draft-ietf-acme-email-smime-05.txt   draft-ietf-acme-email-smime-06.txt 
Network Working Group A. Melnikov Network Working Group A. Melnikov
Internet-Draft Isode Ltd Internet-Draft Isode Ltd
Intended status: Informational July 8, 2019 Intended status: Informational November 1, 2019
Expires: January 9, 2020 Expires: May 4, 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-05 draft-ietf-acme-email-smime-06
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
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This Internet-Draft will expire on January 9, 2020. This Internet-Draft will expire on May 4, 2020.
Copyright Notice Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the Copyright (c) 2019 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 12 skipping to change at page 2, line 12
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 . . . . . . . . . . . . . . . . . . . 4 3.2. ACME response email . . . . . . . . . . . . . . . . . . . 4
4. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 5 4. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 5. Internationalization Considerations . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. Normative References . . . . . . . . . . . . . . . . . . . . 6 7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 8 8. Normative References . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 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.
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 [RFC8550] Section 3 defines extensions for issuing end user S/MIME [RFC8550]
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3. Use of ACME for issuing end user S/MIME certificates 3. Use of ACME for issuing end user S/MIME certificates
ACME [RFC8555] defines "dns" Identifier Type that is used to verify ACME [RFC8555] defines "dns" Identifier Type that is used to verify
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]. This can be used with S/MIME or other similar addresses [RFC6531]. (When Internationalized Email addresses are
service that requires posession of a certificate tied to an email used, both U-labels and A-labels [RFC5890] are allowed in the domain
address. part.) 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 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 [RFC4648] first part of the token, which
least 64 bit of entropy. The challenge email message structure contains at least 64 bit of entropy. The challenge email message
is described in more details in Section 3.1. The second part of structure is described in more details in Section 3.1. The
the token (token-part2, which also contains at least 64 bit of second part of the token (token-part2, which also contains at
entropy) is returned over HTTPS [RFC2818] to the ACME client. least 64 bit of entropy) is returned over HTTPS [RFC2818] to the
ACME client.
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
[RFC8555]), then includes the base64url encoded SHA-256 digest [RFC8555]), then includes the base64url encoded SHA-256 digest
[FIPS180-4] of the key authorization in the body of a response [FIPS180-4] of the key authorization in the body of a response
email message containing a single text/plain MIME body part email message containing a single text/plain MIME body part
[RFC2045]. The response email message structure is described in [RFC2045]. The response email message structure is described in
more details in Section 3.2 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
skipping to change at page 3, line 45 skipping to change at page 3, line 49
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 octet long after decoding. Due to recommended 78 octet least 64 octet 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 message MUST include the "Auto-Submitted: auto-generated"
header field [RFC3834]. It MAY include optional parameters as header field [RFC3834]. The "Auto-Submitted" header field SHOULD
allowed by syntax of Auto-Submitted header field. include "type=acme" parameter. It MAY include other optional
parameters as allowed by syntax of Auto-Submitted header field.
3. In order to prove authenticity of a challenge message, it MUST be 3. The message MAY contain Reply-To header field.
4. 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", "To", contain the "h=" tag that includes at least "From", "Sender",
"Subject", "Content-Type" and "Message-ID" header fields. "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].
4. If S/MIME signing is not used to prove authenticity of the 5. If S/MIME signing is not used to prove authenticity of the
challenge message, then the message MUST have a single text/plain challenge message, then the message MUST have a single text/plain
MIME body part [RFC2045], that contains human readable MIME body part [RFC2045], that contains 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, then the text/plain message is used to construct a used, then the text/plain message is used to construct a
multipart/signed or "application/pkcs7-mime; smime-type=signed- multipart/signed or "application/pkcs7-mime; smime-type=signed-
data;". Either way, it MUST use S/MIME header protection. data;". Either way, it MUST use S/MIME header protection.
Example ACME "challenge" email Example ACME "challenge" email (note that DKIM related header fields
are not included for simplicity).
Auto-Submitted: auto-generated Auto-Submitted: auto-generated; type=acme
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 MIME-Version: 1.0
This is an automatically generated ACME challenge for email address This is an automatically generated ACME challenge for email address
"alexey@example.com". If you haven't requested an S/MIME "alexey@example.com". If you haven't requested an S/MIME
skipping to change at page 4, line 37 skipping to change at page 4, line 50
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:
ACME: <token-part1>", where the string "ACME:" is followed by "<Reply-prefix> ACME: <token-part1>", where <Reply-prefix> is
folding white space (FWS, see [RFC5322]) and then by <token- typically the reply prefix "Re: " and the string "ACME:" is
part1> is the base64url encoded first part of the ACME token that followed by folding white space (FWS, see [RFC5322]) and then by
MUST be at least 64 octet long after decoding. Due to <token-part1>. <token-part1> is the base64url encoded first part
recommended 78 octet line length limit in [RFC5322], the subject of the ACME token (as received in the ACME challenge) that MUST
line can be folded, so whitespaces (if any) within the <token- be at least 64 octet long after decoding. Due to recommended 78
part1> MUST be ignored. [RFC2231] encoding of subject MUST be octet line length limit in [RFC5322], the subject line can be
supported, but when used, only "UTF-8" and "US-ASCII" charsets folded, so whitespaces (if any) within the <token-part1> MUST be
MUST be used (i.e. other charsets MUST NOT be used). 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 2. The From: header field contains the email address of the user
that is requesting S/MIME certificate issuance. that is requesting S/MIME certificate issuance.
3. The To: header field of the response contains the value from the 3. The To: header field of the response contains the value from the
From: header field of the challenge email. 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" 4. The Cc: header field is ignored if present in the "response"
email message. email message.
5. The message MUST have a single text/plain MIME body part 5. The In-Reply-To: header field SHOULD be set to the Message-ID
[RFC2045], containing base64url encoded SHA-256 digest header field of the challenge message according to rules in
[FIPS180-4] of the key authorization, calculated based on token- Section 3.6.4 of [RFC5322].
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 6. Media type of the "response" email message is either text/plain
7bit, however use of Quoted-Printable or base64 is not prohibited or multipart/alternative containing text/plain as one of the
in a "response" email message. alternatives. The text/plain body part MUST start 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-----". There should not be any text after the
terminating line, but if any text is found, it is ignored.
Example ACME "response" email 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.
Date: Sat, 1 Sep 2018 11:12:00 +0100 8. In order to prove authenticity of a response message, it MUST be
Message-ID: <111-22222-3333333@example.com> DKIM [RFC6376] signed. The resulting DKIM-Signature header field
From: alexey@example.com MUST contain the "h=" tag that includes at least "From",
To: acme-generator@example.org "Sender", "Reply-To", "To", "CC", "Subject", "Date", "In-Reply-
Subject: Re: ACME: <base64url-encoded-token-with-64-octets-of-entropy> To", "References", "Message-ID", "Content-Type" and "Content-
Content-Type: text/plain Transfer-Encoding" header fields.
MIME-Version: 1.0
LoqXcYV8q5ONbJQxbmR7SCTNo3tiAXDfowy Example ACME "response" email (note that DKIM related header fields
jxAjEuX0.9jg46WB3rR_AHD-EBXdN7cBkH1WOu0tA3M9 are not included for simplicity).
fm21mqTI
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 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". Also, for automated
processing it might be better to use at least multipart/mixed processing it might be better to define a special MIME type that
with a special MIME type. is included as one of body parts inside multipart/mixed
container.
2. How to verify authenticity of "response" email messages? We 5. Internationalization Considerations
can't require use of S/MIME, as this protocol is used for S/MIME
user enrollment. However DKIM/SPF/DMARC can be recommended.
3. Define a new parameter to "Auto-Submitted: auto-generated", so [RFC8616] updated/clarified use of DKIM/SPF/DMARC with
that it is easier to figure out that a particilar message is an Internationalized Email addresses [RFC6531]. Please consult RFC 8616
ACME challenge message? in regards to any changes that need to be implemented.
5. IANA Considerations Use of non ASCII characters in left hand sides of Internationalized
Email addresses requires putting Internationalized Email Addresses in
X.509 Certificates [RFC8398].
6. 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
challenge types that are used with Identifier Type "email": "email- challenge types that are used with Identifier Type "email": "email-
reply". The reference for it is this document. reply". The reference for it is this document.
6. Security Considerations 7. Security Considerations
TBD. Please see Security Considerations of [RFC8555] for general security
considerations related to use of ACME.
7. Normative References 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 messages (by posessing user's password or a secret derived from
it that can give read and reply access ("password equivalent"
information), or by being given permissions to act on user's behalf
using email delegation feature) 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.
8. 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,
skipping to change at page 7, line 14 skipping to change at page 8, line 18
[RFC2985] Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object [RFC2985] Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object
Classes and Attribute Types Version 2.0", RFC 2985, Classes and Attribute Types Version 2.0", RFC 2985,
DOI 10.17487/RFC2985, November 2000, DOI 10.17487/RFC2985, November 2000,
<https://www.rfc-editor.org/info/rfc2985>. <https://www.rfc-editor.org/info/rfc2985>.
[RFC3834] Moore, K., "Recommendations for Automatic Responses to [RFC3834] Moore, K., "Recommendations for Automatic Responses to
Electronic Mail", RFC 3834, DOI 10.17487/RFC3834, August Electronic Mail", RFC 3834, DOI 10.17487/RFC3834, August
2004, <https://www.rfc-editor.org/info/rfc3834>. 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, [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008, DOI 10.17487/RFC5321, October 2008,
<https://www.rfc-editor.org/info/rfc5321>. <https://www.rfc-editor.org/info/rfc5321>.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008, DOI 10.17487/RFC5322, October 2008,
<https://www.rfc-editor.org/info/rfc5322>. <https://www.rfc-editor.org/info/rfc5322>.
[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., [RFC6376] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
"DomainKeys Identified Mail (DKIM) Signatures", STD 76, "DomainKeys Identified Mail (DKIM) Signatures", STD 76,
RFC 6376, DOI 10.17487/RFC6376, September 2011, RFC 6376, DOI 10.17487/RFC6376, September 2011,
<https://www.rfc-editor.org/info/rfc6376>. <https://www.rfc-editor.org/info/rfc6376>.
[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 [RFC7208] Kitterman, S., "Sender Policy Framework (SPF) for
Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May Authorizing Use of Domains in Email, Version 1", RFC 7208,
2015, <https://www.rfc-editor.org/info/rfc7515>. 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/ [RFC8550] Schaad, J., Ramsdell, B., and S. Turner, "Secure/
Multipurpose Internet Mail Extensions (S/MIME) Version 4.0 Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
Certificate Handling", RFC 8550, DOI 10.17487/RFC8550, Certificate Handling", RFC 8550, DOI 10.17487/RFC8550,
April 2019, <https://www.rfc-editor.org/info/rfc8550>. April 2019, <https://www.rfc-editor.org/info/rfc8550>.
[RFC8551] Schaad, J., Ramsdell, B., and S. Turner, "Secure/ [RFC8551] Schaad, J., Ramsdell, B., and S. Turner, "Secure/
Multipurpose Internet Mail Extensions (S/MIME) Version 4.0 Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
Message Specification", RFC 8551, DOI 10.17487/RFC8551, Message Specification", RFC 8551, DOI 10.17487/RFC8551,
April 2019, <https://www.rfc-editor.org/info/rfc8551>. April 2019, <https://www.rfc-editor.org/info/rfc8551>.
[RFC8555] Barnes, R., Hoffman-Andrews, J., McCarney, D., and J. [RFC8555] Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
Kasten, "Automatic Certificate Management Environment Kasten, "Automatic Certificate Management Environment
(ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019, (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,
<https://www.rfc-editor.org/info/rfc8555>. <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>.
Appendix A. Acknowledgements Appendix A. Acknowledgements
Thank you to Andreas Schulze and Gerd v. Egidy for suggestions, Thank you to Andreas Schulze, Gerd v. Egidy and James A Baker for
comments and corrections on this document. 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
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