idnits 2.17.1
draft-ietf-acme-client-02.txt:
Checking boilerplate required by RFC 5378 and the IETF Trust (see
https://trustee.ietf.org/license-info):
----------------------------------------------------------------------------
No issues found here.
Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt:
----------------------------------------------------------------------------
No issues found here.
Checking nits according to https://www.ietf.org/id-info/checklist :
----------------------------------------------------------------------------
No issues found here.
Miscellaneous warnings:
----------------------------------------------------------------------------
== The copyright year in the IETF Trust and authors Copyright Line does not
match the current year
== The document seems to lack the recommended RFC 2119 boilerplate, even if
it appears to use RFC 2119 keywords.
(The document does seem to have the reference to RFC 2119 which the
ID-Checklist requires).
-- The document date (October 5, 2020) is 1299 days in the past. Is this
intentional?
Checking references for intended status: Proposed Standard
----------------------------------------------------------------------------
(See RFCs 3967 and 4897 for information about using normative references
to lower-maturity documents in RFCs)
== Missing Reference: 'NIST800-63r3' is mentioned on line 599, but not
defined
== Missing Reference: 'NIST800-63A' is mentioned on line 584, but not
defined
== Missing Reference: 'NIST800-63C' is mentioned on line 594, but not
defined
== Missing Reference: 'RFC6844' is mentioned on line 262, but not defined
** Obsolete undefined reference: RFC 6844 (Obsoleted by RFC 8659)
== Missing Reference: 'NIST800-63B' is mentioned on line 589, but not
defined
== Unused Reference: 'RFC2119' is defined on line 547, but no explicit
reference was found in the text
== Unused Reference: 'RFC7030' is defined on line 562, but no explicit
reference was found in the text
== Unused Reference: 'RFC8174' is defined on line 567, but no explicit
reference was found in the text
== Unused Reference: 'I-D.ietf-acme-ip' is defined on line 578, but no
explicit reference was found in the text
** Downref: Normative reference to an Informational RFC: RFC 4226
** Downref: Normative reference to an Informational RFC: RFC 6238
Summary: 3 errors (**), 0 flaws (~~), 11 warnings (==), 1 comment (--).
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