ACME for Subdomains
draft-ietf-acme-subdomains-00

Network Working Group                                           O. Friel
Internet-Draft                                                 R. Barnes
Intended status: Standards Track                                   Cisco
Expires: 28 April 2022                                      T. Hollebeek
                                                                DigiCert
                                                           M. Richardson
                                                Sandelman Software Works
                                                         25 October 2021

                          ACME for Subdomains
                     draft-ietf-acme-subdomains-00

Abstract

   This document outlines how ACME can be used by a client to obtain a
   certificate for a subdomain identifier from a certification
   authority.  The client has fulfilled a challenge against a parent
   domain but does not need to fulfill a challenge against the explicit
   subdomain as certificate policy allows issuance of the subdomain
   certificate without explicit subdomain ownership proof.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   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 28 April 2022.

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

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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  ACME Workflow and Identifier Requirements . . . . . . . . . .   4
   4.  ACME Issuance of Subdomain Certificates . . . . . . . . . . .   5
     4.1.  ACME Challenge Type . . . . . . . . . . . . . . . . . . .   6
     4.2.  Authorization Object  . . . . . . . . . . . . . . . . . .   6
     4.3.  Pre-Authorization . . . . . . . . . . . . . . . . . . . .   7
     4.4.  New Orders  . . . . . . . . . . . . . . . . . . . . . . .   8
     4.5.  Directory Object Metadata . . . . . . . . . . . . . . . .  10
   5.  Illustrative Call Flow  . . . . . . . . . . . . . . . . . . .  10
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
     6.1.  Authorization Object Fields Registry  . . . . . . . . . .  16
     6.2.  Directory Object Metadata Fields Registry . . . . . . . .  16
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
     7.1.  ACME Server Policy Considerations . . . . . . . . . . . .  18
   8.  Informative References  . . . . . . . . . . . . . . . . . . .  18
   Appendix A.  CA Browser Forum Baseline Requirements Extracts  . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

1.  Introduction

   ACME [RFC8555] defines a protocol that a certification authority (CA)
   and an applicant can use to automate the process of domain name
   ownership validation and X.509v3 (PKIX) [RFC5280] certificate
   issuance.  This document outlines how ACME can be used to issue
   subdomain certificates, without requiring the ACME client to
   explicitly fulfill an ownership challenge against the subdomain
   identifiers - the ACME client need only fulfill an ownership
   challenge against a parent domain identifier.

2.  Terminology

   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.

   The following terms are defined in DNS Terminology [RFC8499] and are
   reproduced here:

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   *  Label: An ordered list of zero or more octets that makes up a
      portion of a domain name.  Using graph theory, a label identifies
      one node in a portion of the graph of all possible domain names.

   *  Domain Name: An ordered list of one or more labels.

   *  Subdomain: "A domain is a subdomain of another domain if it is
      contained within that domain.  This relationship can be tested by
      seeing if the subdomain's name ends with the containing domain's
      name."  (Quoted from [RFC1034], Section 3.1) For example, in the
      host name "nnn.mmm.example.com", both "mmm.example.com" and
      "nnn.mmm.example.com" are subdomains of "example.com".  Note that
      the comparisons here are done on whole labels; that is,
      "ooo.example.com" is not a subdomain of "oo.example.com".

   *  Fully-Qualified Domain Name (FQDN): This is often just a clear way
      of saying the same thing as "domain name of a node", as outlined
      above.  However, the term is ambiguous.  Strictly speaking, a
      fully-qualified domain name would include every label, including
      the zero-length label of the root: such a name would be written
      "www.example.net." (note the terminating dot).  But, because every
      name eventually shares the common root, names are often written
      relative to the root (such as "www.example.net") and are still
      called "fully qualified".  This term first appeared in [RFC0819].
      In this document, names are often written relative to the root.

   The following terms are defined in the CA/Browser Forum Baseline
   Requirements [CAB] version 1.7.1 and are reproduced here:

   *  Authorization Domain Name (ADN): The Domain Name used to obtain
      authorization for certificate issuance for a given FQDN.  The CA
      may use the FQDN returned from a DNS CNAME lookup as the FQDN for
      the purposes of domain validation.  If the FQDN contains a
      wildcard character, then the CA MUST remove all wildcard labels
      from the left most portion of requested FQDN.  The CA may prune
      zero or more labels from left to right until encountering a Base
      Domain Name and may use any one of the intermediate values for the
      purpose of domain validation

   *  Base Domain Name: The portion of an applied-for FQDN that is the
      first domain name node left of a registry-controlled or public
      suffix plus the registry-controlled or public suffix (e.g.
      "example.co.uk" or "example.com").  For FQDNs where the right-most
      domain name node is a gTLD having ICANN Specification 13 in its
      registry agreement, the gTLD itself may be used as the Base Domain
      Name.

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   *  Certification Authority (CA): An organization that is responsible
      for the creation, issuance, revocation, and management of
      Certificates.  The term applies equally to both Roots CAs and
      Subordinate CAs

   *  Domain Namespace: The set of all possible Domain Names that are
      subordinate to a single node in the Domain Name System

   The following additional terms are used in this document:

   *  Certification Authority (CA): An organization that is responsible
      for the creation, issuance, revocation, and management of
      Certificates.  The term applies equally to both Roots CAs and
      Subordinate CAs

   *  CSR: Certificate Signing Request

   *  Parent Domain: a domain is a parent domain of a subdomain if it
      contains that subdomain, as per the [RFC8499] definition of
      subdomain.  For example, for the host name "nnn.mmm.example.com",
      both "mmm.example.com" and "example.com" are parent domains of
      "nnn.mmm.example.com".

3.  ACME Workflow and Identifier Requirements

   A typical ACME workflow for issuance of certificates is as follows:

   1.  client POSTs a newOrder request that contains a set of
       "identifiers"

   2.  server replies with a set of "authorizations" and a "finalize"
       URI

   3.  client sends POST-as-GET requests to retrieve the
       "authorizations", with the downloaded "authorization" object(s)
       containing the "identifier" that the client must prove that they
       control, and a set of associated "challenges", one of which the
       the client must fulfil

   4.  client proves control over the "identifier" in the
       "authorization" object by completing one of the specified
       challenges, for example, by publishing a DNS TXT record

   5.  client POSTs a CSR to the "finalize" API

   6.  server replies with an updated order object that includes a
       "certificate" URI

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   7.  client sends POST-as-GET request to the "certificate" URI to
       download the certificate

   ACME places the following restrictions on "identifiers":

   *  [RFC8555] section 7.1.3: The authorizations required are dictated
      by server policy; there may not be a 1:1 relationship between the
      order identifiers and the authorizations required.

   *  [RFC8555] section 7.1.4: the only type of "identifier" defined by
      the ACME specification is an FQDN: "The only type of identifier
      defined by this specification is a fully qualified domain name
      (type: "dns").  The domain name MUST be encoded in the form in
      which it would appear in a certificate."

   *  [RFC8555] section 7.4: the "identifier" in the CSR request must
      match the "identifier" in the newOrder request: "The CSR MUST
      indicate the exact same set of requested identifiers as the
      initial newOrder request."

   *  [RFC8555] section 8.3: the "identifier", or FQDN, in the
      "authorization" object must be used when fulfilling challenges via
      HTTP: "Construct a URL by populating the URL template ... where
      the domain field is set to the domain name being verified"

   *  [RFC8555] section 8.4: the "identifier", or FQDN, in the
      "authorization" object must be used when fulfilling challenges via
      DNS: "The client constructs the validation domain name by
      prepending the label "_acme-challenge" to the domain name being
      validated."

   ACME does not mandate that the "identifier" in a newOrder request
   matches the "identifier" in "authorization" objects.

4.  ACME Issuance of Subdomain Certificates

   As noted in the previous section, ACME does not mandate that the
   "identifier" in a newOrder request matches the "identifier" in
   "authorization" objects.  This means that the ACME specification does
   not preclude an ACME server processing newOrder requests and issuing
   certificates for a subdomain without requiring a challenge to be
   fulfilled against that explicit subdomain.

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   ACME server policy could allow issuance of certificates for a
   subdomain to a client where the client only has to fulfill an
   authorization challenge for a parent domain of that subdomain.  This
   allows a flow where a client proves ownership of, for example,
   "example.org" and then successfully obtains a certificate for
   "sub.example.org".

   ACME server policy is out of scope of this document, however some
   commentary is provided in Section 7.1.

   Clients need a mechanism to instruct the ACME server that they are
   requesting authorization for a Domain Namespace subordinate to a
   given ADN, as opposed to just requesting authorization for an
   explicit ADN identifier.  Clients need a mechanism to do this in both
   newAuthz and newOrder requests.  ACME servers need a mechanism to
   indicate to clients that authorization objects are valid for an
   entire Domain Namespace.  These are described in this section.

4.1.  ACME Challenge Type

   ACME for subdomains is restricted for use with "dns-01" challenges.
   If a server policy allows a client to fulfill a challenge against a
   parent ADN of a requested certificate FQDN identifier, then the
   server MUST issue a "dns-01" challenge against that parent ADN.

4.2.  Authorization Object

   ACME [RFC8555] section 7.1.4 defines the authorization object.  When
   ACME server policy allows authorization for Domain Namespaces
   subordinate to an ADN, the server indicates this by including the
   "domainNamespace" flag in the authorization object for that ADN
   identifier:

   domainNamespace (optional, boolean):  This field MUST be present
      and true for authorizations where ACME server policy allows
      certificates to be issued for any Domain Name in the Domain
      Namespace subordinate to the ADN specified in the 'identifier'
      field of the authorization object.

   The following example shows an authorization object for the ADN
   example.org where the authorization covers the Domain Namespace
   subordinate to example.org.

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      {
        "status": "valid",
        "expires": "2015-03-01T14:09:07.99Z",

        "identifier": {
          "type": "dns",
          "value": "example.org"
        },

        "challenges": [
          {
            "url": "https://example.com/acme/chall/prV_B7yEyA4",
            "type": "http-01",
            "status": "valid",
            "token": "DGyRejmCefe7v4NfDGDKfA",
            "validated": "2014-12-01T12:05:58.16Z"
          }
        ],

        "domainNamespace": true
      }

   If the "domainNamespace" field is not included, then the assumed
   default value is false.

4.3.  Pre-Authorization

   The standard ACME workflow has authorization objects created
   reactively in response to a certificate order.  ACME also allows for
   pre-authorization, where clients obtain authorization for an
   identifier proactively, outside of the context of a specific
   issuance.  With the ACME pre-authorization flow, a client can pre-
   authorize for a parent ADN once, and then issue multiple newOrder
   requests for certificates with identifiers in the Domain Namespace
   subordinate to that ADN.

   ACME [RFC8555] section 7.4.1 defines the "identifier" object for
   newAuthz requests.  One additional field for the "identifier" object
   is defined:

   domainNamespace (optional, boolean): An ACME client sets this flag
      to indicate to the server that it is requesting an authorization
      for the Domain Namespace subordinate to the specified ADN
      identifier value

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   Clients include the flag in the "identifier" object of newAuthz
   requests to indicate that they are requesting a Domain Namespace
   authorization.  In the following example newAuthz payload, the client
   is requesting pre-authorization for the Domain Namespace subordinate
   to example.org.

        "payload": base64url({
          "identifier": {
            "type": "dns",
            "value": "example.org",
            "domainNamespace": true
          }
        })

   If the server is willing to allow a single authorization for the
   Domain Namespace, and there is not an existing authorization object
   for the identifier, then it will create an authorization object and
   include the "domainNamespace" flag with value of true.  If the server
   policy does not allow creation of Domain Namespace authorizations
   subordinate to that ADN, the server can create an authorization
   object for the indicated identifier, and include the
   "domainNamespace" flag with value of false.  In both scenarios,
   handling of the pre-authorization follows the process documented in
   ACME section 7.4.1.

4.4.  New Orders

   Clients need a mechanism to optionally indicate to servers whether or
   not they are authorized to fulfill challenges against parent ADNs for
   a given identifier FQDN.  For example, if a client places an order
   for an identifier foo.bar.example.org, and is authorized to update
   DNS TXT records against the parent ADNs bar.example.org or
   example.org, then the client needs a mechanism to indicate control
   over the parent ADNs to the ACME server.

   This can be achieved by adding an optional field "domainNamespace" to
   the "identifiers" field in the order object:

   domainNamespace (optional, string): This is the parent ADN of a
      Domain Namespace that the requested identifier belongs to. The
      client MUST have DNS control over the parent ADN.

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   This field specifies the ADN of the Domain Namespace that the client
   has DNS control over, and is capable of fulfilling challenges
   against.  Based on server policy, the server can choose to issue a
   challenge against any parent domain of the identifier in the Domain
   Namespace up to and including the specified "domainNamespace", and
   create a corresponding authorization object against the chosen
   identifier.

   In the following example newOrder payload, the client requests a
   certificate for identifier foo.bar.example.org and indicates that it
   can fulfill a challenge against the parent ADN and the Domain
   Namespace subordinate to bar.example.org.  The server can then choose
   to issue a challenge against either foo.bar.example.org or
   bar.example.org identifiers.

   "payload": base64url({
          "identifiers": [
            { "type": "dns",
              "value": "foo.bar.example.org",
              "domainNamespace": "bar.example.org"  }
          ],
          "notBefore": "2016-01-01T00:04:00+04:00",
          "notAfter": "2016-01-08T00:04:00+04:00"
        })

   In the following example newOrder payload, the client requests a
   certificate for identifier foo.bar.example.org and indicates that it
   can fulfill a challenge against the parent ADN and the Domain
   Namespace subordinate to example.org.  The server can then choose to
   issue a challenge against any one of foo.bar.example.org,
   bar.example.org or example.org identifiers.

   "payload": base64url({
          "identifiers": [
            { "type": "dns",
              "value": "foo.bar.example.org",
              "domainNamespace": "example.org"  }
          ],
          "notBefore": "2016-01-01T00:04:00+04:00",
          "notAfter": "2016-01-08T00:04:00+04:00"
        })

   If the client is unable to fulfill authorizations against parent
   ADNs, the client should not include the "domainNamespace" field.

   Server newOrder handling generally follows the process documented
   ACME section 7.4.  If the server is willing to allow Domain Namespace
   authorizations for the ADN specified in "domainNamespace", then it

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   creates an authorization object against that ADN and includes the
   "domainNamespace" flag with a value of true.  If the server policy
   does not allow creation of Domain Namespace authorizations against
   that ADN, then it can create an authorization object for the
   indicated identifier value, and include the "domainNamespace" flag
   with value of false.

4.5.  Directory Object Metadata

   An ACME server can advertise support for authorization of Domain
   Namespaces by including the following boolean flag in its "ACME
   Directory Metadata Fields" registry:

   domainNamespace (optional, bool): Indicates if an ACME server
      supports authorization of Domain Namespaces.

   If not specified, then no default value is assumed.  If an ACME
   server supports authorization of Domain Namespaces, it can indicate
   this by including this field with a value of "true".

5.  Illustrative Call Flow

   The call flow illustrated here uses the ACME pre-authorization flow
   using DNS-based proof of ownership.

   +--------+                   +------+     +-----+
   | Client |                   | ACME |     | DNS |
   +--------+                   +------+     +-----+
       |                            |           |
    STEP 1: Pre-Authorization of parent domain
       |                            |           |
       | POST /newAuthz             |           |
       | "example.org"              |           |
       |--------------------------->|           |
       |                            |           |
       | 201 authorizations         |           |
       |<---------------------------|           |
       |                            |           |
       | Publish DNS TXT            |           |
       | "example.org"              |           |
       |--------------------------------------->|
       |                            |           |
       | POST /challenge            |           |
       |--------------------------->|           |
       |                            | Verify    |
       |                            |---------->|
       | 200 status=valid           |           |
       |<---------------------------|           |

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       |                            |           |
       | Delete DNS TXT             |           |
       | "example.org"              |           |
       |--------------------------------------->|
       |                            |           |
    STEP 2: Place order for sub1.example.org
       |                            |           |
       | POST /newOrder             |           |
       | "sub1.example.org"         |           |
       |--------------------------->|           |
       |                            |           |
       | 201 status=ready           |           |
       |<---------------------------|           |
       |                            |           |
       | POST /finalize             |           |
       | CSR SAN "sub1.example.org" |           |
       |--------------------------->|           |
       |                            |           |
       | 200 OK status=valid        |           |
       |<---------------------------|           |
       |                            |           |
       | POST /certificate          |           |
       |--------------------------->|           |
       |                            |           |
       | 200 OK                     |           |
       | PEM SAN "sub1.example.org" |           |
       |<---------------------------|           |
       |                            |           |
    STEP 3: Place order for sub2.example.org
       |                            |           |
       | POST /newOrder             |           |
       | "sub2.example.org"         |           |
       |--------------------------->|           |
       |                            |           |
       | 201 status=ready           |           |
       |<---------------------------|           |
       |                            |           |
       | POST /finalize             |           |
       | CSR SAN "sub2.example.org" |           |
       |--------------------------->|           |
       |                            |           |
       | 200 OK status=valid        |           |
       |<---------------------------|           |
       |                            |           |
       | POST /certificate          |           |
       |--------------------------->|           |
       |                            |           |
       | 200 OK                     |           |

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       | PEM SAN "sub2.example.org" |           |
       |<---------------------------|           |

   *  STEP 1: Pre-authorization of Domain Namespace

      The client sends a newAuthz request for the parent ADN of the
      Domain Namespace including the "domainNamespace" flag in the
      identifier object.

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

      {
        "protected": base64url({
          "alg": "ES256",
          "kid": "https://example.com/acme/acct/evOfKhNU60wg",
          "nonce": "uQpSjlRb4vQVCjVYAyyUWg",
          "url": "https://example.com/acme/new-authz"
        }),
        "payload": base64url({
          "identifier": {
            "type": "dns",
            "value": "example.org",
            "domainNamespace": true
          }
        }),
        "signature": "nuSDISbWG8mMgE7H...QyVUL68yzf3Zawps"
      }

   The server creates and returns an authorization object for the
   identifier including the "domainNamespace" flag.  The object is
   initially in "pending" state.  Once the client completes the
   challenge, the server will transition the authorization object and
   associated challenge object status to "valid".

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      {
        "status": "pending",
        "expires": "2015-03-01T14:09:07.99Z",

        "identifier": {
          "type": "dns",
          "value": "example.org"
        },

        "challenges": [
          {
            "url": "https://example.com/acme/chall/prV_B7yEyA4",
            "type": "http-01",
            "status": "pending",
            "token": "DGyRejmCefe7v4NfDGDKfA",
            "validated": "2014-12-01T12:05:58.16Z"
          }
        ],

        "domainNamespace": true
      }

   *  STEP 2: The client places a newOrder for sub1.example.org

      The client sends a newOrder request to the server and includes the
      subdomain identifier.  Note that the identifier is in the Domain
      Namespace that has been pre-authorised in step 1.  The client does
      not need to include the "domainNamespace" field in the
      "identifier" object as it has already pre-authorized the Domain
      Namespace.

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

      {
        "protected": base64url({
          "alg": "ES256",
          "kid": "https://example.com/acme/acct/evOfKhNU60wg",
          "nonce": "5XJ1L3lEkMG7tR6pA00clA",
          "url": "https://example.com/acme/new-order"
        }),
        "payload": base64url({
          "identifiers": [
            { "type": "dns", "value": "sub1.example.org" }
          ],
          "notBefore": "2016-01-01T00:04:00+04:00",
          "notAfter": "2016-01-08T00:04:00+04:00"
        }),
        "signature": "H6ZXtGjTZyUnPeKn...wEA4TklBdh3e454g"
      }

   As an authorization object already exists for the parent ADN of the
   Domain Namespace, the server replies with an order object with a
   status of "valid" that includes a link to the existing "valid"
   authorization object.

      HTTP/1.1 201 Created
      Replay-Nonce: MYAuvOpaoIiywTezizk5vw
      Link: <https://example.com/acme/directory>;rel="index"
      Location: https://example.com/acme/order/TOlocE8rfgo

      {
        "status": "valid",
        "expires": "2016-01-05T14:09:07.99Z",

        "notBefore": "2016-01-01T00:00:00Z",
        "notAfter": "2016-01-08T00:00:00Z",

        "identifiers": [
          { "type": "dns", "value": "sub1.example.org" }
        ],

        "authorizations": [
          "https://example.com/acme/authz/PAniVnsZcis"
        ],

        "finalize": "https://example.com/acme/order/TOlocrfgo/finalize"
      }

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   The client can proceed to finalize the order and download the
   certificate for sub1.example.org.

   *  STEP 3: The client places a newOrder for sub2.example.org

      The client sends a newOrder request to the server and includes the
      subdomain identifier.  Note that the identifier is in the Domain
      Namespace that has been pre-authorised in step 1.  The client does
      not need to include the "domainNamespace" field in the
      "identifier" object as it has already pre-authorized the Domain
      Namespace.

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

      {
        "protected": base64url({
          "alg": "ES256",
          "kid": "https://example.com/acme/acct/evOfKhNU60wg",
          "nonce": "5XJ1L3lEkMG7tR6pA00clA",
          "url": "https://example.com/acme/new-order"
        }),
        "payload": base64url({
          "identifiers": [
            { "type": "dns", "value": "sub2.example.org" }
          ],
          "notBefore": "2016-01-01T00:04:00+04:00",
          "notAfter": "2016-01-08T00:04:00+04:00"
        }),
        "signature": "H6ZXtGjTZyUnPeKn...wEA4TklBdh3e454g"
      }

   As an authorization object already exists for the parent ADN of the
   Domain Namespace, the server replies with an order object with a
   status of "valid" that includes a link to the existing "valid"
   authorization object.

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      HTTP/1.1 201 Created
      Replay-Nonce: MYAuvOpaoIiywTezizk5vw
      Link: <https://example.com/acme/directory>;rel="index"
      Location: https://example.com/acme/order/TOlocE8rfgo

      {
        "status": "valid",
        "expires": "2016-01-05T14:09:07.99Z",

        "notBefore": "2016-01-01T00:00:00Z",
        "notAfter": "2016-01-08T00:00:00Z",

        "identifiers": [
          { "type": "dns", "value": "sub1.example.org" }
        ],

        "authorizations": [
          "https://example.com/acme/authz/PAniVnsZcis"
        ],

        "finalize": "https://example.com/acme/order/ROni7rdde/finalize"
      }

   The client can proceed to finalize the order and download the
   certificate for sub2.example.org.

6.  IANA Considerations

6.1.  Authorization Object Fields Registry

   The following field is added to the "ACME Authorization Object
   Fields" registry defined in ACME [RFC8555].

       +-----------------+------------+--------------+-----------+
       | Field Name      | Field Type | Configurable | Reference |
       +-----------------+------------+--------------+-----------+
       | domainNamespace | boolean    | false        | RFC XXXX  |
       +-----------------+------------+--------------+-----------+

6.2.  Directory Object Metadata Fields Registry

   The following field is added to the "ACME Directory Metadata Fields"
   registry defined in ACME [RFC8555].

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        +-----------------+------------+-----------+
        | Field Name      | Field Type | Reference |
        +-----------------+------------+-----------+
        | domainNamespace | boolean    | RFC XXXX  |
        +-----------------+------------+-----------+

7.  Security Considerations

   This document documents enhancements to ACME [RFC8555] that optimize
   the protocol flows for issuance of certificates for subdomains.  The
   underlying goal of ACME for Subdomains remains the same as that of
   ACME: managing certificates that attest to identifier/key bindings
   for these subdomains.  Thus, ACME for Subdomains has the same two
   security goals as ACME:

   1.  Only an entity that controls an identifier can get an
       authorization for that identifier

   2.  Once authorized, an account key's authorizations cannot be
       improperly used by another account

   ACME for Subdomains makes no changes to:

   *  account or account key management

   *  ACME channel establishment, security mechanisms or threat model

   *  Validation channel establishment, security mechanisms or threat
      model

   Therefore, all Security Considerations in ACME in the following areas
   are equally applicable to ACME for Subdomains:

   *  Threat Model

   *  Integrity of Authorizations

   *  Denial-of-Service Considerations

   *  Server-Side Request Forgery

   *  CA Policy Considerations

   Some additional comments on ACME server policy are given in the
   following section.

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7.1.  ACME Server Policy Considerations

   The ACME for Subdomains and the ACME specifications do not mandate
   any specific ACME server or CA policies, or any specific use cases
   for issuance of certificates.  For example, an ACME server could be
   used:

   *  to issue Web PKI certificates where the ACME server must comply
      with CA/Browser Forum [CAB] Baseline Requirements.

   *  as a Private CA for issuance of certificates within an
      organisation.  The organisation could enforce whatever policies
      they desire on the ACME server.

   *  for issuance of IoT device certificates.  There are currently no
      IoT device certificate policies that are generally enforced across
      the industry.  Organizations issuing IoT device certificates can
      enforce whatever policies they desire on the ACME server.

   ACME server policy could specify whether:

   *  issuance of subdomain certificates is allowed based on proof of
      ownership of a parent domain

   *  issuance of subdomain certificates is allowed, but only for a
      specific set of parent domains

   *  whether DNS based proof of ownership, or HTTP based proof of
      ownership, or both, are allowed

   ACME server policy specification is explicitly out of scope of this
   document.  For reference, extracts from CA/Browser Forum Baseline
   Requirements are given in the appendices.

8.  Informative References

   [CAB]      CA/Browser Forum, "Baseline Requirements for the Issuance
              and Management of Publicly-Trusted Certificates", n.d.,
              <https://cabforum.org/wp-content/uploads/CA-Browser-Forum-
              BR-1.7.1.pdf>.

   [RFC0819]  Su, Z. and J. Postel, "The Domain Naming Convention for
              Internet User Applications", RFC 819,
              DOI 10.17487/RFC0819, August 1982,
              <https://www.rfc-editor.org/info/rfc819>.

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   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
              <https://www.rfc-editor.org/info/rfc1034>.

   [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>.

   [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>.

   [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>.

   [RFC8499]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
              January 2019, <https://www.rfc-editor.org/info/rfc8499>.

   [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>.

Appendix A.  CA Browser Forum Baseline Requirements Extracts

   The CA/Browser Forum Baseline Requirements [CAB] allow issuance of
   subdomain certificates where authorization is only required for a
   parent domain.  Baseline Requirements version 1.7.1 states:

   *  Section: "1.6.1 Definitions": Authorization Domain Name: The
      Domain Name used to obtain authorization for certificate issuance
      for a given FQDN.  The CA may use the FQDN returned from a DNS
      CNAME lookup as the FQDN for the purposes of domain validation.
      If the FQDN contains a wildcard character, then the CA MUST remove
      all wildcard labels from the left most portion of requested FQDN.
      The CA may prune zero or more labels from left to right until
      encountering a Base Domain Name and may use any one of the
      intermediate values for the purpose of domain validation.

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   *  Section: "3.2.2.4.6 Agreed-Upon Change to Website": Once the FQDN
      has been validated using this method, the CA MAY also issue
      Certificates for other FQDNs that end with all the labels of the
      validated FQDN.  This method is suitable for validating Wildcard
      Domain Names.

   *  Section: "3.2.2.4.7 DNS Change": Once the FQDN has been validated
      using this method, the CA MAY also issue Certificates for other
      FQDNs that end with all the labels of the validated FQDN.  This
      method is suitable for validating Wildcard Domain Names.

Authors' Addresses

   Owen Friel
   Cisco

   Email: ofriel@cisco.com

   Richard Barnes
   Cisco

   Email: rlb@ipv.sx

   Tim Hollebeek
   DigiCert

   Email: tim.hollebeek@digicert.com

   Michael Richardson
   Sandelman Software Works

   Email: mcr+ietf@sandelman.ca

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