Network Working Group A. Melnikov
Internet-Draft Isode Ltd
Intended status: Informational July 25, 2018
Expires: January 26, 2019
Extensions to Automatic Certificate Management Environment for email TLS
draft-ietf-acme-email-tls-05
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, IMAP and POP3
services . . . . . . . . . . . . . . . . . . . . . . . . . . 2
3.1. "service" field in JSON payload . . . . . . . . . . . . . 3
3.2. "port" field in JSON payload . . . . . . . . . . . . . . 4
3.3. DNS challenge for email services . . . . . . . . . . . . 4
3.4. CAPABILITY challenge for email services . . . . . . . . . 4
3.4.1. Registration of the ACME SMTP extension . . . . . . . 6
4. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. Normative References . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
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 and POP3) 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, IMAP and POP3 services
SMTP [RFC5321] (including SMTP Submission [RFC6409]), IMAP [RFC3501]
and POP3 [RFC2449] 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 [RFC8314].
[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, IMAP and POP3.
In order to use these challenges JWS [RFC7515] object used by
[I-D.ietf-acme-acme] is extended. The following extra requirements
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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. If this JWS header parameter is not included,
the default assigned IANA port for the corresponding "service" is
assumed.
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"
}),
"payload": base64url({
"type": "dns-email-00",
"service": "smtp",
"port": 25,
"keyAuthorization": "IlirfxKKXA...vb29HhjjLPSggQiE"
}),
"signature": "7cbg5JO1Gf5YLjjF...SpkUfcdPai9uVYYU"
}
Figure 1
3.1. "service" field in JSON payload
The "service" field in JSON payload 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" field in JSON payload
The "port" field in JSON payload 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 IANA 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 IMAPS 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.
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
The ACME SMTP extension is formerly defined in Section 3.4.1.
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
Similarly, "capability-pop-00" challenge, ACME client (== POP3
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. POP3 server than returns the base64url encoding of
this digest as a value of the "ACME" capability in response to CAPA
command [RFC2449]:
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C: CAPA
S: +OK Capability list follows
S: TOP
S: SASL CRAM-MD5 KERBEROS_V4
S: UIDL
S: ACME gfj9Xq...Rg85nM
S: IMPLEMENTATION Shlemazle-Plotz-v915
S: .
Note that in the above example only presence of the ACME capability
token is relevant as far as this document is concerned.
Figure 3
3.4.1. Registration of the ACME SMTP extension
The ACME SMTP service extension is defined as follows:
1. The textual name of this extension is "ACME for SMTP".
2. The EHLO keyword value associated with this extension is "ACME".
3. The EHLO keyword has a single required parameter which is a
base64url encoded SHA-256 hash, which is 44 octets in length.
4. This extension doesn't define any new SMTP verbs.
5. This extension doesn't add any new parameters to MAIL FROM or
RCPT TO commands.
6. The ACME extension is valid for the submission service [RFC6409]
(default port number 587) or its version running directly over
TLS [RFC8314] ("submissions" service, default port number 465) .
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. The cleanest way is to change
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"service" to "services", change "port" to "ports" and make both
of them arrays.
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",
"capability-imap" and "capability-pop". The reference for all of
them is this document.
6. Security Considerations
Security Considerations from [I-D.ietf-acme-acme] relevant to the DNS
challenge type are also relevant to "dns-email".
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., McCarney, D., and J.
Kasten, "Automatic Certificate Management Environment
(ACME)", draft-ietf-acme-acme-12 (work in progress), April
2018.
[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>.
[RFC2449] Gellens, R., Newman, C., and L. Lundblade, "POP3 Extension
Mechanism", RFC 2449, DOI 10.17487/RFC2449, November 1998,
<https://www.rfc-editor.org/info/rfc2449>.
[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>.
[RFC8314] Moore, K. and C. Newman, "Cleartext Considered Obsolete:
Use of Transport Layer Security (TLS) for Email Submission
and Access", RFC 8314, DOI 10.17487/RFC8314, January 2018,
<https://www.rfc-editor.org/info/rfc8314>.
Author's Address
Alexey Melnikov
Isode Ltd
14 Castle Mews
Hampton, Middlesex TW12 2NP
UK
EMail: Alexey.Melnikov@isode.com
Melnikov Expires January 26, 2019 [Page 8]
