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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group R. Barnes 3 Internet-Draft Mozilla 4 Intended status: Standards Track J. Hoffman-Andrews 5 Expires: January 9, 2017 EFF 6 J. Kasten 7 University of Michigan 8 July 08, 2016 10 Automatic Certificate Management Environment (ACME) 11 draft-ietf-acme-acme-03 13 Abstract 15 Certificates in the Web's X.509 PKI (PKIX) are used for a number of 16 purposes, the most significant of which is the authentication of 17 domain names. Thus, certificate authorities in the Web PKI are 18 trusted to verify that an applicant for a certificate legitimately 19 represents the domain name(s) in the certificate. Today, this 20 verification is done through a collection of ad hoc mechanisms. This 21 document describes a protocol that a certificate authority (CA) and 22 an applicant can use to automate the process of verification and 23 certificate issuance. The protocol also provides facilities for 24 other certificate management functions, such as certificate 25 revocation. 27 DISCLAIMER: This is a work in progress draft of ACME and has not yet 28 had a thorough security analysis. 30 RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH: The source for 31 this draft is maintained in GitHub. Suggested changes should be 32 submitted as pull requests at https://github.com/ietf-wg-acme/acme . 33 Instructions are on that page as well. Editorial changes can be 34 managed in GitHub, but any substantive change should be discussed on 35 the ACME mailing list (acme@ietf.org). 37 Status of This Memo 39 This Internet-Draft is submitted in full conformance with the 40 provisions of BCP 78 and BCP 79. 42 Internet-Drafts are working documents of the Internet Engineering 43 Task Force (IETF). Note that other groups may also distribute 44 working documents as Internet-Drafts. The list of current Internet- 45 Drafts is at http://datatracker.ietf.org/drafts/current/. 47 Internet-Drafts are draft documents valid for a maximum of six months 48 and may be updated, replaced, or obsoleted by other documents at any 49 time. It is inappropriate to use Internet-Drafts as reference 50 material or to cite them other than as "work in progress." 52 This Internet-Draft will expire on January 9, 2017. 54 Copyright Notice 56 Copyright (c) 2016 IETF Trust and the persons identified as the 57 document authors. All rights reserved. 59 This document is subject to BCP 78 and the IETF Trust's Legal 60 Provisions Relating to IETF Documents 61 (http://trustee.ietf.org/license-info) in effect on the date of 62 publication of this document. Please review these documents 63 carefully, as they describe your rights and restrictions with respect 64 to this document. Code Components extracted from this document must 65 include Simplified BSD License text as described in Section 4.e of 66 the Trust Legal Provisions and are provided without warranty as 67 described in the Simplified BSD License. 69 Table of Contents 71 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 72 2. Deployment Model and Operator Experience . . . . . . . . . . 4 73 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6 74 4. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 6 75 5. Message Transport . . . . . . . . . . . . . . . . . . . . . . 8 76 5.1. HTTPS Requests . . . . . . . . . . . . . . . . . . . . . 9 77 5.2. Request Authentication . . . . . . . . . . . . . . . . . 9 78 5.3. Request URI Integrity . . . . . . . . . . . . . . . . . . 10 79 5.3.1. "url" (URL) JWS header parameter . . . . . . . . . . 10 80 5.4. Replay protection . . . . . . . . . . . . . . . . . . . . 11 81 5.4.1. Replay-Nonce . . . . . . . . . . . . . . . . . . . . 11 82 5.4.2. "nonce" (Nonce) JWS header parameter . . . . . . . . 12 83 5.5. Rate limits . . . . . . . . . . . . . . . . . . . . . . . 12 84 5.6. Errors . . . . . . . . . . . . . . . . . . . . . . . . . 12 85 6. Certificate Management . . . . . . . . . . . . . . . . . . . 14 86 6.1. Resources . . . . . . . . . . . . . . . . . . . . . . . . 14 87 6.1.1. Directory . . . . . . . . . . . . . . . . . . . . . . 16 88 6.1.2. Registration Objects . . . . . . . . . . . . . . . . 17 89 6.1.3. Application Objects . . . . . . . . . . . . . . . . . 19 90 6.1.4. Authorization Objects . . . . . . . . . . . . . . . . 21 91 6.2. Registration . . . . . . . . . . . . . . . . . . . . . . 23 92 6.2.1. Account Key Roll-over . . . . . . . . . . . . . . . . 25 93 6.2.2. Account deactivation . . . . . . . . . . . . . . . . 27 94 6.3. Applying for Certificate Issuance . . . . . . . . . . . . 28 95 6.3.1. Downloading the Certificate . . . . . . . . . . . . . 30 96 6.4. Identifier Authorization . . . . . . . . . . . . . . . . 31 97 6.4.1. Responding to Challenges . . . . . . . . . . . . . . 33 98 6.4.2. Deactivating an Authorization . . . . . . . . . . . . 35 99 6.5. Certificate Revocation . . . . . . . . . . . . . . . . . 36 100 7. Identifier Validation Challenges . . . . . . . . . . . . . . 38 101 7.1. Key Authorizations . . . . . . . . . . . . . . . . . . . 39 102 7.2. HTTP . . . . . . . . . . . . . . . . . . . . . . . . . . 40 103 7.3. TLS with Server Name Indication (TLS SNI) . . . . . . . . 42 104 7.4. DNS . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 105 7.5. Out-of-Band . . . . . . . . . . . . . . . . . . . . . . . 45 106 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 46 107 8.1. Well-Known URI for the HTTP Challenge . . . . . . . . . . 46 108 8.2. Replay-Nonce HTTP Header . . . . . . . . . . . . . . . . 47 109 8.3. "url" JWS Header Parameter . . . . . . . . . . . . . . . 47 110 8.4. "nonce" JWS Header Parameter . . . . . . . . . . . . . . 47 111 8.5. URN Sub-namespace for ACME (urn:ietf:params:acme) . . . . 48 112 8.6. New Registries . . . . . . . . . . . . . . . . . . . . . 48 113 8.6.1. Error Codes . . . . . . . . . . . . . . . . . . . . . 48 114 8.6.2. Resource Types . . . . . . . . . . . . . . . . . . . 49 115 8.6.3. Identifier Types . . . . . . . . . . . . . . . . . . 49 116 8.6.4. Challenge Types . . . . . . . . . . . . . . . . . . . 50 117 9. Security Considerations . . . . . . . . . . . . . . . . . . . 50 118 9.1. Threat model . . . . . . . . . . . . . . . . . . . . . . 51 119 9.2. Integrity of Authorizations . . . . . . . . . . . . . . . 52 120 9.3. Denial-of-Service Considerations . . . . . . . . . . . . 54 121 9.4. Server-Side Request Forgery . . . . . . . . . . . . . . . 55 122 9.5. CA Policy Considerations . . . . . . . . . . . . . . . . 55 123 10. Operational Considerations . . . . . . . . . . . . . . . . . 56 124 10.1. DNS over TCP . . . . . . . . . . . . . . . . . . . . . . 56 125 10.2. Default Virtual Hosts . . . . . . . . . . . . . . . . . 56 126 10.3. Use of DNSSEC Resolvers . . . . . . . . . . . . . . . . 57 127 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 57 128 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 58 129 12.1. Normative References . . . . . . . . . . . . . . . . . . 58 130 12.2. Informative References . . . . . . . . . . . . . . . . . 60 131 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 61 133 1. Introduction 135 Certificates in the Web PKI [RFC5280] are most commonly used to 136 authenticate domain names. Thus, certificate authorities in the Web 137 PKI are trusted to verify that an applicant for a certificate 138 legitimately represents the domain name(s) in the certificate. 140 Existing Web PKI certificate authorities tend to run on a set of ad 141 hoc protocols for certificate issuance and identity verification. A 142 typical user experience is something like: 144 o Generate a PKCS#10 [RFC2986] Certificate Signing Request (CSR). 146 o Cut-and-paste the CSR into a CA web page. 148 o Prove ownership of the domain by one of the following methods: 150 * Put a CA-provided challenge at a specific place on the web 151 server. 153 * Put a CA-provided challenge at a DNS location corresponding to 154 the target domain. 156 * Receive CA challenge at a (hopefully) administrator-controlled 157 e-mail address corresponding to the domain and then respond to 158 it on the CA's web page. 160 o Download the issued certificate and install it on their Web 161 Server. 163 With the exception of the CSR itself and the certificates that are 164 issued, these are all completely ad hoc procedures and are 165 accomplished by getting the human user to follow interactive natural- 166 language instructions from the CA rather than by machine-implemented 167 published protocols. In many cases, the instructions are difficult 168 to follow and cause significant confusion. Informal usability tests 169 by the authors indicate that webmasters often need 1-3 hours to 170 obtain and install a certificate for a domain. Even in the best 171 case, the lack of published, standardized mechanisms presents an 172 obstacle to the wide deployment of HTTPS and other PKIX-dependent 173 systems because it inhibits mechanization of tasks related to 174 certificate issuance, deployment, and revocation. 176 This document describes an extensible framework for automating the 177 issuance and domain validation procedure, thereby allowing servers 178 and infrastructural software to obtain certificates without user 179 interaction. Use of this protocol should radically simplify the 180 deployment of HTTPS and the practicality of PKIX authentication for 181 other protocols based on TLS [RFC5246]. 183 2. Deployment Model and Operator Experience 185 The major guiding use case for ACME is obtaining certificates for Web 186 sites (HTTPS [RFC2818]). In that case, the server is intended to 187 speak for one or more domains, and the process of certificate 188 issuance is intended to verify that the server actually speaks for 189 the domain(s). 191 Different types of certificates reflect different kinds of CA 192 verification of information about the certificate subject. "Domain 193 Validation" (DV) certificates are by far the most common type. For 194 DV validation, the CA merely verifies that the requester has 195 effective control of the web server and/or DNS server for the domain, 196 but does not explicitly attempt to verify their real-world identity. 197 (This is as opposed to "Organization Validation" (OV) and "Extended 198 Validation" (EV) certificates, where the process is intended to also 199 verify the real-world identity of the requester.) 201 DV certificate validation commonly checks claims about properties 202 related to control of a domain name - properties that can be observed 203 by the issuing authority in an interactive process that can be 204 conducted purely online. That means that under typical 205 circumstances, all steps in the request, verification, and issuance 206 process can be represented and performed by Internet protocols with 207 no out-of-band human intervention. 209 When deploying a current HTTPS server, an operator generally gets a 210 prompt to generate a self-signed certificate. When an operator 211 deploys an ACME-compatible web server, the experience would be 212 something like this: 214 o The ACME client prompts the operator for the intended domain 215 name(s) that the web server is to stand for. 217 o The ACME client presents the operator with a list of CAs from 218 which it could get a certificate. (This list will change over 219 time based on the capabilities of CAs and updates to ACME 220 configuration.) The ACME client might prompt the operator for 221 payment information at this point. 223 o The operator selects a CA. 225 o In the background, the ACME client contacts the CA and requests 226 that a certificate be issued for the intended domain name(s). 228 o Once the CA is satisfied, the certificate is issued and the ACME 229 client automatically downloads and installs it, potentially 230 notifying the operator via e-mail, SMS, etc. 232 o The ACME client periodically contacts the CA to get updated 233 certificates, stapled OCSP responses, or whatever else would be 234 required to keep the server functional and its credentials up-to- 235 date. 237 The overall idea is that it's nearly as easy to deploy with a CA- 238 issued certificate as a self-signed certificate, and that once the 239 operator has done so, the process is self-sustaining with minimal 240 manual intervention. Close integration of ACME with HTTPS servers, 241 for example, can allow the immediate and automated deployment of 242 certificates as they are issued, optionally sparing the human 243 administrator from additional configuration work. 245 3. Terminology 247 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 248 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 249 document are to be interpreted as described in RFC 2119 [RFC2119]. 251 The two main roles in ACME are "client" and "server". The ACME 252 client uses the protocol to request certificate management actions, 253 such as issuance or revocation. An ACME client therefore typically 254 runs on a web server, mail server, or some other server system which 255 requires valid TLS certificates. The ACME server runs at a 256 certificate authority, and responds to client requests, performing 257 the requested actions if the client is authorized. 259 An ACME client is represented by an "account key pair". The client 260 uses the private key of this key pair to sign all messages sent to 261 the server. The server uses the public key to verify the 262 authenticity and integrity of messages from the client. 264 4. Protocol Overview 266 ACME allows a client to request certificate management actions using 267 a set of JSON messages carried over HTTPS. In some ways, ACME 268 functions much like a traditional CA, in which a user creates an 269 account, adds identifiers to that account (proving control of the 270 domains), and requests certificate issuance for those domains while 271 logged in to the account. 273 In ACME, the account is represented by an account key pair. The "add 274 a domain" function is accomplished by authorizing the key pair for a 275 given domain. Certificate issuance and revocation are authorized by 276 a signature with the key pair. 278 The first phase of ACME is for the client to register with the ACME 279 server. The client generates an asymmetric key pair and associates 280 this key pair with a set of contact information by signing the 281 contact information. The server acknowledges the registration by 282 replying with a registration object echoing the client's input. The 283 server can also provide terms of service at this stage, which the 284 client can present to a human user. 286 Client Server 288 Contact Information 289 Signature -------> 291 <------- Registration 292 Terms of Service 294 Once the client is registered, there are three major steps it needs 295 to take to get a certificate: 297 1. Apply for a certificate to be issued 299 2. Fulfill the server's requirements for issuance 301 3. Finalize the application and request issuance 303 The client's application for a certificate describes the desired 304 certificate using a PKCS#10 Certificate Signing Request (CSR) plus a 305 few additional fields that capture semantics that are not supported 306 in the CSR format. If the server is willing to consider issuing such 307 a certificate, it responds with a list of requirements that the 308 client must satisfy before the certificate will be issued. 310 For example, in most cases, the server will require the client to 311 demonstrate that it controls the identifiers in the requested 312 certificate. Because there are many different ways to validate 313 possession of different types of identifiers, the server will choose 314 from an extensible set of challenges that are appropriate for the 315 identifier being claimed. The client responds with a set of 316 responses that tell the server which challenges the client has 317 completed. The server then validates the challenges to check that 318 the client has accomplished the challenge. 320 Once the validation process is complete and the server is satisfied 321 that the client has met its requirements, the server can either 322 proactively issue the requested certificate or wait for the client to 323 request that the application be "finalized", at which point the 324 certificate will be issued and provided to the client. 326 Application 327 Signature -------> 328 <------- Requirements 329 (e.g., Challenges) 331 Responses 332 Signature -------> 334 <~~~~~~~~Validation~~~~~~~~> 336 Finalize application 337 Signature -------> 338 <------- Certificate 340 To revoke a certificate, the client simply sends a revocation request 341 indicating the certificate to be revoked, signed with an authorized 342 key pair. The server indicates whether the request has succeeded. 344 Client Server 346 Revocation request 347 Signature --------> 349 <-------- Result 351 Note that while ACME is defined with enough flexibility to handle 352 different types of identifiers in principle, the primary use case 353 addressed by this document is the case where domain names are used as 354 identifiers. For example, all of the identifier validation 355 challenges described in Section 7 below address validation of domain 356 names. The use of ACME for other protocols will require further 357 specification, in order to describe how these identifiers are encoded 358 in the protocol, and what types of validation challenges the server 359 might require. 361 5. Message Transport 363 Communications between an ACME client and an ACME server are done 364 over HTTPS, using JWS to provide some additional security properties 365 for messages sent from the client to the server. HTTPS provides 366 server authentication and confidentiality. With some ACME-specific 367 extensions, JWS provides authentication of the client's request 368 payloads, anti-replay protection, and integrity for the HTTPS request 369 URI. 371 5.1. HTTPS Requests 373 Each ACME function is accomplished by the client sending a sequence 374 of HTTPS requests to the server, carrying JSON messages 375 [RFC2818][RFC7159]. Use of HTTPS is REQUIRED. Clients SHOULD 376 support HTTP public key pinning [RFC7469], and servers SHOULD emit 377 pinning headers. Each subsection of Section 6 below describes the 378 message formats used by the function, and the order in which messages 379 are sent. 381 In all HTTPS transactions used by ACME, the ACME client is the HTTPS 382 client and the ACME server is the HTTPS server. 384 ACME servers that are intended to be generally accessible need to use 385 Cross-Origin Resource Sharing (CORS) in order to be accessible from 386 browser-based clients [W3C.CR-cors-20130129]. Such servers SHOULD 387 set the Access-Control-Allow-Origin header field to the value "*". 389 Binary fields in the JSON objects used by ACME are encoded using 390 base64url encoding described in [RFC4648] Section 5, according to the 391 profile specified in JSON Web Signature [RFC7515] Section 2. This 392 encoding uses a URL safe character set. Trailing '=' characters MUST 393 be stripped. 395 5.2. Request Authentication 397 All ACME requests with a non-empty body MUST encapsulate the body in 398 a JWS object, signed using the account key pair. The server MUST 399 verify the JWS before processing the request. (For readability, 400 however, the examples below omit this encapsulation.) Encapsulating 401 request bodies in JWS provides a simple authentication of requests by 402 way of key continuity. 404 JWS objects sent in ACME requests MUST meet the following additional 405 criteria: 407 o The JWS MUST be encoded using UTF-8 409 o The JWS MUST NOT have the value "none" in its "alg" field 411 o The JWS Protected Header MUST include the following fields: 413 * "alg" 415 * "jwk" 417 * "nonce" (defined below) 418 * "url" (defined below) 420 Note that this implies that GET requests are not authenticated. 421 Servers MUST NOT respond to GET requests for resources that might be 422 considered sensitive. 424 In the examples below, JWS objects are shown in the JSON or flattened 425 JSON serialization, with the protected header and payload expressed 426 as base64url(content) instead of the actual base64-encoded value, so 427 that the content is readable. Some fields are omitted for brevity, 428 marked with "...". 430 5.3. Request URI Integrity 432 It is common in deployment the entity terminating TLS for HTTPS to be 433 different from the entity operating the logical HTTPS server, with a 434 "request routing" layer in the middle. For example, an ACME CA might 435 have a content delivery network terminate TLS connections from 436 clients so that it can inspect client requests for denial-of-service 437 protection. 439 These intermediaries can also change values in the request that are 440 not signed in the HTTPS request, e.g., the request URI and headers. 441 ACME uses JWS to provide a limited integrity mechanism, which 442 protects against an intermediary changing the request URI to another 443 ACME URI of a different type. (It does not protect against changing 444 between URIs of the same type, e.g., from one authorization URI to 445 another). 447 As noted above, all ACME request object carry a "url" parameter in 448 their protected header. This header parameter encodes the URL to 449 which the client is directing the request. On receiving such an 450 object in an HTTP request, the server MUST compare the "url" 451 parameter to the request URI. If the two do not match, then the 452 server MUST reject the request as unauthorized. 454 Except for the directory resource, all ACME resources are addressed 455 with URLs provided to the client by the server. In such cases, the 456 client MUST set the "url" field to the exact string provided by the 457 server (rather than performing any re-encoding on the URL). 459 5.3.1. "url" (URL) JWS header parameter 461 The "url" header parameter specifies the URL to which this JWS object 462 is directed [RFC3986]. The "url" parameter MUST be carried in the 463 protected header of the JWS. The value of the "nonce" header MUST be 464 a JSON string representing the URL. 466 5.4. Replay protection 468 In order to protect ACME resources from any possible replay attacks, 469 ACME requests have a mandatory anti-replay mechanism. This mechanism 470 is based on the server maintaining a list of nonces that it has 471 issued to clients, and requiring any signed request from the client 472 to carry such a nonce. 474 An ACME server MUST include a Replay-Nonce header field in each 475 successful response it provides to a client, with contents as 476 specified below. In particular, the ACME server MUST provide a 477 Replay-Nonce header field in response to a HEAD request for any valid 478 resource. (This allows clients to easily obtain a fresh nonce.) It 479 MAY also provide nonces in error responses. 481 Every JWS sent by an ACME client MUST include, in its protected 482 header, the "nonce" header parameter, with contents as defined below. 483 As part of JWS verification, the ACME server MUST verify that the 484 value of the "nonce" header is a value that the server previously 485 provided in a Replay-Nonce header field. Once a nonce value has 486 appeared in an ACME request, the server MUST consider it invalid, in 487 the same way as a value it had never issued. 489 When a server rejects a request because its nonce value was 490 unacceptable (or not present), it SHOULD provide HTTP status code 400 491 (Bad Request), and indicate the ACME error code 492 "urn:ietf:params:acme:error:badNonce". 494 The precise method used to generate and track nonces is up to the 495 server. For example, the server could generate a random 128-bit 496 value for each response, keep a list of issued nonces, and strike 497 nonces from this list as they are used. 499 5.4.1. Replay-Nonce 501 The "Replay-Nonce" header field includes a server-generated value 502 that the server can use to detect unauthorized replay in future 503 client requests. The server should generate the value provided in 504 Replay-Nonce in such a way that they are unique to each message, with 505 high probability. 507 The value of the Replay-Nonce field MUST be an octet string encoded 508 according to the base64url encoding described in Section 2 of 509 [RFC7515]. Clients MUST ignore invalid Replay-Nonce values. 511 base64url = [A-Z] / [a-z] / [0-9] / "-" / "_" 513 Replay-Nonce = *base64url 515 The Replay-Nonce header field SHOULD NOT be included in HTTP request 516 messages. 518 5.4.2. "nonce" (Nonce) JWS header parameter 520 The "nonce" header parameter provides a unique value that enables the 521 verifier of a JWS to recognize when replay has occurred. The "nonce" 522 header parameter MUST be carried in the protected header of the JWS. 524 The value of the "nonce" header parameter MUST be an octet string, 525 encoded according to the base64url encoding described in Section 2 of 526 [RFC7515]. If the value of a "nonce" header parameter is not valid 527 according to this encoding, then the verifier MUST reject the JWS as 528 malformed. 530 5.5. Rate limits 532 Creation of resources can be rate limited to ensure fair usage and 533 prevent abuse. Once the rate limit is exceeded, the server MUST 534 respond with an error with the code "rateLimited". Additionally, the 535 server SHOULD send a "Retry-After" header indicating when the current 536 request may succeed again. If multiple rate limits are in place, 537 that is the time where all rate limits allow access again for the 538 current request with exactly the same parameters. 540 In addition to the human readable "detail" field of the error 541 response, the server MAY send one or multiple tokens in the "Link" 542 header pointing to documentation about the specific hit rate limits 543 using the "rate-limit" relation. 545 5.6. Errors 547 Errors can be reported in ACME both at the HTTP layer and within ACME 548 payloads. ACME servers can return responses with an HTTP error 549 response code (4XX or 5XX). For example: If the client submits a 550 request using a method not allowed in this document, then the server 551 MAY return status code 405 (Method Not Allowed). 553 When the server responds with an error status, it SHOULD provide 554 additional information using problem document [RFC7807]. To 555 facilitate automatic response to errors, this document defines the 556 following standard tokens for use in the "type" field (within the 557 "urn:ietf:params:acme:error:" namespace): 559 +-----------------------+-------------------------------------------+ 560 | Code | Description | 561 +-----------------------+-------------------------------------------+ 562 | badCSR | The CSR is unacceptable (e.g., due to a | 563 | | short key) | 564 | | | 565 | badNonce | The client sent an unacceptable anti- | 566 | | replay nonce | 567 | | | 568 | connection | The server could not connect to | 569 | | validation target | 570 | | | 571 | dnssec | DNSSEC validation failed | 572 | | | 573 | caa | CAA records forbid the CA from issuing | 574 | | | 575 | malformed | The request message was malformed | 576 | | | 577 | serverInternal | The server experienced an internal error | 578 | | | 579 | tls | The server received a TLS error during | 580 | | validation | 581 | | | 582 | unauthorized | The client lacks sufficient authorization | 583 | | | 584 | unknownHost | The server could not resolve a domain | 585 | | name | 586 | | | 587 | rateLimited | The request exceeds a rate limit | 588 | | | 589 | invalidContact | The contact URI for a registration was | 590 | | invalid | 591 | | | 592 | rejectedIdentifier | The server will not issue for the | 593 | | identifier | 594 | | | 595 | unsupportedIdentifier | Identifier is not supported, but may be | 596 | | in future | 597 +-----------------------+-------------------------------------------+ 599 This list is not exhaustive. The server MAY return errors whose 600 "type" field is set to a URI other than those defined above. Servers 601 MUST NOT use the ACME URN namespace for errors other than the 602 standard types. Clients SHOULD display the "detail" field of such 603 errors. 605 Authorization and challenge objects can also contain error 606 information to indicate why the server was unable to validate 607 authorization. 609 6. Certificate Management 611 In this section, we describe the certificate management functions 612 that ACME enables: 614 o Account Key Registration 616 o Application for a Certificate 618 o Account Key Authorization 620 o Certificate Issuance 622 o Certificate Revocation 624 6.1. Resources 626 ACME is structured as a REST application with a few types of 627 resources: 629 o Registration resources, representing information about an account 631 o Application resources, represnting an account's requests to issue 632 certificates 634 o Authorization resources, representing an account's authorization 635 to act for an identifier 637 o Challenge resources, representing a challenge to prove control of 638 an identifier 640 o Certificate resources, representing issued certificates 642 o A "directory" resource 644 o A "new-registration" resource 646 o A "new-application" resource 648 o A "revoke-certificate" resource 650 o A "key-change" resource 651 For the "new-X" resources above, the server MUST have exactly one 652 resource for each function. This resource may be addressed by 653 multiple URIs, but all must provide equivalent functionality. 655 ACME uses different URIs for different management functions. Each 656 function is listed in a directory along with its corresponding URI, 657 so clients only need to be configured with the directory URI. These 658 URIs are connected by a few different link relations [RFC5988]. 660 The "up" link relation is used with challenge resources to indicate 661 the authorization resource to which a challenge belongs. It is also 662 used from certificate resources to indicate a resource from which the 663 client may fetch a chain of CA certificates that could be used to 664 validate the certificate in the original resource. 666 The "directory" link relation is present on all resources other than 667 the directory and indicates the directory URL. 669 The following diagram illustrates the relations between resources on 670 an ACME server. For the most part, these relations are expressed by 671 URLs provided as strings in the resources' JSON representations. 672 Lines with labels in quotes indicate HTTP link relations 674 directory 675 | 676 | 677 ---------------------------------------------------- 678 | | | 679 | | | 680 V V V 681 new-reg new-app revoke-cert 682 | | ^ 683 | | | "revoke" 684 V V | 685 reg -------------> app -------------> cert ---------+ 686 | ^ | 687 | | "up" | "up" 688 V | V 689 authz cert-chain 690 | ^ 691 | | "up" 692 V | 693 challenge 695 The following table illustrates a typical sequence of requests 696 required to establish a new account with the server, prove control of 697 an identifier, issue a certificate, and fetch an updated certificate 698 some time after issuance. The "->" is a mnemonic for a Location 699 header pointing to a created resource. 701 +--------------------+----------------+------------+ 702 | Action | Request | Response | 703 +--------------------+----------------+------------+ 704 | Register | POST new-reg | 201 -> reg | 705 | | | | 706 | Apply for a cert | POST new-app | 201 -> app | 707 | | | | 708 | Fetch challenges | GET authz | 200 | 709 | | | | 710 | Answer challenges | POST challenge | 200 | 711 | | | | 712 | Poll for status | GET authz | 200 | 713 | | | | 714 | Request issuance | POST app | 200 | 715 | | | | 716 | Check for new cert | GET cert | 200 | 717 +--------------------+----------------+------------+ 719 The remainder of this section provides the details of how these 720 resources are structured and how the ACME protocol makes use of them. 722 6.1.1. Directory 724 In order to help clients configure themselves with the right URIs for 725 each ACME operation, ACME servers provide a directory object. This 726 should be the only URL needed to configure clients. It is a JSON 727 dictionary, whose keys are drawn from the following table and whose 728 values are the corresponding URLs. 730 +-------------+--------------------+ 731 | Key | URL in value | 732 +-------------+--------------------+ 733 | new-reg | New registration | 734 | | | 735 | new-app | New application | 736 | | | 737 | revoke-cert | Revoke certificate | 738 | | | 739 | key-change | Key change | 740 +-------------+--------------------+ 742 There is no constraint on the actual URI of the directory except that 743 it should be different from the other ACME server resources' URIs, 744 and that it should not clash with other services. For instance: 746 o a host which function as both an ACME and Web server may want to 747 keep the root path "/" for an HTML "front page", and and place the 748 ACME directory under path "/acme". 750 o a host which only functions as an ACME server could place the 751 directory under path "/". 753 The dictionary MAY additionally contain a key "meta". If present, it 754 MUST be a JSON dictionary; each item in the dictionary is an item of 755 metadata relating to the service provided by the ACME server. 757 The following metadata items are defined, all of which are OPTIONAL: 759 "terms-of-service" (optional, string): A URI identifying the current 760 terms of service. 762 "website" (optional, string)): An HTTP or HTTPS URL locating a 763 website providing more information about the ACME server. 765 "caa-identities" (optional, array of string): Each string MUST be a 766 lowercase hostname which the ACME server recognises as referring 767 to itself for the purposes of CAA record validation as defined in 768 [RFC6844]. This allows clients to determine the correct issuer 769 domain name to use when configuring CAA record. 771 Clients access the directory by sending a GET request to the 772 directory URI. 774 HTTP/1.1 200 OK 775 Content-Type: application/json 777 { 778 "new-reg": "https://example.com/acme/new-reg", 779 "new-app": "https://example.com/acme/new-app", 780 "revoke-cert": "https://example.com/acme/revoke-cert", 781 "key-change": "https://example.com/acme/key-change", 782 "meta": { 783 "terms-of-service": "https://example.com/acme/terms", 784 "website": "https://www.example.com/", 785 "caa-identities": ["example.com"] 786 } 787 } 789 6.1.2. Registration Objects 791 An ACME registration resource represents a set of metadata associated 792 to an account key pair. Registration resources have the following 793 structure: 795 key (required, dictionary): The public key of the account key pair, 796 encoded as a JSON Web Key object [RFC7517]. 798 status (required, string): "good" or "deactivated" 800 contact (optional, array of string): An array of URIs that the 801 server can use to contact the client for issues related to this 802 authorization. For example, the server may wish to notify the 803 client about server-initiated revocation. 805 agreement (optional, string): A URI referring to a subscriber 806 agreement or terms of service provided by the server (see below). 807 Including this field indicates the client's agreement with the 808 referenced terms. 810 applications (required, string): A URI from which a list of 811 authorizations submitted by this account can be fetched via a GET 812 request. The result of the GET request MUST be a JSON object 813 whose "applications" field is an array of strings, where each 814 string is the URI of an authorization belonging to this 815 registration. The server SHOULD include pending applications, and 816 SHOULD NOT include applications that are invalid. The server MAY 817 return an incomplete list, along with a Link header with link 818 relation "next" indicating a URL to retrieve further entries. 820 certificates (required, string): A URI from which a list of 821 certificates issued for this account can be fetched via a GET 822 request. The result of the GET request MUST be a JSON object 823 whose "certificates" field is an array of strings, where each 824 string is the URI of a certificate. The server SHOULD NOT include 825 expired or revoked certificates. The server MAY return an 826 incomplete list, along with a Link header with link relation 827 "next" indicating a URL to retrieve further entries. 829 { 830 "contact": [ 831 "mailto:cert-admin@example.com", 832 "tel:+12025551212" 833 ], 834 "agreement": "https://example.com/acme/terms", 835 "authorizations": "https://example.com/acme/reg/1/authz", 836 "certificates": "https://example.com/acme/reg/1/cert" 837 } 839 6.1.3. Application Objects 841 An ACME registration resource represents a client's request for a 842 certificate, and is used to track the progress of that application 843 through to issuance. Thus, the object contains information about the 844 requested certificate, the server's requirements, and any 845 certificates that have resulted from this application. 847 status (required, string): The status of this authorization. 848 Possible values are: "unknown", "pending", "processing", "valid", 849 and "invalid". 851 expires (optional, string): The timestamp after which the server 852 will consider this application invalid, encoded in the format 853 specified in RFC 3339 [RFC3339]. This field is REQUIRED for 854 objects with "pending" or "valid" in the status field. 856 csr (required, string): A CSR encoding the parameters for the 857 certificate being requested [RFC2986]. The CSR is sent in the 858 Base64url-encoded version of the DER format. (Note: This field 859 uses the same modified Base64 encoding rules used elsewhere in 860 this document, so it is different from PEM.) 862 notBefore (optional, string): The requested value of the notBefore 863 field in the certificate, in the date format defined in [RFC3339] 865 notAfter (optional, string): The requested value of the notAfter 866 field in the certificate, in the date format defined in [RFC3339] 868 requirements (required, array): The requirements that the client 869 needs to fulfill before the requested certificate can be granted 870 (for pending applications). For final applications, the 871 requirements that were met. Each entry is a dictionary with 872 parameters describing the requirement (see below). 874 certificate (optional, string): A URL for the certificate that has 875 been issued in response to this application. 877 { 878 "status": "pending", 879 "expires": "2015-03-01T14:09:00Z", 881 "csr": "jcRf4uXra7FGYW5ZMewvV...rhlnznwy8YbpMGqwidEXfE", 882 "notBefore": "2016-01-01T00:00:00Z", 883 "notAfter": "2016-01-08T00:00:00Z", 885 "requirements": [ 886 { 887 "type": "authorization", 888 "status": "valid", 889 "url": "https://example.com/acme/authz/1234" 890 }, 891 { 892 "type": "out-of-band", 893 "status": "pending", 894 "url": "https://example.com/acme/payment/1234" 895 } 896 ] 898 "certificate": "https://example.com/acme/cert/1234" 899 } 901 [[ Open issue: There are two possible behaviors for the CA here. 902 Either (a) the CA automatically issues once all the requirements are 903 fulfilled, or (b) the CA waits for confirmation from the client that 904 it should issue. If we allow both, we will need a signal in the 905 application object of whether confirmation is required. I would 906 prefer that auto-issue be the default, which would imply a syntax 907 like "confirm": true ]] 909 [[ Open issue: Should this syntax allow multiple certificates? That 910 would support reissuance / renewal in a straightforward way, 911 especially if the CSR / notBefore / notAfter could be updated. ]] 913 The elements of the "requirements" array are immutable once set, 914 except for their "status" fields. If any other part of the object 915 changes after the object is created, the client MUST consider the 916 application invalid. 918 The "requirements" array in the challenge SHOULD reflect everything 919 that the CA required the client to do before issuance, even if some 920 requirements were fulfilled in earlier applications. For example, if 921 a CA allows multiple applications to be fufilled based on a single 922 authorization transaction, then it must reflect that authorization in 923 all of the applications. 925 Each entry in the "requirements" array expresses a requirement from 926 the CA for the client to takek a particular action. All requirements 927 objects have the following basic fields: 929 type (required, string): The type of requirement (see below for 930 defined types) 932 status (required, string): The status of this requirement. Possible 933 values are: "pending", "valid", and "invalid". 935 All additional fields are specified by the requirement type. 937 6.1.3.1. Authorization Requirement 939 A requirement with type "authorization" requests that the ACME client 940 complete an authorization transaction. The server specifies the 941 authorization by pre-provisioning a pending authorization resource 942 and providing the URI for this resource in the requirement. 944 url (required, string): The URL for the authorization resource 946 To fulfill this requirement, the ACME client should fetch the 947 authorization object from the indicated URL, then follow the process 948 for obtaining authorization as specified in Section 6.4. 950 6.1.3.2. Out-of-Band Requirement 952 A requirement with type "out-of-band" requests that the ACME client 953 have a human user visit a web page in order to receive further 954 instructions for how to fulfill the requirement. The requirement 955 object provides a URI for the web page to be visited. 957 url (required, string): The URL to be visited. The scheme of this 958 URL MUST be "http" or "https" 960 To fulfill this requirement, the ACME client should direct the user 961 to the indicated web page. 963 6.1.4. Authorization Objects 965 An ACME authorization object represents server's authorization for an 966 account to represent an identifier. In addition to the identifier, 967 an authorization includes several metadata fields, such as the status 968 of the authorization (e.g., "pending", "valid", or "revoked") and 969 which challenges were used to validate possession of the identifier. 971 The structure of an ACME authorization resource is as follows: 973 identifier (required, dictionary of string): The identifier that the 974 account is authorized to represent 976 type (required, string): The type of identifier. 978 value (required, string): The identifier itself. 980 status (required, string): The status of this authorization. 981 Possible values are: "unknown", "pending", "processing", "valid", 982 "invalid" and "revoked". If this field is missing, then the 983 default value is "pending". 985 expires (optional, string): The timestamp after which the server 986 will consider this authorization invalid, encoded in the format 987 specified in RFC 3339 [RFC3339]. This field is REQUIRED for 988 objects with "valid" in the "status field. 990 scope (optional, string): If this field is present, then it MUST 991 contain a URI for an application resource, such that this 992 authorization is only valid for that resource. If this field is 993 absent, then the CA MUST consider this authorization valid for all 994 applications until the authorization expires. [[ Open issue: More 995 flexible scoping? ]] 997 challenges (required, array): The challenges that the client needs 998 to fulfill in order to prove possession of the identifier (for 999 pending authorizations). For final authorizations, the challenges 1000 that were used. Each array entry is a dictionary with parameters 1001 required to validate the challenge, as specified in Section 7. 1003 combinations (optional, array of arrays of integers): A collection 1004 of sets of challenges, each of which would be sufficient to prove 1005 possession of the identifier. Clients complete a set of 1006 challenges that covers at least one set in this array. Challenges 1007 are identified by their indices in the challenges array. If no 1008 "combinations" element is included in an authorization object, the 1009 client completes all challenges. 1011 The only type of identifier defined by this specification is a fully- 1012 qualified domain name (type: "dns"). The value of the identifier 1013 MUST be the ASCII representation of the domain name. Wildcard domain 1014 names (with "*" as the first label) MUST NOT be included in 1015 authorization requests. 1017 { 1018 "status": "valid", 1019 "expires": "2015-03-01T14:09:00Z", 1021 "identifier": { 1022 "type": "dns", 1023 "value": "example.org" 1024 }, 1026 "challenges": [ 1027 { 1028 "type": "http-01", 1029 "status": "valid", 1030 "validated": "2014-12-01T12:05:00Z", 1031 "keyAuthorization": "SXQe-2XODaDxNR...vb29HhjjLPSggwiE" 1032 } 1033 ] 1034 } 1036 6.2. Registration 1038 A client creates a new account with the server by sending a POST 1039 request to the server's new-registration URI. The body of the 1040 request is a stub registration object containing only the "contact" 1041 field. 1043 POST /acme/new-reg HTTP/1.1 1044 Host: example.com 1045 Content-Type: application/jose+json 1047 { 1048 "protected": base64url({ 1049 "alg": "ES256", 1050 "jwk": {...}, 1051 "nonce": "6S8IqOGY7eL2lsGoTZYifg", 1052 "url": "https://example.com/acme/new-reg" 1053 }) 1054 "payload": base64url({ 1055 "contact": [ 1056 "mailto:cert-admin@example.com", 1057 "tel:+12025551212" 1058 ] 1059 }), 1060 "signature": "RZPOnYoPs1PhjszF...-nh6X1qtOFPB519I" 1061 } 1063 The server MUST ignore any values provided in the "key", 1064 "authorizations", and "certificates" fields in registration bodies 1065 sent by the client, as well as any other fields that it does not 1066 recognize. If new fields are specified in the future, the 1067 specification of those fields MUST describe whether they may be 1068 provided by the client. 1070 The server creates a registration object with the included contact 1071 information. The "key" element of the registration is set to the 1072 public key used to verify the JWS (i.e., the "jwk" element of the JWS 1073 header). The server returns this registration object in a 201 1074 (Created) response, with the registration URI in a Location header 1075 field. 1077 If the server already has a registration object with the provided 1078 account key, then it MUST return a 409 (Conflict) response and 1079 provide the URI of that registration in a Location header field. 1080 This allows a client that has an account key but not the 1081 corresponding registration URI to recover the registration URI. 1083 If the server wishes to present the client with terms under which the 1084 ACME service is to be used, it MUST indicate the URI where such terms 1085 can be accessed in a Link header with link relation "terms-of- 1086 service". As noted above, the client may indicate its agreement with 1087 these terms by updating its registration to include the "agreement" 1088 field, with the terms URI as its value. When these terms change in a 1089 way that requires an agreement update, the server MUST use a 1090 different URI in the Link header. 1092 HTTP/1.1 201 Created 1093 Content-Type: application/json 1094 Location: https://example.com/acme/reg/asdf 1095 Link: ;rel="terms-of-service" 1096 Link: ;rel="directory" 1098 { 1099 "key": { /* JWK from JWS header */ }, 1100 "status": "good", 1102 "contact": [ 1103 "mailto:cert-admin@example.com", 1104 "tel:+12025551212" 1105 ] 1106 } 1108 If the client wishes to update this information in the future, it 1109 sends a POST request with updated information to the registration 1110 URI. The server MUST ignore any updates to the "key", 1111 "authorizations, or "certificates" fields, and MUST verify that the 1112 request is signed with the private key corresponding to the "key" 1113 field of the request before updating the registration. 1115 For example, to update the contact information in the above 1116 registration, the client could send the following request: 1118 POST /acme/reg/asdf HTTP/1.1 1119 Host: example.com 1120 Content-Type: application/jose+json 1122 { 1123 "protected": base64url({ 1124 "alg": "ES256", 1125 "jwk": {...}, 1126 "nonce": "ax5RnthDqp_Yf4_HZnFLmA", 1127 "url": "https://example.com/acme/reg/asdf" 1128 }) 1129 "payload": base64url({ 1130 "contact": [ 1131 "mailto:certificates@example.com", 1132 "tel:+12125551212" 1133 ] 1134 }), 1135 "signature": "hDXzvcj8T6fbFbmn...rDzXzzvzpRy64N0o" 1136 } 1138 Servers SHOULD NOT respond to GET requests for registration resources 1139 as these requests are not authenticated. If a client wishes to query 1140 the server for information about its account (e.g., to examine the 1141 "contact" or "certificates" fields), then it SHOULD do so by sending 1142 a POST request with an empty update. That is, it should send a JWS 1143 whose payload is trivial ({}). 1145 6.2.1. Account Key Roll-over 1147 A client may wish to change the public key that is associated with a 1148 registration in order to recover from a key compromise or proactively 1149 mitigate the impact of an unnoticed key compromise. 1151 To change the key associate with an account, the client sends a POST 1152 request containing a key-change object with the following fields: 1154 oldKey (required, JWK): The JWK representation of the original key 1155 (i.e., the client's current account key) 1157 newKey (requrired, JWK): The JWK representation of the new key 1158 The JWS of this POST must have two signatures: one signature from the 1159 existing key on the account, and one signature from the new key that 1160 the client proposes to use. This demonstrates that the client 1161 actually has control of the private key corresponding to the new 1162 public key. The protected header must contain a JWK field containing 1163 the current account key. 1165 POST /acme/key-change HTTP/1.1 1166 Host: example.com 1167 Content-Type: application/jose+json 1169 { 1170 "payload": base64url({ 1171 "oldKey": /* Old key in JWK form */ 1172 "newKey": /* New key in JWK form */ 1173 }), 1174 "signatures": [{ 1175 "protected": base64url({ 1176 "alg": "ES256", 1177 "jwk": /* old key */, 1178 "nonce": "pq00v-D1KB0sReG4jFfqVg", 1179 "url": "https://example.com/acme/key-change" 1180 }), 1181 "signature": "XFvVbo9diBlIBvhE...UI62sNT6MZsCJpQo" 1182 }, { 1183 "protected": base64url({ 1184 "alg": "ES256", 1185 "jwk": /* new key */, 1186 "nonce": "vYjyueEYhMjpVQHe_unw4g", 1187 "url": "https://example.com/acme/key-change" 1188 }), 1189 "signature": "q20gG1f1r9cD6tBM...a48h0CkP11tl5Doo" 1190 }] 1191 } 1193 On receiving key-change request, the server MUST perform the 1194 following steps in addition to the typical JWS validation: 1196 1. Check that the JWS protected header container a "jwk" field 1197 containing a key that matches a currently active account. 1199 2. Check that there are exactly two signatures on the JWS. 1201 3. Check that one of the signatures validates using the account key 1202 from (1). 1204 4. Check that the "key" field contains a well-formed JWK that meets 1205 key strength requirements. 1207 5. Check that the "key" field is not equivalent to the current 1208 account key or any other currently active account key. 1210 6. Check that one of the two signatures on the JWS validates using 1211 the JWK from the "key" field. 1213 If all of these checks pass, then the server updates the 1214 corresponding registration by replacing the old account key with the 1215 new public key and returns status code 200. Otherwise, the server 1216 responds with an error status code and a problem document describing 1217 the error. 1219 6.2.2. Account deactivation 1221 A client may deactivate an account by posting a signed update to the 1222 server with a status field of "deactivated." Clients may wish to do 1223 this when the account key is compromised. 1225 POST /acme/reg/asdf HTTP/1.1 1226 Host: example.com 1227 Content-Type: application/jose+json 1229 { 1230 "protected": base64url({ 1231 "alg": "ES256", 1232 "jwk": {...}, 1233 "nonce": "ntuJWWSic4WVNSqeUmshgg", 1234 "url": "https://example.com/acme/reg/asdf" 1235 }) 1236 "payload": base64url({ 1237 "status": "deactivated" 1238 }), 1239 "signature": "earzVLd3m5M4xJzR...bVTqn7R08AKOVf3Y" 1240 } 1242 The server MUST verify that the request is signed by the account key. 1243 If the server accepts the deactivation request, it should reply with 1244 a 200 (OK) status code and the current contents of the registration 1245 object. 1247 Once an account is deactivated, the server MUST NOT accept further 1248 requests authorized by that account's key. It is up to server policy 1249 how long to retain data related to that account, whether to revoke 1250 certificates issued by that account, and whether to send email to 1251 that account's contacts. ACME does not provide a way to reactivate a 1252 deactivated account. 1254 6.3. Applying for Certificate Issuance 1256 The holder of an account key pair may use ACME to submit an 1257 application for a certificate to be issued. The client makes this 1258 request by sending a POST request to the server's new-application 1259 resource. The body of the POST is a JWS object whose JSON payload is 1260 a subset of the application object defined in Section 6.1.3, 1261 containing the fields that describe the certificate to be issued: 1263 csr (required, string): A CSR encoding the parameters for the 1264 certificate being requested [RFC2986]. The CSR is sent in the 1265 Base64url-encoded version of the DER format. (Note: This field 1266 uses the same modified Base64 encoding rules used elsewhere in 1267 this document, so it is different from PEM.) 1269 notBefore (optional, string): The requested value of the notBefore 1270 field in the certificate, in the date format defined in [RFC3339] 1272 notAfter (optional, string): The requested value of the notAfter 1273 field in the certificate, in the date format defined in [RFC3339] 1275 POST /acme/new-app HTTP/1.1 1276 Host: example.com 1277 Content-Type: application/jose+json 1279 { 1280 "protected": base64url({ 1281 "alg": "ES256", 1282 "jwk": {...}, 1283 "nonce": "5XJ1L3lEkMG7tR6pA00clA", 1284 "url": "https://example.com/acme/new-app" 1285 }) 1286 "payload": base64url({ 1287 "csr": "5jNudRx6Ye4HzKEqT5...FS6aKdZeGsysoCo4H9P", 1288 "notBefore": "2016-01-01T00:00:00Z", 1289 "notAfter": "2016-01-08T00:00:00Z" 1290 }), 1291 "signature": "H6ZXtGjTZyUnPeKn...wEA4TklBdh3e454g" 1292 } 1294 The CSR encodes the client's requests with regard to the content of 1295 the certificate to be issued. The CSR MUST indicate the requested 1296 identifiers, either in the commonName portion of the requested 1297 subject name, or in an extensionRequest attribute [RFC2985] 1298 requesting a subjectAltName extension. 1300 The server MUST return an error if it cannot fulfil the request as 1301 specified, and MUST NOT issue a certificate with contents other than 1302 those requested. If the server requires the request to be modified 1303 in a certain way, it should indicate the required changes using an 1304 appropriate error code and description. 1306 If the server is willing to issue the requested certificate, it 1307 responds with a 201 (Created) response. The body of this response is 1308 an application object reflecting the client's request and any 1309 requirements the client must fulfill before the certificate will be 1310 issued. 1312 HTTP/1.1 201 Created 1313 Location: https://example.com/acme/app/asdf 1315 { 1316 "status": "pending", 1317 "expires": "2015-03-01T14:09:00Z", 1319 "csr": "jcRf4uXra7FGYW5ZMewvV...rhlnznwy8YbpMGqwidEXfE", 1320 "notBefore": "2016-01-01T00:00:00Z", 1321 "notAfter": "2016-01-08T00:00:00Z", 1323 "requirements": [ 1324 { 1325 "type": "authorization", 1326 "status": "valid", 1327 "url": "https://example.com/acme/authz/1234" 1328 }, 1329 { 1330 "type": "out-of-band", 1331 "status": "pending", 1332 "url": "https://example.com/acme/payment/1234" 1333 } 1334 ] 1335 } 1337 The application object returned by the server represents a promise 1338 that if the client fulfills the server's requirements before the 1339 "expires" time, then the server will issue the requested certificate. 1340 In the application object, any object in the "requirements" array 1341 whose status is "pending" represents an action that the client must 1342 perform before the server will issue the certificate. If the client 1343 fails to complete the required actions before the "expires" time, 1344 then the server SHOULD change the status of the application to 1345 "invalid" and MAY delete the application resource. 1347 The server SHOULD issue the requested certificate and update the 1348 application resource with a URL for the certificate as soon as the 1349 client has fulfilled the server's requirements. If the client has 1350 already satisfied the server's requirements at the time of this 1351 request (e.g., by obtaining authorization for all of the identifiers 1352 in the certificate in previous transactions), then the server MAY 1353 proactively issue the requested certificate and provide a URL for it 1354 in the "certificate" field of the application. The server MUST, 1355 however, still list the satisfied requirements in the "requirements" 1356 array, with the state "valid". 1358 Once the client believes it has fulfilled the server's requirements, 1359 it should send a GET request to the application resource to obtain 1360 its current state. The status of the application will indicate what 1361 action the client should take: 1363 o "invalid": The certificate will not be issued. Consider this 1364 application process abandoned. 1366 o "pending": The server does not believe that the client has 1367 fulfilled the requirements. Check the "requirements" array for 1368 requirements that are still pending. 1370 o "processing": The server agrees that the requirements have been 1371 fulfilled, and is in the process of generating the certificate. 1372 Retry after the time given in the "Retry-After" header field of 1373 the response, if any. 1375 o "valid": The server has issued the certificate and provisioned its 1376 URL to the "certificate" field of the application. Download the 1377 certificate. 1379 6.3.1. Downloading the Certificate 1381 To download the issued certificate, the client simply sends a GET 1382 request to the certificate URL. 1384 The default format of the certificate is DER (application/pkix-cert). 1385 The client may request other formats by including an Accept header in 1386 its request. For example, the client may use the media type 1387 application/x-pem-file to request the certificate in PEM format. 1389 The server provides metadata about the certificate in HTTP headers. 1390 In particular, the server MUST send one or more link relation header 1391 fields [RFC5988] with relation "up", each indicating a single 1392 certificate resource for the issuer of this certificate. The server 1393 MAY also include the "up" links from these resources to enable the 1394 client to build a full certificate chain. 1396 The server MUST also provide a link relation header field with 1397 relation "author" to indicate the application under which this 1398 certificate was issued. 1400 If the CA participates in Certificate Transparency (CT) [RFC6962], 1401 then they may want to provide the client with a Signed Certificate 1402 Timestamp (SCT) that can be used to prove that a certificate was 1403 submitted to a CT log. An SCT can be included as an extension in the 1404 certificate or as an extension to OCSP responses for the certificate. 1405 The server can also provide the client with direct access to an SCT 1406 for a certificate using a Link relation header field with relation 1407 "ct-sct". 1409 GET /acme/cert/asdf HTTP/1.1 1410 Host: example.com 1411 Accept: application/pkix-cert 1413 HTTP/1.1 200 OK 1414 Content-Type: application/pkix-cert 1415 Link: ;rel="up";title="issuer" 1416 Link: ;rel="revoke" 1417 Link: ;rel="author" 1418 Link: ;rel="ct-sct" 1419 Link: ;rel="directory" 1421 [DER-encoded certificate] 1423 A certificate resource represents a single, immutable certificate. 1424 If the client wishes to obtain a renewed certificate, the client 1425 initiates a new application process to request one. 1427 Because certificate resources are immutable once issuance is 1428 complete, the server MAY enable the caching of the resource by adding 1429 Expires and Cache-Control headers specifying a point in time in the 1430 distant future. These headers have no relation to the certificate's 1431 period of validity. 1433 6.4. Identifier Authorization 1435 The identifier authorization process establishes the authorization of 1436 an account to manage certificates for a given identifier. This 1437 process must assure the server of two things: First, that the client 1438 controls the private key of the account key pair, and second, that 1439 the client holds the identifier in question. This process may be 1440 repeated to associate multiple identifiers to a key pair (e.g., to 1441 request certificates with multiple identifiers), or to associate 1442 multiple accounts with an identifier (e.g., to allow multiple 1443 entities to manage certificates). The server may declare that an 1444 authorization is only valid for a specific application by setting the 1445 "scope" field of the authorization to the URI for that application. 1447 Authorization resources are created by the server in response to 1448 certificate applications submitted by an account key holder; their 1449 URLs are provided to the client in "authorization" requirement 1450 objects. The authorization object is implicitly tied to the account 1451 key used to sign the new-application request. 1453 When a client receives an application from the server with an 1454 "authorization" requirement, it downloads the authorization resource 1455 by sending a GET request to the indicated URL. 1457 GET /acme/authz/1234 HTTP/1.1 1458 Host: example.com 1460 HTTP/1.1 200 OK 1461 Content-Type: application/json 1462 Link: ;rel="directory" 1464 { 1465 "status": "pending", 1467 "identifier": { 1468 "type": "dns", 1469 "value": "example.org" 1470 }, 1472 "challenges": [ 1473 { 1474 "type": "http-01", 1475 "uri": "https://example.com/authz/asdf/0", 1476 "token": "IlirfxKKXAsHtmzK29Pj8A" 1477 }, 1478 { 1479 "type": "dns-01", 1480 "uri": "https://example.com/authz/asdf/1", 1481 "token": "DGyRejmCefe7v4NfDGDKfA" 1482 } 1483 ], 1485 "combinations": [[0], [1]] 1486 } 1488 6.4.1. Responding to Challenges 1490 To prove control of the identifier and receive authorization, the 1491 client needs to respond with information to complete the challenges. 1492 To do this, the client updates the authorization object received from 1493 the server by filling in any required information in the elements of 1494 the "challenges" dictionary. (This is also the stage where the 1495 client should perform any actions required by the challenge.) 1497 The client sends these updates back to the server in the form of a 1498 JSON object with the response fields required by the challenge type, 1499 carried in a POST request to the challenge URI (not authorization 1500 URI). This allows the client to send information only for challenges 1501 it is responding to. 1503 For example, if the client were to respond to the "http-01" challenge 1504 in the above authorization, it would send the following request: 1506 POST /acme/authz/asdf/0 HTTP/1.1 1507 Host: example.com 1508 Content-Type: application/jose+json 1510 { 1511 "protected": base64url({ 1512 "alg": "ES256", 1513 "jwk": {...}, 1514 "nonce": "Q_s3MWoqT05TrdkM2MTDcw", 1515 "url": "https://example.com/acme/authz/asdf/0" 1516 }) 1517 "payload": base64url({ 1518 "type": "http-01", 1519 "keyAuthorization": "IlirfxKKXA...vb29HhjjLPSggwiE" 1520 }), 1521 "signature": "9cbg5JO1Gf5YLjjz...SpkUfcdPai9uVYYQ" 1522 } 1524 The server updates the authorization document by updating its 1525 representation of the challenge with the response fields provided by 1526 the client. The server MUST ignore any fields in the response object 1527 that are not specified as response fields for this type of challenge. 1528 The server provides a 200 (OK) response with the updated challenge 1529 object as its body. 1531 If the client's response is invalid for some reason, or does not 1532 provide the server with appropriate information to validate the 1533 challenge, then the server MUST return an HTTP error. On receiving 1534 such an error, the client SHOULD undo any actions that have been 1535 taken to fulfill the challenge, e.g., removing files that have been 1536 provisioned to a web server. 1538 Presumably, the client's responses provide the server with enough 1539 information to validate one or more challenges. The server is said 1540 to "finalize" the authorization when it has completed all the 1541 validations it is going to complete, and assigns the authorization a 1542 status of "valid" or "invalid", corresponding to whether it considers 1543 the account authorized for the identifier. If the final state is 1544 "valid", the server MUST add an "expires" field to the authorization. 1545 When finalizing an authorization, the server MAY remove the 1546 "combinations" field (if present) or remove any challenges still 1547 pending. The server SHOULD NOT remove challenges with status 1548 "invalid". 1550 Usually, the validation process will take some time, so the client 1551 will need to poll the authorization resource to see when it is 1552 finalized. For challenges where the client can tell when the server 1553 has validated the challenge (e.g., by seeing an HTTP or DNS request 1554 from the server), the client SHOULD NOT begin polling until it has 1555 seen the validation request from the server. 1557 To check on the status of an authorization, the client sends a GET 1558 request to the authorization URI, and the server responds with the 1559 current authorization object. In responding to poll requests while 1560 the validation is still in progress, the server MUST return a 202 1561 (Accepted) response, and MAY include a Retry-After header field to 1562 suggest a polling interval to the client. 1564 GET /acme/authz/asdf HTTP/1.1 1565 Host: example.com 1567 HTTP/1.1 200 OK 1569 { 1570 "status": "valid", 1571 "expires": "2015-03-01T14:09:00Z", 1573 "identifier": { 1574 "type": "dns", 1575 "value": "example.org" 1576 }, 1578 "challenges": [ 1579 { 1580 "type": "http-01" 1581 "status": "valid", 1582 "validated": "2014-12-01T12:05:00Z", 1583 "token": "IlirfxKKXAsHtmzK29Pj8A", 1584 "keyAuthorization": "IlirfxKKXA...vb29HhjjLPSggwiE" 1585 } 1586 ] 1587 } 1589 6.4.2. Deactivating an Authorization 1591 If a client wishes to relinquish its authorization to issue 1592 certificates for an identifier, then it may request that the server 1593 deactivate each authorization associated with that identifier by 1594 sending a POST request with the static object {"status": 1595 "deactivated"}. 1597 POST /acme/authz/asdf HTTP/1.1 1598 Host: example.com 1599 Content-Type: application/jose+json 1601 { 1602 "protected": base64url({ 1603 "alg": "ES256", 1604 "jwk": {...}, 1605 "nonce": "xWCM9lGbIyCgue8di6ueWQ", 1606 "url": "https://example.com/acme/authz/asdf" 1607 }) 1608 "payload": base64url({ 1609 "status": "deactivated" 1610 }), 1611 "signature": "srX9Ji7Le9bjszhu...WTFdtujObzMtZcx4" 1612 } 1614 The server MUST verify that the request is signed by the account key 1615 corresponding to the account that owns the authorization. If the 1616 server accepts the deactivation, it should reply with a 200 (OK) 1617 status code and the current contents of the registration object. 1619 The server MUST NOT treat deactivated authorization objects as 1620 sufficient for issuing certificates. 1622 6.5. Certificate Revocation 1624 To request that a certificate be revoked, the client sends a POST 1625 request to the ACME server's revoke-cert URI. The body of the POST 1626 is a JWS object whose JSON payload contains the certificate to be 1627 revoked: 1629 certificate (required, string): The certificate to be revoked, in 1630 the base64url-encoded version of the DER format. (Note: This 1631 field uses the same modified Base64 encoding rules used elsewhere 1632 in this document, so it is different from PEM.) 1634 reason (optional, int): One of the revocation reasonCodes defined in 1635 RFC 5280 [RFC5280] Section 5.3.1 to be used when generating OCSP 1636 responses and CRLs. If this field is not set the server SHOULD 1637 use the unspecified (0) reasonCode value when generating OCSP 1638 responses and CRLs. The server MAY disallow a subset of 1639 reasonCodes from being used by the user. 1641 POST /acme/revoke-cert HTTP/1.1 1642 Host: example.com 1643 Content-Type: application/jose+json 1645 { 1646 "protected": base64url({ 1647 "alg": "ES256", 1648 "jwk": {...}, 1649 "nonce": "JHb54aT_KTXBWQOzGYkt9A", 1650 "url": "https://example.com/acme/revoke-cert" 1651 }) 1652 "payload": base64url({ 1653 "certificate": "MIIEDTCCAvegAwIBAgIRAP8...", 1654 "reason": 1 1655 }), 1656 "signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4" 1657 } 1659 Revocation requests are different from other ACME request in that 1660 they can be signed either with an account key pair or the key pair in 1661 the certificate. Before revoking a certificate, the server MUST 1662 verify that the key used to sign the request is authorized to revoke 1663 the certificate. The server SHOULD consider at least the following 1664 keys authorized for a given certificate: 1666 o the public key in the certificate. 1668 o an account key that is authorized to act for all of the 1669 identifier(s) in the certificate. 1671 If the revocation succeeds, the server responds with status code 200 1672 (OK). If the revocation fails, the server returns an error. 1674 HTTP/1.1 200 OK 1675 Content-Length: 0 1677 --- or --- 1679 HTTP/1.1 403 Forbidden 1680 Content-Type: application/problem+json 1681 Content-Language: en 1683 { 1684 "type": "urn:ietf:params:acme:error:unauthorized" 1685 "detail": "No authorization provided for name example.net" 1686 "instance": "http://example.com/doc/unauthorized" 1687 } 1689 7. Identifier Validation Challenges 1691 There are few types of identifiers in the world for which there is a 1692 standardized mechanism to prove possession of a given identifier. In 1693 all practical cases, CAs rely on a variety of means to test whether 1694 an entity applying for a certificate with a given identifier actually 1695 controls that identifier. 1697 Challenges provide the server with assurance that an account key 1698 holder is also the entity that controls an identifier. For each type 1699 of challenge, it must be the case that in order for an entity to 1700 successfully complete the challenge the entity must both: 1702 o Hold the private key of the account key pair used to respond to 1703 the challenge 1705 o Control the identifier in question 1707 Section 9 documents how the challenges defined in this document meet 1708 these requirements. New challenges will need to document how they 1709 do. 1711 ACME uses an extensible challenge/response framework for identifier 1712 validation. The server presents a set of challenges in the 1713 authorization object it sends to a client (as objects in the 1714 "challenges" array), and the client responds by sending a response 1715 object in a POST request to a challenge URI. 1717 This section describes an initial set of challenge types. Each 1718 challenge must describe: 1720 1. Content of challenge objects 1722 2. Content of response objects 1724 3. How the server uses the challenge and response to verify control 1725 of an identifier 1727 Challenge objects all contain the following basic fields: 1729 type (required, string): The type of challenge encoded in the 1730 object. 1732 uri (required, string): The URI to which a response can be posted. 1734 status (required, string): The status of this authorization. 1735 Possible values are: "pending", "valid", and "invalid". 1737 validated (optional, string): The time at which this challenge was 1738 completed by the server, encoded in the format specified in RFC 1739 3339 [RFC3339]. This field is REQUIRED if the "status" field is 1740 "valid". 1742 error (optional, dictionary of string): The error that occurred 1743 while the server was validating the challenge, if any. This field 1744 is structured as a problem document [RFC7807]. 1746 All additional fields are specified by the challenge type. If the 1747 server sets a challenge's "status" to "invalid", it SHOULD also 1748 include the "error" field to help the client diagnose why they failed 1749 the challenge. 1751 Different challenges allow the server to obtain proof of different 1752 aspects of control over an identifier. In some challenges, like HTTP 1753 and TLS SNI, the client directly proves its ability to do certain 1754 things related to the identifier. The choice of which challenges to 1755 offer to a client under which circumstances is a matter of server 1756 policy. 1758 The identifier validation challenges described in this section all 1759 relate to validation of domain names. If ACME is extended in the 1760 future to support other types of identifier, there will need to be 1761 new challenge types, and they will need to specify which types of 1762 identifier they apply to. 1764 [[ Editor's Note: In pre-RFC versions of this specification, 1765 challenges are labeled by type, and with the version of the draft in 1766 which they were introduced. For example, if an HTTP challenge were 1767 introduced in version -03 and a breaking change made in version -05, 1768 then there would be a challenge labeled "http-03" and one labeled 1769 "http-05" - but not one labeled "http-04", since challenge in version 1770 -04 was compatible with one in version -04. ]] 1772 [[ Editor's Note: Operators SHOULD NOT issue "combinations" arrays in 1773 authorization objects that require the client to perform multiple 1774 challenges over the same type, e.g., ["http-03", "http-05"]. 1775 Challenges within a type are testing the same capability of the 1776 domain owner, and it may not be possible to satisfy both at once. ]] 1778 7.1. Key Authorizations 1780 Several of the challenges in this document makes use of a key 1781 authorization string. A key authorization is a string that expresses 1782 a domain holder's authorization for a specified key to satisfy a 1783 specified challenge, by concatenating the token for the challenge 1784 with a key fingerprint, separated by a "." character: 1786 key-authz = token || '.' || base64url(JWK\_Thumbprint(accountKey)) 1788 The "JWK_Thumbprint" step indicates the computation specified in 1789 [RFC7638], using the SHA-256 digest. As specified in the individual 1790 challenges below, the token for a challenge is a JSON string 1791 comprised entirely of characters in the URL-safe Base64 alphabet. 1792 The "||" operator indicates concatenation of strings. 1794 In computations involving key authorizations, such as the digest 1795 computations required for the DNS and TLS SNI challenges, the key 1796 authorization string MUST be represented in UTF-8 form (or, 1797 equivalently, ASCII). 1799 An example of how to compute a JWK thumbprint can be found in 1800 Section 3.1 of [RFC7638]. Note that some cryptographic libraries 1801 prepend a zero octet to the representation of the RSA public key 1802 parameters N and E, in order to avoid ambiguity with regard to the 1803 sign of the number. As noted in JWA [RFC7518], a JWK object MUST NOT 1804 include this zero octet. That is, any initial zero octets MUST be 1805 stripped before the values are base64url-encoded. 1807 7.2. HTTP 1809 With HTTP validation, the client in an ACME transaction proves its 1810 control over a domain name by proving that it can provision resources 1811 on an HTTP server that responds for that domain name. The ACME 1812 server challenges the client to provision a file at a specific path, 1813 with a specific string as its content. 1815 As a domain may resolve to multiple IPv4 and IPv6 addresses, the 1816 server will connect to at least one of the hosts found in A and AAAA 1817 records, at its discretion. Because many webservers allocate a 1818 default HTTPS virtual host to a particular low-privilege tenant user 1819 in a subtle and non-intuitive manner, the challenge must be completed 1820 over HTTP, not HTTPS. 1822 type (required, string): The string "http-01" 1824 token (required, string): A random value that uniquely identifies 1825 the challenge. This value MUST have at least 128 bits of entropy, 1826 in order to prevent an attacker from guessing it. It MUST NOT 1827 contain any characters outside the URL-safe Base64 alphabet and 1828 MUST NOT contain any padding characters ("="). 1830 { 1831 "type": "http-01", 1832 "token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA" 1833 } 1834 A client responds to this challenge by constructing a key 1835 authorization from the "token" value provided in the challenge and 1836 the client's account key. The client then provisions the key 1837 authorization as a resource on the HTTP server for the domain in 1838 question. 1840 The path at which the resource is provisioned is comprised of the 1841 fixed prefix ".well-known/acme-challenge/", followed by the "token" 1842 value in the challenge. The value of the resource MUST be the ASCII 1843 representation of the key authorization. 1845 .well-known/acme-challenge/evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA 1847 The client's response to this challenge indicates its agreement to 1848 this challenge by sending the server the key authorization covering 1849 the challenge's token and the client's account key. In addition, the 1850 client MAY advise the server at which IP the challenge is 1851 provisioned. 1853 keyAuthorization (required, string): The key authorization for this 1854 challenge. This value MUST match the token from the challenge and 1855 the client's account key. 1857 /* BEGIN JWS-signed content */ 1858 { 1859 "keyAuthorization": "evaGxfADs...62jcerQ" 1860 } 1861 /* END JWS-signed content */ 1863 On receiving a response, the server MUST verify that the key 1864 authorization in the response matches the "token" value in the 1865 challenge and the client's account key. If they do not match, then 1866 the server MUST return an HTTP error in response to the POST request 1867 in which the client sent the challenge. 1869 Given a challenge/response pair, the server verifies the client's 1870 control of the domain by verifying that the resource was provisioned 1871 as expected. 1873 1. Form a URI by populating the URI template [RFC6570] 1874 "http://{domain}/.well-known/acme-challenge/{token}", where: 1876 * the domain field is set to the domain name being verified; and 1878 * the token field is set to the token in the challenge. 1880 2. Verify that the resulting URI is well-formed. 1882 3. Dereference the URI using an HTTP GET request. 1884 4. Verify that the body of the response is well-formed key 1885 authorization. The server SHOULD ignore whitespace characters at 1886 the end of the body. 1888 5. Verify that key authorization provided by the server matches the 1889 token for this challenge and the client's account key. 1891 If all of the above verifications succeed, then the validation is 1892 successful. If the request fails, or the body does not pass these 1893 checks, then it has failed. 1895 7.3. TLS with Server Name Indication (TLS SNI) 1897 The TLS with Server Name Indication (TLS SNI) validation method 1898 proves control over a domain name by requiring the client to 1899 configure a TLS server referenced by an A/AAAA record under the 1900 domain name to respond to specific connection attempts utilizing the 1901 Server Name Indication extension [RFC6066]. The server verifies the 1902 client's challenge by accessing the reconfigured server and verifying 1903 a particular challenge certificate is presented. 1905 type (required, string): The string "tls-sni-02" 1907 token (required, string): A random value that uniquely identifies 1908 the challenge. This value MUST have at least 128 bits of entropy, 1909 in order to prevent an attacker from guessing it. It MUST NOT 1910 contain any characters outside the URL-safe Base64 alphabet and 1911 MUST NOT contain any padding characters ("="). 1913 { 1914 "type": "tls-sni-02", 1915 "token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA" 1916 } 1918 A client responds to this challenge by constructing a self-signed 1919 certificate which the client MUST provision at the domain name 1920 concerned in order to pass the challenge. 1922 The certificate may be constructed arbitrarily, except that each 1923 certificate MUST have exactly two subjectAlternativeNames, SAN A and 1924 SAN B. Both MUST be dNSNames. 1926 SAN A MUST be constructed as follows: compute the SHA-256 digest of 1927 the UTF-8-encoded challenge token and encode it in lowercase 1928 hexadecimal form. The dNSName is "x.y.token.acme.invalid", where x 1929 is the first half of the hexadecimal representation and y is the 1930 second half. 1932 SAN B MUST be constructed as follows: compute the SHA-256 digest of 1933 the UTF-8 encoded key authorization and encode it in lowercase 1934 hexadecimal form. The dNSName is "x.y.ka.acme.invalid" where x is 1935 the first half of the hexadecimal representation and y is the second 1936 half. 1938 The client MUST ensure that the certificate is served to TLS 1939 connections specifying a Server Name Indication (SNI) value of SAN A. 1941 The response to the TLS-SNI challenge simply acknowledges that the 1942 client is ready to fulfill this challenge. 1944 keyAuthorization (required, string): The key authorization for this 1945 challenge. This value MUST match the token from the challenge and 1946 the client's account key. 1948 /* BEGIN JWS-signed content */ 1949 { 1950 "keyAuthorization": "evaGxfADs...62jcerQ" 1951 } 1952 /* END JWS-signed content */ 1954 On receiving a response, the server MUST verify that the key 1955 authorization in the response matches the "token" value in the 1956 challenge and the client's account key. If they do not match, then 1957 the server MUST return an HTTP error in response to the POST request 1958 in which the client sent the challenge. 1960 Given a challenge/response pair, the ACME server verifies the 1961 client's control of the domain by verifying that the TLS server was 1962 configured appropriately, using these steps: 1964 1. Compute SAN A and SAN B in the same way as the client. 1966 2. Open a TLS connection to the domain name being validated on the 1967 requested port, presenting SAN A in the SNI field. In the 1968 ClientHello initiating the TLS handshake, the server MUST include 1969 a server_name extension (i.e., SNI) containing SAN A. The server 1970 SHOULD ensure that it does not reveal SAN B in any way when 1971 making the TLS connection, such that the presentation of SAN B in 1972 the returned certificate proves association with the client. 1974 3. Verify that the certificate contains a subjectAltName extension 1975 containing dNSName entries of SAN A and SAN B and no other 1976 entries. The comparison MUST be insensitive to case and ordering 1977 of names. 1979 It is RECOMMENDED that the ACME server validation TLS connections 1980 from multiple vantage points to reduce the risk of DNS hijacking 1981 attacks. 1983 If all of the above verifications succeed, then the validation is 1984 successful. Otherwise, the validation fails. 1986 7.4. DNS 1988 When the identifier being validated is a domain name, the client can 1989 prove control of that domain by provisioning a resource record under 1990 it. The DNS challenge requires the client to provision a TXT record 1991 containing a designated value under a specific validation domain 1992 name. 1994 type (required, string): The string "dns-01" 1996 token (required, string): A random value that uniquely identifies 1997 the challenge. This value MUST have at least 128 bits of entropy, 1998 in order to prevent an attacker from guessing it. It MUST NOT 1999 contain any characters outside the URL-safe Base64 alphabet and 2000 MUST NOT contain any padding characters ("="). 2002 { 2003 "type": "dns-01", 2004 "token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA" 2005 } 2007 A client responds to this challenge by constructing a key 2008 authorization from the "token" value provided in the challenge and 2009 the client's account key. The client then computes the SHA-256 2010 digest of the key authorization. 2012 The record provisioned to the DNS is the base64url encoding of this 2013 digest. The client constructs the validation domain name by 2014 prepending the label "_acme-challenge" to the domain name being 2015 validated, then provisions a TXT record with the digest value under 2016 that name. For example, if the domain name being validated is 2017 "example.com", then the client would provision the following DNS 2018 record: 2020 _acme-challenge.example.com. 300 IN TXT "gfj9Xq...Rg85nM" 2021 The response to the DNS challenge provides the computed key 2022 authorization to acknowledge that the client is ready to fulfill this 2023 challenge. 2025 keyAuthorization (required, string): The key authorization for this 2026 challenge. This value MUST match the token from the challenge and 2027 the client's account key. 2029 /* BEGIN JWS-signed content */ 2030 { 2031 "keyAuthorization": "evaGxfADs...62jcerQ" 2032 } 2033 /* END JWS-signed content */ 2035 On receiving a response, the server MUST verify that the key 2036 authorization in the response matches the "token" value in the 2037 challenge and the client's account key. If they do not match, then 2038 the server MUST return an HTTP error in response to the POST request 2039 in which the client sent the challenge. 2041 To validate a DNS challenge, the server performs the following steps: 2043 1. Compute the SHA-256 digest of the key authorization 2045 2. Query for TXT records under the validation domain name 2047 3. Verify that the contents of one of the TXT records matches the 2048 digest value 2050 If all of the above verifications succeed, then the validation is 2051 successful. If no DNS record is found, or DNS record and response 2052 payload do not pass these checks, then the validation fails. 2054 7.5. Out-of-Band 2056 There may be cases where a server cannot perform automated validation 2057 of an identifier, for example if validation requires some manual 2058 steps. In such cases, the server may provide an "out of band" (OOB) 2059 challenge to request that the client perform some action outside of 2060 ACME in order to validate possession of the identifier. 2062 The OOB challenge requests that the client have a human user visit a 2063 web page to receive instructions on how to validate possession of the 2064 identifier, by providing a URL for that web page. 2066 type (required, string): The string "oob-01" 2067 url (required, string): The URL to be visited. The scheme of this 2068 URL MUST be "http" or "https" 2070 { 2071 "type": "oob-01", 2072 "url": "https://example.com/validate/evaGxfADs6pSRb2LAv9IZ" 2073 } 2075 A client responds to this challenge by presenting the indicated URL 2076 for a human user to navigate to. If the user choses to complete this 2077 challege (by vising the website and completing its instructions), the 2078 client indicates this by sending a simple acknowledgement response to 2079 the server. 2081 type (required, string): The string "oob-01" 2083 /* BEGIN JWS-signed content */ 2084 { 2085 "type": "oob-01" 2086 } 2087 /* END JWS-signed content */ 2089 On receiving a response, the server MUST verify that the value of the 2090 "type" field is as required. Otherwise, the steps the server takes 2091 to validate identifier possession are determined by the server's 2092 local policy. 2094 8. IANA Considerations 2096 [[ Editor's Note: Should we create a registry for tokens that go into 2097 the various JSON objects used by this protocol, i.e., the field names 2098 in the JSON objects? ]] 2100 8.1. Well-Known URI for the HTTP Challenge 2102 The "Well-Known URIs" registry should be updated with the following 2103 additional value (using the template from [RFC5785]): 2105 URI suffix: acme-challenge 2107 Change controller: IETF 2109 Specification document(s): This document, Section Section 7.2 2111 Related information: N/A 2113 8.2. Replay-Nonce HTTP Header 2115 The "Message Headers" registry should be updated with the following 2116 additional value: 2118 | Header Field Name | Protocol | Status | Reference | 2119 +:------------+:------+:------+:-----------+ | Replay-Nonce | http | 2120 standard | Section 5.4.1 | 2122 8.3. "url" JWS Header Parameter 2124 The "JSON Web Signature and Encryption Header Parameters" registry 2125 should be updated with the following additional value: 2127 o Header Parameter Name: "url" 2129 o Header Parameter Description: URL 2131 o Header Parameter Usage Location(s): JWE, JWS 2133 o Change Controller: IESG 2135 o Specification Document(s): Section 5.3.1 of RFC XXXX 2137 [[ RFC EDITOR: Please replace XXXX above with the RFC number assigned 2138 to this document ]] 2140 8.4. "nonce" JWS Header Parameter 2142 The "JSON Web Signature and Encryption Header Parameters" registry 2143 should be updated with the following additional value: 2145 o Header Parameter Name: "nonce" 2147 o Header Parameter Description: Nonce 2149 o Header Parameter Usage Location(s): JWE, JWS 2151 o Change Controller: IESG 2153 o Specification Document(s): Section 5.4.2 of RFC XXXX 2155 [[ RFC EDITOR: Please replace XXXX above with the RFC number assigned 2156 to this document ]] 2158 8.5. URN Sub-namespace for ACME (urn:ietf:params:acme) 2160 The "IETF URN Sub-namespace for Registered Protocol Parameter 2161 Identifiers" registry should be updated with the following additional 2162 value, following the template in [RFC3553]: 2164 Registry name: acme 2166 Specification: RFC XXXX 2168 Repository: URL-TBD 2170 Index value: No transformation needed. The 2172 [[ RFC EDITOR: Please replace XXXX above with the RFC number assigned 2173 to this document, and replace URL-TBD with the URL assigned by IANA 2174 for registries of ACME parameters. ]] 2176 8.6. New Registries 2178 This document requests that IANA create the following new registries: 2180 1. ACME Error Codes 2182 2. ACME Resource Types 2184 3. ACME Identifier Types 2186 4. ACME Challenge Types 2188 All of these registries should be administered under a Specification 2189 Required policy [RFC5226]. 2191 8.6.1. Error Codes 2193 This registry lists values that are used within URN values that are 2194 provided in the "type" field of problem documents in ACME. 2196 Template: 2198 o Code: The label to be included in the URN for this error, 2199 following "urn:ietf:params:acme:" 2201 o Description: A human-readable description of the error 2203 o Reference: Where the error is defined 2204 Initial contents: The codes and descriptions in the table in 2205 Section 5.6 above, with the Reference field set to point to this 2206 specification. 2208 8.6.2. Resource Types 2210 This registry lists the types of resources that ACME servers may list 2211 in their directory objects. 2213 Template: 2215 o Key: The value to be used as a dictionary key in the directory 2216 object 2218 o Resource type: The type of resource labeled by the key 2220 o Reference: Where the identifier type is defined 2222 Initial contents: 2224 +-------------+--------------------+-----------+ 2225 | Key | Resource type | Reference | 2226 +-------------+--------------------+-----------+ 2227 | new-reg | New registration | RFC XXXX | 2228 | | | | 2229 | new-app | New application | RFC XXXX | 2230 | | | | 2231 | revoke-cert | Revoke certificate | RFC XXXX | 2232 | | | | 2233 | key-change | Key change | RFC XXXX | 2234 +-------------+--------------------+-----------+ 2236 [[ RFC EDITOR: Please replace XXXX above with the RFC number assigned 2237 to this document ]] 2239 8.6.3. Identifier Types 2241 This registry lists the types of identifiers that ACME clients may 2242 request authorization to issue in certificates. 2244 Template: 2246 o Label: The value to be put in the "type" field of the identifier 2247 object 2249 o Reference: Where the identifier type is defined 2251 Initial contents: 2253 +-------+-----------+ 2254 | Label | Reference | 2255 +-------+-----------+ 2256 | dns | RFC XXXX | 2257 +-------+-----------+ 2259 [[ RFC EDITOR: Please replace XXXX above with the RFC number assigned 2260 to this document ]] 2262 8.6.4. Challenge Types 2264 This registry lists the ways that ACME servers can offer to validate 2265 control of an identifier. The "Identifier Type" field in template 2266 MUST be contained in the Label column of the ACME Identifier Types 2267 registry. 2269 Template: 2271 o Label: The value to be put in the "type" field of challenge 2272 objects using this validation mechanism 2274 o Identifier Type: The type of identifier that this mechanism 2275 applies to 2277 o Reference: Where the challenge type is defined 2279 Initial Contents 2281 +---------+-----------------+-----------+ 2282 | Label | Identifier Type | Reference | 2283 +---------+-----------------+-----------+ 2284 | http | dns | RFC XXXX | 2285 | | | | 2286 | tls-sni | dns | RFC XXXX | 2287 | | | | 2288 | dns | dns | RFC XXXX | 2289 +---------+-----------------+-----------+ 2291 [[ RFC EDITOR: Please replace XXXX above with the RFC number assigned 2292 to this document ]] 2294 9. Security Considerations 2296 ACME is a protocol for managing certificates that attest to 2297 identifier/key bindings. Thus the foremost security goal of ACME is 2298 to ensure the integrity of this process, i.e., to ensure that the 2299 bindings attested by certificates are correct, and that only 2300 authorized entities can manage certificates. ACME identifies clients 2301 by their account keys, so this overall goal breaks down into two more 2302 precise goals: 2304 1. Only an entity that controls an identifier can get an account key 2305 authorized for that identifier 2307 2. Once authorized, an account key's authorizations cannot be 2308 improperly transferred to another account key 2310 In this section, we discuss the threat model that underlies ACME and 2311 the ways that ACME achieves these security goals within that threat 2312 model. We also discuss the denial-of-service risks that ACME servers 2313 face, and a few other miscellaneous considerations. 2315 9.1. Threat model 2317 As a service on the Internet, ACME broadly exists within the Internet 2318 threat model [RFC3552]. In analyzing ACME, it is useful to think of 2319 an ACME server interacting with other Internet hosts along three 2320 "channels": 2322 o An ACME channel, over which the ACME HTTPS requests are exchanged 2324 o A validation channel, over which the ACME server performs 2325 additional requests to validate a client's control of an 2326 identifier 2328 o A contact channel, over which the ACME server sends messages to 2329 the registered contacts for ACME clients 2331 +------------+ 2332 | ACME | ACME Channel 2333 | Client |--------------------+ 2334 +------------+ | 2335 ^ V 2336 | Contact Channel +------------+ 2337 +--------------------| ACME | 2338 | Server | 2339 +------------+ 2340 +------------+ | 2341 | Validation |<-------------------+ 2342 | Server | Validation Channel 2343 +------------+ 2345 In practice, the risks to these channels are not entirely separate, 2346 but they are different in most cases. Each of the three channels, 2347 for example, uses a different communications pattern: the ACME 2348 channel will comprise inbound HTTPS connections to the ACME server, 2349 the validation channel outbound HTTP or DNS requests, and the contact 2350 channel will use channels such as email and PSTN. 2352 Broadly speaking, ACME aims to be secure against active and passive 2353 attackers on any individual channel. Some vulnerabilities arise 2354 (noted below), when an attacker can exploit both the ACME channel and 2355 one of the others. 2357 On the ACME channel, in addition to network-layer attackers, we also 2358 need to account for application-layer man in the middle attacks, and 2359 for abusive use of the protocol itself. Protection against 2360 application-layer MitM addresses potential attackers such as Content 2361 Distribution Networks (CDNs) and middleboxes with a TLS MitM 2362 function. Preventing abusive use of ACME means ensuring that an 2363 attacker with access to the validation or contact channels can't 2364 obtain illegitimate authorization by acting as an ACME client 2365 (legitimately, in terms of the protocol). 2367 9.2. Integrity of Authorizations 2369 ACME allows anyone to request challenges for an identifier by 2370 registering an account key and sending a new-application request 2371 under that account key. The integrity of the authorization process 2372 thus depends on the identifier validation challenges to ensure that 2373 the challenge can only be completed by someone who both (1) holds the 2374 private key of the account key pair, and (2) controls the identifier 2375 in question. 2377 Validation responses need to be bound to an account key pair in order 2378 to avoid situations where an ACME MitM can switch out a legitimate 2379 domain holder's account key for one of his choosing, e.g.: 2381 o Legitimate domain holder registers account key pair A 2383 o MitM registers account key pair B 2385 o Legitimate domain holder sends a new-application request signed 2386 under account key A 2388 o MitM suppresses the legitimate request, but sends the same request 2389 signed under account key B 2391 o ACME server issues challenges and MitM forwards them to the 2392 legitimate domain holder 2394 o Legitimate domain holder provisions the validation response 2395 o ACME server performs validation query and sees the response 2396 provisioned by the legitimate domain holder 2398 o Because the challenges were issued in response to a message signed 2399 account key B, the ACME server grants authorization to account key 2400 B (the MitM) instead of account key A (the legitimate domain 2401 holder) 2403 All of the challenges above have a binding between the account 2404 private key and the validation query made by the server, via the key 2405 authorization. The key authorization is signed by the account 2406 private key, reflects the corresponding public key, and is provided 2407 to the server in the validation response. 2409 The association of challenges to identifiers is typically done by 2410 requiring the client to perform some action that only someone who 2411 effectively controls the identifier can perform. For the challenges 2412 in this document, the actions are: 2414 o HTTP: Provision files under .well-known on a web server for the 2415 domain 2417 o TLS SNI: Configure a TLS server for the domain 2419 o DNS: Provision DNS resource records for the domain 2421 There are several ways that these assumptions can be violated, both 2422 by misconfiguration and by attack. For example, on a web server that 2423 allows non-administrative users to write to .well-known, any user can 2424 claim to own the server's hostname by responding to an HTTP 2425 challenge, and likewise for TLS configuration and TLS SNI. 2427 The use of hosting providers is a particular risk for ACME 2428 validation. If the owner of the domain has outsourced operation of 2429 DNS or web services to a hosting provider, there is nothing that can 2430 be done against tampering by the hosting provider. As far as the 2431 outside world is concerned, the zone or web site provided by the 2432 hosting provider is the real thing. 2434 More limited forms of delegation can also lead to an unintended party 2435 gaining the ability to successfully complete a validation 2436 transaction. For example, suppose an ACME server follows HTTP 2437 redirects in HTTP validation and a web site operator provisions a 2438 catch-all redirect rule that redirects requests for unknown resources 2439 to a different domain. Then the target of the redirect could use 2440 that to get a certificate through HTTP validation, since the 2441 validation path will not be known to the primary server. 2443 The DNS is a common point of vulnerability for all of these 2444 challenges. An entity that can provision false DNS records for a 2445 domain can attack the DNS challenge directly, and can provision false 2446 A/AAAA records to direct the ACME server to send its TLS SNI or HTTP 2447 validation query to a server of the attacker's choosing. There are a 2448 few different mitigations that ACME servers can apply: 2450 o Always querying the DNS using a DNSSEC-validating resolver 2451 (enhancing security for zones that are DNSSEC-enabled) 2453 o Querying the DNS from multiple vantage points to address local 2454 attackers 2456 o Applying mitigations against DNS off-path attackers, e.g., adding 2457 entropy to requests [I-D.vixie-dnsext-dns0x20] or only using TCP 2459 Given these considerations, the ACME validation process makes it 2460 impossible for any attacker on the ACME channel, or a passive 2461 attacker on the validation channel to hijack the authorization 2462 process to authorize a key of the attacker's choice. 2464 An attacker that can only see the ACME channel would need to convince 2465 the validation server to provide a response that would authorize the 2466 attacker's account key, but this is prevented by binding the 2467 validation response to the account key used to request challenges. A 2468 passive attacker on the validation channel can observe the correct 2469 validation response and even replay it, but that response can only be 2470 used with the account key for which it was generated. 2472 An active attacker on the validation channel can subvert the ACME 2473 process, by performing normal ACME transactions and providing a 2474 validation response for his own account key. The risks due to 2475 hosting providers noted above are a particular case. For identifiers 2476 where the server already has some public key associated with the 2477 domain this attack can be prevented by requiring the client to prove 2478 control of the corresponding private key. 2480 9.3. Denial-of-Service Considerations 2482 As a protocol run over HTTPS, standard considerations for TCP-based 2483 and HTTP-based DoS mitigation also apply to ACME. 2485 At the application layer, ACME requires the server to perform a few 2486 potentially expensive operations. Identifier validation transactions 2487 require the ACME server to make outbound connections to potentially 2488 attacker-controlled servers, and certificate issuance can require 2489 interactions with cryptographic hardware. 2491 In addition, an attacker can also cause the ACME server to send 2492 validation requests to a domain of its choosing by submitting 2493 authorization requests for the victim domain. 2495 All of these attacks can be mitigated by the application of 2496 appropriate rate limits. Issues closer to the front end, like POST 2497 body validation, can be addressed using HTTP request limiting. For 2498 validation and certificate requests, there are other identifiers on 2499 which rate limits can be keyed. For example, the server might limit 2500 the rate at which any individual account key can issue certificates, 2501 or the rate at which validation can be requested within a given 2502 subtree of the DNS. 2504 9.4. Server-Side Request Forgery 2506 Server-Side Request Forgery (SSRF) attacks can arise when an attacker 2507 can cause a server to perform HTTP requests to an attacker-chosen 2508 URL. In the ACME HTTP challenge validation process, the ACME server 2509 performs an HTTP GET request to a URL in which the attacker can 2510 choose the domain. This request is made before the server has 2511 verified that the client controls the domain, so any client can cause 2512 a query to any domain. 2514 Some server implementations include information from the validation 2515 server's response (in order to facilitate debugging). Such 2516 implementations enable an attacker to extract this information from 2517 any web server that is accessible to the ACME server, even if it is 2518 not accessible to the ACME client. 2520 It might seem that the risk of SSRF through this channel is limited 2521 by the fact that the attacker can only control the domain of the URL, 2522 not the path. However, if the attacker first sets the domain to one 2523 they control, then they can send the server an HTTP redirect (e.g., a 2524 302 response) which will cause the server to query an arbitrary URI. 2526 In order to further limit the SSRF risk, ACME server operators should 2527 ensure that validation queries can only be sent to servers on the 2528 public Internet, and not, say, web services within the server 2529 operator's internal network. Since the attacker could make requests 2530 to these public servers himself, he can't gain anything extra through 2531 an SSRF attack on ACME aside from a layer of anonymization. 2533 9.5. CA Policy Considerations 2535 The controls on issuance enabled by ACME are focused on validating 2536 that a certificate applicant controls the identifier he claims. 2537 Before issuing a certificate, however, there are many other checks 2538 that a CA might need to perform, for example: 2540 o Has the client agreed to a subscriber agreement? 2542 o Is the claimed identifier syntactically valid? 2544 o For domain names: 2546 * If the leftmost label is a '*', then have the appropriate 2547 checks been applied? 2549 * Is the name on the Public Suffix List? 2551 * Is the name a high-value name? 2553 * Is the name a known phishing domain? 2555 o Is the key in the CSR sufficiently strong? 2557 o Is the CSR signed with an acceptable algorithm? 2559 CAs that use ACME to automate issuance will need to ensure that their 2560 servers perform all necessary checks before issuing. 2562 10. Operational Considerations 2564 There are certain factors that arise in operational reality that 2565 operators of ACME-based CAs will need to keep in mind when 2566 configuring their services. For example: 2568 10.1. DNS over TCP 2570 As noted above, DNS forgery attacks against the ACME server can 2571 result in the server making incorrect decisions about domain control 2572 and thus mis-issuing certificates. Servers SHOULD verify DNSSEC when 2573 it is available for a domain. When DNSSEC is not available, servers 2574 SHOULD perform DNS queries over TCP, which provides better resistance 2575 to some forgery attacks than DNS over UDP. 2577 10.2. Default Virtual Hosts 2579 In many cases, TLS-based services are deployed on hosted platforms 2580 that use the Server Name Indication (SNI) TLS extension to 2581 distinguish between different hosted services or "virtual hosts". 2582 When a client initiates a TLS connection with an SNI value indicating 2583 a provisioned host, the hosting platform routes the connection to 2584 that host. 2586 When a connection comes in with an unknown SNI value, one might 2587 expect the hosting platform to terminate the TLS connection. 2589 However, some hosting platforms will choose a virtual host to be the 2590 "default", and route connections with unknown SNI values to that 2591 host. 2593 In such cases, the owner of the default virtual host can complete a 2594 TLS-based challenge (e.g., "tls-sni-02") for any domain with an A 2595 record that points to the hosting platform. This could result in 2596 mis-issuance in cases where there are multiple hosts with different 2597 owners resident on the hosting platform. 2599 A CA that accepts TLS-based proof of domain control should attempt to 2600 check whether a domain is hosted on a domain with a default virtual 2601 host before allowing an authorization request for this host to use a 2602 TLS-based challenge. A default virtual host can be detected by 2603 initiating TLS connections to the host with random SNI values within 2604 the namespace used for the TLS-based challenge (the "acme.invalid" 2605 namespace for "tls-sni-02"). 2607 10.3. Use of DNSSEC Resolvers 2609 An ACME-based CA will often need to make DNS queries, e.g., to 2610 validate control of DNS names. Because the security of such 2611 validations ultimately depends on the authenticity of DNS data, every 2612 possible precaution should be taken to secure DNS queries done by the 2613 CA. It is therefore RECOMMENDED that ACME-based CAs make all DNS 2614 queries via DNSSEC-validating stub or recursive resolvers. This 2615 provides additional protection to domains which choose to make use of 2616 DNSSEC. 2618 An ACME-based CA must use only a resolver if it trusts the resolver 2619 and every component of the network route by which it is accessed. It 2620 is therefore RECOMMENDED that ACME-based CAs operate their own 2621 DNSSEC-validating resolvers within their trusted network and use 2622 these resolvers both for both CAA record lookups and all record 2623 lookups in furtherance of a challenge scheme (A, AAAA, TXT, etc.). 2625 11. Acknowledgements 2627 In addition to the editors listed on the front page, this document 2628 has benefited from contributions from a broad set of contributors, 2629 all the way back to its inception. 2631 o Peter Eckersley, EFF 2633 o Eric Rescorla, Mozilla 2635 o Seth Schoen, EFF 2636 o Alex Halderman, University of Michigan 2638 o Martin Thomson, Mozilla 2640 o Jakub Warmuz, University of Oxford 2642 This document draws on many concepts established by Eric Rescorla's 2643 "Automated Certificate Issuance Protocol" draft. Martin Thomson 2644 provided helpful guidance in the use of HTTP. 2646 12. References 2648 12.1. Normative References 2650 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 2651 Requirement Levels", BCP 14, RFC 2119, 2652 DOI 10.17487/RFC2119, March 1997, 2653 . 2655 [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, 2656 DOI 10.17487/RFC2818, May 2000, 2657 . 2659 [RFC2985] Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object 2660 Classes and Attribute Types Version 2.0", RFC 2985, 2661 DOI 10.17487/RFC2985, November 2000, 2662 . 2664 [RFC2986] Nystrom, M. and B. Kaliski, "PKCS #10: Certification 2665 Request Syntax Specification Version 1.7", RFC 2986, 2666 DOI 10.17487/RFC2986, November 2000, 2667 . 2669 [RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet: 2670 Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002, 2671 . 2673 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 2674 Resource Identifier (URI): Generic Syntax", STD 66, 2675 RFC 3986, DOI 10.17487/RFC3986, January 2005, 2676 . 2678 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data 2679 Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006, 2680 . 2682 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 2683 (TLS) Protocol Version 1.2", RFC 5246, 2684 DOI 10.17487/RFC5246, August 2008, 2685 . 2687 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 2688 Housley, R., and W. Polk, "Internet X.509 Public Key 2689 Infrastructure Certificate and Certificate Revocation List 2690 (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, 2691 . 2693 [RFC5988] Nottingham, M., "Web Linking", RFC 5988, 2694 DOI 10.17487/RFC5988, October 2010, 2695 . 2697 [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) 2698 Extensions: Extension Definitions", RFC 6066, 2699 DOI 10.17487/RFC6066, January 2011, 2700 . 2702 [RFC6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M., 2703 and D. Orchard, "URI Template", RFC 6570, 2704 DOI 10.17487/RFC6570, March 2012, 2705 . 2707 [RFC6844] Hallam-Baker, P. and R. Stradling, "DNS Certification 2708 Authority Authorization (CAA) Resource Record", RFC 6844, 2709 DOI 10.17487/RFC6844, January 2013, 2710 . 2712 [RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data 2713 Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March 2714 2014, . 2716 [RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web 2717 Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May 2718 2015, . 2720 [RFC7517] Jones, M., "JSON Web Key (JWK)", RFC 7517, 2721 DOI 10.17487/RFC7517, May 2015, 2722 . 2724 [RFC7518] Jones, M., "JSON Web Algorithms (JWA)", RFC 7518, 2725 DOI 10.17487/RFC7518, May 2015, 2726 . 2728 [RFC7638] Jones, M. and N. Sakimura, "JSON Web Key (JWK) 2729 Thumbprint", RFC 7638, DOI 10.17487/RFC7638, September 2730 2015, . 2732 [RFC7807] Nottingham, M. and E. Wilde, "Problem Details for HTTP 2733 APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016, 2734 . 2736 12.2. Informative References 2738 [I-D.vixie-dnsext-dns0x20] 2739 Vixie, P. and D. Dagon, "Use of Bit 0x20 in DNS Labels to 2740 Improve Transaction Identity", draft-vixie-dnsext- 2741 dns0x20-00 (work in progress), March 2008. 2743 [RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC 2744 Text on Security Considerations", BCP 72, RFC 3552, 2745 DOI 10.17487/RFC3552, July 2003, 2746 . 2748 [RFC3553] Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An 2749 IETF URN Sub-namespace for Registered Protocol 2750 Parameters", BCP 73, RFC 3553, DOI 10.17487/RFC3553, June 2751 2003, . 2753 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 2754 IANA Considerations Section in RFCs", BCP 26, RFC 5226, 2755 DOI 10.17487/RFC5226, May 2008, 2756 . 2758 [RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known 2759 Uniform Resource Identifiers (URIs)", RFC 5785, 2760 DOI 10.17487/RFC5785, April 2010, 2761 . 2763 [RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate 2764 Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013, 2765 . 2767 [RFC7469] Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning 2768 Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April 2769 2015, . 2771 [W3C.CR-cors-20130129] 2772 Kesteren, A., "Cross-Origin Resource Sharing", World Wide 2773 Web Consortium CR CR-cors-20130129, January 2013, 2774 . 2776 Authors' Addresses 2778 Richard Barnes 2779 Mozilla 2781 Email: rlb@ipv.sx 2783 Jacob Hoffman-Andrews 2784 EFF 2786 Email: jsha@eff.org 2788 James Kasten 2789 University of Michigan 2791 Email: jdkasten@umich.edu