[Docs] [txt|pdf|xml|html] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: (draft-newman-email-deep) 00 01 02 03 04 05 06 07 08 09 10 11 12 RFC 8314

Network Working Group                                           K. Moore
Internet-Draft                                          Network Heretics
Updates: 1939, 2595, 3464, 3501, 5068,                         C. Newman
6186, 6409 (if approved)                                          Oracle
Intended status: Standards Track                          March 10, 2016
Expires: September 11, 2016


                Deployable Enhanced Email Privacy (DEEP)
                    draft-ietf-uta-email-deep-02.txt

Abstract

   This specification defines a set of requirements and facilities
   designed to improve email confidentiality between a mail user agent
   (MUA) and a mail submission or mail access server.  This provides
   mechanisms intended to increase use of already deployed Transport
   Layer Security (TLS) technology, provide a model for mail user
   agent's confidentiality assurance, and enable mail service providers
   to advertise improved TLS confidentiality facilities.

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 http://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 September 11, 2016.

Copyright Notice

   Copyright (c) 2016 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
   (http://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



Moore & Newman         Expires September 11, 2016               [Page 1]


Internet-Draft  Deployable Enhanced Email Privacy (DEEP)      March 2016


   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.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Conventions and Terminology Used in This Document  . . . . . .  4
   3.  Mail Account Confidentiality Assurance Level . . . . . . . . .  5
     3.1.   High Confidentiality Assurance  . . . . . . . . . . . . .  6
     3.2.   No Confidentiality Assurance  . . . . . . . . . . . . . .  6
     3.3.   Other Confidentiality Assurance Levels  . . . . . . . . .  7
   4.  Implicit TLS . . . . . . . . . . . . . . . . . . . . . . . . .  7
     4.1.   Implicit TLS for POP  . . . . . . . . . . . . . . . . . .  7
     4.2.   Implicit TLS for IMAP . . . . . . . . . . . . . . . . . .  7
     4.3.   Implicit TLS for SMTP Submission  . . . . . . . . . . . .  8
     4.4.   Implicit TLS Connection Closure for POP, IMAP and SMTP  .  9
   5.  Email Security Upgrading Using Security Latches  . . . . . . .  9
     5.1.   Email Security Tags . . . . . . . . . . . . . . . . . . . 10
     5.2.   Initial Set of Email Security Tags  . . . . . . . . . . . 10
     5.3.   Server DEEP Status  . . . . . . . . . . . . . . . . . . . 10
     5.4.   Email Security Tag Latch Failures . . . . . . . . . . . . 11
   6.  Recording TLS Cipher Suite in Received Header  . . . . . . . . 11
   7.  Extensions for DEEP Status and Reporting . . . . . . . . . . . 12
     7.1.   IMAP DEEP Extension . . . . . . . . . . . . . . . . . . . 12
     7.2.   POP DEEP Extension  . . . . . . . . . . . . . . . . . . . 14
     7.3.   SMTP DEEP Extension . . . . . . . . . . . . . . . . . . . 15
     7.4.   SMTP Error Extension  . . . . . . . . . . . . . . . . . . 17
   8.  Account Setup Considerations . . . . . . . . . . . . . . . . . 17
     8.1.   Use of SRV records in Establishing Configuration  . . . . 17
     8.2.   Certificate Pinning . . . . . . . . . . . . . . . . . . . 18
   9.  Implementation Requirements  . . . . . . . . . . . . . . . . . 18
     9.1.   All Implementations (Client and Server) . . . . . . . . . 19
       9.1.1.  Client Certificate Authentication  . . . . . . . . . . 19
     9.2.   Mail Server Implementation Requirements . . . . . . . . . 20
     9.3.   Mail User Agent Implementation Requirements . . . . . . . 20
     9.4.   Non-configurable MUAs and nonstandard access protocols  . 21
     9.5.   DEEP Compliance for Anti-Virus/Anti-Spam Software and
            Services  . . . . . . . . . . . . . . . . . . . . . . . . 21
   10. Mail Service Provider Requirements . . . . . . . . . . . . . . 22
     10.1.  Server Requirements . . . . . . . . . . . . . . . . . . . 22
     10.2.  MSPs MUST provide Submission Servers  . . . . . . . . . . 22
     10.3.  TLS Server Certificate Requirements . . . . . . . . . . . 22
     10.4.  Recommended DNS records for mail protocol servers . . . . 23
       10.4.1. MX records . . . . . . . . . . . . . . . . . . . . . . 23
       10.4.2. SRV records  . . . . . . . . . . . . . . . . . . . . . 23



Moore & Newman         Expires September 11, 2016               [Page 2]


Internet-Draft  Deployable Enhanced Email Privacy (DEEP)      March 2016


       10.4.3. DNSSEC . . . . . . . . . . . . . . . . . . . . . . . . 23
       10.4.4. TLSA records . . . . . . . . . . . . . . . . . . . . . 23
     10.5.  MSP Server Monitoring . . . . . . . . . . . . . . . . . . 23
     10.6.  Advertisement of DEEP status  . . . . . . . . . . . . . . 24
     10.7.  Require TLS . . . . . . . . . . . . . . . . . . . . . . . 24
     10.8.  Changes to Internet Facing Servers  . . . . . . . . . . . 24
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 24
     11.1.  Security Tag Registry . . . . . . . . . . . . . . . . . . 24
     11.2.  Initial Set of Security Tags  . . . . . . . . . . . . . . 25
     11.3.  POP3S Port Registration Update  . . . . . . . . . . . . . 27
     11.4.  IMAPS Port Registration Update  . . . . . . . . . . . . . 27
     11.5.  Submissions Port Registration . . . . . . . . . . . . . . 28
     11.6.  DEEP IMAP Capability  . . . . . . . . . . . . . . . . . . 28
     11.7.  DEEP POP3 Capability  . . . . . . . . . . . . . . . . . . 29
     11.8.  DEEP SMTP EHLO Keyword  . . . . . . . . . . . . . . . . . 29
     11.9.  SMTP Enhanced Status Code . . . . . . . . . . . . . . . . 29
     11.10. MAIL Parameters Additional-registered-clauses
            Sub-Registry  . . . . . . . . . . . . . . . . . . . . . . 30
   12. Security Considerations  . . . . . . . . . . . . . . . . . . . 30
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
     13.1.  Normative References  . . . . . . . . . . . . . . . . . . 30
     13.2.  Informative References  . . . . . . . . . . . . . . . . . 32
   Appendix A.  Design Considerations . . . . . . . . . . . . . . . . 33
   Appendix B.  Change Log  . . . . . . . . . . . . . . . . . . . . . 35
   Appendix C.  Acknowledgements  . . . . . . . . . . . . . . . . . . 37
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 37

























Moore & Newman         Expires September 11, 2016               [Page 3]


Internet-Draft  Deployable Enhanced Email Privacy (DEEP)      March 2016


1.  Introduction

   Software that provides email service via Internet Message Access
   Protocol (IMAP) [RFC3501], Post Office Protocol (POP) [RFC1939]
   and/or Simple Mail Transfer Protocol (SMTP) Submission [RFC6409]
   usually has Transport Layer Security (TLS) [RFC5246] support but
   often does not use it in a way that maximizes end-user
   confidentiality.  This specification proposes changes to email
   software and deployments intended to increase the use of TLS and
   record when that use occurs.

   In brief, this memo now recommends that:

   o  MUAs associate a confidentiality assurance level with each mail
      account, and the default level requires use of TLS with
      certificate validation for all TCP connections;

   o  TLS on a well-known port ("Implicit TLS") be supported for IMAP,
      POP, and SMTP Submission [RFC6409] for all electronic mail user
      agents (MUAs), servers, and service providers;

   o  MUAs and mail protocol servers cooperate (via mechanisms defined
      in this specification) to upgrade security feature use and record/
      indicate that usage appropriately.

   This does not address use of TLS with SMTP for message relay (where
   Message Submission [RFC6409] does not apply).  Improved use of TLS
   with SMTP for message relay requires a different approach.  Future
   work may address that topic and one approach is described in
   [I-D.ietf-dane-smtp-with-dane].

   The recommendations in this memo do not replace the functionality of,
   and are not intended as a substitute for, end-to-end encryption of
   electronic mail.

   This draft is subject to change.  Implementation of this proposal is
   not recommended at this time.  Please discuss this proposal on the
   ietf-uta mailing list.


2.  Conventions and Terminology 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].

   This specification expresses syntax using the Augmented Backus-Naur
   Form (ABNF) as described in [RFC5234], including the core rules in



Moore & Newman         Expires September 11, 2016               [Page 4]


Internet-Draft  Deployable Enhanced Email Privacy (DEEP)      March 2016


   Appendix B and rules from [RFC5322].

   In examples, "C:" and "S:" indicate lines sent by the client and
   server respectively.  If a single "C:" or "S:" label applies to
   multiple lines, then the line breaks between those lines are for
   editorial clarity only and are not part of the actual protocol
   exchange.


3.  Mail Account Confidentiality Assurance Level

   A "mail account" refers to the network services an end user uses to
   read, submit and manage email communications on the Internet.  This
   typically involves at least one mail access server (IMAP or POP) and
   at least one SMTP submission server.  An end users uses a mail user
   agent (MUA) to access a mail account and most MUAs support one or
   more mail accounts.  This document uses the term "confidentiality
   assurance level" to indicate the degree to which the network
   connections between an MUA and a mail account have confidentiality
   protection from both passive and active attackers on the network.

   The configuration necessary for a mail account includes an email
   address, connection information and authentication credentials for
   network services.  MUAs compliant with this specification MUST also
   associate a confidentiality assurance level with each mail account.
   MUAs MUST implement a high confidentiality assurance level as
   described in the next section.

   MUAs SHOULD continuously indicate to the user the confidentiality
   assurance level of the account currently in use when reading,
   submitting and managing mail (e.g., via a lock icon, background
   colors and indications similar to those commonly used in web browsers
   for a similar purpose) and SHOULD indicate the confidentiality
   assurance level for each account whenever displaying a list of mail
   accounts.  Note that the displayed confidentiality assurance level
   could be higher than the level set at account configuration but never
   lower.  If multiple active connections are associated with an account
   or view, the indication should match the level provided by the least
   confidential connection.

   Account configuration occurs when an MUA is first used to access a
   particular service, when a user wishes to access or submit mail
   through servers in addition to those specified or found during first
   use, or when a user explicitly requests to change account
   configuration parameters such as server names, user names, passwords,
   client certificates, etc.  Account configuration can be entirely
   manual (entering server names explicitly) or partially automated via
   a mechanism such as DNS SRV records [RFC6186].  MUAs SHOULD use the



Moore & Newman         Expires September 11, 2016               [Page 5]


Internet-Draft  Deployable Enhanced Email Privacy (DEEP)      March 2016


   high confidentiality assurance level as the default for newly
   configured accounts.

3.1.  High Confidentiality Assurance

   A mail account has a high confidentiality assurance when the
   following conditions are met on all TCP server connections associated
   with an account.  This includes connections to POP, IMAP and SMTP
   submission servers as well as any other associated protocols defined
   now or in the future.  Examples of protocols associated with a mail
   account include managesieve [RFC5804] and MTQP [RFC3887].

   o  TCP connections MUST attempt to negotiate TLS via either Implicit
      TLS Section 4 or STARTTLS.

   o  MUAs MUST implement [I-D.ietf-uta-email-tls-certs] and PKIX
      [RFC5280].

   o  MUAs MAY implement DANE [RFC6698].

   o  User agents MUST abort a TLS session if the TLS negotiation fails
      or the server's certificate or identity fails to verify.  A user
      may reconfigure the account to lower the expected level of
      confidentiality if he/she chooses.  Reduction of expected account
      confidentiality MUST NOT be done on a click-through basis.

   The end user is part of the system that protects the user's
   confidentiality and security.  As a result, it's critical not to
   present the end user with a simple action that reduces their
   confidentiality in response to certificate validation failure.  An
   MUA which offers a user actions such as "connect anyway", "trust
   certificate for future connections" or "lower confidentiality
   assurance for this account" in response to certificate validation
   failure is not providing a high confidentiality assurance as defined
   in this section and thus does not comply with this document.
   Examples of acceptable actions to offer would be "work offline", "try
   again later", and "open service provider status web page".

3.2.  No Confidentiality Assurance

   MUAs MAY implement a no confidentiality assurance level for accounts.
   At this level, the MUA MUST attempt to negotiate TLS, but MAY ignore
   server certificate validation failures.  MUAs MAY support use of
   connections without TLS, but if they do they SHOULD attempt TLS first
   if available and MUST implement code to reconnect without TLS if TLS
   negotiation fails for reasons other than server certificate validity.

   Note that if the TLS certificate is not successfully validated as



Moore & Newman         Expires September 11, 2016               [Page 6]


Internet-Draft  Deployable Enhanced Email Privacy (DEEP)      March 2016


   described in Section 3.1 or a version of SSL/TLS prior to TLS 1.0 is
   used, the client MUST NOT present a high confidentiality indication
   for the account or connection.

3.3.  Other Confidentiality Assurance Levels

   This specification is not intended to limit experimentation and
   innovation with respect to user confidentiality.  As a result more
   confidentiality assurance levels are permitted.  However, levels
   below "no confidentiality assurance" described in the previous
   section are discouraged and implementers are cautioned that end users
   may be confused by too many confidentiality assurance levels.


4.  Implicit TLS

   Previous standards for use of email protocols with TLS used the
   STARTTLS mechanism: [RFC2595], [RFC3207], and [RFC3501].  With
   STARTTLS, the client establishes a clear text application session and
   determines whether to issue a STARTTLS command based on server
   capabilities and client configuration.  If the client issues a
   STARTTLS command, a TLS handshake follows that can upgrade the
   connection.  While this mechanism has been deployed, an alternate
   mechanism where TLS is negotiated immediately at connection start on
   a separate port (referred to in this document as "Implicit TLS") has
   been deployed more successfully.  To increase use of TLS, this
   specification recommends use of implicit TLS by new POP, IMAP and
   SMTP Submission software.

4.1.  Implicit TLS for POP

   When a TCP connection is established for the "pop3s" service (default
   port 995), a TLS handshake begins immediately.  Clients MUST
   implement the certificate validation mechanism described in
   [I-D.ietf-uta-email-tls-certs].  Once the TLS session is established,
   POP3 [RFC1939] protocol messages are exchanged as TLS application
   data for the remainder of the TCP connection.  After the server sends
   a +OK greeting, the server and client MUST enter AUTHORIZATION state,
   even if client credentials were supplied during the TLS handshake.

   See Section 9.1.1 for additional information on client certificate
   authentication.  See Section 11.3 for port registration information.

4.2.  Implicit TLS for IMAP

   When a TCP connection is established for the "imaps" service (default
   port 993), a TLS handshake begins immediately.  Clients MUST
   implement the certificate validation mechanism described in [RFC3501]



Moore & Newman         Expires September 11, 2016               [Page 7]


Internet-Draft  Deployable Enhanced Email Privacy (DEEP)      March 2016


   and SHOULD implement the certificate validation mechanism described
   in [I-D.ietf-uta-email-tls-certs].  Once the TLS session is
   established, IMAP [RFC3501] protocol messages are exchanged as TLS
   application data for the remainder of the TCP connection.  If client
   credentials were provided during the TLS handshake that the server
   finds acceptable, the server MAY issue a PREAUTH greeting in which
   case both the server and client enter AUTHENTICATED state.  If the
   server issues an OK greeting then both server and client enter NOT
   AUTHENTICATED state.

   See Section 9.1.1 for additional information on client certificate
   authentication.  See Section 11.4 for port registration information.

4.3.  Implicit TLS for SMTP Submission

   When a TCP connection is established for the "submissions" service
   (default port 465), a TLS handshake begins immediately.  Clients MUST
   implement the certificate validation mechanism described in
   [I-D.ietf-uta-email-tls-certs].  Once a TLS session is established,
   message submission protocol data [RFC6409] is exchanged as TLS
   application data for the remainder of the TCP connection.  (Note: the
   "submissions" service name is defined in section 10.3 of this
   document, and follows the usual convention that the name of a service
   layered on top of Implicit TLS consists of the name of the service as
   used without TLS, with an "s" appended.)

   The STARTTLS mechanism on port 587 is relatively widely deployed due
   to the situation with port 465 (discussed in Section 11.5).  This
   differs from IMAP and POP services where implicit TLS is more widely
   deployed on servers than STARTTLS.  It is desirable to migrate MUA
   software to implicit TLS over time for consistency as well as the
   reasons discussed elsewhere in this document.  However, to maximize
   use of encryption for submission it is desirable to support both
   mechanisms for Message Submission over TLS for a transition period of
   several years.  As a result, clients and servers SHOULD implement
   both STARTTLS on port 587 and implicit TLS on port 465 for this
   transition period.  Note that there is no significant difference
   between the security properties of STARTTLS on port 587 and implicit
   TLS on port 465 if the implementations are correct and both client
   and server are configured to require successful negotiation of TLS
   prior to message submission (as required in Section 9.1).

   Note that the submissions port provides access to a Mail Submission
   Agent (MSA) as defined in [RFC6409] so requirements and
   recommendations for MSAs in that document apply to the submissions
   port, including the requirement to implement SMTP AUTH [RFC4954].

   See Section 9.1.1 for additional information on client certificate



Moore & Newman         Expires September 11, 2016               [Page 8]


Internet-Draft  Deployable Enhanced Email Privacy (DEEP)      March 2016


   authentication.  See Section 11.5 for port registration information.

4.4.  Implicit TLS Connection Closure for POP, IMAP and SMTP

   When a client or server wishes to close the connection, it SHOULD
   initiate the exchange of TLS close alerts before TCP connection
   termination.  The client MAY, after sending a TLS close alert,
   gracefully close the TCP connection without waiting for a TLS
   response from the server.


5.  Email Security Upgrading Using Security Latches

   Once an improved email security mechanism is deployed and ready for
   general use, it is desirable to continue using it for all future
   email service.  For example, TLS is widely deployed in email
   software, but use of TLS is often not required.  At the time this is
   written, deployed mail user agents (MUAs) [RFC5598] usually make a
   determination if TLS is available when an account is first configured
   and may require use of TLS with that account if and only if it was
   initially available.  If the service provider makes TLS available
   after initial client configuration, many MUAs will not notice the
   change.

   Alternatively, a security feature may be purely opportunistic and
   thus subject to downgrade attacks.  For example, at the time this was
   written, most TLS stacks that support TLS 1.2 will use an older TLS
   version if the peer does not support TLS 1.2 and some do so without
   alerting the client of the reduced security.  Thus a variety of
   active attacks could cause the loss of TLS 1.2 benefits.  Only if
   client policy is upgraded to require TLS 1.2 can the client prevent
   all downgrade attacks.  However, this sort of security policy upgrade
   will be ignored by most users unless it is automated.

   This section describes a mechanism, called "security latches", which
   is designed to permit an MUA to recognize when a service provider has
   committed to provide certain server security features, and that it's
   safe for the client to change its configuration for that account to
   require that such features be present in future sessions with that
   server.  When an MUA implements both confidentiality assurance levels
   and security latches, then both the end-user and the service provider
   independently have the ability to improve the end-user's
   confidentiality.

   Note that security latches are a mechanism similar to HTTP Strict
   Transport Security (HSTS) [RFC6797] but are extensible.





Moore & Newman         Expires September 11, 2016               [Page 9]


Internet-Draft  Deployable Enhanced Email Privacy (DEEP)      March 2016


5.1.  Email Security Tags

   Each security latch is given a name known as an email security tag.
   An email security tag is a short alphanumeric token that represents a
   security facility that can be used by an IMAP, POP or SMTP Submission
   session.  When a server advertises a security tag it is making a
   commitment to support that security facility indefinitely and
   recommending that the client save that security tag with the account
   configuration and require that security feature for future
   connections to that server.  When a security tag is saved by the
   client in this way, it is then considered latched.  For the "tls11"
   and/or "tls12" tags, the client SHOULD refuse to connect to the
   server unless the appropriate level of TLS is successfully
   negotiated.  The client SHOULD NOT latch tags unless they are
   advertised by the server, TLS is active and the client successfully
   authenticates the server with the TLS session.  Once a security tag
   is latched, all subsequent connections to that host require that
   security feature.  For this confidentiality protection to work as
   desired clients MUST NOT offer a click-through-to-connect action when
   unable to achieve connection security matching the latched security
   tags.

   An identifier for a security tag has the following formal syntax:

     security-tag  =  ALPHA *63(ALPHA / DIGIT / "-" / "_")

5.2.  Initial Set of Email Security Tags

   This section describes an initial set of email security tags.  The
   IANA Considerations Section 11 defines a registry so that more tags
   can be defined in the future.  The initial set of tags are defined in
   Section 11.2 and include tls11, tls12, tls-cert and tls-dane-tlsa.

5.3.  Server DEEP Status

   Servers supporting this extension MUST advertise a DEEP status.  This
   status includes a list of security-tags the server administrator has
   explicitly configured as recommended for use by end-users (the list
   MAY be empty), an optional https Uniform Resource Locator (URL)
   [RFC2818] that the client can save and subsequently resolve for the
   user in the event of a security connection problem, and the DEEP
   status can be extended by future updates to this specification.  DEEP
   status has the following formal syntax:








Moore & Newman         Expires September 11, 2016              [Page 10]


Internet-Draft  Deployable Enhanced Email Privacy (DEEP)      March 2016


     EXTCHAR      = 0x20-21 / 0x23-2E / 0x30-3B / 0x3D-40
                    / 0x5B-60 / 0x7B-7E
                  ; printable characters excluding " \ < and ALPHA

     deep-extend  = EXTCHAR *(EXTCHAR / ALPHA / "<")
                  ; clients MUST ignore, for future extensibility

     deep-status  = [deep-tag *(SP deep-tag)]

     deep-tag     = deep-https / security-tag / deep-extend

     deep-https   =  "<" <URI from RFC 3986 with https scheme> ">"

   The syntax for a Uniform Resource Identifier (URI) is defined in
   [RFC3986].  Protocol extensions to advertise DEEP status are defined
   in Section 7.

   If the client successfully negotiates TLS and authenticates the
   server (e.g., via tls-cert, tls-dane-tlsa or SCRAM-SHA1-PLUS with
   channel bindings [RFC5802]), then the client SHOULD record the
   server's DEEP status information in the account configuration with
   the server's hostname.  Otherwise, the client SHOULD ignore the
   server-provided DEEP status.

5.4.  Email Security Tag Latch Failures

   When a security tag latch has been set for connections from a client
   to a server and the property identified by that tag is no longer
   available, this results in a connection failure.  An MUA SHOULD
   inform the user of a potential threat to their confidentiality and
   offer to resolve a previously-recorded DEEP status https URL if one
   is available.  MUAs are discouraged from offering a lightweight
   option to reset or ignore latches as this defeats the benefit they
   provide to end users.


6.  Recording TLS Cipher Suite in Received Header

   The ESMTPS transmission type [RFC3848] provides trace information
   that can indicate TLS was used when transferring mail.  However, TLS
   usage by itself is not a guarantee of confidentiality or security.
   The TLS cipher suite provides additional information about the level
   of security made available for a connection.  This defines a new SMTP
   "tls" Received header additional-registered-clause that is used to
   record the TLS cipher suite that was negotiated for the connection.
   The value included in this additional clause SHOULD be the registered
   cipher suite name (e.g., TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256)
   included in the TLS cipher suite registry.  In the event the



Moore & Newman         Expires September 11, 2016              [Page 11]


Internet-Draft  Deployable Enhanced Email Privacy (DEEP)      March 2016


   implementation does not know the name of the cipher suite (a
   situation that should be remedied promptly), a four-digit hexadecimal
   cipher suite identifier MAY be used.  The ABNF for the field follows:

     tls-cipher-clause  =  CFWS "tls" FWS tls-cipher

     tls-cipher         =  tls-cipher-suite-name / tls-cipher-suite-hex

     tls-cipher-name    =  ALPHA *(ALPHA / DIGIT / "_")
                           ; as registered in IANA cipher suite registry

     tls-cipher-hex     =  "0x" 4HEXDIG


7.  Extensions for DEEP Status and Reporting

   This memo defines optional mechanisms for use by MUAs to communicate
   DEEP status to servers and for servers to advertise available
   latches.  One purpose of such mechanisms is to permit servers to
   determine which and how many clients have latched security
   facilities, and thus, to permit operators to be aware of potential
   impact to their users should support for such facilities be changed.
   For IMAP, the existing ID command is extended to provide this
   capability.  For SMTP Submission, a new CLIENT command is defined.
   No similar mechanism is defined for POP in this version of the memo
   to keep POP simpler, but one may be added in the future if deemed
   necessary.

   In addition, for each of IMAP, POP, and SMTP, a new DEEP capability
   is defined so the client can access the server's DEEP status.

7.1.  IMAP DEEP Extension

   When an IMAP server advertises the DEEP capability, that indicates
   the IMAP server implements IMAP4 ID [RFC2971] with additional field
   values defined here.  This is grouped with the ID command because
   that is the existing IMAP mechanism for clients to report data for
   server logging, and provides a way for the server to report the DEEP
   status.

   deep  From server to client, the argument to this ID field is the
      server DEEP status.  Servers MUST provide this information in
      response to an ID command.

   latch  From client to server, this is a space-separated list of
      security tags the client has latched for this server.  Servers MAY
      record this information so administrators know the expected latch-
      related security properties of the client and can thus act to



Moore & Newman         Expires September 11, 2016              [Page 12]


Internet-Draft  Deployable Enhanced Email Privacy (DEEP)      March 2016


      avoid security latch failures (e.g., by renewing server
      certificates on time, etc).

   latch-fail  From client to server, a space-separated list including
      one or more security tag the client has latched that the client
      was unable to achieve.  This allows clients to report errors to
      the server prior to terminating the connection to the server in
      the event an acceptable security level is unavailable.

   security-tags  From client to server, this is a space-separated list
      of security tags the client supports that are not latched.

   tls  Server-side IMAP proxies that accept TLS connections from
      clients and connect in-the-clear over a fully private secure
      network to the server SHOULD use this field to report the tls-
      cipher (syntax as defined in Section 6) to the server.

   IMAP clients SHOULD use the IMAP ID command to report latch failures
   and determine the server DEEP status.  Clients MAY use the ID command
   to report other latch or security tag information.  IMAP servers MUST
   implement the ID command at least to report DEEP status to clients.

     <client connected to port 993 and negotiated TLS successfully>
     S: * OK [CAPABILITY IMAP4rev1 DEEP ID AUTH=PLAIN
             AUTH=SCRAM-SHA-1] hello
     C: a001 ID ("name" "Demo Mail" "version" "1.5" "latch"
             "tls11 tls-cert" "security-tags" "tls12")
     S: * ID ("name" "Demo Server" "version" "1.7" "deep-status"
              "<https://www.example.com/security-support.html>")
     S: a001 OK ID completed

                                 Example 1

   This example shows a client that successfully negotiated TLS version
   1.0 or later and verified the server's certificate as required by
   IMAP.  The client supports TLS 1.2.  However, even if the client
   successfully negotiated TLS 1.2, it will not latch that security tag
   automatically because the server did not advertise that tag.  If the
   client successfully validated the server certificate, it will latch
   the provided URL.











Moore & Newman         Expires September 11, 2016              [Page 13]


Internet-Draft  Deployable Enhanced Email Privacy (DEEP)      March 2016


     <client connected to port 993 and negotiated TLS successfully>
     S: * OK [CAPABILITY IMAP4rev1 DEEP ID AUTH=PLAIN
             AUTH=SCRAM-SHA-1] hello
     C: a001 ID ("name" "Demo Mail" "version" "1.5" "latch-failure"
             "tls-cert")
     S: * ID ("name" "Demo Server" "version" "1.7" "deep-status"
              "tls11 <https://www.example.com/security-support.html>")
     S: a001 OK ID completed
     C: a002 LOGOUT

                                 Example 2

   This example shows a client that negotiated TLS, but was unable to
   verify the server's certificate.  The latch-failure informs the
   server of this problem, at which point the client can disconnect.  If
   the client had previously latched a URI for security problems from
   this server, it could offer to resolve that URI.  However, the deep-
   status in this exchange is ignored due to the latch failure.

     <IMAP Proxy connected over private network on port 143, there is
     a client connected to the proxy on port 993 that negotiated TLS>
     S: * OK [CAPABILITY IMAP4rev1 DEEP ID AUTH=PLAIN
             AUTH=SCRAM-SHA-1] hello
     C: a001 ID ("name" "Demo Mail" "version" "1.5" "latch"
             "tls11 tls-cert" "security-tags" "tls12"
             "tls" "TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256")
     S: * ID ("name" "Demo Server" "version" "1.7" "deep-status"
              "tls11 tls-cert <https://www.example.com/support.html>")
     S: a001 OK ID completed

                                 Example 3

   This example shows the connection from an IMAP proxy to a back-end
   server.  The client connected to the proxy and sent the ID command
   shown in example 1, and the proxy has added the "tls" item to the ID
   command so the back-end server can log the cipher suite that was used
   on the connection from the client.

7.2.  POP DEEP Extension

   POP servers supporting this specification MUST implement the POP3
   extension mechanism [RFC2449].  POP servers MUST advertise the DEEP
   capability with an argument indicating the server's DEEP status.








Moore & Newman         Expires September 11, 2016              [Page 14]


Internet-Draft  Deployable Enhanced Email Privacy (DEEP)      March 2016


     <client connected to port 995 and negotiated TLS successfully>
     S: +OK POP server ready
     C: CAPA
     S: +OK Capability list follows
     S: TOP
     S: SASL PLAIN SCRAM-SHA-1
     S: RESP-CODES
     S: PIPELINING
     S: UIDL
     S: DEEP tls11 tls12 <https://www.example.com/security-support.html>
     S: .

                                 Example 4

   After verifying the TLS server certificate and issuing CAPA, the
   client can latch any or all of the DEEP status.  If the client
   connects to this same server later and has a security failure, the
   client can direct the user's browser to the previously-latched URI
   where the service provider may provide advice to the end user.

7.3.  SMTP DEEP Extension

   SMTP Submission servers supporting this specification MUST implement
   the DEEP SMTP extension.  The name of this extension is DEEP.  The
   EHLO keyword value is DEEP and the deep-status ABNF is the syntax of
   the EHLO keyword parameters.  This does not add parameters to the
   MAIL FROM or RCPT TO commands.  This also adds a CLIENT command to
   SMTP which is used to report client information to the server.  The
   formal syntax for the command follows:






















Moore & Newman         Expires September 11, 2016              [Page 15]


Internet-Draft  Deployable Enhanced Email Privacy (DEEP)      March 2016


     deep-cmd             = "CLIENT" 1*(SP deep-parameter)

     deep-parameter       = name / version / latch / latch-fail
                            / security-tags / tls / future-extension

     name                 = "name=" esmtp-value

     version              = "version=" esmtp-value

     latch                = "latch=" security-tag *("," security-tag)

     latch-fail           = "latch-fail=" security-tag
                            *("," security-tag)

     security-tags        = "security-tags=" security-tag
                            *("," security-tag)

     tls                  = "tls=" tls-cipher

     future-extension     = esmtp-param

     esmtp-param          = <as defined in RFC 5321>

     esmtp-value          = <as defined in RFC 5321>

   The CLIENT command parameters listed here have the same meaning as
   the parameters used in the IMAP DEEP extension (Section 7.1).  The
   server responds to the CLIENT command with a "250" if the command has
   correct syntax and a "501" if the command has incorrect syntax.

     <client connected to port 465 and negotiated TLS successfully>
     S: 220 example.com Demo SMTP Submission Server
     C: EHLO client.example.com
     S: 250-example.com
     S: 250-8BITMIME
     S: 250-PIPELINING
     S: 250-DSN
     S: 250-AUTH PLAIN LOGIN
     S: 250-DEEP tls12 tls-cert <https://www.example.com/status.html>
     S: 250-BURL imap
     S: 250 SIZE 0
     C: CLIENT name=demo_submit version=1.5 latch=tls11,tls-cert
               security-tags=tls12
     S: 250 OK

                                 Example 5





Moore & Newman         Expires September 11, 2016              [Page 16]


Internet-Draft  Deployable Enhanced Email Privacy (DEEP)      March 2016


7.4.  SMTP Error Extension

   Although this document focuses on SMTP Submission, it is possible to
   use security latches for SMTP transport as well.  When MTA transport
   fails due to a security latch, the MTA MUST use the SMTP enhanced
   status code X.7.TBD (RFC Editor note: update this TBD).  The SMTP
   notary response [RFC3464] for a security latch failure MUST include
   an additional "SMTP-Security-Latch" recipient-specific header field
   that includes a space-delimited list including one or more security
   latch that failed.  The ABNF for this new field follows:

     CFWS                 =  <defined in RFC 5322>

     FWS                  =  <defined in RFC 5322>

     smtp-security-latch  =  "SMTP-Security-Latch:" CFWS
                             security-tag *(FWS security-tag)


8.  Account Setup Considerations

8.1.  Use of SRV records in Establishing Configuration

   This section updates [RFC6186] by changing the preference rules and
   adding a new SRV service label _submissions._tcp to refer to Message
   Submission with implicit TLS.

   User-configurable MUAs SHOULD support use of [RFC6186] for account
   setup.  However, when using configuration information obtained by
   this method, MUAs SHOULD default to a high confidentiality assurance
   level, unless the user has explicitly requested reduced
   confidentiality.  This will have the effect of causing the MUA to
   ignore advertised configurations that do not support TLS, even when
   those advertised configurations have a higher priority than other
   advertised configurations.

   When using [RFC6186] configuration information, Mail User Agents
   SHOULD NOT automatically establish new configurations that do not
   require TLS for all servers, unless there are no advertised
   configurations using TLS.  If such a configuration is chosen, prior
   to attempting to authenticate to the server or use the server for
   message submission, the MUA SHOULD warn the user that traffic to that
   server will not be encrypted and that it will therefore likely be
   intercepted by unauthorized parties.  The specific wording is to be
   determined by the implementation, but it should adequately capture
   the sense of risk given the widespread incidence of mass surveillance
   of email traffic.




Moore & Newman         Expires September 11, 2016              [Page 17]


Internet-Draft  Deployable Enhanced Email Privacy (DEEP)      March 2016


   When establishing a new configuration for connecting to an IMAP, POP,
   or SMTP Submission server, an MUA SHOULD NOT blindly trust SRV
   records unless they are signed by DNSSEC and have a valid signature.
   Instead, the MUA SHOULD warn the user that the DNS-advertised
   mechanism for connecting to the server is not authenticated, and
   request the user to manually verify the connection details by
   reference to his or her mail service provider's documentation.

   Similarly, an MUA MUST NOT consult SRV records to determine which
   servers to use on every connection attempt, unless those SRV records
   are signed by DNSSEC and have a valid signature.  However, an MUA MAY
   consult SRV records from time to time to determine if an MSP's server
   configuration has changed, and alert the user if it appears that this
   has happened.  This can also serve as a means to encourage users to
   upgrade their configurations to require TLS if and when their MSPs
   support it.

8.2.  Certificate Pinning

   During account setup, the MUA will identify servers that provide
   account services such as mail access and mail submission (the
   previous section describes one way to do this).  The certificates for
   these servers are verified using the rules described in
   [I-D.ietf-uta-email-tls-certs] and PKIX [RFC5280].  In the event the
   certificate does not validate due to an expired certificate, lack of
   appropriate chain of trust or lack of identifier match, the MUA MAY
   create a persistent binding between that certificate and the saved
   host name for the server.  This is called certificate pinning.
   Certificate pinning is only appropriate during account setup and MUST
   NOT be offered in response to a failed certificate validation for an
   existing account.  An MUA that allows certificate pinning MUST NOT
   allow a certificate pinned for one account to validate connections
   for other accounts.

   A pinned certificate is subject to a man-in-the-middle attack at
   account setup time, and lacks a mechanism to revoke or securely
   refresh the certificate.  Therefore use of a pinned certificate does
   not provide a high confidentiality assurance and an MUA MUST NOT
   indicate a high level for an account or connection using a pinned
   certificate.  Additional advice on certificate pinning is present in
   [RFC6125].


9.  Implementation Requirements

   This section details requirements for implementations of electronic
   mail protocol clients and servers.  A requirement for a client or
   server implementation to support a particular feature is not the same



Moore & Newman         Expires September 11, 2016              [Page 18]


Internet-Draft  Deployable Enhanced Email Privacy (DEEP)      March 2016


   thing as a requirement that a client or server running a conforming
   implementation be configured to use that feature.  Requirements for
   Mail Service Providers (MSPs) are distinct from requirements for
   protocol implementations, and are listed in a separate section.

9.1.  All Implementations (Client and Server)

   These requirements apply to MUAs as well as POP, IMAP and SMTP
   Submission servers.

   o  All implementations MUST be configurable to support implicit TLS
      using the TLS 1.2 protocol or later [RFC5246].

   o  All implementations MUST implement the recommended cipher suites
      described in [RFC7525] or a future BCP or standards track revision
      of that document.

   o  All implementations MUST be configurable to require TLS before
      performing any operation other than capability discovery and
      STARTTLS.

   o  The IMAP specification [RFC3501] is hereby modified to revoke the
      second paragraph of section 11.1 and replace it with the text from
      the first three bullet items in this list.

   o  The standard for use of TLS with IMAP, POP3 and ACAP [RFC2595] is
      modified to revoke section 2.1 and replace it with the text from
      the first three bullet items in this list.

   o  The standard for Message Submission [RFC6409] is updated to add
      the first three bullet items above to section 4.3 as well as to
      require implementation of the TLS server identity check as
      described in [I-D.ietf-uta-email-tls-certs] and PKIX [RFC5280].

9.1.1.  Client Certificate Authentication

   MUAs and mail servers MAY implement client certificate authentication
   on the implicit TLS port.  Servers MUST NOT request a client
   certificate during the TLS handshake unless the server is configured
   to accept some client certificates as sufficient for authentication
   and the server has the ability to determine a mail server
   authorization identity matching such certificates.  How to make this
   determination is presently implementation specific.  Clients MUST NOT
   provide a client certificate during the TLS handshake unless the
   server requests one and the client has determined the certificate can
   be safely used with that specific server, OR the client has been
   explicitly configured by the user to use that particular certificate
   with that server.  How to make this determination is presently



Moore & Newman         Expires September 11, 2016              [Page 19]


Internet-Draft  Deployable Enhanced Email Privacy (DEEP)      March 2016


   implementation specific.  If the server accepts the client's
   certificate as sufficient for authorization, it MUST enable the SASL
   EXTERNAL [RFC4422] mechanism.  An IMAPS server MAY issue a PREAUTH
   greeting instead of enabling SASL EXTERNAL.  A client supporting
   client certificate authentication with implicit TLS MUST implement
   the SASL EXTERNAL [RFC4422] mechanism using the appropriate
   authentication command (AUTH for POP3 [RFC5034], AUTH for SMTP
   Submission [RFC4954], AUTHENTICATE for IMAP [RFC3501]).

9.2.  Mail Server Implementation Requirements

   These requirements apply to servers that implement POP, IMAP or SMTP
   Submission.

   o  Servers MUST implement the DEEP extension described in Section 7

   o  IMAP and SMTP submission servers SHOULD implement and be
      configurable to support STARTTLS.  This enables discovery of new
      TLS availability, and can increase usage of TLS by legacy clients.

   o  Servers MUST NOT advertise STARTTLS if it is unlikely to succeed
      based on server configuration (e.g., there is no server
      certificate installed).

   o  SMTP message submission servers that have negotiated TLS SHOULD
      add a Received header field to the message including the tls
      clause described in Section 6.

   o  Servers MUST be configurable to include the TLS cipher information
      in any connection or user logging or auditing facility they
      provide.

9.3.  Mail User Agent Implementation Requirements

   This section describes requirements on Mail User Agents (MUAs) using
   IMAP, POP, and/or Submission protocols.  Note: Requirements
   pertaining to use of Submission servers are also applicable to use of
   SMTP servers (e.g., port 25) for mail submission.

   o  User agents SHOULD indicate to users at configuration time, the
      expected level of confidentiality based on appropriate security
      inputs such as which security latches are pre-set, the number of
      trust anchors, certificate validity, use of an extended validation
      certificate, TLS version supported, and TLS cipher suites
      supported by both server and client.  This indication SHOULD also
      be present when editing or viewing account configuration.





Moore & Newman         Expires September 11, 2016              [Page 20]


Internet-Draft  Deployable Enhanced Email Privacy (DEEP)      March 2016


   o  MUAs SHOULD detect when STARTTLS and/or implicit TLS becomes
      available for a protocol and set the tls11 latch if the server
      advertises the tls11 security tag after a successful TLS
      negotiation.

   o  Whenever requested to establish any configuration that does not
      require both TLS and server certificate verification to talk to a
      server or account, an MUA SHOULD warn its user that his or her
      mail traffic (including password, if applicable) will be exposed
      to attackers, and give the user an opportunity to abort the
      connection prior to transmission of any such password or traffic.

   o  MUAs SHOULD implement the "tls12" security latch (the TLS library
      has to provide an API that controls permissible TLS versions and
      communicates the negotiated TLS protocol version to the
      application for this to be possible).

   o  See Section 3 for additional requirements.

9.4.  Non-configurable MUAs and nonstandard access protocols

   MUAs which are not configurable to use user-specified servers MUST
   implement TLS or similarly other strong encryption mechanism when
   communicating with their mail servers.  This generally applies to
   MUAs that are pre-configured to operate with one or more specific
   services, whether or not supplied by the vendor of those services.

   MUAs using protocols other than IMAP, POP, and Submission to
   communicate with mail servers, MUST implement TLS or other similarly
   robust encryption mechanism in conjunction with those protocols.

9.5.  DEEP Compliance for Anti-Virus/Anti-Spam Software and Services

   There are multiple ways to connect an Anti-Virus and/or Anti-Spam
   (AVAS) service to a mail server.  Some mechanisms, such as the de-
   facto milter protocol do not impact DEEP.  However, some services use
   an SMTP relay proxy that intercepts mail at the application layer to
   perform a scan and proxy or forward to another MTA.  Deploying AVAS
   services in this way can cause many problems [RFC2979] including
   direct interference with DEEP and confidentiality or security
   reduction.  An AVAS product or service is considered DEEP compliant
   if all IMAP, POP and SMTP-related software it includes is DEEP
   compliant and it advertises and supports all security latches that
   the actual servers advertise.

   Note that end-to-end email encryption prevents AVAS software and
   services from using email content as part of a spam or virus
   assessment.  Furthermore, while DEEP high confidentiality assurance



Moore & Newman         Expires September 11, 2016              [Page 21]


Internet-Draft  Deployable Enhanced Email Privacy (DEEP)      March 2016


   can prevent a man-in-the-middle from introducing spam or virus
   content between the MUA and Submission server, it does not prevent
   other forms of client or account compromise so use of AVAS services
   for submitted email remains necessary.


10.  Mail Service Provider Requirements

   This section details requirements for providers of IMAP, POP, and/or
   SMTP submission services, for providers who claim to conform to this
   specification.

10.1.  Server Requirements

   Mail Service Providers MUST use server implementations that conform
   to this specification.

10.2.  MSPs MUST provide Submission Servers

   This document updates the advice in [RFC5068] by making Implicit TLS
   on port 465 the preferred submission port.

   Mail Service Providers that accept mail submissions from end-users
   using the Internet Protocol MUST provide one or more SMTP Submission
   servers for this purpose, separate from the SMTP servers used to
   process incoming mail.  Those submission servers MUST be configured
   to support Implicit TLS on port 465 and SHOULD support STARTTLS if
   port 587 is used.

   MSPs MAY also support submission of messages via one or more
   designated SMTP servers to facilitate compatibility with legacy MUAs.

   Discussion: SMTP servers used to accept incoming mail or to relay
   mail are expected to accept mail in cleartext.  This is incompatible
   with the purpose of this memo which is to encourage encryption of
   traffic between mail servers.  There is no such requirement for mail
   submission servers to accept mail in cleartext or without
   authentication.  For other reasons, use of separate SMTP submission
   servers has been best practice for many years.

10.3.  TLS Server Certificate Requirements

   MSPs MUST maintain valid server certificates for all servers.  Those
   server certificates SHOULD present DNS-IDs and SRV-IDs conforming to
   [RFC6125] and which will be recognized by MUAs meeting the
   requirements of that specification.  In addition, those server
   certificates MAY provide other DNS-IDs, SRV-IDs, or CN-IDs needed for
   compatibility with existing MUAs.



Moore & Newman         Expires September 11, 2016              [Page 22]


Internet-Draft  Deployable Enhanced Email Privacy (DEEP)      March 2016


   If a protocol server provides service for more than one mail domain,
   it MAY use a separate IP address for each domain and/or a server
   certificate that advertises multiple domains.  This will generally be
   necessary unless and until it is acceptable to impose the constraint
   that the server and all clients support the Server Name Indication
   extension to TLS [RFC6066].

10.4.  Recommended DNS records for mail protocol servers

   This section discusses not only the DNS records that are recommended,
   but also implications of DNS records for server configuration and TLS
   server certificates.

10.4.1.  MX records

   It is recommended that MSPs advertise MX records for handling of
   inbound mail (instead of relying entirely on A or AAAA records), and
   that those MX records be signed using DNSSEC.  This is mentioned here
   only for completeness, as handling of inbound mail is out of scope
   for this document.

10.4.2.  SRV records

   MSPs SHOULD advertise SRV records to aid MUAs in determination of
   proper configuration of servers, per the instructions in [RFC6186].

   MSPs SHOULD advertise servers that support Implicit TLS in preference
   to those which support cleartext and/or STARTTLS operation.

10.4.3.  DNSSEC

   All DNS records advertised by an MSP as a means of aiding clients in
   communicating with the MSP's servers, SHOULD be signed using DNSSEC.

10.4.4.  TLSA records

   MSPs SHOULD advertise TLSA records to provide an additional trust
   anchor for public keys used in TLS server certificates.  However,
   TLSA records MUST NOT be advertised unless they are signed using
   DNSSEC.

10.5.  MSP Server Monitoring

   MSPs SHOULD regularly and frequently monitor their various servers to
   make sure that: TLS server certificates remain valid and are not
   about to expire, TLSA records match the public keys advertised in
   server certificates, are signed using DNSSEC, server configurations
   are consistent with SRV advertisements, and DNSSEC signatures are



Moore & Newman         Expires September 11, 2016              [Page 23]


Internet-Draft  Deployable Enhanced Email Privacy (DEEP)      March 2016


   valid and verifiable.  Failure to detect expired certificates and DNS
   configuration errors in a timely fashion can result in significant
   loss of service for an MSP's users and a significant support burden
   for the MSP.

10.6.  Advertisement of DEEP status

   MSPs SHOULD advertise a DEEP status that includes tls11, tls-cert and
   an HTTPS URL that can be used to inform clients of service outages or
   problems impacting client confidentiality.  Note that advertising
   tls-cert is a commitment to maintain and renew server certificates.

10.7.  Require TLS

   New servers and services SHOULD be configured to require TLS unless
   it's necessary to support legacy clients or existing client
   configurations.

10.8.  Changes to Internet Facing Servers

   When an MSP changes the Internet Facing Servers providing mail access
   and mail submission services, including SMTP-based spam/virus
   filters, it is generally necessary to support the same and/or a newer
   version of TLS and the same security tags that were previously
   advertised.


11.  IANA Considerations

11.1.  Security Tag Registry

   IANA shall create (has created) the registry "Email Security Tags".
   This registry is a single table and will use an expert review process
   [RFC5226].  Each registration will contain the following fields:

   Name:  The name of the security tag.  This follows the security-tag
      ABNF.

   Description:  This describes the meaning of the security tag and the
      conditions under which the tag is latched.

   Intended Usage:  One of COMMON, LIMITED USE or OBSOLETE.

   Reference:  Optional reference to specification.







Moore & Newman         Expires September 11, 2016              [Page 24]


Internet-Draft  Deployable Enhanced Email Privacy (DEEP)      March 2016


   Submitter:  The identify of the submitter or submitters.

   Change Controller:  The identity of the change controller for the
      registration.  This will be "IESG" in case of registrations in
      IETF-produced documents.

   The expert reviewer will verify the tag name follows the ABNF, and
   that the description field is clear, unambiguous, does not overlap
   existing deployed technology, does not create security problems and
   appropriately considers interoperability issues.  Email security tags
   intended for LIMITED USE have a lower review bar (interoperability
   and overlap issues are less of a concern).  The reviewer may approve
   a registration, reject for a stated reason or recommend the proposal
   have standards track review due to importance or difficult
   subtleties.

   Standards-track registrations may be updated if the relevant
   standards are updated as a consequence of that action.  Non-
   standards-track entries may be updated by the listed change
   controller.  The entry's name and submitter may not be changed.  In
   exceptional cases, any aspect of any registered entity may be updated
   at the direction of the IESG (for example, to correct a conflict).

11.2.  Initial Set of Security Tags

   This document defines four initial security tags for the security tag
   registry as follows:

   Name:  tls11

   Description:  This indicates TLS version 1.1 [RFC4346] or later was
      negotiated successfully including negotiation of a strong
      encryption layer with a symmetric key of at least 128 bits.  This
      tag indicates the server certificate was valid but does not
      indicate the validation mechanism (e.g., PKIX [RFC5280] or DANE
      [RFC6698]).  This tag is latched if the client sees this tag in
      the advertised server DEEP status provided after successfully
      negotiating TLS version 1.0 or later.

   Intended Usage:  COMMON

   Reference:  RFC XXXX (this document once published)

   Submitter:  Authors of this document







Moore & Newman         Expires September 11, 2016              [Page 25]


Internet-Draft  Deployable Enhanced Email Privacy (DEEP)      March 2016


   Change Controller:  IESG

   Name:  tls12

   Description:  This indicates TLS version 1.2 [RFC5246] or later was
      negotiated successfully including negotiation of a strong
      encryption layer with a symmetric key of at least 128 bits.  This
      tag indicates the server certificate was valid but does not
      indicate the validation mechanism (e.g., PKIX [RFC5280] or DANE
      [RFC6698]).  This tag is latched if the client sees this tag in
      the advertised server DEEP status provided after successfully
      negotiating TLS version 1.2 or later.

   Intended Usage:  COMMON

   Reference:  RFC XXXX (this document once published)

   Submitter:  Authors of this document

   Change Controller:  IESG

   Name:  tls-cert

   Description:  This tag indicates that TLS was successfully negotiated
      and the server certificate was successfully verified by the client
      using PKIX [RFC5280] and the server certificate identity was
      verified using the algorithm appropriate for the protocol (see
      Section 4).  This tag is latched if the client sees this tag in
      the advertised server DEEP status after successfully negotiating
      TLS and verifying the certificate and server identity.

   Intended Usage:  COMMON

   Reference:  RFC XXXX (this document once published)

   Submitter:  Authors of this document

   Change Controller:  IESG

   Name:  tls-dane-tlsa

   Description:  This tag indicates that TLS was successfully negotiated
      and the server certificate was successfully verified by the client
      using the procedures described in [RFC6698] and the server
      certificate identity was verified using the algorithm appropriate
      for the protocol (see Section 4).  This tag is latched if the
      client sees this tag in the advertised server DEEP status after
      successfully negotiating TLS and verifying the certificate and



Moore & Newman         Expires September 11, 2016              [Page 26]


Internet-Draft  Deployable Enhanced Email Privacy (DEEP)      March 2016


      server identity.

   Intended Usage:  COMMON

   Reference:  RFC XXXX (this document once published)

   Submitter:  Authors of this document

   Change Controller:  IESG

11.3.  POP3S Port Registration Update

   IANA is asked to update the registration of the TCP well-known port
   995 using the following template ([RFC6335]):

     Service Name: pop3s
     Transport Protocol: TCP
     Assignee: IETF <iesg@ietf.org>
     Contact: IESG <iesg@ietf.org>
     Description: POP3 over TLS protocol
     Reference: RFC XXXX (this document once published)

     Service Name: pop3s
     Transport Protocol: TCP
     Assignee: IETF <iesg@ietf.org>
     Contact: IESG <iesg@ietf.org>
     Description: POP3 over TLS protocol
     Reference: RFC XXXX (this document once published)

11.4.  IMAPS Port Registration Update

   IANA is asked to update the registration of the TCP well-known port
   993 using the following template ([RFC6335]):

     Service Name: imaps
     Transport Protocol: TCP
     Assignee: IETF <iesg@ietf.org>
     Contact: IESG <iesg@ietf.org>
     Description: IMAP over TLS protocol
     Reference: RFC XXXX (this document once published)

     Service Name: imaps
     Transport Protocol: TCP
     Assignee: IETF <iesg@ietf.org>
     Contact: IESG <iesg@ietf.org>
     Description: IMAP over TLS protocol
     Reference: RFC XXXX (this document once published)




Moore & Newman         Expires September 11, 2016              [Page 27]


Internet-Draft  Deployable Enhanced Email Privacy (DEEP)      March 2016


11.5.  Submissions Port Registration

   IANA is asked to assign an alternate usage of port 465 in addition to
   the current assignment using the following template ([RFC6335]):

     Service Name: submissions
     Transport Protocol: TCP
     Assignee: IETF <iesg@ietf.org>
     Contact: IESG <iesg@ietf.org>
     Description: Message Submission over TLS protocol
     Reference: RFC XXXX (this document once published)

     Service Name: submissions
     Transport Protocol: TCP
     Assignee: IETF <iesg@ietf.org>
     Contact: IESG <iesg@ietf.org>
     Description: Message Submission over TLS protocol
     Reference: RFC XXXX (this document once published)

   This is a one time procedural exception to the rules in RFC 6335.
   This requires explicit IESG approval and does not set a precedent.
   Historically, port 465 was briefly registered as the "smtps" port.
   This registration made no sense as the SMTP transport MX
   infrastructure has no way to specify a port so port 25 is always
   used.  As a result, the registration was revoked and was subsequently
   reassigned to a different service.  In hindsight, the "smtps"
   registration should have been renamed or reserved rather than
   revoked.  Unfortunately, some widely deployed mail software
   interpreted "smtps" as "submissions" [RFC6409] and used that port for
   email submission by default when an end-user requests security during
   account setup.  If a new port is assigned for the submissions
   service, email software will either continue with unregistered use of
   port 465 (leaving the port registry inaccurate relative to de-facto
   practice and wasting a well-known port), or confusion between the de-
   facto and registered ports will cause harmful interoperability
   problems that will deter use of TLS for message submission.  The
   authors believe both of these outcomes are less desirable than a wart
   in the registry documenting real-world usage of a port for two
   purposes.  Although STARTTLS-on-port-587 has deployed, it has not
   replaced deployed use of implicit TLS submission on port 465.

11.6.  DEEP IMAP Capability

   This document adds the DEEP capability to the IMAP capabilities
   registry.  This is described in Section 7.1.






Moore & Newman         Expires September 11, 2016              [Page 28]


Internet-Draft  Deployable Enhanced Email Privacy (DEEP)      March 2016


11.7.  DEEP POP3 Capability

   This document adds the DEEP capability to the POP3 capabilities
   registry.

   CAPA Tag:  DEEP

   Arguments:  deep-status

   Added Commands:  none

   Standard Commands affected:  none

   Announced status / possible differences:  both / may change after
      STLS

   Commands Valid in States:  N/A

   Specification Reference:  This document

   Discussion:  See Section 7.2.

11.8.  DEEP SMTP EHLO Keyword

   This document adds the DEEP EHLO Keyword to the SMTP Service
   Extension registry.  This is described in Section 7.3.

11.9.  SMTP Enhanced Status Code

   This document adds the following entry to the "SMTP Enhanced Status
   Codes" registry created by [RFC5248].

   Code:  X.7.TBD (IANA, please assign the next available number)

   Sample Text:  Message Transport Failed due to missing required
      security.

   Associated Basic Status Code:  450, 454, 550, 554

   Description  This code indicates an SMTP server was unable to forward
      a message to the next host necessary for delivery because it
      required a higher level of transport security or confidentiality
      than was available.  The temporary form of this error is preferred
      in case the problem is caused by a temporary administrative error
      such as an expired server certificate.






Moore & Newman         Expires September 11, 2016              [Page 29]


Internet-Draft  Deployable Enhanced Email Privacy (DEEP)      March 2016


   Reference  This document

   Submitter  C. Newman

   Change Controller  IESG

11.10.  MAIL Parameters Additional-registered-clauses Sub-Registry

   This document adds the following entry to the "Additional-registered-
   clauses" sub-registry of the "MAIL Parameters" registry, created by
   [RFC5321]:

   Clause Name:  tls

   Description:  Indicates the TLS cipher suite used for a transport
      connection.

   Syntax Summary:  See tls-cipher ABNF Section 6

   Reference:  This document.


12.  Security Considerations

   This entire document is about security considerations.  In general,
   this is targeted to improve mail confidentiality and to mitigate
   threats external to the email system such as network-level snooping
   or interception; this is not intended to mitigate active attackers
   who have compromised service provider systems.

   It could be argued that sharing the name and version of the client
   software with the server has privacy implications.  Although
   providing this information is not required, it is encouraged so that
   mail service providers can more effectively inform end-users running
   old clients that they need to upgrade to protect their security, or
   know which clients to use in a test deployment prior to upgrading a
   server to have higher security requirements.


13.  References

13.1.  Normative References

   [RFC1939]  Myers, J. and M. Rose, "Post Office Protocol - Version 3",
              STD 53, RFC 1939, May 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.



Moore & Newman         Expires September 11, 2016              [Page 30]


Internet-Draft  Deployable Enhanced Email Privacy (DEEP)      March 2016


   [RFC2449]  Gellens, R., Newman, C., and L. Lundblade, "POP3 Extension
              Mechanism", RFC 2449, November 1998.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [RFC2971]  Showalter, T., "IMAP4 ID extension", RFC 2971,
              October 2000.

   [RFC3207]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
              Transport Layer Security", RFC 3207, February 2002.

   [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, March 2003.

   [RFC3464]  Moore, K. and G. Vaudreuil, "An Extensible Message Format
              for Delivery Status Notifications", RFC 3464,
              January 2003.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC5034]  Siemborski, R. and A. Menon-Sen, "The Post Office Protocol
              (POP3) Simple Authentication and Security Layer (SASL)
              Authentication Mechanism", RFC 5034, July 2007.

   [RFC5068]  Hutzler, C., Crocker, D., Resnick, P., Allman, E., and T.
              Finch, "Email Submission Operations: Access and
              Accountability Requirements", BCP 134, RFC 5068,
              November 2007.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5248]  Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced
              Mail System Status Codes", BCP 138, RFC 5248, June 2008.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, May 2008.



Moore & Newman         Expires September 11, 2016              [Page 31]


Internet-Draft  Deployable Enhanced Email Privacy (DEEP)      March 2016


   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              October 2008.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              October 2008.

   [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, March 2011.

   [RFC6186]  Daboo, C., "Use of SRV Records for Locating Email
              Submission/Access Services", RFC 6186, March 2011.

   [RFC6409]  Gellens, R. and J. Klensin, "Message Submission for Mail",
              STD 72, RFC 6409, November 2011.

   [I-D.ietf-uta-email-tls-certs]
              Melnikov, A., "Updated TLS Server Identity Check Procedure
              for Email Related Protocols",
              draft-ietf-uta-email-tls-certs-03 (work in progress),
              June 2015.

   [I-D.ietf-dane-smtp-with-dane]
              Dukhovni, V. and W. Hardaker, "SMTP security via
              opportunistic DANE TLS", draft-ietf-dane-smtp-with-dane-02
              (work in progress), October 2013.

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, May 2015.

13.2.  Informative References

   [RFC2595]  Newman, C., "Using TLS with IMAP, POP3 and ACAP",
              RFC 2595, June 1999.

   [RFC2979]  Freed, N., "Behavior of and Requirements for Internet
              Firewalls", RFC 2979, October 2000.

   [RFC3848]  Newman, C., "ESMTP and LMTP Transmission Types
              Registration", RFC 3848, July 2004.

   [RFC3887]  Hansen, T., "Message Tracking Query Protocol", RFC 3887,
              September 2004.




Moore & Newman         Expires September 11, 2016              [Page 32]


Internet-Draft  Deployable Enhanced Email Privacy (DEEP)      March 2016


   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346, April 2006.

   [RFC4422]  Melnikov, A. and K. Zeilenga, "Simple Authentication and
              Security Layer (SASL)", RFC 4422, June 2006.

   [RFC4954]  Siemborski, R. and A. Melnikov, "SMTP Service Extension
              for Authentication", RFC 4954, July 2007.

   [RFC5598]  Crocker, D., "Internet Mail Architecture", RFC 5598,
              July 2009.

   [RFC5802]  Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams,
              "Salted Challenge Response Authentication Mechanism
              (SCRAM) SASL and GSS-API Mechanisms", RFC 5802, July 2010.

   [RFC5804]  Melnikov, A. and T. Martin, "A Protocol for Remotely
              Managing Sieve Scripts", RFC 5804, July 2010.

   [RFC6066]  Eastlake, D., "Transport Layer Security (TLS) Extensions:
              Extension Definitions", RFC 6066, January 2011.

   [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
              Cheshire, "Internet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", BCP 165,
              RFC 6335, August 2011.

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, August 2012.

   [RFC6797]  Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
              Transport Security (HSTS)", RFC 6797, November 2012.


Appendix A.  Design Considerations

   This section is not normative.

   The first version of this was written independently from
   draft-moore-email-tls-00.txt; subsequent versions merge ideas from
   both drafts.

   One author of this document was also the author of RFC 2595 that
   became the standard for TLS usage with POP and IMAP, and the other
   author was perhaps the first to propose that idea.  In hindsight both
   authors now believe that that approach was a mistake.  At this point



Moore & Newman         Expires September 11, 2016              [Page 33]


Internet-Draft  Deployable Enhanced Email Privacy (DEEP)      March 2016


   the authors believe that while anything that makes it easier to
   deploy TLS is good, the desirable end state is that these protocols
   always use TLS, leaving no need for a separate port for cleartext
   operation except to support legacy clients while they continue to be
   used.  The separate port model for TLS is inherently simpler to
   implement, debug and deploy.  It also enables a "generic TLS load-
   balancer" that accepts secure client connections for arbitrary foo-
   over-TLS protocols and forwards them to a server that may or may not
   support TLS.  Such load-balancers cause many problems because they
   violate the end-to-end principle and the server loses the ability to
   log security-relevant information about the client unless the
   protocol is designed to forward that information (as this
   specification does for the cipher suite).  However, they can result
   in TLS deployment where it would not otherwise happen which is a
   sufficiently important goal that it overrides the problems.

   Although STARTTLS appears only slightly more complex than separate-
   port TLS, we again learned the lesson that complexity is the enemy of
   security in the form of the STARTTLS command injection vulnerability
   (CERT vulnerability ID #555316).  Although there's nothing inherently
   wrong with STARTTLS, the fact it resulted in a common implementation
   error (made independently by multiple implementers) suggests it is a
   less secure architecture than Implicit TLS.

   Section 7 of RFC 2595 critiques the separate-port approach to TLS.
   The first bullet was a correct critique.  There are proposals in the
   http community to address that, and use of SRV records as described
   in RFC 6186 resolves that critique for email.  The second bullet is
   correct as well, but not very important because useful deployment of
   security layers other than TLS in email is small enough to be
   effectively irrelevant.  The third bullet is incorrect because it
   misses the desirable option of "use and latch-on TLS if available".
   The fourth bullet may be correct, but is not a problem yet with
   current port consumption rates.  The fundamental error was
   prioritizing a perceived better design based on a mostly valid
   critique over real-world deployability.  But getting security and
   confidentiality facilities actually deployed is so important it
   should trump design purity considerations.

   Port 465 is presently used for two purposes: for submissions by a
   large number of clients and service providers and for the "urd"
   protocol by one vendor.  Actually documenting this current state is
   controversial as discussed in the IANA considerations section.
   However, there is no good alternative.  Registering a new port for
   submissions when port 465 is widely used for that purpose already
   will just create interoperability problems.  Registering a port
   that's only used if advertised by an SRV record (RFC 6186) would not
   create interoperability problems but would require all client and



Moore & Newman         Expires September 11, 2016              [Page 34]


Internet-Draft  Deployable Enhanced Email Privacy (DEEP)      March 2016


   server deployments and software to change significantly which is
   contrary to the goal of promoting more TLS use.  Encouraging use of
   STARTTLS on port 587 would not create interoperability problems, but
   is unlikely to have impact on current undocumented use of port 465
   and makes the guidance in this document less consistent.  The
   remaining option is to document the current state of the world and
   support future use of port 465 for submission as this increases
   consistency and ease-of-deployment for TLS email submission.


Appendix B.  Change Log

   Changes since draft-ietf-uta-email-deep-01:

   o  Change text in tls11 and tls12 registrations to clarify
      certificate rules, including additional PKIX and DANE references.

   o  Change from tls10 to tls11 (including reference) as the minimum.

   o  Fix typo in example 5.

   o  Remove open issues section; enough time has passed so not worth
      waiting for more input.

   Changes since draft-ietf-uta-email-deep-00:

   o  Update and clarify abstract

   o  use term confidentiality instead of privacy in most cases.

   o  update open issues to request input for missing text.

   o  move certificate pinning sub-section to account setup section and
      attempt to define it more precisely.

   o  Add note about end-to-end encryption in AVAS section.

   o  swap order of DNSSEC and TLSA sub-sections.

   o  change meaning of 'tls10' and 'tls12' latches to require
      certificate validation.

   o  Replace cipher suite advice with reference to RFC 7525.  Change
      examples to use TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 as cipher
      suite.

   o  Add text to update IMAP, POP3 and Message Submission standards
      with newer TLS advice.



Moore & Newman         Expires September 11, 2016              [Page 35]


Internet-Draft  Deployable Enhanced Email Privacy (DEEP)      March 2016


   o  Add clearer text in introduction that this does not cover SMTP
      relay.

   o  Update references to uta-tls-certs.

   o  Add paragraph to Implicit TLS for SMTP Submission section
      recommending that STARTTLS also be implemented.

   Changes since draft-newman-email-deep-02:

   o  Changed "privacy assurance" to "confidentiality assurance"

   o  Changed "low privacy assurance" to "no confidentiality assurance"

   o  Attempt to improve definition of confidentiality assurance level.

   o  Add SHOULD indicate when MUA is showing list of mail accounts.

   o  Add SHOULD NOT latch tls10, tls12 tags until TLS negotiated.

   o  Removed sentence about deleting and re-creating the account in
      latch failure section.

   o  Remove use of word "fallback" with respect to TLS version
      negotiation.

   o  Added bullet about changes to Internet facing servers to MSP
      section.

   o  minor wording improvements based on feedback

   Changes since -01:

   o  Updated abstract, introduction and document structure to focus
      more on mail user agent privacy assurance.

   o  Added email account privacy section, also moving section on
      account setup using SRV records to that section.

   o  Finished writing IANA considerations section

   o  Remove provisional concept and instead have server explicitly list
      security tags clients should latch.

   o  Added note that rules for the submissions port follow the same
      rules as those for the submit port.





Moore & Newman         Expires September 11, 2016              [Page 36]


Internet-Draft  Deployable Enhanced Email Privacy (DEEP)      March 2016


   o  Reference and update advice in [RFC5068].

   o  Fixed typo in Client Certificate Authentication section.

   o  Removed tls-pfs security latch and all mention of perfect forward
      secrecy as it was controversial.

   o  Added reference to HSTS.

   Changes since -00:

   o  Rewrote introduction to merge ideas from draft-moore-email-tls-00.

   o  Added Implicit TLS section, Account configuration section and IANA
      port registration updates based on draft-moore-email-tls-00.

   o  Add protocol details necessary to standardize implicit TLS for
      POP/IMAP/submission, using ideas from
      draft-melnikov-pop3-over-tls.

   o  Reduce initial set of security tags based on feedback.

   o  Add deep status concept to allow a window for software updates to
      be backed out before latches make that problematic, as well as to
      provide service providers with a mechanism they can use to assist
      customers in the event of a privacy failure.

   o  Add DNS SRV section from draft-moore-email-tls-00.

   o  Write most of the missing IANA considerations section.

   o  Rewrite most of implementation requirements section based more on
      draft-moore-email-tls-00.  Remove new cipher requirements for now
      because those may be dealt with elsewhere.


Appendix C.  Acknowledgements

   Thanks to Ned Freed for discussion of the initial latch concepts in
   this document.  Thanks to Alexey Melnikov for
   draft-melnikov-pop3-over-tls-02, which was the basis of the POP3
   implicit TLS text.  Thanks to Russ Housley, Alexey Melnikov and Dan
   Newman for review feedback.  Thanks to Paul Hoffman for interesting
   feedback in initial conversations about this idea.







Moore & Newman         Expires September 11, 2016              [Page 37]


Internet-Draft  Deployable Enhanced Email Privacy (DEEP)      March 2016


Authors' Addresses

   Keith Moore
   Network Heretics
   PO Box 1934
   Knoxville, TN  37901
   US

   Email: moore@network-heretics.com


   Chris Newman
   Oracle
   440 E. Huntington Dr., Suite 400
   Arcadia, CA  91006
   US

   Email: chris.newman@oracle.com

































Moore & Newman         Expires September 11, 2016              [Page 38]


Html markup produced by rfcmarkup 1.128b, available from https://tools.ietf.org/tools/rfcmarkup/