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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 ACME Working Group R. Barnes 3 Internet-Draft Cisco 4 Intended status: Standards Track J. Hoffman-Andrews 5 Expires: February 11, 2019 EFF 6 D. McCarney 7 Let's Encrypt 8 J. Kasten 9 University of Michigan 10 August 10, 2018 12 Automatic Certificate Management Environment (ACME) 13 draft-ietf-acme-acme-14 15 Abstract 17 Public Key Infrastructure X.509 (PKIX) certificates are used for a 18 number of purposes, the most significant of which is the 19 authentication of domain names. Thus, certification authorities 20 (CAs) in the Web PKI are trusted to verify that an applicant for a 21 certificate legitimately represents the domain name(s) in the 22 certificate. Today, this verification is done through a collection 23 of ad hoc mechanisms. This document describes a protocol that a CA 24 and an applicant can use to automate the process of verification and 25 certificate issuance. The protocol also provides facilities for 26 other certificate management functions, such as certificate 27 revocation. 29 RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH: The source for 30 this draft is maintained in GitHub. Suggested changes should be 31 submitted as pull requests at https://github.com/ietf-wg-acme/acme 32 [1]. Instructions are on that page as well. Editorial changes can 33 be managed in GitHub, but any substantive change should be discussed 34 on the ACME mailing list (acme@ietf.org). 36 Status of This Memo 38 This Internet-Draft is submitted in full conformance with the 39 provisions of BCP 78 and BCP 79. 41 Internet-Drafts are working documents of the Internet Engineering 42 Task Force (IETF). Note that other groups may also distribute 43 working documents as Internet-Drafts. The list of current Internet- 44 Drafts is at https://datatracker.ietf.org/drafts/current/. 46 Internet-Drafts are draft documents valid for a maximum of six months 47 and may be updated, replaced, or obsoleted by other documents at any 48 time. It is inappropriate to use Internet-Drafts as reference 49 material or to cite them other than as "work in progress." 51 This Internet-Draft will expire on February 11, 2019. 53 Copyright Notice 55 Copyright (c) 2018 IETF Trust and the persons identified as the 56 document authors. All rights reserved. 58 This document is subject to BCP 78 and the IETF Trust's Legal 59 Provisions Relating to IETF Documents 60 (https://trustee.ietf.org/license-info) in effect on the date of 61 publication of this document. Please review these documents 62 carefully, as they describe your rights and restrictions with respect 63 to this document. Code Components extracted from this document must 64 include Simplified BSD License text as described in Section 4.e of 65 the Trust Legal Provisions and are provided without warranty as 66 described in the Simplified BSD License. 68 Table of Contents 70 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 71 2. Deployment Model and Operator Experience . . . . . . . . . . 5 72 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 7 73 4. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 7 74 5. Character Encoding . . . . . . . . . . . . . . . . . . . . . 10 75 6. Message Transport . . . . . . . . . . . . . . . . . . . . . . 10 76 6.1. HTTPS Requests . . . . . . . . . . . . . . . . . . . . . 10 77 6.2. Request Authentication . . . . . . . . . . . . . . . . . 11 78 6.3. Request URL Integrity . . . . . . . . . . . . . . . . . . 12 79 6.3.1. "url" (URL) JWS Header Parameter . . . . . . . . . . 13 80 6.4. Replay protection . . . . . . . . . . . . . . . . . . . . 13 81 6.4.1. Replay-Nonce . . . . . . . . . . . . . . . . . . . . 14 82 6.4.2. "nonce" (Nonce) JWS Header Parameter . . . . . . . . 14 83 6.5. Rate Limits . . . . . . . . . . . . . . . . . . . . . . . 14 84 6.6. Errors . . . . . . . . . . . . . . . . . . . . . . . . . 15 85 6.6.1. Subproblems . . . . . . . . . . . . . . . . . . . . . 17 86 7. Certificate Management . . . . . . . . . . . . . . . . . . . 18 87 7.1. Resources . . . . . . . . . . . . . . . . . . . . . . . . 18 88 7.1.1. Directory . . . . . . . . . . . . . . . . . . . . . . 21 89 7.1.2. Account Objects . . . . . . . . . . . . . . . . . . . 23 90 7.1.3. Order Objects . . . . . . . . . . . . . . . . . . . . 24 91 7.1.4. Authorization Objects . . . . . . . . . . . . . . . . 27 92 7.1.5. Challenge Objects . . . . . . . . . . . . . . . . . . 29 93 7.1.6. Status Changes . . . . . . . . . . . . . . . . . . . 29 94 7.2. Getting a Nonce . . . . . . . . . . . . . . . . . . . . . 31 95 7.3. Account Creation . . . . . . . . . . . . . . . . . . . . 32 96 7.3.1. Finding an Account URL Given a Key . . . . . . . . . 34 97 7.3.2. Account Update . . . . . . . . . . . . . . . . . . . 35 98 7.3.3. Account Information . . . . . . . . . . . . . . . . . 35 99 7.3.4. Changes of Terms of Service . . . . . . . . . . . . . 36 100 7.3.5. External Account Binding . . . . . . . . . . . . . . 36 101 7.3.6. Account Key Roll-over . . . . . . . . . . . . . . . . 38 102 7.3.7. Account Deactivation . . . . . . . . . . . . . . . . 41 103 7.4. Applying for Certificate Issuance . . . . . . . . . . . . 42 104 7.4.1. Pre-Authorization . . . . . . . . . . . . . . . . . . 47 105 7.4.2. Downloading the Certificate . . . . . . . . . . . . . 49 106 7.5. Identifier Authorization . . . . . . . . . . . . . . . . 50 107 7.5.1. Responding to Challenges . . . . . . . . . . . . . . 51 108 7.5.2. Deactivating an Authorization . . . . . . . . . . . . 53 109 7.6. Certificate Revocation . . . . . . . . . . . . . . . . . 54 110 8. Identifier Validation Challenges . . . . . . . . . . . . . . 56 111 8.1. Key Authorizations . . . . . . . . . . . . . . . . . . . 58 112 8.2. Retrying Challenges . . . . . . . . . . . . . . . . . . . 58 113 8.3. HTTP Challenge . . . . . . . . . . . . . . . . . . . . . 59 114 8.4. DNS Challenge . . . . . . . . . . . . . . . . . . . . . . 61 115 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 63 116 9.1. MIME Type: application/pem-certificate-chain . . . . . . 63 117 9.2. Well-Known URI for the HTTP Challenge . . . . . . . . . . 64 118 9.3. Replay-Nonce HTTP Header . . . . . . . . . . . . . . . . 64 119 9.4. "url" JWS Header Parameter . . . . . . . . . . . . . . . 64 120 9.5. "nonce" JWS Header Parameter . . . . . . . . . . . . . . 65 121 9.6. URN Sub-namespace for ACME (urn:ietf:params:acme) . . . . 65 122 9.7. New Registries . . . . . . . . . . . . . . . . . . . . . 65 123 9.7.1. Fields in Account Objects . . . . . . . . . . . . . . 66 124 9.7.2. Fields in Order Objects . . . . . . . . . . . . . . . 67 125 9.7.3. Fields in Authorization Objects . . . . . . . . . . . 68 126 9.7.4. Error Types . . . . . . . . . . . . . . . . . . . . . 69 127 9.7.5. Resource Types . . . . . . . . . . . . . . . . . . . 69 128 9.7.6. Fields in the "meta" Object within a Directory Object 70 129 9.7.7. Identifier Types . . . . . . . . . . . . . . . . . . 71 130 9.7.8. Validation Methods . . . . . . . . . . . . . . . . . 71 131 10. Security Considerations . . . . . . . . . . . . . . . . . . . 73 132 10.1. Threat Model . . . . . . . . . . . . . . . . . . . . . . 73 133 10.2. Integrity of Authorizations . . . . . . . . . . . . . . 74 134 10.3. Denial-of-Service Considerations . . . . . . . . . . . . 78 135 10.4. Server-Side Request Forgery . . . . . . . . . . . . . . 78 136 10.5. CA Policy Considerations . . . . . . . . . . . . . . . . 79 137 11. Operational Considerations . . . . . . . . . . . . . . . . . 80 138 11.1. Key Selection . . . . . . . . . . . . . . . . . . . . . 80 139 11.2. DNS security . . . . . . . . . . . . . . . . . . . . . . 81 140 11.3. Token Entropy . . . . . . . . . . . . . . . . . . . . . 81 141 11.4. Malformed Certificate Chains . . . . . . . . . . . . . . 82 142 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 82 143 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 83 144 13.1. Normative References . . . . . . . . . . . . . . . . . . 83 145 13.2. Informative References . . . . . . . . . . . . . . . . . 86 146 13.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 87 147 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 87 149 1. Introduction 151 Certificates [RFC5280] in the Web PKI are most commonly used to 152 authenticate domain names. Thus, certification authorities (CAs) in 153 the Web PKI are trusted to verify that an applicant for a certificate 154 legitimately represents the domain name(s) in the certificate. 156 Different types of certificates reflect different kinds of CA 157 verification of information about the certificate subject. "Domain 158 Validation" (DV) certificates are by far the most common type. The 159 only validation the CA is required to perform in the DV issuance 160 process is to verify that the requester has effective control of the 161 domain. The CA is not required to attempt to verify the requester's 162 real-world identity. (This is as opposed to "Organization 163 Validation" (OV) and "Extended Validation" (EV) certificates, where 164 the process is intended to also verify the real-world identity of the 165 requester.) 167 Existing Web PKI certificate authorities tend to use a set of ad hoc 168 protocols for certificate issuance and identity verification. In the 169 case of DV certificates, a typical user experience is something like: 171 o Generate a PKCS#10 [RFC2986] Certificate Signing Request (CSR). 173 o Cut-and-paste the CSR into a CA's web page. 175 o Prove ownership of the domain by one of the following methods: 177 * Put a CA-provided challenge at a specific place on the web 178 server. 180 * Put a CA-provided challenge in a DNS record corresponding to 181 the target domain. 183 * Receive a CA-provided challenge at a (hopefully) administrator- 184 controlled email address corresponding to the domain and then 185 respond to it on the CA's web page. 187 o Download the issued certificate and install it on their Web 188 Server. 190 With the exception of the CSR itself and the certificates that are 191 issued, these are all completely ad hoc procedures and are 192 accomplished by getting the human user to follow interactive natural- 193 language instructions from the CA rather than by machine-implemented 194 published protocols. In many cases, the instructions are difficult 195 to follow and cause significant frustration and confusion. Informal 196 usability tests by the authors indicate that webmasters often need 197 1-3 hours to obtain and install a certificate for a domain. Even in 198 the best case, the lack of published, standardized mechanisms 199 presents an obstacle to the wide deployment of HTTPS and other PKIX- 200 dependent systems because it inhibits mechanization of tasks related 201 to certificate issuance, deployment, and revocation. 203 This document describes an extensible framework for automating the 204 issuance and domain validation procedure, thereby allowing servers 205 and infrastructure software to obtain certificates without user 206 interaction. Use of this protocol should radically simplify the 207 deployment of HTTPS and the practicality of PKIX-based authentication 208 for other protocols based on Transport Layer Security (TLS) 209 [RFC5246]. 211 It should be noted that while the focus of this document is on 212 validating domain names for purposes of issuing certificates in the 213 Web PKI, ACME supports extensions for uses with other identifiers in 214 other PKI contexts. For example, as of this writing, there is 215 ongoing work to use ACME for issuance of Web PKI certificates 216 attesting to IP addresses [I-D.ietf-acme-ip] and STIR certificates 217 attesting to telephone numbers [I-D.ietf-acme-telephone]. 219 ACME can also be used to automate some aspects of certificate 220 management even where non-automated processes are still needed. For 221 example, the external account binding feature (see Section 7.3.5) can 222 allow an ACME account to use authorizations that have been granted to 223 an external, non-ACME account. This allows ACME to address issuance 224 scenarios that cannot yet be fully automated, such as the issuance of 225 Extended Validation certificates. 227 2. Deployment Model and Operator Experience 229 The guiding use case for ACME is obtaining certificates for websites 230 (HTTPS [RFC2818]). In this case, the user's web server is intended 231 to speak for one or more domains, and the process of certificate 232 issuance is intended to verify that this web server actually speaks 233 for the domain(s). 235 DV certificate validation commonly checks claims about properties 236 related to control of a domain name - properties that can be observed 237 by the certificate issuer in an interactive process that can be 238 conducted purely online. That means that under typical 239 circumstances, all steps in the request, verification, and issuance 240 process can be represented and performed by Internet protocols with 241 no out-of-band human intervention. 243 Prior to ACME, when deploying an HTTPS server, a server operator 244 typically gets a prompt to generate a self-signed certificate. If 245 the operator were instead deploying an HTTPS server using ACME, the 246 experience would be something like this: 248 o The operator's ACME client prompts the operator for the intended 249 domain name(s) that the web server is to stand for. 251 o The ACME client presents the operator with a list of CAs from 252 which it could get a certificate. (This list will change over 253 time based on the capabilities of CAs and updates to ACME 254 configuration.) The ACME client might prompt the operator for 255 payment information at this point. 257 o The operator selects a CA. 259 o In the background, the ACME client contacts the CA and requests 260 that it issue a certificate for the intended domain name(s). 262 o The CA verifies that the client controls the requested domain 263 name(s) by having the ACME client perform some action(s) that can 264 only be done with control of the domain name(s). For example, the 265 CA might require a client requesting example.com to provision DNS 266 record under example.com or an HTTP resource under 267 http://example.com. 269 o Once the CA is satisfied, it issues the certificate and the ACME 270 client automatically downloads and installs it, potentially 271 notifying the operator via email, SMS, etc. 273 o The ACME client periodically contacts the CA to get updated 274 certificates, stapled OCSP responses, or whatever else would be 275 required to keep the web server functional and its credentials up- 276 to-date. 278 In this way, it would be nearly as easy to deploy with a CA-issued 279 certificate as with a self-signed certificate. Furthermore, the 280 maintenance of that CA-issued certificate would require minimal 281 manual intervention. Such close integration of ACME with HTTPS 282 servers allows the immediate and automated deployment of certificates 283 as they are issued, sparing the human administrator from much of the 284 time-consuming work described in the previous section. 286 3. Terminology 288 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 289 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 290 "OPTIONAL" in this document are to be interpreted as described in BCP 291 14 [RFC2119] [RFC8174] when, and only when, they appear in all 292 capitals, as shown here. 294 The two main roles in ACME are "client" and "server". The ACME 295 client uses the protocol to request certificate management actions, 296 such as issuance or revocation. An ACME client may run on a web 297 server, mail server, or some other server system which requires valid 298 X.509 certificates. Or, it may run on a separate server that does 299 not consume the certificate, but is authorized to respond to a CA- 300 provided challenge. The ACME server runs at a certification 301 authority, and responds to client requests, performing the requested 302 actions if the client is authorized. 304 An ACME client authenticates to the server by means of an "account 305 key pair". The client uses the private key of this key pair to sign 306 all messages sent to the server. The server uses the public key to 307 verify the authenticity and integrity of messages from the client. 309 4. Protocol Overview 311 ACME allows a client to request certificate management actions using 312 a set of JavaScript Object Notation (JSON) messages carried over 313 HTTPS. Issuance using ACME resembles a traditional CA's issuance 314 process, in which a user creates an account, requests a certificate, 315 and proves control of the domain(s) in that certificate in order for 316 the CA to issue the requested certificate. 318 The first phase of ACME is for the client to request an account with 319 the ACME server. The client generates an asymmetric key pair and 320 requests a new account, optionally providing contact information, 321 agreeing to terms of service, and/or associating the account with an 322 existing account in another system. The creation request is signed 323 with the generated private key to prove that the client controls it. 325 Client Server 327 [Contact Information] 328 [ToS Agreement] 329 [Additional Data] 330 Signature -------> 331 Account URL 332 <------- Account Object 334 [] Information covered by request signatures 336 Account Creation 338 Once an account is registered, there are four major steps the client 339 needs to take to get a certificate: 341 1. Submit an order for a certificate to be issued 343 2. Prove control of any identifiers requested in the certificate 345 3. Finalize the order by submitting a CSR 347 4. Await issuance and download the issued certificate 349 The client's order for a certificate describes the desired 350 identifiers plus a few additional fields that capture semantics that 351 are not supported in the CSR format. If the server is willing to 352 consider issuing such a certificate, it responds with a list of 353 requirements that the client must satisfy before the certificate will 354 be issued. 356 For example, in most cases, the server will require the client to 357 demonstrate that it controls the identifiers in the requested 358 certificate. Because there are many different ways to validate 359 possession of different types of identifiers, the server will choose 360 from an extensible set of challenges that are appropriate for the 361 identifier being claimed. The client responds with a set of 362 responses that tell the server which challenges the client has 363 completed. The server then validates that the client has completed 364 the challenges. 366 Once the validation process is complete and the server is satisfied 367 that the client has met its requirements, the client finalizes the 368 order by submitting a PKCS#10 Certificate Signing Request (CSR). The 369 server will issue the requested certificate and make it available to 370 the client. 372 Client Server 374 [Order] 375 Signature -------> 376 <------- Required Authorizations 378 [Responses] 379 Signature -------> 381 <~~~~~~~~Validation~~~~~~~~> 383 [CSR] 384 Signature -------> 385 <------- Acknowledgement 387 <~~~~~~Await issuance~~~~~~> 389 GET request -------> 390 <------- Certificate 392 [] Information covered by request signatures 394 Certificate Issuance 396 To revoke a certificate, the client sends a signed revocation request 397 indicating the certificate to be revoked: 399 Client Server 401 [Revocation request] 402 Signature --------> 404 <-------- Result 406 [] Information covered by request signatures 408 Certificate Revocation 410 Note that while ACME is defined with enough flexibility to handle 411 different types of identifiers in principle, the primary use case 412 addressed by this document is the case where domain names are used as 413 identifiers. For example, all of the identifier validation 414 challenges described in Section 8 below address validation of domain 415 names. The use of ACME for other identifiers will require further 416 specification in order to describe how these identifiers are encoded 417 in the protocol and what types of validation challenges the server 418 might require. 420 5. Character Encoding 422 All requests and responses sent via HTTP by ACME clients, ACME 423 servers, and validation servers as well as any inputs for digest 424 computations MUST be encoded using the UTF-8 [RFC3629] character set. 426 6. Message Transport 428 Communications between an ACME client and an ACME server are done 429 over HTTPS, using JSON Web Signature (JWS) [RFC7515] to provide some 430 additional security properties for messages sent from the client to 431 the server. HTTPS provides server authentication and 432 confidentiality. With some ACME-specific extensions, JWS provides 433 authentication of the client's request payloads, anti-replay 434 protection, and integrity for the HTTPS request URL. 436 6.1. HTTPS Requests 438 Each ACME function is accomplished by the client sending a sequence 439 of HTTPS requests to the server, carrying JSON messages 440 [RFC2818][RFC7159]. Use of HTTPS is REQUIRED. Each subsection of 441 Section 7 below describes the message formats used by the function 442 and the order in which messages are sent. 444 In most HTTPS transactions used by ACME, the ACME client is the HTTPS 445 client and the ACME server is the HTTPS server. The ACME server acts 446 as an HTTP and HTTPS client when validating challenges via HTTP. 448 ACME servers SHOULD follow the recommendations of [RFC7525] when 449 configuring their TLS implementations. ACME servers that support TLS 450 1.3 MAY allow clients to send early data (0-RTT). This is safe 451 because the ACME protocol itself includes anti-replay protections 452 (see Section 6.4). 454 ACME clients MUST send a User-Agent header, in accordance with 455 [RFC7231]. This header SHOULD include the name and version of the 456 ACME software in addition to the name and version of the underlying 457 HTTP client software. 459 ACME clients SHOULD send an Accept-Language header in accordance with 460 [RFC7231] to enable localization of error messages. 462 ACME servers that are intended to be generally accessible need to use 463 Cross-Origin Resource Sharing (CORS) in order to be accessible from 464 browser-based clients [W3C.CR-cors-20130129]. Such servers SHOULD 465 set the Access-Control-Allow-Origin header field to the value "*". 467 Binary fields in the JSON objects used by ACME are encoded using 468 base64url encoding described in [RFC4648] Section 5, according to the 469 profile specified in JSON Web Signature [RFC7515] Section 2. This 470 encoding uses a URL safe character set. Trailing '=' characters MUST 471 be stripped. Encoded values that include trailing '=' characters 472 MUST be rejected as improperly encoded. 474 6.2. Request Authentication 476 All ACME requests with a non-empty body MUST encapsulate their 477 payload in a JSON Web Signature (JWS) [RFC7515] object, signed using 478 the account's private key unless otherwise specified. The server 479 MUST verify the JWS before processing the request. Encapsulating 480 request bodies in JWS provides authentication of requests. 482 JWS objects sent in ACME requests MUST meet the following additional 483 criteria: 485 o The JWS MUST be in the Flattened JSON Serialization [RFC7515] 487 o The JWS MUST NOT have multiple signatures 489 o The JWS Unencoded Payload Option [RFC7797] MUST NOT be used 491 o The JWS Unprotected Header [RFC7515] MUST NOT be used 493 o The JWS Payload MUST NOT be detached 495 o The JWS Protected Header MUST include the following fields: 497 * "alg" (Algorithm) 499 + This field MUST NOT contain "none" or a Message 500 Authentication Code (MAC)-based algorithm 502 * "nonce" (defined in Section 6.4 below) 504 * "url" (defined in Section 6.3 below) 506 * Either "jwk" (JSON Web Key) or "kid" (Key ID) as specified 507 below 509 An ACME server MUST implement the "ES256" signature algorithm 510 [RFC7518] and SHOULD implement the "EdDSA" signature algorithm using 511 the "Ed25519" variant (indicated by "crv") [RFC8037]. 513 The "jwk" and "kid" fields are mutually exclusive. Servers MUST 514 reject requests that contain both. 516 For newAccount requests, and for revokeCert requests authenticated by 517 a certificate key, there MUST be a "jwk" field. This field MUST 518 contain the public key corresponding to the private key used to sign 519 the JWS. 521 For all other requests, the request is signed using an existing 522 account and there MUST be a "kid" field. This field MUST contain the 523 account URL received by POSTing to the newAccount resource. 525 Note that authentication via signed JWS request bodies implies that 526 GET requests are not authenticated. Servers MUST NOT respond to GET 527 requests for resources that might be considered sensitive. Account 528 resources are the only sensitive resources defined in this 529 specification. 531 If the client sends a JWS signed with an algorithm that the server 532 does not support, then the server MUST return an error with status 533 code 400 (Bad Request) and type 534 "urn:ietf:params:acme:error:badSignatureAlgorithm". The problem 535 document returned with the error MUST include an "algorithms" field 536 with an array of supported "alg" values. See Section 6.6 for more 537 details on the structure of error responses. 539 Because client requests in ACME carry JWS objects in the Flattened 540 JSON Serialization, they must have the "Content-Type" header field 541 set to "application/jose+json". If a request does not meet this 542 requirement, then the server MUST return a response with status code 543 415 (Unsupported Media Type). 545 6.3. Request URL Integrity 547 It is common in deployment for the entity terminating TLS for HTTPS 548 to be different from the entity operating the logical HTTPS server, 549 with a "request routing" layer in the middle. For example, an ACME 550 CA might have a content delivery network terminate TLS connections 551 from clients so that it can inspect client requests for denial-of- 552 service protection. 554 These intermediaries can also change values in the request that are 555 not signed in the HTTPS request, e.g., the request URL and headers. 556 ACME uses JWS to provide an integrity mechanism, which protects 557 against an intermediary changing the request URL to another ACME URL. 559 As noted in Section 6.2 above, all ACME request objects carry a "url" 560 header parameter in their protected header. This header parameter 561 encodes the URL to which the client is directing the request. On 562 receiving such an object in an HTTP request, the server MUST compare 563 the "url" header parameter to the request URL. If the two do not 564 match, then the server MUST reject the request as unauthorized. 566 Except for the directory resource, all ACME resources are addressed 567 with URLs provided to the client by the server. In requests sent to 568 these resources, the client MUST set the "url" header parameter to 569 the exact string provided by the server (rather than performing any 570 re-encoding on the URL). The server SHOULD perform the corresponding 571 string equality check, configuring each resource with the URL string 572 provided to clients and having the resource check that requests have 573 the same string in their "url" header parameter. 575 6.3.1. "url" (URL) JWS Header Parameter 577 The "url" header parameter specifies the URL [RFC3986] to which this 578 JWS object is directed. The "url" header parameter MUST be carried 579 in the protected header of the JWS. The value of the "url" header 580 parameter MUST be a string representing the target URL. 582 6.4. Replay protection 584 In order to protect ACME resources from any possible replay attacks, 585 ACME requests have a mandatory anti-replay mechanism. This mechanism 586 is based on the server maintaining a list of nonces that it has 587 issued to clients, and requiring any signed request from the client 588 to carry such a nonce. 590 An ACME server provides nonces to clients using the HTTP Replay-Nonce 591 header field, as specified in Section 6.4.1 below. The server MUST 592 include a Replay-Nonce header field in every successful response to a 593 POST request and SHOULD provide it in error responses as well. 595 Every JWS sent by an ACME client MUST include, in its protected 596 header, the "nonce" header parameter, with contents as defined in 597 Section 6.4.2 below. As part of JWS verification, the ACME server 598 MUST verify that the value of the "nonce" header is a value that the 599 server previously provided in a Replay-Nonce header field. Once a 600 nonce value has appeared in an ACME request, the server MUST consider 601 it invalid, in the same way as a value it had never issued. 603 When a server rejects a request because its nonce value was 604 unacceptable (or not present), it MUST provide HTTP status code 400 605 (Bad Request), and indicate the ACME error type 606 "urn:ietf:params:acme:error:badNonce". An error response with the 607 "badNonce" error type MUST include a Replay-Nonce header with a fresh 608 nonce. On receiving such a response, a client SHOULD retry the 609 request using the new nonce. 611 The precise method used to generate and track nonces is up to the 612 server. For example, the server could generate a random 128-bit 613 value for each response, keep a list of issued nonces, and strike 614 nonces from this list as they are used. 616 6.4.1. Replay-Nonce 618 The "Replay-Nonce" header field includes a server-generated value 619 that the server can use to detect unauthorized replay in future 620 client requests. The server MUST generate the value provided in 621 Replay-Nonce in such a way that they are unique to each message, with 622 high probability. For instance, it is acceptable to generate Replay- 623 Nonces randomly. 625 The value of the Replay-Nonce field MUST be an octet string encoded 626 according to the base64url encoding described in Section 2 of 627 [RFC7515]. Clients MUST ignore invalid Replay-Nonce values. The 628 ABNF [RFC5234] for the Replay-Nonce header field follows: 630 base64url = [A-Z] / [a-z] / [0-9] / "-" / "_" 632 Replay-Nonce = *base64url 634 The Replay-Nonce header field SHOULD NOT be included in HTTP request 635 messages. 637 6.4.2. "nonce" (Nonce) JWS Header Parameter 639 The "nonce" header parameter provides a unique value that enables the 640 verifier of a JWS to recognize when replay has occurred. The "nonce" 641 header parameter MUST be carried in the protected header of the JWS. 643 The value of the "nonce" header parameter MUST be an octet string, 644 encoded according to the base64url encoding described in Section 2 of 645 [RFC7515]. If the value of a "nonce" header parameter is not valid 646 according to this encoding, then the verifier MUST reject the JWS as 647 malformed. 649 6.5. Rate Limits 651 Creation of resources can be rate limited by ACME servers to ensure 652 fair usage and prevent abuse. Once the rate limit is exceeded, the 653 server MUST respond with an error with the type 654 "urn:ietf:params:acme:error:rateLimited". Additionally, the server 655 SHOULD send a "Retry-After" header [RFC7231] indicating when the 656 current request may succeed again. If multiple rate limits are in 657 place, that is the time where all rate limits allow access again for 658 the current request with exactly the same parameters. 660 In addition to the human-readable "detail" field of the error 661 response, the server MAY send one or multiple link relations in the 662 "Link" header [RFC8288] pointing to documentation about the specific 663 rate limit that was hit, using the "help" link relation type. 665 6.6. Errors 667 Errors can be reported in ACME both at the HTTP layer and within 668 challenge objects as defined in Section 8. ACME servers can return 669 responses with an HTTP error response code (4XX or 5XX). For 670 example: If the client submits a request using a method not allowed 671 in this document, then the server MAY return status code 405 (Method 672 Not Allowed). 674 When the server responds with an error status, it SHOULD provide 675 additional information using a problem document [RFC7807]. To 676 facilitate automatic response to errors, this document defines the 677 following standard tokens for use in the "type" field (within the 678 ACME URN namespace "urn:ietf:params:acme:error:"): 680 +-------------------------+-----------------------------------------+ 681 | Type | Description | 682 +-------------------------+-----------------------------------------+ 683 | accountDoesNotExist | The request specified an account that | 684 | | does not exist | 685 | | | 686 | alreadyRevoked | The request specified a certificate to | 687 | | be revoked that has already been | 688 | | revoked | 689 | | | 690 | badCSR | The CSR is unacceptable (e.g., due to a | 691 | | short key) | 692 | | | 693 | badNonce | The client sent an unacceptable anti- | 694 | | replay nonce | 695 | | | 696 | badRevocationReason | The revocation reason provided is not | 697 | | allowed by the server | 698 | | | 699 | badSignatureAlgorithm | The JWS was signed with an algorithm | 700 | | the server does not support | 701 | | | 702 | caa | Certification Authority Authorization | 703 | | (CAA) records forbid the CA from | 704 | | issuing | 705 | | | 706 | compound | Specific error conditions are indicated | 707 | | in the "subproblems" array. | 708 | | | 709 | connection | The server could not connect to | 710 | | validation target | 711 | | | 712 | dns | There was a problem with a DNS query | 713 | | during identifier validation | 714 | | | 715 | externalAccountRequired | The request must include a value for | 716 | | the "externalAccountBinding" field | 717 | | | 718 | incorrectResponse | Response received didn't match the | 719 | | challenge's requirements | 720 | | | 721 | invalidContact | A contact URL for an account was | 722 | | invalid | 723 | | | 724 | malformed | The request message was malformed | 725 | | | 726 | rateLimited | The request exceeds a rate limit | 727 | | | 728 | rejectedIdentifier | The server will not issue for the | 729 | | identifier | 730 | | | 731 | serverInternal | The server experienced an internal | 732 | | error | 733 | | | 734 | tls | The server received a TLS error during | 735 | | validation | 736 | | | 737 | unauthorized | The client lacks sufficient | 738 | | authorization | 739 | | | 740 | unsupportedContact | A contact URL for an account used an | 741 | | unsupported protocol scheme | 742 | | | 743 | unsupportedIdentifier | Identifier is not supported, but may be | 744 | | in future | 745 | | | 746 | userActionRequired | Visit the "instance" URL and take | 747 | | actions specified there | 748 +-------------------------+-----------------------------------------+ 750 This list is not exhaustive. The server MAY return errors whose 751 "type" field is set to a URI other than those defined above. Servers 752 MUST NOT use the ACME URN namespace Section 9.6 for errors other than 753 the standard types. Clients SHOULD display the "detail" field of all 754 errors. 756 In the remainder of this document, we use the tokens in the table 757 above to refer to error types, rather than the full URNs. For 758 example, an "error of type 'badCSR'" refers to an error document with 759 "type" value "urn:ietf:params:acme:error:badCSR". 761 6.6.1. Subproblems 763 Sometimes a CA may need to return multiple errors in response to a 764 request. Additionally, the CA may need to attribute errors to 765 specific identifiers. For instance, a new-order request may contain 766 multiple identifiers for which the CA cannot issue. In this 767 situation, an ACME problem document MAY contain the "subproblems" 768 field, containing a JSON array of problem documents, each of which 769 MAY contain an "identifier" field. If present, the "identifier" 770 field MUST contain an ACME identifier (Section 9.7.7). The 771 "identifier" field MUST NOT be present at the top level in ACME 772 problem documents. It can only be present in subproblems. 773 Subproblems need not all have the same type, and do not need to match 774 the top level type. 776 ACME clients may choose to use the "identifier" field of a subproblem 777 as a hint that an operation would succeed if that identifier were 778 omitted. For instance, if an order contains ten DNS identifiers, and 779 the new-order request returns a problem document with two 780 subproblems, referencing two of those identifiers, the ACME client 781 may choose to submit another order containing only the eight 782 identifiers not listed in the problem document. 784 HTTP/1.1 403 Forbidden 785 Content-Type: application/problem+json 787 { 788 "type": "urn:ietf:params:acme:error:malformed", 789 "detail": "Some of the identifiers requested were rejected", 790 "subproblems": [ 791 { 792 "type": "urn:ietf:params:acme:error:malformed", 793 "detail": "Invalid underscore in DNS name \"_example.com\"", 794 "identifier": { 795 "type": "dns", 796 "value": "_example.com" 797 } 798 }, 799 { 800 "type": "urn:ietf:params:acme:error:rejectedIdentifier", 801 "detail": "This CA will not issue for \"example.net\"", 802 "identifier": { 803 "type": "dns", 804 "value": "example.net" 805 } 806 } 807 ] 808 } 810 7. Certificate Management 812 In this section, we describe the certificate management functions 813 that ACME enables: 815 o Account Creation 817 o Ordering a Certificate 819 o Identifier Authorization 821 o Certificate Issuance 823 o Certificate Revocation 825 7.1. Resources 827 ACME is structured as a REST [REST] application with the following 828 types of resources: 830 o Account resources, representing information about an account 831 (Section 7.1.2, Section 7.3) 833 o Order resources, representing an account's requests to issue 834 certificates (Section 7.1.3) 836 o Authorization resources, representing an account's authorization 837 to act for an identifier (Section 7.1.4) 839 o Challenge resources, representing a challenge to prove control of 840 an identifier (Section 7.5, Section 8) 842 o Certificate resources, representing issued certificates 843 (Section 7.4.2) 845 o A "directory" resource (Section 7.1.1) 847 o A "newNonce" resource (Section 7.2) 849 o A "newAccount" resource (Section 7.3) 851 o A "newOrder" resource (Section 7.4) 853 o A "revokeCert" resource (Section 7.6) 855 o A "keyChange" resource (Section 7.3.6) 857 The server MUST provide "directory" and "newNonce" resources. 859 ACME uses different URLs for different management functions. Each 860 function is listed in a directory along with its corresponding URL, 861 so clients only need to be configured with the directory URL. These 862 URLs are connected by a few different link relations [RFC5988]. 864 The "up" link relation is used with challenge resources to indicate 865 the authorization resource to which a challenge belongs. It is also 866 used from certificate resources to indicate a resource from which the 867 client may fetch a chain of CA certificates that could be used to 868 validate the certificate in the original resource. 870 The "index" link relation is present on all resources other than the 871 directory and indicates the URL of the directory. 873 The following diagram illustrates the relations between resources on 874 an ACME server. For the most part, these relations are expressed by 875 URLs provided as strings in the resources' JSON representations. 876 Lines with labels in quotes indicate HTTP link relations. 878 directory 879 | 880 +--> newNonce 881 | 882 +----------+----------+-----+-----+------------+ 883 | | | | | 884 | | | | | 885 V V V V V 886 newAccount newAuthz newOrder revokeCert keyChange 887 | | | 888 | | | 889 V | V 890 account | order --+--> finalize 891 | | | 892 | | +--> cert 893 | V 894 +---> authorization 895 | ^ 896 | | "up" 897 V | 898 challenge 900 ACME Resources and Relationships 902 The following table illustrates a typical sequence of requests 903 required to establish a new account with the server, prove control of 904 an identifier, issue a certificate, and fetch an updated certificate 905 some time after issuance. The "->" is a mnemonic for a Location 906 header pointing to a created resource. 908 +-----------------------+--------------------------+----------------+ 909 | Action | Request | Response | 910 +-----------------------+--------------------------+----------------+ 911 | Get directory | GET directory | 200 | 912 | | | | 913 | Get nonce | HEAD newNonce | 200 | 914 | | | | 915 | Create account | POST newAccount | 201 -> account | 916 | | | | 917 | Submit order | POST newOrder | 201 -> order | 918 | | | | 919 | Fetch challenges | GET order | 200 | 920 | | authorizations | | 921 | | | | 922 | Respond to challenges | POST challenge urls | 200 | 923 | | | | 924 | Poll for status | GET order | 200 | 925 | | | | 926 | Finalize order | POST order finalize | 200 | 927 | | | | 928 | Poll for status | GET order | 200 | 929 | | | | 930 | Download certificate | GET order certificate | 200 | 931 +-----------------------+--------------------------+----------------+ 933 The remainder of this section provides the details of how these 934 resources are structured and how the ACME protocol makes use of them. 936 7.1.1. Directory 938 In order to help clients configure themselves with the right URLs for 939 each ACME operation, ACME servers provide a directory object. This 940 should be the only URL needed to configure clients. It is a JSON 941 object, whose field names are drawn from the following table and 942 whose values are the corresponding URLs. 944 +------------+--------------------+ 945 | Field | URL in value | 946 +------------+--------------------+ 947 | newNonce | New nonce | 948 | | | 949 | newAccount | New account | 950 | | | 951 | newOrder | New order | 952 | | | 953 | newAuthz | New authorization | 954 | | | 955 | revokeCert | Revoke certificate | 956 | | | 957 | keyChange | Key Change | 958 +------------+--------------------+ 960 There is no constraint on the URL of the directory except that it 961 should be different from the other ACME server resources' URLs, and 962 that it should not clash with other services. For instance: 964 o a host which functions as both an ACME and a Web server may want 965 to keep the root path "/" for an HTML "front page", and place the 966 ACME directory under the path "/acme". 968 o a host which only functions as an ACME server could place the 969 directory under the path "/". 971 If the ACME server does not implement pre-authorization 972 (Section 7.4.1) it MUST omit the "newAuthz" field of the directory. 974 The object MAY additionally contain a field "meta". If present, it 975 MUST be a JSON object; each field in the object is an item of 976 metadata relating to the service provided by the ACME server. 978 The following metadata items are defined, all of which are OPTIONAL: 980 termsOfService (optional, string): A URL identifying the current 981 terms of service. 983 website (optional, string): An HTTP or HTTPS URL locating a website 984 providing more information about the ACME server. 986 caaIdentities (optional, array of string): Each string MUST be a 987 lowercase hostname which the ACME server recognizes as referring 988 to itself for the purposes of CAA record validation as defined in 989 [RFC6844]. This allows clients to determine the correct issuer 990 domain name to use when configuring CAA records. 992 externalAccountRequired (optional, boolean): If this field is 993 present and set to "true", then the CA requires that all new- 994 account requests include an "externalAccountBinding" field 995 associating the new account with an external account. 997 Clients access the directory by sending a GET request to the 998 directory URL. 1000 HTTP/1.1 200 OK 1001 Content-Type: application/json 1003 { 1004 "newNonce": "https://example.com/acme/new-nonce", 1005 "newAccount": "https://example.com/acme/new-account", 1006 "newOrder": "https://example.com/acme/new-order", 1007 "newAuthz": "https://example.com/acme/new-authz", 1008 "revokeCert": "https://example.com/acme/revoke-cert", 1009 "keyChange": "https://example.com/acme/key-change", 1010 "meta": { 1011 "termsOfService": "https://example.com/acme/terms/2017-5-30", 1012 "website": "https://www.example.com/", 1013 "caaIdentities": ["example.com"], 1014 "externalAccountRequired": false 1015 } 1016 } 1018 7.1.2. Account Objects 1020 An ACME account resource represents a set of metadata associated with 1021 an account. Account resources have the following structure: 1023 status (required, string): The status of this account. Possible 1024 values are: "valid", "deactivated", and "revoked". The value 1025 "deactivated" should be used to indicate client-initiated 1026 deactivation whereas "revoked" should be used to indicate server- 1027 initiated deactivation. (See Section 7.1.6) 1029 contact (optional, array of string): An array of URLs that the 1030 server can use to contact the client for issues related to this 1031 account. For example, the server may wish to notify the client 1032 about server-initiated revocation or certificate expiration. 1034 termsOfServiceAgreed (optional, boolean): Including this field in a 1035 new-account request, with a value of true, indicates the client's 1036 agreement with the terms of service. This field is not updateable 1037 by the client. 1039 orders (required, string): A URL from which a list of orders 1040 submitted by this account can be fetched via a GET request, as 1041 described in Section 7.1.2.1. 1043 { 1044 "status": "valid", 1045 "contact": [ 1046 "mailto:cert-admin@example.com", 1047 "mailto:admin@example.com" 1048 ], 1049 "termsOfServiceAgreed": true, 1050 "orders": "https://example.com/acme/acct/1/orders" 1051 } 1053 7.1.2.1. Orders List 1055 Each account object includes an "orders" URL from which a list of 1056 orders created by the account can be fetched via GET request. The 1057 result of the GET request MUST be a JSON object whose "orders" field 1058 is an array of URLs, each identifying an order belonging to the 1059 account. The server SHOULD include pending orders, and SHOULD NOT 1060 include orders that are invalid in the array of URLs. The server MAY 1061 return an incomplete list, along with a Link header with a "next" 1062 link relation indicating where further entries can be acquired. 1064 HTTP/1.1 200 OK 1065 Content-Type: application/json 1066 Link: ;rel="next" 1068 { 1069 "orders": [ 1070 "https://example.com/acme/acct/1/order/1", 1071 "https://example.com/acme/acct/1/order/2", 1072 /* 47 more URLs not shown for example brevity */ 1073 "https://example.com/acme/acct/1/order/50" 1074 ] 1075 } 1077 7.1.3. Order Objects 1079 An ACME order object represents a client's request for a certificate 1080 and is used to track the progress of that order through to issuance. 1081 Thus, the object contains information about the requested 1082 certificate, the authorizations that the server requires the client 1083 to complete, and any certificates that have resulted from this order. 1085 status (required, string): The status of this order. Possible 1086 values are: "pending", "ready", "processing", "valid", and 1087 "invalid". (See Section 7.1.6) 1089 expires (optional, string): The timestamp after which the server 1090 will consider this order invalid, encoded in the format specified 1091 in RFC 3339 [RFC3339]. This field is REQUIRED for objects with 1092 "pending" or "valid" in the status field. 1094 identifiers (required, array of object): An array of identifier 1095 objects that the order pertains to. 1097 type (required, string): The type of identifier. This document 1098 defines the "dns" identifier type. See the registry defined in 1099 Section 9.7.7 for any others. 1101 value (required, string): The identifier itself. 1103 notBefore (optional, string): The requested value of the notBefore 1104 field in the certificate, in the date format defined in [RFC3339]. 1106 notAfter (optional, string): The requested value of the notAfter 1107 field in the certificate, in the date format defined in [RFC3339]. 1109 error (optional, object): The error that occurred while processing 1110 the order, if any. This field is structured as a problem document 1111 [RFC7807]. 1113 authorizations (required, array of string): For pending orders, the 1114 authorizations that the client needs to complete before the 1115 requested certificate can be issued (see Section 7.5), including 1116 unexpired authorizations that the client has completed in the past 1117 for identifiers specified in the order. The authorizations 1118 required are dictated by server policy and there may not be a 1:1 1119 relationship between the order identifiers and the authorizations 1120 required. For final orders (in the "valid" or "invalid" state), 1121 the authorizations that were completed. Each entry is a URL from 1122 which an authorization can be fetched with a GET request. 1124 finalize (required, string): A URL that a CSR must be POSTed to once 1125 all of the order's authorizations are satisfied to finalize the 1126 order. The result of a successful finalization will be the 1127 population of the certificate URL for the order. 1129 certificate (optional, string): A URL for the certificate that has 1130 been issued in response to this order. 1132 { 1133 "status": "valid", 1134 "expires": "2015-03-01T14:09:00Z", 1136 "identifiers": [ 1137 { "type": "dns", "value": "example.com" }, 1138 { "type": "dns", "value": "www.example.com" } 1139 ], 1141 "notBefore": "2016-01-01T00:00:00Z", 1142 "notAfter": "2016-01-08T00:00:00Z", 1144 "authorizations": [ 1145 "https://example.com/acme/authz/1234", 1146 "https://example.com/acme/authz/2345" 1147 ], 1149 "finalize": "https://example.com/acme/acct/1/order/1/finalize", 1151 "certificate": "https://example.com/acme/cert/1234" 1152 } 1154 Any identifier of type "dns" in a new-order request MAY have a 1155 wildcard domain name as its value. A wildcard domain name consists 1156 of a single asterisk character followed by a single full stop 1157 character ("*.") followed by a domain name as defined for use in the 1158 Subject Alternate Name Extension by RFC 5280 [RFC5280]. An 1159 authorization returned by the server for a wildcard domain name 1160 identifier MUST NOT include the asterisk and full stop ("*.") prefix 1161 in the authorization identifier value. The returned authorization 1162 MUST include the optional "wildcard" field, with a value of true. 1164 The elements of the "authorizations" and "identifiers" array are 1165 immutable once set. The server MUST NOT change the contents of 1166 either array after they are created. If a client observes a change 1167 in the contents of either array, then it SHOULD consider the order 1168 invalid. 1170 The "authorizations" array of the order SHOULD reflect all 1171 authorizations that the CA takes into account in deciding to issue, 1172 even if some authorizations were fulfilled in earlier orders or in 1173 pre-authorization transactions. For example, if a CA allows multiple 1174 orders to be fulfilled based on a single authorization transaction, 1175 then it SHOULD reflect that authorization in all of the orders. 1177 Note that just because an authorization URL is listed in the 1178 "authorizations" array of an order object doesn't mean that the 1179 client is required to take action. There are several reasons that 1180 the referenced authorizations may already be valid: 1182 o The client completed the authorization as part of a previous order 1184 o The client previously pre-authorized the identifier (see 1185 Section 7.4.1) 1187 o The server granted the client authorization based on an external 1188 account 1190 Clients SHOULD check the "status" field of an order to determine 1191 whether they need to take any action. 1193 7.1.4. Authorization Objects 1195 An ACME authorization object represents a server's authorization for 1196 an account to represent an identifier. In addition to the 1197 identifier, an authorization includes several metadata fields, such 1198 as the status of the authorization (e.g., "pending", "valid", or 1199 "revoked") and which challenges were used to validate possession of 1200 the identifier. 1202 The structure of an ACME authorization resource is as follows: 1204 identifier (required, object): The identifier that the account is 1205 authorized to represent 1207 type (required, string): The type of identifier. (See below and 1208 Section 9.7.7) 1210 value (required, string): The identifier itself. 1212 status (required, string): The status of this authorization. 1213 Possible values are: "pending", "valid", "invalid", "deactivated", 1214 "expired", and "revoked". (See Section 7.1.6) 1216 expires (optional, string): The timestamp after which the server 1217 will consider this authorization invalid, encoded in the format 1218 specified in RFC 3339 [RFC3339]. This field is REQUIRED for 1219 objects with "valid" in the "status" field. 1221 challenges (required, array of objects): For pending authorizations, 1222 the challenges that the client can fulfill in order to prove 1223 possession of the identifier. For final authorizations (in the 1224 "valid" or "invalid" state), the challenges that were used. Each 1225 array entry is an object with parameters required to validate the 1226 challenge. A client should attempt to fulfill one of these 1227 challenges, and a server should consider any one of the challenges 1228 sufficient to make the authorization valid. 1230 wildcard (optional, boolean): For authorizations created as a result 1231 of a newOrder request containing a DNS identifier with a value 1232 that contained a wildcard prefix this field MUST be present, and 1233 true. 1235 The only type of identifier defined by this specification is a fully- 1236 qualified domain name (type: "dns"). If a domain name contains non- 1237 ASCII Unicode characters it MUST be encoded using the rules defined 1238 in [RFC3492]. Servers MUST verify any identifier values that begin 1239 with the ASCII Compatible Encoding prefix "xn--" as defined in 1240 [RFC5890] are properly encoded. Wildcard domain names (with "*" as 1241 the first label) MUST NOT be included in authorization objects. If 1242 an authorization object conveys authorization for the base domain of 1243 a newOrder DNS type identifier with a wildcard prefix then the 1244 optional authorizations "wildcard" field MUST be present with a value 1245 of true. 1247 Section 8 describes a set of challenges for domain name validation. 1249 { 1250 "status": "valid", 1251 "expires": "2015-03-01T14:09:00Z", 1253 "identifier": { 1254 "type": "dns", 1255 "value": "example.org" 1256 }, 1258 "challenges": [ 1259 { 1260 "url": "https://example.com/acme/authz/1234/0", 1261 "type": "http-01", 1262 "status": "valid", 1263 "token": "DGyRejmCefe7v4NfDGDKfA", 1264 "validated": "2014-12-01T12:05:00Z" 1265 } 1266 ], 1268 "wildcard": false 1269 } 1271 7.1.5. Challenge Objects 1273 An ACME challenge object represents a server's offer to validate a 1274 client's possession of an identifier in a specific way. Unlike the 1275 other objects listed above, there is not a single standard structure 1276 for a challenge object. The contents of a challenge object depend on 1277 the validation method being used. The general structure of challenge 1278 objects and an initial set of validation methods are described in 1279 Section 8. 1281 7.1.6. Status Changes 1283 Each ACME object type goes through a simple state machine over its 1284 lifetime. The "status" field of the object indicates which state the 1285 object is currently in. 1287 Challenge objects are created in the "pending" state. They 1288 transition to the "processing" state when the client responds to the 1289 challenge (see Section 7.5.1) and the server begins attempting to 1290 validate that the client has completed the challenge. Note that 1291 within the "processing" state, the server may attempt to validate the 1292 challenge multiple times (see Section 8.2). Likewise, client 1293 requests for retries do not cause a state change. If validation is 1294 successful, the challenge moves to the "valid" state; if there is an 1295 error, the challenge moves to the "invalid" state. 1297 pending 1298 | 1299 | Receive 1300 | response 1301 V 1302 processing <-+ 1303 | | | Server retry or 1304 | | | client retry request 1305 | +----+ 1306 | 1307 | 1308 Successful | Failed 1309 validation | validation 1310 +---------+---------+ 1311 | | 1312 V V 1313 valid invalid 1315 State Transitions for Challenge Objects 1317 Authorization objects are created in the "pending" state. If one of 1318 the challenges listed in the authorization transitions to the "valid" 1319 state, then the authorization also changes to the "valid" state. If 1320 there is an error while the authorization is still pending, then the 1321 authorization transitions to the "invalid" state. Once the 1322 authorization is in the valid state, it can expire ("expired"), be 1323 deactivated by the client ("deactivated", see Section 7.5.2), or 1324 revoked by the server ("revoked"). 1326 pending --------------------+ 1327 | | 1328 | | 1329 Error | Challenge valid | 1330 +---------+---------+ | 1331 | | | 1332 V V | 1333 invalid valid | 1334 | | 1335 | | 1336 | | 1337 +--------------+--------------+ 1338 | | | 1339 | | | 1340 Server | Client | Time after | 1341 revoke | deactivate | "expires" | 1342 V V V 1343 revoked deactivated expired 1345 State Transitions for Authorization Objects 1347 Order objects are created in the "pending" state. Once all of the 1348 authorizations listed in the order object are in the "valid" state, 1349 the order transitions to the "ready" state. The order moves to the 1350 "processing" state after the client submits a request to the order's 1351 "finalize" URL and the CA begins the issuance process for the 1352 certificate. Once the certificate is issued, the order enters the 1353 "valid" state. If an error occurs at any of these stages, the order 1354 moves to the "invalid" state. The order also moves to the "invalid" 1355 state if it expires, or one of its authorizations enters a final 1356 state other than "valid" ("expired", "revoked", "deactivated"). 1358 pending --------------+ 1359 | | 1360 | All authz | 1361 | "valid" | 1362 V | 1363 ready ---------------+ 1364 | | 1365 | Receive | 1366 | finalize | 1367 | request | 1368 V | 1369 processing ------------+ 1370 | | 1371 | Certificate | Error or 1372 | issued | Authorization failure 1373 V V 1374 valid invalid 1376 State Transitions for Order Objects 1378 Account objects are created in the "valid" state, since no further 1379 action is required to create an account after a successful newAccount 1380 request. If the account is deactivated by the client or revoked by 1381 the server, it moves to the corresponding state. 1383 valid 1384 | 1385 | 1386 +-----------+-----------+ 1387 Client | Server | 1388 deactiv.| revoke | 1389 V V 1390 deactivated revoked 1392 State Transitions for Account Objects 1394 Note that some of these states may not ever appear in a "status" 1395 field, depending on server behavior. For example, a server that 1396 issues synchronously will never show an order in the "processing" 1397 state. A server that deletes expired authorizations immediately will 1398 never show an authorization in the "expired" state. 1400 7.2. Getting a Nonce 1402 Before sending a POST request to the server, an ACME client needs to 1403 have a fresh anti-replay nonce to put in the "nonce" header of the 1404 JWS. In most cases, the client will have gotten a nonce from a 1405 previous request. However, the client might sometimes need to get a 1406 new nonce, e.g., on its first request to the server or if an existing 1407 nonce is no longer valid. 1409 To get a fresh nonce, the client sends a HEAD request to the new- 1410 nonce resource on the server. The server's response MUST include a 1411 Replay-Nonce header field containing a fresh nonce, and SHOULD have 1412 status code 200 (OK). The server MUST also respond to GET requests 1413 for this resource, returning an empty body (while still providing a 1414 Replay-Nonce header) with a 204 (No Content) status. 1416 HEAD /acme/new-nonce HTTP/1.1 1417 Host: example.com 1419 HTTP/1.1 200 OK 1420 Replay-Nonce: oFvnlFP1wIhRlYS2jTaXbA 1421 Cache-Control: no-store 1423 Proxy caching of responses from the new-nonce resource can cause 1424 clients receive the same nonce repeatedly, leading to badNonce 1425 errors. The server MUST include a Cache-Control header field with 1426 the "no-store" directive in responses for the new-nonce resource, in 1427 order to prevent caching of this resource. 1429 7.3. Account Creation 1431 A client creates a new account with the server by sending a POST 1432 request to the server's new-account URL. The body of the request is 1433 a stub account object optionally containing the "contact" and 1434 "termsOfServiceAgreed" fields. 1436 contact (optional, array of string): Same meaning as the 1437 corresponding server field defined in Section 7.1.2 1439 termsOfServiceAgreed (optional, boolean): Same meaning as the 1440 corresponding server field defined in Section 7.1.2 1442 onlyReturnExisting (optional, boolean): If this field is present 1443 with the value "true", then the server MUST NOT create a new 1444 account if one does not already exist. This allows a client to 1445 look up an account URL based on an account key (see 1446 Section 7.3.1). 1448 externalAccountBinding (optional, object): An optional field for 1449 binding the new account with an existing non-ACME account (see 1450 Section 7.3.5). 1452 POST /acme/new-account HTTP/1.1 1453 Host: example.com 1454 Content-Type: application/jose+json 1456 { 1457 "protected": base64url({ 1458 "alg": "ES256", 1459 "jwk": {...}, 1460 "nonce": "6S8IqOGY7eL2lsGoTZYifg", 1461 "url": "https://example.com/acme/new-account" 1462 }), 1463 "payload": base64url({ 1464 "termsOfServiceAgreed": true, 1465 "contact": [ 1466 "mailto:cert-admin@example.com", 1467 "mailto:admin@example.com" 1468 ] 1469 }), 1470 "signature": "RZPOnYoPs1PhjszF...-nh6X1qtOFPB519I" 1471 } 1473 The server MUST ignore any values provided in the "orders" fields in 1474 account bodies sent by the client, as well as any other fields that 1475 it does not recognize. If new fields are specified in the future, 1476 the specification of those fields MUST describe whether they can be 1477 provided by the client. 1479 In general, the server MUST ignore any fields in the request object 1480 that it does not recognize. In particular, it MUST NOT reflect 1481 unrecognized fields in the resulting account object. This allows 1482 clients to detect when servers do not support an extension field. 1484 The server SHOULD validate that the contact URLs in the "contact" 1485 field are valid and supported by the server. If the server validates 1486 contact URLs it MUST support the "mailto" scheme. Clients MUST NOT 1487 provide a "mailto" URL in the "contact" field that contains "hfields" 1488 [RFC6068], or more than one "addr-spec" in the "to" component. If a 1489 server encounters a "mailto" contact URL that does not meet these 1490 criteria, then it SHOULD reject it as invalid. 1492 If the server rejects a contact URL for using an unsupported scheme 1493 it MUST return an error of type "unsupportedContact", with a 1494 description describing the error and what types of contact URLs the 1495 server considers acceptable. If the server rejects a contact URL for 1496 using a supported scheme but an invalid value then the server MUST 1497 return an error of type "invalidContact". 1499 If the server wishes to present the client with terms under which the 1500 ACME service is to be used, it MUST indicate the URL where such terms 1501 can be accessed in the "termsOfService" subfield of the "meta" field 1502 in the directory object, and the server MUST reject new-account 1503 requests that do not have the "termsOfServiceAgreed" field set to 1504 "true". Clients SHOULD NOT automatically agree to terms by default. 1505 Rather, they SHOULD require some user interaction for agreement to 1506 terms. 1508 The server creates an account and stores the public key used to 1509 verify the JWS (i.e., the "jwk" element of the JWS header) to 1510 authenticate future requests from the account. The server returns 1511 this account object in a 201 (Created) response, with the account URL 1512 in a Location header field. The account URL is used as the "kid" 1513 value in the JWS authenticating subsequent requests by this account 1514 (See Section 6.2). 1516 HTTP/1.1 201 Created 1517 Content-Type: application/json 1518 Replay-Nonce: D8s4D2mLs8Vn-goWuPQeKA 1519 Location: https://example.com/acme/acct/1 1520 Link: ;rel="index" 1522 { 1523 "status": "valid", 1525 "contact": [ 1526 "mailto:cert-admin@example.com", 1527 "mailto:admin@example.com" 1528 ], 1530 "orders": "https://example.com/acme/acct/1/orders" 1531 } 1533 7.3.1. Finding an Account URL Given a Key 1535 If the server receives a newAccount request signed with a key for 1536 which it already has an account registered with the provided account 1537 key, then it MUST return a response with a 200 (OK) status code and 1538 provide the URL of that account in the Location header field. The 1539 body of this response represents the account object as it existed on 1540 the server before this request; any fields in the request object MUST 1541 be ignored. This allows a client that has an account key but not the 1542 corresponding account URL to recover the account URL. 1544 If a client wishes to find the URL for an existing account and does 1545 not want an account to be created if one does not already exist, then 1546 it SHOULD do so by sending a POST request to the new-account URL with 1547 a JWS whose payload has an "onlyReturnExisting" field set to "true" 1548 ({"onlyReturnExisting": true}). If a client sends such a request and 1549 an account does not exist, then the server MUST return an error 1550 response with status code 400 (Bad Request) and type 1551 "urn:ietf:params:acme:error:accountDoesNotExist". 1553 7.3.2. Account Update 1555 If the client wishes to update this information in the future, it 1556 sends a POST request with updated information to the account URL. 1557 The server MUST ignore any updates to the "orders" field, 1558 "termsOfServiceAgreed" field (see Section 7.3.4), or any other fields 1559 it does not recognize. If the server accepts the update, it MUST 1560 return a response with a 200 (OK) status code and the resulting 1561 account object. 1563 For example, to update the contact information in the above account, 1564 the client could send the following request: 1566 POST /acme/acct/1 HTTP/1.1 1567 Host: example.com 1568 Content-Type: application/jose+json 1570 { 1571 "protected": base64url({ 1572 "alg": "ES256", 1573 "kid": "https://example.com/acme/acct/1", 1574 "nonce": "ax5RnthDqp_Yf4_HZnFLmA", 1575 "url": "https://example.com/acme/acct/1" 1576 }), 1577 "payload": base64url({ 1578 "contact": [ 1579 "mailto:certificates@example.com", 1580 "mailto:admin@example.com" 1581 ] 1582 }), 1583 "signature": "hDXzvcj8T6fbFbmn...rDzXzzvzpRy64N0o" 1584 } 1586 7.3.3. Account Information 1588 Servers MUST NOT respond to GET requests for account resources as 1589 these requests are not authenticated. If a client wishes to query 1590 the server for information about its account (e.g., to examine the 1591 "contact" or "orders" fields), then it SHOULD do so by sending a POST 1592 request with an empty update. That is, it should send a JWS whose 1593 payload is an empty object ({}). 1595 7.3.4. Changes of Terms of Service 1597 As described above, a client can indicate its agreement with the CA's 1598 terms of service by setting the "termsOfServiceAgreed" field in its 1599 account object to "true". 1601 If the server has changed its terms of service since a client 1602 initially agreed, and the server is unwilling to process a request 1603 without explicit agreement to the new terms, then it MUST return an 1604 error response with status code 403 (Forbidden) and type 1605 "urn:ietf:params:acme:error:userActionRequired". This response MUST 1606 include a Link header with link relation "terms-of-service" and the 1607 latest terms-of-service URL. 1609 The problem document returned with the error MUST also include an 1610 "instance" field, indicating a URL that the client should direct a 1611 human user to visit in order for instructions on how to agree to the 1612 terms. 1614 HTTP/1.1 403 Forbidden 1615 Replay-Nonce: IXVHDyxIRGcTE0VSblhPzw 1616 Link: ;rel="terms-of-service" 1617 Content-Type: application/problem+json 1618 Content-Language: en 1620 { 1621 "type": "urn:ietf:params:acme:error:userActionRequired", 1622 "detail": "Terms of service have changed", 1623 "instance": "https://example.com/acme/agreement/?token=W8Ih3PswD-8" 1624 } 1626 7.3.5. External Account Binding 1628 The server MAY require a value for the "externalAccountBinding" field 1629 to be present in "newAccount" requests. This can be used to 1630 associate an ACME account with an existing account in a non-ACME 1631 system, such as a CA customer database. 1633 To enable ACME account binding, the CA operating the ACME server 1634 needs to provide the ACME client with a MAC key and a key identifier, 1635 using some mechanism outside of ACME. The key identifier MUST be an 1636 ASCII string. The MAC key SHOULD be provided in base64url-encoded 1637 form, to maximize compatibility between non-ACME provisioning systems 1638 and ACME clients. 1640 The ACME client then computes a binding JWS to indicate the external 1641 account holder's approval of the ACME account key. The payload of 1642 this JWS is the account key being registered, in JWK form. The 1643 protected header of the JWS MUST meet the following criteria: 1645 o The "alg" field MUST indicate a MAC-based algorithm 1647 o The "kid" field MUST contain the key identifier provided by the CA 1649 o The "nonce" field MUST NOT be present 1651 o The "url" field MUST be set to the same value as the outer JWS 1653 The "signature" field of the JWS will contain the MAC value computed 1654 with the MAC key provided by the CA. 1656 POST /acme/new-account HTTP/1.1 1657 Host: example.com 1658 Content-Type: application/jose+json 1660 { 1661 "protected": base64url({ 1662 "alg": "ES256", 1663 "jwk": /* account key */, 1664 "nonce": "K60BWPrMQG9SDxBDS_xtSw", 1665 "url": "https://example.com/acme/new-account" 1666 }), 1667 "payload": base64url({ 1668 "contact": ["mailto:example@anonymous.invalid"], 1669 "termsOfServiceAgreed": true, 1671 "externalAccountBinding": { 1672 "protected": base64url({ 1673 "alg": "HS256", 1674 "kid": /* key identifier from CA */, 1675 "url": "https://example.com/acme/new-account" 1676 }), 1677 "payload": base64url(/* same as in "jwk" above */), 1678 "signature": /* MAC using MAC key from CA */ 1679 } 1680 }), 1681 "signature": "5TWiqIYQfIDfALQv...x9C2mg8JGPxl5bI4" 1682 } 1684 If a CA requires that new-account requests contain an 1685 "externalAccountBinding" field, then it MUST provide the value "true" 1686 in the "externalAccountRequired" subfield of the "meta" field in the 1687 directory object. If the CA receives a new-account request without 1688 an "externalAccountBinding" field, then it SHOULD reply with an error 1689 of type "externalAccountRequired". 1691 When a CA receives a new-account request containing an 1692 "externalAccountBinding" field, it decides whether or not to verify 1693 the binding. If the CA does not verify the binding, then it MUST NOT 1694 reflect the "externalAccountBinding" field in the resulting account 1695 object (if any). To verify the account binding, the CA MUST take the 1696 following steps: 1698 1. Verify that the value of the field is a well-formed JWS 1700 2. Verify that the JWS protected field meets the above criteria 1702 3. Retrieve the MAC key corresponding to the key identifier in the 1703 "kid" field 1705 4. Verify that the MAC on the JWS verifies using that MAC key 1707 5. Verify that the payload of the JWS represents the same key as was 1708 used to verify the outer JWS (i.e., the "jwk" field of the outer 1709 JWS) 1711 If all of these checks pass and the CA creates a new account, then 1712 the CA may consider the new account associated with the external 1713 account corresponding to the MAC key. The account object the CA 1714 returns MUST include an "externalAccountBinding" field with the same 1715 value as the field in the request. If any of these checks fail, then 1716 the CA MUST reject the new-account request. 1718 7.3.6. Account Key Roll-over 1720 A client may wish to change the public key that is associated with an 1721 account in order to recover from a key compromise or proactively 1722 mitigate the impact of an unnoticed key compromise. 1724 To change the key associated with an account, the client sends a 1725 request to the server containing signatures by both the old and new 1726 keys. The signature by the new key covers the account URL and the 1727 old key, signifying a request by the new key holder to take over the 1728 account from the old key holder. The signature by the old key covers 1729 this request and its signature, and indicates the old key holder's 1730 assent to the roll-over request. 1732 To create this request object, the client first constructs a key- 1733 change object describing the account to be updated and its account 1734 key: 1736 account (required, string): The URL for the account being modified. 1737 The content of this field MUST be the exact string provided in the 1738 Location header field in response to the new-account request that 1739 created the account. 1741 oldKey (required, JWK): The JWK representation of the old key 1743 The client then encapsulates the key-change object in an "inner" JWS, 1744 signed with the requested new account key. This "inner" JWS becomes 1745 the payload for the "outer" JWS that is the body of the ACME request. 1747 The outer JWS MUST meet the normal requirements for an ACME JWS (see 1748 Section 6.2). The inner JWS MUST meet the normal requirements, with 1749 the following differences: 1751 o The inner JWS MUST have a "jwk" header parameter, containing the 1752 public key of the new key pair. 1754 o The inner JWS MUST have the same "url" header parameter as the 1755 outer JWS. 1757 o The inner JWS is NOT REQUIRED to have a "nonce" header parameter. 1758 The server MUST ignore any value provided for the "nonce" header 1759 parameter. 1761 This transaction has signatures from both the old and new keys so 1762 that the server can verify that the holders of the two keys both 1763 agree to the change. The signatures are nested to preserve the 1764 property that all signatures on POST messages are signed by exactly 1765 one key. The "inner" JWS effectively represents a request by the 1766 holder of the new key to take over the account form the holder of the 1767 old key. The "outer" JWS represents the current account holder's 1768 assent to this request. 1770 POST /acme/key-change HTTP/1.1 1771 Host: example.com 1772 Content-Type: application/jose+json 1774 { 1775 "protected": base64url({ 1776 "alg": "ES256", 1777 "kid": "https://example.com/acme/acct/1", 1778 "nonce": "K60BWPrMQG9SDxBDS_xtSw", 1779 "url": "https://example.com/acme/key-change" 1780 }), 1781 "payload": base64url({ 1782 "protected": base64url({ 1783 "alg": "ES256", 1784 "jwk": /* new key */, 1785 "url": "https://example.com/acme/key-change" 1786 }), 1787 "payload": base64url({ 1788 "account": "https://example.com/acme/acct/1", 1789 "oldKey": /* old key */ 1790 }), 1791 "signature": "Xe8B94RD30Azj2ea...8BmZIRtcSKPSd8gU" 1792 }), 1793 "signature": "5TWiqIYQfIDfALQv...x9C2mg8JGPxl5bI4" 1794 } 1796 On receiving key-change request, the server MUST perform the 1797 following steps in addition to the typical JWS validation: 1799 1. Validate the POST request belongs to a currently active account, 1800 as described in Section 6. 1802 2. Check that the payload of the JWS is a well-formed JWS object 1803 (the "inner JWS"). 1805 3. Check that the JWS protected header of the inner JWS has a "jwk" 1806 field. 1808 4. Check that the inner JWS verifies using the key in its "jwk" 1809 field. 1811 5. Check that the payload of the inner JWS is a well-formed key- 1812 change object (as described above). 1814 6. Check that the "url" parameters of the inner and outer JWSs are 1815 the same. 1817 7. Check that the "account" field of the key-change object contains 1818 the URL for the account matching the old key (i.e., the "kid" 1819 field in the outer JWS). 1821 8. Check that the "oldKey" field of the key-change object is the 1822 same as the account key for the account in question. 1824 9. Check that no account exists whose account key is the same as the 1825 key in the "jwk" header parameter of the inner JWS. 1827 If all of these checks pass, then the server updates the 1828 corresponding account by replacing the old account key with the new 1829 public key and returns status code 200 (OK). Otherwise, the server 1830 responds with an error status code and a problem document describing 1831 the error. If there is an existing account with the new key 1832 provided, then the server SHOULD use status code 409 (Conflict) and 1833 provide the URL of that account in the Location header field. 1835 Note that changing the account key for an account SHOULD NOT have any 1836 other impact on the account. For example, the server MUST NOT 1837 invalidate pending orders or authorization transactions based on a 1838 change of account key. 1840 7.3.7. Account Deactivation 1842 A client can deactivate an account by posting a signed update to the 1843 server with a status field of "deactivated." Clients may wish to do 1844 this when the account key is compromised or decommissioned. 1846 POST /acme/acct/1 HTTP/1.1 1847 Host: example.com 1848 Content-Type: application/jose+json 1850 { 1851 "protected": base64url({ 1852 "alg": "ES256", 1853 "kid": "https://example.com/acme/acct/1", 1854 "nonce": "ntuJWWSic4WVNSqeUmshgg", 1855 "url": "https://example.com/acme/acct/1" 1856 }), 1857 "payload": base64url({ 1858 "status": "deactivated" 1859 }), 1860 "signature": "earzVLd3m5M4xJzR...bVTqn7R08AKOVf3Y" 1861 } 1862 The server MUST verify that the request is signed by the account key. 1863 If the server accepts the deactivation request, it replies with a 200 1864 (OK) status code and the current contents of the account object. 1866 Once an account is deactivated, the server MUST NOT accept further 1867 requests authorized by that account's key. The server SHOULD cancel 1868 any pending operations authorized by the account's key, such as 1869 certificate orders. A server may take a variety of actions in 1870 response to an account deactivation, e.g., deleting data related to 1871 that account or sending mail to the account's contacts. Servers 1872 SHOULD NOT revoke certificates issued by the deactivated account, 1873 since this could cause operational disruption for servers using these 1874 certificates. ACME does not provide a way to reactivate a 1875 deactivated account. 1877 7.4. Applying for Certificate Issuance 1879 The client begins the certificate issuance process by sending a POST 1880 request to the server's new-order resource. The body of the POST is 1881 a JWS object whose JSON payload is a subset of the order object 1882 defined in Section 7.1.3, containing the fields that describe the 1883 certificate to be issued: 1885 identifiers (required, array of object): An array of identifier 1886 objects that the client wishes to submit an order for. 1888 type (required, string): The type of identifier. 1890 value (required, string): The identifier itself. 1892 notBefore (optional, string): The requested value of the notBefore 1893 field in the certificate, in the date format defined in [RFC3339]. 1895 notAfter (optional, string): The requested value of the notAfter 1896 field in the certificate, in the date format defined in [RFC3339]. 1898 POST /acme/new-order HTTP/1.1 1899 Host: example.com 1900 Content-Type: application/jose+json 1902 { 1903 "protected": base64url({ 1904 "alg": "ES256", 1905 "kid": "https://example.com/acme/acct/1", 1906 "nonce": "5XJ1L3lEkMG7tR6pA00clA", 1907 "url": "https://example.com/acme/new-order" 1908 }), 1909 "payload": base64url({ 1910 "identifiers": [ 1911 { "type": "dns", "value": "example.com" } 1912 ], 1913 "notBefore": "2016-01-01T00:00:00Z", 1914 "notAfter": "2016-01-08T00:00:00Z" 1915 }), 1916 "signature": "H6ZXtGjTZyUnPeKn...wEA4TklBdh3e454g" 1917 } 1919 The server MUST return an error if it cannot fulfill the request as 1920 specified, and MUST NOT issue a certificate with contents other than 1921 those requested. If the server requires the request to be modified 1922 in a certain way, it should indicate the required changes using an 1923 appropriate error type and description. 1925 If the server is willing to issue the requested certificate, it 1926 responds with a 201 (Created) response. The body of this response is 1927 an order object reflecting the client's request and any 1928 authorizations the client must complete before the certificate will 1929 be issued. 1931 HTTP/1.1 201 Created 1932 Replay-Nonce: MYAuvOpaoIiywTezizk5vw 1933 Location: https://example.com/acme/order/asdf 1935 { 1936 "status": "pending", 1937 "expires": "2016-01-01T00:00:00Z", 1939 "notBefore": "2016-01-01T00:00:00Z", 1940 "notAfter": "2016-01-08T00:00:00Z", 1942 "identifiers": [ 1943 { "type": "dns", "value": "example.com" }, 1944 { "type": "dns", "value": "www.example.com" } 1945 ], 1947 "authorizations": [ 1948 "https://example.com/acme/authz/1234", 1949 "https://example.com/acme/authz/2345" 1950 ], 1952 "finalize": "https://example.com/acme/order/asdf/finalize" 1953 } 1955 The order object returned by the server represents a promise that if 1956 the client fulfills the server's requirements before the "expires" 1957 time, then the server will be willing to finalize the order upon 1958 request and issue the requested certificate. In the order object, 1959 any authorization referenced in the "authorizations" array whose 1960 status is "pending" represents an authorization transaction that the 1961 client must complete before the server will issue the certificate 1962 (see Section 7.5). If the client fails to complete the required 1963 actions before the "expires" time, then the server SHOULD change the 1964 status of the order to "invalid" and MAY delete the order resource. 1965 Clients SHOULD NOT make any assumptions about the sort order of 1966 "identifiers" or "authorizations" elements in the returned order 1967 object. 1969 Once the client believes it has fulfilled the server's requirements, 1970 it should send a POST request to the order resource's finalize URL. 1971 The POST body MUST include a CSR: 1973 csr (required, string): A CSR encoding the parameters for the 1974 certificate being requested [RFC2986]. The CSR is sent in the 1975 base64url-encoded version of the DER format. (Note: Because this 1976 field uses base64url, and does not include headers, it is 1977 different from PEM.). 1979 POST /acme/order/asdf/finalize HTTP/1.1 1980 Host: example.com 1981 Content-Type: application/jose+json 1983 { 1984 "protected": base64url({ 1985 "alg": "ES256", 1986 "kid": "https://example.com/acme/acct/1", 1987 "nonce": "MSF2j2nawWHPxxkE3ZJtKQ", 1988 "url": "https://example.com/acme/order/asdf/finalize" 1989 }), 1990 "payload": base64url({ 1991 "csr": "MIIBPTCBxAIBADBFMQ...FS6aKdZeGsysoCo4H9P", 1992 }), 1993 "signature": "uOrUfIIk5RyQ...nw62Ay1cl6AB" 1994 } 1996 The CSR encodes the client's requests with regard to the content of 1997 the certificate to be issued. The CSR MUST indicate the exact same 1998 set of requested identifiers as the initial new-order request. 1999 Identifiers of type "dns" MUST appear either in the commonName 2000 portion of the requested subject name, or in an extensionRequest 2001 attribute [RFC2985] requesting a subjectAltName extension. (These 2002 identifiers may appear in any sort order.) Specifications that 2003 define new identifier types must specify where in the certificate 2004 signing request these identifiers can appear. 2006 A request to finalize an order will result in error if the CA is 2007 unwilling to issue a certificate corresponding to the submitted CSR. 2008 For example: 2010 o If the order indicated does not have status "ready" 2012 o If the CSR and order identifiers differ 2014 o If the account is not authorized for the identifiers indicated in 2015 the CSR 2017 o If the CSR requests extensions that the CA is not willing to 2018 include 2020 In such cases, the problem document returned by the server SHOULD use 2021 error code "badCSR", and describe specific reasons the CSR was 2022 rejected in its "details" field. After returning such an error, the 2023 server SHOULD leave the order in the "ready" state, to allow the 2024 client to submit a new finalize request with an amended CSR. 2026 A valid request to finalize an order will return the order to be 2027 finalized. The client should begin polling the order by sending a 2028 GET request to the order resource to obtain its current state. The 2029 status of the order will indicate what action the client should take: 2031 o "invalid": The certificate will not be issued. Consider this 2032 order process abandoned. 2034 o "pending": The server does not believe that the client has 2035 fulfilled the requirements. Check the "authorizations" array for 2036 entries that are still pending. 2038 o "ready": The server agrees that the requirements have been 2039 fulfilled, and is awaiting finalization. Submit a finalization 2040 request. 2042 o "processing": The certificate is being issued. Send a GET request 2043 after the time given in the "Retry-After" header field of the 2044 response, if any. 2046 o "valid": The server has issued the certificate and provisioned its 2047 URL to the "certificate" field of the order. Download the 2048 certificate. 2050 HTTP/1.1 200 OK 2051 Replay-Nonce: CGf81JWBsq8QyIgPCi9Q9X 2052 Location: https://example.com/acme/order/asdf 2054 { 2055 "status": "valid", 2056 "expires": "2016-01-01T00:00:00Z", 2058 "notBefore": "2016-01-01T00:00:00Z", 2059 "notAfter": "2016-01-08T00:00:00Z", 2061 "identifiers": [ 2062 { "type": "dns", "value": "example.com" }, 2063 { "type": "dns", "value": "www.example.com" } 2064 ], 2066 "authorizations": [ 2067 "https://example.com/acme/authz/1234", 2068 "https://example.com/acme/authz/2345" 2069 ], 2071 "finalize": "https://example.com/acme/order/asdf/finalize", 2073 "certificate": "https://example.com/acme/cert/asdf" 2074 } 2076 7.4.1. Pre-Authorization 2078 The order process described above presumes that authorization objects 2079 are created reactively, in response to a certificate order. Some 2080 servers may also wish to enable clients to obtain authorization for 2081 an identifier proactively, outside of the context of a specific 2082 issuance. For example, a client hosting virtual servers for a 2083 collection of names might wish to obtain authorization before any 2084 virtual servers are created and only create a certificate when a 2085 virtual server starts up. 2087 In some cases, a CA running an ACME server might have a completely 2088 external, non-ACME process for authorizing a client to issue 2089 certificates for an identifier. In these cases, the CA should 2090 provision its ACME server with authorization objects corresponding to 2091 these authorizations and reflect them as already valid in any orders 2092 submitted by the client. 2094 If a CA wishes to allow pre-authorization within ACME, it can offer a 2095 "new authorization" resource in its directory by adding the field 2096 "newAuthz" with a URL for the new authorization resource. 2098 To request authorization for an identifier, the client sends a POST 2099 request to the new-authorization resource specifying the identifier 2100 for which authorization is being requested. 2102 identifier (required, object): The identifier to appear in the 2103 resulting authorization object (see Section 7.1.4) 2105 POST /acme/new-authz HTTP/1.1 2106 Host: example.com 2107 Content-Type: application/jose+json 2109 { 2110 "protected": base64url({ 2111 "alg": "ES256", 2112 "kid": "https://example.com/acme/acct/1", 2113 "nonce": "uQpSjlRb4vQVCjVYAyyUWg", 2114 "url": "https://example.com/acme/new-authz" 2115 }), 2116 "payload": base64url({ 2117 "identifier": { 2118 "type": "dns", 2119 "value": "example.net" 2120 } 2121 }), 2122 "signature": "nuSDISbWG8mMgE7H...QyVUL68yzf3Zawps" 2123 } 2125 Note that because the identifier in a pre-authorization request is 2126 the exact identifier to be included in the authorization object, pre- 2127 authorization cannot be used to authorize issuance with wildcard DNS 2128 identifiers. 2130 Before processing the authorization request, the server SHOULD 2131 determine whether it is willing to issue certificates for the 2132 identifier. For example, the server should check that the identifier 2133 is of a supported type. Servers might also check names against a 2134 blacklist of known high-value identifiers. If the server is 2135 unwilling to issue for the identifier, it SHOULD return a 403 2136 (Forbidden) error, with a problem document describing the reason for 2137 the rejection. 2139 If the server is willing to proceed, it builds a pending 2140 authorization object from the inputs submitted by the client: 2142 o "identifier" the identifier submitted by the client 2144 o "status" MUST be "pending" unless the server has out-of-band 2145 information about the client's authorization status 2147 o "challenges" as selected by the server's policy for this 2148 identifier 2150 The server allocates a new URL for this authorization, and returns a 2151 201 (Created) response, with the authorization URL in the Location 2152 header field, and the JSON authorization object in the body. The 2153 client then follows the process described in Section 7.5 to complete 2154 the authorization process. 2156 7.4.2. Downloading the Certificate 2158 To download the issued certificate, the client simply sends a GET 2159 request to the certificate URL. 2161 The default format of the certificate is application/pem-certificate- 2162 chain (see Section 9). 2164 The server MAY provide one or more link relation header fields 2165 [RFC5988] with relation "alternate". Each such field SHOULD express 2166 an alternative certificate chain starting with the same end-entity 2167 certificate. This can be used to express paths to various trust 2168 anchors. Clients can fetch these alternates and use their own 2169 heuristics to decide which is optimal. 2171 GET /acme/cert/asdf HTTP/1.1 2172 Host: example.com 2173 Accept: application/pkix-cert 2175 HTTP/1.1 200 OK 2176 Content-Type: application/pem-certificate-chain 2177 Link: ;rel="index" 2179 -----BEGIN CERTIFICATE----- 2180 [End-entity certificate contents] 2181 -----END CERTIFICATE----- 2182 -----BEGIN CERTIFICATE----- 2183 [Issuer certificate contents] 2184 -----END CERTIFICATE----- 2185 -----BEGIN CERTIFICATE----- 2186 [Other certificate contents] 2187 -----END CERTIFICATE----- 2189 A certificate resource represents a single, immutable certificate. 2190 If the client wishes to obtain a renewed certificate, the client 2191 initiates a new order process to request one. 2193 Because certificate resources are immutable once issuance is 2194 complete, the server MAY enable the caching of the resource by adding 2195 Expires and Cache-Control headers specifying a point in time in the 2196 distant future. These headers have no relation to the certificate's 2197 period of validity. 2199 The ACME client MAY request other formats by including an Accept 2200 header [RFC7231] in its request. For example, the client could use 2201 the media type "application/pkix-cert" [RFC2585] or "applicaiton/ 2202 pkcs7-mime" [RFC5751] to request the end-entity certificate in DER 2203 format. Server support for alternate formats is OPTIONAL. For 2204 formats that can only express a single certificate, the server SHOULD 2205 provide one or more "Link: rel="up"" headers pointing to an issuer or 2206 issuers so that ACME clients can build a certificate chain as defined 2207 in TLS. 2209 7.5. Identifier Authorization 2211 The identifier authorization process establishes the authorization of 2212 an account to manage certificates for a given identifier. This 2213 process assures the server of two things: 2215 1. That the client controls the private key of the account key pair, 2216 and 2218 2. That the client controls the identifier in question. 2220 This process may be repeated to associate multiple identifiers to a 2221 key pair (e.g., to request certificates with multiple identifiers), 2222 or to associate multiple accounts with an identifier (e.g., to allow 2223 multiple entities to manage certificates). 2225 Authorization resources are created by the server in response to 2226 certificate orders or authorization requests submitted by an account 2227 key holder; their URLs are provided to the client in the responses to 2228 these requests. The authorization object is implicitly tied to the 2229 account key used to sign the request. 2231 When a client receives an order from the server it downloads the 2232 authorization resources by sending GET requests to the indicated 2233 URLs. If the client initiates authorization using a request to the 2234 new authorization resource, it will have already received the pending 2235 authorization object in the response to that request. 2237 GET /acme/authz/1234 HTTP/1.1 2238 Host: example.com 2240 HTTP/1.1 200 OK 2241 Content-Type: application/json 2242 Link: ;rel="index" 2244 { 2245 "status": "pending", 2246 "expires": "2018-03-03T14:09:00Z", 2248 "identifier": { 2249 "type": "dns", 2250 "value": "example.org" 2251 }, 2253 "challenges": [ 2254 { 2255 "type": "http-01", 2256 "url": "https://example.com/acme/authz/1234/0", 2257 "token": "DGyRejmCefe7v4NfDGDKfA" 2258 }, 2259 { 2260 "type": "dns-01", 2261 "url": "https://example.com/acme/authz/1234/2", 2262 "token": "DGyRejmCefe7v4NfDGDKfA" 2263 } 2264 ], 2266 "wildcard": false 2267 } 2269 7.5.1. Responding to Challenges 2271 To prove control of the identifier and receive authorization, the 2272 client needs to respond with information to complete the challenges. 2273 To do this, the client updates the authorization object received from 2274 the server by filling in any required information in the elements of 2275 the "challenges" dictionary. 2277 The client sends these updates back to the server in the form of a 2278 JSON object with contents as specified by the challenge type, carried 2279 in a POST request to the challenge URL (not authorization URL) once 2280 it is ready for the server to attempt validation. 2282 For example, if the client were to respond to the "http-01" challenge 2283 in the above authorization, it would send the following request: 2285 POST /acme/authz/1234/0 HTTP/1.1 2286 Host: example.com 2287 Content-Type: application/jose+json 2289 { 2290 "protected": base64url({ 2291 "alg": "ES256", 2292 "kid": "https://example.com/acme/acct/1", 2293 "nonce": "Q_s3MWoqT05TrdkM2MTDcw", 2294 "url": "https://example.com/acme/authz/1234/0" 2295 }), 2296 "payload": base64url({}), 2297 "signature": "9cbg5JO1Gf5YLjjz...SpkUfcdPai9uVYYQ" 2298 } 2300 The server updates the authorization document by updating its 2301 representation of the challenge with the response object provided by 2302 the client. The server MUST ignore any fields in the response object 2303 that are not specified as response fields for this type of challenge. 2304 The server provides a 200 (OK) response with the updated challenge 2305 object as its body. 2307 If the client's response is invalid for any reason or does not 2308 provide the server with appropriate information to validate the 2309 challenge, then the server MUST return an HTTP error. On receiving 2310 such an error, the client SHOULD undo any actions that have been 2311 taken to fulfill the challenge, e.g., removing files that have been 2312 provisioned to a web server. 2314 The server is said to "finalize" the authorization when it has 2315 completed one of the validations, by assigning the authorization a 2316 status of "valid" or "invalid", corresponding to whether it considers 2317 the account authorized for the identifier. If the final state is 2318 "valid", then the server MUST include an "expires" field. When 2319 finalizing an authorization, the server MAY remove challenges other 2320 than the one that was completed, and may modify the "expires" field. 2321 The server SHOULD NOT remove challenges with status "invalid". 2323 Usually, the validation process will take some time, so the client 2324 will need to poll the authorization resource to see when it is 2325 finalized. For challenges where the client can tell when the server 2326 has validated the challenge (e.g., by seeing an HTTP or DNS request 2327 from the server), the client SHOULD NOT begin polling until it has 2328 seen the validation request from the server. 2330 To check on the status of an authorization, the client sends a GET 2331 request to the authorization URL, and the server responds with the 2332 current authorization object. In responding to poll requests while 2333 the validation is still in progress, the server MUST return a 200 2334 (OK) response and MAY include a Retry-After header field to suggest a 2335 polling interval to the client. 2337 GET /acme/authz/1234 HTTP/1.1 2338 Host: example.com 2340 HTTP/1.1 200 OK 2341 Content-Type: application/json 2343 { 2344 "status": "valid", 2345 "expires": "2018-09-09T14:09:00Z", 2347 "identifier": { 2348 "type": "dns", 2349 "value": "example.org" 2350 }, 2352 "challenges": [ 2353 { 2354 "type": "http-01", 2355 "url": "https://example.com/acme/authz/1234/0", 2356 "status": "valid", 2357 "validated": "2014-12-01T12:05:00Z", 2358 "token": "IlirfxKKXAsHtmzK29Pj8A" 2359 } 2360 ], 2362 "wildcard": false 2363 } 2365 7.5.2. Deactivating an Authorization 2367 If a client wishes to relinquish its authorization to issue 2368 certificates for an identifier, then it may request that the server 2369 deactivates each authorization associated with it by sending POST 2370 requests with the static object {"status": "deactivated"} to each 2371 authorization URL. 2373 POST /acme/authz/1234 HTTP/1.1 2374 Host: example.com 2375 Content-Type: application/jose+json 2377 { 2378 "protected": base64url({ 2379 "alg": "ES256", 2380 "kid": "https://example.com/acme/acct/1", 2381 "nonce": "xWCM9lGbIyCgue8di6ueWQ", 2382 "url": "https://example.com/acme/authz/1234" 2383 }), 2384 "payload": base64url({ 2385 "status": "deactivated" 2386 }), 2387 "signature": "srX9Ji7Le9bjszhu...WTFdtujObzMtZcx4" 2388 } 2390 The server MUST verify that the request is signed by the account key 2391 corresponding to the account that owns the authorization. If the 2392 server accepts the deactivation, it should reply with a 200 (OK) 2393 status code and the updated contents of the authorization object. 2395 The server MUST NOT treat deactivated authorization objects as 2396 sufficient for issuing certificates. 2398 7.6. Certificate Revocation 2400 To request that a certificate be revoked, the client sends a POST 2401 request to the ACME server's revokeCert URL. The body of the POST is 2402 a JWS object whose JSON payload contains the certificate to be 2403 revoked: 2405 certificate (required, string): The certificate to be revoked, in 2406 the base64url-encoded version of the DER format. (Note: Because 2407 this field uses base64url, and does not include headers, it is 2408 different from PEM.) 2410 reason (optional, int): One of the revocation reasonCodes defined in 2411 Section 5.3.1 of [RFC5280] to be used when generating OCSP 2412 responses and CRLs. If this field is not set the server SHOULD 2413 omit the reasonCode CRL entry extension when generating OCSP 2414 responses and CRLs. The server MAY disallow a subset of 2415 reasonCodes from being used by the user. If a request contains a 2416 disallowed reasonCode the server MUST reject it with the error 2417 type "urn:ietf:params:acme:error:badRevocationReason". The 2418 problem document detail SHOULD indicate which reasonCodes are 2419 allowed. 2421 Revocation requests are different from other ACME requests in that 2422 they can be signed either with an account key pair or the key pair in 2423 the certificate. 2425 Example using an account key pair for the signature: 2427 POST /acme/revoke-cert HTTP/1.1 2428 Host: example.com 2429 Content-Type: application/jose+json 2431 { 2432 "protected": base64url({ 2433 "alg": "ES256", 2434 "kid": "https://example.com/acme/acct/1", 2435 "nonce": "JHb54aT_KTXBWQOzGYkt9A", 2436 "url": "https://example.com/acme/revoke-cert" 2437 }), 2438 "payload": base64url({ 2439 "certificate": "MIIEDTCCAvegAwIBAgIRAP8...", 2440 "reason": 4 2441 }), 2442 "signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4" 2443 } 2445 Example using the certificate key pair for the signature: 2447 POST /acme/revoke-cert HTTP/1.1 2448 Host: example.com 2449 Content-Type: application/jose+json 2451 { 2452 "protected": base64url({ 2453 "alg": "RS256", 2454 "jwk": /* certificate's public key */, 2455 "nonce": "JHb54aT_KTXBWQOzGYkt9A", 2456 "url": "https://example.com/acme/revoke-cert" 2457 }), 2458 "payload": base64url({ 2459 "certificate": "MIIEDTCCAvegAwIBAgIRAP8...", 2460 "reason": 1 2461 }), 2462 "signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4" 2463 } 2465 Before revoking a certificate, the server MUST verify that the key 2466 used to sign the request is authorized to revoke the certificate. 2467 The server MUST consider at least the following accounts authorized 2468 for a given certificate: 2470 o the account that issued the certificate. 2472 o an account that holds authorizations for all of the identifiers in 2473 the certificate. 2475 The server MUST also consider a revocation request valid if it is 2476 signed with the private key corresponding to the public key in the 2477 certificate. 2479 If the revocation succeeds, the server responds with status code 200 2480 (OK). If the revocation fails, the server returns an error. For 2481 example, if the certificate has already been revoked the server 2482 returns an error response with status code 400 (Bad Request) and type 2483 "urn:ietf:params:acme:error:alreadyRevoked". 2485 HTTP/1.1 200 OK 2486 Replay-Nonce: IXVHDyxIRGcTE0VSblhPzw 2487 Content-Length: 0 2489 --- or --- 2491 HTTP/1.1 403 Forbidden 2492 Replay-Nonce: IXVHDyxIRGcTE0VSblhPzw 2493 Content-Type: application/problem+json 2494 Content-Language: en 2496 { 2497 "type": "urn:ietf:params:acme:error:unauthorized", 2498 "detail": "No authorization provided for name example.net" 2499 } 2501 8. Identifier Validation Challenges 2503 There are few types of identifiers in the world for which there is a 2504 standardized mechanism to prove possession of a given identifier. In 2505 all practical cases, CAs rely on a variety of means to test whether 2506 an entity applying for a certificate with a given identifier actually 2507 controls that identifier. 2509 Challenges provide the server with assurance that an account holder 2510 is also the entity that controls an identifier. For each type of 2511 challenge, it must be the case that in order for an entity to 2512 successfully complete the challenge the entity must both: 2514 o Hold the private key of the account key pair used to respond to 2515 the challenge 2517 o Control the identifier in question 2518 Section 10 documents how the challenges defined in this document meet 2519 these requirements. New challenges will need to document how they 2520 do. 2522 ACME uses an extensible challenge/response framework for identifier 2523 validation. The server presents a set of challenges in the 2524 authorization object it sends to a client (as objects in the 2525 "challenges" array), and the client responds by sending a response 2526 object in a POST request to a challenge URL. 2528 This section describes an initial set of challenge types. The 2529 definition of a challenge type includes: 2531 1. Content of challenge objects 2533 2. Content of response objects 2535 3. How the server uses the challenge and response to verify control 2536 of an identifier 2538 Challenge objects all contain the following basic fields: 2540 type (required, string): The type of challenge encoded in the 2541 object. 2543 url (required, string): The URL to which a response can be posted. 2545 status (required, string): The status of this challenge. Possible 2546 values are: "pending", "processing", "valid", and "invalid". (See 2547 Section 7.1.6) 2549 validated (optional, string): The time at which the server validated 2550 this challenge, encoded in the format specified in RFC 3339 2551 [RFC3339]. This field is REQUIRED if the "status" field is 2552 "valid". 2554 error (optional, object): Error that occurred while the server was 2555 validating the challenge, if any, structured as a problem document 2556 [RFC7807]. Multiple errors can be indicated by using subproblems 2557 Section 6.6.1. 2559 All additional fields are specified by the challenge type. If the 2560 server sets a challenge's "status" to "invalid", it SHOULD also 2561 include the "error" field to help the client diagnose why the 2562 challenge failed. 2564 Different challenges allow the server to obtain proof of different 2565 aspects of control over an identifier. In some challenges, like HTTP 2566 and DNS, the client directly proves its ability to do certain things 2567 related to the identifier. The choice of which challenges to offer 2568 to a client under which circumstances is a matter of server policy. 2570 The identifier validation challenges described in this section all 2571 relate to validation of domain names. If ACME is extended in the 2572 future to support other types of identifiers, there will need to be 2573 new challenge types, and they will need to specify which types of 2574 identifier they apply to. 2576 8.1. Key Authorizations 2578 All challenges defined in this document make use of a key 2579 authorization string. A key authorization is a string that expresses 2580 a domain holder's authorization for a specified key to satisfy a 2581 specified challenge, by concatenating the token for the challenge 2582 with a key fingerprint, separated by a "." character: 2584 keyAuthorization = token || '.' || base64url(JWK_Thumbprint(accountKey)) 2586 The "JWK_Thumbprint" step indicates the computation specified in 2587 [RFC7638], using the SHA-256 digest [FIPS180-4]. As noted in 2588 [RFC7518] any prepended zero octets in the fields of a JWK object 2589 MUST be stripped before doing the computation. 2591 As specified in the individual challenges below, the token for a 2592 challenge is a string comprised entirely of characters in the URL- 2593 safe base64 alphabet. The "||" operator indicates concatenation of 2594 strings. 2596 8.2. Retrying Challenges 2598 ACME challenges typically require the client to set up some network- 2599 accessible resource that the server can query in order to validate 2600 that the client controls an identifier. In practice it is not 2601 uncommon for the server's queries to fail while a resource is being 2602 set up, e.g., due to information propagating across a cluster or 2603 firewall rules not being in place. 2605 Clients SHOULD NOT respond to challenges until they believe that the 2606 server's queries will succeed. If a server's initial validation 2607 query fails, the server SHOULD retry the query after some time, in 2608 order to account for delay in setting up responses such as DNS 2609 records or HTTP resources. The precise retry schedule is up to the 2610 server, but server operators should keep in mind the operational 2611 scenarios that the schedule is trying to accommodate. Given that 2612 retries are intended to address things like propagation delays in 2613 HTTP or DNS provisioning, there should not usually be any reason to 2614 retry more often than every 5 or 10 seconds. While the server is 2615 still trying, the status of the challenge remains "processing"; it is 2616 only marked "invalid" once the server has given up. 2618 The server MUST provide information about its retry state to the 2619 client via the "error" field in the challenge and the Retry-After 2620 HTTP header field in response to requests to the challenge resource. 2621 The server MUST add an entry to the "error" field in the challenge 2622 after each failed validation query. The server SHOULD set the Retry- 2623 After header field to a time after the server's next validation 2624 query, since the status of the challenge will not change until that 2625 time. 2627 Clients can explicitly request a retry by re-sending their response 2628 to a challenge in a new POST request (with a new nonce, etc.). This 2629 allows clients to request a retry when the state has changed (e.g., 2630 after firewall rules have been updated). Servers SHOULD retry a 2631 request immediately on receiving such a POST request. In order to 2632 avoid denial-of-service attacks via client-initiated retries, servers 2633 SHOULD rate-limit such requests. 2635 8.3. HTTP Challenge 2637 With HTTP validation, the client in an ACME transaction proves its 2638 control over a domain name by proving that it can provision HTTP 2639 resources on a server accessible under that domain name. The ACME 2640 server challenges the client to provision a file at a specific path, 2641 with a specific string as its content. 2643 As a domain may resolve to multiple IPv4 and IPv6 addresses, the 2644 server will connect to at least one of the hosts found in the DNS A 2645 and AAAA records, at its discretion. Because many web servers 2646 allocate a default HTTPS virtual host to a particular low-privilege 2647 tenant user in a subtle and non-intuitive manner, the challenge must 2648 be completed over HTTP, not HTTPS. 2650 type (required, string): The string "http-01" 2652 token (required, string): A random value that uniquely identifies 2653 the challenge. This value MUST have at least 128 bits of entropy. 2654 It MUST NOT contain any characters outside the base64url alphabet, 2655 and MUST NOT include base64 padding characters ("="). See 2656 [RFC4086] for additional information on randomness requirements. 2658 GET /acme/authz/1234/0 HTTP/1.1 2659 Host: example.com 2661 HTTP/1.1 200 OK 2662 Content-Type: application/json 2664 { 2665 "type": "http-01", 2666 "url": "https://example.com/acme/authz/0", 2667 "status": "pending", 2668 "token": "LoqXcYV8q5ONbJQxbmR7SCTNo3tiAXDfowyjxAjEuX0" 2669 } 2671 A client fulfills this challenge by constructing a key authorization 2672 from the "token" value provided in the challenge and the client's 2673 account key. The client then provisions the key authorization as a 2674 resource on the HTTP server for the domain in question. 2676 The path at which the resource is provisioned is comprised of the 2677 fixed prefix "/.well-known/acme-challenge/", followed by the "token" 2678 value in the challenge. The value of the resource MUST be the ASCII 2679 representation of the key authorization. 2681 GET /.well-known/acme-challenge/LoqXcYV8q5ONbJQxbmR7SCTNo3tiAXDfowyjxAjEuX0 2682 Host: example.org 2684 HTTP/1.1 200 OK 2685 Content-Type: application/octet-stream 2687 LoqXcYV8q5ONbJQxbmR7SCTNo3tiAXDfowyjxAjEuX0.9jg46WB3rR_AHD-EBXdN7cBkH1WOu0tA3M9fm21mqTI 2689 A client responds with an empty object ({}) to acknowledge that the 2690 challenge can be validated by the server. 2692 POST /acme/authz/1234/0 2693 Host: example.com 2694 Content-Type: application/jose+json 2696 { 2697 "protected": base64url({ 2698 "alg": "ES256", 2699 "kid": "https://example.com/acme/acct/1", 2700 "nonce": "JHb54aT_KTXBWQOzGYkt9A", 2701 "url": "https://example.com/acme/authz/1234/0" 2702 }), 2703 "payload": base64url({}), 2704 "signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4" 2705 } 2706 On receiving a response, the server constructs and stores the key 2707 authorization from the challenge "token" value and the current client 2708 account key. 2710 Given a challenge/response pair, the server verifies the client's 2711 control of the domain by verifying that the resource was provisioned 2712 as expected. 2714 1. Construct a URL by populating the URL template [RFC6570] 2715 "http://{domain}/.well-known/acme-challenge/{token}", where: 2717 * the domain field is set to the domain name being verified; and 2719 * the token field is set to the token in the challenge. 2721 2. Verify that the resulting URL is well-formed. 2723 3. Dereference the URL using an HTTP GET request. This request MUST 2724 be sent to TCP port 80 on the HTTP server. 2726 4. Verify that the body of the response is well-formed key 2727 authorization. The server SHOULD ignore whitespace characters at 2728 the end of the body. 2730 5. Verify that key authorization provided by the HTTP server matches 2731 the key authorization stored by the server. 2733 The server SHOULD follow redirects when dereferencing the URL. 2735 If all of the above verifications succeed, then the validation is 2736 successful. If the request fails, or the body does not pass these 2737 checks, then it has failed. 2739 8.4. DNS Challenge 2741 When the identifier being validated is a domain name, the client can 2742 prove control of that domain by provisioning a TXT resource record 2743 containing a designated value for a specific validation domain name. 2745 type (required, string): The string "dns-01" 2747 token (required, string): A random value that uniquely identifies 2748 the challenge. This value MUST have at least 128 bits of entropy. 2749 It MUST NOT contain any characters outside the base64url alphabet, 2750 including padding characters ("="). 2752 GET /acme/authz/1234/2 HTTP/1.1 2753 Host: example.com 2755 HTTP/1.1 200 OK 2756 Content-Type: application/json 2758 { 2759 "type": "dns-01", 2760 "url": "https://example.com/acme/authz/1234/2", 2761 "status": "pending", 2762 "token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA" 2763 } 2765 A client fulfills this challenge by constructing a key authorization 2766 from the "token" value provided in the challenge and the client's 2767 account key. The client then computes the SHA-256 digest [FIPS180-4] 2768 of the key authorization. 2770 The record provisioned to the DNS contains the base64url encoding of 2771 this digest. The client constructs the validation domain name by 2772 prepending the label "_acme-challenge" to the domain name being 2773 validated, then provisions a TXT record with the digest value under 2774 that name. For example, if the domain name being validated is 2775 "example.org", then the client would provision the following DNS 2776 record: 2778 _acme-challenge.example.org. 300 IN TXT "gfj9Xq...Rg85nM" 2780 A client responds with an empty object ({}) to acknowledge that the 2781 challenge can be validated by the server. 2783 POST /acme/authz/1234/2 2784 Host: example.com 2785 Content-Type: application/jose+json 2787 { 2788 "protected": base64url({ 2789 "alg": "ES256", 2790 "kid": "https://example.com/acme/acct/1", 2791 "nonce": "JHb54aT_KTXBWQOzGYkt9A", 2792 "url": "https://example.com/acme/authz/1234/2" 2793 }), 2794 "payload": base64url({}), 2795 "signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4" 2796 } 2797 On receiving a response, the server constructs and stores the key 2798 authorization from the challenge "token" value and the current client 2799 account key. 2801 To validate a DNS challenge, the server performs the following steps: 2803 1. Compute the SHA-256 digest [FIPS180-4] of the stored key 2804 authorization 2806 2. Query for TXT records for the validation domain name 2808 3. Verify that the contents of one of the TXT records match the 2809 digest value 2811 If all of the above verifications succeed, then the validation is 2812 successful. If no DNS record is found, or DNS record and response 2813 payload do not pass these checks, then the validation fails. 2815 9. IANA Considerations 2817 9.1. MIME Type: application/pem-certificate-chain 2819 The "Media Types" registry should be updated with the following 2820 additional value: 2822 MIME media type name: application 2824 MIME subtype name: pem-certificate-chain 2826 Required parameters: None 2828 Optional parameters: None 2830 Encoding considerations: None 2832 Security considerations: Carries a cryptographic certificate and its 2833 associated certificate chain 2835 Interoperability considerations: None 2837 Published specification: draft-ietf-acme-acme [[ RFC EDITOR: Please 2838 replace draft-ietf-acme-acme above with the RFC number assigned to 2839 this ]] 2841 Applications which use this media type: Any MIME-compliant transport 2843 Additional information: 2845 File contains one or more certificates encoded with the PEM textual 2846 encoding, according to RFC 7468 [RFC7468]. In order to provide easy 2847 interoperation with TLS, the first certificate MUST be an end-entity 2848 certificate. Each following certificate SHOULD directly certify the 2849 one preceding it. Because certificate validation requires that trust 2850 anchors be distributed independently, a certificate that specifies a 2851 trust anchor MAY be omitted from the chain, provided that supported 2852 peers are known to possess any omitted certificates. 2854 9.2. Well-Known URI for the HTTP Challenge 2856 The "Well-Known URIs" registry should be updated with the following 2857 additional value (using the template from [RFC5785]): 2859 URI suffix: acme-challenge 2861 Change controller: IETF 2863 Specification document(s): This document, Section Section 8.3 2865 Related information: N/A 2867 9.3. Replay-Nonce HTTP Header 2869 The "Message Headers" registry should be updated with the following 2870 additional value: 2872 +------------------+----------+----------+--------------------------+ 2873 | Header Field | Protocol | Status | Reference | 2874 | Name | | | | 2875 +------------------+----------+----------+--------------------------+ 2876 | Replay-Nonce | http | standard | [[this-RFC, Section | 2877 | | | | 6.4.1] | 2878 +------------------+----------+----------+--------------------------+ 2880 9.4. "url" JWS Header Parameter 2882 The "JSON Web Signature and Encryption Header Parameters" registry 2883 should be updated with the following additional value: 2885 o Header Parameter Name: "url" 2887 o Header Parameter Description: URL 2889 o Header Parameter Usage Location(s): JWE, JWS 2891 o Change Controller: IESG 2892 o Specification Document(s): Section 6.3.1 of RFC XXXX 2894 [[ RFC EDITOR: Please replace XXXX above with the RFC number assigned 2895 to this document ]] 2897 9.5. "nonce" JWS Header Parameter 2899 The "JSON Web Signature and Encryption Header Parameters" registry 2900 should be updated with the following additional value: 2902 o Header Parameter Name: "nonce" 2904 o Header Parameter Description: Nonce 2906 o Header Parameter Usage Location(s): JWE, JWS 2908 o Change Controller: IESG 2910 o Specification Document(s): Section 6.4.2 of RFC XXXX 2912 [[ RFC EDITOR: Please replace XXXX above with the RFC number assigned 2913 to this document ]] 2915 9.6. URN Sub-namespace for ACME (urn:ietf:params:acme) 2917 The "IETF URN Sub-namespace for Registered Protocol Parameter 2918 Identifiers" registry should be updated with the following additional 2919 value, following the template in [RFC3553]: 2921 Registry name: acme 2923 Specification: RFC XXXX 2925 Repository: URL-TBD 2927 Index value: No transformation needed. 2929 [[ RFC EDITOR: Please replace XXXX above with the RFC number assigned 2930 to this document, and replace URL-TBD with the URL assigned by IANA 2931 for registries of ACME parameters. ]] 2933 9.7. New Registries 2935 This document requests that IANA create the following new registries: 2937 1. ACME Account Object Fields (Section 9.7.1) 2939 2. ACME Order Object Fields (Section 9.7.2) 2940 3. ACME Authorization Object Fields (Section 9.7.3) 2942 4. ACME Error Types (Section 9.7.4) 2944 5. ACME Resource Types (Section 9.7.5) 2946 6. ACME Directory Metadata Fields (Section 9.7.6) 2948 7. ACME Identifier Types (Section 9.7.7) 2950 8. ACME Validation Methods (Section 9.7.8) 2952 All of these registries are under a heading of "Automated Certificate 2953 Management Environment (ACME) Protocol" and are administered under a 2954 Specification Required policy [RFC8126]. 2956 9.7.1. Fields in Account Objects 2958 This registry lists field names that are defined for use in ACME 2959 account objects. Fields marked as "configurable" may be included in 2960 a new-account request. 2962 Template: 2964 o Field name: The string to be used as a field name in the JSON 2965 object 2967 o Field type: The type of value to be provided, e.g., string, 2968 boolean, array of string 2970 o Client configurable: Boolean indicating whether the server should 2971 accept values provided by the client 2973 o Reference: Where this field is defined 2975 Initial contents: The fields and descriptions defined in 2976 Section 7.1.2. 2978 +------------------------+---------------+--------------+-----------+ 2979 | Field Name | Field Type | Configurable | Reference | 2980 +------------------------+---------------+--------------+-----------+ 2981 | status | string | false | RFC XXXX | 2982 | | | | | 2983 | contact | array of | true | RFC XXXX | 2984 | | string | | | 2985 | | | | | 2986 | externalAccountBinding | object | true | RFC XXXX | 2987 | | | | | 2988 | termsOfServiceAgreed | boolean | true | RFC XXXX | 2989 | | | | | 2990 | orders | string | false | RFC XXXX | 2991 +------------------------+---------------+--------------+-----------+ 2993 [[ RFC EDITOR: Please replace XXXX above with the RFC number assigned 2994 to this document ]] 2996 9.7.2. Fields in Order Objects 2998 This registry lists field names that are defined for use in ACME 2999 order objects. Fields marked as "configurable" may be included in a 3000 new-order request. 3002 Template: 3004 o Field name: The string to be used as a field name in the JSON 3005 object 3007 o Field type: The type of value to be provided, e.g., string, 3008 boolean, array of string 3010 o Client configurable: Boolean indicating whether the server should 3011 accept values provided by the client 3013 o Reference: Where this field is defined 3015 Initial contents: The fields and descriptions defined in 3016 Section 7.1.3. 3018 +----------------+-----------------+--------------+-----------+ 3019 | Field Name | Field Type | Configurable | Reference | 3020 +----------------+-----------------+--------------+-----------+ 3021 | status | string | false | RFC XXXX | 3022 | | | | | 3023 | expires | string | false | RFC XXXX | 3024 | | | | | 3025 | identifiers | array of object | true | RFC XXXX | 3026 | | | | | 3027 | notBefore | string | true | RFC XXXX | 3028 | | | | | 3029 | notAfter | string | true | RFC XXXX | 3030 | | | | | 3031 | authorizations | array of string | false | RFC XXXX | 3032 | | | | | 3033 | finalize | string | false | RFC XXXX | 3034 | | | | | 3035 | certificate | string | false | RFC XXXX | 3036 +----------------+-----------------+--------------+-----------+ 3038 [[ RFC EDITOR: Please replace XXXX above with the RFC number assigned 3039 to this document ]] 3041 9.7.3. Fields in Authorization Objects 3043 This registry lists field names that are defined for use in ACME 3044 authorization objects. Fields marked as "configurable" may be 3045 included in a new-authorization request. 3047 Template: 3049 o Field name: The string to be used as a field name in the JSON 3050 object 3052 o Field type: The type of value to be provided, e.g., string, 3053 boolean, array of string 3055 o Client configurable: Boolean indicating whether the server should 3056 accept values provided by the client 3058 o Reference: Where this field is defined 3060 Initial contents: The fields and descriptions defined in 3061 Section 7.1.4. 3063 +------------+-----------------+--------------+-----------+ 3064 | Field Name | Field Type | Configurable | Reference | 3065 +------------+-----------------+--------------+-----------+ 3066 | identifier | object | true | RFC XXXX | 3067 | | | | | 3068 | status | string | false | RFC XXXX | 3069 | | | | | 3070 | expires | string | false | RFC XXXX | 3071 | | | | | 3072 | challenges | array of object | false | RFC XXXX | 3073 | | | | | 3074 | wildcard | boolean | false | RFC XXXX | 3075 +------------+-----------------+--------------+-----------+ 3077 [[ RFC EDITOR: Please replace XXXX above with the RFC number assigned 3078 to this document ]] 3080 9.7.4. Error Types 3082 This registry lists values that are used within URN values that are 3083 provided in the "type" field of problem documents in ACME. 3085 Template: 3087 o Type: The label to be included in the URN for this error, 3088 following "urn:ietf:params:acme:error:" 3090 o Description: A human-readable description of the error 3092 o Reference: Where the error is defined 3094 Initial contents: The types and descriptions in the table in 3095 Section 6.6 above, with the Reference field set to point to this 3096 specification. 3098 9.7.5. Resource Types 3100 This registry lists the types of resources that ACME servers may list 3101 in their directory objects. 3103 Template: 3105 o Field name: The value to be used as a field name in the directory 3106 object 3108 o Resource type: The type of resource labeled by the field 3110 o Reference: Where the resource type is defined 3111 Initial contents: 3113 +------------+--------------------+-----------+ 3114 | Field Name | Resource Type | Reference | 3115 +------------+--------------------+-----------+ 3116 | newNonce | New nonce | RFC XXXX | 3117 | | | | 3118 | newAccount | New account | RFC XXXX | 3119 | | | | 3120 | newOrder | New order | RFC XXXX | 3121 | | | | 3122 | newAuthz | New authorization | RFC XXXX | 3123 | | | | 3124 | revokeCert | Revoke certificate | RFC XXXX | 3125 | | | | 3126 | keyChange | Key change | RFC XXXX | 3127 | | | | 3128 | meta | Metadata object | RFC XXXX | 3129 +------------+--------------------+-----------+ 3131 [[ RFC EDITOR: Please replace XXXX above with the RFC number assigned 3132 to this document ]] 3134 9.7.6. Fields in the "meta" Object within a Directory Object 3136 This registry lists field names that are defined for use in the JSON 3137 object included in the "meta" field of an ACME directory object. 3139 Template: 3141 o Field name: The string to be used as a field name in the JSON 3142 object 3144 o Field type: The type of value to be provided, e.g., string, 3145 boolean, array of string 3147 o Reference: Where this field is defined 3149 Initial contents: The fields and descriptions defined in 3150 Section 7.1.2. 3152 +-------------------------+-----------------+-----------+ 3153 | Field Name | Field Type | Reference | 3154 +-------------------------+-----------------+-----------+ 3155 | termsOfService | string | RFC XXXX | 3156 | | | | 3157 | website | string | RFC XXXX | 3158 | | | | 3159 | caaIdentities | array of string | RFC XXXX | 3160 | | | | 3161 | externalAccountRequired | boolean | RFC XXXX | 3162 +-------------------------+-----------------+-----------+ 3164 [[ RFC EDITOR: Please replace XXXX above with the RFC number assigned 3165 to this document ]] 3167 9.7.7. Identifier Types 3169 This registry lists the types of identifiers that can be present in 3170 ACME authorization objects. 3172 Template: 3174 o Label: The value to be put in the "type" field of the identifier 3175 object 3177 o Reference: Where the identifier type is defined 3179 Initial contents: 3181 +-------+-----------+ 3182 | Label | Reference | 3183 +-------+-----------+ 3184 | dns | RFC XXXX | 3185 +-------+-----------+ 3187 [[ RFC EDITOR: Please replace XXXX above with the RFC number assigned 3188 to this document ]] 3190 9.7.8. Validation Methods 3192 This registry lists identifiers for the ways that CAs can validate 3193 control of identifiers. Each method's entry must specify whether it 3194 corresponds to an ACME challenge type. The "Identifier Type" field 3195 must be contained in the Label column of the ACME Identifier Types 3196 registry. 3198 Template: 3200 o Label: The identifier for this validation method 3202 o Identifier Type: The type of identifier that this method applies 3203 to 3205 o ACME: "Y" if the validation method corresponds to an ACME 3206 challenge type; "N" otherwise. 3208 o Reference: Where the validation method is defined 3210 Initial Contents 3212 +------------+-----------------+------+-----------+ 3213 | Label | Identifier Type | ACME | Reference | 3214 +------------+-----------------+------+-----------+ 3215 | http-01 | dns | Y | RFC XXXX | 3216 | | | | | 3217 | dns-01 | dns | Y | RFC XXXX | 3218 | | | | | 3219 | tls-sni-01 | RESERVED | N | RFC XXXX | 3220 | | | | | 3221 | tls-sni-02 | RESERVED | N | RFC XXXX | 3222 +------------+-----------------+------+-----------+ 3224 When evaluating a request for an assignment in this registry, the 3225 designated expert should ensure that the method being registered has 3226 a clear, interoperable definition and does not overlap with existing 3227 validation methods. That is, it should not be possible for a client 3228 and server to follow the same set of actions to fulfill two different 3229 validation methods. 3231 The values "tls-sni-01" and "tls-sni-02" are reserved because they 3232 were used in pre-RFC versions of this specification to denote 3233 validation methods that were removed because they were found not to 3234 be secure in some cases. 3236 Validation methods do not have to be compatible with ACME in order to 3237 be registered. For example, a CA might wish to register a validation 3238 method in order to support its use with the ACME extensions to CAA 3239 [I-D.ietf-acme-caa]. 3241 [[ RFC EDITOR: Please replace XXXX above with the RFC number assigned 3242 to this document ]] 3244 10. Security Considerations 3246 ACME is a protocol for managing certificates that attest to 3247 identifier/key bindings. Thus the foremost security goal of ACME is 3248 to ensure the integrity of this process, i.e., to ensure that the 3249 bindings attested by certificates are correct and that only 3250 authorized entities can manage certificates. ACME identifies clients 3251 by their account keys, so this overall goal breaks down into two more 3252 precise goals: 3254 1. Only an entity that controls an identifier can get an 3255 authorization for that identifier 3257 2. Once authorized, an account key's authorizations cannot be 3258 improperly used by another account 3260 In this section, we discuss the threat model that underlies ACME and 3261 the ways that ACME achieves these security goals within that threat 3262 model. We also discuss the denial-of-service risks that ACME servers 3263 face, and a few other miscellaneous considerations. 3265 10.1. Threat Model 3267 As a service on the Internet, ACME broadly exists within the Internet 3268 threat model [RFC3552]. In analyzing ACME, it is useful to think of 3269 an ACME server interacting with other Internet hosts along two 3270 "channels": 3272 o An ACME channel, over which the ACME HTTPS requests are exchanged 3274 o A validation channel, over which the ACME server performs 3275 additional requests to validate a client's control of an 3276 identifier 3278 +------------+ 3279 | ACME | ACME Channel 3280 | Client |--------------------+ 3281 +------------+ | 3282 V 3283 +------------+ 3284 | ACME | 3285 | Server | 3286 +------------+ 3287 +------------+ | 3288 | Validation |<-------------------+ 3289 | Server | Validation Channel 3290 +------------+ 3292 Communications Channels Used by ACME 3294 In practice, the risks to these channels are not entirely separate, 3295 but they are different in most cases. Each channel, for example, 3296 uses a different communications pattern: the ACME channel will 3297 comprise inbound HTTPS connections to the ACME server and the 3298 validation channel outbound HTTP or DNS requests. 3300 Broadly speaking, ACME aims to be secure against active and passive 3301 attackers on any individual channel. Some vulnerabilities arise 3302 (noted below) when an attacker can exploit both the ACME channel and 3303 one of the others. 3305 On the ACME channel, in addition to network layer attackers, we also 3306 need to account for man-in-the-middle (MitM) attacks at the 3307 application layer, and for abusive use of the protocol itself. 3308 Protection against application layer MitM addresses potential 3309 attackers such as Content Distribution Networks (CDNs) and 3310 middleboxes with a TLS MitM function. Preventing abusive use of ACME 3311 means ensuring that an attacker with access to the validation channel 3312 can't obtain illegitimate authorization by acting as an ACME client 3313 (legitimately, in terms of the protocol). 3315 10.2. Integrity of Authorizations 3317 ACME allows anyone to request challenges for an identifier by 3318 registering an account key and sending a new-order request using that 3319 account key. The integrity of the authorization process thus depends 3320 on the identifier validation challenges to ensure that the challenge 3321 can only be completed by someone who both (1) holds the private key 3322 of the account key pair, and (2) controls the identifier in question. 3324 Validation responses need to be bound to an account key pair in order 3325 to avoid situations where a MitM on ACME HTTPS requests can switch 3326 out a legitimate domain holder's account key for one of his choosing. 3327 Such MitMs can arise, for example, if a CA uses a CDN or third-party 3328 reverse proxy in front of its ACME interface. An attack by such an 3329 MitM could have the following form: 3331 o Legitimate domain holder registers account key pair A 3333 o MitM registers account key pair B 3335 o Legitimate domain holder sends a new-order request signed using 3336 account key A 3338 o MitM suppresses the legitimate request but sends the same request 3339 signed using account key B 3341 o ACME server issues challenges and MitM forwards them to the 3342 legitimate domain holder 3344 o Legitimate domain holder provisions the validation response 3346 o ACME server performs validation query and sees the response 3347 provisioned by the legitimate domain holder 3349 o Because the challenges were issued in response to a message signed 3350 account key B, the ACME server grants authorization to account key 3351 B (the MitM) instead of account key A (the legitimate domain 3352 holder) 3354 Domain ACME 3355 Holder MitM Server 3356 | | | 3357 | newAccount(A) | | 3358 |--------------------->|--------------------->| 3359 | | | 3360 | | newAccount(B) | 3361 | |--------------------->| 3362 | newOrder(domain, A) | | 3363 |--------------------->| | 3364 | | newOrder(domain, B) | 3365 | |--------------------->| 3366 | | | 3367 | authz, challenges | authz, challenges | 3368 |<---------------------|<---------------------| 3369 | | | 3370 | response(chall, A) | response(chall, B) | 3371 |--------------------->|--------------------->| 3372 | | | 3373 | validation request | | 3374 |<--------------------------------------------| 3375 | | | 3376 | validation response | | 3377 |-------------------------------------------->| 3378 | | | 3379 | | | Considers challenge 3380 | | | fulfilled by B. 3381 | | | 3383 Man-in-the-Middle Attack Exploiting a Validation Method without 3384 Account Key Binding 3386 All of the challenges defined in this document have a binding between 3387 the account private key and the validation query made by the server, 3388 via the key authorization. The key authorization reflects the 3389 account public key, is provided to the server in the validation 3390 response over the validation channel and signed afterwards by the 3391 corresponding private key in the challenge response over the ACME 3392 channel. 3394 The association of challenges to identifiers is typically done by 3395 requiring the client to perform some action that only someone who 3396 effectively controls the identifier can perform. For the challenges 3397 in this document, the actions are: 3399 o HTTP: Provision files under .well-known on a web server for the 3400 domain 3402 o DNS: Provision DNS resource records for the domain 3404 There are several ways that these assumptions can be violated, both 3405 by misconfiguration and by attacks. For example, on a web server 3406 that allows non-administrative users to write to .well-known, any 3407 user can claim to own the web server's hostname by responding to an 3408 HTTP challenge. Similarly, if a server that can be used for ACME 3409 validation is compromised by a malicious actor, then that malicious 3410 actor can use that access to obtain certificates via ACME. 3412 The use of hosting providers is a particular risk for ACME 3413 validation. If the owner of the domain has outsourced operation of 3414 DNS or web services to a hosting provider, there is nothing that can 3415 be done against tampering by the hosting provider. As far as the 3416 outside world is concerned, the zone or website provided by the 3417 hosting provider is the real thing. 3419 More limited forms of delegation can also lead to an unintended party 3420 gaining the ability to successfully complete a validation 3421 transaction. For example, suppose an ACME server follows HTTP 3422 redirects in HTTP validation and a website operator provisions a 3423 catch-all redirect rule that redirects requests for unknown resources 3424 to a different domain. Then the target of the redirect could use 3425 that to get a certificate through HTTP validation since the 3426 validation path will not be known to the primary server. 3428 The DNS is a common point of vulnerability for all of these 3429 challenges. An entity that can provision false DNS records for a 3430 domain can attack the DNS challenge directly and can provision false 3431 A/AAAA records to direct the ACME server to send its HTTP validation 3432 query to a remote server of the attacker's choosing. There are a few 3433 different mitigations that ACME servers can apply: 3435 o Always querying the DNS using a DNSSEC-validating resolver 3436 (enhancing security for zones that are DNSSEC-enabled) 3438 o Querying the DNS from multiple vantage points to address local 3439 attackers 3441 o Applying mitigations against DNS off-path attackers, e.g., adding 3442 entropy to requests [I-D.vixie-dnsext-dns0x20] or only using TCP 3444 Given these considerations, the ACME validation process makes it 3445 impossible for any attacker on the ACME channel or a passive attacker 3446 on the validation channel to hijack the authorization process to 3447 authorize a key of the attacker's choice. 3449 An attacker that can only see the ACME channel would need to convince 3450 the validation server to provide a response that would authorize the 3451 attacker's account key, but this is prevented by binding the 3452 validation response to the account key used to request challenges. A 3453 passive attacker on the validation channel can observe the correct 3454 validation response and even replay it, but that response can only be 3455 used with the account key for which it was generated. 3457 An active attacker on the validation channel can subvert the ACME 3458 process, by performing normal ACME transactions and providing a 3459 validation response for his own account key. The risks due to 3460 hosting providers noted above are a particular case. 3462 It is RECOMMENDED that the server perform DNS queries and make HTTP 3463 connections from various network perspectives, in order to make MitM 3464 attacks harder. 3466 10.3. Denial-of-Service Considerations 3468 As a protocol run over HTTPS, standard considerations for TCP-based 3469 and HTTP-based DoS mitigation also apply to ACME. 3471 At the application layer, ACME requires the server to perform a few 3472 potentially expensive operations. Identifier validation transactions 3473 require the ACME server to make outbound connections to potentially 3474 attacker-controlled servers, and certificate issuance can require 3475 interactions with cryptographic hardware. 3477 In addition, an attacker can also cause the ACME server to send 3478 validation requests to a domain of its choosing by submitting 3479 authorization requests for the victim domain. 3481 All of these attacks can be mitigated by the application of 3482 appropriate rate limits. Issues closer to the front end, like POST 3483 body validation, can be addressed using HTTP request limiting. For 3484 validation and certificate requests, there are other identifiers on 3485 which rate limits can be keyed. For example, the server might limit 3486 the rate at which any individual account key can issue certificates 3487 or the rate at which validation can be requested within a given 3488 subtree of the DNS. And in order to prevent attackers from 3489 circumventing these limits simply by minting new accounts, servers 3490 would need to limit the rate at which accounts can be registered. 3492 10.4. Server-Side Request Forgery 3494 Server-Side Request Forgery (SSRF) attacks can arise when an attacker 3495 can cause a server to perform HTTP requests to an attacker-chosen 3496 URL. In the ACME HTTP challenge validation process, the ACME server 3497 performs an HTTP GET request to a URL in which the attacker can 3498 choose the domain. This request is made before the server has 3499 verified that the client controls the domain, so any client can cause 3500 a query to any domain. 3502 Some server implementations include information from the validation 3503 server's response (in order to facilitate debugging). Such 3504 implementations enable an attacker to extract this information from 3505 any web server that is accessible to the ACME server, even if it is 3506 not accessible to the ACME client. For example, the ACME server 3507 might be able to access servers behind a firewall that would prevent 3508 access by the ACME client. 3510 It might seem that the risk of SSRF through this channel is limited 3511 by the fact that the attacker can only control the domain of the URL, 3512 not the path. However, if the attacker first sets the domain to one 3513 they control, then they can send the server an HTTP redirect (e.g., a 3514 302 response) which will cause the server to query an arbitrary URL. 3516 In order to further limit the SSRF risk, ACME server operators should 3517 ensure that validation queries can only be sent to servers on the 3518 public Internet, and not, say, web services within the server 3519 operator's internal network. Since the attacker could make requests 3520 to these public servers himself, he can't gain anything extra through 3521 an SSRF attack on ACME aside from a layer of anonymization. 3523 10.5. CA Policy Considerations 3525 The controls on issuance enabled by ACME are focused on validating 3526 that a certificate applicant controls the identifier he claims. 3527 Before issuing a certificate, however, there are many other checks 3528 that a CA might need to perform, for example: 3530 o Has the client agreed to a subscriber agreement? 3532 o Is the claimed identifier syntactically valid? 3534 o For domain names: 3536 * If the leftmost label is a '*', then have the appropriate 3537 checks been applied? 3539 * Is the name on the Public Suffix List? 3541 * Is the name a high-value name? 3543 * Is the name a known phishing domain? 3545 o Is the key in the CSR sufficiently strong? 3547 o Is the CSR signed with an acceptable algorithm? 3549 o Has issuance been authorized or forbidden by a Certificate 3550 Authority Authorization (CAA) record? [RFC6844] 3552 CAs that use ACME to automate issuance will need to ensure that their 3553 servers perform all necessary checks before issuing. 3555 CAs using ACME to allow clients to agree to terms of service should 3556 keep in mind that ACME clients can automate this agreement, possibly 3557 not involving a human user. 3559 11. Operational Considerations 3561 There are certain factors that arise in operational reality that 3562 operators of ACME-based CAs will need to keep in mind when 3563 configuring their services. For example: 3565 11.1. Key Selection 3567 ACME relies on two different classes of key pair: 3569 o Account key pairs, which are used to authenticate account holders 3571 o Certificate key pairs, which are used to sign and verify CSRs (and 3572 whose public keys are included in certificates) 3574 Compromise of the private key of an account key pair has more serious 3575 consequences than compromise of a private key corresponding to a 3576 certificate. While the compromise of a certificate key pair allows 3577 the attacker to impersonate the entities named in the certificate for 3578 the lifetime of the certificate, the compromise of an account key 3579 pair allows the attacker to take full control of the victim's ACME 3580 account, and take any action that the legitimate account holder could 3581 take within the scope of ACME: 3583 1. Issuing certificates using existing authorizations 3585 2. Revoking existing certificates 3587 3. Accessing and changing account information (e.g., contacts) 3589 4. Changing the account key pair for the account, locking out the 3590 legitimate account holder 3592 For this reason, it is RECOMMENDED that account key pairs be used for 3593 no other purpose besides ACME authentication. For example, the 3594 public key of an account key pair SHOULD NOT be included in a 3595 certificate. ACME clients and servers SHOULD verify that a CSR 3596 submitted in a finalize request does not contain a public key for any 3597 known account key pair. In particular, when a server receives a 3598 finalize request, it MUST verify that the public key in a CSR is not 3599 the same as the public key of the account key pair used to 3600 authenticate that request. This assures that vulnerabilities in the 3601 protocols with which the certificate is used (e.g., signing oracles 3602 in TLS [JSS15]) do not result in compromise of the ACME account. 3604 11.2. DNS security 3606 As noted above, DNS forgery attacks against the ACME server can 3607 result in the server making incorrect decisions about domain control 3608 and thus mis-issuing certificates. Servers SHOULD perform DNS 3609 queries over TCP, which provides better resistance to some forgery 3610 attacks than DNS over UDP. 3612 An ACME-based CA will often need to make DNS queries, e.g., to 3613 validate control of DNS names. Because the security of such 3614 validations ultimately depends on the authenticity of DNS data, every 3615 possible precaution should be taken to secure DNS queries done by the 3616 CA. It is therefore RECOMMENDED that ACME-based CAs make all DNS 3617 queries via DNSSEC-validating stub or recursive resolvers. This 3618 provides additional protection to domains which choose to make use of 3619 DNSSEC. 3621 An ACME-based CA must use only a resolver if it trusts the resolver 3622 and every component of the network route by which it is accessed. It 3623 is therefore RECOMMENDED that ACME-based CAs operate their own 3624 DNSSEC-validating resolvers within their trusted network and use 3625 these resolvers both for both CAA record lookups and all record 3626 lookups in furtherance of a challenge scheme (A, AAAA, TXT, etc.). 3628 11.3. Token Entropy 3630 The http-01, and dns-01 validation methods mandate the usage of a 3631 random token value to uniquely identify the challenge. The value of 3632 the token is required to contain at least 128 bits of entropy for the 3633 following security properties. First, the ACME client should not be 3634 able to influence the ACME server's choice of token as this may allow 3635 an attacker to reuse a domain owner's previous challenge responses 3636 for a new validation request. Secondly, the entropy requirement 3637 prevents ACME clients from implementing a "naive" validation server 3638 that automatically replies to challenges without participating in the 3639 creation of the initial authorization request. 3641 11.4. Malformed Certificate Chains 3643 ACME provides certificate chains in the widely-used format known 3644 colloquially as PEM (though it may diverge from the actual Privacy 3645 Enhanced Mail specifications [RFC1421], as noted in [RFC7468]). Some 3646 current software will allow the configuration of a private key and a 3647 certificate in one PEM file, by concatenating the textual encodings 3648 of the two objects. In the context of ACME, such software might be 3649 vulnerable to "key replacement" attacks. A malicious ACME server 3650 could cause a client to use a private key of its choosing by 3651 including the key in the PEM file returned in response to a query for 3652 a certificate URL. 3654 When processing an file of type "application/pem-certificate-chain", 3655 a client SHOULD verify that the file contains only encoded 3656 certificates. If anything other than a certificate is found (i.e., 3657 if the string "-----BEGIN" is ever followed by anything other than 3658 "CERTIFICATE"), then the client MUST reject the file as invalid. 3660 12. Acknowledgements 3662 In addition to the editors listed on the front page, this document 3663 has benefited from contributions from a broad set of contributors, 3664 all the way back to its inception. 3666 o Andrew Ayer, SSLMate 3668 o Karthik Bhargavan, INRIA 3670 o Peter Eckersley, EFF 3672 o Alex Halderman, University of Michigan 3674 o Sophie Herold, Hemio 3676 o Eric Rescorla, Mozilla 3678 o Seth Schoen, EFF 3680 o Martin Thomson, Mozilla 3682 o Jakub Warmuz, University of Oxford 3684 This document draws on many concepts established by Eric Rescorla's 3685 "Automated Certificate Issuance Protocol" draft. Martin Thomson 3686 provided helpful guidance in the use of HTTP. 3688 13. References 3690 13.1. Normative References 3692 [FIPS180-4] 3693 Department of Commerce, National., "NIST FIPS 180-4, 3694 Secure Hash Standard", March 2012, 3695 . 3698 [JSS15] Somorovsky, J., "On the Security of TLS 1.3 and QUIC 3699 Against Weaknesses in PKCS#1 v1.5 Encryption", n.d., 3700 . 3702 [REST] Fielding, R., "Architectural Styles and the Design of 3703 Network-based Software Architectures", 2000, 3704 . 3707 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 3708 Requirement Levels", BCP 14, RFC 2119, 3709 DOI 10.17487/RFC2119, March 1997, 3710 . 3712 [RFC2585] Housley, R. and P. Hoffman, "Internet X.509 Public Key 3713 Infrastructure Operational Protocols: FTP and HTTP", 3714 RFC 2585, DOI 10.17487/RFC2585, May 1999, 3715 . 3717 [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, 3718 DOI 10.17487/RFC2818, May 2000, 3719 . 3721 [RFC2985] Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object 3722 Classes and Attribute Types Version 2.0", RFC 2985, 3723 DOI 10.17487/RFC2985, November 2000, 3724 . 3726 [RFC2986] Nystrom, M. and B. Kaliski, "PKCS #10: Certification 3727 Request Syntax Specification Version 1.7", RFC 2986, 3728 DOI 10.17487/RFC2986, November 2000, 3729 . 3731 [RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet: 3732 Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002, 3733 . 3735 [RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode 3736 for Internationalized Domain Names in Applications 3737 (IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003, 3738 . 3740 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 3741 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November 3742 2003, . 3744 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 3745 Resource Identifier (URI): Generic Syntax", STD 66, 3746 RFC 3986, DOI 10.17487/RFC3986, January 2005, 3747 . 3749 [RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker, 3750 "Randomness Requirements for Security", BCP 106, RFC 4086, 3751 DOI 10.17487/RFC4086, June 2005, 3752 . 3754 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data 3755 Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006, 3756 . 3758 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 3759 Specifications: ABNF", STD 68, RFC 5234, 3760 DOI 10.17487/RFC5234, January 2008, 3761 . 3763 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 3764 (TLS) Protocol Version 1.2", RFC 5246, 3765 DOI 10.17487/RFC5246, August 2008, 3766 . 3768 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 3769 Housley, R., and W. Polk, "Internet X.509 Public Key 3770 Infrastructure Certificate and Certificate Revocation List 3771 (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, 3772 . 3774 [RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet 3775 Mail Extensions (S/MIME) Version 3.2 Message 3776 Specification", RFC 5751, DOI 10.17487/RFC5751, January 3777 2010, . 3779 [RFC5890] Klensin, J., "Internationalized Domain Names for 3780 Applications (IDNA): Definitions and Document Framework", 3781 RFC 5890, DOI 10.17487/RFC5890, August 2010, 3782 . 3784 [RFC5988] Nottingham, M., "Web Linking", RFC 5988, 3785 DOI 10.17487/RFC5988, October 2010, 3786 . 3788 [RFC6068] Duerst, M., Masinter, L., and J. Zawinski, "The 'mailto' 3789 URI Scheme", RFC 6068, DOI 10.17487/RFC6068, October 2010, 3790 . 3792 [RFC6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M., 3793 and D. Orchard, "URI Template", RFC 6570, 3794 DOI 10.17487/RFC6570, March 2012, 3795 . 3797 [RFC6844] Hallam-Baker, P. and R. Stradling, "DNS Certification 3798 Authority Authorization (CAA) Resource Record", RFC 6844, 3799 DOI 10.17487/RFC6844, January 2013, 3800 . 3802 [RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data 3803 Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March 3804 2014, . 3806 [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 3807 Protocol (HTTP/1.1): Semantics and Content", RFC 7231, 3808 DOI 10.17487/RFC7231, June 2014, 3809 . 3811 [RFC7468] Josefsson, S. and S. Leonard, "Textual Encodings of PKIX, 3812 PKCS, and CMS Structures", RFC 7468, DOI 10.17487/RFC7468, 3813 April 2015, . 3815 [RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web 3816 Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May 3817 2015, . 3819 [RFC7518] Jones, M., "JSON Web Algorithms (JWA)", RFC 7518, 3820 DOI 10.17487/RFC7518, May 2015, 3821 . 3823 [RFC7638] Jones, M. and N. Sakimura, "JSON Web Key (JWK) 3824 Thumbprint", RFC 7638, DOI 10.17487/RFC7638, September 3825 2015, . 3827 [RFC7797] Jones, M., "JSON Web Signature (JWS) Unencoded Payload 3828 Option", RFC 7797, DOI 10.17487/RFC7797, February 2016, 3829 . 3831 [RFC7807] Nottingham, M. and E. Wilde, "Problem Details for HTTP 3832 APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016, 3833 . 3835 [RFC8037] Liusvaara, I., "CFRG Elliptic Curve Diffie-Hellman (ECDH) 3836 and Signatures in JSON Object Signing and Encryption 3837 (JOSE)", RFC 8037, DOI 10.17487/RFC8037, January 2017, 3838 . 3840 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for 3841 Writing an IANA Considerations Section in RFCs", BCP 26, 3842 RFC 8126, DOI 10.17487/RFC8126, June 2017, 3843 . 3845 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 3846 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 3847 May 2017, . 3849 [RFC8288] Nottingham, M., "Web Linking", RFC 8288, 3850 DOI 10.17487/RFC8288, October 2017, 3851 . 3853 13.2. Informative References 3855 [I-D.ietf-acme-caa] 3856 Landau, H., "CAA Record Extensions for Account URI and 3857 ACME Method Binding", draft-ietf-acme-caa-05 (work in 3858 progress), June 2018. 3860 [I-D.ietf-acme-ip] 3861 Shoemaker, R., "ACME IP Identifier Validation Extension", 3862 draft-ietf-acme-ip-04 (work in progress), July 2018. 3864 [I-D.ietf-acme-telephone] 3865 Peterson, J. and R. Barnes, "ACME Identifiers and 3866 Challenges for Telephone Numbers", draft-ietf-acme- 3867 telephone-01 (work in progress), October 2017. 3869 [I-D.vixie-dnsext-dns0x20] 3870 Vixie, P. and D. Dagon, "Use of Bit 0x20 in DNS Labels to 3871 Improve Transaction Identity", draft-vixie-dnsext- 3872 dns0x20-00 (work in progress), March 2008. 3874 [RFC1421] Linn, J., "Privacy Enhancement for Internet Electronic 3875 Mail: Part I: Message Encryption and Authentication 3876 Procedures", RFC 1421, DOI 10.17487/RFC1421, February 3877 1993, . 3879 [RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC 3880 Text on Security Considerations", BCP 72, RFC 3552, 3881 DOI 10.17487/RFC3552, July 2003, 3882 . 3884 [RFC3553] Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An 3885 IETF URN Sub-namespace for Registered Protocol 3886 Parameters", BCP 73, RFC 3553, DOI 10.17487/RFC3553, June 3887 2003, . 3889 [RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known 3890 Uniform Resource Identifiers (URIs)", RFC 5785, 3891 DOI 10.17487/RFC5785, April 2010, 3892 . 3894 [RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre, 3895 "Recommendations for Secure Use of Transport Layer 3896 Security (TLS) and Datagram Transport Layer Security 3897 (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May 3898 2015, . 3900 [W3C.CR-cors-20130129] 3901 Kesteren, A., "Cross-Origin Resource Sharing", World Wide 3902 Web Consortium CR CR-cors-20130129, January 2013, 3903 . 3905 13.3. URIs 3907 [1] https://github.com/ietf-wg-acme/acme 3909 Authors' Addresses 3911 Richard Barnes 3912 Cisco 3914 Email: rlb@ipv.sx 3916 Jacob Hoffman-Andrews 3917 EFF 3919 Email: jsha@eff.org 3921 Daniel McCarney 3922 Let's Encrypt 3924 Email: cpu@letsencrypt.org 3925 James Kasten 3926 University of Michigan 3928 Email: jdkasten@umich.edu