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Checking references for intended status: Experimental ---------------------------------------------------------------------------- -- Obsolete informational reference (is this intentional?): RFC 6982 (Obsoleted by RFC 7942) Summary: 0 errors (**), 0 flaws (~~), 1 warning (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group P. Wouters 3 Internet-Draft Red Hat 4 Intended status: Experimental April 19, 2016 5 Expires: October 21, 2016 7 Using DANE to Associate OpenPGP public keys with email addresses 8 draft-ietf-dane-openpgpkey-10 10 Abstract 12 OpenPGP is a message format for email (and file) encryption that 13 lacks a standardized lookup mechanism to securely obtain OpenPGP 14 public keys. DNS-Based Authentication of Named Entities ("DANE") is 15 a method for publishing public keys in DNS. This document specifies 16 a DANE method for publishing and locating OpenPGP public keys in DNS 17 for a specific email address using a new OPENPGPKEY DNS Resource 18 Record. Security is provided via Secure DNS, however the OPENPGPKEY 19 record is not a replacement for verification of authenticity via the 20 "Web Of Trust" or manual verification. The OPENPGPKEY record can be 21 used to encrypt an email that would otherwise have to be sent 22 unencrypted. 24 Status of This Memo 26 This Internet-Draft is submitted in full conformance with the 27 provisions of BCP 78 and BCP 79. 29 Internet-Drafts are working documents of the Internet Engineering 30 Task Force (IETF). Note that other groups may also distribute 31 working documents as Internet-Drafts. The list of current Internet- 32 Drafts is at http://datatracker.ietf.org/drafts/current/. 34 Internet-Drafts are draft documents valid for a maximum of six months 35 and may be updated, replaced, or obsoleted by other documents at any 36 time. It is inappropriate to use Internet-Drafts as reference 37 material or to cite them other than as "work in progress." 39 This Internet-Draft will expire on October 21, 2016. 41 Copyright Notice 43 Copyright (c) 2016 IETF Trust and the persons identified as the 44 document authors. All rights reserved. 46 This document is subject to BCP 78 and the IETF Trust's Legal 47 Provisions Relating to IETF Documents 48 (http://trustee.ietf.org/license-info) in effect on the date of 49 publication of this document. Please review these documents 50 carefully, as they describe your rights and restrictions with respect 51 to this document. Code Components extracted from this document must 52 include Simplified BSD License text as described in Section 4.e of 53 the Trust Legal Provisions and are provided without warranty as 54 described in the Simplified BSD License. 56 Table of Contents 58 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 59 1.1. Experiment goal . . . . . . . . . . . . . . . . . . . . . 3 60 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 61 2. The OPENPGPKEY Resource Record . . . . . . . . . . . . . . . 4 62 2.1. The OPENPGPKEY RDATA component . . . . . . . . . . . . . 5 63 2.1.1. The OPENPGPKEY RDATA content . . . . . . . . . . . . 5 64 2.1.2. Reducing the Transferable Public Key size . . . . . . 6 65 2.2. The OPENPGPKEY RDATA wire format . . . . . . . . . . . . 6 66 2.3. The OPENPGPKEY RDATA presentation format . . . . . . . . 6 67 3. Location of the OPENPGPKEY record . . . . . . . . . . . . . . 6 68 4. Email address variants and internationalization 69 considerations . . . . . . . . . . . . . . . . . . . . . . . 7 70 5. Application use of OPENPGPKEY . . . . . . . . . . . . . . . . 8 71 5.1. Obtaining an OpenPGP key for a specific email address . . 8 72 5.2. Confirming that an OpenPGP key is current . . . . . . . . 9 73 5.3. Public Key UIDs and query names . . . . . . . . . . . . . 9 74 6. OpenPGP Key size and DNS . . . . . . . . . . . . . . . . . . 9 75 7. Security Considerations . . . . . . . . . . . . . . . . . . . 10 76 7.1. MTA behaviour . . . . . . . . . . . . . . . . . . . . . . 11 77 7.2. MUA behaviour . . . . . . . . . . . . . . . . . . . . . . 11 78 7.3. Response size . . . . . . . . . . . . . . . . . . . . . . 12 79 7.4. Email address information leak . . . . . . . . . . . . . 12 80 7.5. Storage of OPENPGPKEY data . . . . . . . . . . . . . . . 12 81 7.6. Security of OpenPGP versus DNSSEC . . . . . . . . . . . . 13 82 8. Implementation Status . . . . . . . . . . . . . . . . . . . . 13 83 8.1. The GNU Privacy Guard (GNUpg) . . . . . . . . . . . . . . 13 84 8.2. hash-slinger . . . . . . . . . . . . . . . . . . . . . . 14 85 8.3. openpgpkey-milter . . . . . . . . . . . . . . . . . . . . 15 86 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 87 9.1. OPENPGPKEY RRtype . . . . . . . . . . . . . . . . . . . . 15 88 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15 89 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 16 90 11.1. Normative References . . . . . . . . . . . . . . . . . . 16 91 11.2. Informative References . . . . . . . . . . . . . . . . . 17 92 Appendix A. Generating OPENPGPKEY records . . . . . . . . . . . 18 93 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 19 95 1. Introduction 96 OpenPGP [RFC4880] public keys are used to encrypt or sign email 97 messages and files. To encrypt an email message, or verify a 98 sender's OpenPGP signature, the email client (MUA) or the email 99 server (MTA) needs to locate the recipient's OpenPGP public key. 101 OpenPGP clients have relied on centralized "well-known" key servers 102 that are accessed using the HTTP Keyserver Protocol [HKP]. 103 Alternatively, users need to manually browse a variety of different 104 front-end websites. These key servers do not require a confirmation 105 of the email address used in the User ID of the uploaded OpenPGP 106 public key. Attackers can - and have - uploaded rogue public keys 107 with other people's email addresses to these key servers. 109 Once uploaded, public keys cannot be deleted. People who did not 110 pre-sign a key revocation can never remove their OpenPGP public key 111 from these key servers once they have lost access to their private 112 key. This results in receiving encrypted email that cannot be 113 decrypted. 115 Therefore, these keyservers are not well suited to support MUAs and 116 MTA's to automatically encrypt email - especially in the absence of 117 an interactive user. 119 This document describes a mechanism to associate a user's OpenPGP 120 public key with their email address, using the OPENPGPKEY DNS RRtype. 121 These records are published in the DNS zone of the user's email 122 address. If the user loses their private key, the OPENPGPKEY DNS 123 record can simply be updated or removed from the zone. 125 The OPENPGPKEY data is secured using Secure DNS [RFC4035] 127 The main goal of the OPENPGPKEY resource record is to stop passive 128 attacks against plaintext emails. While it can also thwart some 129 active attacks (such as people uploading rogue keys to keyservers in 130 the hopes that others will encrypt to these rogue keys), this 131 resource record is not a replacement for verifying OpenPGP public 132 keys via the web of trust signatures, or manually via a fingerprint 133 verification. 135 1.1. Experiment goal 137 This specification is one experiment in improving access to public 138 keys for end-to-end email security. There are a range of ways in 139 which this can reasonably be done, for OpenPGP or S/MIME, for example 140 using the DNS, or SMTP, or HTTP. Proposals for each of these have 141 been made with various levels of support in terms of implementation 142 and deployment. For each such experiment, specifications such as 143 this will enable experiments to be carried out that may succeed or 144 that may uncover technical or other impediments to large- or small- 145 scale deployments. The IETF encourages those implementing and 146 deploying such experiments to publicly document their experiences so 147 that future specifications in this space can benefit. 149 This document defines an RRtype whose use is Experimental. The goal 150 of the experiment is to see whether encrypted email usage will 151 increase if an automated discovery method is available to MTA's and 152 MUA's to help the enduser with email encryption key management. 154 It is unclear if this RRtype will scale to some of the larger email 155 service deployments. Concerns have been raised about the size of the 156 OPENPGPKEY record and the size of the resulting DNS zone files. This 157 experiment hopefully will give the working group some insight into 158 whether this is a problem or not. 160 If the experiment is successful, it is expected that the findings of 161 the experiment will result in an updated document for standards track 162 approval. 164 The OPENPGPKEY RRtype somewhat resembles the generic CERT record 165 defined in [RFC4398]. However, the CERT record uses sub-typing with 166 many different types of keys and certificates. It is suspected that 167 its general application of very different protocols (PKIX versus 168 OpenPGP) has been the cause for lack of implementation and 169 deployment. Furthermore, the CERT record uses sub-typing, which is 170 now considered to be a bad idea for DNS. 172 1.2. Terminology 174 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 175 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 176 document are to be interpreted as described in RFC 2119 [RFC2119]. 178 This document also makes use of standard DNSSEC and DANE terminology. 179 See DNSSEC [RFC4033], [RFC4034], [RFC4035], and DANE [RFC6698] for 180 these terms. 182 2. The OPENPGPKEY Resource Record 184 The OPENPGPKEY DNS resource record (RR) is used to associate an end 185 entity OpenPGP Transferable Public Key (see Section 11.1 of [RFC4880] 186 with an email address, thus forming a "OpenPGP public key 187 association". A user that wishes to specify more than one OpenPGP 188 key, for example because they are transitioning to a newer stronger 189 key, can do so by adding multiple OPENPGPKEY records. A single 190 OPENPGPKEY DNS record MUST only contain one OpenPGP key. 192 The type value allocated for the OPENPGPKEY RR type is 61. The 193 OPENPGPKEY RR is class independent. 195 2.1. The OPENPGPKEY RDATA component 197 The RDATA portion of an OPENPGPKEY Resource Record contains a single 198 value consisting of a [RFC4880] formatted Transferable Public Key. 200 2.1.1. The OPENPGPKEY RDATA content 202 An OpenPGP Transferable Public Key can be arbitrarily large. DNS 203 records are limited in size. When creating OPENPGPKEY DNS records, 204 the OpenPGP Transferable Public Key should be filtered to only 205 contain appropriate and useful data. At a minimum, an OPENPGPKEY 206 Transferable Public Key for the user hugh@example.com should contain: 208 o The primary key X 209 o One User ID Y, which SHOULD match 'hugh@example.com' 210 o self-signature from X, binding X to Y 212 If the primary key is not encryption-capable, a relevant subkey 213 should be included resulting in an OPENPGPKEY Transferable Public Key 214 containing: 216 o The primary key X 217 o One User ID Y, which SHOULD match 'hugh@example.com' 218 o self-signature from X, binding X to Y 219 o encryption-capable subkey Z 220 o self-signature from X, binding Z to X 221 o [ other subkeys if relevant ... ] 223 The user can also elect to add a few third-party certifications which 224 they believe would be helpful for validation in the traditional Web 225 Of Trust. The resulting OPENPGPKEY Transferable Public Key would 226 then look like: 228 o The primary key X 229 o One User ID Y, which SHOULD match 'hugh@example.com' 230 o self-signature from X, binding X to Y 231 o third-party certification from V, binding Y to X 232 o [ other third-party certifications if relevant ... ] 233 o encryption-capable subkey Z 234 o self-signature from X, binding Z to X 235 o [ other subkeys if relevant ... ] 237 2.1.2. Reducing the Transferable Public Key size 239 When preparing a Transferable Public Key for a specific OPENPGPKEY 240 RDATA format with the goal of minimizing certificate size, a user 241 would typically want to: 243 o Where one User ID from the certifications matches the looked-up 244 address, strip away non-matching User IDs and any associated 245 certifications (self-signatures or third-party certifications). 247 o Strip away all User Attribute packets and associated 248 certifications. 250 o Strip away all expired subkeys. The user may want to keep revoked 251 subkeys if these were revoked prior to their preferred expiration 252 time to ensure that correspondents know about these earlier than 253 expected revocations. 255 o Strip away all but the most recent self-signature for the 256 remaining user IDs and subkeys. 258 o Optionally strip away any uninteresting or unimportant third-party 259 User ID certifications. This is a value judgment by the user that 260 is difficult to automate. At the very least, expired and 261 superseded third-party certifcations should be stripped out. The 262 user should attempt to keep the most recent and most well 263 connected certifications in the Web Of Trust in their Transferable 264 Public Key. 266 2.2. The OPENPGPKEY RDATA wire format 268 The RDATA Wire Format consists of a single OpenPGP Transferable 269 Public Key as defined in Section 11.1 of [RFC4880]. Note that this 270 format is without ASCII armor or base64 encoding. 272 2.3. The OPENPGPKEY RDATA presentation format 274 The RDATA Presentation Format, as visible in master files [RFC1035], 275 consists of a single OpenPGP Transferable Public Key as defined in 276 Section 11.1 of [RFC4880] encoded in base64 as defined in Section 4 277 of [RFC4648]. 279 3. Location of the OPENPGPKEY record 281 The DNS does not allow the use of all characters that are supported 282 in the "local-part" of email addresses as defined in [RFC5322] and 283 [RFC6530]. Therefore, email addresses are mapped into DNS using the 284 following method: 286 o The "left-hand side" of the email address, called the "local-part" 287 in both the mail message format definition [RFC5322] and in the 288 specification for internationalized email [RFC6530]) is encoded in 289 UTF-8 (or its subset ASCII). If the local-part is written in 290 another charset it MUST be converted to UTF-8. 292 o The local-part is first canonicalized using the following rules. 293 If the local-part is unquoted, any whitespace (CFWS) around dots 294 (".") is removed. Any enclosing double quotes are removed. Any 295 literal quoting is removed. 297 o If the local-part contains any non-ASCII characters, it SHOULD be 298 normalized using the Unicode Normalization Form C from 299 [Unicode52]. Recommended normalization rules can be found in 300 Section 10.1 of [RFC6530]. 302 o The local-part is hashed using the SHA2-256 [RFC5754] algorithm, 303 with the hash truncated to 28 octets and represented in its 304 hexadecimal representation, to become the left-most label in the 305 prepared domain name. 307 o The string "_openpgpkey" becomes the second left-most label in the 308 prepared domain name. 310 o The domain name (the "right-hand side" of the email address, 311 called the "domain" in [RFC5322]) is appended to the result of 312 step 2 to complete the prepared domain name. 314 For example, to request an OPENPGPKEY resource record for a user 315 whose email address is "hugh@example.com", an OPENPGPKEY query would 316 be placed for the following QNAME: "c93f1e400f26708f98cb19d936620da35 317 eec8f72e57f9eec01c1afd6._openpgpkey.example.com". The corresponding 318 RR in the example.com zone might look like (key shortened for 319 formatting): 321 c9[..]d6._openpgpkey.example.com. IN OPENPGPKEY 323 4. Email address variants and internationalization considerations 325 Mail systems usually handle variant forms of local-parts. The most 326 common variants are upper and lower case, often automatically 327 corrected when a name is recognized as such. Other variants include 328 systems that ignore "noise" characters such as dots, so that local 329 parts johnsmith and John.Smith would be equivalent. Many systems 330 allow "extensions" such as john-ext or mary+ext where john or mary is 331 treated as the effective local-part, and the ext is passed to the 332 recipient for further handling. This can complicate finding the 333 OPENPGPKEY record associated with the dynamically created email 334 address. 336 [RFC5321] and its predecessors have always made it clear that only 337 the recipient MTA is allowed to interpret the local-part of an 338 address. MUA's and MTA's supporting OPENPGPKEY therefore MUST NOT 339 perform any kind of mapping rules based on the email address. 341 Section 3 above defines how the local-part is used to determine the 342 location in which one looks for an OPENPGPKEY record. Given the 343 variety of local-parts seen in email, designing a good experiment for 344 this is difficult as: a) some current implementations are known to 345 lowercase at least US-ASCII local-parts, b) we know from (many) other 346 situations that any strategy based on guessing and making multiple 347 DNS queries is not going to achieve consensus for good reasons, and 348 c) the underlying issues are just hard - see Section 10.1 of 349 [RFC6530] for discussion of just some of the issues that would need 350 to be tackled to fully address this problem. 352 However, while this specification is not the place to try to address 353 these issues with local-parts, doing so is also not required to 354 determine the outcome of this experiment. If this experiment 355 succeeds then further work on email addresses with non-ASCII local- 356 parts will be needed and that would be better based on the findings 357 from this experiment, rather than doing nothing or starting this 358 experiment based on a speculative approach to what is a very complex 359 topic. 361 5. Application use of OPENPGPKEY 363 The OPENPGPKEY record allows an application or service to obtain an 364 OpenPGP public key and use it for verifying a digital signature or 365 encrypting a message to the public key. The DNS answer MUST pass 366 DNSSEC validation; if DNSSEC validation reaches any state other than 367 "Secure" (as specified in [RFC4035]), the DNSSEC validation MUST be 368 treated as a failure. 370 5.1. Obtaining an OpenPGP key for a specific email address 372 If no OpenPGP public keys are known for an email address, an 373 OPENPGPKEY DNS lookup MAY be performed to seek the OpenPGP public key 374 that corresponds to that email address. This public key can then be 375 used to verify a received signed message or can be used to send out 376 an encrypted email message. An application whose attempt fails to 377 retrieve a DNSSEC verified OPENPGPKEY RR from the DNS should remember 378 that failure for some time to avoid sending out a DNS request for 379 each email message the application is sending out; such DNS requests 380 constitute a privacy leak 382 5.2. Confirming that an OpenPGP key is current 384 Locally stored OpenPGP public keys are not automatically refreshed. 385 If the owner of that key creates a new OpenPGP public key, that owner 386 is unable to securely notify all users and applications that have its 387 old OpenPGP public key. Applications and users can perform an 388 OPENPGPKEY lookup to confirm the locally stored OpenPGP public key is 389 still the correct key to use. If the locally stored OpenPGP public 390 key is different from the DNSSEC validated OpenPGP public key 391 currently published in DNS, the confirmation MUST be treated as a 392 failure unless the locally stored OpenPGP key signed the newly 393 published OpenPGP public key found in DNS. An application that can 394 interact with the user MAY ask the user for guidance, otherwise the 395 application will have to apply local policy. For privacy reasons, an 396 application MUST NOT attempt to lookup an OpenPGP key from DNSSEC at 397 every use of that key. 399 5.3. Public Key UIDs and query names 401 An OpenPGP public key can be associated with multiple email addresses 402 by specifying multiple key uids. The OpenPGP public key obtained 403 from a OPENPGPKEY RR can be used as long as the query and resulting 404 data form a proper email to uid identity association. 406 CNAME's (see [RFC2181]) and DNAME's (see [RFC6672]) can be followed 407 to obtain an OPENPGPKEY RR, as long as the original recipient's email 408 address appears as one of the OpenPGP public key uids. For example, 409 if the OPENPGPKEY RR query for hugh@example.com 410 (8d57[...]b7._openpgpkey.example.com) yields a CNAME to 411 8d57[...]b7._openpgpkey.example.net, and an OPENPGPKEY RR for 412 8d57[...]b7._openpgpkey.example.net exists, then this OpenPGP public 413 key can be used, provided one of the key uids contains 414 "hugh@example.com". This public key cannot be used if it would only 415 contain the key uid "hugh@example.net". 417 If one of the OpenPGP key uids contains only a single wildcard as the 418 LHS of the email address, such as "*@example.com", the OpenPGP public 419 key may be used for any email address within that domain. Wildcards 420 at other locations (eg hugh@*.com) or regular expressions in key uids 421 are not allowed, and any OPENPGPKEY RR containing these MUST be 422 ignored. 424 6. OpenPGP Key size and DNS 426 Due to the expected size of the OPENPGPKEY record, applications 427 SHOULD use TCP - not UDP - to perform queries for the OPENPGPKEY 428 Resource Record. 430 Although the reliability of the transport of large DNS Resource 431 Records has improved in the last years, it is still recommended to 432 keep the DNS records as small as possible without sacrificing the 433 security properties of the public key. The algorithm type and key 434 size of OpenPGP keys should not be modified to accommodate this 435 section. 437 OpenPGP supports various attributes that do not contribute to the 438 security of a key, such as an embedded image file. It is recommended 439 that these properties not be exported to OpenPGP public keyrings that 440 are used to create OPENPGPKEY Resource Records. Some OpenPGP 441 software, for example GnuPG, support a "minimal key export" that is 442 well suited to use as OPENPGPKEY RDATA. See Appendix A. 444 7. Security Considerations 446 DNSSEC is not an alternative for the "web of trust" or for manual 447 fingerprint verification by users. DANE for OpenPGP as specified in 448 this document is a solution aimed to ease obtaining someone's public 449 key. Without manual verification of the OpenPGP key obtained via 450 DANE, this retrieved key should only be used for encryption if the 451 only other alternative is sending the message in plaintext. While 452 this thwarts all passive attacks that simply capture and log all 453 plaintext email content, it is not a security measure against active 454 attacks. A user who publishes an OPENPGPKEY record in DNS still 455 expects senders to perform their due diligence by additional (non- 456 DNSSEC) verification of their public key via other out-of-band 457 methods before sending any confidential or sensitive information. 459 In other words, the OPENPGPKEY record MUST NOT be used to send 460 sensitive information without additional verification or confirmation 461 that the OpenPGP key actually belongs to the target recipient. 463 Various components could be responsible for encrypting an email 464 message to a target recipient. It could be done by the sender's MUA 465 or a MUA plugin or the sender's MTA. Each of these have their own 466 characteristics. A MUA can ask the user to make a decision before 467 continuing. The MUA can either accept or refuse a message. The MTA 468 must deliver the message as-is, or encrypt the message before 469 delivering. Each of these components should attempt to encrypt an 470 unencrypted outgoing message whenever possible. 472 In theory, two different local-parts could hash to the same value. 473 This document assumes that such a hash collision has a negliable 474 chance of happening. 476 Organisations that are required to be able to read everyone's 477 encrypted email should publish the escrow key as the OPENPGPKEY 478 record. Mail servers of such organizations MAY optionally re-encrypt 479 the message to the individual's OpenPGP key. 481 7.1. MTA behaviour 483 An MTA could be operating in a stand-alone mode, without access to 484 the sender's OpenPGP public keyring, or in a way where it can access 485 the user's OpenPGP public keyring. Regardless, the MTA MUST NOT 486 modify the user's OpenPGP keyring. 488 An MTA sending an email MUST NOT add the public key obtained from an 489 OPENPGPKEY resource record to a permanent public keyring for future 490 use beyond the TTL. 492 If the obtained public key is revoked, the MTA MUST NOT use the key 493 for encryption, even if that would result in sending the message in 494 plaintext. 496 If a message is already encrypted, the MTA SHOULD NOT re-encrypt the 497 message, even if different encryption schemes or different encryption 498 keys would be used. 500 If the DNS request for an OPENPGPKEY record returned an Indeterminate 501 or Bogus answer as specified in [RFC4035], the MTA MUST NOT send the 502 message and queue the plaintext message for encrypted delivery at a 503 later time. If the problem persists, the email should be returned 504 via the regular bounce methods. 506 If multiple non-revoked OPENPGPKEY resource records are found, the 507 MTA SHOULD pick the most secure RR based on its local policy. 509 7.2. MUA behaviour 511 If the public key for a recipient obtained from the locally stored 512 sender's public keyring differs from the recipient's OPENPGPKEY RR, 513 the MUA SHOULD halt processing the message and interact with the user 514 to resolve the conflict before continuing to process the message. 516 If the public key for a recipient obtained from the locally stored 517 sender's public keyring contains contradicting properties for the 518 same key obtained from an OPENPGPKEY RR, the MUA SHOULD NOT accept 519 the message for delivery. 521 If multiple non-revoked OPENPGPKEY resource records are found, the 522 MUA SHOULD pick the most secure OpenPGP public key based on its local 523 policy. 525 The MUA MAY interact with the user to resolve any conflicts between 526 locally stored keyrings and OPENPGPKEY RRdata. 528 A MUA that is encrypting a message SHOULD clearly indicate to the 529 user the difference between encrypting to a locally stored and 530 previously user-verified public key and encrypting to public key 531 obtained via an OPENPGPKEY resource record that was not manually 532 verified by the user in the past. 534 7.3. Response size 536 To prevent amplification attacks, an Authoritative DNS server MAY 537 wish to prevent returning OPENPGPKEY records over UDP unless the 538 source IP address has been confirmed with [EDNS-COOKIE]. Such 539 servers MUST NOT return REFUSED, but answer the query with an empty 540 Answer Section and the truncation flag set ("TC=1"). 542 7.4. Email address information leak 544 The hashing of the local-part in this document is not a security 545 feature. Publishing OPENPGPKEY records however, will create a list 546 of hashes of valid email addresses, which could simplify obtaining a 547 list of valid email addresses for a particular domain. It is 548 desirable to not ease the harvesting of email addresses where 549 possible. 551 The domain name part of the email address is not used as part of the 552 hash so that hashes can be used in multiple zones deployed using 553 DNAME [RFC6672]. This does makes it slightly easier and cheaper to 554 brute-force the SHA2-256 hashes into common and short local-parts, as 555 single rainbow tables can be re-used across domains. This can be 556 somewhat countered by using NSEC3. 558 DNS zones that are signed with DNSSEC using NSEC for denial of 559 existence are susceptible to zone-walking, a mechanism that allows 560 someone to enumerate all the OPENPGPKEY hashes in a zone. This can 561 be used in combination with previously hashed common or short local- 562 parts (in rainbow tables) to deduce valid email addresses. DNSSEC- 563 signed zones using NSEC3 for denial of existence instead of NSEC are 564 significantly harder to brute-force after performing a zone-walk. 566 7.5. Storage of OPENPGPKEY data 568 Users may have a local key store with OpenPGP public keys. An 569 application supporting the use of OPENPGPKEY DNS records MUST NOT 570 modify the local key store without explicit confirmation of the user, 571 as the application is unaware of the user's personal policy for 572 adding, removing or updating their local key store. An application 573 MAY warn the user if an OPENPGPKEY record does not match the OpenPGP 574 public key in the local key store. 576 Applications that cannot interact with users, such as daemon 577 processes, SHOULD store OpenPGP public keys obtained via OPENPGPKEY 578 up to their DNS TTL value. This avoids repeated DNS lookups that 579 third parties could monitor to determine when an email is being sent 580 to a particular user. 582 7.6. Security of OpenPGP versus DNSSEC 584 Anyone who can obtain a DNSSEC private key of a domain name via 585 coercion, theft or brute force calculations, can replace any 586 OPENPGPKEY record in that zone and all of the delegated child zones. 587 Any future messages encrypted with the malicious OpenPGP key could 588 then be read. 590 Therefore, an OpenPGP key obtained via a DNSSEC validated OPENPGPKEY 591 record can only be trusted as much as the DNS domain can be trusted, 592 and is no substitute for in-person OpenPGP key verification or 593 additional Openpgp verification via "Web Of Trust" signatures present 594 on the OpenPGP in question. 596 8. Implementation Status 598 [RFC Editor Note: Please remove this entire seciton prior to 599 publication as an RFC.] 601 This section records the status of known implementations of the 602 protocol defined by this specification at the time of posting of this 603 Internet-Draft, and is based on a proposal described in [RFC6982]. 604 The description of implementations in this section is intended to 605 assist the IETF in its decision processes in progressing drafts to 606 RFCs. Please note that the listing of any individual implementation 607 here does not imply endorsement by the IETF. Furthermore, no effort 608 has been spent to verify the information presented here that was 609 supplied by IETF contributors. This is not intended as, and must not 610 be construed to be, a catalog of available implementations or their 611 features. Readers are advised to note that other implementations may 612 exist. According to RFC 6982, "this will allow reviewers and working 613 groups to assign due consideration to documents that have the benefit 614 of running code, which may serve as evidence of valuable 615 experimentation and feedback that have made the implemented protocols 616 more mature. It is up to the individual working groups to use this 617 information as they see fit." 619 8.1. The GNU Privacy Guard (GNUpg) 620 Implementation Name and Details: The GNUpg software, more commonly 621 known as "gpg", is is available at https://gnupg.org/ 623 Brief Description: Support has been added to gnupg in their git 624 repository. This code is expected to be part of the next official 625 release. 627 Level of Maturity: The implementation has just been added and has 628 not seen widespread deployment. 630 Coverage: The implementation follows the latest draft with the 631 exception that it first performs a lowercase of the local-part 632 before hashing. This is done because other parts in the code that 633 perform a lookup of uid already performed a localcasing to ensure 634 case insensitivity. The implementors are tracking the development 635 of this draft in particular with respect to the lowercase issue. 637 Licensing: All code is covered under the GNU Public License version 638 3 or later. 640 Implementation Experience: Currrent experience limited to small test 641 networks only 643 Contact Information: https://gnupg.org/ 645 Interoperability: No report. 647 8.2. hash-slinger 649 Implementation Name and Details: The hash-slinger software is a 650 collection of tools to generate, download and verify application 651 public keys and application fingerprints. It uses DNSSEC 652 validation. The tool is written by the author of this document. 653 It is available at http://people.redhat.com/pwouters/ 655 Brief Description: Support has been added in the form of an 656 "openpgpkey" command that can generate, fetch, validate the DNSSEC 657 authentication and verify OPENPGPKEY records. 659 Level of Maturity: The implementation has been around for a few 660 months but has not seen widespread deployment. 662 Coverage: The implementation follows the latest draft with the 663 exception that it first performs a lowercase of the local-part 664 before hashing. 666 Licensing: All code is covered under the GNU Public License version 667 3 or later. 669 Implementation Experience: Currrent experience limited to small test 670 networks only 672 Contact Information: pwouters@redhat.com 674 Interoperability: No report. 676 8.3. openpgpkey-milter 678 Implementation Name and Details: The openpgpkey-milter is a Postfix 679 and Sendmail Mail server plugin (milter) that automatically 680 encrypts email before sending further to other SMTP servers. It 681 is written by the author of this document. It is available at 682 http://github.com/letoams/openpgpkey-milter/ 684 Brief Description: Before forwarding an unencrypted email, the 685 plugin looks for the presence of an OPENPGPKEY record. When 686 available, it will encrypt the email message and send out the 687 encrypted email. 689 Level of Maturity: The implementation has been around for a few 690 months but has not seen widespread deployment. 692 Coverage: The implementation follows the latest draft with the 693 exception that it first performs a lowercase of the local-part 694 before hashing. 696 Licensing: All code is covered under the GNU Public License version 697 3 or later. 699 Implementation Experience: Currrent experience limited to small test 700 networks only 702 Contact Information: pwouters@redhat.com 704 Interoperability: No report. 706 9. IANA Considerations 708 9.1. OPENPGPKEY RRtype 710 This document uses a new DNS RR type, OPENPGPKEY, whose value 61 has 711 been allocated by IANA from the Resource Record (RR) TYPEs 712 subregistry of the Domain Name System (DNS) Parameters registry. 714 10. Acknowledgments 715 This document is based on RFC-4255 and draft-ietf-dane-smime whose 716 authors are Paul Hoffman, Jacob Schlyter and W. Griffin. Olafur 717 Gudmundsson provided feedback and suggested various improvements. 718 Willem Toorop contributed the gpg and hexdump command options. 719 Daniel Kahn Gillmor provided the text describing the OpenPGP packet 720 formats and filtering options. Edwin Taylor contributed language 721 improvements for various iterations of this document. Text regarding 722 email mappings was taken from draft-levine-dns-mailbox whose author 723 is John Levine. 725 11. References 727 11.1. Normative References 729 [RFC1035] Mockapetris, P., "Domain names - implementation and 730 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 731 November 1987, . 733 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 734 Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/ 735 RFC2119, March 1997, 736 . 738 [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS 739 Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997, 740 . 742 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 743 Rose, "DNS Security Introduction and Requirements", RFC 744 4033, DOI 10.17487/RFC4033, March 2005, 745 . 747 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. 748 Rose, "Resource Records for the DNS Security Extensions", 749 RFC 4034, DOI 10.17487/RFC4034, March 2005, 750 . 752 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 753 Rose, "Protocol Modifications for the DNS Security 754 Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005, 755 . 757 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data 758 Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006, 759 . 761 [RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. 762 Thayer, "OpenPGP Message Format", RFC 4880, DOI 10.17487/ 763 RFC4880, November 2007, 764 . 766 [RFC5754] Turner, S., "Using SHA2 Algorithms with Cryptographic 767 Message Syntax", RFC 5754, DOI 10.17487/RFC5754, January 768 2010, . 770 11.2. Informative References 772 [EDNS-COOKIE] 773 Eastlake, Donald., "Domain Name System (DNS) Cookies", 774 draft-ietf-dnsop-cookies (work in progress), August 2015. 776 [HKP] Shaw, D., "The OpenPGP HTTP Keyserver Protocol (HKP)", 777 draft-shaw-openpgp-hkp (work in progress), March 2013. 779 [RFC3597] Gustafsson, A., "Handling of Unknown DNS Resource Record 780 (RR) Types", RFC 3597, DOI 10.17487/RFC3597, September 781 2003, . 783 [RFC4398] Josefsson, S., "Storing Certificates in the Domain Name 784 System (DNS)", RFC 4398, DOI 10.17487/RFC4398, March 2006, 785 . 787 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 788 DOI 10.17487/RFC5321, October 2008, 789 . 791 [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, DOI 792 10.17487/RFC5322, October 2008, 793 . 795 [RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for 796 Internationalized Email", RFC 6530, DOI 10.17487/RFC6530, 797 February 2012, . 799 [RFC6672] Rose, S. and W. Wijngaards, "DNAME Redirection in the 800 DNS", RFC 6672, DOI 10.17487/RFC6672, June 2012, 801 . 803 [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication 804 of Named Entities (DANE) Transport Layer Security (TLS) 805 Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August 806 2012, . 808 [RFC6982] Sheffer, Y. and A. Farrel, "Improving Awareness of Running 809 Code: The Implementation Status Section", RFC 6982, DOI 810 10.17487/RFC6982, July 2013, 811 . 813 [Unicode52] 814 The Unicode Consortium, "The Unicode Standard, Version 815 5.2.0, defined by: "The Unicode Standard, Version 5.2.0", 816 (Mountain View, CA: The Unicode Consortium, 2009. ISBN 817 978-1-936213-00-9).", October 2009. 819 Appendix A. Generating OPENPGPKEY records 821 The commonly available GnuPG software can be used to generate a 822 minimum Transferable Public Key for the RRdata portion of an 823 OPENPGPKEY record: 825 gpg --export --export-options export-minimal,no-export-attributes \ 826 hugh@example.com | base64 828 The --armor or -a option of the gpg command should NOT be used, as it 829 adds additional markers around the armored key. 831 When DNS software reading or signing the zone file does not yet 832 support the OPENPGPKEY RRtype, the Generic Record Syntax of [RFC3597] 833 can be used to generate the RDATA. One needs to calculate the number 834 of octets and the actual data in hexadecimal: 836 gpg --export --export-options export-minimal,no-export-attributes \ 837 hugh@example.com | wc -c 839 gpg --export --export-options export-minimal,no-export-attributes \ 840 hugh@example.com | hexdump -e \ 841 '"\t" /1 "%.2x"' -e '/32 "\n"' 843 These values can then be used to generate a generic record (line 844 break has been added for formatting): 846 ._openpgpkey.example.com. IN TYPE61 \# \ 847 849 The openpgpkey command in the hash-slinger software can be used to 850 generate complete OPENPGPKEY records 852 ~> openpgpkey --output rfc hugh@example.com 853 c9[..]d6._openpgpkey.example.com. IN OPENPGPKEY mQCNAzIG[...] 855 ~> openpgpkey --output generic hugh@example.com 856 c9[..]d6._openpgpkey.example.com. IN TYPE61 \# 2313 99008d03[...] 858 Author's Address 860 Paul Wouters 861 Red Hat 863 Email: pwouters@redhat.com