Extensions to Automatic Certificate Management Environment for end user S/MIME certificates
draft-ietf-acme-email-smime-05
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| Document | Type | Active Internet-Draft (acme WG) | |
|---|---|---|---|
| Author | Alexey Melnikov | ||
| Last updated | 2019-07-08 | ||
| Replaces | draft-melnikov-acme-email-smime | ||
| Stream | Internet Engineering Task Force (IETF) | ||
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draft-ietf-acme-email-smime-05
Network Working Group A. Melnikov
Internet-Draft Isode Ltd
Intended status: Informational July 8, 2019
Expires: January 9, 2020
Extensions to Automatic Certificate Management Environment for end user
S/MIME certificates
draft-ietf-acme-email-smime-05
Abstract
This document specifies identifiers and challenges required to enable
the Automated Certificate Management Environment (ACME) to issue
certificates for use by email users that want to use S/MIME.
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
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 9, 2020.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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publication of this document. Please review these documents
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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 issuing end user S/MIME certificates . . . . 2
3.1. ACME challenge email . . . . . . . . . . . . . . . . . . 3
3.2. ACME response email . . . . . . . . . . . . . . . . . . . 4
4. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. Normative References . . . . . . . . . . . . . . . . . . . . 6
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
ACME [RFC8555] 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 an extension to ACME for use by S/MIME.
Section 3 defines extensions for issuing end user S/MIME [RFC8550]
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 issuing end user S/MIME certificates
ACME [RFC8555] 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] or Internationalized Email
addresses [RFC6531]. This can be used with S/MIME or other similar
service that requires posession of a certificate tied to an email
address.
Any identifier of type "email" in a newOrder request MUST NOT have a
wildcard ("*") character in its value.
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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:
1. ACME server generates a "challenge" email message with the
subject "ACME: <token-part1>", where <token-part1> is the
base64url encoded first part of the token, which contains at
least 64 bit of entropy. The challenge email message structure
is described in more details in Section 3.1. The second part of
the token (token-part2, which also contains at least 64 bit of
entropy) is returned over HTTPS [RFC2818] to the ACME client.
2. ACME client concatenates "token-part1" and "token-part2" to
create "token", calculates key-authz (as per Section 8.1 of
[RFC8555]), then includes the base64url encoded SHA-256 digest
[FIPS180-4] of the key authorization in the body of a response
email message containing a single text/plain MIME body part
[RFC2045]. The response email message structure is described in
more details in Section 3.2
For an identifier of type "email", CSR MUST contain the request email
address in an extensionRequest attribute [RFC2985] requesting a
subjectAltName extension.
3.1. ACME challenge email
A "challenge" email message MUST have the following structure:
1. The message Subject header field has the following syntax: "ACME:
<token-part1>", where the prefix "ACME:" is followed by folding
white space (FWS, see [RFC5322]) and then by <token-part1> is the
base64url encoded first part of the ACME token that MUST be at
least 64 octet long after decoding. Due to recommended 78 octet
line length limit in [RFC5322], the subject line can be folded,
so whitespaces (if any) within the <token-part1> MUST be ignored.
[RFC2231] encoding of subject MUST be supported, but when used,
only "UTF-8" and "US-ASCII" charsets MUST be used (i.e. other
charsets MUST NOT be used).
2. The message MUST include the "Auto-Submitted: auto-generated"
header field [RFC3834]. It MAY include optional parameters as
allowed by syntax of Auto-Submitted header field.
3. In order to prove authenticity of a challenge message, it MUST be
either DKIM [RFC6376] signed or S/MIME [RFC8551] signed. If DKIM
signing is used, the resulting DKIM-Signature header field MUST
contain the "h=" tag that includes at least "From", "To",
"Subject", "Content-Type" and "Message-ID" header fields.
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4. If S/MIME signing is not used to prove authenticity of the
challenge message, then the message MUST have a single text/plain
MIME body part [RFC2045], that contains human readable
explanation of the purpose of the message. If S/MIME signing is
used, then the text/plain message is used to construct a
multipart/signed or "application/pkcs7-mime; smime-type=signed-
data;". Either way, it MUST use S/MIME header protection.
Example ACME "challenge" email
Auto-Submitted: auto-generated
Date: Sat, 1 Sep 2018 10:08:55 +0100
Message-ID: <A2299BB.FF7788@example.org>
From: acme-generator@example.org
To: alexey@example.com
Subject: ACME: <base64url-encoded-token-with-64-octets-of-entropy>
Content-Type: text/plain
MIME-Version: 1.0
This is an automatically generated ACME challenge for email address
"alexey@example.com". If you haven't requested an S/MIME
certificate generation for this email address, be very afraid.
If you did request it, your email client might be able to process
this request automatically, or you might have to paste the first
token part into an external program.
Figure 1
3.2. ACME response email
A "response" email message MUST have the following structure:
1. The message Subject header field has the following syntax: "Re:
ACME: <token-part1>", where the string "ACME:" is followed by
folding white space (FWS, see [RFC5322]) and then by <token-
part1> is the base64url encoded first part of the ACME token that
MUST be at least 64 octet long after decoding. Due to
recommended 78 octet line length limit in [RFC5322], the subject
line can be folded, so whitespaces (if any) within the <token-
part1> MUST be ignored. [RFC2231] encoding of subject MUST be
supported, but when used, only "UTF-8" and "US-ASCII" charsets
MUST be used (i.e. other charsets MUST NOT be used).
2. The From: header field contains the email address of the user
that is requesting S/MIME certificate issuance.
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3. The To: header field of the response contains the value from the
From: header field of the challenge email.
4. The Cc: header field is ignored if present in the "response"
email message.
5. The message MUST have a single text/plain MIME body part
[RFC2045], containing base64url encoded SHA-256 digest
[FIPS180-4] of the key authorization, calculated based on token-
part1 (received over email) and token-part2 (received over
HTTPS). Note that due to historic line length limitations in
email, line endings (CRLFs) can be freely inserted in the middle
of the encoded digest, so they MUST be ignored when processing
it.
6. There is no need to use any Content-Transfer-Encoding other than
7bit, however use of Quoted-Printable or base64 is not prohibited
in a "response" email message.
Example ACME "response" email
Date: Sat, 1 Sep 2018 11:12:00 +0100
Message-ID: <111-22222-3333333@example.com>
From: alexey@example.com
To: acme-generator@example.org
Subject: Re: ACME: <base64url-encoded-token-with-64-octets-of-entropy>
Content-Type: text/plain
MIME-Version: 1.0
LoqXcYV8q5ONbJQxbmR7SCTNo3tiAXDfowy
jxAjEuX0.9jg46WB3rR_AHD-EBXdN7cBkH1WOu0tA3M9
fm21mqTI
Figure 2
4. Open Issues
[[This section should be empty before publication]]
1. Do we need to handle text/html or multipart/alternative in email
challenge? Simplicity suggests "no". However, for automated
processing it might be better to use at least multipart/mixed
with a special MIME type.
2. How to verify authenticity of "response" email messages? We
can't require use of S/MIME, as this protocol is used for S/MIME
user enrollment. However DKIM/SPF/DMARC can be recommended.
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3. Define a new parameter to "Auto-Submitted: auto-generated", so
that it is easier to figure out that a particilar message is an
ACME challenge message?
5. IANA Considerations
IANA is requested to register a new Identifier Type "email" which
corresponds to an (all ASCII) email address [RFC5321] or
Internationalized Email addresses [RFC6531].
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.
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>.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
<https://www.rfc-editor.org/info/rfc2045>.
[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>.
[RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and Encoded
Word Extensions: Character Sets, Languages, and
Continuations", RFC 2231, DOI 10.17487/RFC2231, November
1997, <https://www.rfc-editor.org/info/rfc2231>.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000,
<https://www.rfc-editor.org/info/rfc2818>.
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[RFC2985] Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object
Classes and Attribute Types Version 2.0", RFC 2985,
DOI 10.17487/RFC2985, November 2000,
<https://www.rfc-editor.org/info/rfc2985>.
[RFC3834] Moore, K., "Recommendations for Automatic Responses to
Electronic Mail", RFC 3834, DOI 10.17487/RFC3834, August
2004, <https://www.rfc-editor.org/info/rfc3834>.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008,
<https://www.rfc-editor.org/info/rfc5321>.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008,
<https://www.rfc-editor.org/info/rfc5322>.
[RFC6376] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
"DomainKeys Identified Mail (DKIM) Signatures", STD 76,
RFC 6376, DOI 10.17487/RFC6376, September 2011,
<https://www.rfc-editor.org/info/rfc6376>.
[RFC6531] Yao, J. and W. Mao, "SMTP Extension for Internationalized
Email", RFC 6531, DOI 10.17487/RFC6531, February 2012,
<https://www.rfc-editor.org/info/rfc6531>.
[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>.
[RFC8550] Schaad, J., Ramsdell, B., and S. Turner, "Secure/
Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
Certificate Handling", RFC 8550, DOI 10.17487/RFC8550,
April 2019, <https://www.rfc-editor.org/info/rfc8550>.
[RFC8551] Schaad, J., Ramsdell, B., and S. Turner, "Secure/
Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
Message Specification", RFC 8551, DOI 10.17487/RFC8551,
April 2019, <https://www.rfc-editor.org/info/rfc8551>.
[RFC8555] Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
Kasten, "Automatic Certificate Management Environment
(ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,
<https://www.rfc-editor.org/info/rfc8555>.
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Appendix A. Acknowledgements
Thank you to Andreas Schulze and Gerd v. Egidy for suggestions,
comments and corrections on this document.
Author's Address
Alexey Melnikov
Isode Ltd
14 Castle Mews
Hampton, Middlesex TW12 2NP
UK
EMail: alexey.melnikov@isode.com
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