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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: August 7, 2017 EFF 6 J. Kasten 7 University of Michigan 8 February 03, 2017 10 Automatic Certificate Management Environment (ACME) 11 draft-ietf-acme-acme-05 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 August 7, 2017. 54 Copyright Notice 56 Copyright (c) 2017 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 . . . . . . . . . . 5 73 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6 74 4. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 6 75 5. Message Transport . . . . . . . . . . . . . . . . . . . . . . 8 76 5.1. HTTPS Requests . . . . . . . . . . . . . . . . . . . . . 8 77 5.2. Request Authentication . . . . . . . . . . . . . . . . . 9 78 5.3. Equivalence of JWKs . . . . . . . . . . . . . . . . . . . 11 79 5.4. Request URI Integrity . . . . . . . . . . . . . . . . . . 11 80 5.4.1. "url" (URL) JWS header parameter . . . . . . . . . . 12 81 5.5. Replay protection . . . . . . . . . . . . . . . . . . . . 12 82 5.5.1. Replay-Nonce . . . . . . . . . . . . . . . . . . . . 13 83 5.5.2. "nonce" (Nonce) JWS header parameter . . . . . . . . 13 84 5.6. Rate limits . . . . . . . . . . . . . . . . . . . . . . . 13 85 5.7. Errors . . . . . . . . . . . . . . . . . . . . . . . . . 14 86 6. Certificate Management . . . . . . . . . . . . . . . . . . . 16 87 6.1. Resources . . . . . . . . . . . . . . . . . . . . . . . . 16 88 6.1.1. Directory . . . . . . . . . . . . . . . . . . . . . . 18 89 6.1.2. Account Objects . . . . . . . . . . . . . . . . . . . 20 90 6.1.3. Order Objects . . . . . . . . . . . . . . . . . . . . 21 91 6.1.4. Authorization Objects . . . . . . . . . . . . . . . . 23 92 6.2. Getting a Nonce . . . . . . . . . . . . . . . . . . . . . 24 93 6.3. Account Creation . . . . . . . . . . . . . . . . . . . . 25 94 6.3.1. Changes of Terms of Service . . . . . . . . . . . . . 27 95 6.3.2. External Account Binding . . . . . . . . . . . . . . 28 96 6.3.3. Account Key Roll-over . . . . . . . . . . . . . . . . 30 97 6.3.4. Account deactivation . . . . . . . . . . . . . . . . 32 98 6.4. Applying for Certificate Issuance . . . . . . . . . . . . 33 99 6.4.1. Pre-Authorization . . . . . . . . . . . . . . . . . . 35 100 6.4.2. Downloading the Certificate . . . . . . . . . . . . . 37 101 6.5. Identifier Authorization . . . . . . . . . . . . . . . . 39 102 6.5.1. Responding to Challenges . . . . . . . . . . . . . . 40 103 6.5.2. Deactivating an Authorization . . . . . . . . . . . . 42 104 6.6. Certificate Revocation . . . . . . . . . . . . . . . . . 43 105 7. Identifier Validation Challenges . . . . . . . . . . . . . . 45 106 7.1. Key Authorizations . . . . . . . . . . . . . . . . . . . 47 107 7.2. HTTP . . . . . . . . . . . . . . . . . . . . . . . . . . 47 108 7.3. TLS with Server Name Indication (TLS SNI) . . . . . . . . 50 109 7.4. DNS . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 110 7.5. Out-of-Band . . . . . . . . . . . . . . . . . . . . . . . 54 111 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 55 112 8.1. Well-Known URI for the HTTP Challenge . . . . . . . . . . 55 113 8.2. Replay-Nonce HTTP Header . . . . . . . . . . . . . . . . 55 114 8.3. "url" JWS Header Parameter . . . . . . . . . . . . . . . 56 115 8.4. "nonce" JWS Header Parameter . . . . . . . . . . . . . . 56 116 8.5. URN Sub-namespace for ACME (urn:ietf:params:acme) . . . . 56 117 8.6. New Registries . . . . . . . . . . . . . . . . . . . . . 57 118 8.6.1. Fields in Account Objects . . . . . . . . . . . . . . 57 119 8.6.2. Fields in Order Objects . . . . . . . . . . . . . . . 58 120 8.6.3. Error Codes . . . . . . . . . . . . . . . . . . . . . 59 121 8.6.4. Resource Types . . . . . . . . . . . . . . . . . . . 59 122 8.6.5. Identifier Types . . . . . . . . . . . . . . . . . . 60 123 8.6.6. Challenge Types . . . . . . . . . . . . . . . . . . . 60 124 9. Security Considerations . . . . . . . . . . . . . . . . . . . 61 125 9.1. Threat model . . . . . . . . . . . . . . . . . . . . . . 62 126 9.2. Integrity of Authorizations . . . . . . . . . . . . . . . 63 127 9.3. Denial-of-Service Considerations . . . . . . . . . . . . 65 128 9.4. Server-Side Request Forgery . . . . . . . . . . . . . . . 65 129 9.5. CA Policy Considerations . . . . . . . . . . . . . . . . 66 130 10. Operational Considerations . . . . . . . . . . . . . . . . . 67 131 10.1. DNS over TCP . . . . . . . . . . . . . . . . . . . . . . 67 132 10.2. Default Virtual Hosts . . . . . . . . . . . . . . . . . 67 133 10.3. Use of DNSSEC Resolvers . . . . . . . . . . . . . . . . 68 134 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 68 135 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 69 136 12.1. Normative References . . . . . . . . . . . . . . . . . . 69 137 12.2. Informative References . . . . . . . . . . . . . . . . . 71 138 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 72 140 1. Introduction 142 Certificates in the Web PKI [RFC5280] are most commonly used to 143 authenticate domain names. Thus, certificate authorities in the Web 144 PKI are trusted to verify that an applicant for a certificate 145 legitimately represents the domain name(s) in the certificate. 147 Different types of certificates reflect different kinds of CA 148 verification of information about the certificate subject. "Domain 149 Validation" (DV) certificates are by far the most common type. For 150 DV validation, the CA merely verifies that the requester has 151 effective control of the web server and/or DNS server for the domain, 152 but does not explicitly attempt to verify their real-world identity. 153 (This is as opposed to "Organization Validation" (OV) and "Extended 154 Validation" (EV) certificates, where the process is intended to also 155 verify the real-world identity of the requester.) 157 Existing Web PKI certificate authorities tend to run on a set of ad 158 hoc protocols for certificate issuance and identity verification. In 159 the case of DV certificates, a typical user experience is something 160 like: 162 o Generate a PKCS#10 [RFC2986] Certificate Signing Request (CSR). 164 o Cut-and-paste the CSR into a CA web page. 166 o Prove ownership of the domain by one of the following methods: 168 * Put a CA-provided challenge at a specific place on the web 169 server. 171 * Put a CA-provided challenge at a DNS location corresponding to 172 the target domain. 174 * Receive CA challenge at a (hopefully) administrator-controlled 175 e-mail address corresponding to the domain and then respond to 176 it on the CA's web page. 178 o Download the issued certificate and install it on their Web 179 Server. 181 With the exception of the CSR itself and the certificates that are 182 issued, these are all completely ad hoc procedures and are 183 accomplished by getting the human user to follow interactive natural- 184 language instructions from the CA rather than by machine-implemented 185 published protocols. In many cases, the instructions are difficult 186 to follow and cause significant confusion. Informal usability tests 187 by the authors indicate that webmasters often need 1-3 hours to 188 obtain and install a certificate for a domain. Even in the best 189 case, the lack of published, standardized mechanisms presents an 190 obstacle to the wide deployment of HTTPS and other PKIX-dependent 191 systems because it inhibits mechanization of tasks related to 192 certificate issuance, deployment, and revocation. 194 This document describes an extensible framework for automating the 195 issuance and domain validation procedure, thereby allowing servers 196 and infrastructural software to obtain certificates without user 197 interaction. Use of this protocol should radically simplify the 198 deployment of HTTPS and the practicality of PKIX authentication for 199 other protocols based on TLS [RFC5246]. 201 2. Deployment Model and Operator Experience 203 The guiding use case for ACME is obtaining certificates for Web sites 204 (HTTPS [RFC2818]). In this case, the user's web server is intended 205 to speak for one or more domains, and the process of certificate 206 issuance is intended to verify that this server actually speaks for 207 the domain(s). 209 DV certificate validation commonly checks claims about properties 210 related to control of a domain name - properties that can be observed 211 by the issuing authority in an interactive process that can be 212 conducted purely online. That means that under typical 213 circumstances, all steps in the request, verification, and issuance 214 process can be represented and performed by Internet protocols with 215 no out-of-band human intervention. 217 At time of writing, when deploying an HTTPS server, an operator 218 typically gets a prompt to generate a self-signed certificate. If 219 the operator were instead deploying an ACME-compatible web server, 220 the experience would be something like this: 222 o The ACME client prompts the operator for the intended domain 223 name(s) that the web server is to stand for. 225 o The ACME client presents the operator with a list of CAs from 226 which it could get a certificate. (This list will change over 227 time based on the capabilities of CAs and updates to ACME 228 configuration.) The ACME client might prompt the operator for 229 payment information at this point. 231 o The operator selects a CA. 233 o In the background, the ACME client contacts the CA and requests 234 that a certificate be issued for the intended domain name(s). 236 o Once the CA is satisfied, the certificate is issued and the ACME 237 client automatically downloads and installs it, potentially 238 notifying the operator via e-mail, SMS, etc. 240 o The ACME client periodically contacts the CA to get updated 241 certificates, stapled OCSP responses, or whatever else would be 242 required to keep the server functional and its credentials up-to- 243 date. 245 In this way, it would be nearly as easy to deploy with a CA-issued 246 certificate as with a self-signed certificate. Furthermore, the 247 maintenance of that CA-issued certificate would require minimal 248 manual intervention. Such close integration of ACME with HTTPS 249 servers would allow the immediate and automated deployment of 250 certificates as they are issued, sparing the human administrator from 251 much of the time-consuming work described in the previous section. 253 3. Terminology 255 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 256 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 257 document are to be interpreted as described in RFC 2119 [RFC2119]. 259 The two main roles in ACME are "client" and "server". The ACME 260 client uses the protocol to request certificate management actions, 261 such as issuance or revocation. An ACME client therefore typically 262 runs on a web server, mail server, or some other server system which 263 requires valid TLS certificates. The ACME server runs at a 264 certificate authority, and responds to client requests, performing 265 the requested actions if the client is authorized. 267 An ACME client is represented by an "account key pair". The client 268 uses the private key of this key pair to sign all messages sent to 269 the server. The server uses the public key to verify the 270 authenticity and integrity of messages from the client. 272 4. Protocol Overview 274 ACME allows a client to request certificate management actions using 275 a set of JSON messages carried over HTTPS. In many ways, ACME 276 functions much like a traditional CA, in which a user creates an 277 account, requests a certificate, and proves control of the domains in 278 that certificate in order for the CA to sign the requested 279 certificate. 281 The first phase of ACME is for the client to request an account with 282 the ACME server. The client generates an asymmetric key pair and 283 requests a new account, optionally providing contact information, 284 agreeing to terms of service, and/or associating the account with an 285 existing account in another system. The creation request is signed 286 with the generated private key to prove that the client controls it. 288 Client Server 290 Contact Information 291 ToS Agreement 292 Additional Data 293 Signature -------> 295 <------- Account 297 Once an account is registered, there are three major steps the client 298 needs to take to get a certificate: 300 1. Submit an order for a certificate to be issued 302 2. Prove control of any identifiers requested in the certificate 304 3. Await issuance and download the issued certificate 306 The client's order for a certificate describes the desired 307 certificate using a PKCS#10 Certificate Signing Request (CSR) plus a 308 few additional fields that capture semantics that are not supported 309 in the CSR format. If the server is willing to consider issuing such 310 a certificate, it responds with a list of requirements that the 311 client must satisfy before the certificate will be issued. 313 For example, in most cases, the server will require the client to 314 demonstrate that it controls the identifiers in the requested 315 certificate. Because there are many different ways to validate 316 possession of different types of identifiers, the server will choose 317 from an extensible set of challenges that are appropriate for the 318 identifier being claimed. The client responds with a set of 319 responses that tell the server which challenges the client has 320 completed. The server then validates the challenges to check that 321 the client has accomplished the challenge. 323 Once the validation process is complete and the server is satisfied 324 that the client has met its requirements, the server will issue the 325 requested certificate and make it available to the client. 327 Order 328 Signature -------> 329 Required 330 <------- Authorizations 332 Responses 333 Signature -------> 335 <~~~~~~~~Validation~~~~~~~~> 337 <------- Certificate 339 To revoke a certificate, the client sends a signed revocation request 340 indicating the certificate to be revoked: 342 Client Server 344 Revocation request 345 Signature --------> 347 <-------- Result 349 Note that while ACME is defined with enough flexibility to handle 350 different types of identifiers in principle, the primary use case 351 addressed by this document is the case where domain names are used as 352 identifiers. For example, all of the identifier validation 353 challenges described in Section 7 below address validation of domain 354 names. The use of ACME for other identifiers will require further 355 specification, in order to describe how these identifiers are encoded 356 in the protocol, and what types of validation challenges the server 357 might require. 359 5. Message Transport 361 Communications between an ACME client and an ACME server are done 362 over HTTPS, using JSON Web Signature (JWS) [RFC7515] to provide some 363 additional security properties for messages sent from the client to 364 the server. HTTPS provides server authentication and 365 confidentiality. With some ACME-specific extensions, JWS provides 366 authentication of the client's request payloads, anti-replay 367 protection, and integrity for the HTTPS request URI. 369 5.1. HTTPS Requests 371 Each ACME function is accomplished by the client sending a sequence 372 of HTTPS requests to the server, carrying JSON messages 373 [RFC2818][RFC7159]. Use of HTTPS is REQUIRED. Clients SHOULD 374 support HTTP public key pinning [RFC7469], and servers SHOULD emit 375 pinning headers. Each subsection of Section 6 below describes the 376 message formats used by the function, and the order in which messages 377 are sent. 379 In most HTTPS transactions used by ACME, the ACME client is the HTTPS 380 client and the ACME server is the HTTPS server. The ACME server acts 381 as an HTTP and HTTPS client when validating challenges via HTTP. 383 ACME clients SHOULD send a User-Agent header in accordance with 384 [RFC7231], including the name and version of the ACME software in 385 addition to the name and version of the underlying HTTP client 386 software. 388 ACME clients SHOULD send an Accept-Language header in accordance with 389 [RFC7231] to enable localization of error messages. 391 ACME servers that are intended to be generally accessible need to use 392 Cross-Origin Resource Sharing (CORS) in order to be accessible from 393 browser-based clients [W3C.CR-cors-20130129]. Such servers SHOULD 394 set the Access-Control-Allow-Origin header field to the value "*". 396 Binary fields in the JSON objects used by ACME are encoded using 397 base64url encoding described in [RFC4648] Section 5, according to the 398 profile specified in JSON Web Signature [RFC7515] Section 2. This 399 encoding uses a URL safe character set. Trailing '=' characters MUST 400 be stripped. 402 5.2. Request Authentication 404 All ACME requests with a non-empty body MUST encapsulate their 405 payload in a JWS object, signed (in most cases) using the account's 406 private key. The server MUST verify the JWS before processing the 407 request. Encapsulating request bodies in JWS provides a simple 408 authentication of requests. 410 JWS objects sent in ACME requests MUST meet the following additional 411 criteria: 413 o The JWS MUST be encoded using UTF-8 415 o The JWS MUST NOT have the value "none" in its "alg" field 417 o The JWS MUST NOT have a MAC-based algorithm in its "alg" field 419 o The JWS Protected Header MUST include the following fields: 421 * "alg" 422 * "jwk" (only for requests to new-reg and revoke-cert resources) 424 * "kid" (for all other requests). 426 * "nonce" (defined below) 428 * "url" (defined below) 430 The "jwk" and "kid" fields are mutually exclusive. Servers MUST 431 reject requests that contain both. 433 For new-reg requests, and for revoke-cert requests authenticated by 434 certificate key, there MUST be a "jwk" field. 436 For all other requests, there MUST be a "kid" field. This field must 437 contain the account URI received by POSTing to the new-reg resource. 439 Note that authentication via signed JWS request bodies implies that 440 GET requests are not authenticated. Servers MUST NOT respond to GET 441 requests for resources that might be considered sensitive. 443 If the client sends a JWS signed with an algorithm that the server 444 does not support, then the server MUST return an error with status 445 code 400 (Bad Request) and type 446 "urn:ietf:params:acme:error:badSignatureAlgorithm". The problem 447 document returned with the error MUST include an "algorithms" field 448 with an array of supported "alg" values. 450 HTTP/1.1 400 Bad Request 451 Replay-Nonce: IXVHDyxIRGcTE0VSblhPzw 452 Content-Type: application/problem+json 453 Content-Language: en 455 { 456 "type": "urn:ietf:params:acme:error:badSignatureAlgorithm", 457 "detail": "Algorithm 'ES384' is not supported", 458 "algorithms": ["RS256", "RS384", "ES256"] 459 } 461 In the examples below, JWS objects are shown in the JSON or flattened 462 JSON serialization, with the protected header and payload expressed 463 as base64url(content) instead of the actual base64-encoded value, so 464 that the content is readable. Some fields are omitted for brevity, 465 marked with "...". 467 5.3. Equivalence of JWKs 469 At some points in the protocol, it is necessary for the server to 470 determine whether two JSON Web Key (JWK) [RFC7517] objects represent 471 the same key. In performing these checks, the server MUST consider 472 two JWKs to match if and only if they have the identical values in 473 all fields included in the computation of a JWK thumbprint for that 474 key. That is, the keys must have the same "kty" value and contain 475 identical values in the fields used in the computation of a JWK 476 thumbprint for that key type: 478 o "RSA": "n", "e" 480 o "EC": "crv", "x", "y" 482 Note that this comparison is equivalent to computing the JWK 483 thumbprints of the two keys and comparing thumbprints. The only 484 difference is that there is no requirement for a hash computation 485 (and thus it is independent of the choice of hash function) and no 486 risk of hash collision. 488 5.4. Request URI Integrity 490 It is common in deployment for the entity terminating TLS for HTTPS 491 to be different from the entity operating the logical HTTPS server, 492 with a "request routing" layer in the middle. For example, an ACME 493 CA might have a content delivery network terminate TLS connections 494 from clients so that it can inspect client requests for denial-of- 495 service protection. 497 These intermediaries can also change values in the request that are 498 not signed in the HTTPS request, e.g., the request URI and headers. 499 ACME uses JWS to provide an integrity mechanism, which protects 500 against an intermediary changing the request URI to another ACME URI. 502 As noted above, all ACME request objects carry a "url" parameter in 503 their protected header. This header parameter encodes the URL to 504 which the client is directing the request. On receiving such an 505 object in an HTTP request, the server MUST compare the "url" 506 parameter to the request URI. If the two do not match, then the 507 server MUST reject the request as unauthorized. 509 Except for the directory resource, all ACME resources are addressed 510 with URLs provided to the client by the server. For these resources, 511 the client MUST set the "url" field to the exact string provided by 512 the server (rather than performing any re-encoding on the URL). The 513 server SHOULD perform the corresponding string equality check, 514 configuring each resource with the URL string provided to clients and 515 having the resource check that requests have the same string in their 516 "url" fields. 518 5.4.1. "url" (URL) JWS header parameter 520 The "url" header parameter specifies the URL [RFC3986] to which this 521 JWS object is directed. The "url" parameter MUST be carried in the 522 protected header of the JWS. The value of the "url" header MUST be a 523 JSON string representing the URL. 525 5.5. Replay protection 527 In order to protect ACME resources from any possible replay attacks, 528 ACME requests have a mandatory anti-replay mechanism. This mechanism 529 is based on the server maintaining a list of nonces that it has 530 issued to clients, and requiring any signed request from the client 531 to carry such a nonce. 533 An ACME server provides nonces to clients using the Replay-Nonce 534 header field, as specified below. The server MUST include a Replay- 535 Nonce header field in every successful response to a POST request, 536 and SHOULD provide it in error responses as well. 538 Every JWS sent by an ACME client MUST include, in its protected 539 header, the "nonce" header parameter, with contents as defined below. 540 As part of JWS verification, the ACME server MUST verify that the 541 value of the "nonce" header is a value that the server previously 542 provided in a Replay-Nonce header field. Once a nonce value has 543 appeared in an ACME request, the server MUST consider it invalid, in 544 the same way as a value it had never issued. 546 When a server rejects a request because its nonce value was 547 unacceptable (or not present), it MUST provide HTTP status code 400 548 (Bad Request), and indicate the ACME error code 549 "urn:ietf:params:acme:error:badNonce". An error response with the 550 "badNonce" error code MUST include a Replay-Nonce header with a fresh 551 nonce. On receiving such a response, a client SHOULD retry the 552 request using the new nonce. 554 The precise method used to generate and track nonces is up to the 555 server. For example, the server could generate a random 128-bit 556 value for each response, keep a list of issued nonces, and strike 557 nonces from this list as they are used. 559 5.5.1. Replay-Nonce 561 The "Replay-Nonce" header field includes a server-generated value 562 that the server can use to detect unauthorized replay in future 563 client requests. The server should generate the value provided in 564 Replay-Nonce in such a way that they are unique to each message, with 565 high probability. 567 The value of the Replay-Nonce field MUST be an octet string encoded 568 according to the base64url encoding described in Section 2 of 569 [RFC7515]. Clients MUST ignore invalid Replay-Nonce values. 571 base64url = [A-Z] / [a-z] / [0-9] / "-" / "_" 573 Replay-Nonce = *base64url 575 The Replay-Nonce header field SHOULD NOT be included in HTTP request 576 messages. 578 5.5.2. "nonce" (Nonce) JWS header parameter 580 The "nonce" header parameter provides a unique value that enables the 581 verifier of a JWS to recognize when replay has occurred. The "nonce" 582 header parameter MUST be carried in the protected header of the JWS. 584 The value of the "nonce" header parameter MUST be an octet string, 585 encoded according to the base64url encoding described in Section 2 of 586 [RFC7515]. If the value of a "nonce" header parameter is not valid 587 according to this encoding, then the verifier MUST reject the JWS as 588 malformed. 590 5.6. Rate limits 592 Creation of resources can be rate limited to ensure fair usage and 593 prevent abuse. Once the rate limit is exceeded, the server MUST 594 respond with an error with the type 595 "urn:ietf:params:acme:error:rateLimited". Additionally, the server 596 SHOULD send a "Retry-After" header indicating when the current 597 request may succeed again. If multiple rate limits are in place, 598 that is the time where all rate limits allow access again for the 599 current request with exactly the same parameters. 601 In addition to the human readable "detail" field of the error 602 response, the server MAY send one or multiple tokens in the "Link" 603 header pointing to documentation about the specific hit rate limits 604 using the "rate-limit" relation. 606 5.7. Errors 608 Errors can be reported in ACME both at the HTTP layer and within ACME 609 payloads. ACME servers can return responses with an HTTP error 610 response code (4XX or 5XX). For example: If the client submits a 611 request using a method not allowed in this document, then the server 612 MAY return status code 405 (Method Not Allowed). 614 When the server responds with an error status, it SHOULD provide 615 additional information using problem document [RFC7807]. To 616 facilitate automatic response to errors, this document defines the 617 following standard tokens for use in the "type" field (within the 618 "urn:ietf:params:acme:error:" namespace): 620 +-----------------------+-------------------------------------------+ 621 | Code | Description | 622 +-----------------------+-------------------------------------------+ 623 | badCSR | The CSR is unacceptable (e.g., due to a | 624 | | short key) | 625 | | | 626 | badNonce | The client sent an unacceptable anti- | 627 | | replay nonce | 628 | | | 629 | badSignatureAlgorithm | The JWS was signed with an algorithm the | 630 | | server does not support | 631 | | | 632 | caa | CAA records forbid the CA from issuing | 633 | | | 634 | connection | The server could not connect to | 635 | | validation target | 636 | | | 637 | dnssec | DNSSEC validation failed | 638 | | | 639 | invalidContact | The contact URI for a registration was | 640 | | invalid | 641 | | | 642 | malformed | The request message was malformed | 643 | | | 644 | rateLimited | The request exceeds a rate limit | 645 | | | 646 | rejectedIdentifier | The server will not issue for the | 647 | | identifier | 648 | | | 649 | serverInternal | The server experienced an internal error | 650 | | | 651 | tls | The server received a TLS error during | 652 | | validation | 653 | | | 654 | unauthorized | The client lacks sufficient authorization | 655 | | | 656 | unknownHost | The server could not resolve a domain | 657 | | name | 658 | | | 659 | unsupportedIdentifier | Identifier is not supported, but may be | 660 | | in future | 661 | | | 662 | userActionRequired | The user visit the "instance" URL and | 663 | | take actions specified there | 664 +-----------------------+-------------------------------------------+ 666 This list is not exhaustive. The server MAY return errors whose 667 "type" field is set to a URI other than those defined above. Servers 668 MUST NOT use the ACME URN namespace for errors other than the 669 standard types. Clients SHOULD display the "detail" field of all 670 errors. 672 Authorization and challenge objects can also contain error 673 information to indicate why the server was unable to validate 674 authorization. 676 6. Certificate Management 678 In this section, we describe the certificate management functions 679 that ACME enables: 681 o Account Creation 683 o Ordering a Certificate 685 o Identifier Authorization 687 o Certificate Issuance 689 o Certificate Revocation 691 6.1. Resources 693 ACME is structured as a REST application with a few types of 694 resources: 696 o Account resources, representing information about an account 698 o Order resources, representing an account's requests to issue 699 certificates 701 o Authorization resources, representing an account's authorization 702 to act for an identifier 704 o Challenge resources, representing a challenge to prove control of 705 an identifier 707 o Certificate resources, representing issued certificates 709 o A "directory" resource 711 o A "new-nonce" resource 713 o A "new-account" resource 715 o A "new-order" resource 716 o A "revoke-certificate" resource 718 o A "key-change" resource 720 The server MUST provide "directory" and "new-nonce" resources. 722 ACME uses different URIs for different management functions. Each 723 function is listed in a directory along with its corresponding URI, 724 so clients only need to be configured with the directory URI. These 725 URIs are connected by a few different link relations [RFC5988]. 727 The "up" link relation is used with challenge resources to indicate 728 the authorization resource to which a challenge belongs. It is also 729 used from certificate resources to indicate a resource from which the 730 client may fetch a chain of CA certificates that could be used to 731 validate the certificate in the original resource. 733 The "directory" link relation is present on all resources other than 734 the directory and indicates the directory URL. 736 The following diagram illustrates the relations between resources on 737 an ACME server. For the most part, these relations are expressed by 738 URLs provided as strings in the resources' JSON representations. 739 Lines with labels in quotes indicate HTTP link relations 741 directory 742 | 743 |--> new-nonce 744 | 745 --------------------------------------------------+ 746 | | | | 747 | | | | 748 V V V V 749 new-account new-authz new-order revoke-cert 750 | | | ^ 751 | | | "author" | "revoke" 752 V | V <-------- | 753 acct | order --------> cert ---------+ 754 | | ^ | 755 | | | "up" | "up" 756 | V | V 757 +------> authz cert-chain 758 | ^ 759 | | "up" 760 V | 761 challenge 763 The following table illustrates a typical sequence of requests 764 required to establish a new account with the server, prove control of 765 an identifier, issue a certificate, and fetch an updated certificate 766 some time after issuance. The "->" is a mnemonic for a Location 767 header pointing to a created resource. 769 +----------------------+------------------+----------------+ 770 | Action | Request | Response | 771 +----------------------+------------------+----------------+ 772 | Get a nonce | HEAD new-nonce | 204 | 773 | | | | 774 | Create account | POST new-account | 201 -> account | 775 | | | | 776 | Submit an order | POST new-order | 201 -> order | 777 | | | | 778 | Fetch challenges | GET authz | 200 | 779 | | | | 780 | Respond to challenge | POST challenge | 200 | 781 | | | | 782 | Poll for status | GET authz | 200 | 783 | | | | 784 | Check for new cert | GET cert | 200 | 785 +----------------------+------------------+----------------+ 787 The remainder of this section provides the details of how these 788 resources are structured and how the ACME protocol makes use of them. 790 6.1.1. Directory 792 In order to help clients configure themselves with the right URIs for 793 each ACME operation, ACME servers provide a directory object. This 794 should be the only URL needed to configure clients. It is a JSON 795 dictionary, whose keys are drawn from the following table and whose 796 values are the corresponding URLs. 798 +-------------+--------------------+ 799 | Key | URL in value | 800 +-------------+--------------------+ 801 | new-nonce | New nonce | 802 | | | 803 | new-account | New account | 804 | | | 805 | new-order | New order | 806 | | | 807 | new-authz | New authorization | 808 | | | 809 | revoke-cert | Revoke certificate | 810 | | | 811 | key-change | Key change | 812 +-------------+--------------------+ 814 There is no constraint on the actual URI of the directory except that 815 it should be different from the other ACME server resources' URIs, 816 and that it should not clash with other services. For instance: 818 o a host which function as both an ACME and Web server may want to 819 keep the root path "/" for an HTML "front page", and and place the 820 ACME directory under path "/acme". 822 o a host which only functions as an ACME server could place the 823 directory under path "/". 825 The dictionary MAY additionally contain a key "meta". If present, it 826 MUST be a JSON dictionary; each item in the dictionary is an item of 827 metadata relating to the service provided by the ACME server. 829 The following metadata items are defined, all of which are OPTIONAL: 831 "terms-of-service" (optional, string): A URI identifying the current 832 terms of service. 834 "website" (optional, string): An HTTP or HTTPS URL locating a 835 website providing more information about the ACME server. 837 "caa-identities" (optional, array of string): Each string MUST be a 838 lowercase hostname which the ACME server recognises as referring 839 to itself for the purposes of CAA record validation as defined in 840 [RFC6844]. This allows clients to determine the correct issuer 841 domain name to use when configuring CAA record. 843 Clients access the directory by sending a GET request to the 844 directory URI. 846 HTTP/1.1 200 OK 847 Content-Type: application/json 849 { 850 "new-nonce": "https://example.com/acme/new-nonce", 851 "new-account": "https://example.com/acme/new-account", 852 "new-order": "https://example.com/acme/new-order", 853 "new-authz": "https://example.com/acme/new-authz", 854 "revoke-cert": "https://example.com/acme/revoke-cert", 855 "key-change": "https://example.com/acme/key-change", 856 "meta": { 857 "terms-of-service": "https://example.com/acme/terms", 858 "website": "https://www.example.com/", 859 "caa-identities": ["example.com"] 860 } 861 } 863 6.1.2. Account Objects 865 An ACME account resource represents a set of metadata associated to 866 an account. Account resources have the following structure: 868 key (required, dictionary): The public key of the account's key 869 pair, encoded as a JSON Web Key object [RFC7517]. The client may 870 not directly update this field, but must use the key-change 871 resource instead. 873 status (required, string): The status of this account. Possible 874 values are: "valid", "deactivated", and "revoked". The value 875 "deactivated" should be used to indicate user initiated 876 deactivation whereas "revoked" should be used to indicate 877 administratively initiated deactivation. 879 contact (optional, array of string): An array of URIs that the 880 server can use to contact the client for issues related to this 881 account. For example, the server may wish to notify the client 882 about server-initiated revocation or certificate expiration. 884 terms-of-service-agreed (optional, boolean): Including this field in 885 a new-account request, with a value of true, indicates the 886 client's agreement with the terms of service. This field is not 887 updateable by the client. 889 orders (required, string): A URI from which a list of orders 890 submitted by this account can be fetched via a GET request, as 891 described in Section 6.1.2.1 893 { 894 "contact": [ 895 "mailto:cert-admin@example.com", 896 "tel:+12025551212" 897 ], 898 "terms-of-service-agreed": true, 899 "orders": "https://example.com/acme/acct/1/orders" 900 } 902 6.1.2.1. Orders List 904 Each account object includes an "orders" URI from which a list of 905 orders created by the account can be fetched via GET request. The 906 result of the GET request MUST be a JSON object whose "orders" field 907 is an array of URIs, each identifying an order belonging to the 908 account. The server SHOULD include pending orders, and SHOULD NOT 909 include orders that are invalid in the array of URIs. The server MAY 910 return an incomplete list, along with a Link header with link 911 relation "next" indicating a URL to retrieve further entries. 913 HTTP/1.1 200 OK 914 Content-Type: application/json 915 Link: href="/acme/acct/1/orders?cursor=2", rel="next" 917 { 918 "orders": [ 919 "https://example.com/acme/acct/1/order/1", 920 "https://example.com/acme/acct/1/order/2", 921 /* 47 more URLs not shown for example brevity */ 922 "https://example.com/acme/acct/1/order/50" 923 ] 924 } 926 6.1.3. Order Objects 928 An ACME order object represents a client's request for a certificate, 929 and is used to track the progress of that order through to issuance. 930 Thus, the object contains information about the requested 931 certificate, the authorizations that the server requires the client 932 to complete, and any certificates that have resulted from this order. 934 status (required, string): The status of this order. Possible 935 values are: "pending", "processing", "valid", and "invalid". 937 expires (optional, string): The timestamp after which the server 938 will consider this order invalid, encoded in the format specified 939 in RFC 3339 [RFC3339]. This field is REQUIRED for objects with 940 "pending" or "valid" in the status field. 942 csr (required, string): A CSR encoding the parameters for the 943 certificate being requested [RFC2986]. The CSR is sent in the 944 base64url-encoded version of the DER format. (Note: Because this 945 field uses base64url, and does not include headers, it is 946 different from PEM.) 948 notBefore (optional, string): The requested value of the notBefore 949 field in the certificate, in the date format defined in [RFC3339] 951 notAfter (optional, string): The requested value of the notAfter 952 field in the certificate, in the date format defined in [RFC3339] 954 authorizations (required, array of string): For pending orders, the 955 authorizations that the client needs to complete before the 956 requested certificate can be issued (see Section 6.5). For final 957 orders, the authorizations that were completed. Each entry is a 958 URL from which an authorization can be fetched with a GET request. 960 certificate (optional, string): A URL for the certificate that has 961 been issued in response to this order. 963 { 964 "status": "pending", 965 "expires": "2015-03-01T14:09:00Z", 967 "csr": "jcRf4uXra7FGYW5ZMewvV...rhlnznwy8YbpMGqwidEXfE", 968 "notBefore": "2016-01-01T00:00:00Z", 969 "notAfter": "2016-01-08T00:00:00Z", 971 "authorizations": [ 972 "https://example.com/acme/authz/1234", 973 "https://example.com/acme/authz/2345" 974 ], 976 "certificate": "https://example.com/acme/cert/1234" 977 } 979 The elements of the "authorizations" array are immutable once set. 980 The server MUST NOT change the contents of the "authorizations" array 981 after it is created. If a client observes a change in the contents 982 of the "authorizations" array, then it SHOULD consider the order 983 invalid. 985 The "authorizations" array in the challenge SHOULD reflect all 986 authorizations that the CA takes into account in deciding to issue, 987 even if some authorizations were fulfilled in earlier orders or in 988 pre-authorization transactions. For example, if a CA allows multiple 989 orders to be fufilled based on a single authorization transaction, 990 then it SHOULD reflect that authorization in all of the order. 992 6.1.4. Authorization Objects 994 An ACME authorization object represents a server's authorization for 995 an account to represent an identifier. In addition to the 996 identifier, an authorization includes several metadata fields, such 997 as the status of the authorization (e.g., "pending", "valid", or 998 "revoked") and which challenges were used to validate possession of 999 the identifier. 1001 The structure of an ACME authorization resource is as follows: 1003 identifier (required, dictionary of string): The identifier that the 1004 account is authorized to represent 1006 type (required, string): The type of identifier. 1008 value (required, string): The identifier itself. 1010 status (required, string): The status of this authorization. 1011 Possible values are: "pending", "processing", "valid", "invalid" 1012 and "revoked". If this field is missing, then the default value 1013 is "pending". 1015 expires (optional, string): The timestamp after which the server 1016 will consider this authorization invalid, encoded in the format 1017 specified in RFC 3339 [RFC3339]. This field is REQUIRED for 1018 objects with "valid" in the "status" field. 1020 scope (optional, string): If this field is present, then it MUST 1021 contain a URI for an order resource, such that this authorization 1022 is only valid for that resource. If this field is absent, then 1023 the CA MUST consider this authorization valid for all orders until 1024 the authorization expires. [[ Open issue: More flexible scoping? 1025 ]] 1027 challenges (required, array): The challenges that the client can 1028 fulfill in order to prove possession of the identifier (for 1029 pending authorizations). For final authorizations, the challenges 1030 that were used. Each array entry is a dictionary with parameters 1031 required to validate the challenge, as specified in Section 7. A 1032 client should attempt to fulfill at most one of these challenges, 1033 and a server should consider any one of the challenges sufficient 1034 to make the authorization valid. 1036 The only type of identifier defined by this specification is a fully- 1037 qualified domain name (type: "dns"). The value of the identifier 1038 MUST be the ASCII representation of the domain name. If a domain 1039 name contains Unicode characters it MUST be encoded using the rules 1040 defined in [RFC3492]. Servers MUST verify any identifier values that 1041 begin with the ASCII Compatible Encoding prefix "xn-" as defined in 1042 [RFC5890] are properly encoded. Wildcard domain names (with "*" as 1043 the first label) MUST NOT be included in authorization objects. 1045 { 1046 "status": "valid", 1047 "expires": "2015-03-01T14:09:00Z", 1049 "identifier": { 1050 "type": "dns", 1051 "value": "example.org" 1052 }, 1054 "challenges": [ 1055 { 1056 "type": "http-01", 1057 "status": "valid", 1058 "validated": "2014-12-01T12:05:00Z", 1059 "keyAuthorization": "SXQe-2XODaDxNR...vb29HhjjLPSggwiE" 1060 } 1061 ] 1062 } 1064 6.2. Getting a Nonce 1066 Before sending a POST request to the server, an ACME client needs to 1067 have a fresh anti-replay nonce to put in the "nonce" header of the 1068 JWS. In most cases, the client will have gotten a nonce from a 1069 previous request. However, the client might sometimes need to get a 1070 new nonce, e.g., on its first request to the server or if an existing 1071 nonce is no longer valid. 1073 To get a fresh nonce, the client sends a HEAD request to the new- 1074 nonce resource on the server. The server's response MUST include a 1075 Replay-Nonce header field containing a fresh nonce, and SHOULD have 1076 status code 204 (No Content). The server SHOULD also respond to GET 1077 requests for this resource, returning an empty body (while still 1078 providing a Replay-Nonce header). 1080 HEAD /acme/new-nonce HTTP/1.1 1081 Host: example.com 1083 HTTP/1.1 204 No Content 1084 Replay-Nonce: oFvnlFP1wIhRlYS2jTaXbA 1085 Cache-Control: no-store 1087 Proxy caching of responses from the new-nonce resource can cause 1088 clients receive the same nonce repeatedly, leading to badNonce 1089 errors. The server MUST include a Cache-Control header field with 1090 the "no-store" directive in responses for the new-nonce resource, in 1091 order to prevent caching of this resource. 1093 6.3. Account Creation 1095 A client creates a new account with the server by sending a POST 1096 request to the server's new-account URI. The body of the request is 1097 a stub account object containing only the "contact" field. 1099 POST /acme/new-account HTTP/1.1 1100 Host: example.com 1101 Content-Type: application/jose+json 1103 { 1104 "protected": base64url({ 1105 "alg": "ES256", 1106 "jwk": {...}, 1107 "nonce": "6S8IqOGY7eL2lsGoTZYifg", 1108 "url": "https://example.com/acme/new-account" 1109 }), 1110 "payload": base64url({ 1111 "terms-of-service-agreed": true, 1112 "contact": [ 1113 "mailto:cert-admin@example.com", 1114 "tel:+12025551212" 1115 ] 1116 }), 1117 "signature": "RZPOnYoPs1PhjszF...-nh6X1qtOFPB519I" 1118 } 1120 The server MUST ignore any values provided in the "key", and "orders" 1121 fields in account bodies sent by the client, as well as any other 1122 fields that it does not recognize. If new fields are specified in 1123 the future, the specification of those fields MUST describe whether 1124 they may be provided by the client. 1126 In general, the server MUST ignore any fields in the request object 1127 that it does not recognize. In particular, it MUST NOT reflect 1128 unrecognized fields in the resulting account object. This allows 1129 clients to detect when servers do not support an extension field. 1131 The server SHOULD validate that the contact URLs in the "contact" 1132 field are valid and supported by the server. If the client provides 1133 the server with an invalid or unsupported contact URL, then the 1134 server MUST return an error of type "invalidContact", with a 1135 description describing the error and what types of contact URL the 1136 server considers acceptable. 1138 The server creates an account object with the included contact 1139 information. The "key" element of the account is set to the public 1140 key used to verify the JWS (i.e., the "jwk" element of the JWS 1141 header). The server returns this account object in a 201 (Created) 1142 response, with the account URI in a Location header field. 1144 If the server already has an account registered with the provided 1145 account key, then it MUST return a 200 (OK) response and provide the 1146 URI of that account in a Content-Location header field. This allows 1147 a client that has an account key but not the corresponding account 1148 URI to recover the account URI. 1150 If the server wishes to present the client with terms under which the 1151 ACME service is to be used, it MUST indicate the URI where such terms 1152 can be accessed in the "terms-of-service" subfield of the "meta" 1153 field in the directory object, and the server MUST reject new-account 1154 requests that do not have the "terms-of-service-agreed" set to 1155 "true". Clients SHOULD NOT automatically agree to terms by default. 1156 Rather, they SHOULD require some user interaction for agreement to 1157 terms. 1159 HTTP/1.1 201 Created 1160 Content-Type: application/json 1161 Replay-Nonce: D8s4D2mLs8Vn-goWuPQeKA 1162 Location: https://example.com/acme/acct/1 1163 Link: ;rel="directory" 1165 { 1166 "key": { /* JWK from JWS header */ }, 1167 "status": "valid", 1169 "contact": [ 1170 "mailto:cert-admin@example.com", 1171 "tel:+12025551212" 1172 ] 1173 } 1174 If the client wishes to update this information in the future, it 1175 sends a POST request with updated information to the account URI. 1176 The server MUST ignore any updates to the "key", or "order" fields or 1177 any other fields it does not recognize. The server MUST verify that 1178 the request is signed with the private key corresponding to the "key" 1179 field of the request before updating the registration. 1181 For example, to update the contact information in the above account, 1182 the client could send the following request: 1184 POST /acme/acct/1 HTTP/1.1 1185 Host: example.com 1186 Content-Type: application/jose+json 1188 { 1189 "protected": base64url({ 1190 "alg": "ES256", 1191 "kid": "https://example.com/acme/acct/1", 1192 "nonce": "ax5RnthDqp_Yf4_HZnFLmA", 1193 "url": "https://example.com/acme/acct/1" 1194 }), 1195 "payload": base64url({ 1196 "contact": [ 1197 "mailto:certificates@example.com", 1198 "tel:+12125551212" 1199 ] 1200 }), 1201 "signature": "hDXzvcj8T6fbFbmn...rDzXzzvzpRy64N0o" 1202 } 1204 Servers SHOULD NOT respond to GET requests for account resources as 1205 these requests are not authenticated. If a client wishes to query 1206 the server for information about its account (e.g., to examine the 1207 "contact" or "certificates" fields), then it SHOULD do so by sending 1208 a POST request with an empty update. That is, it should send a JWS 1209 whose payload is trivial ({}). 1211 6.3.1. Changes of Terms of Service 1213 As described above, a client can indicate its agreement with the CA's 1214 terms of service by setting the "terms-of-service-agreed" field in 1215 its account object to "true". 1217 If the server has changed its terms of service since a client 1218 initially agreed, and the server is unwilling to process a request 1219 without explicit agreement to the new terms, then it MUST return an 1220 error response with status code 403 (Forbidden) and type 1221 "urn:ietf:params:acme:error:userActionRequired". This response MUST 1222 include a Link header with link relation "terms-of-service" and the 1223 latest terms-of-service URL. 1225 The problem document returned with the error MUST also include an 1226 "instance" field, indicating a URL that the client should direct a 1227 human user to visit in order for instructions on how to agree to the 1228 terms. 1230 HTTP/1.1 403 Forbidden 1231 Replay-Nonce: IXVHDyxIRGcTE0VSblhPzw 1232 Content-Type: application/problem+json 1233 Content-Language: en 1235 { 1236 "type": "urn:ietf:params:acme:error:userActionRequired", 1237 "detail": "Terms of service have changed", 1238 "instance": "http://example.com/agreement/?token=W8Ih3PswD-8" 1239 } 1241 6.3.2. External Account Binding 1243 The server MAY require a value to be present for the "external- 1244 account-binding" field. This can be used to an ACME account with an 1245 existing account in a non-ACME system, such as a CA customer 1246 database. 1248 To enable ACME account binding, a CA needs to provision the ACME 1249 client with a MAC key and a key identifier. The key identifier MUST 1250 be an ASCII string. The MAC key SHOULD be provided in base64url- 1251 encoded form, to maximize compatibility between provisioning systems 1252 and ACME clients. 1254 The ACME client then computes a binding JWS to indicate the external 1255 account's approval of the ACME account key. The payload of this JWS 1256 is the account key being registered, in JWK form. The protected 1257 header of the JWS MUST meet the following criteria: 1259 o The "alg" field MUST indicate a MAC-based algorithm 1261 o The "kid" field MUST contain the key identifier provided by the CA 1263 o The "nonce" field MUST NOT be present 1265 o The "url" field MUST be set to the same value as the outer JWS 1267 The "signature" field of the JWS will contain the MAC value computed 1268 with the MAC key provided by the CA. 1270 POST /acme/new-reg HTTP/1.1 1271 Host: example.com 1272 Content-Type: application/jose+json 1274 { 1275 "protected": base64url({ 1276 "alg": "ES256", 1277 "jwk": /* account key */, 1278 "nonce": "K60BWPrMQG9SDxBDS_xtSw", 1279 "url": "https://example.com/acme/new-account" 1280 }), 1281 "payload": base64url({ 1282 "contact": ["mailto:example@anonymous.invalid"], 1283 "terms-of-service-agreed": true, 1285 "external-account-binding": { 1286 "protected": base64url({ 1287 "alg": "HS256", 1288 "kid": /* key identifier from CA */, 1289 "url": "https://example.com/acme/new-account" 1290 }), 1291 "payload": base64url(/* same as in "jwk" above */), 1292 "signature": /* MAC using MAC key from CA */ 1293 } 1294 }), 1295 "signature": "5TWiqIYQfIDfALQv...x9C2mg8JGPxl5bI4" 1296 } 1298 When a CA receives a new-account request containing an "external- 1299 account-binding" field, it must decide whether or not to verify the 1300 binding. If the CA does not verify the binding, then it MUST NOT 1301 reflect the "external-account-binding" field in the resulting account 1302 object (if any). To verify the account binding, the CA MUST take the 1303 following steps: 1305 1. Verify that the value of the field is a well-formed JWS 1307 2. Verify that the JWS protected meets the above criteria 1309 3. Retrieve the MAC key corresponding to the key identifier in the 1310 "kid" field 1312 4. Verify that the MAC on the JWS verifies using that MAC key 1314 5. Verify that the payload of the JWS represents the same key as was 1315 used to verify the outer JWS (i.e., the "jwk" field of the outer 1316 JWS) 1318 If all of these checks pass and the CA creates a new account, then 1319 the CA may consider the new account associated with the external 1320 account corresponding to the MAC key, and MUST reflect value of the 1321 "external-account-binding" field in the resulting account object. If 1322 any of these checks fail, then the CA MUST reject the new- 1323 registration request. 1325 6.3.3. Account Key Roll-over 1327 A client may wish to change the public key that is associated with a 1328 account in order to recover from a key compromise or proactively 1329 mitigate the impact of an unnoticed key compromise. 1331 To change the key associated with an account, the client first 1332 constructs a key-change object describing the change that it would 1333 like the server to make: 1335 account (required, string): The URL for account being modified. The 1336 content of this field MUST be the exact string provided in the 1337 Location header field in response to the new-account request that 1338 created the account. 1340 newKey (required, JWK): The JWK representation of the new key 1342 The client then encapsulates the key-change object in a JWS, signed 1343 with the requested new account key (i.e., the key matching the 1344 "newKey" value). 1346 The outer JWS MUST meet the normal requirements for an ACME JWS (see 1347 Section 5.2). The inner JWS MUST meet the normal requirements, with 1348 the following exceptions: 1350 o The inner JWS MUST have the same "url" parameter as the outer JWS. 1352 o The inner JWS is NOT REQUIRED to have a "nonce" parameter. The 1353 server MUST ignore any value provided for the "nonce" header 1354 parameter. 1356 This transaction has signatures from both the old and new keys so 1357 that the server can verify that the holders of the two keys both 1358 agree to the change. The signatures are nested to preserve the 1359 property that all signatures on POST messages are signed by exactly 1360 one key. 1362 POST /acme/key-change HTTP/1.1 1363 Host: example.com 1364 Content-Type: application/jose+json 1366 { 1367 "protected": base64url({ 1368 "alg": "ES256", 1369 "jwk": /* old key */, 1370 "nonce": "K60BWPrMQG9SDxBDS_xtSw", 1371 "url": "https://example.com/acme/key-change" 1372 }), 1373 "payload": base64url({ 1374 "protected": base64url({ 1375 "alg": "ES256", 1376 "jwk": /* new key */, 1377 "url": "https://example.com/acme/key-change" 1378 }), 1379 "payload": base64url({ 1380 "account": "https://example.com/acme/acct/1", 1381 "newKey": /* new key */ 1382 }), 1383 "signature": "Xe8B94RD30Azj2ea...8BmZIRtcSKPSd8gU" 1384 }), 1385 "signature": "5TWiqIYQfIDfALQv...x9C2mg8JGPxl5bI4" 1386 } 1388 On receiving key-change request, the server MUST perform the 1389 following steps in addition to the typical JWS validation: 1391 1. Validate the POST request belongs to a currently active account, 1392 as described in Message Transport. 1394 2. Check that the payload of the JWS is a well-formed JWS object 1395 (the "inner JWS") 1397 3. Check that the JWS protected header of the inner JWS has a "jwk" 1398 field. 1400 4. Check that the inner JWS verifies using the key in its "jwk" 1401 field 1403 5. Check that the payload of the inner JWS is a well-formed key- 1404 change object (as described above) 1406 6. Check that the "url" parameters of the inner and outer JWSs are 1407 the same 1409 7. Check that the "account" field of the key-change object contains 1410 the URL for the account matching the old key 1412 8. Check that the "newKey" field of the key-change object contains 1413 the key used to sign the inner JWS. 1415 If all of these checks pass, then the server updates the 1416 corresponding account by replacing the old account key with the new 1417 public key and returns status code 200. Otherwise, the server 1418 responds with an error status code and a problem document describing 1419 the error. 1421 6.3.4. Account deactivation 1423 A client may deactivate an account by posting a signed update to the 1424 server with a status field of "deactivated." Clients may wish to do 1425 this when the account key is compromised. 1427 POST /acme/acct/1 HTTP/1.1 1428 Host: example.com 1429 Content-Type: application/jose+json 1431 { 1432 "protected": base64url({ 1433 "alg": "ES256", 1434 "kid": "https://example.com/acme/acct/1", 1435 "nonce": "ntuJWWSic4WVNSqeUmshgg", 1436 "url": "https://example.com/acme/acct/1" 1437 }), 1438 "payload": base64url({ 1439 "status": "deactivated" 1440 }), 1441 "signature": "earzVLd3m5M4xJzR...bVTqn7R08AKOVf3Y" 1442 } 1444 The server MUST verify that the request is signed by the account key. 1445 If the server accepts the deactivation request, it should reply with 1446 a 200 (OK) status code and the current contents of the account 1447 object. 1449 Once an account is deactivated, the server MUST NOT accept further 1450 requests authorized by that account's key. It is up to server policy 1451 how long to retain data related to that account, whether to revoke 1452 certificates issued by that account, and whether to send email to 1453 that account's contacts. ACME does not provide a way to reactivate a 1454 deactivated account. 1456 6.4. Applying for Certificate Issuance 1458 A client may use ACME to submit an order for a certificate to be 1459 issued. The client makes this request by sending a POST request to 1460 the server's new-order resource. The body of the POST is a JWS 1461 object whose JSON payload is a subset of the order object defined in 1462 Section 6.1.3, containing the fields that describe the certificate to 1463 be issued: 1465 csr (required, string): A CSR encoding the parameters for the 1466 certificate being requested [RFC2986]. The CSR is sent in the 1467 base64url-encoded version of the DER format. (Note: Because this 1468 field uses base64url, and does not include headers, it is 1469 different from PEM.) 1471 notBefore (optional, string): The requested value of the notBefore 1472 field in the certificate, in the date format defined in [RFC3339] 1474 notAfter (optional, string): The requested value of the notAfter 1475 field in the certificate, in the date format defined in [RFC3339] 1477 POST /acme/new-order HTTP/1.1 1478 Host: example.com 1479 Content-Type: application/jose+json 1481 { 1482 "protected": base64url({ 1483 "alg": "ES256", 1484 "kid": "https://example.com/acme/acct/1", 1485 "nonce": "5XJ1L3lEkMG7tR6pA00clA", 1486 "url": "https://example.com/acme/new-order" 1487 }), 1488 "payload": base64url({ 1489 "csr": "5jNudRx6Ye4HzKEqT5...FS6aKdZeGsysoCo4H9P", 1490 "notBefore": "2016-01-01T00:00:00Z", 1491 "notAfter": "2016-01-08T00:00:00Z" 1492 }), 1493 "signature": "H6ZXtGjTZyUnPeKn...wEA4TklBdh3e454g" 1494 } 1496 The CSR encodes the client's requests with regard to the content of 1497 the certificate to be issued. The CSR MUST indicate the requested 1498 identifiers, either in the commonName portion of the requested 1499 subject name, or in an extensionRequest attribute [RFC2985] 1500 requesting a subjectAltName extension. 1502 The server MUST return an error if it cannot fulfil the request as 1503 specified, and MUST NOT issue a certificate with contents other than 1504 those requested. If the server requires the request to be modified 1505 in a certain way, it should indicate the required changes using an 1506 appropriate error code and description. 1508 If the server is willing to issue the requested certificate, it 1509 responds with a 201 (Created) response. The body of this response is 1510 an order object reflecting the client's request and any 1511 authorizations the client must complete before the certificate will 1512 be issued. 1514 HTTP/1.1 201 Created 1515 Replay-Nonce: MYAuvOpaoIiywTezizk5vw 1516 Location: https://example.com/acme/order/asdf 1518 { 1519 "status": "pending", 1520 "expires": "2016-01-01T00:00:00Z", 1522 "csr": "jcRf4uXra7FGYW5ZMewvV...rhlnznwy8YbpMGqwidEXfE", 1523 "notBefore": "2016-01-01T00:00:00Z", 1524 "notAfter": "2016-01-08T00:00:00Z", 1526 "authorizations": [ 1527 "https://example.com/acme/authz/1234", 1528 "https://example.com/acme/authz/2345" 1529 ] 1530 } 1532 The order object returned by the server represents a promise that if 1533 the client fulfills the server's requirements before the "expires" 1534 time, then the server will issue the requested certificate. In the 1535 order object, any authorization referenced in the "authorizations" 1536 array whose status is "pending" represents an authorization 1537 transaction that the client must complete before the server will 1538 issue the certificate (see Section 6.5). If the client fails to 1539 complete the required actions before the "expires" time, then the 1540 server SHOULD change the status of the order to "invalid" and MAY 1541 delete the order resource. 1543 The server MUST issue the requested certificate and update the order 1544 resource with a URL for the certificate shortly after the client has 1545 fulfilled the server's requirements. If the client has already 1546 satisfied the server's requirements at the time of this request 1547 (e.g., by obtaining authorization for all of the identifiers in the 1548 certificate in previous transactions), then the server MUST 1549 proactively issue the requested certificate and provide a URL for it 1550 in the "certificate" field of the order. The server MUST, however, 1551 still list the completed authorizations in the "authorizations" 1552 array. 1554 Once the client believes it has fulfilled the server's requirements, 1555 it should send a GET request to the order resource to obtain its 1556 current state. The status of the order will indicate what action the 1557 client should take: 1559 o "invalid": The certificate will not be issued. Consider this 1560 order process abandoned. 1562 o "pending": The server does not believe that the client has 1563 fulfilled the requirements. Check the "authorizations" array for 1564 entries that are still pending. 1566 o "processing": The server agrees that the requirements have been 1567 fulfilled, and is in the process of generating the certificate. 1568 Retry after the time given in the "Retry-After" header field of 1569 the response, if any. 1571 o "valid": The server has issued the certificate and provisioned its 1572 URL to the "certificate" field of the order. Download the 1573 certificate. 1575 6.4.1. Pre-Authorization 1577 The order process described above presumes that authorization objects 1578 are created reactively, in response to a certificate order. Some 1579 servers may also wish to enable clients to obtain authorization for 1580 an identifier proactively, outside of the context of a specific 1581 issuance. For example, a client hosting virtual servers for a 1582 collection of names might wish to obtain authorization before any 1583 servers are created, and only create a certificate when a server 1584 starts up. 1586 In some cases, a CA running an ACME server might have a completely 1587 external, non-ACME process for authorizing a client to issue for an 1588 identifier. In these case, the CA should provision its ACME server 1589 with authorization objects corresponding to these authorizations and 1590 reflect them as already valid in any orders submitted by the client. 1592 If a CA wishes to allow pre-authorization within ACME, it can offer a 1593 "new authorization" resource in its directory by adding the key "new- 1594 authz" with a URL for the new authorization resource. 1596 To request authorization for an identifier, the client sends a POST 1597 request to the new-authorization resource specifying the identifier 1598 for which authorization is being requested and how the server should 1599 behave with respect to existing authorizations for this identifier. 1601 identifier (required, dictionary of string): The identifier that the 1602 account is authorized to represent 1604 type (required, string): The type of identifier. 1606 value (required, string): The identifier itself. 1608 existing (optional, string): How an existing authorization should be 1609 handled. Possible values are "accept" and "require". 1611 POST /acme/new-authz HTTP/1.1 1612 Host: example.com 1613 Content-Type: application/jose+json 1615 { 1616 "protected": base64url({ 1617 "alg": "ES256", 1618 "jwk": {...}, 1619 "nonce": "uQpSjlRb4vQVCjVYAyyUWg", 1620 "url": "https://example.com/acme/new-authz" 1621 }), 1622 "payload": base64url({ 1623 "identifier": { 1624 "type": "dns", 1625 "value": "example.net" 1626 }, 1627 "existing": "accept" 1628 }), 1629 "signature": "nuSDISbWG8mMgE7H...QyVUL68yzf3Zawps" 1630 } 1632 Before processing the authorization request, the server SHOULD 1633 determine whether it is willing to issue certificates for the 1634 identifier. For example, the server should check that the identifier 1635 is of a supported type. Servers might also check names against a 1636 blacklist of known high-value identifiers. If the server is 1637 unwilling to issue for the identifier, it SHOULD return a 403 1638 (Forbidden) error, with a problem document describing the reason for 1639 the rejection. 1641 If the authorization request specifies "existing" with a value of 1642 "accept" or "require", before proceeding, the server SHOULD determine 1643 whether there are any existing, valid authorization resources for the 1644 account and given identifier. If one or more such authorizations 1645 exists, a response SHOULD returned with status code 303 (See Other) 1646 and a Location header pointing to the existing resource URL; 1647 processing of the request then stops. If there are multiple such 1648 authorizations, the authorization with the latest expiry date SHOULD 1649 be returned. If no existing authorizations were found and the value 1650 for "existing" was "require", then the server MUST return status code 1651 404 (Not Found); if it was "accept" or was any other value or was 1652 absent, processing continues as follows. 1654 If the server is willing to proceed, it builds a pending 1655 authorization object from the inputs submitted by the client. 1657 o "identifier" the identifier submitted by the client 1659 o "status": MUST be "pending" unless the server has out-of-band 1660 information about the client's authorization status 1662 o "challenges" and "combinations": As selected by the server's 1663 policy for this identifier 1665 The server allocates a new URI for this authorization, and returns a 1666 201 (Created) response, with the authorization URI in a Location 1667 header field, and the JSON authorization object in the body. The 1668 client then follows the process described in Section 6.5 to complete 1669 the authorization process. 1671 6.4.2. Downloading the Certificate 1673 To download the issued certificate, the client simply sends a GET 1674 request to the certificate URL. 1676 The default format of the certificate is PEM (application/x-pem-file) 1677 as specified by [RFC7468]. This format should contain the end-entity 1678 certificate first, followed by any intermediate certificates that are 1679 needed to build a path to a trusted root. Servers SHOULD NOT include 1680 self-signed trust anchors. The client may request other formats by 1681 including an Accept header in its request. For example, the client 1682 may use the media type application/pkix-cert to request the end- 1683 entity certificate in DER format. 1685 The server MAY provide one or more link relation header fields 1686 [RFC5988] with relation "alternate". Each such field should express 1687 an alternative certificate chain starting with the same end-entity 1688 certificate. This can be used to express paths to various trust 1689 anchors. Clients can fetch these alternates and use their own 1690 heuristics to decide which is optimal. 1692 The server MUST also provide a link relation header field with 1693 relation "author" to indicate the order under which this certificate 1694 was issued. 1696 If the CA participates in Certificate Transparency (CT) [RFC6962], 1697 then they may want to provide the client with a Signed Certificate 1698 Timestamp (SCT) that can be used to prove that a certificate was 1699 submitted to a CT log. An SCT can be included as an extension in the 1700 certificate or as an extension to OCSP responses for the certificate. 1701 The server can also provide the client with direct access to an SCT 1702 for a certificate using a Link relation header field with relation 1703 "ct-sct". 1705 GET /acme/cert/asdf HTTP/1.1 1706 Host: example.com 1707 Accept: application/pkix-cert 1709 HTTP/1.1 200 OK 1710 Content-Type: application/pkix-cert 1711 Link: ;rel="up";title="issuer" 1712 Link: ;rel="revoke" 1713 Link: ;rel="author" 1714 Link: ;rel="ct-sct" 1715 Link: ;rel="directory" 1717 -----BEGIN CERTIFICATE----- 1718 [End-entity certificate contents] 1719 -----END CERTIFICATE----- 1720 -----BEGIN CERTIFICATE----- 1721 [Issuer certificate contents] 1722 -----END CERTIFICATE----- 1723 -----BEGIN CERTIFICATE----- 1724 [Other certificate contents] 1725 -----END CERTIFICATE----- 1727 A certificate resource represents a single, immutable certificate. 1728 If the client wishes to obtain a renewed certificate, the client 1729 initiates a new order process to request one. 1731 Because certificate resources are immutable once issuance is 1732 complete, the server MAY enable the caching of the resource by adding 1733 Expires and Cache-Control headers specifying a point in time in the 1734 distant future. These headers have no relation to the certificate's 1735 period of validity. 1737 6.5. Identifier Authorization 1739 The identifier authorization process establishes the authorization of 1740 an account to manage certificates for a given identifier. This 1741 process must assure the server of two things: First, that the client 1742 controls the private key of the account key pair, and second, that 1743 the client controls the identifier in question. This process may be 1744 repeated to associate multiple identifiers to a key pair (e.g., to 1745 request certificates with multiple identifiers), or to associate 1746 multiple accounts with an identifier (e.g., to allow multiple 1747 entities to manage certificates). The server may declare that an 1748 authorization is only valid for a specific order by setting the 1749 "scope" field of the authorization to the URI for that order. 1751 Authorization resources are created by the server in response to 1752 certificate orders or authorization requests submitted by an account 1753 key holder; their URLs are provided to the client in the responses to 1754 these requests. The authorization object is implicitly tied to the 1755 account key used to sign the request. 1757 When a client receives an order from the server it downloads the 1758 authorization resource by sending a GET request to the indicated URL. 1759 If the client initiates authorization using a request to the new 1760 authorization resource, it will have already recevied the pending 1761 authorization object in the response to that request. 1763 GET /acme/authz/1234 HTTP/1.1 1764 Host: example.com 1766 HTTP/1.1 200 OK 1767 Content-Type: application/json 1768 Link: ;rel="directory" 1770 { 1771 "status": "pending", 1772 "expires": "2018-03-03T14:09:00Z", 1774 "identifier": { 1775 "type": "dns", 1776 "value": "example.org" 1777 }, 1779 "challenges": [ 1780 { 1781 "type": "http-01", 1782 "url": "https://example.com/authz/1234/0", 1783 "token": "DGyRejmCefe7v4NfDGDKfA" 1784 }, 1785 { 1786 "type": "tls-sni-02", 1787 "url": "https://example.com/authz/1234/1", 1788 "token": "DGyRejmCefe7v4NfDGDKfA" 1789 }, 1790 { 1791 "type": "dns-01", 1792 "url": "https://example.com/authz/1234/2", 1793 "token": "DGyRejmCefe7v4NfDGDKfA" 1794 } 1795 ] 1796 } 1798 6.5.1. Responding to Challenges 1800 To prove control of the identifier and receive authorization, the 1801 client needs to respond with information to complete the challenges. 1802 To do this, the client updates the authorization object received from 1803 the server by filling in any required information in the elements of 1804 the "challenges" dictionary. (This is also the stage where the 1805 client should perform any actions required by the challenge.) 1807 The client sends these updates back to the server in the form of a 1808 JSON object with the response fields required by the challenge type, 1809 carried in a POST request to the challenge URI (not authorization 1810 URI). This allows the client to send information only for challenges 1811 it is responding to. 1813 For example, if the client were to respond to the "http-01" challenge 1814 in the above authorization, it would send the following request: 1816 POST /acme/authz/asdf/0 HTTP/1.1 1817 Host: example.com 1818 Content-Type: application/jose+json 1820 { 1821 "protected": base64url({ 1822 "alg": "ES256", 1823 "kid": "https://example.com/acme/acct/1", 1824 "nonce": "Q_s3MWoqT05TrdkM2MTDcw", 1825 "url": "https://example.com/acme/authz/asdf/0" 1826 }), 1827 "payload": base64url({ 1828 "type": "http-01", 1829 "keyAuthorization": "IlirfxKKXA...vb29HhjjLPSggwiE" 1830 }), 1831 "signature": "9cbg5JO1Gf5YLjjz...SpkUfcdPai9uVYYQ" 1832 } 1834 The server updates the authorization document by updating its 1835 representation of the challenge with the response fields provided by 1836 the client. The server MUST ignore any fields in the response object 1837 that are not specified as response fields for this type of challenge. 1838 The server provides a 200 (OK) response with the updated challenge 1839 object as its body. 1841 If the client's response is invalid for some reason, or does not 1842 provide the server with appropriate information to validate the 1843 challenge, then the server MUST return an HTTP error. On receiving 1844 such an error, the client SHOULD undo any actions that have been 1845 taken to fulfill the challenge, e.g., removing files that have been 1846 provisioned to a web server. 1848 The server is said to "finalize" the authorization when it has 1849 completed one of the validations, by assigning the authorization a 1850 status of "valid" or "invalid", corresponding to whether it considers 1851 the account authorized for the identifier. If the final state is 1852 "valid", then the server MUST include an "expires" field. When 1853 finalizing an authorization, the server MAY remove challenges other 1854 than the one that was completed, and may modify the "expires" field. 1855 The server SHOULD NOT remove challenges with status "invalid". 1857 Usually, the validation process will take some time, so the client 1858 will need to poll the authorization resource to see when it is 1859 finalized. For challenges where the client can tell when the server 1860 has validated the challenge (e.g., by seeing an HTTP or DNS request 1861 from the server), the client SHOULD NOT begin polling until it has 1862 seen the validation request from the server. 1864 To check on the status of an authorization, the client sends a GET 1865 request to the authorization URI, and the server responds with the 1866 current authorization object. In responding to poll requests while 1867 the validation is still in progress, the server MUST return a 202 1868 (Accepted) response, and MAY include a Retry-After header field to 1869 suggest a polling interval to the client. 1871 GET /acme/authz/asdf HTTP/1.1 1872 Host: example.com 1874 HTTP/1.1 200 OK 1876 { 1877 "status": "valid", 1878 "expires": "2018-09-09T14:09:00Z", 1880 "identifier": { 1881 "type": "dns", 1882 "value": "example.org" 1883 }, 1885 "challenges": [ 1886 { 1887 "type": "http-01" 1888 "url": "https://example.com/authz/asdf/0", 1889 "status": "valid", 1890 "validated": "2014-12-01T12:05:00Z", 1891 "token": "IlirfxKKXAsHtmzK29Pj8A", 1892 "keyAuthorization": "IlirfxKKXA...vb29HhjjLPSggwiE" 1893 } 1894 ] 1895 } 1897 6.5.2. Deactivating an Authorization 1899 If a client wishes to relinquish its authorization to issue 1900 certificates for an identifier, then it may request that the server 1901 deactivate each authorization associated with that identifier by 1902 sending a POST request with the static object {"status": 1903 "deactivated"}. 1905 POST /acme/authz/asdf HTTP/1.1 1906 Host: example.com 1907 Content-Type: application/jose+json 1909 { 1910 "protected": base64url({ 1911 "alg": "ES256", 1912 "kid": "https://example.com/acme/acct/1", 1913 "nonce": "xWCM9lGbIyCgue8di6ueWQ", 1914 "url": "https://example.com/acme/authz/asdf" 1915 }), 1916 "payload": base64url({ 1917 "status": "deactivated" 1918 }), 1919 "signature": "srX9Ji7Le9bjszhu...WTFdtujObzMtZcx4" 1920 } 1922 The server MUST verify that the request is signed by the account key 1923 corresponding to the account that owns the authorization. If the 1924 server accepts the deactivation, it should reply with a 200 (OK) 1925 status code and the current contents of the authorization object. 1927 The server MUST NOT treat deactivated authorization objects as 1928 sufficient for issuing certificates. 1930 6.6. Certificate Revocation 1932 To request that a certificate be revoked, the client sends a POST 1933 request to the ACME server's revoke-cert URI. The body of the POST 1934 is a JWS object whose JSON payload contains the certificate to be 1935 revoked: 1937 certificate (required, string): The certificate to be revoked, in 1938 the base64url-encoded version of the DER format. (Note: Because 1939 this field uses base64url, and does not include headers, it is 1940 different from PEM.) 1942 reason (optional, int): One of the revocation reasonCodes defined in 1943 [RFC5280] Section 5.3.1 to be used when generating OCSP responses 1944 and CRLs. If this field is not set the server SHOULD use the 1945 unspecified (0) reasonCode value when generating OCSP responses 1946 and CRLs. The server MAY disallow a subset of reasonCodes from 1947 being used by the user. 1949 POST /acme/revoke-cert HTTP/1.1 1950 Host: example.com 1951 Content-Type: application/jose+json 1953 { 1954 "protected": base64url({ 1955 "alg": "ES256", 1956 "kid": "https://example.com/acme/acct/1", // OR "jwk" 1957 "nonce": "JHb54aT_KTXBWQOzGYkt9A", 1958 "url": "https://example.com/acme/revoke-cert" 1959 }), 1960 "payload": base64url({ 1961 "certificate": "MIIEDTCCAvegAwIBAgIRAP8...", 1962 "reason": 1 1963 }), 1964 "signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4" 1965 } 1967 Revocation requests are different from other ACME request in that 1968 they can be signed either with an account key pair or the key pair in 1969 the certificate. Before revoking a certificate, the server MUST 1970 verify that the key used to sign the request is authorized to revoke 1971 the certificate. The server SHOULD consider at least the following 1972 accounts authorized for a given certificate: 1974 o the account that issued the certificate. 1976 o an account that holds authorizations for all of the identifiers in 1977 the certificate. 1979 The server SHOULD also consider a revocation request valid if it is 1980 signed with the private key corresponding to the public key in the 1981 certificate. 1983 If the revocation succeeds, the server responds with status code 200 1984 (OK). If the revocation fails, the server returns an error. 1986 HTTP/1.1 200 OK 1987 Replay-Nonce: IXVHDyxIRGcTE0VSblhPzw 1988 Content-Length: 0 1990 --- or --- 1992 HTTP/1.1 403 Forbidden 1993 Replay-Nonce: IXVHDyxIRGcTE0VSblhPzw 1994 Content-Type: application/problem+json 1995 Content-Language: en 1997 { 1998 "type": "urn:ietf:params:acme:error:unauthorized", 1999 "detail": "No authorization provided for name example.net", 2000 "instance": "http://example.com/doc/unauthorized" 2001 } 2003 7. Identifier Validation Challenges 2005 There are few types of identifiers in the world for which there is a 2006 standardized mechanism to prove possession of a given identifier. In 2007 all practical cases, CAs rely on a variety of means to test whether 2008 an entity applying for a certificate with a given identifier actually 2009 controls that identifier. 2011 Challenges provide the server with assurance that an account holder 2012 is also the entity that controls an identifier. For each type of 2013 challenge, it must be the case that in order for an entity to 2014 successfully complete the challenge the entity must both: 2016 o Hold the private key of the account key pair used to respond to 2017 the challenge 2019 o Control the identifier in question 2021 Section 9 documents how the challenges defined in this document meet 2022 these requirements. New challenges will need to document how they 2023 do. 2025 ACME uses an extensible challenge/response framework for identifier 2026 validation. The server presents a set of challenges in the 2027 authorization object it sends to a client (as objects in the 2028 "challenges" array), and the client responds by sending a response 2029 object in a POST request to a challenge URI. 2031 This section describes an initial set of challenge types. Each 2032 challenge must describe: 2034 1. Content of challenge objects 2036 2. Content of response objects 2038 3. How the server uses the challenge and response to verify control 2039 of an identifier 2041 Challenge objects all contain the following basic fields: 2043 type (required, string): The type of challenge encoded in the 2044 object. 2046 url (required, string): The URL to which a response can be posted. 2048 status (required, string): The status of this authorization. 2049 Possible values are: "pending", "valid", and "invalid". 2051 validated (optional, string): The time at which this challenge was 2052 completed by the server, encoded in the format specified in RFC 2053 3339 [RFC3339]. This field is REQUIRED if the "status" field is 2054 "valid". 2056 error (optional, dictionary of string): The error that occurred 2057 while the server was validating the challenge, if any. This field 2058 is structured as a problem document [RFC7807]. 2060 All additional fields are specified by the challenge type. If the 2061 server sets a challenge's "status" to "invalid", it SHOULD also 2062 include the "error" field to help the client diagnose why they failed 2063 the challenge. 2065 Different challenges allow the server to obtain proof of different 2066 aspects of control over an identifier. In some challenges, like 2067 HTTP, TLS SNI, and DNS, the client directly proves its ability to do 2068 certain things related to the identifier. The choice of which 2069 challenges to offer to a client under which circumstances is a matter 2070 of server policy. 2072 The identifier validation challenges described in this section all 2073 relate to validation of domain names. If ACME is extended in the 2074 future to support other types of identifier, there will need to be 2075 new challenge types, and they will need to specify which types of 2076 identifier they apply to. 2078 [[ Editor's Note: In pre-RFC versions of this specification, 2079 challenges are labeled by type, and with the version of the draft in 2080 which they were introduced. For example, if an HTTP challenge were 2081 introduced in version -03 and a breaking change made in version -05, 2082 then there would be a challenge labeled "http-03" and one labeled 2083 "http-05" - but not one labeled "http-04", since challenge in version 2084 -04 was compatible with one in version -04. ]] 2086 7.1. Key Authorizations 2088 Several of the challenges in this document makes use of a key 2089 authorization string. A key authorization is a string that expresses 2090 a domain holder's authorization for a specified key to satisfy a 2091 specified challenge, by concatenating the token for the challenge 2092 with a key fingerprint, separated by a "." character: 2094 key-authz = token || '.' || base64url(JWK\_Thumbprint(accountKey)) 2096 The "JWK_Thumbprint" step indicates the computation specified in 2097 [RFC7638], using the SHA-256 digest. As specified in the individual 2098 challenges below, the token for a challenge is a JSON string 2099 comprised entirely of characters in the URL-safe base64 alphabet. 2100 The "||" operator indicates concatenation of strings. 2102 In computations involving key authorizations, such as the digest 2103 computations required for the DNS and TLS SNI challenges, the key 2104 authorization string MUST be represented in UTF-8 form (or, 2105 equivalently, ASCII). 2107 An example of how to compute a JWK thumbprint can be found in 2108 Section 3.1 of [RFC7638]. Note that some cryptographic libraries 2109 prepend a zero octet to the representation of the RSA public key 2110 parameters N and E, in order to avoid ambiguity with regard to the 2111 sign of the number. As noted in JWA [RFC7518], a JWK object MUST NOT 2112 include this zero octet. That is, any initial zero octets MUST be 2113 stripped before the values are base64url-encoded. 2115 7.2. HTTP 2117 With HTTP validation, the client in an ACME transaction proves its 2118 control over a domain name by proving that it can provision resources 2119 on an HTTP server that responds for that domain name. The ACME 2120 server challenges the client to provision a file at a specific path, 2121 with a specific string as its content. 2123 As a domain may resolve to multiple IPv4 and IPv6 addresses, the 2124 server will connect to at least one of the hosts found in A and AAAA 2125 records, at its discretion. Because many webservers allocate a 2126 default HTTPS virtual host to a particular low-privilege tenant user 2127 in a subtle and non-intuitive manner, the challenge must be completed 2128 over HTTP, not HTTPS. 2130 type (required, string): The string "http-01" 2132 token (required, string): A random value that uniquely identifies 2133 the challenge. This value MUST have at least 128 bits of entropy, 2134 in order to prevent an attacker from guessing it. It MUST NOT 2135 contain any characters outside the base64url alphabet and MUST NOT 2136 contain any padding characters ("="). 2138 GET /acme/authz/1234/0 HTTP/1.1 2139 Host: example.com 2141 HTTP/1.1 200 OK 2142 { 2143 "type": "http-01", 2144 "url": "https://example.com/acme/authz/0", 2145 "status": "pending", 2146 "token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA" 2147 } 2149 A client responds to this challenge by constructing a key 2150 authorization from the "token" value provided in the challenge and 2151 the client's account key. The client then provisions the key 2152 authorization as a resource on the HTTP server for the domain in 2153 question. 2155 The path at which the resource is provisioned is comprised of the 2156 fixed prefix ".well-known/acme-challenge/", followed by the "token" 2157 value in the challenge. The value of the resource MUST be the ASCII 2158 representation of the key authorization. 2160 .well-known/acme-challenge/evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA 2162 The client's response to this challenge indicates its agreement to 2163 this challenge by sending the server the key authorization covering 2164 the challenge's token and the client's account key. 2166 keyAuthorization (required, string): The key authorization for this 2167 challenge. This value MUST match the token from the challenge and 2168 the client's account key. 2170 POST /acme/authz/1234/0 2171 Host: example.com 2172 Content-Type: application/jose+json 2174 { 2175 "protected": base64url({ 2176 "alg": "ES256", 2177 "kid": "https://example.com/acme/acct/1", 2178 "nonce": "JHb54aT_KTXBWQOzGYkt9A", 2179 "url": "https://example.com/acme/authz/1234/0" 2180 }), 2181 "payload": base64url({ 2182 "keyAuthorization": "evaGxfADs...62jcerQ" 2183 }), 2184 "signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4" 2185 } 2187 On receiving a response, the server MUST verify that the key 2188 authorization in the response matches the "token" value in the 2189 challenge and the client's account key. If they do not match, then 2190 the server MUST return an HTTP error in response to the POST request 2191 in which the client sent the challenge. 2193 Given a challenge/response pair, the server verifies the client's 2194 control of the domain by verifying that the resource was provisioned 2195 as expected. 2197 1. Form a URI by populating the URI template [RFC6570] 2198 "http://{domain}/.well-known/acme-challenge/{token}", where: 2200 * the domain field is set to the domain name being verified; and 2202 * the token field is set to the token in the challenge. 2204 2. Verify that the resulting URI is well-formed. 2206 3. Dereference the URI using an HTTP GET request. This request MUST 2207 be sent to TCP port 80 on the server. 2209 4. Verify that the body of the response is well-formed key 2210 authorization. The server SHOULD ignore whitespace characters at 2211 the end of the body. 2213 5. Verify that key authorization provided by the server matches the 2214 token for this challenge and the client's account key. 2216 If all of the above verifications succeed, then the validation is 2217 successful. If the request fails, or the body does not pass these 2218 checks, then it has failed. 2220 7.3. TLS with Server Name Indication (TLS SNI) 2222 The TLS with Server Name Indication (TLS SNI) validation method 2223 proves control over a domain name by requiring the client to 2224 configure a TLS server referenced by an A/AAAA record under the 2225 domain name to respond to specific connection attempts utilizing the 2226 Server Name Indication extension [RFC6066]. The server verifies the 2227 client's challenge by accessing the reconfigured server and verifying 2228 a particular challenge certificate is presented. 2230 type (required, string): The string "tls-sni-02" 2232 token (required, string): A random value that uniquely identifies 2233 the challenge. This value MUST have at least 128 bits of entropy, 2234 in order to prevent an attacker from guessing it. It MUST NOT 2235 contain any characters outside the base64url alphabet and MUST NOT 2236 contain any padding characters ("="). 2238 GET /acme/authz/1234/1 HTTP/1.1 2239 Host: example.com 2241 HTTP/1.1 200 OK 2242 { 2243 "type": "tls-sni-02", 2244 "url": "https://example.com/acme/authz/1234/1", 2245 "status": "pending", 2246 "token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA" 2247 } 2249 A client responds to this challenge by constructing a self-signed 2250 certificate which the client MUST provision at the domain name 2251 concerned in order to pass the challenge. 2253 The certificate may be constructed arbitrarily, except that each 2254 certificate MUST have exactly two subjectAlternativeNames, SAN A and 2255 SAN B. Both MUST be dNSNames. 2257 SAN A MUST be constructed as follows: compute the SHA-256 digest of 2258 the UTF-8-encoded challenge token and encode it in lowercase 2259 hexadecimal form. The dNSName is "x.y.token.acme.invalid", where x 2260 is the first half of the hexadecimal representation and y is the 2261 second half. 2263 SAN B MUST be constructed as follows: compute the SHA-256 digest of 2264 the UTF-8 encoded key authorization and encode it in lowercase 2265 hexadecimal form. The dNSName is "x.y.ka.acme.invalid" where x is 2266 the first half of the hexadecimal representation and y is the second 2267 half. 2269 The client MUST ensure that the certificate is served to TLS 2270 connections specifying a Server Name Indication (SNI) value of SAN A. 2272 The response to the TLS-SNI challenge simply acknowledges that the 2273 client is ready to fulfill this challenge. 2275 keyAuthorization (required, string): The key authorization for this 2276 challenge. This value MUST match the token from the challenge and 2277 the client's account key. 2279 POST /acme/authz/1234/1 2280 Host: example.com 2281 Content-Type: application/jose+json 2283 { 2284 "protected": base64url({ 2285 "alg": "ES256", 2286 "kid": "https://example.com/acme/acct/1", 2287 "nonce": "JHb54aT_KTXBWQOzGYkt9A", 2288 "url": "https://example.com/acme/authz/1234/1" 2289 }), 2290 "payload": base64url({ 2291 "keyAuthorization": "evaGxfADs...62jcerQ" 2292 }), 2293 "signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4" 2294 } 2296 On receiving a response, the server MUST verify that the key 2297 authorization in the response matches the "token" value in the 2298 challenge and the client's account key. If they do not match, then 2299 the server MUST return an HTTP error in response to the POST request 2300 in which the client sent the challenge. 2302 Given a challenge/response pair, the ACME server verifies the 2303 client's control of the domain by verifying that the TLS server was 2304 configured appropriately, using these steps: 2306 1. Compute SAN A and SAN B in the same way as the client. 2308 2. Open a TLS connection to the domain name being validated, 2309 presenting SAN A in the SNI field. This connection MUST be sent 2310 to TCP port 443 on the server. In the ClientHello initiating the 2311 TLS handshake, the server MUST include a server_name extension 2312 (i.e., SNI) containing SAN A. The server SHOULD ensure that it 2313 does not reveal SAN B in any way when making the TLS connection, 2314 such that the presentation of SAN B in the returned certificate 2315 proves association with the client. 2317 3. Verify that the certificate contains a subjectAltName extension 2318 containing dNSName entries of SAN A and SAN B and no other 2319 entries. The comparison MUST be insensitive to case and ordering 2320 of names. 2322 It is RECOMMENDED that the server open multiple TLS connections from 2323 various network perspectives, in order to make MitM attacks harder. 2325 If all of the above verifications succeed, then the validation is 2326 successful. Otherwise, the validation fails. 2328 7.4. DNS 2330 When the identifier being validated is a domain name, the client can 2331 prove control of that domain by provisioning a resource record under 2332 it. The DNS challenge requires the client to provision a TXT record 2333 containing a designated value under a specific validation domain 2334 name. 2336 type (required, string): The string "dns-01" 2338 token (required, string): A random value that uniquely identifies 2339 the challenge. This value MUST have at least 128 bits of entropy, 2340 in order to prevent an attacker from guessing it. It MUST NOT 2341 contain any characters outside the base64url alphabet and MUST NOT 2342 contain any padding characters ("="). 2344 GET /acme/authz/1234/2 HTTP/1.1 2345 Host: example.com 2347 HTTP/1.1 200 OK 2348 { 2349 "type": "dns-01", 2350 "url": "https://example.com/acme/authz/1234/2", 2351 "status": "pending", 2352 "token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA" 2353 } 2355 A client responds to this challenge by constructing a key 2356 authorization from the "token" value provided in the challenge and 2357 the client's account key. The client then computes the SHA-256 2358 digest of the key authorization. 2360 The record provisioned to the DNS is the base64url encoding of this 2361 digest. The client constructs the validation domain name by 2362 prepending the label "_acme-challenge" to the domain name being 2363 validated, then provisions a TXT record with the digest value under 2364 that name. For example, if the domain name being validated is 2365 "example.com", then the client would provision the following DNS 2366 record: 2368 _acme-challenge.example.com. 300 IN TXT "gfj9Xq...Rg85nM" 2370 The response to the DNS challenge provides the computed key 2371 authorization to acknowledge that the client is ready to fulfill this 2372 challenge. 2374 keyAuthorization (required, string): The key authorization for this 2375 challenge. This value MUST match the token from the challenge and 2376 the client's account key. 2378 POST /acme/authz/1234/2 2379 Host: example.com 2380 Content-Type: application/jose+json 2382 { 2383 "protected": base64url({ 2384 "alg": "ES256", 2385 "kid": "https://example.com/acme/acct/1", 2386 "nonce": "JHb54aT_KTXBWQOzGYkt9A", 2387 "url": "https://example.com/acme/authz/1234/2" 2388 }), 2389 "payload": base64url({ 2390 "keyAuthorization": "evaGxfADs...62jcerQ" 2391 }), 2392 "signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4" 2393 } 2395 On receiving a response, the server MUST verify that the key 2396 authorization in the response matches the "token" value in the 2397 challenge and the client's account key. If they do not match, then 2398 the server MUST return an HTTP error in response to the POST request 2399 in which the client sent the challenge. 2401 To validate a DNS challenge, the server performs the following steps: 2403 1. Compute the SHA-256 digest of the key authorization 2405 2. Query for TXT records under the validation domain name 2406 3. Verify that the contents of one of the TXT records matches the 2407 digest value 2409 It is RECOMMENDED that the server perform multiple DNS queries from 2410 various network perspectives, in order to make MitM attacks harder. 2412 If all of the above verifications succeed, then the validation is 2413 successful. If no DNS record is found, or DNS record and response 2414 payload do not pass these checks, then the validation fails. 2416 7.5. Out-of-Band 2418 There may be cases where a server cannot perform automated validation 2419 of an identifier, for example if validation requires some manual 2420 steps. In such cases, the server may provide an "out of band" (OOB) 2421 challenge to request that the client perform some action outside of 2422 ACME in order to validate possession of the identifier. 2424 The OOB challenge requests that the client have a human user visit a 2425 web page to receive instructions on how to validate possession of the 2426 identifier, by providing a URL for that web page. 2428 type (required, string): The string "oob-01" 2430 href (required, string): The URL to be visited. The scheme of this 2431 URL MUST be "http" or "https". Note that this field is distinct 2432 from the "url" field of the challenge, which identifies the 2433 challenge itself. 2435 GET /acme/authz/1234/3 HTTP/1.1 2436 Host: example.com 2438 HTTP/1.1 200 OK 2439 { 2440 "type": "oob-01", 2441 "href": "https://example.com/validate/evaGxfADs6pSRb2LAv9IZ" 2442 } 2444 A client responds to this challenge by presenting the indicated URL 2445 for a human user to navigate to. If the user choses to complete this 2446 challege (by vising the website and completing its instructions), the 2447 client indicates this by sending a simple acknowledgement response to 2448 the server. 2450 type (required, string): The string "oob-01" 2451 POST /acme/authz/1234/3 2452 Host: example.com 2453 Content-Type: application/jose+json 2455 { 2456 "protected": base64url({ 2457 "alg": "ES256", 2458 "kid": "https://example.com/acme/acct/1", 2459 "nonce": "JHb54aT_KTXBWQOzGYkt9A", 2460 "url": "https://example.com/acme/authz/1234/3" 2461 }), 2462 "payload": base64url({ 2463 "type": "oob-01" 2464 }), 2465 "signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4" 2466 } 2468 On receiving a response, the server MUST verify that the value of the 2469 "type" field is "oob-01". Otherwise, the steps the server takes to 2470 validate identifier possession are determined by the server's local 2471 policy. 2473 8. IANA Considerations 2475 8.1. Well-Known URI for the HTTP Challenge 2477 The "Well-Known URIs" registry should be updated with the following 2478 additional value (using the template from [RFC5785]): 2480 URI suffix: acme-challenge 2482 Change controller: IETF 2484 Specification document(s): This document, Section Section 7.2 2486 Related information: N/A 2488 8.2. Replay-Nonce HTTP Header 2490 The "Message Headers" registry should be updated with the following 2491 additional value: 2493 | Header Field Name | Protocol | Status | Reference | 2494 +:------------+:------+:------+:-----------+ | Replay-Nonce | http | 2495 standard | Section 5.5.1 | 2497 8.3. "url" JWS Header Parameter 2499 The "JSON Web Signature and Encryption Header Parameters" registry 2500 should be updated with the following additional value: 2502 o Header Parameter Name: "url" 2504 o Header Parameter Description: URL 2506 o Header Parameter Usage Location(s): JWE, JWS 2508 o Change Controller: IESG 2510 o Specification Document(s): Section 5.4.1 of RFC XXXX 2512 [[ RFC EDITOR: Please replace XXXX above with the RFC number assigned 2513 to this document ]] 2515 8.4. "nonce" JWS Header Parameter 2517 The "JSON Web Signature and Encryption Header Parameters" registry 2518 should be updated with the following additional value: 2520 o Header Parameter Name: "nonce" 2522 o Header Parameter Description: Nonce 2524 o Header Parameter Usage Location(s): JWE, JWS 2526 o Change Controller: IESG 2528 o Specification Document(s): Section 5.5.2 of RFC XXXX 2530 [[ RFC EDITOR: Please replace XXXX above with the RFC number assigned 2531 to this document ]] 2533 8.5. URN Sub-namespace for ACME (urn:ietf:params:acme) 2535 The "IETF URN Sub-namespace for Registered Protocol Parameter 2536 Identifiers" registry should be updated with the following additional 2537 value, following the template in [RFC3553]: 2539 Registry name: acme 2541 Specification: RFC XXXX 2543 Repository: URL-TBD 2544 Index value: No transformation needed. 2546 [[ RFC EDITOR: Please replace XXXX above with the RFC number assigned 2547 to this document, and replace URL-TBD with the URL assigned by IANA 2548 for registries of ACME parameters. ]] 2550 8.6. New Registries 2552 This document requests that IANA create the following new registries: 2554 1. ACME Error Codes 2556 2. ACME Resource Types 2558 3. ACME Identifier Types 2560 4. ACME Challenge Types 2562 All of these registries should be administered under a Specification 2563 Required policy [RFC5226]. 2565 8.6.1. Fields in Account Objects 2567 This registry lists field names that are defined for use in ACME 2568 account objects. Fields marked as "configurable" may be included in 2569 a new-account request. 2571 Template: 2573 o Field name: The string to be used as a key in the JSON dictionary 2575 o Field type: The type of value to be provided, e.g., string, 2576 boolean, array of string 2578 o Client configurable: Boolean indicating whether the server should 2579 accept values provided by the client 2581 o Reference: Where this field is defined 2583 Initial contents: The fields and descriptions defined in 2584 Section 6.1.2. 2586 +--------------------------+-------------+--------------+-----------+ 2587 | Field Name | Field Type | Configurable | Reference | 2588 +--------------------------+-------------+--------------+-----------+ 2589 | key | dictionary | false | RFC XXXX | 2590 | | | | | 2591 | status | string | false | RFC XXXX | 2592 | | | | | 2593 | contact | array of | true | RFC XXXX | 2594 | | string | | | 2595 | | | | | 2596 | external-account-binding | dictionary | true | RFC XXXX | 2597 | | | | | 2598 | terms-of-service-agreed | boolean | false | RFC XXXX | 2599 | | | | | 2600 | orders | array of | false | RFC XXXX | 2601 | | string | | | 2602 +--------------------------+-------------+--------------+-----------+ 2604 8.6.2. Fields in Order Objects 2606 This registry lists field names that are defined for use in ACME 2607 order objects. Fields marked as "configurable" may be included in a 2608 new-order request. 2610 Template: 2612 o Field name: The string to be used as a key in the JSON dictionary 2614 o Field type: The type of value to be provided, e.g., string, 2615 boolean, array of string 2617 o Client configurable: Boolean indicating whether the server should 2618 accept values provided by the client 2620 o Reference: Where this field is defined 2622 Initial contents: The fields and descriptions defined in 2623 Section 6.1.3. 2625 +----------------+-------------------+--------------+-----------+ 2626 | Field Name | Field Type | Configurable | Reference | 2627 +----------------+-------------------+--------------+-----------+ 2628 | status | string | false | RFC XXXX | 2629 | | | | | 2630 | expires | string | false | RFC XXXX | 2631 | | | | | 2632 | csr | string | true | RFC XXXX | 2633 | | | | | 2634 | notBefore | string | true | RFC XXXX | 2635 | | | | | 2636 | notAfter | string | true | RFC XXXX | 2637 | | | | | 2638 | authorizations | array of string | false | RFC XXXX | 2639 | | | | | 2640 | certificate | string | false | RFC XXXX | 2641 +----------------+-------------------+--------------+-----------+ 2643 8.6.3. Error Codes 2645 This registry lists values that are used within URN values that are 2646 provided in the "type" field of problem documents in ACME. 2648 Template: 2650 o Code: The label to be included in the URN for this error, 2651 following "urn:ietf:params:acme:" 2653 o Description: A human-readable description of the error 2655 o Reference: Where the error is defined 2657 Initial contents: The codes and descriptions in the table in 2658 Section 5.7 above, with the Reference field set to point to this 2659 specification. 2661 8.6.4. Resource Types 2663 This registry lists the types of resources that ACME servers may list 2664 in their directory objects. 2666 Template: 2668 o Key: The value to be used as a dictionary key in the directory 2669 object 2671 o Resource type: The type of resource labeled by the key 2672 o Reference: Where the identifier type is defined 2674 Initial contents: 2676 +-------------+--------------------+-----------+ 2677 | Key | Resource type | Reference | 2678 +-------------+--------------------+-----------+ 2679 | new-account | New account | RFC XXXX | 2680 | | | | 2681 | new-order | New order | RFC XXXX | 2682 | | | | 2683 | revoke-cert | Revoke certificate | RFC XXXX | 2684 | | | | 2685 | key-change | Key change | RFC XXXX | 2686 +-------------+--------------------+-----------+ 2688 [[ RFC EDITOR: Please replace XXXX above with the RFC number assigned 2689 to this document ]] 2691 8.6.5. Identifier Types 2693 This registry lists the types of identifiers that ACME clients may 2694 request authorization to issue in certificates. 2696 Template: 2698 o Label: The value to be put in the "type" field of the identifier 2699 object 2701 o Reference: Where the identifier type is defined 2703 Initial contents: 2705 +-------+-----------+ 2706 | Label | Reference | 2707 +-------+-----------+ 2708 | dns | RFC XXXX | 2709 +-------+-----------+ 2711 [[ RFC EDITOR: Please replace XXXX above with the RFC number assigned 2712 to this document ]] 2714 8.6.6. Challenge Types 2716 This registry lists the ways that ACME servers can offer to validate 2717 control of an identifier. The "Identifier Type" field in template 2718 must be contained in the Label column of the ACME Identifier Types 2719 registry. 2721 Template: 2723 o Label: The value to be put in the "type" field of challenge 2724 objects using this validation mechanism 2726 o Identifier Type: The type of identifier that this mechanism 2727 applies to 2729 o Reference: Where the challenge type is defined 2731 Initial Contents 2733 +---------+-----------------+-----------+ 2734 | Label | Identifier Type | Reference | 2735 +---------+-----------------+-----------+ 2736 | http | dns | RFC XXXX | 2737 | | | | 2738 | tls-sni | dns | RFC XXXX | 2739 | | | | 2740 | dns | dns | RFC XXXX | 2741 +---------+-----------------+-----------+ 2743 [[ RFC EDITOR: Please replace XXXX above with the RFC number assigned 2744 to this document ]] 2746 9. Security Considerations 2748 ACME is a protocol for managing certificates that attest to 2749 identifier/key bindings. Thus the foremost security goal of ACME is 2750 to ensure the integrity of this process, i.e., to ensure that the 2751 bindings attested by certificates are correct, and that only 2752 authorized entities can manage certificates. ACME identifies clients 2753 by their account keys, so this overall goal breaks down into two more 2754 precise goals: 2756 1. Only an entity that controls an identifier can get an account key 2757 authorized for that identifier 2759 2. Once authorized, an account key's authorizations cannot be 2760 improperly transferred to another account key 2762 In this section, we discuss the threat model that underlies ACME and 2763 the ways that ACME achieves these security goals within that threat 2764 model. We also discuss the denial-of-service risks that ACME servers 2765 face, and a few other miscellaneous considerations. 2767 9.1. Threat model 2769 As a service on the Internet, ACME broadly exists within the Internet 2770 threat model [RFC3552]. In analyzing ACME, it is useful to think of 2771 an ACME server interacting with other Internet hosts along two 2772 "channels": 2774 o An ACME channel, over which the ACME HTTPS requests are exchanged 2776 o A validation channel, over which the ACME server performs 2777 additional requests to validate a client's control of an 2778 identifier 2780 +------------+ 2781 | ACME | ACME Channel 2782 | Client |--------------------+ 2783 +------------+ | 2784 V 2785 +------------+ 2786 | ACME | 2787 | Server | 2788 +------------+ 2789 +------------+ | 2790 | Validation |<-------------------+ 2791 | Server | Validation Channel 2792 +------------+ 2794 In practice, the risks to these channels are not entirely separate, 2795 but they are different in most cases. Each channel, for example, 2796 uses a different communications pattern: the ACME channel will 2797 comprise inbound HTTPS connections to the ACME server and the 2798 validation channel outbound HTTP or DNS requests. 2800 Broadly speaking, ACME aims to be secure against active and passive 2801 attackers on any individual channel. Some vulnerabilities arise 2802 (noted below), when an attacker can exploit both the ACME channel and 2803 one of the others. 2805 On the ACME channel, in addition to network-layer attackers, we also 2806 need to account for application-layer man in the middle attacks, and 2807 for abusive use of the protocol itself. Protection against 2808 application-layer MitM addresses potential attackers such as Content 2809 Distribution Networks (CDNs) and middleboxes with a TLS MitM 2810 function. Preventing abusive use of ACME means ensuring that an 2811 attacker with access to the validation channel can't obtain 2812 illegitimate authorization by acting as an ACME client (legitimately, 2813 in terms of the protocol). 2815 9.2. Integrity of Authorizations 2817 ACME allows anyone to request challenges for an identifier by 2818 registering an account key and sending a new-order request under that 2819 account key. The integrity of the authorization process thus depends 2820 on the identifier validation challenges to ensure that the challenge 2821 can only be completed by someone who both (1) holds the private key 2822 of the account key pair, and (2) controls the identifier in question. 2824 Validation responses need to be bound to an account key pair in order 2825 to avoid situations where an ACME MitM can switch out a legitimate 2826 domain holder's account key for one of his choosing, e.g.: 2828 o Legitimate domain holder registers account key pair A 2830 o MitM registers account key pair B 2832 o Legitimate domain holder sends a new-order request signed under 2833 account key A 2835 o MitM suppresses the legitimate request, but sends the same request 2836 signed under account key B 2838 o ACME server issues challenges and MitM forwards them to the 2839 legitimate domain holder 2841 o Legitimate domain holder provisions the validation response 2843 o ACME server performs validation query and sees the response 2844 provisioned by the legitimate domain holder 2846 o Because the challenges were issued in response to a message signed 2847 account key B, the ACME server grants authorization to account key 2848 B (the MitM) instead of account key A (the legitimate domain 2849 holder) 2851 All of the challenges above have a binding between the account 2852 private key and the validation query made by the server, via the key 2853 authorization. The key authorization is signed by the account 2854 private key, reflects the corresponding public key, and is provided 2855 to the server in the validation response. 2857 The association of challenges to identifiers is typically done by 2858 requiring the client to perform some action that only someone who 2859 effectively controls the identifier can perform. For the challenges 2860 in this document, the actions are: 2862 o HTTP: Provision files under .well-known on a web server for the 2863 domain 2865 o TLS SNI: Configure a TLS server for the domain 2867 o DNS: Provision DNS resource records for the domain 2869 There are several ways that these assumptions can be violated, both 2870 by misconfiguration and by attack. For example, on a web server that 2871 allows non-administrative users to write to .well-known, any user can 2872 claim to own the server's hostname by responding to an HTTP 2873 challenge, and likewise for TLS configuration and TLS SNI. 2875 The use of hosting providers is a particular risk for ACME 2876 validation. If the owner of the domain has outsourced operation of 2877 DNS or web services to a hosting provider, there is nothing that can 2878 be done against tampering by the hosting provider. As far as the 2879 outside world is concerned, the zone or web site provided by the 2880 hosting provider is the real thing. 2882 More limited forms of delegation can also lead to an unintended party 2883 gaining the ability to successfully complete a validation 2884 transaction. For example, suppose an ACME server follows HTTP 2885 redirects in HTTP validation and a web site operator provisions a 2886 catch-all redirect rule that redirects requests for unknown resources 2887 to a different domain. Then the target of the redirect could use 2888 that to get a certificate through HTTP validation, since the 2889 validation path will not be known to the primary server. 2891 The DNS is a common point of vulnerability for all of these 2892 challenges. An entity that can provision false DNS records for a 2893 domain can attack the DNS challenge directly, and can provision false 2894 A/AAAA records to direct the ACME server to send its TLS SNI or HTTP 2895 validation query to a server of the attacker's choosing. There are a 2896 few different mitigations that ACME servers can apply: 2898 o Always querying the DNS using a DNSSEC-validating resolver 2899 (enhancing security for zones that are DNSSEC-enabled) 2901 o Querying the DNS from multiple vantage points to address local 2902 attackers 2904 o Applying mitigations against DNS off-path attackers, e.g., adding 2905 entropy to requests [I-D.vixie-dnsext-dns0x20] or only using TCP 2907 Given these considerations, the ACME validation process makes it 2908 impossible for any attacker on the ACME channel, or a passive 2909 attacker on the validation channel to hijack the authorization 2910 process to authorize a key of the attacker's choice. 2912 An attacker that can only see the ACME channel would need to convince 2913 the validation server to provide a response that would authorize the 2914 attacker's account key, but this is prevented by binding the 2915 validation response to the account key used to request challenges. A 2916 passive attacker on the validation channel can observe the correct 2917 validation response and even replay it, but that response can only be 2918 used with the account key for which it was generated. 2920 An active attacker on the validation channel can subvert the ACME 2921 process, by performing normal ACME transactions and providing a 2922 validation response for his own account key. The risks due to 2923 hosting providers noted above are a particular case. For identifiers 2924 where the server already has some public key associated with the 2925 domain this attack can be prevented by requiring the client to prove 2926 control of the corresponding private key. 2928 9.3. Denial-of-Service Considerations 2930 As a protocol run over HTTPS, standard considerations for TCP-based 2931 and HTTP-based DoS mitigation also apply to ACME. 2933 At the application layer, ACME requires the server to perform a few 2934 potentially expensive operations. Identifier validation transactions 2935 require the ACME server to make outbound connections to potentially 2936 attacker-controlled servers, and certificate issuance can require 2937 interactions with cryptographic hardware. 2939 In addition, an attacker can also cause the ACME server to send 2940 validation requests to a domain of its choosing by submitting 2941 authorization requests for the victim domain. 2943 All of these attacks can be mitigated by the application of 2944 appropriate rate limits. Issues closer to the front end, like POST 2945 body validation, can be addressed using HTTP request limiting. For 2946 validation and certificate requests, there are other identifiers on 2947 which rate limits can be keyed. For example, the server might limit 2948 the rate at which any individual account key can issue certificates, 2949 or the rate at which validation can be requested within a given 2950 subtree of the DNS. 2952 9.4. Server-Side Request Forgery 2954 Server-Side Request Forgery (SSRF) attacks can arise when an attacker 2955 can cause a server to perform HTTP requests to an attacker-chosen 2956 URL. In the ACME HTTP challenge validation process, the ACME server 2957 performs an HTTP GET request to a URL in which the attacker can 2958 choose the domain. This request is made before the server has 2959 verified that the client controls the domain, so any client can cause 2960 a query to any domain. 2962 Some server implementations include information from the validation 2963 server's response (in order to facilitate debugging). Such 2964 implementations enable an attacker to extract this information from 2965 any web server that is accessible to the ACME server, even if it is 2966 not accessible to the ACME client. 2968 It might seem that the risk of SSRF through this channel is limited 2969 by the fact that the attacker can only control the domain of the URL, 2970 not the path. However, if the attacker first sets the domain to one 2971 they control, then they can send the server an HTTP redirect (e.g., a 2972 302 response) which will cause the server to query an arbitrary URI. 2974 In order to further limit the SSRF risk, ACME server operators should 2975 ensure that validation queries can only be sent to servers on the 2976 public Internet, and not, say, web services within the server 2977 operator's internal network. Since the attacker could make requests 2978 to these public servers himself, he can't gain anything extra through 2979 an SSRF attack on ACME aside from a layer of anonymization. 2981 9.5. CA Policy Considerations 2983 The controls on issuance enabled by ACME are focused on validating 2984 that a certificate applicant controls the identifier he claims. 2985 Before issuing a certificate, however, there are many other checks 2986 that a CA might need to perform, for example: 2988 o Has the client agreed to a subscriber agreement? 2990 o Is the claimed identifier syntactically valid? 2992 o For domain names: 2994 * If the leftmost label is a '*', then have the appropriate 2995 checks been applied? 2997 * Is the name on the Public Suffix List? 2999 * Is the name a high-value name? 3001 * Is the name a known phishing domain? 3003 o Is the key in the CSR sufficiently strong? 3004 o Is the CSR signed with an acceptable algorithm? 3006 CAs that use ACME to automate issuance will need to ensure that their 3007 servers perform all necessary checks before issuing. 3009 CAs using ACME to allow clients to agree to terms of service should 3010 keep in mind that ACME clients can automate this agreement, possibly 3011 not involving a human user. If a CA wishes to have stronger evidence 3012 of user consent, it may present an out-of-band requirement or 3013 challenge to require human involvement. 3015 10. Operational Considerations 3017 There are certain factors that arise in operational reality that 3018 operators of ACME-based CAs will need to keep in mind when 3019 configuring their services. For example: 3021 10.1. DNS over TCP 3023 As noted above, DNS forgery attacks against the ACME server can 3024 result in the server making incorrect decisions about domain control 3025 and thus mis-issuing certificates. Servers SHOULD verify DNSSEC when 3026 it is available for a domain. When DNSSEC is not available, servers 3027 SHOULD perform DNS queries over TCP, which provides better resistance 3028 to some forgery attacks than DNS over UDP. 3030 10.2. Default Virtual Hosts 3032 In many cases, TLS-based services are deployed on hosted platforms 3033 that use the Server Name Indication (SNI) TLS extension to 3034 distinguish between different hosted services or "virtual hosts". 3035 When a client initiates a TLS connection with an SNI value indicating 3036 a provisioned host, the hosting platform routes the connection to 3037 that host. 3039 When a connection comes in with an unknown SNI value, one might 3040 expect the hosting platform to terminate the TLS connection. 3041 However, some hosting platforms will choose a virtual host to be the 3042 "default", and route connections with unknown SNI values to that 3043 host. 3045 In such cases, the owner of the default virtual host can complete a 3046 TLS-based challenge (e.g., "tls-sni-02") for any domain with an A 3047 record that points to the hosting platform. This could result in 3048 mis-issuance in cases where there are multiple hosts with different 3049 owners resident on the hosting platform. 3051 A CA that accepts TLS-based proof of domain control should attempt to 3052 check whether a domain is hosted on a domain with a default virtual 3053 host before allowing an authorization request for this host to use a 3054 TLS-based challenge. A default virtual host can be detected by 3055 initiating TLS connections to the host with random SNI values within 3056 the namespace used for the TLS-based challenge (the "acme.invalid" 3057 namespace for "tls-sni-02"). 3059 10.3. Use of DNSSEC Resolvers 3061 An ACME-based CA will often need to make DNS queries, e.g., to 3062 validate control of DNS names. Because the security of such 3063 validations ultimately depends on the authenticity of DNS data, every 3064 possible precaution should be taken to secure DNS queries done by the 3065 CA. It is therefore RECOMMENDED that ACME-based CAs make all DNS 3066 queries via DNSSEC-validating stub or recursive resolvers. This 3067 provides additional protection to domains which choose to make use of 3068 DNSSEC. 3070 An ACME-based CA must use only a resolver if it trusts the resolver 3071 and every component of the network route by which it is accessed. It 3072 is therefore RECOMMENDED that ACME-based CAs operate their own 3073 DNSSEC-validating resolvers within their trusted network and use 3074 these resolvers both for both CAA record lookups and all record 3075 lookups in furtherance of a challenge scheme (A, AAAA, TXT, etc.). 3077 11. Acknowledgements 3079 In addition to the editors listed on the front page, this document 3080 has benefited from contributions from a broad set of contributors, 3081 all the way back to its inception. 3083 o Peter Eckersley, EFF 3085 o Eric Rescorla, Mozilla 3087 o Seth Schoen, EFF 3089 o Alex Halderman, University of Michigan 3091 o Martin Thomson, Mozilla 3093 o Jakub Warmuz, University of Oxford 3095 This document draws on many concepts established by Eric Rescorla's 3096 "Automated Certificate Issuance Protocol" draft. Martin Thomson 3097 provided helpful guidance in the use of HTTP. 3099 12. References 3101 12.1. Normative References 3103 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 3104 Requirement Levels", BCP 14, RFC 2119, 3105 DOI 10.17487/RFC2119, March 1997, 3106 . 3108 [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, 3109 DOI 10.17487/RFC2818, May 2000, 3110 . 3112 [RFC2985] Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object 3113 Classes and Attribute Types Version 2.0", RFC 2985, 3114 DOI 10.17487/RFC2985, November 2000, 3115 . 3117 [RFC2986] Nystrom, M. and B. Kaliski, "PKCS #10: Certification 3118 Request Syntax Specification Version 1.7", RFC 2986, 3119 DOI 10.17487/RFC2986, November 2000, 3120 . 3122 [RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet: 3123 Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002, 3124 . 3126 [RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode 3127 for Internationalized Domain Names in Applications 3128 (IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003, 3129 . 3131 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 3132 Resource Identifier (URI): Generic Syntax", STD 66, 3133 RFC 3986, DOI 10.17487/RFC3986, January 2005, 3134 . 3136 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data 3137 Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006, 3138 . 3140 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 3141 (TLS) Protocol Version 1.2", RFC 5246, 3142 DOI 10.17487/RFC5246, August 2008, 3143 . 3145 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 3146 Housley, R., and W. Polk, "Internet X.509 Public Key 3147 Infrastructure Certificate and Certificate Revocation List 3148 (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, 3149 . 3151 [RFC5890] Klensin, J., "Internationalized Domain Names for 3152 Applications (IDNA): Definitions and Document Framework", 3153 RFC 5890, DOI 10.17487/RFC5890, August 2010, 3154 . 3156 [RFC5988] Nottingham, M., "Web Linking", RFC 5988, 3157 DOI 10.17487/RFC5988, October 2010, 3158 . 3160 [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) 3161 Extensions: Extension Definitions", RFC 6066, 3162 DOI 10.17487/RFC6066, January 2011, 3163 . 3165 [RFC6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M., 3166 and D. Orchard, "URI Template", RFC 6570, 3167 DOI 10.17487/RFC6570, March 2012, 3168 . 3170 [RFC6844] Hallam-Baker, P. and R. Stradling, "DNS Certification 3171 Authority Authorization (CAA) Resource Record", RFC 6844, 3172 DOI 10.17487/RFC6844, January 2013, 3173 . 3175 [RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data 3176 Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March 3177 2014, . 3179 [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 3180 Protocol (HTTP/1.1): Semantics and Content", RFC 7231, 3181 DOI 10.17487/RFC7231, June 2014, 3182 . 3184 [RFC7468] Josefsson, S. and S. Leonard, "Textual Encodings of PKIX, 3185 PKCS, and CMS Structures", RFC 7468, DOI 10.17487/RFC7468, 3186 April 2015, . 3188 [RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web 3189 Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May 3190 2015, . 3192 [RFC7517] Jones, M., "JSON Web Key (JWK)", RFC 7517, 3193 DOI 10.17487/RFC7517, May 2015, 3194 . 3196 [RFC7518] Jones, M., "JSON Web Algorithms (JWA)", RFC 7518, 3197 DOI 10.17487/RFC7518, May 2015, 3198 . 3200 [RFC7638] Jones, M. and N. Sakimura, "JSON Web Key (JWK) 3201 Thumbprint", RFC 7638, DOI 10.17487/RFC7638, September 3202 2015, . 3204 [RFC7807] Nottingham, M. and E. Wilde, "Problem Details for HTTP 3205 APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016, 3206 . 3208 12.2. Informative References 3210 [I-D.vixie-dnsext-dns0x20] 3211 Vixie, P. and D. Dagon, "Use of Bit 0x20 in DNS Labels to 3212 Improve Transaction Identity", draft-vixie-dnsext- 3213 dns0x20-00 (work in progress), March 2008. 3215 [RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC 3216 Text on Security Considerations", BCP 72, RFC 3552, 3217 DOI 10.17487/RFC3552, July 2003, 3218 . 3220 [RFC3553] Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An 3221 IETF URN Sub-namespace for Registered Protocol 3222 Parameters", BCP 73, RFC 3553, DOI 10.17487/RFC3553, June 3223 2003, . 3225 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 3226 IANA Considerations Section in RFCs", BCP 26, RFC 5226, 3227 DOI 10.17487/RFC5226, May 2008, 3228 . 3230 [RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known 3231 Uniform Resource Identifiers (URIs)", RFC 5785, 3232 DOI 10.17487/RFC5785, April 2010, 3233 . 3235 [RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate 3236 Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013, 3237 . 3239 [RFC7469] Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning 3240 Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April 3241 2015, . 3243 [W3C.CR-cors-20130129] 3244 Kesteren, A., "Cross-Origin Resource Sharing", World Wide 3245 Web Consortium CR CR-cors-20130129, January 2013, 3246 . 3248 Authors' Addresses 3250 Richard Barnes 3251 Mozilla 3253 Email: rlb@ipv.sx 3255 Jacob Hoffman-Andrews 3256 EFF 3258 Email: jsha@eff.org 3260 James Kasten 3261 University of Michigan 3263 Email: jdkasten@umich.edu