draft-ietf-acme-email-smime-08.txt   draft-ietf-acme-email-smime-09.txt 
Network Working Group A. Melnikov Network Working Group A. Melnikov
Internet-Draft Isode Ltd Internet-Draft Isode Ltd
Intended status: Informational June 16, 2020 Intended status: Informational October 27, 2020
Expires: December 18, 2020 Expires: April 30, 2021
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-08 draft-ietf-acme-email-smime-09
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
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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 December 18, 2020. This Internet-Draft will expire on April 30, 2021.
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.
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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. Internationalization Considerations . . . . . . . . . . . . . 6 4. Internationalization Considerations . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
5.1. ACME Identifier Type . . . . . . . . . . . . . . . . . . 7 5.1. ACME Identifier Type . . . . . . . . . . . . . . . . . . 7
5.2. ACME Challenge Type . . . . . . . . . . . . . . . . . . . 7 5.2. ACME Challenge Type . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7 6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. Normative References . . . . . . . . . . . . . . . . . . . . 8 7. Normative References . . . . . . . . . . . . . . . . . . . . 9
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 10 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12
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]
certificates. certificates.
2. Conventions Used in This Document 2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
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 a "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]. (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 possession 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. An end-user initiates issuance of an S/MIME certificate for one
subject "ACME: <token-part1>", where <token-part1> is the of her email addresses. This might be done using email client UI
base64url encoded [RFC4648] first part of the token, which (and might use some HTTP API underneath), by visiting a
contains at least 64 bit of entropy. The challenge email message Certificate Authority web page or by sending an email to a well
structure is described in more details in Section 3.1. The known Certificate Authority's email address. This document
second part of the token (token-part2, which also contains at doesn't prescribe how exactly S/MIME certificate issuance is
least 64 bit of entropy) is returned over HTTPS [RFC2818] to the initiated.
ACME client.
2. ACME client concatenates "token-part1" and "token-part2" to 2. The ACME server (run by the Certificate Authority or their
create "token", calculates key-authz (as per Section 8.1 of authorized third party) generates a "challenge" email message
[RFC8555]), then includes the base64url encoded SHA-256 digest with the subject "ACME: <token-part1>", where <token-part1> is
the base64url encoded [RFC4648] first part of the token, which
contains at least 64 bits of entropy. (ACME server MUST generate
token afresh for each S/MIME issuance request.) 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 bits of entropy) is returned over HTTPS
[RFC2818] to the ACME client.
3. The ACME client concatenates "token-part1" and "token-part2" to
create "token", calculates keyAuthorization (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 [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", the PKCS#10 [RFC2986] Certificate
address in an extensionRequest attribute [RFC2985] requesting a Signing Request (CSR) MUST contain the requested email address in an
subjectAltName extension. extensionRequest attribute [RFC2985] requesting a subjectAltName
extension.
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 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>, which
base64url encoded first part of the ACME token that MUST be at is the base64url encoded first part of the ACME token that MUST
least 64 bit long after decoding. Due to recommended 78 octet be at least 64 bits long after decoding. Due to the recommended
line length limit in [RFC5322], the subject line can be folded, 78-octet line length limit in [RFC5322], the subject line can be
so whitespaces (if any) within the <token-part1> MUST be ignored. folded, so whitespaces (if any) within the <token-part1> MUST be
[RFC2231] encoding of subject MUST be supported, but when used, ignored. [RFC2231] encoding of the message Subject header field
only "UTF-8" and "US-ASCII" charsets MUST be used (i.e. other MUST be supported, but when used, only "UTF-8" and "US-ASCII"
charsets MUST NOT be used). 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 2. The To header field MUST be the email address of the entity that
requested S/MIME certificate to be generated. requested the S/MIME certificate to be generated.
3. The message MAY contain Reply-To header field. 3. The message MAY contain a Reply-To header field.
4. The message MUST include the "Auto-Submitted: auto-generated" 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 the "type=acme" parameter. It MAY include other optional
parameters as allowed by syntax of Auto-Submitted header field. parameters as allowed by the syntax of the 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 5. 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-
Encoding" header fields. The message MUST also pass DMARC Transfer-Encoding" header fields. The message MUST also pass
validation [RFC7489], which implies DKIM and SPF validation DMARC validation [RFC7489], which implies DKIM and SPF validation
[RFC7208]. [RFC7208].
7. The body of the challenge message is not used for automated 6. 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 a human-
explanation of the purpose of the message. If S/MIME signing is readable explanation of the purpose of the message. If S/MIME
used to prove authenticity of the challenge message, then signing is used to prove authenticity of the challenge message,
multipart/signed or "application/pkcs7-mime; smime-type=signed- then the multipart/signed or "application/pkcs7-mime; smime-
data;" media type should be used. Either way, it MUST use S/MIME type=signed-data;" media type should be used. Either way, it
header protection. MUST use S/MIME header protection.
Example ACME "challenge" email (note that DKIM related header fields An example ACME "challenge" email (note that DKIM related header
are not included for simplicity). fields are not included for simplicity).
Auto-Submitted: auto-generated; type=acme 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-bits-of-entropy> Subject: ACME: <base64url-encoded-token-with-64-bits-of-entropy>
Content-Type: text/plain Content-Type: text/plain
MIME-Version: 1.0 MIME-Version: 1.0
skipping to change at page 5, line 7 skipping to change at page 5, line 28
"alexey@example.com". 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 valid "response" email message MUST have the following structure:
1. The message Subject header field has the following syntax: 1. The message Subject header field has the following syntax:
"<Reply-prefix> ACME: <token-part1>", where <Reply-prefix> is "<Reply-prefix> ACME: <token-part1>", where <Reply-prefix> is
typically the reply prefix "Re: " and the string "ACME:" is typically the reply prefix "Re:" and the string "ACME:" is
followed by folding white space (FWS, see [RFC5322]) and then by preceded and followed by folding white space (FWS, see [RFC5322])
<token-part1>. <token-part1> is the base64url encoded first part and then by <token-part1>. <token-part1> is the base64url encoded
of the ACME token (as received in the ACME challenge) that MUST first part of the ACME token (as received in the ACME challenge)
be at least 64 bit long after decoding. Due to recommended 78 that MUST be at least 64 bits long after decoding. Due to
octet line length limit in [RFC5322], the subject line can be recommended 78 octet line length limit in [RFC5322], the subject
folded, so whitespaces (if any) within the <token-part1> MUST be line can be folded, so whitespaces (if any) within the <token-
ignored. [RFC2231] encoding of subject MUST be supported, but part1> MUST be ignored. [RFC2231] encoding of the Subject header
when used, only "UTF-8" and "US-ASCII" charsets MUST be used field MUST be supported, but when used, only "UTF-8" and "US-
(i.e. other charsets MUST NOT be used). ASCII" charsets MUST be used (i.e. other charsets MUST NOT be
used). When parsing subjects, ACME servers must decode [RFC2231]
encoding (if any) and then they can ignore any prefix before the
"ACME:" label.
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
Reply-To: header field from the challenge message (if set) or Reply-To: header field from the challenge message (if set) or
from the From: header field of the challenge message otherwise. 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 In-Reply-To: header field SHOULD be set to the Message-ID 5. The In-Reply-To: header field SHOULD be set to the Message-ID
header field of the challenge message according to rules in header field of the challenge message according to rules in
Section 3.6.4 of [RFC5322]. Section 3.6.4 of [RFC5322].
6. Media type of the "response" email message is either text/plain 6. List-* header fields [RFC4021][RFC8058] MUST be absent (i.e., the
or multipart/alternative containing text/plain as one of the reply can't come from a mailing list)
alternatives. The text/plain body part (whether or not it is
7. The 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 inside multipart/alternative) MUST contain a block of lines
starting with the line "-----BEGIN ACME RESPONSE-----", followed starting with the line "-----BEGIN ACME RESPONSE-----", followed
by one or more line containing base64url encoded SHA-256 digest by one or more line containing the base64url-encoded SHA-256
[FIPS180-4] of the key authorization, calculated based on token- digest [FIPS180-4] of the key authorization, calculated from
part1 (received over email) and token-part2 (received over concatenated token-part1 (received over email) and token-part2
HTTPS). (Note that due to historic line length limitations in (received over HTTPS). See the 3rd bullet point in Section 3 for
email, line endings (CRLFs) can be freely inserted in the middle more details. (Note that due to historical line length
of the encoded digest, so they MUST be ignored when processing limitations in email, line endings (CRLFs) can be freely inserted
it.). The final line of the encoded digest is followed by the in the middle of the encoded digest, so they MUST be ignored when
line containing "-----END ACME RESPONSE-----". Any text before processing it.) The final line of the encoded digest is followed
and after this block is ignored. For example such text might by a line containing "-----END ACME RESPONSE-----". Any text
explain what to do with it for ACME-unaware clients. before and after this block is ignored. For example such text
might explain what to do with it for ACME-unaware clients.
7. There is no need to use any Content-Transfer-Encoding other than 8. There is no need to use any Content-Transfer-Encoding other than
7bit for the text/plain body part, however use of Quoted- 7bit for the text/plain body part, however use of Quoted-
Printable or base64 is not prohibited in a "response" email Printable or base64 is not prohibited in a "response" email
message. message.
8. In order to prove authenticity of a response message, it MUST be 9. In order to prove authenticity of a response message, it MUST be
DKIM [RFC6376] signed. The resulting DKIM-Signature header field DKIM [RFC6376] signed. The resulting DKIM-Signature header field
MUST contain the "h=" tag that includes at least "From", MUST contain the "h=" tag that includes at least "From",
"Sender", "Reply-To", "To", "CC", "Subject", "Date", "In-Reply- "Sender", "Reply-To", "To", "CC", "Subject", "Date", "In-Reply-
To", "References", "Message-ID", "Content-Type" and "Content- To", "References", "Message-ID", "Content-Type", and "Content-
Transfer-Encoding" header fields. Transfer-Encoding" header fields. The message MUST also pass
DMARC validation [RFC7489], which implies DKIM and SPF validation
[RFC7208].
Example ACME "response" email (note that DKIM related header fields Example ACME "response" email (note that DKIM related header fields
are not included for simplicity). are not included for simplicity).
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-enough-entropy> Subject: Re: ACME: <base64url-encoded-token-with-enough-entropy>
Content-Type: text/plain Content-Type: text/plain
skipping to change at page 7, line 32 skipping to change at page 8, line 19
+----------------+-----------------+------+-----------+ +----------------+-----------------+------+-----------+
6. Security Considerations 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.
I.e. ACME server is only vouching that the requested email address
seem to belong to the entity that requested the certificate.
Security of "email-reply-00" challenge type depends on security of The security of the "email-reply-00" challenge type depends on the
email system. A third party that can read and reply to user's email security of the email system. A third party that can read and reply
messages (by possessing a user's password or a secret derived from it to user's email messages (by possessing a user's password or a secret
that can give read and reply access, such as "password equivalent" derived from it that can give read and reply access, such as
information; or by being given permissions to act on user's behalf "password equivalent" information; or by being given permissions to
using email delegation feature common in some email systems) can act on a user's behalf using email delegation feature common in some
request S/MIME certificates and is indistinguishable from the email email systems) can request S/MIME certificates using the protocol
account owner. specified in this document and is indistinguishable from the email
account owner. This has several possible implications:
Email system in its turn depends on DNS. A third party that can 1. an entity that compromised an email account would be able to
request S/MIME certificates using the protocol specified in this
document and such entity couldn't be distinguished from the
legitimate email account owner (unless some external sources of
information are consulted);
2. for email addresses with legitimate shared access/control by
multiple users, any such user would be able to request S/MIME
certificates using the protocol specified in this document and
such requests can't be attributed to a specific user without
consulting external systems (such as IMAP/SMTP access logs);
3. protocol specified in this document is not suitable for use with
email addresses associated with mailing lists [RFC5321].
An 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.
7. Normative References 7. Normative References
[FIPS180-4] [FIPS180-4]
National Institute of Standards and Technology, "Secure National Institute of Standards and Technology, "Secure
skipping to change at page 8, line 37 skipping to change at page 9, line 37
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000, DOI 10.17487/RFC2818, May 2000,
<https://www.rfc-editor.org/info/rfc2818>. <https://www.rfc-editor.org/info/rfc2818>.
[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>.
[RFC2986] Nystrom, M. and B. Kaliski, "PKCS #10: Certification
Request Syntax Specification Version 1.7", RFC 2986,
DOI 10.17487/RFC2986, November 2000,
<https://www.rfc-editor.org/info/rfc2986>.
[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>.
[RFC4021] Klyne, G. and J. Palme, "Registration of Mail and MIME
Header Fields", RFC 4021, DOI 10.17487/RFC4021, March
2005, <https://www.rfc-editor.org/info/rfc4021>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006, Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>. <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,
skipping to change at page 9, line 29 skipping to change at page 10, line 37
[RFC7208] Kitterman, S., "Sender Policy Framework (SPF) for [RFC7208] Kitterman, S., "Sender Policy Framework (SPF) for
Authorizing Use of Domains in Email, Version 1", RFC 7208, Authorizing Use of Domains in Email, Version 1", RFC 7208,
DOI 10.17487/RFC7208, April 2014, DOI 10.17487/RFC7208, April 2014,
<https://www.rfc-editor.org/info/rfc7208>. <https://www.rfc-editor.org/info/rfc7208>.
[RFC7489] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based [RFC7489] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
Message Authentication, Reporting, and Conformance Message Authentication, Reporting, and Conformance
(DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015, (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
<https://www.rfc-editor.org/info/rfc7489>. <https://www.rfc-editor.org/info/rfc7489>.
[RFC8058] Levine, J. and T. Herkula, "Signaling One-Click
Functionality for List Email Headers", RFC 8058,
DOI 10.17487/RFC8058, January 2017,
<https://www.rfc-editor.org/info/rfc8058>.
[RFC8398] Melnikov, A., Ed. and W. Chuang, Ed., "Internationalized [RFC8398] Melnikov, A., Ed. and W. Chuang, Ed., "Internationalized
Email Addresses in X.509 Certificates", RFC 8398, Email Addresses in X.509 Certificates", RFC 8398,
DOI 10.17487/RFC8398, May 2018, DOI 10.17487/RFC8398, May 2018,
<https://www.rfc-editor.org/info/rfc8398>. <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>.
skipping to change at page 10, line 7 skipping to change at page 12, line 7
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 [RFC8616] Levine, J., "Email Authentication for Internationalized
Mail", RFC 8616, DOI 10.17487/RFC8616, June 2019, Mail", RFC 8616, DOI 10.17487/RFC8616, June 2019,
<https://www.rfc-editor.org/info/rfc8616>. <https://www.rfc-editor.org/info/rfc8616>.
Appendix A. Acknowledgements Appendix A. Acknowledgements
Thank you to Andreas Schulze, Gerd v. Egidy, James A Baker, Ben Thank you to Andreas Schulze, Gerd v. Egidy, James A. Baker, Ben
Schwartz and Michael Jenkins for suggestions, comments and Schwartz, Peter Yee and Michael Jenkins for suggestions, comments,
corrections on this document. 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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