Network Working Group O. Friel Internet-Draft R. Barnes Intended status: Standards Track Cisco Expires: April 12, 2021 T. Hollebeek DigiCert M. Richardson Sandelman Software Works October 09, 2020 ACME for Subdomains draft-friel-acme-subdomains-03 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 fulfil 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 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 April 12, 2021. Copyright Notice Copyright (c) 2020 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 Friel, et al. Expires April 12, 2021 [Page 1] Internet-Draft ACME-SUBDOMAINS October 2020 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 . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2 3. ACME Workflow and Identifier Requirements . . . . . . . . . . 3 4. Open Items . . . . . . . . . . . . . . . . . . . . . . . . . 4 5. ACME Issuance of Subdomain Certificates . . . . . . . . . . . 5 5.1. Pre-Authorization . . . . . . . . . . . . . . . . . . . . 5 5.2. Illustrative Call Flow . . . . . . . . . . . . . . . . . 6 5.3. newOrder and newAuthz Handling . . . . . . . . . . . . . 7 5.4. Examples . . . . . . . . . . . . . . . . . . . . . . . . 8 6. Resource Enhancements . . . . . . . . . . . . . . . . . . . . 9 6.1. Authorization Object . . . . . . . . . . . . . . . . . . 9 6.2. Directory Object Metadata . . . . . . . . . . . . . . . . 9 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 7.1. Authorization Object Fields Registry . . . . . . . . . . 9 7.2. Directory Object Metadata Fields Registry . . . . . . . . 9 8. Security Considerations . . . . . . . . . . . . . . . . . . . 10 8.1. ACME Server Policy Considerations . . . . . . . . . . . . 11 9. Informative References . . . . . . . . . . . . . . . . . . . 11 Appendix A. CA Browser Forum Baseline Requirements Extracts . . 12 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12 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 fulfil an ownership challenge against the subdomain identifiers - the ACME client need only fulfil 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. Friel, et al. Expires April 12, 2021 [Page 2] Internet-Draft ACME-SUBDOMAINS October 2020 The following terms are defined in the CA/Browser Baseline Requirements [CAB] and are reproduced here: o 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. o Domain Name: The label assigned to a node in the Domain Name System o Domain Namespace: The set of all possible Domain Names that are subordinate to a single node in the Domain Name System The following terms are used in this document: o CA: Certification Authority o CSR: Certificate Signing Request o FQDN: Fully Qualified Domain Name o Parent Domain: a node in the Domain Name System that has a Domain Name o Subdomain: a Domain Name that is in the Domain Namespace of a given Parent Domain 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 Friel, et al. Expires April 12, 2021 [Page 3] Internet-Draft ACME-SUBDOMAINS October 2020 4. client proves control over the "identifier" in the "authorization" object by completing the specified challenge, 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 7. client sends POST-as-GET request to the "certificate" URI to download the certificate ACME places the following restrictions on "identifiers": o section 7.1.4: the only type of "identifier" defined by the ACME specification is a fully qualified domain name: "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." o 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." o Sections 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" o 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. Open Items 1. Does the client need a mechanism to indicate that they want to authorize a parent domain and not the explicit subdomain identifier? Or a mechanism to indicate that they are happy to authorize against a choice of identifiers? E.g. for foo1.foo2.bar.example.com, should the client be able to specify anywhere from 1 to 4 identifiers they are willing to fulfil challenges for? Friel, et al. Expires April 12, 2021 [Page 4] Internet-Draft ACME-SUBDOMAINS October 2020 2. Does the server need a mechanism to provide a choice of identifiers to the client and let the client chose which challenge to fulfil? E.g. for foo1.foo2.bar.example.com, should the server be able to specify anywhere from 1 to 4 identifiers that the client can pick from to fulfil? Both 1 and 2 would require changes to the JSON object definitions. For 1, each identifier in the newOrder or newAuthz requests would need a child array of alternative identifiers the client is willing to fulfil. For 2, the current order object contains a set of authorizations that must all be completed, the authorization object contains a single identifier that all challenges are against, so therefore its not possible for the server to give the client a choice of identifiers to pick from. This document does not currently define how 1 or 2 could be accomplished. This document merely defines how a client can submit a newOrder / newAuthz for one identifier (e.g. foo1.foo2.bar.example.com), and the server to choose a parent identifier (e.g. example.com) that it requires challenge fulfilment on, and specify that identifier in the authorization object. 5. 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. ACME server policy could allow issuance of certificates for a subdomain to a client where the client only has to fulfil 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 8.1. 5.1. 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. This document allows for both workflows, and Section 5.3 Friel, et al. Expires April 12, 2021 [Page 5] Internet-Draft ACME-SUBDOMAINS October 2020 outlines how the ACME server handles newOrder and newAuthz requests for both workflows. It may make sense to use the ACME pre-authorization flow for the subdomain use case, however, that is an operator implementation and deployment decision. With the ACME pre-authorization flow, the client could pre-authorize for the parent domain once, and then issue multiple newOrder requests for certificates for multiple subdomains. 5.2. Illustrative Call Flow The call flow illustrated here uses the ACME pre-authorization flow. The call flow also illustrates the DNS-based proof of ownership mechanism, but the subdomain workflow is equally valid for HTTP 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 | | |<---------------------| | | | | | Delete DNS TXT | | | "example.org" | | |--------------------------------->| | | | STEP 2: Place order for subdomain | | | | POST /newOrder | | | "sub.example.org" | | Friel, et al. Expires April 12, 2021 [Page 6] Internet-Draft ACME-SUBDOMAINS October 2020 |--------------------->| | | | | | 201 status=ready | | |<---------------------| | | | | | POST /finalize | | | CSR "sub.example.org"| | |--------------------->| | | | | | 200 OK status=valid | | |<---------------------| | | | | | POST /certificate | | |--------------------->| | | | | | 200 OK | | | PKI "sub.example.org"| | |<---------------------| | 5.3. newOrder and newAuthz Handling Servers may consider validation of a parent domain sufficient authorization for a subdomain. If a server has such a policy and a client is already authorized for the parent domain then: o If the client submits a newAuthz request for a subdomain: The server MUST return status 200 (OK) response. The response body is the existing authorization object for the parent domain with status set to "valid". o If the client submits a newOrder request for a subdomain: The server MUST return a 201 (Created) response. The response body is an order object with status set to "ready" and links to the unexpired authorizations against the parent domain. If a server has such a policy and a client is not authorized for the parent domain then: o If the client submits a newAuthz request for a subdomain: The server MUST return a status 201 (Created) response. The response body is a newly created authorization object for the parent domain with status set to "pending". o If the client submits a newOrder request for a subdomain: The server MUST return a status 201 (Created) response. The response body is an order object with status set to "pending" and links to newly created authorizations objects against the parent domain. Friel, et al. Expires April 12, 2021 [Page 7] Internet-Draft ACME-SUBDOMAINS October 2020 5.4. Examples In order to illustrate subdomain behaviour, let us assume that a client wishes to get certificates for subdomain identifiers "sub0.example.org", "sub1.example.org" and "sub2.example.org" under parent domain "example.org", and CA policy allows certificate issuance of these subdomain identifiers while only requiring the client to fulfil an ownership challenge for parent domain "example.org". Let us also assume that the client has not yet proven ownership of parent domain "example.org". 1. The client POSTs a newOrder request for identifier "sub0.example.org" The server creates an authorization object for identifier "example.org". The server replies with a 201 (Created) response. The response body is an order object with status set to "pending" and a link to newly created authorization object against the parent domain "example.org". Therefore, the server is instructing the client to fulfil a challenge against domain identifier "example.org" in order to obtain a certificate including identifier "sub0.example.org". The client completes the challenge for "example.org", POSTs a CSR to the order finalize URI, and downloads the certificate. 2. The client POSTs a newOrder request for identifier "sub1.example.org" The server replies with a 201 (Created) response. The response body is an order object with status set to "ready" and a link to the unexpired authorization against the parent domain "example.org". The client POSTs a CSR to the order finalize URI, and downloads the certificate. 3. The client POSTs a newAuthz request for identifier "sub2.example.org" The server replies with a 200 (OK) response. The response body is the previously created authorization object for "example.org" with status set to "valid". Friel, et al. Expires April 12, 2021 [Page 8] Internet-Draft ACME-SUBDOMAINS October 2020 6. Resource Enhancements This document defines enhancements to the authorization and directory objects. 6.1. Authorization Object If an ACME server allows issuance of certificates for subdomains of a parent domain, then the authorization object for the parent domain MUST include the optional "includeSubDomains" field, with a value of true. The structure of an ACME authorization resource is enhanced to include the following optional field: includeSubDomains (optional, boolean): This field MUST be present and true for authorizations where ACME server policy allows certificates to to be issued for subdomains of the identifier in the authorization object without explicit authorization of the subdomain 6.2. Directory Object Metadata An ACME server can advertise support of issuance of subdomain certificates by including the boolean field "includeSubDomainsAuthorization" in its "ACME Directory Metadata Fields" registry. If not specified, then no default value is assumed. If an ACME server supports issuance of subdomain certificates, it can indicate this by including this field with a value of "true". 7. IANA Considerations 7.1. 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 | +-------------------+------------+--------------+-----------+ | includeSubDomains | boolean | false | RFC XXXX | +-------------------+------------+--------------+-----------+ 7.2. Directory Object Metadata Fields Registry The following field is added to the "ACME Directory Metadata Fields" registry defined in ACME [RFC8555]. Friel, et al. Expires April 12, 2021 [Page 9] Internet-Draft ACME-SUBDOMAINS October 2020 +--------------------------------+------------+-----------+ | Field Name | Field Type | Reference | +--------------------------------+------------+-----------+ | includeSubDomainsAuthorization | boolean | RFC XXXX | +--------------------------------+------------+-----------+ 8. 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: o account or account key management o ACME channel establishment, security mechanisms or threat model o 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: o Threat Model o Integrity of Authorizations o Denial-of-Service Considerations o Server-Side Request Forgery o CA Policy Considerations Some additional comments on ACME server opicy are given in the following section. Friel, et al. Expires April 12, 2021 [Page 10] Internet-Draft ACME-SUBDOMAINS October 2020 8.1. 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: o to issue Web PKI certificates where the ACME server must comply with CA/Browser Forum [CAB] Baseline Requirements. o as a Private CA for issuance of certificates within an organisation. The organisation could enforce whatever policies they desire on the ACME server. o for issuance of IoT device certificates. There are currently no IoT device certificate policies that are generally enforced across the industry. Organsations issuing IoT device certificates can enforce whatever policies they desire on the ACME server. ACME server policy could specify whether: o issuance of subdomain certificates is allowed based on proof of ownership of a parent domain o issuance of subdomain certificates is allowed, but only for a specific set of parent domains o whether DNS based proof of ownership, or HTTP based proof of ownership, or both, are allowed ACME server policy specification is exlpicitly out of scope of this document. For reference, extracts from CA/Browser Forum Baseline Requirements are given in the appendices. 9. Informative References [CAB] CA/Browser Forum, "Baseline Requirements for the Issuance and Management of Publicly-Trusted Certificates", n.d., . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . Friel, et al. Expires April 12, 2021 [Page 11] Internet-Draft ACME-SUBDOMAINS October 2020 [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, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC8555] Barnes, R., Hoffman-Andrews, J., McCarney, D., and J. Kasten, "Automatic Certificate Management Environment (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019, . 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: o 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. o 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. o 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 Friel, et al. Expires April 12, 2021 [Page 12] Internet-Draft ACME-SUBDOMAINS October 2020 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 Friel, et al. Expires April 12, 2021 [Page 13]