Extensions to Automatic Certificate Management Environment for email TLS
draft-ietf-acme-email-tls-03

Abstract

This document specifies identifiers and challenges required to enable the Automated Certificate Management Environment (ACME) to issue certificates for use by TLS email services.

Status of This Memo

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This Internet-Draft will expire on September 5, 2018.

Copyright Notice

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Table of Contents

• 1. Introduction
• 2. Conventions Used in This Document
• 3. Use of ACME for use by TLS-protected SMTP and IMAP services
• 3.1. "service" JWS header parameter
• 3.2. "port" JWS header parameter
• 3.3. DNS challenge for email services
• 3.4. CAPABILITY challenge for email services
• 4. Open Issues
• 5. IANA Considerations
• 6. Security Considerations
• 7. Normative References
• Author's Address

1. Introduction

[I-D.ietf-acme-acme] 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. Section 3 defines extensions for how email services (such as SMTP, IMAP) can get certificates for use with TLS.

2. Conventions Used in This Document

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 [RFC2119].

3. Use of ACME for use by TLS-protected SMTP and IMAP services

SMTP [RFC5321] (including SMTP Submission [RFC6409]) and IMAP [RFC3501] servers use TLS [RFC5246] 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 .

[I-D.ietf-acme-acme] 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 [RFC7515] object used by [I-D.ietf-acme-acme] is extended. The following extra requirements are in addition to requirements on JWS objects sent in ACME defined in Section 6.2 of [I-D.ietf-acme-acme]:

1. "service" JWS header parameter MUST be included. See Section 3.1 for more details.
2. "port" JWS header parameter SHOULD be included. See Section 3.2 for more details.

For example, if the ACME client were to respond to the "dns-email-00" challenge, it would send the following request:

   POST /acme/authz/asdf/0 HTTP/1.1
   Host: example.com
   Content-Type: application/jose+json

   {
     "protected": base64url({
       "alg": "ES256",
       "kid": "https://example.com/acme/acct/1",
       "nonce": "Q_s3MWoqT05TrdkM2MTDcw",
       "url": "https://example.com/acme/authz/asdf/0",
       "service": "smtp",
       "port": 25
     }),
     "payload": base64url({
       "type": "dns-email-00",
       "keyAuthorization": "IlirfxKKXA...vb29HhjjLPSggQiE"
     }),
     "signature": "7cbg5JO1Gf5YLjjF...SpkUfcdPai9uVYYU"
   }

Figure 1

3.1. "service" JWS header parameter

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 [RFC7817] (in particular see Section 4 of that document).

[[This parameter might have applicability beyond email services.]]

3.2. "port" JWS header parameter

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.]]

3.3. DNS challenge for email services

"dns-email-00" is very similar to "dns-01" defined in Section 8.4 of [I-D.ietf-acme-acme].

The difference between processing of "dns-email-00" and "dns-01" are listed below:

1. 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.
2. [[TODO: Make sure that both service name and port number are incorporated into the hash]]

3.4. CAPABILITY challenge for email services

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 [FIPS180-4] of the key authorization. SMTP server than returns the base64url encoding of this digest as a value of the "ACME" EHLO capability. For example:

250-smtp.example.com
250-SIZE
250-8BITMIME
250-BINARYMIME
250-PIPELINING
250-HELP
250-DSN
250-CHUNKING
250-AUTH SCRAM-SHA-1
250-AUTH=SCRAM-SHA-1
250-STARTTLS
250-ACME gfj9Xq...Rg85nM
250-MT-PRIORITY
250 ENHANCEDSTATUSCODES

Note that in the above example only presence of the ACME is relevant as far as this document is concerned.

Figure 2

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 [FIPS180-4] of the key authorization. IMAP server than returns the base64url encoding of this digest as a value of the "ACME" capability:

* OK [CAPABILITY IMAP4rev1 LOGINDISABLED LITERAL+ ENABLE STARTTLS ACME=gfj9Xq...Rg85nM] Example IMAP4rev1 server ready

 or

* CAPABILITY IMAP4rev1 LOGINDISABLED LITERAL+ ENABLE STARTTLS ACME=gfj9Xq...Rg85nM

Note that in the above example only presence of the ACME capability token is relevant as far as this document is concerned.

Figure 3

[[TODO: Add support for POP3 as per John Levine]]

4. Open Issues

[[This section should be empty before publication]]

1. 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.
2. Add support for LMTP (RFC 2033)?

5. IANA Considerations

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.

6. Security Considerations

TBD.

7. Normative References

Author's Address