idnits 2.17.1 draft-ietf-dkim-deployment-08.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- ** The document seems to lack a License Notice according IETF Trust Provisions of 28 Dec 2009, Section 6.b.i or Provisions of 12 Sep 2009 Section 6.b -- however, there's a paragraph with a matching beginning. Boilerplate error? (You're using the IETF Trust Provisions' Section 6.b License Notice from 12 Feb 2009 rather than one of the newer Notices. See https://trustee.ietf.org/license-info/.) Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- ** The abstract seems to contain references ([RFC4871]), which it shouldn't. Please replace those with straight textual mentions of the documents in question. ** The document seems to lack a both a reference to RFC 2119 and the recommended RFC 2119 boilerplate, even if it appears to use RFC 2119 keywords. RFC 2119 keyword, line 277: '...s to a responsible identity. DKIM MAY...' RFC 2119 keyword, line 279: '...ide DKIM verifier MUST communicate the...' RFC 2119 keyword, line 281: '... module and MAY communicate the U...' RFC 2119 keyword, line 291: '...ide DKIM verifier MUST communicate the...' RFC 2119 keyword, line 293: '... module and MAY communicate the U...' (59 more instances...) Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year == Using lowercase 'not' together with uppercase 'MUST', 'SHALL', 'SHOULD', or 'RECOMMENDED' is not an accepted usage according to RFC 2119. Please use uppercase 'NOT' together with RFC 2119 keywords (if that is what you mean). Found 'SHOULD not' in this paragraph: If key management hardware support is available, it SHOULD be used. If keys are stored in software, appropriate file control protections MUST be employed, and any location in which the private key is stored in plaintext form SHOULD be excluded from regular backup processes and SHOULD not be accessible through any form of network including private local area networks. Auditing software SHOULD be used periodically to verify that the permissions on the private key files remain secure. == Using lowercase 'not' together with uppercase 'MUST', 'SHALL', 'SHOULD', or 'RECOMMENDED' is not an accepted usage according to RFC 2119. Please use uppercase 'NOT' together with RFC 2119 keywords (if that is what you mean). Found 'SHOULD not' in this paragraph: In the most restrictive case a domain holder maintains full control over the creation of key records and employs appropriate key record restrictions to enforce restrictions on the messages for which the third party signer is able to sign. If such restrictions are impractical, the domain holder SHOULD delegate a DNS subzone for publishing key records to the third party signer. The domain holder SHOULD not allow a third party signer unrestricted access to its DNS service for the purpose of publishing key records. -- The document seems to lack a disclaimer for pre-RFC5378 work, but may have content which was first submitted before 10 November 2008. If you have contacted all the original authors and they are all willing to grant the BCP78 rights to the IETF Trust, then this is fine, and you can ignore this comment. If not, you may need to add the pre-RFC5378 disclaimer. (See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (August 25, 2009) is 5356 days in the past. Is this intentional? 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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 DomainKeys Identified Mail T. Hansen 3 Internet-Draft AT&T Laboratories 4 Intended status: Informational E. Siegel 5 Expires: February 26, 2010 6 P. Hallam-Baker 7 Default Deny Security, Inc. 8 D. Crocker 9 Brandenburg InternetWorking 10 August 25, 2009 12 DomainKeys Identified Mail (DKIM) Development, Deployment and Operations 13 draft-ietf-dkim-deployment-08 15 Status of this Memo 17 This Internet-Draft is submitted to IETF in full conformance with the 18 provisions of BCP 78 and BCP 79. 20 Internet-Drafts are working documents of the Internet Engineering 21 Task Force (IETF), its areas, and its working groups. Note that 22 other groups may also distribute working documents as Internet- 23 Drafts. 25 Internet-Drafts are draft documents valid for a maximum of six months 26 and may be updated, replaced, or obsoleted by other documents at any 27 time. It is inappropriate to use Internet-Drafts as reference 28 material or to cite them other than as "work in progress." 30 The list of current Internet-Drafts can be accessed at 31 http://www.ietf.org/ietf/1id-abstracts.txt. 33 The list of Internet-Draft Shadow Directories can be accessed at 34 http://www.ietf.org/shadow.html. 36 This Internet-Draft will expire on February 26, 2010. 38 Copyright Notice 40 Copyright (c) 2009 IETF Trust and the persons identified as the 41 document authors. All rights reserved. 43 This document is subject to BCP 78 and the IETF Trust's Legal 44 Provisions Relating to IETF Documents in effect on the date of 45 publication of this document (http://trustee.ietf.org/license-info). 46 Please review these documents carefully, as they describe your rights 47 and restrictions with respect to this document. 49 Abstract 51 DomainKeys Identified Mail (DKIM) allows an organization to claim 52 responsibility for transmitting a message, in a way that can be 53 validated by a recipient. The organization can be the author's, the 54 originating sending site, an intermediary, or one of their agents. A 55 message can contain multiple signatures, from the same or different 56 organizations involved with the message. DKIM defines a domain-level 57 digital signature authentication framework for email, using public 58 key cryptography, using the domain name service as its key server 59 technology [RFC4871]. This permits verification of a responsible 60 organization, as well as the integrity of the message contents. DKIM 61 will also provide a mechanism that permits potential email signers to 62 publish information about their email signing practices; this will 63 permit email receivers to make additional assessments about messages. 64 DKIM's authentication of email identity can assist in the global 65 control of "spam" and "phishing". This document provides 66 implementation, deployment, operational and migration considerations 67 for DKIM. 69 Table of Contents 71 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 72 2. Using DKIM as Part of Trust Assessment . . . . . . . . . . . . 5 73 2.1. A Systems View of Email Trust Assessment . . . . . . . . . 5 74 2.2. Choosing a DKIM Tag for the Assessment Identifier . . . . 7 75 2.3. Choosing the Signing Domain Name . . . . . . . . . . . . . 9 76 2.4. Recipient-based Assessments . . . . . . . . . . . . . . . 11 77 2.5. Filtering . . . . . . . . . . . . . . . . . . . . . . . . 12 78 3. DKIM Key Generation, Storage, and Management . . . . . . . . . 14 79 3.1. Private Key Management: Deployment and Ongoing 80 Operations . . . . . . . . . . . . . . . . . . . . . . . . 15 81 3.2. Storing Public Keys: DNS Server Software Considerations . 16 82 3.3. Per User Signing Key Management Issues . . . . . . . . . . 17 83 3.4. Third Party Signer Key Management and Selector 84 Administration . . . . . . . . . . . . . . . . . . . . . . 17 85 3.5. Key Pair / Selector Lifecycle Management . . . . . . . . . 18 86 4. Signing . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 87 4.1. DNS Records . . . . . . . . . . . . . . . . . . . . . . . 20 88 4.2. Signing Module . . . . . . . . . . . . . . . . . . . . . . 20 89 4.3. Signing Policies and Practices . . . . . . . . . . . . . . 21 90 5. Verifying . . . . . . . . . . . . . . . . . . . . . . . . . . 21 91 5.1. Intended Scope of Use . . . . . . . . . . . . . . . . . . 21 92 5.2. Signature Scope . . . . . . . . . . . . . . . . . . . . . 22 93 5.3. Design Scope of Use . . . . . . . . . . . . . . . . . . . 22 94 5.4. Inbound Mail Filtering . . . . . . . . . . . . . . . . . . 23 95 5.5. Messages sent through Mailing Lists and other 96 Intermediaries . . . . . . . . . . . . . . . . . . . . . . 23 97 5.6. Generation, Transmission and Use of Results Headers . . . 24 98 6. Taxonomy of Signatures . . . . . . . . . . . . . . . . . . . . 24 99 6.1. Single Domain Signature . . . . . . . . . . . . . . . . . 25 100 6.2. Parent Domain Signature . . . . . . . . . . . . . . . . . 25 101 6.3. Third Party Signature . . . . . . . . . . . . . . . . . . 26 102 6.4. Using Trusted Third Party Senders . . . . . . . . . . . . 27 103 6.5. Multiple Signatures . . . . . . . . . . . . . . . . . . . 28 104 7. Example Usage Scenarios . . . . . . . . . . . . . . . . . . . 30 105 7.1. Author's Organization - Simple . . . . . . . . . . . . . . 30 106 7.2. Author's Organization - Differentiated Types of Mail . . . 30 107 7.3. Author Domain Signing Practices . . . . . . . . . . . . . 31 108 7.4. Delegated Signing . . . . . . . . . . . . . . . . . . . . 32 109 7.5. Independent Third Party Service Providers . . . . . . . . 33 110 7.6. Mail Streams Based on Behavioral Assessment . . . . . . . 34 111 7.7. Agent or Mediator Signatures . . . . . . . . . . . . . . . 34 112 8. Usage Considerations . . . . . . . . . . . . . . . . . . . . . 35 113 8.1. Non-standard Submission and Delivery Scenarios . . . . . . 35 114 8.2. Protection of Internal Mail . . . . . . . . . . . . . . . 36 115 8.3. Signature Granularity . . . . . . . . . . . . . . . . . . 36 116 8.4. Email Infrastructure Agents . . . . . . . . . . . . . . . 37 117 8.5. Mail User Agent . . . . . . . . . . . . . . . . . . . . . 39 118 9. Other Considerations . . . . . . . . . . . . . . . . . . . . . 40 119 9.1. Security Considerations . . . . . . . . . . . . . . . . . 40 120 9.2. IANA Considerations . . . . . . . . . . . . . . . . . . . 40 121 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 40 122 11. Informative References . . . . . . . . . . . . . . . . . . . . 40 123 Appendix A. Migration Strategies . . . . . . . . . . . . . . . . 42 124 A.1. Migrating from DomainKeys . . . . . . . . . . . . . . . . 42 125 A.2. Migrating Hash Algorithms . . . . . . . . . . . . . . . . 47 126 A.3. Migrating Signing Algorithms . . . . . . . . . . . . . . . 48 127 Appendix B. General Coding Criteria for Cryptographic 128 Applications . . . . . . . . . . . . . . . . . . . . 48 129 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 49 131 1. Introduction 133 DomainKeys Identified Mail (DKIM) allows an organization to claim 134 responsibility for transmitting a message, in a way that can be 135 validated by a recipient. This document provides practical tips for: 136 those who are developing DKIM software, mailing list managers, 137 filtering strategies based on the output from DKIM verification, and 138 DNS servers; those who are deploying DKIM software, keys, mailing 139 list software, and migrating from DomainKeys; and those who are 140 responsible for the on-going operations of an email infrastructure 141 that has deployed DKIM. 143 The document is organized around the key concepts related to DKIM. 144 Within each section, additional considerations specific to 145 development, deployment, or ongoing operations are highlighted where 146 appropriate. The possibility of use of DKIM results as input to a 147 local reputation database is also discussed. 149 2. Using DKIM as Part of Trust Assessment 151 2.1. A Systems View of Email Trust Assessment 153 DKIM participates in a trust-oriented enhancement to the Internet's 154 email service, to facilitate message handling decisions, such as for 155 delivery and for content display. Trust-oriented message handling 156 has substantial differences from approaches that consider messages in 157 terms of risk and abuse. With trust, there is a collaborative 158 exchange between a willing participant along the sending path and a 159 willing participant at the recipient site. In contrast, the risk 160 model entails independent action by the recipient site, in the face 161 of a potentially unknown, hostile and deceptive sender. This 162 translates into a very basic technical difference: In the face of 163 unilateral action by the recipient and even antagonistic efforts by 164 the sender, risk-oriented mechanisms will be based on heuristics, 165 that is, on guessing. Guessing produces statistical results with 166 some false negatives and some false positives. For trust-based 167 exchanges, the goal is the deterministic exchange of information. 168 For DKIM, that information is the one identifier that represents a 169 stream of mail for which an independent assessment is sought (by the 170 signer.) 172 A trust-based service is built upon a validated Responsible 173 Identifier that labels a stream of mail and is controlled by an 174 identity (role, person or organization). The identity is 175 acknowledging some degree of responsibility for the message stream. 176 Given a basis for believing that an identifier is being used in an 177 authorized manner, the recipient site can make and use an assessment 178 of the associated identity. An identity can use different 179 identifiers, on the assumption that the different streams might 180 produce different assessments. For example, even the best-run 181 marketing campaigns will tend to produce some complaints that can 182 affect the reputation of the associated identifier, whereas a stream 183 of transactional messages is likely to have a more pristine 184 reputation. 186 Determining that the identifier's use is valid is quite different 187 from determining that the content of a message is valid. The former 188 means only that the identifier for the responsible role, person or 189 organization has been legitimately associated with a message. The 190 latter means that the content of the message can be believed and, 191 typically, that the claimed author of the content is correct. DKIM 192 validates only the presence of the identifier used to sign the 193 message. Even when this identifier is validated, DKIM carries no 194 implication that any of the message content, including the 195 RFC5322.From field, is valid. Surprisingly, this limit to the 196 semantics of a DKIM signature applies even when the validated signing 197 identifier is the same domain name as is used in the From: field! 198 DKIM's only claim about message content is that the content cited in 199 the DKIM-Signature: field's h= tag has been delivered without 200 modification. That is, it asserts message content integrity, not 201 message content validity. 203 As shown in Figure 1, this enhancement is a communication between a 204 responsible role, person or organization that signs the message and a 205 recipient organization that assesses its trust in the signer and then 206 makes handling decisions based on a collection of assessments, of 207 which the DKIM mechanism is only a part. In this model, validation 208 is an intermediary step, having the sole task of passing a validated 209 Responsible Identifier to the Identity Assessor. The communication 210 is of a single Responsible Identifier that the Responsible Identity 211 wishes to have used by the Identity Assessor. The Identifier is the 212 sole, formal input and output value of DKIM signing. The Identity 213 Assessor uses this single, provided Identifier for consulting 214 whatever assessment data bases are deemed appropriate by the 215 assessing entity. In turn, output from the Identity Assessor is fed 216 into a Handling Filter engine that considers a range of factors, 217 along with this single output value; the range of factors can include 218 ancillary information from the DKIM validation. 220 Identity Assessment covers a range of possible functions. It can be 221 as simple as determining whether the identifier is a member of some 222 list, such as authorized operators or participants in a group that 223 might be of interest for recipient assessment. Equally, it can 224 indicate a degree of trust (reputation) that is to be afforded the 225 actor using that identifier. The extent to which the assessment 226 affects handling of the message is, of course, determined later, by 227 the Handling Filter. 229 +------+------+ +------+------+ 230 | Author | | Recipient | 231 +------+------+ +------+------+ 232 | ^ 233 | | 234 | +------+------+ 235 | -->| Handling |<-- 236 | -->| Filter |<-- 237 | +-------------+ 238 | ^ 239 V Responsible | 240 +-------------+ Identifier +------+------+ 241 | Responsible |. . . . . . . . . . .>| Identity | 242 | Identity | . . | Assessor | 243 +------+------+ . . +-------------+ 244 | . . ^ ^ 245 V . . | | 246 +------------------.-------.--------------------+ | | 247 | +------+------+ . . . . . +-------------+ | | | +-----------+ 248 | | Identifier | | Identifier +--|--+ +--+ Assessment| 249 | | Signer +------------->| Validator | | | Databases | 250 | +-------------+ +-------------+ | +-----------+ 251 | DKIM Service | 252 +-----------------------------------------------+ 254 Figure 1: Actors in a Trust Sequence using DKIM 256 2.2. Choosing a DKIM Tag for the Assessment Identifier 258 The signer of a message needs to be able to provide precise data and 259 know what that data will mean upon delivery to the Assessor. If 260 there is ambiguity in the choice that will be made on the receive 261 side, then the sender cannot know what basis for assessment will be 262 used. DKIM has three values that specify identification information 263 and it is easy to confuse their use, although only one defines the 264 formal input and output of DKIM, with the other two being used for 265 internal protocol functioning and adjunct purposes, such as auditing 266 and debugging. 268 The salient values include the s=, d= and i= parameters in the DKIM- 269 Signature: header field. In order to achieve the end-to-end 270 determinism needed for this collaborative exchange from the signer to 271 the assessor, the core model needs to specify what the signer is 272 required to provide to the assessor. The Update to RFC4871 273 [I-D.ietf-dkim-rfc4871-errata]now specifies: 275 DKIM's primary task is to communicate from the Signer to a 276 recipient-side Identity Assessor a single Signing Domain 277 Identifier (SDID) that refers to a responsible identity. DKIM MAY 278 optionally provide a single responsible Agent or User Identifier 279 (AUID)... A receive-side DKIM verifier MUST communicate the 280 Signing Domain Identifier (d=) to a consuming Identity Assessor 281 module and MAY communicate the User Agent Identifier (i=) if 282 present.... To the extent that a receiver attempts to intuit any 283 structured semantics for either of the identifiers, this is a 284 heuristic function that is outside the scope of DKIM's 285 specification and semantics. 287 The single, mandatory value that DKIM supplies as its output is: 289 d= This specifies the "domain of the signing entity." It is a 290 domain name and is combined with the Selector to form a DNS 291 query... A receive-side DKIM verifier MUST communicate the 292 Signing Domain Identifier (d=) to a consuming Identity Assessor 293 module and MAY communicate the User Agent Identifier (i=) if 294 present. 296 The adjunct values are: 298 s= This tag specifies the Selector. It is used to discriminate 299 among different keys that can be used for the same d= domain 300 name. As discussed in Section 4.3 of [RFC5585]: "If verifiers 301 were to employ the selector as part of a name assessment 302 mechanism, then there would be no remaining mechanism for 303 making a transition from an old, or compromised, key to a new 304 one." Consequently, the Selector is not appropriate for use as 305 part or all of the identifier used to make assessments. 307 i= This tag is optional and provides the "[i]dentity of the 308 user or agent (e.g., a mailing list manager) on behalf of which 309 this message is signed." The identity can be in the syntax of 310 an entire email address or only a domain name. The domain name 311 can be the same as for d= or it can be a sub-name of the d= 312 name. 314 NOTE: Although the i= identity has the syntax of an email 315 address, it is not required to have that semantics. That is, 316 "the identity of the user" need not be the same as the user's 317 mailbox. For example the signer might wish to use i= to encode 318 user-related audit information, such as how they were accessing 319 the service at the time of message posting. Therefore it is 320 not possible to conclude anything from the i= string's 321 (dis)similarity to email addresses elsewhere in the header 323 So, i= can have any of these properties: 325 * Be a valid domain when it is the same as d= 327 * Appear to be a sub-domain of d= but might not even exist 329 * Look like a mailbox address but might have different semantics 330 and therefore not function as a valid email address 332 * Be unique for each message, such as indicating access details 333 of the user for the specific posting 335 This underscores why the tag needs to be treated as being opaque, 336 since it can represent any semantics, known only to the signer. 338 Hence, i= serves well as a token that is usable like a Web cookie, 339 for return to the signing ADMD -- such as for auditing and debugging. 340 Of course in some scenarios the i= string might provide a useful 341 adjunct value for additional (heuristic) processing by the Handling 342 Filter. 344 2.3. Choosing the Signing Domain Name 346 A DKIM signing entity can serve different roles, such as being the 347 author of content, or the operator of the mail service, or the 348 operator of a reputation service that also provides signing services 349 on behalf of its customers. In these different roles, the basis for 350 distinguishing among portions of email traffic can vary. For an 351 entity creating DKIM signatures it is likely that different portions 352 of its mail will warrant different levels of trust. For example: 354 * Mail is sent for different purposes, such as marketing vs. 355 transactional, and recipients demonstrate different patterns of 356 acceptance between these. 358 * For an operator of an email service, there often are distinct 359 sub-populations of users warranting different levels of trust 360 or privilege, such as paid vs. free users, or users engaged in 361 direct correspondence vs. users sending bulk mail. 363 * Mail originating outside an operator's system, such as when it 364 is redistributed by a mailing list service run by the operator, 365 will warrant a different reputation from mail submitted by 366 users authenticated with the operator. 368 It is therefore likely to be useful for a signer to use different d= 369 sub-domain names, for different message traffic streams, so that 370 receivers can make differential assessments. However, too much 371 differentiation -- that is, too fine a granularity of signing domains 372 -- makes it difficult for the receiver to discern a sufficiently 373 stable pattern of traffic for developing an accurate and reliable 374 assessment. So the differentiation needs to achieve a balance. 375 Generally in a trust system, legitimate signers have an incentive to 376 pick a small stable set of identities, so that recipients and others 377 can attribute reputations to them. The set of these identities a 378 receiver trusts is likely to be quite a bit smaller than the set it 379 views as risky. 381 The challenge in using additional layers of sub-domains is whether 382 the extra granularity will be useful for the assessor. In fact, 383 potentially excessive levels invites ambiguity: if the assessor does 384 not take advantage of the added granularity, then what granularity 385 will it use? That ambiguity would move the use of DKIM back to the 386 realm of heuristics, rather than the deterministic processing that is 387 its goal. 389 Hence the challenge is to determine a useful scheme for labeling 390 different traffic streams. The most obvious choices are among 391 different types of content and/or different types of authors. 392 Although stability is essential, it is likely that the choices will 393 change, over time, so the scheme needs to be flexible. 395 For those originating message content, the most likely choice of sub- 396 domain naming scheme will by based upon type of content, which can 397 use content-oriented labels or service-oriented labels. For example: 399 transaction.example.com 400 newsletter.example.com 401 bugreport.example.com 402 support.example.com 403 sales.example.com 404 marketing.example.com 406 where the choices are best dictated by whether they provide the 407 Identity Assessor with the ability to discriminate usefully among 408 streams of mail that demonstrate significantly different degrees of 409 recipient acceptance or safety. Again, the danger in providing too 410 fine a granularity is that related message streams that are labeled 411 separately will not benefit from an aggregate reputation. 413 For those operating messaging services on behalf of a variety of 414 customers, an obvious scheme to use has a different sub-domain label 415 for each customer. For example: 417 widgetco.example.net 418 moviestudio.example.net 419 bigbank.example.net 421 However it can also be appropriate to label by the class of service 422 or class of customer, such as: 424 premier.example.net 425 free.example.net 426 certified.example.net 428 Prior to using domain names for distinguishing among sources of data, 429 IP Addresses have been the basis for distinction. Service operators 430 typically have done this by dedicating specific outbound IP Addresses 431 to specific mail streams -- typically to specific customers. For 432 example, a university might want to distinguish mail from the 433 Administration, versus mail from the student dorms. In order to make 434 adoption of a DKIM-based service easier, it can be reasonable to 435 translate the same partitioning of traffic, using domain names in 436 place of the different IP Addresses. 438 2.4. Recipient-based Assessments 440 DKIM gives the recipient site's Identity Assessor a verifiable 441 identifier to use for analysis. Although the mechanism does not make 442 claims that the signer is a Good Actor or a Bad Actor, it does make 443 it possible to know that use of the identifier is valid. This is in 444 marked contrast with schemes that do not have authentication. 445 Without verification, it is not possible to know whether the 446 identifier -- whether taken from the RFC5322.From field, 447 RFC5321.MailFrom command, or the like -- is being used by an 448 authorized agent. DKIM solves this problem. Hence with DKIM, the 449 Assessor can know that two messages with the same DKIM d= identifier 450 are, in fact, signed by the same person or organization. This 451 permits a far more stable and accurate assessment of mail traffic 452 using that identifier. 454 DKIM is distinctive, in that it provides an identifier which is not 455 necessarily related to any other identifier in the message. Hence, 456 the signer might be the author's ADMD, one of the operators along the 457 transit path, or a reputation service being used by one of those 458 handling services. In fact, a message can have multiple signatures, 459 possibly by any number of these actors. 461 As discussed above, the choice of identifiers needs to be based on 462 differences that the signer thinks will be useful for the recipient 463 Assessor. Over time, industry practices establish norms for these 464 choices. 466 Absent such norms, it is best for signers to distinguish among 467 streams that have significant differences, while consuming the 468 smallest number of identifiers possible. This will limit the 469 burden on recipient Assessors. 471 A common view about a DKIM signature is that it carries a degree of 472 assurance about some or all of the message contents, and in 473 particular that the RFC5322.From field is likely to be valid. In 474 fact, DKIM makes assurances only about the integrity of the data and 475 not about its validity. Still, presumptions of From: field validity 476 remain a concern. Hence a signer using a domain name that is 477 unrelated to the domain name in the From: field can reasonably expect 478 that the disparity will warrant some curiosity, at least until 479 signing by independent operators has produced some established 480 practice among recipient Assessors. 482 With the identifier(s) supplied by DKIM, the Assessor can consult an 483 independent assessment service about the entity associated with the 484 identifier(s). Another possibility is that the Assessor can develop 485 its own reputation rating for the identifier(s). That is, over time, 486 the Assessor can observe the stream of messages associated with the 487 identifier(s) developing a reaction to associated content. For 488 example, if there is a high percentage of user complaints regarding 489 signed mail with a "d=" value of "widgetco.example.net", the Assessor 490 might include that fact in the vector of data it provides to the 491 Handling Filter. This is also discussed briefly in Section 5.4. 493 2.5. Filtering 495 After assessing the signer of a message, each receiving site creates 496 and tunes its own Handling Filter according to criteria specific for 497 that site. Still, there are commonalities across sites, and this 498 section offers a discussion, rather than a specification, of some 499 types of input to that process and how they can be used. 501 The discussion focuses on variations in Organizational Trust versus 502 Message Risk, that is, the degree of positive assessment of a DKIM- 503 signing organization, and the potential danger present in the message 504 stream signed by that organization. While it might seem that higher 505 trust automatically means lower risk, the experience with real-world 506 operations provides examples of every combination of the two factors, 507 as shown in Table 1. Only three levels of granularity are listed, in 508 order to keep discussion manageable. This also ensures extensive 509 flexibility for each site's detailed choices. 511 +---+---------------------+--------------------+--------------------+ 512 | | Low | Medium | High | 513 | | | | | 514 | | | | | 515 | | | | | 516 | | | | | 517 | O | | | | 518 | R | | | | 519 | G | | | | 520 | | | | | 521 | T | | | | 522 | R | | | | 523 | U | | | | 524 | S | | | | 525 | T | | | | 526 | | | | | 527 | M | | | | 528 +---+---------------------+--------------------+--------------------+ 529 | * | Unknown org, | Registered org, | Good Org, | 530 | L | Few msgs: | New Identifier: | Good msgs: | 531 | o | _Mild filtering_ | _Medium filtering_ | _Avoid FP(!)_ | 532 | w | | | | 533 | * | Unknown org, | Registered org, | Good org, Bad msg | 534 | M | New Identifier: | Mixed msgs: | burst: | 535 | e | _Default filtering_ | _Medium filtering_ | _Accept & Contact_ | 536 | d | | | | 537 | i | | | | 538 | u | | | | 539 | * | Black-Listed org, | Registered org, | Good org, | 540 | H | Bad msgs: | Bad msgs: | Compromised: | 541 | i | _Avoid FN(!)_ | _Strong filtering_ | _Fully blocked_ | 542 | g | | | | 543 | h | | | | 544 +---+---------------------+--------------------+--------------------+ 546 Table 1: Organizational Trust vs. Message Risk 548 The table indicates preferences for different handling of different 549 combinations, such as tuning filtering to avoid False Positives (FP) 550 or avoiding False Negatives (FN). Perhaps unexpectedly, it also 551 lists a case in which the receiving site might wish to deliver 552 problematic mail, rather than redirecting it, but also of course 553 contacting the signing organization, seeking resolution of the 554 problem. 556 3. DKIM Key Generation, Storage, and Management 558 By itself, verification of a digital signature only allows the 559 verifier to conclude with a very high degree of certainty that the 560 signature was created by a party with access to the corresponding 561 private signing key. It follows that a verifier requires means to 562 (1) obtain the public key for the purpose of verification and (2) 563 infer useful attributes of the key holder. 565 In a traditional Public Key Infrastructure (PKI), the functions of 566 key distribution and key accreditation are separated. In DKIM 567 [RFC4871], these functions are both performed through the DNS. 569 In either case, the ability to infer semantics from a digital 570 signature depends on the assumption that the corresponding private 571 key is only accessible to a party with a particular set of 572 attributes. In traditional PKI, a Trusted Third Party (TTP) vouches 573 that the key holder has been validated with respect to a specified 574 set of attributes. The range of attributes that may be attested in 575 such a scheme is thus limited only to the type of attributes that a 576 TTP can establish effective processes for validating. In DKIM, 577 Trusted Third parties are not employed and the functions of key 578 distribution and accreditation are combined. 580 Consequently there are only two types of inference that a signer may 581 make from a key published in a DKIM Key Record: 583 1. That a party with the ability to control DNS records within a DNS 584 zone intends to claim responsibility for messages signed using 585 the corresponding private signature key. 587 2. That use of a specific key is restricted to the particular subset 588 of messages identified by the selector. 590 The ability to draw any useful conclusion from verification of a 591 digital signature relies on the assumption that the corresponding 592 private key is only accessible to a party with a particular set of 593 attributes. In the case of DKIM, this means that the party that 594 created the corresponding DKIM key record in the specific zone 595 intended to claim responsibility for the signed message. 597 Ideally we would like to draw a stronger conclusion, that if we 598 obtain a DKIM key record from the DNS zone example.com, that the 599 legitimate holder of the DNS zone example.com claims responsibility 600 for the signed message. In order for this conclusion to be drawn it 601 is necessary for the verifier to assume that the operational security 602 of the DNS zone and corresponding private key are adequate. 604 3.1. Private Key Management: Deployment and Ongoing Operations 606 Access to signing keys MUST be carefully managed to prevent use by 607 unauthorized parties and to minimize the consequences if a compromise 608 were to occur. 610 While a DKIM signing key is used to sign messages on behalf of many 611 mail users, the signing key itself SHOULD be under direct control of 612 as few key holders as possible. If a key holder were to leave the 613 organization, all signing keys held by that key holder SHOULD be 614 withdrawn from service and if appropriate, replaced. 616 If key management hardware support is available, it SHOULD be used. 617 If keys are stored in software, appropriate file control protections 618 MUST be employed, and any location in which the private key is stored 619 in plaintext form SHOULD be excluded from regular backup processes 620 and SHOULD not be accessible through any form of network including 621 private local area networks. Auditing software SHOULD be used 622 periodically to verify that the permissions on the private key files 623 remain secure. 625 Wherever possible a signature key SHOULD exist in exactly one 626 location and be erased when no longer used. Ideally a signature key 627 pair SHOULD be generated as close to the signing point as possible 628 and only the public key component transferred to another party. If 629 this is not possible, the private key MUST be transported in an 630 encrypted format that protects the confidentiality of the signing 631 key. A shared directory on a local file system does not provide 632 adequate security for distribution of signing keys in plaintext form. 634 Key escrow schemes are not necessary and SHOULD NOT be used. In the 635 unlikely event of a signing key becomming lost, a new signature key 636 pair may be generated as easily as recovery from a key escrow scheme. 638 To enable accountability and auditing: 640 o Responsibility for the security of a signing key SHOULD ultimately 641 vest in a single named individual. 643 o Where multiple parties are authorized to sign messages, each 644 signer SHOULD use a different key to enable accountability and 645 auditing. 647 Best practices for management of cryptographic keying material 648 require keying material to be refreshed at regular intervals, 649 particularly where key management is achieved through software. 650 While this practice is highly desirable it is of considerably less 651 importance than the requirement to maintain the secrecy of the 652 corresponding private key. An operational practice in which the 653 private key is stored in tamper proof hardware and changed once a 654 year is considerably more desirable than one in which the signature 655 key is changed on an hourly basis but maintained in software. 657 3.2. Storing Public Keys: DNS Server Software Considerations 659 In order to use DKIM a DNS domain holder requires (1) the ability to 660 create the necessary DKIM DNS records and (2) sufficient operational 661 security controls to prevent insertion of spurious DNS records by an 662 attacker. 664 DNS record management is often operated by an administrative staff 665 that is different from those who operate an organization's email 666 service. In order to ensure that DKIM DNS records are accurate, this 667 imposes a requirement for careful coordination between the two 668 operations groups. If the best practices for private key management 669 described above are observed, such deployment is not a one time 670 event; DNS DKIM selectors will be changed over time signing keys are 671 terminated and replaced. 673 At a minimum, a DNS server that handles queries for DKIM key records 674 MUST allow the server administrators to add free-form TXT records. 675 It would be better if the the DKIM records could be entered using a 676 structured form, supporting the DKIM-specific fields. 678 Ideally DNSSEC [RFC4034] SHOULD be employed in a configuration that 679 provides protection against record insertion attacks and zone 680 enumeration. In the case that NSEC3 [RFC5155] records are employed 681 to prevent insertion attack, the OPT-OUT flag MUST be set clear. 683 3.2.1. Assignment of Selectors 685 Selectors are assigned according to the administrative needs of the 686 signing domain, such as for rolling over to a new key or for 687 delegating of the right to authenticate a portion of the namespace to 688 a trusted third party. Examples include: 690 jun2005.eng._domainkey.example.com 692 widget.promotion._domainkey.example.com 694 It is intended that assessments of DKIM identities be based on the 695 domain name, and not include the selector. While past practice of a 696 signer may permit a verifier to infer additional properties of 697 particular messages from the structure DKIM key selector, unannounced 698 administrative changes such as a change of signing softeware may 699 cause such heuristics to fail at any time. 701 3.3. Per User Signing Key Management Issues 703 While a signer may establish business rules, such as issue of 704 individual signature keys for each end-user, DKIM makes no provision 705 for communicating these to other parties. Out of band distribution 706 of such business rules is outside the scope of DKIM. Consequently 707 there is no means by which external parties may make use of such keys 708 to attribute messages with any greater granularity than a DNS domain. 710 If per-user signing keys are assigned for internal purposes (e.g. 711 authenticating messages sent to an MTA for distribution), the 712 following issues need to be considered before using such signatures 713 as an alternative to traditional edge signing at the outbound MTA: 715 External verifiers will be unable to make use of the additional 716 signature granularity without access to additional information 717 passed out of band with respect to [RFC4871]. 719 If the number of user keys is large, the efficiency of local 720 caching of key records by verifiers will be lower. 722 A large number of end users may be less likely to be able to 723 manage private key data securely on their personal computer than 724 an administrator running an edge MTA. 726 3.4. Third Party Signer Key Management and Selector Administration 728 A DKIM key record only asserts that the holder of the corresponding 729 domain name makes a claim of responsibility for messages signed under 730 the corresponding key. In some applications, such as bulk mail 731 delivery, it is desirable to delegate the ability to make a claim of 732 responsibility to another party. In this case the trust relationship 733 is established between the domain holder and the verifier but the 734 private signature key is held by a third party. 736 Signature keys used by a third party signer SHOULD be kept entirely 737 separate from those used by the domain holder and other third party 738 signers. To limit potential exposure of the private key, the 739 signature key pair SHOULD be generated by the third party signer and 740 the public component of the key transmitted to the domain holder, 741 rather than have the domain holder generate the key pair and transmit 742 the private component to the third party signer. 744 Domain holders SHOULD adopt a least privilege approach and grant 745 third party signers the minimum access necessary to perform the 746 desired function. Limiting the access granted to Third Party Signers 747 serves to protect the interests of both parties. The domain holder 748 minimizes its security risk and the Trusted Third Party Signer avoids 749 unnecessary liability. 751 In the most restrictive case a domain holder maintains full control 752 over the creation of key records and employs appropriate key record 753 restrictions to enforce restrictions on the messages for which the 754 third party signer is able to sign. If such restrictions are 755 impractical, the domain holder SHOULD delegate a DNS subzone for 756 publishing key records to the third party signer. The domain holder 757 SHOULD not allow a third party signer unrestricted access to its DNS 758 service for the purpose of publishing key records. 760 3.5. Key Pair / Selector Lifecycle Management 762 Deployments SHOULD establish, document and observe processes for 763 managing the entire lifecycle of a public key pair. 765 3.5.1. Example Key Deployment Process 767 When it is determined that a new key pair is required: 769 1. A Key Pair is generated by the signing device. 771 2. A proposed key selector record is generated and transmitted to 772 the DNS administration infrasrtructure. 774 3. The DNS administration infrastructure verifies the authenticity 775 of the key selector registration request. If accepted 777 1. A key selector is assigned. 779 2. The corresponding key record published in the DNS. 781 3. Wait for DNS updates to propagate (if necessary). 783 4. Report assigned key selector to signing device. 785 4. Signer verifies correct registration of the key record. 787 5. Signer begins generating signatures using the new key pair. 789 6. Signer terminates any private keys that are no longer required 790 due to issue of replacement. 792 3.5.2. Example Key Termination Process 794 When it is determined that a private signature key is no longer 795 required: 797 1. Signer stops using the private key for signature operations. 799 2. Signer deletes all records of the private key, including in- 800 memory copies at the signing device. 802 3. Signer notifies the DNS administration infrasrtructure that the 803 signing key is withdrawn from service and that the corresponding 804 key records may be withdrawn from service at a specified future 805 date. 807 4. The DNS administration infrastructure verifies the authenticity 808 of the key selector termination request. If accepted, 810 1. The key selector is scheduled for deletion at a future time 811 determined by site policy. 813 2. Wait for deletion time to arrive. 815 3. The signer either publishes a revocation key selector with an 816 empty "p=" field, or deletes the key selector record 817 entirely. 819 5. As far as the verifier is concerned, there is no functional 820 difference between verifying against a key selector with an empty 821 "p=" field, and verifying against a missing key selector: both 822 result in a failed signature and the signature should be treated 823 as if it had not been there. However, there is a minor semantic 824 difference: with the empty "p=" field, the signer is explicitly 825 stating that the key has been revoked. The empty "p=" record 826 provides a gravestone for an old selector, making it less likely 827 that the selector might be accidently reused with a different 828 public key. 830 4. Signing 832 Creating messages that have one or more DKIM signatures, requires 833 support in only two outbound email service components: 835 o A DNS Administrative interface that can create and maintain the 836 relevant DNS names -- including names with underscores -- and 837 resource records (RR). 839 o A trusted module, called the Signing Module, which is within the 840 organization's outbound email handling service and which creates 841 and adds the DKIM-Signature: header field(s) to the message. 843 If the module creates more than one signature, there needs to be the 844 appropriate means of telling it which one(s) to use. If a large 845 number of names is used for signing, it will help to have the 846 administrative tool support a batch processing mode. 848 4.1. DNS Records 850 A receiver attempting to verify a DKIM signature obtains the public 851 key that is associated with the signature for that message. The 852 DKIM-Signature: header in the message contains the d= tag with the 853 basic domain name doing the signing and serving as output to the 854 Identity Assessor, and the s= tag with the selector that is added to 855 the name, for finding the specific public key. Hence, the relevant 856 ._domainkey. DNS record needs to contain a 857 DKIM-related RR that provides the public key information. 859 The administrator of the zone containing the relevant domain name 860 adds this information. Initial DKIM DNS information is contained 861 within TXT RRs. DNS administrative software varies considerably in 862 its abilities to support DKIM names, such as with underscores, and to 863 add new types of DNS information. 865 4.2. Signing Module 867 The module doing signing can be placed anywhere within an 868 organization's trusted Administrative Management Domain (ADMD); 869 obvious choices include department-level posting agents, as well as 870 outbound boundary MTAs to the open Internet. However any other 871 module, including the author's MUA, is potentially acceptable, as 872 long as the signature survives any remaining handling within the 873 ADMD. Hence the choice among the modules depends upon software 874 development, administrative overhead, security exposures and transit 875 handling tradeoffs. One perspective that helps to resolve this 876 choice is the difference between the increased flexibility, from 877 placement at (or close to) the MUA, versus the streamlined 878 administration and operation, that is more easily obtained by 879 implementing the mechanism "deeper" into the organization's email 880 infrastructure, such as at its boundary MTA. 882 Note the discussion in Section 2.2, concerning use of the i= tag. 884 The signing module uses the appropriate private key to create one or 885 more signatures. The means by which the signing module obtains the 886 private key(s) is not specified by DKIM. Given that DKIM is intended 887 for use during email transit, rather than for long-term storage, it 888 is expected that keys will be changed regularly. For administrative 889 convenience, key information SHOULD NOT be hard-coded into software. 891 4.3. Signing Policies and Practices 893 Every organization (ADMD) will have its own policies and practices 894 for deciding when to sign messages (message stream) and with what 895 domain name, selector and key. Examples of particular message 896 streams include all mail sent from the ADMD, versus mail from 897 particular types of user accounts, versus mail having particular 898 types of content. Given this variability, and the likelihood that 899 signing practices will change over time, it will be useful to have 900 these decisions represented through run-time configuration 901 information, rather than being hard-coded into the signing software. 903 As noted in Section 2.3, the choice of signing name granularity 904 requires balancing administrative convenience and utility for 905 recipients. Too much granularity is higher administrative overhead 906 and well might attempt to impose more differential analysis on the 907 recipient than they wish to support. In such cases, they are likely 908 to use only a super-name -- right-hand substring -- of the signing 909 name. When this occurs, the signer will not know what portion is 910 being used; this then moves DKIM back to the non-deterministic world 911 of heuristics, rather than the mechanistic world of signer/recipient 912 collaboration that DKIM seeks. 914 5. Verifying 916 A message recipient may verify a DKIM signature to determine if a 917 claim of responsibility has been made for the message by a trusted 918 domain. 920 Access control requires two components: authentication and 921 authorization. By design, verification of a DKIM signature only 922 provides the authentication component of an access control decision 923 and MUST be combined with additional sources of information such as 924 reputation data to arrive at an access control decision. 926 5.1. Intended Scope of Use 928 DKIM requires that a message with a signature that is found to be 929 invalid is to be treated as if the message had not been signed at 930 all. 932 If a DKIM signature fails to verify, it is entirely possible that the 933 message is valid and that either there is a configuration error in 934 the signer's system (e.g. a missing key record) or that the message 935 was inadvertently modified in transit. It is thus undesirable for 936 mail infrastructure to treat messages with invalid signatures less 937 favorably than those with no signatures whatsoever. Contrariwise, 938 creation of an invalid signature requires a trivial amount of effort 939 on the part of an attacker. If messages with invalid signatures were 940 to be treated preferentially to messages with no signatures 941 whatsoever, attackers will simply add invalid signature blocks to 942 gain the preferential treatment. It follows that messages with 943 invalid signatures SHOULD be treated no better and no worse than 944 those with no signature at all. 946 5.2. Signature Scope 948 As with any other digital signature scheme, verifiers MUST only 949 consider the part of the message that is inside the scope of the 950 message as being authenticated by the signature. 952 For example, if the l= option is employed to specify a content length 953 for the scope of the signature, only the part of the message that is 954 within the scope of the content signature would be considered 955 authentic. 957 5.3. Design Scope of Use 959 Public Key cryptography provides an exceptionally high degree of 960 assurance, bordering on absolute certainty, that the party that 961 created a valid digital signature had access to the private key 962 corresponding to the public key indicated in the signature. 964 In order to make useful conclusions from the verification of a valid 965 digital signature, the verifier is obliged to make assumptions that 966 fall far short of absolute certainty. Consequently, mere validation 967 of a DKIM signature does not represent proof positive that a valid 968 claim of responsibility was made for it by the indicated party, that 969 the message is authentic, or that the message is not abusive. In 970 particular: 972 o The legitimate private key holder may have lost control of its 973 private key. 975 o The legitimate domain holder may have lost control of the DNS 976 server for the zone from which the key record was retrieved. 978 o The key record may not have been delivered from the legitimate DNS 979 server for the zone from which the key record was retrieved. 981 o Ownership of the DNS zone may have changed. 983 In practice these limitations have little or no impact on the field 984 of use for which DKIM is designed but may have a bearing if use is 985 made of the DKIM message signature format or key retrieval mechanism 986 in other specifications. 988 In particular the DKIM key retrieval mechanism is designed for ease 989 of use and deployment rather than to provide a high assurance Public 990 Key Infrastructure suitable for purposes that require robust non- 991 repudiation such as establishing legally binding contracts. 992 Developers seeking to extend DKIM beyond its design application 993 SHOULD consider replacing or supplementing the DNS key retreival 994 mechanism with one that is designed to meet the intended purposes. 996 5.4. Inbound Mail Filtering 998 DKIM is frequently employed in a mail filtering strategy to avoid 999 performing content analysis on email originating from trusted 1000 sources. Messages that carry a valid DKIM signature from a trusted 1001 source may be whitelisted, avoiding the need to perform computation 1002 and hence energy intensive content analysis to determine the 1003 disposition of the message. 1005 Mail sources may be determined to be trusted by means of previously 1006 observed behavior and/or reference to external reputation or 1007 accreditation services. The precise means by which this is 1008 acomplished is outside the scope of DKIM. 1010 5.4.1. Non-Verifying Adaptive Spam Filtering Systems 1012 Adaptive (or learning) spam filtering mechanisms that are not capable 1013 of verifying DKIM signatures SHOULD at minimum be configured to 1014 ignore DKIM header data entirely. 1016 5.5. Messages sent through Mailing Lists and other Intermediaries 1018 Intermediaries such as mailing lists pose a particular challenge for 1019 DKIM implementations as the message processing steps performed by the 1020 intermediary may cause the message content to change in ways that 1021 prevent the signature passing verification. 1023 Such intermediaries are strongly encouraged to deploy DKIM signing so 1024 that a verifiable claim of responsibility remains available to 1025 parties attempting to verify the modified message. 1027 5.6. Generation, Transmission and Use of Results Headers 1029 In many deployments it is desirable to separate signature 1030 verification from the application relying on the verification. A 1031 system may choose to relay information indicating the results of its 1032 message authentication efforts using various means; adding a "results 1033 header" to the message is one such mechanism. [RFC5451] For example, 1034 consider the cases where: 1036 o The application relying on DKIM signature verification is not 1037 capable of performing the verification. 1039 o The message may be modified after the signature verification is 1040 performed. 1042 o The signature key may not be available by the time that the 1043 message is read. 1045 In such cases it is important that the communication link between the 1046 signature verifier and the relying application be sufficiently secure 1047 to prevent insertion of a message that carries a bogus results 1048 header. 1050 An intermediary that generates results headers SHOULD ensure that 1051 relying applications are able to distinguish valid results headers 1052 issued by the intermediary from those introduced by an attacker. For 1053 example, this can be accomplished by signing the results header. At 1054 a minimum, results headers on incoming messages SHOULD be removed if 1055 they purport to have been issued by the intermediary but cannot be 1056 verified as authentic. 1058 Further discussion on trusting the results as relayed from a verifier 1059 to something downstream can be found in [RFC5451] 1061 6. Taxonomy of Signatures 1063 As described in section Section 2.1, a DKIM signature tells the 1064 signature verifier that the owner of a particular domain name accepts 1065 some responsibility for the message. It does not, in and of itself, 1066 provide any information about the trustworthiness or behavior of that 1067 identity. What it does provide is a verified identity to which such 1068 behavioral information can be associated, so that those who collect 1069 and use such information can be assured that it truly pertains to the 1070 identity in question. 1072 This section lays out a taxonomy of some of the different identities, 1073 or combinations of identities, that might usefully be represented by 1074 a DKIM signature. 1076 6.1. Single Domain Signature 1078 Perhaps the simplest case is when an organization signs its own 1079 outbound email using its own domain in the SDID 1080 [I-D.ietf-dkim-rfc4871-errata] of the signature. For example, 1081 Company A would sign the outbound mail from its employees with 1082 d=companyA.example. 1084 In the most straightforward configuration, the addresses in the 1085 RFC5322.From would also be in the companyA.example domain, but that 1086 direct correlation is not required. 1088 A special case of the Single Domain Signature is an Author Signature 1089 as defined by the Author Domain Signing Practices specification 1090 [RFC5617]. Author signatures are signatures from an author's 1091 organization that have an SDID value that matches that of an 1092 RFC5322.From address of the signed message. 1094 Although an author signature might in some cases be proof against 1095 spoofing the domain name of the RFC5322.From address, it is important 1096 to note that the DKIM and ADSP validation apply only to the exact 1097 address string and not to look-alike addresses nor to the human- 1098 friendly "display-name" or names and addresses used within the body 1099 of the message. That is, it protects only against the misuse of a 1100 precise address string within the RFC5322.From field and nothing 1101 else. For example, a message from bob@domain.example with a valid 1102 signature where d=d0main.example would fail an ADSP check because the 1103 signature domain, however similar, is distinct; however a message 1104 from bob@d0main.example with a valid signature where d=d0main.example 1105 would pass an ADSP check, even though to a human it might be obvious 1106 that d0main.example is likely a malicious attempt to spoof the domain 1107 domain.example. This example highlights that ADSP, like DKIM, is 1108 only able to validate a signing identifier: it still requires some 1109 external process to attach a meaningful reputation to that 1110 identifier. 1112 6.2. Parent Domain Signature 1114 Another approach that might be taken by an organization with multiple 1115 active subdomains is to apply the same (single) signature domain to 1116 mail from all subdomains. In this case, the signature chosen would 1117 usually be the signature of a parent domain common to all subdomains. 1118 For example, mail from marketing.domain.example, 1119 sales.domain.example, and engineering.domain.example might all use a 1120 signature where d=domain.example. 1122 This approach has the virtue of simplicity, but it is important to 1123 consider the implications of such a choice. As discussed in 1124 Section 2.3, if the type of mail sent from the different subdomains 1125 is significantly different or if there is reason to believe that the 1126 reputation of the subdomains would differ, then it may be a good idea 1127 to acknowledge this and provide distinct signatures for each of the 1128 subdomains (d=marketing.domain.example, sales.domain.example, etc.). 1129 However, if the mail and reputations are likely to be similar, then 1130 the simpler approach of using a single common parent domain in the 1131 signature may work well. 1133 Another approach to distinguishing the streams using a single DKIM 1134 key would be to leverage the AUID [I-D.ietf-dkim-rfc4871-errata] (i= 1135 tag) in the DKIM signature to differentiate the mail streams. For 1136 example, marketing email would be signed with 1137 i=marketing.domain.example and d=domain.example. 1139 It's important to remember, however, that under core DKIM semantics 1140 the AUID is opaque to receivers. That means that it will only be an 1141 effective differentiator if there is an out of band agreement about 1142 the i= semantics. 1144 6.3. Third Party Signature 1146 A signature whose domain does not match the domain of the 1147 RFC5322.From address is sometimes referred to as a third party 1148 signature. In certain cases even the parent domain signature 1149 described above would be considered a third party signature because 1150 it would not be an exact match for the domain in the From: address. 1152 Although there is often heated debate about the value of third party 1153 signatures, it is important to note that the DKIM specification 1154 attaches no particular significance to the identity in a DKIM 1155 signature. The identity specified within the signature is the 1156 identity that is taking responsibility for the message, and it is 1157 only the interpretation of a given receiver that gives one identity 1158 more or less significance than another. In particular, most 1159 independent reputation services assign trust based on the specific 1160 identifier string, not its "role": in general they make no 1161 distinction between, for example, an author signature and a third 1162 party signature. 1164 For some, a signature unrelated to the author domain (the domain in 1165 the RFC5322.From address) is less valuable because there is an 1166 assumption that the presence of an author signature guarantees that 1167 the use of the address in the From: header is authorized. 1169 For others, that relevance is tied strictly to the recorded 1170 behavioral data assigned to the identity in question, i.e. its trust 1171 assessment or reputation. The reasoning here is that an identity 1172 with a good reputation is unlikely to maintain that good reputation 1173 if it is in the habit of vouching for messages that are unwanted or 1174 abusive; in fact, doing so will rapidly degrade its reputation so 1175 that future messages will no longer benefit from it. It is therefore 1176 low risk to facilitate the delivery of messages that contain a valid 1177 signature of a domain with a strong positive reputation, independent 1178 of whether or not that domain is associated with the address in the 1179 RFC5322.From header field of the message. 1181 Third party signatures encompass a wide range of identities. Some of 1182 the more common are: 1184 Service Provider: In cases where email is outsourced to an Email 1185 Service Provider (ESP), Internet Service Provider (ISP), or other 1186 type of service provider, that service provider may choose to DKIM 1187 sign outbound mail with either its own identifier -- relying on 1188 its own, aggregate reputation -- or with a subdomain of the 1189 provider that is unique to the message author but still part of 1190 the provider's aggregate reputation. Such service providers may 1191 also encompass delegated business functions such as benefit 1192 management, although these will more often be treated as trusted 1193 third party senders (see below). 1195 Parent Domain. As discussed above, organizations choosing to apply a 1196 parent domain signature to mail originating from subdomains may 1197 have their signatures treated as third party by some verifiers, 1198 depending on whether or not the "t=s" tag is used to constrain the 1199 parent signature to apply only to its own specific domain. The 1200 default is to consider a parent domain signature valid for its 1201 subdomains. 1203 Reputation Provider: Another possible category of third party 1204 signature would be the identity of a third party reputation 1205 provider. Such a signature would indicate to receivers that the 1206 message was being vouched for by that third party. 1208 6.4. Using Trusted Third Party Senders 1210 For most of the cases described so far, there has been an assumption 1211 that the signing agent was responsible for creating and maintaining 1212 its own DKIM signing infrastructure, including its own keys, and 1213 signing with its own identity. 1215 A different model arises when an organization uses a trusted third 1216 party sender for certain key business functions, but still wants that 1217 email to benefit from the organization's own identity and reputation: 1219 in other words, the mail would come out of the trusted third party's 1220 mail servers, but the signature applied would be that of the 1221 controlling organization. 1223 This can be done by having the third party generate a key pair that 1224 is designated uniquely for use by that trusted third party and 1225 publishing the public key in the controlling organization's DNS 1226 domain, thus enabling the third party to sign mail using the 1227 signature of the controlling organization. For example, if Company A 1228 outsources its employee benefits to a third party, it can use a 1229 special key pair that enables the benefits company to sign mail as 1230 "companyA.example". Because the key pair is unique to that trusted 1231 third party, it is easy for Company A to revoke the authorization if 1232 necessary by simply removing the public key from the companyA.example 1233 DNS. 1235 A more cautious approach would be to create a dedicated subdomain 1236 (e.g. benefits.companyA.example) to segment the outsourced mail 1237 stream, and to publish the public key there; the signature would then 1238 use d=benefits.companyA.example. 1240 6.4.1. DNS Delegation 1242 Another possibility for configuring trusted third party access, as 1243 discussed in section 3.4, is to have Company A use DNS delegation and 1244 have the designated subdomain managed directly by the trusted third 1245 party. In this case, Company A would create a subdomain 1246 benefits.companya.example, and delegate the DNS management of that 1247 subdomain to the benefits company so it could maintain its own key 1248 records. Should revocation become necessary, Company A could simply 1249 remove the DNS delegation record. 1251 6.5. Multiple Signatures 1253 A simple configuration for DKIM-signed mail is to have a single 1254 signature on a given message. This works well for domains that 1255 manage and send all of their own email from single sources, or for 1256 cases where multiple email streams exist but each has its own unique 1257 key pair. It also represents the case in which only one of the 1258 participants in an email sequence is able to sign, no matter whether 1259 it represents the author or one of the operators. 1261 The examples thus far have considered the implications of using 1262 different identities in DKIM signatures, but have used only one such 1263 identity for any given message. In some cases, it may make sense to 1264 have more than one identity claiming responsibility for the same 1265 message. 1267 There are a number of situations where applying more than one DKIM 1268 signature to the same message might make sense. A few examples are: 1270 Companies with multiple subdomain identities: A company that has 1271 multiple subdomains sending distinct categories of mail might 1272 choose to sign with distinct subdomain identities to enable each 1273 subdomain to manage its own identity. However, it might also want 1274 to provide a common identity that cuts across all of the distinct 1275 subdomains. For example, Company A may sign mail for its sales 1276 department with a signature where d=sales.companya.example, and a 1277 second signature where d=companya.example 1279 Service Providers: A service providers may, as described above, 1280 choose to sign outbound messages with either its own identity or 1281 with an identity unique to each of its clients (possibly 1282 delegated). However, it may also do both: sign each outbound 1283 message with its own identity as well as with the identity of each 1284 individual client. For example, ESP A might sign mail for its 1285 client Company B with its service provider signature 1286 d=espa.example, and a second client-specific signature where d= 1287 either companyb.example, or companyb.espa.example. The existence 1288 of the service provider signature could, for example, help cover a 1289 new client while it establishes its own reputation, or help a very 1290 small volume client who might never reach a volume threshold 1291 sufficient to establish an individual reputation. 1293 Forwarders Forwarded mail poses a number of challenges to email 1294 authentication. DKIM is relatively robust in the presence of 1295 forwarders as long as the signature is designed to avoid message 1296 parts that are likely to be modified; however, some forwarders do 1297 make modifications that can invalidate a DKIM signature. 1299 Some forwarders such as mailing lists or "forward article to a 1300 friend" services might choose to add their own signatures to 1301 outbound messages to vouch for them having legitimately originated 1302 from the designated service. In this case, the signature would be 1303 added even in the presence of a preexisting signature, and both 1304 signatures would be relevant to the verifier. 1306 Any forwarder that modifies messages in ways that will break 1307 preexisting DKIM signatures SHOULD always sign its forwarded 1308 messages. 1310 Reputation Providers: Although third party reputation providers 1311 today use a variety of protocols to communicate their information 1312 to receivers, it is possible that they, or other organizations 1313 willing to put their "seal of approval" on an email stream might 1314 choose to use a DKIM signature to do it. In nearly all cases, 1315 this "reputation" signature would be in addition to the author or 1316 originator signature. 1318 One important caveat to the use of multiple signatures is that there 1319 is currently no clear consensus among receivers on how they plan to 1320 handle them. The opinions range from ignoring all but one signature 1321 (and the specification of which of them is verified differs from 1322 receiver to receiver), to verifying all signatures present and 1323 applying a weighted blend of the trust assessments for those 1324 identifiers, to verifying all signatures present and simply using the 1325 identifier that represents the most positive trust assessment. It is 1326 likely that the industry will evolve to accept multiple signatures 1327 using either the second or third of these, but it may take some time 1328 before one approach becomes pervasive. 1330 7. Example Usage Scenarios 1332 Signatures are created by different types of email actors, based on 1333 different criteria, such as where the actor operates in the sequence 1334 from author to recipient, whether they want different messages to be 1335 evaluated under the same reputation or a different one, and so on. 1336 This section provides some examples of usage scenarios for DKIM 1337 deployments; the selection is not intended to be exhaustive, but to 1338 illustrate a set of key deployment considerations. 1340 7.1. Author's Organization - Simple 1342 The simplest DKIM configuration is to have some mail from a given 1343 organization (Company A) be signed with the same d= value (e.g. 1344 d=companya.example). If there is a desire to associate additional 1345 information, the AUID [I-D.ietf-dkim-rfc4871-errata] value can become 1346 uniqueID@companya.example, or @uniqueID.companya.example. 1348 In this scenario, Company A need only generate a single signing key 1349 and publish it under their top level domain (companya.example); the 1350 signing module would then tailor the AUID value as needed at signing 1351 time. 1353 7.2. Author's Organization - Differentiated Types of Mail 1355 A slight variation of the one signature case is where Company A signs 1356 some of its mail, but it wants to differentiate different categories 1357 of its outbound mail by using different identifiers. For example, it 1358 might choose to distinguish marketing mail, billing or transactional 1359 mail, and individual corporate email into marketing.companya.example, 1360 billing.companya.example, and companya.example, where each category 1361 is assigned a unique subdomain and unique signing keys. 1363 7.3. Author Domain Signing Practices 1365 7.3.1. Introduction 1367 Some domains may decide to sign all of their outgoing mail. If all 1368 of the legitimate mail for a domain is signed, recipients can be more 1369 aggressive in their filtering of mail that uses the domain but does 1370 not have a valid signature from the domain; in such a configuration, 1371 the absence of a signature would be more significant than for the 1372 general case. It might be desirable for such domains to be able to 1373 advertise their intent to other receivers: this is the topic of 1374 Author Domain Signing Practices (ADSP). 1376 Note that ADSP is not for everyone. Sending domains that do not 1377 control all legitimate outbound mail purporting to be from their 1378 domain (i.e., with a RFC5322.From address in their domain) are likely 1379 to experience delivery problems with some percentage of that mail. 1380 Administrators evaluating ADSP for their domains SHOULD carefully 1381 weigh the risk of phishing attacks against the likelihood of 1382 undelivered mail. 1384 This section covers some examples of ADSP usage: for the complete 1385 specification, see [RFC5617] 1387 7.3.2. A Few Definitions 1389 In the ADSP specification, an address in the From header field of a 1390 message [RFC5322] is defined as an "Author Address", and an "Author 1391 Domain" is defined as anything to the right of the '@' in an Author 1392 Address. 1394 An "Author Signature" is thus any valid signature where the value of 1395 the SDID matches an Author Domain in the message. 1397 It is important to note that unlike the DKIM specification which 1398 makes no correlation between the signature domain and any message 1399 headers, the ADSP specification applies only to the author domain. 1400 In essence, under ADSP, any non-author signatures are ignored 1401 (treated as if they are not present). 1403 Signers wishing to publish an Author Domain Signing Practices (ADSP) 1404 [RFC5617] record describing their signing practices will thus want to 1405 include an author signature on their outbound mail to avoid ADSP 1406 verification failures. For example, if the address in the 1407 RFC5322.From is bob@company.example, the SDID value of the author 1408 signature must be company.example. 1410 7.3.3. Some ADSP Examples 1412 An organization (Company A) may specify its signing practices by 1413 publishing an ADSP record with "dkim=all" or "dkim=discardable". In 1414 order to avoid misdelivery of its mail at receivers that are 1415 validating ADSP, Company A MUST first have done an exhaustive 1416 analysis to determine all sources of outbound mail from its domain 1417 (companyA.example) and ensure that they all have valid author 1418 signatures from that domain. 1420 For example, email with an RFC5322.From address of bob@ 1421 companyA.example MUST have an author signature where the SDID value 1422 is "companyA.example" or it will fail an ADSP validation. 1424 Note that once an organization publishes an ADSP record using 1425 dkim=all or dkim=discardable, any email with a RFC5322.From address 1426 that uses the domain where the ADSP record is published that does not 1427 have a valid author signature is at risk of being misdelivered or 1428 discarded. For example, if a message with an RFC5322.From address of 1429 newsletter@companyA.example has a signature with 1430 d=marketing.companyA.example, that message will fail the ADSP check 1431 because the signature would not be considered a valid author 1432 signature. 1434 Because the semantics of an ADSP author signature are more 1435 constrained than the semantics of a "pure" DKIM signature, it is 1436 important to make sure the nuances are well understood before 1437 deploying an ADSP record. The ADSP specification [RFC5617] provides 1438 some fairly extensive lookup examples (in Appendix A) and usage 1439 examples (in Appendix B). 1441 In particular, in order to prevent mail from being negatively 1442 impacted or even discarded at the receiver, it is essential to 1443 perform a thorough survey of outbound mail from a domain before 1444 publishing an ADSP policy of anything stronger than "unknown". This 1445 includes mail that might be sent from external sources that may not 1446 be authorized to use the domain signature, as well as mail that risks 1447 modification in transit that might invalidate an otherwise valid 1448 author signature (e.g. mailing lists, courtesy forwarders, and other 1449 paths that could add or modify headers, or modify the message body). 1451 7.4. Delegated Signing 1453 An organization may choose to outsource certain key services to an 1454 independent company. For example, Company A might outsource its 1455 benefits management, or Organization B might outsource its marketing 1456 email. 1458 If Company A wants to ensure that all of the mail sent on its behalf 1459 through the benefits providers email servers shares the Company A 1460 reputation, as discussed in Section 6.4 it can either publish keys 1461 designated for the use of the benefits provider under 1462 companyA.example (preferably under a designated subdomain of 1463 companyA.example), or it can delegate a subdomain (e.g. 1464 benefits.companyA.example) to the provider and enable the provider to 1465 generate the keys and manage the DNS for the designated subdomain. 1467 In both of these cases, mail would be physically going out of the 1468 benefit provider's mail servers with a signature of e.g. 1469 d=benefits.companya.example. Note that the From: address is not 1470 constrained: it could either be affiliated with the benefits company 1471 (e.g. benefits-admin@benefitprovider.example, or 1472 benefits-provider@benefits.companya.example), or with the companyA 1473 domain. 1475 Note that in both of the above scenarios, as discussed in 1476 Section 3.4, security concerns dictate that the keys be generated by 1477 the organization that plans to do the signing so that there is no 1478 need to transfer the private key. In other words, the benefits 1479 provider would generate keys for both of the above scenarios. 1481 7.5. Independent Third Party Service Providers 1483 Another way to manage the service provider configuration would be to 1484 have the service provider sign the outgoing mail on behalf of its 1485 client Company A with its own (provider) identifier. For example, an 1486 Email Service Provider (ESP A) might want to share its own mailing 1487 reputation with its clients, and may sign all outgoing mail from its 1488 clients with its own d= domain (e.g. d=espa.example). 1490 Should the ESP want to distinguish among its clients, it has two 1491 options: 1493 o Share the SDID domain, and use the AUID value to distinguish among 1494 the clients: e.g. a signature on behalf of client A would have 1495 d=espa.example and i=clienta.espa.example (or 1496 i=clienta@espa.example) 1498 o Extend the SDID domain, so there is a unique value (and subdomain) 1499 for each client: e.g. a signature on behalf of client A would have 1500 d=clienta.espa.example. 1502 Note that this scenario and the delegation scenario are not mutually 1503 exclusive: in some cases, it may be desirable to sign the same 1504 message with both the ESP and the ESP client identities. 1506 7.6. Mail Streams Based on Behavioral Assessment 1508 An ISP (ISP A) might want to assign signatures to outbound mail from 1509 its users according to each user's past sending behavior 1510 (reputation). In other words, the ISP would segment its outbound 1511 traffic according to its own assessment of message quality, to aid 1512 recipients in differentiating among these different streams. Since 1513 the semantics of behavioral assessments are not valid AUID values, 1514 ISP A (ispa.example) may configure subdomains corresponding to the 1515 assessment categories (e.g. good.ispa.example, neutral.ispa.example, 1516 bad.ispa.example), and use these subdomains in the d= value of the 1517 signature. 1519 The signing module may also set the AUID value to have a unique user 1520 id (distinct from the local-part of the user's email address), for 1521 example user3456@neutral.domain.example. Using a userid that is 1522 distinct from a given email alias is useful in environments where a 1523 single user might register multiple email aliases. 1525 Note that in this case the AUID values are only partially stable. 1526 They are stable in the sense that a given i= value will always 1527 represent the same identity, but they are unstable in the sense that 1528 a given user can migrate among the assessment subdomains depending on 1529 their sending behavior (i.e., the same user might have multiple AUID 1530 values over the lifetime of a single account). 1532 In this scenario, ISP A may generate as many keys as there are 1533 assessment subdomains (SDID values), so that each assessment 1534 subdomain has its own key. The signing module would then choose its 1535 signing key based on the assessment of the user whose mail was being 1536 signed, and if desired include the user id in the AUID of the 1537 signature. As discussed earlier, the per-user granularity of the 1538 AUID may be ignored by many verifiers, so organizations choosing to 1539 use it should not rely on its use for receiver side filtering 1540 results; however, some organizations may also find the information 1541 useful for their own purposes in processing bounces or abuse reports. 1543 7.7. Agent or Mediator Signatures 1545 Another scenario is that of an agent, usually a re-mailer of some 1546 kind, that signs on behalf of the service or organization that it 1547 represents. Some examples of agents might be a mailing list manager, 1548 or the "forward article to a friend" service that many online 1549 publications offer. In most of these cases, the signature is 1550 asserting that the message originated with, or was relayed by, the 1551 service asserting responsibility. In general, if the service is 1552 configured in such a way that its forwarding would break existing 1553 DKIM signatures, it SHOULD always add its own signature. 1555 8. Usage Considerations 1557 8.1. Non-standard Submission and Delivery Scenarios 1559 The robustness of DKIM's verification mechanism is based on the fact 1560 that only authorized signing modules have access to the designated 1561 private key. This has the side effect that email submission and 1562 delivery scenarios that originate or relay messages from outside the 1563 domain of the authorized signing module will not have access to that 1564 protected private key, and thus will be unable to attach the expected 1565 domain signature to those messages. Such scenarios include mailing 1566 lists, courtesy forwarders, MTAs at hotels, hotspot networks used by 1567 travelling users, and other paths that could add or modify headers, 1568 or modify the message body. 1570 For example, assume Joe works for Company A and has an email address 1571 joe@companya.example. Joe also has a ISP-1 account 1572 joe@isp1.example.com, and he uses ISP-1's multiple address feature to 1573 attach his work email joe@companya.example to his ISP-1 account. 1574 When Joe sends email from his ISP-1 account and uses 1575 joe@companya.example as his designated From: address, that email 1576 cannot have a signature with d=companya.example because the ISP-1 1577 servers have no access to Company A's private key. In ISP-1's case 1578 it will have a ISP-1 signature, but for some other mail clients 1579 offering the same multiple address feature there may be no signature 1580 at all on the message. 1582 Another example might be the use of a forward article to a friend 1583 service. Most instances of these services today allow someone to 1584 send an article with their email address in the RFC5322.From to their 1585 designated recipient. If Joe used either of his two addresses 1586 (joe@companya.example or joe@isp1.example.com), the forwarder would 1587 be equally unable to sign with a corresponding domain . As in the 1588 mail client case, the forwarder may either sign as its own domain, or 1589 may put no signature on the message. 1591 A third example is the use of privately configured forwarding. 1592 Assume that Joe has another account at ISP-2, joe@isp-2.example.com, 1593 but he'd prefer to read his ISP-2 mail from his ISP-1 account. He 1594 sets up his ISP-2 account to forward all incoming mail to 1595 joe@isp1.example.com. Assume alice@companyb.example sends 1596 joe@isp-2.example.com an email. Depending on how companyb.example 1597 configured its signature, and depending on whether or not ISP-2 1598 modifies messages that it forwards, it is possible that when Alice's 1599 message is received in Joe's ISP-1 account the original signature 1600 fails verification. 1602 8.2. Protection of Internal Mail 1604 One identity is particularly amenable to easy and accurate 1605 assessment: the organization's own identity. Members of an 1606 organization tend to trust messages that purport to be from within 1607 that organization. However Internet Mail does not provide a 1608 straightforward means of determining whether such mail is, in fact, 1609 from within the organization. DKIM can be used to remedy this 1610 exposure. If the organization signs all of its mail, then its 1611 boundary MTAs can look for mail purporting to be from the 1612 organization that does not contain a verifiable signature. 1614 Such mail can in most cases be presumed to be spurious. However, 1615 domain managers are advised to consider the ways that mail processing 1616 can modify messages in ways that will invalidate an existing DKIM 1617 signature: mailing lists, courtesy forwarders, and other paths that 1618 could add or modify headers or modify the message body (e.g. MTAs at 1619 hotels, hotspot networks used by travelling users, and other 1620 scenarios described in the previous section). Such breakage is 1621 particularly relevant in the presence of Author Domain Signing 1622 Practices. 1624 8.3. Signature Granularity 1626 Although DKIM's use of domain names is optimized for a scope of 1627 organization-level signing, it is possible to administer sub-domains 1628 or otherwise adjust signatures in a way that supports per-user 1629 identification. This user level granularity can be specified in two 1630 ways: either by sharing the signing identity and specifying an 1631 extension to the i= value that has a per-user granularity, or by 1632 creating and signing with unique per-user keys. 1634 A subdomain or local part in the i= tag SHOULD be treated as an 1635 opaque identifier and thus need not correspond directly to a DNS 1636 subdomain or be a specific user address. 1638 The primary way to sign with per-user keys requires each user to have 1639 a distinct DNS (sub)domain, where each distinct d= value has a key 1640 published. (It is possible, although not recommended, to publish the 1641 same key in more than one distinct domain.) 1643 It is technically possible to publish per-user keys within a single 1644 domain or subdomain by utilizing different selector values. This is 1645 not recommended and is unlikely to be treated uniquely by Assessors: 1646 the primary purpose of selectors is to facilitate key management, and 1647 the DKIM specification recommends against using them in determining 1648 or assessing identies. 1650 In most cases, it would be impractical to sign email on a per-user 1651 granularity. Such an approach would be 1653 likely to be ignored: In most cases today, if receivers are 1654 verifying DKIM signatures they are in general taking the simplest 1655 possible approach. In many cases maintaining reputation 1656 information at a per user granularity is not interesting to them, 1657 in large part because the per user volume is too small to be 1658 useful or interesting. So even if senders take on the complexity 1659 necessary to support per user signatures, receivers are unlikely 1660 to retain anything more than the base domain reputation. 1662 difficult to manage: Any scheme that involves maintenance of a 1663 significant number of public keys may require infrastructure 1664 enhancements or extensive administrative expertise. For domains 1665 of any size, maintaining a valid per-user keypair, knowing when 1666 keys need to be revoked or added due to user attrition or 1667 onboarding, and the overhead of having the signing engine 1668 constantly swapping keys can create significant and often 1669 unnecessary managment complexity. It is also important to note 1670 that there is no way within the scope of the DKIM specification 1671 for a receiver to infer that a sender intends a per-user 1672 granularity. 1674 As mentioned before, what may make sense, however, is to use the 1675 infrastructure that enables finer granularity in signatures to 1676 identify segments smaller than a domain but much larger than a per- 1677 user segmentation. For example, a university might want to segment 1678 student, staff, and faculty mail into three distinct streams with 1679 differing reputations. This can be done by creating seperate sub- 1680 domains for the desired segments, and either specifying the 1681 subdomains in the i= tag of the DKIM Signature or by adding 1682 subdomains to the d= tag and assigning and signing with different 1683 keys for each subdomain. 1685 For those who choose to represent user level granularity in 1686 signatures, the performance and management considerations above 1687 suggest that it would be more effective to do it by specifying a 1688 local part or subdomain extension in the i= tag rather than by 1689 extending the d= domain and publishing individual keys. 1691 8.4. Email Infrastructure Agents 1693 It is expected that the most common venue for a DKIM implementation 1694 will be within the infrastructure of an organization's email service, 1695 such as a department or a boundary MTA. What follows are some 1696 general recommendations for the Email Infrastructure. 1698 Outbound: An MSA or an Outbound MTA used for mail submission 1699 SHOULD ensure that the message sent is in compliance with the 1700 advertised email sending policy. It SHOULD also be able to 1701 generate an operator alert if it determines that the email 1702 messages do not comply with the published DKIM sending policy. 1704 An MSA SHOULD be aware that some MUAs may add their own 1705 signatures. If the MSA needs to perform operations on a 1706 message to make it comply with its email sending policy, if at 1707 all possible, it SHOULD do so in a way that would not break 1708 those signatures. 1710 MUAs equipped with the ability to sign SHOULD NOT be 1711 encouraged. In terms of security, MUAs are generally not under 1712 the direct control of those in responsible roles within an 1713 organization and are thus more vulnerable to attack and 1714 compromise, which would expose private signing keys to 1715 intruders and thus jeopardize the integrity and reputation of 1716 the organization. 1718 Inbound: When an organization deploys DKIM, it needs to make 1719 sure that its email infrastructure components that do not have 1720 primary roles in DKIM handling do not modify message in ways 1721 that prevent subsequent verification. 1723 An inbound MTA or an MDA may incorporate an indication of the 1724 verification results into the message, such as using an 1725 Authentication-Results header field. [RFC5451] 1727 Intermediaries: An email intermediary is both an inbound and 1728 outbound MTA. Each of the requirements outlined in the 1729 sections relating to MTAs apply. If the intermediary modifies 1730 a message in a way that breaks the signature, the intermediary 1732 + SHOULD deploy abuse filtering measures on the inbound mail, 1733 and 1735 + MAY remove all signatures that will be broken 1737 In addition the intermediary MAY: 1739 + Verify the message signature prior to modification. 1741 + Incorporate an indication of the verification results into 1742 the message, such as using an Authentication-Results header 1743 field. [RFC5451] 1745 + Sign the modified message including the verification results 1746 (e.g., the Authentication-Results header field). 1748 8.5. Mail User Agent 1750 The DKIM specification is expected to be used primarily between 1751 Boundary MTAs, or other infrastructure components of the originating 1752 and receiving ADMDs. However there is nothing in DKIM that is 1753 specific to those venues. In particular, MUAs MAY also support DKIM 1754 signing and verifying directly. 1756 Outbound: An MUA MAY support signing even if mail is to be 1757 relayed through an outbound MSA. In this case the signature 1758 applied by the MUA will be in addition to any signature added 1759 by the MSA. However, the warnings in the previous section 1760 should be taken into consideration. 1762 Some user software goes beyond simple user functionality and 1763 also perform MSA and MTA functions. When this is employed for 1764 sending directly to a receiving ADMD, the user software SHOULD 1765 be considered an outbound MTA. 1767 Inbound: An MUA MAY rely on a report of a DKIM signature 1768 verification that took place at some point in the inbound MTA/ 1769 MDA path (e.g., an Authentication-Results header field), or an 1770 MUA MAY perform DKIM signature verification directly. A 1771 verifying MUA SHOULD allow for the case where mail has modified 1772 in the inbound MTA path; if a signature fails, the message 1773 SHOULD NOT be treated any different than if it did not have a 1774 signature. 1776 An MUA that looks for an Authentication-Results header field 1777 MUST be configurable to choose which Authentication-Results are 1778 considered trustable. The MUA developer is encouraged to re- 1779 read the Security Considerations of [RFC5451]. 1781 DKIM requires that all verifiers treat messages with signatures 1782 that do not verify as if they are unsigned. 1784 If verification in the client is to be acceptable to users, it 1785 is essential that successful verification of a signature not 1786 result in a less than satisfactory user experience compared to 1787 leaving the message unsigned. The mere presence of a verified 1788 DKIM signature MUST NOT by itself be used by an MUA to indicate 1789 that a message is to be treated better than a message without a 1790 verified DKIM signature. However, the fact that a DKIM 1791 signature was verified MAY be used as input into a reputation 1792 system (i.e., a whitelist of domains and users) for 1793 presentation of such indicators. 1795 It is common for components of an ADMD's email infrastructure to do 1796 violence to a message, such that a DKIM signature might be rendered 1797 invalid. Hence, users of MUAs that support DKIM signing and/or 1798 verifying need a basis for knowing that their associated email 1799 infrastructure will not break a signature. 1801 9. Other Considerations 1803 9.1. Security Considerations 1805 The security considerations of the DKIM protocol are described in the 1806 DKIM base specification [RFC4871]. 1808 9.2. IANA Considerations 1810 This document has no considerations for IANA. 1812 10. Acknowledgements 1814 TBD 1816 11. Informative References 1818 [I-D.ietf-dkim-rfc4871-errata] 1819 Crocker, D., "RFC 4871 DomainKeys Identified Mail (DKIM) 1820 Signatures -- Update", draft-ietf-dkim-rfc4871-errata-07 1821 (work in progress), June 2009. 1823 [RFC0989] Linn, J. and IAB Privacy Task Force, "Privacy enhancement 1824 for Internet electronic mail: Part I: Message encipherment 1825 and authentication procedures", RFC 989, February 1987. 1827 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", 1828 STD 13, RFC 1034, November 1987. 1830 [RFC1848] Crocker, S., Galvin, J., Murphy, S., and N. Freed, "MIME 1831 Object Security Services", RFC 1848, October 1995. 1833 [RFC1991] Atkins, D., Stallings, W., and P. Zimmermann, "PGP Message 1834 Exchange Formats", RFC 1991, August 1996. 1836 [RFC2440] Callas, J., Donnerhacke, L., Finney, H., and R. Thayer, 1837 "OpenPGP Message Format", RFC 2440, November 1998. 1839 [RFC3156] Elkins, M., Del Torto, D., Levien, R., and T. Roessler, 1840 "MIME Security with OpenPGP", RFC 3156, August 2001. 1842 [RFC3164] Lonvick, C., "The BSD Syslog Protocol", RFC 3164, 1843 August 2001. 1845 [RFC3851] Ramsdell, B., "Secure/Multipurpose Internet Mail 1846 Extensions (S/MIME) Version 3.1 Message Specification", 1847 RFC 3851, July 2004. 1849 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. 1850 Rose, "Resource Records for the DNS Security Extensions", 1851 RFC 4034, March 2005. 1853 [RFC4686] Fenton, J., "Analysis of Threats Motivating DomainKeys 1854 Identified Mail (DKIM)", RFC 4686, September 2006. 1856 [RFC4870] Delany, M., "Domain-Based Email Authentication Using 1857 Public Keys Advertised in the DNS (DomainKeys)", RFC 4870, 1858 May 2007. 1860 [RFC4871] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, 1861 J., and M. Thomas, "DomainKeys Identified Mail (DKIM) 1862 Signatures", RFC 4871, May 2007. 1864 [RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. 1865 Thayer, "OpenPGP Message Format", RFC 4880, November 2007. 1867 [RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS 1868 Security (DNSSEC) Hashed Authenticated Denial of 1869 Existence", RFC 5155, March 2008. 1871 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 1872 October 2008. 1874 [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, 1875 October 2008. 1877 [RFC5451] Kucherawy, M., "Message Header Field for Indicating 1878 Message Authentication Status", RFC 5451, April 2009. 1880 [RFC5585] Hansen, T., Crocker, D., and P. Hallam-Baker, "DomainKeys 1881 Identified Mail (DKIM) Service Overview", RFC 5585, 1882 July 2009. 1884 [RFC5617] Allman, E., Fenton, J., Delany, M., and J. Levine, 1885 "DomainKeys Identified Mail (DKIM) Author Domain Signing 1886 Practices (ADSP)", RFC 5617, August 2009. 1888 Appendix A. Migration Strategies 1890 There are three migration occassions worth noting in particular for 1891 DKIM: 1893 1. Migrating from Domain Keys to DKIM. 1895 2. Migrating from a current hash algorithm to a new standardized 1896 hash algorithm. 1898 3. Migrating from a current signing algorithm to a new standardized 1899 signing algorithm. 1901 The case of deploying a new key selector record is described 1902 elsewhere (Section 3.5). 1904 As with any migration, the steps required will be determined by who 1905 is doing the migration and their assessment of 1907 o the users of what they are generating, or 1909 o the providers of what they are consuming. 1911 Signers and verifiers have different considerations. 1913 A.1. Migrating from DomainKeys 1915 DKIM replaces the earlier DomainKeys (DK) specification. Selector 1916 files are mostly compatible between the two specifications. 1918 A.1.1. Signers 1920 A signer that currently signs with DK will go through various stages 1921 as it migrates to using DKIM, not all of which are required for all 1922 signers. The real questions that a signer must ask are: 1924 1. how many receivers or what types of receivers are *only* looking 1925 at the DK signatures and not the DKIM signatures, and 1927 2. how much does the signer care about those receivers? 1929 If no one is looking at the DK signature any more, then it's no 1930 longer necessary to sign with DK. Or if all "large players" are 1931 looking at DKIM in addition to or instead of DK, a signer MAY choose 1932 to stop signing with DK. 1934 With respect to signing policies, a reasonable, initial approach is 1935 to use DKIM signatures in the same way as DomainKeys signatures are 1936 already being used. In particular, the same selectors and DNS Key 1937 Records may be used for both, after verifying that they are 1938 compatible as discussed below. 1940 Each secondary step in all of the following scenarios is to be 1941 prefaced with the gating factor "test, then when comfortable with the 1942 previous step's results, continue". 1944 One migration strategy is to: 1946 o ensure that the current selector DNS key record is compatible with 1947 both DK and DKIM 1949 o sign messages with both DK and DKIM signatures 1951 o when it's decided that DK signatures are no longer necessary, stop 1952 signing with DK 1954 Another migration strategy is to: 1956 o add a new selector DNS key record only for DKIM signatures 1958 o sign messages with both DK (using the old DNS key record) and DKIM 1959 signatures (using the new DNS key record) 1961 o when it's decided that DK signatures are no longer necessary, stop 1962 signing with DK 1964 o eventually remove the old DK selector DNS record 1966 A combined migration strategy is to: 1968 o ensure that the current selector DNS key record is compatible with 1969 both DK and DKIM 1971 o start signing messages with both DK and DKIM signatures 1973 o add a new selector DNS key record for DKIM signatures 1975 o switch the DKIM signatures to use the new selector 1977 o when it's decided that DK signatures are no longer necessary, stop 1978 signing with DK 1980 o eventually remove the old DK selector DNS record 1982 Another migration strategy is to: 1984 o add a new selector DNS key record for DKIM signatures 1986 o do a flash cut and replace the DK signatures with DKIM signatures 1988 o eventually remove the old DK selector DNS record 1990 Another migration strategy is to: 1992 o ensure that the current selector DNS key record is compatible with 1993 both DK and DKIM 1995 o do a flash cut and replace the DK signatures with DKIM signatures 1997 Note that when you have separate key records for DK and DKIM, you can 1998 use the same public key for both. 2000 A.1.1.1. DNS Selector Key Records 2002 The first step in some of the above scenarios is ensuring that the 2003 selector DNS key records are compatible for both DK and DKIM. The 2004 format of the DNS key record was intentionally meant to be backwardly 2005 compatible between the two systems, but not necessarily upwardly 2006 compatible. DKIM has enhanced the DK DNS key record format by adding 2007 several optional parameters, which DK must ignore. However, there is 2008 one critical difference between DK and DKIM DNS key records: the 2009 definitions of the "g" fields: 2011 g= granularity of the key In both DK and DKIM, this is an optional 2012 field that is used to constrain which sending address(es) can 2013 legitimately use this selector. Unfortunately, the treatment of 2014 an empty field ("g=;") is different. DKIM allows wildcards where 2015 DK does not. For DK, an empty field is the same as a missing 2016 value, and is treated as allowing any sending address. For DKIM, 2017 an empty field only matches an empty local part. In DKIM, both a 2018 missing value and "g=*;" mean to allow any sending address. 2020 If your DK DNS key record has an empty "g" field in it ("g=;"), 2021 your best course of action is to modify the record to remove the 2022 empty field. In that way, the DK semantics will remain the same, 2023 and the DKIM semantics will match. 2025 If your DNS key record does not have an empty "g" field in it 2026 ("g=;"), it's probable that the record can be left alone. But your 2027 best course of action would still be to make sure it has a "v" field. 2029 When the decision is made to stop supporting DomainKeys and to only 2030 support DKIM, you MUST verify that the "g" field is compatible with 2031 DKIM, and it SHOULD have "v=DKIM1;" in it. It is highly RECOMMENDED 2032 that if you want to use an empty "g" field in your DKIM selector, you 2033 also include the "v" field. 2035 A.1.1.2. Removing DomainKeys Signatures 2037 The principal use of DomainKeys is at Boundary MTAs. Because no 2038 operational transition is ever instantaneous, it is advisable to 2039 continue performing DomainKeys signing until it is determined that 2040 DomainKeys receive-side support is no longer used, or is sufficiently 2041 reduced. That is, a signer SHOULD add a DKIM signature to a message 2042 that also has a DomainKeys signature and keep it there until you 2043 decide it is deemed no longer useful. The signer may do its 2044 transitions in a straightforward manner, or more gradually. Note 2045 that because digital signatures are not free, there is a cost to 2046 performing both signing algorithms, so signing with both algorithms 2047 should not be needlessly prolonged. 2049 The tricky part is deciding when DK signatures are no longer 2050 necessary. The real questions are: how many DomainKeys verifiers are 2051 there that do *not* also do DKIM verification, which of those are 2052 important, and how can you track their usage? Most of the early 2053 adopters of DK verification have added DKIM verification, but not all 2054 yet. If a verifier finds a message with both DK and DKIM, it may 2055 choose to verify both signatures, or just one or the other. 2057 Many DNS services offer tracking statistics so it can be determined 2058 how often a DNS record has been accessed. By using separate DNS 2059 selector key records for your signatures, you can chart the usage of 2060 your records over time, and watch the trends. An additional 2061 distinguishing factor to track would take into account the verifiers 2062 that verify both the DK and DKIM signatures, and discount those from 2063 counts of DK selector usage. When the number for DK selector access 2064 reaches a low-enough level, that's the time to consider discontinuing 2065 signing with DK. 2067 Note, this level of rigor is not required. It is perfectly 2068 reasonable for a DK signer to decide to follow the "flash cut" 2069 scenario described above. 2071 A.1.2. Verifiers 2073 As a verifier, several issues must be considered: 2075 A.1.2.1. Should DK signature verification be performed? 2077 At the time of writing, there is still a significant number of sites 2078 that are only producing DK signatures. Over time, it is expected 2079 that this number will go to zero, but it may take several years. So 2080 it would be prudent for the foreseeable future for a verifier to look 2081 for and verify both DKIM and DK signatures. 2083 A.1.2.2. Should both DK and DKIM signatures be evaluated on a single 2084 message? 2086 For a period of time, there will be sites that sign with both DK and 2087 DKIM. A verifier receiving a message that has both types of 2088 signatures may verify both signatures, or just one. One disadvantage 2089 of verifying both signatures is that signers will have a more 2090 difficult time deciding how many verifiers are still using their DK 2091 selectors. One transition strategy is to verify the DKIM signature, 2092 then only verify the DK signature if the DKIM verification fails. 2094 A.1.2.3. DNS Selector Key Records 2096 The format of the DNS key record was intentionally meant to be 2097 backwardly compatible between DK and DKIM, but not necessarily 2098 upwardly compatible. DKIM has enhanced the DK DNS key record format 2099 by adding several optional parameters, which DK must ignore. 2100 However, there is one key difference between DK and DKIM DNS key 2101 records: the definitions of the g fields: 2103 g= granularity of the key In both DK and DKIM, this is an optional 2104 field that is used to constrain which sending address(es) can 2105 legitimately use this selector. Unfortunately, the treatment of 2106 an empty field ("g=;") is different. For DK, an empty field is 2107 the same as a missing value, and is treated as allowing any 2108 sending address. For DKIM, an empty field only matches an empty 2109 local part. 2111 v= version of the selector It is recommended that a DKIM selector 2112 have "v=DKIM1;" at its beginning, but it is not required. 2114 If a DKIM verifier finds a selector record that has an empty "g" 2115 field ("g=;") and it does not have a "v" field ("v=DKIM1;") at its 2116 beginning, it is faced with deciding if this record was 2118 1. from a DK signer that transitioned to supporting DKIM but forgot 2119 to remove the "g" field (so that it could be used by both DK and 2120 DKIM verifiers), or 2122 2. from a DKIM signer that truly meant to use the empty "g" field 2123 but forgot to put in the "v" field. It is RECOMMENDED that you 2124 treat such records using the first interpretation, and treat such 2125 records as if the signer did not have a "g" field in the record. 2127 A.2. Migrating Hash Algorithms 2129 [RFC4871] defines the use of two hash algorithms, SHA-1 and SHA-256. 2130 The security of all hash algorithms is constantly under attack, and 2131 SHA-1 has already shown weaknesses as of this writing. Migrating 2132 from SHA-1 to SHA-256 is not an issue, because all verifiers are 2133 already required to support SHA-256. But when it becomes necessary 2134 to replace SHA-256 with a more secure algorithm, there will be a 2135 migratory period. In the following, "NEWHASH" is used to represent a 2136 new hash algorithm. Section 4.1 of [RFC4871] briefly discusses this 2137 scenario. 2139 A.2.1. Signers 2141 As with migrating from DK to DKIM, migrating hash algorithms is 2142 dependent on the signer's best guess as to the utility of continuing 2143 to sign with the older algorithms and the expected support for the 2144 newer algorithm by verifiers. The utility of continuing to sign with 2145 the older algorithms is also based on how broken the existing hash 2146 algorithms are considered and how important that is to the signers. 2148 One strategy is to wait until it's determined that there is a large 2149 enough base of verifiers available that support NEWHASH, and then 2150 flash cut to the new algorithm. 2152 Another strategy is to sign with both the old and new hash algorithms 2153 for a period of time. This is particularly useful for testing the 2154 new code to support the new hash algorithm, as verifiers will 2155 continue to accept the signature for the older hash algorithm and 2156 should ignore any signature that fails because the code is slightly 2157 wrong. Once the signer has determined that the new code is correct 2158 AND it's determined that there is a large enough base of verifiers 2159 available that support NEWHASH, the signer can flash cut to the new 2160 algorithm. 2162 One advantage migrating hash algorithms has is that the selector can 2163 be completely compatible for all hash algorithms. The key selector 2164 has an optional "h=" field that may be used to list the hash 2165 algorithms being used; it also is used to limit the algorithms that a 2166 verifier will accept. If the signer is not currently using the key- 2167 selector "h=" field, no change is required. If the signer is 2168 currently using the key-selector "h=" field, NEWHASH will need to be 2169 added to the list, as in "h=sha256:NEWHASH;". (When the signer is no 2170 longer using sha256, it can be removed from the "h=" list.) 2172 A.2.2. Verifiers 2174 When a new hash algorithm becomes standardized, it is best for a 2175 verifier to start supporting it as quickly as possible. 2177 A.3. Migrating Signing Algorithms 2179 [RFC4871] defines the use of the RSA signing algorithm. Similar to 2180 hashes, signing algorithms are constantly under attack, and when it 2181 becomes necessary to replace RSA with a newer signing algorithm, 2182 there will be a migratory period. In the following, "NEWALG" is used 2183 to represent a new signing algorithm. 2185 A.3.1. Signers 2187 As with the other migration issues discussed above, migrating signing 2188 algorithms is dependent on the signer's best guess as to the utility 2189 of continuing to sign with the older algorithms and the expected 2190 support for the newer algorithm by verifiers. The utility of 2191 continuing to sign with the older algorithms is also based on how 2192 broken the existing signing algorithms are considered and how 2193 important that is to the signers. 2195 As before, the two basic strategies are to 1) wait until there is 2196 sufficient base of verifiers available that support NEWALG and then 2197 do a flash cut to NEWALG, and 2) using a phased approach by signing 2198 with both the old and new algorithms before removing support for the 2199 old algorithm. 2201 It is unlikely that a new algorithm would be able to use the same 2202 public key as "rsa", so using the same selector DNS record for both 2203 algorithms' keys is ruled out. Therefore, in order to use the new 2204 algorithm, a new DNS selector record would need to be deployed in 2205 parallel with the existing DNS selector record for the existing 2206 algorithm. The new DNS selector record would specify a different 2207 "k=" value to reflect the use of NEWALG. 2209 A.3.2. Verifiers 2211 When a new hash algorithm becomes standardized, it is best for a 2212 verifier to start supporting it as quickly as possible. 2214 Appendix B. General Coding Criteria for Cryptographic Applications 2216 NOTE: This section could possibly be changed into a reference to 2217 something else, such as another rfc. 2219 Correct implementation of a cryptographic algorithm is a necessary 2220 but not a sufficient condition for the coding of cryptographic 2221 applications. Coding of cryptographic libraries requires close 2222 attention to security considerations that are unique to cryptographic 2223 applications. 2225 In addition to the usual security coding considerations, such as 2226 avoiding buffer or integer overflow and underflow, implementers 2227 should pay close attention to management of cryptographic private 2228 keys and session keys, ensuring that these are correctly initialized 2229 and disposed of. 2231 Operating system mechanisms that permit the confidentiality of 2232 private keys to be protected against other processes should be used 2233 when available. In particular, great care must be taken when 2234 releasing memory pages to the operating system to ensure that private 2235 key information is not disclosed to other processes. 2237 Certain implementations of public key algorithms such as RSA may be 2238 vulnerable to a timing analysis attack. 2240 Support for cryptographic hardware providing key management 2241 capabilities is strongly encouraged. In addition to offering 2242 performance benefits, many cryptographic hardware devices provide 2243 robust and verifiable management of private keys. 2245 Fortunately appropriately designed and coded cryptographic libraries 2246 are available for most operating system platforms under license terms 2247 compatible with commercial, open source and free software license 2248 terms. Use of standard cryptographic libraries is strongly 2249 encouraged. These have been extensively tested, reduce development 2250 time and support a wide range of cryptographic hardware. 2252 Authors' Addresses 2254 Tony Hansen 2255 AT&T Laboratories 2256 200 Laurel Ave. South 2257 Middletown, NJ 07748 2258 USA 2260 Email: tony+dkimov@maillennium.att.com 2261 Ellen Siegel 2263 Email: dkim@esiegel.net 2265 Phillip Hallam-Baker 2266 Default Deny Security, Inc. 2268 Email: phillip@hallambaker.com 2270 Dave Crocker 2271 Brandenburg InternetWorking 2272 675 Spruce Dr. 2273 Sunnyvale, CA 94086 2274 USA 2276 Email: dcrocker@bbiw.net