Network Working Group A. Melnikov
Internet-Draft Isode Ltd
Intended status: Informational November 12, 2017
Expires: May 16, 2018

Extensions to Automatic Certificate Management Environment for email TLS
draft-ietf-acme-email-tls-02

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

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

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This Internet-Draft will expire on May 16, 2018.

Copyright Notice

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Table of Contents

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.

[[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. [[OPEN ISSUE: Should service name and port number be 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

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?)
  2. Add support for LMTP (RFC 2033)?
  3. One possible alternative for issuing TLS certificates for email services is to define a new Identifier Type that specifies service@domain. The current version of the document just reuses "dns".

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

[FIPS180-4] National Institute of Standards and Technology, "Secure Hash Standard (SHS)", FIPS PUB 180-4, August 2015.
[I-D.ietf-acme-acme] Barnes, R., Hoffman-Andrews, J. and J. Kasten, "Automatic Certificate Management Environment (ACME)", Internet-Draft draft-ietf-acme-acme-06, March 2017.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, August 2008.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, DOI 10.17487/RFC5321, October 2008.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March 2011.
[RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail", STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011.
[RFC7515] Jones, M., Bradley, J. and N. Sakimura, "JSON Web Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May 2015.
[RFC7817] Melnikov, A., "Updated Transport Layer Security (TLS) Server Identity Check Procedure for Email-Related Protocols", RFC 7817, DOI 10.17487/RFC7817, March 2016.

Author's Address

Alexey Melnikov Isode Ltd 14 Castle Mews Hampton, Middlesex TW12 2NP UK EMail: Alexey.Melnikov@isode.com