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Versions: (draft-sheffer-acme-star-delegation) 00 01 02 03

ACME                                                          Y. Sheffer
Internet-Draft                                                    Intuit
Intended status: Standards Track                                D. Lopez
Expires: 9 September 2020                              A. Pastor Perales
                                                          Telefonica I+D
                                                              T. Fossati
                                                                     ARM
                                                            8 March 2020


       An ACME Profile for Generating Delegated STAR Certificates
                   draft-ietf-acme-star-delegation-03

Abstract

   This memo proposes a profile of the ACME protocol that allows the
   owner of an identifier (e.g., a domain name) to delegate to a third
   party access to a certificate associated with said identifier.  A
   primary use case is that of a 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 by
   cancelling the associated STAR certificate renewal with the ACME CA.
   Another 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 9 September 2020.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   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 . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Conventions used in this document . . . . . . . . . . . .   4
   2.  Protocol Flow . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Preconditions . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.3.  Delegated Identity Profile  . . . . . . . . . . . . . . .   6
       2.3.1.  Order Object on the NDC-IdO side  . . . . . . . . . .   7
       2.3.2.  Order Object on the IdO-CA side . . . . . . . . . . .   9
       2.3.3.  Capability Discovery  . . . . . . . . . . . . . . . .   9
       2.3.4.  On Cancellation . . . . . . . . . . . . . . . . . . .  10
     2.4.  Proxy Behavior  . . . . . . . . . . . . . . . . . . . . .  10
   3.  CSR Template  . . . . . . . . . . . . . . . . . . . . . . . .  11
     3.1.  Template Syntax . . . . . . . . . . . . . . . . . . . . .  11
     3.2.  Example . . . . . . . . . . . . . . . . . . . . . . . . .  12
   4.  Further Use Cases . . . . . . . . . . . . . . . . . . . . . .  12
     4.1.  CDNI  . . . . . . . . . . . . . . . . . . . . . . . . . .  13
       4.1.1.  Multiple Parallel Delegates . . . . . . . . . . . . .  13
       4.1.2.  Chained Delegation  . . . . . . . . . . . . . . . . .  13
     4.2.  STIR  . . . . . . . . . . . . . . . . . . . . . . . . . .  16
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
     5.1.  New Fields in the "auto-renewal" Object within a
           Directory Metadata Object . . . . . . . . . . . . . . . .  17
     5.2.  New Fields for Identifiers  . . . . . . . . . . . . . . .  18
     5.3.  CSR Template Registry . . . . . . . . . . . . . . . . . .  18
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
     6.1.  Restricting CDNs to the Delegation Mechanism  . . . . . .  18
     6.2.  TBC . . . . . . . . . . . . . . . . . . . . . . . . . . .  19
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  19
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  19
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  20
   Appendix A.  Document History . . . . . . . . . . . . . . . . . .  21
     A.1.  draft-ietf-acme-star-delegation-03  . . . . . . . . . . .  21
     A.2.  draft-ietf-acme-star-delegation-02  . . . . . . . . . . .  21
     A.3.  draft-ietf-acme-star-delegation-01  . . . . . . . . . . .  21
     A.4.  draft-ietf-acme-star-delegation-00  . . . . . . . . . . .  21



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     A.5.  draft-sheffer-acme-star-delegation-01 . . . . . . . . . .  21
     A.6.  draft-sheffer-acme-star-delegation-00 . . . . . . . . . .  21
   Appendix B.  CSR Template Schema  . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  25

1.  Introduction

   This document is a companion document to [I-D.ietf-acme-star].  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
   [I-D.ietf-acme-star].

   An Identifier Owner (IdO), that we can associate in the primary use
   case to a content provider (also referred to as Domain Name Owner,
   DNO), has agreements in place with one or more NDC (Name Delegation
   Consumer) to use and attest its identity.  In the primary use case,
   we consider a CDN provider contracted to serve the IdO 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 authority
   of the URL that is requested, i.e., that of the IdO.  Understandably,
   most IdOs 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 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 [I-D.ietf-acme-star]) to request issuance of a 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.

   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.

1.1.  Terminology




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   IdO  Identifier Owner, the owner of an identifier (e.g., a domain
      name) that needs to be delegated.
   DNO  Domain Name Owner, a specific kind of IdO whose identifier is a
      domain name
   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  The IETF Automated Certificate Management Environment, a
      certificate management protocol.
   CA  A Certificate Authority that implements the ACME protocol.

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 draft as well as the
   extended ACME protocol described in [I-D.ietf-acme-star].

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 Certificate 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




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   NDC's key-pair and is responsible for completing the identity
   verification process towards the ACME CA.

2.2.  Overview

   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 STAR [I-D.ietf-acme-star]
   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 is as follow (Figure 1):

   *  NDC sends an Order for the delegated identifier to IdO;
   *  IdO creates an Order 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, the Order is moved to an "invalid"
      state and everything stops;
   *  If the CSR verification is successful, IdO moves the Order to
      state "processing", and sends an Order' (using its own account)
      for the delegated identifier to the ACME STAR CA;
   *  If the ACME STAR protocol fails, Order' moves to "invalid" and the
      same state is reflected in the NDC Order;
   *  If the ACME STAR run is successful (i.e., Order' is "valid"), IdO
      copies the "star-certificate" URL from Order' to Order and moves
      its state to "valid".

   The NDC can now download, install and use the short-term certificate
   bearing the name delegated by the IdO.  This sequence of actions is
   repeated until the STAR certificate expires or the IdO decides to
   cancel the automatic renewal process with the ACME STAR CA.

   Note that, because the identity validation is suppressed, the NDC
   sends the finalize request, including the CSR, to the IdO immediately
   after the Order has been acknowledged.  The IdO must buffer a (valid)
   CSR until the Validation phase completes successfully.

    .------.            .---------------.            .------.
   |  NDC   |          |       IdO       |          |   CA   |
   +--------+          +--------+--------+          +--------+
   | Client |          | Server | Client |          | Server |
   '---+----'          '----+---+---+----'          '----+---'
       |                    |       |                    |
       |   Order            |       |                    |
       |   Signature        |       |                    |



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       o------------------->|       |                    |
       |                    |       |                    |
       |   [ No identity ]  |       |                    |
       |   [ validation  ]  |       |                    |
       |                    |       |                    |
       |   CSR              |       |                    |
       |   Signature        |       |                    |
       o------------------->|       |                    |
       |   Acknowledgement  |       |   Order'           |
       |<-------------------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






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2.3.1.  Order Object on the NDC-IdO side

   The Order object created by the NDC:

   *  MUST contain identifiers with the new "delegated" field set to
      true;
   *  MUST NOT contain the notBefore and notAfter fields;
   *  MAY contain an "auto-renewal" object and inside it, any of the
      fields listed in Section 3.1.1 of [I-D.ietf-acme-star];
   *  In case the identifier type is "dns", it MAY contain a "cname"
      field with the alias of the identifier in the NDC domain.  This
      field is used by the IdO to create the DNS aliasing needed to
      redirect the resolvers to the delegated entity.

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

   {
     "protected": base64url({
       "alg": "ES256",
       "kid": "https://acme.dno.example/acme/acct/evOfKhNU60wg",
       "nonce": "5XJ1L3lEkMG7tR6pA00clA",
       "url": "https://acme.dno.example/acme/new-order"
     }),
     "payload": base64url({
       "identifiers": [
         {
           "type": "dns",
           "value": "abc.ndc.dno.example.",
           "delegated": true,
           "cname": "abc.ndc.example."
         }
       ],
     }),
     "signature": "H6ZXtGjTZyUnPeKn...wEA4TklBdh3e454g"
   }

   The Order object that is created on the IdO:

   *  MUST start in the "ready" state;
   *  MUST contain an "authorizations" array with zero elements;
   *  MUST NOT contain the "notBefore" and "notAfter" fields.








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   {
     "status": "ready",
     "expires": "2016-01-01T00:00:00Z",

     "identifiers": [
      {
        "type": "dns",
        "value": "abc.ndc.dno.example.",
        "delegated": true,
        "cname": "abc.ndc.example."
      }
     ],

     "authorizations": [],

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

   The IdO SHOULD copy the "auto-renewal" object (if it exists) from the
   NDC request into the related STAR request to the ACME CA.

   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 "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.





















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   {
     "status": "valid",
     "expires": "2016-01-01T00:00:00Z",

     "identifiers": [
      {
        "type": "dns",
        "value": "abc.ndc.dno.example.",
        "delegated": true,
        "cname": "abc.ndc.example."
      }
     ],

     "authorizations": [],

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

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

   If an "identifier" object of type "dns" was included, the IdO MUST
   validate the specified CNAME at this point in the flow.  The NDC and
   IdO may have a pre-established list of valid CNAME values.  At the
   minimum, the IdO MUST verify that both DNS names are syntactically
   valid.

   Following this validation, the IdO can add the CNAME records to its
   zone:

      abc.ndc.dno.example. CNAME abc.ndc.example.

2.3.2.  Order Object on the IdO-CA side

   When sending the Order to the ACME CA, the IdO SHOULD strip the
   "delegated" and "cname" attributes sent by the NDC (Section 2.3.1).
   The IdO MUST add the necessary STAR extensions to the Order.  In
   addition, to allow the NDC to download the certificate using
   unauthenticated GET, the IdO MUST add the allow-certificate-get
   attribute and set it to true.  This implies that an "auto-renewal"
   object must be included in the Order.

2.3.3.  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 inside the "auto-renewal" object in the "meta" field:





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   *  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.

2.3.4.  On Cancellation

   It is worth noting that cancellation of the ACME STAR certificate is
   a prerogative of the IdO.  The NDC does not own the relevant account
   key on the ACME CA, therefore it can't issue a cancellation request
   for the STAR cert.  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.

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 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 the server 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.

   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 is rewritten.
   *  Get Order response:
      -  The "status", "expires", "authorizations", "identifiers" and
         "auto-renewal" attributes/objects MUST be copied as-is.
      -  Similarly, the "star-certificate" URL 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.



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   *  Finalize request:
      -  The CSR MUST be copied as-is.
   *  Finalize response:
      -  Both the Location header and the "finalize" URLs are rewritten.

   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.

3.1.  Template Syntax

   The template is a JSON document.  Each field 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.dno.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 what its value is.  This is denoted by
      "*".

   The NDC MUST NOT include in the CSR any fields that are not specified
   in the template, and in particular MUST NOT add any extensions unless
   those were previously negotiated out of band with the IdO.

   The mapping between X.509 CSR fields and the template will be defined
   in a future revision of this document.

   When the CSR is received by the IdO, it MUST verify that the CSR is
   consistent with the template that the IdO sent earlier.  The IdO MAY
   enforce additional constraints, e.g. by restricting field lengths.







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3.2.  Example

   The CSR template in Figure 2 represents one possible CSR template
   governing the delegation exchanges provided in the rest of this
   document.

   {
     "keyTypes": [
       {
         "PublicKeyType": "RSA",
         "PublicKeyLength": 4096,
         "SignatureType": "sha256WithRSAEncryption"
       }
     ],
     "subject": {
       "country": "CA",
       "stateOrProvince": "**",
       "locality": "**",
       "commonName": "**"
     },
     "extensions": {
       "subjectAltName": {
         "DNS": [
           "client1.ndc.dno.example"
         ],
         "IP": [
           "1.2.3.4",
           "13::17"
         ]
       },
       "keyUsage": [
         "digitalSignature"
       ],
       "extendedKeyUsage": [
         "serverAuth",
         "timeStamping"
       ]
     }
   }

                       Figure 2: Example CSR template

   The template syntax is defined in Appendix B.

4.  Further Use Cases






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4.1.  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.

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 DNO 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 DNO, 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.









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4.1.2.1.  Two-Level Delegation in CDNI

   A User Agent (browser or set-top-box) wants to fetch the video
   resource at the following URI: "https://video.cp.example/movie".
   Redirection between Content Provider, upstream, and downstream CDNs
   is arranged as a CNAME-based aliasing chain as illustrated in
   Figure 3.

                                                    .------------.
                            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 3: 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




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   certificate whose SAN matches "video.cp.example", i.e., a Content
   Provider's name.

   Figure 4 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 |
   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 4: Two levels delegation in CDNI

   TBD bootstrap, see https://github.com/yaronf/I-D/issues/47

   1.   dCDN requests CDNI path metadata to uCDN;
   2.   uCDN replies with, among other CDNI things, the STAR delegation




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        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
        ACME 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 cert 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 cert using unauthenticated GET from the
        ACME 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.

4.2.  STIR

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

   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] cert extension.

   In details (Figure 5):

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




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   5.  The Order contents 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.

         .-------------------.
        |     .------.------. |
        |     | 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 5: 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 Fields in the "auto-renewal" Object within a Directory
      Metadata Object

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







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              +--------------------+------------+-----------+
              | Field Name         | Field Type | Reference |
              +====================+============+===========+
              | delegation-enabled | boolean    | RFC XXXX  |
              +--------------------+------------+-----------+

                                  Table 1

5.2.  New Fields for Identifiers

   *  "delegated", TBD.

5.3.  CSR Template Registry

   TODO

6.  Security Considerations

6.1.  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.

   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.

   Following is the proposed solution where 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 [RFC6844] 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.




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   We note that the above solution may need to be tweaked depending on
   the exact capabilities and authorisation flows supported by the
   selected CAs.

6.2.  TBC

   *  CSR validation
   *  CNAME mappings
   *  Composition with ACME STAR
   *  Composition with other ACME extensions
   *  Channel security

7.  Acknowledgments

   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

   [I-D.handrews-json-schema]
              Wright, A., Andrews, H., Hutton, B., and G. Dennis, "JSON
              Schema: A Media Type for Describing JSON Documents", Work
              in Progress, Internet-Draft, draft-handrews-json-schema-
              02, 17 September 2019,
              <http://www.ietf.org/internet-drafts/draft-handrews-json-
              schema-02.txt>.

   [I-D.ietf-acme-star]
              Sheffer, Y., Lopez, D., Dios, O., Pastor, A., and T.
              Fossati, "Support for Short-Term, Automatically-Renewed
              (STAR) Certificates in Automated Certificate Management
              Environment (ACME)", Work in Progress, Internet-Draft,
              draft-ietf-acme-star-11, 24 October 2019,
              <http://www.ietf.org/internet-drafts/draft-ietf-acme-star-
              11.txt>.

   [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>.

   [RFC6844]  Hallam-Baker, P. and R. Stradling, "DNS Certification
              Authority Authorization (CAA) Resource Record", RFC 6844,




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              DOI 10.17487/RFC6844, January 2013,
              <https://www.rfc-editor.org/info/rfc6844>.

   [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>.

   [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>.

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-05, 4 November 2019,
              <http://www.ietf.org/internet-drafts/draft-ietf-acme-
              authority-token-tnauthlist-05.txt>.

   [I-D.ietf-cdni-interfaces-https-delegation]
              Fieau, F., Emile, S., and S. Mishra, "CDNI extensions for
              HTTPS delegation", Work in Progress, Internet-Draft,
              draft-ietf-cdni-interfaces-https-delegation-02, 4 November
              2019,
              <http://www.ietf.org/internet-drafts/draft-ietf-cdni-
              interfaces-https-delegation-02.txt>.

   [I-D.ietf-stir-cert-delegation]
              Peterson, J., "STIR Certificate Delegation", Work in
              Progress, Internet-Draft, draft-ietf-stir-cert-delegation-
              01, 4 November 2019,
              <http://www.ietf.org/internet-drafts/draft-ietf-stir-cert-
              delegation-01.txt>.

   [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>.





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   [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-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.2.  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.3.  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.4.  draft-ietf-acme-star-delegation-00

   *  Republished as a working group draft.

A.5.  draft-sheffer-acme-star-delegation-01

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

A.6.  draft-sheffer-acme-star-delegation-00

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






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Appendix B.  CSR Template Schema

   Following is a JSON Schema definition of the CSR template.  The
   syntax used is that of draft 7 of JSON Schema, which may not be the
   latest version of the corresponding Internet Draft
   [I-D.handrews-json-schema] at the time of publication.

   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",
     "$def": {
       "distinguished-name": {
         "$id": "#distinguished-name",
         "type": "object",
         "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": {



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             "type": "string",
             "const": "RSA"
           },
           "PublicKeyLength": {
             "type": "integer"
           },
           "SignatureType": {
             "type": "string",
             "enum": [
               "sha256WithRSAEncryption"
             ]
           }
         },
         "additionalProperties": false
       },
       "ecKeyTYpe": {
         "$id": "#ecKeyType",
         "type": "object",
         "properties": {
           "PublicKeyType": {
             "type": "string",
             "const": "ecPublicKey"
           },
           "Curve": {
             "type": "string",
             "enum": [
               "secp521r1"
             ]
           },
           "SignatureType": {
             "type": "string",
             "enum": [
               "ecdsa-with-SHA256"
             ]
           }
         },
         "additionalProperties": false
       }
     },
     "type": "object",
     "properties": {
       "keyTypes": {
         "type": "array",
         "items": {
           "oneOf": [
             {
               "$ref": "#rsaKeyType"
             },



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             {
               "$ref": "#ecKeyType"
             }
           ]
         }
       },
       "subject": {
         "$ref": "#distinguished-name"
       },
       "extensions": {
         "type": "object",
         "properties": {
           "keyUsage": {
             "type": "array",
             "items": {
               "type": "string",
               "enum": [
                 "digitalSignature",
                 "nonRepudiation",
                 "keyEncipherment",
                 "dataEncipherment",
                 "keyAgreement",
                 "keyCertSign",
                 "cRLSign",
                 "encipherOnly",
                 "decipherOnly"
               ]
             }
           },
           "extendedKeyUsage": {
             "type": "array",
             "items": {
               "type": "string",
               "enum": [
                 "serverAuth",
                 "clientAuth",
                 "codeSigning",
                 "emailProtection",
                 "timeStamping",
                 "OCSPSigning"
               ]
             }
           },
           "subjectAltName": {
             "type": "object",
             "properties": {
               "DNS": {
                 "type": "array",



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                 "items": {
                   "type": "string",
                   "format": "hostname"
                 }
               },
               "IP": {
                 "type": "array",
                 "items": {
                   "oneOf": [
                     {
                       "type": "string",
                       "format": "ipv4"
                     },
                     {
                       "type": "string",
                       "format": "ipv6"
                     }
                   ]
                 }
               },
               "Email": {
                 "type": "array",
                 "items": {
                   "type": "string",
                   "format": "email"
                 }
               }
             },
             "additionalProperties": false
           }
         },
         "additionalProperties": false
       }
     },
     "additionalProperties": false
   }

Authors' Addresses

   Yaron Sheffer
   Intuit

   Email: yaronf.ietf@gmail.com


   Diego Lopez
   Telefonica I+D




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   Email: diego.r.lopez@telefonica.com


   Antonio Agustin Pastor Perales
   Telefonica I+D

   Email: antonio.pastorperales@telefonica.com


   Thomas Fossati
   ARM

   Email: thomas.fossati@arm.com






































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