Network Working Group A. Melnikov
Internet-Draft Isode Ltd
Intended status: Informational March 4, 2018
Expires: September 5, 2018
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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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. 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:
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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
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
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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
[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>.
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[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008,
<https://www.rfc-editor.org/info/rfc5321>.
[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
Melnikov Expires September 5, 2018 [Page 7]
