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Checking references for intended status: Experimental ---------------------------------------------------------------------------- == Missing Reference: 'FWS' is mentioned on line 312, but not defined == Missing Reference: 'THIS MEMO' is mentioned on line 493, 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 1, 2012 5 Expires: July 4, 2012 7 DKIM Authorized Third-Party Signers 8 draft-kucherawy-dkim-atps-14 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 4, 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 . . . . . . . . . . . . . . . . . . . . . . 12 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. Acknowledgements . . . . . . . . . . . . . . . . . . 14 81 1. Introduction 83 [DKIM] defines a mechanism for transparent domain-level signing of 84 messages for the purpose of declaring that a particular 85 Administrative Mail Domain (ADMD) takes some responsibility for a 86 message. 88 DKIM, however, deliberately makes no binding between the DNS domain 89 of the signer and any other identity found in the message. Despite 90 this, there is an automatic human perception that an author domain 91 signature (one for which the RFC5322.From domain matches the DNS 92 domain of the signer) is more valuable or trustworthy than any other. 94 To enable a third party to apply DKIM signatures to messages, the 95 DKIM specification suggests delegation to a third party of either 96 subdomains or private keys, so that the third party can add DKIM 97 signatures that appear to have been added by the Author ADMD. Absent 98 is a protocol by which an Author ADMD can announce that messages 99 bearing specific valid DKIM signatures on its mail, which are added 100 by other ADMDs, are to be treated as if they were signed by the 101 Author ADMD itself. This memo presents an experimental mechanism for 102 doing so. 104 ATPS augments the semantics of DKIM by providing to the verifier 105 multiple identifiers rather than one. Specifically, it validates the 106 identifier found in the DKIM signature, and then provides the 107 RFC5322.From domain for evaluation. 109 This memo also registers, per [AUTHRES], the means to indicate to 110 agents downstream of the Verifier that a third-party signature 111 verification occurred. 113 2. Definitions 115 2.1. Keywords 117 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 118 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 119 document are to be interpreted as described in [KEYWORDS]. 121 2.2. E-Mail Architecture Terminology 123 Readers are advised to be familiar with the material and terminology 124 discussed in [MAIL] and [EMAIL-ARCH]. 126 3. Roles and Scope 128 The context of this protocol involves the following roles: 130 o ADministrative Mail Domain (ADMD)s, whose DNS domain name(s) 131 appear in the RFC5322.From field of a [MAIL] message; 133 o ATPS Signers, which apply [DKIM] signatures using their own 134 domains, but on behalf of the message Author's ADMD; and 136 o the Verifier, who implements the signature validation procedures 137 described in [DKIM]. 139 An ADMD implements this protocol if it wishes to announce that a 140 signature from any in a set of specified DNS domains is to be 141 considered equivalent to one from the ADMD itself. One might, for 142 example, wish to delegate signing authority for its DNS domain to an 143 approved messaging service provider without doing the work of key 144 transfer described in Appendix B.1.1 of [DKIM]. An authorized ATPS 145 Signer makes a claim of this relationship via new tags in the DKIM 146 signature, and the ADMD confirms this claim by publishing a specific 147 TXT record in its DNS. 149 A Verifier implements this protocol if it wishes to ensure that a 150 message bears one or more signatures from sources authorized to sign 151 mail on behalf of the ADMD, and identify for special treatment mail 152 that meets (or does not meet) that criterion. It will do so by 153 treating the signer's authorization on behalf of the author's ADMD to 154 mean that the signer's signature is equivalent to one affixed by the 155 author's ADMD. 157 4. Queries and Replies 159 This section describes in detail the queries issued, the replies 160 received, and how they should be interpreted and applied. 162 4.1. Hash Selection 164 The author's ADMD will indicate authorization of a third party to 165 sign its mail via the presence of a DNS TXT record that contains an 166 encoding of the third party's DNS domain name. The encoding 167 mechanism is constructed so that any domain name can be added to the 168 DNS in a fixed length, so that longer third-party domain names are 169 not excluded from participation because of the overall length limit 170 on a DNS query. 172 Part of the mechanism requires constructing a digest of the third 173 party's DNS domain name. The author ADMD MUST select a digest 174 ("hash") method currently supported by DKIM (see Section 7.7 of 175 [DKIM]), and this selection needs to be communicated to the ATPS 176 Signer as it is used in generation of its the third party signatures. 178 The full DNS mechanism is specified in Section 4.3. 180 4.2. Extension to DKIM 182 [DKIM] signatures contain a "tag=value" sequence. This protocol will 183 add additional tags called "atps" and "atpsh". 185 When the ATPS Signer generates a DKIM signature for another ADMD, it 186 MUST put its own domain in the signature's "d" tag, and include an 187 "atps" tag that has as its value the domain name of the ADMD on whose 188 behalf it is signing. 190 The tag name that carries the name of the selected hash algorithm is 191 "atpsh". This tag MUST also be included, as it is required as part 192 of the algorithm that will be enacted by the Verifier. 194 The formal syntax definition, per [ABNF]: 196 dkim-atps-tag = %x61.74.70.73 *WSP "=" *WSP domain-name 198 dkim-atpsh-tag = %x61.74.70.73.68 *WSP "=" *WSP key-h-tag-alg 200 "domain-name" and "key-h-tag-alg" are defined in [DKIM]. 202 The registration for these tags can be found in Section 8. 204 4.3. ATPS Query Details 206 When a [DKIM] signature including an "atps" tag is successfully 207 verified, and is considered acceptable to the Verifier according to 208 any local policy requirements (which are not discussed here or in 209 [DKIM]), the Verifier compares the domain name in the value of that 210 tag with the one found in the RFC5322.From field of the message. The 211 match MUST be done in a case-insensitive manner. 213 If they do not match, the "atps" tag MUST be ignored. 215 If they do match, the Verifier issues a DNS TXT query, as specified 216 below, looking for confirmation by the Author ADMD that the ATPS 217 Signer is authorized by that ADMD to sign mail on its behalf. Where 218 multiple DKIM signatures are present including valid "atps" tags, 219 these queries MAY be done in any order or MAY be done in parallel. 221 Where the RFC5322.From field contains multiple addresses, this 222 process SHOULD be applied if the "atps" tag's value matches any of 223 the domains found in that field. These MAY be done in any order. 225 Note that the algorithm uses hashing, but this is not a security 226 mechanism. See Section 9.2 for discussion. 228 The name for the query is constructed as follows: 230 1. Select the hash algorithm from the "atpsh" tag in the signature. 231 If the hash algorithm specified does not appear in the list 232 registered with IANA as one valid for use with DKIM (see Section 233 7.7 of [DKIM]), abort the query. 235 2. Extract the value of the "d=" tag from the signature. 237 3. Convert any upper-case characters in that string to their lower- 238 case equivalents. 240 4. Feed the resulting string to the selected hash algorithm. 242 5. Convert the output of the hash to a string of printable ASCII 243 characters by applying base32 encoding as defined in Section 6 of 244 [BASE32]. The base32 encoding is used because its output is 245 restricted to characters that are legal for use in labels in the 246 DNS, and evaluates the same way in the DNS whether encoded using 247 uppercase or lowercase characters. 249 6. Append the string "._atps." 251 7. Append the domain name found in the "atps" tag of the validated 252 signature. 254 The query's formal syntax definition, per [ABNF]: 256 atps-query = 1*BASE32 %x2e.5f.61.74.70.73.2e domain-name 258 BASE32 = ( ALPHA / %x32-37 ) 260 See Appendix A for an example of a query construction. 262 4.4. ATPS Reply Details 264 In the descriptions below, the label NOERROR symbolizes DNS response 265 code ("rcode") 0, and NXDOMAIN represents rcode 3. See Section 4.1.1 266 of [DNS] for further details. 268 At this time, only three possibilities need to be identified in this 269 specification: 271 o An answer is returned (i.e. [DNS] reply code NOERROR with at 272 least one answer) containing a valid ATPS reply. In this case, 273 the protocol has been satisfied and the Verifier can conclude that 274 the signing domain is authorized by the ADMD to sign its mail. 275 Further queries SHOULD NOT be initiated. 277 o No answer is returned (i.e. [DNS] reply code NXDOMAIN, or NOERROR 278 with no answers), or one or more answers have been returned as 279 described above but none contain a valid ATPS reply. In this 280 case, the Signer has not been authorized to act as a third-party 281 signer for this ADMD, and thus the Verifier MUST continue to the 282 next query, if any. 284 o An error is returned (i.e. any other [DNS] reply code). It is no 285 longer possible to determine whether or not this message satisfies 286 the ADMD's list of authorized third-party signers. The Verifier 287 SHOULD stop processing and defer the message for later processing, 288 such as requesting temporary failure code from the MTA. 290 If all queries are completed and return either NXDOMAIN or NOERROR 291 with no answers, then the Signer was not authorized by the ADMD. 293 A valid ATPS reply consists of a sequence of tag-value pairs as 294 described in Section 3.2 of [DKIM]. The following tags and values 295 are currently supported in ATPS records: 297 d: Domain (plain-text; RECOMMENDED) This tag includes a plain-text 298 copy of the DNS domain being authorized as an ATPS Signer. This 299 is included to assist with collision detections; for example, if 300 the base32 encoding of this name is not the same as the base32 301 portion of the query, or more simply if this name is not the same 302 as that found in the "atps" tag, a hash collision could have 303 occurred. ABNF: 305 atps-d-tag = %x64 [FWS] "=" [FWS] domain-name 306 ; FWS is defined in [DKIM] 308 v: Version (plain-text; REQUIRED) This tag defines the version of 309 this specification that applies to the ATPS record. The record 310 MUST be ignored if the value is not "ATPS1". ABNF: 312 atps-v-tag = %x76 [FWS] "=" [FWS] %x41.54.50.53.31 313 ; FWS is defined in [DKIM] 315 5. Interpretation 317 For each DKIM signature that verifies (see Section 6 of [DKIM]), if a 318 Verifier succeeds in confirming that the Author's ADMD authorized the 319 ATPS Signer using this protocol, then the Verifier SHOULD evaluate 320 the message as though it contained a valid signature from the 321 Author's ADMD. It MAY also independently evaluate the ATPS Signer 322 when determining message disposition. 324 This assertion is based on the fact that the ADMD explicitly endorsed 325 the ATPS Signer. Therefore, a module assessing reputation that is 326 based on DKIM signature verification SHOULD apply the reputation of 327 the Author's ADMD domain instead of, or in addition to, that of the 328 ATPS Signer domain. 330 6. Relationship to ADSP 332 [ADSP] defined a protocol by which the owner of an Author Domain can 333 advertise a request to message receivers that messages bearing no 334 valid author signature be treated with suspicion or even discarded. 336 A Verifier implementing both ADSP and ATPS MUST test ATPS first. If 337 ATPS indicates a valid delegation, the verifier MUST act, with 338 respect to ADSP, as though the message has a valid Author Domain 339 Signature (because that's what the delegation means), and no ADSP 340 test is required. 342 7. Experiment Process 344 The working group that developed DKIM considered a third-party 345 mechanism such as this one to be controversial, in terms of need and 346 practicality, and decided that an alternative mechanism was suffient. 347 However, this was not based on actual experience as there is no 348 specific history on this question. Thus, this experiment was 349 devised. 351 The experimental protocol described here has been implemented as an 352 extension to DKIM in two software products, one of which is open 353 source and seeing increasingly wide use. It is included there to 354 allow customers of those systems to make use of it if they believe 355 such third party assertions are useful to the overall DKIM mechanism. 356 Further adoption as part of the experiment is welcome and encouraged. 358 Use of the protocol and anecdotes of how it affects the overall DKIM 359 experience will be collected by those implementers and the author of 360 this memo. Those participating in the experiment are also advised to 361 observe and report the impact of what is discussed in Section 9.4, 362 especially with respect to MTA latency that may be introduced. 364 If the response is substantial and positive, advancement along the 365 Standards Track might be warranted. 367 8. IANA Considerations 369 Section 8.2, Section 8.3, and Section 8.4, below, specify requested 370 IANA actions. Section 8.1 needs no IANA action at this time. 372 8.1. ATPS Tag Registry 374 NOTE: NO IANA ACTION IS REQUIRED BY THIS SECTION AT THIS TIME. [RFC 375 EDITOR: Please remove this paragraph before publication.] 377 If this specification is ever moved to the Standards Track, IANA will 378 then be asked to create an Authorized Third Party Signature (ATPS) 379 Tag Registry to enumerate the tags that are valid for use in ATPS 380 records. This section documents that future registry in advance. 382 New registrations or updates MUST be published in accordance with the 383 "Specification Required" guidelines described in [IANA]. Such 384 registry changes MUST contain the following information: 386 1. Name of the tag being registered or updated 388 2. The document where the specification is created or updated 390 3. The status of the tag, one of "current" (tag is in current use), 391 "deprecated" (tag is in current use but its use is discouraged), 392 or "historic" (tag is no longer in use) 394 The registry's sole initial entry is: 396 +-----+--------------+---------+ 397 | Tag | Specified In | Status | 398 +-----+--------------+---------+ 399 | v | [this memo] | current | 400 +-----+--------------+---------+ 402 8.2. Email Authentication Method Name Registry Update 404 The following is to be added to the Email Authentication Methods 405 Registry (in the Email Authentication Parameters group) established 406 by [AUTHRES] as per [IANA]: 408 Method: dkim-atps 409 Defined In: [THIS MEMO] 411 ptype: header 413 property: from 415 value: contents of the [MAIL] From: header field, with comments 416 removed 418 8.3. Email Authentication Result Name Registry Update 420 The following are to be added to the Email Authentication Result 421 Names Registry (in the Email Authentication Parameters group) 422 established by [AUTHRES] as per [IANA]: 424 Code: none 426 Existing/New Code: existing 428 Defined In: [AUTHRES] 430 Auth Method: dkim-atps 432 Meaning: No valid DKIM signatures were found on the message bearing 433 "atps" tags. 435 Code: pass 437 Existing/New Code: existing 439 Defined In: [AUTHRES] 441 Auth Method: dkim-atps 443 Meaning: An ATPS evaluation was performed and a valid signature from 444 an authorized third-party was found on the message. 446 Code: fail 448 Existing/New Code: existing 450 Defined In: [AUTHRES] 452 Auth Method: dkim-atps 453 Meaning: All valid DKIM signatures bearing an "atps" tag either did 454 not reference a domain name found in the RFC5322.From field, or 455 the ATPS test(s) performed failed to confirm a third-party 456 authorization. 458 Code: temperror 460 Existing/New Code: existing 462 Defined In: [AUTHRES] 464 Auth Method: dkim-atps 466 Meaning: An ATPS evaluation could not be completed due to some error 467 that is likely transient in nature, such as a temporary DNS error. 468 A later attempt might produce a final result. 470 Code: permerror 472 Existing/New Code: existing 474 Defined In: [AUTHRES] 476 Auth Method: dkim-atps 478 Meaning: An ATPS evaluation could not be completed due to some error 479 that is not likely transient in nature, such as a permanent DNS 480 error. A later attempt is unlikely to produce a final result. 482 8.4. DKIM-Signature Tag Specification Registry 484 The following are to be added to the DKIM-Signature Tag Speficication 485 Registry (in the DomainKeys Identifie Mail (DKIM) Parameters group) 486 established by [DKIM] as per [IANA]: 488 +-------+-------------+---------+ 489 | TYPE | REFERENCE | STATUS | 490 +-------+-------------+---------+ 491 | atps | [THIS MEMO] | current | 492 +-------+-------------+---------+ 493 | atpsh | [THIS MEMO] | current | 494 +-------+-------------+---------+ 496 9. Security Considerations 498 This section discusses potential security issues related to this 499 experimental protocol. 501 9.1. Hash Selection 503 The selection of the hash algorithm to be used (see Section 4.1) has 504 security implications, as weaker algorithms have more risk of 505 collision, meaning a second DNS domain name could in theory be 506 constructed to appear to have been authorized by the Author ADMD. 508 At the time of publication of [DKIM], use of SHA256 was preferred 509 over SHA1 for this reason, though support for both has been 510 maintained. See Section 3.3 of [DKIM] for additional discussion. 512 9.2. False Privacy 514 The fact that the authorized third-party domain name is hashed and 515 then encoded with base32 might give some the false sense that the 516 relationship between the two parties is somehow protected. This is 517 not the case. Indeed, the very purpose of this protocol is to make 518 it possible for such relationships to be discovered, so such an 519 obscuration would make that process more difficult without a shared 520 secret delivered out-of-band to message verifiers (which also adds 521 further complexity. Rather, the hash and encode steps are done 522 merely to convert any third-party domain name to a fixed width in the 523 construction of the DNS query. 525 9.3. Transient Security Failures 527 Approving a third party signer exposes the ADMD to the risk that the 528 third party signer becomes compromised and then begins to sign 529 malicious or nuisance messages on behalf of the ADMD. This can 530 obviously reflect negatively on the ADMD, and the impact of this can 531 become more severe as automated domain reputation systems are 532 developed and deployed. Thorough vetting and monitoring practices by 533 ADMDs of third party signers will likely need to become the norm. 535 9.4. Load on the DNS 537 A Verifier participating in DKIM, ADSP and ATPS will now issue a 538 number of TXT queries to the DNS equal to as many as one (for the 539 ADSP query) plus the number of signatures on the message (one for 540 each key that is to be verified) plus the number of signatures that 541 validated which also bear an "atps" tag. This is in addition to any 542 PTR and A queries the MTA might issue at the time the actual message 543 relaying or delivery is initiated. Obviously this can be burdensome 544 on the DNS at both ends, and waiting for that number of queries to 545 return when they are issued in parallel could trigger timeouts in the 546 MTA. 548 An alternative to this that has not yet been explored is the storage 549 of the ATPS data at a specific URL tied to the Author's domain name. 550 This would alleviate pressure on the DNS at the expense of requiring 551 the ADMD to operate a web server (which has its own security 552 implications) and the addition of the establishment of a TCP 553 connection. Moreover, the Verifier would be well advised to 554 implement caching of this data to prevent ATPS from being used as a 555 denial-of-service vector. 557 See Section 8.5 of [DKIM] for further discussion of DNS-related 558 issues. 560 10. References 562 10.1. Normative References 564 [ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax 565 Specifications: ABNF", STD 68, RFC 5234, January 2008. 567 [AUTHRES] Kucherawy, M., "Message Header Field for Indicating 568 Message Authentication Status", RFC 5451, April 2009. 570 [BASE32] Josefsson, S., "The Base16, Base32, and Base64 Data 571 Encodings", RFC 4648, October 2006. 573 [DKIM] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, 574 Ed., "DomainKeys Identified Mail (DKIM) Signatures", 575 RFC 6376, September 2011. 577 [DNS] Mockapetris, P., "Domain names - implementation and 578 specification", STD 13, RFC 1035, November 1987. 580 [EMAIL-ARCH] Crocker, D., "Internet Mail Architecture", RFC 5598, 581 October 2008. 583 [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate 584 Requirement Levels", BCP 14, RFC 2119, March 1997. 586 [MAIL] Resnick, P., Ed., "Internet Message Format", RFC 5322, 587 October 2008. 589 10.2. Informative References 591 [ADSP] Allman, E., Fenton, J., Delany, M., and J. Levine, 592 "DomainKeys Identified Mail (DKIM) Author Domain 593 Signing Practices (ADSP)", RFC 5617, August 2009. 595 [IANA] Narten, T. and H. Alvestrand, "Guidelines for Writing 596 an IANA Considerations Section in RFCs", BCP 26, 597 RFC 5226, May 2008. 599 Appendix A. Example Query and Reply 601 This section presents an example of the use of this protocol to query 602 for a third-party authorization and discusses the interpretation of 603 the result. 605 Presume a message for which the RFC5322.From domain is "example.com", 606 and it bears two valid signatures, from "one.example.net" and from 607 "two.example.net", each with an "atps" tag whose value is 608 "example.com", and no "atpsh" tag is present in either. The 609 following actions are taken: 611 1. A SHA1 hash of "one.example.net" is computed and then converted 612 to printable form using base32 encoding, resulting in the string 613 "QSP4I4D24CRHOPDZ3O3ZIU2KSGS3X6Z6". 615 2. A TXT query is issued to 616 "QSP4I4D24CRHOPDZ3O3ZIU2KSGS3X6Z6._atps.example.com". 618 3. If a valid reply arrives, the algorithm stops with [AUTHRES] 619 result "pass". If a DNS error code other than NXDOMAIN is 620 returned, the algorithm stops with a result of "temperror" or 621 "permerror" as appropriate. 623 4. A SHA1 hash of "two.example.net" is computed and then converted 624 to printable form using base32 encoding, resulting in the string 625 "ZTZGRRV3F45A4U6HLDKBF3ZCOW4V2AJX". 627 5. A TXT query is issued to 628 "ZTZGRRV3F45A4U6HLDKBF3ZCOW4V2AJX._atps.example.com". 630 6. If a valid reply arrives, the algorithm stops with [AUTHRES] 631 result "pass". If a DNS error code other than NXDOMAIN is 632 returned, the algorithm stops with a result of "temperror" or 633 "permerror" as appropriate. 635 7. As there are no valid signatures left to test, the algorithm 636 stops with an "unknown" result. 638 Appendix B. Acknowledgements 640 The author wishes to acknowledge Dave Crocker, Frank Ellermann, Mark 641 Martinec and Phil Pennock for their review and constructive criticism 642 of this proposal. 644 The author also wishes to acknowledge Doug Otis and Daniel Black for 645 their original draft upon which this work was based. 647 Author's Address 649 Murray S. Kucherawy 650 Cloudmark, Inc. 651 128 King St., 2nd Floor 652 San Francisco, CA 94107 653 US 655 Phone: +1 415 946 3800 656 EMail: msk@cloudmark.com