Extensions to Automatic Certificate Management Environment for email TLS
Isode Ltd14 Castle MewsHamptonMiddlesexTW12 2NPUKAlexey.Melnikov@isode.comACMEIMAPSMTP
This document specifies identifiers and challenges required to enable
the Automated Certificate Management Environment (ACME) to issue
certificates for use by TLS email services.
is a mechanism for automating certificate
management on the Internet. It enables administrative entities to
prove effective control over resources like domain names, and
automates the process of generating and issuing certificates.
This document describes extensions to ACME for use by email services.
defines extensions for how email services
(such as SMTP, IMAP) can get certificates for use with TLS.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in
.SMTP (including SMTP Submission ) and IMAP servers
use TLS to provide server identity authentication, data confidentiality and integrity services.
Such TLS protected email services either use STARTTLS command or run on a separate TLS-protected port
.
defines several challenge types that can be extended for use by email services.
This document also defines some new challenge types specific to SMTP and IMAP.
In order to use these challenges JWS object used by
is extended. The following extra requirements are in addition to requirements on JWS objects sent in ACME
defined in Section 6.2 of :
"service" JWS header parameter MUST be included. See for more details.
"port" JWS header parameter SHOULD be included. See for more details.
The "service" JWS header parameter specifies the service for which TLS server certificate should be issued.
Valid values come from "Service Names and Transport Protocol Port Numbers" IANA registry
<https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml>.
ACME servers compliant with this specification MUST support (in particular see Section 4 of that document).
[[This parameter might have applicability beyond email services.]]The "port" JWS header parameter specifies the TCP port number where the corresponding
service is running. ACME server MAY check that the TCP port corresponds to the requested
"service", for example that the port is the assigned default port for the service.
[[This parameter might have applicability beyond email services.]]"dns-email-00" is very similar to "dns-01" defined in Section 8.4 of .The difference between processing of "dns-email-00" and "dns-01" are listed below:
The TXT record used to validate this challenge is _<port>._<service>._acme-challenge.<domain>.
For example, for domain "example.com" and IMAP service running on port 993, the TXT record name is
_993._imaps._acme-challenge.example.com. For domain "example.net" and IMAP service running on port 143,
the TXT record name is _143._imap._acme-challenge.example.next.
[[TODO: Make sure that both service name and port number are incorporated into the hash]]
For "capability-smtp-00" challenge, ACME client (== SMTP server)
constructs a key authorization from the "token" value provided in the challenge
and the client's account key. The client then computes the SHA-256
digest of the key authorization. SMTP server than
returns the base64url encoding of this digest as a value of the "ACME" EHLO capability. For example:
Similarly, "capability-imap-00" challenge, ACME client (== IMAP server)
constructs a key authorization from the "token" value provided in the challenge
and the client's account key. The client then computes the SHA-256
digest of the key authorization. IMAP server than
returns the base64url encoding of this digest as a value of the "ACME" capability:
[[TODO: Add support for POP3 as per John Levine]]
[[This section should be empty before publication]]
Should the same certificate be allowed to be used on both IMAP (143) and IMAPS (993) ports?
(These ports have different service names associated with them.
Is 1 service/port per ACME certificate a restriction imposed by this document?)
Maybe if the ACME server sees a request for port 143 (or 993), it can include SRV-ID for the
other port, if it can verify that both are running? (How can this be done reliably?)
Many email servers don't allow different certificates
to be configured for different ports they are listening on.Add support for LMTP (RFC 2033)?
IANA is requested to register the following ACME challenge types that are used with Identifier Type "dns":
"dns-email", "capability-smtp" and "capability-imap". The reference for all of them is this document.
TBD.
Automatic Certificate Management Environment (ACME)
Certificates in PKI using X.509 (PKIX) are used for a number of purposes, the most significant of which is the authentication of domain names. Thus, certificate authorities in the Web PKI are trusted to verify that an applicant for a certificate legitimately represents the domain name(s) in the certificate. Today, this verification is done through a collection of ad hoc mechanisms. This document describes a protocol that a certification authority (CA) and an applicant can use to automate the process of verification and certificate issuance. The protocol also provides facilities for other certificate management functions, such as certificate revocation.
Secure Hash Standard (SHS)National Institute of Standards and Technology