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Melnikov 3 Internet-Draft Isode Ltd 4 Intended status: Standards Track January 12, 2016 5 Expires: July 15, 2016 7 Updated DNSSEC-based TLS Server Identity Check Procedure for Email 8 Related Protocols 9 draft-melnikov-uta-dnssec-email-tls-certs-00 11 Abstract 13 This document describes DNSSEC-based TLS server identity verification 14 procedure for SMTP Submission, IMAP, POP and ManageSieve clients. 16 Status of This Memo 18 This Internet-Draft is submitted in full conformance with the 19 provisions of BCP 78 and BCP 79. 21 Internet-Drafts are working documents of the Internet Engineering 22 Task Force (IETF). Note that other groups may also distribute 23 working documents as Internet-Drafts. The list of current Internet- 24 Drafts is at http://datatracker.ietf.org/drafts/current/. 26 Internet-Drafts are draft documents valid for a maximum of six months 27 and may be updated, replaced, or obsoleted by other documents at any 28 time. It is inappropriate to use Internet-Drafts as reference 29 material or to cite them other than as "work in progress." 31 This Internet-Draft will expire on July 15, 2016. 33 Copyright Notice 35 Copyright (c) 2016 IETF Trust and the persons identified as the 36 document authors. All rights reserved. 38 This document is subject to BCP 78 and the IETF Trust's Legal 39 Provisions Relating to IETF Documents 40 (http://trustee.ietf.org/license-info) in effect on the date of 41 publication of this document. Please review these documents 42 carefully, as they describe your rights and restrictions with respect 43 to this document. Code Components extracted from this document must 44 include Simplified BSD License text as described in Section 4.e of 45 the Trust Legal Provisions and are provided without warranty as 46 described in the Simplified BSD License. 48 Internet-DrafDNSSEC-based TLS Server Identity Check for Ema January 2016 50 Table of Contents 52 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 53 2. Conventions Used in This Document . . . . . . . . . . . . . . 3 54 3. Email Server Certificate Verification Rules . . . . . . . . . 3 55 4. Compliance Checklist for Certification Authorities . . . . . 5 56 4.1. Notes on handling of delegated email services by 57 Certification Authorities . . . . . . . . . . . . . . . . 5 58 5. Compliance Checklist for Mail Service Providers and 59 Certificate Signing Request generation tools . . . . . . . . 6 60 5.1. Notes on hosting multiple domains . . . . . . . . . . . . 7 61 6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 8 62 7. Operational Considerations . . . . . . . . . . . . . . . . . 9 63 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 64 9. Security Considerations . . . . . . . . . . . . . . . . . . . 9 65 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 66 10.1. Normative References . . . . . . . . . . . . . . . . . . 9 67 10.2. Informative References . . . . . . . . . . . . . . . . . 11 68 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 13 69 Appendix B. Changes since draft-melnikov-uta-dnssec-email-tls- 70 certs-00 . . . . . . . . . . . . . . . . . . . . . . 13 71 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 13 73 1. Introduction 75 Use of TLS by SMTP Submission, IMAP, POP and ManageSieve clients is 76 described in [RFC3207], [RFC3501], [RFC2595] and [RFC5804] 77 respectively. Each of the documents describes slightly different 78 rules for server certificate identity verification (or doesn't define 79 any rules at all). In reality, email client and server developers 80 implement many of these protocols at the same time, so it would be 81 good to define modern and consistent rules for verifying email server 82 identities using TLS. 84 This document describes the updated TLS server identity verification 85 procedure for SMTP Submission [RFC6409] [RFC3207], IMAP [RFC3501], 86 POP [RFC1939] and ManageSieve [RFC5804] clients. It replaces 87 Section 2.4 of RFC 2595. 89 Note that this document doesn't apply to use of TLS in MTA-to-MTA 90 SMTP. 92 This document provides a consistent TLS server identity verification 93 procedure across multiple email related protocols. This should make 94 it easier for Certification Authorities and ISPs to deploy TLS for 95 email use, and would enable email client developers to write more 96 secure code. 98 Internet-DrafDNSSEC-based TLS Server Identity Check for Ema January 2016 100 2. Conventions Used in This Document 102 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 103 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 104 document are to be interpreted as described in [RFC2119]. 106 The following terms or concepts are used through the document: 108 reference identifier: (formally defined in [RFC6125]) One of the 109 domain names that the email client (an SMTP, IMAP, POP3 or 110 ManageSieve client) associates with the target email server. For 111 some identifier types, the identifier also includes an application 112 service type. Reference identifiers are used for performing name 113 checks on server certificates. 115 CN-ID, DNS-ID, SRV-ID and URI-ID are identifier types (see [RFC6125] 116 for details). For convenience, their short definitions from 117 [RFC6125] are listed below: 119 CN-ID = a Relative Distinguished Name (RDN) in the certificate 120 subject field that contains one and only one attribute-type-and- 121 value pair of type Common Name (CN), where the value matches the 122 overall form of a domain name (informally, dot- separated letter- 123 digit-hyphen labels). 125 DNS-ID = a subjectAltName entry of type dNSName 127 SRV-ID = a subjectAltName entry of type otherName whose name form 128 is SRVName 130 URI-ID = a subjectAltName entry of type uniformResourceIdentifier 131 whose value includes both (i) a "scheme" and (ii) a "host" 132 component (or its equivalent) that matches the "reg-name" rule 133 (where the quoted terms represent the associated [RFC5234] 134 productions from [RFC3986]). 136 This documents uses the phrase 'RRSet is "insecure"' as defined in 137 Section 2.1.1 of [RFC7672]. Similarly, 'RRSet is "secure"' if it is 138 not "insecure". 140 3. Email Server Certificate Verification Rules 142 During a TLS negotiation, an email client (i.e., an SMTP, IMAP, POP3 143 or ManageSieve client) MUST check its understanding of the server 144 identity (client's reference identifiers) against the server's 145 identity as presented in the server Certificate message, in order to 146 prevent man-in-the-middle attacks. This check is only performed 147 after the server certificate passes certification path validation as 149 Internet-DrafDNSSEC-based TLS Server Identity Check for Ema January 2016 151 described in Section 6 of [RFC5280]. Matching is performed according 152 to the rules specified in Section 6 of [RFC6125], including the 153 relative order of matching of different identifier types, 154 "certificate pinning" and the procedure on failure to match. The 155 following inputs are used by the verification procedure used in 156 [RFC6125]: 158 1. For DNS-ID and CN-ID identifier types the client MUST use one or 159 more of the following as "reference identifiers": (a) the domain 160 portion of the user's email address, (b) the hostname it used to 161 open the connection (without CNAME canonicalization). The client 162 MAY also use (c) a value securely derived from (a) or (b), such 163 as using "secure" DNSSEC [RFC4033][RFC4034][RFC4035] validated 164 lookup or a value obtained from the local hostname file. 166 2. When using email service discovery procedure specified in 167 [RFC6186] the client MUST also use the domain portion of the 168 user's email address as another "reference identifier" to compare 169 against SRV-ID identifier in the server certificate. If DNSSEC 170 protected SRV lookup (and all CNAME leading to it) are "secure", 171 the email client MAY also use the resulting hostname from such 172 lookup as DNS-ID/CN-ID reference identifier types. (This also 173 corresponds to the case (c) above.) 175 The rules and guidelines defined in [RFC6125] apply to an email 176 server certificate, with the following supplemental rules: 178 1. Support for the DNS-ID identifier type (subjectAltName of dNSName 179 type [RFC5280]) is REQUIRED in Email client software 180 implementations. 182 2. Support for the SRV-ID identifier type (subjectAltName of SRVName 183 type [RFC4985]) is REQUIRED for email client software 184 implementations that support [RFC6186] and don't rely on DNSSEC 185 protection of DNS SRV records. List of SRV-ID types for email 186 services is specified in [RFC6186]. For the ManageSieve protocol 187 the service name "sieve" is used. 189 3. URI-ID identifier type (subjectAltName of 190 uniformResourceIdentifier type [RFC5280]) MUST NOT be used by 191 clients for server verification, as URI-ID were not historically 192 used for email. 194 4. For backward compatibility with deployed software CN-ID 195 identifier type (CN attribute from the subject name, see 196 [RFC6125]) MAY be used for server identity verification. 198 Internet-DrafDNSSEC-based TLS Server Identity Check for Ema January 2016 200 5. Email protocols allow use of certain wildcards in identifiers 201 presented by email servers. The "*" wildcard character MAY be 202 used as the left-most name component of DNS-ID or CN-ID in the 203 certificate. For example, a DNS-ID of *.example.com would match 204 a.example.com, foo.example.com, etc. but would not match 205 example.com. Note that the wildcard character MUST NOT be used 206 as a fragment of the left-most name component (e.g., 207 *oo.example.com, f*o.example.com, or foo*.example.com). 209 4. Compliance Checklist for Certification Authorities 211 1. CA MUST support issuance of server certificates with DNS-ID 212 identifier type (subjectAltName of dNSName type [RFC5280]). 213 (Note that some DNS-IDs may refer to domain portions of email 214 addresses, so they might not have corresponding A/AAAA DNS 215 records.) 217 2. CA MUST support issuance of server certificates with SRV-ID 218 identifier type (subjectAltName of SRVName type [RFC4985]) for 219 each type of email service. 221 3. For backward compatibility with deployed client base, CA MUST 222 support issuance of server certificates with CN-ID identifier 223 type (CN attribute from the subject name, see [RFC6125]). 225 4. CA MAY allow "*" (wildcard) as the left-most name component of 226 DNS-ID or CN-ID in server certificates it issues. 228 4.1. Notes on handling of delegated email services by Certification 229 Authorities 231 [RFC6186] provides an easy way for organizations to autoconfigure 232 email clients. It also allows for delegation of email services to an 233 email hosting provider. When connecting to such delegated hosting 234 service an email client that attempts to verify TLS server identity 235 needs to know that if it connects to imap.hosting.example.net that 236 such server is authorized to provide email access for an email such 237 as alice@example.org. In absence of SRV-IDs, users of compliant 238 email clients would be forced to manually confirm exception, because 239 the TLS server certificate verification procedures specified in this 240 document would result in failure to match the TLS server certificate 241 against the expected domain(s). One way to provide such 242 authorization is for the TLS certificate for imap.hosting.example.net 243 to include SRV-ID(s) (or DNS-ID) for the example.org domain. Another 244 way is for DNS SRV lookups to be protected by DNSSEC. 246 A certification authority that receives a Certificate Signing Request 247 containing multiple unrelated DNS-IDs and/or SRV-IDs (e.g. DNS-ID of 249 Internet-DrafDNSSEC-based TLS Server Identity Check for Ema January 2016 251 example.org and DNS-ID of example.com) needs to verify that the 252 entity that supplied such Certificate Signing Request is authorized 253 to provide email service for all requested domains. 255 The ability to issue certificates that contain an SRV-ID (or a DNS-ID 256 for the domain part of email addresses) implies the ability to verify 257 that entities requesting them are authorized to run email service for 258 these SRV-IDs/DNS-IDs. In particular, certification authorities that 259 can't verify such authorization (whether for a particular domain or 260 in general) MUST NOT include such email SRV-IDs/DNS-IDs in 261 certificates they issue. This document doesn't specify exact 262 mechanism(s) that can be used to achieve this. However, a few 263 special case recommendations are listed below. 265 A certification authority willing to sign a certificate containing a 266 particular DNS-ID SHOULD also support signing a certificate 267 containing one or more of email SRV-IDs for the same domain, because 268 the SRV-ID effectively provides more restricted access to an email 269 service for the domain (as opposed to unrestricted use of any 270 services for the same domain, as specified by DNS-ID). 272 A certification authority which also provides DNS service for a 273 domain can use DNS information to validate SRV-IDs/DNS-IDs for the 274 domain. 276 A certification authority which is also a Mail Service Provider for a 277 hosted domain can use that knowdledge to validate SRV-IDs/DNS-IDs for 278 the domain. 280 5. Compliance Checklist for Mail Service Providers and Certificate 281 Signing Request generation tools 283 Mail Service Providers and Certificate Signing Request generation 284 tools 286 1. MUST include the DNS-ID identifier type in Certificate Signing 287 Requests for the host name(s) where the email server(s) are 288 running. They SHOULD include the DNS-ID identifier type in 289 Certificate Signing Requests for the domain portion of served 290 email addresses. 292 2. If the email services provided are discoverable using DNS SRV as 293 specified in [RFC6186], the Mail Service Provider MUST (a) 294 include the SRV-ID identifier type for each type of email service 295 in Certificate Signing Requests and/or (b) make sure that 296 relevant SRV records are DNSSEC protected and "secure". 298 Internet-DrafDNSSEC-based TLS Server Identity Check for Ema January 2016 300 3. SHOULD include CN-ID identifier type for the host name where the 301 email server(s) is running in Certificate Signing Requests for 302 backward compatibility with deployed email clients. (Note, a 303 certificate can only include a single CN-ID, so if a mail service 304 is running on multiple hosts, either each host has to use 305 different certificate with its own CN-ID, a single certificate 306 with multiple DNS-IDs, or a single certificate with wildcard in 307 CN-ID can be used). 309 4. MAY include "*" (wildcard) as the left-most name component of 310 DNS-ID or CN-ID in Certificate Signing Requests. 312 5.1. Notes on hosting multiple domains 314 A server that hosts multiple domains needs to do one of the following 315 (or some combination thereof): 317 1. Use DNS SRV records to redirect each hosted email service to a 318 fixed domain, deploy TLS certificate(s) for that single domain, 319 and instruct users to configure their clients with appropriate 320 pinning (unless the SRV records can always be obtained via 321 DNSSEC). Some email clients come with preloaded list of pinned 322 certificates for some popular domains, which can avoid the need 323 for manual confirmation. 325 2. Use a single TLS certificate that includes a complete list of all 326 the domains it is serving. 328 3. Serve each domain on its own IP/port, using separate TLS 329 certificates on each IP/port. 331 4. Use Server Name Indication (SNI) TLS extension [RFC6066] to 332 select the right certificate to return during TLS negotiation. 333 Each domain has its own TLS certificate in this case. 335 Each of these deployment choices have their scaling or operational 336 disadvantages when the list of domains changes. Use of DNS SRV 337 without SRV-ID requires manual confirmation from users or ubiquitous 338 availability of DNSSEC and its APIs. A single certificate (the 339 second choice) requires that when a domain is added, then a new 340 Certificate Signing Request that includes a complete list of all the 341 domains needs to be issued and passed to a CA in order to generate a 342 new certificate. Separate IP/port can avoid regenerating the 343 certificate, but requires more transport layer resources. Use of TLS 344 SNI requires each email client to use it. 346 Internet-DrafDNSSEC-based TLS Server Identity Check for Ema January 2016 348 Several Mail Service Providers host hundreds and even thousands of 349 domains. DNSSEC protected SRV records can address scaling issues 350 caused by use of TLS in multi-tenanted environments. 352 6. Examples 354 Consider an IMAP-accessible email server which supports both IMAP and 355 IMAPS (IMAP-over-TLS) at the host "mail.example.net" servicing email 356 addresses of the form "user@example.net". A certificate for this 357 service needs to include DNS-IDs of "example.net" (because it is the 358 domain portion of emails) and "mail.example.net" (this is what a user 359 of this server enters manually, if not using [RFC6186]). It might 360 also include CN-ID of "mail.example.net" for backward compatibility 361 with deployed infrastructure. 363 Consider the IMAP-accessible email server from the previous paragraph 364 which is additionally discoverable via DNS SRV lookups in domain 365 "example.net" (DNS SRV records "_imap._tcp.example.net" and 366 "_imaps._tcp.example.net"). In addition to DNS-ID/CN-ID identity 367 types specified above, a certificate for this service also needs to 368 include SRV-IDs of "_imap.example.net" (when STARTTLS is used on the 369 IMAP port) and "_imaps.example.net" (when TLS is used on IMAPS port). 370 See [RFC6186] for more details. (Note that unlike DNS SRV there is 371 no "_tcp" component in SRV-IDs). If DNS SRV are DNSSEC protected, 372 email clients that perform DNSSEC validation of SRV records would 373 check for DNS-IDs that contain the target of SRV records, instead of 374 SRV-IDs. 376 Consider the IMAP-accessible email server from the first paragraph 377 which is running on a host also known as "mycompany.example.com". In 378 addition to DNS-ID identity types specified above, a certificate for 379 this service also needs to include DNS-ID of "mycompany.example.com" 380 (this is what a user of this server enters manually, if not using 381 [RFC6186]). It might also include CN-ID of "mycompany.example.com" 382 instead of the CN-ID "mail.example.net" for backward compatibility 383 with deployed infrastructure. (This is so, because a certificate can 384 only include a single CN-ID) 386 Consider an SMTP Submission server at the host "submit.example.net" 387 servicing email addresses of the form "user@example.net" and 388 discoverable via DNS SRV lookups in domain "example.net" (DNS SRV 389 records "_submission._tcp.example.net"). A certificate for this 390 service needs to include SRV-IDs of "_submission.example.net" (see 391 [RFC6186]) along with DNS-IDs of "example.net" and 392 "submit.example.net". It might also include CN-ID of 393 "submit.example.net" for backward compatibility with deployed 394 infrastructure. 396 Internet-DrafDNSSEC-based TLS Server Identity Check for Ema January 2016 398 Consider a host "mail.example.net" servicing email addresses of the 399 form "user@example.net" and discoverable via DNS SRV lookups in 400 domain "example.net", which runs SMTP Submission, IMAPS and POP3S 401 (POP3-over-TLS) and ManageSieve services. Each of the servers can 402 use their own certificate specific to their service (see examples 403 above). Alternatively they can all share a single certificate that 404 would include SRV-IDs of "_submission.example.net", 405 "_imaps.example.net", "_pop3s.example.net" and "_sieve.example.net" 406 along with DNS-IDs of "example.net" and "mail.example.net". It might 407 also include CN-ID of "mail.example.net" for backward compatibility 408 with deployed infrastructure. 410 7. Operational Considerations 412 Section 5 covers operational considerations (in particular use of DNS 413 SRV for autoconfiguration) related to generating TLS certificiates 414 for email servers so that they can be successfully verified by email 415 clients. Additionally, Section 5.1 talks about operational 416 considerations related to hosting multiple domains. 418 8. IANA Considerations 420 This document doesn't require any action from IANA. 422 9. Security Considerations 424 The goal of this document is to improve interoperability and thus 425 security of email clients wishing to access email servers over TLS 426 protected email protocols, by specifying a consistent set of rules 427 that email service providers, email client writers and Certification 428 Authorities can use when creating server certificates. 430 TLS Server Identity Check for Email relies on use of trustworthy DNS 431 hostnames when constructing "reference identifiers" that are checked 432 against an email server certificate. Such trustworthy names are 433 either entered manually (for example if they are advertised on a Mail 434 Service Provider's website), explicitly confirmed by the user (e.g. 435 if they are a target of a DNS SRV lookup) or derived using a secure 436 third party service (e.g. DNSSEC-protected SRV records which are 437 verified by the client or trusted local resolver). 439 10. References 441 10.1. Normative References 443 [RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3", 444 STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996, 445 . 447 Internet-DrafDNSSEC-based TLS Server Identity Check for Ema January 2016 449 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 450 Requirement Levels", BCP 14, RFC 2119, 451 DOI 10.17487/RFC2119, March 1997, 452 . 454 [RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over 455 Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207, 456 February 2002, . 458 [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 459 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, 460 . 462 [RFC4985] Santesson, S., "Internet X.509 Public Key Infrastructure 463 Subject Alternative Name for Expression of Service Name", 464 RFC 4985, DOI 10.17487/RFC4985, August 2007, 465 . 467 [RFC5804] Melnikov, A., Ed. and T. Martin, "A Protocol for Remotely 468 Managing Sieve Scripts", RFC 5804, DOI 10.17487/RFC5804, 469 July 2010, . 471 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 472 Housley, R., and W. Polk, "Internet X.509 Public Key 473 Infrastructure Certificate and Certificate Revocation List 474 (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, 475 . 477 [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and 478 Verification of Domain-Based Application Service Identity 479 within Internet Public Key Infrastructure Using X.509 480 (PKIX) Certificates in the Context of Transport Layer 481 Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March 482 2011, . 484 [RFC6186] Daboo, C., "Use of SRV Records for Locating Email 485 Submission/Access Services", RFC 6186, 486 DOI 10.17487/RFC6186, March 2011, 487 . 489 [RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail", 490 STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011, 491 . 493 Internet-DrafDNSSEC-based TLS Server Identity Check for Ema January 2016 495 [RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via 496 Opportunistic DNS-Based Authentication of Named Entities 497 (DANE) Transport Layer Security (TLS)", RFC 7672, 498 DOI 10.17487/RFC7672, October 2015, 499 . 501 10.2. Informative References 503 [RFC2595] Newman, C., "Using TLS with IMAP, POP3 and ACAP", 504 RFC 2595, DOI 10.17487/RFC2595, June 1999, 505 . 507 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 508 Resource Identifier (URI): Generic Syntax", STD 66, 509 RFC 3986, DOI 10.17487/RFC3986, January 2005, 510 . 512 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 513 Rose, "DNS Security Introduction and Requirements", 514 RFC 4033, DOI 10.17487/RFC4033, March 2005, 515 . 517 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. 518 Rose, "Resource Records for the DNS Security Extensions", 519 RFC 4034, DOI 10.17487/RFC4034, March 2005, 520 . 522 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 523 Rose, "Protocol Modifications for the DNS Security 524 Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005, 525 . 527 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 528 Specifications: ABNF", STD 68, RFC 5234, 529 DOI 10.17487/RFC5234, January 2008, 530 . 532 [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) 533 Extensions: Extension Definitions", RFC 6066, 534 DOI 10.17487/RFC6066, January 2011, 535 . 537 [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication 538 of Named Entities (DANE) Transport Layer Security (TLS) 539 Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August 540 2012, . 542 Internet-DrafDNSSEC-based TLS Server Identity Check for Ema January 2016 544 [RFC7711] Miller, M. and P. Saint-Andre, "PKIX over Secure HTTP 545 (POSH)", RFC 7711, DOI 10.17487/RFC7711, November 2015, 546 . 548 Internet-DrafDNSSEC-based TLS Server Identity Check for Ema January 2016 550 Appendix A. Acknowledgements 552 The editor of this document copied lots of text from RFC 2595 and RFC 553 6125, RFC 7672, so the hard work of editors of these document is 554 appreciated. 556 Appendix B. Changes since draft-melnikov-uta-dnssec-email-tls-certs-00 558 [[Note to RFC Editor: Please delete this section before publication]] 560 TBD 562 Author's Address 564 Alexey Melnikov 565 Isode Ltd 566 14 Castle Mews 567 Hampton, Middlesex TW12 2NP 568 UK 570 EMail: Alexey.Melnikov@isode.com