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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) ** Obsolete normative reference: RFC 3501 (Obsoleted by RFC 9051) ** Obsolete normative reference: RFC 5246 (Obsoleted by RFC 8446) ** Obsolete normative reference: RFC 7525 (Obsoleted by RFC 9325) -- Obsolete informational reference (is this intentional?): RFC 4346 (Obsoleted by RFC 5246) -- Obsolete informational reference (is this intentional?): RFC 6125 (Obsoleted by RFC 9525) Summary: 3 errors (**), 0 flaws (~~), 1 warning (==), 9 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group K. Moore 3 Internet-Draft Windrock 4 Updates: 1939, 2595, 3464, 3501, 5068, C. Newman 5 6186, 6409 (if approved) Oracle 6 Intended status: Standards Track July 21, 2017 7 Expires: January 22, 2018 9 Cleartext Considered Obsolete: Use of TLS for Email Submission and 10 Access 11 draft-ietf-uta-email-deep-08 13 Abstract 15 This specification outlines current recommendations for use of 16 Transport Layer Security (TLS) to provide confidentiality of email 17 traffic between a mail user agent (MUA) and a mail submission or mail 18 access server. 20 Status of This Memo 22 This Internet-Draft is submitted in full conformance with the 23 provisions of BCP 78 and BCP 79. 25 Internet-Drafts are working documents of the Internet Engineering 26 Task Force (IETF). Note that other groups may also distribute 27 working documents as Internet-Drafts. The list of current Internet- 28 Drafts is at http://datatracker.ietf.org/drafts/current/. 30 Internet-Drafts are draft documents valid for a maximum of six months 31 and may be updated, replaced, or obsoleted by other documents at any 32 time. It is inappropriate to use Internet-Drafts as reference 33 material or to cite them other than as "work in progress." 35 This Internet-Draft will expire on January 22, 2018. 37 Copyright Notice 39 Copyright (c) 2017 IETF Trust and the persons identified as the 40 document authors. All rights reserved. 42 This document is subject to BCP 78 and the IETF Trust's Legal 43 Provisions Relating to IETF Documents 44 (http://trustee.ietf.org/license-info) in effect on the date of 45 publication of this document. Please review these documents 46 carefully, as they describe your rights and restrictions with respect 47 to this document. Code Components extracted from this document must 48 include Simplified BSD License text as described in Section 4.e of 49 the Trust Legal Provisions and are provided without warranty as 50 described in the Simplified BSD License. 52 Table of Contents 54 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 55 2. Conventions and Terminology Used in This Document . . . . . . 3 56 3. Implicit TLS . . . . . . . . . . . . . . . . . . . . . . . . 4 57 3.1. Implicit TLS for POP . . . . . . . . . . . . . . . . . . 5 58 3.2. Implicit TLS for IMAP . . . . . . . . . . . . . . . . . . 5 59 3.3. Implicit TLS for SMTP Submission . . . . . . . . . . . . 5 60 3.4. Implicit TLS Connection Closure for POP, IMAP and SMTP 61 Submission . . . . . . . . . . . . . . . . . . . . . . . 6 62 4. Recommendations for Use of TLS by Mail Access 63 Services and Message Submission Services . . . . . . . . . . 6 64 4.1. Deprecation of Services Using Cleartext and TLS Versions 65 < 1.1 . . . . . . . . . . . . . . . . . . . . . . . . . . 8 66 4.2. Mail Server Use of Client Certificate Authentication . . 8 67 4.3. Recording TLS Cipher Suite in Received Header . . . . . . 9 68 4.4. TLS Server Certificate Requirements . . . . . . . . . . . 9 69 4.5. Recommended DNS records for mail protocol servers . . . . 10 70 4.5.1. MX records . . . . . . . . . . . . . . . . . . . . . 10 71 4.5.2. SRV records . . . . . . . . . . . . . . . . . . . . . 10 72 4.5.3. DNSSEC . . . . . . . . . . . . . . . . . . . . . . . 10 73 4.5.4. TLSA records . . . . . . . . . . . . . . . . . . . . 10 74 4.6. Changes to Internet Facing Servers . . . . . . . . . . . 10 75 5. Recommendations for use of TLS by Mail User Agents . . . . . 10 76 5.1. Use of SRV records in Establishing Configuration . . . . 12 77 5.2. Minimum Confidentiality Level . . . . . . . . . . . . . . 13 78 5.3. Certificiate Validation . . . . . . . . . . . . . . . . . 14 79 5.4. Certificate Pinning . . . . . . . . . . . . . . . . . . . 14 80 5.5. Client Certificate Authentication . . . . . . . . . . . . 15 81 6. Considerations related to Anti-Virus/Anti-Spam Software and 82 Services . . . . . . . . . . . . . . . . . . . . . . . . . . 15 83 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 84 7.1. POP3S Port Registration Update . . . . . . . . . . . . . 15 85 7.2. IMAPS Port Registration Update . . . . . . . . . . . . . 16 86 7.3. Submissions Port Registration . . . . . . . . . . . . . . 16 87 8. Security Considerations . . . . . . . . . . . . . . . . . . . 17 88 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 89 9.1. Normative References . . . . . . . . . . . . . . . . . . 17 90 9.2. Informative References . . . . . . . . . . . . . . . . . 19 91 Appendix A. Design Considerations . . . . . . . . . . . . . . . 20 92 Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 21 93 Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 27 94 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27 96 1. Introduction 98 Software that provides email service via Internet Message Access 99 Protocol (IMAP) [RFC3501], Post Office Protocol (POP) [RFC1939] and/ 100 or Simple Mail Transfer Protocol (SMTP) Submission [RFC6409] usually 101 has Transport Layer Security (TLS) [RFC5246] support but often does 102 not use it in a way that maximizes end-user confidentiality. This 103 specification describes current recommendations for use of TLS in 104 interactions between Mail User Agents and Mail Access Services, and 105 between Mail User Agents and Mail Submission Services. 107 In brief, this memo now recommends that: 109 o TLS version 1.2 or greater be used for all traffic between mail 110 user agents (MUAs) and mail submission servers, and also between 111 MUAs and mail access servers. 113 o MUAs and mail service providers discourage use of cleartext 114 protocols for mail access and mail submission, and deprecate use 115 of cleartext protocols for these purposes as soon as practicable. 117 o Use of "Implicit TLS" on ports reserved for that purpose, in 118 preference to STARTTLS on a port that otherwise supports 119 cleartext. 121 This memo does not address use of TLS with SMTP for message relay 122 (where Message Submission [RFC6409] does not apply). Improved use of 123 TLS with SMTP for message relay requires a different approach. One 124 approach to address that topic is described in [RFC7672]. 126 The recommendations in this memo do not replace the functionality of, 127 and are not intended as a substitute for, end-to-end encryption of 128 electronic mail. 130 2. Conventions and Terminology Used in This Document 132 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 133 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 134 document are to be interpreted as described in [RFC2119]. 136 The term "Implicit TLS" refers to the automatic negotiation of TLS 137 whenever a TCP connection is made on a particular TCP port that is 138 used exclusively by that server for TLS connections. The term 139 "Implicit TLS" is intended to contrast with use of STARTTLS and 140 similar commands in POP, IMAP, SMTP message submission, and other 141 protocols, that are used by client and server to explicitly negotiate 142 TLS on an established cleartext TCP connection. 144 The term "Mail Access Services" includes POP, IMAP and any other 145 protocol used to access or modify received messages, or to access or 146 modify a mail user's account configuration. 148 "Mail Submission Service" refers to the use of the protocol specified 149 in [RFC6409] (or one of its predecessors or successors) for 150 submission of outgoing messages for delivery to recipients. 152 The term "Mail Service Provider" (MSP) refers to a provider of Mail 153 Access Services and/or Mail Submission Services. 155 The term "Mail Account" refers to a user's identity with a Mail 156 Service Provider, that user's authentication credentials, any user 157 email that is stored by the MSP, and any other per-user configuration 158 information maintained by the MSP (for example, spam filtering 159 instructions). Most Mail User Agents (MUAs) support the ability to 160 access multiple Mail Accounts. 162 For each account that an MUA accesses on its user's behalf, it must 163 have the server names, ports, authentication credentials, and other 164 configuration information specified by the user. This information 165 which is used by the MUA is referred to as "Mail Account 166 Configuration" 168 This specification expresses syntax using the Augmented Backus-Naur 169 Form (ABNF) as described in [RFC5234], including the core rules in 170 Appendix B and rules from [RFC5322]. 172 3. Implicit TLS 174 Previous standards for use of email protocols with TLS used the 175 STARTTLS mechanism: [RFC2595], [RFC3207], and [RFC3501]. With 176 STARTTLS, the client establishes a cleartext application session and 177 determines whether to issue a STARTTLS command based on server 178 capabilities and client configuration. If the client issues a 179 STARTTLS command, a TLS handshake follows that can upgrade the 180 connection. While this mechanism has been deployed, an alternate 181 mechanism where TLS is negotiated immediately at connection start on 182 a separate port (referred to in this document as "Implicit TLS") has 183 been deployed more successfully. To encourage more widespread use of 184 TLS, and to encourage a greater consistency for how TLS is used, this 185 specification now recommends use of Implicit TLS for POP, IMAP, SMTP 186 Submission, and all other protocols used between a Mail User Agent 187 and a mail service. 189 3.1. Implicit TLS for POP 191 When a TCP connection is established for the "pop3s" service (default 192 port 995), a TLS handshake begins immediately. Clients MUST 193 implement the certificate validation mechanism described in 194 [RFC7817]. Once the TLS session is established, POP3 [RFC1939] 195 protocol messages are exchanged as TLS application data for the 196 remainder of the TCP connection. After the server sends a +OK 197 greeting, the server and client MUST enter AUTHORIZATION state, even 198 if client credentials were supplied during the TLS handshake. 200 See Section 5.5 and Section 4.2 for additional information on client 201 certificate authentication. See Section 7.1 for port registration 202 information. 204 3.2. Implicit TLS for IMAP 206 When a TCP connection is established for the "imaps" service (default 207 port 993), a TLS handshake begins immediately. Clients MUST 208 implement the certificate validation mechanism described in [RFC3501] 209 and SHOULD implement the certificate validation mechanism described 210 in [RFC7817]. Once the TLS session is established, IMAP [RFC3501] 211 protocol messages are exchanged as TLS application data for the 212 remainder of the TCP connection. If client credentials were provided 213 during the TLS handshake that the server finds acceptable, the server 214 MAY issue a PREAUTH greeting in which case both the server and client 215 enter AUTHENTICATED state. If the server issues an OK greeting then 216 both server and client enter NOT AUTHENTICATED state. 218 See Section 5.5 and Section 4.2 for additional information on client 219 certificate authentication. See Section 7.1 and Section 7.2 for port 220 registration information. 222 3.3. Implicit TLS for SMTP Submission 224 When a TCP connection is established for the "submissions" service 225 (default port 465), a TLS handshake begins immediately. Clients MUST 226 implement the certificate validation mechanism described in 227 [RFC7817]. Once a TLS session is established, message submission 228 protocol data [RFC6409] is exchanged as TLS application data for the 229 remainder of the TCP connection. (Note: the "submissions" service 230 name is defined in section 10.3 of this document, and follows the 231 usual convention that the name of a service layered on top of 232 Implicit TLS consists of the name of the service as used without TLS, 233 with an "s" appended.) 235 The STARTTLS mechanism on port 587 is relatively widely deployed due 236 to the situation with port 465 (discussed in Section 7.3). This 237 differs from IMAP and POP services where Implicit TLS is more widely 238 deployed on servers than STARTTLS. It is desirable to migrate core 239 protocols used by MUA software to Implicit TLS over time for 240 consistency as well as the additional reasons discussed in 241 Appendix A. However, to maximize use of encryption for submission it 242 is desirable to support both mechanisms for Message Submission over 243 TLS for a transition period of several years. As a result, clients 244 and servers SHOULD implement both STARTTLS on port 587 and Implicit 245 TLS on port 465 for this transition period. Note that there is no 246 significant difference between the security properties of STARTTLS on 247 port 587 and Implicit TLS on port 465 if the implementations are 248 correct and both client and server are configured to require 249 successful negotiation of TLS prior to message submission. 251 Note that the "submissions" port provides access to a Mail Submission 252 Agent (MSA) as defined in [RFC6409] so requirements and 253 recommendations for MSAs in that document apply to the submissions 254 port, including the requirement to implement SMTP AUTH [RFC4954]. 256 See Section 5.5 and Section 4.2 for additional information on client 257 certificate authentication. See Section 7.3 for port registration 258 information. 260 3.4. Implicit TLS Connection Closure for POP, IMAP and SMTP Submission 262 When a client or server wishes to close the connection, it SHOULD 263 initiate the exchange of TLS close alerts before TCP connection 264 termination. The client MAY, after sending a TLS close alert, 265 gracefully close the TCP connection without waiting for a TLS 266 response from the server. 268 4. Recommendations for Use of TLS by Mail Access Services and Message 269 Submission Services 271 The following practices are recommended for Mail Access Services and 272 Mail Submission Services: 274 o Mail Service Providers (MSPs) which support POP, IMAP, and/or 275 Message Submission, MUST support TLS access for those services. 277 o Other services than POP, IMAP and/or Message Submission provided 278 by MSPs SHOULD support TLS access, and MUST support TLS access for 279 those services which support authentication via username and 280 password. 282 o MSPs which support POP, IMAP, and/or Message Submission, SHOULD 283 provide and support instances of those services which use Implicit 284 TLS. (See Section 3.) 286 o For compatibility with existing MUAs and existing MUA 287 configurations, MSPs SHOULD also, in the near term, provide 288 instances of these services which support STARTTLS. This will 289 permit legacy MUAs to discover new availability of TLS capability 290 on servers, and may increase use of TLS by such MUAs. However, 291 servers SHOULD NOT advertise STARTTLS if use of the STARTTLS 292 command by a client is likely to fail (for example, if the server 293 has no server certificate configured.) 295 o MSPs SHOULD advertise their Mail Access Services and Mail 296 Submission Services using DNS SRV records according to [RFC6186]. 297 (In addition to making correct configuration easier for MUAs, this 298 provides a way by which MUAs can discover when an MSP begins to 299 offer TLS-based services.) Services supporting TLS SHOULD be 300 advertised in preference to cleartext services (if offered). In 301 addition, services using Implicit TLS SHOULD be advertised in 302 preference to services supporting STARTTLS (if offered). (See 303 also Section 4.5.) 305 o MSPs SHOULD deprecate use of cleartext Mail Access Services and 306 Mail Submission Services as soon as practicable. (See 307 Section 4.1.) 309 o MSPs currently supporting such use of cleartext SMTP (on port 25) 310 as a means of message submission by their users (whether or not 311 requiring authentication) SHOULD transition their users to using 312 TLS (either Implicit TLS or STARTTLS) as soon as practicable. 314 o Mail services MUST support TLS 1.2 or later. 316 o All Mail services SHOULD implement the recommended TLS cipher 317 suites described in [RFC7525] or a future BCP or standards track 318 revision of that document. 320 o Mail services currently supporting SSL 2.x, SSL 3.0, or TLS 1.0 321 SHOULD transition their users to later versions of TLS, and 322 discontinue support for those versions of SSL and TLS, as soon as 323 practicable. 325 o Mail Submission Servers accepting mail using TLS SHOULD include 326 the TLS ciphersuite of the session in which the mail was received, 327 in the Received field of the outgoing message. (See Section 4.3.) 329 o All Mail services implementing TLS SHOULD log TLS cipher 330 information along with any connection or authentication logs that 331 they maintain. 333 Additional considerations and details appear below. 335 4.1. Deprecation of Services Using Cleartext and TLS Versions < 1.1 337 The specific means employed for deprecation of cleartext Mail Access 338 Services and Mail Submission Services this MAY vary from one MSP to 339 the next in light of their user communities' needs and constraints. 340 For example, an MSP MAY implement a gradual transition in which, over 341 time, more and more users are forbidden to authenticate to cleartext 342 instances of these services, thus encouraging those users to migrate 343 to Implicit TLS. Access to cleartext services should eventually be 344 either disabled, or limited strictly for use by legacy systems which 345 cannot be upgraded. 347 After a user's ability to authenticate to a service using cleartext 348 is revoked, the server denying such access MUST NOT provide any 349 indication over a cleartext channel of whether the user's 350 authentication credentials were valid. An attempt to authenticate as 351 such a user using either invalid credentials or valid credentials 352 MUST both result in the same indication of access being denied. 354 Also, users authenticating with passwords SHOULD be required to 355 change those passwords when migrating from cleartext to TLS, since 356 the old passwords were likely to have been compromised. 358 Transition of users from SSL or TLS 1.0 to later versions of TLS MAY 359 be accomplished by a means similar to that described above. There 360 are multiple ways to accomplish this. One way is for the server to 361 refuse a ClientHello message from any client sending a protocol 362 version number corresponding to any version of SSL or TLS 1.0. 363 Another way is for the server to accept ClientHello messages from 364 some client versions that it does not wish to support, but later 365 refuse to allow the user to authenticate. The latter method may 366 provide a better indication to the user of the reason for the failure 367 but (depending on the protocol and method of authentication used) may 368 also risk exposure of the user's password over an channel which is 369 known to not provide adequate confidentiality. 371 It is RECOMMENDED that new users be required to use TLS version 1.1 372 or greater from the start. However an MSP may find it necessary to 373 make exceptions to accommodate some legacy systems which support only 374 earlier versions of TLS, or only cleartext. 376 4.2. Mail Server Use of Client Certificate Authentication 378 Mail servers MAY implement client certificate authentication on the 379 Implicit TLS port. Servers MUST NOT request a client certificate 380 during the TLS handshake unless the server is configured to accept 381 some client certificates as sufficient for authentication and the 382 server has the ability to determine a mail server authorization 383 identity matching such certificates. How to make this determination 384 is presently implementation specific. 386 If the server accepts the client's certificate as sufficient for 387 authorization, it MUST enable the SASL EXTERNAL [RFC4422] mechanism. 388 An IMAPS server MAY issue a PREAUTH greeting instead of enabling SASL 389 EXTERNAL. 391 4.3. Recording TLS Cipher Suite in Received Header 393 The ESMTPS transmission type [RFC3848] provides trace information 394 that can indicate TLS was used when transferring mail. However, TLS 395 usage by itself is not a guarantee of confidentiality or security. 396 The TLS cipher suite provides additional information about the level 397 of security made available for a connection. This defines a new SMTP 398 "tls" Received header additional-registered-clause that is used to 399 record the TLS cipher suite that was negotiated for the connection. 400 The value included in this additional clause SHOULD be the registered 401 cipher suite name (e.g., TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) 402 included in the TLS cipher suite registry. In the event the 403 implementation does not know the name of the cipher suite (a 404 situation that should be remedied promptly), a four-digit hexadecimal 405 cipher suite identifier MAY be used. The ABNF for the field follows: 407 tls-cipher-clause = CFWS "tls" FWS tls-cipher 409 tls-cipher = tls-cipher-suite-name / tls-cipher-suite-hex 411 tls-cipher-name = ALPHA *(ALPHA / DIGIT / "_") 412 ; as registered in IANA cipher suite registry 414 tls-cipher-hex = "0x" 4HEXDIG 416 4.4. TLS Server Certificate Requirements 418 MSPs MUST maintain valid server certificates for all servers. See 419 [RFC7817] for the recommendations and requirements necessary to 420 achieve this. 422 If a protocol server provides service for more than one mail domain, 423 it MAY use a separate IP address for each domain and/or a server 424 certificate that advertises multiple domains. This will generally be 425 necessary unless and until it is acceptable to impose the constraint 426 that the server and all clients support the Server Name Indication 427 extension to TLS [RFC6066]. For more discussion of this problem, see 428 section 5.1 of [RFC7817]. 430 4.5. Recommended DNS records for mail protocol servers 432 This section discusses not only the DNS records that are recommended, 433 but also implications of DNS records for server configuration and TLS 434 server certificates. 436 4.5.1. MX records 438 It is recommended that MSPs advertise MX records for handling of 439 inbound mail (instead of relying entirely on A or AAAA records), and 440 that those MX records be signed using DNSSEC. This is mentioned here 441 only for completeness, as handling of inbound mail is out of scope 442 for this document. 444 4.5.2. SRV records 446 MSPs SHOULD advertise SRV records to aid MUAs in determination of 447 proper configuration of servers, per the instructions in [RFC6186]. 449 MSPs SHOULD advertise servers that support Implicit TLS in preference 450 to those which support cleartext and/or STARTTLS operation. 452 4.5.3. DNSSEC 454 All DNS records advertised by an MSP as a means of aiding clients in 455 communicating with the MSP's servers, SHOULD be signed using DNSSEC. 457 4.5.4. TLSA records 459 MSPs SHOULD advertise TLSA records to provide an additional trust 460 anchor for public keys used in TLS server certificates. However, 461 TLSA records MUST NOT be advertised unless they are signed using 462 DNSSEC. 464 4.6. Changes to Internet Facing Servers 466 When an MSP changes the Internet Facing Servers providing mail access 467 and mail submission services, including SMTP-based spam/virus 468 filters, it is generally necessary to support the same and/or a newer 469 version of TLS and the same security directives that were previously 470 advertised. 472 5. Recommendations for use of TLS by Mail User Agents 474 It is recommended that Mail User Agents implement the following 475 practices: 477 o MUAs SHOULD be capable of using DNS SRV records to discover Mail 478 Access Services and Mail Submission Services that are advertised 479 by a MSP for an account being configured. Other means of 480 discovering server configuration information (e.g. a database 481 maintained by the MUA vendor) MAY also be supported. (See 482 Section 5.1 for more information.) 484 o MUAs SHOULD be configurable to require a minimum level of 485 confidentiality for any particular Mail Account, and refuse to 486 exchange information via any service associated with that Mail 487 Account if the session does not provide that minimum level of 488 confidentiality. (See Section 5.2.) 490 o MUAs MUST NOT consider a session as meeting a minimum level of 491 confidentiality if the server's TLS certificate cannot be 492 validated. (See Section 5.3.) 494 o MUAs MAY impose other minimum confidentiality requirements in the 495 future, e.g. in order to discourage use of TLS versions or 496 cryptographic algorithms in which weaknesses have been discovered. 498 o MUAs SHOULD provide a prominent visual indication of the level of 499 confidentiality associated with an account configuration (for 500 example, indications such as "lock" icons or changed background 501 colors similar to those used by some browsers), at appropriate 502 times and locations in order to inform the user of the 503 confidentiality of the communications associated with that 504 account. For example, this might be done whenever (a) prompting 505 the user for authentication credentials, (b) the user is composing 506 mail that will be sent to a particular submission server, (c) a 507 list of accounts is displayed (particularly if the user can select 508 from that list to read mail), or (d) the user is requesting to 509 view or update any configuration data that will be stored on a 510 remote server. 512 o MUAs MUST implement TLS 1.2 [RFC5246] or later. Earlier TLS and 513 SSL versions MAY also be supported so long as the MUA requires at 514 least TLS 1.1 [RFC4346] when accessing accounts that are 515 configured to impose minimum confidentiality requirements. Per 516 [RFC7525], TLS 1.1 (or earlier) SHOULD NOT be used unless no 517 higher version is available during TLS protocol negotiation. 519 o All MUAs SHOULD implement the recommended TLS cipher suites 520 described in [RFC7525] or a future BCP or standards track revision 521 of that document. 523 o MUAs that are configured to not require minimum confidentiality 524 for one or more accounts SHOULD detect when TLS becomes available 525 on those accounts (using [RFC6186] or other means), and offer to 526 upgrade the account to require TLS. 528 Additional considerations and details appear below. 530 5.1. Use of SRV records in Establishing Configuration 532 This section updates [RFC6186] by changing the preference rules and 533 adding a new SRV service label _submissions._tcp to refer to Message 534 Submission with Implicit TLS. 536 User-configurable MUAs SHOULD support use of [RFC6186] for account 537 setup. However, when using configuration information obtained by 538 this method, MUAs SHOULD ignore advertised services that do not 539 satisfy minimum confidentiality requirements, unless the user has 540 explicitly requested reduced confidentiality. This will have the 541 effect of causing the MUA to default to ignoring advertised 542 configurations that do not support TLS, even when those advertised 543 configurations have a higher priority than other advertised 544 configurations. 546 When using [RFC6186] configuration information, Mail User Agents 547 SHOULD NOT automatically establish new configurations that do not 548 require TLS for all servers, unless there are no advertised 549 configurations using TLS. If such a configuration is chosen, prior 550 to attempting to authenticate to the server or use the server for 551 message submission, the MUA SHOULD warn the user that traffic to that 552 server will not be encrypted and that it will therefore likely be 553 intercepted by unauthorized parties. The specific wording is to be 554 determined by the implementation, but it should adequately capture 555 the sense of risk given the widespread incidence of mass surveillance 556 of email traffic. 558 Similarly, a MUA MUST NOT attempt to "test" a particular mail account 559 configuration by submitting the user's authentication credentials to 560 a server, unless a TLS session meeting minimum confidentiality levels 561 has been established with that server. If minimum confidentiality 562 requirements have not been satisfied, the MUA must explicitly warn 563 the user that his password may be exposed to attackers before testing 564 the new configuration. 566 When establishing a new configuration for connecting to an IMAP, POP, 567 or SMTP submission server, based on SRV records, an MUA SHOULD either 568 verify that the SRV records are verifiably signed using DNSSEC, or 569 that the target FQDN of the SRV record matches the original server 570 FQDN for which the SRV queries were made. If the target FQDN is not 571 in the queried domain, the MUA SHOULD verify with the user that the 572 SRV target FQDN is suitable for use, before executing any connections 573 to the host. (See [RFC6186] section 6). 575 An MUA MUST NOT consult SRV records to determine which servers to use 576 on every connection attempt, unless those SRV records are signed by 577 DNSSEC and have a valid signature. However, an MUA MAY consult SRV 578 records from time to time to determine if an MSP's server 579 configuration has changed, and alert the user if it appears that this 580 has happened. This can also serve as a means to encourage users to 581 upgrade their configurations to require TLS if and when their MSPs 582 support it. 584 5.2. Minimum Confidentiality Level 586 MUAs SHOULD, by default, require a minimum level of confidentiality 587 for services accessed by each account. For MUAs supporting the 588 ability to access multiple mail accounts, this requirement SHOULD be 589 configurable on a per-account basis. 591 The default minimum expected level of confidentiality for all new 592 accounts SHOULD be at least use of TLS version 1.1 or greater, and 593 successful validation of the server's certificate. (Future revisions 594 to this specification may raise these requirements or impose 595 additional requirements to address newly-discovered weaknesses in 596 protocols or cryptographic algorithms.) 598 MUAs MAY permit the user to disable this minimum confidentiality 599 requirement during initial account configuration, or subsequently 600 editing an account configuration, but MUST warn users that such a 601 configuration will not assure privacy for either passwords or 602 messages. 604 An MUA which is configured to require a minimum level of 605 confidentiality for a mail account MUST NOT attempt to perform any 606 operation other than capability discovery, or STARTTLS for servers 607 not using Implicit TLS, unless the minimum level of confidentiality 608 is provided by that connection. 610 MUAs SHOULD NOT allow users to "click through" to access or send mail 611 via an connection, or to authenticate to any service using a 612 password, if that account is configured to impose minimum 613 confidentiality requirements and that connection does not meet all of 614 those requirements. Experience indicates that users presented with 615 such an option often "click through" without understanding the risks 616 that they're accepting by doing so. Furthermore, users who 617 frequently find the need to "click through" to use an insecure 618 connection may become conditioned to do so as a matter of habit, 619 before considering whether the risks are reasonable in each specific 620 instance. 622 An MUA which is not configured to require a minimum level of 623 confidentiality for a mail account SHOULD still attempt to connect to 624 the services associated with that account using the most secure means 625 available, e.g. by using Implicit TLS or STARTTLS. 627 5.3. Certificiate Validation 629 MUAs MUST validate TLS server certificates according to [RFC7817] and 630 PKIX [RFC5280]. 632 MUAs MAY also support DANE [RFC6698] as a means of validating server 633 certificates in order to meet minimum confidentiality requirements. 635 MUAs MAY support use of certificate pinning but MUST NOT consider a 636 connection in which the server's authenticity relies on certificate 637 pinning, as providing the minimum level of confidentiality. (See 638 Section 5.4.) 640 5.4. Certificate Pinning 642 During account setup, the MUA will identify servers that provide 643 account services such as mail access and mail submission (the 644 previous section describes one way to do this). The certificates for 645 these servers are verified using the rules described in [RFC7817] and 646 PKIX [RFC5280]. In the event the certificate does not validate due 647 to an expired certificate, lack of appropriate chain of trust, or 648 lack of identifier match, the MUA MAY offer to create a persistent 649 binding between that certificate and the saved host name for the 650 server, for use when accessing that account's servers. This is 651 called certificate pinning. 653 Certificate pinning is only appropriate during mail account setup and 654 MUST NOT be offered as an option in response to a failed certificate 655 validation for an existing mail account. An MUA that allows 656 certificate pinning MUST NOT allow a certificate pinned for one 657 account to validate connections for other accounts. 659 A pinned certificate is subject to a man-in-the-middle attack at 660 account setup time, and lacks a mechanism to revoke or securely 661 refresh the certificate. Note also that a man-in-the-middle attack 662 at account setup time will expose the user's password to the attacker 663 (if a password is used). Therefore use of a pinned certificate does 664 not meet the requirement for a minimum confidentiality level, and an 665 MUA MUST NOT indicate to the user that the such confidentiality is 666 provided. Additional advice on certificate pinning is present in 667 [RFC6125]. 669 5.5. Client Certificate Authentication 671 MUAs MAY implement client certificate authentication on the Implicit 672 TLS port. An MUA MUST NOT provide a client certificate during the 673 TLS handshake unless the server requests one and the client has 674 determined the certificate can be safely used with that specific 675 server, OR the client has been explicitly configured by the user to 676 use that particular certificate with that server. How to make this 677 determination is presently implementation specific. 679 A client supporting client certificate authentication with Implicit 680 TLS MUST implement the SASL EXTERNAL [RFC4422] mechanism using the 681 appropriate authentication command (AUTH for POP3 [RFC5034], AUTH for 682 SMTP Submission [RFC4954], AUTHENTICATE for IMAP [RFC3501]). 684 6. Considerations related to Anti-Virus/Anti-Spam Software and Services 686 There are multiple ways to connect an Anti-Virus and/or Anti-Spam 687 (AVAS) service to a mail server. Some mechanisms, such as the de- 688 facto milter protocol, are out of scope for this specification. 689 However, some services use an SMTP relay proxy that intercepts mail 690 at the application layer to perform a scan and proxy or forward to 691 another MTA. Deploying AVAS services in this way can cause many 692 problems [RFC2979] including direct interference with this 693 specification, and other forms of confidentiality or security 694 reduction. An AVAS product or service is considered compatible with 695 this specification if all IMAP, POP and SMTP-related software 696 (including proxies) it includes are compliant with this 697 specification. 699 Note that end-to-end email encryption prevents AVAS software and 700 services from using email content as part of a spam or virus 701 assessment. Furthermore, while a minimum confidentiality level can 702 prevent a man-in-the-middle from introducing spam or virus content 703 between the MUA and Submission server, it does not prevent other 704 forms of client or account compromise. Use of AVAS services for 705 submitted email therefore remains necessary. 707 7. IANA Considerations 709 7.1. POP3S Port Registration Update 711 IANA is asked to update the registration of the TCP well-known port 712 995 using the following template ([RFC6335]): 714 Service Name: pop3s 715 Transport Protocol: TCP 716 Assignee: IETF 717 Contact: IESG 718 Description: POP3 over TLS protocol 719 Reference: RFC XXXX (this document once published) 720 Port Number: 995 722 7.2. IMAPS Port Registration Update 724 IANA is asked to update the registration of the TCP well-known port 725 993 using the following template ([RFC6335]): 727 Service Name: imaps 728 Transport Protocol: TCP 729 Assignee: IETF 730 Contact: IESG 731 Description: IMAP over TLS protocol 732 Reference: RFC XXXX (this document once published) 733 Port Number: 993 735 7.3. Submissions Port Registration 737 IANA is asked to assign an alternate usage of port 465 in addition to 738 the current assignment using the following template ([RFC6335]): 740 Service Name: submissions 741 Transport Protocol: TCP 742 Assignee: IETF 743 Contact: IESG 744 Description: Message Submission over TLS protocol 745 Reference: RFC XXXX (this document once published) 746 Port Number: 465 748 This is a one-time procedural exception to the rules in RFC 6335. 749 This requires explicit IESG approval and does not set a precedent. 751 Historically, port 465 was briefly registered as the "smtps" port. 752 This registration made no sense as the SMTP transport MX 753 infrastructure has no way to specify a port, so port 25 is always 754 used. As a result, the registration was revoked and was subsequently 755 reassigned to a different service. In hindsight, the "smtps" 756 registration should have been renamed or reserved rather than 757 revoked. Unfortunately, some widely deployed mail software 758 interpreted "smtps" as "submissions" [RFC6409] and used that port for 759 email submission by default when an end-user requests security during 760 account setup. If a new port is assigned for the submissions 761 service, email software will either continue with unregistered use of 762 port 465 (leaving the port registry inaccurate relative to de-facto 763 practice and wasting a well-known port), or confusion between the de- 764 facto and registered ports will cause harmful interoperability 765 problems that will deter use of TLS for message submission. The 766 authors believe both of these outcomes are less desirable than a wart 767 in the registry documenting real-world usage of a port for two 768 purposes. Although STARTTLS-on-port-587 has deployed, it has not 769 replaced deployed use of Implicit TLS submission on port 465. 771 8. Security Considerations 773 This entire document is about security considerations. In general, 774 this is targeted to improve mail confidentiality and to mitigate 775 threats external to the email system such as network-level snooping 776 or interception; this is not intended to mitigate active attackers 777 who have compromised service provider systems. 779 It could be argued that sharing the name and version of the client 780 software with the server has privacy implications. Although 781 providing this information is not required, it is encouraged so that 782 mail service providers can more effectively inform end-users running 783 old clients that they need to upgrade to protect their security, or 784 know which clients to use in a test deployment prior to upgrading a 785 server to have higher security requirements. 787 9. References 789 9.1. Normative References 791 [RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3", 792 STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996, 793 . 795 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 796 Requirement Levels", BCP 14, RFC 2119, 797 DOI 10.17487/RFC2119, March 1997, 798 . 800 [RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over 801 Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207, 802 February 2002, . 804 [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 805 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, 806 . 808 [RFC5034] Siemborski, R. and A. Menon-Sen, "The Post Office Protocol 809 (POP3) Simple Authentication and Security Layer (SASL) 810 Authentication Mechanism", RFC 5034, DOI 10.17487/RFC5034, 811 July 2007, . 813 [RFC5068] Hutzler, C., Crocker, D., Resnick, P., Allman, E., and T. 814 Finch, "Email Submission Operations: Access and 815 Accountability Requirements", BCP 134, RFC 5068, 816 DOI 10.17487/RFC5068, November 2007, 817 . 819 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 820 Specifications: ABNF", STD 68, RFC 5234, 821 DOI 10.17487/RFC5234, January 2008, 822 . 824 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 825 (TLS) Protocol Version 1.2", RFC 5246, 826 DOI 10.17487/RFC5246, August 2008, 827 . 829 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 830 Housley, R., and W. Polk, "Internet X.509 Public Key 831 Infrastructure Certificate and Certificate Revocation List 832 (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, 833 . 835 [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, 836 DOI 10.17487/RFC5322, October 2008, 837 . 839 [RFC6186] Daboo, C., "Use of SRV Records for Locating Email 840 Submission/Access Services", RFC 6186, 841 DOI 10.17487/RFC6186, March 2011, 842 . 844 [RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail", 845 STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011, 846 . 848 [RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre, 849 "Recommendations for Secure Use of Transport Layer 850 Security (TLS) and Datagram Transport Layer Security 851 (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May 852 2015, . 854 [RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via 855 Opportunistic DNS-Based Authentication of Named Entities 856 (DANE) Transport Layer Security (TLS)", RFC 7672, 857 DOI 10.17487/RFC7672, October 2015, 858 . 860 [RFC7817] Melnikov, A., "Updated Transport Layer Security (TLS) 861 Server Identity Check Procedure for Email-Related 862 Protocols", RFC 7817, DOI 10.17487/RFC7817, March 2016, 863 . 865 9.2. Informative References 867 [RFC2595] Newman, C., "Using TLS with IMAP, POP3 and ACAP", 868 RFC 2595, DOI 10.17487/RFC2595, June 1999, 869 . 871 [RFC2979] Freed, N., "Behavior of and Requirements for Internet 872 Firewalls", RFC 2979, DOI 10.17487/RFC2979, October 2000, 873 . 875 [RFC3848] Newman, C., "ESMTP and LMTP Transmission Types 876 Registration", RFC 3848, DOI 10.17487/RFC3848, July 2004, 877 . 879 [RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security 880 (TLS) Protocol Version 1.1", RFC 4346, 881 DOI 10.17487/RFC4346, April 2006, 882 . 884 [RFC4422] Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple 885 Authentication and Security Layer (SASL)", RFC 4422, 886 DOI 10.17487/RFC4422, June 2006, 887 . 889 [RFC4954] Siemborski, R., Ed. and A. Melnikov, Ed., "SMTP Service 890 Extension for Authentication", RFC 4954, 891 DOI 10.17487/RFC4954, July 2007, 892 . 894 [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) 895 Extensions: Extension Definitions", RFC 6066, 896 DOI 10.17487/RFC6066, January 2011, 897 . 899 [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and 900 Verification of Domain-Based Application Service Identity 901 within Internet Public Key Infrastructure Using X.509 902 (PKIX) Certificates in the Context of Transport Layer 903 Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March 904 2011, . 906 [RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S. 907 Cheshire, "Internet Assigned Numbers Authority (IANA) 908 Procedures for the Management of the Service Name and 909 Transport Protocol Port Number Registry", BCP 165, 910 RFC 6335, DOI 10.17487/RFC6335, August 2011, 911 . 913 [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication 914 of Named Entities (DANE) Transport Layer Security (TLS) 915 Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August 916 2012, . 918 Appendix A. Design Considerations 920 This section is not normative. 922 The first version of this was written independently from draft-moore- 923 email-tls-00.txt; subsequent versions merge ideas from both drafts. 925 One author of this document was also the author of RFC 2595 that 926 became the standard for TLS usage with POP and IMAP, and the other 927 author was perhaps the first to propose that idea. In hindsight both 928 authors now believe that that approach was a mistake. At this point 929 the authors believe that while anything that makes it easier to 930 deploy TLS is good, the desirable end state is that these protocols 931 always use TLS, leaving no need for a separate port for cleartext 932 operation except to support legacy clients while they continue to be 933 used. The separate port model for TLS is inherently simpler to 934 implement, debug and deploy. It also enables a "generic TLS load- 935 balancer" that accepts secure client connections for arbitrary foo- 936 over-TLS protocols and forwards them to a server that may or may not 937 support TLS. Such load-balancers cause many problems because they 938 violate the end-to-end principle and the server loses the ability to 939 log security-relevant information about the client unless the 940 protocol is designed to forward that information (as this 941 specification does for the cipher suite). However, they can result 942 in TLS deployment where it would not otherwise happen which is a 943 sufficiently important goal that it overrides the problems. 945 Although STARTTLS appears only slightly more complex than separate- 946 port TLS, we again learned the lesson that complexity is the enemy of 947 security in the form of the STARTTLS command injection vulnerability 948 (CERT vulnerability ID #555316). Although there's nothing inherently 949 wrong with STARTTLS, the fact it resulted in a common implementation 950 error (made independently by multiple implementers) suggests it is a 951 less secure architecture than Implicit TLS. 953 Section 7 of RFC 2595 critiques the separate-port approach to TLS. 954 The first bullet was a correct critique. There are proposals in the 955 http community to address that, and use of SRV records as described 956 in RFC 6186 resolves that critique for email. The second bullet is 957 correct as well, but not very important because useful deployment of 958 security layers other than TLS in email is small enough to be 959 effectively irrelevant. The third bullet is incorrect because it 960 misses the desirable option of "use and latch-on TLS if available". 961 The fourth bullet may be correct, but is not a problem yet with 962 current port consumption rates. The fundamental error was 963 prioritizing a perceived better design based on a mostly valid 964 critique over real-world deployability. But getting security and 965 confidentiality facilities actually deployed is so important it 966 should trump design purity considerations. 968 Port 465 is presently used for two purposes: for submissions by a 969 large number of clients and service providers and for the "urd" 970 protocol by one vendor. Actually documenting this current state is 971 controversial as discussed in the IANA considerations section. 972 However, there is no good alternative. Registering a new port for 973 submissions when port 465 is widely used for that purpose already 974 will just create interoperability problems. Registering a port 975 that's only used if advertised by an SRV record (RFC 6186) would not 976 create interoperability problems but would require all client and 977 server deployments and software to change significantly which is 978 contrary to the goal of promoting more TLS use. Encouraging use of 979 STARTTLS on port 587 would not create interoperability problems, but 980 is unlikely to have impact on current undocumented use of port 465 981 and makes the guidance in this document less consistent. The 982 remaining option is to document the current state of the world and 983 support future use of port 465 for submission as this increases 984 consistency and ease-of-deployment for TLS email submission. 986 Appendix B. Change Log 988 Changes since draft-ietf-uta-email-deep-07: 990 o After discussion with the WG in Prague, removed BCP language and 991 once again made unambiguous that this is intended as a standards- 992 track document. 994 o Server implementations now MUST implement TLS 1.2, consistent with 995 RFC 7525. MUAs may still consider a TLS 1.1 session as meeting 996 minimum confidentiality requirements. 998 o MSPs now MUST support TLS for POP, IMAP, Submission, and any other 999 services that use username/password authentication. 1001 o Added text to clarify the purpose of recommending that MSPs use 1002 DNS SRV records to advertise services. 1004 o Changed text about MUAs not blindly trusting unsigned SRV records, 1005 to instead restate RFC 6186 requirements. 1007 Changes since draft-ietf-uta-email-deep-06: 1009 o On the recommendation of one of the co-chairs and some working 1010 group members, rewrote document with the intended status of BCP. 1011 This involved removing a great deal of text that consisted 1012 essentially of new protocol specification, especially the STS 1013 features, on the theory that a BCP should base its recommendations 1014 on current practice, and that new protocol features should be 1015 subject to the interoperability test requirements associated with 1016 normal standards-track documents. 1018 Changes since draft-ietf-uta-email-deep-05: 1020 o Clarify throughout that the confidentiality assurance level 1021 associated with a mail account is a minimum level; attempt to 1022 distinguish this from the current confidentiality level provided 1023 by a connection between client and server. 1025 o Change naming for confidentiality assurance levels: instead of 1026 "high" or "no" confidence, assign numbers 1 and 0 to them 1027 respectively. This because it seems likely that in the not-too- 1028 distant future, what was defined in -05 as "high" confidence will 1029 be considered insufficient, and calling that "high" confidence 1030 will become misleading. For example, relying entirely on a list 1031 of trusted CAs to validate server certificates from arbitrary 1032 parties, appears to be less and less reliable in practice at 1033 thwarting MITM attacks. 1035 o Clarify that if some services associated with a mail account don't 1036 meet the minimum confidentiality assurance level assigned to that 1037 account, other services that do meet that minimum confidentiality 1038 assurance level may continue to be used. 1040 o Clarify that successful negotiation of at least TLS version 1.1 is 1041 required as a condition of meeting confidentiality assurance level 1042 1. 1044 o Clarify that validation of a server certificate using either DANE 1045 or PKIX is sufficient to meet the certificate validation 1046 requirement of confidentiality assurance level 1. 1048 o Clarify that minimum confidentiality assurance levels are separate 1049 from security directives, and that the requirements of both 1050 mechanisms must be met. 1052 o Explicitly cite an example that a security directive of tls- 1053 version=1.2 won't be saved if the currently negotiated tls-version 1054 is 1.1. (This example already appeared a bit later in the text, 1055 but for author KM it seemed to make the mechanism clearer to use 1056 this example earlier.) 1058 o Clarify some protocol examples as to whether PKIX or DANE was used 1059 to verify a server's certificate. 1061 o Remove most references to DEEP as the conversion from DEEP to MUA- 1062 STS seemed incomplete, but kept the DEEP command for use in POP3 1063 on the assumption that author CN wanted it that way. 1065 o Removed most references to "latch" and derivative words. 1067 o Added pkix+dane as a value for the tls-cert directive, to indicate 1068 (from a server) that both PKIX and DANE validation will be 1069 supported, or (from a client) that both PKIX and DANE were used to 1070 validate a certificate. Also clarified what each of any, pkix, 1071 dane, and pkix+dane mean when advertised by a server and in 1072 particular that tls-cert=any provides no assurance of future PKIX 1073 verifiability in contrast to tls-cert=pkix or tls-cert=pkix+dane. 1074 It seemed important to support the ability to evolve to using 1075 multiple trust anchors for certificate validation, but also to 1076 allow servers to have the option to migrate from PKIX to DANE if 1077 that made sense for them. This change seemed less disruptive than 1078 either defining additional directives, or allowing multiple 1079 instances of the same directive with different values to appear in 1080 the same advertisement. 1082 o Clarify interaction of this specification with anti-virus / anti- 1083 spam mechanisms. 1085 Changes since draft-ietf-uta-email-deep-04: 1087 o Swap sections 5.1 and 5.3 ("Email Security Tags" and "Server DEEP 1088 Status") as that order may aid understanding of the model. Also 1089 rewrote parts of these two sections to try to make the model 1090 clearer. 1092 o Add text about versioning of security tags to make the model 1093 clearer. 1095 o Add example of security tag upgrade. 1097 o Convert remaining mention of TLS 1.0 to TLS 1.1. 1099 o Change document title from DEEP to MUA STS to align with SMTP 1100 relay STS. 1102 * Slight updates to abstract and introductions. 1104 * Rename security latches/tags to security directives. 1106 * Rename server DEEP status to STS policy. 1108 * Change syntax to use directive-style HSTS syntax. 1110 o Make HSTS reference normative. 1112 o Remove SMTP DSN header as that belongs in SMTP relay STS document. 1114 Changes since draft-ietf-uta-email-deep-03: 1116 o Add more references to ietf-uta-email-tls-certs in implementation 1117 requirements section. 1119 o Replace primary reference to RFC 6125 with ietf-uta-email-tls- 1120 certs, so move RFC 6125 to informative list for this 1121 specification. 1123 Changes since draft-ietf-uta-email-deep-02: 1125 o Make reference to design considerations explicit rather than 1126 "elsewhere in this document". 1128 o Change provider requirement so SMTP submission services are 1129 separate from SMTP MTA services as opposed to the previous 1130 phrasing that required the servers be separate (which is too 1131 restrictive). 1133 o Update DANE SMTP reference 1134 Changes since draft-ietf-uta-email-deep-01: 1136 o Change text in tls11 and tls12 registrations to clarify 1137 certificate rules, including additional PKIX and DANE references. 1139 o Change from tls10 to tls11 (including reference) as the minimum. 1141 o Fix typo in example 5. 1143 o Remove open issues section; enough time has passed so not worth 1144 waiting for more input. 1146 Changes since draft-ietf-uta-email-deep-00: 1148 o Update and clarify abstract 1150 o use term confidentiality instead of privacy in most cases. 1152 o update open issues to request input for missing text. 1154 o move certificate pinning sub-section to account setup section and 1155 attempt to define it more precisely. 1157 o Add note about end-to-end encryption in AVAS section. 1159 o swap order of DNSSEC and TLSA sub-sections. 1161 o change meaning of 'tls10' and 'tls12' latches to require 1162 certificate validation. 1164 o Replace cipher suite advice with reference to RFC 7525. Change 1165 examples to use TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 as cipher 1166 suite. 1168 o Add text to update IMAP, POP3 and Message Submission standards 1169 with newer TLS advice. 1171 o Add clearer text in introduction that this does not cover SMTP 1172 relay. 1174 o Update references to uta-tls-certs. 1176 o Add paragraph to Implicit TLS for SMTP Submission section 1177 recommending that STARTTLS also be implemented. 1179 Changes since draft-newman-email-deep-02: 1181 o Changed "privacy assurance" to "confidentiality assurance" 1182 o Changed "low privacy assurance" to "no confidentiality assurance" 1184 o Attempt to improve definition of confidentiality assurance level. 1186 o Add SHOULD indicate when MUA is showing list of mail accounts. 1188 o Add SHOULD NOT latch tls10, tls12 tags until TLS negotiated. 1190 o Removed sentence about deleting and re-creating the account in 1191 latch failure section. 1193 o Remove use of word "fallback" with respect to TLS version 1194 negotiation. 1196 o Added bullet about changes to Internet facing servers to MSP 1197 section. 1199 o minor wording improvements based on feedback 1201 Changes since -01: 1203 o Updated abstract, introduction and document structure to focus 1204 more on mail user agent privacy assurance. 1206 o Added email account privacy section, also moving section on 1207 account setup using SRV records to that section. 1209 o Finished writing IANA considerations section 1211 o Remove provisional concept and instead have server explicitly list 1212 security tags clients should latch. 1214 o Added note that rules for the submissions port follow the same 1215 rules as those for the submit port. 1217 o Reference and update advice in [RFC5068]. 1219 o Fixed typo in Client Certificate Authentication section. 1221 o Removed tls-pfs security latch and all mention of perfect forward 1222 secrecy as it was controversial. 1224 o Added reference to HSTS. 1226 Changes since -00: 1228 o Rewrote introduction to merge ideas from draft-moore-email-tls-00. 1230 o Added Implicit TLS section, Account configuration section and IANA 1231 port registration updates based on draft-moore-email-tls-00. 1233 o Add protocol details necessary to standardize implicit TLS for 1234 POP/IMAP/submission, using ideas from draft-melnikov-pop3-over- 1235 tls. 1237 o Reduce initial set of security tags based on feedback. 1239 o Add deep status concept to allow a window for software updates to 1240 be backed out before latches make that problematic, as well as to 1241 provide service providers with a mechanism they can use to assist 1242 customers in the event of a privacy failure. 1244 o Add DNS SRV section from draft-moore-email-tls-00. 1246 o Write most of the missing IANA considerations section. 1248 o Rewrite most of implementation requirements section based more on 1249 draft-moore-email-tls-00. Remove new cipher requirements for now 1250 because those may be dealt with elsewhere. 1252 Appendix C. Acknowledgements 1254 Thanks to Ned Freed for discussion of the initial latch concepts in 1255 this document. Thanks to Alexey Melnikov for draft-melnikov-pop3- 1256 over-tls-02, which was the basis of the POP3 Implicit TLS text. 1257 Thanks to Russ Housley, Alexey Melnikov and Dan Newman for review 1258 feedback. Thanks to Paul Hoffman for interesting feedback in initial 1259 conversations about this idea. 1261 Authors' Addresses 1263 Keith Moore 1264 Windrock 1265 PO Box 1934 1266 Knoxville, TN 37901 1267 US 1269 Email: moore@network-heretics.com 1270 Chris Newman 1271 Oracle 1272 440 E. Huntington Dr., Suite 400 1273 Arcadia, CA 91006 1274 US 1276 Email: chris.newman@oracle.com