RFC 8555: Automatic Certificate Management Environment (ACME)
- R. Barnes,
- J. Hoffman-Andrews,
- D. McCarney,
- J. Kasten
Proposed Standard
Internet Engineering Task Force (IETF) R. Barnes
Request for Comments: 8555 Cisco
Category: Standards Track J. Hoffman-Andrews
ISSN: 2070-1721 EFF
D. McCarney
Let's Encrypt
J. Kasten
University of Michigan
March 2019
Automatic Certificate Management Environment (ACME)
Abstract
Public Key Infrastructure using X.509 (PKIX) certificates are used
for a number of purposes, the most significant of which is the
authentication of domain names. Thus, certification authorities
(CAs) in the Web PKI are trusted to verify that an applicant for a
certificate legitimately represents the domain name(s) in the
certificate. As of this writing, this verification is done through a
collection of ad hoc mechanisms. This document describes a protocol
that a CA and an applicant can use to automate the process of
verification and certificate issuance. The protocol also provides
facilities for other certificate management functions, such as
certificate revocation.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8555.
Barnes, et al. Standards Track [Page 1]
RFC 8555 ACME March 2019 Copyright Notice Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction ....................................................4 2. Deployment Model and Operator Experience ........................5 3. Terminology .....................................................7 4. Protocol Overview ...............................................7 5. Character Encoding .............................................10 6. Message Transport ..............................................10 6.1. HTTPS Requests ............................................10 6.2. Request Authentication ....................................11 6.3. GET and POST-as-GET Requests ..............................13 6.4. Request URL Integrity .....................................13 6.4.1. "url" (URL) JWS Header Parameter ...................14 6.5. Replay Protection .........................................14 6.5.1. Replay-Nonce .......................................15 6.5.2. "nonce" (Nonce) JWS Header Parameter ...............16 6.6. Rate Limits ...............................................16 6.7. Errors ....................................................16 6.7.1. Subproblems ........................................18 7. Certificate Management .........................................20 7.1. Resources .................................................20 7.1.1. Directory ..........................................23 7.1.2. Account Objects ....................................24 7.1.3. Order Objects ......................................26 7.1.4. Authorization Objects ..............................28 7.1.5. Challenge Objects ..................................30 7.1.6. Status Changes .....................................30 7.2. Getting a Nonce ...........................................34 7.3. Account Management ........................................34 7.3.1. Finding an Account URL Given a Key .................36 7.3.2. Account Update .....................................37 7.3.3. Changes of Terms of Service ........................38 7.3.4. External Account Binding ...........................38 Barnes, et al. Standards Track [Page 2]
RFC 8555 ACME March 2019 7.3.5. Account Key Rollover ...............................40 7.3.6. Account Deactivation ...............................43 7.4. Applying for Certificate Issuance .........................44 7.4.1. Pre-authorization ..................................49 7.4.2. Downloading the Certificate ........................51 7.5. Identifier Authorization ..................................53 7.5.1. Responding to Challenges ...........................54 7.5.2. Deactivating an Authorization ......................57 7.6. Certificate Revocation ....................................58 8. Identifier Validation Challenges ...............................60 8.1. Key Authorizations ........................................62 8.2. Retrying Challenges .......................................63 8.3. HTTP Challenge ............................................63 8.4. DNS Challenge .............................................66 9. IANA Considerations ............................................68 9.1. Media Type: application/pem-certificate-chain .............68 9.2. Well-Known URI for the HTTP Challenge .....................69 9.3. Replay-Nonce HTTP Header ..................................69 9.4. "url" JWS Header Parameter ................................70 9.5. "nonce" JWS Header Parameter ..............................70 9.6. URN Sub-namespace for ACME (urn:ietf:params:acme) .........70 9.7. New Registries ............................................71 9.7.1. Fields in Account Objects ..........................71 9.7.2. Fields in Order Objects ............................72 9.7.3. Fields in Authorization Objects ....................73 9.7.4. Error Types ........................................74 9.7.5. Resource Types .....................................74 9.7.6. Fields in the "meta" Object within a Directory Object ...................................75 9.7.7. Identifier Types ...................................76 9.7.8. Validation Methods .................................76 10. Security Considerations .......................................78 10.1. Threat Model .............................................78 10.2. Integrity of Authorizations ..............................80 10.3. Denial-of-Service Considerations .........................83 10.4. Server-Side Request Forgery ..............................84 10.5. CA Policy Considerations .................................84 11. Operational Considerations ....................................86 11.1. Key Selection ............................................86 11.2. DNS Security .............................................87 11.3. Token Entropy ............................................88 11.4. Malformed Certificate Chains .............................88 12. References ....................................................88 12.1. Normative References .....................................88 12.2. Informative References ...................................92 Acknowledgements ..................................................94 Authors' Addresses ................................................95 Barnes, et al. Standards Track [Page 3]
RFC 8555 ACME March 2019 1. Introduction Certificates [RFC5280] in the Web PKI are most commonly used to authenticate domain names. Thus, certification authorities (CAs) in the Web PKI are trusted to verify that an applicant for a certificate legitimately represents the domain name(s) in the certificate. Different types of certificates reflect different kinds of CA verification of information about the certificate subject. "Domain Validation" (DV) certificates are by far the most common type. The only validation the CA is required to perform in the DV issuance process is to verify that the requester has effective control of the domain [CABFBR]. The CA is not required to attempt to verify the requester's real-world identity. (This is as opposed to "Organization Validation" (OV) and "Extended Validation" (EV) certificates, where the process is intended to also verify the real- world identity of the requester.) Existing Web PKI certification authorities tend to use a set of ad hoc protocols for certificate issuance and identity verification. In the case of DV certificates, a typical user experience is something like: o Generate a PKCS#10 [RFC2986] Certificate Signing Request (CSR). o Cut and paste the CSR into a CA's web page. o Prove ownership of the domain(s) in the CSR by one of the following methods: * Put a CA-provided challenge at a specific place on the web server. * Put a CA-provided challenge in a DNS record corresponding to the target domain. * Receive a CA-provided challenge at (hopefully) an administrator-controlled email address corresponding to the domain, and then respond to it on the CA's web page. o Download the issued certificate and install it on the user's Web Server. With the exception of the CSR itself and the certificates that are issued, these are all completely ad hoc procedures and are accomplished by getting the human user to follow interactive natural- language instructions from the CA rather than by machine-implemented published protocols. In many cases, the instructions are difficult Barnes, et al. Standards Track [Page 4]
RFC 8555 ACME March 2019 to follow and cause significant frustration and confusion. Informal usability tests by the authors indicate that webmasters often need 1-3 hours to obtain and install a certificate for a domain. Even in the best case, the lack of published, standardized mechanisms presents an obstacle to the wide deployment of HTTPS and other PKIX- dependent systems because it inhibits mechanization of tasks related to certificate issuance, deployment, and revocation. This document describes an extensible framework for automating the issuance and domain validation procedure, thereby allowing servers and infrastructure software to obtain certificates without user interaction. Use of this protocol should radically simplify the deployment of HTTPS and the practicality of PKIX-based authentication for other protocols based on Transport Layer Security (TLS) [RFC8446]. It should be noted that while the focus of this document is on validating domain names for purposes of issuing certificates in the Web PKI, ACME supports extensions for uses with other identifiers in other PKI contexts. For example, as of this writing, there is ongoing work to use ACME for issuance of Web PKI certificates attesting to IP addresses [ACME-IP] and Secure Telephone Identity Revisited (STIR) certificates attesting to telephone numbers [ACME-TELEPHONE]. ACME can also be used to automate some aspects of certificate management even where non-automated processes are still needed. For example, the external account binding feature (see Section 7.3.4) can allow an ACME account to use authorizations that have been granted to an external, non-ACME account. This allows ACME to address issuance scenarios that cannot yet be fully automated, such as the issuance of "Extended Validation" certificates. 2. Deployment Model and Operator Experience The guiding use case for ACME is obtaining certificates for websites (HTTPS [RFC2818]). In this case, a web server is intended to speak for one or more domains, and the process of certificate issuance is intended to verify that this web server actually speaks for the domain(s). DV certificate validation commonly checks claims about properties related to control of a domain name -- properties that can be observed by the certificate issuer in an interactive process that can be conducted purely online. That means that under typical circumstances, all steps in the request, verification, and issuance process can be represented and performed by Internet protocols with no out-of-band human intervention. Barnes, et al. Standards Track [Page 5]
RFC 8555 ACME March 2019 Prior to ACME, when deploying an HTTPS server, a server operator typically gets a prompt to generate a self-signed certificate. If the operator were instead deploying an HTTPS server using ACME, the experience would be something like this: o The operator's ACME client prompts the operator for the intended domain name(s) that the web server is to stand for. o The ACME client presents the operator with a list of CAs from which it could get a certificate. (This list will change over time based on the capabilities of CAs and updates to ACME configuration.) The ACME client might prompt the operator for payment information at this point. o The operator selects a CA. o In the background, the ACME client contacts the CA and requests that it issue a certificate for the intended domain name(s). o The CA verifies that the client controls the requested domain name(s) by having the ACME client perform some action(s) that can only be done with control of the domain name(s). For example, the CA might require a client requesting example.com to provision a DNS record under example.com or an HTTP resource under http://example.com. o Once the CA is satisfied, it issues the certificate and the ACME client automatically downloads and installs it, potentially notifying the operator via email, SMS, etc. o The ACME client periodically contacts the CA to get updated certificates, stapled Online Certificate Status Protocol (OCSP) responses [RFC6960], or whatever else would be required to keep the web server functional and its credentials up to date. In this way, it would be nearly as easy to deploy with a CA-issued certificate as with a self-signed certificate. Furthermore, the maintenance of that CA-issued certificate would require minimal manual intervention. Such close integration of ACME with HTTPS servers allows the immediate and automated deployment of certificates as they are issued, sparing the human administrator from much of the time-consuming work described in the previous section. Barnes, et al. Standards Track [Page 6]
RFC 8555 ACME March 2019 3. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. The two main roles in ACME are "client" and "server". The ACME client uses the protocol to request certificate management actions, such as issuance or revocation. An ACME client may run on a web server, mail server, or some other server system that requires valid X.509 certificates. Or, it may run on a separate server that does not consume the certificate but is authorized to respond to a CA- provided challenge. The ACME server runs at a certification authority and responds to client requests, performing the requested actions if the client is authorized. An ACME client authenticates to the server by means of an "account key pair". The client uses the private key of this key pair to sign all messages sent to the server. The server uses the public key to verify the authenticity and integrity of messages from the client. 4. Protocol Overview ACME allows a client to request certificate management actions using a set of JavaScript Object Notation (JSON) messages [RFC8259] carried over HTTPS [RFC2818]. Issuance using ACME resembles a traditional CA's issuance process, in which a user creates an account, requests a certificate, and proves control of the domain(s) in that certificate in order for the CA to issue the requested certificate. The first phase of ACME is for the client to request an account with the ACME server. The client generates an asymmetric key pair and requests a new account, optionally providing contact information, agreeing to terms of service (ToS), and/or associating the account with an existing account in another system. The creation request is signed with the generated private key to prove that the client controls it. Barnes, et al. Standards Track [Page 7]
RFC 8555 ACME March 2019 Client Server [Contact Information] [ToS Agreement] [Additional Data] Signature -------> Account URL <------- Account Object [] Information covered by request signatures Account Creation Once an account is registered, there are four major steps the client needs to take to get a certificate: 1. Submit an order for a certificate to be issued 2. Prove control of any identifiers requested in the certificate 3. Finalize the order by submitting a CSR 4. Await issuance and download the issued certificate The client's order for a certificate describes the desired identifiers plus a few additional fields that capture semantics that are not supported in the CSR format. If the server is willing to consider issuing such a certificate, it responds with a list of requirements that the client must satisfy before the certificate will be issued. For example, in most cases, the server will require the client to demonstrate that it controls the identifiers in the requested certificate. Because there are many different ways to validate possession of different types of identifiers, the server will choose from an extensible set of challenges that are appropriate for the identifier being claimed. The client responds with a set of responses that tell the server which challenges the client has completed. The server then validates that the client has completed the challenges. Once the validation process is complete and the server is satisfied that the client has met its requirements, the client finalizes the order by submitting a PKCS#10 Certificate Signing Request (CSR). The server will issue the requested certificate and make it available to the client. Barnes, et al. Standards Track [Page 8]
RFC 8555 ACME March 2019 Client Server [Order] Signature -------> <------- Required Authorizations [Responses] Signature -------> <~~~~~~~~Validation~~~~~~~~> [CSR] Signature -------> <------- Acknowledgement <~~~~~~Await issuance~~~~~~> [POST-as-GET request] Signature -------> <------- Certificate [] Information covered by request signatures Certificate Issuance To revoke a certificate, the client sends a signed revocation request indicating the certificate to be revoked: Client Server [Revocation request] Signature --------> <-------- Result [] Information covered by request signatures Certificate Revocation Note that while ACME is defined with enough flexibility to handle different types of identifiers in principle, the primary use case addressed by this document is the case where domain names are used as identifiers. For example, all of the identifier validation challenges described in Section 8 address validation of domain names. The use of ACME for other identifiers will require further specification in order to describe how these identifiers are encoded in the protocol and what types of validation challenges the server might require. Barnes, et al. Standards Track [Page 9]
RFC 8555 ACME March 2019 5. Character Encoding All requests and responses sent via HTTP by ACME clients, ACME servers, and validation servers as well as any inputs for digest computations MUST be encoded using the UTF-8 character set [RFC3629]. Note that identifiers that appear in certificates may have their own encoding considerations (e.g., DNS names containing non-ASCII characters are expressed as A-labels rather than U-labels). Any such encoding considerations are to be applied prior to the aforementioned UTF-8 encoding. 6. Message Transport Communications between an ACME client and an ACME server are done over HTTPS, using JSON Web Signature (JWS) [RFC7515] to provide some additional security properties for messages sent from the client to the server. HTTPS provides server authentication and confidentiality. With some ACME-specific extensions, JWS provides authentication of the client's request payloads, anti-replay protection, and integrity for the HTTPS request URL. 6.1. HTTPS Requests Each ACME function is accomplished by the client sending a sequence of HTTPS requests to the server [RFC2818], carrying JSON messages [RFC8259]. Use of HTTPS is REQUIRED. Each subsection of Section 7 below describes the message formats used by the function and the order in which messages are sent. In most HTTPS transactions used by ACME, the ACME client is the HTTPS client and the ACME server is the HTTPS server. The ACME server acts as a client when validating challenges: an HTTP client when validating an 'http-01' challenge, a DNS client with 'dns-01', etc. ACME servers SHOULD follow the recommendations of [RFC7525] when configuring their TLS implementations. ACME servers that support TLS 1.3 MAY allow clients to send early data (0-RTT). This is safe because the ACME protocol itself includes anti-replay protections (see Section 6.5) in all cases where they are required. For this reason, there are no restrictions on what ACME data can be carried in 0-RTT. ACME clients MUST send a User-Agent header field, in accordance with [RFC7231]. This header field SHOULD include the name and version of the ACME software in addition to the name and version of the underlying HTTP client software. Barnes, et al. Standards Track [Page 10]
RFC 8555 ACME March 2019 ACME clients SHOULD send an Accept-Language header field in accordance with [RFC7231] to enable localization of error messages. ACME servers that are intended to be generally accessible need to use Cross-Origin Resource Sharing (CORS) in order to be accessible from browser-based clients [W3C.REC-cors-20140116]. Such servers SHOULD set the Access-Control-Allow-Origin header field to the value "*". Binary fields in the JSON objects used by ACME are encoded using base64url encoding described in Section 5 of [RFC4648] according to the profile specified in JSON Web Signature in Section 2 of [RFC7515]. This encoding uses a URL safe character set. Trailing '=' characters MUST be stripped. Encoded values that include trailing '=' characters MUST be rejected as improperly encoded. 6.2. Request Authentication All ACME requests with a non-empty body MUST encapsulate their payload in a JSON Web Signature (JWS) [RFC7515] object, signed using the account's private key unless otherwise specified. The server MUST verify the JWS before processing the request. Encapsulating request bodies in JWS provides authentication of requests. A JWS object sent as the body of an ACME request MUST meet the following additional criteria: o The JWS MUST be in the Flattened JSON Serialization [RFC7515] o The JWS MUST NOT have multiple signatures o The JWS Unencoded Payload Option [RFC7797] MUST NOT be used o The JWS Unprotected Header [RFC7515] MUST NOT be used o The JWS Payload MUST NOT be detached o The JWS Protected Header MUST include the following fields: * "alg" (Algorithm) + This field MUST NOT contain "none" or a Message Authentication Code (MAC) algorithm (e.g. one in which the algorithm registry description mentions MAC/HMAC). * "nonce" (defined in Section 6.5) * "url" (defined in Section 6.4) Barnes, et al. Standards Track [Page 11]
RFC 8555 ACME March 2019 * Either "jwk" (JSON Web Key) or "kid" (Key ID) as specified below An ACME server MUST implement the "ES256" signature algorithm [RFC7518] and SHOULD implement the "EdDSA" signature algorithm using the "Ed25519" variant (indicated by "crv") [RFC8037]. The "jwk" and "kid" fields are mutually exclusive. Servers MUST reject requests that contain both. For newAccount requests, and for revokeCert requests authenticated by a certificate key, there MUST be a "jwk" field. This field MUST contain the public key corresponding to the private key used to sign the JWS. For all other requests, the request is signed using an existing account, and there MUST be a "kid" field. This field MUST contain the account URL received by POSTing to the newAccount resource. If the client sends a JWS signed with an algorithm that the server does not support, then the server MUST return an error with status code 400 (Bad Request) and type "urn:ietf:params:acme:error:badSignatureAlgorithm". The problem document returned with the error MUST include an "algorithms" field with an array of supported "alg" values. See Section 6.7 for more details on the structure of error responses. If the server supports the signature algorithm "alg" but either does not support or chooses to reject the public key "jwk", then the server MUST return an error with status code 400 (Bad Request) and type "urn:ietf:params:acme:error:badPublicKey". The problem document detail SHOULD describe the reason for rejecting the public key; some example reasons are: o "alg" is "RS256" but the modulus "n" is too small (e.g., 512-bit) o "alg" is "ES256" but "jwk" does not contain a valid P-256 public key o "alg" is "EdDSA" and "crv" is "Ed448", but the server only supports "EdDSA" with "Ed25519" o the corresponding private key is known to have been compromised Barnes, et al. Standards Track [Page 12]
RFC 8555 ACME March 2019 Because client requests in ACME carry JWS objects in the Flattened JSON Serialization, they must have the Content-Type header field set to "application/jose+json". If a request does not meet this requirement, then the server MUST return a response with status code 415 (Unsupported Media Type). 6.3. GET and POST-as-GET Requests Note that authentication via signed JWS request bodies implies that requests without an entity body are not authenticated, in particular GET requests. Except for the cases described in this section, if the server receives a GET request, it MUST return an error with status code 405 (Method Not Allowed) and type "malformed". If a client wishes to fetch a resource from the server (which would otherwise be done with a GET), then it MUST send a POST request with a JWS body as described above, where the payload of the JWS is a zero-length octet string. In other words, the "payload" field of the JWS object MUST be present and set to the empty string (""). We will refer to these as "POST-as-GET" requests. On receiving a request with a zero-length (and thus non-JSON) payload, the server MUST authenticate the sender and verify any access control rules. Otherwise, the server MUST treat this request as having the same semantics as a GET request for the same resource. The server MUST allow GET requests for the directory and newNonce resources (see Section 7.1), in addition to POST-as-GET requests for these resources. This enables clients to bootstrap into the ACME authentication system. 6.4. Request URL Integrity It is common in deployment for the entity terminating TLS for HTTPS to be different from the entity operating the logical HTTPS server, with a "request routing" layer in the middle. For example, an ACME CA might have a content delivery network terminate TLS connections from clients so that it can inspect client requests for denial-of- service (DoS) protection. These intermediaries can also change values in the request that are not signed in the HTTPS request, e.g., the request URL and header fields. ACME uses JWS to provide an integrity mechanism, which protects against an intermediary changing the request URL to another ACME URL. Barnes, et al. Standards Track [Page 13]
RFC 8555 ACME March 2019 As noted in Section 6.2, all ACME request objects carry a "url" header parameter in their protected header. This header parameter encodes the URL to which the client is directing the request. On receiving such an object in an HTTP request, the server MUST compare the "url" header parameter to the request URL. If the two do not match, then the server MUST reject the request as unauthorized. Except for the directory resource, all ACME resources are addressed with URLs provided to the client by the server. In POST requests sent to these resources, the client MUST set the "url" header parameter to the exact string provided by the server (rather than performing any re-encoding on the URL). The server SHOULD perform the corresponding string equality check, configuring each resource with the URL string provided to clients and having the resource check that requests have the same string in their "url" header parameter. The server MUST reject the request as unauthorized if the string equality check fails. 6.4.1. "url" (URL) JWS Header Parameter The "url" header parameter specifies the URL [RFC3986] to which this JWS object is directed. The "url" header parameter MUST be carried in the protected header of the JWS. The value of the "url" header parameter MUST be a string representing the target URL. 6.5. Replay Protection In order to protect ACME resources from any possible replay attacks, ACME POST requests have a mandatory anti-replay mechanism. This mechanism is based on the server maintaining a list of nonces that it has issued, and requiring any signed request from the client to carry such a nonce. An ACME server provides nonces to clients using the HTTP Replay-Nonce header field, as specified in Section 6.5.1. The server MUST include a Replay-Nonce header field in every successful response to a POST request and SHOULD provide it in error responses as well. Every JWS sent by an ACME client MUST include, in its protected header, the "nonce" header parameter, with contents as defined in Section 6.5.2. As part of JWS verification, the ACME server MUST verify that the value of the "nonce" header is a value that the server previously provided in a Replay-Nonce header field. Once a nonce value has appeared in an ACME request, the server MUST consider it invalid, in the same way as a value it had never issued. Barnes, et al. Standards Track [Page 14]
RFC 8555 ACME March 2019 When a server rejects a request because its nonce value was unacceptable (or not present), it MUST provide HTTP status code 400 (Bad Request), and indicate the ACME error type "urn:ietf:params:acme:error:badNonce". An error response with the "badNonce" error type MUST include a Replay-Nonce header field with a fresh nonce that the server will accept in a retry of the original query (and possibly in other requests, according to the server's nonce scoping policy). On receiving such a response, a client SHOULD retry the request using the new nonce. The precise method used to generate and track nonces is up to the server. For example, the server could generate a random 128-bit value for each response, keep a list of issued nonces, and strike nonces from this list as they are used. Other than the constraint above with regard to nonces issued in "badNonce" responses, ACME does not constrain how servers scope nonces. Clients MAY assume that nonces have broad scope, e.g., by having a single pool of nonces used for all requests. However, when retrying in response to a "badNonce" error, the client MUST use the nonce provided in the error response. Servers should scope nonces broadly enough that retries are not needed very often. 6.5.1. Replay-Nonce The Replay-Nonce HTTP header field includes a server-generated value that the server can use to detect unauthorized replay in future client requests. The server MUST generate the values provided in Replay-Nonce header fields in such a way that they are unique to each message, with high probability, and unpredictable to anyone besides the server. For instance, it is acceptable to generate Replay-Nonces randomly. The value of the Replay-Nonce header field MUST be an octet string encoded according to the base64url encoding described in Section 2 of [RFC7515]. Clients MUST ignore invalid Replay-Nonce values. The ABNF [RFC5234] for the Replay-Nonce header field follows: base64url = ALPHA / DIGIT / "-" / "_" Replay-Nonce = 1*base64url The Replay-Nonce header field SHOULD NOT be included in HTTP request messages. Barnes, et al. Standards Track [Page 15]
RFC 8555 ACME March 2019 6.5.2. "nonce" (Nonce) JWS Header Parameter The "nonce" header parameter provides a unique value that enables the verifier of a JWS to recognize when replay has occurred. The "nonce" header parameter MUST be carried in the protected header of the JWS. The value of the "nonce" header parameter MUST be an octet string, encoded according to the base64url encoding described in Section 2 of [RFC7515]. If the value of a "nonce" header parameter is not valid according to this encoding, then the verifier MUST reject the JWS as malformed. 6.6. Rate Limits Creation of resources can be rate limited by ACME servers to ensure fair usage and prevent abuse. Once the rate limit is exceeded, the server MUST respond with an error with the type "urn:ietf:params:acme:error:rateLimited". Additionally, the server SHOULD send a Retry-After header field [RFC7231] indicating when the current request may succeed again. If multiple rate limits are in place, that is the time where all rate limits allow access again for the current request with exactly the same parameters. In addition to the human-readable "detail" field of the error response, the server MAY send one or multiple link relations in the Link header field [RFC8288] pointing to documentation about the specific rate limit that was hit, using the "help" link relation type. 6.7. Errors Errors can be reported in ACME both at the HTTP layer and within challenge objects as defined in Section 8. ACME servers can return responses with an HTTP error response code (4XX or 5XX). For example, if the client submits a request using a method not allowed in this document, then the server MAY return status code 405 (Method Not Allowed). When the server responds with an error status, it SHOULD provide additional information using a problem document [RFC7807]. To facilitate automatic response to errors, this document defines the following standard tokens for use in the "type" field (within the ACME URN namespace "urn:ietf:params:acme:error:"): Barnes, et al. Standards Track [Page 16]
RFC 8555 ACME March 2019 +-------------------------+-----------------------------------------+ | Type | Description | +-------------------------+-----------------------------------------+ | accountDoesNotExist | The request specified an account that | | | does not exist | | | | | alreadyRevoked | The request specified a certificate to | | | be revoked that has already been | | | revoked | | | | | badCSR | The CSR is unacceptable (e.g., due to a | | | short key) | | | | | badNonce | The client sent an unacceptable anti- | | | replay nonce | | | | | badPublicKey | The JWS was signed by a public key the | | | server does not support | | | | | badRevocationReason | The revocation reason provided is not | | | allowed by the server | | | | | badSignatureAlgorithm | The JWS was signed with an algorithm | | | the server does not support | | | | | caa | Certification Authority Authorization | | | (CAA) records forbid the CA from | | | issuing a certificate | | | | | compound | Specific error conditions are indicated | | | in the "subproblems" array | | | | | connection | The server could not connect to | | | validation target | | | | | dns | There was a problem with a DNS query | | | during identifier validation | | | | | externalAccountRequired | The request must include a value for | | | the "externalAccountBinding" field | | | | | incorrectResponse | Response received didn't match the | | | challenge's requirements | | | | | invalidContact | A contact URL for an account was | | | invalid | | | | | malformed | The request message was malformed | Barnes, et al. Standards Track [Page 17]
RFC 8555 ACME March 2019 | | | | orderNotReady | The request attempted to finalize an | | | order that is not ready to be finalized | | | | | rateLimited | The request exceeds a rate limit | | | | | rejectedIdentifier | The server will not issue certificates | | | for the identifier | | | | | serverInternal | The server experienced an internal | | | error | | | | | tls | The server received a TLS error during | | | validation | | | | | unauthorized | The client lacks sufficient | | | authorization | | | | | unsupportedContact | A contact URL for an account used an | | | unsupported protocol scheme | | | | | unsupportedIdentifier | An identifier is of an unsupported type | | | | | userActionRequired | Visit the "instance" URL and take | | | actions specified there | +-------------------------+-----------------------------------------+ This list is not exhaustive. The server MAY return errors whose "type" field is set to a URI other than those defined above. Servers MUST NOT use the ACME URN namespace for errors not listed in the appropriate IANA registry (see Section 9.6). Clients SHOULD display the "detail" field of all errors. In the remainder of this document, we use the tokens in the table above to refer to error types, rather than the full URNs. For example, an "error of type 'badCSR'" refers to an error document with "type" value "urn:ietf:params:acme:error:badCSR". 6.7.1. Subproblems Sometimes a CA may need to return multiple errors in response to a request. Additionally, the CA may need to attribute errors to specific identifiers. For instance, a newOrder request may contain multiple identifiers for which the CA cannot issue certificates. In this situation, an ACME problem document MAY contain the "subproblems" field, containing a JSON array of problem documents, each of which MAY contain an "identifier" field. If present, the "identifier" field MUST contain an ACME identifier (Section 9.7.7). Barnes, et al. Standards Track [Page 18]
RFC 8555 ACME March 2019 The "identifier" field MUST NOT be present at the top level in ACME problem documents. It can only be present in subproblems. Subproblems need not all have the same type, and they do not need to match the top level type. ACME clients may choose to use the "identifier" field of a subproblem as a hint that an operation would succeed if that identifier were omitted. For instance, if an order contains ten DNS identifiers, and the newOrder request returns a problem document with two subproblems (referencing two of those identifiers), the ACME client may choose to submit another order containing only the eight identifiers not listed in the problem document. HTTP/1.1 403 Forbidden Content-Type: application/problem+json Link: <https://example.com/acme/directory>;rel="index" { "type": "urn:ietf:params:acme:error:malformed", "detail": "Some of the identifiers requested were rejected", "subproblems": [ { "type": "urn:ietf:params:acme:error:malformed", "detail": "Invalid underscore in DNS name \"_example.org\"", "identifier": { "type": "dns", "value": "_example.org" } }, { "type": "urn:ietf:params:acme:error:rejectedIdentifier", "detail": "This CA will not issue for \"example.net\"", "identifier": { "type": "dns", "value": "example.net" } } ] } Barnes, et al. Standards Track [Page 19]
RFC 8555 ACME March 2019 7. Certificate Management In this section, we describe the certificate management functions that ACME enables: o Account Creation o Ordering a Certificate o Identifier Authorization o Certificate Issuance o Certificate Revocation 7.1. Resources ACME is structured as an HTTP-based application with the following types of resources: o Account resources, representing information about an account (Section 7.1.2, Section 7.3) o Order resources, representing an account's requests to issue certificates (Section 7.1.3) o Authorization resources, representing an account's authorization to act for an identifier (Section 7.1.4) o Challenge resources, representing a challenge to prove control of an identifier (Section 7.5, Section 8) o Certificate resources, representing issued certificates (Section 7.4.2) o A "directory" resource (Section 7.1.1) o A "newNonce" resource (Section 7.2) o A "newAccount" resource (Section 7.3) o A "newOrder" resource (Section 7.4) o A "revokeCert" resource (Section 7.6) o A "keyChange" resource (Section 7.3.5) The server MUST provide "directory" and "newNonce" resources. Barnes, et al. Standards Track [Page 20]
RFC 8555 ACME March 2019 ACME uses different URLs for different management functions. Each function is listed in a directory along with its corresponding URL, so clients only need to be configured with the directory URL. These URLs are connected by a few different link relations [RFC8288]. The "up" link relation is used with challenge resources to indicate the authorization resource to which a challenge belongs. It is also used, with some media types, from certificate resources to indicate a resource from which the client may fetch a chain of CA certificates that could be used to validate the certificate in the original resource. The "index" link relation is present on all resources other than the directory and indicates the URL of the directory. The following diagram illustrates the relations between resources on an ACME server. For the most part, these relations are expressed by URLs provided as strings in the resources' JSON representations. Lines with labels in quotes indicate HTTP link relations. directory | +--> newNonce | +----------+----------+-----+-----+------------+ | | | | | | | | | | V V V V V newAccount newAuthz newOrder revokeCert keyChange | | | | | | V | V account | order --+--> finalize | | | | | +--> cert | V +---> authorization | ^ | | "up" V | challenge ACME Resources and Relationships Barnes, et al. Standards Track [Page 21]
RFC 8555 ACME March 2019 The following table illustrates a typical sequence of requests required to establish a new account with the server, prove control of an identifier, issue a certificate, and fetch an updated certificate some time after issuance. The "->" is a mnemonic for a Location header field pointing to a created resource. +-------------------+--------------------------------+--------------+ | Action | Request | Response | +-------------------+--------------------------------+--------------+ | Get directory | GET directory | 200 | | | | | | Get nonce | HEAD newNonce | 200 | | | | | | Create account | POST newAccount | 201 -> | | | | account | | | | | | Submit order | POST newOrder | 201 -> order | | | | | | Fetch challenges | POST-as-GET order's | 200 | | | authorization urls | | | | | | | Respond to | POST authorization challenge | 200 | | challenges | urls | | | | | | | Poll for status | POST-as-GET order | 200 | | | | | | Finalize order | POST order's finalize url | 200 | | | | | | Poll for status | POST-as-GET order | 200 | | | | | | Download | POST-as-GET order's | 200 | | certificate | certificate url | | +-------------------+--------------------------------+--------------+ The remainder of this section provides the details of how these resources are structured and how the ACME protocol makes use of them. Barnes, et al. Standards Track [Page 22]
RFC 8555 ACME March 2019 7.1.1. Directory In order to help clients configure themselves with the right URLs for each ACME operation, ACME servers provide a directory object. This should be the only URL needed to configure clients. It is a JSON object, whose field names are drawn from the resource registry (Section 9.7.5) and whose values are the corresponding URLs. +------------+--------------------+ | Field | URL in Value | +------------+--------------------+ | newNonce | New nonce | | | | | newAccount | New account | | | | | newOrder | New order | | | | | newAuthz | New authorization | | | | | revokeCert | Revoke certificate | | | | | keyChange | Key change | +------------+--------------------+ There is no constraint on the URL of the directory except that it should be different from the other ACME server resources' URLs, and that it should not clash with other services. For instance: o a host that functions as both an ACME and a Web server may want to keep the root path "/" for an HTML "front page" and place the ACME directory under the path "/acme". o a host that only functions as an ACME server could place the directory under the path "/". If the ACME server does not implement pre-authorization (Section 7.4.1), it MUST omit the "newAuthz" field of the directory. The object MAY additionally contain a "meta" field. If present, it MUST be a JSON object; each field in the object is an item of metadata relating to the service provided by the ACME server. The following metadata items are defined (Section 9.7.6), all of which are OPTIONAL: termsOfService (optional, string): A URL identifying the current terms of service. Barnes, et al. Standards Track [Page 23]
RFC 8555 ACME March 2019 website (optional, string): An HTTP or HTTPS URL locating a website providing more information about the ACME server. caaIdentities (optional, array of string): The hostnames that the ACME server recognizes as referring to itself for the purposes of CAA record validation as defined in [RFC6844]. Each string MUST represent the same sequence of ASCII code points that the server will expect to see as the "Issuer Domain Name" in a CAA issue or issuewild property tag. This allows clients to determine the correct issuer domain name to use when configuring CAA records. externalAccountRequired (optional, boolean): If this field is present and set to "true", then the CA requires that all newAccount requests include an "externalAccountBinding" field associating the new account with an external account. Clients access the directory by sending a GET request to the directory URL. HTTP/1.1 200 OK Content-Type: application/json { "newNonce": "https://example.com/acme/new-nonce", "newAccount": "https://example.com/acme/new-account", "newOrder": "https://example.com/acme/new-order", "newAuthz": "https://example.com/acme/new-authz", "revokeCert": "https://example.com/acme/revoke-cert", "keyChange": "https://example.com/acme/key-change", "meta": { "termsOfService": "https://example.com/acme/terms/2017-5-30", "website": "https://www.example.com/", "caaIdentities": ["example.com"], "externalAccountRequired": false } } 7.1.2. Account Objects An ACME account resource represents a set of metadata associated with an account. Account resources have the following structure: status (required, string): The status of this account. Possible values are "valid", "deactivated", and "revoked". The value "deactivated" should be used to indicate client-initiated deactivation whereas "revoked" should be used to indicate server- initiated deactivation. See Section 7.1.6. Barnes, et al. Standards Track [Page 24]
RFC 8555 ACME March 2019 contact (optional, array of string): An array of URLs that the server can use to contact the client for issues related to this account. For example, the server may wish to notify the client about server-initiated revocation or certificate expiration. For information on supported URL schemes, see Section 7.3. termsOfServiceAgreed (optional, boolean): Including this field in a newAccount request, with a value of true, indicates the client's agreement with the terms of service. This field cannot be updated by the client. externalAccountBinding (optional, object): Including this field in a newAccount request indicates approval by the holder of an existing non-ACME account to bind that account to this ACME account. This field is not updateable by the client (see Section 7.3.4). orders (required, string): A URL from which a list of orders submitted by this account can be fetched via a POST-as-GET request, as described in Section 7.1.2.1. { "status": "valid", "contact": [ "mailto:cert-admin@example.org", "mailto:admin@example.org" ], "termsOfServiceAgreed": true, "orders": "https://example.com/acme/orders/rzGoeA" } 7.1.2.1. Orders List Each account object includes an "orders" URL from which a list of orders created by the account can be fetched via POST-as-GET request. The result of the request MUST be a JSON object whose "orders" field is an array of URLs, each identifying an order belonging to the account. The server SHOULD include pending orders and SHOULD NOT include orders that are invalid in the array of URLs. The server MAY return an incomplete list, along with a Link header field with a "next" link relation indicating where further entries can be acquired. Barnes, et al. Standards Track [Page 25]
RFC 8555 ACME March 2019 HTTP/1.1 200 OK Content-Type: application/json Link: <https://example.com/acme/directory>;rel="index" Link: <https://example.com/acme/orders/rzGoeA?cursor=2>;rel="next" { "orders": [ "https://example.com/acme/order/TOlocE8rfgo", "https://example.com/acme/order/4E16bbL5iSw", /* more URLs not shown for example brevity */ "https://example.com/acme/order/neBHYLfw0mg" ] } 7.1.3. Order Objects An ACME order object represents a client's request for a certificate and is used to track the progress of that order through to issuance. Thus, the object contains information about the requested certificate, the authorizations that the server requires the client to complete, and any certificates that have resulted from this order. status (required, string): The status of this order. Possible values are "pending", "ready", "processing", "valid", and "invalid". See Section 7.1.6. expires (optional, string): The timestamp after which the server will consider this order invalid, encoded in the format specified in [RFC3339]. This field is REQUIRED for objects with "pending" or "valid" in the status field. identifiers (required, array of object): An array of identifier objects that the order pertains to. type (required, string): The type of identifier. This document defines the "dns" identifier type. See the registry defined in Section 9.7.7 for any others. value (required, string): The identifier itself. notBefore (optional, string): The requested value of the notBefore field in the certificate, in the date format defined in [RFC3339]. notAfter (optional, string): The requested value of the notAfter field in the certificate, in the date format defined in [RFC3339]. Barnes, et al. Standards Track [Page 26]
RFC 8555 ACME March 2019 error (optional, object): The error that occurred while processing the order, if any. This field is structured as a problem document [RFC7807]. authorizations (required, array of string): For pending orders, the authorizations that the client needs to complete before the requested certificate can be issued (see Section 7.5), including unexpired authorizations that the client has completed in the past for identifiers specified in the order. The authorizations required are dictated by server policy; there may not be a 1:1 relationship between the order identifiers and the authorizations required. For final orders (in the "valid" or "invalid" state), the authorizations that were completed. Each entry is a URL from which an authorization can be fetched with a POST-as-GET request. finalize (required, string): A URL that a CSR must be POSTed to once all of the order's authorizations are satisfied to finalize the order. The result of a successful finalization will be the population of the certificate URL for the order. certificate (optional, string): A URL for the certificate that has been issued in response to this order. { "status": "valid", "expires": "2016-01-20T14:09:07.99Z", "identifiers": [ { "type": "dns", "value": "www.example.org" }, { "type": "dns", "value": "example.org" } ], "notBefore": "2016-01-01T00:00:00Z", "notAfter": "2016-01-08T00:00:00Z", "authorizations": [ "https://example.com/acme/authz/PAniVnsZcis", "https://example.com/acme/authz/r4HqLzrSrpI" ], "finalize": "https://example.com/acme/order/TOlocE8rfgo/finalize", "certificate": "https://example.com/acme/cert/mAt3xBGaobw" } Any identifier of type "dns" in a newOrder request MAY have a wildcard domain name as its value. A wildcard domain name consists of a single asterisk character followed by a single full stop Barnes, et al. Standards Track [Page 27]
RFC 8555 ACME March 2019 character ("*.") followed by a domain name as defined for use in the Subject Alternate Name Extension by [RFC5280]. An authorization returned by the server for a wildcard domain name identifier MUST NOT include the asterisk and full stop ("*.") prefix in the authorization identifier value. The returned authorization MUST include the optional "wildcard" field, with a value of true. The elements of the "authorizations" and "identifiers" arrays are immutable once set. The server MUST NOT change the contents of either array after they are created. If a client observes a change in the contents of either array, then it SHOULD consider the order invalid. The "authorizations" array of the order SHOULD reflect all authorizations that the CA takes into account in deciding to issue, even if some authorizations were fulfilled in earlier orders or in pre-authorization transactions. For example, if a CA allows multiple orders to be fulfilled based on a single authorization transaction, then it SHOULD reflect that authorization in all of the orders. Note that just because an authorization URL is listed in the "authorizations" array of an order object doesn't mean that the client is required to take action. There are several reasons that the referenced authorizations may already be valid: o The client completed the authorization as part of a previous order o The client previously pre-authorized the identifier (see Section 7.4.1) o The server granted the client authorization based on an external account Clients SHOULD check the "status" field of an order to determine whether they need to take any action. 7.1.4. Authorization Objects An ACME authorization object represents a server's authorization for an account to represent an identifier. In addition to the identifier, an authorization includes several metadata fields, such as the status of the authorization (e.g., "pending", "valid", or "revoked") and which challenges were used to validate possession of the identifier. Barnes, et al. Standards Track [Page 28]
RFC 8555 ACME March 2019 The structure of an ACME authorization resource is as follows: identifier (required, object): The identifier that the account is authorized to represent. type (required, string): The type of identifier (see below and Section 9.7.7). value (required, string): The identifier itself. status (required, string): The status of this authorization. Possible values are "pending", "valid", "invalid", "deactivated", "expired", and "revoked". See Section 7.1.6. expires (optional, string): The timestamp after which the server will consider this authorization invalid, encoded in the format specified in [RFC3339]. This field is REQUIRED for objects with "valid" in the "status" field. challenges (required, array of objects): For pending authorizations, the challenges that the client can fulfill in order to prove possession of the identifier. For valid authorizations, the challenge that was validated. For invalid authorizations, the challenge that was attempted and failed. Each array entry is an object with parameters required to validate the challenge. A client should attempt to fulfill one of these challenges, and a server should consider any one of the challenges sufficient to make the authorization valid. wildcard (optional, boolean): This field MUST be present and true for authorizations created as a result of a newOrder request containing a DNS identifier with a value that was a wildcard domain name. For other authorizations, it MUST be absent. Wildcard domain names are described in Section 7.1.3. The only type of identifier defined by this specification is a fully qualified domain name (type: "dns"). The domain name MUST be encoded in the form in which it would appear in a certificate. That is, it MUST be encoded according to the rules in Section 7 of [RFC5280]. Servers MUST verify any identifier values that begin with the ASCII- Compatible Encoding prefix "xn--" as defined in [RFC5890] are properly encoded. Wildcard domain names (with "*" as the first label) MUST NOT be included in authorization objects. If an authorization object conveys authorization for the base domain of a newOrder DNS identifier containing a wildcard domain name, then the optional authorizations "wildcard" field MUST be present with a value of true. Barnes, et al. Standards Track [Page 29]
RFC 8555 ACME March 2019 Section 8 describes a set of challenges for domain name validation. { "status": "valid", "expires": "2015-03-01T14:09:07.99Z", "identifier": { "type": "dns", "value": "www.example.org" }, "challenges": [ { "url": "https://example.com/acme/chall/prV_B7yEyA4", "type": "http-01", "status": "valid", "token": "DGyRejmCefe7v4NfDGDKfA", "validated": "2014-12-01T12:05:58.16Z" } ], "wildcard": false } 7.1.5. Challenge Objects An ACME challenge object represents a server's offer to validate a client's possession of an identifier in a specific way. Unlike the other objects listed above, there is not a single standard structure for a challenge object. The contents of a challenge object depend on the validation method being used. The general structure of challenge objects and an initial set of validation methods are described in Section 8. 7.1.6. Status Changes Each ACME object type goes through a simple state machine over its lifetime. The "status" field of the object indicates which state the object is currently in. Challenge objects are created in the "pending" state. They transition to the "processing" state when the client responds to the challenge (see Section 7.5.1) and the server begins attempting to validate that the client has completed the challenge. Note that within the "processing" state, the server may attempt to validate the challenge multiple times (see Section 8.2). Likewise, client Barnes, et al. Standards Track [Page 30]
RFC 8555 ACME March 2019 requests for retries do not cause a state change. If validation is successful, the challenge moves to the "valid" state; if there is an error, the challenge moves to the "invalid" state. pending | | Receive | response V processing <-+ | | | Server retry or | | | client retry request | +----+ | | Successful | Failed validation | validation +---------+---------+ | | V V valid invalid State Transitions for Challenge Objects Authorization objects are created in the "pending" state. If one of the challenges listed in the authorization transitions to the "valid" state, then the authorization also changes to the "valid" state. If the client attempts to fulfill a challenge and fails, or if there is an error while the authorization is still pending, then the authorization transitions to the "invalid" state. Once the authorization is in the "valid" state, it can expire ("expired"), be deactivated by the client ("deactivated", see Section 7.5.2), or revoked by the server ("revoked"). Barnes, et al. Standards Track [Page 31]
RFC 8555 ACME March 2019 pending --------------------+ | | Challenge failure | | or | | Error | Challenge valid | +---------+---------+ | | | | V V | invalid valid | | | | | | | +--------------+--------------+ | | | | | | Server | Client | Time after | revoke | deactivate | "expires" | V V V revoked deactivated expired State Transitions for Authorization Objects Order objects are created in the "pending" state. Once all of the authorizations listed in the order object are in the "valid" state, the order transitions to the "ready" state. The order moves to the "processing" state after the client submits a request to the order's "finalize" URL and the CA begins the issuance process for the certificate. Once the certificate is issued, the order enters the "valid" state. If an error occurs at any of these stages, the order moves to the "invalid" state. The order also moves to the "invalid" state if it expires or one of its authorizations enters a final state other than "valid" ("expired", "revoked", or "deactivated"). Barnes, et al. Standards Track [Page 32]
RFC 8555 ACME March 2019 pending --------------+ | | | All authz | | "valid" | V | ready ---------------+ | | | Receive | | finalize | | request | V | processing ------------+ | | | Certificate | Error or | issued | Authorization failure V V valid invalid State Transitions for Order Objects Account objects are created in the "valid" state, since no further action is required to create an account after a successful newAccount request. If the account is deactivated by the client or revoked by the server, it moves to the corresponding state. valid | | +-----------+-----------+ Client | Server | deactiv.| revoke | V V deactivated revoked State Transitions for Account Objects Note that some of these states may not ever appear in a "status" field, depending on server behavior. For example, a server that issues synchronously will never show an order in the "processing" state. A server that deletes expired authorizations immediately will never show an authorization in the "expired" state. Barnes, et al. Standards Track [Page 33]
RFC 8555 ACME March 2019 7.2. Getting a Nonce Before sending a POST request to the server, an ACME client needs to have a fresh anti-replay nonce to put in the "nonce" header of the JWS. In most cases, the client will have gotten a nonce from a previous request. However, the client might sometimes need to get a new nonce, e.g., on its first request to the server or if an existing nonce is no longer valid. To get a fresh nonce, the client sends a HEAD request to the newNonce resource on the server. The server's response MUST include a Replay- Nonce header field containing a fresh nonce and SHOULD have status code 200 (OK). The server MUST also respond to GET requests for this resource, returning an empty body (while still providing a Replay- Nonce header) with a status code of 204 (No Content). HEAD /acme/new-nonce HTTP/1.1 Host: example.com HTTP/1.1 200 OK Replay-Nonce: oFvnlFP1wIhRlYS2jTaXbA Cache-Control: no-store Link: <https://example.com/acme/directory>;rel="index" Proxy caching of responses from the newNonce resource can cause clients to receive the same nonce repeatedly, leading to "badNonce" errors. The server MUST include a Cache-Control header field with the "no-store" directive in responses for the newNonce resource, in order to prevent caching of this resource. 7.3. Account Management In this section, we describe how an ACME client can create an account on an ACME server and perform some modifications to the account after it has been created. A client creates a new account with the server by sending a POST request to the server's newAccount URL. The body of the request is a stub account object containing some subset of the following fields: contact (optional, array of string): Same meaning as the corresponding server field defined in Section 7.1.2. termsOfServiceAgreed (optional, boolean): Same meaning as the corresponding server field defined in Section 7.1.2. Barnes, et al. Standards Track [Page 34]
RFC 8555 ACME March 2019 onlyReturnExisting (optional, boolean): If this field is present with the value "true", then the server MUST NOT create a new account if one does not already exist. This allows a client to look up an account URL based on an account key (see Section 7.3.1). externalAccountBinding (optional, object): Same meaning as the corresponding server field defined in Section 7.1.2 POST /acme/new-account HTTP/1.1 Host: example.com Content-Type: application/jose+json { "protected": base64url({ "alg": "ES256", "jwk": {...}, "nonce": "6S8IqOGY7eL2lsGoTZYifg", "url": "https://example.com/acme/new-account" }), "payload": base64url({ "termsOfServiceAgreed": true, "contact": [ "mailto:cert-admin@example.org", "mailto:admin@example.org" ] }), "signature": "RZPOnYoPs1PhjszF...-nh6X1qtOFPB519I" } The server MUST ignore any values provided in the "orders" fields in account objects sent by the client, as well as any other fields that it does not recognize. If new fields are specified in the future, the specification of those fields MUST describe whether they can be provided by the client. The server MUST NOT reflect the "onlyReturnExisting" field or any unrecognized fields in the resulting account object. This allows clients to detect when servers do not support an extension field. The server SHOULD validate that the contact URLs in the "contact" field are valid and supported by the server. If the server validates contact URLs, it MUST support the "mailto" scheme. Clients MUST NOT provide a "mailto" URL in the "contact" field that contains "hfields" [RFC6068] or more than one "addr-spec" in the "to" component. If a server encounters a "mailto" contact URL that does not meet these criteria, then it SHOULD reject it as invalid. Barnes, et al. Standards Track [Page 35]
RFC 8555 ACME March 2019 If the server rejects a contact URL for using an unsupported scheme, it MUST return an error of type "unsupportedContact", with a description of the error and what types of contact URLs the server considers acceptable. If the server rejects a contact URL for using a supported scheme but an invalid value, then the server MUST return an error of type "invalidContact". If the server wishes to require the client to agree to terms under which the ACME service is to be used, it MUST indicate the URL where such terms can be accessed in the "termsOfService" subfield of the "meta" field in the directory object, and the server MUST reject newAccount requests that do not have the "termsOfServiceAgreed" field set to "true". Clients SHOULD NOT automatically agree to terms by default. Rather, they SHOULD require some user interaction for agreement to terms. The server creates an account and stores the public key used to verify the JWS (i.e., the "jwk" element of the JWS header) to authenticate future requests from the account. The server returns this account object in a 201 (Created) response, with the account URL in a Location header field. The account URL is used as the "kid" value in the JWS authenticating subsequent requests by this account (see Section 6.2). The account URL is also used for requests for management actions on this account, as described below. HTTP/1.1 201 Created Content-Type: application/json Replay-Nonce: D8s4D2mLs8Vn-goWuPQeKA Link: <https://example.com/acme/directory>;rel="index" Location: https://example.com/acme/acct/evOfKhNU60wg { "status": "valid", "contact": [ "mailto:cert-admin@example.org", "mailto:admin@example.org" ], "orders": "https://example.com/acme/acct/evOfKhNU60wg/orders" } 7.3.1. Finding an Account URL Given a Key If the server receives a newAccount request signed with a key for which it already has an account registered with the provided account key, then it MUST return a response with status code 200 (OK) and provide the URL of that account in the Location header field. The Barnes, et al. Standards Track [Page 36]
RFC 8555 ACME March 2019 body of this response represents the account object as it existed on the server before this request; any fields in the request object MUST be ignored. This allows a client that has an account key but not the corresponding account URL to recover the account URL. If a client wishes to find the URL for an existing account and does not want an account to be created if one does not already exist, then it SHOULD do so by sending a POST request to the newAccount URL with a JWS whose payload has an "onlyReturnExisting" field set to "true" ({"onlyReturnExisting": true}). If a client sends such a request and an account does not exist, then the server MUST return an error response with status code 400 (Bad Request) and type "urn:ietf:params:acme:error:accountDoesNotExist". 7.3.2. Account Update If the client wishes to update this information in the future, it sends a POST request with updated information to the account URL. The server MUST ignore any updates to the "orders" field, "termsOfServiceAgreed" field (see Section 7.3.3), the "status" field (except as allowed by Section 7.3.6), or any other fields it does not recognize. If the server accepts the update, it MUST return a response with a 200 (OK) status code and the resulting account object. For example, to update the contact information in the above account, the client could send the following request: POST /acme/acct/evOfKhNU60wg HTTP/1.1 Host: example.com Content-Type: application/jose+json { "protected": base64url({ "alg": "ES256", "kid": "https://example.com/acme/acct/evOfKhNU60wg", "nonce": "ax5RnthDqp_Yf4_HZnFLmA", "url": "https://example.com/acme/acct/evOfKhNU60wg" }), "payload": base64url({ "contact": [ "mailto:certificates@example.org", "mailto:admin@example.org" ] }), "signature": "hDXzvcj8T6fbFbmn...rDzXzzvzpRy64N0o" } Barnes, et al. Standards Track [Page 37]
RFC 8555 ACME March 2019 7.3.3. Changes of Terms of Service As described above, a client can indicate its agreement with the CA's terms of service by setting the "termsOfServiceAgreed" field in its account object to "true". If the server has changed its terms of service since a client initially agreed, and the server is unwilling to process a request without explicit agreement to the new terms, then it MUST return an error response with status code 403 (Forbidden) and type "urn:ietf:params:acme:error:userActionRequired". This response MUST include a Link header field with link relation "terms-of-service" and the latest terms-of-service URL. The problem document returned with the error MUST also include an "instance" field, indicating a URL that the client should direct a human user to visit in order for instructions on how to agree to the terms. HTTP/1.1 403 Forbidden Replay-Nonce: T81bdZroZ2ITWSondpTmAw Link: <https://example.com/acme/directory>;rel="index" Link: <https://example.com/acme/terms/2017-6-02>;rel="terms-of-service" Content-Type: application/problem+json Content-Language: en { "type": "urn:ietf:params:acme:error:userActionRequired", "detail": "Terms of service have changed", "instance": "https://example.com/acme/agreement/?token=W8Ih3PswD-8" } 7.3.4. External Account Binding The server MAY require a value for the "externalAccountBinding" field to be present in "newAccount" requests. This can be used to associate an ACME account with an existing account in a non-ACME system, such as a CA customer database. To enable ACME account binding, the CA operating the ACME server needs to provide the ACME client with a MAC key and a key identifier, using some mechanism outside of ACME. The key identifier MUST be an ASCII string. The MAC key SHOULD be provided in base64url-encoded form, to maximize compatibility between non-ACME provisioning systems and ACME clients. Barnes, et al. Standards Track [Page 38]
RFC 8555 ACME March 2019 The ACME client then computes a binding JWS to indicate the external account holder's approval of the ACME account key. The payload of this JWS is the ACME account key being registered, in JWK form. The protected header of the JWS MUST meet the following criteria: o The "alg" field MUST indicate a MAC-based algorithm o The "kid" field MUST contain the key identifier provided by the CA o The "nonce" field MUST NOT be present o The "url" field MUST be set to the same value as the outer JWS The "signature" field of the JWS will contain the MAC value computed with the MAC key provided by the CA. POST /acme/new-account HTTP/1.1 Host: example.com Content-Type: application/jose+json { "protected": base64url({ "alg": "ES256", "jwk": /* account key */, "nonce": "K60BWPrMQG9SDxBDS_xtSw", "url": "https://example.com/acme/new-account" }), "payload": base64url({ "contact": [ "mailto:cert-admin@example.org", "mailto:admin@example.org" ], "termsOfServiceAgreed": true, "externalAccountBinding": { "protected": base64url({ "alg": "HS256", "kid": /* key identifier from CA */, "url": "https://example.com/acme/new-account" }), "payload": base64url(/* same as in "jwk" above */), "signature": /* MAC using MAC key from CA */ } }), "signature": "5TWiqIYQfIDfALQv...x9C2mg8JGPxl5bI4" } Barnes, et al. Standards Track [Page 39]
RFC 8555 ACME March 2019 If such a CA requires that newAccount requests contain an "externalAccountBinding" field, then it MUST provide the value "true" in the "externalAccountRequired" subfield of the "meta" field in the directory object. If the CA receives a newAccount request without an "externalAccountBinding" field, then it SHOULD reply with an error of type "externalAccountRequired". When a CA receives a newAccount request containing an "externalAccountBinding" field, it decides whether or not to verify the binding. If the CA does not verify the binding, then it MUST NOT reflect the "externalAccountBinding" field in the resulting account object (if any). To verify the account binding, the CA MUST take the following steps: 1. Verify that the value of the field is a well-formed JWS 2. Verify that the JWS protected field meets the above criteria 3. Retrieve the MAC key corresponding to the key identifier in the "kid" field 4. Verify that the MAC on the JWS verifies using that MAC key 5. Verify that the payload of the JWS represents the same key as was used to verify the outer JWS (i.e., the "jwk" field of the outer JWS) If all of these checks pass and the CA creates a new account, then the CA may consider the new account associated with the external account corresponding to the MAC key. The account object the CA returns MUST include an "externalAccountBinding" field with the same value as the field in the request. If any of these checks fail, then the CA MUST reject the newAccount request. 7.3.5. Account Key Rollover A client may wish to change the public key that is associated with an account in order to recover from a key compromise or proactively mitigate the impact of an unnoticed key compromise. To change the key associated with an account, the client sends a request to the server containing signatures by both the old and new keys. The signature by the new key covers the account URL and the old key, signifying a request by the new key holder to take over the account from the old key holder. The signature by the old key covers this request and its signature, and indicates the old key holder's assent to the rollover request. Barnes, et al. Standards Track [Page 40]
RFC 8555 ACME March 2019 To create this request object, the client first constructs a keyChange object describing the account to be updated and its account key: account (required, string): The URL for the account being modified. The content of this field MUST be the exact string provided in the Location header field in response to the newAccount request that created the account. oldKey (required, JWK): The JWK representation of the old key. The client then encapsulates the keyChange object in an "inner" JWS, signed with the requested new account key. This "inner" JWS becomes the payload for the "outer" JWS that is the body of the ACME request. The outer JWS MUST meet the normal requirements for an ACME JWS request body (see Section 6.2). The inner JWS MUST meet the normal requirements, with the following differences: o The inner JWS MUST have a "jwk" header parameter, containing the public key of the new key pair. o The inner JWS MUST have the same "url" header parameter as the outer JWS. o The inner JWS MUST omit the "nonce" header parameter. This transaction has signatures from both the old and new keys so that the server can verify that the holders of the two keys both agree to the change. The signatures are nested to preserve the property that all signatures on POST messages are signed by exactly one key. The "inner" JWS effectively represents a request by the holder of the new key to take over the account form the holder of the old key. The "outer" JWS represents the current account holder's assent to this request. Barnes, et al. Standards Track [Page 41]
RFC 8555 ACME March 2019 POST /acme/key-change HTTP/1.1 Host: example.com Content-Type: application/jose+json { "protected": base64url({ "alg": "ES256", "kid": "https://example.com/acme/acct/evOfKhNU60wg", "nonce": "S9XaOcxP5McpnTcWPIhYuB", "url": "https://example.com/acme/key-change" }), "payload": base64url({ "protected": base64url({ "alg": "ES256", "jwk": /* new key */, "url": "https://example.com/acme/key-change" }), "payload": base64url({ "account": "https://example.com/acme/acct/evOfKhNU60wg", "oldKey": /* old key */ }), "signature": "Xe8B94RD30Azj2ea...8BmZIRtcSKPSd8gU" }), "signature": "5TWiqIYQfIDfALQv...x9C2mg8JGPxl5bI4" } On receiving a keyChange request, the server MUST perform the following steps in addition to the typical JWS validation: 1. Validate the POST request belongs to a currently active account, as described in Section 6. 2. Check that the payload of the JWS is a well-formed JWS object (the "inner JWS"). 3. Check that the JWS protected header of the inner JWS has a "jwk" field. 4. Check that the inner JWS verifies using the key in its "jwk" field. 5. Check that the payload of the inner JWS is a well-formed keyChange object (as described above). 6. Check that the "url" parameters of the inner and outer JWSs are the same. Barnes, et al. Standards Track [Page 42]
RFC 8555 ACME March 2019 7. Check that the "account" field of the keyChange object contains the URL for the account matching the old key (i.e., the "kid" field in the outer JWS). 8. Check that the "oldKey" field of the keyChange object is the same as the account key for the account in question. 9. Check that no account exists whose account key is the same as the key in the "jwk" header parameter of the inner JWS. If all of these checks pass, then the server updates the corresponding account by replacing the old account key with the new public key and returns status code 200 (OK). Otherwise, the server responds with an error status code and a problem document describing the error. If there is an existing account with the new key provided, then the server SHOULD use status code 409 (Conflict) and provide the URL of that account in the Location header field. Note that changing the account key for an account SHOULD NOT have any other impact on the account. For example, the server MUST NOT invalidate pending orders or authorization transactions based on a change of account key. 7.3.6. Account Deactivation A client can deactivate an account by posting a signed update to the account URL with a status field of "deactivated". Clients may wish to do this when the account key is compromised or decommissioned. A deactivated account can no longer request certificate issuance or access resources related to the account, such as orders or authorizations. If a server receives a POST or POST-as-GET from a deactivated account, it MUST return an error response with status code 401 (Unauthorized) and type "urn:ietf:params:acme:error:unauthorized". Barnes, et al. Standards Track [Page 43]
RFC 8555 ACME March 2019 POST /acme/acct/evOfKhNU60wg HTTP/1.1 Host: example.com Content-Type: application/jose+json { "protected": base64url({ "alg": "ES256", "kid": "https://example.com/acme/acct/evOfKhNU60wg", "nonce": "ntuJWWSic4WVNSqeUmshgg", "url": "https://example.com/acme/acct/evOfKhNU60wg" }), "payload": base64url({ "status": "deactivated" }), "signature": "earzVLd3m5M4xJzR...bVTqn7R08AKOVf3Y" } The server MUST verify that the request is signed by the account key. If the server accepts the deactivation request, it replies with a 200 (OK) status code and the current contents of the account object. Once an account is deactivated, the server MUST NOT accept further requests authorized by that account's key. The server SHOULD cancel any pending operations authorized by the account's key, such as certificate orders. A server may take a variety of actions in response to an account deactivation, e.g., deleting data related to that account or sending mail to the account's contacts. Servers SHOULD NOT revoke certificates issued by the deactivated account, since this could cause operational disruption for servers using these certificates. ACME does not provide a way to reactivate a deactivated account. 7.4. Applying for Certificate Issuance The client begins the certificate issuance process by sending a POST request to the server's newOrder resource. The body of the POST is a JWS object whose JSON payload is a subset of the order object defined in Section 7.1.3, containing the fields that describe the certificate to be issued: identifiers (required, array of object): An array of identifier objects that the client wishes to submit an order for. type (required, string): The type of identifier. value (required, string): The identifier itself. Barnes, et al. Standards Track [Page 44]
RFC 8555 ACME March 2019 notBefore (optional, string): The requested value of the notBefore field in the certificate, in the date format defined in [RFC3339]. notAfter (optional, string): The requested value of the notAfter field in the certificate, in the date format defined in [RFC3339]. POST /acme/new-order HTTP/1.1 Host: example.com Content-Type: application/jose+json { "protected": base64url({ "alg": "ES256", "kid": "https://example.com/acme/acct/evOfKhNU60wg", "nonce": "5XJ1L3lEkMG7tR6pA00clA", "url": "https://example.com/acme/new-order" }), "payload": base64url({ "identifiers": [ { "type": "dns", "value": "www.example.org" }, { "type": "dns", "value": "example.org" } ], "notBefore": "2016-01-01T00:04:00+04:00", "notAfter": "2016-01-08T00:04:00+04:00" }), "signature": "H6ZXtGjTZyUnPeKn...wEA4TklBdh3e454g" } The server MUST return an error if it cannot fulfill the request as specified, and it MUST NOT issue a certificate with contents other than those requested. If the server requires the request to be modified in a certain way, it should indicate the required changes using an appropriate error type and description. If the server is willing to issue the requested certificate, it responds with a 201 (Created) response. The body of this response is an order object reflecting the client's request and any authorizations the client must complete before the certificate will be issued. Barnes, et al. Standards Track [Page 45]
RFC 8555 ACME March 2019 HTTP/1.1 201 Created Replay-Nonce: MYAuvOpaoIiywTezizk5vw Link: <https://example.com/acme/directory>;rel="index" Location: https://example.com/acme/order/TOlocE8rfgo { "status": "pending", "expires": "2016-01-05T14:09:07.99Z", "notBefore": "2016-01-01T00:00:00Z", "notAfter": "2016-01-08T00:00:00Z", "identifiers": [ { "type": "dns", "value": "www.example.org" }, { "type": "dns", "value": "example.org" } ], "authorizations": [ "https://example.com/acme/authz/PAniVnsZcis", "https://example.com/acme/authz/r4HqLzrSrpI" ], "finalize": "https://example.com/acme/order/TOlocE8rfgo/finalize" } The order object returned by the server represents a promise that if the client fulfills the server's requirements before the "expires" time, then the server will be willing to finalize the order upon request and issue the requested certificate. In the order object, any authorization referenced in the "authorizations" array whose status is "pending" represents an authorization transaction that the client must complete before the server will issue the certificate (see Section 7.5). If the client fails to complete the required actions before the "expires" time, then the server SHOULD change the status of the order to "invalid" and MAY delete the order resource. Clients MUST NOT make any assumptions about the sort order of "identifiers" or "authorizations" elements in the returned order object. Barnes, et al. Standards Track [Page 46]
RFC 8555 ACME March 2019 Once the client believes it has fulfilled the server's requirements, it should send a POST request to the order resource's finalize URL. The POST body MUST include a CSR: csr (required, string): A CSR encoding the parameters for the certificate being requested [RFC2986]. The CSR is sent in the base64url-encoded version of the DER format. (Note: Because this field uses base64url, and does not include headers, it is different from PEM.) POST /acme/order/TOlocE8rfgo/finalize HTTP/1.1 Host: example.com Content-Type: application/jose+json { "protected": base64url({ "alg": "ES256", "kid": "https://example.com/acme/acct/evOfKhNU60wg", "nonce": "MSF2j2nawWHPxxkE3ZJtKQ", "url": "https://example.com/acme/order/TOlocE8rfgo/finalize" }), "payload": base64url({ "csr": "MIIBPTCBxAIBADBFMQ...FS6aKdZeGsysoCo4H9P", }), "signature": "uOrUfIIk5RyQ...nw62Ay1cl6AB" } The CSR encodes the client's requests with regard to the content of the certificate to be issued. The CSR MUST indicate the exact same set of requested identifiers as the initial newOrder request. Identifiers of type "dns" MUST appear either in the commonName portion of the requested subject name or in an extensionRequest attribute [RFC2985] requesting a subjectAltName extension, or both. (These identifiers may appear in any sort order.) Specifications that define new identifier types must specify where in the certificate signing request these identifiers can appear. A request to finalize an order will result in an error if the CA is unwilling to issue a certificate corresponding to the submitted CSR. For example: o If the CSR and order identifiers differ o If the account is not authorized for the identifiers indicated in the CSR o If the CSR requests extensions that the CA is not willing to include Barnes, et al. Standards Track [Page 47]
RFC 8555 ACME March 2019 In such cases, the problem document returned by the server SHOULD use error code "badCSR" and describe specific reasons the CSR was rejected in its "detail" field. After returning such an error, the server SHOULD leave the order in the "ready" state, to allow the client to submit a new finalize request with an amended CSR. A request to finalize an order will result in error if the order is not in the "ready" state. In such cases, the server MUST return a 403 (Forbidden) error with a problem document of type "orderNotReady". The client should then send a POST-as-GET request to the order resource to obtain its current state. The status of the order will indicate what action the client should take (see below). If a request to finalize an order is successful, the server will return a 200 (OK) with an updated order object. The status of the order will indicate what action the client should take: o "invalid": The certificate will not be issued. Consider this order process abandoned. o "pending": The server does not believe that the client has fulfilled the requirements. Check the "authorizations" array for entries that are still pending. o "ready": The server agrees that the requirements have been fulfilled, and is awaiting finalization. Submit a finalization request. o "processing": The certificate is being issued. Send a POST-as-GET request after the time given in the Retry-After header field of the response, if any. o "valid": The server has issued the certificate and provisioned its URL to the "certificate" field of the order. Download the certificate. Barnes, et al. Standards Track [Page 48]
RFC 8555 ACME March 2019 HTTP/1.1 200 OK Replay-Nonce: CGf81JWBsq8QyIgPCi9Q9X Link: <https://example.com/acme/directory>;rel="index" Location: https://example.com/acme/order/TOlocE8rfgo { "status": "valid", "expires": "2016-01-20T14:09:07.99Z", "notBefore": "2016-01-01T00:00:00Z", "notAfter": "2016-01-08T00:00:00Z", "identifiers": [ { "type": "dns", "value": "www.example.org" }, { "type": "dns", "value": "example.org" } ], "authorizations": [ "https://example.com/acme/authz/PAniVnsZcis", "https://example.com/acme/authz/r4HqLzrSrpI" ], "finalize": "https://example.com/acme/order/TOlocE8rfgo/finalize", "certificate": "https://example.com/acme/cert/mAt3xBGaobw" } 7.4.1. Pre-authorization The order process described above presumes that authorization objects are created reactively, in response to a certificate order. Some servers may also wish to enable clients to obtain authorization for an identifier proactively, outside of the context of a specific issuance. For example, a client hosting virtual servers for a collection of names might wish to obtain authorization before any virtual servers are created and only create a certificate when a virtual server starts up. In some cases, a CA running an ACME server might have a completely external, non-ACME process for authorizing a client to issue certificates for an identifier. In these cases, the CA should provision its ACME server with authorization objects corresponding to these authorizations and reflect them as already valid in any orders submitted by the client. If a CA wishes to allow pre-authorization within ACME, it can offer a "new authorization" resource in its directory by adding the field "newAuthz" with a URL for the newAuthz resource. Barnes, et al. Standards Track [Page 49]