ipr: trust200902 stand_alone: yes title: "Automated Certificate Management Environment (ACME) for Subdomains" ipr: trust200902 abbrev: ACME-SUBDOMAINS area: Security wg: ACME Working Group cat: std consensus: 'yes' docname: draft-ietf-acme-subdomains-latest coding: utf-8 pi: rfcedstyle: 'yes' strict: 'yes' toc: 'yes' tocdepth: '2' symrefs: 'yes' sortrefs: 'yes' compact: 'no' subcompact: 'no' comments: 'yes' inline: 'yes'
author:
- ins: O. Friel name: Owen Friel org: Cisco email: ofriel@cisco.com
- ins: R. Barnes name: Richard Barnes org: Cisco email: rlb@ipv.sx
- ins: T. Hollebeek name: Tim Hollebeek org: DigiCert email: tim.hollebeek@digicert.com
- ins: M. Richardson name: Michael Richardson org: Sandelman Software Works email: mcr+ietf@sandelman.ca
informative: CAB: author: org: CA/Browser Forum title: Baseline Requirements for the Issuance and Management of Publicly-Trusted Certificates target: https://cabforum.org/baseline-requirements-documents/
ACME-Identifier-Types: author: org: IANA title: ACME Identifier Types target: https://www.iana.org/assignments/acme/acme.xhtml#acme-identifier-types
ACME-Validation-Methods: author: org: IANA title: ACME Validation Methods target: https://www.iana.org/assignments/acme/acme.xhtml#acme-validation-methods
--- abstract
This document specifies how Automated Certificate Management Environment (ACME) can be used by a client to obtain a certificate for a subdomain identifier from a certification authority. This document specifies how a client can fulfill a challenge against an ancestor domain but may not need to fulfill a challenge against the explicit subdomain if certification authority policy allows issuance of the subdomain certificate without explicit subdomain ownership proof.
--- middle
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. The CA is the ACME server and the applicant is the ACME client, and the client uses the ACME protocol to request certificate issuance from the server. 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 an ancestor domain identifier.
{::boilerplate bcp14-tagged}
The following terms are defined in DNS Terminology {{!RFC8499}} and are reproduced here:
-
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.
-
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 definition for "subdomain" is taken from DNS Terminology {{!RFC8499}} and reproduced here, however the definition is ambiguous and is further clarified below:
- 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 {{Section 3.1 of ?RFC1034}}.) 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".
The definition is ambiguous as it appears to allow a subdomain to include the given domain. That is, "mmm.example.com" ends with "mmm.example.com" and thus is a subdomain of itself. This document interprets the first sentence of the above definition as meaning "A domain is a subdomain of a different domain if it is contained within that different domain.". A domain cannot be a subdomain of itself. For example, "mmm.example.com" is not a subdomain of "mmm.example.com".
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 Root CAs and Subordinate CAs. Refer to {{?RFC5280}} for detailed information on Certification Authorities.
-
CSR: Certificate Signing Request as defined in {{?RFC2986}}
-
Ancestor Domain: a domain is an ancestor domain of a subdomain if it contains that subdomain and has less labels than that subdomain. A domain cannot be an ancestor domain of itself. For example, for the host name "nnn.mmm.example.com", both "mmm.example.com" and "example.com" are ancestor domains of "nnn.mmm.example.com". However, "nnn.mmm.example.com" is not an ancestor domain of "nnn.mmm.example.com". Note that the comparisons here are done on whole labels; that is, "oo.example.com" is not an ancestor domain of "ooo.example.com".
ACME {{!RFC8555}} defines the following object types which are used in this document:
-
Order Object: An ACME order object represents a client's request for a certificate and is used to track the progress of that order through to issuance.
-
Authorization Object: An ACME authorization object represents a server's authorization for an account to represent an identifier.
-
Challenge Object: An ACME challenge object represents a server's offer to validate a client's possession of an identifier in a specific way.
ACME {{!RFC8555}} Section 6.3 introduces the following term which is used in this document:
- POST-as-GET Request: When a client wishes to fetch a resource from the server, then it MUST send a POST request with a signed JWS body, where the JWS body is specified in ACME {{!RFC8555}} Section 6.2. ACME refers to these as "POST-as-GET" requests.
A typical ACME {{!RFC8555}} workflow for issuance of certificates is as follows:
-
client POSTs a newOrder request that contains a set of "identifiers"
-
server replies with an order object that contains a set of links to authorization object(s) and a "finalize" URI
-
client sends POST-as-GET requests to retrieve the authorization object(s), with the downloaded authorization object(s) containing the "identifier" that the client must prove that they control, and a set of links to associated challenges objects, one of which the client must fulfill
-
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
-
client POSTs a CSR to the "finalize" API
-
server replies with an updated order object that includes a "certificate" URI
-
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 match the "identifier" in authorization objects.
The base ACME {{!RFC8555}} document only specifies the "dns" identifier type. Additional identifiers may be defined and registered in the IANA {{ACME-Identifier-Types}} registry. For example, {{?RFC8738}} specifies the "ip" identifier type. This document is only relevant for the "dns" identifier type.
Note also that ACME supports multiple different validation methods that can be used to fulfill challenges and prove ownership of identifiers. Validation methods are registered in the IANA {{ACME-Validation-Methods}} registry. This document does not mandate use of any particular validation method or methods. ACME server policy dictates which validation methods are supported. See {{acme-server-policy-considerations}} for more information on ACME server policy.
As noted in the previous section, ACME {{!RFC8555}} does not mandate that the "identifier" in a newOrder request match 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 fulfill an authorization challenge for an ancestor 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 {{acme-server-policy-considerations}}.
Clients need a mechanism to instruct the ACME server that they are requesting authorization for all subdomains subordinate to the specified domain, as opposed to just requesting authorization for an explicit domain 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 all subdomains under the specified domain. These are described in this section.
ACME ({{!RFC8555, Section 7.1.4}}) defines the authorization object. This document defines a new "subdomainAuthAllowed" field for the authorization object. When ACME server policy allows authorization for subdomains subordinate to a domain, the server indicates this by including the new "subdomainAuthAllowed" field in the authorization object for that domain identifier:
subdomainAuthAllowed (optional, boolean): If present, this field
MUST be true for authorizations where ACME server policy
allows certificates to be issued for any subdomain subordinate
to the domain specified in the 'identifier' field of the
authorization object.
The following example shows an authorization object for the domain example.org where the authorization covers the subdomains subordinate to example.org.
{
"status": "valid",
"expires": "2023-09-01T14:09:07.99Z",
"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"
}
],
"subdomainAuthAllowed": true
}
If the "subdomainAuthAllowed" field is not included, then the assumed default value is false.
If ACME server policy allows issuance of certificates containing wildcard identifiers under that authorization object, then the server SHOULD include the "wildcard" field with a value of true, as per {{!RFC8555, Section 7.1.4}}.
The basic 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 domain once, and then issue multiple newOrder requests for certificates with identifiers in the subdomains subordinate to that domain.
ACME {{RFC8555, Section 7.4.1}} defines the "identifier" object for newAuthz requests. This document defines a new "subdomainAuthAllowed" field for the "identifier" object:
subdomainAuthAllowed (optional, boolean): An ACME client sets
this flag to indicate to the server that it is requesting an
authorization for the subdomains subordinate to the specified
domain identifier value
Clients include the new "subdomainAuthAllowed" field in the "identifier" object of newAuthz requests to indicate that they are requesting a subdomain authorization. In the following example newAuthz payload, the client is requesting pre-authorization for the subdomains subordinate to example.org.
"payload": base64url({
"identifier": {
"type": "dns",
"value": "example.org",
"subdomainAuthAllowed": true
}
})
If the server is willing to allow a single authorization for the subdomains, and there is not an existing authorization object for the identifier, then it will create an authorization object and include the "subdomainAuthAllowed" flag with value of true.
If the server policy does not allow creation of subdomain authorizations subordinate to that domain, the server can create an authorization object for the indicated identifier, and MAY include the "subdomainAuthAllowed" flag with value of false. If the server creates an authorization object and does not include the "subdomainAuthAllowed" flag, then the assumed value is false.
In both scenarios, handling of the pre-authorization follows the process documented in ACME {{RFC8555, Section 7.4.1}}.
Clients need a mechanism to optionally indicate to servers whether or not they are authorized to fulfill challenges against an ancestor domain for a given identifier. For example, if a client places an order for an identifier foo.bar.example.org, and is authorized to fulfill a challenge against the ancestor domains bar.example.org or example.org, then the client needs a mechanism to indicate control over the ancestor domains to the ACME server.
In order to accomplish this, this document defines a new "ancestorDomain" field for the identifier that is included in order objects.
ancestorDomain (optional, string): This is an ancestor domain of
the requested identifier. The client MUST be able to fulfill
a challenge against the ancestor domain.
This field specifies an ancestor domain of the identifier 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 ancestor domain of the identifier up to and including the specified "ancestorDomain", 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 ancestor domain 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",
"ancestorDomain": "bar.example.org" }
],
"notBefore": "2023-09-01T00:04:00+04:00",
"notAfter": "2023-09-08T00:04:00+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 ancestor domain 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",
"ancestorDomain": "example.org" }
],
"notBefore": "2023-09-01T00:04:00+04:00",
"notAfter": "2023-09-08T00:04:00+04:00"
})
If the client is unable to fulfill authorizations against an ancestor domain, the client should not include the "ancestorDomain" field.
Server newOrder handling generally follows the process documented in ACME, {{Section 7.4 of RFC8555}}. If the server is willing to allow subdomain authorizations for the domain specified in "ancestorDomain", then it creates an authorization object against that ancestor domain and includes the "subdomainAuthAllowed" flag with a value of true.
If the server policy does not allow creation of subdomain authorizations against that ancestor domain, then it can create an authorization object for the indicated identifier value, and SHOULD NOT include the "subdomainAuthAllowed" flag. As the client requested a subdomain authorization for the ancestor domain, and not for the indicated identifier, there is no need for the server to include the "subdomainAuthAllowed" flag in the authorization object for the indicated identifier.
This document defines a new "subdomainAuthAllowed" ACME directory metadata field. An ACME server can advertise support for authorization of subdomains by including the "subdomainAuthAllowed" boolean flag in its "ACME Directory Metadata Fields" registry:
subdomainAuthAllowed (optional, bool): Indicates if an ACME
server supports authorization of subdomains.
If not specified, then the assumed default value is false. If an ACME server supports authorization of subdomains, it can indicate this by including this field with a value of "true".
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 ancestor 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 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 | |
| PEM SAN "sub2.example.org" | |
|<---------------------------| |
-
STEP 1: Pre-authorization of ancestor domain
The client sends a newAuthz request for the ancestor domain including the "subdomainAuthAllowed" 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",
"subdomainAuthAllowed": true
}
}),
"signature": "nuSDISbWG8mMgE7H...QyVUL68yzf3Zawps"
}
The server creates and returns an authorization object for the identifier including the "subdomainAuthAllowed" flag. The object is initially in "pending" state.
{
"status": "pending",
"expires": "2023-09-01T14:09:07.99Z",
"identifier": {
"type": "dns",
"value": "example.org"
},
"challenges": [
{
"url": "https://example.com/acme/chall/prV_B7yEyA4",
"type": "dns-01",
"status": "pending",
"token": "DGyRejmCefe7v4NfDGDKfA",
"validated": "2023-08-01T12:05:58.16Z"
}
],
"subdomainAuthAllowed": true
}
The example illustrates the client completing a DNS challenge by publishing a DNS TXT record. The client then posts to the challenge resource to inform the server that it can validate the challenge.
Once the server validates the challenge by checking the DNS TXT record, the server will transition the authorization object and associated challenge object status to "valid".
The call flow above illustrates the ACME server replying to the client's challenge with status of "valid" after the ACME server has validated the DNS challenge. However, the validation flow may take some time. If this is the case, the ACME server may reply to the client's challenge immediately with a status of "processing", and the client will then need to poll the authorization resource to see when it is finalized. Refer to ACME {{RFC8555, Section 7.5.1}} for more details.
-
STEP 2: The client places a newOrder for
sub1.example.orgThe client sends a newOrder request to the server and includes the subdomain identifier. Note that the identifier is a subdomain of the ancestor domain that has been pre-authorized in step 1. The client does not need to include the "subdomainAuthAllowed" field in the "identifier" object as it has already pre-authorized the ancestor domain.
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": "2023-09-01T00:04:00+04:00",
"notAfter": "2023-09-08T00:04:00+04:00"
}),
"signature": "H6ZXtGjTZyUnPeKn...wEA4TklBdh3e454g"
}
As an authorization object already exists for the ancestor domain, the server replies with an order object with a status of "ready" 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": "ready",
"expires": "2023-09-01T14:09:07.99Z",
"notBefore": "2023-09-01T00:00:00Z",
"notAfter": "2023-09-08T00:00:00Z",
"identifiers": [
{ "type": "dns", "value": "sub1.example.org" }
],
"authorizations": [
"https://example.com/acme/authz/PAniVnsZcis"
],
"finalize": "https://example.com/acme/order/TOlocrfgo/finalize"
}
The client can proceed to finalize the order by posting a CSR to the "finalize" resource. The client can then download the certificate for sub1.example.org.
-
STEP 3: The client places a newOrder for
sub2.example.orgThe client sends a newOrder request to the server and includes the subdomain identifier. Note that the identifier is a subdomain of the ancestor domain that has been pre-authorized in step 1. The client does not need to include the "subdomainAuthAllowed" field in the "identifier" object as it has already pre-authorized the ancestor domain.
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": "2023-09-01T00:04:00+04:00",
"notAfter": "2023-09-08T00:04:00+04:00"
}),
"signature": "H6ZXtGjTZyUnPeKn...wEA4TklBdh3e454g"
}
As an authorization object already exists for the ancestor domain, the server replies with an order object with a status of "ready" 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": "ready",
"expires": "2023-09-01T14:09:07.99Z",
"notBefore": "2023-09-01T00:00:00Z",
"notAfter": "2023-09-08T00:00:00Z",
"identifiers": [
{ "type": "dns", "value": "sub2.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 by posting a CSR to the "finalize" resource. The client can then download the certificate for sub2.example.org.
The following field is added to the "ACME Authorization Object Fields" registry defined in ACME {{!RFC8555}}.
+----------------------+------------+--------------+-----------+
| Field Name | Field Type | Configurable | Reference |
+----------------------+------------+--------------+-----------+
| subdomainAuthAllowed | boolean | false | RFC XXXX |
+----------------------+------------+--------------+-----------+
The following field is added to the "ACME Directory Metadata Fields" registry defined in ACME {{!RFC8555}}.
+----------------------+------------+-----------+
| Field Name | Field Type | Reference |
+----------------------+------------+-----------+
| subdomainAuthAllowed | boolean | RFC XXXX |
+----------------------+------------+-----------+
This document specifies 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:
-
Only an entity that controls an identifier can get an authorization for that identifier
-
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
The only exception is that in order to satisfy goal (1) above, this draft assumes that control over a domain may imply control over a subdomain, and therefore authorization for certificate issuance for the former may imply authorization for certificate issuance for the latter. In many ecosystems, this is a safe assumption, especially because control over the domain can often be leveraged to successfully demonstrate control over subdomains anyway, for example by temporarily modifying DNS for the subdomain to point to a server the ancestor domain owner controls, rendering the distinction moot. For example, the CA/Browser Forum Baseline Requirements may consider control of an ancestor domain sufficient for issuance of certificates for subdomains, but only if specific processes and procedures are used for validating ownership of the ancestor domain.
In ecosystems where control of an ancestor domain may not imply control over subdomains or authorization for issuance of certificates for subdomains, a more complicated threat analysis and server policy might be needed.
Some additional comments on ACME server policy are given later in this section.
There may be scenarios were a client wishes to deactivate an authorization object for an ancestor domain, or deactivate its account completely. For example, a client may want to do this if an account key is compromised, or if a authorization object covering domains subordinate to an ancestor domain is no longer needed. The client can deactivate an authorization using the mechanism specified in {{!RFC8555, Section 7.5.2}} and can deactivate an account using the mechanism specified in {{!RFC8555, Section 7.3.6}}.
The {{!RFC8499}} definition of a subdomain is reproduced in {{terminology}}. When comparing domains to determine if one is a subdomain of the other, it is important to compare entire labels, and not rely on a string prefix match. Relying on string prefix matches may yield incorrect results.
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 organization. The organization 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 an ancestor domain
-
issuance of subdomain certificates is allowed, but only for a specific set of ancestor domains
-
DNS based proof of ownership, or HTTP based proof of ownership, or both, are allowed
The CA policy considerations listed in {{!RFC8555, Section 10.5}} are equally applicable here. These include, but are not limited to:
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Is the claimed identifier syntactically valid?
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For domain names:
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Is the name on the Public Suffix List?
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Is the name a high-value name?
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Is the key in the CSR sufficiently strong?
Refer to {{!RFC8555, Section 10.5}} for more CA policy considerations.
ACME server policy specification is explicitly out of scope of this document.