Network Working Group                                        A. Melnikov
Internet-Draft                                                 Isode Ltd
Intended status: Informational                          October 19, 2018
Expires: April 22,                              July 8, 2019
Expires: January 9, 2020

Extensions to Automatic Certificate Management Environment for end user
                          S/MIME certificates
                     draft-ietf-acme-email-smime-04
                     draft-ietf-acme-email-smime-05

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

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   This Internet-Draft will expire on April 22, 2019. January 9, 2020.

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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 S/MIME certificates  . . . .   2
     3.1.  ACME challenge email  . . . . . . . . . . . . . . . . . .   3
     3.2.  ACME response email . . . . . . . . . . . . . . . . . . .   4
   4.  Open Issues . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .   6
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   [I-D.ietf-acme-acme]

   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 [RFC5750] [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

   [I-D.ietf-acme-acme]

   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].  This can be used with S/MIME or other similar
   service that requires posession 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
   email address:

   1.  ACME server generates a "challenge" email message with the
       subject "ACME: <token-part1>", where <token-part1> is the
       base64url encoded 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
       [I-D.ietf-acme-acme]),
       [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 exactly
       one SP character. 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 octet long after decoding.  [[Alexey: is the following going to be problematic due  Due to the total subject lenght and recommended 78 octet
       line length
       limit?]] For ease of implementation, no RFC 2231 limit in [RFC5322], the subject line can be folded,
       so whitespaces (if any) within the <token-part1> MUST be ignored.
       [RFC2231] encoding of subject is allowed in the "challenge" email message. 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 message MUST include the "Auto-Submitted: auto-generated"
       header field [RFC3834].  It MAY include optional parameters as
       allowed by syntax of Auto-Submitted header field.

   3.  The  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", "To",
       "Subject", "Content-Type" and "Message-ID" header fields.

   4.  If S/MIME signing is not used to prove authenticity of the
       challenge message, then the message MUST have a single text/plain
       MIME body part [RFC2045], that contains human readable
       explanation of the purpose of the message.  If S/MIME signing is
       used, then the text/plain message is used to construct a
       multipart/signed or "application/pkcs7-mime; smime-type=signed-
       data;".  Either way, it MUST use S/MIME header protection.

   Example ACME "challenge" email

     Auto-Submitted: auto-generated
     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-octets-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 "response" email message MUST have the following structure:

   1.  The message Subject header field has the following syntax: "Re:
       ACME: <token-part1>", where the string "ACME:" is followed by
       exactly one SP character.  <token-part1>
       folding white space (FWS, see [RFC5322]) and then by <token-
       part1> is the base64url encoded first part of the ACME token.  (Note token that this would be a subject
       generated by any email client replying to the "challenge" email
       message.)  [[Alexey: is the following going to
       MUST be problematic due at least 64 octet long after decoding.  Due to the total subject lenght and
       recommended 78 octet line length
       limit?]] For ease of implementation, no RFC 2231 limit in [RFC5322], the subject
       line can be folded, so whitespaces (if any) within the <token-
       part1> MUST be ignored.  [RFC2231] encoding of subject is allowed in the "response" email message. 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
       From: header field of the challenge email.

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

   5.  The message MUST have a single text/plain MIME body part
       [RFC2045], 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 need to MUST be ignored when processing
       it.

   6.  There is no need to use any Content-Transfer-Encoding other than
       7bit, however use of Quoted-Printable or base64 is not prohibited
       in a "response" email message.

   Example ACME "response" email

   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-64-octets-of-entropy>
   Content-Type: text/plain
   MIME-Version: 1.0

   LoqXcYV8q5ONbJQxbmR7SCTNo3tiAXDfowy
   jxAjEuX0.9jg46WB3rR_AHD-EBXdN7cBkH1WOu0tA3M9
   fm21mqTI

                                 Figure 2

4.  Open Issues

   [[This section should be empty before publication]]

   1.  Do we need to handle text/html or multipart/alternative in email
       challenge?  Simplicity suggests "no".  However, for automated
       processing it might be better to use at least multipart/mixed
       with a special MIME type.

   2.  How to verify authenticity of "challenge" email messages?  The
       document can recommend either S/MIME signing (in which case the
       "challenge" email message should be allowed to be S/MIME signed
       multipart/signed or application/pkcs7-mime containing SignedData)
       and/or presence of DKIM signature covering at least From/To/
       Subject, with working SPF and DMARC.

   3.  Similarly to the above: how to verify authenticity of "response" email messages?  We
       can't require use of S/MIME, as this protocol is used for S/MIME
       user enrollment.  However DKIM/SPF/DMARC can be recommended.

   4.

   3.  Define a new parameter to "Auto-Submitted: auto-generated", so
       that it is easier to figure out that a particilar message is an
       ACME challenge message?

5.  IANA Considerations

   IANA is requested to register a new Identifier Type "email" which
   corresponds to an (all ASCII) email address [RFC5321] or
   Internationalized Email addresses [RFC6531].

   And finally, IANA is requested to register the following ACME
   challenge types that are used with Identifier Type "email": "email-
   reply".  The reference for it is this document.

6.  Security Considerations

   TBD.

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>.

   [I-D.ietf-acme-acme]
              Barnes, R., Hoffman-Andrews, J., and J. Kasten, "Automatic
              Certificate Management Environment (ACME)", draft-ietf-
              acme-acme-14 (work in progress), August 2018.

   [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>.

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              DOI 10.17487/RFC5321, October 2008,
              <https://www.rfc-editor.org/info/rfc5321>.

   [RFC5750]  Ramsdell, B.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008,
              <https://www.rfc-editor.org/info/rfc5322>.

   [RFC6376]  Crocker, D., Ed., Hansen, T., Ed., and S. Turner, "Secure/Multipurpose Internet M. Kucherawy, Ed.,
              "DomainKeys Identified Mail Extensions (S/MIME) Version 3.2 Certificate
              Handling", (DKIM) Signatures", STD 76,
              RFC 5750, 6376, DOI 10.17487/RFC5750, January 2010,
              <https://www.rfc-editor.org/info/rfc5750>. 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>.

   [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
              2015, <https://www.rfc-editor.org/info/rfc7515>.

   [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>.

Appendix A.  Acknowledgements

   Thank you to Andreas Schulze and Gerd v.  Egidy 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