Internet DRAFT - draft-ietf-acme-email-smime

draft-ietf-acme-email-smime







Network Working Group                                        A. Melnikov
Internet-Draft                                                 Isode Ltd
Intended status: Informational                             June 16, 2020
Expires: December 18, 2020


Extensions to Automatic Certificate Management Environment for end user
                          S/MIME certificates
                     draft-ietf-acme-email-smime-08

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 is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 18, 2020.

Copyright Notice

   Copyright (c) 2020 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.




Melnikov                Expires December 18, 2020               [Page 1]

Internet-Draft               ACME for S/MIME                   June 2020


Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   2
   3.  Use of ACME for issuing end user S/MIME certificates  . . . .   2
     3.1.  ACME challenge email  . . . . . . . . . . . . . . . . . .   3
     3.2.  ACME response email . . . . . . . . . . . . . . . . . . .   5
   4.  Internationalization Considerations . . . . . . . . . . . . .   6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  ACME Identifier Type  . . . . . . . . . . . . . . . . . .   7
     5.2.  ACME Challenge Type . . . . . . . . . . . . . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .   8
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  10

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 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.

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 S/MIME certificates

   ACME [RFC8555] defines "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.




Melnikov                Expires December 18, 2020               [Page 2]

Internet-Draft               ACME for S/MIME                   June 2020


   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
   email address:

   1.  ACME server 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 of entropy.  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 of entropy) is returned over HTTPS [RFC2818] to the
       ACME client.

   2.  ACME client concatenates "token-part1" and "token-part2" to
       create "token", calculates key-authz (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

   For an identifier of type "email", CSR MUST contain the request email
   address in an extensionRequest attribute [RFC2985] requesting a
   subjectAltName extension.

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> is the
       base64url encoded first part of the ACME token that MUST be at
       least 64 bit 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 subject 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 S/MIME certificate to be generated.

   3.  The message MAY contain Reply-To header field.




Melnikov                Expires December 18, 2020               [Page 3]

Internet-Draft               ACME for S/MIME                   June 2020


   4.  The message MUST include the "Auto-Submitted: auto-generated"
       header field [RFC3834].  The "Auto-Submitted" header field SHOULD
       include "type=acme" parameter.  It MAY include other optional
       parameters as allowed by syntax of 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
       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" and "Content-Transfer-
       Encoding" header fields.  The message MUST also pass DMARC
       validation [RFC7489], which implies DKIM and SPF validation
       [RFC7208].

   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 human readable
       explanation of the purpose of the message.  If S/MIME signing is
       used to prove authenticity of the challenge message, then
       multipart/signed or "application/pkcs7-mime; smime-type=signed-
       data;" media type should be used.  Either way, it MUST use S/MIME
       header protection.

   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



Melnikov                Expires December 18, 2020               [Page 4]

Internet-Draft               ACME for S/MIME                   June 2020


3.2.  ACME response email

   A "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
       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 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 subject 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 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"
       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.  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 base64url encoded SHA-256 digest
       [FIPS180-4] of the key authorization, calculated based on token-
       part1 (received over email) and token-part2 (received over
       HTTPS).  (Note that due to historic 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 the
       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.





Melnikov                Expires December 18, 2020               [Page 5]

Internet-Draft               ACME for S/MIME                   June 2020


   7.  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.

   8.  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.

   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








Melnikov                Expires December 18, 2020               [Page 6]

Internet-Draft               ACME for S/MIME                   June 2020


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.

   Security of "email-reply-00" challenge type depends on security of
   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 user's behalf
   using email delegation feature common in some email systems) can
   request S/MIME certificates and is indistinguishable from the email
   account owner.

   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.







Melnikov                Expires December 18, 2020               [Page 7]

Internet-Draft               ACME for S/MIME                   June 2020


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>.

   [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>.




Melnikov                Expires December 18, 2020               [Page 8]

Internet-Draft               ACME for S/MIME                   June 2020


   [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>.

   [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>.




Melnikov                Expires December 18, 2020               [Page 9]

Internet-Draft               ACME for S/MIME                   June 2020


Appendix A.  Acknowledgements

   Thank you to Andreas Schulze, Gerd v.  Egidy, James A Baker, Ben
   Schwartz 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




































Melnikov                Expires December 18, 2020              [Page 10]