Network Working Group A. Melnikov
Internet-Draft Isode Ltd
Intended status: Informational June 16, 2020
Expires: December 18, 2020
Extensions to Automatic Certificate Management Environment for end user
S/MIME certificates
draft-ietf-acme-email-smime-08
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
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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 . . . . . . . . . . . . . . . . . . . 5
4. Internationalization Considerations . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
5.1. ACME Identifier Type . . . . . . . . . . . . . . . . . . 7
5.2. ACME Challenge Type . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. Normative References . . . . . . . . . . . . . . . . . . . . 8
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
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]. (When Internationalized Email addresses are
used, both U-labels and A-labels [RFC5890] are allowed in the domain
part.) This can be used with S/MIME or other similar service that
requires possession of a certificate tied to an email address.
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Any identifier of type "email" in a newOrder request MUST NOT have a
wildcard ("*") character in its value.
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 [RFC4648] 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 bit 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 To header field MUST be the email address of the entity that
requested S/MIME certificate to be generated.
3. The message MAY contain Reply-To header field.
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4. The message MUST include the "Auto-Submitted: auto-generated"
header field [RFC3834]. The "Auto-Submitted" header field SHOULD
include "type=acme" parameter. It MAY include other optional
parameters as allowed by syntax of Auto-Submitted header field.
5. The message MAY contain Reply-To header field.
6. 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", "Sender",
"Reply-To", "To", "CC", "Subject", "Date", "In-Reply-To",
"References", "Message-ID", "Content-Type" and "Content-Transfer-
Encoding" header fields. The message MUST also pass DMARC
validation [RFC7489], which implies DKIM and SPF validation
[RFC7208].
7. The body of the challenge message is not used for automated
processing, so it can be any media type. (However there are
extra requirements on S/MIME signing, if used. See below.)
Typically it is text/plain or text/html containing human readable
explanation of the purpose of the message. If S/MIME signing is
used to prove authenticity of the challenge message, then
multipart/signed or "application/pkcs7-mime; smime-type=signed-
data;" media type should be used. Either way, it MUST use S/MIME
header protection.
Example ACME "challenge" email (note that DKIM related header fields
are not included for simplicity).
Auto-Submitted: auto-generated; type=acme
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-bits-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
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3.2. ACME response email
A "response" email message MUST have the following structure:
1. The message Subject header field has the following syntax:
"<Reply-prefix> ACME: <token-part1>", where <Reply-prefix> is
typically the reply prefix "Re: " and the string "ACME:" is
followed by folding white space (FWS, see [RFC5322]) and then by
<token-part1>. <token-part1> is the base64url encoded first part
of the ACME token (as received in the ACME challenge) that MUST
be at least 64 bit 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.
3. The To: header field of the response contains the value from the
Reply-To: header field from the challenge message (if set) or
from the From: header field of the challenge message otherwise.
4. The Cc: header field is ignored if present in the "response"
email message.
5. The In-Reply-To: header field SHOULD be set to the Message-ID
header field of the challenge message according to rules in
Section 3.6.4 of [RFC5322].
6. Media type of the "response" email message is either text/plain
or multipart/alternative containing text/plain as one of the
alternatives. The text/plain body part (whether or not it is
inside multipart/alternative) MUST contain a block of lines
starting with the line "-----BEGIN ACME RESPONSE-----", followed
by one or more line 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.). The final line of the encoded digest is followed by the
line containing "-----END ACME RESPONSE-----". Any text before
and after this block is ignored. For example such text might
explain what to do with it for ACME-unaware clients.
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7. There is no need to use any Content-Transfer-Encoding other than
7bit for the text/plain body part, however use of Quoted-
Printable or base64 is not prohibited in a "response" email
message.
8. In order to prove authenticity of a response message, it MUST be
DKIM [RFC6376] signed. The resulting DKIM-Signature header field
MUST contain the "h=" tag that includes at least "From",
"Sender", "Reply-To", "To", "CC", "Subject", "Date", "In-Reply-
To", "References", "Message-ID", "Content-Type" and "Content-
Transfer-Encoding" header fields.
Example ACME "response" email (note that DKIM related header fields
are not included for simplicity).
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-enough-entropy>
Content-Type: text/plain
MIME-Version: 1.0
-----BEGIN ACME RESPONSE-----
LoqXcYV8q5ONbJQxbmR7SCTNo3tiAXDfowy
jxAjEuX0.9jg46WB3rR_AHD-EBXdN7cBkH1WOu0tA3M9
fm21mqTI
-----END ACME RESPONSE-----
Figure 2
4. Internationalization Considerations
[RFC8616] updated/clarified use of DKIM/SPF/DMARC with
Internationalized Email addresses [RFC6531]. Please consult RFC 8616
in regards to any changes that need to be implemented.
Use of non ASCII characters in left hand sides of Internationalized
Email addresses requires putting Internationalized Email Addresses in
X.509 Certificates [RFC8398].
5. IANA Considerations
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5.1. ACME Identifier Type
IANA is requested to register a new Identifier type in the "ACME
Identifier Types" registry defined in Section 9.7.7 of [RFC8555] with
Label "email" and a Reference to [RFCXXXX], [RFC5321] and [RFC6531].
The new Identifier Type corresponds to an (all ASCII) email address
[RFC5321] or Internationalized Email addresses [RFC6531].
5.2. ACME Challenge Type
IANA is also requested to register a new entry in the "ACME
Validation Methods" registry defined in Section 9.7.8 of [RFC8555].
This entry is as follows:
+----------------+-----------------+------+-----------+
| Label | Identifier Type | ACME | Reference |
+----------------+-----------------+------+-----------+
| email-reply-00 | email | Y | [RFCXXXX] |
+----------------+-----------------+------+-----------+
6. Security Considerations
Please see Security Considerations of [RFC8555] for general security
considerations related to use of ACME. This challenge/response
protocol demonstrates that an entity that controls the private key
(corresponding to the public key in the certificate) also controls
the named email account. Any claims about the correctness or
fitness-for-purpose of the email address must be otherwise assured.
Security of "email-reply-00" challenge type depends on security of
email system. A third party that can read and reply to user's email
messages (by possessing a user's password or a secret derived from it
that can give read and reply access, such as "password equivalent"
information; or by being given permissions to act on user's behalf
using email delegation feature common in some email systems) can
request S/MIME certificates and is indistinguishable from the email
account owner.
Email system in its turn depends on DNS. A third party that can
manipulate DNS MX records for a domain might be able to redirect
email and can get (at least temporary) read and reply access to it.
Similar considerations apply to SPF and DMARC TXT records in DNS.
Use of DNSSEC by email system administrators is recommended to avoid
easy spoofing of DNS records affecting email system.
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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>.
[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>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.
[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>.
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[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, DOI 10.17487/RFC5890, August 2010,
<https://www.rfc-editor.org/info/rfc5890>.
[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>.
[RFC7208] Kitterman, S., "Sender Policy Framework (SPF) for
Authorizing Use of Domains in Email, Version 1", RFC 7208,
DOI 10.17487/RFC7208, April 2014,
<https://www.rfc-editor.org/info/rfc7208>.
[RFC7489] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
Message Authentication, Reporting, and Conformance
(DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
<https://www.rfc-editor.org/info/rfc7489>.
[RFC8398] Melnikov, A., Ed. and W. Chuang, Ed., "Internationalized
Email Addresses in X.509 Certificates", RFC 8398,
DOI 10.17487/RFC8398, May 2018,
<https://www.rfc-editor.org/info/rfc8398>.
[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>.
[RFC8616] Levine, J., "Email Authentication for Internationalized
Mail", RFC 8616, DOI 10.17487/RFC8616, June 2019,
<https://www.rfc-editor.org/info/rfc8616>.
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Appendix A. Acknowledgements
Thank you to Andreas Schulze, Gerd v. Egidy, James A Baker, Ben
Schwartz and Michael Jenkins 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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