1 Internet Engineering Task Force (IETF) P. Hoffman
2 Request for Comments: 8162 ICANN
3 Category: Experimental J. Schlyter
4 ISSN: 2070-1721 Kirei AB
5 May 2017
6
7
8 Using Secure DNS to Associate Certificates with Domain Names for S/MIME
9
10 Abstract
11
12 This document describes how to use secure DNS to associate an S/MIME
13 user's certificate with the intended domain name, similar to the way
14 that DNS-Based Authentication of Named Entities (DANE), RFC 6698,
15 does for TLS.
16
17 Status of This Memo
18
19 This document is not an Internet Standards Track specification; it is
20 published for examination, experimental implementation, and
21 evaluation.
22
23 This document defines an Experimental Protocol for the Internet
24 community. This document is a product of the Internet Engineering
25 Task Force (IETF). It represents the consensus of the IETF
26 community. It has received public review and has been approved for
27 publication by the Internet Engineering Steering Group (IESG). Not
28 all documents approved by the IESG are a candidate for any level of
29 Internet Standard; see Section 2 of RFC 7841.
30
31 Information about the current status of this document, any errata,
32 and how to provide feedback on it may be obtained at
33 http://www.rfc-editor.org/info/rfc8162.
34
35 Copyright Notice
36
37 Copyright (c) 2017 IETF Trust and the persons identified as the
38 document authors. All rights reserved.
39
40 This document is subject to BCP 78 and the IETF Trust's Legal
41 Provisions Relating to IETF Documents
42 (http://trustee.ietf.org/license-info) in effect on the date of
43 publication of this document. Please review these documents
44 carefully, as they describe your rights and restrictions with respect
45 to this document. Code Components extracted from this document must
46 include Simplified BSD License text as described in Section 4.e of
47 the Trust Legal Provisions and are provided without warranty as
48 described in the Simplified BSD License.
49
50
51
52 Hoffman & Schlyter Experimental [Page 1]
53 RFC 8162 DNS-Based Authentication for S/MIME May 2017
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55
56 Table of Contents
57
58 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
59 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
60 1.2. Experiment Goal . . . . . . . . . . . . . . . . . . . . . 3
61 2. The SMIMEA Resource Record . . . . . . . . . . . . . . . . . 4
62 3. Location of the SMIMEA Record . . . . . . . . . . . . . . . . 4
63 4. Email Address Variants and Internationalization
64 Considerations . . . . . . . . . . . . . . . . . . . . . . . 5
65 5. Mandatory-to-Implement Features . . . . . . . . . . . . . . . 6
66 6. Application Use of S/MIME Certificate Associations . . . . . 6
67 7. Certificate Size and DNS . . . . . . . . . . . . . . . . . . 7
68 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
69 9. Security Considerations . . . . . . . . . . . . . . . . . . . 7
70 9.1. Response Size . . . . . . . . . . . . . . . . . . . . . . 8
71 9.2. Email Address Information Leak . . . . . . . . . . . . . 8
72 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
73 10.1. Normative References . . . . . . . . . . . . . . . . . . 9
74 10.2. Informative References . . . . . . . . . . . . . . . . . 10
75 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 11
76 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
77
78 1. Introduction
79
80 S/MIME [RFC5751] messages often contain a certificate (some messages
81 contain more than one certificate). These certificates assist in
82 authenticating the sender of the message and can be used for
83 encrypting messages that will be sent in reply. In order for the
84 S/MIME receiver to authenticate that a message is from the sender
85 identified in the message, the receiver's Mail User Agent (MUA) must
86 validate that this certificate is associated with the purported
87 sender. Currently, the MUA must trust a trust anchor upon which the
88 sender's certificate is rooted and must successfully validate the
89 certificate. There are other requirements on the MUA, such as
90 associating the identity in the certificate with that of the message,
91 that are out of scope for this document.
92
93 Some people want to authenticate the association of the sender's
94 certificate with the sender without trusting a configured trust
The IETF is responsible for the creation and maintenance of the DNS RFCs. The ICANN DNS RFC annotation project provides a forum for collecting community annotations on these RFCs as an aid to understanding for implementers and any interested parties. The annotations displayed here are not the result of the IETF consensus process.
This RFC is included in the DNS RFCs annotation project whose home page is here.
95 anchor. Others to want mitigate the difficulty of finding
96 certificates from outside the enterprise. Given that the DNS
97 administrator for a domain name is authorized to give identifying
98 information about the zone, it makes sense to allow that
99 administrator to also make an authoritative binding between email
100 messages purporting to come from the domain name and a certificate
101 that might be used by someone authorized to send mail from those
102 servers. The easiest way to do this is to use the DNS.
103
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110
111 This document describes a mechanism for associating a user's
112 certificate with the domain that is similar to that described in DANE
113 itself [RFC6698], as updated by [RFC7218] and [RFC7671]; it is also
114 similar to the mechanism given in [RFC7929] for OpenPGP. Most of the
115 operational and security considerations for using the mechanism in
116 this document are described in RFC 6698 and are not described here at
117 all. Only the major differences between this mechanism and those
118 used in RFC 6698 are described here. Thus, the reader must be
119 familiar with RFC 6698 before reading this document.
120
121 1.1. Terminology
122
123 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
124 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
125 "OPTIONAL" in this document are to be interpreted as described in
126 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
127 capitals, as shown here.
128
129 This document also makes use of standard PKIX, DNSSEC, and S/MIME
130 terminology. See PKIX [RFC5280], DNSSEC [RFC4033] [RFC4034]
131 [RFC4035], and S/MIME [RFC5751] for these terms.
132
133 1.2. Experiment Goal
134
135 This specification is one experiment in improving access to public
136 keys for end-to-end email security. There are a range of ways in
137 which this can reasonably be done for OpenPGP or S/MIME, for example,
138 using the DNS, SMTP, or HTTP. Proposals for each of these have been
139 made with various levels of support in terms of implementation and
140 deployment. For each such experiment, specifications such as this
141 will enable experiments to be carried out that may succeed or that
142 may uncover technical or other impediments to large- or small-scale
143 deployments. The IETF encourages those implementing and deploying
144 such experiments to publicly document their experiences so that
145 future specifications in this space can benefit.
146
147 This document defines an RRtype whose use is Experimental. The goal
148 of the experiment is to see whether encrypted email usage will
149 increase if an automated discovery method is available to Mail
150 Transfer Agents (MTAs) and if MUAs help the end user with email
151 encryption key management.
152
153 It is unclear if this RRtype will scale to some of the larger email
154 service deployments. Concerns have been raised about the size of the
155 SMIMEA record and the size of the resulting DNS zone files. This
156 experiment hopefully will give the IETF some insight into whether or
157 not this is a problem.
158
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165
166 If the experiment is successful, it is expected that the findings of
167 the experiment will result in an updated document for Standards Track
168 approval.
169
170 2. The SMIMEA Resource Record
171
172 The SMIMEA DNS resource record (RR) is used to associate an end
173 entity certificate or public key with the associated email address,
174 thus forming a "SMIMEA certificate association". The semantics of
175 how the SMIMEA resource record is interpreted are given later in this
176 document. Note that the information returned in the SMIMEA record
177 might be for the end entity certificate, or it might be for the trust
178 anchor or an intermediate certificate. This mechanism is similar to
179 the one given in [RFC7929] for OpenPGP.
180
181 The type value for the SMIMEA RRtype is defined in Section 8. The
182 SMIMEA resource record is class independent.
183
184 The SMIMEA wire format and presentation format are the same as for
185 the TLSA record as described in Section 2.1 of [RFC6698]. The
186 certificate usage field, the selector field, and the matching type
187 field have the same format; the semantics are also the same except
188 where RFC 6698 talks about TLS as the target protocol for the
189 certificate information.
190
191 3. Location of the SMIMEA Record
192
193 The DNS does not allow the use of all characters that are supported
194 in the "local-part" of email addresses as defined in [RFC5322] and
195 [RFC6530]. Therefore, email addresses are mapped into DNS using the
196 following method:
197
198 1. The "left-hand side" of the email address, called the "local-
199 part" in both the mail message format definition [RFC5322] and in
200 the specification for internationalized email [RFC6530]) is
201 encoded in UTF-8 (or its subset ASCII). If the local-part is
202 written in another charset, it MUST be converted to UTF-8.
203
204 2. The local-part is first canonicalized using the following rules.
205 If the local-part is unquoted, any comments and/or folding
206 whitespace (CFWS) around dots (".") is removed. Any enclosing
207 double quotes are removed. Any literal quoting is removed.
208
209 3. If the local-part contains any non-ASCII characters, it SHOULD be
210 normalized using the Unicode Normalization Form C from [UNICODE].
211 Recommended normalization rules can be found in Section 10.1 of
212 [RFC6530].
213
214
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220
221 4. The local-part is hashed using the SHA2-256 [RFC5754] algorithm,
222 with the hash truncated to 28 octets and represented in its
223 hexadecimal representation, to become the left-most label in the
224 prepared domain name.
225
226 5. The string "_smimecert" becomes the second left-most label in the
227 prepared domain name.
228
229 6. The domain name (the "right-hand side" of the email address,
230 called the "domain" in [RFC5322]) is appended to the result of
231 step 5 to complete the prepared domain name.
232
233 For example, to request an SMIMEA resource record for a user whose
234 email address is "hugh@example.com", an SMIMEA query would be placed
235 for the following QNAME: "c93f1e400f26708f98cb19d936620da35eec8f72e57
236 f9eec01c1afd6._smimecert.example.com".
237
238 4. Email Address Variants and Internationalization Considerations
239
240 Mail systems usually handle variant forms of local-parts. The most
241 common variants are upper and lower case, often automatically
242 corrected when a name is recognized as such. Other variants include
243 systems that ignore "noise" characters such as dots, so that local-
244 parts 'johnsmith' and 'John.Smith' would be equivalent. Many systems
245 allow "extensions" such as 'john-ext' or 'mary+ext' where 'john' or
246 'mary' is treated as the effective local-part, and the 'ext' is
247 passed to the recipient for further handling. This can complicate
248 finding the SMIMEA record associated with the dynamically created
249 email address.
250
251 [RFC5321] and its predecessors have always made it clear that only
252 the recipient MTA is allowed to interpret the local-part of an
253 address. Therefore, sending MUAs and MTAs supporting this
254 specification MUST NOT perform any kind of mapping rules based on the
255 email address. In order to improve the chances of finding SMIMEA
256 resource records for a particular local-part, domains that allow
257 variant forms (such as treating local-parts as case-insensitive)
258 might publish SMIMEA resource records for all variants of local-
259 parts, might publish variants on first use (for example, a webmail
260 provider that also controls DNS for a domain can publish variants as
261 used by owner of a particular local-part), or might just publish
262 SMIMEA resource records for the most common variants.
263
264 Section 3 above defines how the local-part is used to determine the
265 location in which one looks for an SMIMEA resource record. Given the
266 variety of local-parts seen in email, designing a good experiment for
267 this is difficult as a) some current implementations are known to
268 lowercase at least US-ASCII local-parts, b) we know from (many) other
269
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276 situations that any strategy based on guessing and making multiple
277 DNS queries is not going to achieve consensus for good reasons, and
278 c) the underlying issues are just hard -- see Section 10.1 of
279 [RFC6530] for discussion of just some of the issues that would need
280 to be tackled to fully address this problem.
281
282 However, while this specification is not the place to try to address
283 these issues with local-parts, doing so is also not required to
284 determine the outcome of this experiment. If this experiment
285 succeeds, then further work on email addresses with non-ASCII local-
286 parts will be needed, and that would be better based on the findings
287 from this experiment, rather than doing nothing or starting this
288 experiment based on a speculative approach to what is a very complex
289 topic.
290
291 5. Mandatory-to-Implement Features
292
293 S/MIME MUAs conforming to this specification MUST be able to
294 correctly interpret SMIMEA records with certificate usages 0, 1, 2,
295 and 3. S/MIME MUAs conforming to this specification MUST be able to
296 compare a certificate association with a certificate offered by
297 another S/MIME MUA using selector types 0 and 1, and matching type 0
298 (no hash used) and matching type 1 (SHA-256), and SHOULD be able to
299 make such comparisons with matching type 2 (SHA-512).
300
301 S/MIME MUAs conforming to this specification MUST be able to
302 interpret any S/MIME capabilities (defined in [RFC4262]) in any
303 certificates that it receives through SMIMEA records.
304
305 6. Application Use of S/MIME Certificate Associations
306
307 The SMIMEA record allows an application or service to obtain an
308 S/MIME certificate or public key and use it for verifying a digital
309 signature or encrypting a message to the public key. The DNS answer
310 MUST pass DNSSEC validation; if DNSSEC validation reaches any state
311 other than "Secure" (as specified in [RFC4035]), the DNSSEC
312 validation MUST be treated as a failure.
313
314 If no S/MIME certificates are known for an email address, an SMIMEA
315 DNS lookup MAY be performed to seek the certificate or public key
316 that corresponds to that email address. This can then be used to
317 verify a received signed message or can be used to send out an
318 encrypted email message. An application whose attempt fails to
319 retrieve a DNSSEC-verified SMIMEA resource record from the DNS should
320 remember that failed attempt and not retry it for some time. This
321 will avoid sending out a DNS request for each email message the
322 application is sending out; such DNS requests constitute a privacy
323 leak.
324
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330
331 7. Certificate Size and DNS
332
333 Due to the expected size of the SMIMEA record, applications SHOULD
334 use TCP -- not UDP -- to perform queries for the SMIMEA resource
335 record.
336
337 Although the reliability of the transport of large DNS resource
338 records has improved in the last years, it is still recommended to
339 keep the DNS records as small as possible without sacrificing the
340 security properties of the public key. The algorithm type and key
341 size of certificates should not be modified to accommodate this
342 section.
343
344 8. IANA Considerations
345
346 This document uses a new DNS RRtype, SMIMEA, whose value (53) was
347 allocated by IANA from the "Resource Record (RR) TYPEs" subregistry
348 of the "Domain Name System (DNS) Parameters" registry.
349
350 9. Security Considerations
351
352 Client treatment of any information included in the trust anchor is a
353 matter of local policy. This specification does not mandate that
354 such information be inspected or validated by the domain name
355 administrator.
356
357 DNSSEC does not protect the queries from pervasive monitoring as
358 defined in [RFC7258]. Since DNS queries are currently mostly
359 unencrypted, a query to look up a target SMIMEA record could reveal
360 that a user using the (monitored) recursive DNS server is attempting
361 to send encrypted email to a target.
362
363 Various components could be responsible for encrypting an email
364 message to a target recipient. It could be done by the sender's MUA,
365 an MUA plugin, or the sender's MTA. Each of these have their own
366 characteristics. An MUA can ask the user to make a decision before
367 continuing. The MUA can either accept or refuse a message. The MTA
368 might deliver the message as is or encrypt the message before
369 delivering. Each of these components should attempt to encrypt an
370 unencrypted outgoing message whenever possible.
371
372 In theory, two different local-parts could hash to the same value.
373 This document assumes that such a hash collision has a negligible
374 chance of happening.
375
376 If an obtained S/MIME certificate is revoked or expired, that
377 certificate MUST NOT be used, even if that would result in sending a
378 message in plaintext.
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385
386 Anyone who can obtain a DNSSEC private key of a domain name via
387 coercion, theft, or brute-force calculations can replace any SMIMEA
388 record in that zone and all of the delegated child zones. Any future
389 messages encrypted with the malicious SMIMEA key could then be read.
390 Therefore, a certificate or key obtained from a DNSSEC-validated
391 SMIMEA record can only be trusted as much as the DNS domain can be
392 trusted.
393
394 Organizations that are required to be able to read everyone's
395 encrypted email should publish the escrow key as the SMIMEA record.
396 Mail servers of such organizations MAY optionally re-encrypt the
397 message to the individual's S/MIME key. This case can be considered
398 a special case of the key-replacement attack described above.
399
400 9.1. Response Size
401
402 To prevent amplification attacks, an Authoritative DNS server MAY
403 wish to prevent returning SMIMEA records over UDP unless the source
404 IP address has been confirmed with DNS Cookies [RFC7873]. If a query
405 is received via UDP without source IP address verification, the
406 server MUST NOT return REFUSED but answer the query with an empty
407 answer section and the truncation flag set ("TC=1").
408
409 9.2. Email Address Information Leak
410
411 The hashing of the local-part in this document is not a security
412 feature. Publishing SMIMEA records will create a list of hashes of
413 valid email addresses, which could simplify obtaining a list of valid
414 email addresses for a particular domain. It is desirable to not ease
415 the harvesting of email addresses where possible.
416
417 The domain name part of the email address is not used as part of the
418 hash so that hashes can be used in multiple zones deployed using
419 DNAME [RFC6672]. This makes it slightly easier and cheaper to brute-
420 force the SHA2-256 hashes into common and short local-parts, as
421 single rainbow tables [Rainbow] can be reused across domains. This
422 can be somewhat countered by using NSEC3 [RFC5155].
423
424 DNS zones that are signed with DNSSEC using NSEC [RFC4033] for denial
425 of existence are susceptible to zone walking, a mechanism that allows
426 someone to enumerate all the SMIMEA hashes in a zone. This can be
427 used in combination with previously hashed common or short local-
428 parts (in rainbow tables) to deduce valid email addresses. DNSSEC-
429 signed zones using NSEC3 for denial of existence instead of NSEC are
430 significantly harder to brute-force after performing a zone walk.
431
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440
441 10. References
442
443 10.1. Normative References
444
445 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
446 Requirement Levels", BCP 14, RFC 2119,
447 DOI 10.17487/RFC2119, March 1997,
448 <http://www.rfc-editor.org/info/rfc2119>.
449
450 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
451 Rose, "DNS Security Introduction and Requirements",
452 RFC 4033, DOI 10.17487/RFC4033, March 2005,
453 <http://www.rfc-editor.org/info/rfc4033>.
454
455 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
456 Rose, "Resource Records for the DNS Security Extensions",
457 RFC 4034, DOI 10.17487/RFC4034, March 2005,
458 <http://www.rfc-editor.org/info/rfc4034>.
459
460 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
461 Rose, "Protocol Modifications for the DNS Security
462 Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
463 <http://www.rfc-editor.org/info/rfc4035>.
464
465 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
466 Housley, R., and W. Polk, "Internet X.509 Public Key
467 Infrastructure Certificate and Certificate Revocation List
468 (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
469 <http://www.rfc-editor.org/info/rfc5280>.
470
471 [RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
472 Mail Extensions (S/MIME) Version 3.2 Message
473 Specification", RFC 5751, DOI 10.17487/RFC5751, January
474 2010, <http://www.rfc-editor.org/info/rfc5751>.
475
476 [RFC5754] Turner, S., "Using SHA2 Algorithms with Cryptographic
477 Message Syntax", RFC 5754, DOI 10.17487/RFC5754, January
478 2010, <http://www.rfc-editor.org/info/rfc5754>.
479
480 [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
481 of Named Entities (DANE) Transport Layer Security (TLS)
482 Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
483 2012, <http://www.rfc-editor.org/info/rfc6698>.
484
485 [RFC7671] Dukhovni, V. and W. Hardaker, "The DNS-Based
486 Authentication of Named Entities (DANE) Protocol: Updates
487 and Operational Guidance", RFC 7671, DOI 10.17487/RFC7671,
488 October 2015, <http://www.rfc-editor.org/info/rfc7671>.
489
490
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495
496 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
497 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
498 May 2017, <http://www.rfc-editor.org/info/rfc8174>.
499
500 10.2. Informative References
501
502 [Rainbow] Oechslin, P., "Making a Faster Cryptanalytic Time-Memory
503 Trade-Off", DOI 10.1007/978-3-540-45146-4_36, 2003,
504 <http://www.iacr.org/cryptodb/archive/2003/
505 CRYPTO/1615/1615.ps>.
506
507 [RFC4262] Santesson, S., "X.509 Certificate Extension for Secure/
508 Multipurpose Internet Mail Extensions (S/MIME)
509 Capabilities", RFC 4262, DOI 10.17487/RFC4262, December
510 2005, <http://www.rfc-editor.org/info/rfc4262>.
511
512 [RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
513 Security (DNSSEC) Hashed Authenticated Denial of
514 Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008,
515 <http://www.rfc-editor.org/info/rfc5155>.
516
517 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
518 DOI 10.17487/RFC5321, October 2008,
519 <http://www.rfc-editor.org/info/rfc5321>.
520
521 [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
522 DOI 10.17487/RFC5322, October 2008,
523 <http://www.rfc-editor.org/info/rfc5322>.
524
525 [RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for
526 Internationalized Email", RFC 6530, DOI 10.17487/RFC6530,
527 February 2012, <http://www.rfc-editor.org/info/rfc6530>.
528
529 [RFC6672] Rose, S. and W. Wijngaards, "DNAME Redirection in the
530 DNS", RFC 6672, DOI 10.17487/RFC6672, June 2012,
531 <http://www.rfc-editor.org/info/rfc6672>.
532
533 [RFC7218] Gudmundsson, O., "Adding Acronyms to Simplify
534 Conversations about DNS-Based Authentication of Named
535 Entities (DANE)", RFC 7218, DOI 10.17487/RFC7218, April
536 2014, <http://www.rfc-editor.org/info/rfc7218>.
537
538 [RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
539 Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
540 2014, <http://www.rfc-editor.org/info/rfc7258>.
541
542
543
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550
551 [RFC7873] Eastlake 3rd, D. and M. Andrews, "Domain Name System (DNS)
552 Cookies", RFC 7873, DOI 10.17487/RFC7873, May 2016,
553 <http://www.rfc-editor.org/info/rfc7873>.
554
555 [RFC7929] Wouters, P., "DNS-Based Authentication of Named Entities
556 (DANE) Bindings for OpenPGP", RFC 7929,
557 DOI 10.17487/RFC7929, August 2016,
558 <http://www.rfc-editor.org/info/rfc7929>.
559
560 [UNICODE] The Unicode Consortium, "The Unicode Standard",
561 <http://www.unicode.org/versions/latest/>.
562
563 Acknowledgements
564
565 A great deal of material in this document is copied from [RFC7929].
566 That material was created by Paul Wouters and other participants in
567 the IETF DANE WG.
568
569 Brian Dickson, Stephen Farrell, Miek Gieben, Martin Pels, and Jim
570 Schaad contributed technical ideas and support to this document.
571
572 Authors' Addresses
573
574 Paul Hoffman
575 ICANN
576
577 Email: paul.hoffman@icann.org
578
579
580 Jakob Schlyter
581 Kirei AB
582
583 Email: jakob@kirei.se
584
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Others to want mitigate the difficulty of finding certificates from outside the enterprise.
Otherstowant to mitigate the difficulty of finding certificates from outside the enterprise.