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

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Document	Type	This is an older version of an Internet-Draft whose latest revision state is "Expired".
	Author	Alexey Melnikov
	Last updated	2017-11-12
	Replaces	draft-melnikov-acme-email-tls
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draft-ietf-acme-email-tls-02

Network Working Group A. Melnikov
Internet-Draft Isode Ltd
Intended status: Informational November 11, 2017
Expires: May 15, 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.

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 May 15, 2018.

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
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.

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Table of Contents

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

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]:

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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.]]

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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:

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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

* 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)?

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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,
<https://csrc.nist.gov/publications/detail/fips/180/4/
final>.

[I-D.ietf-acme-acme]
Barnes, R., Hoffman-Andrews, J., and J. Kasten, "Automatic
Certificate Management Environment (ACME)", draft-ietf-
acme-acme-06 (work in progress), 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,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
<https://www.rfc-editor.org/info/rfc3501>.

[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.

[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008,
<https://www.rfc-editor.org/info/rfc5321>.

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[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, <https://www.rfc-editor.org/info/rfc6125>.

[RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail",
STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011,
<https://www.rfc-editor.org/info/rfc6409>.

[RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
2015, <https://www.rfc-editor.org/info/rfc7515>.

[RFC7817] Melnikov, A., "Updated Transport Layer Security (TLS)
Server Identity Check Procedure for Email-Related
Protocols", RFC 7817, DOI 10.17487/RFC7817, March 2016,
<https://www.rfc-editor.org/info/rfc7817>.

Author's Address

Alexey Melnikov
Isode Ltd
14 Castle Mews
Hampton, Middlesex TW12 2NP
UK

EMail: Alexey.Melnikov@isode.com

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

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