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Versions: 00 draft-ietf-acme-email-tls

Network Working Group                                        A. Melnikov
Internet-Draft                                                 Isode Ltd
Intended status: Informational                              June 7, 2017
Expires: December 9, 2017


Extensions to Automatic Certificate Management Environment for email TLS
                    draft-melnikov-acme-email-tls-00

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.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on December 9, 2017.

Copyright Notice

   Copyright (c) 2017 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
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   described in the Simplified BSD License.





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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
     3.3.  TLS with Server Name Indication (SNI) challenge for email
           services  . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.4.  DNS challenge for email services  . . . . . . . . . . . .   4
     3.5.  CAPABILITY challenge for email services . . . . . . . . .   5
   4.  Open Issues . . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   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) and IMAP [RFC3501] servers
   use TLS 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



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   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?) be included.  See
       Section 3.2 for more details.

   For example, if the client were to respond to the "tls-sni-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": "tls-sni-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.

3.2.  "port" JWS header parameter

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




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

3.3.  TLS with Server Name Indication (SNI) challenge for email services

   "tls-sni-email-00" is very similar to "tls-sni-01" defined in
   Section 8.3 of [I-D.ietf-acme-acme].

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

   1.  SAN A MUST be constructed as follows: compute the SHA-256 digest
       [FIPS180-4] of the challenge token and encode it in lowercase
       hexadecimal form.  The dNSName is "<x>.<y>.<token>.acme.invalid",
       where <x> is the first half of the hexadecimal representation and
       <y> is the second half, and <token> was generated by the ACME
       server.  SAN B MUST be constructed as follows: compute the
       SHA-256 digest of the key authorization and encode it in
       lowercase hexadecimal form.  The dNSName is
       "<x>.<y>.<ka>.acme.invalid" where <x> is the first half of the
       hexadecimal representation and <x> is the second half, and <ka>
       is the key authorization.  [[OPEN ISSUE: Should service name and
       port number be incorporated into SAN A and B?]]

   2.  When verifying the client's control of the domain/service, ACME
       server connects to port as specified in "port" JWS header
       parameter (Section 3.2), instead of port 443.  When connecting to
       ports 25, 143 and 587, ACME server needs to use STARTTLS command.
       When connecting to ports 465 or 993, ACME server initiate TLS
       negotiation immediately upon connection to the corresponding
       ports.  In all cases ACME server presents SAN A in the SNI field,
       constructed as specified above.

3.4.  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>.  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.





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

3.5.  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:

   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

                                 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 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

                                 Figure 3








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4.  Open Issues

   [[This section should be empty before publication]]

   1.  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": "tls-sni-email", "dns-email",
   "capability-smtp" and "capability-imap".  The reference for all of
   them is this document.

6.  Security Considerations

   TBD.

7.  Normative References

   [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,
              <http://www.rfc-editor.org/info/rfc2119>.

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

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              DOI 10.17487/RFC5321, October 2008,
              <http://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, <http://www.rfc-editor.org/info/rfc6125>.





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

Author's Address

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

   EMail: Alexey.Melnikov@isode.com






































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