Internet DRAFT - draft-melnikov-acme-email-tls-smime
draft-melnikov-acme-email-tls-smime
Network Working Group A. Melnikov
Internet-Draft Isode Ltd
Intended status: Informational June 2, 2017
Expires: December 4, 2017
Extensions to Automatic Certificate Management Environment for email TLS
and S/MIME
draft-melnikov-acme-email-tls-smime-00
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 and for email recipients
that want to use S/MIME.
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. TLS with Server Name Indication (SNI) challenge for email
services . . . . . . . . . . . . . . . . . . . . . . . . 4
3.4. DNS challenge for email services . . . . . . . . . . . . 4
3.5. CAPABILITY challenge for email services . . . . . . . . . 5
4. Use of ACME for issuing end user S/MIME certificates . . . . 6
5. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8. 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) can get certificates for use with TLS. Section 4 defines
extensions for issuing end user S/MIME [RFC5751] certificates.
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) and IMAP [RFC3501] servers
use TLS 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.
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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 MUST (SHOULD?) be included. See
Section 3.2 for more details.
For example, if the client were to respond to the "tls-sni-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": "tls-sni-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 server MAY include SRV-ID [RFC6125]
subjectAltNames in issued certificates.
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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. TLS with Server Name Indication (SNI) challenge for email services
"tls-sni-email-00" is very similar to "tls-sni-01" defined in
Section 8.3 of [I-D.ietf-acme-acme].
The difference between processing of "tls-sni-email-00" and "tls-sni-
01" are listed below:
1. SAN A MUST be constructed as follows: compute the SHA-256 digest
[FIPS180-4] of the challenge token and encode it in lowercase
hexadecimal form. The dNSName is "<x>.<y>.<token>.acme.invalid",
where <x> is the first half of the hexadecimal representation and
<y> is the second half, and <token> was generated by the ACME
server. SAN B MUST be constructed as follows: compute the
SHA-256 digest of the key authorization and encode it in
lowercase hexadecimal form. The dNSName is
"<x>.<y>.<ka>.acme.invalid" where <x> is the first half of the
hexadecimal representation and <x> is the second half, and <ka>
is the key authorization. [[OPEN ISSUE: Should service name and
port number be incorporated into SAN A and B?]]
2. When verifying the client's control of the domain/service, ACME
server connects to port as specified in "port" JWS header
parameter (Section 3.2), instead of port 443. When connecting to
ports 25, 143 and 587, ACME server needs to use STARTTLS command.
When connecting to ports 465 or 993, ACME server initiate TLS
negotiation immediately upon connection to the corresponding
ports. In all cases ACME server presents SAN A in the SNI field,
constructed as specified above.
3.4. 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
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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?]]
3.5. 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:
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
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. SMTP 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
Figure 3
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4. Use of ACME for issuing end user S/MIME certificates
[I-D.ietf-acme-acme] defines "dns" Identifier Type that is used to
verify that a particular entity has control over a domain or specific
service associated with the domain. In order to be able to issue
end-user S/MIME certificates, ACME needs a new Identifier Type that
proves ownership of an email address.
This document defines a new Identifier Type "email" which corresponds
to an (all ASCII) email address [RFC5321]. This can be used with
S/MIME or other similar service that requires posession of a
certificate tied to an email address.
A new challenge type "email-reply-00" is used with "email" Identifier
Type, which provides proof that an ACME client has control over an
email address: [[Very rough outline follows]]
1. ACME server generates an email message with the subject
containing "ACME <token-part1>", where <token-part1> is the
base64url encoded first part of the token, which contains at
least 64 bit of entropy. The second part of the token (token-
part2, which also contains at least 64 bit of entropy) is
returned over HTTPS to the ACME client. ACME client concatenates
"token-part1" and "token-part2" to create "token", calculates
key-authz (as per Section 8.1 of [I-D.ietf-acme-acme]), then
included the base64url encoded SHA-256 digest [FIPS180-4] of the
key authorization in a response email message. The response
email message has a single text/plain MIME body part. [[Do we
need to handle text/html or multipart/alternative? Simplicity
suggests "no".]]
[[Do we need a proof that ACME client can submit email on behalf of
the user, not just read the challenge using IMAP?]]
5. Open Issues
[[This section should be empty before publication]]
1. 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".
6. IANA Considerations
IANA is requested to register the following ACME challenge types that
are used with Identifier Type "dns": "tls-sni-email", "dns-email",
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"capability-smtp" and "capability-imap". The reference for all of
them is this document.
IANA is requested to register a new Identifier Type "email" which
corresponds to an (all ASCII) email address [RFC5321].
And finally, IANA is requested to register the following ACME
challenge types that are used with Identifier Type "email": "email-
reply". The reference for it is this document.
7. Security Considerations
TBD.
8. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008,
<http://www.rfc-editor.org/info/rfc5321>.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
<http://www.rfc-editor.org/info/rfc3501>.
[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, DOI 10.17487/RFC5751, January
2010, <http://www.rfc-editor.org/info/rfc5751>.
[RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
2015, <http://www.rfc-editor.org/info/rfc7515>.
[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, <http://www.rfc-editor.org/info/rfc6125>.
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[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.
Author's Address
Alexey Melnikov
Isode Ltd
14 Castle Mews
Hampton, Middlesex TW12 2NP
UK
EMail: Alexey.Melnikov@isode.com
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