPublicKey"
           },
           "namedCurve": {
             "type": "string",
             "enum": [
               "secp256r1",
               "secp384r1",
               "secp521r1"
             ]
           },
           "SignatureType": {
             "type": "string",
             "enum": [
               "ecdsa-with-SHA256",
               "ecdsa-with-SHA384",
               "ecdsa-with-SHA512"
             ]
           }
         },
         "required": [
           "PublicKeyType",
           "namedCurve",
           "SignatureType"
         ],
         "additionalProperties": false
       }
     },
     "type": "object",
     "properties": {
       "keyTypes": {
         "type": "array",
         "minItems": 1,
         "items": {
           "anyOf": [
             {
               "$ref": "#rsaKeyType"
             },
             {
               "$ref": "#ecdsaKeyType"
             }
           ]
         }
       },
       "subject": {
         "$ref": "#distinguished-name"
       },
       "extensions": {
         "type": "object",
         "properties": {
           "keyUsage": {
             "type": "array",
             "minItems": 1,
             "items": {
               "type": "string",
               "enum": [
                 "digitalSignature",
                 "nonRepudiation",
                 "keyEncipherment",
                 "dataEncipherment",
                 "keyAgreement",
                 "keyCertSign",
                 "cRLSign",
                 "encipherOnly",
                 "decipherOnly"
               ]
             }
           },
           "extendedKeyUsage": {
             "type": "array",
             "minItems": 1,
             "items": {
               "anyOf": [
                 {
                   "type": "string",
                   "enum": [
                     "serverAuth",
                     "clientAuth",
                     "codeSigning",
                     "emailProtection",
                     "timeStamping",
                     "OCSPSigning"
                   ]
                 },
                 {
                   "type": "string",
                   "pattern": "^([0-2])((\\.0)|(\\.[1-9][0-9]*))*$",
                   "description": "Used for OID values"
                 }
               ]
             }
           },
           "subjectAltName": {
             "type": "object",
             "minProperties": 1,
             "properties": {
               "DNS": {
                 "type": "array",
                 "minItems": 1,
                 "items": {
                   "anyOf": [
                     {
                       "type": "string",
                       "enum": [
                         "*",
                         "**"
                       ]
                     },
                     {
                       "type": "string",
                       "format": "hostname"
                     }
                   ]
                 }
               },
               "Email": {
                 "type": "array",
                 "minItems": 1,
                 "items": {
                   "type": "string",
                   "format": "email"
                 }
               },
               "URI": {
                 "type": "array",
                 "minItems": 1,
                 "items": {
                   "type": "string",
                   "format": "uri"
                 }
               }
             },
             "additionalProperties": false
           }
         },
         "required": [
           "subjectAltName"
         ],
         "additionalProperties": false
       }
     },
     "required": [
       "extensions",
       "keyTypes"
     ],
     "additionalProperties": false
   }

Acknowledgements

   We would like to thank the following people who contributed
   significantly to this document with their review comments and design
   proposals: Richard Barnes, Carsten Bormann, Roman Danyliw, Lars
   Eggert, Frédéric Fieau, Russ Housley, Ben Kaduk, Eric Kline, Sanjay
   Mishra, Francesca Palombini, Jon Peterson, Ryan Sleevi, Emile
   Stephan, and Éric Vyncke.

   This work is partially supported by the European Commission under
   Horizon 2020 grant agreement no. 688421 Measurement and Architecture
   for a Middleboxed Internet (MAMI).  This support does not imply
   endorsement.

Authors' Addresses

   Yaron Sheffer
   Intuit

   Email: yaronf.ietf@gmail.com


   Diego López
   Telefonica I+D

   Email: diego.r.lopez@telefonica.com


   Antonio Agustín Pastor Perales
   Telefonica I+D

   Email: antonio.pastorperales@telefonica.com


   Thomas Fossati
   ARM

   Email: thomas.fossati@arm.com