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Network Working Group                                          R. Barnes
Internet-Draft                                                   Mozilla
Intended status: Standards Track                      J. Hoffman-Andrews
Expires: August 7, 2017                                              EFF
                                                               J. Kasten
                                                  University of Michigan
                                                       February 03, 2017


          Automatic Certificate Management Environment (ACME)
                        draft-ietf-acme-acme-05

Abstract

   Certificates in the Web's X.509 PKI (PKIX) are used for a number of
   purposes, the most significant of which is the authentication of
   domain names.  Thus, certificate authorities in the Web PKI are
   trusted to verify that an applicant for a certificate legitimately
   represents the domain name(s) in the certificate.  Today, this
   verification is done through a collection of ad hoc mechanisms.  This
   document describes a protocol that a certificate authority (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.

   DISCLAIMER: This is a work in progress draft of ACME and has not yet
   had a thorough security analysis.

   RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH: The source for
   this draft is maintained in GitHub.  Suggested changes should be
   submitted as pull requests at https://github.com/ietf-wg-acme/acme .
   Instructions are on that page as well.  Editorial changes can be
   managed in GitHub, but any substantive change should be discussed on
   the ACME mailing list (acme@ietf.org).

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any



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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 7, 2017.

Copyright Notice

   Copyright (c) 2017 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
   (http://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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Deployment Model and Operator Experience  . . . . . . . . . .   5
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Message Transport . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  HTTPS Requests  . . . . . . . . . . . . . . . . . . . . .   8
     5.2.  Request Authentication  . . . . . . . . . . . . . . . . .   9
     5.3.  Equivalence of JWKs . . . . . . . . . . . . . . . . . . .  11
     5.4.  Request URI Integrity . . . . . . . . . . . . . . . . . .  11
       5.4.1.  "url" (URL) JWS header parameter  . . . . . . . . . .  12
     5.5.  Replay protection . . . . . . . . . . . . . . . . . . . .  12
       5.5.1.  Replay-Nonce  . . . . . . . . . . . . . . . . . . . .  13
       5.5.2.  "nonce" (Nonce) JWS header parameter  . . . . . . . .  13
     5.6.  Rate limits . . . . . . . . . . . . . . . . . . . . . . .  13
     5.7.  Errors  . . . . . . . . . . . . . . . . . . . . . . . . .  14
   6.  Certificate Management  . . . . . . . . . . . . . . . . . . .  16
     6.1.  Resources . . . . . . . . . . . . . . . . . . . . . . . .  16
       6.1.1.  Directory . . . . . . . . . . . . . . . . . . . . . .  18
       6.1.2.  Account Objects . . . . . . . . . . . . . . . . . . .  20
       6.1.3.  Order Objects . . . . . . . . . . . . . . . . . . . .  21
       6.1.4.  Authorization Objects . . . . . . . . . . . . . . . .  23
     6.2.  Getting a Nonce . . . . . . . . . . . . . . . . . . . . .  24
     6.3.  Account Creation  . . . . . . . . . . . . . . . . . . . .  25
       6.3.1.  Changes of Terms of Service . . . . . . . . . . . . .  27
       6.3.2.  External Account Binding  . . . . . . . . . . . . . .  28
       6.3.3.  Account Key Roll-over . . . . . . . . . . . . . . . .  30



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       6.3.4.  Account deactivation  . . . . . . . . . . . . . . . .  32
     6.4.  Applying for Certificate Issuance . . . . . . . . . . . .  33
       6.4.1.  Pre-Authorization . . . . . . . . . . . . . . . . . .  35
       6.4.2.  Downloading the Certificate . . . . . . . . . . . . .  37
     6.5.  Identifier Authorization  . . . . . . . . . . . . . . . .  39
       6.5.1.  Responding to Challenges  . . . . . . . . . . . . . .  40
       6.5.2.  Deactivating an Authorization . . . . . . . . . . . .  42
     6.6.  Certificate Revocation  . . . . . . . . . . . . . . . . .  43
   7.  Identifier Validation Challenges  . . . . . . . . . . . . . .  45
     7.1.  Key Authorizations  . . . . . . . . . . . . . . . . . . .  47
     7.2.  HTTP  . . . . . . . . . . . . . . . . . . . . . . . . . .  47
     7.3.  TLS with Server Name Indication (TLS SNI) . . . . . . . .  50
     7.4.  DNS . . . . . . . . . . . . . . . . . . . . . . . . . . .  52
     7.5.  Out-of-Band . . . . . . . . . . . . . . . . . . . . . . .  54
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  55
     8.1.  Well-Known URI for the HTTP Challenge . . . . . . . . . .  55
     8.2.  Replay-Nonce HTTP Header  . . . . . . . . . . . . . . . .  55
     8.3.  "url" JWS Header Parameter  . . . . . . . . . . . . . . .  56
     8.4.  "nonce" JWS Header Parameter  . . . . . . . . . . . . . .  56
     8.5.  URN Sub-namespace for ACME (urn:ietf:params:acme) . . . .  56
     8.6.  New Registries  . . . . . . . . . . . . . . . . . . . . .  57
       8.6.1.  Fields in Account Objects . . . . . . . . . . . . . .  57
       8.6.2.  Fields in Order Objects . . . . . . . . . . . . . . .  58
       8.6.3.  Error Codes . . . . . . . . . . . . . . . . . . . . .  59
       8.6.4.  Resource Types  . . . . . . . . . . . . . . . . . . .  59
       8.6.5.  Identifier Types  . . . . . . . . . . . . . . . . . .  60
       8.6.6.  Challenge Types . . . . . . . . . . . . . . . . . . .  60
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  61
     9.1.  Threat model  . . . . . . . . . . . . . . . . . . . . . .  62
     9.2.  Integrity of Authorizations . . . . . . . . . . . . . . .  63
     9.3.  Denial-of-Service Considerations  . . . . . . . . . . . .  65
     9.4.  Server-Side Request Forgery . . . . . . . . . . . . . . .  65
     9.5.  CA Policy Considerations  . . . . . . . . . . . . . . . .  66
   10. Operational Considerations  . . . . . . . . . . . . . . . . .  67
     10.1.  DNS over TCP . . . . . . . . . . . . . . . . . . . . . .  67
     10.2.  Default Virtual Hosts  . . . . . . . . . . . . . . . . .  67
     10.3.  Use of DNSSEC Resolvers  . . . . . . . . . . . . . . . .  68
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  68
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  69
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  69
     12.2.  Informative References . . . . . . . . . . . . . . . . .  71
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  72

1.  Introduction

   Certificates in the Web PKI [RFC5280] are most commonly used to
   authenticate domain names.  Thus, certificate authorities in the Web




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   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.  For
   DV validation, the CA merely verifies that the requester has
   effective control of the web server and/or DNS server for the domain,
   but does not explicitly attempt to verify their 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 certificate authorities tend to run on 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 web page.

   o  Prove ownership of the domain by one of the following methods:

      *  Put a CA-provided challenge at a specific place on the web
         server.

      *  Put a CA-provided challenge at a DNS location corresponding to
         the target domain.

      *  Receive CA challenge at a (hopefully) administrator-controlled
         e-mail 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 their 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
   to follow and cause significant 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




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   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 infrastructural software to obtain certificates without user
   interaction.  Use of this protocol should radically simplify the
   deployment of HTTPS and the practicality of PKIX authentication for
   other protocols based on TLS [RFC5246].

2.  Deployment Model and Operator Experience

   The guiding use case for ACME is obtaining certificates for Web sites
   (HTTPS [RFC2818]).  In this case, the user's web server is intended
   to speak for one or more domains, and the process of certificate
   issuance is intended to verify that this 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 issuing authority 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.

   At time of writing, when deploying an HTTPS server, an operator
   typically gets a prompt to generate a self-signed certificate.  If
   the operator were instead deploying an ACME-compatible web server,
   the experience would be something like this:

   o  The 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 a certificate be issued for the intended domain name(s).

   o  Once the CA is satisfied, the certificate is issued and the ACME
      client automatically downloads and installs it, potentially
      notifying the operator via e-mail, SMS, etc.



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   o  The ACME client periodically contacts the CA to get updated
      certificates, stapled OCSP responses, or whatever else would be
      required to keep the 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 would allow 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.

3.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   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 therefore typically
   runs on a web server, mail server, or some other server system which
   requires valid TLS certificates.  The ACME server runs at a
   certificate authority, and responds to client requests, performing
   the requested actions if the client is authorized.

   An ACME client is represented by 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 JSON messages carried over HTTPS.  In many ways, ACME
   functions much like a traditional CA, in which a user creates an
   account, requests a certificate, and proves control of the domains in
   that certificate in order for the CA to sign 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, 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

   Once an account is registered, there are three 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.  Await issuance and download the issued certificate

   The client's order for a certificate describes the desired
   certificate using a PKCS#10 Certificate Signing Request (CSR) 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 the challenges to check that
   the client has accomplished the challenge.

   Once the validation process is complete and the server is satisfied
   that the client has met its requirements, the server will issue the
   requested certificate and make it available to the client.













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         Order
         Signature                     ------->
                                                                Required
                                       <-------           Authorizations

         Responses
         Signature                     ------->

                             <~~~~~~~~Validation~~~~~~~~>

                                       <-------             Certificate

   To revoke a certificate, the client sends a signed revocation request
   indicating the certificate to be revoked:

         Client                                                 Server

         Revocation request
         Signature                    -------->

                                      <--------                 Result

   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 7 below 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.

5.  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 URI.

5.1.  HTTPS Requests

   Each ACME function is accomplished by the client sending a sequence
   of HTTPS requests to the server, carrying JSON messages
   [RFC2818][RFC7159].  Use of HTTPS is REQUIRED.  Clients SHOULD
   support HTTP public key pinning [RFC7469], and servers SHOULD emit



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   pinning headers.  Each subsection of Section 6 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 an HTTP and HTTPS client when validating challenges via HTTP.

   ACME clients SHOULD send a User-Agent header in accordance with
   [RFC7231], including the name and version of the ACME software in
   addition to the name and version of the underlying HTTP client
   software.

   ACME clients SHOULD send an Accept-Language header 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.CR-cors-20130129].  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 [RFC4648] Section 5, according to the
   profile specified in JSON Web Signature [RFC7515] Section 2.  This
   encoding uses a URL safe character set.  Trailing '=' characters MUST
   be stripped.

5.2.  Request Authentication

   All ACME requests with a non-empty body MUST encapsulate their
   payload in a JWS object, signed (in most cases) using the account's
   private key.  The server MUST verify the JWS before processing the
   request.  Encapsulating request bodies in JWS provides a simple
   authentication of requests.

   JWS objects sent in ACME requests MUST meet the following additional
   criteria:

   o  The JWS MUST be encoded using UTF-8

   o  The JWS MUST NOT have the value "none" in its "alg" field

   o  The JWS MUST NOT have a MAC-based algorithm in its "alg" field

   o  The JWS Protected Header MUST include the following fields:

      *  "alg"




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      *  "jwk" (only for requests to new-reg and revoke-cert resources)

      *  "kid" (for all other requests).

      *  "nonce" (defined below)

      *  "url" (defined below)

   The "jwk" and "kid" fields are mutually exclusive.  Servers MUST
   reject requests that contain both.

   For new-reg requests, and for revoke-cert requests authenticated by
   certificate key, there MUST be a "jwk" field.

   For all other requests, there MUST be a "kid" field.  This field must
   contain the account URI received by POSTing to the new-reg resource.

   Note that authentication via signed JWS request bodies implies that
   GET requests are not authenticated.  Servers MUST NOT respond to GET
   requests for resources that might be considered sensitive.

   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.

   HTTP/1.1 400 Bad Request
   Replay-Nonce: IXVHDyxIRGcTE0VSblhPzw
   Content-Type: application/problem+json
   Content-Language: en

   {
     "type": "urn:ietf:params:acme:error:badSignatureAlgorithm",
     "detail": "Algorithm 'ES384' is not supported",
     "algorithms": ["RS256", "RS384", "ES256"]
   }

   In the examples below, JWS objects are shown in the JSON or flattened
   JSON serialization, with the protected header and payload expressed
   as base64url(content) instead of the actual base64-encoded value, so
   that the content is readable.  Some fields are omitted for brevity,
   marked with "...".







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5.3.  Equivalence of JWKs

   At some points in the protocol, it is necessary for the server to
   determine whether two JSON Web Key (JWK) [RFC7517] objects represent
   the same key.  In performing these checks, the server MUST consider
   two JWKs to match if and only if they have the identical values in
   all fields included in the computation of a JWK thumbprint for that
   key.  That is, the keys must have the same "kty" value and contain
   identical values in the fields used in the computation of a JWK
   thumbprint for that key type:

   o  "RSA": "n", "e"

   o  "EC": "crv", "x", "y"

   Note that this comparison is equivalent to computing the JWK
   thumbprints of the two keys and comparing thumbprints.  The only
   difference is that there is no requirement for a hash computation
   (and thus it is independent of the choice of hash function) and no
   risk of hash collision.

5.4.  Request URI 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 protection.

   These intermediaries can also change values in the request that are
   not signed in the HTTPS request, e.g., the request URI and headers.
   ACME uses JWS to provide an integrity mechanism, which protects
   against an intermediary changing the request URI to another ACME URI.

   As noted above, all ACME request objects carry a "url" 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"
   parameter to the request URI.  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.  For these resources,
   the client MUST set the "url" field 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



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   having the resource check that requests have the same string in their
   "url" fields.

5.4.1.  "url" (URL) JWS header parameter

   The "url" header parameter specifies the URL [RFC3986] to which this
   JWS object is directed.  The "url" parameter MUST be carried in the
   protected header of the JWS.  The value of the "url" header MUST be a
   JSON string representing the URL.

5.5.  Replay protection

   In order to protect ACME resources from any possible replay attacks,
   ACME requests have a mandatory anti-replay mechanism.  This mechanism
   is based on the server maintaining a list of nonces that it has
   issued to clients, and requiring any signed request from the client
   to carry such a nonce.

   An ACME server provides nonces to clients using the Replay-Nonce
   header field, as specified below.  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 below.
   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.

   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 code
   "urn:ietf:params:acme:error:badNonce".  An error response with the
   "badNonce" error code MUST include a Replay-Nonce header with a fresh
   nonce.  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.








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5.5.1.  Replay-Nonce

   The "Replay-Nonce" header field includes a server-generated value
   that the server can use to detect unauthorized replay in future
   client requests.  The server should generate the value provided in
   Replay-Nonce in such a way that they are unique to each message, with
   high probability.

   The value of the Replay-Nonce 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.

     base64url = [A-Z] / [a-z] / [0-9] / "-" / "_"

     Replay-Nonce = *base64url

   The Replay-Nonce header field SHOULD NOT be included in HTTP request
   messages.

5.5.2.  "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.

5.6.  Rate limits

   Creation of resources can be rate limited 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 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 tokens in the "Link"
   header pointing to documentation about the specific hit rate limits
   using the "rate-limit" relation.





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5.7.  Errors

   Errors can be reported in ACME both at the HTTP layer and within ACME
   payloads.  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 problem document [RFC7807].  To
   facilitate automatic response to errors, this document defines the
   following standard tokens for use in the "type" field (within the
   "urn:ietf:params:acme:error:" namespace):






































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   +-----------------------+-------------------------------------------+
   | Code                  | Description                               |
   +-----------------------+-------------------------------------------+
   | badCSR                | The CSR is unacceptable (e.g., due to a   |
   |                       | short key)                                |
   |                       |                                           |
   | badNonce              | The client sent an unacceptable anti-     |
   |                       | replay nonce                              |
   |                       |                                           |
   | badSignatureAlgorithm | The JWS was signed with an algorithm the  |
   |                       | server does not support                   |
   |                       |                                           |
   | caa                   | CAA records forbid the CA from issuing    |
   |                       |                                           |
   | connection            | The server could not connect to           |
   |                       | validation target                         |
   |                       |                                           |
   | dnssec                | DNSSEC validation failed                  |
   |                       |                                           |
   | invalidContact        | The contact URI for a registration was    |
   |                       | invalid                                   |
   |                       |                                           |
   | malformed             | The request message was malformed         |
   |                       |                                           |
   | rateLimited           | The request exceeds a rate limit          |
   |                       |                                           |
   | rejectedIdentifier    | The server will not issue 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 |
   |                       |                                           |
   | unknownHost           | The server could not resolve a domain     |
   |                       | name                                      |
   |                       |                                           |
   | unsupportedIdentifier | Identifier is not supported, but may be   |
   |                       | in future                                 |
   |                       |                                           |
   | userActionRequired    | The user 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



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   MUST NOT use the ACME URN namespace for errors other than the
   standard types.  Clients SHOULD display the "detail" field of all
   errors.

   Authorization and challenge objects can also contain error
   information to indicate why the server was unable to validate
   authorization.

6.  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

6.1.  Resources

   ACME is structured as a REST application with a few types of
   resources:

   o  Account resources, representing information about an account

   o  Order resources, representing an account's requests to issue
      certificates

   o  Authorization resources, representing an account's authorization
      to act for an identifier

   o  Challenge resources, representing a challenge to prove control of
      an identifier

   o  Certificate resources, representing issued certificates

   o  A "directory" resource

   o  A "new-nonce" resource

   o  A "new-account" resource

   o  A "new-order" resource



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   o  A "revoke-certificate" resource

   o  A "key-change" resource

   The server MUST provide "directory" and "new-nonce" resources.

   ACME uses different URIs for different management functions.  Each
   function is listed in a directory along with its corresponding URI,
   so clients only need to be configured with the directory URI.  These
   URIs are connected by a few different link relations [RFC5988].

   The "up" link relation is used with challenge resources to indicate
   the authorization resource to which a challenge belongs.  It is also
   used 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 "directory" link relation is present on all resources other than
   the directory and indicates the directory URL.

   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
                                      |
                                      |--> new-nonce
                                      |
          --------------------------------------------------+
          |          |          |                           |
          |          |          |                           |
          V          V          V                           V
    new-account  new-authz  new-order                  revoke-cert
          |          |          |                           ^
          |          |          |    "author"               | "revoke"
          V          |          V   <--------               |
         acct        |        order --------> cert ---------+
                     |         | ^              |
                     |         | | "up"         | "up"
                     |         V |              V
                     +------> authz         cert-chain
                               | ^
                               | | "up"
                               V |
                             challenge





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   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 pointing to a created resource.

       +----------------------+------------------+----------------+
       | Action               | Request          | Response       |
       +----------------------+------------------+----------------+
       | Get a nonce          | HEAD new-nonce   | 204            |
       |                      |                  |                |
       | Create account       | POST new-account | 201 -> account |
       |                      |                  |                |
       | Submit an order      | POST new-order   | 201 -> order   |
       |                      |                  |                |
       | Fetch challenges     | GET  authz       | 200            |
       |                      |                  |                |
       | Respond to challenge | POST challenge   | 200            |
       |                      |                  |                |
       | Poll for status      | GET  authz       | 200            |
       |                      |                  |                |
       | Check for new cert   | GET  cert        | 200            |
       +----------------------+------------------+----------------+

   The remainder of this section provides the details of how these
   resources are structured and how the ACME protocol makes use of them.

6.1.1.  Directory

   In order to help clients configure themselves with the right URIs for
   each ACME operation, ACME servers provide a directory object.  This
   should be the only URL needed to configure clients.  It is a JSON
   dictionary, whose keys are drawn from the following table and whose
   values are the corresponding URLs.

















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                   +-------------+--------------------+
                   | Key         | URL in value       |
                   +-------------+--------------------+
                   | new-nonce   | New nonce          |
                   |             |                    |
                   | new-account | New account        |
                   |             |                    |
                   | new-order   | New order          |
                   |             |                    |
                   | new-authz   | New authorization  |
                   |             |                    |
                   | revoke-cert | Revoke certificate |
                   |             |                    |
                   | key-change  | Key change         |
                   +-------------+--------------------+

   There is no constraint on the actual URI of the directory except that
   it should be different from the other ACME server resources' URIs,
   and that it should not clash with other services.  For instance:

   o  a host which function as both an ACME and Web server may want to
      keep the root path "/" for an HTML "front page", and and place the
      ACME directory under path "/acme".

   o  a host which only functions as an ACME server could place the
      directory under path "/".

   The dictionary MAY additionally contain a key "meta".  If present, it
   MUST be a JSON dictionary; each item in the dictionary is an item of
   metadata relating to the service provided by the ACME server.

   The following metadata items are defined, all of which are OPTIONAL:

   "terms-of-service" (optional, string):  A URI identifying the current
      terms of service.

   "website" (optional, string):  An HTTP or HTTPS URL locating a
      website providing more information about the ACME server.

   "caa-identities" (optional, array of string):  Each string MUST be a
      lowercase hostname which the ACME server recognises as referring
      to itself for the purposes of CAA record validation as defined in
      [RFC6844].  This allows clients to determine the correct issuer
      domain name to use when configuring CAA record.

   Clients access the directory by sending a GET request to the
   directory URI.




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   HTTP/1.1 200 OK
   Content-Type: application/json

   {
     "new-nonce": "https://example.com/acme/new-nonce",
     "new-account": "https://example.com/acme/new-account",
     "new-order": "https://example.com/acme/new-order",
     "new-authz": "https://example.com/acme/new-authz",
     "revoke-cert": "https://example.com/acme/revoke-cert",
     "key-change": "https://example.com/acme/key-change",
     "meta": {
       "terms-of-service": "https://example.com/acme/terms",
       "website": "https://www.example.com/",
       "caa-identities": ["example.com"]
     }
   }

6.1.2.  Account Objects

   An ACME account resource represents a set of metadata associated to
   an account.  Account resources have the following structure:

   key (required, dictionary):  The public key of the account's key
      pair, encoded as a JSON Web Key object [RFC7517].  The client may
      not directly update this field, but must use the key-change
      resource instead.

   status (required, string):  The status of this account.  Possible
      values are: "valid", "deactivated", and "revoked".  The value
      "deactivated" should be used to indicate user initiated
      deactivation whereas "revoked" should be used to indicate
      administratively initiated deactivation.

   contact (optional, array of string):  An array of URIs 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.

   terms-of-service-agreed (optional, boolean):  Including this field in
      a new-account request, with a value of true, indicates the
      client's agreement with the terms of service.  This field is not
      updateable by the client.

   orders (required, string):  A URI from which a list of orders
      submitted by this account can be fetched via a GET request, as
      described in Section 6.1.2.1





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   {
     "contact": [
       "mailto:cert-admin@example.com",
       "tel:+12025551212"
     ],
     "terms-of-service-agreed": true,
     "orders": "https://example.com/acme/acct/1/orders"
   }

6.1.2.1.  Orders List

   Each account object includes an "orders" URI from which a list of
   orders created by the account can be fetched via GET request.  The
   result of the GET request MUST be a JSON object whose "orders" field
   is an array of URIs, 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 URIs.  The server MAY
   return an incomplete list, along with a Link header with link
   relation "next" indicating a URL to retrieve further entries.

   HTTP/1.1 200 OK
   Content-Type: application/json
   Link: href="/acme/acct/1/orders?cursor=2", rel="next"

   {
     "orders": [
       "https://example.com/acme/acct/1/order/1",
       "https://example.com/acme/acct/1/order/2",
       /* 47 more URLs not shown for example brevity */
       "https://example.com/acme/acct/1/order/50"
     ]
   }

6.1.3.  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", "processing", "valid", and "invalid".

   expires (optional, string):  The timestamp after which the server
      will consider this order invalid, encoded in the format specified
      in RFC 3339 [RFC3339].  This field is REQUIRED for objects with
      "pending" or "valid" in the status field.



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   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.)

   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]

   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 6.5).  For final
      orders, the authorizations that were completed.  Each entry is a
      URL from which an authorization can be fetched with a GET request.

   certificate (optional, string):  A URL for the certificate that has
      been issued in response to this order.

   {
     "status": "pending",
     "expires": "2015-03-01T14:09:00Z",

     "csr": "jcRf4uXra7FGYW5ZMewvV...rhlnznwy8YbpMGqwidEXfE",
     "notBefore": "2016-01-01T00:00:00Z",
     "notAfter": "2016-01-08T00:00:00Z",

     "authorizations": [
       "https://example.com/acme/authz/1234",
       "https://example.com/acme/authz/2345"
     ],

     "certificate": "https://example.com/acme/cert/1234"
   }

   The elements of the "authorizations" array are immutable once set.
   The server MUST NOT change the contents of the "authorizations" array
   after it is created.  If a client observes a change in the contents
   of the "authorizations" array, then it SHOULD consider the order
   invalid.

   The "authorizations" array in the challenge 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




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   orders to be fufilled based on a single authorization transaction,
   then it SHOULD reflect that authorization in all of the order.

6.1.4.  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.

   The structure of an ACME authorization resource is as follows:

   identifier (required, dictionary of string):  The identifier that the
      account is authorized to represent

      type (required, string):  The type of identifier.

      value (required, string):  The identifier itself.

   status (required, string):  The status of this authorization.
      Possible values are: "pending", "processing", "valid", "invalid"
      and "revoked".  If this field is missing, then the default value
      is "pending".

   expires (optional, string):  The timestamp after which the server
      will consider this authorization invalid, encoded in the format
      specified in RFC 3339 [RFC3339].  This field is REQUIRED for
      objects with "valid" in the "status" field.

   scope (optional, string):  If this field is present, then it MUST
      contain a URI for an order resource, such that this authorization
      is only valid for that resource.  If this field is absent, then
      the CA MUST consider this authorization valid for all orders until
      the authorization expires. [[ Open issue: More flexible scoping?
      ]]

   challenges (required, array):  The challenges that the client can
      fulfill in order to prove possession of the identifier (for
      pending authorizations).  For final authorizations, the challenges
      that were used.  Each array entry is a dictionary with parameters
      required to validate the challenge, as specified in Section 7.  A
      client should attempt to fulfill at most one of these challenges,
      and a server should consider any one of the challenges sufficient
      to make the authorization valid.





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   The only type of identifier defined by this specification is a fully-
   qualified domain name (type: "dns").  The value of the identifier
   MUST be the ASCII representation of the domain name.  If a domain
   name contains Unicode characters it MUST be encoded using the rules
   defined in [RFC3492].  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.

   {
     "status": "valid",
     "expires": "2015-03-01T14:09:00Z",

     "identifier": {
       "type": "dns",
       "value": "example.org"
     },

     "challenges": [
       {
         "type": "http-01",
         "status": "valid",
         "validated": "2014-12-01T12:05:00Z",
         "keyAuthorization": "SXQe-2XODaDxNR...vb29HhjjLPSggwiE"
       }
     ]
   }

6.2.  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 new-
   nonce resource on the server.  The server's response MUST include a
   Replay-Nonce header field containing a fresh nonce, and SHOULD have
   status code 204 (No Content).  The server SHOULD also respond to GET
   requests for this resource, returning an empty body (while still
   providing a Replay-Nonce header).








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   HEAD /acme/new-nonce HTTP/1.1
   Host: example.com

   HTTP/1.1 204 No Content
   Replay-Nonce: oFvnlFP1wIhRlYS2jTaXbA
   Cache-Control: no-store

   Proxy caching of responses from the new-nonce resource can cause
   clients 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 new-nonce resource, in
   order to prevent caching of this resource.

6.3.  Account Creation

   A client creates a new account with the server by sending a POST
   request to the server's new-account URI.  The body of the request is
   a stub account object containing only the "contact" field.

   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({
       "terms-of-service-agreed": true,
       "contact": [
         "mailto:cert-admin@example.com",
         "tel:+12025551212"
       ]
     }),
     "signature": "RZPOnYoPs1PhjszF...-nh6X1qtOFPB519I"
   }

   The server MUST ignore any values provided in the "key", and "orders"
   fields in account bodies 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 may be provided by the client.

   In general, the server MUST ignore any fields in the request object
   that it does not recognize.  In particular, it MUST NOT reflect



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   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 client provides
   the server with an invalid or unsupported contact URL, then the
   server MUST return an error of type "invalidContact", with a
   description describing the error and what types of contact URL the
   server considers acceptable.

   The server creates an account object with the included contact
   information.  The "key" element of the account is set to the public
   key used to verify the JWS (i.e., the "jwk" element of the JWS
   header).  The server returns this account object in a 201 (Created)
   response, with the account URI in a Location header field.

   If the server already has an account registered with the provided
   account key, then it MUST return a 200 (OK) response and provide the
   URI of that account in a Content-Location header field.  This allows
   a client that has an account key but not the corresponding account
   URI to recover the account URI.

   If the server wishes to present the client with terms under which the
   ACME service is to be used, it MUST indicate the URI where such terms
   can be accessed in the "terms-of-service" subfield of the "meta"
   field in the directory object, and the server MUST reject new-account
   requests that do not have the "terms-of-service-agreed" set to
   "true".  Clients SHOULD NOT automatically agree to terms by default.
   Rather, they SHOULD require some user interaction for agreement to
   terms.

   HTTP/1.1 201 Created
   Content-Type: application/json
   Replay-Nonce: D8s4D2mLs8Vn-goWuPQeKA
   Location: https://example.com/acme/acct/1
   Link: <https://example.com/acme/some-directory>;rel="directory"

   {
     "key": { /* JWK from JWS header */ },
     "status": "valid",

     "contact": [
       "mailto:cert-admin@example.com",
       "tel:+12025551212"
     ]
   }





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   If the client wishes to update this information in the future, it
   sends a POST request with updated information to the account URI.
   The server MUST ignore any updates to the "key", or "order" fields or
   any other fields it does not recognize.  The server MUST verify that
   the request is signed with the private key corresponding to the "key"
   field of the request before updating the registration.

   For example, to update the contact information in the above account,
   the client could send the following request:

   POST /acme/acct/1 HTTP/1.1
   Host: example.com
   Content-Type: application/jose+json

   {
     "protected": base64url({
       "alg": "ES256",
       "kid": "https://example.com/acme/acct/1",
       "nonce": "ax5RnthDqp_Yf4_HZnFLmA",
       "url": "https://example.com/acme/acct/1"
     }),
     "payload": base64url({
       "contact": [
         "mailto:certificates@example.com",
         "tel:+12125551212"
       ]
     }),
     "signature": "hDXzvcj8T6fbFbmn...rDzXzzvzpRy64N0o"
   }

   Servers SHOULD NOT respond to GET requests for account resources as
   these requests are not authenticated.  If a client wishes to query
   the server for information about its account (e.g., to examine the
   "contact" or "certificates" fields), then it SHOULD do so by sending
   a POST request with an empty update.  That is, it should send a JWS
   whose payload is trivial ({}).

6.3.1.  Changes of Terms of Service

   As described above, a client can indicate its agreement with the CA's
   terms of service by setting the "terms-of-service-agreed" 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



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   include a Link header 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: IXVHDyxIRGcTE0VSblhPzw
   Content-Type: application/problem+json
   Content-Language: en

   {
     "type": "urn:ietf:params:acme:error:userActionRequired",
     "detail": "Terms of service have changed",
     "instance": "http://example.com/agreement/?token=W8Ih3PswD-8"
   }

6.3.2.  External Account Binding

   The server MAY require a value to be present for the "external-
   account-binding" field.  This can be used to an ACME account with an
   existing account in a non-ACME system, such as a CA customer
   database.

   To enable ACME account binding, a CA needs to provision the ACME
   client with a MAC key and a key identifier.  The key identifier MUST
   be an ASCII string.  The MAC key SHOULD be provided in base64url-
   encoded form, to maximize compatibility between provisioning systems
   and ACME clients.

   The ACME client then computes a binding JWS to indicate the external
   account's approval of the ACME account key.  The payload of this JWS
   is the 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.




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   POST /acme/new-reg 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:example@anonymous.invalid"],
       "terms-of-service-agreed": true,

       "external-account-binding": {
         "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"
   }

   When a CA receives a new-account request containing an "external-
   account-binding" field, it must decide whether or not to verify the
   binding.  If the CA does not verify the binding, then it MUST NOT
   reflect the "external-account-binding" 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 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)




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   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, and MUST reflect value of the
   "external-account-binding" field in the resulting account object.  If
   any of these checks fail, then the CA MUST reject the new-
   registration request.

6.3.3.  Account Key Roll-over

   A client may wish to change the public key that is associated with a
   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 first
   constructs a key-change object describing the change that it would
   like the server to make:

   account (required, string):  The URL for account being modified.  The
      content of this field MUST be the exact string provided in the
      Location header field in response to the new-account request that
      created the account.

   newKey (required, JWK):  The JWK representation of the new key

   The client then encapsulates the key-change object in a JWS, signed
   with the requested new account key (i.e., the key matching the
   "newKey" value).

   The outer JWS MUST meet the normal requirements for an ACME JWS (see
   Section 5.2).  The inner JWS MUST meet the normal requirements, with
   the following exceptions:

   o  The inner JWS MUST have the same "url" parameter as the outer JWS.

   o  The inner JWS is NOT REQUIRED to have a "nonce" parameter.  The
      server MUST ignore any value provided for 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.








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   POST /acme/key-change HTTP/1.1
   Host: example.com
   Content-Type: application/jose+json

   {
     "protected": base64url({
       "alg": "ES256",
       "jwk": /* old key */,
       "nonce": "K60BWPrMQG9SDxBDS_xtSw",
       "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/1",
         "newKey": /* new key */
       }),
       "signature": "Xe8B94RD30Azj2ea...8BmZIRtcSKPSd8gU"
     }),
     "signature": "5TWiqIYQfIDfALQv...x9C2mg8JGPxl5bI4"
   }

   On receiving key-change 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 Message Transport.

   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 key-
       change object (as described above)

   6.  Check that the "url" parameters of the inner and outer JWSs are
       the same





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   7.  Check that the "account" field of the key-change object contains
       the URL for the account matching the old key

   8.  Check that the "newKey" field of the key-change object contains
       the key used to sign 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.  Otherwise, the server
   responds with an error status code and a problem document describing
   the error.

6.3.4.  Account deactivation

   A client may deactivate an account by posting a signed update to the
   server with a status field of "deactivated."  Clients may wish to do
   this when the account key is compromised.

   POST /acme/acct/1 HTTP/1.1
   Host: example.com
   Content-Type: application/jose+json

   {
     "protected": base64url({
       "alg": "ES256",
       "kid": "https://example.com/acme/acct/1",
       "nonce": "ntuJWWSic4WVNSqeUmshgg",
       "url": "https://example.com/acme/acct/1"
     }),
     "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 should reply 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.  It is up to server policy
   how long to retain data related to that account, whether to revoke
   certificates issued by that account, and whether to send email to
   that account's contacts.  ACME does not provide a way to reactivate a
   deactivated account.





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6.4.  Applying for Certificate Issuance

   A client may use ACME to submit an order for a certificate to be
   issued.  The client makes this request by sending a POST request to
   the server's new-order resource.  The body of the POST is a JWS
   object whose JSON payload is a subset of the order object defined in
   Section 6.1.3, containing the fields that describe the certificate to
   be issued:

   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.)

   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/1",
       "nonce": "5XJ1L3lEkMG7tR6pA00clA",
       "url": "https://example.com/acme/new-order"
     }),
     "payload": base64url({
       "csr": "5jNudRx6Ye4HzKEqT5...FS6aKdZeGsysoCo4H9P",
       "notBefore": "2016-01-01T00:00:00Z",
       "notAfter": "2016-01-08T00:00:00Z"
     }),
     "signature": "H6ZXtGjTZyUnPeKn...wEA4TklBdh3e454g"
   }

   The CSR encodes the client's requests with regard to the content of
   the certificate to be issued.  The CSR MUST indicate the requested
   identifiers, either in the commonName portion of the requested
   subject name, or in an extensionRequest attribute [RFC2985]
   requesting a subjectAltName extension.

   The server MUST return an error if it cannot fulfil the request as
   specified, and MUST NOT issue a certificate with contents other than



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   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 code 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.

   HTTP/1.1 201 Created
   Replay-Nonce: MYAuvOpaoIiywTezizk5vw
   Location: https://example.com/acme/order/asdf

   {
     "status": "pending",
     "expires": "2016-01-01T00:00:00Z",

     "csr": "jcRf4uXra7FGYW5ZMewvV...rhlnznwy8YbpMGqwidEXfE",
     "notBefore": "2016-01-01T00:00:00Z",
     "notAfter": "2016-01-08T00:00:00Z",

     "authorizations": [
       "https://example.com/acme/authz/1234",
       "https://example.com/acme/authz/2345"
     ]
   }

   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 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 6.5).  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.

   The server MUST issue the requested certificate and update the order
   resource with a URL for the certificate shortly after the client has
   fulfilled the server's requirements.  If the client has already
   satisfied the server's requirements at the time of this request
   (e.g., by obtaining authorization for all of the identifiers in the
   certificate in previous transactions), then the server MUST
   proactively issue the requested certificate and provide a URL for it
   in the "certificate" field of the order.  The server MUST, however,




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   still list the completed authorizations in the "authorizations"
   array.

   Once the client believes it has fulfilled the server's requirements,
   it should send a GET request to the order resource to obtain its
   current state.  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  "processing": The server agrees that the requirements have been
      fulfilled, and is in the process of generating the certificate.
      Retry 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.

6.4.1.  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
   servers are created, and only create a certificate when a 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 for an
   identifier.  In these case, 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 key "new-
   authz" with a URL for the new authorization resource.

   To request authorization for an identifier, the client sends a POST
   request to the new-authorization resource specifying the identifier




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   for which authorization is being requested and how the server should
   behave with respect to existing authorizations for this identifier.

   identifier (required, dictionary of string):  The identifier that the
      account is authorized to represent

      type (required, string):  The type of identifier.

      value (required, string):  The identifier itself.

   existing (optional, string):  How an existing authorization should be
      handled.  Possible values are "accept" and "require".

   POST /acme/new-authz HTTP/1.1
   Host: example.com
   Content-Type: application/jose+json

   {
     "protected": base64url({
       "alg": "ES256",
       "jwk": {...},
       "nonce": "uQpSjlRb4vQVCjVYAyyUWg",
       "url": "https://example.com/acme/new-authz"
     }),
     "payload": base64url({
       "identifier": {
         "type": "dns",
         "value": "example.net"
       },
       "existing": "accept"
     }),
     "signature": "nuSDISbWG8mMgE7H...QyVUL68yzf3Zawps"
   }

   Before processing the authorization request, the server SHOULD
   determine whether it is willing to issue certificates for the
   identifier.  For example, the server should check that the identifier
   is of a supported type.  Servers might also check names against a
   blacklist of known high-value identifiers.  If the server is
   unwilling to issue for the identifier, it SHOULD return a 403
   (Forbidden) error, with a problem document describing the reason for
   the rejection.

   If the authorization request specifies "existing" with a value of
   "accept" or "require", before proceeding, the server SHOULD determine
   whether there are any existing, valid authorization resources for the
   account and given identifier.  If one or more such authorizations
   exists, a response SHOULD returned with status code 303 (See Other)



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   and a Location header pointing to the existing resource URL;
   processing of the request then stops.  If there are multiple such
   authorizations, the authorization with the latest expiry date SHOULD
   be returned.  If no existing authorizations were found and the value
   for "existing" was "require", then the server MUST return status code
   404 (Not Found); if it was "accept" or was any other value or was
   absent, processing continues as follows.

   If the server is willing to proceed, it builds a pending
   authorization object from the inputs submitted by the client.

   o  "identifier" the identifier submitted by the client

   o  "status": MUST be "pending" unless the server has out-of-band
      information about the client's authorization status

   o  "challenges" and "combinations": As selected by the server's
      policy for this identifier

   The server allocates a new URI for this authorization, and returns a
   201 (Created) response, with the authorization URI in a Location
   header field, and the JSON authorization object in the body.  The
   client then follows the process described in Section 6.5 to complete
   the authorization process.

6.4.2.  Downloading the Certificate

   To download the issued certificate, the client simply sends a GET
   request to the certificate URL.

   The default format of the certificate is PEM (application/x-pem-file)
   as specified by [RFC7468].  This format should contain the end-entity
   certificate first, followed by any intermediate certificates that are
   needed to build a path to a trusted root.  Servers SHOULD NOT include
   self-signed trust anchors.  The client may request other formats by
   including an Accept header in its request.  For example, the client
   may use the media type application/pkix-cert to request the end-
   entity certificate in DER format.

   The server MAY provide one or more link relation header fields
   [RFC5988] with relation "alternate".  Each such field should express
   an alternative certificate chain starting with the same end-entity
   certificate.  This can be used to express paths to various trust
   anchors.  Clients can fetch these alternates and use their own
   heuristics to decide which is optimal.






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   The server MUST also provide a link relation header field with
   relation "author" to indicate the order under which this certificate
   was issued.

   If the CA participates in Certificate Transparency (CT) [RFC6962],
   then they may want to provide the client with a Signed Certificate
   Timestamp (SCT) that can be used to prove that a certificate was
   submitted to a CT log.  An SCT can be included as an extension in the
   certificate or as an extension to OCSP responses for the certificate.
   The server can also provide the client with direct access to an SCT
   for a certificate using a Link relation header field with relation
   "ct-sct".

   GET /acme/cert/asdf HTTP/1.1
   Host: example.com
   Accept: application/pkix-cert

   HTTP/1.1 200 OK
   Content-Type: application/pkix-cert
   Link: <https://example.com/acme/ca-cert>;rel="up";title="issuer"
   Link: <https://example.com/acme/revoke-cert>;rel="revoke"
   Link: <https://example.com/acme/order/asdf>;rel="author"
   Link: <https://example.com/acme/sct/asdf>;rel="ct-sct"
   Link: <https://example.com/acme/some-directory>;rel="directory"

   -----BEGIN CERTIFICATE-----
   [End-entity certificate contents]
   -----END CERTIFICATE-----
   -----BEGIN CERTIFICATE-----
   [Issuer certificate contents]
   -----END CERTIFICATE-----
   -----BEGIN CERTIFICATE-----
   [Other certificate contents]
   -----END CERTIFICATE-----

   A certificate resource represents a single, immutable certificate.
   If the client wishes to obtain a renewed certificate, the client
   initiates a new order process to request one.

   Because certificate resources are immutable once issuance is
   complete, the server MAY enable the caching of the resource by adding
   Expires and Cache-Control headers specifying a point in time in the
   distant future.  These headers have no relation to the certificate's
   period of validity.







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6.5.  Identifier Authorization

   The identifier authorization process establishes the authorization of
   an account to manage certificates for a given identifier.  This
   process must assure the server of two things: First, that the client
   controls the private key of the account key pair, and second, that
   the client controls the identifier in question.  This process may be
   repeated to associate multiple identifiers to a key pair (e.g., to
   request certificates with multiple identifiers), or to associate
   multiple accounts with an identifier (e.g., to allow multiple
   entities to manage certificates).  The server may declare that an
   authorization is only valid for a specific order by setting the
   "scope" field of the authorization to the URI for that order.

   Authorization resources are created by the server in response to
   certificate orders or authorization requests submitted by an account
   key holder; their URLs are provided to the client in the responses to
   these requests.  The authorization object is implicitly tied to the
   account key used to sign the request.

   When a client receives an order from the server it downloads the
   authorization resource by sending a GET request to the indicated URL.
   If the client initiates authorization using a request to the new
   authorization resource, it will have already recevied the pending
   authorization object in the response to that request.


























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   GET /acme/authz/1234 HTTP/1.1
   Host: example.com

   HTTP/1.1 200 OK
   Content-Type: application/json
   Link: <https://example.com/acme/some-directory>;rel="directory"

   {
     "status": "pending",
     "expires": "2018-03-03T14:09:00Z",

     "identifier": {
       "type": "dns",
       "value": "example.org"
     },

     "challenges": [
       {
         "type": "http-01",
         "url": "https://example.com/authz/1234/0",
         "token": "DGyRejmCefe7v4NfDGDKfA"
       },
       {
         "type": "tls-sni-02",
         "url": "https://example.com/authz/1234/1",
         "token": "DGyRejmCefe7v4NfDGDKfA"
       },
       {
         "type": "dns-01",
         "url": "https://example.com/authz/1234/2",
         "token": "DGyRejmCefe7v4NfDGDKfA"
       }
     ]
   }

6.5.1.  Responding to Challenges

   To prove control of the identifier and receive authorization, the
   client needs to respond with information to complete the challenges.
   To do this, the client updates the authorization object received from
   the server by filling in any required information in the elements of
   the "challenges" dictionary.  (This is also the stage where the
   client should perform any actions required by the challenge.)

   The client sends these updates back to the server in the form of a
   JSON object with the response fields required by the challenge type,
   carried in a POST request to the challenge URI (not authorization




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   URI).  This allows the client to send information only for challenges
   it is responding to.

   For example, if the client were to respond to the "http-01" challenge
   in the above authorization, it would send the following request:

   POST /acme/authz/asdf/0 HTTP/1.1
   Host: example.com
   Content-Type: application/jose+json

   {
     "protected": base64url({
       "alg": "ES256",
       "kid": "https://example.com/acme/acct/1",
       "nonce": "Q_s3MWoqT05TrdkM2MTDcw",
       "url": "https://example.com/acme/authz/asdf/0"
     }),
     "payload": base64url({
       "type": "http-01",
       "keyAuthorization": "IlirfxKKXA...vb29HhjjLPSggwiE"
     }),
     "signature": "9cbg5JO1Gf5YLjjz...SpkUfcdPai9uVYYQ"
   }

   The server updates the authorization document by updating its
   representation of the challenge with the response fields provided by
   the client.  The server MUST ignore any fields in the response object
   that are not specified as response fields for this type of challenge.
   The server provides a 200 (OK) response with the updated challenge
   object as its body.

   If the client's response is invalid for some reason, or does not
   provide the server with appropriate information to validate the
   challenge, then the server MUST return an HTTP error.  On receiving
   such an error, the client SHOULD undo any actions that have been
   taken to fulfill the challenge, e.g., removing files that have been
   provisioned to a web server.

   The server is said to "finalize" the authorization when it has
   completed one of the validations, by assigning the authorization a
   status of "valid" or "invalid", corresponding to whether it considers
   the account authorized for the identifier.  If the final state is
   "valid", then the server MUST include an "expires" field.  When
   finalizing an authorization, the server MAY remove challenges other
   than the one that was completed, and may modify the "expires" field.
   The server SHOULD NOT remove challenges with status "invalid".





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   Usually, the validation process will take some time, so the client
   will need to poll the authorization resource to see when it is
   finalized.  For challenges where the client can tell when the server
   has validated the challenge (e.g., by seeing an HTTP or DNS request
   from the server), the client SHOULD NOT begin polling until it has
   seen the validation request from the server.

   To check on the status of an authorization, the client sends a GET
   request to the authorization URI, and the server responds with the
   current authorization object.  In responding to poll requests while
   the validation is still in progress, the server MUST return a 202
   (Accepted) response, and MAY include a Retry-After header field to
   suggest a polling interval to the client.

   GET /acme/authz/asdf HTTP/1.1
   Host: example.com

   HTTP/1.1 200 OK

   {
     "status": "valid",
     "expires": "2018-09-09T14:09:00Z",

     "identifier": {
       "type": "dns",
       "value": "example.org"
     },

     "challenges": [
       {
         "type": "http-01"
         "url": "https://example.com/authz/asdf/0",
         "status": "valid",
         "validated": "2014-12-01T12:05:00Z",
         "token": "IlirfxKKXAsHtmzK29Pj8A",
         "keyAuthorization": "IlirfxKKXA...vb29HhjjLPSggwiE"
       }
     ]
   }

6.5.2.  Deactivating an Authorization

   If a client wishes to relinquish its authorization to issue
   certificates for an identifier, then it may request that the server
   deactivate each authorization associated with that identifier by
   sending a POST request with the static object {"status":
   "deactivated"}.




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   POST /acme/authz/asdf HTTP/1.1
   Host: example.com
   Content-Type: application/jose+json

   {
     "protected": base64url({
       "alg": "ES256",
       "kid": "https://example.com/acme/acct/1",
       "nonce": "xWCM9lGbIyCgue8di6ueWQ",
       "url": "https://example.com/acme/authz/asdf"
     }),
     "payload": base64url({
       "status": "deactivated"
     }),
     "signature": "srX9Ji7Le9bjszhu...WTFdtujObzMtZcx4"
   }

   The server MUST verify that the request is signed by the account key
   corresponding to the account that owns the authorization.  If the
   server accepts the deactivation, it should reply with a 200 (OK)
   status code and the current contents of the authorization object.

   The server MUST NOT treat deactivated authorization objects as
   sufficient for issuing certificates.

6.6.  Certificate Revocation

   To request that a certificate be revoked, the client sends a POST
   request to the ACME server's revoke-cert URI.  The body of the POST
   is a JWS object whose JSON payload contains the certificate to be
   revoked:

   certificate (required, string):  The certificate to be revoked, 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.)

   reason (optional, int):  One of the revocation reasonCodes defined in
      [RFC5280] Section 5.3.1 to be used when generating OCSP responses
      and CRLs.  If this field is not set the server SHOULD use the
      unspecified (0) reasonCode value when generating OCSP responses
      and CRLs.  The server MAY disallow a subset of reasonCodes from
      being used by the user.








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   POST /acme/revoke-cert HTTP/1.1
   Host: example.com
   Content-Type: application/jose+json

   {
     "protected": base64url({
       "alg": "ES256",
       "kid": "https://example.com/acme/acct/1", // OR "jwk"
       "nonce": "JHb54aT_KTXBWQOzGYkt9A",
       "url": "https://example.com/acme/revoke-cert"
     }),
     "payload": base64url({
       "certificate": "MIIEDTCCAvegAwIBAgIRAP8...",
       "reason": 1
     }),
     "signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4"
   }

   Revocation requests are different from other ACME request in that
   they can be signed either with an account key pair or the key pair in
   the certificate.  Before revoking a certificate, the server MUST
   verify that the key used to sign the request is authorized to revoke
   the certificate.  The server SHOULD consider at least the following
   accounts authorized for a given certificate:

   o  the account that issued the certificate.

   o  an account that holds authorizations for all of the identifiers in
      the certificate.

   The server SHOULD also consider a revocation request valid if it is
   signed with the private key corresponding to the public key in the
   certificate.

   If the revocation succeeds, the server responds with status code 200
   (OK).  If the revocation fails, the server returns an error.















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   HTTP/1.1 200 OK
   Replay-Nonce: IXVHDyxIRGcTE0VSblhPzw
   Content-Length: 0

   --- or ---

   HTTP/1.1 403 Forbidden
   Replay-Nonce: IXVHDyxIRGcTE0VSblhPzw
   Content-Type: application/problem+json
   Content-Language: en

   {
     "type": "urn:ietf:params:acme:error:unauthorized",
     "detail": "No authorization provided for name example.net",
     "instance": "http://example.com/doc/unauthorized"
   }

7.  Identifier Validation Challenges

   There are few types of identifiers in the world for which there is a
   standardized mechanism to prove possession of a given identifier.  In
   all practical cases, CAs rely on a variety of means to test whether
   an entity applying for a certificate with a given identifier actually
   controls that identifier.

   Challenges provide the server with assurance that an account holder
   is also the entity that controls an identifier.  For each type of
   challenge, it must be the case that in order for an entity to
   successfully complete the challenge the entity must both:

   o  Hold the private key of the account key pair used to respond to
      the challenge

   o  Control the identifier in question

   Section 9 documents how the challenges defined in this document meet
   these requirements.  New challenges will need to document how they
   do.

   ACME uses an extensible challenge/response framework for identifier
   validation.  The server presents a set of challenges in the
   authorization object it sends to a client (as objects in the
   "challenges" array), and the client responds by sending a response
   object in a POST request to a challenge URI.

   This section describes an initial set of challenge types.  Each
   challenge must describe:




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   1.  Content of challenge objects

   2.  Content of response objects

   3.  How the server uses the challenge and response to verify control
       of an identifier

   Challenge objects all contain the following basic fields:

   type (required, string):  The type of challenge encoded in the
      object.

   url (required, string):  The URL to which a response can be posted.

   status (required, string):  The status of this authorization.
      Possible values are: "pending", "valid", and "invalid".

   validated (optional, string):  The time at which this challenge was
      completed by the server, encoded in the format specified in RFC
      3339 [RFC3339].  This field is REQUIRED if the "status" field is
      "valid".

   error (optional, dictionary of string):  The error that occurred
      while the server was validating the challenge, if any.  This field
      is structured as a problem document [RFC7807].

   All additional fields are specified by the challenge type.  If the
   server sets a challenge's "status" to "invalid", it SHOULD also
   include the "error" field to help the client diagnose why they failed
   the challenge.

   Different challenges allow the server to obtain proof of different
   aspects of control over an identifier.  In some challenges, like
   HTTP, TLS SNI, and DNS, the client directly proves its ability to do
   certain things related to the identifier.  The choice of which
   challenges to offer to a client under which circumstances is a matter
   of server policy.

   The identifier validation challenges described in this section all
   relate to validation of domain names.  If ACME is extended in the
   future to support other types of identifier, there will need to be
   new challenge types, and they will need to specify which types of
   identifier they apply to.

   [[ Editor's Note: In pre-RFC versions of this specification,
   challenges are labeled by type, and with the version of the draft in
   which they were introduced.  For example, if an HTTP challenge were
   introduced in version -03 and a breaking change made in version -05,



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   then there would be a challenge labeled "http-03" and one labeled
   "http-05" - but not one labeled "http-04", since challenge in version
   -04 was compatible with one in version -04. ]]

7.1.  Key Authorizations

   Several of the challenges in this document makes use of a key
   authorization string.  A key authorization is a string that expresses
   a domain holder's authorization for a specified key to satisfy a
   specified challenge, by concatenating the token for the challenge
   with a key fingerprint, separated by a "." character:

   key-authz = token || '.' || base64url(JWK\_Thumbprint(accountKey))

   The "JWK_Thumbprint" step indicates the computation specified in
   [RFC7638], using the SHA-256 digest.  As specified in the individual
   challenges below, the token for a challenge is a JSON string
   comprised entirely of characters in the URL-safe base64 alphabet.
   The "||" operator indicates concatenation of strings.

   In computations involving key authorizations, such as the digest
   computations required for the DNS and TLS SNI challenges, the key
   authorization string MUST be represented in UTF-8 form (or,
   equivalently, ASCII).

   An example of how to compute a JWK thumbprint can be found in
   Section 3.1 of [RFC7638].  Note that some cryptographic libraries
   prepend a zero octet to the representation of the RSA public key
   parameters N and E, in order to avoid ambiguity with regard to the
   sign of the number.  As noted in JWA [RFC7518], a JWK object MUST NOT
   include this zero octet.  That is, any initial zero octets MUST be
   stripped before the values are base64url-encoded.

7.2.  HTTP

   With HTTP validation, the client in an ACME transaction proves its
   control over a domain name by proving that it can provision resources
   on an HTTP server that responds for that domain name.  The ACME
   server challenges the client to provision a file at a specific path,
   with a specific string as its content.

   As a domain may resolve to multiple IPv4 and IPv6 addresses, the
   server will connect to at least one of the hosts found in A and AAAA
   records, at its discretion.  Because many webservers allocate a
   default HTTPS virtual host to a particular low-privilege tenant user
   in a subtle and non-intuitive manner, the challenge must be completed
   over HTTP, not HTTPS.




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   type (required, string):  The string "http-01"

   token (required, string):  A random value that uniquely identifies
      the challenge.  This value MUST have at least 128 bits of entropy,
      in order to prevent an attacker from guessing it.  It MUST NOT
      contain any characters outside the base64url alphabet and MUST NOT
      contain any padding characters ("=").

   GET /acme/authz/1234/0 HTTP/1.1
   Host: example.com

   HTTP/1.1 200 OK
   {
     "type": "http-01",
     "url": "https://example.com/acme/authz/0",
     "status": "pending",
     "token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA"
   }

   A client responds to this challenge by constructing a key
   authorization from the "token" value provided in the challenge and
   the client's account key.  The client then provisions the key
   authorization as a resource on the HTTP server for the domain in
   question.

   The path at which the resource is provisioned is comprised of the
   fixed prefix ".well-known/acme-challenge/", followed by the "token"
   value in the challenge.  The value of the resource MUST be the ASCII
   representation of the key authorization.

  .well-known/acme-challenge/evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA

   The client's response to this challenge indicates its agreement to
   this challenge by sending the server the key authorization covering
   the challenge's token and the client's account key.

   keyAuthorization (required, string):  The key authorization for this
      challenge.  This value MUST match the token from the challenge and
      the client's account key.












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   POST /acme/authz/1234/0
   Host: example.com
   Content-Type: application/jose+json

   {
     "protected": base64url({
       "alg": "ES256",
       "kid": "https://example.com/acme/acct/1",
       "nonce": "JHb54aT_KTXBWQOzGYkt9A",
       "url": "https://example.com/acme/authz/1234/0"
     }),
     "payload": base64url({
       "keyAuthorization": "evaGxfADs...62jcerQ"
     }),
     "signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4"
   }

   On receiving a response, the server MUST verify that the key
   authorization in the response matches the "token" value in the
   challenge and the client's account key.  If they do not match, then
   the server MUST return an HTTP error in response to the POST request
   in which the client sent the challenge.

   Given a challenge/response pair, the server verifies the client's
   control of the domain by verifying that the resource was provisioned
   as expected.

   1.  Form a URI by populating the URI template [RFC6570]
       "http://{domain}/.well-known/acme-challenge/{token}", where:

       *  the domain field is set to the domain name being verified; and

       *  the token field is set to the token in the challenge.

   2.  Verify that the resulting URI is well-formed.

   3.  Dereference the URI using an HTTP GET request.  This request MUST
       be sent to TCP port 80 on the server.

   4.  Verify that the body of the response is well-formed key
       authorization.  The server SHOULD ignore whitespace characters at
       the end of the body.

   5.  Verify that key authorization provided by the server matches the
       token for this challenge and the client's account key.






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   If all of the above verifications succeed, then the validation is
   successful.  If the request fails, or the body does not pass these
   checks, then it has failed.

7.3.  TLS with Server Name Indication (TLS SNI)

   The TLS with Server Name Indication (TLS SNI) validation method
   proves control over a domain name by requiring the client to
   configure a TLS server referenced by an A/AAAA record under the
   domain name to respond to specific connection attempts utilizing the
   Server Name Indication extension [RFC6066].  The server verifies the
   client's challenge by accessing the reconfigured server and verifying
   a particular challenge certificate is presented.

   type (required, string):  The string "tls-sni-02"

   token (required, string):  A random value that uniquely identifies
      the challenge.  This value MUST have at least 128 bits of entropy,
      in order to prevent an attacker from guessing it.  It MUST NOT
      contain any characters outside the base64url alphabet and MUST NOT
      contain any padding characters ("=").

   GET /acme/authz/1234/1 HTTP/1.1
   Host: example.com

   HTTP/1.1 200 OK
   {
     "type": "tls-sni-02",
     "url": "https://example.com/acme/authz/1234/1",
     "status": "pending",
     "token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA"
   }

   A client responds to this challenge by constructing a self-signed
   certificate which the client MUST provision at the domain name
   concerned in order to pass the challenge.

   The certificate may be constructed arbitrarily, except that each
   certificate MUST have exactly two subjectAlternativeNames, SAN A and
   SAN B.  Both MUST be dNSNames.

   SAN A MUST be constructed as follows: compute the SHA-256 digest of
   the UTF-8-encoded challenge token and encode it in lowercase
   hexadecimal form.  The dNSName is "x.y.token.acme.invalid", where x
   is the first half of the hexadecimal representation and y is the
   second half.





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   SAN B MUST be constructed as follows: compute the SHA-256 digest of
   the UTF-8 encoded key authorization and encode it in lowercase
   hexadecimal form.  The dNSName is "x.y.ka.acme.invalid" where x is
   the first half of the hexadecimal representation and y is the second
   half.

   The client MUST ensure that the certificate is served to TLS
   connections specifying a Server Name Indication (SNI) value of SAN A.

   The response to the TLS-SNI challenge simply acknowledges that the
   client is ready to fulfill this challenge.

   keyAuthorization (required, string):  The key authorization for this
      challenge.  This value MUST match the token from the challenge and
      the client's account key.

   POST /acme/authz/1234/1
   Host: example.com
   Content-Type: application/jose+json

   {
     "protected": base64url({
       "alg": "ES256",
       "kid": "https://example.com/acme/acct/1",
       "nonce": "JHb54aT_KTXBWQOzGYkt9A",
       "url": "https://example.com/acme/authz/1234/1"
     }),
     "payload": base64url({
       "keyAuthorization": "evaGxfADs...62jcerQ"
     }),
     "signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4"
   }

   On receiving a response, the server MUST verify that the key
   authorization in the response matches the "token" value in the
   challenge and the client's account key.  If they do not match, then
   the server MUST return an HTTP error in response to the POST request
   in which the client sent the challenge.

   Given a challenge/response pair, the ACME server verifies the
   client's control of the domain by verifying that the TLS server was
   configured appropriately, using these steps:

   1.  Compute SAN A and SAN B in the same way as the client.

   2.  Open a TLS connection to the domain name being validated,
       presenting SAN A in the SNI field.  This connection MUST be sent
       to TCP port 443 on the server.  In the ClientHello initiating the



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       TLS handshake, the server MUST include a server_name extension
       (i.e., SNI) containing SAN A.  The server SHOULD ensure that it
       does not reveal SAN B in any way when making the TLS connection,
       such that the presentation of SAN B in the returned certificate
       proves association with the client.

   3.  Verify that the certificate contains a subjectAltName extension
       containing dNSName entries of SAN A and SAN B and no other
       entries.  The comparison MUST be insensitive to case and ordering
       of names.

   It is RECOMMENDED that the server open multiple TLS connections from
   various network perspectives, in order to make MitM attacks harder.

   If all of the above verifications succeed, then the validation is
   successful.  Otherwise, the validation fails.

7.4.  DNS

   When the identifier being validated is a domain name, the client can
   prove control of that domain by provisioning a resource record under
   it.  The DNS challenge requires the client to provision a TXT record
   containing a designated value under a specific validation domain
   name.

   type (required, string):  The string "dns-01"

   token (required, string):  A random value that uniquely identifies
      the challenge.  This value MUST have at least 128 bits of entropy,
      in order to prevent an attacker from guessing it.  It MUST NOT
      contain any characters outside the base64url alphabet and MUST NOT
      contain any padding characters ("=").

   GET /acme/authz/1234/2 HTTP/1.1
   Host: example.com

   HTTP/1.1 200 OK
   {
     "type": "dns-01",
     "url": "https://example.com/acme/authz/1234/2",
     "status": "pending",
     "token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA"
   }

   A client responds to this challenge by constructing a key
   authorization from the "token" value provided in the challenge and
   the client's account key.  The client then computes the SHA-256
   digest of the key authorization.



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   The record provisioned to the DNS is the base64url encoding of this
   digest.  The client constructs the validation domain name by
   prepending the label "_acme-challenge" to the domain name being
   validated, then provisions a TXT record with the digest value under
   that name.  For example, if the domain name being validated is
   "example.com", then the client would provision the following DNS
   record:

   _acme-challenge.example.com. 300 IN TXT "gfj9Xq...Rg85nM"

   The response to the DNS challenge provides the computed key
   authorization to acknowledge that the client is ready to fulfill this
   challenge.

   keyAuthorization (required, string):  The key authorization for this
      challenge.  This value MUST match the token from the challenge and
      the client's account key.

   POST /acme/authz/1234/2
   Host: example.com
   Content-Type: application/jose+json

   {
     "protected": base64url({
       "alg": "ES256",
       "kid": "https://example.com/acme/acct/1",
       "nonce": "JHb54aT_KTXBWQOzGYkt9A",
       "url": "https://example.com/acme/authz/1234/2"
     }),
     "payload": base64url({
       "keyAuthorization": "evaGxfADs...62jcerQ"
     }),
     "signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4"
   }

   On receiving a response, the server MUST verify that the key
   authorization in the response matches the "token" value in the
   challenge and the client's account key.  If they do not match, then
   the server MUST return an HTTP error in response to the POST request
   in which the client sent the challenge.

   To validate a DNS challenge, the server performs the following steps:

   1.  Compute the SHA-256 digest of the key authorization

   2.  Query for TXT records under the validation domain name





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   3.  Verify that the contents of one of the TXT records matches the
       digest value

   It is RECOMMENDED that the server perform multiple DNS queries from
   various network perspectives, in order to make MitM attacks harder.

   If all of the above verifications succeed, then the validation is
   successful.  If no DNS record is found, or DNS record and response
   payload do not pass these checks, then the validation fails.

7.5.  Out-of-Band

   There may be cases where a server cannot perform automated validation
   of an identifier, for example if validation requires some manual
   steps.  In such cases, the server may provide an "out of band" (OOB)
   challenge to request that the client perform some action outside of
   ACME in order to validate possession of the identifier.

   The OOB challenge requests that the client have a human user visit a
   web page to receive instructions on how to validate possession of the
   identifier, by providing a URL for that web page.

   type (required, string):  The string "oob-01"

   href (required, string):  The URL to be visited.  The scheme of this
      URL MUST be "http" or "https".  Note that this field is distinct
      from the "url" field of the challenge, which identifies the
      challenge itself.

   GET /acme/authz/1234/3 HTTP/1.1
   Host: example.com

   HTTP/1.1 200 OK
   {
     "type": "oob-01",
     "href": "https://example.com/validate/evaGxfADs6pSRb2LAv9IZ"
   }

   A client responds to this challenge by presenting the indicated URL
   for a human user to navigate to.  If the user choses to complete this
   challege (by vising the website and completing its instructions), the
   client indicates this by sending a simple acknowledgement response to
   the server.

   type (required, string):  The string "oob-01"






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   POST /acme/authz/1234/3
   Host: example.com
   Content-Type: application/jose+json

   {
     "protected": base64url({
       "alg": "ES256",
       "kid": "https://example.com/acme/acct/1",
       "nonce": "JHb54aT_KTXBWQOzGYkt9A",
       "url": "https://example.com/acme/authz/1234/3"
     }),
     "payload": base64url({
       "type": "oob-01"
     }),
     "signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4"
   }

   On receiving a response, the server MUST verify that the value of the
   "type" field is "oob-01".  Otherwise, the steps the server takes to
   validate identifier possession are determined by the server's local
   policy.

8.  IANA Considerations

8.1.  Well-Known URI for the HTTP Challenge

   The "Well-Known URIs" registry should be updated with the following
   additional value (using the template from [RFC5785]):

   URI suffix: acme-challenge

   Change controller: IETF

   Specification document(s): This document, Section Section 7.2

   Related information: N/A

8.2.  Replay-Nonce HTTP Header

   The "Message Headers" registry should be updated with the following
   additional value:

   | Header Field Name | Protocol | Status | Reference |
   +:------------+:------+:------+:-----------+ | Replay-Nonce | http |
   standard | Section 5.5.1 |






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8.3.  "url" JWS Header Parameter

   The "JSON Web Signature and Encryption Header Parameters" registry
   should be updated with the following additional value:

   o  Header Parameter Name: "url"

   o  Header Parameter Description: URL

   o  Header Parameter Usage Location(s): JWE, JWS

   o  Change Controller: IESG

   o  Specification Document(s): Section 5.4.1 of RFC XXXX

   [[ RFC EDITOR: Please replace XXXX above with the RFC number assigned
   to this document ]]

8.4.  "nonce" JWS Header Parameter

   The "JSON Web Signature and Encryption Header Parameters" registry
   should be updated with the following additional value:

   o  Header Parameter Name: "nonce"

   o  Header Parameter Description: Nonce

   o  Header Parameter Usage Location(s): JWE, JWS

   o  Change Controller: IESG

   o  Specification Document(s): Section 5.5.2 of RFC XXXX

   [[ RFC EDITOR: Please replace XXXX above with the RFC number assigned
   to this document ]]

8.5.  URN Sub-namespace for ACME (urn:ietf:params:acme)

   The "IETF URN Sub-namespace for Registered Protocol Parameter
   Identifiers" registry should be updated with the following additional
   value, following the template in [RFC3553]:

   Registry name:  acme

   Specification:  RFC XXXX

   Repository:  URL-TBD




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   Index value:  No transformation needed.

   [[ RFC EDITOR: Please replace XXXX above with the RFC number assigned
   to this document, and replace URL-TBD with the URL assigned by IANA
   for registries of ACME parameters. ]]

8.6.  New Registries

   This document requests that IANA create the following new registries:

   1.  ACME Error Codes

   2.  ACME Resource Types

   3.  ACME Identifier Types

   4.  ACME Challenge Types

   All of these registries should be administered under a Specification
   Required policy [RFC5226].

8.6.1.  Fields in Account Objects

   This registry lists field names that are defined for use in ACME
   account objects.  Fields marked as "configurable" may be included in
   a new-account request.

   Template:

   o  Field name: The string to be used as a key in the JSON dictionary

   o  Field type: The type of value to be provided, e.g., string,
      boolean, array of string

   o  Client configurable: Boolean indicating whether the server should
      accept values provided by the client

   o  Reference: Where this field is defined

   Initial contents: The fields and descriptions defined in
   Section 6.1.2.










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   +--------------------------+-------------+--------------+-----------+
   | Field Name               | Field Type  | Configurable | Reference |
   +--------------------------+-------------+--------------+-----------+
   | key                      | dictionary  | false        | RFC XXXX  |
   |                          |             |              |           |
   | status                   | string      | false        | RFC XXXX  |
   |                          |             |              |           |
   | contact                  | array of    | true         | RFC XXXX  |
   |                          | string      |              |           |
   |                          |             |              |           |
   | external-account-binding | dictionary  | true         | RFC XXXX  |
   |                          |             |              |           |
   | terms-of-service-agreed  | boolean     | false        | RFC XXXX  |
   |                          |             |              |           |
   | orders                   | array of    | false        | RFC XXXX  |
   |                          | string      |              |           |
   +--------------------------+-------------+--------------+-----------+

8.6.2.  Fields in Order Objects

   This registry lists field names that are defined for use in ACME
   order objects.  Fields marked as "configurable" may be included in a
   new-order request.

   Template:

   o  Field name: The string to be used as a key in the JSON dictionary

   o  Field type: The type of value to be provided, e.g., string,
      boolean, array of string

   o  Client configurable: Boolean indicating whether the server should
      accept values provided by the client

   o  Reference: Where this field is defined

   Initial contents: The fields and descriptions defined in
   Section 6.1.3.













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     +----------------+-------------------+--------------+-----------+
     | Field Name     | Field Type        | Configurable | Reference |
     +----------------+-------------------+--------------+-----------+
     | status         | string            | false        | RFC XXXX  |
     |                |                   |              |           |
     | expires        | string            | false        | RFC XXXX  |
     |                |                   |              |           |
     | csr            | string            | true         | RFC XXXX  |
     |                |                   |              |           |
     | notBefore      | string            | true         | RFC XXXX  |
     |                |                   |              |           |
     | notAfter       | string            | true         | RFC XXXX  |
     |                |                   |              |           |
     | authorizations | array of string   | false        | RFC XXXX  |
     |                |                   |              |           |
     | certificate    | string            | false        | RFC XXXX  |
     +----------------+-------------------+--------------+-----------+

8.6.3.  Error Codes

   This registry lists values that are used within URN values that are
   provided in the "type" field of problem documents in ACME.

   Template:

   o  Code: The label to be included in the URN for this error,
      following "urn:ietf:params:acme:"

   o  Description: A human-readable description of the error

   o  Reference: Where the error is defined

   Initial contents: The codes and descriptions in the table in
   Section 5.7 above, with the Reference field set to point to this
   specification.

8.6.4.  Resource Types

   This registry lists the types of resources that ACME servers may list
   in their directory objects.

   Template:

   o  Key: The value to be used as a dictionary key in the directory
      object

   o  Resource type: The type of resource labeled by the key




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   o  Reference: Where the identifier type is defined

   Initial contents:

             +-------------+--------------------+-----------+
             | Key         | Resource type      | Reference |
             +-------------+--------------------+-----------+
             | new-account | New account        | RFC XXXX  |
             |             |                    |           |
             | new-order   | New order          | RFC XXXX  |
             |             |                    |           |
             | revoke-cert | Revoke certificate | RFC XXXX  |
             |             |                    |           |
             | key-change  | Key change         | RFC XXXX  |
             +-------------+--------------------+-----------+

   [[ RFC EDITOR: Please replace XXXX above with the RFC number assigned
   to this document ]]

8.6.5.  Identifier Types

   This registry lists the types of identifiers that ACME clients may
   request authorization to issue in certificates.

   Template:

   o  Label: The value to be put in the "type" field of the identifier
      object

   o  Reference: Where the identifier type is defined

   Initial contents:

                           +-------+-----------+
                           | Label | Reference |
                           +-------+-----------+
                           | dns   | RFC XXXX  |
                           +-------+-----------+

   [[ RFC EDITOR: Please replace XXXX above with the RFC number assigned
   to this document ]]

8.6.6.  Challenge Types

   This registry lists the ways that ACME servers can offer to validate
   control of an identifier.  The "Identifier Type" field in template
   must be contained in the Label column of the ACME Identifier Types
   registry.



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   Template:

   o  Label: The value to be put in the "type" field of challenge
      objects using this validation mechanism

   o  Identifier Type: The type of identifier that this mechanism
      applies to

   o  Reference: Where the challenge type is defined

   Initial Contents

                 +---------+-----------------+-----------+
                 | Label   | Identifier Type | Reference |
                 +---------+-----------------+-----------+
                 | http    | dns             | RFC XXXX  |
                 |         |                 |           |
                 | tls-sni | dns             | RFC XXXX  |
                 |         |                 |           |
                 | dns     | dns             | RFC XXXX  |
                 +---------+-----------------+-----------+

   [[ RFC EDITOR: Please replace XXXX above with the RFC number assigned
   to this document ]]

9.  Security Considerations

   ACME is a protocol for managing certificates that attest to
   identifier/key bindings.  Thus the foremost security goal of ACME is
   to ensure the integrity of this process, i.e., to ensure that the
   bindings attested by certificates are correct, and that only
   authorized entities can manage certificates.  ACME identifies clients
   by their account keys, so this overall goal breaks down into two more
   precise goals:

   1.  Only an entity that controls an identifier can get an account key
       authorized for that identifier

   2.  Once authorized, an account key's authorizations cannot be
       improperly transferred to another account key

   In this section, we discuss the threat model that underlies ACME and
   the ways that ACME achieves these security goals within that threat
   model.  We also discuss the denial-of-service risks that ACME servers
   face, and a few other miscellaneous considerations.






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9.1.  Threat model

   As a service on the Internet, ACME broadly exists within the Internet
   threat model [RFC3552].  In analyzing ACME, it is useful to think of
   an ACME server interacting with other Internet hosts along two
   "channels":

   o  An ACME channel, over which the ACME HTTPS requests are exchanged

   o  A validation channel, over which the ACME server performs
      additional requests to validate a client's control of an
      identifier

   +------------+
   |    ACME    |     ACME Channel
   |   Client   |--------------------+
   +------------+                    |
                                     V
                               +------------+
                               |    ACME    |
                               |   Server   |
                               +------------+
   +------------+                    |
   | Validation |<-------------------+
   |   Server   |  Validation Channel
   +------------+

   In practice, the risks to these channels are not entirely separate,
   but they are different in most cases.  Each channel, for example,
   uses a different communications pattern: the ACME channel will
   comprise inbound HTTPS connections to the ACME server and the
   validation channel outbound HTTP or DNS requests.

   Broadly speaking, ACME aims to be secure against active and passive
   attackers on any individual channel.  Some vulnerabilities arise
   (noted below), when an attacker can exploit both the ACME channel and
   one of the others.

   On the ACME channel, in addition to network-layer attackers, we also
   need to account for application-layer man in the middle attacks, and
   for abusive use of the protocol itself.  Protection against
   application-layer MitM addresses potential attackers such as Content
   Distribution Networks (CDNs) and middleboxes with a TLS MitM
   function.  Preventing abusive use of ACME means ensuring that an
   attacker with access to the validation channel can't obtain
   illegitimate authorization by acting as an ACME client (legitimately,
   in terms of the protocol).




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9.2.  Integrity of Authorizations

   ACME allows anyone to request challenges for an identifier by
   registering an account key and sending a new-order request under that
   account key.  The integrity of the authorization process thus depends
   on the identifier validation challenges to ensure that the challenge
   can only be completed by someone who both (1) holds the private key
   of the account key pair, and (2) controls the identifier in question.

   Validation responses need to be bound to an account key pair in order
   to avoid situations where an ACME MitM can switch out a legitimate
   domain holder's account key for one of his choosing, e.g.:

   o  Legitimate domain holder registers account key pair A

   o  MitM registers account key pair B

   o  Legitimate domain holder sends a new-order request signed under
      account key A

   o  MitM suppresses the legitimate request, but sends the same request
      signed under account key B

   o  ACME server issues challenges and MitM forwards them to the
      legitimate domain holder

   o  Legitimate domain holder provisions the validation response

   o  ACME server performs validation query and sees the response
      provisioned by the legitimate domain holder

   o  Because the challenges were issued in response to a message signed
      account key B, the ACME server grants authorization to account key
      B (the MitM) instead of account key A (the legitimate domain
      holder)

   All of the challenges above have a binding between the account
   private key and the validation query made by the server, via the key
   authorization.  The key authorization is signed by the account
   private key, reflects the corresponding public key, and is provided
   to the server in the validation response.

   The association of challenges to identifiers is typically done by
   requiring the client to perform some action that only someone who
   effectively controls the identifier can perform.  For the challenges
   in this document, the actions are:





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   o  HTTP: Provision files under .well-known on a web server for the
      domain

   o  TLS SNI: Configure a TLS server for the domain

   o  DNS: Provision DNS resource records for the domain

   There are several ways that these assumptions can be violated, both
   by misconfiguration and by attack.  For example, on a web server that
   allows non-administrative users to write to .well-known, any user can
   claim to own the server's hostname by responding to an HTTP
   challenge, and likewise for TLS configuration and TLS SNI.

   The use of hosting providers is a particular risk for ACME
   validation.  If the owner of the domain has outsourced operation of
   DNS or web services to a hosting provider, there is nothing that can
   be done against tampering by the hosting provider.  As far as the
   outside world is concerned, the zone or web site provided by the
   hosting provider is the real thing.

   More limited forms of delegation can also lead to an unintended party
   gaining the ability to successfully complete a validation
   transaction.  For example, suppose an ACME server follows HTTP
   redirects in HTTP validation and a web site operator provisions a
   catch-all redirect rule that redirects requests for unknown resources
   to a different domain.  Then the target of the redirect could use
   that to get a certificate through HTTP validation, since the
   validation path will not be known to the primary server.

   The DNS is a common point of vulnerability for all of these
   challenges.  An entity that can provision false DNS records for a
   domain can attack the DNS challenge directly, and can provision false
   A/AAAA records to direct the ACME server to send its TLS SNI or HTTP
   validation query to a server of the attacker's choosing.  There are a
   few different mitigations that ACME servers can apply:

   o  Always querying the DNS using a DNSSEC-validating resolver
      (enhancing security for zones that are DNSSEC-enabled)

   o  Querying the DNS from multiple vantage points to address local
      attackers

   o  Applying mitigations against DNS off-path attackers, e.g., adding
      entropy to requests [I-D.vixie-dnsext-dns0x20] or only using TCP

   Given these considerations, the ACME validation process makes it
   impossible for any attacker on the ACME channel, or a passive




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   attacker on the validation channel to hijack the authorization
   process to authorize a key of the attacker's choice.

   An attacker that can only see the ACME channel would need to convince
   the validation server to provide a response that would authorize the
   attacker's account key, but this is prevented by binding the
   validation response to the account key used to request challenges.  A
   passive attacker on the validation channel can observe the correct
   validation response and even replay it, but that response can only be
   used with the account key for which it was generated.

   An active attacker on the validation channel can subvert the ACME
   process, by performing normal ACME transactions and providing a
   validation response for his own account key.  The risks due to
   hosting providers noted above are a particular case.  For identifiers
   where the server already has some public key associated with the
   domain this attack can be prevented by requiring the client to prove
   control of the corresponding private key.

9.3.  Denial-of-Service Considerations

   As a protocol run over HTTPS, standard considerations for TCP-based
   and HTTP-based DoS mitigation also apply to ACME.

   At the application layer, ACME requires the server to perform a few
   potentially expensive operations.  Identifier validation transactions
   require the ACME server to make outbound connections to potentially
   attacker-controlled servers, and certificate issuance can require
   interactions with cryptographic hardware.

   In addition, an attacker can also cause the ACME server to send
   validation requests to a domain of its choosing by submitting
   authorization requests for the victim domain.

   All of these attacks can be mitigated by the application of
   appropriate rate limits.  Issues closer to the front end, like POST
   body validation, can be addressed using HTTP request limiting.  For
   validation and certificate requests, there are other identifiers on
   which rate limits can be keyed.  For example, the server might limit
   the rate at which any individual account key can issue certificates,
   or the rate at which validation can be requested within a given
   subtree of the DNS.

9.4.  Server-Side Request Forgery

   Server-Side Request Forgery (SSRF) attacks can arise when an attacker
   can cause a server to perform HTTP requests to an attacker-chosen
   URL.  In the ACME HTTP challenge validation process, the ACME server



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   performs an HTTP GET request to a URL in which the attacker can
   choose the domain.  This request is made before the server has
   verified that the client controls the domain, so any client can cause
   a query to any domain.

   Some server implementations include information from the validation
   server's response (in order to facilitate debugging).  Such
   implementations enable an attacker to extract this information from
   any web server that is accessible to the ACME server, even if it is
   not accessible to the ACME client.

   It might seem that the risk of SSRF through this channel is limited
   by the fact that the attacker can only control the domain of the URL,
   not the path.  However, if the attacker first sets the domain to one
   they control, then they can send the server an HTTP redirect (e.g., a
   302 response) which will cause the server to query an arbitrary URI.

   In order to further limit the SSRF risk, ACME server operators should
   ensure that validation queries can only be sent to servers on the
   public Internet, and not, say, web services within the server
   operator's internal network.  Since the attacker could make requests
   to these public servers himself, he can't gain anything extra through
   an SSRF attack on ACME aside from a layer of anonymization.

9.5.  CA Policy Considerations

   The controls on issuance enabled by ACME are focused on validating
   that a certificate applicant controls the identifier he claims.
   Before issuing a certificate, however, there are many other checks
   that a CA might need to perform, for example:

   o  Has the client agreed to a subscriber agreement?

   o  Is the claimed identifier syntactically valid?

   o  For domain names:

      *  If the leftmost label is a '*', then have the appropriate
         checks been applied?

      *  Is the name on the Public Suffix List?

      *  Is the name a high-value name?

      *  Is the name a known phishing domain?

   o  Is the key in the CSR sufficiently strong?




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   o  Is the CSR signed with an acceptable algorithm?

   CAs that use ACME to automate issuance will need to ensure that their
   servers perform all necessary checks before issuing.

   CAs using ACME to allow clients to agree to terms of service should
   keep in mind that ACME clients can automate this agreement, possibly
   not involving a human user.  If a CA wishes to have stronger evidence
   of user consent, it may present an out-of-band requirement or
   challenge to require human involvement.

10.  Operational Considerations

   There are certain factors that arise in operational reality that
   operators of ACME-based CAs will need to keep in mind when
   configuring their services.  For example:

10.1.  DNS over TCP

   As noted above, DNS forgery attacks against the ACME server can
   result in the server making incorrect decisions about domain control
   and thus mis-issuing certificates.  Servers SHOULD verify DNSSEC when
   it is available for a domain.  When DNSSEC is not available, servers
   SHOULD perform DNS queries over TCP, which provides better resistance
   to some forgery attacks than DNS over UDP.

10.2.  Default Virtual Hosts

   In many cases, TLS-based services are deployed on hosted platforms
   that use the Server Name Indication (SNI) TLS extension to
   distinguish between different hosted services or "virtual hosts".
   When a client initiates a TLS connection with an SNI value indicating
   a provisioned host, the hosting platform routes the connection to
   that host.

   When a connection comes in with an unknown SNI value, one might
   expect the hosting platform to terminate the TLS connection.
   However, some hosting platforms will choose a virtual host to be the
   "default", and route connections with unknown SNI values to that
   host.

   In such cases, the owner of the default virtual host can complete a
   TLS-based challenge (e.g., "tls-sni-02") for any domain with an A
   record that points to the hosting platform.  This could result in
   mis-issuance in cases where there are multiple hosts with different
   owners resident on the hosting platform.





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   A CA that accepts TLS-based proof of domain control should attempt to
   check whether a domain is hosted on a domain with a default virtual
   host before allowing an authorization request for this host to use a
   TLS-based challenge.  A default virtual host can be detected by
   initiating TLS connections to the host with random SNI values within
   the namespace used for the TLS-based challenge (the "acme.invalid"
   namespace for "tls-sni-02").

10.3.  Use of DNSSEC Resolvers

   An ACME-based CA will often need to make DNS queries, e.g., to
   validate control of DNS names.  Because the security of such
   validations ultimately depends on the authenticity of DNS data, every
   possible precaution should be taken to secure DNS queries done by the
   CA.  It is therefore RECOMMENDED that ACME-based CAs make all DNS
   queries via DNSSEC-validating stub or recursive resolvers.  This
   provides additional protection to domains which choose to make use of
   DNSSEC.

   An ACME-based CA must use only a resolver if it trusts the resolver
   and every component of the network route by which it is accessed.  It
   is therefore RECOMMENDED that ACME-based CAs operate their own
   DNSSEC-validating resolvers within their trusted network and use
   these resolvers both for both CAA record lookups and all record
   lookups in furtherance of a challenge scheme (A, AAAA, TXT, etc.).

11.  Acknowledgements

   In addition to the editors listed on the front page, this document
   has benefited from contributions from a broad set of contributors,
   all the way back to its inception.

   o  Peter Eckersley, EFF

   o  Eric Rescorla, Mozilla

   o  Seth Schoen, EFF

   o  Alex Halderman, University of Michigan

   o  Martin Thomson, Mozilla

   o  Jakub Warmuz, University of Oxford

   This document draws on many concepts established by Eric Rescorla's
   "Automated Certificate Issuance Protocol" draft.  Martin Thomson
   provided helpful guidance in the use of HTTP.




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12.  References

12.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000,
              <http://www.rfc-editor.org/info/rfc2818>.

   [RFC2985]  Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object
              Classes and Attribute Types Version 2.0", RFC 2985,
              DOI 10.17487/RFC2985, November 2000,
              <http://www.rfc-editor.org/info/rfc2985>.

   [RFC2986]  Nystrom, M. and B. Kaliski, "PKCS #10: Certification
              Request Syntax Specification Version 1.7", RFC 2986,
              DOI 10.17487/RFC2986, November 2000,
              <http://www.rfc-editor.org/info/rfc2986>.

   [RFC3339]  Klyne, G. and C. Newman, "Date and Time on the Internet:
              Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
              <http://www.rfc-editor.org/info/rfc3339>.

   [RFC3492]  Costello, A., "Punycode: A Bootstring encoding of Unicode
              for Internationalized Domain Names in Applications
              (IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003,
              <http://www.rfc-editor.org/info/rfc3492>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <http://www.rfc-editor.org/info/rfc3986>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <http://www.rfc-editor.org/info/rfc4648>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <http://www.rfc-editor.org/info/rfc5246>.






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   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <http://www.rfc-editor.org/info/rfc5280>.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, DOI 10.17487/RFC5890, August 2010,
              <http://www.rfc-editor.org/info/rfc5890>.

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988,
              DOI 10.17487/RFC5988, October 2010,
              <http://www.rfc-editor.org/info/rfc5988>.

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,
              <http://www.rfc-editor.org/info/rfc6066>.

   [RFC6570]  Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
              and D. Orchard, "URI Template", RFC 6570,
              DOI 10.17487/RFC6570, March 2012,
              <http://www.rfc-editor.org/info/rfc6570>.

   [RFC6844]  Hallam-Baker, P. and R. Stradling, "DNS Certification
              Authority Authorization (CAA) Resource Record", RFC 6844,
              DOI 10.17487/RFC6844, January 2013,
              <http://www.rfc-editor.org/info/rfc6844>.

   [RFC7159]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
              2014, <http://www.rfc-editor.org/info/rfc7159>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <http://www.rfc-editor.org/info/rfc7231>.

   [RFC7468]  Josefsson, S. and S. Leonard, "Textual Encodings of PKIX,
              PKCS, and CMS Structures", RFC 7468, DOI 10.17487/RFC7468,
              April 2015, <http://www.rfc-editor.org/info/rfc7468>.

   [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
              2015, <http://www.rfc-editor.org/info/rfc7515>.





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   [RFC7517]  Jones, M., "JSON Web Key (JWK)", RFC 7517,
              DOI 10.17487/RFC7517, May 2015,
              <http://www.rfc-editor.org/info/rfc7517>.

   [RFC7518]  Jones, M., "JSON Web Algorithms (JWA)", RFC 7518,
              DOI 10.17487/RFC7518, May 2015,
              <http://www.rfc-editor.org/info/rfc7518>.

   [RFC7638]  Jones, M. and N. Sakimura, "JSON Web Key (JWK)
              Thumbprint", RFC 7638, DOI 10.17487/RFC7638, September
              2015, <http://www.rfc-editor.org/info/rfc7638>.

   [RFC7807]  Nottingham, M. and E. Wilde, "Problem Details for HTTP
              APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016,
              <http://www.rfc-editor.org/info/rfc7807>.

12.2.  Informative References

   [I-D.vixie-dnsext-dns0x20]
              Vixie, P. and D. Dagon, "Use of Bit 0x20 in DNS Labels to
              Improve Transaction Identity", draft-vixie-dnsext-
              dns0x20-00 (work in progress), March 2008.

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              DOI 10.17487/RFC3552, July 2003,
              <http://www.rfc-editor.org/info/rfc3552>.

   [RFC3553]  Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An
              IETF URN Sub-namespace for Registered Protocol
              Parameters", BCP 73, RFC 3553, DOI 10.17487/RFC3553, June
              2003, <http://www.rfc-editor.org/info/rfc3553>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.

   [RFC5785]  Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
              Uniform Resource Identifiers (URIs)", RFC 5785,
              DOI 10.17487/RFC5785, April 2010,
              <http://www.rfc-editor.org/info/rfc5785>.

   [RFC6962]  Laurie, B., Langley, A., and E. Kasper, "Certificate
              Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
              <http://www.rfc-editor.org/info/rfc6962>.





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Internet-Draft                    ACME                     February 2017


   [RFC7469]  Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
              Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April
              2015, <http://www.rfc-editor.org/info/rfc7469>.

   [W3C.CR-cors-20130129]
              Kesteren, A., "Cross-Origin Resource Sharing", World Wide
              Web Consortium CR CR-cors-20130129, January 2013,
              <http://www.w3.org/TR/2013/CR-cors-20130129>.

Authors' Addresses

   Richard Barnes
   Mozilla

   Email: rlb@ipv.sx


   Jacob Hoffman-Andrews
   EFF

   Email: jsha@eff.org


   James Kasten
   University of Michigan

   Email: jdkasten@umich.edu
























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