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Versions: 01 02 03 04

Network Working Group                                          R. Barnes
Internet-Draft                                                   Mozilla
Intended status: Standards Track                      J. Hoffman-Andrews
Expires: January 22, 2016                                            EFF
                                                               J. Kasten
                                                  University of Michigan
                                                           July 21, 2015


          Automatic Certificate Management Environment (ACME)
                          draft-barnes-acme-04

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.

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
   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 January 22, 2016.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   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  . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Protocol Elements . . . . . . . . . . . . . . . . . . . . . .   9
     5.1.  HTTPS Requests  . . . . . . . . . . . . . . . . . . . . .   9
     5.2.  Registration Objects  . . . . . . . . . . . . . . . . . .  10
     5.3.  Authorization Objects . . . . . . . . . . . . . . . . . .  11
     5.4.  Errors  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     5.5.  Replay protection . . . . . . . . . . . . . . . . . . . .  14
       5.5.1.  Replay-Nonce  . . . . . . . . . . . . . . . . . . . .  14
       5.5.2.  "nonce" (Nonce) JWS header parameter  . . . . . . . .  15
     5.6.  Key Agreement . . . . . . . . . . . . . . . . . . . . . .  15
   6.  Certificate Management  . . . . . . . . . . . . . . . . . . .  16
     6.1.  Resources . . . . . . . . . . . . . . . . . . . . . . . .  16
     6.2.  Directory . . . . . . . . . . . . . . . . . . . . . . . .  18
     6.3.  Registration  . . . . . . . . . . . . . . . . . . . . . .  18
       6.3.1.  Recovery Keys . . . . . . . . . . . . . . . . . . . .  20
     6.4.  Account Recovery  . . . . . . . . . . . . . . . . . . . .  22
       6.4.1.  MAC-Based Recovery  . . . . . . . . . . . . . . . . .  23
       6.4.2.  Contact-Based Recovery  . . . . . . . . . . . . . . .  25
     6.5.  Identifier Authorization  . . . . . . . . . . . . . . . .  27
     6.6.  Certificate Issuance  . . . . . . . . . . . . . . . . . .  31
     6.7.  Certificate Revocation  . . . . . . . . . . . . . . . . .  34
   7.  Identifier Validation Challenges  . . . . . . . . . . . . . .  35
     7.1.  Simple HTTP . . . . . . . . . . . . . . . . . . . . . . .  37
     7.2.  Domain Validation with Server Name Indication (DVSNI) . .  39
     7.3.  Proof of Possession of a Prior Key  . . . . . . . . . . .  41
     7.4.  DNS . . . . . . . . . . . . . . . . . . . . . . . . . . .  44
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  46
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  46
     9.1.  Threat model  . . . . . . . . . . . . . . . . . . . . . .  47
     9.2.  Integrity of Authorizations . . . . . . . . . . . . . . .  48
     9.3.  Preventing Authorization Hijacking  . . . . . . . . . . .  50
     9.4.  Denial-of-Service Considerations  . . . . . . . . . . . .  52
     9.5.  CA Policy Considerations  . . . . . . . . . . . . . . . .  52



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   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  53
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  53
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  53
     11.2.  Informative References . . . . . . . . . . . . . . . . .  55
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  55

1.  Introduction

   Certificates in the Web PKI are most commonly used to authenticate
   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.

   Existing Web PKI certificate authorities tend to run on a set of ad
   hoc protocols for certificate issuance and identity verification.  A
   typical user experience is something like:

   o  Generate a PKCS#10 [RFC2314] 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
   systems because it inhibits mechanization of tasks related to
   certificate issuance, deployment, and revocation.



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   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 major guiding use case for ACME is obtaining certificates for Web
   sites (HTTPS [RFC2818]).  In that case, the server is intended to
   speak for one or more domains, and the process of certificate
   issuance is intended to verify that the server actually speaks for
   the domain.

   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.)

   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.

   When an operator deploys a current HTTPS server, it generally prompts
   him to generate a self-signed certificate.  When an operator deploys
   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.



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   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.

   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.

   The overall idea is that it's nearly as easy to deploy with a CA-
   issued certificate as a self-signed certificate, and that once the
   operator has done so, the process is self-sustaining with minimal
   manual intervention.  Close integration of ACME with HTTPS servers,
   for example, can allow the immediate and automated deployment of
   certificates as they are issued, optionally sparing the human
   administrator from additional configuration work.

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.

   For simplicity, in all HTTPS transactions used by ACME, the ACME
   client is the HTTPS client and the ACME server is the HTTPS server.

   In the discussion below, we will refer to three different types of
   keys / key pairs:

   Subject Public Key:  A public key to be included in a certificate.

   Account Key Pair:  A key pair for which the ACME server considers the
      holder of the private key authorized to manage certificates for a
      given identifier.  The same key pair may be authorized for
      multiple identifiers.





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   Recovery Key:  A MAC key that a client can use to demonstrate that it
      participated in a prior registration transaction.

   ACME messaging is based on HTTPS [RFC2818] and JSON [RFC7159].  Since
   JSON is a text-based format, binary fields are Base64-encoded.  For
   Base64 encoding, we use the variant defined in [RFC7515].  The
   important features of this encoding are (1) that it uses the URL-safe
   character set, and (2) that "=" padding characters are stripped.

   Some HTTPS bodies in ACME are authenticated and integrity-protected
   by being encapsulated in a JSON Web Signature (JWS) object [RFC7515].
   ACME uses a profile of JWS, with the following restrictions:

   o  The JWS MUST use the Flattened JSON Serialization

   o  The JWS MUST be encoded using UTF-8

   o  The JWS Header or Protected Header MUST include "alg" and "jwk"
      fields

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

   Additionally, JWS objects used in ACME MUST include the "nonce"
   header parameter, defined below.

4.  Protocol Overview

   ACME allows a client to request certificate management actions using
   a set of JSON messages carried over HTTPS.  In some ways, ACME
   functions much like a traditional CA, in which a user creates an
   account, adds identifiers to that account (proving control of the
   domains), and requests certificate issuance for those domains while
   logged in to the account.

   In ACME, the account is represented by an account key pair.  The "add
   a domain" function is accomplished by authorizing the key pair for a
   given domain.  Certificate issuance and revocation are authorized by
   a signature with the key pair.

   The first phase of ACME is for the client to register with the ACME
   server.  The client generates an asymmetric key pair and associates
   this key pair with a set of contact information by signing the
   contact information.  The server acknowledges the registration by
   replying with a registration object echoing the client's input.







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         Client                                                  Server

         Contact Information
         Signature                     ------->

                                       <-------            Registration

   Before a client can issue certificates, it must establish an
   authorization with the server for an account key pair to act for the
   identifier(s) that it wishes to include in the certificate.  To do
   this, the client must demonstrate to the server both (1) that it
   holds the private key of the account key pair, and (2) that it has
   authority over the identifier being claimed.

   Proof of possession of the account key is built into the ACME
   protocol.  All messages from the client to the server are signed by
   the client, and the server verifies them using the public key of the
   account key pair.

   To verify that the client controls the identifier being claimed, the
   server issues the client a set of challenges.  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.

   For example, if the client requests a domain name, the server might
   challenge the client to provision a record in the DNS under that
   name, or to provision a file on a web server referenced by an A or
   AAAA record under that name.  The server would then query the DNS for
   the record in question, or send an HTTP request for the file.  If the
   client provisioned the DNS or the web server as expected, then the
   server considers the client authorized for the domain name.















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         Client                                                  Server

         Identifier
         Signature                     ------->

                                       <-------              Challenges

         Responses
         Signature                     ------->

                                       <-------       Updated Challenge

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

         Poll                          ------->

                                       <-------           Authorization

   Once the client has authorized an account key pair for an identifier,
   it can use the key pair to authorize the issuance of certificates for
   the identifier.  To do this, the client sends a PKCS#10 Certificate
   Signing Request (CSR) to the server (indicating the identifier(s) to
   be included in the issued certificate) and a signature over the CSR
   by the private key of the account key pair.

   If the server agrees to issue the certificate, then it creates the
   certificate and provides it in its response.  The certificate is
   assigned a URI, which the client can use to fetch updated versions of
   the certificate.

         Client                                                 Server

         CSR
         Signature                    -------->

                                      <--------            Certificate

   To revoke a certificate, the client simply sends a revocation
   request, signed with an authorized key pair, and the server indicates
   whether the request has succeeded.

         Client                                                 Server

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

                                      <--------                 Result




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   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 protocols 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.  Protocol Elements

   This section describes several components that are used by ACME, and
   general rules that apply to ACME transactions.

5.1.  HTTPS Requests

   Each ACME function is accomplished by the client sending a sequence
   of HTTPS requests to the server, carrying JSON messages.  Use of
   HTTPS is REQUIRED.  Clients SHOULD support HTTP public key pinning
   [RFC7469], and servers SHOULD emit pinning headers.  Each subsection
   of Section 6 below describes the message formats used by the
   function, and the order in which messages are sent.

   All ACME requests with a non-empty body MUST encapsulate the body in
   a JWS object, signed using the account key pair.  The server MUST
   verify the JWS before processing the request.  (For readability,
   however, the examples below omit this encapsulation.)  Encapsulating
   request bodies in JWS provides a simple authentication of requests by
   way of key continuity.

   Note that this implies that GET requests are not authenticated.
   Servers MUST NOT respond to GET requests for resources that might be
   considered sensitive.

   An ACME request carries a JSON dictionary that provides the details
   of the client's request to the server.  In order to avoid attacks
   that might arise from sending a request object to a resource of the
   wrong type, each request object MUST have a "resource" field that
   indicates what type of resource the request is addressed to, as
   defined in the below table:










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                +----------------------+------------------+
                | Resource type        | "resource" value |
                +----------------------+------------------+
                | New registration     | new-reg          |
                |                      |                  |
                | Recover registration | recover-reg      |
                |                      |                  |
                | New authorization    | new-authz        |
                |                      |                  |
                | New certificate      | new-cert         |
                |                      |                  |
                | Revoke certificate   | revoke-cert      |
                |                      |                  |
                | Registration         | reg              |
                |                      |                  |
                | Authorization        | authz            |
                |                      |                  |
                | Challenge            | challenge        |
                |                      |                  |
                | Certificate          | cert             |
                +----------------------+------------------+

   Other fields in ACME request bodies are described below.

   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 "*".

5.2.  Registration Objects

   An ACME registration resource represents a set of metadata associated
   to an account key pair.  Registration resources have the following
   structure:

   key (required, dictionary):  The public key of the account key pair,
      encoded as a JSON Web Key object [RFC7517].

   contact (optional, array of string):  An array of URIs that the
      server can use to contact the client for issues related to this
      authorization.  For example, the server may wish to notify the
      client about server-initiated revocation.

   agreement (optional, string):  A URI referring to a subscriber
      agreement or terms of service provided by the server (see below).
      Including this field indicates the client's agreement with the
      referenced terms.




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   authorizations (optional, string):  A URI from which a list of
      authorizations granted to this account can be fetched via a GET
      request.  The result of the GET request MUST be a JSON object
      whose "authorizations" field is an array of strings, where each
      string is the URI of an authorization belonging to this
      registration.  The server SHOULD include pending authorizations,
      and SHOULD NOT include authorizations that are invalid or expired.

   certificates (optional, string):  A URI from which a list of
      certificates issued for this account can be fetched via a GET
      request.  The result of the GET request MUST be a JSON object
      whose "certificates" field is an array of strings, where each
      string is the URI of a certificate.  The server SHOULD NOT include
      expired certificates.

   {
     "resource": "new-reg",
     "contact": [
       "mailto:cert-admin@example.com",
       "tel:+12025551212"
     ],
     "agreement": "https://example.com/acme/terms",
     "authorizations": "https://example.com/acme/reg/1/authz",
     "certificates": "https://example.com/acme/reg/1/cert",
   }

5.3.  Authorization Objects

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




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   expires (optional, string):  The date after which the server will
      consider this authorization invalid, encoded in the format
      specified in RFC 3339 [RFC3339].

   challenges (required, array):  The challenges that the client needs
      to 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.

   combinations (optional, array of arrays of integers):  A collection
      of sets of challenges, each of which would be sufficient to prove
      possession of the identifier.  Clients complete a set of
      challenges that that covers at least one set in this array.
      Challenges are identified by their indices in the challenges
      array.  If no "combinations" element is included in an
      authorization object, the client completes all challenges.

   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.  Wildcard domain
   names (with "*" as the first label) MUST NOT be included in
   authorization requests.  See Section 6.6 below for more information
   about wildcard domains.

   {
     "status": "valid",
     "expires": "2015-03-01",

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

     "challenges": [
       {
         "type": "simpleHttp",
         "status": "valid",
         "validated": "2014-12-01T12:05Z",
         "token": "IlirfxKKXAsHtmzK29Pj8A"
         "path": "Hf5GrX4Q7EBax9hc2jJnfw"
       }
     ],
   }







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5.4.  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
   [I-D.ietf-appsawg-http-problem].  The "type" and "detail" fields MUST
   be populated.  To facilitate automatic response to errors, this
   document defines the following standard tokens for use in the "type"
   field (within the "urn:acme:" namespace):

   +----------------+--------------------------------------------------+
   | Code           | Semantic                                         |
   +----------------+--------------------------------------------------+
   | badCSR         | The CSR is unacceptable (e.g., due to a short    |
   |                | key)                                             |
   |                |                                                  |
   | badNonce       | The client sent an unacceptable anti-replay      |
   |                | nonce                                            |
   |                |                                                  |
   | connection     | The server could not connect to the client for   |
   |                | DV                                               |
   |                |                                                  |
   | dnssec         | The server could not validate a DNSSEC signed    |
   |                | domain                                           |
   |                |                                                  |
   | malformed      | The request message was malformed                |
   |                |                                                  |
   | serverInternal | The server experienced an internal error         |
   |                |                                                  |
   | tls            | The server experienced a TLS error during DV     |
   |                |                                                  |
   | unauthorized   | The client lacks sufficient authorization        |
   |                |                                                  |
   | unknownHost    | The server could not resolve a domain name       |
   +----------------+--------------------------------------------------+

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

   TODO: Flesh out errors and syntax for them





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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 MUST include a Replay-Nonce header field in each
   successful response it provides to a client, with contents as
   specified below.  In particular, the ACME server MUST provide a
   Replay-Nonce header field in response to a HEAD request for any valid
   resource.  (This allows clients to easily obtain a fresh nonce.)  It
   MAY also provide nonces in error responses.

   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 SHOULD provide HTTP status code 400
   (Bad Request), and indicate the ACME error code "urn:acme:badNonce".

   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.

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




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   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.  Key Agreement

   Certain elements of the protocol will require the establishment of a
   shared secret between the client and the server, in such a way that
   an entity observing the ACME protocol cannot derive the secret.  In
   these cases, we use a simple ECDH key exchange, based on the system
   used by CMS [RFC5753]:

   o  Inputs:

      *  Client-generated key pair

      *  Server-generated key pair

      *  Length of the shared secret to be derived

      *  Label

   o  Perform the ECDH primitive operation to obtain Z (Section 3.3.1 of
      [SEC1])

   o  Select a hash algorithm according to the curve being used:

      *  For "P-256", use SHA-256

      *  For "P-384", use SHA-384

      *  For "P-521", use SHA-512

   o  Derive the shared secret value using the KDF in Section 3.6.1 of
      [SEC1] using Z and the selected hash algorithm, and with the UTF-8
      encoding of the label as the SharedInfo value




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   In cases where the length of the derived secret is shorter than the
   output length of the chosen hash algorithm, the KDF referenced above
   reduces to a single hash invocation.  The shared secret is equal to
   the leftmost octets of the following:

   H( Z || 00000001 || label )

6.  Certificate Management

   In this section, we describe the certificate management functions
   that ACME enables:

   o  Account Key Registration

   o  Account Recovery

   o  Account Key Authorization

   o  Certificate Issuance

   o  Certificate Renewal

   o  Certificate Revocation

6.1.  Resources

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

   o  Registration resources, representing information about an account

   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-registration" resource

   o  A "new-authorization" resource

   o  A "new-certificate" resource

   o  A "revoke-certificate" resource



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   For the "new-X" resources above, the server MUST have exactly one
   resource for each function.  This resource may be addressed by
   multiple URIs, but all must provide equivalent functionality.

   In general, the intent is for authorization and certificate resources
   to contain only public information, so that CAs may publish these
   resources to document what certificates have been issued and how they
   were authorized.  Non-public information, such as contact
   information, is stored in registration 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.

   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 following diagram illustrates the relations between resources on
   an ACME server.  The solid lines indicate link relations, and the
   dotted lines correspond to relationships expressed in other ways,
   e.g., the Location header in a 201 (Created) response.

                                  directory
                                      .
                                      .
          ....................................................
          .                  .                  .            .
          .                  .                  .            .
          V     "next"       V      "next"      V            V
       new-reg ---+----> new-authz ---+----> new-cert    revoke-cert
          .       |          .        |         .            ^
          .       |          .        |         .            | "revoke"
          V       |          V        |         V            |
         reg* ----+        authz -----+       cert-----------+
                            . ^                 |
                            . | "up"            | "up"
                            V |                 V
                          challenge         cert-chain

   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.




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          +--------------------+----------------+--------------+
          | Action             | Request        | Response     |
          +--------------------+----------------+--------------+
          | Register           | POST new-reg   | 201 -> reg   |
          |                    |                |              |
          | Request challenges | POST new-authz | 201 -> authz |
          |                    |                |              |
          | Answer challenges  | POST challenge | 200          |
          |                    |                |              |
          | Poll for status    | GET  authz     | 200          |
          |                    |                |              |
          | Request issuance   | POST new-cert  | 201 -> cert  |
          |                    |                |              |
          | 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.

6.2.  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 root URL with which clients are configured.  It is a
   JSON dictionary, whose keys are the "resource" values listed in
   Section 5.1, and whose values are the URIs used to accomplish the
   corresponding function.

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

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

   {
     "new-reg": "https://example.com/acme/new-reg",
     "recover-reg": "https://example.com/acme/recover-reg",
     "new-authz": "https://example.com/acme/new-authz",
     "new-cert": "https://example.com/acme/new-cert",
     "revoke-cert": "https://example.com/acme/revoke-cert"
   }

6.3.  Registration

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



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   POST /acme/new-registration HTTP/1.1
   Host: example.com

   {
     "resource": "new-reg",
     "contact": [
       "mailto:cert-admin@example.com",
       "tel:+12025551212"
     ],
   }
   /* Signed as JWS */

   The server MUST ignore any values provided in the "key",
   "authorizations", and "certificates" fields in registration 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.

   The server creates a registration object with the included contact
   information.  The "key" element of the registration is set to the
   public key used to verify the JWS (i.e., the "jwk" element of the JWS
   header).  The server returns this registration object in a 201
   (Created) response, with the registration URI in a Location header
   field.  The server MUST also indicate its new-authorization URI using
   the "next" link relation.

   If the server already has a registration object with the provided
   account key, then it MUST return a 409 (Conflict) response and
   provide the URI of that registration in a Location header field.
   This allows a client that has an account key but not the
   corresponding registration URI to recover the registration 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 a Link header with link relation "terms-of-
   service".  As noted above, the client may indicate its agreement with
   these terms by updating its registration to include the "agreement"
   field, with the terms URI as its value.












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   HTTP/1.1 201 Created
   Content-Type: application/json
   Location: https://example.com/acme/reg/asdf
   Link: <https://example.com/acme/new-authz>;rel="next"
   Link: <https://example.com/acme/recover-reg>;rel="recover"
   Link: <https://example.com/acme/terms>;rel="terms-of-service"

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

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

   If the client wishes to update this information in the future, it
   sends a POST request with updated information to the registration
   URI.  The server MUST ignore any updates to the "key",
   "authorizations, or "certificates" fields, and MUST verify that the
   request is signed with the private key corresponding to the "key"
   field of the request before updating the registration.

   Servers SHOULD NOT respond to GET requests for registration 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 ({"resource":"reg"}).

6.3.1.  Recovery Keys

   If the client wishes to establish a secret key with the server that
   it can use to recover this account later (a "recovery key"), then it
   must perform a simple key agreement protocol as part of the new-
   registration transaction.  The client and server perform an ECDH
   exchange through the new-registration transaction (using the
   technique in Section 5.6), and the result is the recovery key.

   To request a recovery key, the client includes a "recoveryKey" field
   in its new-registration request.  The value of this field is a JSON
   object.

   client (required, JWK):  The client's ECDH public key

   length (required, number):  The length of the derived secret, in
      octets.




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   In the client's request, this object contains a JWK for a random ECDH
   public key generated by the client and the client-selected length
   value.  Clients need to choose length values that balance security
   and usability.  On the one hand, a longer secret makes it more
   difficult for an attacker to recover the secret when it is used for
   recovery (see Section 6.4.1).  On the other hand, clients may wish to
   make the recovery key short enough for a user to easily write it
   down.

   POST /acme/new-registration HTTP/1.1
   Host: example.com

   {
     "resource": "new-reg",
     "contact": [
       "mailto:cert-admin@example.com",
       "tel:+12025551212"
     ],
     "recoveryKey": {
       "client": { "kty": "EC", ... },
       "length": 128
     }
   }
   /* Signed as JWS */

   The server MUST validate that the elliptic curve ("crv") and length
   value chosen by the client are acceptable, and that it is otherwise
   willing to create a recovery key.  If not, then it MUST reject the
   new-registration request.

   If the server agrees to create a recovery key, then it generates its
   own random ECDH key pair and combines it with with the client's
   public key as described in Section 5.6 above, using the label
   "recovery".  The derived secret value is the recovery key.  The
   server then returns to the client the ECDH key that it generated.
   The server MUST generate a fresh key pair for every transaction.

   server (required, JWK):  The server's ECDH public key













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   HTTP/1.1 201 Created
   Content-Type: application/json
   Location: https://example.com/acme/reg/asdf

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

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

     "recoveryKey": {
       "server": { "kty": "EC", ... }
     }
   }

   On receiving the server's response, the client can compute the
   recovery key by combining the server's public key together with the
   private key corresponding to the public key that it sent to the
   server.

   Clients may refresh the recovery key associated with a registration
   by sending a POST request with a new recoveryKey object.  If the
   server agrees to refresh the recovery key, then it responds in the
   same way as to a new registration request that asks for a recovery
   key.

   POST /acme/reg/asdf HTTP/1.1
   Host: example.com

   {
     "resource": "reg",
     "recoveryKey": {
       "client": { "kty": "EC", ... }
     }
   }
   /* Signed as JWS */

6.4.  Account Recovery

   Once a client has created an account with an ACME server, it is
   possible that the private key for the account will be lost.  The
   recovery contacts included in the registration allows the client to
   recover from this situation, as long as it still has access to these
   contacts.





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   By "recovery", we mean that the information associated with an old
   account key is bound to a new account key.  When a recovery process
   succeeds, the server provides the client with a new registration
   whose contents are the same as base registration object - except for
   the "key" field, which is set to the new account key.  The server
   reassigns resources associated with the base registration to the new
   registration (e.g., authorizations and certificates).  The server
   SHOULD delete the old registration resource after it has been used as
   a base for recovery.

   In addition to the recovery mechanisms defined by ACME, individual
   client implementations may also offer implementation-specific
   recovery mechanisms.  For example, if a client creates account keys
   deterministically from a seed value, then this seed could be used to
   recover the account key by re-generating it.  Or an implementation
   could escrow an encrypted copy of the account key with a cloud
   storage provider, and give the encryption key to the user as a
   recovery value.

6.4.1.  MAC-Based Recovery

   With MAC-based recovery, the client proves to the server that it
   holds a secret value established in the initial registration
   transaction.  The client requests MAC-based recovery by sending a MAC
   over the new account key, using the recovery key from the initial
   registration.

   method (required, string):  The string "mac"

   base (required, string):  The URI for the registration to be
      recovered.

   mac (required, string):  A JSON-formatted JWS object using an HMAC
      algorithm, whose payload is the JWK representation of the public
      key of the new account key pair.
















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   POST /acme/recover-reg HTTP/1.1
   Host: example.com

   {
     "resource": "recover-reg",
     "method": "mac",
     "base": "https://example.com/acme/reg/asdf",
     "mac": {
       "header": { "alg": "HS256" },
       "payload": base64(JWK(newAccountKey)),
       "signature": "5wUrDI3eAaV4wl2Rfj3aC0Pp--XB3t4YYuNgacv_D3U"
     }
   }
   /* Signed as JWS, with new account key */

   On receiving such a request the server MUST verify that:

   o  The base registration has a recovery key associated with it

   o  The "alg" value in the "mac" JWS represents a MAC algorithm

   o  The "mac" JWS is valid according to the validation rules in
      [RFC7515], using the recovery key as the MAC key

   o  The JWK in the payload represents the new account key (i.e. the
      key used to verify the ACME message)

   If those conditions are met, and the recovery request is otherwise
   acceptable to the server, then the recovery process has succeeded.
   The server creates a new registration resource based on the base
   registration and the new account key, and returns it on a 201
   (Created) response, together with a Location header indicating a URI
   for the new registration.  If the recovery request is unsuccessful,
   the server returns an error response, such as 403 (Forbidden).

















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   HTTP/1.1 201 Created
   Content-Type: application/json
   Location: https://example.com/acme/reg/asdf
   Link: <https://example.com/acme/new-authz>;rel="next"
   Link: <https://example.com/acme/recover-reg>;rel="recover"
   Link: <https://example.com/acme/terms>;rel="terms-of-service"

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

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

     "authorizations": "...",
     "certificate": "..."
   }

6.4.2.  Contact-Based Recovery

   In the contact-based recovery process, the client requests that the
   server send a message to one of the contact URIs registered for the
   account.  That message indicates some action that the server requires
   the client's user to perform, e.g., clicking a link in an email.  If
   the user successfully completes the server's required actions, then
   the server will bind the account to the new account key.

   (Note that this process is almost entirely out of band with respect
   to ACME.  ACME only allows the client to initiate the process, and
   the server to indicate the result.)

   To initiate contact-based recovery, the client sends a POST request
   to the server's recover-registration URI, with a body specifying
   which registration is to be recovered.  The body of the request MUST
   be signed by the client's new account key pair.

   method (required, string):  The string "contact"

   base (required, string):  The URI for the registration to be
      recovered.










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   POST /acme/recover-reg HTTP/1.1
   Host: example.com

   {
     "resource": "recover-reg",
     "method": "contact",
     "base": "https://example.com/acme/reg/asdf",
     "contact": [
       "mailto:forgetful@example.net"
     ]
   }
   /* Signed as JWS, with new account key */

   If the server agrees to attempt contact-based recovery, then it
   creates a new registration resource containing a stub registration
   object.  The stub registration has the client's new account key and
   contacts, but no authorizations or certificates associated.  The
   server returns the stub contact in a 201 (Created) response, along
   with a Location header field indicating the URI for the new
   registration resource (which will be the registration URI if the
   recovery succeeds).

   HTTP/1.1 201 Created
   Content-Type: application/json
   Location: https://example.com/acme/reg/qwer

   {
     "key": { /* new account key from JWS header */ },

     "contact": [
       "mailto:forgetful@example.net"
     ]
   }

   After recovery has been initiated, the server follows its chosen
   recovery process, out-of-band to ACME.  While the recovery process is
   ongoing, the client may poll the registration resource's URI for
   status, by sending a POST request with a trivial body
   ({"resource":"reg"}).  If the recovery process is still pending, the
   server sends a 202 (Accepted) status code, and a Retry-After header
   field.  If the recovery process has failed, the server sends an error
   code (e.g., 404), and SHOULD delete the stub registration resource.

   If the recovery process has succeeded, then the server will send a
   200 (OK) response, containing the full registration object, with any
   necessary information copied from the old registration).  The client
   may now use this in the same way as if he had gotten it from a new-
   registration transaction.



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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 holds 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).

   As illustrated by the figure in the overview section above, the
   authorization process proceeds in two phases.  The client first
   requests a new authorization, and the server issues challenges, then
   the client responds to those challenges and the server validates the
   client's responses.

   To begin the key authorization process, the client sends a POST
   request to the server's new-authorization resource.  The body of the
   POST request MUST contain a JWS object, whose payload is a partial
   authorization object.  This JWS object MUST contain only the
   "identifier" field, so that the server knows what identifier is being
   authorized.  The server MUST ignore any other fields present in the
   client's request object.

   The authorization object is implicitly tied to the account key used
   to sign the request.  Once created, the authorization may only be
   updated by that account.

   POST /acme/new-authorization HTTP/1.1
   Host: example.com

   {
     "resource": "new-authz",
     "identifier": {
       "type": "dns",
       "value": "example.org"
     }
   }
   /* Signed as JWS */

   Before processing the authorization further, 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



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   (Forbidden) error, with a problem document describing the reason for
   the rejection.

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

   o  "identifier" the identifier submitted by the client.

   o  "status": MUST be "pending"

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

   o  The "expires" field MUST be absent.

   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.
































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   HTTP/1.1 201 Created
   Content-Type: application/json
   Location: https://example.com/authz/asdf
   Link: <https://example.com/acme/new-cert>;rel="next"

   {
     "status": "pending",

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

     "challenges": [
       {
         "type": "simpleHttp",
         "uri": "https://example.com/authz/asdf/0",
         "token": "IlirfxKKXAsHtmzK29Pj8A"
       },
       {
         "type": "dns",
         "uri": "https://example.com/authz/asdf/1"
         "token": "DGyRejmCefe7v4NfDGDKfA"
       }
     },

     "combinations": [
       [0, 2],
       [1, 2]
     ]
   }

   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.  For example, if the
   client wishes to complete the "simpleHttp" challenge, it needs to
   provide the "path" component.  (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 URI
   or the new-authorization URI).  This allows the client to send
   information only for challenges it is responding to.






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

   POST /acme/authz/asdf/0 HTTP/1.1
   Host: example.com

   {
     "resource": "challenge",
     "type": "simpleHttp",
     "path": "Hf5GrX4Q7EBax9hc2jJnfw"
   }
   /* Signed as JWS */

   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.

   Presumably, the client's responses provide the server with enough
   information to validate one or more challenges.  The server is said
   to "finalize" the authorization when it has completed all the
   validations it is going to complete, and assigns 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", the server MUST add an "expires" field to the authorization.
   When finalizing an authorization, the server MAY remove the
   "combinations" field (if present) or remove any challenges still
   pending.  The server SHOULD NOT remove challenges with status
   "invalid".

   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 with a Retry-After header field.






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

   HTTP/1.1 200 OK

   {
     "status": "valid",
     "expires": "2015-03-01",

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

     "challenges": [
       {
         "type": "simpleHttp"
         "status": "valid",
         "validated": "2014-12-01T12:05Z",
         "token": "IlirfxKKXAsHtmzK29Pj8A"
         "path": "Hf5GrX4Q7EBax9hc2jJnfw"
       }
     ]
   }

6.6.  Certificate Issuance

   The holder of an authorized key pair for an identifier may use ACME
   to request that a certificate be issued for that identifier.  The
   client makes this request by sending a POST request to the server's
   new-certificate resource.  The body of the POST is a JWS object whose
   JSON payload contains a Certificate Signing Request (CSR) [RFC2986].
   The CSR encodes the parameters of the requested certificate;
   authority to issue is demonstrated by the JWS signature by an account
   key, from which the server can look up related authorizations.

   csr (required, string):  A CSR encoding the parameters for the
      certificate being requested.  The CSR is sent in the
      Base64-encoded version of the DER format.  (Note: This field uses
      the same modified Base64-encoding rules used elsewhere in this
      document, so it is different from PEM.)










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   POST /acme/new-cert HTTP/1.1
   Host: example.com
   Accept: application/pkix-cert

   {
     "resource": "new-cert",
     "csr": "5jNudRx6Ye4HzKEqT5...FS6aKdZeGsysoCo4H9P",
   }
   /* Signed as JWS */

   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 values provided in the CSR are only a request, and are not
   guaranteed.  The server or CA may alter any fields in the certificate
   before issuance.  For example, the CA may remove identifiers that are
   not authorized for the account key that signed the request.

   It is up to the server's local policy to decide which names are
   acceptable in a certificate, given the authorizations that the server
   associates with the client's account key.  A server MAY consider a
   client authorized for a wildcard domain if it is authorized for the
   underlying domain name (without the "*" label).  Servers SHOULD NOT
   extend authorization across identifier types.  For example, if a
   client is authorized for "example.com", then the server should not
   allow the client to issue a certificate with an iPAddress
   subjectAltName, even if it contains an IP address to which
   example.com resolves.

   If the CA decides to issue a certificate, then the server creates a
   new certificate resource and returns a URI for it in the Location
   header field of a 201 (Created) response.

   HTTP/1.1 201 Created
   Location: https://example.com/acme/cert/asdf

   If the certificate is available at the time of the response, it is
   provided in the body of the response.  If the CA has not yet issued
   the certificate, the body of this response will be empty.  The client
   should then send a GET request to the certificate URI to poll for the
   certificate.  As long as the certificate is unavailable, the server
   MUST provide a 202 (Accepted) response and include a Retry-After
   header to indicate when the server believes the certificate will be
   issued (as in the example above).




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

   HTTP/1.1 202 Accepted
   Retry-After: 120

   The default format of the certificate is DER (application/pkix-cert).
   The client may request other formats by including an Accept header in
   its request.

   The server provides metadata about the certificate in HTTP headers.
   In particular, the server MUST include a Link relation header field
   [RFC5988] with relation "up" to provide a certificate under which
   this certificate was issued, and one with relation "author" to
   indicate the registration under which this certicate was issued.  The
   server MAY also include an Expires header as a hint to the client
   about when to renew the certificate.  (Of course, the real expiration
   of the certificate is controlled by the notAfter time in the
   certificate itself.)

   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/reg/asdf>;rel="author"
   Location: https://example.com/acme/cert/asdf
   Content-Location: https://example.com/acme/cert-seq/12345

   [DER-encoded certificate]

   A certificate resource always represents the most recent certificate
   issued for the name/key binding expressed in the CSR.  If the CA
   allows a certificate to be renewed, then it publishes renewed
   versions of the certificate through the same certificate URI.

   Clients retrieve renewed versions of the certificate using a GET
   query to the certificate URI, which the server should then return in
   a 200 (OK) response.  The server SHOULD provide a stable URI for each
   specific certificate in the Content-Location header field, as shown
   above.  Requests to stable certificate URIs MUST always result in the
   same certificate.





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   To avoid unnecessary renewals, the CA may choose not to issue a
   renewed certificate until it receives such a request (if it even
   allows renewal at all).  In such cases, if the CA requires some time
   to generate the new certificate, the CA MUST return a 202 (Accepted)
   response, with a Retry-After header field that indicates when the new
   certificate will be available.  The CA MAY include the current (non-
   renewed) certificate as the body of the response.

   Likewise, in order to prevent unnecessary renewal due to queries by
   parties other than the account key holder, certificate URIs should be
   structured as capability URLs [W3C.WD-capability-urls-20140218].

   From the client's perspective, there is no difference between a
   certificate URI that allows renewal and one that does not.  If the
   client wishes to obtain a renewed certificate, and a GET request to
   the certificate URI does not yield one, then the client may initiate
   a new-certificate transaction to request one.

6.7.  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 Base64-encoded version of the DER format.  (Note: This field
      uses the same modified Base64-encoding rules used elsewhere in
      this document, so it is different from PEM.)

   POST /acme/revoke-cert HTTP/1.1
   Host: example.com

   {
     "resource": "revoke-cert",
     "certificate": "MIIEDTCCAvegAwIBAgIRAP8..."
   }
   /* Signed as JWS */

   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 at least one of these conditions applies:

   o  the public key of the key pair signing the request matches the
      public key in the certificate.





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   o  the key pair signing the request is an account key, and the
      corresponding account is authorized to act for all of the
      identifier(s) in the certificate.

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

   HTTP/1.1 200 OK
   Content-Length: 0

   --- or ---

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

   {
     "type": "urn: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 identifier 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 the an account key
   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.

   To accommodate this reality, ACME includes an extensible challenge/
   response framework for identifier validation.  This section describes
   an initial set of Challenge types.  Each challenge must describe:




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   o  Content of Challenge payloads (in Challenge messages)

   o  Content of Response payloads (in authorizationRequest messages)

   o  How the server uses the Challenge and Response to verify control
      of an identifier

   The general structure of Challenge and Response payloads is as
   follows:

   type (required, string):  The type of Challenge or Response encoded
      in the object.

   uri (required, string):  The URI to which a response can be posted.

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

   validated (optional, string): : The time at which this challenge was
   completed by the server, encoded in the format specified in RFC 3339
   [RFC3339].

   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 [I-D.ietf-appsawg-http-problem].

   All additional fields are specified by the Challenge type.  The
   server MUST ignore any values provided in the "uri", "status",
   "validated", and "error" fields of a Response payload.  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
   Simple HTTP and DVSNI, the client directly proves its ability to do
   certain things related to the identifier.  In the Proof of Possession
   challenge, the client proves historical control of the identifier, by
   reference to a prior authorization transaction or certificate.

   The choice of which Challenges to offer to a client under which
   circumstances is a matter of server policy.  A CA may choose
   different sets of challenges depending on whether it has interacted
   with a domain before, and how.  For example:





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   o  New domain with no known certificates: Domain Validation (DVSNI or
      Simple HTTP)

   o  Domain for which known certs exist from other CAs: DV + Proof of
      Possession of previous CA-signed key

   o  Domain with a cert from this CA, lost account key: DV + PoP of
      ACME-certified Subject key

   o  Domain with a cert from this CA, all keys and recovery mechanisms
      lost: Out of band proof of authority for the domain

   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.

7.1.  Simple HTTP

   With Simple 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 with a specific
   string as its contents.

   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.  The HTTP server may be made available
   over either HTTPS or unencrypted HTTP; the client tells the server in
   its response which to check.

   type (required, string):  The string "simpleHttp"

   token (required, string):  The value to be provisioned in the file.
      This value MUST have at least 128 bits of entropy, in order to
      prevent an attacker from guessing it.  It MUST NOT contain any
      non-ASCII characters.

   {
     "type": "simpleHttp",
     "token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ+PCt92wr+oA"
   }

   A client responds to this challenge by signing a JWS object and
   provisioning it as a resource on the HTTP server for the domain in
   question.  The payload of the JWS MUST be a JSON dictionary
   containing the fields "type", "token", "path", and "tls" from the



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   ACME challenge and response (see below), and no other fields.  If the
   "tls" field is not included in the response, then validation object
   MUST have its "tls" field set to "true".  The JWS MUST be signed with
   the client's account key pair.  This JWS is NOT REQUIRED to have a
   "nonce" header parameter (as with the JWS objects that carry ACME
   request objects), but MUST otherwise meet the guidelines laid out in
   Section 3.

   {
     "type": "simpleHttp",
     "token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA",
     "tls": false
   }

   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.

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

   The client's response to this challenge indicates whether it would
   prefer for the validation request to be sent over TLS:

   type (required, string):  The string "simpleHttp"

   tls (optional, boolean, default true):  If this attribute is present
      and set to "false", the server will perform its validation check
      over unencrypted HTTP (on port 80) rather than over HTTPS.
      Otherwise the check will be done over HTTPS, on port 443.

   {
     "type": "simpleHttp",
     "token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA",
     "tls": false
   }
   /* Signed as JWS */

   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]
       "{scheme}://{domain}/.well-known/acme-challenge/{token}", where:

       *  the scheme field is set to "http" if the "tls" field in the
          response is present and set to false, and "https" otherwise;

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



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       *  the token field is the token provided in the challenge.

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

   3.  Dereference the URI using an HTTP or HTTPS GET request.  If using
       HTTPS, the ACME server MUST ignore the certificate provided by
       the HTTPS server.

   4.  Verify that the Content-Type header of the response is either
       absent, or has the value "application/jose+json"

   5.  Verify that the body of the response is a valid JWS of the type
       indicated by the Content-Type header (if present), signed with
       the client's account key

   6.  Verify that the payload of the JWS meets the following criteria:

       *  It is a valid JSON dictionary

       *  It has exactly four fields

       *  Its "type" field is set to "simpleHttp"

       *  Its "token" field is equal to the "token" field in the
          challenge

       *  Its "path" field is equal to the "path" field in the response

       *  Its "tls" field is equal to the "tls" field in the response,
          or "true" if the "tls" field was absent

   Comparisons of the "path" and "token" fields MUST be performed in
   terms of Unicode code points, taking into account the encodings of
   the stored nonce and the body of the request.

   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.

7.2.  Domain Validation with Server Name Indication (DVSNI)

   The Domain Validation with Server Name Indication (DVSNI) 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.



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   type (required, string):  The string "dvsni"

   token (required, string):  A random value with at least 128 bits of
      entropy, base64-encoded

   {
     "type": "dvsni",
     "token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJyPCt92wrDoA",
   }

   In response to the challenge, the client uses its account private key
   to sign a JWS over a JSON object describing the challenge.  The
   validation object covered by the signature MUST have the following
   fields and no others:

   type (required, string):  The string "dvsni"

   token (required, string):  A random value with at least 128 bits of
      entropy, base64-encoded

   {
     "type": "dvsni",
     "token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJyPCt92wrDoA",
   }

   The client serializes the validation object to UTF-8, then uses its
   account private key to sign a JWS with the serialized JSON object as
   its payload.  This JWS is NOT REQUIRED to have the "nonce" header
   parameter.

   The client will compute Z, the SHA-256 of the "signature" value from
   the JWS.  The hash is calculated over the base64-encoded signature
   string.  Z is encoded in hexadecimal form.

   The client will generate a self-signed certificate with the
   subjectAlternativeName extension containing the dNSName
   "<Z[0:32]>.<Z[32:64]>.acme.invalid".  The client will then configure
   the TLS server at the domain such that when a handshake is initiated
   with the Server Name Indication extension set to
   "<Z[0:32]>.<Z[32:64]>.acme.invalid", the generated test certificate
   is presented.

   The response to the DVSNI challenge provides the validation JWS to
   the server.

   type (required, string):  The string "dvsni"





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   validation (required, string):  The JWS object computed with the
      validation object and the account key

   {
     "type": "dvsni",
     "validation": {
       "header": { "alg": "RS256" },
       "payload": "qzu9...6bjn",
       "signature": "gfj9XqFv07e1wU66hSLYkiFqYakPSjAu8TsyXRg85nM"
     }
   }

   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.

   1.  Verify the validation JWS using the account key for which the
       challenge was issued.

   2.  Decode the payload of the JWS as UTF-8 encoded JSON.

   3.  Verify that there are exactly two fields in the decoded object,
       and that:

       *  The "type" field is set to "dvsni"

       *  The "token" field matches the "token" value in the challenge

   4.  Open a TLS connection to the domain name being validated on port
       443, presenting the value "<Z[0:32]>.<Z[32:64]>.acme.invalid" in
       the SNI field.

   5.  Verify that the certificate contains a subjectAltName extension
       with the dNSName of "<Z[0:32]>.<Z[32:64]>.acme.invalid".

   It is RECOMMENDED that the ACME server validation TLS connections
   from multiple vantage points to reduce the risk of DNS hijacking
   attacks.

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

7.3.  Proof of Possession of a Prior Key

   The Proof of Possession challenge verifies that a client possesses a
   private key corresponding to a server-specified public key, as
   demonstrated by its ability to sign with that key.  This challenge is
   meant to be used when the server knows of a public key that is



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   already associated with the identifier being claimed, and wishes for
   new authorizations to be authorized by the holder of the
   corresponding private key.  For DNS identifiers, for example, this
   can help guard against domain hijacking.

   This method is useful if a server policy calls for issuing a
   certificate only to an entity that already possesses the subject
   private key of a particular prior related certificate (perhaps issued
   by a different CA).  It may also help enable other kinds of server
   policy that are related to authenticating a client's identity using
   digital signatures.

   This challenge proceeds in much the same way as the proof of
   possession of the authorized key pair in the main ACME flow
   (challenge + authorizationRequest).  The server provides a nonce and
   the client signs over the nonce.  The main difference is that rather
   than signing with the private key of the key pair being authorized,
   the client signs with a private key specified by the server.  The
   server can specify which key pair(s) are acceptable directly (by
   indicating a public key), or by asking for the key corresponding to a
   certificate.

   The server provides the following fields as part of the challenge:

   type (required, string):  The string "proofOfPossession"

   certs (optional, array of string):  An array of certificates, in
      Base64-encoded DER format, that contain acceptable public keys.

   {
     "type": "proofOfPossession",
     "certs": ["MIIF7z...bYVQLY"]
   }

   In response to this challenge, the client uses the private key
   corresponding to one of the acceptable public keys to sign a JWS
   object including data related to the challenge.  The validation
   object covered by the signature has the following fields:

   type (required, string):  The string "proofOfPossession"

   identifiers (required, identifier):  A list of identifiers for which
      the holder of the prior key authorizes the new key

   accountKey (required, JWK):  The client's account public key






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   {
     "type": "proofOfPossession",
     "identifiers: [{"type": "dns", "value": "example.com"}],
     "accountKey": { "kty": "RSA", ... }
   }

   This JWS is NOT REQUIRED to have a "nonce" header parameter (as with
   the JWS objects that carry ACME request objects).  This allows proof-
   of-possession response objects to be computed off-line.  For example,
   as part of a domain transfer, the new domain owner might require the
   old domain owner to sign a proof-of-possession validation object, so
   that the new domain owner can present that in an ACME transaction
   later.

   The validation JWS MUST contain a "jwk" header parameter indicating
   the public key under which the server should verify the JWS.

   The client's response includes the server-provided nonce, together
   with a signature over that nonce by one of the private keys requested
   by the server.

   type (required, string):  The string "proofOfPossession"

   authorization (required, JWS):  The validation JWS

   {
     "type": "proofOfPossession",
     "authorization": {
       "header": {
         "alg": "RS256",
         "jwk": {
           "kty": "RSA",
           "e": "AQAB",
           "n": "AMswMT...3aVtjE"
         }
       },
       "payload": "SfiR1...gSAl7A",
       "signature": "XcQLfL...cW5beg"
     }
   }

   To validate a proof-of-possession challenge, the server performs the
   following steps:

   1.  Verify that the public key in the "jwk" header of the
       "authorization" JWS corresponds to one of the certificates in the
       "certs" field of the challenge




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   2.  Verify the "authorization" JWS using the key indicated in its
       "jwk" header

   3.  Decode the payload of the JWS as UTF-8 encoded JSON

   4.  Verify that there are exactly three fields in the decoded object,
       and that:

       *  The "type" field is set to "proofOfPossession"

       *  The "identifier" field contains the identifier for which
          authorization is being validated

       *  The "accountKey" field matches the account key for which the
          challenge was issued

   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 resource records 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"

   token (required, string):  A random value with at least 128 bits of
      entropy.  It MUST NOT contain any characters outside the URL-safe
      Base64 alphabet.

   {
     "type": "dns",
     "token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ+PCt92wr+oA",
   }

   In response to this challenge, the client uses its account private
   key to sign a JWS over a JSON object describing the challenge.  The
   validation object covered by the signature MUST have the following
   fields and no others:

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

   token (required, string):  The token value in the challenge





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   {
     "type": "dns",
     "token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ+PCt92wr+oA",
   }

   The client serializes the validation object to UTF-8, then uses its
   account private key to sign a JWS with the serialized JSON object as
   its payload.  This JWS is NOT REQUIRED to have the "nonce" header
   parameter.

   The record provisioned to the DNS is the "signature" value from the
   JWS, i.e., the base64-encoded signature value.  The client constructs
   the validation domain name by appending the label "_acme-challenge"
   to the domain name being validated, then provisions a TXT record with
   the signature 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 a DNS challenge provides the validation JWS to the
   server.

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

   validation (required, JWS):  The JWS object computed with the
      validation object and the account key

   {
     "type": "dns"
     "clientPublicKey": { "kty": "EC", ... },
     "validation": {
       "header": { "alg": "HS256" },
       "payload": "qzu9...6bjn",
       "signature": "gfj9XqFv07e1wU66hSLYkiFqYakPSjAu8TsyXRg85nM"
     }
   }

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

   1.  Verify the validation JWS using the account key for which this
       challenge was issued

   2.  Decode the payload of the JWS as UTF-8 encoded JSON

   3.  Verify that there are exactly two fields in the decoded object,
       and that:




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       *  The "type" field is set to "dns"

       *  The "token" field matches the "token" value in the challenge

   4.  Query for TXT records under the validation domain name

   5.  Verify that the contents of one of the TXT records match the
       "signature" value in the "validation" JWS

   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.

8.  IANA Considerations

   TODO

   o  Register .well-known path

   o  Register Replay-Nonce HTTP header

   o  Register "nonce" JWS header parameter

   o  Register "urn:acme" namespace

   o  Create identifier validation method registry

   o  Registries of syntax tokens, e.g., message types / error types?

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 a 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




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   model.  We also discuss the denial-of-service risks that ACME servers
   face, and a few other miscellaneous considerations.

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 three
   "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

   o  A contact channel, over which the ACME server sends messages to
      the registered contacts for ACME clients

   +------------+
   |    ACME    |     ACME Channel
   |   Client   |--------------------+
   +------------+                    |
          ^                          V
          |   Contact Channel  +------------+
          +--------------------|    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 of the three channels,
   for example, uses a different communications pattern: the ACME
   channel will comprise inbound HTTPS connections to the ACME server,
   the validation channel outbound HTTP or DNS requests, and the contact
   channel will use channels such as email and PSTN.

   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



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   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 or contact channels can't
   obtain illegitimate authorization by acting as an ACME client
   (legitimately, in terms of the protocol).

9.2.  Integrity of Authorizations

   ACME allows anyone to request challenges for an identifier by
   registering an account key and sending a new-authorization 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-authorization 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 that require an out-of-band query by the
   server have a binding to the account private key, such that the only
   the account private key holder can successfully respond to the
   validation query:



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   o  Simple HTTP: The value provided in the validation request is
      signed by the account private key.

   o  DVSNI: The validation TLS request uses the account key pair as the
      server's key pair.

   o  DNS: The MAC covers the account key, and the MAC key is derived
      from an ECDH public key signed with the account private key.

   o  Proof of possession of a prior key: The signature by the prior key
      covers the account public key.

   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:

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

   o  DVSNI: Configure a TLS server for the domain

   o  DNS: Provision DNS resource records for the domain

   o  Proof of possession of a prior key: Sign using the private key
      specified by the server

   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 a Simple HTTP
   challenge, and likewise for TLS configuration and DVSNI.

   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 Simple HTTP validation and a web site operator
   provisions a catch-all redirect rule that redirects requests for
   unknown resources to different domain.  Then the target of the
   redirect could use that to get a certificate through Simple HTTP




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   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 DVSNI or Simple
   HTTP validation query to a server of the attacker's choosing.  There
   are a few different mitigations that ACME servers can apply:

   o  Checking the DNSSEC status of DNS records used in ACME validation
      (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
   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 credential associated with the
   domain this attack can be prevented by requiring the client to
   complete a proof-of-possession challenge.

9.3.  Preventing Authorization Hijacking

   The account recovery processes described in Section 6.4 allow
   authorization to be transferred from one account key to another, in
   case the former account key pair's private key is lost.  ACME needs
   to prevent these processes from being exploited by an attacker to
   hijack the authorizations attached to one key and assign them to a
   key of the attacker's choosing.



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   Recovery takes place in two steps: 1.  Provisioning recovery
   information (contact or recovery key) 2.  Using recovery information
   to recover an account

   The provisioning process needs to ensure that only the account key
   holder ends up with information that is useful for recovery.  The
   recovery process needs to assure that only the (now former) account
   key holder can successfully execute recovery, i.e., that this entity
   is the only one that can choose the new account key that receives the
   capabilities held by the account being recovered.

   MAC-based recovery can be performed if the attacker knows the account
   key and registration URI for the account being recovered.  Both of
   these are difficult to obtain for a network attacker, because ACME
   uses HTTPS, though if the recovery key and registration URI are
   sufficiently predictable, the attacker might be able to guess them.
   An ACME MitM can see the registration URI, but still has to guess the
   recovery key, since neither the ECDH in the provisioning phase nor
   HMAC in the recovery phase will reveal it to him.

   ACME clients can thus mitigate problems with MAC-based recovery by
   using long recovery keys.  ACME servers should enforce a minimum
   recovery key length, and impose rate limits on recovery to limit an
   attacker's ability to test different guesses about the recovery key.

   Contact-based recovery uses both the ACME channel and the contact
   channel.  The provisioning process is only visible to an ACME MitM,
   and even then, the MitM can only observe the contact information
   provided.  If the ACME attacker does not also have access to the
   contact channel, there is no risk.

   The security of the contact-based recovery process is entirely
   dependent on the security of the contact channel.  The details of
   this will depend on the specific out-of-band technique used by the
   server.  For example:

   o  If the server requires a user to click a link in a message sent to
      a contact address, then the contact channel will need to ensure
      that the message is only available to the legitimate owner of the
      contact address.  Otherwise, a passive attacker could see the link
      and click it first, or an active attacker could redirect the
      message.

   o  If the server requires a user to respond to a message sent to a
      contact address containing a secret value, then the contact
      channel will need to ensure that an attacker cannot observe the
      secret value and spoof a message from the contact address.




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   In practice, many contact channels that can be used to reach many
   clients do not provide strong assurances of the types noted above.
   In designing and deploying contact-based recovery schemes, ACME
   servers operators will need to find an appropriate balance between
   using contact channels that can reach many clients and using contact-
   based recovery schemes that achieve an appropriate level of risk
   using those contact channels.

9.4.  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.

9.5.  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?




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      *  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?

   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.

10.  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.

11.  References

11.1.  Normative References

   [I-D.ietf-appsawg-http-problem]
              Nottingham, M. and E. Wilde, "Problem Details for HTTP
              APIs", draft-ietf-appsawg-http-problem-00 (work in
              progress), September 2014.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.





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   [RFC2314]  Kaliski, B., "PKCS #10: Certification Request Syntax
              Version 1.5", RFC 2314, March 1998.

   [RFC2985]  Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object
              Classes and Attribute Types Version 2.0", RFC 2985,
              November 2000.

   [RFC2986]  Nystrom, M. and B. Kaliski, "PKCS #10: Certification
              Request Syntax Specification Version 1.7", RFC 2986,
              November 2000.

   [RFC3339]  Klyne, G., Ed. and C. Newman, "Date and Time on the
              Internet: Timestamps", RFC 3339, July 2002.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66, RFC
              3986, January 2005.

   [RFC4514]  Zeilenga, K., "Lightweight Directory Access Protocol
              (LDAP): String Representation of Distinguished Names", RFC
              4514, June 2006.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, October 2006.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [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, May 2008.

   [RFC5753]  Turner, S. and D. Brown, "Use of Elliptic Curve
              Cryptography (ECC) Algorithms in Cryptographic Message
              Syntax (CMS)", RFC 5753, January 2010.

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988, October 2010.

   [RFC6066]  Eastlake, D., "Transport Layer Security (TLS) Extensions:
              Extension Definitions", RFC 6066, January 2011.

   [RFC6570]  Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
              and D. Orchard, "URI Template", RFC 6570, March 2012.



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   [RFC7159]  Bray, T., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, March 2014.

   [RFC7469]  Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
              Extension for HTTP", RFC 7469, April 2015.

   [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, May 2015.

   [RFC7517]  Jones, M., "JSON Web Key (JWK)", RFC 7517, May 2015.

   [RFC7518]  Jones, M., "JSON Web Algorithms (JWA)", RFC 7518, May
              2015.

   [SEC1]     Standards for Efficient Cryptography Group, "SEC 1:
              Elliptic Curve Cryptography", May 2009,
              <http://www.secg.org/sec1-v2.pdf>.

11.2.  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.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552, July
              2003.

   [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>.

   [W3C.WD-capability-urls-20140218]
              Tennison, J., "Good Practices for Capability URLs", World
              Wide Web Consortium WD WD-capability-urls-20140218,
              February 2014,
              <http://www.w3.org/TR/2014/WD-capability-urls-20140218>.

Authors' Addresses

   Richard Barnes
   Mozilla

   Email: rlb@ipv.sx



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   Jacob Hoffman-Andrews
   EFF

   Email: jsha@eff.org


   James Kasten
   University of Michigan

   Email: jdkasten@umich.edu









































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