draft-ietf-uta-email-deep-06.txt   draft-ietf-uta-email-deep-07.txt 
Network Working Group K. Moore Network Working Group K. Moore
Internet-Draft Network Heretics Internet-Draft Network Heretics
Updates: 1939, 2595, 3464, 3501, 5068, C. Newman Updates: 1939, 2595, 3464, 3501, 5068, C. Newman
6186, 6409 (if approved) Oracle 6186, 6409 (if approved) Oracle
Intended status: Standards Track March 13, 2017 Intended status: Standards Track July 3, 2017
Expires: September 14, 2017 Expires: January 4, 2018
Mail User Agent Strict Transport Security (MUA-STS) Cleartext Considered Obsolete: Best Current Practices for Use of TLS for
draft-ietf-uta-email-deep-06 Email Submission and Access
draft-ietf-uta-email-deep-07
Abstract Abstract
This specification defines a set of requirements and facilities This specification outlines best current practices for use of
designed to improve email confidentiality between a mail user agent Transport Layer Security (TLS) to provide confidentiality of email
(MUA) and a mail submission or mail access server. This provides traffic between a mail user agent (MUA) and a mail submission or mail
mechanisms intended to increase use of already deployed Transport access server.
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 Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 14, 2017. This Internet-Draft will expire on January 4, 2018.
Copyright Notice Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Terminology Used in This Document . . . . . . 4 2. Conventions and Terminology Used in This Document . . . . . . 3
3. Mail Account Confidentiality Assurance Level . . . . . . . . 4 3. Implicit TLS . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Confidentiality Assurance Level 1 . . . . . . . . . . . . 6 3.1. Implicit TLS for POP . . . . . . . . . . . . . . . . . . 5
3.2. Confidentiality Assurance Level 0 . . . . . . . . . . . . 7 3.2. Implicit TLS for IMAP . . . . . . . . . . . . . . . . . . 5
3.3. Other Confidentiality Assurance Levels . . . . . . . . . 7 3.3. Implicit TLS for SMTP Submission . . . . . . . . . . . . 5
4. Implicit TLS . . . . . . . . . . . . . . . . . . . . . . . . 7 3.4. Implicit TLS Connection Closure for POP, IMAP and SMTP
4.1. Implicit TLS for POP . . . . . . . . . . . . . . . . . . 8 Submission . . . . . . . . . . . . . . . . . . . . . . . 6
4.2. Implicit TLS for IMAP . . . . . . . . . . . . . . . . . . 8 4. Best Current Practices for Use of TLS by Mail Access
4.3. Implicit TLS for SMTP Submission . . . . . . . . . . . . 8 Services and Message Submission Services . . . . . . . . . . 6
4.4. Implicit TLS Connection Closure for POP, IMAP and SMTP . 9 4.1. Deprecation of Services Using Cleartext and TLS Versions
5. Email Security Upgrading Using Security Directives . . . . . 9 < 1.1 . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Server Strict Transport Security Policy . . . . . . . . . . . 11 4.2. Mail Server Use of Client Certificate Authentication . . 8
7. Client Storage of Email Security Directives . . . . . . . . . 11 4.3. Recording TLS Cipher Suite in Received Header . . . . . . 9
7.1. Security Directive Upgrade Example . . . . . . . . . . . 12 4.4. TLS Server Certificate Requirements . . . . . . . . . . . 9
7.2. Security Policy Failures . . . . . . . . . . . . . . . . 12 4.5. Recommended DNS records for mail protocol servers . . . . 9
8. Recording TLS Cipher Suite in Received Header . . . . . . . . 12 4.5.1. MX records . . . . . . . . . . . . . . . . . . . . . 10
9. Extensions for STS Policy and Reporting . . . . . . . . . . . 13 4.5.2. SRV records . . . . . . . . . . . . . . . . . . . . . 10
9.1. IMAP STS Extension . . . . . . . . . . . . . . . . . . . 13 4.5.3. DNSSEC . . . . . . . . . . . . . . . . . . . . . . . 10
9.2. POP DEEP Extension . . . . . . . . . . . . . . . . . . . 15 4.5.4. TLSA records . . . . . . . . . . . . . . . . . . . . 10
9.3. SMTP MSTS Extension . . . . . . . . . . . . . . . . . . . 16 4.6. Changes to Internet Facing Servers . . . . . . . . . . . 10
10. Account Setup Considerations . . . . . . . . . . . . . . . . 18 5. Best Current Practices for use of TLS by Mail User Agents . . 10
10.1. Use of SRV records in Establishing Configuration . . . . 18 5.1. Use of SRV records in Establishing Configuration . . . . 12
10.2. Certificate Pinning . . . . . . . . . . . . . . . . . . 19 5.2. Minimum Confidentiality Level . . . . . . . . . . . . . . 13
11. Implementation Requirements . . . . . . . . . . . . . . . . . 19 5.3. Certificiate Validation . . . . . . . . . . . . . . . . . 14
11.1. All Implementations (Client and Server) . . . . . . . . 19 5.4. Certificate Pinning . . . . . . . . . . . . . . . . . . . 14
11.1.1. Client Certificate Authentication . . . . . . . . . 20 5.5. Client Certificate Authentication . . . . . . . . . . . . 14
11.2. Mail Server Implementation Requirements . . . . . . . . 21 6. Considerations related to Anti-Virus/Anti-Spam Software and
11.3. Mail User Agent Implementation Requirements . . . . . . 21 Services . . . . . . . . . . . . . . . . . . . . . . . . . . 15
11.4. Non-configurable MUAs and nonstandard access protocols . 22 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
11.5. Compliance for Anti-Virus/Anti-Spam Software and 7.1. POP3S Port Registration Update . . . . . . . . . . . . . 15
Services . . . . . . . . . . . . . . . . . . . . . . . . 22 7.2. IMAPS Port Registration Update . . . . . . . . . . . . . 16
12. Mail Service Provider Requirements . . . . . . . . . . . . . 23 7.3. Submissions Port Registration . . . . . . . . . . . . . . 16
12.1. Server Requirements . . . . . . . . . . . . . . . . . . 23 8. Security Considerations . . . . . . . . . . . . . . . . . . . 17
12.2. MSPs MUST provide Submission Servers . . . . . . . . . . 23 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
12.3. TLS Server Certificate Requirements . . . . . . . . . . 23 9.1. Normative References . . . . . . . . . . . . . . . . . . 17
12.4. Recommended DNS records for mail protocol servers . . . 24 9.2. Informative References . . . . . . . . . . . . . . . . . 19
12.4.1. MX records . . . . . . . . . . . . . . . . . . . . . 24 Appendix A. Design Considerations . . . . . . . . . . . . . . . 20
12.4.2. SRV records . . . . . . . . . . . . . . . . . . . . 24 Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 21
12.4.3. DNSSEC . . . . . . . . . . . . . . . . . . . . . . . 24 Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 26
12.4.4. TLSA records . . . . . . . . . . . . . . . . . . . . 24 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
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 1. Introduction
Software that provides email service via Internet Message Access Software that provides email service via Internet Message Access
Protocol (IMAP) [RFC3501], Post Office Protocol (POP) [RFC1939] and/ Protocol (IMAP) [RFC3501], Post Office Protocol (POP) [RFC1939] and/
or Simple Mail Transfer Protocol (SMTP) Submission [RFC6409] usually or Simple Mail Transfer Protocol (SMTP) Submission [RFC6409] usually
has Transport Layer Security (TLS) [RFC5246] support but often does has Transport Layer Security (TLS) [RFC5246] support but often does
not use it in a way that maximizes end-user confidentiality. This not use it in a way that maximizes end-user confidentiality. This
specification proposes changes to email software and deployments specification describes best current practices for use of TLS in
intended to increase the use of TLS and record when that use occurs. interactions between Mail User Agents and Mail Access Services, and
This adapts the strict transport security (STS) model described in between Mail User Agents and Mail Submission Services.
[RFC6797] to cover mail user agents (MUAs).
In brief, this memo now recommends that: In brief, this memo now recommends that:
o MUAs associate a minimum confidentiality assurance level with each o TLS version 1.1 or greater be used for all traffic between mail
mail account, and disconnections associated with that account that user agents (MUAs) and mail submission servers, and also between
do not provide the minimum confidentiality assurance level MUAs and mail access servers.
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;
o TLS on a well-known port ("Implicit TLS") be supported for IMAP, o MUAs and mail service providers discourage use of cleartext
POP, and SMTP Submission [RFC6409] for all electronic mail user protocols for mail access and mail submission, and deprecate use
agents (MUAs), servers, and service providers; of cleartext protocols for these purposes as soon as practicable.
o MUAs and mail protocol servers cooperate (via mechanisms defined o Use of "Implicit TLS" on ports reserved for that purpose, in
in this specification) to upgrade security feature use and record/ preference to STARTTLS on a port that otherwise supports
indicate that usage appropriately. The security upgrade model is cleartext.
aligned with the HTTP STS specification [RFC6797].
This does not address use of TLS with SMTP for message relay (where This memo does not address use of TLS with SMTP for message relay
Message Submission [RFC6409] does not apply). Improved use of TLS (where Message Submission [RFC6409] does not apply). Improved use of
with SMTP for message relay requires a different approach. One TLS with SMTP for message relay requires a different approach. One
approach to address that topic is described in [RFC7672]. approach to address that topic is described in [RFC7672].
The recommendations in this memo do not replace the functionality of, The recommendations in this memo do not replace the functionality of,
and are not intended as a substitute for, end-to-end encryption of and are not intended as a substitute for, end-to-end encryption of
electronic mail. 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 2. Conventions and Terminology Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
This specification expresses syntax using the Augmented Backus-Naur The term "Implicit TLS" refers to the automatic negotiation of TLS
Form (ABNF) as described in [RFC5234], including the core rules in whenever a TCP connection is made on a particular TCP port that is
Appendix B and rules from [RFC5322]. used exclusively by that server for TLS connections. The term
"Implicit TLS" is intended to contrast with use of STARTTLS and
In examples, "C:" and "S:" indicate lines sent by the client and similar commands in POP, IMAP, SMTP message submission, and other
server respectively. If a single "C:" or "S:" label applies to protocols, that are used by client and server to explicitly negotiate
multiple lines, then the line breaks between those lines are for TLS on an established cleartext TCP connection.
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
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
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
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 The term "Mail Access Services" includes POP, IMAP and any other
protocol used to access or modify received messages, or to access or
modify a mail user's account configuration.
MUAs MAY support the ability to configure accounts with a minimum "Mail Submission Service" refers to the use of the protocol specified
confidentiality assurance level of 0. At this level, the MUA MUST in [RFC6409] (or one of its predecessors or successors) for
attempt to negotiate TLS, but MAY ignore server certificate submission of outgoing messages for delivery to recipients.
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 The term "Mail Service Provider" (MSP) refers to a provider of Mail
negotiated, or the TLS server certificate is not successfully Access Services and/or Mail Submission Services.
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 The term "Mail Account" refers to a user's identity with a Mail
Service Provider, that user's authentication credentials, any user
email that is stored by the MSP, and any other per-user configuration
information maintained by the MSP (for example, spam filtering
instructions). Most Mail User Agents (MUAs) support the ability to
access multiple Mail Accounts.
This specification is not intended to limit experimentation and For each account that an MUA accesses on its user's behalf, it must
innovation with respect to user confidentiality. As a result, an have the server names, ports, authentication credentials, and other
implementation MAY implement confidentiality assurance levels other configuration information specified by the user. This information
than those defined in this document, as long as those levels are which is used by the MUA is referred to as "Mail Account
distinguished in user interfaces from those defined in this document, Configuration"
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 This specification expresses syntax using the Augmented Backus-Naur
standardized in the future. For example, a future confidentiality Form (ABNF) as described in [RFC5234], including the core rules in
assurance levels might require multiple independent trust anchors for Appendix B and rules from [RFC5322].
server certificate validation.
4. Implicit TLS 3. Implicit TLS
Previous standards for use of email protocols with TLS used the Previous standards for use of email protocols with TLS used the
STARTTLS mechanism: [RFC2595], [RFC3207], and [RFC3501]. With STARTTLS mechanism: [RFC2595], [RFC3207], and [RFC3501]. With
STARTTLS, the client establishes a clear text application session and STARTTLS, the client establishes a cleartext application session and
determines whether to issue a STARTTLS command based on server determines whether to issue a STARTTLS command based on server
capabilities and client configuration. If the client issues a capabilities and client configuration. If the client issues a
STARTTLS command, a TLS handshake follows that can upgrade the STARTTLS command, a TLS handshake follows that can upgrade the
connection. While this mechanism has been deployed, an alternate connection. While this mechanism has been deployed, an alternate
mechanism where TLS is negotiated immediately at connection start on mechanism where TLS is negotiated immediately at connection start on
a separate port (referred to in this document as "Implicit TLS") has a separate port (referred to in this document as "Implicit TLS") has
been deployed more successfully. To increase use of TLS, this been deployed more successfully. To encourage more widespread use of
specification recommends use of implicit TLS by new POP, IMAP and TLS, and to encourage a greater consistency for how TLS is used, this
SMTP Submission software. specification now recommends use of Implicit TLS for POP, IMAP, SMTP
Submission, and all other protocols used between a Mail User Agent
and a mail service.
4.1. Implicit TLS for POP 3.1. Implicit TLS for POP
When a TCP connection is established for the "pop3s" service (default When a TCP connection is established for the "pop3s" service (default
port 995), a TLS handshake begins immediately. Clients MUST port 995), a TLS handshake begins immediately. Clients MUST
implement the certificate validation mechanism described in implement the certificate validation mechanism described in
[RFC7817]. Once the TLS session is established, POP3 [RFC1939] [RFC7817]. Once the TLS session is established, POP3 [RFC1939]
protocol messages are exchanged as TLS application data for the protocol messages are exchanged as TLS application data for the
remainder of the TCP connection. After the server sends a +OK remainder of the TCP connection. After the server sends a +OK
greeting, the server and client MUST enter AUTHORIZATION state, even greeting, the server and client MUST enter AUTHORIZATION state, even
if client credentials were supplied during the TLS handshake. if client credentials were supplied during the TLS handshake.
See Section 11.1.1 for additional information on client certificate See Section 5.5 and Section 4.2 for additional information on client
authentication. See Section 13.3 for port registration information. certificate authentication. See Section 7.1 for port registration
information.
4.2. Implicit TLS for IMAP 3.2. Implicit TLS for IMAP
When a TCP connection is established for the "imaps" service (default When a TCP connection is established for the "imaps" service (default
port 993), a TLS handshake begins immediately. Clients MUST port 993), a TLS handshake begins immediately. Clients MUST
implement the certificate validation mechanism described in [RFC3501] implement the certificate validation mechanism described in [RFC3501]
and SHOULD implement the certificate validation mechanism described and SHOULD implement the certificate validation mechanism described
in [RFC7817]. Once the TLS session is established, IMAP [RFC3501] in [RFC7817]. Once the TLS session is established, IMAP [RFC3501]
protocol messages are exchanged as TLS application data for the protocol messages are exchanged as TLS application data for the
remainder of the TCP connection. If client credentials were provided remainder of the TCP connection. If client credentials were provided
during the TLS handshake that the server finds acceptable, the server during the TLS handshake that the server finds acceptable, the server
MAY issue a PREAUTH greeting in which case both the server and client MAY issue a PREAUTH greeting in which case both the server and client
enter AUTHENTICATED state. If the server issues an OK greeting then enter AUTHENTICATED state. If the server issues an OK greeting then
both server and client enter NOT AUTHENTICATED state. both server and client enter NOT AUTHENTICATED state.
See Section 11.1.1 for additional information on client certificate See Section 5.5 and Section 4.2 for additional information on client
authentication. See Section 13.4 for port registration information. certificate authentication. See Section 7.1 for port registration
information. See Section 7.2 for port registration information.
4.3. Implicit TLS for SMTP Submission 3.3. Implicit TLS for SMTP Submission
When a TCP connection is established for the "submissions" service When a TCP connection is established for the "submissions" service
(default port 465), a TLS handshake begins immediately. Clients MUST (default port 465), a TLS handshake begins immediately. Clients MUST
implement the certificate validation mechanism described in implement the certificate validation mechanism described in
[RFC7817]. Once a TLS session is established, message submission [RFC7817]. Once a TLS session is established, message submission
protocol data [RFC6409] is exchanged as TLS application data for the protocol data [RFC6409] is exchanged as TLS application data for the
remainder of the TCP connection. (Note: the "submissions" service remainder of the TCP connection. (Note: the "submissions" service
name is defined in section 10.3 of this document, and follows the 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 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, Implicit TLS consists of the name of the service as used without TLS,
with an "s" appended.) with an "s" appended.)
The STARTTLS mechanism on port 587 is relatively widely deployed due The STARTTLS mechanism on port 587 is relatively widely deployed due
to the situation with port 465 (discussed in Section 13.5). This to the situation with port 465 (discussed in Section 7.3). This
differs from IMAP and POP services where implicit TLS is more widely differs from IMAP and POP services where Implicit TLS is more widely
deployed on servers than STARTTLS. It is desirable to migrate core deployed on servers than STARTTLS. It is desirable to migrate core
protocols used by MUA software to implicit TLS over time for protocols used by MUA software to Implicit TLS over time for
consistency as well as the additional reasons discussed in consistency as well as the additional reasons discussed in
Appendix A. However, to maximize use of encryption for submission it Appendix A. However, to maximize use of encryption for submission it
is desirable to support both mechanisms for Message Submission over is desirable to support both mechanisms for Message Submission over
TLS for a transition period of several years. As a result, clients TLS for a transition period of several years. As a result, clients
and servers SHOULD implement both STARTTLS on port 587 and implicit and servers SHOULD implement both STARTTLS on port 587 and Implicit
TLS on port 465 for this transition period. Note that there is no TLS on port 465 for this transition period. Note that there is no
significant difference between the security properties of STARTTLS on significant difference between the security properties of STARTTLS on
port 587 and implicit TLS on port 465 if the implementations are port 587 and Implicit TLS on port 465 if the implementations are
correct and both client and server are configured to require correct and both client and server are configured to require
successful negotiation of TLS prior to message submission (as successful negotiation of TLS prior to message submission.
required in Section 11.1).
Note that the submissions port provides access to a Mail Submission Note that the "submissions" port provides access to a Mail Submission
Agent (MSA) as defined in [RFC6409] so requirements and Agent (MSA) as defined in [RFC6409] so requirements and
recommendations for MSAs in that document apply to the submissions recommendations for MSAs in that document apply to the submissions
port, including the requirement to implement SMTP AUTH [RFC4954]. port, including the requirement to implement SMTP AUTH [RFC4954].
See Section 11.1.1 for additional information on client certificate See Section 5.5 and Section 4.2 for additional information on client
authentication. See Section 13.5 for port registration information. certificate authentication. See Section 7.3 for port registration
information.
4.4. Implicit TLS Connection Closure for POP, IMAP and SMTP 3.4. Implicit TLS Connection Closure for POP, IMAP and SMTP Submission
When a client or server wishes to close the connection, it SHOULD When a client or server wishes to close the connection, it SHOULD
initiate the exchange of TLS close alerts before TCP connection initiate the exchange of TLS close alerts before TCP connection
termination. The client MAY, after sending a TLS close alert, termination. The client MAY, after sending a TLS close alert,
gracefully close the TCP connection without waiting for a TLS gracefully close the TCP connection without waiting for a TLS
response from the server. response from the server.
5. Email Security Upgrading Using Security Directives 4. Best Current Practices for Use of TLS by Mail Access Services and
Message Submission Services
Once an improved email security mechanism is deployed and ready for
general use, it is desirable to continue using it for all future
email service. For example, TLS is widely deployed in email
software, but use of TLS is often not required. At the time this is
written, deployed mail user agents (MUAs) [RFC5598] usually make a
determination if TLS is available when an account is first configured
and may require use of TLS with that account if and only if it was
initially available. If the service provider makes TLS available
after initial client configuration, many MUAs will not notice the
change.
Alternatively, a security feature may be purely opportunistic and
thus subject to downgrade attacks. For example, at the time this was
written, most TLS stacks that support TLS 1.2 will use an older TLS
version if the peer does not support TLS 1.2 and 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 The following practices are recommended for Mail Access Services and
confidentiality assurance levels. A connection between a client and Mail Submission Services:
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: o Mail Service Providers (MSPs) which support POP, IMAP, and/or
Message Submission, SHOULD provide and support instances of those
services which use Implicit TLS. (See Section 3.)
directive = directive-name [ "=" directive-value ] o For compatibility with existing MUAs and existing MUA
configurations, MSPs SHOULD also, in the near term, provide
instances of these services which support STARTTLS. This will
permit legacy MUAs to discover new availability of TLS capability
on servers, and may increase use of TLS by such MUAs. However,
servers SHOULD NOT advertise STARTTLS if use of the STARTTLS
command by a client is likely to fail (for example, if the server
has no server certificate configured.)
directive-name = token o MSPs SHOULD advertise their Mail Access Services and Mail
Submission Services using DNS SRV records according to [RFC6186].
Services supporting TLS SHOULD be advertised in preference to
cleartext services (if offered). In addition, services using
Implicit TLS SHOULD be advertised in preference to services
supporting STARTTLS (if offered). (See also Section 4.5.)
directive-value = token o MSPs SHOULD deprecate use of cleartext Mail Access Services and
Mail Submission Services as soon as practicable. (See
Section 4.1.)
token = <As defined in RFC 7230> o MSPs that provide mail submission as a service, SHOULD support
Mail Submission services using Implicit TLS.
This is a subset of the syntax used by HSTS [RFC6797] as revised in o MSPs currently supporting such use of cleartext SMTP (on port 25)
[RFC7230]; but simplified for use by protocols other than HTTP. as a means of message submission by their users (whether or not
requiring authentication) SHOULD transition their users to using
TLS (either Implicit TLS or STARTTLS) as soon as practicable.
6. Server Strict Transport Security Policy o Mail services SHOULD support TLS 1.2 or later.
Servers supporting this extension MUST advertise an STS policy. This o All Mail services SHOULD implement the recommended TLS cipher
includes a list of security directives the server administrator has suites described in [RFC7525] or a future BCP or standards track
explicitly configured as recommended for use by clients (the list MAY revision of that document.
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 o Mail services currently supporting SSL 2.x, SSL 3.0, or TLS 1.0
containing an https Uniform Resource Locator (URL) [RFC2818] that the SHOULD transition their users to later versions of TLS, and
client can save and subsequently resolve for the user in the event of discontinue support for those versions of SSL and TLS, as soon as
a security connection problem. Server STS policy has the following practicable.
formal syntax:
sts-policy = [directive *(";" [SP] directive)] o Mail Submission Servers accepting mail using TLS SHOULD include
the TLS ciphersuite of the session in which the mail was received,
in the Received field of the outgoing message. (See Section 4.3.)
Protocol extensions to advertise STS policy for email servers are o All Mail services implementing TLS SHOULD log TLS cipher
defined in Section 9. information along with any connection or authentication logs that
they maintain.
The IANA Considerations Section 13 defines a registry so that more Additional considerations and details appear below.
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 4.1. Deprecation of Services Using Cleartext and TLS Versions < 1.1
Before a client can consider storing any security directives, it MUST The specific means employed for deprecation of cleartext Mail Access
verify that the connection to the server uses TLS, the server has Services and Mail Submission Services this MAY vary from one MSP to
been authenticated, and any requirements for any previously saved the next in light of their user communities' needs and constraints.
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 For example, an MSP MAY implement a gradual transition in which, over
to the next directive. time, more and more users are forbidden to authenticate to cleartext
instances of these services, thus encouraging those users to migrate
to Implicit TLS. Access to cleartext services should eventually be
either disabled, or limited strictly for use by legacy systems which
cannot be upgraded.
2. If the security directive is already saved with the same value After a user's ability to authenticate to a service using cleartext
(or a value considered greater than the current value in the is revoked, the server denying such access MUST NOT provide any
directive's definition), the client skips the security directive indication of whether the user's authentication credentials were
and moves on to the next one. valid. An attempt to authenticate as such a user using either
invalid credentials or valid credentials MUST both result in the same
indication of access being denied.
3. The client verifies the connection meets the requirements of the Also, users authenticating with passwords SHOULD be required to
security directive. If the connection does not, then the change those passwords when migrating from cleartext to TLS, since
directive will not be saved. For example, a security directive the old passwords were likely to have been compromised.
claiming that the server supports tls-version 1.2 will not be
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 Transition of users from SSL or TLS 1.0 to later versions of TLS MAY
account configuration to save the security directive. be accomplished by a means similar to that described above. There
are multiple ways to accomplish this. One way is for the server to
refuse a ClientHello message from any client sending a protocol
version number corresponding to any version of SSL or TLS 1.0.
Another way is for the server to accept ClientHello messages from
some client versions that it does not wish to support, but later
refuse to allow the user to authenticate. The latter method may
provide a better indication to the user of the reason for the failure
but (depending on the protocol and method of authentication used) may
also risk exposure of the user's password over an channel which is
known to not provide adequate confidentiality.
Once a security directive is saved, all subsequent connections to It is RECOMMENDED that new users be required to use TLS version 1.1
that host require any associated security feature. For this or greater from the start. However an MSP may find it necessary to
confidentiality protection to work as desired clients MUST NOT offer make exceptions to accommodate some legacy systems which support only
a click-through-to-connect action when unable to achieve connection earlier versions of TLS, or only cleartext.
security matching the saved security directives.
7.1. Security Directive Upgrade Example 4.2. Mail Server Use of Client Certificate Authentication
Suppose a server advertises the "tls-version" directive name with Mail servers MAY implement client certificate authentication on the
value "1.1". A client that successfully negotiates either TLS 1.1 or Implicit TLS port. Servers MUST NOT request a client certificate
TLS 1.2 SHOULD save this directive. The server may subsequently during the TLS handshake unless the server is configured to accept
change the value to "1.2". When a client with "1.1" saved value some client certificates as sufficient for authentication and the
connects and negotiates TLS 1.2, it will upgrade the saved directive server has the ability to determine a mail server authorization
value to "1.2". However, a client that only supports TLS 1.1 will identity matching such certificates. How to make this determination
continue to require use of TLS 1.1 and work with that server as long is presently implementation specific.
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 If the server accepts the client's certificate as sufficient for
authorization, it MUST enable the SASL EXTERNAL [RFC4422] mechanism.
When a security directive has been saved for connections from a An IMAPS server MAY issue a PREAUTH greeting instead of enabling SASL
client to a server and the facility identified by that directive is EXTERNAL.
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 4.3. Recording TLS Cipher Suite in Received Header
The ESMTPS transmission type [RFC3848] provides trace information The ESMTPS transmission type [RFC3848] provides trace information
that can indicate TLS was used when transferring mail. However, TLS that can indicate TLS was used when transferring mail. However, TLS
usage by itself is not a guarantee of confidentiality or security. usage by itself is not a guarantee of confidentiality or security.
The TLS cipher suite provides additional information about the level The TLS cipher suite provides additional information about the level
of security made available for a connection. This defines a new SMTP of security made available for a connection. This defines a new SMTP
"tls" Received header additional-registered-clause that is used to "tls" Received header additional-registered-clause that is used to
record the TLS cipher suite that was negotiated for the connection. record the TLS cipher suite that was negotiated for the connection.
The value included in this additional clause SHOULD be the registered The value included in this additional clause SHOULD be the registered
cipher suite name (e.g., TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) cipher suite name (e.g., TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256)
included in the TLS cipher suite registry. In the event the included in the TLS cipher suite registry. In the event the
implementation does not know the name of the cipher suite (a implementation does not know the name of the cipher suite (a
situation that should be remedied promptly), a four-digit hexadecimal situation that should be remedied promptly), a four-digit hexadecimal
cipher suite identifier MAY be used. The ABNF for the field follows: cipher suite identifier MAY be used. The ABNF for the field follows:
tls-cipher-clause = CFWS "tls" FWS tls-cipher 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
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 tls-cipher = tls-cipher-suite-name / tls-cipher-suite-hex
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.
<client connected to port 993 and negotiated TLS successfully> tls-cipher-name = ALPHA *(ALPHA / DIGIT / "_")
S: * OK [CAPABILITY IMAP4rev1 DEEP ID AUTH=PLAIN ; as registered in IANA cipher suite registry
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 tls-cipher-hex = "0x" 4HEXDIG
This example shows a client that negotiated TLS, but was unable to 4.4. TLS Server Certificate Requirements
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 MSPs MUST maintain valid server certificates for all servers. See
a client connected to the proxy on port 993 that negotiated TLS> [RFC7817] for the recommendations and requirements necessary to
S: * OK [CAPABILITY IMAP4rev1 DEEP ID AUTH=PLAIN achieve this.
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 If a protocol server provides service for more than one mail domain,
it MAY use a separate IP address for each domain and/or a server
certificate that advertises multiple domains. This will generally be
necessary unless and until it is acceptable to impose the constraint
that the server and all clients support the Server Name Indication
extension to TLS [RFC6066]. For more discussion of this problem, see
section 5.1 of [RFC7817].
This example shows the connection from an IMAP proxy to a back-end 4.5. Recommended DNS records for mail protocol servers
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 This section discusses not only the DNS records that are recommended,
but also implications of DNS records for server configuration and TLS
server certificates.
POP servers supporting this specification MUST implement the POP3 4.5.1. MX records
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
specification, before the terms were changed to align better with
those used in HSTS.)
<client connected to port 995 and negotiated TLS successfully> It is recommended that MSPs advertise MX records for handling of
S: +OK POP server ready inbound mail (instead of relying entirely on A or AAAA records), and
C: CAPA that those MX records be signed using DNSSEC. This is mentioned here
S: +OK Capability list follows only for completeness, as handling of inbound mail is out of scope
S: TOP for this document.
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 4.5.2. SRV records
After verifying the TLS server certificate and issuing CAPA, the MSPs SHOULD advertise SRV records to aid MUAs in determination of
client can save any or all of the STS policy. If the client connects proper configuration of servers, per the instructions in [RFC6186].
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 MSPs SHOULD advertise servers that support Implicit TLS in preference
to those which support cleartext and/or STARTTLS operation.
SMTP Submission servers supporting this specification MUST implement 4.5.3. DNSSEC
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:
deep-cmd = "CLIENT" 1*(SP deep-parameter) 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.
deep-parameter = name / version / policy-fail 4.5.4. TLSA records
/ directives / tls / future-extension
name = "name" SP esmtp-value 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.
version = "version" SP esmtp-value 4.6. Changes to Internet Facing Servers
saved = "saved" SP directive-list 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.
policy-fail = "policy-fail" SP directive-list 5. Best Current Practices for use of TLS by Mail User Agents
directive-list = DQUOTE [directive It is recommended that Mail User Agents implement the following
*(";" [SP] directive)] DQUOTE practices:
directives = "directives" SP directive-list o MUAs SHOULD be capable of using DNS SRV records to discover Mail
Access Services and Mail Submission Services that are advertised
by a MSP for an account being configured. Other means of
discovering server configuration information (e.g. a database
maintained by the MUA vendor) MAY also be supported. (See
Section 5.1 for more information.)
tls = "tls" SP tls-cipher o MUAs SHOULD be configurable to require a minimum level of
confidentiality for any particular Mail Account, and refuse to
exchange information via any service associated with that Mail
Account if the session does not provide that minimum level of
confidentiality. (See Section 5.2.)
future-extension = Atom SP String o MUAs MUST NOT consider a session as meeting a minimum level of
confidentiality if the server's TLS certificate cannot be
validated. (See Section 5.3.)
Atom = <as defined in RFC 5321> o MUAs MAY impose other minimum confidentiality requirements in the
future, e.g. in order to discourage use of TLS versions or
cryptographic algorithms in which weaknesses have been discovered.
String = <as defined in RFC 5321> o MUAs SHOULD provide a prominent visual indication of the level of
confidentiality associated with an account configuration (for
example, indications such as "lock" icons or changed background
colors similar to those used by some browsers), at appropriate
times and locations in order to inform the user of the
confidentiality of the communications associated with that
account. For example, this might be done whenever (a) prompting
the user for authentication credentials, (b) the user is composing
mail that will be sent to a particular submission server, (c) a
list of accounts is displayed (particularly if the user can select
from that list to read mail), or (d) the user is requesting to
view or update any configuration data that will be stored on a
remote server.
The CLIENT command parameters listed here have the same meaning as o MUAs SHOULD implement TLS 1.2 or later. Earlier TLS and SSL
the parameters used in the IMAP STS extension (Section 9.1). The versions MAY also be supported so long as the MUA requires at
server responds to the CLIENT command with a "250" if the command has least TLS 1.1 when accessing accounts that are configured to
correct syntax and a "501" if the command has incorrect syntax. impose minimum confidentiality requirements.
<client connected to port 465 and negotiated TLS successfully> o All MUAs SHOULD implement the recommended TLS cipher suites
S: 220 example.com Demo SMTP Submission Server described in [RFC7525] or a future BCP or standards track revision
C: EHLO client.example.com of that document.
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 o MUAs that are configured to not require minimum confidentiality
for one or more accounts SHOULD detect when TLS becomes available
on those accounts, and offer to upgrade the account to impose
minimum confidentiality requirements.
10. Account Setup Considerations Additional considerations and details appear below.
10.1. Use of SRV records in Establishing Configuration 5.1. Use of SRV records in Establishing Configuration
This section updates [RFC6186] by changing the preference rules and This section updates [RFC6186] by changing the preference rules and
adding a new SRV service label _submissions._tcp to refer to Message adding a new SRV service label _submissions._tcp to refer to Message
Submission with implicit TLS. Submission with Implicit TLS.
User-configurable MUAs SHOULD support use of [RFC6186] for account User-configurable MUAs SHOULD support use of [RFC6186] for account
setup. However, when using configuration information obtained by setup. However, when using configuration information obtained by
this method, MUAs SHOULD default to a minimum confidentiality this method, MUAs SHOULD ignore advertised services that do not
assurance level of 1, unless the user has explicitly requested satisfy minimum confidentiality requirements, unless the user has
reduced confidentiality. This will have the effect of causing the explicitly requested reduced confidentiality. This will have the
MUA to ignore advertised configurations that do not support TLS, even effect of causing the MUA to default to ignoring advertised
when those advertised configurations have a higher priority than configurations that do not support TLS, even when those advertised
other advertised configurations. configurations have a higher priority than other advertised
configurations.
When using [RFC6186] configuration information, Mail User Agents When using [RFC6186] configuration information, Mail User Agents
SHOULD NOT automatically establish new configurations that do not SHOULD NOT automatically establish new configurations that do not
require TLS for all servers, unless there are no advertised require TLS for all servers, unless there are no advertised
configurations using TLS. If such a configuration is chosen, prior configurations using TLS. If such a configuration is chosen, prior
to attempting to authenticate to the server or use the server for to attempting to authenticate to the server or use the server for
message submission, the MUA SHOULD warn the user that traffic to that message submission, the MUA SHOULD warn the user that traffic to that
server will not be encrypted and that it will therefore likely be server will not be encrypted and that it will therefore likely be
intercepted by unauthorized parties. The specific wording is to be intercepted by unauthorized parties. The specific wording is to be
determined by the implementation, but it should adequately capture determined by the implementation, but it should adequately capture
the sense of risk given the widespread incidence of mass surveillance the sense of risk given the widespread incidence of mass surveillance
of email traffic. of email traffic.
Similarly, a MUA MUST NOT attempt to "test" a particular mail account
configuration by submitting the user's authentication credentials to
a server, unless a TLS session meeting minimum confidentiality levels
has been established with that server. If minimum confidentiality
requirements have not been satisfied, the MUA must explicitly warn
the user that his password may be exposed to attackers before testing
the new configuration.
When establishing a new configuration for connecting to an IMAP, POP, When establishing a new configuration for connecting to an IMAP, POP,
or SMTP Submission server, an MUA SHOULD NOT blindly trust SRV or SMTP Submission server, an MUA SHOULD NOT blindly trust SRV
records unless they are signed by DNSSEC and have a valid signature. records unless they are signed by DNSSEC and have a valid signature.
Instead, the MUA SHOULD warn the user that the DNS-advertised Instead, the MUA SHOULD warn the user that the DNS-advertised
mechanism for connecting to the server is not authenticated, and mechanism for connecting to the server is not authenticated, and
request the user to manually verify the connection details by request the user to manually verify the connection details by
reference to his or her mail service provider's documentation. reference to his or her mail service provider's documentation.
Similarly, an MUA MUST NOT consult SRV records to determine which Similarly, an MUA MUST NOT consult SRV records to determine which
servers to use on every connection attempt, unless those SRV records servers to use on every connection attempt, unless those SRV records
are signed by DNSSEC and have a valid signature. However, an MUA MAY 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 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 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 has happened. This can also serve as a means to encourage users to
upgrade their configurations to require TLS if and when their MSPs upgrade their configurations to require TLS if and when their MSPs
support it. support it.
10.2. Certificate Pinning 5.2. Minimum Confidentiality Level
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.
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]).
11.2. Mail Server Implementation Requirements MUAs SHOULD, by default, require a minimum level of confidentiality
for services accessed by each account. For MUAs supporting the
ability to access multiple mail accounts, this requirement SHOULD be
configurable on a per-account basis.
These requirements apply to servers that implement POP, IMAP or SMTP The default minimum expected level of confidentiality for all new
Submission. accounts SHOULD be at least use of TLS version 1.1 or greater, and
successful validation of the server's certificate. (Future revisions
to this specification may raise these requirements or impose
additional requirements to address newly-discovered weaknesses in
protocols or cryptographic algorithms.)
o Servers MUST implement the appropriate STS Policy and Reporting MUAs MAY permit the user to disable this minimum confidentiality
extensions described in Section 9 requirement during initial account configuration, or subsequently
editing an account configuration, but MUST warn users that such a
configuration will not assure privacy for either passwords or
messages.
o IMAP and SMTP submission servers SHOULD implement and be An MUA which is configured to require a minimum level of
configurable to support STARTTLS. This enables discovery of new confidentiality for a mail account MUST NOT attempt to perform any
TLS availability, and can increase usage of TLS by legacy clients. operation other than capability discovery, or STARTTLS for servers
not using Implicit TLS, unless the minimum level of confidentiality
is provided by that connection.
o Servers MUST NOT advertise STARTTLS capability if it is unlikely MUAs SHOULD NOT allow users to "click through" to access or send mail
to succeed based on server configuration (e.g., there is no server via an connection, or to authenticate to any service using a
certificate installed). password, if that account is configured to impose minimum
confidentiality requirements and that connection does not meet all of
those requirements. Experience indicates that users presented with
such an option often "click through" without understanding the risks
that they're accepting by doing so. Furthermore, users who
frequently find the need to "click through" to use an insecure
connection may become conditioned to do so as a matter of habit,
before considering whether the risks are reasonable in each specific
instance.
o SMTP message submission servers that have negotiated TLS SHOULD An MUA which is not configured to require a minimum level of
add a Received header field to the message including the tls confidentiality for a mail account SHOULD still attempt to connect to
clause described in Section 8. the services associated with that account using the most secure means
available, e.g. by using Implicit TLS or STARTTLS.
o Servers MUST be configurable to include the TLS cipher information 5.3. Certificiate Validation
in any connection or user logging or auditing facility they
provide.
11.3. Mail User Agent Implementation Requirements MUAs MUST validate TLS server certificates according to [RFC7817] and
PKIX [RFC5280].
This section describes requirements on Mail User Agents (MUAs) using MUAs MAY also support DANE [RFC6698] as a means of validating server
IMAP, POP, and/or Submission protocols. Note: Requirements certificates in order to meet minimum confidentiality 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 MUAs MAY support use of certificate pinning but MUST NOT consider a
minimum expected level of confidentiality based on appropriate connection in which the server's authenticity relies on certificate
security inputs such as which security directives are pre-set, the pinning, as providing the minimum level of confidentiality. (See
number of trust anchors, certificate validity, use of an extended Section 5.4.)
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 5.4. Certificate Pinning
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.
o Whenever requested to establish any configuration that does not During account setup, the MUA will identify servers that provide
require both TLS and server certificate verification to talk to a account services such as mail access and mail submission (the
server or account, an MUA SHOULD warn its user that his or her previous section describes one way to do this). The certificates for
mail traffic (including password, if applicable) will be exposed these servers are verified using the rules described in [RFC7817] and
to attackers, and give the user an opportunity to abort the PKIX [RFC5280]. In the event the certificate does not validate due
connection prior to transmission of any such password or traffic. to an expired certificate, lack of appropriate chain of trust, or
lack of identifier match, the MUA MAY offer to create a persistent
binding between that certificate and the saved host name for the
server, for use when accessing that account's servers. This is
called certificate pinning.
o MUAs SHOULD support the ability to save the "tls-version=1.2" Certificate pinning is only appropriate during mail account setup and
security directive (the TLS library has to provide an API that MUST NOT be offered as an option in response to a failed certificate
controls permissible TLS versions, and communicates the negotiated validation for an existing mail account. An MUA that allows
TLS protocol version to the application, for this to be possible). certificate pinning MUST NOT allow a certificate pinned for one
account to validate connections for other accounts.
o See Section 3 for additional requirements. 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. Note also that a man-in-the-middle attack
at account setup time will expose the user's password to the attacker
(if a password is used). Therefore use of a pinned certificate does
not meet the requirement for a minimum confidentiality level, and an
MUA MUST NOT indicate to the user that the such confidentiality is
provided. Additional advice on certificate pinning is present in
[RFC6125].
11.4. Non-configurable MUAs and nonstandard access protocols 5.5. Client Certificate Authentication
MUAs which are not configurable to use user-specified servers MUST MUAs MAY implement client certificate authentication on the Implicit
implement TLS or similarly other strong encryption mechanism when TLS port. An MUA MUST NOT provide a client certificate during the
communicating with their mail servers. This generally applies to TLS handshake unless the server requests one and the client has
MUAs that are pre-configured to operate with one or more specific determined the certificate can be safely used with that specific
services, whether or not supplied by the vendor of those services. 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.
MUAs using protocols other than IMAP, POP, and Submission to A client supporting client certificate authentication with Implicit
communicate with mail servers, MUST implement TLS or other similarly TLS MUST implement the SASL EXTERNAL [RFC4422] mechanism using the
robust encryption mechanism in conjunction with those protocols. appropriate authentication command (AUTH for POP3 [RFC5034], AUTH for
SMTP Submission [RFC4954], AUTHENTICATE for IMAP [RFC3501]).
11.5. Compliance for Anti-Virus/Anti-Spam Software and Services 6. Considerations related to Anti-Virus/Anti-Spam Software and Services
There are multiple ways to connect an Anti-Virus and/or Anti-Spam 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- (AVAS) service to a mail server. Some mechanisms, such as the de-
facto milter protocol, are out of scope for this specification. facto milter protocol, are out of scope for this specification.
However, some services use an SMTP relay proxy that intercepts mail However, some services use an SMTP relay proxy that intercepts mail
at the application layer to perform a scan and proxy or forward to at the application layer to perform a scan and proxy or forward to
another MTA. Deploying AVAS services in this way can cause many another MTA. Deploying AVAS services in this way can cause many
problems [RFC2979] including direct interference with this problems [RFC2979] including direct interference with this
specification, and other forms of confidentiality or security specification, and other forms of confidentiality or security
reduction. An AVAS product or service is considered compliant with reduction. An AVAS product or service is considered compatible with
this specification if all IMAP, POP and SMTP-related software this specification if all IMAP, POP and SMTP-related software
(including proxies) it includes are compliant with this (including proxies) it includes are compliant with this
specification, and each of these services advertise and support all specification.
security directives that the actual end-servers advertise.
Note that end-to-end email encryption prevents AVAS software and Note that end-to-end email encryption prevents AVAS software and
services from using email content as part of a spam or virus services from using email content as part of a spam or virus
assessment. Furthermore, while a minimum confidentiality assurance assessment. Furthermore, while a minimum confidentiality level can
level of 1 or better can prevent a man-in-the-middle from introducing prevent a man-in-the-middle from introducing spam or virus content
spam or virus content between the MUA and Submission server, it does between the MUA and Submission server, it does not prevent other
not prevent other forms of client or account compromise. Use of AVAS forms of client or account compromise. Use of AVAS services for
services for submitted email therefore remains necessary. 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
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.
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.
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
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
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 7. IANA Considerations
Change Controller: IESG
13.3. POP3S Port Registration Update 7.1. POP3S Port Registration Update
IANA is asked to update the registration of the TCP well-known port IANA is asked to update the registration of the TCP well-known port
995 using the following template ([RFC6335]): 995 using the following template ([RFC6335]):
Service Name: pop3s Service Name: pop3s
Transport Protocol: TCP Transport Protocol: TCP
Assignee: IETF <iesg@ietf.org> Assignee: IETF <iesg@ietf.org>
Contact: IESG <iesg@ietf.org> Contact: IESG <iesg@ietf.org>
Description: POP3 over TLS protocol Description: POP3 over TLS protocol
Reference: RFC XXXX (this document once published) Reference: RFC XXXX (this document once published)
Port Number: 995 Port Number: 995
13.4. IMAPS Port Registration Update 7.2. IMAPS Port Registration Update
IANA is asked to update the registration of the TCP well-known port IANA is asked to update the registration of the TCP well-known port
993 using the following template ([RFC6335]): 993 using the following template ([RFC6335]):
Service Name: imaps Service Name: imaps
Transport Protocol: TCP Transport Protocol: TCP
Assignee: IETF <iesg@ietf.org> Assignee: IETF <iesg@ietf.org>
Contact: IESG <iesg@ietf.org> Contact: IESG <iesg@ietf.org>
Description: IMAP over TLS protocol Description: IMAP over TLS protocol
Reference: RFC XXXX (this document once published) Reference: RFC XXXX (this document once published)
Port Number: 993 Port Number: 993
13.5. Submissions Port Registration 7.3. Submissions Port Registration
IANA is asked to assign an alternate usage of port 465 in addition to IANA is asked to assign an alternate usage of port 465 in addition to
the current assignment using the following template ([RFC6335]): the current assignment using the following template ([RFC6335]):
Service Name: submissions Service Name: submissions
Transport Protocol: TCP Transport Protocol: TCP
Assignee: IETF <iesg@ietf.org> Assignee: IETF <iesg@ietf.org>
Contact: IESG <iesg@ietf.org> Contact: IESG <iesg@ietf.org>
Description: Message Submission over TLS protocol Description: Message Submission over TLS protocol
Reference: RFC XXXX (this document once published) Reference: RFC XXXX (this document once published)
Port Number: 465 Port Number: 465
This is a one time procedural exception to the rules in RFC 6335. This is a one-time procedural exception to the rules in RFC 6335.
This requires explicit IESG approval and does not set a precedent. This requires explicit IESG approval and does not set a precedent.
Historically, port 465 was briefly registered as the "smtps" port. Historically, port 465 was briefly registered as the "smtps" port.
This registration made no sense as the SMTP transport MX This registration made no sense as the SMTP transport MX
infrastructure has no way to specify a port so port 25 is always infrastructure has no way to specify a port, so port 25 is always
used. As a result, the registration was revoked and was subsequently used. As a result, the registration was revoked and was subsequently
reassigned to a different service. In hindsight, the "smtps" reassigned to a different service. In hindsight, the "smtps"
registration should have been renamed or reserved rather than registration should have been renamed or reserved rather than
revoked. Unfortunately, some widely deployed mail software revoked. Unfortunately, some widely deployed mail software
interpreted "smtps" as "submissions" [RFC6409] and used that port for interpreted "smtps" as "submissions" [RFC6409] and used that port for
email submission by default when an end-user requests security during email submission by default when an end-user requests security during
account setup. If a new port is assigned for the submissions account setup. If a new port is assigned for the submissions
service, email software will either continue with unregistered use of service, email software will either continue with unregistered use of
port 465 (leaving the port registry inaccurate relative to de-facto port 465 (leaving the port registry inaccurate relative to de-facto
practice and wasting a well-known port), or confusion between the de- practice and wasting a well-known port), or confusion between the de-
facto and registered ports will cause harmful interoperability facto and registered ports will cause harmful interoperability
problems that will deter use of TLS for message submission. The problems that will deter use of TLS for message submission. The
authors believe both of these outcomes are less desirable than a wart authors believe both of these outcomes are less desirable than a wart
in the registry documenting real-world usage of a port for two in the registry documenting real-world usage of a port for two
purposes. Although STARTTLS-on-port-587 has deployed, it has not purposes. Although STARTTLS-on-port-587 has deployed, it has not
replaced deployed use of implicit TLS submission on port 465. 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.
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 8. Security Considerations
This entire document is about security considerations. In general, This entire document is about security considerations. In general,
this is targeted to improve mail confidentiality and to mitigate this is targeted to improve mail confidentiality and to mitigate
threats external to the email system such as network-level snooping threats external to the email system such as network-level snooping
or interception; this is not intended to mitigate active attackers or interception; this is not intended to mitigate active attackers
who have compromised service provider systems. who have compromised service provider systems.
It could be argued that sharing the name and version of the client It could be argued that sharing the name and version of the client
software with the server has privacy implications. Although software with the server has privacy implications. Although
providing this information is not required, it is encouraged so that providing this information is not required, it is encouraged so that
mail service providers can more effectively inform end-users running mail service providers can more effectively inform end-users running
old clients that they need to upgrade to protect their security, or 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 know which clients to use in a test deployment prior to upgrading a
server to have higher security requirements. server to have higher security requirements.
15. References 9. References
15.1. Normative References 9.1. Normative References
[RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3", [RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3",
STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996, STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996,
<http://www.rfc-editor.org/info/rfc1939>. <http://www.rfc-editor.org/info/rfc1939>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>. <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>.
[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 [RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207, Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
February 2002, <http://www.rfc-editor.org/info/rfc3207>. February 2002, <http://www.rfc-editor.org/info/rfc3207>.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
<http://www.rfc-editor.org/info/rfc3501>. <http://www.rfc-editor.org/info/rfc3501>.
[RFC5034] Siemborski, R. and A. Menon-Sen, "The Post Office Protocol [RFC5034] Siemborski, R. and A. Menon-Sen, "The Post Office Protocol
(POP3) Simple Authentication and Security Layer (SASL) (POP3) Simple Authentication and Security Layer (SASL)
skipping to change at page 32, line 42 skipping to change at page 18, line 15
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008, DOI 10.17487/RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>. <http://www.rfc-editor.org/info/rfc5234>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, (TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008, DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>. <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., [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<http://www.rfc-editor.org/info/rfc5280>. <http://www.rfc-editor.org/info/rfc5280>.
[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, [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008, DOI 10.17487/RFC5322, October 2008,
<http://www.rfc-editor.org/info/rfc5322>. <http://www.rfc-editor.org/info/rfc5322>.
[RFC6186] Daboo, C., "Use of SRV Records for Locating Email [RFC6186] Daboo, C., "Use of SRV Records for Locating Email
Submission/Access Services", RFC 6186, Submission/Access Services", RFC 6186,
DOI 10.17487/RFC6186, March 2011, DOI 10.17487/RFC6186, March 2011,
<http://www.rfc-editor.org/info/rfc6186>. <http://www.rfc-editor.org/info/rfc6186>.
[RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail", [RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail",
STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011, STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011,
<http://www.rfc-editor.org/info/rfc6409>. <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, [RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer "Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
2015, <http://www.rfc-editor.org/info/rfc7525>. 2015, <http://www.rfc-editor.org/info/rfc7525>.
[RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via [RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via
Opportunistic DNS-Based Authentication of Named Entities Opportunistic DNS-Based Authentication of Named Entities
(DANE) Transport Layer Security (TLS)", RFC 7672, (DANE) Transport Layer Security (TLS)", RFC 7672,
DOI 10.17487/RFC7672, October 2015, DOI 10.17487/RFC7672, October 2015,
<http://www.rfc-editor.org/info/rfc7672>. <http://www.rfc-editor.org/info/rfc7672>.
[RFC7817] Melnikov, A., "Updated Transport Layer Security (TLS) [RFC7817] Melnikov, A., "Updated Transport Layer Security (TLS)
Server Identity Check Procedure for Email-Related Server Identity Check Procedure for Email-Related
Protocols", RFC 7817, DOI 10.17487/RFC7817, March 2016, Protocols", RFC 7817, DOI 10.17487/RFC7817, March 2016,
<http://www.rfc-editor.org/info/rfc7817>. <http://www.rfc-editor.org/info/rfc7817>.
15.2. Informative References 9.2. Informative References
[RFC2595] Newman, C., "Using TLS with IMAP, POP3 and ACAP", [RFC2595] Newman, C., "Using TLS with IMAP, POP3 and ACAP",
RFC 2595, DOI 10.17487/RFC2595, June 1999, RFC 2595, DOI 10.17487/RFC2595, June 1999,
<http://www.rfc-editor.org/info/rfc2595>. <http://www.rfc-editor.org/info/rfc2595>.
[RFC2979] Freed, N., "Behavior of and Requirements for Internet [RFC2979] Freed, N., "Behavior of and Requirements for Internet
Firewalls", RFC 2979, DOI 10.17487/RFC2979, October 2000, Firewalls", RFC 2979, DOI 10.17487/RFC2979, October 2000,
<http://www.rfc-editor.org/info/rfc2979>. <http://www.rfc-editor.org/info/rfc2979>.
[RFC3848] Newman, C., "ESMTP and LMTP Transmission Types [RFC3848] Newman, C., "ESMTP and LMTP Transmission Types
Registration", RFC 3848, DOI 10.17487/RFC3848, July 2004, Registration", RFC 3848, DOI 10.17487/RFC3848, July 2004,
<http://www.rfc-editor.org/info/rfc3848>. <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 [RFC4422] Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
Authentication and Security Layer (SASL)", RFC 4422, Authentication and Security Layer (SASL)", RFC 4422,
DOI 10.17487/RFC4422, June 2006, DOI 10.17487/RFC4422, June 2006,
<http://www.rfc-editor.org/info/rfc4422>. <http://www.rfc-editor.org/info/rfc4422>.
[RFC4954] Siemborski, R., Ed. and A. Melnikov, Ed., "SMTP Service [RFC4954] Siemborski, R., Ed. and A. Melnikov, Ed., "SMTP Service
Extension for Authentication", RFC 4954, Extension for Authentication", RFC 4954,
DOI 10.17487/RFC4954, July 2007, DOI 10.17487/RFC4954, July 2007,
<http://www.rfc-editor.org/info/rfc4954>. <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) [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066, Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011, DOI 10.17487/RFC6066, January 2011,
<http://www.rfc-editor.org/info/rfc6066>. <http://www.rfc-editor.org/info/rfc6066>.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509 within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer (PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
skipping to change at page 36, line 45 skipping to change at page 21, line 27
contrary to the goal of promoting more TLS use. Encouraging use of contrary to the goal of promoting more TLS use. Encouraging use of
STARTTLS on port 587 would not create interoperability problems, but STARTTLS on port 587 would not create interoperability problems, but
is unlikely to have impact on current undocumented use of port 465 is unlikely to have impact on current undocumented use of port 465
and makes the guidance in this document less consistent. The and makes the guidance in this document less consistent. The
remaining option is to document the current state of the world and remaining option is to document the current state of the world and
support future use of port 465 for submission as this increases support future use of port 465 for submission as this increases
consistency and ease-of-deployment for TLS email submission. consistency and ease-of-deployment for TLS email submission.
Appendix B. Change Log Appendix B. Change Log
Changes since draft-ietf-uta-email-deep-06:
o On the recommendation of one of the co-chairs and some working
group members, rewrote document with the intended status of BCP.
This involved removing a great deal of text that consisted
essentially of new protocol specification, especially the STS
features, on the theory that a BCP should base its recommendations
on current practice, and that new protocol features should be
subject to the interoperability test requirements associated with
normal standards-track documents.
Changes since draft-ietf-uta-email-deep-05: Changes since draft-ietf-uta-email-deep-05:
o Clarify throughout that the confidentiality assurance level o Clarify throughout that the confidentiality assurance level
associated with a mail account is a minimum level; attempt to associated with a mail account is a minimum level; attempt to
distinguish this from the current confidentiality level provided distinguish this from the current confidentiality level provided
by a connection between client and server. by a connection between client and server.
o Change naming for confidentiality assurance levels: instead of o Change naming for confidentiality assurance levels: instead of
"high" or "no" confidence, assign numbers 1 and 0 to them "high" or "no" confidence, assign numbers 1 and 0 to them
respectively. This because it seems likely that in the not-too- respectively. This because it seems likely that in the not-too-
distant future, what was defined in -05 as "high" confidence will distant future, what was defined in -05 as "high" confidence will
be considered insufficient, and calling that "high" confidence be considered insufficient, and calling that "high" confidence
will become misleading. For example, relying entirely on a list will become misleading. For example, relying entirely on a list
of trusted CAs to validate server certificates from arbitrary of trusted CAs to validate server certificates from arbitrary
parties, appears to be less and less reliable in practice at parties, appears to be less and less reliable in practice at
thwarting MITM attacks. thwarting MITM attacks.
o Clarify that if some services associated with a mail account don't o Clarify that if some services associated with a mail account don't
meet the minimium confidentiality assurance level assigned to that meet the minimum confidentiality assurance level assigned to that
account, other services that do meet that minimum confidentiality account, other services that do meet that minimum confidentiality
assurance level may continue to be used. assurance level may continue to be used.
o Clarify that successful negotiation of at least TLS version 1.1 is o Clarify that successful negotiation of at least TLS version 1.1 is
required as a condition of meeting confidentiality assurance level required as a condition of meeting confidentiality assurance level
1. 1.
o Clarify that validation of a server certificate using either DANE o Clarify that validation of a server certificate using either DANE
or PKIX is sufficient to meet the certificate validation or PKIX is sufficient to meet the certificate validation
requirement of confidentiality assurance level 1. requirement of confidentiality assurance level 1.
o Clarify that minimum confidentiality assurance levels are separate o Clarify that minimum confidentiality assurance levels are separate
from security directives, and that the requrements of both from security directives, and that the requirements of both
mechanisms must be met. mechanisms must be met.
o Explicitly cite an example that a security directive of tls- o Explicitly cite an example that a security directive of tls-
version=1.2 won't be saved if the currently negotiated tls-version 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, 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 but for author KM it seemed to make the mechanism clearer to use
this example earlier.) this example earlier.)
o Clarify some protocol examples as to whether PKIX or DANE was used o Clarify some protocol examples as to whether PKIX or DANE was used
to verify a server's certificate. to verify a server's certificate.
skipping to change at page 41, line 49 skipping to change at page 26, line 37
o Write most of the missing IANA considerations section. o Write most of the missing IANA considerations section.
o Rewrite most of implementation requirements section based more on o Rewrite most of implementation requirements section based more on
draft-moore-email-tls-00. Remove new cipher requirements for now draft-moore-email-tls-00. Remove new cipher requirements for now
because those may be dealt with elsewhere. because those may be dealt with elsewhere.
Appendix C. Acknowledgements Appendix C. Acknowledgements
Thanks to Ned Freed for discussion of the initial latch concepts in Thanks to Ned Freed for discussion of the initial latch concepts in
this document. Thanks to Alexey Melnikov for draft-melnikov-pop3- this document. Thanks to Alexey Melnikov for draft-melnikov-pop3-
over-tls-02, which was the basis of the POP3 implicit TLS text. over-tls-02, which was the basis of the POP3 Implicit TLS text.
Thanks to Russ Housley, Alexey Melnikov and Dan Newman for review Thanks to Russ Housley, Alexey Melnikov and Dan Newman for review
feedback. Thanks to Paul Hoffman for interesting feedback in initial feedback. Thanks to Paul Hoffman for interesting feedback in initial
conversations about this idea. conversations about this idea.
Authors' Addresses Authors' Addresses
Keith Moore Keith Moore
Network Heretics Network Heretics
PO Box 1934 PO Box 1934
Knoxville, TN 37901 Knoxville, TN 37901
 End of changes. 139 change blocks. 
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