Network Working Group                                        A. Melnikov
Internet-Draft                                                 Isode Ltd
Intended status: Informational                          October 28,                         November 11, 2017
Expires: May 1, 15, 2018

Extensions to Automatic Certificate Management Environment for email TLS
                      draft-ietf-acme-email-tls-01
                      draft-ietf-acme-email-tls-02

Abstract

   This document specifies identifiers and challenges required to enable
   the Automated Certificate Management Environment (ACME) to issue
   certificates for use by TLS email services.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   2
   3.  Use of ACME for use by TLS-protected SMTP and IMAP services .   2
     3.1.  "service" JWS header parameter  . . . . . . . . . . . . .   3
     3.2.  "port" JWS header parameter . . . . . . . . . . . . . . .   3   4
     3.3.  DNS challenge for email services  . . . . . . . . . . . .   4
     3.4.  CAPABILITY challenge for email services . . . . . . . . .   4
   4.  Open Issues . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5   6
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6   7

1.  Introduction

   [I-D.ietf-acme-acme] 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 extensions to ACME for use by email services.
   Section 3 defines extensions for how email services (such as SMTP,
   IMAP) can get certificates for use with TLS.

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 use by TLS-protected SMTP and IMAP services

   SMTP [RFC5321] (including SMTP Submission [RFC6409]) and IMAP
   [RFC3501] servers use TLS [RFC5246] to provide server identity
   authentication, data confidentiality and integrity services.  Such
   TLS protected email services either use STARTTLS command or run on a
   separate TLS-protected port .

   [I-D.ietf-acme-acme] defines several challenge types that can be
   extended for use by email services.  This document also defines some
   new challenge types specific to SMTP and IMAP.

   In order to use these challenges JWS [RFC7515] object used by
   [I-D.ietf-acme-acme] is extended.  The following extra requirements
   are in addition to requirements on JWS objects sent in ACME defined
   in Section 6.2 of [I-D.ietf-acme-acme]:

   1.  "service" JWS header parameter MUST be included.  See Section 3.1
       for more details.

   2.  "port" JWS header parameter MUST (SHOULD?) SHOULD be included.  See Section 3.2
       for more details.

   For example, if the ACME client were to respond to the "dns-email-00"
   challenge, it would send the following request:

      POST /acme/authz/asdf/0 HTTP/1.1
      Host: example.com
      Content-Type: application/jose+json

      {
        "protected": base64url({
          "alg": "ES256",
          "kid": "https://example.com/acme/acct/1",
          "nonce": "Q_s3MWoqT05TrdkM2MTDcw",
          "url": "https://example.com/acme/authz/asdf/0",
          "service": "smtp",
          "port": 25
        }),
        "payload": base64url({
          "type": "dns-email-00",
          "keyAuthorization": "IlirfxKKXA...vb29HhjjLPSggQiE"
        }),
        "signature": "7cbg5JO1Gf5YLjjF...SpkUfcdPai9uVYYU"
      }

                                 Figure 1

3.1.  "service" JWS header parameter

   The "service" JWS header parameter specifies the service for which
   TLS server certificate should be issued.  Valid values come from
   "Service Names and Transport Protocol Port Numbers" IANA registry
   <https://www.iana.org/assignments/service-names-port-numbers/service-
   names-port-numbers.xhtml>.

   ACME server MAY include SRV-ID [RFC6125]
   subjectAltNames in issued certificates. servers compliant with this specification MUST support [RFC7817]
   (in particular see Section 4 of that document).

   [[This parameter might have applicability beyond email services.]]

3.2.  "port" JWS header parameter

   The "port" JWS header parameter specifies the TCP port number where
   the corresponding service is running.

   [[This parameter might have applicability beyond email services.]]

3.3.  DNS challenge for email services

   "dns-email-00" is very similar to "dns-01" defined in Section 8.4 of
   [I-D.ietf-acme-acme].

   The difference between processing of "dns-email-00" and "dns-01" are
   listed below:

   1.  The TXT record used to validate this challenge is
       _<port>._<service>_acme-challenge.<domain>.
       _<port>._<service>._acme-challenge.<domain>.  For example, for
       domain "example.com" and IMAP service running on port 993, the
       TXT record name is _993._imaps._acme-challenge.example.com.  For
       domain "example.net" and IMAP service running on port 143, the
       TXT record name is _143._imap._acme-challenge.example.next.

   2.  [[OPEN ISSUE: Should service name and port number be incorporated
       into the hash?]]

3.4.  CAPABILITY challenge for email services

   For "capability-smtp-00" challenge, ACME client (== SMTP server)
   constructs a key authorization from the "token" value provided in the
   challenge and the client's account key.  The client then computes the
   SHA-256 digest [FIPS180-4] of the key authorization.  SMTP server
   than returns the base64url encoding of this digest as a value of the
   "ACME" EHLO capability.  For example:

   250-smtp.example.com
   250-SIZE
   250-8BITMIME
   250-BINARYMIME
   250-PIPELINING
   250-HELP
   250-DSN
   250-CHUNKING
   250-AUTH SCRAM-SHA-1
   250-AUTH=SCRAM-SHA-1
   250-STARTTLS
   250-ACME gfj9Xq...Rg85nM
   250-MT-PRIORITY
   250 ENHANCEDSTATUSCODES

   Note that in the above example only presence of the ACME and possibly
   STARTTLS capabilities is relevant
   as far as this document is concerned.

                                 Figure 2

   Similarly, "capability-imap-00" challenge, ACME client (== IMAP
   server) constructs a key authorization from the "token" value
   provided in the challenge and the client's account key.  The client
   then computes the SHA-256 digest [FIPS180-4] of the key
   authorization.  SMTP  IMAP server than returns the base64url encoding of
   this digest as a value of the "ACME" capability:

* OK [CAPABILITY IMAP4rev1 LOGINDISABLED LITERAL+ ENABLE STARTTLS ACME=gfj9Xq...Rg85nM] Example IMAP4rev1 server ready

 or

* CAPABILITY IMAP4rev1 LOGINDISABLED LITERAL+ ENABLE STARTTLS ACME=gfj9Xq...Rg85nM

   Note that in the above example only presence of the ACME and possibly
   STARTTLS capabilities capability
   token is relevant as far as this document is concerned.

                                 Figure 3

4.  Open Issues

   [[This section should be empty before publication]]

   1.  Should the same certificate be allowed to be used on both IMAP
       (143) and IMAPS (993) ports?  (These ports have different service
       names associated with them.  Is 1 service/port per ACME
       certificate a restriction imposed by this document?)

   2.  Add support for LMTP? LMTP (RFC 2033)?
   3.  One possible alternative for issuing TLS certificates for email
       services is to define a new Identifier Type that specifies
       service@domain.  The current version of the document just reuses
       "dns".

5.  IANA Considerations

   IANA is requested to register the following ACME challenge types that
   are used with Identifier Type "dns": "dns-email", "capability-smtp"
   and "capability-imap".  The reference for all of them 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-06 (work in progress), March 2017.

   [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>.

   [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
              <https://www.rfc-editor.org/info/rfc3501>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/info/rfc5246>.

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

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
              2011, <https://www.rfc-editor.org/info/rfc6125>.

   [RFC6409]  Gellens, R. and J. Klensin, "Message Submission for Mail",
              STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011,
              <https://www.rfc-editor.org/info/rfc6409>.

   [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>.

   [RFC7817]  Melnikov, A., "Updated Transport Layer Security (TLS)
              Server Identity Check Procedure for Email-Related
              Protocols", RFC 7817, DOI 10.17487/RFC7817, March 2016,
              <https://www.rfc-editor.org/info/rfc7817>.

Author's Address

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

   EMail: Alexey.Melnikov@isode.com