Network Working Group A. Melnikov
Internet-Draft Isode Ltd
Intended status: Informational
June 16, October 27, 2020
Expires: December 18, 2020 April 30, 2021
Extensions to Automatic Certificate Management Environment for end user end-user
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
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provisions of BCP 78 and BCP 79.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . 2
3. Use of ACME for issuing end user end-user S/MIME certificates . . . . 2
3.1. ACME challenge email . . . . . . . . . . . . . . . . . . 3
3.2. ACME response email . . . . . . . . . . . . . . . . . . . 5
4. Internationalization Considerations . . . . . . . . . . . . . 6 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 7
5.1. ACME Identifier Type . . . . . . . . . . . . . . . . . . 7
5.2. ACME Challenge Type . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7 8
7. Normative References . . . . . . . . . . . . . . . . . . . . 8 9
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 10 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10 12
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 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 end-user S/MIME [RFC8550]
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Use of ACME for issuing end user 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 (all ASCII) email address [RFC5321] or Internationalized Email
addresses [RFC6531]. (When Internationalized Email addresses are
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
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 "email" Identifier
Type, which provides proof that an ACME client has control over an
1. An end-user initiates issuance of an S/MIME certificate for one
of her email addresses. This might be done using email client UI
(and might use some HTTP API underneath), by visiting a
Certificate Authority web page or by sending an email to a well
known Certificate Authority's email address. This document
doesn't prescribe how exactly S/MIME certificate issuance is
2. The ACME server (run by the Certificate Authority or their
authorized third party) generates a "challenge" email message
with the subject "ACME: <token-part1>", where <token-part1> is
the base64url encoded [RFC4648] first part of the token, which
contains at least 64 bit 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 bit 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 key-authz 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
email message containing a single text/plain MIME body part
[RFC2045]. The response email message structure is described in
more details in Section 3.2 3.2.
For an identifier of type "email", CSR the PKCS#10 [RFC2986] Certificate
Signing Request (CSR) MUST contain the request requested email address in an
extensionRequest attribute [RFC2985] requesting a subjectAltName
3.1. ACME challenge email
A "challenge" email message MUST have the following structure:
1. The message 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> <token-part1>, which
is the base64url encoded first part of the ACME token that MUST
be at least 64 bit bits long after decoding. Due to the recommended 78 octet
78-octet line length limit in [RFC5322], the subject line can be
folded, so whitespaces (if any) within the <token-part1> MUST be
ignored. [RFC2231] encoding of subject the message Subject header field
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 To header field MUST be the email address of the entity that
requested the S/MIME certificate to be generated.
3. The message MAY contain a Reply-To header field.
4. The message MUST include the "Auto-Submitted: auto-generated"
header field [RFC3834]. 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
5. The message MAY contain Reply-To header field.
6. In order to prove authenticity of a challenge message, it MUST be
either DKIM [RFC6376] signed or S/MIME [RFC8551] signed. 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", "Content-Type" "Content-Type", and "Content-Transfer-
Transfer-Encoding" header fields. The message MUST also pass
DMARC validation [RFC7489], which implies DKIM and SPF validation
6. 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 human 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-
type=signed-data;" media type should be used. Either way, it
MUST use S/MIME header protection.
An example ACME "challenge" email (note that DKIM related header
fields are not included for simplicity).
Auto-Submitted: auto-generated; type=acme
Date: Sat, 1 Sep 2018 10:08:55 +0100
Subject: ACME: <base64url-encoded-token-with-64-bits-of-entropy>
This is an automatically generated ACME challenge for email address
"email@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.
3.2. ACME response email
A valid "response" email message MUST have the following structure:
1. The message Subject header field has the following syntax:
"<Reply-prefix> ACME: <token-part1>", where <Reply-prefix> is
typically the reply prefix "Re: " "Re:" and the string "ACME:" is
preceded and followed by folding white space (FWS, see [RFC5322])
and then by <token-part1>. <token-part1> is the base64url encoded
first part of the ACME token (as received in the ACME challenge)
that MUST be at least 64 bit bits long after decoding. Due to
recommended 78 octet line length limit in [RFC5322], the subject
line can be folded, so whitespaces (if any) within the <token-part1> <token-
part1> MUST be ignored. [RFC2231] encoding of subject the Subject header
field MUST be supported, but when used, only "UTF-8" and "US-ASCII" "US-
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
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) or
from the From: header field of the challenge message otherwise.
4. The Cc: header field is ignored if present in the "response"
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. Media 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 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
starting with the line "-----BEGIN ACME RESPONSE-----", followed
by one or more line containing base64url encoded the base64url-encoded SHA-256
digest [FIPS180-4] of the key authorization, calculated based on token-
concatenated token-part1 (received over email) and token-part2
(received over HTTPS). See the 3rd bullet point in Section 3 for
more details. (Note that due to historic 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
it.). it.) The final line of the encoded digest is followed
by the 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, however use of Quoted-
Printable or base64 is not prohibited in a "response" email
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" "Content-Type", and "Content-
Transfer-Encoding" header fields. The message MUST also pass
DMARC validation [RFC7489], which implies DKIM and SPF validation
Example ACME "response" email (note that DKIM related header fields
are not included for simplicity).
Date: Sat, 1 Sep 2018 11:12:00 +0100
Subject: Re: ACME: <base64url-encoded-token-with-enough-entropy>
-----BEGIN ACME RESPONSE-----
-----END ACME RESPONSE-----
4. Internationalization Considerations
[RFC8616] updated/clarified use of DKIM/SPF/DMARC with
Internationalized Email addresses [RFC6531]. Please consult RFC 8616
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 is requested to register 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 [RFCXXXX], [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 is also requested to register a new entry in the "ACME
Validation Methods" registry defined in Section 9.7.8 of [RFC8555].
This entry is as follows:
| Label | Identifier Type | ACME | Reference |
| email-reply-00 | email | Y | [RFCXXXX] |
6. Security Considerations
Please see Security Considerations of [RFC8555] for general security
considerations related to 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. Any claims about the correctness or
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.
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 feature common in some
email systems) can request S/MIME certificates using the protocol
specified in this document and is indistinguishable from the email
Email 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 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
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.
7. Normative References
National Institute of Standards and Technology, "Secure
Hash Standard (SHS)", FIPS PUB 180-4, August 2015,
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
[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
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000,
[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,
[RFC2986] Nystrom, M. and B. Kaliski, "PKCS #10: Certification
Request Syntax Specification Version 1.7", RFC 2986,
DOI 10.17487/RFC2986, November 2000,
[RFC3834] Moore, K., "Recommendations for Automatic Responses to
Electronic Mail", RFC 3834, DOI 10.17487/RFC3834, August
[RFC4021] Klyne, G. and J. Palme, "Registration of Mail and MIME
Header Fields", RFC 4021, DOI 10.17487/RFC4021, March
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008,
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008,
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, DOI 10.17487/RFC5890, August 2010,
[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,
[RFC6531] Yao, J. and W. Mao, "SMTP Extension for Internationalized
Email", RFC 6531, DOI 10.17487/RFC6531, February 2012,
[RFC7208] Kitterman, S., "Sender Policy Framework (SPF) for
Authorizing Use of Domains in Email, Version 1", RFC 7208,
DOI 10.17487/RFC7208, April 2014,
[RFC7489] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
Message Authentication, Reporting, and Conformance
(DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
[RFC8058] Levine, J. and T. Herkula, "Signaling One-Click
Functionality for List Email Headers", RFC 8058,
DOI 10.17487/RFC8058, January 2017,
[RFC8398] Melnikov, A., Ed. and W. Chuang, Ed., "Internationalized
Email Addresses in X.509 Certificates", RFC 8398,
DOI 10.17487/RFC8398, May 2018,
[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,
[RFC8616] Levine, J., "Email Authentication for Internationalized
Mail", RFC 8616, DOI 10.17487/RFC8616, June 2019,
Appendix A. Acknowledgements
Thank you to Andreas Schulze, Gerd v. Egidy, James A A. Baker, Ben
Schwartz, Peter Yee and Michael Jenkins for suggestions, comments comments,
and corrections on this document.
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