1 Internet Engineering Task Force (IETF) P. Wouters
2 Request for Comments: 7929 Red Hat
3 Category: Experimental August 2016
4 ISSN: 2070-1721
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7 DNS-Based Authentication of Named Entities (DANE) Bindings for OpenPGP
8
9 Abstract
10
11 OpenPGP is a message format for email (and file) encryption that
12 lacks a standardized lookup mechanism to securely obtain OpenPGP
13 public keys. DNS-Based Authentication of Named Entities (DANE) is a
14 method for publishing public keys in DNS. This document specifies a
15 DANE method for publishing and locating OpenPGP public keys in DNS
16 for a specific email address using a new OPENPGPKEY DNS resource
17 record. Security is provided via Secure DNS, however the OPENPGPKEY
18 record is not a replacement for verification of authenticity via the
19 "web of trust" or manual verification. The OPENPGPKEY record can be
20 used to encrypt an email that would otherwise have to be sent
21 unencrypted.
22
23 Status of This Memo
24
25 This document is not an Internet Standards Track specification; it is
26 published for examination, experimental implementation, and
27 evaluation.
28
29 This document defines an Experimental Protocol for the Internet
30 community. This document is a product of the Internet Engineering
31 Task Force (IETF). It represents the consensus of the IETF
32 community. It has received public review and has been approved for
33 publication by the Internet Engineering Steering Group (IESG). Not
34 all documents approved by the IESG are a candidate for any level of
35 Internet Standard; see Section 2 of RFC 7841.
36
37 Information about the current status of this document, any errata,
38 and how to provide feedback on it may be obtained at
39 http://www.rfc-editor.org/info/rfc7929.
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56 Copyright Notice
57
58 Copyright (c) 2016 IETF Trust and the persons identified as the
59 document authors. All rights reserved.
60
61 This document is subject to BCP 78 and the IETF Trust's Legal
62 Provisions Relating to IETF Documents
63 (http://trustee.ietf.org/license-info) in effect on the date of
64 publication of this document. Please review these documents
65 carefully, as they describe your rights and restrictions with respect
66 to this document. Code Components extracted from this document must
67 include Simplified BSD License text as described in Section 4.e of
68 the Trust Legal Provisions and are provided without warranty as
69 described in the Simplified BSD License.
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111 Table of Contents
112
113 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
114 1.1. Experiment Goal . . . . . . . . . . . . . . . . . . . . . 4
115 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
116 2. The OPENPGPKEY Resource Record . . . . . . . . . . . . . . . 5
117 2.1. The OPENPGPKEY RDATA Component . . . . . . . . . . . . . 6
118 2.1.1. The OPENPGPKEY RDATA Content . . . . . . . . . . . . 6
119 2.1.2. Reducing the Transferable Public Key Size . . . . . . 7
120 2.2. The OPENPGPKEY RDATA Wire Format . . . . . . . . . . . . 7
121 2.3. The OPENPGPKEY RDATA Presentation Format . . . . . . . . 7
122 3. Location of the OPENPGPKEY Record . . . . . . . . . . . . . . 8
123 4. Email Address Variants and Internationalization
124 Considerations . . . . . . . . . . . . . . . . . . . . . . . 9
125 5. Application Use of OPENPGPKEY . . . . . . . . . . . . . . . . 10
126 5.1. Obtaining an OpenPGP Key for a Specific Email Address . . 10
127 5.2. Confirming that an OpenPGP Key is Current . . . . . . . . 10
128 5.3. Public Key UIDs and Query Names . . . . . . . . . . . . . 10
129 6. OpenPGP Key Size and DNS . . . . . . . . . . . . . . . . . . 11
130 7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
131 7.1. MTA Behavior . . . . . . . . . . . . . . . . . . . . . . 12
132 7.2. MUA Behavior . . . . . . . . . . . . . . . . . . . . . . 13
133 7.3. Response Size . . . . . . . . . . . . . . . . . . . . . . 14
134 7.4. Email Address Information Leak . . . . . . . . . . . . . 14
135 7.5. Storage of OPENPGPKEY Data . . . . . . . . . . . . . . . 14
136 7.6. Security of OpenPGP versus DNSSEC . . . . . . . . . . . . 15
137 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
138 8.1. OPENPGPKEY RRtype . . . . . . . . . . . . . . . . . . . . 15
139 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
140 9.1. Normative References . . . . . . . . . . . . . . . . . . 15
141 9.2. Informative References . . . . . . . . . . . . . . . . . 16
142 Appendix A. Generating OPENPGPKEY Records . . . . . . . . . . . 18
143 Appendix B. OPENPGPKEY IANA Template . . . . . . . . . . . . . . 19
144 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 20
145 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 20
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166 1. Introduction
167
168 OpenPGP [RFC4880] public keys are used to encrypt or sign email
169 messages and files. To encrypt an email message, or verify a
170 sender's OpenPGP signature, the email client Mail User Agent (MUA) or
171 the email server Mail Transfer Agent (MTA) needs to locate the
172 recipient's OpenPGP public key.
173
174 OpenPGP clients have relied on centralized "well-known" key servers
175 that are accessed using the HTTP Keyserver Protocol [HKP].
176 Alternatively, users need to manually browse a variety of different
177 front-end websites. These key servers do not require a confirmation
178 of the email address used in the User ID (UID) of the uploaded
179 OpenPGP public key. Attackers can -- and have -- uploaded rogue
180 public keys with other people's email addresses to these key servers.
181
182 Once uploaded, public keys cannot be deleted. People who did not
183 pre-sign a key revocation can never remove their OpenPGP public key
184 from these key servers once they have lost access to their private
185 key. This results in receiving encrypted email that cannot be
186 decrypted.
187
188 Therefore, these key servers are not well suited to support MUAs and
189 MTAs to automatically encrypt email -- especially in the absence of
190 an interactive user.
191
192 This document describes a mechanism to associate a user's OpenPGP
193 public key with their email address, using the OPENPGPKEY DNS RRtype.
194 These records are published in the DNS zone of the user's email
195 address. If the user loses their private key, the OPENPGPKEY DNS
196 record can simply be updated or removed from the zone.
197
198 The OPENPGPKEY data is secured using Secure DNS [RFC4035].
199
200 The main goal of the OPENPGPKEY resource record is to stop passive
201 attacks against plaintext emails. While it can also thwart some
202 active attacks (such as people uploading rogue keys to key servers in
203 the hopes that others will encrypt to these rogue keys), this
204 resource record is not a replacement for verifying OpenPGP public
205 keys via the "web of trust" signatures, or manually via a fingerprint
206 verification.
207
208 1.1. Experiment Goal
209
210 This specification is one experiment in improving access to public
211 keys for end-to-end email security. There are a range of ways in
212 which this can reasonably be done for OpenPGP or S/MIME, for example,
213 using the DNS, or SMTP, or HTTP. Proposals for each of these have
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221 been made with various levels of support in terms of implementation
222 and deployment. For each such experiment, specifications such as
223 this will enable experiments to be carried out that may succeed or
224 that may uncover technical or other impediments to large- or small-
225 scale deployments. The IETF encourages those implementing and
226 deploying such experiments to publicly document their experiences so
227 that future specifications in this space can benefit.
228
229 This document defines an RRtype whose use is Experimental. The goal
230 of the experiment is to see whether encrypted email usage will
231 increase if an automated discovery method is available to MTAs and
232 MUAs to help the end user with email encryption key management.
233
234 It is unclear if this RRtype will scale to some of the larger email
235 service deployments. Concerns have been raised about the size of the
236 OPENPGPKEY record and the size of the resulting DNS zone files. This
237 experiment hopefully will give the working group some insight into
238 whether or not this is a problem.
239
240 If the experiment is successful, it is expected that the findings of
241 the experiment will result in an updated document for standards track
242 approval.
243
244 The OPENPGPKEY RRtype somewhat resembles the generic CERT record
245 defined in [RFC4398]. However, the CERT record uses sub-typing with
246 many different types of keys and certificates. It is suspected that
247 its general application of very different protocols (PKIX versus
248 OpenPGP) has been the cause for lack of implementation and
249 deployment. Furthermore, the CERT record uses sub-typing, which is
250 now considered to be a bad idea for DNS.
251
252 1.2. Terminology
253
254 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
255 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
256 document are to be interpreted as described in RFC 2119 [RFC2119].
257
258 This document also makes use of standard DNSSEC and DANE terminology.
259 See DNSSEC [RFC4033], [RFC4034], [RFC4035], and DANE [RFC6698] for
260 these terms.
261
262 2. The OPENPGPKEY Resource Record
263
264 The OPENPGPKEY DNS resource record (RR) is used to associate an end
265 entity OpenPGP Transferable Public Key (see Section 11.1 of
266 [RFC4880]) with an email address, thus forming an "OpenPGP public key
267 association". A user that wishes to specify more than one OpenPGP
268 key, for example, because they are transitioning to a newer stronger
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276 key, can do so by adding multiple OPENPGPKEY records. A single
277 OPENPGPKEY DNS record MUST only contain one OpenPGP key.
278
279 The type value allocated for the OPENPGPKEY RR type is 61. The
280 OPENPGPKEY RR is class independent.
281
282 2.1. The OPENPGPKEY RDATA Component
283
284 The RDATA portion of an OPENPGPKEY resource record contains a single
285 value consisting of a Transferable Public Key formatted as specified
286 in [RFC4880].
287
288 2.1.1. The OPENPGPKEY RDATA Content
289
290 An OpenPGP Transferable Public Key can be arbitrarily large. DNS
291 records are limited in size. When creating OPENPGPKEY DNS records,
292 the OpenPGP Transferable Public Key should be filtered to only
293 contain appropriate and useful data. At a minimum, an OPENPGPKEY
294 Transferable Public Key for the user hugh@example.com should contain:
295
296 o The primary key X
297 o One User ID Y, which SHOULD match 'hugh@example.com'
298 o Self-signature from X, binding X to Y
299
300 If the primary key is not encryption-capable, at least one relevant
301 subkey should be included, resulting in an OPENPGPKEY Transferable
302 Public Key containing:
303
304 o The primary key X
305 o One User ID Y, which SHOULD match 'hugh@example.com'
306 o Self-signature from X, binding X to Y
307 o Encryption-capable subkey Z
308 o Self-signature from X, binding Z to X
309 o (Other subkeys, if relevant)
310
311 The user can also elect to add a few third-party certifications,
312 which they believe would be helpful for validation in the traditional
313 "web of trust". The resulting OPENPGPKEY Transferable Public Key
314 would then look like:
315
316 o The primary key X
317 o One User ID Y, which SHOULD match 'hugh@example.com'
318 o Self-signature from X, binding X to Y
319 o Third-party certification from V, binding Y to X
320 o (Other third-party certifications, if relevant)
321 o Encryption-capable subkey Z
322 o Self-signature from X, binding Z to X
323 o (Other subkeys, if relevant)
324
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331 2.1.2. Reducing the Transferable Public Key Size
332
333 When preparing a Transferable Public Key for a specific OPENPGPKEY
334 RDATA format with the goal of minimizing certificate size, a user
335 would typically want to:
336
337 o Where one User ID from the certifications matches the looked-up
338 address, strip away non-matching User IDs and any associated
339 certifications (self-signatures or third-party certifications).
340
341 o Strip away all User Attribute packets and associated
342 certifications.
343
344 o Strip away all expired subkeys. The user may want to keep revoked
345 subkeys if these were revoked prior to their preferred expiration
346 time to ensure that correspondents know about these earlier than
347 expected revocations.
348
349 o Strip away all but the most recent self-signature for the
350 remaining User IDs and subkeys.
351
352 o Optionally strip away any uninteresting or unimportant third-party
353 User ID certifications. This is a value judgment by the user that
354 is difficult to automate. At the very least, expired and
355 superseded third-party certifications should be stripped out. The
356 user should attempt to keep the most recent and most well-
357 connected certifications in the "web of trust" in their
358 Transferable Public Key.
359
360 2.2. The OPENPGPKEY RDATA Wire Format
361
362 The RDATA Wire Format consists of a single OpenPGP Transferable
363 Public Key as defined in Section 11.1 of [RFC4880]. Note that this
364 format is without ASCII armor or base64 encoding.
365
366 2.3. The OPENPGPKEY RDATA Presentation Format
367
368 The RDATA Presentation Format, as visible in master files [RFC1035],
369 consists of a single OpenPGP Transferable Public Key as defined in
370 Section 11.1 of [RFC4880] encoded in base64 as defined in Section 4
371 of [RFC4648].
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386 3. Location of the OPENPGPKEY Record
387
388 The DNS does not allow the use of all characters that are supported
389 in the "local-part" of email addresses as defined in [RFC5322] and
390 [RFC6530]. Therefore, email addresses are mapped into DNS using the
391 following method:
392
393 1. The "left-hand side" of the email address, called the "local-
394 part" in both the mail message format definition [RFC5322] and in
395 the specification for internationalized email [RFC6530]) is
396 encoded in UTF-8 (or its subset ASCII). If the local-part is
397 written in another charset, it MUST be converted to UTF-8.
398
399 2. The local-part is first canonicalized using the following rules.
400 If the local-part is unquoted, any comments and/or folding
401 whitespace (CFWS) around dots (".") is removed. Any enclosing
402 double quotes are removed. Any literal quoting is removed.
403
404 3. If the local-part contains any non-ASCII characters, it SHOULD be
405 normalized using the Unicode Normalization Form C from
406 [Unicode90]. Recommended normalization rules can be found in
407 Section 10.1 of [RFC6530].
408
409 4. The local-part is hashed using the SHA2-256 [RFC5754] algorithm,
410 with the hash truncated to 28 octets and represented in its
411 hexadecimal representation, to become the left-most label in the
412 prepared domain name.
413
414 5. The string "_openpgpkey" becomes the second left-most label in
415 the prepared domain name.
416
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.
This RFC is implemented in BIND 9.18 (all versions).
417 6. The domain name (the "right-hand side" of the email address,
418 called the "domain" in [RFC5322]) is appended to the result of
419 step 2 to complete the prepared domain name.
420
421 For example, to request an OPENPGPKEY resource record for a user
422 whose email address is "hugh@example.com", an OPENPGPKEY query would
423 be placed for the following QNAME: "c93f1e400f26708f98cb19d936620da35
424 eec8f72e57f9eec01c1afd6._openpgpkey.example.com". The corresponding
425 RR in the example.com zone might look like (key shortened for
426 formatting):
427
428 c9[..]d6._openpgpkey.example.com. IN OPENPGPKEY <base64 public key>
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441 4. Email Address Variants and Internationalization Considerations
442
443 Mail systems usually handle variant forms of local-parts. The most
444 common variants are upper- and lowercase, often automatically
445 corrected when a name is recognized as such. Other variants include
446 systems that ignore "noise" characters such as dots, so that local-
447 parts 'johnsmith' and 'John.Smith' would be equivalent. Many systems
448 allow "extensions" such as 'john-ext' or 'mary+ext' where 'john' or
449 'mary' is treated as the effective local-part, and 'ext' is passed to
450 the recipient for further handling. This can complicate finding the
451 OPENPGPKEY record associated with the dynamically created email
452 address.
453
454 [RFC5321] and its predecessors have always made it clear that only
455 the recipient MTA is allowed to interpret the local-part of an
456 address. Therefore, sending MUAs and MTAs supporting OPENPGPKEY MUST
457 NOT perform any kind of mapping rules based on the email address. In
458 order to improve chances of finding OPENPGP RRs for a particular
459 local-part, domains that allow variant forms (such as treating local-
460 parts as case-insensitive) might publish OPENPGP RRs for all variants
461 of local-parts, might publish variants on first use (for example, a
462 webmail provider that also controls DNS for a domain can publish
463 variants as used by owner of a particular local-part) or just publish
464 OPENPGP RRs for the most common variants.
465
466 Section 3 above defines how the local-part is used to determine the
467 location where one looks for an OPENPGPKEY record. Given the variety
468 of local-parts seen in email, designing a good experiment for this is
469 difficult, as: a) some current implementations are known to lowercase
470 at least US-ASCII local-parts, b) we know from (many) other
471 situations that any strategy based on guessing and making multiple
472 DNS queries is not going to achieve consensus for good reasons, and
473 c) the underlying issues are just hard -- see Section 10.1 of
474 [RFC6530] for discussion of just some of the issues that would need
475 to be tackled to fully address this problem.
476
477 However, while this specification is not the place to try to address
478 these issues with local-parts, doing so is also not required to
479 determine the outcome of this experiment. If this experiment
480 succeeds, then further work on email addresses with non-ASCII local-
481 parts will be needed and, based on the findings from this experiment,
482 that would be better than doing nothing or starting this experiment
483 based on a speculative approach to what is a very complex topic.
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496 5. Application Use of OPENPGPKEY
497
498 The OPENPGPKEY record allows an application or service to obtain an
499 OpenPGP public key and use it for verifying a digital signature or
500 encrypting a message to the public key. The DNS answer MUST pass
501 DNSSEC validation; if DNSSEC validation reaches any state other than
502 "Secure" (as specified in [RFC4035]), the DNSSEC validation MUST be
503 treated as a failure.
504
505 5.1. Obtaining an OpenPGP Key for a Specific Email Address
506
507 If no OpenPGP public keys are known for an email address, an
508 OPENPGPKEY DNS lookup MAY be performed to seek the OpenPGP public key
509 that corresponds to that email address. This public key can then be
510 used to verify a received signed message or can be used to send out
511 an encrypted email message. An application whose attempt fails to
512 retrieve a DNSSEC-verified OPENPGPKEY RR from the DNS should remember
513 that failure for some time to avoid sending out a DNS request for
514 each email message the application is sending out; such DNS requests
515 constitute a privacy leak.
516
517 5.2. Confirming that an OpenPGP Key is Current
518
519 Locally stored OpenPGP public keys are not automatically refreshed.
520 If the owner of that key creates a new OpenPGP public key, that owner
521 is unable to securely notify all users and applications that have its
522 old OpenPGP public key. Applications and users can perform an
523 OPENPGPKEY lookup to confirm that the locally stored OpenPGP public
524 key is still the correct key to use. If the locally stored OpenPGP
525 public key is different from the DNSSEC-validated OpenPGP public key
526 currently published in DNS, the confirmation MUST be treated as a
527 failure unless the locally stored OpenPGP key signed the newly
528 published OpenPGP public key found in DNS. An application that can
529 interact with the user MAY ask the user for guidance; otherwise, the
530 application will have to apply local policy. For privacy reasons, an
531 application MUST NOT attempt to look up an OpenPGP key from DNSSEC at
532 every use of that key.
533
534 5.3. Public Key UIDs and Query Names
535
536 An OpenPGP public key can be associated with multiple email addresses
537 by specifying multiple key UIDs. The OpenPGP public key obtained
538 from an OPENPGPKEY RR can be used as long as the query and resulting
539 data form a proper email to the UID identity association.
540
541 CNAMEs (see [RFC2181]) and DNAMEs (see [RFC6672]) can be followed to
542 obtain an OPENPGPKEY RR, as long as the original recipient's email
6. The domain name (the "right-hand side" of the email address,
called the "domain" in [RFC5322]) is appended to the result of
step 2 to complete the prepared domain name.
6. The domain name (the "right-hand side" of the email address,
called the "domain" in [RFC5322]) is appended to the result of
step 25 to complete the prepared domain name.
543 address appears as one of the OpenPGP public key UIDs. For example,
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551 if the OPENPGPKEY RR query for hugh@example.com
552 (8d57[...]b7._openpgpkey.example.com) yields a CNAME to
553 8d57[...]b7._openpgpkey.example.net, and an OPENPGPKEY RR for
554 8d57[...]b7._openpgpkey.example.net exists, then this OpenPGP public
555 key can be used, provided one of the key UIDs contains
556 "hugh@example.com". This public key cannot be used if it would only
557 contain the key UID "hugh@example.net".
558
559 If one of the OpenPGP key UIDs contains only a single wildcard as the
560 left-hand side of the email address, such as "*@example.com", the
561 OpenPGP public key may be used for any email address within that
562 domain. Wildcards at other locations (e.g., "hugh@*.com") or regular
563 expressions in key UIDs are not allowed, and any OPENPGPKEY RR
564 containing these MUST be ignored.
565
566 6. OpenPGP Key Size and DNS
567
568 Due to the expected size of the OPENPGPKEY record, applications
569 SHOULD use TCP -- not UDP -- to perform queries for the OPENPGPKEY
570 resource record.
571
572 Although the reliability of the transport of large DNS resource
573 records has improved in the last years, it is still recommended to
574 keep the DNS records as small as possible without sacrificing the
575 security properties of the public key. The algorithm type and key
576 size of OpenPGP keys should not be modified to accommodate this
577 section.
578
579 OpenPGP supports various attributes that do not contribute to the
580 security of a key, such as an embedded image file. It is recommended
581 that these properties not be exported to OpenPGP public keyrings that
582 are used to create OPENPGPKEY resource records. Some OpenPGP
583 software (for example, GnuPG) supports a "minimal key export" that is
584 well suited to use as OPENPGPKEY RDATA. See Appendix A.
585
586 7. Security Considerations
587
588 DNSSEC is not an alternative for the "web of trust" or for manual
589 fingerprint verification by users. DANE for OpenPGP, as specified in
590 this document, is a solution aimed to ease obtaining someone's public
591 key. Without manual verification of the OpenPGP key obtained via
592 DANE, this retrieved key should only be used for encryption if the
593 only other alternative is sending the message in plaintext. While
594 this thwarts all passive attacks that simply capture and log all
595 plaintext email content, it is not a security measure against active
596 attacks. A user who publishes an OPENPGPKEY record in DNS still
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606 expects senders to perform their due diligence by additional (non-
607 DNSSEC) verification of their public key via other out-of-band
608 methods before sending any confidential or sensitive information.
609
610 In other words, the OPENPGPKEY record MUST NOT be used to send
611 sensitive information without additional verification or confirmation
612 that the OpenPGP key actually belongs to the target recipient.
613
614 DNSSEC does not protect the queries from Pervasive Monitoring as
615 defined in [RFC7258]. Since DNS queries are currently mostly
616 unencrypted, a query to look up a target OPENPGPKEY record could
617 reveal that a user using the (monitored) recursive DNS server is
618 attempting to send encrypted email to a target. This information is
619 normally protected by the MUAs and MTAs by using Transport Layer
620 Security (TLS) encryption using STARTTLS. The DNS itself can
621 mitigate some privacy concerns, but the user needs to select a
622 trusted DNS server that supports these privacy-enhancing features.
623 Recursive DNS servers can support DNS Query Name Minimalisation
624 [RFC7816], which limits leaking the QNAME to only the recursive DNS
625 server and the nameservers of the actual zone being queried for.
626 Recursive DNS servers can also support TLS [RFC7858] to ensure that
627 the path between the end user and the recursive DNS server is
628 encrypted.
629
630 Various components could be responsible for encrypting an email
631 message to a target recipient. It could be done by the sender's MUA
632 or a MUA plug-in or the sender's MTA. Each of these have their own
633 characteristics. A MUA can ask the user to make a decision before
634 continuing. The MUA can either accept or refuse a message. The MTA
635 must deliver the message as-is, or encrypt the message before
636 delivering. Each of these components should attempt to encrypt an
637 unencrypted outgoing message whenever possible.
638
639 In theory, two different local-parts could hash to the same value.
640 This document assumes that such a hash collision has a negligible
641 chance of happening.
642
643 Organizations that are required to be able to read everyone's
644 encrypted email should publish the escrow key as the OPENPGPKEY
645 record. Mail servers of such organizations MAY optionally re-encrypt
646 the message to the individual's OpenPGP key.
647
648 7.1. MTA Behavior
649
650 An MTA could be operating in a stand-alone mode, without access to
651 the sender's OpenPGP public keyring, or in a way where it can access
652 the user's OpenPGP public keyring. Regardless, the MTA MUST NOT
653 modify the user's OpenPGP keyring.
654
655
656
657 Wouters Experimental [Page 12]
658 RFC 7929 DANE for OpenPGP Keys August 2016
659
660
661 An MTA sending an email MUST NOT add the public key obtained from an
662 OPENPGPKEY resource record to a permanent public keyring for future
663 use beyond the TTL.
664
665 If the obtained public key is revoked, the MTA MUST NOT use the key
666 for encryption, even if that would result in sending the message in
667 plaintext.
668
669 If a message is already encrypted, the MTA SHOULD NOT re-encrypt the
670 message, even if different encryption schemes or different encryption
671 keys would be used.
672
673 If the DNS request for an OPENPGPKEY record returned an Indeterminate
674 or Bogus answer as specified in [RFC4035], the MTA MUST NOT send the
675 message and queue the plaintext message for encrypted delivery at a
676 later time. If the problem persists, the email should be returned
677 via the regular bounce methods.
678
679 If multiple non-revoked OPENPGPKEY resource records are found, the
680 MTA SHOULD pick the most secure RR based on its local policy.
681
682 7.2. MUA Behavior
683
684 If the public key for a recipient obtained from the locally stored
685 sender's public keyring differs from the recipient's OPENPGPKEY RR,
686 the MUA SHOULD halt processing the message and interact with the user
687 to resolve the conflict before continuing to process the message.
688
689 If the public key for a recipient obtained from the locally stored
690 sender's public keyring contains contradicting properties for the
691 same key obtained from an OPENPGPKEY RR, the MUA SHOULD NOT accept
692 the message for delivery.
693
694 If multiple non-revoked OPENPGPKEY resource records are found, the
695 MUA SHOULD pick the most secure OpenPGP public key based on its local
696 policy.
697
698 The MUA MAY interact with the user to resolve any conflicts between
699 locally stored keyrings and OPENPGPKEY RRdata.
700
701 A MUA that is encrypting a message SHOULD clearly indicate to the
702 user the difference between encrypting to a locally stored and
703 previously user-verified public key and encrypting to a public key
704 obtained via an OPENPGPKEY resource record that was not manually
705 verified by the user in the past.
706
707
708
709
710
711
712 Wouters Experimental [Page 13]
713 RFC 7929 DANE for OpenPGP Keys August 2016
714
715
716 7.3. Response Size
717
718 To prevent amplification attacks, an Authoritative DNS server MAY
719 wish to prevent returning OPENPGPKEY records over UDP unless the
720 source IP address has been confirmed with [RFC7873]. Such servers
721 MUST NOT return REFUSED, but answer the query with an empty answer
722 section and the truncation flag set ("TC=1").
723
724 7.4. Email Address Information Leak
725
726 The hashing of the local-part in this document is not a security
727 feature. Publishing OPENPGPKEY records will create a list of hashes
728 of valid email addresses, which could simplify obtaining a list of
729 valid email addresses for a particular domain. It is desirable to
730 not ease the harvesting of email addresses where possible.
731
732 The domain name part of the email address is not used as part of the
733 hash so that hashes can be used in multiple zones deployed using
734 DNAME [RFC6672]. This does makes it slightly easier and cheaper to
735 brute-force the SHA2-256 hashes into common and short local-parts, as
736 single rainbow tables can be re-used across domains. This can be
737 somewhat countered by using NextSECure version 3 (NSEC3).
738
739 DNS zones that are signed with DNSSEC using NSEC for denial of
740 existence are susceptible to zone walking, a mechanism that allows
741 someone to enumerate all the OPENPGPKEY hashes in a zone. This can
742 be used in combination with previously hashed common or short local-
743 parts (in rainbow tables) to deduce valid email addresses. DNSSEC-
744 signed zones using NSEC3 for denial of existence instead of NSEC are
745 significantly harder to brute-force after performing a zone walk.
746
747 7.5. Storage of OPENPGPKEY Data
748
749 Users may have a local key store with OpenPGP public keys. An
750 application supporting the use of OPENPGPKEY DNS records MUST NOT
751 modify the local key store without explicit confirmation of the user,
752 as the application is unaware of the user's personal policy for
753 adding, removing, or updating their local key store. An application
754 MAY warn the user if an OPENPGPKEY record does not match the OpenPGP
755 public key in the local key store.
756
757 Applications that cannot interact with users, such as daemon
758 processes, SHOULD store OpenPGP public keys obtained via OPENPGPKEY
759 up to their DNS TTL value. This avoids repeated DNS lookups that
760 third parties could monitor to determine when an email is being sent
761 to a particular user.
762
763
764
765
766
767 Wouters Experimental [Page 14]
768 RFC 7929 DANE for OpenPGP Keys August 2016
769
770
771 7.6. Security of OpenPGP versus DNSSEC
772
773 Anyone who can obtain a DNSSEC private key of a domain name via
774 coercion, theft, or brute-force calculations, can replace any
775 OPENPGPKEY record in that zone and all of the delegated child zones.
776 Any future messages encrypted with the malicious OpenPGP key could
777 then be read.
778
779 Therefore, an OpenPGP key obtained via a DNSSEC-validated OPENPGPKEY
780 record can only be trusted as much as the DNS domain can be trusted,
781 and is no substitute for in-person OpenPGP key verification or
782 additional OpenPGP verification via "web of trust" signatures present
783 on the OpenPGP in question.
784
785 8. IANA Considerations
786
787 8.1. OPENPGPKEY RRtype
788
789 This document uses a new DNS RR type, OPENPGPKEY, whose value 61 has
790 been allocated by IANA from the "Resource Record (RR) TYPEs"
791 subregistry of the "Domain Name System (DNS) Parameters" registry.
792
793 The IANA template for OPENPGPKEY is listed in Appendix B. It was
794 submitted to IANA for review on July 23, 2014 and approved on August
795 12, 2014.
796
797 9. References
798
799 9.1. Normative References
800
801 [RFC1035] Mockapetris, P., "Domain names - implementation and
802 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
803 November 1987, <http://www.rfc-editor.org/info/rfc1035>.
804
805 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
806 Requirement Levels", BCP 14, RFC 2119,
807 DOI 10.17487/RFC2119, March 1997,
808 <http://www.rfc-editor.org/info/rfc2119>.
809
810 [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
811 Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
812 <http://www.rfc-editor.org/info/rfc2181>.
813
814 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
815 Rose, "DNS Security Introduction and Requirements",
816 RFC 4033, DOI 10.17487/RFC4033, March 2005,
817 <http://www.rfc-editor.org/info/rfc4033>.
818
819
820
821
822 Wouters Experimental [Page 15]
823 RFC 7929 DANE for OpenPGP Keys August 2016
824
825
826 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
827 Rose, "Resource Records for the DNS Security Extensions",
828 RFC 4034, DOI 10.17487/RFC4034, March 2005,
829 <http://www.rfc-editor.org/info/rfc4034>.
830
831 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
832 Rose, "Protocol Modifications for the DNS Security
833 Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
834 <http://www.rfc-editor.org/info/rfc4035>.
835
836 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
837 Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
838 <http://www.rfc-editor.org/info/rfc4648>.
839
840 [RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
841 Thayer, "OpenPGP Message Format", RFC 4880,
842 DOI 10.17487/RFC4880, November 2007,
843 <http://www.rfc-editor.org/info/rfc4880>.
844
845 [RFC5754] Turner, S., "Using SHA2 Algorithms with Cryptographic
846 Message Syntax", RFC 5754, DOI 10.17487/RFC5754, January
847 2010, <http://www.rfc-editor.org/info/rfc5754>.
848
849 9.2. Informative References
850
851 [HKP] Shaw, D., "The OpenPGP HTTP Keyserver Protocol (HKP)",
852 Work in Progress, draft-shaw-openpgp-hkp-00, March 2003.
853
854 [MAILBOX] Levine, J., "Encoding mailbox local-parts in the DNS",
855 Work in Progress, draft-levine-dns-mailbox-01, September
856 2015.
857
858 [RFC3597] Gustafsson, A., "Handling of Unknown DNS Resource Record
859 (RR) Types", RFC 3597, DOI 10.17487/RFC3597, September
860 2003, <http://www.rfc-editor.org/info/rfc3597>.
861
862 [RFC4255] Schlyter, J. and W. Griffin, "Using DNS to Securely
863 Publish Secure Shell (SSH) Key Fingerprints", RFC 4255,
864 DOI 10.17487/RFC4255, January 2006,
865 <http://www.rfc-editor.org/info/rfc4255>.
866
867 [RFC4398] Josefsson, S., "Storing Certificates in the Domain Name
868 System (DNS)", RFC 4398, DOI 10.17487/RFC4398, March 2006,
869 <http://www.rfc-editor.org/info/rfc4398>.
870
871 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
872 DOI 10.17487/RFC5321, October 2008,
873 <http://www.rfc-editor.org/info/rfc5321>.
874
875
876
877 Wouters Experimental [Page 16]
878 RFC 7929 DANE for OpenPGP Keys August 2016
879
880
881 [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
882 DOI 10.17487/RFC5322, October 2008,
883 <http://www.rfc-editor.org/info/rfc5322>.
884
885 [RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for
886 Internationalized Email", RFC 6530, DOI 10.17487/RFC6530,
887 February 2012, <http://www.rfc-editor.org/info/rfc6530>.
888
889 [RFC6672] Rose, S. and W. Wijngaards, "DNAME Redirection in the
890 DNS", RFC 6672, DOI 10.17487/RFC6672, June 2012,
891 <http://www.rfc-editor.org/info/rfc6672>.
892
893 [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
894 of Named Entities (DANE) Transport Layer Security (TLS)
895 Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
896 2012, <http://www.rfc-editor.org/info/rfc6698>.
897
898 [RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
899 Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
900 2014, <http://www.rfc-editor.org/info/rfc7258>.
901
902 [RFC7816] Bortzmeyer, S., "DNS Query Name Minimisation to Improve
903 Privacy", RFC 7816, DOI 10.17487/RFC7816, March 2016,
904 <http://www.rfc-editor.org/info/rfc7816>.
905
906 [RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
907 and P. Hoffman, "Specification for DNS over Transport
908 Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
909 2016, <http://www.rfc-editor.org/info/rfc7858>.
910
911 [RFC7873] Eastlake 3rd, D. and M. Andrews, "Domain Name System (DNS)
912 Cookies", RFC 7873, DOI 10.17487/RFC7873, May 2016,
913 <http://www.rfc-editor.org/info/rfc7873>.
914
915 [SMIME] Hoffman, P. and J. Schlyter, "Using Secure DNS to
916 Associate Certificates with Domain Names For S/MIME", Work
917 in Progress, draft-ietf-dane-smime-12, July 2016.
918
919 [Unicode90]
920 The Unicode Consortium, "The Unicode Standard, Version
921 9.0.0", (Mountain View, CA: The Unicode Consortium,
922 2016. ISBN 978-1-936213-13-9),
923 <http://www.unicode.org/versions/Unicode9.0.0/>.
924
925
926
927
928
929
930
931
932 Wouters Experimental [Page 17]
933 RFC 7929 DANE for OpenPGP Keys August 2016
934
935
936 Appendix A. Generating OPENPGPKEY Records
937
938 The commonly available GnuPG software can be used to generate a
939 minimum Transferable Public Key for the RRdata portion of an
940 OPENPGPKEY record:
941
942 gpg --export --export-options export-minimal,no-export-attributes \
943 hugh@example.com | base64
944
945 The --armor or -a option of the gpg command should not be used, as it
946 adds additional markers around the armored key.
947
948 When DNS software reading or signing of the zone file does not yet
949 support the OPENPGPKEY RRtype, the Generic Record Syntax of [RFC3597]
950 can be used to generate the RDATA. One needs to calculate the number
951 of octets and the actual data in hexadecimal:
952
953 gpg --export --export-options export-minimal,no-export-attributes \
954 hugh@example.com | wc -c
955 gpg --export --export-options export-minimal,no-export-attributes \
956 hugh@example.com | hexdump -e \
957 '"\t" /1 "%.2x"' -e '/32 "\n"'
958
959 These values can then be used to generate a generic record (line
960 break has been added for formatting):
961
962 <SHA2-256-trunc(hugh)>._openpgpkey.example.com. IN TYPE61 \# \
963 <numOctets> <keydata in hex>
964
965 The openpgpkey command in the hash-slinger software can be used to
966 generate complete OPENPGPKEY records
967
968 ~> openpgpkey --output rfc hugh@example.com
969 c9[..]d6._openpgpkey.example.com. IN OPENPGPKEY mQCNAzIG[...]
970
971 ~> openpgpkey --output generic hugh@example.com
972 c9[..]d6._openpgpkey.example.com. IN TYPE61 \# 2313 99008d03[...]
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987 Wouters Experimental [Page 18]
988 RFC 7929 DANE for OpenPGP Keys August 2016
989
990
991 Appendix B. OPENPGPKEY IANA Template
992
993 This is a copy of the original registration template submitted to
994 IANA; the text (including the references) has not been updated.
995
996 A. Submission Date: 23-07-2014
997
998 B.1 Submission Type: [x] New RRTYPE [ ] Modification to RRTYPE
999 B.2 Kind of RR: [x] Data RR [ ] Meta-RR
1000
1001 C. Contact Information for submitter (will be publicly posted):
1002 Name: Paul Wouters Email Address: pwouters@redhat.com
1003 International telephone number: +1-647-896-3464
1004 Other contact handles: paul@nohats.ca
1005
1006 D. Motivation for the new RRTYPE application.
1007
1008 Publishing RFC-4880 OpenPGP formatted keys in DNS with DNSSEC
1009 protection to faciliate automatic encryption of emails in
1010 defense against pervasive monitoring.
1011
1012 E. Description of the proposed RR type.
1013
1014 http://tools.ietf.org/html/draft-ietf-dane-openpgpkey-00#section-2
1015
1016 F. What existing RRTYPE or RRTYPEs come closest to filling that need
1017 and why are they unsatisfactory?
1018
1019 The CERT RRtype is the closest match. It unfortunately depends on
1020 subtyping, and its use in general is no longer recommended. It
1021 also has no human usable presentation format. Some usage types of
1022 CERT require external URI's which complicates the security model.
1023 This was discussed in the dane working group.
1024
1025 G. What mnemonic is requested for the new RRTYPE (optional)?
1026
1027 OPENPGPKEY
1028
1029 H. Does the requested RRTYPE make use of any existing IANA registry
1030 or require the creation of a new IANA subregistry in DNS
1031 Parameters? If so, please indicate which registry is to be used
1032 or created. If a new subregistry is needed, specify the
1033 allocation policy for it and its initial contents. Also include
1034 what the modification procedures will be.
1035
1036 The RDATA part uses the key format specified in RFC-4880, which
1037 itself use
1038 https://www.iana.org/assignments/pgp-parameters/pgp-parameters.xhtm
1039
1040
1041
1042 Wouters Experimental [Page 19]
1043 RFC 7929 DANE for OpenPGP Keys August 2016
1044
1045
1046 This RRcode just uses the formats specified in those registries for
1047 its RRdata part.
1048
1049 I. Does the proposal require/expect any changes in DNS
1050 servers/resolvers that prevent the new type from being processed
1051 as an unknown RRTYPE (see [RFC3597])?
1052
1053 No.
1054
1055 J. Comments:
1056
1057 Currently, three software implementations of
1058 draft-ietf-dane-openpgpkey are using a private number.
1059
1060 Acknowledgments
1061
1062 This document is based on [RFC4255] and [SMIME] whose authors are
1063 Paul Hoffman, Jakob Schlyter, and W. Griffin. Olafur Gudmundsson
1064 provided feedback and suggested various improvements. Willem Toorop
1065 contributed the gpg and hexdump command options. Daniel Kahn Gillmor
1066 provided the text describing the OpenPGP packet formats and filtering
1067 options. Edwin Taylor contributed language improvements for various
1068 iterations of this document. Text regarding email mappings was taken
1069 from [MAILBOX] whose author is John Levine.
1070
1071 Author's Address
1072
1073 Paul Wouters
1074 Red Hat
1075
1076 Email: pwouters@redhat.com
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097 Wouters Experimental [Page 20]
1098
For example, if the OPENPGPKEY RR query for hugh@example.com (8d57[...]b7._openpgpkey.example.com) yields a CNAME to 8d57[...]b7._openpgpkey.example.net, and an OPENPGPKEY RR for 8d57[...]b7._openpgpkey.example.net exists,
For example, if the OPENPGPKEY RR query for hugh@example.com (8d57[...]b7c93f[...]d6._openpgpkey.example.com) yields a CNAME to8d57[...]b7c93f[...]d6._openpgpkey.example.net, and an OPENPGPKEY RR for8d57[...]b7c93f[...]d6._openpgpkey.example.net exists,