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'OPENPGPKEY' -- Obsolete informational reference (is this intentional?): RFC 2822 (Obsoleted by RFC 5322) Summary: 1 error (**), 0 flaws (~~), 6 warnings (==), 4 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: Standards Track October 27, 2014 5 Expires: April 30, 2015 7 Best Common Practise for using OPENPGPKEY records 8 draft-ietf-dane-openpgpkey-usage-01 10 Abstract 12 The OPENPGPKEY DNS Resource Record can be used to match an email 13 address to an OpenPGP key. This document specifies a Best Common 14 Practise ("BCP") for email clients, MUA's and MTA's for using the 15 OPENPGPKEY DNS Resource Record. 17 Status of This Memo 19 This Internet-Draft is submitted in full conformance with the 20 provisions of BCP 78 and BCP 79. 22 Internet-Drafts are working documents of the Internet Engineering 23 Task Force (IETF). Note that other groups may also distribute 24 working documents as Internet-Drafts. The list of current Internet- 25 Drafts is at http://datatracker.ietf.org/drafts/current/. 27 Internet-Drafts are draft documents valid for a maximum of six months 28 and may be updated, replaced, or obsoleted by other documents at any 29 time. It is inappropriate to use Internet-Drafts as reference 30 material or to cite them other than as "work in progress." 32 This Internet-Draft will expire on April 30, 2015. 34 Copyright Notice 36 Copyright (c) 2014 IETF Trust and the persons identified as the 37 document authors. All rights reserved. 39 This document is subject to BCP 78 and the IETF Trust's Legal 40 Provisions Relating to IETF Documents 41 (http://trustee.ietf.org/license-info) in effect on the date of 42 publication of this document. Please review these documents 43 carefully, as they describe your rights and restrictions with respect 44 to this document. Code Components extracted from this document must 45 include Simplified BSD License text as described in Section 4.e of 46 the Trust Legal Provisions and are provided without warranty as 47 described in the Simplified BSD License. 49 Table of Contents 51 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 52 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 2 53 2. The OPENPGPKEY record presence . . . . . . . . . . . . . . . 2 54 3. OpenPGP public key considerations . . . . . . . . . . . . . . 3 55 3.1. Public Key UIDs and email addresses . . . . . . . . . . . 3 56 3.2. Public Key UIDs and IDNA . . . . . . . . . . . . . . . . 3 57 3.3. Public Key UIDs and synthesized DNS records . . . . . . . 3 58 3.4. OpenPGP Key size and DNS . . . . . . . . . . . . . . . . 4 59 4. Security Considerations . . . . . . . . . . . . . . . . . . . 4 60 4.1. Email address information leak . . . . . . . . . . . . . 4 61 4.2. OpenPGP security and DNSSEC . . . . . . . . . . . . . . . 5 62 4.3. MTA behaviour . . . . . . . . . . . . . . . . . . . . . . 5 63 4.4. MUA behaviour . . . . . . . . . . . . . . . . . . . . . . 6 64 4.5. Email client behaviour . . . . . . . . . . . . . . . . . 6 65 5. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 66 5.1. Normative References . . . . . . . . . . . . . . . . . . 7 67 5.2. Informative References . . . . . . . . . . . . . . . . . 7 68 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8 70 1. Introduction 72 This document describes a Best Current Practise ("BCP") for using 73 OPENPGPKEY DNS Resource Records xref target="OPENPGPKEY"/ in email 74 clients, MUA's and MTA. 76 1.1. Terminology 78 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 79 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 80 document are to be interpreted as described in RFC 2119 [RFC2119]. 82 This document also makes use of standard DNSSEC and DANE terminology. 83 See DNSSEC [RFC4033], [RFC4034], [RFC4035], and DANE [RFC6698] for 84 these terms. 86 2. The OPENPGPKEY record presence 88 A user who publishes an OPENPGPKEY record in DNS explicitly prefers 89 receiving encrypted email over receiving unencrypted email. 91 A user who publishes an OPENPGPKEY record in DNS still expects 92 senders to perform their due diligence by additional verification of 93 their public key via other out-of-band methods before sending any 94 confidential or sensitive information 95 In other words, the OPENPGPKEY record in DNS, without any additional 96 verification, should be used only as an alternative to sending 97 plaintext email. It SHOULD NOT be used to change one's opinion on 98 whether it is safe or appropriate to sent the content via email in 99 the first place. 101 3. OpenPGP public key considerations 103 Once an OPENPGPKEY resource record has been found and the OpenPGP 104 public keyring has been decoded, the right public key must be located 105 inside the keyring. For a public key in the keyring to be usable, 106 the public key has to have a key uid as specified in [RFC4648] that 107 matches the email address for which the OPENPGPKEY RR lookup was 108 performed. 110 3.1. Public Key UIDs and email addresses 112 An OpenPGP public key can be associated with multiple email addresses 113 by specifying multiple key uids. The OpenPGP public key obtained 114 from a OPENPGPKEY RR can be used as long as the target recipient's 115 email address appears as one of the OpenPGP public key uids. The 116 name part (left of the @) should appear in the native format, not its 117 SHA2-224 hash that was used to lookup the OPENPGPKEY RR. 119 3.2. Public Key UIDs and IDNA 121 Internationalized domains that use non-ascii characters (U-label) are 122 encoded in DNS using IDNA [RFC5891] - also referred to as punycode or 123 A-label. When matching OpenPGP public key uids, both the email 124 address specified using U-label and A-label should be considered as 125 valid public key uids. 127 3.3. Public Key UIDs and synthesized DNS records 129 CNAME's (see [RFC2181]) and DNAME's (see [RFC6672]) can be followed 130 to obtain an OPENPGPKEY RR, as long as the original recipient's email 131 address appears as one of the OpenPGP public key uids. For example, 132 if the OPENPGPKEY RR query for hugh@example.com 133 (8d57[...]b7._openpgpkey.example.com) yields a CNAME to 134 8d57[...]b7._openpgpkey.example.net, and an OPENPGPKEY RR for 135 8d57[...]b7._openpgpkey.example.net exists, then this OpenPGP public 136 key can be used, provided one of the key uids contains 137 "hugh@example.com". This public key cannot be used if it would only 138 contain the key uid "hugh@example.net". 140 If one of the OpenPGP key uids contains only a single wildcard as the 141 LHS of the email address, such as "*@example.com", the OpenPGP public 142 key may be used for any email address within that domain. Wildcards 143 at other locations (eg hugh@*.com) or regular expressions in key uids 144 are not allowed, and any OPENPGPKEY RR containing these should be 145 ignored. 147 3.4. OpenPGP Key size and DNS 149 Although the reliability of the transport of large DNS Resoruce 150 Records has improved in the last years, it is still recommended to 151 keep the DNS records as small as possible without sacrificing the 152 security properties of the public key. The algorithm type and key 153 size of OpenPGP keys should not be modified to accomodate this 154 section. 156 OpenPGP supports various attributes that do not contribute to the 157 security of a key, such as an embedded image file. It is recommended 158 that these properties are not exported to OpenPGP public keyrings 159 that are used to create OPENPGPKEY Resource Records. Some OpenPGP 160 software, for example GnuPG, have support for a "minimal key export" 161 that is well suited to use as OPENPGPKEY RDATA. 163 4. Security Considerations 165 The main goal of the OPENPGPKEY resource record is to stop passive 166 attacks against plaintext emails. While it can also twart some 167 active attacks (such as people uploading rogue keys to keyservers in 168 the hopes that others will encrypt to these rogue keys), this 169 resource record is not a replacement for verifying OpenPGP public 170 keys via the web of trust signatures, or manually via a fingerprint 171 verification. 173 Various components could be responsible for encrypting an email 174 message to a target recipient. It could be done by the sender's 175 email client or software plugin, the sender's Mail User Agent (MUA) 176 or the sender's Mail Transfer Agent (MTA). Each of these have their 177 own characteristics. An email client can direct the human to make a 178 decision before continuing. The MUA can either accept or refuse a 179 message. The MTA must deliver the message as-is, or encrypt the 180 message before delivering. Each of these programs should ensure that 181 the security of an email message is never downgraded, and that an 182 unencrypted received message will be encrypted whenever possible. 184 Organisations that require to be able to read everyone's encrypted 185 email should publish the escrow key as the OPENPGPKEY record. Upon 186 receipt, such mail servers can optionally re-encrypt the message to 187 the individual's OpenPGP key. 189 4.1. Email address information leak 190 DNS zones that are signed with DNSSEC using NSEC for denial of 191 existence are susceptible to zone-walking, a mechanism that allow 192 someone to enumerate all the names in the zone. Someone who wanted 193 to collect email addresses from a zone that uses OPENPGPKEY might use 194 such a mechanism. DNSSEC-signed zones using NSEC3 for denial of 195 existence are significantly less susceptible to zone-walking. 196 Someone could still attempt a dictionary attack on the zone to find 197 OPENPGPKEY records, just as they can use dictionary attacks on an 198 SMTP server or grab the entire contents of existing PGP key servers 199 to see which addresses are valid. 201 4.2. OpenPGP security and DNSSEC 203 DNSSEC key sizes are chosen based on the fact that these keys can be 204 rolled with next to no requirement for security in the future. If 205 one doubts the strength or security of the DNSSEC key for whatever 206 reason, one simply rolls to a new DNSSEC key with a stronger 207 algorithm or larger key size. 209 This effectively means that anyone who can obtain a DNSSEC private 210 key of a domain name via coercion, theft or brute force calculations, 211 can replace any OPENPGPKEY record in that zone and all of the 212 delegated child zones, irrespective of the key length strength of the 213 OpenPGP keypair. 215 Therefor, DNSSEC is not an alternative for the "web of trust" or for 216 manual fingerprint verification by humans. It is a solution aimed to 217 ease obtaining someone's public key, and without manual verification 218 should be treated as "better then plaintext" only. While this twarts 219 all passive attacks that simply capture and log all plaintext email 220 content, it is not a security measure against active attacks. 222 4.3. MTA behaviour 224 An MTA could be operating in a stand-alone mode, without access to 225 the sender's OpenPGP public keyring, or in a way where it can access 226 the user's OpenPGP public keyring. Regardless, the MTA MUST NOT 227 modify the user's OpenPGP keyring. 229 An MTA sending an email MUST NOT add the public key obtained from an 230 OPENPGPKEY resource record to a permanent public keyring for future 231 use beyond the TTL. 233 If the obtained public key is revoked, the MTA MUST NOT use the key 234 for encryption, even if that would result in sending the message in 235 plaintext. 237 If a message is already encrypted, the MTA SHOULD NOT re-encrypt the 238 message, even if different encryption schemes or different encryption 239 keys were used. 241 If an OPENPGPKEY resource record is received without DNSSEC 242 protection, it MAY still be used for encryption. 244 If the DNS request for an OPENPGPKEY record returned an 245 "indeterminate" or "bogus" answer, the MTA MUST NOT sent the message 246 and queue the plaintext message for delivery at a later time. If the 247 problem persists, the email should be returned via the regular bounce 248 methods. 250 If multiple non-revoked OPENPGPKEY resource records are found, the 251 MTA SHOULD pick the most secure RR based on its local policy. [or 252 should it encrypt to both?] 254 4.4. MUA behaviour 256 If the public key for a recipient obtained from the locally stored 257 sender's public keyring differs from the recipient's OPENPGPKEY RR, 258 the MUA MUST NOT accept the message for delivery. 260 If the public key for a recipient obtained from the locally stored 261 sender's public keyring contains contradicting properties for the 262 same key obtained from an OPENPGPKEY RR, the MUA SHOULD NOT accept 263 the message for delivery. 265 If multiple non-revoked OPENPGPKEY resource records are found, the 266 MUA SHOULD pick the most secure OpenPGP public key based on its local 267 policy. 269 4.5. Email client behaviour 271 Email clients should adhere to the above listed MUA behaviour. 272 Additionally, an email client MAY interact with the user to resolve 273 any conflicts between locally stored keyrings and OPENPGPKEY RRdata. 275 An email client that is encrypting a message SHOULD clearly indicate 276 to the user the difference between encrypting to a locally stored and 277 humanly verified public key and encrypting to an unverified (by the 278 human sender) public key obtained via an OPENPGPKEY resource record. 280 5. References 282 5.1. Normative References 284 [OPENPGPKEY] 285 Wouters, P., "DANE for OpenPGP public keys", draft-ietf- 286 dane-openpgp (work in progress), October 2014. 288 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 289 Requirement Levels", BCP 14, RFC 2119, March 1997. 291 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 292 Rose, "DNS Security Introduction and Requirements", RFC 293 4033, March 2005. 295 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. 296 Rose, "Resource Records for the DNS Security Extensions", 297 RFC 4034, March 2005. 299 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 300 Rose, "Protocol Modifications for the DNS Security 301 Extensions", RFC 4035, March 2005. 303 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data 304 Encodings", RFC 4648, October 2006. 306 [RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. 307 Thayer, "OpenPGP Message Format", RFC 4880, November 2007. 309 [RFC5891] Klensin, J., "Internationalized Domain Names in 310 Applications (IDNA): Protocol", RFC 5891, August 2010. 312 5.2. Informative References 314 [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS 315 Specification", RFC 2181, July 1997. 317 [RFC2822] Resnick, P., "Internet Message Format", RFC 2822, April 318 2001. 320 [RFC4255] Schlyter, J. and W. Griffin, "Using DNS to Securely 321 Publish Secure Shell (SSH) Key Fingerprints", RFC 4255, 322 January 2006. 324 [RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for 325 Internationalized Email", RFC 6530, February 2012. 327 [RFC6672] Rose, S. and W. Wijngaards, "DNAME Redirection in the 328 DNS", RFC 6672, June 2012. 330 [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication 331 of Named Entities (DANE) Transport Layer Security (TLS) 332 Protocol: TLSA", RFC 6698, August 2012. 334 Author's Address 336 Paul Wouters 337 Red Hat 339 Email: pwouters@redhat.com