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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 ACME Working Group Y. Sheffer 3 Internet-Draft Intuit 4 Intended status: Standards Track D. Lopez 5 Expires: January 2, 2020 O. Gonzalez de Dios 6 A. Pastor Perales 7 Telefonica I+D 8 T. Fossati 9 ARM 10 July 01, 2019 12 Support for Short-Term, Automatically-Renewed (STAR) Certificates in 13 Automated Certificate Management Environment (ACME) 14 draft-ietf-acme-star-06 16 Abstract 18 Public-key certificates need to be revoked when they are compromised, 19 that is, when the associated private key is exposed to an 20 unauthorized entity. However the revocation process is often 21 unreliable. An alternative to revocation is issuing a sequence of 22 certificates, each with a short validity period, and terminating this 23 sequence upon compromise. This memo proposes an ACME extension to 24 enable the issuance of short-term and automatically renewed (STAR) 25 X.509 certificates. 27 [RFC Editor: please remove before publication] 29 While the draft is being developed, the editor's version can be found 30 at https://github.com/yaronf/I-D/tree/master/STAR. 32 Status of This Memo 34 This Internet-Draft is submitted in full conformance with the 35 provisions of BCP 78 and BCP 79. 37 Internet-Drafts are working documents of the Internet Engineering 38 Task Force (IETF). Note that other groups may also distribute 39 working documents as Internet-Drafts. The list of current Internet- 40 Drafts is at https://datatracker.ietf.org/drafts/current/. 42 Internet-Drafts are draft documents valid for a maximum of six months 43 and may be updated, replaced, or obsoleted by other documents at any 44 time. It is inappropriate to use Internet-Drafts as reference 45 material or to cite them other than as "work in progress." 47 This Internet-Draft will expire on January 2, 2020. 49 Copyright Notice 51 Copyright (c) 2019 IETF Trust and the persons identified as the 52 document authors. All rights reserved. 54 This document is subject to BCP 78 and the IETF Trust's Legal 55 Provisions Relating to IETF Documents 56 (https://trustee.ietf.org/license-info) in effect on the date of 57 publication of this document. Please review these documents 58 carefully, as they describe your rights and restrictions with respect 59 to this document. Code Components extracted from this document must 60 include Simplified BSD License text as described in Section 4.e of 61 the Trust Legal Provisions and are provided without warranty as 62 described in the Simplified BSD License. 64 Table of Contents 66 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 67 1.1. Name Delegation Use Case . . . . . . . . . . . . . . . . 4 68 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 69 1.3. Conventions used in this document . . . . . . . . . . . . 4 70 2. Protocol Flow . . . . . . . . . . . . . . . . . . . . . . . . 4 71 2.1. Bootstrap . . . . . . . . . . . . . . . . . . . . . . . . 5 72 2.2. Refresh . . . . . . . . . . . . . . . . . . . . . . . . . 5 73 2.3. Termination . . . . . . . . . . . . . . . . . . . . . . . 6 74 3. Protocol Details . . . . . . . . . . . . . . . . . . . . . . 7 75 3.1. ACME Extensions . . . . . . . . . . . . . . . . . . . . . 7 76 3.1.1. Extending the Order Resource . . . . . . . . . . . . 7 77 3.1.2. Canceling a Recurrent Order . . . . . . . . . . . . . 8 78 3.2. Capability Discovery . . . . . . . . . . . . . . . . . . 9 79 3.3. Fetching the Certificates . . . . . . . . . . . . . . . . 10 80 3.4. Negotiating an unauthenticated GET . . . . . . . . . . . 12 81 3.5. Computing notBefore and notAfter of STAR Certificates . . 12 82 3.5.1. Example . . . . . . . . . . . . . . . . . . . . . . . 13 83 4. Operational Considerations . . . . . . . . . . . . . . . . . 14 84 4.1. The Meaning of "Short Term" and the Impact of Skewed 85 Clocks . . . . . . . . . . . . . . . . . . . . . . . . . 14 86 4.2. Impact on Certificate Transparency (CT) Logs . . . . . . 15 87 4.3. Dependability . . . . . . . . . . . . . . . . . . . . . . 15 88 5. Implementation Status . . . . . . . . . . . . . . . . . . . . 15 89 5.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 16 90 5.1.1. ACME Server with STAR extension . . . . . . . . . . . 16 91 5.1.2. STAR Proxy . . . . . . . . . . . . . . . . . . . . . 16 92 5.2. Level of Maturity . . . . . . . . . . . . . . . . . . . . 17 93 5.3. Coverage . . . . . . . . . . . . . . . . . . . . . . . . 17 94 5.4. Version Compatibility . . . . . . . . . . . . . . . . . . 17 95 5.5. Licensing . . . . . . . . . . . . . . . . . . . . . . . . 17 96 5.6. Implementation experience . . . . . . . . . . . . . . . . 17 97 5.7. Contact Information . . . . . . . . . . . . . . . . . . . 18 98 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 99 6.1. New Error Types . . . . . . . . . . . . . . . . . . . . . 18 100 6.2. New fields in Order Objects . . . . . . . . . . . . . . . 19 101 6.3. New fields in the "meta" Object within a Directory Object 19 102 6.4. Not-Before and Not-After HTTP Headers . . . . . . . . . . 19 103 7. Security Considerations . . . . . . . . . . . . . . . . . . . 20 104 7.1. No revocation . . . . . . . . . . . . . . . . . . . . . . 20 105 7.2. Denial of Service Considerations . . . . . . . . . . . . 20 106 7.3. Privacy Considerations . . . . . . . . . . . . . . . . . 21 107 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21 108 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 21 109 9.1. Normative References . . . . . . . . . . . . . . . . . . 21 110 9.2. Informative References . . . . . . . . . . . . . . . . . 22 111 Appendix A. Document History . . . . . . . . . . . . . . . . . . 24 112 A.1. draft-ietf-acme-star-06 . . . . . . . . . . . . . . . . . 24 113 A.2. draft-ietf-acme-star-05 . . . . . . . . . . . . . . . . . 24 114 A.3. draft-ietf-acme-star-04 . . . . . . . . . . . . . . . . . 24 115 A.4. draft-ietf-acme-star-03 . . . . . . . . . . . . . . . . . 24 116 A.5. draft-ietf-acme-star-02 . . . . . . . . . . . . . . . . . 24 117 A.6. draft-ietf-acme-star-01 . . . . . . . . . . . . . . . . . 24 118 A.7. draft-ietf-acme-star-00 . . . . . . . . . . . . . . . . . 25 119 A.8. draft-sheffer-acme-star-02 . . . . . . . . . . . . . . . 25 120 A.9. draft-sheffer-acme-star-01 . . . . . . . . . . . . . . . 25 121 A.10. draft-sheffer-acme-star-00 . . . . . . . . . . . . . . . 25 122 A.11. draft-sheffer-acme-star-lurk-00 . . . . . . . . . . . . . 25 123 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25 125 1. Introduction 127 The ACME protocol [RFC8555] automates the process of issuing a 128 certificate to a named entity (an Identifier Owner or IdO). 129 Typically, but not always, the identifier is a domain name. 131 If the IdO wishes to obtain a string of short-term certificates 132 originating from the same private key (see [Topalovic] about why 133 using short-lived certificates might be preferable to explicit 134 revocation), she must go through the whole ACME protocol each time a 135 new short-term certificate is needed - e.g., every 2-3 days. If done 136 this way, the process would involve frequent interactions between the 137 registration function of the ACME Certification Authority (CA) and 138 the identity provider infrastructure (e.g.: DNS, web servers), 139 therefore making the issuance of short-term certificates exceedingly 140 dependent on the reliability of both. 142 This document presents an extension of the ACME protocol that 143 optimizes this process by making short-term certificates first class 144 objects in the ACME ecosystem. Once the order for a string of short- 145 term certificates is accepted, the CA is responsible for publishing 146 the next certificate at an agreed upon URL before the previous one 147 expires. The IdO can terminate the automatic renewal before the 148 negotiated deadline, if needed - e.g., on key compromise. 150 For a more generic treatment of STAR certificates, readers are 151 referred to [I-D.nir-saag-star]. 153 1.1. Name Delegation Use Case 155 The proposed mechanism can be used as a building block of an 156 efficient name-delegation protocol, for example one that exists 157 between a CDN or a cloud provider and its customers 158 [I-D.ietf-acme-star-delegation]. At any time, the service customer 159 (i.e., the IdO) can terminate the delegation by simply instructing 160 the CA to stop the automatic renewal and letting the currently active 161 certificate expire shortly thereafter. Note that in this case the 162 delegated entity needs to access the auto-renewed certificate without 163 being in possession of the ACME account key that was used for 164 initiating the STAR issuance. 166 1.2. Terminology 168 IdO Identifier Owner, the owner of an identifier, e.g.: a domain 169 name, a telephone number. 170 STAR Short-Term and Automatically Renewed X.509 certificates. 172 1.3. Conventions used in this document 174 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 175 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 176 "OPTIONAL" in this document are to be interpreted as described in 177 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all 178 capitals, as shown here. 180 2. Protocol Flow 182 The following subsections describe the three main phases of the 183 protocol: 185 o Bootstrap: the IdO asks an ACME CA to create a short-term and 186 automatically-renewed (STAR) certificate (Section 2.1); 187 o Auto-renewal: the ACME CA periodically re-issues the short-term 188 certificate and posts it to the star-certificate URL 189 (Section 2.2); 190 o Termination: the IdO requests the ACME CA to discontinue the 191 automatic renewal of the certificate (Section 2.3). 193 2.1. Bootstrap 195 The IdO, in its role as an ACME client, requests the CA to issue a 196 STAR certificate, i.e., one that: 198 o Has a short validity, e.g., 24 to 72 hours. Note that the exact 199 definition of "short" depends on the use case; 200 o Is automatically renewed by the CA for a certain period of time; 201 o Is downloadable from a (highly available) location. 203 Other than that, the ACME protocol flows as usual between IdO and CA. 204 In particular, IdO is responsible for satisfying the requested ACME 205 challenges until the CA is willing to issue the requested 206 certificate. Per normal ACME processing, the IdO is given back an 207 Order resource associated with the STAR certificate to be used in 208 subsequent interaction with the CA (e.g., if the certificate needs to 209 be terminated.) 211 The bootstrap phase ends when the ACME CA updates the Order resource 212 to include the URL for the issued STAR certificate. 214 2.2. Refresh 216 The CA issues the initial certificate after the authorization 217 completes successfully. It then automatically re-issues the 218 certificate using the same CSR (and therefore the same identifier and 219 public key) before the previous one expires, and publishes it to the 220 URL that was returned to the IdO at the end of the bootstrap phase. 221 The certificate user, which could be either the IdO itself or a 222 delegated third party, as described in 223 [I-D.ietf-acme-star-delegation], obtains the certificate 224 (Section 3.3) and uses it. 226 The refresh process (Figure 1) goes on until either: 228 o IdO explicitly terminates the automatic renewal (Section 2.3); or 229 o Automatic renewal expires. 231 Certificate ACME/STAR 232 User Server 233 | Retrieve cert | [...] 234 |---------------------->| | 235 | +------. / 236 | | | / 237 | | Automatic renewal : 238 | | | \ 239 | |<-----' \ 240 | Retrieve cert | | 241 |---------------------->| short validity period 242 | | | 243 | +------. / 244 | | | / 245 | | Automatic renewal : 246 | | | \ 247 | |<-----' \ 248 | Retrieve cert | | 249 |---------------------->| short validity period 250 | | | 251 | +------. / 252 | | | / 253 | | Automatic renewal : 254 | | | \ 255 | |<-----' \ 256 | | | 257 | [...] | [...] 259 Figure 1: Auto renewal 261 2.3. Termination 263 The IdO may request early termination of the STAR certificate by 264 sending a cancellation request to the Order resource, as described in 265 Section 3.1.2. After the CA receives and verifies the request, it 266 shall: 268 o Cancel the automatic renewal process for the STAR certificate; 269 o Change the certificate publication resource to return an error 270 indicating the termination of the issuance; 271 o Change the status of the Order to "canceled". 273 Note that it is not necessary to explicitly revoke the short-term 274 certificate. 276 Certificate ACME/STAR 277 User IdO Server 278 | | | 279 | | Cancel Order | 280 | +---------------------->| 281 | | +-------. 282 | | | | 283 | | | End auto renewal 284 | | | Remove cert link 285 | | | etc. 286 | | | | 287 | | Done |<------' 288 | |<----------------------+ 289 | | | 290 | | 291 | Retrieve cert | 292 +---------------------------------------------->| 293 | Error: recurrentOrderCanceled | 294 |<----------------------------------------------+ 295 | | 297 Figure 2: Termination 299 3. Protocol Details 301 This section describes the protocol details, namely the extensions to 302 the ACME protocol required to issue STAR certificates. 304 3.1. ACME Extensions 306 This protocol extends the ACME protocol, to allow for recurrent 307 Orders. 309 3.1.1. Extending the Order Resource 311 The Order resource is extended with the following attributes: 313 o recurrent (required, boolean): MUST be true for STAR certificates. 314 o recurrent-start-date (optional, string): the earliest date of 315 validity of the first certificate issued, in [RFC3339] format. 316 When omitted, the start date is as soon as authorization is 317 complete. 318 o recurrent-end-date (required, string): the latest date of validity 319 of the last certificate issued, in [RFC3339] format. 320 o recurrent-certificate-validity (required, integer): the maximum 321 validity period of each STAR certificate, an integer that denotes 322 a number of seconds. This is a nominal value which does not 323 include any extra validity time which is due to pre-dating. The 324 client can use the value reflected by the server (which may be 325 different from the one sent by the client) as a hint to configure 326 its polling timer. 327 o recurrent-certificate-predate (optional, integer): amount of pre- 328 dating added to each STAR certificate, an integer that denotes a 329 number of seconds. The default is 0. If present, the value of 330 the notBefore field that would otherwise appear in the STAR 331 certificates is pre-dated by the specified number of seconds. See 332 also Section 4.1. 333 o recurrent-certificate-get (optional, boolean): see Section 3.4. 335 These attributes are included in a POST message when creating the 336 Order, as part of the "payload" encoded object. They are returned 337 when the Order has been created, and the ACME server MAY adjust them 338 at will, according to its local policy (see also Section 3.2). 340 The optional notBefore and notAfter fields defined in Section 7.1.3 341 of [RFC8555] MUST NOT be present in a STAR Order. If they are 342 included, the server MUST return an error with status code 400 "Bad 343 Request" and type "malformedRequest". 345 Section 7.1.6 of [RFC8555] defines the following values for the Order 346 resource's status: "pending", "ready", "processing", "valid", and 347 "invalid". In the case of recurrent Orders, the status MUST be 348 "valid" as long as STAR certificates are being issued. We add a new 349 status value: "canceled", see Section 3.1.2. 351 A STAR certificate is by definition a mutable resource. Instead of 352 overloading the semantics of the "certificate" attribute, this 353 document defines a new attribute "star-certificate" to be used 354 instead of "certificate". 356 o star-certificate (optional, string): A URL for the (rolling) STAR 357 certificate that has been issued in response to this Order. 359 3.1.2. Canceling a Recurrent Order 361 An important property of the recurrent Order is that it can be 362 canceled by the IdO, with no need for certificate revocation. To 363 cancel the Order, the ACME client sends a POST to the Order URL as 364 shown in Figure 3. 366 POST /acme/order/TOlocE8rfgo HTTP/1.1 367 Host: example.org 368 Content-Type: application/jose+json 370 { 371 "protected": base64url({ 372 "alg": "ES256", 373 "kid": "https://example.com/acme/acct/evOfKhNU60wg", 374 "nonce": "5XJ1L3lEkMG7tR6pA00clA", 375 "url": "https://example.com/acme/order/TOlocE8rfgo" 376 }), 377 "payload": base64url({ 378 "status": "canceled" 379 }), 380 "signature": "H6ZXtGjTZyUnPeKn...wEA4TklBdh3e454g" 381 } 383 Figure 3: Canceling a Recurrent Order 385 After a successful cancellation, the server MUST NOT issue any 386 additional certificates for this order. 388 Immediately after the order is canceled, the server: 390 o MUST update the status of the order resource to "canceled" and 391 MUST set an appropriate "expires" date; 392 o MUST respond with 403 (Forbidden) to any requests to the star- 393 certificate endpoint. The response SHOULD provide additional 394 information using a problem document [RFC7807] with type 395 "urn:ietf:params:acme:error:recurrentOrderCanceled". 397 Issuing a cancellation for an order that is not in "valid" state is 398 not allowed. A client MUST NOT send such a request, and a server 399 MUST return an error response with status code 400 (Bad Request) and 400 type "urn:ietf:params:acme:error:recurrentCancellationInvalid". 402 Explicit certificate revocation using the revokeCert interface 403 (Section 7.6 of [RFC8555]) is not supported for STAR certificates. A 404 server receiving a revocation request for a STAR certificate MUST 405 return an error response with status code 403 (Forbidden) and type 406 "urn:ietf:params:acme:error:recurrentRevocationNotSupported". 408 3.2. Capability Discovery 410 In order to support the discovery of STAR capabilities, the directory 411 object defined in Section 9.7.6 of [RFC8555] is extended with the 412 following attributes inside the "meta" field: 414 o star-enabled (required, boolean): indicates STAR support. An ACME 415 STAR server MUST include this attribute, and MUST set it to true 416 if the feature is enabled. 417 o star-min-cert-validity (required, integer): minimum acceptable 418 value for recurrent-certificate-validity, in seconds. 419 o star-max-renewal (required, integer): maximum delta between 420 recurrent-end-date and recurrent-start-date, in seconds. 421 o star-allow-certificate-get (optional, boolean): see Section 3.4. 423 An example directory object advertising STAR support with one day 424 star-min-cert-validity and one year star-max-renewal, and supporting 425 certificate fetching with an HTTP GET is shown in Figure 4. 427 { 428 "new-nonce": "https://example.com/acme/new-nonce", 429 "new-account": "https://example.com/acme/new-account", 430 "new-order": "https://example.com/acme/new-order", 431 "new-authz": "https://example.com/acme/new-authz", 432 "revoke-cert": "https://example.com/acme/revoke-cert", 433 "key-change": "https://example.com/acme/key-change", 434 "meta": { 435 "terms-of-service": "https://example.com/acme/terms/2017-5-30", 436 "website": "https://www.example.com/", 437 "caa-identities": ["example.com"], 438 "star-enabled": true, 439 "star-min-cert-validity": 86400, 440 "star-max-renewal": 31536000, 441 "star-allow-certificate-get": true 442 } 443 } 445 Figure 4: Directory object with STAR support 447 3.3. Fetching the Certificates 449 The certificate is fetched from the star-certificate endpoint with 450 POST-as-GET as per [RFC8555] Section 7.4.2, unless client and server 451 have successfully negotiated the "unauthenticated GET" option 452 described in Section 3.4. In such case, the client can simply issue 453 a GET to the star-certificate resource without authenticating itself 454 to the server as illustrated in Figure 5. 456 GET /acme/cert/mAt3xBGaobw HTTP/1.1 457 Host: example.org 458 Accept: application/pem-certificate-chain 460 HTTP/1.1 200 OK 461 Content-Type: application/pem-certificate-chain 462 Link: ;rel="index" 463 Not-Before: Mon, 1 Feb 2016 00:00:00 GMT 464 Not-After: Mon, 8 Feb 2016 00:00:00 GMT 466 -----BEGIN CERTIFICATE----- 467 [End-entity certificate contents] 468 -----END CERTIFICATE----- 469 -----BEGIN CERTIFICATE----- 470 [Issuer certificate contents] 471 -----END CERTIFICATE----- 472 -----BEGIN CERTIFICATE----- 473 [Other certificate contents] 474 -----END CERTIFICATE----- 476 Figure 5: Fetching a STAR certificate with unauthenticated GET 478 The Server SHOULD include the "Not-Before" and "Not-After" HTTP 479 headers in the response. When they exist, they MUST be equal to the 480 respective fields inside the end-entity certificate. Their format is 481 "HTTP-date" as defined in Section 7.1.1.2 of [RFC7231]. Their 482 purpose is to enable client implementations that do not parse the 483 certificate. 485 To improve robustness, the next certificate MUST be made available by 486 the ACME CA at the URL pointed by "star-certificate" at the latest 487 halfway through the lifetime of the currently active certificate. It 488 is worth noting that this has an implication in case of cancellation: 489 in fact, from the time the next certificate is made available, the 490 cancellation is not completely effective until the latter also 491 expires. To avoid the client accidentally entering a broken state, 492 the "next" certificate MUST be pre-dated so that it is already valid 493 when it is published at the "star-certificate" URL. Note that the 494 server might need to increase the recurrent-certificate-predate value 495 to satisfy the latter requirement. For further discussion on pre- 496 dating, see Section 4.1. 498 The server MUST NOT issue any additional certificates for this order 499 beyond its recurrent-end-date. 501 Immediately after the order expires, the server MUST respond with 403 502 (Forbidden) to any requests to the star-certificate endpoint. The 503 response SHOULD provide additional information using a problem 504 document [RFC7807] with type 505 "urn:ietf:params:acme:error:recurrentOrderExpired". Note that the 506 Order resource's state remains "valid", as per the base protocol. 508 3.4. Negotiating an unauthenticated GET 510 In order to enable the name delegation workflow defined in 511 [I-D.ietf-acme-star-delegation] as well as to increase the 512 reliability of the STAR ecosystem (see Section 4.3 for details), this 513 document defines a mechanism that allows a server to advertise 514 support for accessing star-certificate resources via unauthenticated 515 GET (instead of, or in addition to, POST-as-GET), and a client to 516 enable this service with per-Order granularity. 518 Specifically, a server states its availability to grant 519 unauthenticated access to a client's Order star-certificate by 520 setting the star-allow-certificate-get attribute to true in the meta 521 field of the Directory object: 523 o star-allow-certificate-get (optional, boolean): If this field is 524 present and set to true, the server allows GET requests to star- 525 certificate URLs. 527 A client states its will to access the issued star-certificate via 528 unauthenticated GET by adding a recurrent-certificate-get attribute 529 to its Order and setting it to true. 531 o recurrent-certificate-get (optional, boolean): If this field is 532 present and set to true, the client requests the server to allow 533 unauthenticated GET to the star-certificate associated with this 534 Order. 536 If the server accepts the request, it MUST reflect the attribute 537 setting in the resulting Order object. 539 3.5. Computing notBefore and notAfter of STAR Certificates 541 We define "nominal renewal date" the point in time when a new short- 542 term certificate for a given STAR Order is due. It is a multiple of 543 the Order's recurrent-certificate-validity that starts with the 544 issuance of the first short-term certificate and is upper-bounded by 545 the Order's recurrent-end-date (Figure 6). 547 rcv - STAR Order's recurrent-certificate-validity 548 red - STAR Order's recurrent-end-date 549 nrd[i] - nominal renewal date of the i-th STAR certificate 551 .-rcv-. .-rcv-. .-rcv-. .__. 552 / \ / \ / \ / red 553 -----------o---------o---------o---------o----X-------> t 554 nrd[0] nrd[1] nrd[2] nrd[3] 556 Figure 6: Nominal Renewal Date 558 The rules to determine the notBefore and notAfter values of the i-th 559 STAR certificate are as follows: 561 notBefore = nrd[i] - predating 562 notAfter = min(nrd[i] + rcv, red) 564 where "predating" is the max between the (optional) recurrent- 565 certificate-predate (rcp) and the amount of pre-dating that the 566 server needs to add to make sure that all certificates being 567 published are valid at the time of publication (Section 3.3). The 568 server pre-dating is a fraction f of rcv (i.e., f * rcv with .5 <= f 569 < 1). 571 predating = max(rcp, f * rcv) 573 3.5.1. Example 575 Given a server that intends to publish the next STAR certificate 576 halfway through the lifetime of the previous one, and a STAR Order 577 with the following attributes: 579 { 580 "recurrent-start-date": "2016-01-10T00:00:00Z", 581 "recurrent-end-date": "2016-01-20T00:00:00Z", 582 "recurrent-certificate-validity": 345600, // 4 days 583 "recurrent-certificate-predate": 518400 // 6 days 584 } 586 The amount of pre-dating that needs to be subtracted from each 587 nominal renewal date is 6 days - i.e., max(518400, 345600 * .5). 589 The notBefore and notAfter of each short-term certificate are: 591 +----------------------+----------------------+ 592 | notBefore | notAfter | 593 +----------------------+----------------------+ 594 | 2016-01-04T00:00:00Z | 2016-01-14T00:00:00Z | 595 | 2016-01-08T00:00:00Z | 2016-01-18T00:00:00Z | 596 | 2016-01-12T00:00:00Z | 2016-01-20T00:00:00Z | 597 +----------------------+----------------------+ 599 A client should expect each certificate to be available from the 600 star-certificate endpoint at the following times: 602 +------------------------------------+ 603 | 2016-01-10T00:00:00Z (or earlier) | 604 | 2016-01-12T00:00:00Z | 605 | 2016-01-16T00:00:00Z | 606 +------------------------------------+ 608 4. Operational Considerations 610 4.1. The Meaning of "Short Term" and the Impact of Skewed Clocks 612 "Short Term" is a relative concept, therefore trying to define a cut- 613 off point that works in all cases would be a useless exercise. In 614 practice, the expected lifetime of a STAR certificate will be counted 615 in minutes, hours or days, depending on different factors: the 616 underlying requirements for revocation, how much clock 617 synchronization is expected among relying parties and the issuing CA, 618 etc. 620 Nevertheless, this section attempts to provide reasonable suggestions 621 for the Web use case, informed by current operational and research 622 experience. 624 Acer et al. [Acer] find that one of the main causes of "HTTPS error" 625 warnings in browsers is misconfigured client clocks. In particular, 626 they observe that roughly 95% of the "severe" clock skews - the 6.7% 627 of clock-related breakage reports which account for clients that are 628 more than 24 hours behind - happen to be within 6-7 days. 630 In order to avoid these spurious warnings about a not (yet) valid 631 server certificate, it is RECOMMENDED that site owners pre-date their 632 Web facing certificates by 5 to 7 days. The exact number depends on 633 the percentage of the "clock-skewed" population that the site owner 634 expects to protect - 5 days cover 97.3%, 7 days cover 99.6%. Note 635 that exact choice is also likely to depend on the kind of clients 636 that is prevalent for a given site or app - for example, Android and 637 Mac OS clients are known to behave better than Windows clients. 639 These considerations are clearly out of scope of the present 640 document. 642 In terms of security, STAR certificates and certificates with OCSP 643 must-staple [RFC7633] can be considered roughly equivalent if the 644 STAR certificate's and the OCSP response's lifetimes are the same. 645 Given OCSP responses can be cached on average for 4 days [Stark], it 646 is RECOMMENDED that a STAR certificate that is used on the Web has an 647 "effective" lifetime (excluding any pre-dating to account for clock 648 skews) no longer than 4 days. 650 4.2. Impact on Certificate Transparency (CT) Logs 652 Provided that the recommendations in Section 4.1 are followed, the 653 increase in Certificate Transparency (CT) [RFC6962] log ingestion 654 should be one order of magnitude in the worst case compared to the 655 current state. 657 The input received from most members of the CT community when the 658 issue was raised was that this should not represent a problem for the 659 CT architecture. 661 4.3. Dependability 663 When using authenticated POST-as-GET, the HTTPS endpoint from where 664 the STAR certificate is fetched can't be easily replicated by an on- 665 path HTTP cache. Reducing the caching properties of the protocol 666 makes STAR clients increasingly dependent on the ACME server 667 availability. This might be problematic given the relatively high 668 rate of client-server interactions in a STAR ecosystem. Clients and 669 servers should consider using the mechanism described in Section 3.4 670 to mitigate the risk. 672 5. Implementation Status 674 Note to RFC Editor: please remove this section before publication, 675 including the reference to [RFC7942]. 677 This section records the status of known implementations of the 678 protocol defined by this specification at the time of posting of this 679 Internet-Draft, and is based on a proposal described in [RFC7942]. 680 The description of implementations in this section is intended to 681 assist the IETF in its decision processes in progressing drafts to 682 RFCs. Please note that the listing of any individual implementation 683 here does not imply endorsement by the IETF. Furthermore, no effort 684 has been spent to verify the information presented here that was 685 supplied by IETF contributors. This is not intended as, and must not 686 be construed to be, a catalog of available implementations or their 687 features. Readers are advised to note that other implementations may 688 exist. 690 According to [RFC7942], "this will allow reviewers and working groups 691 to assign due consideration to documents that have the benefit of 692 running code, which may serve as evidence of valuable experimentation 693 and feedback that have made the implemented protocols more mature. 694 It is up to the individual working groups to use this information as 695 they see fit". 697 5.1. Overview 699 The implementation is constructed around 3 elements: STAR Client for 700 the Name Delegation Client (NDC), STAR Proxy for IdO and ACME Server 701 for CA. The communication between them is over an IP network and the 702 HTTPS protocol. 704 The software of the implementation is available at: 705 https://github.com/mami-project/lurk 707 The following subsections offer a basic description, detailed 708 information is available in https://github.com/mami- 709 project/lurk/blob/master/proxySTAR_v2/README.md 711 5.1.1. ACME Server with STAR extension 713 This is a fork of the Let's Encrypt Boulder project that implements 714 an ACME compliant CA. It includes modifications to extend the ACME 715 protocol as it is specified in this draft, to support recurrent 716 orders and cancelling orders. 718 The implementation understands the new "recurrent" attributes as part 719 of the Certificate issuance in the POST request for a new resource. 720 An additional process "renewalManager.go" has been included in 721 parallel that reads the details of each recurrent request, 722 automatically produces a "cron" Linux based task that issues the 723 recurrent certificates, until the lifetime ends or the order is 724 canceled. This process is also in charge of maintaining a fixed URI 725 to enable the NDC to download certificates, unlike Boulder's regular 726 process of producing a unique URI per certificate. 728 5.1.2. STAR Proxy 730 The STAR Proxy has a double role as ACME client and STAR Server. The 731 former is a fork of the EFF Certbot project that implements an ACME 732 compliant client with the STAR extension. The latter is a basic HTTP 733 REST API server. 735 The STAR Proxy understands the basic API request with a server. The 736 current implementation of the API is defined in draft-ietf-acme-star- 737 01. Registration or order cancellation triggers the modified Certbot 738 client that requests, or cancels, the recurrent generation of 739 certificates using the STAR extension over ACME protocol. The URI 740 with the location of the recurrent certificate is delivered to the 741 STAR client as a response. 743 5.2. Level of Maturity 745 This is a prototype. 747 5.3. Coverage 749 A STAR Client is not included in this implementation, but done by 750 direct HTTP request with any open HTTP REST API tool. This is 751 expected to be covered as part of the [I-D.sheffer-acme-star-request] 752 implementation. 754 This implementation completely covers STAR Proxy and ACME Server with 755 STAR extension. 757 5.4. Version Compatibility 759 The implementation is compatible with version draft-ietf-acme-star- 760 01. The implementation is based on the Boulder and Certbot code 761 release from 7-Aug-2017. 763 5.5. Licensing 765 This implementation inherits the Boulder license (Mozilla Public 766 License 2.0) and Certbot license (Apache License Version 2.0 ). 768 5.6. Implementation experience 770 To prove the concept all the implementation has been done with a 771 self-signed CA, to avoid impact on real domains. To be able to do it 772 we use the FAKE_DNS property of Boulder and static /etc/hosts entries 773 with domains names. Nonetheless this implementation should run with 774 real domains. 776 Most of the implementation has been made to avoid deep changes inside 777 of Boulder or Certbot, for example, the recurrent certificates 778 issuance by the CA is based on an external process that auto- 779 configures the standard Linux "cron" daemon in the ACME CA server. 781 The reference setup recommended is one physical host with 3 virtual 782 machines, one for each of the 3 components (client, proxy and server) 783 and the connectivity based on host bridge. 785 Network security is not enabled (iptables default policies are 786 "accept" and all rules removed) in this implementation to simplify 787 and test the protocol. 789 5.7. Contact Information 791 See author details below. 793 6. IANA Considerations 795 [[RFC Editor: please replace XXXX below by the RFC number.]] 797 6.1. New Error Types 799 This document adds the following entries to the ACME Error Type 800 registry: 802 +---------------------------------+---------------------+-----------+ 803 | Type | Description | Reference | 804 +---------------------------------+---------------------+-----------+ 805 | recurrentOrderCanceled | The short-term | RFC XXXX | 806 | | certificate is no | | 807 | | longer available | | 808 | | because the | | 809 | | recurrent order has | | 810 | | been explicitly | | 811 | | canceled by the IdO | | 812 | recurrentOrderExpired | The short-term | RFC XXXX | 813 | | certificate is no | | 814 | | longer available | | 815 | | because the | | 816 | | recurrent order has | | 817 | | expired | | 818 | recurrentCancellationInvalid | A request to cancel | RFC XXXX | 819 | | a recurrent order | | 820 | | that is not in | | 821 | | state "valid" has | | 822 | | been received | | 823 | recurrentRevocationNotSupported | A request to revoke | RFC XXXX | 824 | | a recurrent order | | 825 | | has been received | | 826 +---------------------------------+---------------------+-----------+ 828 6.2. New fields in Order Objects 830 This document adds the following entries to the ACME Order Object 831 Fields registry: 833 +------------------------------+---------+--------------+-----------+ 834 | Field Name | Field | Configurable | Reference | 835 | | Type | | | 836 +------------------------------+---------+--------------+-----------+ 837 | recurrent | string | true | RFC XXXX | 838 | recurrent-start-date | string | true | RFC XXXX | 839 | recurrent-end-date | string | true | RFC XXXX | 840 | recurrent-certificate- | integer | true | RFC XXXX | 841 | validity | | | | 842 | recurrent-certificate- | integer | true | RFC XXXX | 843 | predate | | | | 844 | recurrent-certificate-get | boolean | true | RFC XXXX | 845 | star-certificate | string | false | RFC XXXX | 846 +------------------------------+---------+--------------+-----------+ 848 6.3. New fields in the "meta" Object within a Directory Object 850 This document adds the following entries to the ACME Directory 851 Metadata Fields: 853 +----------------------------+------------+-----------+ 854 | Field Name | Field Type | Reference | 855 +----------------------------+------------+-----------+ 856 | star-enabled | boolean | RCF XXXX | 857 | star-min-cert-validity | integer | RCF XXXX | 858 | star-max-renewal | integer | RCF XXXX | 859 | star-allow-certificate-get | boolean | RFC XXXX | 860 +----------------------------+------------+-----------+ 862 6.4. Not-Before and Not-After HTTP Headers 864 The "Message Headers" registry should be updated with the following 865 additional values: 867 +-------------------+----------+----------+-----------+ 868 | Header Field Name | Protocol | Status | Reference | 869 +-------------------+----------+----------+-----------+ 870 | Not-Before | http | standard | RFC XXXX | 871 | Not-After | http | standard | RFC XXXX | 872 +-------------------+----------+----------+-----------+ 874 7. Security Considerations 876 7.1. No revocation 878 STAR certificates eliminate an important security feature of PKI 879 which is the ability to revoke certificates. Revocation allows the 880 administrator to limit the damage done by a rogue node or an 881 adversary who has control of the private key. With STAR 882 certificates, expiration replaces revocation so there is a timeliness 883 issue. To that end, see also the discussion on clock skew in 884 Section 4.1. 886 It should be noted that revocation also has timeliness issues, 887 because both CRLs and OCSP responses have nextUpdate fields that tell 888 relying parties (RPs) how long they should trust this revocation 889 data. These fields are typically set to hours, days, or even weeks 890 in the future. Any revocation that happens before the time in 891 nextUpdate goes unnoticed by the RP. 893 More discussion of the security of STAR certificates is available in 894 [Topalovic]. 896 7.2. Denial of Service Considerations 898 STAR adds a new attack vector that increases the threat of denial of 899 service attacks, caused by the change to the CA's behavior. Each 900 STAR request amplifies the resource demands upon the CA, where one 901 order produces not one, but potentially dozens or hundreds of 902 certificates, depending on the "recurrent-certificate-validity" 903 parameter. An attacker can use this property to aggressively reduce 904 the "recurrent-certificate-validity" (e.g. 1 sec.) jointly with other 905 ACME attack vectors identified in Sec. 10 of [RFC8555]. Other 906 collateral impact is related to the certificate endpoint resource 907 where the client can retrieve the certificates periodically. If this 908 resource is external to the CA (e.g. a hosted web server), the 909 previous attack will be reflected to that resource. 911 Mitigation recommendations from ACME still apply, but some of them 912 need to be adjusted. For example, applying rate limiting to the 913 initial request, by the nature of the recurrent behavior cannot solve 914 the above problem. The CA server needs complementary mitigation and 915 specifically, it SHOULD enforce a minimum value on "recurrent- 916 certificate-validity". Alternatively, the CA can set an internal 917 certificate generation processes rate limit. 919 7.3. Privacy Considerations 921 In order to avoid correlation of certificates by account, if 922 unauthenticated GET is negotiated (Section 3.4) the recommendation in 923 Section 10.5 of [RFC8555] regarding the choice of URL structure 924 applies, i.e. servers SHOULD choose URLs of certificate resources in 925 a non-guessable way, for example using capability URLs 926 [W3C.WD-capability-urls-20140218]. 928 8. Acknowledgments 930 This work is partially supported by the European Commission under 931 Horizon 2020 grant agreement no. 688421 Measurement and Architecture 932 for a Middleboxed Internet (MAMI). This support does not imply 933 endorsement. 935 Thanks to Roman Danyliw, Jon Peterson, Eric Rescorla, Sean Turner and 936 Martin Thomson for helpful comments and discussions that have shaped 937 this document. 939 9. References 941 9.1. Normative References 943 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 944 Requirement Levels", BCP 14, RFC 2119, 945 DOI 10.17487/RFC2119, March 1997, 946 . 948 [RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet: 949 Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002, 950 . 952 [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 953 Protocol (HTTP/1.1): Semantics and Content", RFC 7231, 954 DOI 10.17487/RFC7231, June 2014, 955 . 957 [RFC7807] Nottingham, M. and E. Wilde, "Problem Details for HTTP 958 APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016, 959 . 961 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 962 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 963 May 2017, . 965 [RFC8555] Barnes, R., Hoffman-Andrews, J., McCarney, D., and J. 966 Kasten, "Automatic Certificate Management Environment 967 (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019, 968 . 970 9.2. Informative References 972 [Acer] Acer, M., Stark, E., Felt, A., Fahl, S., Bhargava, R., 973 Dev, B., Braithwaite, M., Sleevi, R., and P. Tabriz, 974 "Where the Wild Warnings Are: Root Causes of Chrome HTTPS 975 Certificate Errors", DOI 10.1145/3133956.3134007, 2017, 976 . 978 [I-D.ietf-acme-star-delegation] 979 Sheffer, Y., Lopez, D., Pastor, A., and T. Fossati, "An 980 ACME Profile for Generating Delegated STAR Certificates", 981 draft-ietf-acme-star-delegation-00 (work in progress), 982 December 2018. 984 [I-D.nir-saag-star] 985 Nir, Y., Fossati, T., Sheffer, Y., and T. Eckert, 986 "Considerations For Using Short Term Certificates", draft- 987 nir-saag-star-01 (work in progress), March 2018. 989 [I-D.sheffer-acme-star-request] 990 Sheffer, Y., Lopez, D., Dios, O., Pastor, A., and T. 991 Fossati, "Generating Certificate Requests for Short-Term, 992 Automatically-Renewed (STAR) Certificates", draft-sheffer- 993 acme-star-request-02 (work in progress), June 2018. 995 [RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate 996 Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013, 997 . 999 [RFC7633] Hallam-Baker, P., "X.509v3 Transport Layer Security (TLS) 1000 Feature Extension", RFC 7633, DOI 10.17487/RFC7633, 1001 October 2015, . 1003 [RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running 1004 Code: The Implementation Status Section", BCP 205, 1005 RFC 7942, DOI 10.17487/RFC7942, July 2016, 1006 . 1008 [Stark] Stark, E., Huang, L., Israni, D., Jackson, C., and D. 1009 Boneh, "The case for prefetching and prevalidating TLS 1010 server certificates", 2012, 1011 . 1014 [Topalovic] 1015 Topalovic, E., Saeta, B., Huang, L., Jackson, C., and D. 1016 Boneh, "Towards Short-Lived Certificates", 2012, 1017 . 1020 [W3C.WD-capability-urls-20140218] 1021 Tennison, J., "Good Practices for Capability URLs", World 1022 Wide Web Consortium WD WD-capability-urls-20140218, 1023 February 2014, 1024 . 1026 Appendix A. Document History 1028 [[Note to RFC Editor: please remove before publication.]] 1030 A.1. draft-ietf-acme-star-06 1032 o Roman's AD review 1034 A.2. draft-ietf-acme-star-05 1036 o EKR's AD review 1037 o A detailed example of the timing of certificate issuance and 1038 predating 1039 o Added an explicit client-side parameter for predating 1040 o Security considerations around unauthenticated GET 1042 A.3. draft-ietf-acme-star-04 1044 o WG last call comments by Sean Turner 1045 o revokeCert interface handling 1046 o Allow negotiating plain-GET for certs 1047 o In STAR Orders, use star-certificate instead of certificate 1049 A.4. draft-ietf-acme-star-03 1051 o Clock skew considerations 1052 o Recommendations for "short" in the Web use case 1053 o CT log considerations 1055 A.5. draft-ietf-acme-star-02 1057 o Discovery of STAR capabilities via the directory object 1058 o Use the more generic term Identifier Owner (IdO) instead of Domain 1059 Name Owner (DNO) 1060 o More precision about what goes in the order 1061 o Detail server side behavior on cancellation 1063 A.6. draft-ietf-acme-star-01 1065 o Generalized the introduction, separating out the specifics of 1066 CDNs. 1067 o Clean out LURK-specific text. 1068 o Using a POST to ensure cancellation is authenticated. 1069 o First and last date of recurrent cert, as absolute dates. 1070 Validity of certs in seconds. 1071 o Use RFC7807 "Problem Details" in error responses. 1072 o Add IANA considerations. 1073 o Changed the document's title. 1075 A.7. draft-ietf-acme-star-00 1077 o Initial working group version. 1078 o Removed the STAR interface, the protocol between NDC and DNO. 1079 What remains is only the extended ACME protocol. 1081 A.8. draft-sheffer-acme-star-02 1083 o Using a more generic term for the delegation client, NDC. 1084 o Added an additional use case: public cloud services. 1085 o More detail on ACME authorization. 1087 A.9. draft-sheffer-acme-star-01 1089 o A terminology section. 1090 o Some cleanup. 1092 A.10. draft-sheffer-acme-star-00 1094 o Renamed draft to prevent confusion with other work in this space. 1095 o Added an initial STAR protocol: a REST API. 1096 o Discussion of CDNI use cases. 1098 A.11. draft-sheffer-acme-star-lurk-00 1100 o Initial version. 1102 Authors' Addresses 1104 Yaron Sheffer 1105 Intuit 1107 EMail: yaronf.ietf@gmail.com 1109 Diego Lopez 1110 Telefonica I+D 1112 EMail: diego.r.lopez@telefonica.com 1114 Oscar Gonzalez de Dios 1115 Telefonica I+D 1117 EMail: oscar.gonzalezdedios@telefonica.com 1118 Antonio Agustin Pastor Perales 1119 Telefonica I+D 1121 EMail: antonio.pastorperales@telefonica.com 1123 Thomas Fossati 1124 ARM 1126 EMail: thomas.fossati@arm.com