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2 IETF K. Moriarty
3 Internet-Draft Dell Technologies
4 Intended status: Standards Track November 17, 2019
5 Expires: May 20, 2020
7 ACME End User Client and Code Signing Certificates
8 draft-moriarty-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 May 20, 2020.
34 Copyright Notice
36 Copyright (c) 2019 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) . . . . . . . . . . . 10
61 6.2. Public Key Cryptography . . . . . . . . . . . . . . . . . 10
62 6.3. WebAuthn or Public/Private Key Pairs . . . . . . . . . . 11
63 7. Security Considerations . . . . . . . . . . . . . . . . . . . 12
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 CAA helps as anyone verifying a certificate used for code signing can
270 verify that the CA used has been authorized to issue certificates for
271 that organization. CSR requests for code signing certificates
272 typically contain a Common Name (CN) using a domain name that is
273 replaced with the organization name to have the expected details
274 displayed in the resulting certificate. Since this work flow already
275 occurs, there is a path to automation and validation via an existing
276 ACME type, "dns".
278 As noted in RFC8555, "the external account binding feature (see
279 Section 7.3.4) can allow an ACME account to use authorizations that
280 have been granted to an external, non-ACME account. This allows ACME
281 to address issuance scenarios that cannot yet be fully automated,
282 such as the issuance of "Extended Validation" certificates."
284 The ACME challenge object, [RFC8555] Section 7.1.5 is RECOMMENDED for
285 use for Pre-authorization ([RFC8555] Section 7.4.1). Additional
286 challenge types are added to provide higher levels of security for
287 this issuance verification step. The use of OTP, FIDO credentials
288 (public/private key pairs), or validation from a certificate issued
289 at account setup time are defined in Section 8. Pre-Authoriziation.
291 Questions for reviewers:
293 [Is there interest to set a specific or default challenge object for
294 CodeSigning Certificates? Or should this be left to individual CAs
295 to decide and differentiate? The current challenge types defined in
296 RFC8555 include HTTPS (provisioning HTTP resources) and DNS
297 (provisioning a TXT resource record). Use of DNS may be possible,
298 but the HTTP resource doesn't necessarily make sense. Since the
299 process to retrieve an EV CodeSigning certificate usually requires
300 proof of the organization and validation from one of 2 named
301 administrators, some other challenge type like public/private key
302 pairs or OTP may be needed as defined challenge types. An
303 organization may want to tie this contact to a role rather than a
304 person and that consideration should be made in the design as well as
305 implementation by organizations.]
307 ACME provides an option for notification of the operator via email or
308 SMS upon issuance/renewal of a certificate after the domain has been
309 validated as owned by the requestor. This option is RECOMMENDED due
310 to the security considerations of code signing certificates as a way
311 to limit or reduce the possibility of a third party gaining access to
312 a code signing certificate inappropriately. Development of
313 additional challenge types is included in this document to support
314 this for pre-authorization, which would better match the security
315 considerations for this certificate type. Additional types may be
316 added if agreed upon by the working group.
318 Since DNS is used to identify the organization in the request, the
319 identifier "type" ([RFC8555]Section 7.4) is set to dns, not requiring
320 any additions to the ACME protocol for this type of certificate. The
321 distinction lies in the CSR, where the values are set to request a
322 CodeSigning certificate for a client certificate. [Question: Is it
323 helpful to define an identifier for the administrator or for the
324 developer to distinguish the certificate type in ACME and not just
325 the CSR?]
327 KeyUsage (DigitalSignature) and ExtendedKeyUsage (CodeSigning) in the
328 CSR MUST be set to the correct values for the CA to see the request
329 is for a Code Signing certificate. The Enhanced Key Usage SHOULD be
330 set to show this is a client certificate., using OID
331 "1.3.6.1.5.5.7.3.2". The CN MUST be set to the expected registered
332 domain with the CA account.
334 An advantage of ACME is the ability to automate rollover to allow for
335 easy management of short expiry times on certificates. The lifetime
336 of CodeSigning certificates is typically a year or two, but
337 automation could allow for shorter expiry times becoming feasible.
338 However, lifetimes are less of an issue for code signing certificates
339 than other certificate types. however there is a legitmate case for
340 "one signature per certificate." Automation might be helpful in this
341 case if patches or software updates were frequent or to minimize the
342 knowledge needed for the organization using this method.
344 Automation of storage to an HSM, which typically requires
345 authentication is intentionally left out-of-scope.
347 5. Pre-authorization
349 Additional challenge types are defined here for the verification of
350 administrators at an organization requesting CodeSigning
351 certificates. SMS and email are listed as possible in RFC8555 and
352 may be used singularly or in combination as the ACME protocol allows
353 for multiple pre-authorization challenges to be issued. Additional
354 pre-authorization types are defined that provide a higher level of
355 assurance to authorize a request.
357 6. Challenge Types
359 The challenge types defined in the following subsections are to
360 authenticate individuals or holders of specific pre-issued
361 credentials (users acting in roles for an organization). The
362 challenge types can be used to obtain end user certificate types or
363 as a pre-authorization challenges with certificate types such as the
364 Code Signing Certificate. Please note that the pre-authorization
365 challenge is also coupled with the account certificate in ACME for
366 verification. The process for obtaining EV Code Signing Certificates
367 typically requires authorization from one or more individuals in a
368 role for the organization. The use of pre-issued secure credentials,
369 at an assurance level appropriate for the certificate type being
370 issued, provides a way to automate the issuance and renewal process.
372 6.1. One Time Password (OTP)
374 There are numerous one time password technologies with slight
375 variations between implementations. The response to the challenge is
376 entered in the provided URL, offering flexibility to those using this
377 challenge type to acomodate the specific requirements of their
378 solution. Looking at 2 OTP solutions, the challenge response is
379 provided via a tool or app without any user interaction of
380 information required from the server to generate the challenge. The
381 2 solutions that operate in this manner include SecureID and Duo
382 Security. If a challenge is required to generate the response to be
383 provided in the URL, the token can supply the challenge.
385 type (required, string): The string "otp-01".
387 token (required, string): A random value that uniquely identifies
388 the challenge. OTP types and input vary between technologies.
389 The token value will match the type expected for the pre-issued
390 OTP credential. The user will be able to supply a response in the
391 provided URL from this challenge. It MUST NOT contain any
392 characters outside the base64url alphabet and MUST NOT include
393 base64 padding characters ("=").
395 {
396 "type": "otp-01",
397 "url": "https://example.com/acme/chall/WrV_H87EyD3",
398 "status": "pending",
399 "token": "challenge"
400 }
402 6.1.1. HMAC-Based One-Time Password (HOTP)
404 HOTP([RFC4226]) describes an algorithm for the generation of time-
405 based password values.
407 type (required, string): The string "hotp-01".
409 token (required, string): The HOTP value. This SHOULD be the 6
410 digit representation.
412 {
413 "type": "hotp-01",
414 "url": "https://example.com/acme/chall/WrV_H87EyD3",
415 "status": "pending",
416 "token": "123456"
417 }
419 6.1.2. Time-Based One-Time Password (TOTP)
421 TOTP([RFC6238]) describes an algorithm for the generation of time-
422 based password values, an extension from HOTP.
424 type (required, string): The string "totp-01".
426 token (required, string): The TOTP value. This SHOULD be the 6
427 digit representation.
429 {
430 "type": "totp-01",
431 "url": "https://example.com/acme/chall/WrV_H87EyD3",
432 "status": "pending",
433 "token": "123456"
434 }
436 6.1.3. Generic One Time Password (OTP)
438 There are numerous other one time password technologies with slight
439 variations between implementations. The response to the challenge is
440 entered in the provided URL, offering flexibility to those using this
441 challenge type to acomodate the specific requirements of their
442 solution. Looking at 2 OTP solutions, the challenge response is
443 provided via a tool or app without any user interaction of
444 information required from the server to generate the challenge. The
445 2 solutions that operate in this manner include SecureID and Duo
446 Security. If a challenge is required to generate the response to be
447 provided in the URL, the token can supply the challenge.
449 type (required, string): The string "otp-01".
451 token (required, string): A random value that uniquely identifies
452 the challenge. OTP types and input vary between technologies.
453 The token value will match the type expected for the pre-issued
454 OTP credential. The user will be able to supply a response in the
455 provided URL from this challenge. It MUST NOT contain any
456 characters outside the base64url alphabet and MUST NOT include
457 base64 padding characters ("=").
459 {
460 "type": "otp-01",
461 "url": "https://example.com/acme/chall/WrV_H87EyD3",
462 "status": "pending",
463 "token": "challenge"
464 }
466 6.2. Public Key Cryptography
468 Certificates may be pre-issued and stored according to assurance
469 level requirements for the purpose of identifying a user's identity.
470 If a higher assurance level is needed for a user serving in a
471 specific role or for that individual, it is possible for identity
472 proofing to occur in person using identifiers acceptable for the
473 specified process and the private key stored appropriately for the
474 required assurance level. PKCS#11 software or hardware tokens are
475 both possible options. This model assumes that there may be multiple
476 authorized users with different certificates that can be used for the
477 authorization or pre-authentication challenge. As such, the user
478 first provides the digital signature, so the account management can
479 determine if one of the acceptable certificates was used to digitally
480 sign the token.
482 type (required, string): The string "cert-01".
484 token (required, string): A random value that uniquely identifies
485 the challenge. The token for a certificate authentication
486 challenge includes a value for the recipeint to digitally sign
487 using their private key and post to the provided URL. The ACME
488 server then uses the digitally signed content to verify that the
489 challenge was signed using authorized credentials (certificate
490 issued and authorized for this challenge type). It MUST NOT
491 contain any characters outside the base64url alphabet and MUST NOT
492 include base64 padding characters ("=").
494 {
495 "type": "cert-01",
496 "url": "https://example.com/acme/chall/WrV_H87EyD3",
497 "status": "pending",
498 "token": "Some challenge to digitally sign"
499 }
501 6.3. WebAuthn or Public/Private Key Pairs
503 W3C's WebAuthn uses raw public/private key pairs that are issued
504 specific to a service. If WebAuthn or public/private key pairs
505 (PPKP) are selected as the challenge type, the account and credential
506 issuance will have to occur prior to use of this challenge type. The
507 WebAuthn or public/private key pair credentials would be specific to
508 the certificate management account and would be created by the
509 client, then registered with the service as occurs with normal
510 WebAuthn regisration of credentials. As with normal WebAuthn and
511 public/private key pairs, the token or challenge is digitally signed
512 to prove possession of the private key.
514 type (required, string): The string "ppkp-01".
516 token (required, string): A random value that uniquely identifies
517 the challenge. This challenge will operate much in the same way
518 as the certificate challenge as the operations are largely the
519 same. The user will be able to supply a response in the provided
520 URL from this challenge. It MUST NOT contain any characters
521 outside the base64url alphabet and MUST NOT include base64 padding
522 characters ("=").
524 {
525 "type": "ppkp-01",
526 "url": "https://example.com/acme/chall/WrV_H87EyD3",
527 "status": "pending",
528 "token": "Some challenge to sign"
529 }
531 7. Security Considerations
533 This will likely be full of considerations and is TBD for this
534 revision until challenge types are settled.
536 8. IANA Considerations
538 This memo includes no request to IANA, yet.
540 9. Contributors
542 Thank you to reviewers and contributors who helped to improve this
543 document. Thank you to Thomas Peterson who added the one-time
544 password types, HOTP and TOTP. Thank you to Tim Hollebeek for your
545 early review and added text specific to EV certificate issuance and
546 one time use code signing certificates. Thank you to Andrei Popov
547 and Deb Cooley for your reviews and suggestions made in -04.
549 10. References
551 10.1. Normative References
553 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
554 Requirement Levels", BCP 14, RFC 2119,
555 DOI 10.17487/RFC2119, March 1997,
556 .
558 [RFC4226] M'Raihi, D., Bellare, M., Hoornaert, F., Naccache, D., and
559 O. Ranen, "HOTP: An HMAC-Based One-Time Password
560 Algorithm", RFC 4226, DOI 10.17487/RFC4226, December 2005,
561 .
563 [RFC6238] M'Raihi, D., Machani, S., Pei, M., and J. Rydell, "TOTP:
564 Time-Based One-Time Password Algorithm", RFC 6238,
565 DOI 10.17487/RFC6238, May 2011,
566 .
568 [RFC7030] Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed.,
569 "Enrollment over Secure Transport", RFC 7030,
570 DOI 10.17487/RFC7030, October 2013,
571 .
573 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
574 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
575 May 2017, .
577 [RFC8555] Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
578 Kasten, "Automatic Certificate Management Environment
579 (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,
580 .
582 10.2. Informative References
584 [I-D.ietf-acme-ip]
585 Shoemaker, R., "ACME IP Identifier Validation Extension",
586 draft-ietf-acme-ip-08 (work in progress), October 2019.
588 10.3. URL References
590 [NIST800-63A]
591 US National Institute of Standards and Technology,
592 "https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
593 NIST.SP.800-63a.pdf".
595 [NIST800-63B]
596 US National Institute of Standards and Technology,
597 "https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
598 NIST.SP.800-63b.pdf".
600 [NIST800-63C]
601 US National Institute of Standards and Technology,
602 "https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
603 NIST.SP.800-63c.pdf".
605 [NIST800-63r3]
606 US National Institute of Standards and Technology,
607 "https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
608 NIST.SP.800-63-3.pdf".
610 Appendix A. Change Log
612 Note to RFC Editor: if this document does not obsolete an existing
613 RFC, please remove this appendix before publication as an RFC.
615 02 draft added subsections contributed from Thomas Peterson on HOTP
616 and TOTP.
618 Appendix B. Open Issues
620 Note to RFC Editor: please remove this appendix before publication as
621 an RFC.
623 Author's Address
625 Kathleen M. Moriarty
626 Dell Technologies
627 176 South Street
628 Hopkinton
629 US
631 EMail: Kathleen.Moriarty@dell.com