draft-ietf-acme-email-smime-07.txt   draft-ietf-acme-email-smime-08.txt 
Network Working Group A. Melnikov Network Working Group A. Melnikov
Internet-Draft Isode Ltd Internet-Draft Isode Ltd
Intended status: Informational May 2, 2020 Intended status: Informational June 16, 2020
Expires: November 3, 2020 Expires: December 18, 2020
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-07 draft-ietf-acme-email-smime-08
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
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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-
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Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
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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 November 3, 2020. This Internet-Draft will expire on December 18, 2020.
Copyright Notice Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the Copyright (c) 2020 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
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the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
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 . . . . . . . . . . . . . . . . . . . 5 3.2. ACME response email . . . . . . . . . . . . . . . . . . . 5
4. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 6 4. Internationalization Considerations . . . . . . . . . . . . . 6
5. Internationalization Considerations . . . . . . . . . . . . . 6 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 5.1. ACME Identifier Type . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7 5.2. ACME Challenge Type . . . . . . . . . . . . . . . . . . . 7
8. Normative References . . . . . . . . . . . . . . . . . . . . 7 6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. Normative References . . . . . . . . . . . . . . . . . . . . 8
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 10 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction 1. Introduction
ACME [RFC8555] is a mechanism for automating certificate management ACME [RFC8555] is a mechanism for automating certificate management
on the Internet. It enables administrative entities to prove on the Internet. It enables administrative entities to prove
effective control over resources like domain names, and automates the effective control over resources like domain names, and automates the
process of generating and issuing certificates. process of generating and issuing certificates.
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that a particular entity has control over a domain or specific that a particular entity has control over a domain or specific
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]. (When Internationalized Email addresses are addresses [RFC6531]. (When Internationalized Email addresses are
used, both U-labels and A-labels [RFC5890] are allowed in the domain 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 part.) This can be used with S/MIME or other similar service that
requires posession of a certificate tied to an email address. requires possession of a certificate tied to an email address.
Any identifier of type "email" in a newOrder 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
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<token-part1>", where the prefix "ACME:" is followed by folding <token-part1>", where the prefix "ACME:" is followed by folding
white space (FWS, see [RFC5322]) and then by <token-part1> is the white space (FWS, see [RFC5322]) and then by <token-part1> is the
base64url encoded first part of the ACME token that MUST be at base64url encoded first part of the ACME token that MUST be at
least 64 bit long after decoding. Due to recommended 78 octet least 64 bit long after decoding. Due to recommended 78 octet
line length limit in [RFC5322], the subject line can be folded, line length limit in [RFC5322], the subject line can be folded,
so whitespaces (if any) within the <token-part1> MUST be ignored. so whitespaces (if any) within the <token-part1> MUST be ignored.
[RFC2231] encoding of subject MUST be supported, but when used, [RFC2231] encoding of subject MUST be supported, but when used,
only "UTF-8" and "US-ASCII" charsets MUST be used (i.e. other only "UTF-8" and "US-ASCII" charsets MUST be used (i.e. other
charsets MUST NOT be used). charsets MUST NOT be used).
2. The message MUST include the "Auto-Submitted: auto-generated" 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.
4. The message MUST include the "Auto-Submitted: auto-generated"
header field [RFC3834]. The "Auto-Submitted" header field SHOULD header field [RFC3834]. The "Auto-Submitted" header field SHOULD
include "type=acme" parameter. It MAY include other optional include "type=acme" parameter. It MAY include other optional
parameters as allowed by syntax of Auto-Submitted header field. parameters as allowed by syntax of Auto-Submitted header field.
3. The message MAY contain Reply-To header field. 5. The message MAY contain Reply-To header field.
4. In order to prove authenticity of a challenge message, it MUST be 6. In order to prove authenticity of a challenge message, it MUST be
either DKIM [RFC6376] signed or S/MIME [RFC8551] signed. If DKIM either DKIM [RFC6376] signed or S/MIME [RFC8551] signed. If DKIM
signing is used, the resulting DKIM-Signature header field MUST signing is used, the resulting DKIM-Signature header field MUST
contain the "h=" tag that includes at least "From", "Sender", contain the "h=" tag that includes at least "From", "Sender",
"Reply-To", "To", "CC", "Subject", "Date", "In-Reply-To", "Reply-To", "To", "CC", "Subject", "Date", "In-Reply-To",
"References", "Message-ID", "Content-Type" and "Content-Transfer- "References", "Message-ID", "Content-Type" and "Content-Transfer-
Encoding" header fields. The message MUST also pass DMARC Encoding" header fields. The message MUST also pass DMARC
validation [RFC7489], which implies DKIM and SPF validation validation [RFC7489], which implies DKIM and SPF validation
[RFC7208]. [RFC7208].
5. The body of the challenge message is not used for automated 7. The body of the challenge message is not used for automated
processing, so it can be any media type. (However there are processing, so it can be any media type. (However there are
extra requirements on S/MIME signing, if used. See below.) extra requirements on S/MIME signing, if used. See below.)
Typically it is text/plain or text/html containing human readable Typically it is text/plain or text/html containing human readable
explanation of the purpose of the message. If S/MIME signing is explanation of the purpose of the message. If S/MIME signing is
used to prove authenticity of the challenge message, then used to prove authenticity of the challenge message, then
multipart/signed or "application/pkcs7-mime; smime-type=signed- multipart/signed or "application/pkcs7-mime; smime-type=signed-
data;" media type should be used. Either way, it MUST use S/MIME data;" media type should be used. Either way, it MUST use S/MIME
header protection. header protection.
Example ACME "challenge" email (note that DKIM related header fields Example ACME "challenge" email (note that DKIM related header fields
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MIME-Version: 1.0 MIME-Version: 1.0
-----BEGIN ACME RESPONSE----- -----BEGIN ACME RESPONSE-----
LoqXcYV8q5ONbJQxbmR7SCTNo3tiAXDfowy LoqXcYV8q5ONbJQxbmR7SCTNo3tiAXDfowy
jxAjEuX0.9jg46WB3rR_AHD-EBXdN7cBkH1WOu0tA3M9 jxAjEuX0.9jg46WB3rR_AHD-EBXdN7cBkH1WOu0tA3M9
fm21mqTI fm21mqTI
-----END ACME RESPONSE----- -----END ACME RESPONSE-----
Figure 2 Figure 2
4. Open Issues 4. Internationalization Considerations
[[This section should be empty before publication]]
5. Internationalization Considerations
[RFC8616] updated/clarified use of DKIM/SPF/DMARC with [RFC8616] updated/clarified use of DKIM/SPF/DMARC with
Internationalized Email addresses [RFC6531]. Please consult RFC 8616 Internationalized Email addresses [RFC6531]. Please consult RFC 8616
in regards to any changes that need to be implemented. in regards to any changes that need to be implemented.
Use of non ASCII characters in left hand sides of Internationalized Use of non ASCII characters in left hand sides of Internationalized
Email addresses requires putting Internationalized Email Addresses in Email addresses requires putting Internationalized Email Addresses in
X.509 Certificates [RFC8398]. X.509 Certificates [RFC8398].
6. IANA Considerations 5. IANA Considerations
5.1. ACME Identifier Type
IANA is requested to register a new Identifier Type "email" which IANA is requested to register a new Identifier type in the "ACME
corresponds to an (all ASCII) email address [RFC5321] or Identifier Types" registry defined in Section 9.7.7 of [RFC8555] with
Internationalized Email addresses [RFC6531]. 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].
And finally, IANA is requested to register the following ACME 5.2. ACME Challenge Type
challenge types that are used with Identifier Type "email": "email-
reply". The reference for it is this document.
7. Security Considerations 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 Please see Security Considerations of [RFC8555] for general security
considerations related to use of ACME. This challenge/response considerations related to use of ACME. This challenge/response
protocol demonstrates that an entity that controls the private key protocol demonstrates that an entity that controls the private key
(corresponding to the public key in the certificate) also controls (corresponding to the public key in the certificate) also controls
the named email account. Any claims about the correctness or the named email account. Any claims about the correctness or
fitness-for-purpose of the email address must be otherwise assured. fitness-for-purpose of the email address must be otherwise assured.
Security of "email-reply-00" challenge type depends on security of Security of "email-reply-00" challenge type depends on security of
email system. A third party that can can read and reply to user's email system. A third party that can read and reply to user's email
email messages (by posessing user's password or a secret derived from messages (by possessing a user's password or a secret derived from it
it that can give read and reply access ("password equivalent" that can give read and reply access, such as "password equivalent"
information), or by being given permissions to act on user's behalf information; or by being given permissions to act on user's behalf
using email delegation feature) can request S/MIME certificates and using email delegation feature common in some email systems) can
is indistinguishable from the email account owner. 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 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 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. email and can get (at least temporary) read and reply access to it.
Similar considerations apply to SPF and DMARC TXT records in DNS. Similar considerations apply to SPF and DMARC TXT records in DNS.
Use of DNSSEC by email system administrators is recommended to avoid Use of DNSSEC by email system administrators is recommended to avoid
easy spoofing of DNS records affecting email system. easy spoofing of DNS records affecting email system.
8. Normative References 7. Normative References
[FIPS180-4] [FIPS180-4]
National Institute of Standards and Technology, "Secure National Institute of Standards and Technology, "Secure
Hash Standard (SHS)", FIPS PUB 180-4, August 2015, Hash Standard (SHS)", FIPS PUB 180-4, August 2015,
<https://csrc.nist.gov/publications/detail/fips/180/4/ <https://csrc.nist.gov/publications/detail/fips/180/4/
final>. final>.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996, Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
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