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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 29 September 2021 5 Expires: 2 April 2022 7 ACME End User Client and Code Signing Certificates 8 draft-ietf-acme-client-04 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 2 April 2022. 34 Copyright Notice 36 Copyright (c) 2021 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 (https://trustee.ietf.org/ 41 license-info) in effect on the date of publication of this document. 42 Please review these documents carefully, as they describe your rights 43 and restrictions with respect to this document. Code Components 44 extracted from this document must include Simplified BSD License text 45 as described in Section 4.e of the Trust Legal Provisions and are 46 provided without warranty as described in the Simplified BSD License. 48 Table of Contents 50 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 51 2. Identity Proofing for Client Certificates . . . . . . . . . . 2 52 3. End User Client Certificates . . . . . . . . . . . . . . . . 3 53 4. CodeSigning Certificates . . . . . . . . . . . . . . . . . . 5 54 5. Pre-authorization . . . . . . . . . . . . . . . . . . . . . . 8 55 6. Challenge Types . . . . . . . . . . . . . . . . . . . . . . . 8 56 6.1. One Time Password (OTP) . . . . . . . . . . . . . . . . . 8 57 6.1.1. HMAC-Based One-Time Password (HOTP) . . . . . . . . . 9 58 6.1.2. Time-Based One-Time Password (TOTP) . . . . . . . . . 9 59 6.1.3. Generic One Time Password (OTP) . . . . . . . . . . . 10 60 6.2. Public Key Cryptography . . . . . . . . . . . . . . . . . 10 61 6.3. WebAuthn or Public/Private Key Pairs . . . . . . . . . . 11 62 7. Security Considerations . . . . . . . . . . . . . . . . . . . 12 63 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 64 9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 12 65 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 66 10.1. Normative References . . . . . . . . . . . . . . . . . . 12 67 10.2. Informative References . . . . . . . . . . . . . . . . . 13 68 10.3. URL References . . . . . . . . . . . . . . . . . . . . . 13 69 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 13 70 Appendix B. Open Issues . . . . . . . . . . . . . . . . . . . . 13 71 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 14 73 1. Introduction 75 ACME [RFC8555] is a mechanism for automating certificate management 76 on the Internet. It enables administrative entities to prove 77 effective control over resources like domain names, and automates the 78 process of generating and issuing certificates. 80 The core ACME protocol defined challenge types specific to web server 81 certificates with the possibility to create extensions, or additional 82 challenge types for other use cases and certificate types. Client 83 certificates, such as end user and code signing may also benefit from 84 automated management to ease the deployment and maintenance of these 85 certificate types, thus the definition of this extension defining 86 challenge types specific to that usage. 88 2. Identity Proofing for Client Certificates 90 As with the TLS certificates defined in the core ACME document , identity proofing for ACME issued end user 92 client, device client, and code signing certificates is a separate 93 process outside of the automation of ACME. Identity proofing may be 94 an out-of-band process, if needed, and for this draft is likely tied 95 to the credentials used for the defined challenge types. 97 Identity proofing for these certificate types present some challenges 98 for process automation. NIST SP 800-63 r3 [NIST800-63r3] serves as 99 guidance for identity proofing further detailed in NIST SP 800-63A 100 [NIST800-63A] that may occur prior to the ability to automate 101 certificate management via ACME or may obviate the need for it 102 weighing end user privacy as a higher concern and allowing for 103 credential issuance to be decoupled from identity proofing (IAL1). 104 Using this guidance, a CA might select from the identity proofing 105 levels to assert claims on the issued certificates as described in 106 NIST SP 800-63 r3 [NIST800-63r3]. 108 The certificate issuing CA may make this choice by certificate type 109 issued. Once identity proofing has been performed, in cases where 110 this is part of the process, and certificates have been issued, NIST 111 SP 800-63 r3 [NIST800-63r3] includes recommendations for 112 authentication or in the context of ACME, management of issuance for 113 subsequent client, device, or code-signing certificates: 115 If federations and assertions are used for authorizing certificate 116 issuance, NIST SP 800-63 C [NIST800-63C] may be referenced for 117 guidance on levels of assurance. 119 Existing PKI certification authorities (CAs) tend to use a set of ad 120 hoc protocols for certificate issuance and identity verification. 121 For each certificate usage type, a basic process will be described to 122 obtain an initial certificate and for the certificate renewal 123 process. If higher assurance levels are desired, the guidance from 124 NIST SP 800-63 r3 [NIST800-63r3] may be useful and out-of-band 125 identity proofing options are possible options for pre-authorization 126 challenges or notifications. 128 3. End User Client Certificates 130 A client certificate used to authenticate an end user may be used for 131 mutual authentication in TLS, EAP-TLS, or messaging. The client 132 certificate and key in this case may be stored in a browser, PKCS-#11 133 container, Key Management Interoperability Protocol (KMIP) (possible, 134 but just code signing and device client certificates in practice), or 135 another key container. To obtain an end user client certificate and 136 associated key pair, there are several possibilities to automate 137 authentication of an identity credential intended to be tied to an 138 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 JSON Web Token (JWT) signed 153 OAuth token. 155 Risk based authentication used for identity proofing with red herring 156 questions is a third option that could utilize public information on 157 individuals to authenticate. This would be similar to the signup 158 process used in some financial applications. 160 Existing credentials - for instance, FIDO. FIDO uses a public key 161 pair and does not perform identity proofing. FIDO authentication 162 provides a different key pair to each service using FIDO for 163 authentication, which are generated at the client and registered by 164 the server. This may require using the FIDO credentials from a 165 specific service for authentication to gain ACME issued crededentials 166 (not advised based on how FIDO credentials are supposed to be used). 167 Are there instances where the same provider would issue both sets of 168 credentials? You wouldn't want to expose your FIDO credentials to a 169 different party, that's why each service has their own. Would you 170 set up a mechanism to get FIDO credentials to then obtain a 171 certificate? (What use cases would this be necessary? When do you 172 need a certificate where you already have a specific public/private 173 key pair?) This can be defined as an auth type, but should it be? 175 One-time password (OTP) authentication is a secure option. In cases 176 where a higher assurance level is needed, OTP may be a good choice 177 and many options exist today for OTP that could use an app on a phone 178 for instance tied to an existing (or newly established) password. 179 The OTP may be tied to an out-of-band process and may be associated 180 with a username/password and other accounts. 182 One consideration is to understand if the use case could just use 183 FIDO and not create anything new (ACME client certificates). FIDO 184 provides a mechanism to have unique public key pair based access for 185 client authentication to web sites and they are working on non-web. 186 Identity proofing is intentionally decoupled from authentication in 187 this model as that is in line with NIST 800-63r3 recommendations for 188 privacy protections of the user. The credential in this case is 189 authenticated and would be consistent for it's use, but the identity 190 proofing for that credential is not performed. Obviously, identity 191 proofing is more important for some services, like financial 192 applications where tying the user to the identity for access to 193 financial information is important. However, is automated identity 194 proofing important for any user certificate or should it remain 195 decoupled where it could be automated by a service offering or is 196 there a need for a standardized mechanism to support it for user 197 certificates? 199 Three methods for ACME client authentication, not identity proofing, 200 are proposed in the Challenge Type Section. 202 4. CodeSigning Certificates 204 The process to retrieve a code signing certificate is similar to that 205 of a web server certificate, with differences primarily in the CSR 206 request and the resulting certificate properties. [The storage and 207 access of a code signing certificate must be protected and is 208 typically done through hardware, a hardware security module (HSM), 209 which likely has a PKCS#11 interface. A code signing certificate may 210 either be a standard one or an extended validation (EV) certificate.] 212 For automation purposes, the process described in this document will 213 follow the standard process and any out-of-band preprocessing can 214 increase the level of the issued certificate if the CA offers such 215 options and has additional identity proofing mechanisms (in band or 216 out-of-band). 218 Strict vetting processes are necessary for many code signing 219 certificates to provide a high assurance on the signer. In some 220 cases, issuance of a standard CodeSigning certificate will be 221 appropriate and no additional "challenges" [RFC8555 Section 8] will 222 be necessary. In this case, the standard option could be automated 223 very similar to Web server certificates with the only changes being 224 in the CSR properties. However, this may not apply to all scenarios, 225 such as those requiring EV certificates with the possibility for 226 required out-of-band initial authentication and identity proofing. 228 EV code signing certificates have a distinct set of requirements from 229 EV web certificates. In particular, they don't have associated 230 domain names, nor is CAA checking done. The code signing certificate 231 links a public key to an organization, not a domain. CAs may chose 232 different methods to enable the use of ACME for EV code signing 233 certificates. The intent of this work is to provide additional 234 authentication challenge types that may enable their automation 235 process. 237 Organization validation is required for standard code signing 238 certificates from most issuers. The CSR is used to identify the 239 organization from the included domain name in the request. The 240 resulting certificate, however, instead contains the organization's 241 name and for EV certificates, other identifying information for the 242 organization. For EV certificates, this could require that the 243 domain is registered with the Certificate Authority provider, listed 244 in CAA [RFC6844], and administrators for the account are named with 245 provided portal access for certificate issuance and management 246 options. 248 While ACME allows for the client to directly establish an account 249 with a CA, an initial out-of-band process for this step may assist 250 with the additional requirements for EV certificates and assurance 251 levels typically required for code signing certificates. For 252 standard certificates, with a recommendation for additional vetting 253 through extended challenge options to enable ACME to establish the 254 account directly. In cases where code signing certificates are used 255 heavily for an organization, having the portal access replaced with 256 ACME authenticated client access with extra challenges for 257 authentication may be an option to automate the functionality. 259 [For standard certificates, is it worth defining SMS and email for 260 the challenge? Obviously, EV needs more, so a few choices are 261 suggested in this revision.] 263 To improve the vetting process, ACME's optional use of CAA [RFC6844] 264 with the Directory "meta" data "caaIdentities" ([RFC8555] 265 Section 9.7.6) assists with the validation that a CA may have issue 266 certificates for any particular domain and is RECOMMENDED for use 267 with code signing certificates for this additional level of 268 validation checking on issued certificates. 270 As noted in RFC8555, "the external account binding feature (see 271 Section 7.3.4) can allow an ACME account to use authorizations that 272 have been granted to an external, non-ACME account. This allows ACME 273 to address issuance scenarios that cannot yet be fully automated, 274 such as the issuance of "Extended Validation" certificates." 276 The ACME challenge object, [RFC8555] Section 7.1.5 is RECOMMENDED for 277 use for Pre-authorization ([RFC8555] Section 7.4.1). Additional 278 challenge types are added to provide higher levels of security for 279 this issuance verification step. The use of OTP, FIDO credentials 280 (public/private key pairs), or validation from a certificate issued 281 at account setup time are defined in Section 8. Pre-Authoriziation. 283 Questions for reviewers: 285 [Is there interest to set a specific or default challenge object for 286 CodeSigning Certificates? Or should this be left to individual CAs 287 to decide and differentiate? The current challenge types defined in 288 RFC8555 include HTTPS (provisioning HTTP resources) and DNS 289 (provisioning a TXT resource record). Use of DNS may be possible, 290 but the HTTP resource doesn't necessarily make sense. Since the 291 process to retrieve an EV CodeSigning certificate usually requires 292 proof of the organization and validation from one of 2 named 293 administrators, some other challenge type like public/private key 294 pairs or OTP may be needed as defined challenge types. An 295 organization may want to tie this contact to a role rather than a 296 person and that consideration should be made in the design as well as 297 implementation by organizations.] 299 ACME provides an option for notification of the operator via email or 300 SMS upon issuance/renewal of a certificate after the domain has been 301 validated as owned by the requestor. This option is RECOMMENDED due 302 to the security considerations of code signing certificates as a way 303 to limit or reduce the possibility of a third party gaining access to 304 a code signing certificate inappropriately. Development of 305 additional challenge types is included in this document to support 306 this for pre-authorization, which would better match the security 307 considerations for this certificate type. Additional types may be 308 added if agreed upon by the working group. 310 Since DNS is used to identify the organization in the request, the 311 identifier "type" ([RFC8555]Section 7.4) is set to dns, not requiring 312 any additions to the ACME protocol for this type of certificate. The 313 distinction lies in the CSR, where the values are set to request a 314 CodeSigning certificate for a client certificate. [Question: Is it 315 helpful to define an identifier for the administrator or for the 316 developer to distinguish the certificate type in ACME and not just 317 the CSR?] 319 KeyUsage (DigitalSignature) and ExtendedKeyUsage (CodeSigning) in the 320 CSR MUST be set to the correct values for the CA to see the request 321 is for a Code Signing certificate. The Enhanced Key Usage SHOULD be 322 set to show this is a client certificate., using OID 323 "1.3.6.1.5.5.7.3.2". The CN MUST be set to the expected registered 324 domain with the CA account. 326 An advantage of ACME is the ability to automate rollover to allow for 327 easy management of short expiry times on certificates. The lifetime 328 of CodeSigning certificates is typically a year or two, but 329 automation could allow for shorter expiry times becoming feasible. 330 However, lifetimes are less of an issue for code signing certificates 331 than other certificate types. however there is a legitmate case for 332 "one signature per certificate." Automation might be helpful in this 333 case if patches or software updates were frequent or to minimize the 334 knowledge needed for the organization using this method. 336 Automation of storage to a hardware security module (HSM), which 337 typically requires authentication is intentionally left out-of-scope. 339 5. Pre-authorization 341 Additional challenge types are defined here for the verification of 342 administrators at an organization requesting CodeSigning 343 certificates. Email is listed as possible in RFC8555 and may be used 344 singularly or in combination as the ACME protocol allows for multiple 345 pre-authorization challenges to be issued. Additional pre- 346 authorization types are defined that provide a higher level of 347 assurance to authorize a request. 349 6. Challenge Types 351 The challenge types defined in the following subsections are to 352 authenticate individuals or holders of specific pre-issued 353 credentials (users acting in roles for an organization). The 354 challenge types can be used to obtain end user certificate types or 355 as a pre-authorization challenges with certificate types such as the 356 Code Signing Certificate. Please note that the pre-authorization 357 challenge is also coupled with the account certificate in ACME for 358 verification. The process for obtaining EV Code Signing Certificates 359 typically requires authorization from one or more individuals in a 360 role for the organization. The use of pre-issued secure credentials, 361 at an assurance level appropriate for the certificate type being 362 issued, provides a way to automate the issuance and renewal process. 364 6.1. One Time Password (OTP) 366 There are numerous one time password technologies with slight 367 variations between implementations. The response to the challenge is 368 entered in the provided URL, offering flexibility to those using this 369 challenge type to acomodate the specific requirements of their 370 solution. Looking at 2 OTP solutions, the challenge response is 371 provided via a tool or app without any user interaction of 372 information required from the server to generate the challenge. The 373 2 solutions that operate in this manner include SecureID and Duo 374 Security. If a challenge is required to generate the response to be 375 provided in the URL, the token can supply the challenge. 377 type (required, string): The string "otp-01". 379 token (required, string): A random value that uniquely identifies 380 the challenge. OTP types and input vary between technologies. 381 The token value will match the type expected for the pre-issued 382 OTP credential. The user will be able to supply a response in the 383 provided URL from this challenge. It MUST NOT contain any 384 characters outside the base64url alphabet and MUST NOT include 385 base64 padding characters ("="). 387 { 388 "type": "otp-01", 389 "url": "https://example.com/acme/chall/WrV_H87EyD3", 390 "status": "pending", 391 "token": "challenge" 392 } 394 6.1.1. HMAC-Based One-Time Password (HOTP) 396 HOTP([RFC4226]) describes an algorithm for the generation of time- 397 based password values. 399 type (required, string): The string "hotp-01". 401 token (required, string): The HOTP value. This SHOULD be the 6 402 digit representation. 404 { 405 "type": "hotp-01", 406 "url": "https://example.com/acme/chall/WrV_H87EyD3", 407 "status": "pending", 408 "token": "123456" 409 } 411 6.1.2. Time-Based One-Time Password (TOTP) 413 TOTP([RFC6238]) describes an algorithm for the generation of time- 414 based password values, an extension from HOTP. 416 type (required, string): The string "totp-01". 418 token (required, string): The TOTP value. This SHOULD be the 6 419 digit representation. 421 { 422 "type": "totp-01", 423 "url": "https://example.com/acme/chall/WrV_H87EyD3", 424 "status": "pending", 425 "token": "123456" 426 } 428 6.1.3. Generic One Time Password (OTP) 430 There are numerous other one time password technologies with slight 431 variations between implementations. The response to the challenge is 432 entered in the provided URL, offering flexibility to those using this 433 challenge type to acomodate the specific requirements of their 434 solution. Looking at 2 OTP solutions, the challenge response is 435 provided via a tool or app without any user interaction of 436 information required from the server to generate the challenge. The 437 2 solutions that operate in this manner include SecureID and Duo 438 Security. If a challenge is required to generate the response to be 439 provided in the URL, the token can supply the challenge. 441 type (required, string): The string "otp-01". 443 token (required, string): A random value that uniquely identifies 444 the challenge. OTP types and input vary between technologies. 445 The token value will match the type expected for the pre-issued 446 OTP credential. The user will be able to supply a response in the 447 provided URL from this challenge. It MUST NOT contain any 448 characters outside the base64url alphabet and MUST NOT include 449 base64 padding characters ("="). 451 { 452 "type": "otp-01", 453 "url": "https://example.com/acme/chall/WrV_H87EyD3", 454 "status": "pending", 455 "token": "challenge" 456 } 458 6.2. Public Key Cryptography 460 Certificates may be pre-issued and stored according to assurance 461 level requirements for the purpose of identifying a user's identity. 462 If a higher assurance level is needed for a user serving in a 463 specific role or for that individual, it is possible for identity 464 proofing to occur in person using identifiers acceptable for the 465 specified process and the private key stored appropriately for the 466 required assurance level. PKCS#11 software or hardware tokens are 467 both possible options. This model assumes that there may be multiple 468 authorized users with different certificates that can be used for the 469 authorization or pre-authentication challenge. As such, the user 470 first provides the digital signature, so the account management can 471 determine if one of the acceptable certificates was used to digitally 472 sign the token. 474 type (required, string): The string "cert-01". 476 token (required, string): A random value that uniquely identifies 477 the challenge. The token for a certificate authentication 478 challenge includes a value for the recipeint to digitally sign 479 using their private key and post to the provided URL. The ACME 480 server then uses the digitally signed content to verify that the 481 challenge was signed using authorized credentials (certificate 482 issued and authorized for this challenge type). It MUST NOT 483 contain any characters outside the base64url alphabet and MUST NOT 484 include base64 padding characters ("="). 486 { 487 "type": "cert-01", 488 "url": "https://example.com/acme/chall/WrV_H87EyD3", 489 "status": "pending", 490 "token": "Some challenge to digitally sign" 491 } 493 6.3. WebAuthn or Public/Private Key Pairs 495 W3C's WebAuthn uses raw public/private key pairs that are issued 496 specific to a service. If WebAuthn or public/private key pairs 497 (PPKP) are selected as the challenge type, the account and credential 498 issuance will have to occur prior to use of this challenge type. The 499 WebAuthn or public/private key pair credentials would be specific to 500 the certificate management account and would be created by the 501 client, then registered with the service as occurs with normal 502 WebAuthn regisration of credentials. As with normal WebAuthn and 503 public/private key pairs, the token or challenge is digitally signed 504 to prove possession of the private key. 506 type (required, string): The string "ppkp-01". 508 token (required, string): A random value that uniquely identifies 509 the challenge. This challenge will operate much in the same way 510 as the certificate challenge as the operations are largely the 511 same. The user will be able to supply a response in the provided 512 URL from this challenge. It MUST NOT contain any characters 513 outside the base64url alphabet and MUST NOT include base64 padding 514 characters ("="). 516 { 517 "type": "ppkp-01", 518 "url": "https://example.com/acme/chall/WrV_H87EyD3", 519 "status": "pending", 520 "token": "Some challenge to sign" 521 } 523 7. Security Considerations 525 This will likely be full of considerations and is TBD for this 526 revision until challenge types are settled. 528 8. IANA Considerations 530 This memo includes no request to IANA, yet. 532 9. Contributors 534 Thank you to reviewers and contributors who helped to improve this 535 document. Thank you to Thomas Peterson who added the one-time 536 password types, HOTP and TOTP. Thank you to Tim Hollebeek for your 537 early review and added text specific to EV certificate issuance and 538 one time use code signing certificates. Thank you to Andrei Popov 539 and Deb Cooley for your reviews and suggestions made in -04. Thank 540 you to those who reviewed the CAA text removed in version -05 541 including: Carl Mehner, Roland Shoemaker, Ben Schwartz, and Ryan 542 Sleevi. Posted WG version. -02 updates authors email address. 544 10. References 546 10.1. Normative References 548 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 549 Requirement Levels", BCP 14, RFC 2119, 550 DOI 10.17487/RFC2119, March 1997, 551 . 553 [RFC4226] M'Raihi, D., Bellare, M., Hoornaert, F., Naccache, D., and 554 O. Ranen, "HOTP: An HMAC-Based One-Time Password 555 Algorithm", RFC 4226, DOI 10.17487/RFC4226, December 2005, 556 . 558 [RFC6238] M'Raihi, D., Machani, S., Pei, M., and J. Rydell, "TOTP: 559 Time-Based One-Time Password Algorithm", RFC 6238, 560 DOI 10.17487/RFC6238, May 2011, 561 . 563 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 564 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 565 May 2017, . 567 [RFC8555] Barnes, R., Hoffman-Andrews, J., McCarney, D., and J. 568 Kasten, "Automatic Certificate Management Environment 569 (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019, 570 . 572 [RFC7030] Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed., 573 "Enrollment over Secure Transport", RFC 7030, 574 DOI 10.17487/RFC7030, October 2013, 575 . 577 10.2. Informative References 579 [I-D.ietf-acme-ip] 580 Shoemaker, R. B., "Automated Certificate Management 581 Environment (ACME) IP Identifier Validation Extension", 582 Work in Progress, Internet-Draft, draft-ietf-acme-ip-08, 1 583 October 2019, . 586 10.3. URL References 588 [NIST800-63r3] 589 US National Institute of Standards and Technology, 590 "https://nvlpubs.nist.gov/nistpubs/SpecialPublications/ 591 NIST.SP.800-63-3.pdf". 593 [NIST800-63A] 594 US National Institute of Standards and Technology, 595 "https://nvlpubs.nist.gov/nistpubs/SpecialPublications/ 596 NIST.SP.800-63a.pdf". 598 [NIST800-63B] 599 US National Institute of Standards and Technology, 600 "https://nvlpubs.nist.gov/nistpubs/SpecialPublications/ 601 NIST.SP.800-63b.pdf". 603 [NIST800-63C] 604 US National Institute of Standards and Technology, 605 "https://nvlpubs.nist.gov/nistpubs/SpecialPublications/ 606 NIST.SP.800-63c.pdf". 608 Appendix A. Change Log 610 Note to RFC Editor: if this document does not obsolete an existing 611 RFC, please remove this appendix before publication as an RFC. 613 02 draft added subsections contributed from Thomas Peterson on HOTP 614 and TOTP. 616 Appendix B. Open Issues 618 Note to RFC Editor: please remove this appendix before publication as 619 an RFC. 621 Author's Address 623 Kathleen M. Moriarty 624 Dell Technologies 625 176 South Street 626 Hopkinton, 627 United States of America 629 Email: Kathleen.Moriarty.ietf@gmail.com