draft-ietf-acme-email-smime-03.txt   draft-ietf-acme-email-smime-04.txt 
Network Working Group A. Melnikov Network Working Group A. Melnikov
Internet-Draft Isode Ltd Internet-Draft Isode Ltd
Intended status: Informational September 1, 2018 Intended status: Informational October 19, 2018
Expires: March 5, 2019 Expires: April 22, 2019
Extensions to Automatic Certificate Management Environment for end user Extensions to Automatic Certificate Management Environment for end user
S/MIME certificates S/MIME certificates
draft-ietf-acme-email-smime-03 draft-ietf-acme-email-smime-04
Abstract Abstract
This document specifies identifiers and challenges required to enable This document specifies identifiers and challenges required to enable
the Automated Certificate Management Environment (ACME) to issue the Automated Certificate Management Environment (ACME) to issue
certificates for use by email users that want to use S/MIME. certificates for use by email users that want to use S/MIME.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
skipping to change at page 1, line 33 skipping to change at page 1, line 33
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 5, 2019. This Internet-Draft will expire on April 22, 2019.
Copyright Notice Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
skipping to change at page 2, line 13 skipping to change at page 2, line 13
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . 2 2. Conventions Used in This Document . . . . . . . . . . . . . . 2
3. Use of ACME for issuing end user S/MIME certificates . . . . 2 3. Use of ACME for issuing end user S/MIME certificates . . . . 2
3.1. ACME challenge email . . . . . . . . . . . . . . . . . . 3 3.1. ACME challenge email . . . . . . . . . . . . . . . . . . 3
3.2. ACME response email . . . . . . . . . . . . . . . . . . . 4 3.2. ACME response email . . . . . . . . . . . . . . . . . . . 4
4. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 5 4. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 5 6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. Normative References . . . . . . . . . . . . . . . . . . . . 5 7. Normative References . . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction 1. Introduction
[I-D.ietf-acme-acme] is a mechanism for automating certificate [I-D.ietf-acme-acme] is a mechanism for automating certificate
management on the Internet. It enables administrative entities to management on the Internet. It enables administrative entities to
prove effective control over resources like domain names, and prove effective control over resources like domain names, and
automates the process of generating and issuing certificates. automates the process of generating and issuing certificates.
This document describes an extension to ACME for use by S/MIME. This document describes an extension to ACME for use by S/MIME.
Section 3 defines extensions for issuing end user S/MIME [RFC5750] Section 3 defines extensions for issuing end user S/MIME [RFC5750]
skipping to change at page 2, line 49 skipping to change at page 2, line 50
service associated with the domain. In order to be able to issue service associated with the domain. In order to be able to issue
end-user S/MIME certificates, ACME needs a new Identifier Type that end-user S/MIME certificates, ACME needs a new Identifier Type that
proves ownership of an email address. proves ownership of an email address.
This document defines a new Identifier Type "email" which corresponds This document defines a new Identifier Type "email" which corresponds
to an (all ASCII) email address [RFC5321] or Internationalized Email to an (all ASCII) email address [RFC5321] or Internationalized Email
addresses [RFC6531]. This can be used with S/MIME or other similar addresses [RFC6531]. This can be used with S/MIME or other similar
service that requires posession of a certificate tied to an email service that requires posession of a certificate tied to an email
address. address.
Any identifier of type "email" in a new-order request MUST NOT have a Any identifier of type "email" in a newOrder request MUST NOT have a
wildcard ("*") character in its value. wildcard ("*") character in its value.
A new challenge type "email-reply-00" is used with "email" Identifier A new challenge type "email-reply-00" is used with "email" Identifier
Type, which provides proof that an ACME client has control over an Type, which provides proof that an ACME client has control over an
email address: email address:
1. ACME server generates a "challenge" email message with the 1. ACME server generates a "challenge" email message with the
subject "ACME: <token-part1>", where <token-part1> is the subject "ACME: <token-part1>", where <token-part1> is the
base64url encoded first part of the token, which contains at base64url encoded first part of the token, which contains at
least 64 bit of entropy. The challenge email message structure least 64 bit of entropy. The challenge email message structure
skipping to change at page 3, line 27 skipping to change at page 3, line 27
2. ACME client concatenates "token-part1" and "token-part2" to 2. ACME client concatenates "token-part1" and "token-part2" to
create "token", calculates key-authz (as per Section 8.1 of create "token", calculates key-authz (as per Section 8.1 of
[I-D.ietf-acme-acme]), then includes the base64url encoded [I-D.ietf-acme-acme]), then includes the base64url encoded
SHA-256 digest [FIPS180-4] of the key authorization in the body SHA-256 digest [FIPS180-4] of the key authorization in the body
of a response email message containing a single text/plain MIME of a response email message containing a single text/plain MIME
body part [RFC2045]. The response email message structure is body part [RFC2045]. The response email message structure is
described in more details in Section 3.2 described in more details in Section 3.2
For an identifier of type "email", CSR MUST contain the request email For an identifier of type "email", CSR MUST contain the request email
address in an extensionRequest attribute [RFC2985] requesting a address in an extensionRequest attribute [RFC2985] requesting a
subjectAltName extension. (These identifiers may appear in any sort subjectAltName extension.
order.)
3.1. ACME challenge email 3.1. ACME challenge email
A "challenge" email message MUST have the following structure: A "challenge" email message MUST have the following structure:
1. The message Subject header field has the following syntax: "ACME: 1. The message Subject header field has the following syntax: "ACME:
<token-part1>", where the prefix "ACME:" is followed by at least <token-part1>", where the prefix "ACME:" is followed by exactly
one SP or TAB character. <token-part1> is the base64url encoded one SP character. <token-part1> is the base64url encoded first
first part of the ACME token. part of the ACME token that MUST be at least 64 octet long after
decoding. [[Alexey: is the following going to be problematic due
to the total subject lenght and recommended 78 octet line length
limit?]] For ease of implementation, no RFC 2231 encoding of
subject is allowed in the "challenge" email message.
2. The message MUST include the "Auto-Submitted: auto-generated" 2. The message MUST include the "Auto-Submitted: auto-generated"
header field [RFC3834]. It MAY include optional parameters as header field [RFC3834]. It MAY include optional parameters as
allowed by syntax of Auto-Submitted header field. allowed by syntax of Auto-Submitted header field.
3. The message MUST have a single text/plain MIME body part 3. The message MUST have a single text/plain MIME body part
[RFC2045], that contains human readable explanation of the [RFC2045], that contains human readable explanation of the
purpose of the message. purpose of the message.
Example ACME "challenge" email Example ACME "challenge" email
Auto-Submitted: auto-generated Auto-Submitted: auto-generated
Date: Sat, 1 Sep 2018 10:08:55 +0100 Date: Sat, 1 Sep 2018 10:08:55 +0100
Message-ID: <A2299BB.FF7788@example.org> Message-ID: <A2299BB.FF7788@example.org>
From: acme-generator@example.org From: acme-generator@example.org
To: alexey@example.com To: alexey@example.com
Subject: ACME: <base64url-encoded-token-with-64-octets-of-entropy> Subject: ACME: <base64url-encoded-token-with-64-octets-of-entropy>
Content-Type: text/plain Content-Type: text/plain
MIME-Version: 1.0
This is an automatically generated ACME challenge for email address This is an automatically generated ACME challenge for email address
&lt;alexey@example.com&gt;. If you haven't requested an S/MIME "alexey@example.com". If you haven't requested an S/MIME
certificate generation for this email address, be very afraid. certificate generation for this email address, be very afraid.
If you did request it, your email client might be able to process If you did request it, your email client might be able to process
this request automatically, or you might have to paste the first this request automatically, or you might have to paste the first
token part into an external program. token part into an external program.
Figure 1 Figure 1
3.2. ACME response email 3.2. ACME response email
A "response" email message MUST have the following structure: A "response" email message MUST have the following structure:
1. The message Subject header field has the following syntax: "Re: 1. The message Subject header field has the following syntax: "Re:
ACME: <token-part1>", where the prefix "ACME:" is followed by at ACME: <token-part1>", where the string "ACME:" is followed by
least one SP or TAB character. <token-part1> is the base64url exactly one SP character. <token-part1> is the base64url encoded
encoded first part of the ACME token. first part of the ACME token. (Note that this would be a subject
generated by any email client replying to the "challenge" email
message.) [[Alexey: is the following going to be problematic due
to the total subject lenght and recommended 78 octet line length
limit?]] For ease of implementation, no RFC 2231 encoding of
subject is allowed in the "response" email message.
2. The To: header field of the response contains the value from the 2. The From: header field contains the email address of the user
that is requesting S/MIME certificate issuance.
3. The To: header field of the response contains the value from the
From: header field of the challenge email. From: header field of the challenge email.
3. The message MUST have a single text/plain MIME body part 4. The Cc: header field must be absent in the "response" email
message.
5. The message MUST have a single text/plain MIME body part
[RFC2045], containing base64url encoded SHA-256 digest [RFC2045], containing base64url encoded SHA-256 digest
[FIPS180-4] of the key authorization. Note that due to historic [FIPS180-4] of the key authorization, calculated based on token-
line length limitations in email, line endings (CRLFs) can be part1 (received over email) and token-part2 (received over
freely inserted in the middle of the encoded digest, so they need HTTPS). Note that due to historic line length limitations in
to be ignored when processing it. email, line endings (CRLFs) can be freely inserted in the middle
of the encoded digest, so they need to 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 Example ACME "response" email
Date: Sat, 1 Sep 2018 11:12:00 +0100 Date: Sat, 1 Sep 2018 11:12:00 +0100
Message-ID: <111-22222-3333333@example.com> Message-ID: <111-22222-3333333@example.com>
From: alexey@example.com From: alexey@example.com
To: acme-generator@example.org To: acme-generator@example.org
Subject: Re: ACME: <base64url-encoded-token-with-64-octets-of-entropy> Subject: Re: ACME: <base64url-encoded-token-with-64-octets-of-entropy>
Content-Type: text/plain Content-Type: text/plain
MIME-Version: 1.0
LoqXcYV8q5ONbJQxbmR7SCTNo3tiAXDfowy LoqXcYV8q5ONbJQxbmR7SCTNo3tiAXDfowy
jxAjEuX0.9jg46WB3rR_AHD-EBXdN7cBkH1WOu0tA3M9 jxAjEuX0.9jg46WB3rR_AHD-EBXdN7cBkH1WOu0tA3M9
fm21mqTI fm21mqTI
Figure 2 Figure 2
4. Open Issues 4. Open Issues
[[This section should be empty before publication]] [[This section should be empty before publication]]
1. Do we need to handle text/html or multipart/alternative in email 1. Do we need to handle text/html or multipart/alternative in email
challenge? Simplicity suggests "no". However, for automated challenge? Simplicity suggests "no". However, for automated
processing it might be better to use at least multipart/mixed processing it might be better to use at least multipart/mixed
with a special MIME type. with a special MIME type.
2. Define a new parameter to "Auto-Submitted: auto-generated", so 2. How to verify authenticity of "challenge" email messages? The
document can recommend either S/MIME signing (in which case the
"challenge" email message should be allowed to be S/MIME signed
multipart/signed or application/pkcs7-mime containing SignedData)
and/or presence of DKIM signature covering at least From/To/
Subject, with working SPF and DMARC.
3. Similarly to the above: 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.
4. Define a new parameter to "Auto-Submitted: auto-generated", so
that it is easier to figure out that a particilar message is an that it is easier to figure out that a particilar message is an
ACME challenge message? ACME challenge message?
5. IANA Considerations 5. IANA Considerations
IANA is requested to register a new Identifier Type "email" which IANA is requested to register a new Identifier Type "email" which
corresponds to an (all ASCII) email address [RFC5321] or corresponds to an (all ASCII) email address [RFC5321] or
Internationalized Email addresses [RFC6531]. Internationalized Email addresses [RFC6531].
And finally, IANA is requested to register the following ACME And finally, IANA is requested to register the following ACME
skipping to change at page 7, line 9 skipping to change at page 8, line 5
<https://www.rfc-editor.org/info/rfc5750>. <https://www.rfc-editor.org/info/rfc5750>.
[RFC6531] Yao, J. and W. Mao, "SMTP Extension for Internationalized [RFC6531] Yao, J. and W. Mao, "SMTP Extension for Internationalized
Email", RFC 6531, DOI 10.17487/RFC6531, February 2012, Email", RFC 6531, DOI 10.17487/RFC6531, February 2012,
<https://www.rfc-editor.org/info/rfc6531>. <https://www.rfc-editor.org/info/rfc6531>.
[RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web [RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
2015, <https://www.rfc-editor.org/info/rfc7515>. 2015, <https://www.rfc-editor.org/info/rfc7515>.
Appendix A. Acknowledgements
Thank you to Andreas Schulze and Gerd v. Egidy for suggestions,
comments and corrections on this document.
Author's Address Author's Address
Alexey Melnikov Alexey Melnikov
Isode Ltd Isode Ltd
14 Castle Mews 14 Castle Mews
Hampton, Middlesex TW12 2NP Hampton, Middlesex TW12 2NP
UK UK
EMail: alexey.melnikov@isode.com EMail: alexey.melnikov@isode.com
 End of changes. 16 change blocks. 
25 lines changed or deleted 65 lines changed or added

This html diff was produced by rfcdiff 1.47. The latest version is available from http://tools.ietf.org/tools/rfcdiff/