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Checking references for intended status: Experimental ---------------------------------------------------------------------------- == Outdated reference: draft-ietf-dnsop-cookies has been published as RFC 7873 Summary: 0 errors (**), 0 flaws (~~), 2 warnings (==), 1 comment (--). 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 August 28, 2015 5 Expires: February 29, 2016 7 Using DANE to Associate OpenPGP public keys with email addresses 8 draft-ietf-dane-openpgpkey-05 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. This document specifies a method for publishing and 15 locating OpenPGP public keys in DNS for a specific email address 16 using a new OPENPGPKEY DNS Resource Record. Security is provided via 17 DNSSEC. 19 Status of This Memo 21 This Internet-Draft is submitted in full conformance with the 22 provisions of BCP 78 and BCP 79. 24 Internet-Drafts are working documents of the Internet Engineering 25 Task Force (IETF). Note that other groups may also distribute 26 working documents as Internet-Drafts. The list of current Internet- 27 Drafts is at http://datatracker.ietf.org/drafts/current/. 29 Internet-Drafts are draft documents valid for a maximum of six months 30 and may be updated, replaced, or obsoleted by other documents at any 31 time. It is inappropriate to use Internet-Drafts as reference 32 material or to cite them other than as "work in progress." 34 This Internet-Draft will expire on February 29, 2016. 36 Copyright Notice 38 Copyright (c) 2015 IETF Trust and the persons identified as the 39 document authors. All rights reserved. 41 This document is subject to BCP 78 and the IETF Trust's Legal 42 Provisions Relating to IETF Documents 43 (http://trustee.ietf.org/license-info) in effect on the date of 44 publication of this document. Please review these documents 45 carefully, as they describe your rights and restrictions with respect 46 to this document. Code Components extracted from this document must 47 include Simplified BSD License text as described in Section 4.e of 48 the Trust Legal Provisions and are provided without warranty as 49 described in the Simplified BSD License. 51 Table of Contents 53 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 54 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 55 2. The OPENPGPKEY Resource Record . . . . . . . . . . . . . . . 3 56 2.1. The OPENPGPKEY RDATA component . . . . . . . . . . . . . 4 57 2.1.1. The OPENPGPKEY RDATA content . . . . . . . . . . . . 4 58 2.1.2. Reducing the Transferable Public Key size . . . . . . 5 59 2.2. The OPENPGPKEY RDATA wire format . . . . . . . . . . . . 5 60 2.3. The OPENPGPKEY RDATA presentation format . . . . . . . . 5 61 3. Location of the OPENPGPKEY record . . . . . . . . . . . . . . 5 62 4. Email address variants . . . . . . . . . . . . . . . . . . . 6 63 5. Application use of OPENPGPKEY . . . . . . . . . . . . . . . . 7 64 5.1. Obtaining an OpenPGP key for a specific email address . . 7 65 5.2. Confirming the validity of an OpenPGP key . . . . . . . . 7 66 5.3. Verifying an unknown OpenPGP signature . . . . . . . . . 7 67 6. OpenPGP Key size and DNS . . . . . . . . . . . . . . . . . . 7 68 7. Security Considerations . . . . . . . . . . . . . . . . . . . 8 69 7.1. Response size . . . . . . . . . . . . . . . . . . . . . . 8 70 7.2. Email address information leak . . . . . . . . . . . . . 8 71 7.3. Storage of OPENPGPKEY data . . . . . . . . . . . . . . . 9 72 7.4. Forward security of OpenPGP versus DNSSEC . . . . . . . . 9 73 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 74 8.1. OPENPGPKEY RRtype . . . . . . . . . . . . . . . . . . . . 10 75 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10 76 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 77 10.1. Normative References . . . . . . . . . . . . . . . . . . 10 78 10.2. Informative References . . . . . . . . . . . . . . . . . 11 79 Appendix A. Generating OPENPGPKEY records . . . . . . . . . . . 11 80 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12 82 1. Introduction 84 OpenPGP [RFC4880] public keys are used to encrypt or sign email 85 messages and files. To encrypt an email message, or verify a 86 sender's OpenPGP signature, the email client or MTA needs to locate 87 the recipient's OpenPGP public key. 89 OpenPGP clients have relied on centralized "well-known" key servers 90 that are accessed using either the HTTP Keyserver Protocol [HKP] 91 Alternatively, users need to manually browse a variety of different 92 front-end websites. These key servers do not validate the email 93 address in the User ID of the uploaded OpenPGP public key. Attackers 94 can - and have - uploaded rogue public keys with other people's email 95 addresses to these key servers. 97 Once uploaded, public keys cannot be deleted. People who did not 98 pre-sign a key revocation can never remove their OpenPGP public key 99 from these key servers once they have lost access to their private 100 key. This results in receiving encrypted email that cannot be 101 decrypted. 103 Therefor, these keyservers are not well suited to support email 104 clients and MTA's to automatically encrypt email - especially in the 105 absence of an interactive user. 107 This document describes a mechanism to associate a user's OpenPGP 108 public key with their email address, using the OPENPGPKEY DNS RRtype. 109 These records are published in the DNS zone of the user's email 110 address. If the user loses their private key, the OPENPGPKEY DNS 111 record can simply be updated or removed from the zone. 113 The proposed new DNS Resource Record type is secured using DNSSEC. 114 This trust model is not meant to replace the Web Of Trust model. 116 1.1. Terminology 118 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 119 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 120 document are to be interpreted as described in RFC 2119 [RFC2119]. 122 This document also makes use of standard DNSSEC and DANE terminology. 123 See DNSSEC [RFC4033], [RFC4034], [RFC4035], and DANE [RFC6698] for 124 these terms. 126 2. The OPENPGPKEY Resource Record 128 The OPENPGPKEY DNS resource record (RR) is used to associate an end 129 entity OpenPGP Transferable Public Key (see Section 11.1 of [RFC4880] 130 with an email address, thus forming a "OpenPGP public key 131 association". A user that wishes to specify more than one OpenPGP 132 key, for example because they are transitioning to a newer stronger 133 key, can do so by adding multiple OPENPGPKEY records. A single 134 OPENPGPKEY DNS record MUST only contain one OpenPGP key. 136 The type value allocated for the OPENPGPKEY RR type is 61. The 137 OPENPGPKEY RR is class independent. The OPENPGPKEY RR has no special 138 TTL requirements. 140 2.1. The OPENPGPKEY RDATA component 142 The RDATA portion of an OPENPGPKEY Resource Record contains a single 143 value consisting of a [RFC4880] formatted Transferable Public Key. 145 2.1.1. The OPENPGPKEY RDATA content 147 An OpenPGP Transferable Public Key can be arbitrarily large. DNS 148 records are limited in size. When creating OPENPGPKEY DNS records, 149 the OpenPGP Transferable Public Key should be filtered to only 150 contain appropriate and useful data. At a minimum, an OPENPGPKEY 151 Transferable Public Key for the user hugh@example.com should contain: 153 o The primary key X 154 o One User ID Y, which SHOULD match 'hugh@example.com' 155 o self-signature from X, binding X to Y 157 If the primary key is not encryption-capable, a relevant subkey 158 should be included resulting in an OPENPGPKEY Transferable Public Key 159 containing: 161 o The primary key X 162 o One User ID Y, which SHOULD match 'hugh@example.com' 163 o self-signature from X, binding X to Y 164 o encryption-capable subkey Z 165 o self-signature from X, binding Z to X 166 o [ other subkeys if relevant ... ] 168 The user can also elect to add a few third-party certifications which 169 they believe would be helpful for validation in the traditional Web 170 Of Trust. The resulting OPENPGPKEY Transferable Public Key would 171 then look like: 173 o The primary key X 174 o One User ID Y, which SHOULD match 'hugh@example.com' 175 o self-signature from X, binding X to Y 176 o third-party certification from V, binding Y to X 177 o [ other third-party certifications if relevant ... ] 178 o encryption-capable subkey Z 179 o self-signature from X, binding Z to X 180 o [ other subkeys if relevant ... ] 182 2.1.2. Reducing the Transferable Public Key size 184 When preparing a Transferable Public Key for a specific OPENPGPKEY 185 RDATA format with the goal of minimizing certificate size, a user 186 would typically want to: 188 o Where one User ID from the certifications matches the looked-up 189 address, strip away non-matching User IDs and any associated 190 certifications (self-signatures or third-party certifications) 192 o Strip away all User Attribute packets and associated 193 certifications. 195 o Strip away all expired subkeys. The user may want to keep revoked 196 subkeys if these were revoked prior to their preferred expiration 197 time to ensure that correspondents know about these earlier then 198 expected revocations. 200 o Strip away all but the most recent self-sig for the remaining user 201 IDs and subkeys 203 o Optionally strip away any uninteresting or unimportant third-party 204 User ID certifications. This is a value judgment by the user that 205 is difficult to automate. At the very least, expired and 206 superseded third-party certifcations should be stripped out. The 207 user should attempt to keep the most recent and most well 208 connected certifications in the Web Of Trust in their Transferable 209 Public Key. 211 2.2. The OPENPGPKEY RDATA wire format 213 The RDATA Wire Format consists of a single OpenPGP Transferable 214 Public Key as defined in Section 11.1 of [RFC4880]. Note that this 215 format is without ASCII armor or base64 encoding. 217 2.3. The OPENPGPKEY RDATA presentation format 219 The RDATA Presentation Format, as visible in textual zone files, 220 consists of a single OpenPGP Transferable Public Key as defined in 221 Section 11.1 of [RFC4880] encoded in base64 as defined in Section 4 222 of [RFC4648]. 224 3. Location of the OPENPGPKEY record 226 The DNS does not allow the use of all characters that are supported 227 in the "local-part" of email addresses as defined in [RFC5322] and 228 [RFC6530]. Therefore, email addresses are mapped into DNS using the 229 following method: 231 o The user name (the "left-hand side" of the email address, called 232 the "local-part" in the mail message format definition [RFC5322] 233 and the local-part in the specification for internationalized 234 email [RFC6530]) should already be encoded in UTF-8 (or its subset 235 ASCII). If it is written in another encoding it should be 236 converted to UTF-8 and then hashed using the SHA2-256 [RFC5754] 237 algorithm, with the hash truncated to 28 octets and represented in 238 its hexadecimal representation, to become the left-most label in 239 the prepared domain name. Truncation comes from the right-most 240 octets. This does not include the at symbol ("@") that separates 241 the left and right sides of the email address. 243 o The string "_openpgpkey" becomes the second left-most label in the 244 prepared domain name. 246 o The domain name (the "right-hand side" of the email address, 247 called the "domain" in RFC 5322) is appended to the result of step 248 2 to complete the prepared domain name. 250 For example, to request an OPENPGPKEY resource record for a user 251 whose email address is "hugh@example.com", an OPENPGPKEY query would 252 be placed for the following QNAME: "c93f1e400f26708f98cb19d936620da35 253 eec8f72e57f9eec01c1afd6._openpgpkey.example.com". The corresponding 254 RR in the example.com zone might look like (key shortened for 255 formatting): 257 c9[..]d6._openpgpkey.example.com. IN OPENPGPKEY 259 4. Email address variants 261 Mail systems usually handle variant forms of local-parts. The most 262 common variants are upper and lower case, often automatically 263 corrected when a name is recognized as such. Other variants include 264 systems that ignore "noise" characters such as dots, so that local 265 parts johnsmith and John.Smith would be equivalent. Many systems 266 allow "extensions" such as john-ext or mary+ext where john or mary is 267 treated as the effective local-part, and the ext is passed to the 268 recipient for further handling. This can complicate finding the 269 OPENPGPKEY record associated with the dynamically created email 270 address. 272 [RFC5321] and its predecessors have always made it clear that only 273 the recipient MTA is allowed to interpret the local-part of an 274 address. A client supporting OPENPGPKEY therefor MUST NOT perform 275 any kind of mapping rules based on the email address. 277 5. Application use of OPENPGPKEY 279 The OPENPGPKEY record allows an application or service to obtain or 280 verify an OpenPGP public key. The lookup result MUST pass DNSSEC 281 validation; if validation reaches any state other than "Secure", the 282 verification MUST be treated as a failure. 284 5.1. Obtaining an OpenPGP key for a specific email address 286 If no OpenPGP public keys are known for an email address, an 287 OPENPGPKEY lookup MAY be performed to discover the OpenPGP public key 288 that belongs to a specific email address. This public key can then 289 be used to verify a received signed message or can be used to send 290 out an encrypted email message. An application that confirms the 291 lack of an OPENPGPKEY record SHOULD remember this for some time to 292 avoid sending out a DNS request for each email message that is sent 293 out as this constitutes a privacy leak. 295 5.2. Confirming the validity of an OpenPGP key 297 Locally stored OpenPGP public keys are not automatically refreshed. 298 If the owner of that key creates a new OpenPGP public key, that owner 299 is unable to securely notify all users and applications that have its 300 old OpenPGP public key. Applications and users can perform an 301 OPENPGPKEY lookup to confirm the locally stored OpenPGP public key is 302 still the correct key to use. If verifying a locally stored OpenPGP 303 public key and the OpenPGP public key found through DNS is different 304 from the locally stored OpenPGP public key, the verification MUST be 305 treated as a failure. An application that can interact with the user 306 MAY ask the user for guidance. For privacy reasons, an application 307 MUST NOT attempt to validate a locally stored OpenPGP key using an 308 OPENPGPKEY lookup at every use of that key. 310 5.3. Verifying an unknown OpenPGP signature 312 Storage media can be signed using an OpenPGP public key. Even if the 313 OpenPGP public key is included on the storage media, it needs to be 314 independently validated. OpenPGP public keys contain one or more IDs 315 than can have the syntax of an email address. An application can 316 perform a lookup for an OPENPGPKEY at the expected location for the 317 specific email address to confirm the validity of the OpenPGP public 318 key. Once the key has been validated, all files on the storage media 319 that have been signed by this key can now be verified. 321 6. OpenPGP Key size and DNS 322 Due to the expected size of the OPENPGPKEY record, applications 323 SHOULD use TCP - not UDP - to perform queries for the OPENPGPKEY 324 Resource Record. 326 Although the reliability of the transport of large DNS Resource 327 Records has improved in the last years, it is still recommended to 328 keep the DNS records as small as possible without sacrificing the 329 security properties of the public key. The algorithm type and key 330 size of OpenPGP keys should not be modified to accommodate this 331 section. 333 OpenPGP supports various attributes that do not contribute to the 334 security of a key, such as an embedded image file. It is recommended 335 that these properties are not exported to OpenPGP public keyrings 336 that are used to create OPENPGPKEY Resource Records. Some OpenPGP 337 software, for example GnuPG, have support for a "minimal key export" 338 that is well suited to use as OPENPGPKEY RDATA. See Appendix A. 340 7. Security Considerations 342 OPENPGPKEY usage considerations are published in [OPENPGPKEY-USAGE]. 344 7.1. Response size 346 To prevent amplification attacks, an Authoritative DNS server MAY 347 wish to prevent returning OPENPGPKEY records over UDP unless the 348 source IP address has been verified with [DNS-COOKIES]. Such servers 349 MUST NOT return REFUSED, but answer the query with an empty Answer 350 Section and the truncation flag set ("TC=1"). 352 7.2. Email address information leak 354 The hashing of the user name in this document is not a security 355 feature. Publishing OPENPGPKEY records however, will create a list 356 of hashes of valid email addresses, which could simplify obtaining a 357 list of valid email addresses for a particular domain. It is 358 desirable to not ease the harvesting of email addresses where 359 possible. 361 The domain name part of the email address is not used as part of the 362 hash so that hashes can be used in multiple zones deployed using 363 DNAME [RFC6672]. This does makes it slightly easier and cheaper to 364 brute-force the SHA2-256 hashes into common and short user names, as 365 single rainbow tables can be re-used across domains. This can be 366 somewhat countered by using NSEC3. 368 DNS zones that are signed with DNSSEC using NSEC for denial of 369 existence are susceptible to zone-walking, a mechanism that allows 370 someone to enumerate all the OPENPGPKEY hashes in a zone. This can 371 be used in combination with previously hashed common or short user 372 names (in rainbow tables) to deduce valid email addresses. DNSSEC- 373 signed zones using NSEC3 for denial of existence instead of NSEC are 374 significantly harder to brute-force after performing a zone-walk. 376 7.3. Storage of OPENPGPKEY data 378 Users may have a local key store with OpenPGP public keys. An 379 application supporting the use of OPENPGPKEY DNS records MUST NOT 380 modify the local key store without explicit confirmation of the user, 381 as the application is unaware of the user's personal policy for 382 adding, removing or updating their local key store. An application 383 MAY warn the user if an OPENPGPKEY record does not match the OpenPGP 384 public key in the local key store. 386 Applications that do not have users associated with, such as daemon 387 processes, SHOULD store OpenPGP public keys obtained via OPENPGPKEY 388 up to their DNS TTL value. This avoids repeated DNS lookups that 389 third parties could monitor to determine when an email is being sent 390 to a particular user. If TLS is in use between MTA's, only the DNS 391 lookup could happen unencrypted. 393 7.4. Forward security of OpenPGP versus DNSSEC 395 DNSSEC key sizes are chosen based on the fact that these keys can be 396 rolled with next to no requirement for security in the future. If 397 one doubts the strength or security of the DNSSEC key for whatever 398 reason, one simply rolls to a new DNSSEC key with a stronger 399 algorithm or larger key size. On the other hand, OpenPGP key sizes 400 are chosen based on how many years (or decades) their encryption 401 should remain unbreakable by adversaries that own large scale 402 computational resources. 404 This effectively means that anyone who can obtain a DNSSEC private 405 key of a domain name via coercion, theft or brute force calculations, 406 can replace any OPENPGPKEY record in that zone and all of the 407 delegated child zones, irrespective of the key size of the OpenPGP 408 keypair. Any future messages encrypted with the malicious OpenPGP 409 key could then be read. 411 Therefore, an OpenPGP key obtained via an OPENPGPKEY record can only 412 be trusted as much as the DNS domain can be trusted, and is no 413 substitute for in-person key verification of the "Web of Trust". See 414 [OPENPGPKEY-USAGE] for more in-depth information on safe usage of 415 OPENPGPKEY based OpenPGP keys. 417 8. IANA Considerations 419 8.1. OPENPGPKEY RRtype 421 This document uses a new DNS RR type, OPENPGPKEY, whose value 61 has 422 been allocated by IANA from the Resource Record (RR) TYPEs 423 subregistry of the Domain Name System (DNS) Parameters registry. 425 9. Acknowledgments 427 This document is based on RFC-4255 and draft-ietf-dane-smime whose 428 authors are Paul Hoffman, Jacob Schlyter and W. Griffin. Olafur 429 Gudmundsson provided feedback and suggested various improvements. 430 Willem Toorop contributed the gpg and hexdump command options. 431 Daniel Kahn Gillmor provided the text describing the OpenPGP packet 432 formats and filtering options. Edwin Taylor contributed language 433 improvements for various iterations of this document. Text regarding 434 email mappings was taken from draft-levine-dns-mailbox whose author 435 is John Levine. 437 10. References 439 10.1. Normative References 441 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 442 Requirement Levels", BCP 14, RFC 2119, March 1997. 444 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 445 Rose, "DNS Security Introduction and Requirements", RFC 446 4033, March 2005. 448 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. 449 Rose, "Resource Records for the DNS Security Extensions", 450 RFC 4034, March 2005. 452 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 453 Rose, "Protocol Modifications for the DNS Security 454 Extensions", RFC 4035, March 2005. 456 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data 457 Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006, 458 . 460 [RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. 461 Thayer, "OpenPGP Message Format", RFC 4880, DOI 10.17487/ 462 RFC4880, November 2007, 463 . 465 [RFC5754] Turner, S., "Using SHA2 Algorithms with Cryptographic 466 Message Syntax", RFC 5754, DOI 10.17487/RFC5754, January 467 2010, . 469 10.2. Informative References 471 [DNS-COOKIES] 472 Eastlake, Donald., "Domain Name System (DNS) Cookies", 473 draft-ietf-dnsop-cookies (work in progress), August 2015. 475 [HKP] Shaw, D., "The OpenPGP HTTP Keyserver Protocol (HKP)", 476 draft-shaw-openpgp-hkp (work in progress), March 2013. 478 [OPENPGPKEY-USAGE] 479 Wouters, P., "Usage considerations with the DNS OPENPGPKEY 480 record", draft-ietf-dane-openpgpkey-usage (work in 481 progress), October 2014. 483 [RFC3597] Gustafsson, A., "Handling of Unknown DNS Resource Record 484 (RR) Types", RFC 3597, DOI 10.17487/RFC3597, September 485 2003, . 487 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 488 DOI 10.17487/RFC5321, October 2008, 489 . 491 [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, DOI 492 10.17487/RFC5322, October 2008, 493 . 495 [RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for 496 Internationalized Email", RFC 6530, DOI 10.17487/RFC6530, 497 February 2012, . 499 [RFC6672] Rose, S. and W. Wijngaards, "DNAME Redirection in the 500 DNS", RFC 6672, DOI 10.17487/RFC6672, June 2012, 501 . 503 [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication 504 of Named Entities (DANE) Transport Layer Security (TLS) 505 Protocol: TLSA", RFC 6698, August 2012. 507 Appendix A. Generating OPENPGPKEY records 509 The commonly available GnuPG software can be used to generate a 510 minimum Transferable Public Key for the RRdata portion of an 511 OPENPGPKEY record: 513 gpg --export --export-options export-minimal,no-export-attributes \ 514 hugh@example.com | base64 516 The --armor or -a option of the gpg command should NOT be used, as it 517 adds additional markers around the armored key. 519 When DNS software reading or signing the zone file does not yet 520 support the OPENPGPKEY RRtype, the Generic Record Syntax of [RFC3597] 521 can be used to generate the RDATA. One needs to calculate the number 522 of octets and the actual data in hexadecimal: 524 gpg --export --export-options export-minimal,no-export-attributes \ 525 hugh@example.com | wc -c 527 gpg --export --export-options export-minimal,no-export-attributes \ 528 hugh@example.com | hexdump -e \ 529 '"\t" /1 "%.2x"' -e '/32 "\n"' 531 These values can then be used to generate a generic record (line 532 break has been added for formatting): 534 ._openpgpkey.example.com. IN TYPE61 \# \ 535 537 The openpgpkey command in the hash-slinger software can be used to 538 generate complete OPENPGPKEY records 540 ~> openpgpkey --output rfc hugh@example.com 541 c9[..]d6._openpgpkey.example.com. IN OPENPGPKEY mQCNAzIG[...] 543 ~> openpgpkey --output generic hugh@example.com 544 c9[..]d6._openpgpkey.example.com. IN TYPE61 \# 2313 99008d03[...] 546 Author's Address 547 Paul Wouters 548 Red Hat 550 Email: pwouters@redhat.com