wdiff rfc8823.original rfc8823.txt

Network Working Group

Internet Engineering Task Force (IETF) A. Melnikov
Internet-Draft
Request for Comments: 8823 Isode Ltd
Intended status:
Category: Informational February 15, 2021
Expires: August 19, April 2021
ISSN: 2070-1721

Extensions to Automatic Certificate Management Environment for end-user End-User
S/MIME certificates
draft-ietf-acme-email-smime-14 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 Internet-Draft document is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents not an Internet Standards Track specification; it is
published for informational purposes.

This document is a product of the Internet Engineering Task Force
(IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list It represents the consensus of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents valid
approved by the IESG are candidates for a maximum any level of Internet
Standard; see Section 2 of RFC 7841.

Information about the current status of six months this document, any errata,
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This Internet-Draft will expire on August 19, 2021.
https://www.rfc-editor.org/info/rfc8823.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . 3
3. Use of ACME for issuing end-user Issuing End-User S/MIME certificates . . . . 3 Certificates
3.1. ACME challenge email . . . . . . . . . . . . . . . . . . 5 "Challenge" Email
3.2. ACME response email . . . . . . . . . . . . . . . . . . . 7 "Response" Email
3.3. Generating encryption only Encryption-Only or signing only Signing-Only S/MIME
certificates . . . . . . . . . . . . . . . . . . . . . . 9
Certificates
4. Internationalization Considerations . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
5.1. ACME Identifier Type . . . . . . . . . . . . . . . . . . 10
5.2. ACME Challenge Type . . . . . . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.1. Normative References . . . . . . . . . . . . . . . . . . 12
7.2. Informative References . . . . . . . . . . . . . . . . . 14
Appendix A.
Acknowledgements . . . . . . . . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 15

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) which that has a built-in ACME client which 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
user interface to the user and automate certificate issuance.

2. A An MUA which that is not ACME aware, with a separate ACME client
implemented in a command line 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 [RFC2119].
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

3. Use of ACME for issuing end-user Issuing End-User S/MIME certificates Certificates

ACME [RFC8555] defines a "dns" Identifier Type 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 identifier type that
proves ownership of an email address.

This document defines a new Identifier Type "email" 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 other 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" type, "email-reply-00", is used with the "email" Identifier
Type,
identifier type, which provides proof that an ACME client has control
over an email address.

The process of issing 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 end user initiates issuance of an S/MIME certificate for one
of her their email addresses. This might be done by using an email
client UI, by running a command line command-line tool, by visiting a Certificate
Authority
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 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 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 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
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. 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 which that
indicates that it is an email reply, e.g. 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", "token" and calculates keyAuthorization (as per Section 8.1
of
[RFC8555]), then [RFC8555]). Then, it returns the base64url encoded base64url-encoded SHA-256
digest
[FIPS180-4] [RFC6234] of the key authorization. The MUA returns the
base64url encoded
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. Such 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 signing-only or encryption 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]. [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. 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 "Challenge" Email

A "challenge" email message MUST have the following structure:

1. The message Subject header field has the following syntax: "ACME:
<token-part1>", <token-
part1>", where the prefix "ACME:" is followed by folding white
space (FWS, (FWS; see [RFC5322]) and then by <token-part1>, which is
the base64url encoded 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 line-length limit in [RFC5322], the subject line can be
folded, so whitespaces white spaces (if any) within the <token-part1> MUST be
ignored. [RFC2231] encoding of the message Subject header field MUST be
supported, and and, when used, only the "UTF-8" and "US-ASCII"
charsets are allowed: allowed; other charsets MUST NOT be used. US-ASCII 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 challange 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 fields. field.

5. The message MUST include the "Auto-Submitted: auto-generated" Auto-Submitted header field with the
value "auto-generated" [RFC3834]. To aid in debugging (and in (and, for
some
implementations implementations, to make automated processing easier) easier), the "Auto-
Submitted"
Auto-Submitted header field SHOULD include the "type=acme"
parameter. It MAY include other optional parameters 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 DKIM 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 "From",
"Sender", "Reply-To", "To", "CC", "Subject", "Date", "In-
Reply-To", "References", "Message-ID", "Auto-Submitted",
"Content-Type", the
From, Sender, Reply-To, To, CC, Subject, Date, In-Reply-To,
References, Message-ID, Auto-Submitted, Content-Type, and "Content-Transfer-Encoding"
Content-Transfer-Encoding header fields. The DKIM-Signature
header field's "h=" tag SHOULD also include
"Resent-Date", "Resent-From", "Resent-To", "Resent-Cc", "List-
Id", "List-Help", "List-Unsubscribe", "List-Subscribe", "List-
Post", "List-Owner", "List-Archive" 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" List-Unsubscribe-Post header fields. The domain
from the "d=" tag of DKIM-
Signature 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 (However, there are
extra requirements on S/MIME signing, if used. See below.)
Typically
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 any a "response"
email. To aid in debugging debugging, such failed validations SHOULD be
logged.

Here is an example of an ACME "challenge" email (note that that, for simplicity DKIM
related
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 "Response" Email

A valid "response" email message MUST have the following structure:

1. The message 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 folding white space
(FWS, see FWS (see [RFC5322]),
as is normal in reply messages. When parsing the subject, ACME
servers MUST decode [RFC2231] encoding (if any) any), and then they
can ignore any prefix before the "ACME:" label.

2. The From: From header field contains the email address of the user that
is requesting S/MIME certificate issuance.

3. The To: To header field of the response contains the value from the
Reply-To:
Reply-To header field from the challenge message (if set) or set).
Otherwise, it contains the value from the From: From header field of
the challenge message otherwise. message.

4. The Cc: Cc header field is ignored if present in the "response" email
message.

5. The In-Reply-To: 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) list).

7. The media type of the "response" email message is either text/
plain or multipart/alternative [RFC2046] [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
MUAs, which can't always generate pure text/plain, e.g. 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 line lines containing the base64url-encoded SHA-256
digest [FIPS180-4] [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
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.) it. The final line of the encoded
digest is followed by a line containing "-----END containing:

-----END ACME
RESPONSE-----". RESPONSE-----

Any text before and after this block is ignored. For example example,
such text might explain what to do with it for ACME-
unaware 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 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 "From",
"Sender", "Reply-To", "To", "CC", "Subject", "Date", "In-Reply-
To", "References", "Message-ID", "Content-Type" the From,
Sender, Reply-To, To, CC, Subject, Date, In-Reply-To, References,
Message-ID, Content-Type, and "Content-
Transfer-Encoding" Content-Transfer-Encoding header
fields. The DKIM-Signature header field's "h=" tag SHOULD also
include "Resent-Date", "Resent-
From", "Resent-To", "Resent-Cc", "List-Id", "List-Help", "List-
Unsubscribe", "List-Subscribe", "List-Post", "List-Owner", "List-
Archive" 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" 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.

Example

Here is an example of an ACME "response" email (note that that, for simplicity DKIM related
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 Encryption-Only or signing only Signing-Only S/MIME certificates Certificates

ACME extensions specified in this document can be used to request
signing only
signing-only or encryption only encryption-only S/MIME certificates.

In order to request signing only signing-only S/MIME certificate, 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 encryption-only S/MIME certificate, 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 internationalized email
addresses [RFC6531]. Please consult RFC 8616 [RFC8616] in regards to any
changes that need to be implemented.

Use of non ASCII non-ASCII characters in left hand left-hand sides of Internationalized
Email internationalized
email addresses requires putting Internationalized Email Addresses internationalized email addresses in
X.509 Certificates certificates [RFC8398].

5. IANA Considerations

5.1. ACME Identifier Type

IANA is requested to register has registered a new Identifier identifier type in the "ACME Identifier
Types" registry defined in Section 9.7.7 of [RFC8555] with Label
"email" and a Reference to [RFCXXXX], [RFC5321] this document, [RFC5321], and [RFC6531].
The new Identifier Type identifier type corresponds to an (all ASCII) email address
[RFC5321] or Internationalized Email internationalized email addresses [RFC6531].

5.2. ACME Challenge Type

IANA is also requested to register has registered a new entry in the "ACME Validation Methods"
registry defined in Section 9.7.8 of [RFC8555]. This entry is as
follows:

+----------------+-----------------+------+-----------+

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 of the address. It
fitness for purpose. If such claims are needed 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; information, or by being given permissions to
act on a user's behalf using email delegation feature 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 An entity that compromised an email account would be able to
request S/MIME certificates using the protocol specified in this
document
document, and such entity couldn't be distinguished from the
legitimate email account owner (unless some external sources of
information are consulted); consulted).

2. for 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 document; such
requests can't be attributed to a specific user without
consulting external systems (such as IMAP/SMTP access logs); logs).

3. the 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 S/
MIME certificate request is not from a mailing list address,
prohibition on inclusion of List-* header fields helps
Certificate Issuers
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 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, address -- for example 2 factor
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, authorization would be leaked, not the key
authorization itself, so no parts of the token or the 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

[FIPS180-4]
National Institute of Standards and Technology, "Secure
Hash Standard (SHS)", FIPS PUB 180-4, August 2015,
<https://csrc.nist.gov/publications/detail/fips/180/4/
final>.

[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>.

Appendix A.

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 Gillmor, and Benjamin Kaduk for their
suggestions, comments, and corrections on of this document.

Author's Address

Alexey Melnikov
Isode Ltd
14 Castle Mews
Hampton, Middlesex
TW12 2NP
UK

EMail:
United Kingdom

Email: alexey.melnikov@isode.com