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Checking references for intended status: Experimental ---------------------------------------------------------------------------- == Missing Reference: 'FWS' is mentioned on line 318, but not defined == Missing Reference: 'THIS MEMO' is mentioned on line 495, but not defined ** Obsolete normative reference: RFC 5451 (ref. 'AUTHRES') (Obsoleted by RFC 7001) -- Obsolete informational reference (is this intentional?): RFC 5226 (ref. 'IANA') (Obsoleted by RFC 8126) Summary: 1 error (**), 0 flaws (~~), 3 warnings (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Individual submission M. Kucherawy 3 Internet-Draft Cloudmark, Inc. 4 Intended status: Experimental January 4, 2012 5 Expires: July 7, 2012 7 DKIM Authorized Third-Party Signers 8 draft-kucherawy-dkim-atps-15 10 Abstract 12 This experimental specification proposes a modification to Domain 13 Keys Identified Mail (DKIM) allowing advertisement of third-party 14 signature authorizations that are to be interpreted as equivalent to 15 a signature added by the administrative domain of the message's 16 author. 18 Status of This Memo 20 This Internet-Draft is submitted in full conformance with the 21 provisions of BCP 78 and BCP 79. 23 Internet-Drafts are working documents of the Internet Engineering 24 Task Force (IETF). Note that other groups may also distribute 25 working documents as Internet-Drafts. The list of current Internet- 26 Drafts is at http://datatracker.ietf.org/drafts/current/. 28 Internet-Drafts are draft documents valid for a maximum of six months 29 and may be updated, replaced, or obsoleted by other documents at any 30 time. It is inappropriate to use Internet-Drafts as reference 31 material or to cite them other than as "work in progress." 33 This Internet-Draft will expire on July 7, 2012. 35 Copyright Notice 37 Copyright (c) 2012 IETF Trust and the persons identified as the 38 document authors. All rights reserved. 40 This document is subject to BCP 78 and the IETF Trust's Legal 41 Provisions Relating to IETF Documents 42 (http://trustee.ietf.org/license-info) in effect on the date of 43 publication of this document. Please review these documents 44 carefully, as they describe your rights and restrictions with respect 45 to this document. Code Components extracted from this document must 46 include Simplified BSD License text as described in Section 4.e of 47 the Trust Legal Provisions and are provided without warranty as 48 described in the Simplified BSD License. 50 Table of Contents 52 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 53 2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3 54 2.1. Keywords . . . . . . . . . . . . . . . . . . . . . . . . . 3 55 2.2. E-Mail Architecture Terminology . . . . . . . . . . . . . 3 56 3. Roles and Scope . . . . . . . . . . . . . . . . . . . . . . . 4 57 4. Queries and Replies . . . . . . . . . . . . . . . . . . . . . 4 58 4.1. Hash Selection . . . . . . . . . . . . . . . . . . . . . . 4 59 4.2. Extension to DKIM . . . . . . . . . . . . . . . . . . . . 5 60 4.3. ATPS Query Details . . . . . . . . . . . . . . . . . . . . 5 61 4.4. ATPS Reply Details . . . . . . . . . . . . . . . . . . . . 6 62 5. Interpretation . . . . . . . . . . . . . . . . . . . . . . . . 8 63 6. Relationship to ADSP . . . . . . . . . . . . . . . . . . . . . 8 64 7. Experiment Process . . . . . . . . . . . . . . . . . . . . . . 8 65 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 66 8.1. ATPS Tag Registry . . . . . . . . . . . . . . . . . . . . 9 67 8.2. Email Authentication Method Name Registry Update . . . . . 9 68 8.3. Email Authentication Result Name Registry Update . . . . . 10 69 8.4. DKIM-Signature Tag Specification Registry . . . . . . . . 11 70 9. Security Considerations . . . . . . . . . . . . . . . . . . . 11 71 9.1. Hash Selection . . . . . . . . . . . . . . . . . . . . . . 11 72 9.2. False Privacy . . . . . . . . . . . . . . . . . . . . . . 12 73 9.3. Transient Security Failures . . . . . . . . . . . . . . . 12 74 9.4. Load on the DNS . . . . . . . . . . . . . . . . . . . . . 12 75 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13 76 10.1. Normative References . . . . . . . . . . . . . . . . . . . 13 77 10.2. Informative References . . . . . . . . . . . . . . . . . . 13 78 Appendix A. Example Query and Reply . . . . . . . . . . . . . . . 14 79 Appendix B. Choice of DNS RR Type . . . . . . . . . . . . . . . . 14 80 Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 15 82 1. Introduction 84 [DKIM] defines a mechanism for transparent domain-level signing of 85 messages for the purpose of declaring that a particular 86 Administrative Mail Domain (ADMD) takes some responsibility for a 87 message. 89 DKIM, however, deliberately makes no binding between the DNS domain 90 of the signer and any other identity found in the message. Despite 91 this, there is an automatic human perception that an author domain 92 signature (one for which the RFC5322.From domain matches the DNS 93 domain of the signer) is more valuable or trustworthy than any other. 95 To enable a third party to apply DKIM signatures to messages, the 96 DKIM specification suggests delegation to a third party of either 97 subdomains or private keys, so that the third party can add DKIM 98 signatures that appear to have been added by the Author ADMD. Absent 99 is a protocol by which an Author ADMD can announce that messages 100 bearing specific valid DKIM signatures on its mail, which are added 101 by other ADMDs, are to be treated as if they were signed by the 102 Author ADMD itself. This memo presents an experimental mechanism for 103 doing so, called Authorized Third-Party Signers (ATPS). 105 ATPS augments the semantics of DKIM by providing to the verifier 106 multiple identifiers rather than one. Specifically, it validates the 107 identifier found in the DKIM signature, and then provides the 108 RFC5322.From domain for evaluation. 110 This memo also registers, per [AUTHRES], the means to indicate to 111 agents downstream of the Verifier that a third-party signature 112 verification occurred. 114 2. Definitions 116 2.1. Keywords 118 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 119 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 120 document are to be interpreted as described in [KEYWORDS]. 122 2.2. E-Mail Architecture Terminology 124 Readers are advised to be familiar with the material and terminology 125 discussed in [MAIL] and [EMAIL-ARCH]. 127 3. Roles and Scope 129 The context of this protocol involves the following roles: 131 o ADministrative Mail Domain (ADMD)s, whose DNS domain name(s) 132 appear in the RFC5322.From field of a [MAIL] message; 134 o ATPS Signers, which apply [DKIM] signatures using their own 135 domains, but on behalf of the message Author's ADMD; and 137 o the Verifier, who implements the signature validation procedures 138 described in [DKIM]. 140 An ADMD implements this protocol if it wishes to announce that a 141 signature from any in a set of specified DNS domains is to be 142 considered equivalent to one from the ADMD itself. One might, for 143 example, wish to delegate signing authority for its DNS domain to an 144 approved messaging service provider without doing the work of key 145 transfer described in Appendix B.1.1 of [DKIM]. An authorized ATPS 146 Signer makes a claim of this relationship via new tags in the DKIM 147 signature, and the ADMD confirms this claim by publishing a specific 148 TXT record in its DNS. 150 A Verifier implements this protocol if it wishes to ensure that a 151 message bears one or more signatures from sources authorized to sign 152 mail on behalf of the ADMD, and identify for special treatment mail 153 that meets (or does not meet) that criterion. It will do so by 154 treating the signer's authorization on behalf of the author's ADMD to 155 mean that the signer's signature is equivalent to one affixed by the 156 author's ADMD. 158 4. Queries and Replies 160 This section describes in detail the queries issued, the replies 161 received, and how they should be interpreted and applied. 163 4.1. Hash Selection 165 The author's ADMD will indicate authorization of a third party to 166 sign its mail via the presence of a DNS TXT record that contains an 167 encoding of the third party's DNS domain name. The encoding 168 mechanism is constructed so that any domain name can be added to the 169 DNS in a fixed length, so that longer third-party domain names are 170 not excluded from participation because of the overall length limit 171 on a DNS query. 173 Part of the mechanism requires constructing a digest of the third 174 party's DNS domain name. The author ADMD MUST select a digest 175 ("hash") method currently supported by DKIM (see Section 7.7 of 176 [DKIM]), and this selection needs to be communicated to the ATPS 177 Signer as it is used in generation of its the third party signatures. 179 The full DNS mechanism is specified in Section 4.3. 181 4.2. Extension to DKIM 183 [DKIM] signatures contain a "tag=value" sequence. This protocol will 184 add additional tags called "atps" and "atpsh". 186 When the ATPS Signer generates a DKIM signature for another ADMD, it 187 MUST put its own domain in the signature's "d" tag, and include an 188 "atps" tag that has as its value the domain name of the ADMD on whose 189 behalf it is signing. 191 The tag name that carries the name of the selected hash algorithm is 192 "atpsh". This tag MUST also be included, as it is required as part 193 of the algorithm that will be enacted by the Verifier. 195 The formal syntax definition, per [ABNF]: 197 dkim-atps-tag = %x61.74.70.73 *WSP "=" *WSP domain-name 199 dkim-atpsh-tag = %x61.74.70.73.68 *WSP "=" *WSP key-h-tag-alg 201 "domain-name" and "key-h-tag-alg" are defined in [DKIM]. Note that 202 according to [DKIM], internationalized domain names are to be encoded 203 as A-labels, as described in Section 2.3 of [IDNA]. 205 The registration for these tags can be found in Section 8. 207 4.3. ATPS Query Details 209 When a [DKIM] signature including an "atps" tag is successfully 210 verified, and is considered acceptable to the Verifier according to 211 any local policy requirements (which are not discussed here or in 212 [DKIM]), the Verifier compares the domain name in the value of that 213 tag with the one found in the RFC5322.From field of the message. The 214 match MUST be done in a case-insensitive manner. 216 If they do not match, the "atps" tag MUST be ignored. 218 If they do match, the Verifier issues a DNS TXT query, as specified 219 below, looking for confirmation by the Author ADMD that the ATPS 220 Signer is authorized by that ADMD to sign mail on its behalf. Where 221 multiple DKIM signatures are present including valid "atps" tags, 222 these queries MAY be done in any order or MAY be done in parallel. 224 Where the RFC5322.From field contains multiple addresses, this 225 process SHOULD be applied if the "atps" tag's value matches any of 226 the domains found in that field. These MAY be done in any order. 228 Note that the algorithm uses hashing, but this is not a security 229 mechanism. See Section 9.2 for discussion. 231 The name for the query is constructed as follows: 233 1. Select the hash algorithm from the "atpsh" tag in the signature. 234 If the hash algorithm specified does not appear in the list 235 registered with IANA as one valid for use with DKIM (see Section 236 7.7 of [DKIM]), abort the query. 238 2. Extract the value of the "d=" tag from the signature. 240 3. Convert any upper-case characters in that string to their lower- 241 case equivalents. 243 4. Feed the resulting string to the selected hash algorithm. 245 5. Convert the output of the hash to a string of printable ASCII 246 characters by applying base32 encoding as defined in Section 6 of 247 [BASE32]. The base32 encoding is used because its output is 248 restricted to characters that are legal for use in labels in the 249 DNS, and evaluates the same way in the DNS whether encoded using 250 uppercase or lowercase characters. 252 6. Append the string "._atps." 254 7. Append the domain name found in the "atps" tag of the validated 255 signature. 257 The query's formal syntax definition, per [ABNF]: 259 atps-query = 1*63BASE32 %x2e.5f.61.74.70.73.2e domain-name 261 BASE32 = ( ALPHA / %x32-37 ) 263 The width limit of 63 on the base32 encoding is based on the maximum 264 label limit found in Section 2.3.4 of [DNS]. 266 See Appendix A for an example of a query construction. 268 4.4. ATPS Reply Details 270 In the descriptions below, the label NOERROR symbolizes DNS response 271 code ("rcode") 0, and NXDOMAIN represents rcode 3. See Section 4.1.1 272 of [DNS] for further details. 274 At this time, only three possibilities need to be identified in this 275 specification: 277 o An answer is returned (i.e. [DNS] reply code NOERROR with at 278 least one answer) containing a valid ATPS reply. In this case, 279 the protocol has been satisfied and the Verifier can conclude that 280 the signing domain is authorized by the ADMD to sign its mail. 281 Further queries SHOULD NOT be initiated. 283 o No answer is returned (i.e. [DNS] reply code NXDOMAIN, or NOERROR 284 with no answers), or one or more answers have been returned as 285 described above but none contain a valid ATPS reply. In this 286 case, the Signer has not been authorized to act as a third-party 287 signer for this ADMD, and thus the Verifier MUST continue to the 288 next query, if any. 290 o An error is returned (i.e. any other [DNS] reply code). It is no 291 longer possible to determine whether or not this message satisfies 292 the ADMD's list of authorized third-party signers. The Verifier 293 SHOULD stop processing and defer the message for later processing, 294 such as requesting temporary failure code from the MTA. 296 If all queries are completed and return either NXDOMAIN or NOERROR 297 with no answers, then the Signer was not authorized by the ADMD. 299 A valid ATPS reply consists of a sequence of tag-value pairs as 300 described in Section 3.2 of [DKIM]. The following tags and values 301 are currently supported in ATPS records: 303 d: Domain (plain-text; RECOMMENDED) This tag includes a plain-text 304 copy of the DNS domain being authorized as an ATPS Signer. This 305 is included to assist with collision detections; for example, if 306 the base32 encoding of this name is not the same as the base32 307 portion of the query, or more simply if this name is not the same 308 as that found in the "atps" tag, a hash collision could have 309 occurred. ABNF: 311 atps-d-tag = %x64 [FWS] "=" [FWS] domain-name 312 ; FWS is defined in [DKIM] 314 v: Version (plain-text; REQUIRED) This tag defines the version of 315 this specification that applies to the ATPS record. The record 316 MUST be ignored if the value is not "ATPS1". ABNF: 318 atps-v-tag = %x76 [FWS] "=" [FWS] %x41.54.50.53.31 319 ; FWS is defined in [DKIM] 321 5. Interpretation 323 For each DKIM signature that verifies (see Section 6 of [DKIM]), if a 324 Verifier succeeds in confirming that the Author's ADMD authorized the 325 ATPS Signer using this protocol, then the Verifier SHOULD evaluate 326 the message as though it contained a valid signature from the 327 Author's ADMD. It MAY also independently evaluate the ATPS Signer 328 when determining message disposition. 330 This assertion is based on the fact that the ADMD explicitly endorsed 331 the ATPS Signer. Therefore, a module assessing reputation that is 332 based on DKIM signature verification SHOULD apply the reputation of 333 the Author's ADMD domain instead of, or in addition to, that of the 334 ATPS Signer domain. 336 6. Relationship to ADSP 338 [ADSP] defined a protocol by which the owner of an Author Domain can 339 advertise a request to message receivers that messages bearing no 340 valid author signature be treated with suspicion or even discarded. 342 A Verifier implementing both ADSP and ATPS MUST test ATPS first. If 343 ATPS indicates a valid delegation, the verifier MUST act, with 344 respect to ADSP, as though the message has a valid Author Domain 345 Signature (because that's what the delegation means), and no ADSP 346 test is required. 348 7. Experiment Process 350 The working group that developed DKIM considered a third-party 351 mechanism such as this one to be controversial, in terms of need and 352 practicality, and decided that an alternative mechanism was suffient. 353 However, this was not based on actual experience as there is no 354 specific history on this question. Thus, this experiment was 355 devised. 357 The experimental protocol described here has been implemented as an 358 extension to DKIM in two software products, one of which is open 359 source and seeing increasingly wide use. It is included there to 360 allow customers of those systems to make use of it if they believe 361 such third party assertions are useful to the overall DKIM mechanism. 362 Further adoption as part of the experiment is welcome and encouraged. 364 Use of the protocol and anecdotes of how it affects the overall DKIM 365 experience will be collected by those implementers and the author of 366 this memo. Those participating in the experiment are also advised to 367 observe and report the impact of what is discussed in Section 9.4, 368 especially with respect to MTA latency that may be introduced. 370 If the response is substantial and positive, advancement along the 371 Standards Track might be warranted. 373 8. IANA Considerations 375 This section enumerates requested IANA actions. 377 8.1. ATPS Tag Registry 379 IANA is directed to create an Authorized Third Party Signature (ATPS) 380 Tag Registry to enumerate the tags that are valid for use in ATPS 381 records. 383 New registrations or updates MUST be published in accordance with the 384 "Specification Required" guidelines described in [IANA]. Such 385 registry changes MUST contain the following information: 387 1. Name of the tag being registered or updated 389 2. The document where the specification is created or updated 391 3. The status of the tag, one of "current" (tag is in current use), 392 "deprecated" (tag is in current use but its use is discouraged), 393 or "historic" (tag is no longer in use) 395 The registry's sole initial entry is: 397 +-----+--------------+---------+ 398 | Tag | Specified In | Status | 399 +-----+--------------+---------+ 400 | v | [this memo] | current | 401 +-----+--------------+---------+ 403 8.2. Email Authentication Method Name Registry Update 405 The following is to be added to the Email Authentication Methods 406 Registry (in the Email Authentication Parameters group) established 407 by [AUTHRES] as per [IANA]: 409 Method: dkim-atps 411 Defined In: [THIS MEMO] 413 ptype: header 414 property: from 416 value: contents of the [MAIL] From: header field, with comments 417 removed 419 8.3. Email Authentication Result Name Registry Update 421 The following are to be added to the Email Authentication Result 422 Names Registry (in the Email Authentication Parameters group) 423 established by [AUTHRES] as per [IANA]: 425 Code: none 427 Existing/New Code: existing 429 Defined In: [AUTHRES] 431 Auth Method: dkim-atps 433 Meaning: No valid DKIM signatures were found on the message bearing 434 "atps" tags. 436 Code: pass 438 Existing/New Code: existing 440 Defined In: [AUTHRES] 442 Auth Method: dkim-atps 444 Meaning: An ATPS evaluation was performed and a valid signature from 445 an authorized third-party was found on the message. 447 Code: fail 449 Existing/New Code: existing 451 Defined In: [AUTHRES] 453 Auth Method: dkim-atps 455 Meaning: All valid DKIM signatures bearing an "atps" tag either did 456 not reference a domain name found in the RFC5322.From field, or 457 the ATPS test(s) performed failed to confirm a third-party 458 authorization. 460 Code: temperror 462 Existing/New Code: existing 464 Defined In: [AUTHRES] 466 Auth Method: dkim-atps 468 Meaning: An ATPS evaluation could not be completed due to some error 469 that is likely transient in nature, such as a temporary DNS error. 470 A later attempt might produce a final result. 472 Code: permerror 474 Existing/New Code: existing 476 Defined In: [AUTHRES] 478 Auth Method: dkim-atps 480 Meaning: An ATPS evaluation could not be completed due to some error 481 that is not likely transient in nature, such as a permanent DNS 482 error. A later attempt is unlikely to produce a final result. 484 8.4. DKIM-Signature Tag Specification Registry 486 The following are to be added to the DKIM-Signature Tag Speficication 487 Registry (in the DomainKeys Identifie Mail (DKIM) Parameters group) 488 established by [DKIM] as per [IANA]: 490 +-------+-------------+---------+ 491 | TYPE | REFERENCE | STATUS | 492 +-------+-------------+---------+ 493 | atps | [THIS MEMO] | current | 494 +-------+-------------+---------+ 495 | atpsh | [THIS MEMO] | current | 496 +-------+-------------+---------+ 498 9. Security Considerations 500 This section discusses potential security issues related to this 501 experimental protocol. 503 9.1. Hash Selection 505 The selection of the hash algorithm to be used (see Section 4.1) has 506 security implications, as weaker algorithms have more risk of 507 collision, meaning a second DNS domain name could in theory be 508 constructed to appear to have been authorized by the Author ADMD. 510 At the time of publication of [DKIM], use of SHA256 was preferred 511 over SHA1 for this reason, though support for both has been 512 maintained. See Section 3.3 of [DKIM] for additional discussion. 514 9.2. False Privacy 516 The fact that the authorized third-party domain name is hashed and 517 then encoded with base32 might give some the false sense that the 518 relationship between the two parties is somehow protected. This is 519 not the case. Indeed, the very purpose of this protocol is to make 520 it possible for such relationships to be discovered, so such an 521 obscuration would make that process more difficult without a shared 522 secret delivered out-of-band to message verifiers (which also adds 523 further complexity. Rather, the hash and encode steps are done 524 merely to convert any third-party domain name to a fixed width in the 525 construction of the DNS query. 527 9.3. Transient Security Failures 529 Approving a third party signer exposes the ADMD to the risk that the 530 third party signer becomes compromised and then begins to sign 531 malicious or nuisance messages on behalf of the ADMD. This can 532 obviously reflect negatively on the ADMD, and the impact of this can 533 become more severe as automated domain reputation systems are 534 developed and deployed. Thorough vetting and monitoring practices by 535 ADMDs of third party signers will likely need to become the norm. 537 9.4. Load on the DNS 539 A Verifier participating in DKIM, ADSP and ATPS will now issue a 540 number of TXT queries to the DNS equal to as many as one (for the 541 ADSP query) plus the number of signatures on the message (one for 542 each key that is to be verified) plus the number of signatures that 543 validated which also bear an "atps" tag. This is in addition to any 544 PTR and A queries the MTA might issue at the time the actual message 545 relaying or delivery is initiated. Obviously this can be burdensome 546 on the DNS at both ends, and waiting for that number of queries to 547 return when they are issued in parallel could trigger timeouts in the 548 MTA. 550 An alternative to this that has not yet been explored is the storage 551 of the ATPS data at a specific URL tied to the Author's domain name. 552 This would alleviate pressure on the DNS at the expense of requiring 553 the ADMD to operate a web server (which has its own security 554 implications) and the addition of the establishment of a TCP 555 connection. Moreover, the Verifier would be well advised to 556 implement caching of this data to prevent ATPS from being used as a 557 denial-of-service vector. 559 See Section 8.5 of [DKIM] for further discussion of DNS-related 560 issues. 562 10. References 564 10.1. Normative References 566 [ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax 567 Specifications: ABNF", STD 68, RFC 5234, January 2008. 569 [AUTHRES] Kucherawy, M., "Message Header Field for Indicating 570 Message Authentication Status", RFC 5451, April 2009. 572 [BASE32] Josefsson, S., "The Base16, Base32, and Base64 Data 573 Encodings", RFC 4648, October 2006. 575 [DKIM] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, 576 Ed., "DomainKeys Identified Mail (DKIM) Signatures", 577 RFC 6376, September 2011. 579 [DNS] Mockapetris, P., "Domain names - implementation and 580 specification", STD 13, RFC 1035, November 1987. 582 [EMAIL-ARCH] Crocker, D., "Internet Mail Architecture", RFC 5598, 583 October 2008. 585 [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate 586 Requirement Levels", BCP 14, RFC 2119, March 1997. 588 [MAIL] Resnick, P., Ed., "Internet Message Format", RFC 5322, 589 October 2008. 591 10.2. Informative References 593 [ADSP] Allman, E., Fenton, J., Delany, M., and J. Levine, 594 "DomainKeys Identified Mail (DKIM) Author Domain 595 Signing Practices (ADSP)", RFC 5617, August 2009. 597 [IANA] Narten, T. and H. Alvestrand, "Guidelines for Writing 598 an IANA Considerations Section in RFCs", BCP 26, 599 RFC 5226, May 2008. 601 [IDNA] Klensin, J., "Internationalized Domain Names for 602 Applications (IDNA): Definitions and Document 603 Framework", RFC 5890, August 2010. 605 Appendix A. Example Query and Reply 607 This section presents an example of the use of this protocol to query 608 for a third-party authorization and discusses the interpretation of 609 the result. 611 Presume a message for which the RFC5322.From domain is "example.com", 612 and it bears two valid signatures, from "one.example.net" and from 613 "two.example.net", each with an "atps" tag whose value is 614 "example.com", and no "atpsh" tag is present in either. The 615 following actions are taken: 617 1. A SHA1 hash of "one.example.net" is computed and then converted 618 to printable form using base32 encoding, resulting in the string 619 "QSP4I4D24CRHOPDZ3O3ZIU2KSGS3X6Z6". 621 2. A TXT query is issued to 622 "QSP4I4D24CRHOPDZ3O3ZIU2KSGS3X6Z6._atps.example.com". 624 3. If a valid reply arrives, the algorithm stops with [AUTHRES] 625 result "pass". If a DNS error code other than NXDOMAIN is 626 returned, the algorithm stops with a result of "temperror" or 627 "permerror" as appropriate. 629 4. A SHA1 hash of "two.example.net" is computed and then converted 630 to printable form using base32 encoding, resulting in the string 631 "ZTZGRRV3F45A4U6HLDKBF3ZCOW4V2AJX". 633 5. A TXT query is issued to 634 "ZTZGRRV3F45A4U6HLDKBF3ZCOW4V2AJX._atps.example.com". 636 6. If a valid reply arrives, the algorithm stops with [AUTHRES] 637 result "pass". If a DNS error code other than NXDOMAIN is 638 returned, the algorithm stops with a result of "temperror" or 639 "permerror" as appropriate. 641 7. As there are no valid signatures left to test, the algorithm 642 stops with an "unknown" result. 644 Appendix B. Choice of DNS RR Type 646 It was proposed that this work appear within the DNS under a new 647 Resource Record (RR) Type. Although this is possibly an appropriate 648 thing to do, consideration was also given to the fact that major 649 portions of DKIM already use an ASCII-based "tag=value" syntax, and 650 store key and ADSP data in the DNS using TXT resource records. To 651 enable re-use of existing DKIM code, it was decided to re-use the TXT 652 message scheme. 654 Appendix C. Acknowledgements 656 The author wishes to acknowledge Dave Crocker, Frank Ellermann, Mark 657 Martinec and Phil Pennock for their review and constructive criticism 658 of this proposal. 660 The author also wishes to acknowledge Doug Otis and Daniel Black for 661 their original draft upon which this work was based. 663 Author's Address 665 Murray S. Kucherawy 666 Cloudmark, Inc. 667 128 King St., 2nd Floor 668 San Francisco, CA 94107 669 US 671 Phone: +1 415 946 3800 672 EMail: msk@cloudmark.com