Extensions to Automatic Certificate Management Environment for end-user S/MIME certificates
draft-ietf-acme-email-smime-09

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   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
     Message-ID: <A2299BB.FF7788@example.org>
     From: acme-generator@example.org
     To: alexey@example.com
     Subject: ACME: <base64url-encoded-token-with-64-bits-of-entropy>
     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 message Subject header field has the following syntax:
       "<Reply-prefix> ACME: <token-part1>", where <Reply-prefix> is
       typically the reply prefix "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 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> MUST be ignored.  [RFC2231] encoding of the 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).  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
       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.

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   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 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 the base64url-encoded SHA-256
       digest [FIPS180-4] of the key authorization, calculated from
       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 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, however use of Quoted-
       Printable or base64 is not prohibited in a "response" email
       message.

   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", and "Content-
       Transfer-Encoding" header fields.  The message MUST also pass
       DMARC validation [RFC7489], which implies DKIM and SPF validation
       [RFC7208].

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   Example ACME "response" email (note that DKIM related header fields
   are not included for simplicity).

      Date: Sat, 1 Sep 2018 11:12:00 +0100
      Message-ID: <111-22222-3333333@example.com>
      From: alexey@example.com
      To: acme-generator@example.org
      Subject: Re: ACME: <base64url-encoded-token-with-enough-entropy>
      Content-Type: text/plain
      MIME-Version: 1.0

      -----BEGIN ACME RESPONSE-----
      LoqXcYV8q5ONbJQxbmR7SCTNo3tiAXDfowy
      jxAjEuX0.9jg46WB3rR_AHD-EBXdN7cBkH1WOu0tA3M9
      fm21mqTI
      -----END ACME RESPONSE-----

                                 Figure 2

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:

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          +----------------+-----------------+------+-----------+
          |     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
   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 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.

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7.  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>.

   [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,
              <https://www.rfc-editor.org/info/rfc2045>.

   [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>.

   [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
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

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   [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>.

   [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>.

   [RFC7208]  Kitterman, S., "Sender Policy Framework (SPF) for
              Authorizing Use of Domains in Email, Version 1", RFC 7208,
              DOI 10.17487/RFC7208, April 2014,
              <https://www.rfc-editor.org/info/rfc7208>.

   [RFC7489]  Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
              Message Authentication, Reporting, and Conformance
              (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
              <https://www.rfc-editor.org/info/rfc7489>.

   [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
              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>.

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   [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>.

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Appendix A.  Acknowledgements

   Thank you to Andreas Schulze, Gerd v.  Egidy, James A.  Baker, Ben
   Schwartz, Peter Yee and Michael Jenkins for suggestions, comments,
   and corrections on this document.

Author's Address

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

   EMail: alexey.melnikov@isode.com

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