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Versions: (draft-sheffer-acme-star-delegation)           Standards Track
          00 01 02 03 04 05 06 07 08                                    
ACME                                                          Y. Sheffer
Internet-Draft                                                    Intuit
Intended status: Standards Track                                D. López
Expires: 27 September 2021                             A. Pastor Perales
                                                          Telefonica I+D
                                                              T. Fossati
                                                                     ARM
                                                           26 March 2021


         An ACME Profile for Generating Delegated Certificates
                   draft-ietf-acme-star-delegation-07

Abstract

   This memo defines a profile of the Automatic Certificate Management
   Environment (ACME) protocol by which the owner 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 owner of a domain
   name).  The presented mechanism allows the owner of the identifier to
   retain control over the delegation and revoke it at any time.  A key
   property of this mechanism is it does not require any modification to
   the deployed TLS ecosystem.

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 27 September 2021.







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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  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Conventions used in this document . . . . . . . . . . . .   5
   2.  Protocol Flow . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Preconditions . . . . . . . . . . . . . . . . . . . . . .   5
     2.2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   6
     2.3.  Delegated Identity Profile  . . . . . . . . . . . . . . .   8
       2.3.1.  Delegation Configuration  . . . . . . . . . . . . . .   8
       2.3.2.  Order Object Transmitted from NDC to IdO and to ACME
               Server (STAR) . . . . . . . . . . . . . . . . . . . .  10
       2.3.3.  Order Object Transmitted from NDC to IdO and to ACME
               Server (non-STAR) . . . . . . . . . . . . . . . . . .  13
       2.3.4.  Capability Discovery  . . . . . . . . . . . . . . . .  16
       2.3.5.  Terminating the Delegation  . . . . . . . . . . . . .  16
     2.4.  Proxy Behavior  . . . . . . . . . . . . . . . . . . . . .  17
   3.  CSR Template  . . . . . . . . . . . . . . . . . . . . . . . .  18
     3.1.  Template Syntax . . . . . . . . . . . . . . . . . . . . .  19
     3.2.  Example . . . . . . . . . . . . . . . . . . . . . . . . .  20
   4.  Further Use Cases . . . . . . . . . . . . . . . . . . . . . .  21
     4.1.  CDN Interconnection (CDNI)  . . . . . . . . . . . . . . .  21
       4.1.1.  Multiple Parallel Delegates . . . . . . . . . . . . .  22
       4.1.2.  Chained Delegation  . . . . . . . . . . . . . . . . .  22
     4.2.  Secure Telephone Identity Revisited (STIR)  . . . . . . .  25
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  26
     5.1.  New ACME Identifier Object Fields . . . . . . . . . . . .  26
     5.2.  New Fields in the "meta" Object within a Directory
           Object  . . . . . . . . . . . . . . . . . . . . . . . . .  27
     5.3.  New Fields in the Order Object  . . . . . . . . . . . . .  27
     5.4.  New Fields in the Account Object  . . . . . . . . . . . .  28
     5.5.  New Error Types . . . . . . . . . . . . . . . . . . . . .  28
     5.6.  CSR Template Extensions . . . . . . . . . . . . . . . . .  28
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  29



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     6.1.  Trust Model . . . . . . . . . . . . . . . . . . . . . . .  29
     6.2.  Delegation Security Goal  . . . . . . . . . . . . . . . .  29
     6.3.  New ACME Channels . . . . . . . . . . . . . . . . . . . .  30
     6.4.  Restricting CDNs to the Delegation Mechanism  . . . . . .  31
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  32
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  32
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  32
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  34
   Appendix A.  Document History . . . . . . . . . . . . . . . . . .  35
     A.1.  draft-ietf-acme-star-delegation-07  . . . . . . . . . . .  35
     A.2.  draft-ietf-acme-star-delegation-06  . . . . . . . . . . .  35
     A.3.  draft-ietf-acme-star-delegation-05  . . . . . . . . . . .  35
     A.4.  draft-ietf-acme-star-delegation-04  . . . . . . . . . . .  35
     A.5.  draft-ietf-acme-star-delegation-03  . . . . . . . . . . .  36
     A.6.  draft-ietf-acme-star-delegation-02  . . . . . . . . . . .  36
     A.7.  draft-ietf-acme-star-delegation-01  . . . . . . . . . . .  36
     A.8.  draft-ietf-acme-star-delegation-00  . . . . . . . . . . .  36
     A.9.  draft-sheffer-acme-star-delegation-01 . . . . . . . . . .  36
     A.10. draft-sheffer-acme-star-delegation-00 . . . . . . . . . .  36
   Appendix B.  CSR Template: CDDL . . . . . . . . . . . . . . . . .  36
   Appendix C.  CSR Template: JSON Schema  . . . . . . . . . . . . .  39
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  43

1.  Introduction

   This document is a companion document to [RFC8739].  To avoid
   duplication, we give here a bare-bones description of the motivation
   for this solution.  For more details and further use cases, please
   refer to the introductory sections of [RFC8739].

   An Identifier Owner (IdO) has agreements in place with one or more
   NDC (Name Delegation Consumer) 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 and, equally,
   delegates would rather not have to handle other parties' long-term
   secrets.  Other relevant use cases are discussed in Section 4.

   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



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   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), periodically
   fetched by the NDC and 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 delegation of STAR
   certificates, the mechanism proposed here accommodates also 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 ACME CA and waits for the explicit revocation based on CRL and
   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 ongoing efforts address the problem of certificate
   delegation for TLS connections, specifically [I-D.ietf-tls-subcerts]
   and [I-D.mglt-lurk-tls13].  Compared to these other solutions, the
   current document does not introduce additional latency to the TLS
   connection, nor does it require changes to the TLS network stack of
   either the client or the server.

1.1.  Terminology

   IdO  Identifier Owner, the owner of an identifier (e.g., a domain
      name) that needs to be delegated.

   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 symmetry of the two acronyms).

   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 X.509 certificates.

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



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   CA  A Certification Authority that implements the ACME protocol.  In
      this document, the term is synonymous with "ACME server".

   CSR  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
   extended ACME 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;
   *  NDC and IdO have agreed on a "CSR template" to use, including at a
      minimum: subject name (e.g., "somesite.example.com"), requested
      algorithms and key length, key usage, extensions (e.g.,
      TNAuthList).  The NDC is required to use this template for every
      CSR created under the same delegation;
   *  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 ACME server.









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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] as well as any other ACME extension that applies (e.g.,
   [I-D.ietf-acme-authority-token-tnauthlist] for STIR).

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

   *  NDC sends an order 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.  This can continue until the
   STAR certificate 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 2.3.1).
   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.





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    .------.            .---------------.            .------.
   |  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------------------------------------------------>|



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       |                 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 a 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 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/rzGoeA",
     "delegations": "https://acme.ido.example/acme/delegations/adFqoz"
   }

             Figure 2: Example Account object with delegations

2.3.1.2.  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.




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   A delegation object contains the CSR template (see Section 3) 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 3.
   *  cname-map (optional, object): a map of FQDN pairs.  In each pair,
      the name is the delegated identifier, the value is the
      corresponding IdO 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 is shown in Figure 3.

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



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             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
   identifier in the Order object on the NDC-IdO side (see Figure 4).
   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
   NDC, the IdO MUST return an error response with status code 403
   (Forbidden) and 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 Order object created
   by the NDC:

   *  MUST have the delegated name as the identifier value with a
      "delegation" attribute indicating the configuration used for the
      identifier.
   *  MUST NOT contain the "notBefore" and "notAfter" fields;
   *  MUST contain an "auto-renewal" object and inside it, the fields
      listed in Section 3.1.1 of [RFC8739].



























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   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": "5XJ1L3lEkMG7tR6pA00clA",
       "url": "https://acme.ido.example/acme/new-order"
     }),
     "payload": base64url({
       "identifiers": [
         {
           "type": "dns",
           "value": "abc.ndc.ido.example.",
           "delegation":
              "https://acme.ido.example/acme/delegations/adFqoz/2"
         }
       ],
       "auto-renewal": {
         "end-date": "2020-04-20T00:00:00Z",
         "lifetime": 345600,          // 4 days
         "allow-certificate-get": true
       }
     }),
     "signature": "H6ZXtGjTZyUnPeKn...wEA4TklBdh3e454g"
   }

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














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   {
     "status": "ready",
     "expires": "2019-05-01T00:00:00Z",

     "identifiers": [
      {
        "type": "dns",
        "value": "abc.ndc.ido.example.",
        "delegation":
           "https://acme.ido.example/acme/delegations/adFqoz/2"
      }
     ],

     "auto-renewal": {
       "end-date": "2020-04-20T00:00:00Z",
       "lifetime": 345600,
       "allow-certificate-get": true
     },

     "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 that applies to each delegated identifier, as described
   in Section 3.1.  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 Order object created by the IdO:

   *  MUST copy the identifiers sent by the NDC and strip the
      "delegation" attribute;
   *  MUST carry a copy of the "auto-renewal" object sent by the NDC and
      augment it with an "allow-certificate-get" attribute set to true.

   When the validation of the identifiers has been successfully
   completed and the certificate has been issued by the CA, the IdO:




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   *  MUST move its Order resource status to "valid".
   *  MUST copy the "star-certificate" field from the STAR Order.  The
      latter indirectly includes (via the NotBefore and NotAfter HTTP
      headers) the renewal timers needed by the NDC to inform its
      certificate reload logic.

   {
     "status": "valid",
     "expires": "2019-05-01T00:00:00Z",

     "identifiers": [
      {
        "type": "dns",
        "value": "abc.ndc.ido.example.",
        "delegation":
           "https://acme.ido.example/acme/delegations/adFqoz/2"
      }
     ],

     "auto-renewal": {
       "end-date": "2020-04-20T00:00:00Z",
       "lifetime": 345600,
       "allow-certificate-get": true
     },

     "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

2.3.2.1.  CNAME Installation

   If an identifier object of type "dns" was included, the IdO can add
   the corresponding CNAME records to its zone, e.g.:

      abc.ndc.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 Order object
   created by the NDC:





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   *  MUST have the delegated name as the identifier value with a
      "delegation" attribute indicating the configuration used for the
      identifier.

   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.ndc.ido.example.",
           "delegation":
              "https://acme.ido.example/acme/delegations/adFqoz/2"
         }
       ]
     }),
     "signature": "H6ZyWqg8aaKEkYca...dudoz4igiMvUBJ9j"
   }

                   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" configurations.
















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   {
     "status": "ready",
     "expires": "2019-05-01T00:00:00Z",

     "identifiers": [
      {
        "type": "dns",
        "value": "abc.ndc.ido.example.",
        "delegation":
           "https://acme.ido.example/acme/delegations/adFqoz/2"
      }
     ],

     "authorizations": [],

     "finalize": "https://acme.ido.example/acme/order/to8RFGO/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 Order object created by the IdO:

   *  MUST copy the identifiers sent by the NDC and strip the
      "delegation" attribute;
   *  MUST include the "allow-certificate-get" attribute set to true.

   When the validation of the identifiers has been successfully
   completed and the certificate has been issued by the CA, the IdO:

   *  MUST move its Order resource status to "valid".
   *  MUST copy the "certificate" field from the Order, as well as
      "notBefore" and "notAfter" if these fields exist.













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   {
     "status": "valid",
     "expires": "2019-05-01T00:00:00Z",

     "identifiers": [
      {
        "type": "dns",
        "value": "abc.ndc.ido.example.",
        "delegation":
           "https://acme.ido.example/acme/delegations/adFqoz/2"
      }
     ],

     "allow-certificate-get": true,

     "authorizations": [],

     "finalize": "https://acme.ido.example/acme/order/to8RFGO/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.

2.3.4.  Capability Discovery

   In order to help a client to discover support for this profile, the
   directory object of an ACME server MUST contain the following
   attribute in the "meta" field:

   *  delegation-enabled: 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 "delegation-enabled" flag may be specified regardless of the
   existence or setting of the "auto-renewal" flag.

2.3.5.  Terminating the Delegation

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







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2.3.5.1.  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 ACME
   server, 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.

2.3.5.2.  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
   ACME server.

   According to Section 7.6 of [RFC8555], the revocation endpoint can be
   used with either the account keypair, or the certificate keypair.  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 NDC and IdO expected by the
   delegation trust model (Section 6.1), as well as the lack of
   incentives for the NDC to prematurely terminate the delegation, this
   does not represent a 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 4.1.2.  This section
   describes the behavior of such proxies.

   An entity implementing the IdO server role - an "ACME Delegation
   server" - can decide, on a per-identity case, whether to act as a
   proxy into another ACME Delegation server, or to behave as an IdO and
   obtain a certificate directly.  The determining factor is whether it
   can successfully be authorized by the 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.



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

   *  New-order request:
      -  The complete "identifiers" object 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.
      -  And similarly, any "Link" relations.
   *  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, and therefore
   each proxy must be able to authenticate any subordinate server.

3.  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.






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3.1.  Template Syntax

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

   *  A mandatory field, where the template specifies the literal value
      of that field.  This is denoted by a literal string, such as
      "client1.ndc.ido.example.com".
   *  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 in the CSR any fields, including any
   extensions, unless they are specified in the template.

   The structure of the template object is defined by the CDDL [RFC8610]
   document in Appendix B.

   An alternative, non-normative JSON Schema syntax is given in
   Appendix C.

   The "subject" field and its subfields are mapped into the "subject"
   field of the CSR, as per [RFC5280], Section 4.1.2.6.  Other extension
   fields of the CSR template are mapped into the CSR according to the
   table in Section 5.6.

   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 ACME 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.












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   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 needs to have a further layer of checks on
   top of the control rules already provided by the CSR template to
   fully validate the CSR input.

3.2.  Example

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



































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   {
     "keyTypes": [
       {
         "PublicKeyType": "rsaEncryption",
         "PublicKeyLength": 2048,
         "SignatureType": "sha256WithRSAEncryption"
       },
       {
         "PublicKeyType": "id-ecPublicKey",
         "namedCurve": "secp256r1",
         "SignatureType": "ecdsa-with-SHA256"
       }
     ],
     "subject": {
       "country": "CA",
       "stateOrProvince": "**",
       "locality": "**",
       "commonName": "**"
     },
     "extensions": {
       "subjectAltName": {
         "DNS": [
           "client1.ndc.ido.example"
         ]
       },
       "keyUsage": [
         "digitalSignature"
       ],
       "extendedKeyUsage": [
         "serverAuth",
         "clientAuth"
       ]
     }
   }

                      Figure 10: Example CSR template

4.  Further Use Cases

   This non-normative section describes additional use cases that use
   STAR certificate delegation in non-trivial ways.

4.1.  CDN Interconnection (CDNI)

   [I-D.ietf-cdni-interfaces-https-delegation] discusses several
   solutions addressing different delegation requirements for the CDNI
   (CDN Interconnection) environment.  This section discusses two of the
   stated requirements in the context of the STAR delegation workflow.



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   This section uses specifically CDNI terminology, e.g. "uCDN" and
   "dCDN", as defined in [RFC7336].

4.1.1.  Multiple Parallel Delegates

   In some cases the content owner (IdO) would like to delegate
   authority over a web site 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.

4.1.2.  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 namespace that is made available to the dCDN to mint its
      delegated names;
   *  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.

4.1.2.1.  Two-Level Delegation in CDNI

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





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                                                    .------------.
                            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 an 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 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.






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              .--------------------.
             |      .------.------. |
             |      | STAR | ACME |<-------------.
    .------->|  CP  | dele | STAR | |             |
   |         |      | srv  | cli  +-----.         |
   |         |      '---+--'------' |    |        6
   |          '---------|------^---'     5        |
   |                    |      |         |     .--|-------.
   |                    |      |         |    | .-+----.   |
   |                    7      |          '---->| ACME |   |
   |                    |      |              | | STAR | C |
   0                    |      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 levels 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);





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   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
        signalling is complete;
   9.   dCDN requests the STAR certificate using unauthenticated GET
        from the ACME server;
   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.

4.2.  Secure Telephone Identity Revisited (STIR)

   As a second use case, we consider the delegation of credentials in
   the STIR ecosystem [I-D.ietf-stir-cert-delegation].

   This section uses STIR terminology.  The term PASSPorT is defined in
   [RFC8225], and "TNAuthList" in [RFC8226].

   In the STIR "delegated" mode, a service provider SP2 - the NDC -
   needs to sign PASSPorT's [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 private key, that
   includes TN=+123 in the TNAuthList [RFC8226] certificate extension.

   In details (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: SP1 name, 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;
   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 Order contents are forwarded from SP1 to SP2 by means of the
       paired "delegation" Order.




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

         .-------------------.
        |     .------.------. |
        |     | 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.

5.  IANA Considerations

   [[RFC Editor: please replace XXXX below by the RFC number.]]

5.1.  New ACME Identifier Object Fields

   This document requests that IANA create the following new registry
   under the Automated Certificate Management Environment (ACME)
   Protocol:

   *  ACME Identifier Object Fields

   This registry is administered under a Specification Required policy
   [RFC8126].




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   The "ACME Identifier Object Fields" registry lists field names that
   are defined for use in the ACME identifier object.

   Template:

   *  Field name: The string to be used as a field name in the JSON
      object
   *  Field type: The type of value to be provided, e.g., string,
      boolean, array of string
   *  Reference: Where this field is defined

          +============+============+===========================+
          | Field Name | Field Type | Reference                 |
          +============+============+===========================+
          | type       | string     | Section 7.1.3 of RFC 8555 |
          +------------+------------+---------------------------+
          | value      | string     | Section 7.1.3 of RFC 8555 |
          +------------+------------+---------------------------+
          | delegation | string     | RFC XXXX                  |
          +------------+------------+---------------------------+

                                  Table 1

   Note: this registry was not created at the time [RFC8555] was
   standardized likely because it was not anticipated that the
   identifier object would be extended.  It is retrospectively
   introduced to record the status quo and allow controlled
   extensibility of the identifier object.

5.2.  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 XXXX  |
              +--------------------+------------+-----------+

                                  Table 2

5.3.  New Fields in the Order Object

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





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     +=======================+============+==============+===========+
     | Field Name            | Field Type | Configurable | Reference |
     +=======================+============+==============+===========+
     | allow-certificate-get | boolean    | true         | RFC XXXX  |
     +-----------------------+------------+--------------+-----------+

                                  Table 3

5.4.  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 XXXX  |
            +-------------+------------+----------+-----------+

                                  Table 4

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

5.5.  New Error Types

   This document adds the following entries to the ACME Error Type
   registry:

    +===================+================================+===========+
    | Type              | Description                    | Reference |
    +===================+================================+===========+
    | unknownDelegation | An unknown configuration is    | RFC XXXX  |
    |                   | listed in the "delegations"    |           |
    |                   | attribute of the request Order |           |
    +-------------------+--------------------------------+-----------+

                                 Table 5

5.6.  CSR Template Extensions

   IANA is requested to establish a registry "STAR Delegation CSR
   Template Extensions", with "Expert Review" as its registration
   procedure.

   Each extension registered must specify:

   *  An extension name.



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   *  An extension syntax, as a reference to a JSON Schema document that
      defines this extension.
   *  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 B.

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

                                 Table 6

6.  Security Considerations

6.1.  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 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.

6.2.  Delegation Security Goal

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





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   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.  A process for handling revocation of the delegation;
   5.  A process for 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 ACME server.

   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]).

6.3.  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.



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                                  ACME Channel
                           .------------>------------.
   .-----. ACME Channel .--+--.                   .--+----------.
   | NDC +------------->| IdO |                   | ACME server |
   '--+--'              '--+--'                   '--+-+--------'
      |                    '-----------<-------------' |
      |                         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/
   ACME server 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 NDC and ACME server, 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 regards to the latter, it should
   be noted that there is currently no means for an IdO to disable
   authorising revocation based on certificate private keys.  So, in
   theory, an NDC could use the revocation API directly with the ACME
   server, therefore bypassing the IdO.  The NDC SHOULD NOT directly use
   the revocation interface exposed by the ACME server 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.

6.4.  Restricting CDNs to the Delegation Mechanism

   When a web site is delegated to a CDN, the CDN can in principle
   modify the web site at will, 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 the
   CNAME mechanisms defined in the current document is used or not.






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   In some cases, this is the desired behavior: the domain owner trusts
   the CDN to have full control of the cryptographic credentials for the
   site.  The current document however assumes that the domain owner
   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 owner makes sure that the CDN cannot modify the DNS
      records for the domain.  The domain owner 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 owner uses a 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 owner uses the ACME-specific CAA mechanism [RFC8657] to
      restrict issuance to a specific account key which 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 authorisation flows supported by the
   selected CAs.

7.  Acknowledgments

   We would like to thank the following people who contributed
   significantly to this document with their review comments and design
   proposals: Roman Danyliw, Frédéric Fieau, Russ Housley, Sanjay
   Mishra, Jon Peterson, Ryan Sleevi, Emile Stephan.

   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.

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>.






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   [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>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [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>.

   [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>.

   [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>.



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8.2.  Informative References

   [I-D.ietf-acme-authority-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-07, 22 February 2021,
              <https://www.ietf.org/archive/id/draft-ietf-acme-
              authority-token-tnauthlist-07.txt>.

   [I-D.ietf-cdni-interfaces-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-05, 12 March
              2021, <https://www.ietf.org/archive/id/draft-ietf-cdni-
              interfaces-https-delegation-05.txt>.

   [I-D.ietf-stir-cert-delegation]
              Peterson, J., "STIR Certificate Delegation", Work in
              Progress, Internet-Draft, draft-ietf-stir-cert-delegation-
              04, 22 February 2021, <https://www.ietf.org/archive/id/
              draft-ietf-stir-cert-delegation-04.txt>.

   [I-D.ietf-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://www.ietf.org/archive/id/draft-ietf-tls-
              subcerts-10.txt>.

   [I-D.mglt-lurk-tls13]
              Migault, D., "LURK Extension version 1 for (D)TLS 1.3
              Authentication", Work in Progress, Internet-Draft, draft-
              mglt-lurk-tls13-04, 25 January 2021,
              <https://www.ietf.org/archive/id/draft-mglt-lurk-
              tls13-04.txt>.

   [json-schema-07]
              Wright, A., Andrews, H., and G. Luff, "JSON Schema
              Validation: A Vocabulary for Structural Validation of
              JSON", 2018, <https://datatracker.ietf.org/doc/html/draft-
              handrews-json-schema-validation-01>.









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   [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>.

Appendix A.  Document History

   [[Note to RFC Editor: please remove before publication.]]

A.1.  draft-ietf-acme-star-delegation-07

   *  SecDir comments by Russ Housley.
   *  In particular, reorganized some parts of the document to clarify
      handling of non-STAR certificates.
   *  And changed the document's title accordingly.

A.2.  draft-ietf-acme-star-delegation-06

   *  CDDL schema to address Roman's remaining comments.

A.3.  draft-ietf-acme-star-delegation-05

   *  Detailed AD review by Roman Danyliw.
   *  Some comments that were left unaddressed in Ryan Sleevi's review.
   *  Numerous other edits for clarity and consistency.

A.4.  draft-ietf-acme-star-delegation-04

   *  Delegation of non-STAR certificates.
   *  More IANA clarity, specifically on certificate extensions.
   *  Add delegation configuration object and extend account and order
      objects accordingly.



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   *  A lot more depth on Security Considerations.

A.5.  draft-ietf-acme-star-delegation-03

   *  Consistency with the latest changes in the base ACME STAR
      document, e.g. star-delegation-enabled capability renamed and
      moved.
   *  Proxy use cases (recursive delegation) and the definition of proxy
      behavior.
   *  More detailed analysis of the CDNI and STIR use cases, including
      sequence diagrams.

A.6.  draft-ietf-acme-star-delegation-02

   *  Security considerations: review by Ryan Sleevi.
   *  CSR template simplified: instead of being a JSON Schema document
      itself, it is now a simple JSON document which validates to a JSON
      Schema.

A.7.  draft-ietf-acme-star-delegation-01

   *  Refinement of the CDNI use case.
   *  Addition of the CSR template (partial, more work required).
   *  Further security considerations (work in progress).

A.8.  draft-ietf-acme-star-delegation-00

   *  Republished as a working group draft.

A.9.  draft-sheffer-acme-star-delegation-01

   *  Added security considerations about disallowing CDNs from issuing
      certificates for a delegated domain.

A.10.  draft-sheffer-acme-star-delegation-00

   *  Initial version, some text extracted from draft-sheffer-acme-star-
      requests-02

Appendix B.  CSR Template: CDDL

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







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   An additional constraint that is not expressed in CDDL but MUST be
   validated by the recipient is that all objects (e.g.
   "distinguishedName") MUST NOT be empty when they are included, even
   when each separate property is optional.

   csr-template-schema = {
     keyTypes: [ 1* $keyType ]
     ? subject: distinguishedName
     extensions: extensions
   }

   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 = int .ge 2048

   ; 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



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   $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: [ 1* regtext-or-wildcard ]
     ? Email: [ 1* regtext ]
     ? URI: [ 1* regtext ]
     * $$subjectaltname-extension
   }

   extensions = {
     ? keyUsage: [ 1* keyUsageType ]
     ? extendedKeyUsage: [ 1* extendedKeyUsageType ]
     subjectAltName: 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"



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   extendedKeyUsageType /= "OCSPSigning"
   extendedKeyUsageType /= oid

   oid = text .regexp "[0-9]+(\\.[0-9]+)*"

Appendix C.  CSR Template: JSON Schema

   This appendix includes an alternative, non-normative, 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.

   While the CSR template must follow the syntax defined here, neither
   the IdO nor the NDC are expected to validate it at run-time.

   {
     "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": {



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             "type": "string"
           }
         },
         "additionalProperties": false
       },
       "rsaKeyType": {
         "$id": "#rsaKeyType",
         "type": "object",
         "properties": {
           "PublicKeyType": {
             "type": "string",
             "const": "rsaEncryption"
           },
           "PublicKeyLength": {
             "type": "integer",
             "minimum": 2048,
             "multipleOf": 8
           },
           "SignatureType": {
             "type": "string",
             "enum": [
               "sha256WithRSAEncryption",
               "sha384WithRSAEncryption",
               "sha512WithRSAEncryption",
               "sha256WithRSAandMGF1",
               "sha384WithRSAandMGF1",
               "sha512WithRSAandMGF1"
             ]
           }
         },
         "required": [
           "PublicKeyType",
           "PublicKeyLength",
           "SignatureType"
         ],
         "additionalProperties": false
       },
       "ecdsaKeyType": {
         "$id": "#ecdsaKeyType",
         "type": "object",
         "properties": {
           "PublicKeyType": {
             "type": "string",
             "const": "id-ecPublicKey"
           },
           "namedCurve": {
             "type": "string",
             "enum": [



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               "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",
         "items": {
           "oneOf": [
             {
               "$ref": "#rsaKeyType"
             },
             {
               "$ref": "#ecdsaKeyType"
             }
           ]
         }
       },
       "subject": {
         "$ref": "#distinguished-name"
       },
       "extensions": {
         "type": "object",
         "properties": {
           "keyUsage": {
             "type": "array",
             "minItems": 1,
             "items": {
               "type": "string",



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               "enum": [
                 "digitalSignature",
                 "nonRepudiation",
                 "keyEncipherment",
                 "dataEncipherment",
                 "keyAgreement",
                 "keyCertSign",
                 "cRLSign",
                 "encipherOnly",
                 "decipherOnly"
               ]
             }
           },
           "extendedKeyUsage": {
             "type": "array",
             "minItems": 1,
             "items": {
               "oneOf": [
                 {
                   "type": "string",
                   "enum": [
                     "serverAuth",
                     "clientAuth",
                     "codeSigning",
                     "emailProtection",
                     "timeStamping",
                     "OCSPSigning"
                   ]
                 },
                 {
                   "type": "string",
                   "pattern": "^[0-9]+(\\.[0-9]+)*$",
                   "description": "Used for OID values"
                 }
               ]
             }
           },
           "subjectAltName": {
             "type": "object",
             "minProperties": 1,
             "properties": {
               "DNS": {
                 "type": "array",
                 "minItems": 1,
                 "items": {
                   "type": "string",
                   "format": "hostname"
                 }



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               },
               "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
   }

Authors' Addresses

   Yaron Sheffer
   Intuit

   Email: yaronf.ietf@gmail.com


   Diego López
   Telefonica I+D

   Email: diego.r.lopez@telefonica.com





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   Antonio Agustín Pastor Perales
   Telefonica I+D

   Email: antonio.pastorperales@telefonica.com


   Thomas Fossati
   ARM

   Email: thomas.fossati@arm.com









































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