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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 IETF K. Moriarty 3 Internet-Draft Dell Technologies 4 Intended status: Standards Track October 5, 2020 5 Expires: April 8, 2021 7 ACME End User Client and Code Signing Certificates 8 draft-ietf-acme-client-02 10 Abstract 12 Automated Certificate Management Environment (ACME) core protocol 13 addresses the use case of web server certificates for TLS. This 14 document extends the ACME protocol to support end user client, device 15 client, and code signing certificates. 17 Status of This Memo 19 This Internet-Draft is submitted in full conformance with the 20 provisions of BCP 78 and BCP 79. 22 Internet-Drafts are working documents of the Internet Engineering 23 Task Force (IETF). Note that other groups may also distribute 24 working documents as Internet-Drafts. The list of current Internet- 25 Drafts is at https://datatracker.ietf.org/drafts/current/. 27 Internet-Drafts are draft documents valid for a maximum of six months 28 and may be updated, replaced, or obsoleted by other documents at any 29 time. It is inappropriate to use Internet-Drafts as reference 30 material or to cite them other than as "work in progress." 32 This Internet-Draft will expire on April 8, 2021. 34 Copyright Notice 36 Copyright (c) 2020 IETF Trust and the persons identified as the 37 document authors. All rights reserved. 39 This document is subject to BCP 78 and the IETF Trust's Legal 40 Provisions Relating to IETF Documents 41 (https://trustee.ietf.org/license-info) in effect on the date of 42 publication of this document. Please review these documents 43 carefully, as they describe your rights and restrictions with respect 44 to this document. Code Components extracted from this document must 45 include Simplified BSD License text as described in Section 4.e of 46 the Trust Legal Provisions and are provided without warranty as 47 described in the Simplified BSD License. 49 Table of Contents 51 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 52 2. Identity Proofing for Client Certificates . . . . . . . . . . 2 53 3. End User Client Certificates . . . . . . . . . . . . . . . . 3 54 4. CodeSigning Certificates . . . . . . . . . . . . . . . . . . 5 55 5. Pre-authorization . . . . . . . . . . . . . . . . . . . . . . 8 56 6. Challenge Types . . . . . . . . . . . . . . . . . . . . . . . 8 57 6.1. One Time Password (OTP) . . . . . . . . . . . . . . . . . 8 58 6.1.1. HMAC-Based One-Time Password (HOTP) . . . . . . . . . 9 59 6.1.2. Time-Based One-Time Password (TOTP) . . . . . . . . . 9 60 6.1.3. Generic One Time Password (OTP) . . . . . . . . . . . 9 61 6.2. Public Key Cryptography . . . . . . . . . . . . . . . . . 10 62 6.3. WebAuthn or Public/Private Key Pairs . . . . . . . . . . 11 63 7. Security Considerations . . . . . . . . . . . . . . . . . . . 11 64 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 65 9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 12 66 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 67 10.1. Normative References . . . . . . . . . . . . . . . . . . 12 68 10.2. Informative References . . . . . . . . . . . . . . . . . 13 69 10.3. URL References . . . . . . . . . . . . . . . . . . . . . 13 70 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 14 71 Appendix B. Open Issues . . . . . . . . . . . . . . . . . . . . 14 72 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 14 74 1. Introduction 76 ACME [RFC8555] is a mechanism for automating certificate management 77 on the Internet. It enables administrative entities to prove 78 effective control over resources like domain names, and automates the 79 process of generating and issuing certificates. 81 The core ACME protocol defined challenge types specific to web server 82 certificates with the possibility to create extensions, or additional 83 challenge types for other use cases and certificate types. Client 84 certificates, such as end user and code signing may also benefit from 85 automated management to ease the deployment and maintenance of these 86 certificate types, thus the definition of this extension defining 87 challenge types specific to that usage. 89 2. Identity Proofing for Client Certificates 91 As with the TLS certificates defined in the core ACME document , identity proofing for ACME issued end user 93 client, device client, and code signing certificates is a separate 94 process outside of the automation of ACME. Identity proofing may be 95 an out-of-band process, if needed, and for this draft is likely tied 96 to the credentials used for the defined challenge types. 98 Identity proofing for these certificate types present some challenges 99 for process automation. NIST SP 800-63 r3 [NIST800-63r3] serves as 100 guidance for identity proofing further detailed in NIST SP 800-63A 101 [NIST800-63A] that may occur prior to the ability to automate 102 certificate management via ACME or may obviate the need for it 103 weighing end user privacy as a higher concern and allowing for 104 credential issuance to be decoupled from identity proofing (IAL1). 105 Using this guidance, a CA might select from the identity proofing 106 levels to assert claims on the issued certificates as described in 107 NIST SP 800-63 r3 [NIST800-63r3]. 109 The certificate issuing CA may make this choice by certificate type 110 issued. Once identity proofing has been performed, in cases where 111 this is part of the process, and certificates have been issued, NIST 112 SP 800-63 r3 [NIST800-63r3] includes recommendations for 113 authentication or in the context of ACME, management of issuance for 114 subsequent client, device, or code-signing certificates: 116 If federations and assertions are used for authorizing certificate 117 issuance, NIST SP 800-63 C [NIST800-63C] may be referenced for 118 guidance on levels of assurance. 120 Existing PKI certification authorities (CAs) tend to use a set of ad 121 hoc protocols for certificate issuance and identity verification. 122 For each certificate usage type, a basic process will be described to 123 obtain an initial certificate and for the certificate renewal 124 process. If higher assurance levels are desired, the guidance from 125 NIST SP 800-63 r3 [NIST800-63r3] may be useful and out-of-band 126 identity proofing options are possible options for pre-authorization 127 challenges or notifications. 129 3. End User Client Certificates 131 A client certificate used to authenticate an end user may be used for 132 mutual authentication in TLS, EAP-TLS, or messaging. The client 133 certificate in this case may be stored in a browser, PKCS-#11 134 container, KMIP (possible, but just code signing and device client 135 certificates in practice), or another key container. To obtain an 136 end user client certificate, there are several possibilities to 137 automate authentication of an identity credential intended to be tied 138 to an end user. 140 [We need to determine if it is important in ACME to define an 141 automated method that tests the identity or the user or to just have 142 consistent credentials for the authentication challenges. The 143 credentials may be distributed through an out-of-band method that 144 involves identity proofing.] 146 [Several authentication options with identity proofing are 147 intentionally provided for review and discussion by the ACME working 148 group.] 150 A trusted federated service that ties the user to an email address 151 with a reputation of the user attached to the email may be possible. 152 One such example might be the use of a JWT signed OAuth token. 154 Risk based authentication used for identity proofing with red herring 155 questions is a third option that could utilize public information on 156 individuals to authenticate. This would be similar to the signup 157 process used in some financial applications. 159 Existing credentials - for instance, FIDO. FIDO uses a public key 160 pair and does not perform identity proofing. FIDO authentication 161 provides a different key pair to each service using FIDO for 162 authentication, which are generated at the client and registered by 163 the server. This may require using the FIDO credentials from a 164 specific service for authentication to gain ACME issued crededentials 165 (not advised based on how FIDO credentials are supposed to be used). 166 Are there instances where the same provider would issue both sets of 167 credentials? You wouldn't want to expose your FIDO credentials to a 168 different party, that's why each service has their own. Would you 169 set up a mechanism to get FIDO credentials to then obtain a 170 certificate? (What use cases would this be necessary? When do you 171 need a certificate where you already have a specific public/private 172 key pair?) This can be defined as an auth type, but should it be? 174 One-time password (OTP) authentication is a secure option. In cases 175 where a higher assurance level is needed, OTP may be a good choice 176 and many options exist today for OTP that could use an app on a phone 177 for instance tied to an existing (or newly established) password. 178 The OTP may be tied to an out-of-band process and may be associated 179 with a username/password and other accounts. 181 One consideration is to understand if the use case could just use 182 FIDO and not create anything new (ACME client certificates). FIDO 183 provides a mechanism to have unique public key pair based access for 184 client authentication to web sites and they are working on non-web. 185 Identity proofing is intentionally decoupled from authentication in 186 this model as that is in line with NIST 800-63r3 recommendations for 187 privacy protections of the user. The credential in this case is 188 authenticated and would be consistent for it's use, but the identity 189 proofing for that credential is not performed. Obviously, identity 190 proofing is more important for some services, like financial 191 applications where tying the user to the identity for access to 192 financial information is important. However, is automated identity 193 proofing important for any user certificate or should it remain 194 decoupled where it could be automated by a service offering or is 195 there a need for a standardized mechanism to support it for user 196 certificates? 198 Three methods for ACME client authentication, not identity proofing, 199 are proposed in the Challenge Type Section. 201 4. CodeSigning Certificates 203 The process to retrieve a code signing certificate is similar to that 204 of a web server certificate, with differences primarily in the CSR 205 request and the resulting certificate properties. [The storage and 206 access of a code signing certificate must be protected and is 207 typically done through hardware, a hardware security module (HSM), 208 which likely has a PKCS#11 interface. A code signing certificate may 209 either be a standard one or an extended validation (EV) certificate.] 211 For automation purposes, the process described in this document will 212 follow the standard process and any out-of-band preprocessing can 213 increase the level of the issued certificate if the CA offers such 214 options and has additional identity proofing mechanisms (in band or 215 out-of-band). 217 Strict vetting processes are necessary for many code signing 218 certificates to provide a high assurance on the signer. In some 219 cases, issuance of a standard CodeSigning certificate will be 220 appropriate and no additional "challenges" [RFC8555 Section 8] will 221 be necessary. In this case, the standard option could be automated 222 very similar to Web server certificates with the only changes being 223 in the CSR properties. However, this may not apply to all scenarios, 224 such as those requiring EV certificates with the possibility for 225 required out-of-band initial authentication and identity proofing. 227 EV code signing certificates have a distinct set of requirements from 228 EV web certificates. In particular, they don't have associated 229 domain names, nor is CAA checking done. The code signing certificate 230 links a public key to an organization, not a domain. CAs may chose 231 different methods to enable the use of ACME for EV code signing 232 certificates. The intent of this work is to provide additional 233 authentication challenge types that may enable their automation 234 process. 236 Organization validation is required for standard code signing 237 certificates from most issuers. The CSR is used to identify the 238 organization from the included domain name in the request. The 239 resulting certificate, however, instead contains the organization's 240 name and for EV certificates, other identifying information for the 241 organization. For EV certificates, this could require that the 242 domain is registered with the Certificate Authority provider, listed 243 in CAA [RFC6844], and administrators for the account are named with 244 provided portal access for certificate issuance and management 245 options. 247 While ACME allows for the client to directly establish an account 248 with a CA, an initial out-of-band process for this step may assist 249 with the additional requirements for EV certificates and assurance 250 levels typically required for code signing certificates. For 251 standard certificates, with a recommendation for additional vetting 252 through extended challenge options to enable ACME to establish the 253 account directly. In cases where code signing certificates are used 254 heavily for an organization, having the portal access replaced with 255 ACME authenticated client access with extra challenges for 256 authentication may be an option to automate the functionality. 258 [For standard certificates, is it worth defining SMS and email for 259 the challenge? Obviously, EV needs more, so a few choices are 260 suggested in this revision.] 262 To improve the vetting process, ACME's optional use of CAA [RFC6844] 263 with the Directory "meta" data "caaIdentities" ([RFC8555] 264 Section 9.7.6) assists with the validation that a CA may have issue 265 certificates for any particular domain and is RECOMMENDED for use 266 with code signing certificates for this additional level of 267 validation checking on issued certificates. 269 As noted in RFC8555, "the external account binding feature (see 270 Section 7.3.4) can allow an ACME account to use authorizations that 271 have been granted to an external, non-ACME account. This allows ACME 272 to address issuance scenarios that cannot yet be fully automated, 273 such as the issuance of "Extended Validation" certificates." 275 The ACME challenge object, [RFC8555] Section 7.1.5 is RECOMMENDED for 276 use for Pre-authorization ([RFC8555] Section 7.4.1). Additional 277 challenge types are added to provide higher levels of security for 278 this issuance verification step. The use of OTP, FIDO credentials 279 (public/private key pairs), or validation from a certificate issued 280 at account setup time are defined in Section 8. Pre-Authoriziation. 282 Questions for reviewers: 284 [Is there interest to set a specific or default challenge object for 285 CodeSigning Certificates? Or should this be left to individual CAs 286 to decide and differentiate? The current challenge types defined in 287 RFC8555 include HTTPS (provisioning HTTP resources) and DNS 288 (provisioning a TXT resource record). Use of DNS may be possible, 289 but the HTTP resource doesn't necessarily make sense. Since the 290 process to retrieve an EV CodeSigning certificate usually requires 291 proof of the organization and validation from one of 2 named 292 administrators, some other challenge type like public/private key 293 pairs or OTP may be needed as defined challenge types. An 294 organization may want to tie this contact to a role rather than a 295 person and that consideration should be made in the design as well as 296 implementation by organizations.] 298 ACME provides an option for notification of the operator via email or 299 SMS upon issuance/renewal of a certificate after the domain has been 300 validated as owned by the requestor. This option is RECOMMENDED due 301 to the security considerations of code signing certificates as a way 302 to limit or reduce the possibility of a third party gaining access to 303 a code signing certificate inappropriately. Development of 304 additional challenge types is included in this document to support 305 this for pre-authorization, which would better match the security 306 considerations for this certificate type. Additional types may be 307 added if agreed upon by the working group. 309 Since DNS is used to identify the organization in the request, the 310 identifier "type" ([RFC8555]Section 7.4) is set to dns, not requiring 311 any additions to the ACME protocol for this type of certificate. The 312 distinction lies in the CSR, where the values are set to request a 313 CodeSigning certificate for a client certificate. [Question: Is it 314 helpful to define an identifier for the administrator or for the 315 developer to distinguish the certificate type in ACME and not just 316 the CSR?] 318 KeyUsage (DigitalSignature) and ExtendedKeyUsage (CodeSigning) in the 319 CSR MUST be set to the correct values for the CA to see the request 320 is for a Code Signing certificate. The Enhanced Key Usage SHOULD be 321 set to show this is a client certificate., using OID 322 "1.3.6.1.5.5.7.3.2". The CN MUST be set to the expected registered 323 domain with the CA account. 325 An advantage of ACME is the ability to automate rollover to allow for 326 easy management of short expiry times on certificates. The lifetime 327 of CodeSigning certificates is typically a year or two, but 328 automation could allow for shorter expiry times becoming feasible. 329 However, lifetimes are less of an issue for code signing certificates 330 than other certificate types. however there is a legitmate case for 331 "one signature per certificate." Automation might be helpful in this 332 case if patches or software updates were frequent or to minimize the 333 knowledge needed for the organization using this method. 335 Automation of storage to an HSM, which typically requires 336 authentication is intentionally left out-of-scope. 338 5. Pre-authorization 340 Additional challenge types are defined here for the verification of 341 administrators at an organization requesting CodeSigning 342 certificates. SMS and email are listed as possible in RFC8555 and 343 may be used singularly or in combination as the ACME protocol allows 344 for multiple pre-authorization challenges to be issued. Additional 345 pre-authorization types are defined that provide a higher level of 346 assurance to authorize a request. 348 6. Challenge Types 350 The challenge types defined in the following subsections are to 351 authenticate individuals or holders of specific pre-issued 352 credentials (users acting in roles for an organization). The 353 challenge types can be used to obtain end user certificate types or 354 as a pre-authorization challenges with certificate types such as the 355 Code Signing Certificate. Please note that the pre-authorization 356 challenge is also coupled with the account certificate in ACME for 357 verification. The process for obtaining EV Code Signing Certificates 358 typically requires authorization from one or more individuals in a 359 role for the organization. The use of pre-issued secure credentials, 360 at an assurance level appropriate for the certificate type being 361 issued, provides a way to automate the issuance and renewal process. 363 6.1. One Time Password (OTP) 365 There are numerous one time password technologies with slight 366 variations between implementations. The response to the challenge is 367 entered in the provided URL, offering flexibility to those using this 368 challenge type to acomodate the specific requirements of their 369 solution. Looking at 2 OTP solutions, the challenge response is 370 provided via a tool or app without any user interaction of 371 information required from the server to generate the challenge. The 372 2 solutions that operate in this manner include SecureID and Duo 373 Security. If a challenge is required to generate the response to be 374 provided in the URL, the token can supply the challenge. 376 type (required, string): The string "otp-01". 378 token (required, string): A random value that uniquely identifies 379 the challenge. OTP types and input vary between technologies. 380 The token value will match the type expected for the pre-issued 381 OTP credential. The user will be able to supply a response in the 382 provided URL from this challenge. It MUST NOT contain any 383 characters outside the base64url alphabet and MUST NOT include 384 base64 padding characters ("="). 386 { 387 "type": "otp-01", 388 "url": "https://example.com/acme/chall/WrV_H87EyD3", 389 "status": "pending", 390 "token": "challenge" 391 } 393 6.1.1. HMAC-Based One-Time Password (HOTP) 395 HOTP([RFC4226]) describes an algorithm for the generation of time- 396 based password values. 398 type (required, string): The string "hotp-01". 400 token (required, string): The HOTP value. This SHOULD be the 6 401 digit representation. 403 { 404 "type": "hotp-01", 405 "url": "https://example.com/acme/chall/WrV_H87EyD3", 406 "status": "pending", 407 "token": "123456" 408 } 410 6.1.2. Time-Based One-Time Password (TOTP) 412 TOTP([RFC6238]) describes an algorithm for the generation of time- 413 based password values, an extension from HOTP. 415 type (required, string): The string "totp-01". 417 token (required, string): The TOTP value. This SHOULD be the 6 418 digit representation. 420 { 421 "type": "totp-01", 422 "url": "https://example.com/acme/chall/WrV_H87EyD3", 423 "status": "pending", 424 "token": "123456" 425 } 427 6.1.3. Generic One Time Password (OTP) 429 There are numerous other one time password technologies with slight 430 variations between implementations. The response to the challenge is 431 entered in the provided URL, offering flexibility to those using this 432 challenge type to acomodate the specific requirements of their 433 solution. Looking at 2 OTP solutions, the challenge response is 434 provided via a tool or app without any user interaction of 435 information required from the server to generate the challenge. The 436 2 solutions that operate in this manner include SecureID and Duo 437 Security. If a challenge is required to generate the response to be 438 provided in the URL, the token can supply the challenge. 440 type (required, string): The string "otp-01". 442 token (required, string): A random value that uniquely identifies 443 the challenge. OTP types and input vary between technologies. 444 The token value will match the type expected for the pre-issued 445 OTP credential. The user will be able to supply a response in the 446 provided URL from this challenge. It MUST NOT contain any 447 characters outside the base64url alphabet and MUST NOT include 448 base64 padding characters ("="). 450 { 451 "type": "otp-01", 452 "url": "https://example.com/acme/chall/WrV_H87EyD3", 453 "status": "pending", 454 "token": "challenge" 455 } 457 6.2. Public Key Cryptography 459 Certificates may be pre-issued and stored according to assurance 460 level requirements for the purpose of identifying a user's identity. 461 If a higher assurance level is needed for a user serving in a 462 specific role or for that individual, it is possible for identity 463 proofing to occur in person using identifiers acceptable for the 464 specified process and the private key stored appropriately for the 465 required assurance level. PKCS#11 software or hardware tokens are 466 both possible options. This model assumes that there may be multiple 467 authorized users with different certificates that can be used for the 468 authorization or pre-authentication challenge. As such, the user 469 first provides the digital signature, so the account management can 470 determine if one of the acceptable certificates was used to digitally 471 sign the token. 473 type (required, string): The string "cert-01". 475 token (required, string): A random value that uniquely identifies 476 the challenge. The token for a certificate authentication 477 challenge includes a value for the recipeint to digitally sign 478 using their private key and post to the provided URL. The ACME 479 server then uses the digitally signed content to verify that the 480 challenge was signed using authorized credentials (certificate 481 issued and authorized for this challenge type). It MUST NOT 482 contain any characters outside the base64url alphabet and MUST NOT 483 include base64 padding characters ("="). 485 { 486 "type": "cert-01", 487 "url": "https://example.com/acme/chall/WrV_H87EyD3", 488 "status": "pending", 489 "token": "Some challenge to digitally sign" 490 } 492 6.3. WebAuthn or Public/Private Key Pairs 494 W3C's WebAuthn uses raw public/private key pairs that are issued 495 specific to a service. If WebAuthn or public/private key pairs 496 (PPKP) are selected as the challenge type, the account and credential 497 issuance will have to occur prior to use of this challenge type. The 498 WebAuthn or public/private key pair credentials would be specific to 499 the certificate management account and would be created by the 500 client, then registered with the service as occurs with normal 501 WebAuthn regisration of credentials. As with normal WebAuthn and 502 public/private key pairs, the token or challenge is digitally signed 503 to prove possession of the private key. 505 type (required, string): The string "ppkp-01". 507 token (required, string): A random value that uniquely identifies 508 the challenge. This challenge will operate much in the same way 509 as the certificate challenge as the operations are largely the 510 same. The user will be able to supply a response in the provided 511 URL from this challenge. It MUST NOT contain any characters 512 outside the base64url alphabet and MUST NOT include base64 padding 513 characters ("="). 515 { 516 "type": "ppkp-01", 517 "url": "https://example.com/acme/chall/WrV_H87EyD3", 518 "status": "pending", 519 "token": "Some challenge to sign" 520 } 522 7. Security Considerations 524 This will likely be full of considerations and is TBD for this 525 revision until challenge types are settled. 527 8. IANA Considerations 529 This memo includes no request to IANA, yet. 531 9. Contributors 533 Thank you to reviewers and contributors who helped to improve this 534 document. Thank you to Thomas Peterson who added the one-time 535 password types, HOTP and TOTP. Thank you to Tim Hollebeek for your 536 early review and added text specific to EV certificate issuance and 537 one time use code signing certificates. Thank you to Andrei Popov 538 and Deb Cooley for your reviews and suggestions made in -04. Thank 539 you to those who reviewed the CAA text removed in version -05 540 including: Carl Mehner, Roland Shoemaker, Ben Schwartz, and Ryan 541 Sleevi. Posted WG version. -02 updates authors email address. 543 10. References 545 10.1. Normative References 547 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 548 Requirement Levels", BCP 14, RFC 2119, 549 DOI 10.17487/RFC2119, March 1997, 550 . 552 [RFC4226] M'Raihi, D., Bellare, M., Hoornaert, F., Naccache, D., and 553 O. Ranen, "HOTP: An HMAC-Based One-Time Password 554 Algorithm", RFC 4226, DOI 10.17487/RFC4226, December 2005, 555 . 557 [RFC6238] M'Raihi, D., Machani, S., Pei, M., and J. Rydell, "TOTP: 558 Time-Based One-Time Password Algorithm", RFC 6238, 559 DOI 10.17487/RFC6238, May 2011, 560 . 562 [RFC7030] Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed., 563 "Enrollment over Secure Transport", RFC 7030, 564 DOI 10.17487/RFC7030, October 2013, 565 . 567 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 568 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 569 May 2017, . 571 [RFC8555] Barnes, R., Hoffman-Andrews, J., McCarney, D., and J. 572 Kasten, "Automatic Certificate Management Environment 573 (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019, 574 . 576 10.2. Informative References 578 [I-D.ietf-acme-ip] 579 Shoemaker, R., "ACME IP Identifier Validation Extension", 580 draft-ietf-acme-ip-08 (work in progress), October 2019. 582 10.3. URL References 584 [NIST800-63A] 585 US National Institute of Standards and Technology, 586 "https://nvlpubs.nist.gov/nistpubs/SpecialPublications/ 587 NIST.SP.800-63a.pdf". 589 [NIST800-63B] 590 US National Institute of Standards and Technology, 591 "https://nvlpubs.nist.gov/nistpubs/SpecialPublications/ 592 NIST.SP.800-63b.pdf". 594 [NIST800-63C] 595 US National Institute of Standards and Technology, 596 "https://nvlpubs.nist.gov/nistpubs/SpecialPublications/ 597 NIST.SP.800-63c.pdf". 599 [NIST800-63r3] 600 US National Institute of Standards and Technology, 601 "https://nvlpubs.nist.gov/nistpubs/SpecialPublications/ 602 NIST.SP.800-63-3.pdf". 604 Appendix A. Change Log 606 Note to RFC Editor: if this document does not obsolete an existing 607 RFC, please remove this appendix before publication as an RFC. 609 02 draft added subsections contributed from Thomas Peterson on HOTP 610 and TOTP. 612 Appendix B. Open Issues 614 Note to RFC Editor: please remove this appendix before publication as 615 an RFC. 617 Author's Address 619 Kathleen M. Moriarty 620 Dell Technologies 621 176 South Street 622 Hopkinton 623 US 625 EMail: Kathleen.Moriarty.ietf@gmail.com