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Chuang, Ed. 5 Intended status: Standards Track Google, Inc. 6 Expires: September 5, 2018 March 4, 2018 8 Internationalized Email Addresses in X.509 certificates 9 draft-ietf-lamps-eai-addresses-18 11 Abstract 13 This document defines a new name form for inclusion in the otherName 14 field of an X.509 Subject Alternative Name and Issuer Alternative 15 Name extension that allows a certificate subject to be associated 16 with an Internationalized Email Address. 18 This document updates RFC 5280. 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 https://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 September 5, 2018. 37 Copyright Notice 39 Copyright (c) 2018 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 (https://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 . . . . . . . . . . . . . . . . . . . . . . . . 2 55 2. Conventions Used in This Document . . . . . . . . . . . . . . 2 56 3. Name Definitions . . . . . . . . . . . . . . . . . . . . . . 2 57 4. IDNA2008 . . . . . . . . . . . . . . . . . . . . . . . . . . 4 58 5. Matching of Internationalized Email Addresses in X.509 59 certificates . . . . . . . . . . . . . . . . . . . . . . . . 4 60 6. Name constraints in path validation . . . . . . . . . . . . . 5 61 7. Security Considerations . . . . . . . . . . . . . . . . . . . 7 62 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 63 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 64 9.1. Normative References . . . . . . . . . . . . . . . . . . 8 65 9.2. Informative References . . . . . . . . . . . . . . . . . 9 66 Appendix A. ASN.1 Module . . . . . . . . . . . . . . . . . . . . 9 67 Appendix B. Example of SmtpUTF8Mailbox . . . . . . . . . . . . . 10 68 Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 11 69 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11 71 1. Introduction 73 [RFC5280] defines the rfc822Name subjectAltName name type for 74 representing [RFC5321] email addresses. The syntax of rfc822Name is 75 restricted to a subset of US-ASCII characters and thus can't be used 76 to represent Internationalized Email addresses [RFC6531]. This 77 document defines a new otherName variant to represent 78 Internationalized Email addresses. In addition this document 79 requires all email address domains in X.509 certificates to conform 80 to IDNA2008 [RFC5890]. 82 2. Conventions Used in This Document 84 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 85 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 86 document are to be interpreted as described in [RFC2119]. 88 The formal syntax uses the Augmented Backus-Naur Form (ABNF) 89 [RFC5234] notation. 91 3. Name Definitions 93 The GeneralName structure is defined in [RFC5280], and supports many 94 different name forms including otherName for extensibility. This 95 section specifies the SmtpUTF8Mailbox name form of otherName, so that 96 Internationalized Email addresses can appear in the subjectAltName of 97 a certificate, the issuerAltName of a certificate, or anywhere else 98 that GeneralName is used. 100 id-on-SmtpUTF8Mailbox OBJECT IDENTIFIER ::= { id-on 9 } 102 SmtpUTF8Mailbox ::= UTF8String (SIZE (1..MAX)) 103 -- SmtpUTF8Mailbox conforms to Mailbox as specified 104 -- in Section 3.3 of RFC 6531. 106 When the subjectAltName (or issuerAltName) extension contains an 107 Internationalized Email address with a non-ASCII local-part, the 108 address MUST be stored in the SmtpUTF8Mailbox name form of otherName. 109 The format of SmtpUTF8Mailbox is defined as the ABNF rule 110 SmtpUTF8Mailbox. SmtpUTF8Mailbox is a modified version of the 111 Internationalized Mailbox which was defined in Section 3.3 of 112 [RFC6531] which was itself derived from SMTP Mailbox from 113 Section 4.1.2 of [RFC5321]. [RFC6531] defines the following ABNF 114 rules for Mailbox whose parts are modified for internationalization: 115 , , , , 116 , and . In particular, was updated to 117 also support UTF8-non-ascii. UTF8-non-ascii was described by 118 Section 3.1 of [RFC6532]. Also, domain was extended to support 119 U-labels, as defined in [RFC5890]. 121 This document further refines Internationalized [RFC6531] Mailbox 122 ABNF rules and calls this SmtpUTF8Mailbox. In SmtpUTF8Mailbox, 123 labels that include non-ASCII characters MUST be stored in U-label 124 (rather than A-label) [RFC5890] form. This restriction removes the 125 need to determine which label encoding A- or U-label is present in 126 the Domain. As per Section 2.3.2.1 of [RFC5890], U-label are encoded 127 as UTF-8 [RFC3629] in Normalization Form C and other properties 128 specified there. In SmtpUTF8Mailbox, domain labels that solely use 129 ASCII characters (meaning not A- nor U-labels) SHALL use NR-LDH 130 restrictions as specified by Section 2.3.1 of [RFC5890] and SHALL be 131 restricted to lower case letters. NR-LDH stands for "Non-Reserved 132 Letters Digits Hyphen" and is the set of LDH labels that do not have 133 "--" characters in the third and forth character position, which 134 excludes "tagged domain names" such as A-labels. Consistent with the 135 treatment of rfc822Name in [RFC5280], SmtpUTF8Mailbox is an envelope 136 and has no phrase (such as a common name) before it, has no 137 comment (text surrounded in parentheses) after it, and is not 138 surrounded by "<" and ">". 140 Due to name constraint compatibility reasons described in Section 6, 141 SmtpUTF8Mailbox subjectAltName MUST NOT be used unless the local-part 142 of the email address contains non-ASCII characters. When the local- 143 part is ASCII, rfc822Name subjectAltName MUST be used instead of 144 SmtpUTF8Mailbox. This is compatible with legacy software that 145 supports only rfc822Name (and not SmtpUTF8Mailbox). The appropriate 146 usage of rfc822Name and SmtpUTF8Mailbox is summarized in Table 1 147 below. 149 SmtpUTF8Mailbox is encoded as UTF8String. The UTF8String encoding 150 MUST NOT contain a Byte-Order- Mark (BOM) [RFC3629] to aid 151 consistency across implementations particularly for comparison. 153 +-----------------+-------------+--------------+-----------------+ 154 | local-part char | domain char | domain label | subjectAltName | 155 +-----------------+-------------+--------------+-----------------+ 156 | ASCII-only | ASCII-only | NR-LDH label | rfc822Name | 157 | non-ASCII | ASCII-only | NR-LDH label | SmtpUTF8Mailbox | 158 | ASCII-only | non-ASCII | A-label | rfc822Name | 159 | non-ASCII | non-ASCII | U-label | SmtpUTF8Mailbox | 160 +-----------------+-------------+--------------+-----------------+ 162 non-ASCII may additionally include ASCII characters. 164 Table 1: Email address formatting 166 4. IDNA2008 168 To facilitate comparison between email addresses, all email address 169 domains in X.509 certificates MUST conform to IDNA2008 [RFC5890] (and 170 avoid any "mappings" mentioned in that document). Use of non- 171 conforming email address domains introduces the possibility of 172 conversion errors between alternate forms. This applies to 173 SmtpUTF8Mailbox and rfc822Name in subjectAltName, issuerAltName and 174 anywhere else that these are used. 176 5. Matching of Internationalized Email Addresses in X.509 certificates 178 In equivalence comparison with SmtpUTF8Mailbox, there may be some 179 setup work on one or both inputs depending of whether the input is 180 already in comparison form. Comparing SmtpUTF8Mailboxs consists of a 181 domain part step and a local-part step. The comparison form for 182 local-parts is always UTF-8. The comparison form for domain parts 183 depends on context. While some contexts such as certificate path 184 validation in [RFC5280] specify transforming domain to A-label 185 (Section 7.5 and 7.2 in [RFC5280] as updated by 186 [ID-lamps-rfc5280-i18n-update]), this document recommends 187 transforming to UTF-8 U-label instead. This reduces the likelihood 188 of errors by reducing conversions as more implementations natively 189 support U-label domains. 191 Comparison of two SmtpUTF8Mailbox is straightforward with no setup 192 work needed. They are considered equivalent if there is an exact 193 octet-for-octet match. Comparison with email addresses such as 194 Internationalized email address or rfc822Name requires additional 195 setup steps for domain part and local-part. The initial preparation 196 for the email addresses is to remove any phrases or comments, as well 197 as "<" and ">" present. This document calls for comparison of domain 198 labels that include non-ASCII characters be transformed to U-label if 199 not already in that form. The first step is to detect use of the 200 A-label by using Section 5.1 of [RFC5891]. Next if necessary, 201 transform any A-labels to U-labels Unicode as specified in 202 Section 5.2 of [RFC5891]. Finally if necessary convert the Unicode 203 to UTF-8 as specified in Section 3 of [RFC3629]. For ASCII NR-LDH 204 labels, upper case letters are converted to lower case letters. In 205 setup for SmtpUTF8Mailbox, the email address local-part MUST conform 206 to the requirements of [RFC6530] and [RFC6531], including being a 207 string in UTF-8 form. In particular, the local-part MUST NOT be 208 transformed in any way, such as by doing case folding or 209 normalization of any kind. The part of an 210 Internationalized email address is already in UTF-8. For rfc822Name 211 the local-part, which is IA5String (ASCII), trivially maps to UTF-8 212 without change. Once setup is complete, they are again compared 213 octet-for-octet. 215 To summarize non-normatively, the comparison steps including setup 216 are: 218 1. If the domain contains A-labels, transform them to U-labels. 220 2. If the domain contains ASCII NR-LDH labels, lowercase them. 222 3. Compare strings octet-for-octet for equivalence. 224 This specification expressly does not define any wildcard characters 225 and SmtpUTF8Mailbox comparison implementations MUST NOT interpret any 226 character as wildcards. Instead, to specify multiple email addresses 227 through SmtpUTF8Mailbox, the certificate MUST use multiple 228 subjectAltNames or issuerAltNames to explicitly carry any additional 229 email addresses. 231 6. Name constraints in path validation 233 This section updates Section 4.2.1.10 of [RFC5280] to extend 234 rfc822Name name constraints to SmtpUTF8Mailbox subjectAltNames. A 235 SmtpUTF8Mailbox aware path validators will apply name constraint 236 comparison to the subject distinguished name and both forms of 237 subject alternative name rfc822Name and SmtpUTF8Mailbox. 239 Both rfc822Name and SmtpUTF8Mailbox subject alternative names 240 represent the same underlying email address namespace. Since legacy 241 CAs constrained to issue certificates for a specific set of domains 242 would lack corresponding UTF-8 constraints, 243 [ID-lamps-rfc5280-i18n-update] updates modifies and extends 244 rfc822Name name constraints defined in [RFC5280] to cover 245 SmtpUTF8Mailbox subject alternative names. This ensures that the 246 introduction of SmtpUTF8Mailbox does not violate existing name 247 constraints. Since it is not valid to include non-ASCII UTF-8 248 characters in the local-part of rfc822Name name constraints, and 249 since name constraints that include a local-part are rarely, if at 250 all, used in practice, name constraints updated in 251 [ID-lamps-rfc5280-i18n-update] admit the forms that represent all 252 addresses at a host or all mailboxes in a domain, and deprecates 253 rfc822Name name constraints that represent a particular mailbox. 254 That is, rfc822Name constraints with a local-part SHOULD NOT be used. 256 Constraint comparison with SmtpUTF8Mailbox subjectAltName starts with 257 the setup steps defined by Section 5. Setup converts the inputs of 258 the comparison which is one of a subject distinguished name or a 259 rfc822Name or SmtpUTF8Mailbox subjectAltName, and one of a rfc822Name 260 name constraint, to constraint comparison form. For rfc822Name name 261 constraint, this will convert any domain A-labels to U-labels. For 262 both the name constraint and the subject, this will lower case any 263 domain NR-LDH labels. Strip the local-part and "@" separator from 264 each rfc822Name and SmtpUTF8Mailbox, leaving just the domain-part. 265 After setup, this follows the comparison steps defined in 4.2.1.10 of 266 [RFC5280] as follows. If the resulting name constraint domain starts 267 with a "." character, then for the name constraint to match, a suffix 268 of the resulting subject alternative name domain MUST match the name 269 constraint (including the leading ".") octet for octet. If the 270 resulting name constraint domain does not start with a "." character, 271 then for the name constraint to match, the entire resulting subject 272 alternative name domain MUST match the name constraint octet for 273 octet. 275 Certificate Authorities that wish to issue CA certificates with email 276 address name constraint MUST use rfc822Name subject alternative names 277 only. These MUST be IDNA2008 conformant names with no mappings, and 278 with non-ASCII domains encoded in A-labels only. 280 The name constraint requirement with SmtpUTF8Mailbox subject 281 alternative name is illustrated in the non-normative diagram 282 Figure 1. The first example (1) illustrates a permitted rfc822Name 283 ASCII only hostname name constraint, and the corresponding valid 284 rfc822Name subjectAltName and SmtpUTF8Mailbox subjectAltName email 285 addresses. The second example (2) illustrates a permitted rfc822Name 286 hostname name constraint with A-label, and the corresponding valid 287 rfc822Name subjectAltName and SmtpUTF8Mailbox subjectAltName email 288 addresses. Note that an email address with ASCII only local-part is 289 encoded as rfc822Name despite also having unicode present in the 290 domain. 292 +-------------------------------------------------------------------+ 293 | Root CA Cert | 294 +-------------------------------------------------------------------+ 295 | 296 v 297 +-------------------------------------------------------------------+ 298 | Intermediate CA Cert | 299 | Permitted | 300 | rfc822Name: elementary.school.example.com (1) | 301 | | 302 | rfc822Name: xn--pss25c.example.com (2) | 303 | | 304 +-------------------------------------------------------------------+ 305 | 306 v 307 +-------------------------------------------------------------------+ 308 | Entity Cert (w/explicitly permitted subjects) | 309 | SubjectAltName Extension | 310 | rfc822Name: student@elemenary.school.example.com (1) | 311 | SmtpUTF8Mailbox: u+5B66u+751F@elementary.school.example.com | 312 | (1) | 313 | | 314 | rfc822Name: student@xn--pss25c.example.com (2) | 315 | SmtpUTF8Mailbox: u+533Bu+751F@u+5927u+5B66.example.com (2) | 316 | | 317 +-------------------------------------------------------------------+ 319 Name constraints with SmtpUTF8Name and rfc822Name 321 Figure 1 323 7. Security Considerations 325 Use of SmtpUTF8Mailbox for certificate subjectAltName (and 326 issuerAltName) will incur many of the same security considerations as 327 in Section 8 in [RFC5280], but introduces a new issue by permitting 328 non-ASCII characters in the email address local-part. This issue, as 329 mentioned in Section 4.4 of [RFC5890] and in Section 4 of [RFC6532], 330 is that use of Unicode introduces the risk of visually similar and 331 identical characters which can be exploited to deceive the recipient. 332 The former document references some means to mitigate against these 333 attacks. See [WEBER] for more background on security issues with 334 Unicode. 336 8. IANA Considerations 338 In Section 3 and the ASN.1 module identifier defined in Appendix A. 339 IANA is kindly requested to make the following assignments for: 341 The LAMPS-EaiAddresses-2016 ASN.1 module in the "SMI Security for 342 PKIX Module Identifier" registry (1.3.6.1.5.5.7.0). 344 The SmtpUTF8Mailbox otherName in the "PKIX Other Name Forms" 345 registry (1.3.6.1.5.5.7.8). {{ Note to IANA: id-on-smtputf8Name 346 was assigned based on an earlier version of this document. Please 347 change that entry to id-on-SmtpUTF8Mailbox. }} 349 9. References 351 9.1. Normative References 353 [ID-lamps-rfc5280-i18n-update] 354 Housley, R., "Internationalization Updates to RFC 5280", 355 June 2017, . 358 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 359 Requirement Levels", BCP 14, RFC 2119, 360 DOI 10.17487/RFC2119, March 1997, 361 . 363 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 364 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November 365 2003, . 367 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 368 Specifications: ABNF", STD 68, RFC 5234, 369 DOI 10.17487/RFC5234, January 2008, 370 . 372 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 373 Housley, R., and W. Polk, "Internet X.509 Public Key 374 Infrastructure Certificate and Certificate Revocation List 375 (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, 376 . 378 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 379 DOI 10.17487/RFC5321, October 2008, 380 . 382 [RFC5890] Klensin, J., "Internationalized Domain Names for 383 Applications (IDNA): Definitions and Document Framework", 384 RFC 5890, DOI 10.17487/RFC5890, August 2010, 385 . 387 [RFC5891] Klensin, J., "Internationalized Domain Names in 388 Applications (IDNA): Protocol", RFC 5891, 389 DOI 10.17487/RFC5891, August 2010, 390 . 392 [RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for 393 Internationalized Email", RFC 6530, DOI 10.17487/RFC6530, 394 February 2012, . 396 [RFC6531] Yao, J. and W. Mao, "SMTP Extension for Internationalized 397 Email", RFC 6531, DOI 10.17487/RFC6531, February 2012, 398 . 400 [RFC6532] Yang, A., Steele, S., and N. Freed, "Internationalized 401 Email Headers", RFC 6532, DOI 10.17487/RFC6532, February 402 2012, . 404 9.2. Informative References 406 [RFC5912] Hoffman, P. and J. Schaad, "New ASN.1 Modules for the 407 Public Key Infrastructure Using X.509 (PKIX)", RFC 5912, 408 DOI 10.17487/RFC5912, June 2010, 409 . 411 [WEBER] Weber, C., "Attacking Software Globalization", March 2010, 412 . 415 Appendix A. ASN.1 Module 417 The following ASN.1 module normatively specifies the SmtpUTF8Mailbox 418 structure. This specification uses the ASN.1 definitions from 419 [RFC5912] with the 2002 ASN.1 notation used in that document. 420 [RFC5912] updates normative documents using older ASN.1 notation. 422 LAMPS-EaiAddresses-2016 423 { iso(1) identified-organization(3) dod(6) 424 internet(1) security(5) mechanisms(5) pkix(7) id-mod(0) 425 id-mod-lamps-eai-addresses-2016(TBD) } 427 DEFINITIONS IMPLICIT TAGS ::= 428 BEGIN 430 IMPORTS 431 OTHER-NAME 432 FROM PKIX1Implicit-2009 433 { iso(1) identified-organization(3) dod(6) internet(1) security(5) 434 mechanisms(5) pkix(7) id-mod(0) id-mod-pkix1-implicit-02(59) } 436 id-pkix 437 FROM PKIX1Explicit-2009 438 { iso(1) identified-organization(3) dod(6) internet(1) security(5) 439 mechanisms(5) pkix(7) id-mod(0) id-mod-pkix1-explicit-02(51) } ; 441 -- 442 -- otherName carries additional name types for subjectAltName, 443 -- issuerAltName, and other uses of GeneralNames. 444 -- 446 id-on OBJECT IDENTIFIER ::= { id-pkix 8 } 448 SmtpUtf8OtherNames OTHER-NAME ::= { on-SmtpUTF8Mailbox, ... } 450 on-SmtpUTF8Mailbox OTHER-NAME ::= { 451 SmtpUTF8Mailbox IDENTIFIED BY id-on-SmtpUTF8Mailbox 452 } 454 id-on-SmtpUTF8Mailbox OBJECT IDENTIFIER ::= { id-on 9 } 456 SmtpUTF8Mailbox ::= UTF8String (SIZE (1..MAX)) 457 -- SmtpUTF8Mailbox conforms to Mailbox as specified 458 -- in Section 3.3 of RFC 6531. 460 END 462 Appendix B. Example of SmtpUTF8Mailbox 464 This non-normative example demonstrates using SmtpUTF8Mailbox as an 465 otherName in GeneralName to encode the email address 466 "u+8001u+5E2B@example.com". 468 The hexadecimal DER encoding of the email address is: 469 A022060A 2B060105 05070012 0809A014 0C12E880 81E5B8AB 40657861 470 6D706C65 2E636F6D 472 The text decoding is: 473 0 34: [0] { 474 2 10: OBJECT IDENTIFIER '1 3 6 1 5 5 7 0 18 8 9' 475 14 20: [0] { 476 16 18: UTF8String '..@example.com' 477 : } 478 : } 480 Figure 2 482 The example was encoded on the OSS Nokalva ASN.1 Playground and the 483 above text decoding is an output of Peter Gutmann's "dumpasn1" 484 program. 486 Appendix C. Acknowledgements 488 Thank you to Magnus Nystrom for motivating this document. Thanks to 489 Russ Housley, Nicolas Lidzborski, Laetitia Baudoin, Ryan Sleevi, Sean 490 Leonard, Sean Turner, John Levine, and Patrik Falstrom for their 491 feedback. Also special thanks to John Klensin for his valuable input 492 on internationalization, Unicode and ABNF formatting, to Jim Schaad 493 for his help with the ASN.1 example and his helpful feedback, and 494 especially to Viktor Dukhovni for helping us with name constraints 495 and his many detailed document reviews. 497 Authors' Addresses 499 Alexey Melnikov (editor) 500 Isode Ltd 501 14 Castle Mews 502 Hampton, Middlesex TW12 2NP 503 UK 505 Email: Alexey.Melnikov@isode.com 507 Weihaw Chuang (editor) 508 Google, Inc. 509 1600 Amphitheater Parkway 510 Mountain View, CA 94043 511 US 513 Email: weihaw@google.com