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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 13, 2017
Expires: September 14, 2017


          Mail User Agent Strict Transport Security (MUA-STS)
                      draft-ietf-uta-email-deep-06

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 and provides a model for a mail user
   agent's confidentiality assurance.  This enables mail service
   providers to advertise strict transport security (STS) policies that
   request MUAs increase confidentiality assurance.

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 14, 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
   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



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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions and Terminology Used in This Document . . . . . .   4
   3.  Mail Account Confidentiality Assurance Level  . . . . . . . .   4
     3.1.  Confidentiality Assurance Level 1 . . . . . . . . . . . .   6
     3.2.  Confidentiality Assurance Level 0 . . . . . . . . . . . .   7
     3.3.  Other Confidentiality Assurance Levels  . . . . . . . . .   7
   4.  Implicit TLS  . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Implicit TLS for POP  . . . . . . . . . . . . . . . . . .   8
     4.2.  Implicit TLS for IMAP . . . . . . . . . . . . . . . . . .   8
     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 Directives  . . . . .   9
   6.  Server Strict Transport Security Policy . . . . . . . . . . .  11
   7.  Client Storage of Email Security Directives . . . . . . . . .  11
     7.1.  Security Directive Upgrade Example  . . . . . . . . . . .  12
     7.2.  Security Policy Failures  . . . . . . . . . . . . . . . .  12
   8.  Recording TLS Cipher Suite in Received Header . . . . . . . .  12
   9.  Extensions for STS Policy and Reporting . . . . . . . . . . .  13
     9.1.  IMAP STS Extension  . . . . . . . . . . . . . . . . . . .  13
     9.2.  POP DEEP Extension  . . . . . . . . . . . . . . . . . . .  15
     9.3.  SMTP MSTS Extension . . . . . . . . . . . . . . . . . . .  16
   10. Account Setup Considerations  . . . . . . . . . . . . . . . .  18
     10.1.  Use of SRV records in Establishing Configuration . . . .  18
     10.2.  Certificate Pinning  . . . . . . . . . . . . . . . . . .  19
   11. Implementation Requirements . . . . . . . . . . . . . . . . .  19
     11.1.  All Implementations (Client and Server)  . . . . . . . .  19
       11.1.1.  Client Certificate Authentication  . . . . . . . . .  20
     11.2.  Mail Server Implementation Requirements  . . . . . . . .  21
     11.3.  Mail User Agent Implementation Requirements  . . . . . .  21
     11.4.  Non-configurable MUAs and nonstandard access protocols .  22
     11.5.  Compliance for Anti-Virus/Anti-Spam Software and
            Services . . . . . . . . . . . . . . . . . . . . . . . .  22
   12. Mail Service Provider Requirements  . . . . . . . . . . . . .  23
     12.1.  Server Requirements  . . . . . . . . . . . . . . . . . .  23
     12.2.  MSPs MUST provide Submission Servers . . . . . . . . . .  23
     12.3.  TLS Server Certificate Requirements  . . . . . . . . . .  23
     12.4.  Recommended DNS records for mail protocol servers  . . .  24
       12.4.1.  MX records . . . . . . . . . . . . . . . . . . . . .  24
       12.4.2.  SRV records  . . . . . . . . . . . . . . . . . . . .  24
       12.4.3.  DNSSEC . . . . . . . . . . . . . . . . . . . . . . .  24



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       12.4.4.  TLSA records . . . . . . . . . . . . . . . . . . . .  24
     12.5.  MSP Server Monitoring  . . . . . . . . . . . . . . . . .  24
     12.6.  Advertisement of STS policies  . . . . . . . . . . . . .  25
     12.7.  Require TLS  . . . . . . . . . . . . . . . . . . . . . .  25
     12.8.  Changes to Internet Facing Servers . . . . . . . . . . .  25
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  25
     13.1.  Security Directive Registry  . . . . . . . . . . . . . .  25
     13.2.  Initial Set of Security Directives . . . . . . . . . . .  26
     13.3.  POP3S Port Registration Update . . . . . . . . . . . . .  29
     13.4.  IMAPS Port Registration Update . . . . . . . . . . . . .  29
     13.5.  Submissions Port Registration  . . . . . . . . . . . . .  29
     13.6.  STS IMAP Capability  . . . . . . . . . . . . . . . . . .  30
     13.7.  STS POP3 Capability  . . . . . . . . . . . . . . . . . .  30
     13.8.  MSTS SMTP EHLO Keyword . . . . . . . . . . . . . . . . .  30
     13.9.  MAIL Parameters Additional-registered-clauses Sub-
            Registry . . . . . . . . . . . . . . . . . . . . . . . .  31
   14. Security Considerations . . . . . . . . . . . . . . . . . . .  31
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  31
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  31
     15.2.  Informative References . . . . . . . . . . . . . . . . .  34
   Appendix A.  Design Considerations  . . . . . . . . . . . . . . .  35
   Appendix B.  Change Log . . . . . . . . . . . . . . . . . . . . .  36
   Appendix C.  Acknowledgements . . . . . . . . . . . . . . . . . .  41
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  42

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.
   This adapts the strict transport security (STS) model described in
   [RFC6797] to cover mail user agents (MUAs).

   In brief, this memo now recommends that:

   o  MUAs associate a minimum confidentiality assurance level with each
      mail account, and disconnections associated with that account that
      do not provide the minimum confidentiality assurance level
      associated with that account.

   o  By default, MUAs assign a minimum confidentiality assurance level
      that requires use of TLS with certificate validation for all TCP
      connections;




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   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.  The security upgrade model is
      aligned with the HTTP STS specification [RFC6797].

   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.  One
   approach to address that topic is described in [RFC7672].

   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
   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 user uses a mail user
   agent (MUA) to access a mail account.  (Most MUAs support the ability
   to access multiple 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



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   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 minimum confidentiality assurance level with each mail
   account.  If during a session with a network service, the
   requirements for the minimum confidentiality assurance level
   associated with that mail account are not met, the MUA MUST NOT
   continue the session with the network service.  MUAs MUST support at
   least the ability to detect whether a session with a network service
   implements confidentiality assurance level 1 as described in the next
   section.  Note that the minimum confidentiality assurance level
   associated with an account applies to all protocol interactions and
   all servers associated with the account.

   MUAs SHOULD continuously indicate to the user the current
   confidentiality assurance level of any account currently in use when
   reading, submitting and managing mail (e.g., via a lock icon,
   background colors, or other indications similar to those commonly
   used in web browsers for a similar purpose) and SHOULD indicate the
   minimum confidentiality assurance level for each account whenever
   displaying a list of mail accounts.  Note that the displayed
   confidentiality assurance level for a current session could be higher
   than the minimum confidentiality assurance level set at account
   configuration, but never lower.  If multiple active connections are
   associated with an account or view, the indication of the current
   confidentiality assurance level associated with the account should
   reflect the level provided by the least confidential connection.  It
   is therefore possible that at any given instant some services
   associated with a mail account meet the minimum confidentiality
   assurance level associated with the account, and other services do
   not.  An MUA MAY continue to interact with those services for which
   the minimum confidentiality assurance level is met, while refusing to
   interact with those services for which the minimum confidentiality
   assurance level is not met.  For example, if the IMAP service
   associated with a mail account meets the minimum confidentiality
   assurance level, but the Mail Submission service associated with that
   account does not, the MUA MAY continue to permit reading mail from
   that account but MUST NOT send mail until it can do so using a
   Submission service that meets the minimum confidentiality assurance
   level for that account.

   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



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   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 require a
   minimum confidentiality assurance level of 1 as the default for newly
   configured accounts.

   This document defines two initial confidentiality assurance levels, 1
   and 0.  It is expected that other levels may be defined in the
   future, as needed to thwart increasingly sophisticated and/or
   pervasive attacks.

3.1.  Confidentiality Assurance Level 1

   A mail account has a confidentiality assurance level of 1 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 successfully negotiate TLS via either
      Implicit TLS Section 4 or STARTTLS.

   o  For protocols using TCP, both client and server must support, and
      negotiate, a TLS version of 1.1 or greater.

   o  MUAs MUST implement [RFC7817] and PKIX [RFC5280].

   o  MUAs MAY implement DANE [RFC6698] as an alternate means of
      verifying TLS server certificates.  For confidentiality assurance
      level 1, a certificate may be considered valid if it can be
      validated using either DANE or PKIX.

   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 implementing a minimum confidentiality assurance of 1



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   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.  Confidentiality Assurance Level 0

   MUAs MAY support the ability to configure accounts with a minimum
   confidentiality assurance level of 0.  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, or using TLS versions prior to TLS 1.1, for accounts with a
   minimum confidentiality assurance level of 0.  Even for accounts with
   a minimum confidentiality assurance level of 0, MUAs SHOULD attempt
   TLS first if available, and MUST implement the ability to reconnect
   without TLS if TLS negotiation fails for reasons other than server
   certificate validity.

   Note that if TLS is not used, or a version of TLS prior to TLS 1.1 is
   negotiated, or the TLS server certificate is not successfully
   validated as described in Section 3.1, the client MUST clearly
   indicate to the user that there is currently no assurance of
   confidentiality for the mail 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, an
   implementation MAY implement confidentiality assurance levels other
   than those defined in this document, as long as those levels are
   distinguished in user interfaces from those defined in this document,
   and the ordering associated with them reflects the actual expectation
   of confidentiality provided.  However, implementation of levels below
   confidentiality assurance level 0, as described in the previous
   section, is discouraged.  Implementers are also cautioned that end
   users may be confused by too many confidentiality assurance levels.

   As stated above, higher confidentiality assurance levels may be
   standardized in the future.  For example, a future confidentiality
   assurance levels might require multiple independent trust anchors for
   server certificate validation.

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



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   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
   [RFC7817].  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 11.1.1 for additional information on client certificate
   authentication.  See Section 13.3 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]
   and SHOULD implement the certificate validation mechanism described
   in [RFC7817].  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 11.1.1 for additional information on client certificate
   authentication.  See Section 13.4 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
   [RFC7817].  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



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   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 13.5).  This
   differs from IMAP and POP services where implicit TLS is more widely
   deployed on servers than STARTTLS.  It is desirable to migrate core
   protocols used by MUA software to implicit TLS over time for
   consistency as well as the additional reasons discussed in
   Appendix A.  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 11.1).

   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 11.1.1 for additional information on client certificate
   authentication.  See Section 13.5 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 Directives

   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



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   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 many do so without
   alerting the user 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 directives",
   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.  Once the client has changed the
   configuration for a mail service to require specific server security
   features, those features are said to be "latched".

   Note that security directives are a separate mechanism from minimum
   confidentiality assurance levels.  A connection between a client and
   a service MUST meet the requirements of both the minimum
   confidentiality assurance level associated with the account, and the
   conditions of any security directives established for that service.
   Otherwise the client MUST abandon the connection.  When an MUA
   implements both minimum confidentiality assurance levels and security
   directives, then both the end-user and the service provider
   independently have the ability to improve the end-user's
   confidentiality.

   A security directive has the following formal syntax:

     directive        =  directive-name [ "=" directive-value ]

     directive-name   =  token

     directive-value  =  token

     token            =  <As defined in RFC 7230>

   This is a subset of the syntax used by HSTS [RFC6797] as revised in
   [RFC7230]; but simplified for use by protocols other than HTTP.






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6.  Server Strict Transport Security Policy

   Servers supporting this extension MUST advertise an STS policy.  This
   includes a list of security directives the server administrator has
   explicitly configured as recommended for use by clients (the list MAY
   be empty).  When a server advertises a security directive associated
   with a security facility, it is making a commitment to support that
   facility (or a revised version of that facility) indefinitely and
   recommending that the client save that directive with the account
   configuration and require that security facility for future
   connections to that server.

   Server STS policy may also include a "sts-url" directive with a value
   containing an 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.  Server STS policy has the following
   formal syntax:

     sts-policy        =  [directive *(";" [SP] directive)]

   Protocol extensions to advertise STS policy for email servers are
   defined in Section 9.

   The IANA Considerations Section 13 defines a registry so that more
   directives can be defined in the future.  Three initial directives
   are defined for use by MUAs in Section 13.2: tls-version, sts-url,
   and tls-cert.

7.  Client Storage of Email Security Directives

   Before a client can consider storing any security directives, it MUST
   verify that the connection to the server uses TLS, the server has
   been authenticated, and any requirements for any previously saved
   security directives are met.  Then the client performs the following
   steps for each security directive in the STS policy:

   1.  If the security directive name is not known to the client, skip
       to the next directive.

   2.  If the security directive is already saved with the same value
       (or a value considered greater than the current value in the
       directive's definition), the client skips the security directive
       and moves on to the next one.

   3.  The client verifies the connection meets the requirements of the
       security directive.  If the connection does not, then the
       directive will not be saved.  For example, a security directive
       claiming that the server supports tls-version 1.2 will not be



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       saved by a client if the currently negotiated TLS session is
       using TLS 1.1.

   4.  If previous steps pass, the client SHOULD update the current
       account configuration to save the security directive.

   Once a security directive is saved, all subsequent connections to
   that host require any associated 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 saved security directives.

7.1.  Security Directive Upgrade Example

   Suppose a server advertises the "tls-version" directive name with
   value "1.1".  A client that successfully negotiates either TLS 1.1 or
   TLS 1.2 SHOULD save this directive.  The server may subsequently
   change the value to "1.2".  When a client with "1.1" saved value
   connects and negotiates TLS 1.2, it will upgrade the saved directive
   value to "1.2".  However, a client that only supports TLS 1.1 will
   continue to require use of TLS 1.1 and work with that server as long
   as it permits TLS 1.1.  This way individual clients can require the
   newer/stronger protocol (e.g., TLS 1.2), while older clients can
   continue to communicate securely (albeit potentially less so) using
   the older protocol.

7.2.  Security Policy Failures

   When a security directive has been saved for connections from a
   client to a server and the facility identified by that directive 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 sts-url https URL if one
   is available.  MUAs are discouraged from offering a lightweight
   option to reset or ignore directives as this defeats the benefit they
   provide to end users.

8.  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)



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   included in the TLS cipher suite registry.  In the event the
   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

9.  Extensions for STS Policy and Reporting

   This memo defines optional mechanisms for use by MUAs to communicate
   saved STS policy to servers and for servers to advertise policy.  One
   purpose of such mechanisms is to permit servers to determine which
   and how many clients have saved security directives, 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 STS capability is
   defined so the client can access the server's STS policy.

9.1.  IMAP STS Extension

   When an IMAP server advertises the STS 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 STS
   policy.

   sts  From server to client, the argument to this ID field is the
      server STS policy.  Servers MUST provide this information in
      response to an ID command.

   saved  From client to server, this is a list of security directives
      the client has saved for this server (the client MAY omit the
      value for the sts-url directive in this context).  Servers MAY
      record this information so administrators know the expected
      security properties of the client and can thus act to avoid



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

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

   directives  From client to server, this is a list of security
      directive names the client supports that are not saved.

   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 8) to the server.

   IMAP clients SHOULD use the IMAP ID command to report policy failures
   and determine the server STS policy.  Clients MAY use the ID command
   to report other security directive information.  IMAP servers MUST
   implement the ID command at least to report STS policy to clients.

     <client connected to port 993 and negotiated TLS successfully>
     S: * OK [CAPABILITY IMAP4rev1 STS ID AUTH=PLAIN
             AUTH=SCRAM-SHA-1] hello
     C: a001 ID ("name" "Demo Mail" "version" "1.5" "saved"
             "tls-version=1.1; tls-cert"
             "directives" "tls-version=1.2")
     S: * ID ("name" "Demo Server" "version" "1.7" "sts-policy"
              "tls-version=1.1; tls-cert;
              sts-url=https://www.example.com/security-support.html")
     S: a001 OK ID completed

                                 Example 1

   This example shows a client that successfully negotiated TLS version
   1.1 or later and verified the server's certificate as required by
   IMAP.  Even if the client successfully validates the server
   certificate, it will not require tls-version 1.2 in the future as the
   server does not advertise that version as policy.  The client has not
   yet saved an STS URL, but if the client successfully validated the
   server certificate, it will save the provided URL.









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     <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" "policy-failure"
             "tls-cert=pkix")
     S: * ID ("name" "Demo Server" "version" "1.7" "sts-policy"
              "tls-version=1.1;
               sts-url=<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 using PKIX.  The policy-failure
   informs the server of this problem, at which point the client can
   disconnect.  If the client had previously saved the sts-url security
   directive from this server, it could offer to resolve that URI.
   However, the sts-policy in this exchange is ignored due to the
   failure to meet the conditions of the tls-version security directive.

     <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" "saved"
             "tls-version=1.1; tls-cert=pkix"
             "tls" "TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256")
     S: * ID ("name" "Demo Server" "version" "1.7" "sts-policy"
              "tls-version=1.1; tls-cert=pkix;
              sts-url=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.

9.2.  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.
   (Note: DEEP is an ancronym for the original name of this




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   specification, before the terms were changed to align better with
   those used in HSTS.)

     <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: STS tls-version=1.2
        sts-url=<https://www.example.com/security-support.html>
     S: .

                                 Example 4

   After verifying the TLS server certificate and issuing CAPA, the
   client can save any or all of the STS policy.  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-saved URL where the
   service provider can provide advice to the end user.

9.3.  SMTP MSTS Extension

   SMTP Submission servers supporting this specification MUST implement
   the MSTS SMTP extension.  The name of this extension is MSTS.  The
   EHLO keyword value is MSTS and the sts-policy 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:


















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

     deep-parameter       = name / version / policy-fail
                            / directives / tls / future-extension

     name                 = "name" SP esmtp-value

     version              = "version" SP esmtp-value

     saved                = "saved" SP directive-list

     policy-fail          = "policy-fail" SP directive-list

     directive-list       = DQUOTE [directive
                            *(";" [SP] directive)] DQUOTE

     directives           = "directives" SP directive-list

     tls                  = "tls" SP tls-cipher

     future-extension     = Atom SP String

     Atom                 = <as defined in RFC 5321>

     String               = <as defined in RFC 5321>

   The CLIENT command parameters listed here have the same meaning as
   the parameters used in the IMAP STS extension (Section 9.1).  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-MSTS tls-version=1.2; tls-cert;
        sts-url=<https://www.example.com/status.html>
     S: 250-BURL imap
     S: 250 SIZE 0
     C: CLIENT name demo_submit version 1.5 saved "tls-version=1.1;
        tls-cert=pkix+dane" directives "tls-version=1.2"
     S: 250 OK

                                 Example 5



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10.  Account Setup Considerations

10.1.  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 minimum confidentiality
   assurance level of 1, 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.

   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.






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10.2.  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 [RFC7817] 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 meet the requirement for a minimum confidentiality assurance
   level of 1, and an MUA MUST NOT indicate a confidentiality assurance
   level of 1 for an account or connection using a pinned certificate.
   Additional advice on certificate pinning is present in [RFC6125].

11.  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
   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.

11.1.  All Implementations (Client and Server)

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

   o  All implementations MUST implement TLS 1.2 or later, and 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.





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   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.  See Appendix B of
      [RFC7817] to see additional modifications to IMAP certificate
      validation rules.

   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.  See Appendix B of
      [RFC7817] to see additional modifications to RFC 2595 certificate
      validation rules.

   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 [RFC7817] and PKIX [RFC5280].

11.1.1.  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
   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]).








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11.2.  Mail Server Implementation Requirements

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

   o  Servers MUST implement the appropriate STS Policy and Reporting
      extensions described in Section 9

   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 capability 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 8.

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

11.3.  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 when
   using SMTP servers (e.g., port 25) for mail submission.

   o  User agents SHOULD indicate to users at configuration time, the
      minimum expected level of confidentiality based on appropriate
      security inputs such as which security directives 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.

   o  For any mail service not initially configured to require TLS, MUAs
      SHOULD detect when STARTTLS and/or implicit TLS becomes available
      for a protocol and set the tls-version security directive if the
      server advertises the tls-version=1.1 or higher security policy
      after a successful negotiation (including certificate validation)
      of TLS 1.1.





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   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 support the ability to save the "tls-version=1.2"
      security directive (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.

11.4.  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.

11.5.  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, are out of scope for this specification.
   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 this
   specification, and other forms of confidentiality or security
   reduction.  An AVAS product or service is considered compliant with
   this specification if all IMAP, POP and SMTP-related software
   (including proxies) it includes are compliant with this
   specification, and each of these services advertise and support all
   security directives that the actual end-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 a minimum confidentiality assurance
   level of 1 or better can prevent a man-in-the-middle from introducing
   spam or virus content between the MUA and Submission server, it does




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   not prevent other forms of client or account compromise.  Use of AVAS
   services for submitted email therefore remains necessary.

12.  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.

12.1.  Server Requirements

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

12.2.  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
   services, separate from the SMTP MTA services used to process
   incoming mail.  Those submission services 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.

12.3.  TLS Server Certificate Requirements

   MSPs MUST maintain valid server certificates for all servers.  See
   [RFC7817] for the recommendations and requirements necessary to
   achieve this.

   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



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   extension to TLS [RFC6066].  For more discussion of this problem, see
   section 5.1 of [RFC7817].

12.4.  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.

12.4.1.  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.

12.4.2.  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.

12.4.3.  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.

12.4.4.  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.

12.5.  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
   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.



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12.6.  Advertisement of STS policies

   MSPs SHOULD advertise STS policies that include at least tls11, tls-
   cert and sts-url, with the latter having an associated 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.  A MSP MAY
   also specifically indicate a commitment to support PKIX validation,
   DANE validation, or both, using tls-cert=pkix, tls-cert=dane, or tls-
   cert=pkix+dane, respectively.

12.7.  Require TLS

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

12.8.  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 directives that were previously
   advertised.

13.  IANA Considerations

13.1.  Security Directive Registry

   IANA shall create (has created) the registry "STS Security
   Directives".  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 directive.  This follows the
      directive-name ABNF.

   Value:  The permitted values of the security directive.  This should
      also explain if the value is optional or mandatory and what to do
      if the value is not recognized.

   Description:  This describes the meaning of the security directive
      and the conditions under which the directive is saved.

   Scope:  The protocols to which this security directive applies.
      Presently this may be MSTS (for MUA STS), HSTS (for HTTP STS), or
      ALL.




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

   Reference:  Optional reference to specification.

   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 directive name follows the ABNF,
   and that the value and description fields are clear, unambiguous, do
   not overlap existing deployed technology, do not create security
   problems and appropriately considers interoperability issues.  Email
   security directives 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).

13.2.  Initial Set of Security Directives

   This document defines three initial security directives for the
   registry as follows, and registers the two additional directives
   specified in [RFC6797].

   Name:  tls-version

   Value:  Mandatory; 1.1 refers to [RFC4346] or later and 1.2 refers to
      [RFC5246] or later.  Future versions may be added; this is ignored
      if the version is unrecognized.

   Description:  This directive indicates that the TLS version
      negotiated must be the specified version or later.  In the event
      this directive is saved and only an older TLS version is
      available, that results in STS policy failure.

   Scope:  MUA only

   Intended Usage:  COMMON




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   Reference:  RFC XXXX (this document once published)

   Submitter:  Authors of this document

   Change Controller:  IESG

   Name:  tls-cert

   Value:  Optional; pkix refers to PKIX certificate validation; dane
      refers to DANE certificate validation; pkix+dane refers to use of
      both PKIX and DANE validation; any refers to any validation method
      the client considers acceptable.  If no value is supplied, "any"
      is assumed.

   Description:  This directive indicates that TLS was successfully
      negotiated and the server certificate was successfully verified by
      the client [RFC5280] and the server certificate identity was
      verified using the algorithm appropriate for the protocol (see
      Section 4).  This directive is saved if the client sees this in
      the advertised server STS policy after successfully negotiating
      TLS and verifying the certificate and server identity using a
      means consistent with the associated (or implied) value.  Note
      that an advertisement of either tls-cert=pkix or tls-
      cert=pkix+dane in a server's STS policy indicates that the server
      commits to using certificates that are verifiable using PKIX in
      the future, but tls-cert=pkix implies no commitment regarding DANE
      support.  Similarly, an advertisement of either tls-cert=dane or
      tls-cert=pkix+dane indicates that the server commits to using
      certificates that are verifiable using DANE in the future, but
      tls-cert=dane implies no commitment regarding PKIX support.  An
      advertisement of tls-cert or tls-cert=any indicates only that the
      server will continue to provide valid server certificates, but
      makes no commitment about the means of verifiability.  (For the
      HSTS protocol, the presence of a Strict-Transport-Security
      response header serves as an indication that the certificate
      should be valid, so the tls-cert directive is never specified in
      that protocol.)

   Scope:  MUA only

   Intended Usage:  COMMON

   Reference:  RFC XXXX (this document once published)

   Submitter:  Authors of this document

   Change Controller:  IESG




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   Name:  sts-url

   Value:  Mandatory for server-policy, optional for client reporting.
      The value is an https URL.

   Description:  This directive indicates that the client SHOULD resolve
      (with appropriate certificate validation) and display the URL in
      the event of a policy failure.

   Scope:  MUA only

   Intended Usage:  COMMON

   Reference:  RFC XXXX (this document once published)

   Submitter:  Authors of this document

   Change Controller:  IESG

   Name:  max-age

   Value:  see [RFC6797].

   Description:  see [RFC6797].

   Scope:  HSTS only

   Intended Usage:  COMMON

   Reference:  [RFC6797]

   Submitter:  Authors of this document

   Change Controller:  IESG

   Name:  includeSubDomains

   Value:  None

   Description:  see [RFC6797].

   Scope:  HSTS only

   Intended Usage:  COMMON

   Reference:  [RFC6797]

   Submitter:  Authors of this document



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   Change Controller:  IESG

13.3.  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)
     Port Number: 995

13.4.  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)
     Port Number: 993

13.5.  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)
     Port Number: 465

   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"



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   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.

13.6.  STS IMAP Capability

   This document adds the STS capability to the IMAP capabilities
   registry.  This is described in Section 9.1.

13.7.  STS POP3 Capability

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

   CAPA Tag:  STS

   Arguments:  sts-policy

   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 9.2.

13.8.  MSTS SMTP EHLO Keyword

   This document adds the MSTS EHLO Keyword to the SMTP Service
   Extension registry.  This is described in Section 9.3.





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13.9.  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 8

   Reference:  This document.

14.  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.

15.  References

15.1.  Normative References

   [RFC1939]  Myers, J. and M. Rose, "Post Office Protocol - Version 3",
              STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996,
              <http://www.rfc-editor.org/info/rfc1939>.

   [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>.

   [RFC2449]  Gellens, R., Newman, C., and L. Lundblade, "POP3 Extension
              Mechanism", RFC 2449, DOI 10.17487/RFC2449, November 1998,
              <http://www.rfc-editor.org/info/rfc2449>.




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   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000,
              <http://www.rfc-editor.org/info/rfc2818>.

   [RFC2971]  Showalter, T., "IMAP4 ID extension", RFC 2971,
              DOI 10.17487/RFC2971, October 2000,
              <http://www.rfc-editor.org/info/rfc2971>.

   [RFC3207]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
              Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
              February 2002, <http://www.rfc-editor.org/info/rfc3207>.

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

   [RFC5034]  Siemborski, R. and A. Menon-Sen, "The Post Office Protocol
              (POP3) Simple Authentication and Security Layer (SASL)
              Authentication Mechanism", RFC 5034, DOI 10.17487/RFC5034,
              July 2007, <http://www.rfc-editor.org/info/rfc5034>.

   [RFC5068]  Hutzler, C., Crocker, D., Resnick, P., Allman, E., and T.
              Finch, "Email Submission Operations: Access and
              Accountability Requirements", BCP 134, RFC 5068,
              DOI 10.17487/RFC5068, November 2007,
              <http://www.rfc-editor.org/info/rfc5068>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <http://www.rfc-editor.org/info/rfc5234>.

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

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.

   [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, DOI 10.17487/RFC5280, May 2008,
              <http://www.rfc-editor.org/info/rfc5280>.




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

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

   [RFC6186]  Daboo, C., "Use of SRV Records for Locating Email
              Submission/Access Services", RFC 6186,
              DOI 10.17487/RFC6186, March 2011,
              <http://www.rfc-editor.org/info/rfc6186>.

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

   [RFC6797]  Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
              Transport Security (HSTS)", RFC 6797,
              DOI 10.17487/RFC6797, November 2012,
              <http://www.rfc-editor.org/info/rfc6797>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <http://www.rfc-editor.org/info/rfc7230>.

   [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, DOI 10.17487/RFC7525, May
              2015, <http://www.rfc-editor.org/info/rfc7525>.

   [RFC7672]  Dukhovni, V. and W. Hardaker, "SMTP Security via
              Opportunistic DNS-Based Authentication of Named Entities
              (DANE) Transport Layer Security (TLS)", RFC 7672,
              DOI 10.17487/RFC7672, October 2015,
              <http://www.rfc-editor.org/info/rfc7672>.

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








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15.2.  Informative References

   [RFC2595]  Newman, C., "Using TLS with IMAP, POP3 and ACAP",
              RFC 2595, DOI 10.17487/RFC2595, June 1999,
              <http://www.rfc-editor.org/info/rfc2595>.

   [RFC2979]  Freed, N., "Behavior of and Requirements for Internet
              Firewalls", RFC 2979, DOI 10.17487/RFC2979, October 2000,
              <http://www.rfc-editor.org/info/rfc2979>.

   [RFC3848]  Newman, C., "ESMTP and LMTP Transmission Types
              Registration", RFC 3848, DOI 10.17487/RFC3848, July 2004,
              <http://www.rfc-editor.org/info/rfc3848>.

   [RFC3887]  Hansen, T., "Message Tracking Query Protocol", RFC 3887,
              DOI 10.17487/RFC3887, September 2004,
              <http://www.rfc-editor.org/info/rfc3887>.

   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346,
              DOI 10.17487/RFC4346, April 2006,
              <http://www.rfc-editor.org/info/rfc4346>.

   [RFC4422]  Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
              Authentication and Security Layer (SASL)", RFC 4422,
              DOI 10.17487/RFC4422, June 2006,
              <http://www.rfc-editor.org/info/rfc4422>.

   [RFC4954]  Siemborski, R., Ed. and A. Melnikov, Ed., "SMTP Service
              Extension for Authentication", RFC 4954,
              DOI 10.17487/RFC4954, July 2007,
              <http://www.rfc-editor.org/info/rfc4954>.

   [RFC5598]  Crocker, D., "Internet Mail Architecture", RFC 5598,
              DOI 10.17487/RFC5598, July 2009,
              <http://www.rfc-editor.org/info/rfc5598>.

   [RFC5804]  Melnikov, A., Ed. and T. Martin, "A Protocol for Remotely
              Managing Sieve Scripts", RFC 5804, DOI 10.17487/RFC5804,
              July 2010, <http://www.rfc-editor.org/info/rfc5804>.

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,
              <http://www.rfc-editor.org/info/rfc6066>.






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   [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>.

   [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, DOI 10.17487/RFC6335, August 2011,
              <http://www.rfc-editor.org/info/rfc6335>.

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
              2012, <http://www.rfc-editor.org/info/rfc6698>.

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
   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



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   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
   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-05:

   o  Clarify throughout that the confidentiality assurance level
      associated with a mail account is a minimum level; attempt to
      distinguish this from the current confidentiality level provided
      by a connection between client and server.




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   o  Change naming for confidentiality assurance levels: instead of
      "high" or "no" confidence, assign numbers 1 and 0 to them
      respectively.  This because it seems likely that in the not-too-
      distant future, what was defined in -05 as "high" confidence will
      be considered insufficient, and calling that "high" confidence
      will become misleading.  For example, relying entirely on a list
      of trusted CAs to validate server certificates from arbitrary
      parties, appears to be less and less reliable in practice at
      thwarting MITM attacks.

   o  Clarify that if some services associated with a mail account don't
      meet the minimium confidentiality assurance level assigned to that
      account, other services that do meet that minimum confidentiality
      assurance level may continue to be used.

   o  Clarify that successful negotiation of at least TLS version 1.1 is
      required as a condition of meeting confidentiality assurance level
      1.

   o  Clarify that validation of a server certificate using either DANE
      or PKIX is sufficient to meet the certificate validation
      requirement of confidentiality assurance level 1.

   o  Clarify that minimum confidentiality assurance levels are separate
      from security directives, and that the requrements of both
      mechanisms must be met.

   o  Explicitly cite an example that a security directive of tls-
      version=1.2 won't be saved if the currently negotiated tls-version
      is 1.1.  (This example already appeared a bit later in the text,
      but for author KM it seemed to make the mechanism clearer to use
      this example earlier.)

   o  Clarify some protocol examples as to whether PKIX or DANE was used
      to verify a server's certificate.

   o  Remove most references to DEEP as the conversion from DEEP to MUA-
      STS seemed incomplete, but kept the DEEP command for use in POP3
      on the assumption that author CN wanted it that way.

   o  Removed most references to "latch" and derivative words.

   o  Added pkix+dane as a value for the tls-cert directive, to indicate
      (from a server) that both PKIX and DANE validation will be
      supported, or (from a client) that both PKIX and DANE were used to
      validate a certificate.  Also clarified what each of any, pkix,
      dane, and pkix+dane mean when advertised by a server and in
      particular that tls-cert=any provides no assurance of future PKIX



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      verifiability in contrast to tls-cert=pkix or tls-cert=pkix+dane.
      It seemed important to support the ability to evolve to using
      multiple trust anchors for certificate validation, but also to
      allow servers to have the option to migrate from PKIX to DANE if
      that made sense for them.  This change seemed less disruptive than
      either defining additional directives, or allowing multiple
      instances of the same directive with different values to appear in
      the same advertisement.

   o  Clarify interaction of this specification with anti-virus / anti-
      spam mechanisms.

   Changes since draft-ietf-uta-email-deep-04:

   o  Swap sections 5.1 and 5.3 ("Email Security Tags" and "Server DEEP
      Status") as that order may aid understanding of the model.  Also
      rewrote parts of these two sections to try to make the model
      clearer.

   o  Add text about versioning of security tags to make the model
      clearer.

   o  Add example of security tag upgrade.

   o  Convert remaining mention of TLS 1.0 to TLS 1.1.

   o  Change document title from DEEP to MUA STS to align with SMTP
      relay STS.

      *  Slight updates to abstract and introductions.

      *  Rename security latches/tags to security directives.

      *  Rename server DEEP status to STS policy.

      *  Change syntax to use directive-style HSTS syntax.

   o  Make HSTS reference normative.

   o  Remove SMTP DSN header as that belongs in SMTP relay STS document.

   Changes since draft-ietf-uta-email-deep-03:

   o  Add more references to ietf-uta-email-tls-certs in implementation
      requirements section.






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   o  Replace primary reference to RFC 6125 with ietf-uta-email-tls-
      certs, so move RFC 6125 to informative list for this
      specification.

   Changes since draft-ietf-uta-email-deep-02:

   o  Make reference to design considerations explicit rather than
      "elsewhere in this document".

   o  Change provider requirement so SMTP submission services are
      separate from SMTP MTA services as opposed to the previous
      phrasing that required the servers be separate (which is too
      restrictive).

   o  Update DANE SMTP reference

   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.






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   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.

   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




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   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.

   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




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   feedback.  Thanks to Paul Hoffman for interesting feedback in initial
   conversations about this idea.

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






























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