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RFC 8555

Automatic Certificate Management Environment (ACME)

Pages: 95
Proposed STD
Part 1 of 6 – Pages 1 to 19
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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.
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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
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           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
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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
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   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.
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   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.
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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.
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         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.
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         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.
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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.
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   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)
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      *  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
Top   ToC   Page 13
   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.
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   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.
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   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.
Top   ToC   Page 16
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:"):
Top   ToC   Page 17
   +-------------------------+-----------------------------------------+
   | 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       |
Top   ToC   Page 18
   |                         |                                         |
   | 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).
Top   ToC   Page 19
   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"
            }
        }
    ]
}