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Internet Engineering Task Force (IETF) A. Melnikov
Request for Comments: 8823 Isode Ltd
Category: Informational April 2021
ISSN: 2070-1721
Extensions to Automatic Certificate Management Environment for End-User
S/MIME Certificates
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
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8823.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction
2. Conventions Used in This Document
3. Use of ACME for Issuing End-User S/MIME Certificates
3.1. ACME "Challenge" Email
3.2. ACME "Response" Email
3.3. Generating Encryption-Only or Signing-Only S/MIME
Certificates
4. Internationalization Considerations
5. IANA Considerations
5.1. ACME Identifier Type
5.2. ACME Challenge Type
6. Security Considerations
7. References
7.1. Normative References
7.2. Informative References
Acknowledgements
Author's Address
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 it 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.
This document aims to support both:
1. A Mail User Agent (MUA) that has a built-in ACME client that is
aware of the extension described in this document. (We will call
such MUAs "ACME-email-aware".) Such an MUA can present a nice
user interface to the user and automate certificate issuance.
2. An MUA that is not ACME aware, with a separate ACME client
implemented in a command-line tool or as a part of a website.
While S/MIME certificate issuance is not going to be as painless
as in the case of the ACME-email-aware MUA, the extra burden on a
user is going to be minimal.
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Use of ACME for Issuing End-User S/MIME Certificates
ACME [RFC8555] defines a "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 email address. The address can be all ASCII
[RFC5321] or internationalized [RFC6531]; when an internationalized
email address is used, the domain part can contain both U-labels and
A-labels [RFC5890]. This can be used with S/MIME or another similar
service that requires possession of a certificate tied to an email
address.
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 the "email"
identifier type, which provides proof that an ACME client has control
over an email address.
The process of issuing an S/MIME certificate works as follows. Note
that the ACME client can be a standalone application (if the MUA is
not ACME-email-aware) or can be a component of the MUA.
1. An end user initiates issuance of an S/MIME certificate for one
of their email addresses. This might be done by using an email
client UI, by running a command-line tool, by visiting a
certificate authority web page, etc. This document doesn't
prescribe a specific UI used to initiate S/MIME certificate
issuance or where the ACME client is located.
2. The ACME-email-aware client component begins the certificate
issuance process by sending a POST request to the server's
newOrder resource, including the identifier of type "email".
See Section 7.4 of [RFC8555] for more details.
3. The ACME server responds to the POST request, including an
"authorizations" URL for the requested email address. The ACME
client then retrieves information about the corresponding
"email-reply-00" challenge, as specified in Section 7.5 of
[RFC8555]. The "token" field of the corresponding challenge
object (from the "challenges" array) contains token-part2.
token-part2 should contain at least 128 bits of entropy. The
"type" field of the challenge object is "email-reply-00". The
challenge object also contains the "from" field, with the email
address that would be used in the From header field of the
"challenge" email message (see the next step).
An example challenge object might look like this:
{
"type": "email-reply-00",
"url": "https://example.com/acme/chall/ABprV_B7yEyA4f",
"from": "acme-challenge+2i211oi1204310@example.com",
"token": "DGyRejmCefe7v4NfDGDKfA"
}
4. After responding to the authorization request, the ACME server
generates another token and a "challenge" email message with the
subject "ACME: <token-part1>", where <token-part1> is the
base64url-encoded [RFC4648] form of the token. The ACME server
MUST generate a fresh token for each S/MIME issuance request
(authorization request), and token-part1 MUST contain at least
128 bits of entropy. The "challenge" email message structure is
described in more details in Section 3.1.
5. The MUA retrieves and parses the "challenge" email message. If
the MUA is ACME-email-aware, it ignores any "challenge" email
that is not expected, e.g., if there is no ACME certificate
issuance pending. The ACME-email-aware MUA also ignores any
"challenge" email that has the Subject header field that
indicates that it is an email reply, e.g., a subject starting
with the reply prefix "Re:".
6. The ACME client concatenates "token-part1" (received over email)
and "token-part2" (received over HTTPS [RFC2818]) to create the
ACME "token" and calculates keyAuthorization (as per Section 8.1
of [RFC8555]). Then, it returns the base64url-encoded SHA-256
digest [RFC6234] of the key authorization. The MUA returns the
base64url-encoded SHA-256 digest obtained from the ACME client
in the body of a "response" email message. The "response" email
message structure is described in more details in Section 3.2.
If the MUA is ACME-email-aware, it MUST NOT respond to the same
"challenge" email more than once.
7. Once the MUA sends the "response" email, the ACME client
notifies the ACME server by POST to the challenge URL ("url"
field).
8. The ACME client can start polling the authorization URL (using
POST-as-GET requests) to see if the ACME server received and
validated the "response" email message. (See Section 7.5.1 of
[RFC8555] for more details.) If the "status" field of the
challenge switches to "valid", then the ACME client can proceed
with request finalization. The Certificate Signing Request
(CSR) MUST indicate the exact same set of requested identifiers
as the initial newOrder request. For an identifier of type
"email", the PKCS#10 [RFC2986] CSR MUST contain the requested
email address in an extensionRequest attribute [RFC2985]
requesting a subjectAltName extension. The email address MUST
also match the From header field value of the "response" email
message.
9. In order to request generation of signing-only or encryption-
only S/MIME certificates (as opposed to requesting generation of
S/MIME certificates suitable for both), the CSR needs to include
the key usage extension (see Section 4.4.2 of [RFC8550]). This
is described in more details in Section 3.3.
10. If a request to finalize an order is successful, the ACME server
will return a 200 (OK) with an updated order object. If the
certificate is issued successfully, i.e., if the order "status"
is "valid", then the ACME client can download the issued S/MIME
certificate from the URL specified in the "certificate" field.
3.1. ACME "Challenge" Email
A "challenge" email message MUST have the following structure:
1. The 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>, which is
the base64url-encoded first part of the ACME token that MUST be
at least 128 bits long after decoding. Due to the recommended
78-octet line-length limit in [RFC5322], the subject line can be
folded, so white spaces (if any) within the <token-part1> MUST be
ignored. [RFC2231] encoding of the Subject header field MUST be
supported, and, when used, only the "UTF-8" and "US-ASCII"
charsets are allowed; other charsets MUST NOT be used. The US-
ASCII charset SHOULD be used.
2. The From header field MUST be the same email address as specified
in the "from" field of the challenge object.
3. The To header field MUST be the email address of the entity that
requested the S/MIME certificate to be generated.
4. The message MAY contain a Reply-To and/or CC header field.
5. The message MUST include the Auto-Submitted header field with the
value "auto-generated" [RFC3834]. To aid in debugging (and, for
some implementations, to make automated processing easier), the
Auto-Submitted header field SHOULD include the "type=acme"
parameter. It MAY include other optional parameters, as allowed
by the syntax of the Auto-Submitted header field.
6. In order to prove authenticity of a challenge message, it MUST be
signed using either DomainKeys Identified Mail (DKIM) [RFC6376]
or S/MIME [RFC8551].
* If DKIM signing is used, the resulting DKIM-Signature header
field MUST contain the "h=" tag that includes at least the
From, Sender, Reply-To, To, CC, Subject, Date, In-Reply-To,
References, Message-ID, Auto-Submitted, Content-Type, and
Content-Transfer-Encoding header fields. The DKIM-Signature
header field's "h=" tag SHOULD also include the Resent-Date,
Resent-From, Resent-To, Resent-Cc, List-Id, List-Help, List-
Unsubscribe, List-Subscribe, List-Post, List-Owner, List-
Archive, and List-Unsubscribe-Post header fields. The domain
from the "d=" tag of the DKIM-Signature header field MUST be
the same as the domain from the From header field of the
"challenge" email.
* If S/MIME signing is used, the certificate corresponding to
the signer MUST have an rfc822Name subjectAltName extension
with the value equal to the From header field email address of
the "challenge" email.
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 a human-
readable explanation of the purpose of the message. If S/MIME
signing is used to prove authenticity of the challenge message,
then the multipart/signed or "application/pkcs7-mime; smime-
type=signed-data;" media type should be used. Either way, it
MUST use S/MIME header protection.
An email client compliant with this specification that detects that a
particular "challenge" email fails the validation described above
MUST ignore the challenge and thus will not generate a "response"
email. To aid in debugging, such failed validations SHOULD be
logged.
Here is an example of an ACME "challenge" email (note that, for
simplicity, DKIM-related header fields are not included).
Auto-Submitted: auto-generated; type=acme
Date: Sat, 5 Dec 2020 10:08:55 +0100
Message-ID: <A2299BB.FF7788@example.org>
From: acme-generator@example.org
To: alexey@example.com
Subject: ACME: LgYemJLy3F1LDkiJrdIGbEzyFJyOyf6vBdyZ1TG3sME=
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
3.2. ACME "Response" Email
A valid "response" email message MUST have the following structure:
1. The Subject header field is formed as a reply to the ACME
"challenge" email (see Section 3.1). Its syntax is the same as
that of the challenge message except that it may be prefixed by a
US-ASCII reply prefix (typically "Re:") and FWS (see [RFC5322]),
as is normal in reply messages. When parsing the subject, 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 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).
Otherwise, it contains the value from the From header field of
the challenge message.
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. List-* header fields [RFC4021][RFC8058] MUST be absent (i.e., the
reply can't come from a mailing list).
7. The media type of the "response" email message is either text/
plain or multipart/alternative [RFC2046], containing text/plain
as one of the alternatives. (Note that the requirement to
support multipart/alternative is to allow use of ACME-unaware
MUAs, which can't always generate pure text/plain, e.g., if they
reply to a text/html). 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 lines containing the base64url-encoded SHA-256
digest [RFC6234] of the key authorization, calculated from
concatenated token-part1 (received over email) and token-part2
(received over HTTPS), as outlined in the 5th bullet in
Section 3. (Note that each line of text/plain is terminated by
CRLF. Bare LFs or bare CRs are not allowed.) Due to historical
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 a 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.
8. There is no need to use any Content-Transfer-Encoding other than
7bit for the text/plain body part. Use of quoted-printable or
base64 in a "response" email message is not necessary and should
be avoided, though it is permitted.
9. 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 the From,
Sender, Reply-To, To, CC, Subject, Date, In-Reply-To, References,
Message-ID, Content-Type, and Content-Transfer-Encoding header
fields. The DKIM-Signature header field's "h=" tag SHOULD also
include the Resent-Date, Resent-From, Resent-To, Resent-Cc, List-
Id, List-Help, List-Unsubscribe, List-Subscribe, List-Post, List-
Owner, List-Archive, and List-Unsubscribe-Post header fields.
The domain from the "d=" tag of DKIM-Signature header field MUST
be the same as the domain from the From header field of the
"response" email.
Here is an example of an ACME "response" email (note that, for
simplicity, DKIM-related header fields are not included).
Date: Sat, 5 Dec 2020 12:01:45 +0100
Message-ID: <111-22222-3333333@example.com>
In-Reply-To: <A2299BB.FF7788@example.org>
From: alexey@example.com
To: acme-generator@example.org
Subject: Re: ACME: LgYemJLy3F1LDkiJrdIGbEzyFJyOyf6vBdyZ1TG3sME=
Content-Type: text/plain
MIME-Version: 1.0
-----BEGIN ACME RESPONSE-----
LoqXcYV8q5ONbJQxbmR7SCTNo3tiAXDfowy
jxAjEuX0=
-----END ACME RESPONSE-----
Figure 2
3.3. Generating Encryption-Only or Signing-Only S/MIME Certificates
ACME extensions specified in this document can be used to request
signing-only or encryption-only S/MIME certificates.
In order to request signing-only S/MIME certificates, the CSR MUST
include the key usage extension with digitalSignature and/or
nonRepudiation bits set and no other bits set.
In order to request encryption-only S/MIME certificates, the CSR MUST
include the key usage extension with keyEncipherment or keyAgreement
bits set and no other bits set.
Presence of both of the above sets of key usage bits in the CSR, as
well as absence of the key usage extension in the CSR, signals to the
ACME server to issue an S/MIME certificate suitable for both signing
and encryption.
4. Internationalization Considerations
[RFC8616] updated/clarified use of DKIM with internationalized email
addresses [RFC6531]. Please consult [RFC8616] 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
5.1. ACME Identifier Type
IANA has registered 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 this document, [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 has registered 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 | RFC 8823 |
+----------------+-----------------+------+-----------+
Table 1
6. Security Considerations
Please see the Security Considerations section of [RFC8555] for
general security considerations related to the 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. The ACME server is confirming
that the requested email address belongs to the entity that requested
the certificate, but this makes no claim to address correctness or
fitness for purpose. If such claims are needed, they must be
obtained by some other mechanism.
The security of the "email-reply-00" challenge type depends on the
security of the 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 a user's behalf using email delegation features common in some
email systems) can request S/MIME certificates using the protocol
specified in this document and is indistinguishable from the email
account owner. This has several possible implications:
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; such
requests can't be attributed to a specific user without
consulting external systems (such as IMAP/SMTP access logs).
3. The protocol specified in this document is not suitable for use
with email addresses associated with mailing lists [RFC5321].
While it is not always possible to guarantee that a particular S/
MIME certificate request is not from a mailing list address,
prohibition on inclusion of List-* header fields helps
certificate issuers to handle most common cases.
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 an
email and can get (at least temporary) read and reply access to it.
Similar considerations apply to DKIM TXT records in DNS. Use of
DNSSEC by email system administrators is recommended to avoid making
it easy to spoof DNS records affecting an email system. However, use
of DNSSEC is not ubiquitous at the time of publishing of this
document, so it is not required here. Also, many existing systems
that rely on verification of ownership of an email address -- for
example, 2-factor authentication systems used by banks or traditional
certificate issuance systems -- send email messages to email
addresses, expecting the owner to click on the link supplied in them
(or to reply to a message), without requiring use of DNSSEC. So the
risk of not requiring DNSSEC is presumed acceptable in this document.
An ACME email challenge message can be forged by an attacker. As per
requirements on an ACME-email-aware MUA specified in Section 3, the
MUA will not respond to requests it is not expecting. Even if the
attacker causes the erroneous "response" email to go to an attacker-
controlled email address, very little information is leaked -- the
SHA-256 hash of the key authorization would be leaked, not the key
authorization itself, so no parts of the token or the account key
thumbprint are leaked.
An attacker that can read the "response" email has only one chance to
guess the token-part2. Even if the attacker can guess it right, it
still needs to know the ACME account key to be able to make use of
the intercepted SHA-256 hash of the key authorization.
Also see the Security Considerations section of [RFC6376] for details
on how DKIM depends on the DNS and the respective vulnerabilities
this dependence has.
7. References
7.1. Normative References
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
DOI 10.17487/RFC2046, November 1996,
<https://www.rfc-editor.org/info/rfc2046>.
[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>.
[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
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>.
[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>.
[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234,
DOI 10.17487/RFC6234, May 2011,
<https://www.rfc-editor.org/info/rfc6234>.
[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>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[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>.
7.2. Informative References
[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>.
[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>.
Acknowledgements
Thank you to Andreas Schulze, Gerd v. Egidy, James A. Baker, Ben
Schwartz, Peter Yee, Hilarie Orman, Michael Jenkins, Barry Leiba,
Fraser Tweedale, Daniel Kahn Gillmor, and Benjamin Kaduk for their
suggestions, comments, and corrections of this document.
Author's Address
Alexey Melnikov
Isode Ltd
14 Castle Mews
Hampton, Middlesex
TW12 2NP
United Kingdom
Email: alexey.melnikov@isode.com