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


          Automatic Certificate Management Environment (ACME)
                        draft-ietf-acme-acme-02

Abstract

   Certificates in the Web's X.509 PKI (PKIX) are used for a number of
   purposes, the most significant of which is the authentication of
   domain names.  Thus, certificate authorities in the Web PKI are
   trusted to verify that an applicant for a certificate legitimately
   represents the domain name(s) in the certificate.  Today, this
   verification is done through a collection of ad hoc mechanisms.  This
   document describes a protocol that a certificate authority (CA) and
   an applicant can use to automate the process of verification and
   certificate issuance.  The protocol also provides facilities for
   other certificate management functions, such as certificate
   revocation.

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

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

Status of This Memo

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

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

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



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

   This Internet-Draft will expire on September 22, 2016.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Deployment Model and Operator Experience  . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Message Transport . . . . . . . . . . . . . . . . . . . . . .   9
     5.1.  HTTPS Requests  . . . . . . . . . . . . . . . . . . . . .   9
     5.2.  Request Authentication  . . . . . . . . . . . . . . . . .   9
     5.3.  Request URI Type Integrity  . . . . . . . . . . . . . . .  10
     5.4.  Replay protection . . . . . . . . . . . . . . . . . . . .  11
       5.4.1.  Replay-Nonce  . . . . . . . . . . . . . . . . . . . .  12
       5.4.2.  "nonce" (Nonce) JWS header parameter  . . . . . . . .  12
     5.5.  Errors  . . . . . . . . . . . . . . . . . . . . . . . . .  12
   6.  Certificate Management  . . . . . . . . . . . . . . . . . . .  14
     6.1.  Resources . . . . . . . . . . . . . . . . . . . . . . . .  14
       6.1.1.  Registration Objects  . . . . . . . . . . . . . . . .  16
       6.1.2.  Authorization Objects . . . . . . . . . . . . . . . .  17
     6.2.  Directory . . . . . . . . . . . . . . . . . . . . . . . .  18
     6.3.  Registration  . . . . . . . . . . . . . . . . . . . . . .  20
       6.3.1.  Account Key Roll-over . . . . . . . . . . . . . . . .  22
       6.3.2.  Deleting an Account . . . . . . . . . . . . . . . . .  23
     6.4.  Identifier Authorization  . . . . . . . . . . . . . . . .  24
       6.4.1.  Responding to Challenges  . . . . . . . . . . . . . .  26
       6.4.2.  Deleting an Authorization . . . . . . . . . . . . . .  28
     6.5.  Certificate Issuance  . . . . . . . . . . . . . . . . . .  29
     6.6.  Certificate Revocation  . . . . . . . . . . . . . . . . .  32
   7.  Identifier Validation Challenges  . . . . . . . . . . . . . .  33



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     7.1.  Key Authorizations  . . . . . . . . . . . . . . . . . . .  35
     7.2.  HTTP  . . . . . . . . . . . . . . . . . . . . . . . . . .  35
     7.3.  TLS with Server Name Indication (TLS SNI) . . . . . . . .  38
     7.4.  DNS . . . . . . . . . . . . . . . . . . . . . . . . . . .  39
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  41
   9.  Well-Known URI for the HTTP Challenge . . . . . . . . . . . .  41
     9.1.  Replay-Nonce HTTP Header  . . . . . . . . . . . . . . . .  41
     9.2.  "nonce" JWS Header Parameter  . . . . . . . . . . . . . .  41
     9.3.  URN Sub-namespace for ACME (urn:ietf:params:acme) . . . .  42
     9.4.  New Registries  . . . . . . . . . . . . . . . . . . . . .  42
       9.4.1.  Error Codes . . . . . . . . . . . . . . . . . . . . .  42
       9.4.2.  Identifier Types  . . . . . . . . . . . . . . . . . .  43
       9.4.3.  Challenge Types . . . . . . . . . . . . . . . . . . .  43
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  44
     10.1.  Threat model . . . . . . . . . . . . . . . . . . . . . .  44
     10.2.  Integrity of Authorizations  . . . . . . . . . . . . . .  45
     10.3.  Denial-of-Service Considerations . . . . . . . . . . . .  48
     10.4.  CA Policy Considerations . . . . . . . . . . . . . . . .  49
   11. Operational Considerations  . . . . . . . . . . . . . . . . .  49
     11.1.  Default Virtual Hosts  . . . . . . . . . . . . . . . . .  49
     11.2.  Use of DNSSEC Resolvers  . . . . . . . . . . . . . . . .  50
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  50
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  51
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  51
     13.2.  Informative References . . . . . . . . . . . . . . . . .  53
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  54

1.  Introduction

   Certificates in the Web PKI [RFC5280] 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.





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      *  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.

   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(s).

   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.)




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   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 deploying a current HTTPS server, an operator generally gets a
   prompt 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.

   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.








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3.  Terminology

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

   The two main roles in ACME are "client" and "server".  The ACME
   client uses the protocol to request certificate management actions,
   such as issuance or revocation.  An ACME client therefore typically
   runs on a web server, mail server, or some other server system which
   requires valid TLS certificates.  The ACME server runs at a
   certificate authority, and responds to client requests, performing
   the requested actions if the client is authorized.

   An ACME client is represented by an "account key pair".  The client
   uses the private key of this key pair to sign all messages sent to
   the server.  The server uses the public key to verify the
   authenticity and integrity of messages from the client.

4.  Protocol Overview

   ACME allows a client to request certificate management actions using
   a set of JSON messages carried over HTTPS.  In 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.

         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



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   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.

         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



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   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.

   Note that as a result, the CSR is signed twice: One by the private
   key corresponding to the public key in the CSR, and once by the
   private key of the account key pair.  The former signature indicates
   that the holder of the key in the CSR is willing to act for the
   indicated identifiers, and the latter signature indicates to the
   server that the issuance of the certificate is authorized by the
   client (i.e., the domain holder).

   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
   indicating the certificate to be revoked, signed with an authorized
   key pair.  The server indicates whether the request has succeeded.

         Client                                                 Server

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

                                      <--------                 Result

   Note that while ACME is defined with enough flexibility to handle
   different types of identifiers in principle, the primary use case
   addressed by this document is the case where domain names are used as
   identifiers.  For example, all of the identifier validation
   challenges described in Section 7 below address validation of domain
   names.  The use of ACME for other 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.






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5.  Message Transport

   ACME uses a combination of HTTPS and JWS to create a messaging layer
   with a few important security properties.

   Communications between an ACME client and an ACME server are done
   over HTTPS, using JWS to provide som additional security properties
   for messages sent from the client to the server.  HTTPS provides
   server authentication and confidentiality.  With some ACME-specific
   extensions, JWS provides authentication of the client's request
   payloads, anti-replay protection, and a degree of integrity for the
   HTTPS request URI.

5.1.  HTTPS Requests

   Each ACME function is accomplished by the client sending a sequence
   of HTTPS requests to the server, carrying JSON messages
   [RFC2818][RFC7159].  Use of HTTPS is REQUIRED.  Clients SHOULD
   support HTTP public key pinning [RFC7469], and servers SHOULD emit
   pinning headers.  Each subsection of Section 6 below describes the
   message formats used by the function, and the order in which messages
   are sent.

   In all HTTPS transactions used by ACME, the ACME client is the HTTPS
   client and the ACME server is the HTTPS server.

   ACME servers that are intended to be generally accessible need to use
   Cross-Origin Resource Sharing (CORS) in order to be accessible from
   browser-based clients [W3C.CR-cors-20130129].  Such servers SHOULD
   set the Access-Control-Allow-Origin header field to the value "*".

   Binary fields in the JSON objects used by ACME are encoded using
   base64url encoding described in [RFC4648] Section 5, according to the
   profile specified in JSON Web Signature [RFC7515] Section 2.  This
   encoding uses a URL safe character set.  Trailing '=' characters MUST
   be stripped.

5.2.  Request Authentication

   All ACME requests with a non-empty body MUST encapsulate 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.

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



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   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

   o  The JWS Protected Header MUST include the "nonce" field (defined
      below)

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

5.3.  Request URI Type Integrity

   It is common in deployment the entity terminating TLS for HTTPS to be
   different from the entity operating the logical HTTPS server, with a
   "request routing" layer in the middle.  For example, an ACME CA might
   have a content delivery network terminate TLS connections from
   clients so that it can inspect client requests for denial-of-service
   protection.

   These intermediaries can also change values in the request that are
   not signed in the HTTPS request, e.g., the request URI and headers.
   ACME uses JWS to provides a limited integrity mechanism, which
   protects against an intermediary changing the request URI to anothe
   ACME URI of a different type.  (It does not protect against changing
   between URIs of the same type, e.g., from one authorization URI to
   another).

   An ACME request carries a JSON dictionary that provides the details
   of the client's request to the server.  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          |
                 |                    |                  |
                 | 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.

5.4.  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




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   (Bad Request), and indicate the ACME error code
   "urn:ietf:params:acme:error: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.

5.4.1.  Replay-Nonce

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

   The value of the Replay-Nonce field MUST be an octet string encoded
   according to the base64url encoding described in Section 2 of
   [RFC7515].  Clients MUST ignore invalid Replay-Nonce values.

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

     Replay-Nonce = *base64url

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

5.4.2.  "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.

5.5.  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).





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   When the server responds with an error status, it SHOULD provide
   additional information using problem document
   [I-D.ietf-appsawg-http-problem].  To facilitate automatic response to
   errors, this document defines the following standard tokens for use
   in the "type" field (within the "urn:ietf:params:acme:error:"
   namespace):

   +----------------+--------------------------------------------------+
   | Code           | Description                                      |
   +----------------+--------------------------------------------------+
   | 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   |
   |                | validation                                       |
   |                |                                                  |
   | 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        |
   |                | validation                                       |
   |                |                                                  |
   | unauthorized   | The client lacks sufficient authorization        |
   |                |                                                  |
   | unknownHost    | The server could not resolve a domain name       |
   |                |                                                  |
   | rateLimited    | The request exceeds a rate limit                 |
   |                |                                                  |
   | invalidContact | The provided contact URI for a registration was  |
   |                | invalid                                          |
   +----------------+--------------------------------------------------+

   This list is not exhaustive.  The server MAY return errors whose
   "type" field is set to a URI other than those defined above.  Servers
   MUST NOT use the ACME URN namespace for errors other than the
   standard types.  Clients SHOULD display the "detail" field of such
   errors.

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



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6.  Certificate Management

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

   o  Account Key Registration

   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

   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.

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



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   The "up" link relation is used with challenge resources to indicate
   the authorization resource to which a challenge belongs.  It is also
   used from certificate resources to indicate a resource from which the
   client may fetch a chain of CA certificates that could be used to
   validate the certificate in the original resource.

   The "directory" link relation is present on all resources other than
   the directory and indicates the directory URL.

   The following diagram illustrates the relations between resources on
   an ACME server.  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.

6.1.1.  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.

   authorizations (required, 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.
      The server MAY return an incomplete list, along with a Link header
      with link relation "next" indicating a URL to retrieve further
      entries.



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   certificates (required, 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 or revoked certificates.  The server MAY return an
      incomplete list, along with a Link header with link relation
      "next" indicating a URL to retrieve further entries.

   {
     "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",
   }

6.1.2.  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 (required, 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".

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





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   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 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 6.5 below for more information
   about wildcard domains.

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

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

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

6.2.  Directory

   In order to help clients configure themselves with the right URIs for
   each ACME operation, ACME servers provide a directory object.  This
   should be the only URL needed to configure clients.  It is a JSON
   dictionary, whose keys are the "resource" values listed in
   Section 5.1, and whose values are the URIs used to accomplish the
   corresponding function.



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

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

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

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

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

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

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

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

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

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

   {
     "new-reg": "https://example.com/acme/new-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",
     "meta": {
       "terms-of-service": "https://example.com/acme/terms",
       "website": "https://www.example.com/",
       "caa-identities": ["example.com"]
     }
   }





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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).

   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 SHOULD 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.  When these terms change in a
   way that requires an agreement update, the server MUST use a
   different URI in the Link header.



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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/terms>;rel="terms-of-service"
   Link: <https://example.com/acme/some-directory>;rel="directory"

   {
     "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.

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

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

   {
     "resource": "reg",
     "contact": [
       "mailto:certificates@example.com",
       "tel:+12125551212"
     ],
   }
   /* Signed as JWS */

   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"}).  In this case the
   server reply MUST contain the same link headers sent for a new
   registration, to allow a client to retrieve the "new-authorization"
   and "terms-of-service" URI




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6.3.1.  Account Key Roll-over

   A client may wish to change the public key that is associated with a
   registration, e.g., in order to mitigate the risk of key compromise.
   To do this, the client first constructs a JSON object representing a
   request to update the registration:

   resource (required, string):  The string "reg", indicating an update
      to the registration.

   oldKey (required, string):  The JWK thumbprint of the old key
      [RFC7638], base64url-encoded

   {
     "resource": "reg",
     "oldKey": "D7J9RL1f-RWUl68JP-gW1KSl2TkIrJB7hK6rLFFeYMU"
   }

   The client signs this object with the new key pair and encodes the
   object and signature as a JWS.  The client then sends this JWS to the
   server in the "newKey" field of a request to update the registration.

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

   {
     "resource": "reg",
     "newKey": /* JSON object signed as JWS with new key */
   }
   /* Signed as JWS with original key */

   On receiving a request to the registration URL with the "newKey"
   attribute set, the server MUST perform the following steps:

   1.  Check that the contents of the "newKey" attribute are a valid JWS

   2.  Check that the "newKey" JWS verifies using the key in the "jwk"
       header parameter of the JWS

   3.  Check that the payload of the JWS is a valid JSON object

   4.  Check that the "resource" field of the object has the value "reg"

   5.  Check that the "oldKey" field of the object contains the JWK
       thumbprint of the account key for this registration






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   If all of these checks pass, then the server updates the registration
   by replacing the old account key with the public key carried in the
   "jwk" header parameter of the "newKey" JWS object.

   If the update was successful, then the server sends a response with
   status code 200 (OK) and the updated registration object as its body.
   If the update was not successful, then the server responds with an
   error status code and a problem document describing the error.

6.3.2.  Deleting an Account

   If a client no longer wishes to have an account key registered with
   the server, it may request that the server delete its account by
   sending a POST request to the account URI containing the "delete"
   field.

   delete (required, boolean): The boolean value "true".

   The request object MUST contain the "resource" field as required
   above (with the value "reg").  It MUST NOT contain any fields besides
   "resource" and "delete".

   Note that although this object is very simple, the risk of replay or
   fraudulent generation via signing oracles is mitigated by the need
   for an anti-replay token in the protected header of the JWS.

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

   {
     "resource": "reg",
     "delete": true,
   }
   /* Signed as JWS */

   On receiving a POST to an account URI containing a "delete" field,
   the server MUST verify that no other fields were sent in the object
   (other than "resource"), and it MUST verify that the value of the
   "delete" field is "true" (as a boolean, not a string).  If either of
   these checks fails, then the server MUST reject the request with
   status code 400 (Bad Request).

   If the server accepts the deletion request, then it MUST delete the
   account and all related objects and send a response with a 200 (OK)
   status code and an empty body.  The server SHOULD delete any
   authorization objects related to the deleted account, since they can
   no longer be used.  The server SHOULD NOT delete certificate objects




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   related to the account, since certificates issued under the account
   continue to be valid until they expire or are revoked.

6.4.  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



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   is of a supported type.  Servers might also check names against a
   blacklist of known high-value identifiers.  If the server is
   unwilling to issue for the identifier, it SHOULD return a 403
   (Forbidden) error, with a problem document describing the reason for
   the rejection.

   If the 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" unless the server has out-of-band
      information about the client's authorization status

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

   The server allocates a new URI for this authorization, and returns a
   201 (Created) response, with the authorization URI in a Location
   header field, and the JSON authorization object in the body.






























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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"
   Link: <https://example.com/acme/some-directory>;rel="directory"

   {
     "status": "pending",

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

     "challenges": [
       {
         "type": "http-01",
         "uri": "https://example.com/authz/asdf/0",
         "token": "IlirfxKKXAsHtmzK29Pj8A"
       },
       {
         "type": "dns-01",
         "uri": "https://example.com/authz/asdf/1",
         "token": "DGyRejmCefe7v4NfDGDKfA"
       }
     },

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

6.4.1.  Responding to Challenges

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

   The client sends these updates back to the server in the form of a
   JSON object with the response fields required by the challenge type,
   carried in a POST request to the challenge URI (not authorization URI
   or the new-authorization URI).  This allows the client to send
   information only for challenges it is responding to.

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




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

   {
     "resource": "challenge",
     "type": "http-01",
     "keyAuthorization": "IlirfxKKXA...vb29HhjjLPSggwiE"
   }
   /* 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.

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

   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




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   (Accepted) response, and MAY include a Retry-After header field to
   suggest a polling interval to the client.

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

   HTTP/1.1 200 OK

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

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

     "challenges": [
       {
         "type": "http-01"
         "status": "valid",
         "validated": "2014-12-01T12:05:00Z",
         "token": "IlirfxKKXAsHtmzK29Pj8A",
         "keyAuthorization": "IlirfxKKXA...vb29HhjjLPSggwiE"
       }
     ]
   }

6.4.2.  Deleting an Authorization

   If a client wishes to relinquish its authorization to issue
   certificates for an identifier, then it may request that the server
   delete the authorization.  The client makes this request by sending a
   POST request to the authorization URI containing a payload in the
   same format as in Section 6.3.2.  The only difference is that the
   value of the "resource" field is "authz".

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

   {
     "resource": "authz",
     "delete": true,
   }
   /* Signed as JWS */

   The server MUST perform the same validity checks as in Section 6.3.2
   and reject the request if they fail.  If the server deletes the



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   account then it MUST send a response with a 200 (OK) status code and
   an empty body.

6.5.  Certificate Issuance

   The holder of an account key pair authorized for one or more
   identifiers may use ACME to request that a certificate be issued for
   any subset of those identifiers.  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.  Some attributes which
   cannot be reflected in a CSR are placed directly in the certificate
   request.

   csr (required, string):  A CSR encoding the parameters for the
      certificate being requested.  The CSR is sent in the Base64url-
      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.)

   notBefore (optional, string):  The requested value of the notBefore
      field in the certificate, in the date format defined in [RFC3339]

   notAfter (optional, string):  The requested value of the notAfter
      field in the certificate, in the date format defined in [RFC3339]

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

   {
     "resource": "new-cert",
     "csr": "5jNudRx6Ye4HzKEqT5...FS6aKdZeGsysoCo4H9P",
     "notBefore": "2016-01-01T00:00:00Z",
     "notAfter": "2016-01-08T00:00:00Z"
   }
   /* 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.





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   The values provided in the CSR are only a request, and are not
   guaranteed.  The server SHOULD return an error if it cannot fulfil
   the request as specified, but MAY issue a certificate with contents
   other than those requested, according to its local policy (e.g.,
   removing identifiers for which the client is not authorized).

   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).

   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




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   this certificate was issued, and one with relation "author" to
   indicate the registration under which this certificate was issued.

   The server MAY 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.)

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

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

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

   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



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   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.

6.6.  Certificate Revocation

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

   certificate (required, string):  The certificate to be revoked, in
      the base64url-encoded version of the DER format.  (Note: 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 that the key used to sign the request is authorized to revoke
   the certificate.  The server SHOULD consider at least the following
   keys authorized for a given certificate:

   o  the public key in the certificate.

   o  an account key that is authorized to act for all of the
      identifier(s) in the certificate.




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

7.  Identifier Validation Challenges

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

   Challenges provide the server with assurance that an account 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 10 documents how the challenges defined in this document meet
   these requirements.  New challenges will need to document how they
   do.

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

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



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   1.  Content of challenge objects

   2.  Content of response objects

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

   Challenge objects all contain the following basic fields:

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

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

   status (required, string):  The status of this authorization.
      Possible values are: "pending", "valid", and "invalid".  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].  This field is REQUIRED if the "status" field is
      "valid".

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

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

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

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

   [[ Editor's Note: In pre-RFC versions of this specification,
   challenges are labeled by type, and with the version of the draft in



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   which they were introduced.  For example, if an HTTP challenge were
   introduced in version -03 and a breaking change made in version -05,
   then there would be a challenge labeled "http-03" and one labeled
   "http-05" - but not one labeled "http-04", since challenge in version
   -04 was compatible with one in version -04. ]]

   [[ Editor's Note: Operators SHOULD NOT issue "combinations" arrays in
   authorization objects that require the client to perform multiple
   challenges over the same type, e.g., ["http-03", "http-05"].
   Challenges within a type are testing the same capability of the
   domain owner, and it may not be possible to satisfy both at once. ]]

7.1.  Key Authorizations

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

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

   The "JWK_Thumbprint" step indicates the computation specified in
   [RFC7638], using the SHA-256 digest.  As specified in the individual
   challenges below, the token for a challenge is a JSON string
   comprised entirely of characters in the URL-safe Base64 alphabet.
   The "||" operator indicates concatenation of strings.

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

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

7.2.  HTTP

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



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   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.  Because many web servers allocate a default HTTPS virtual
   host to a particular low-privilege tenant user in a subtle and non-
   intuitive manner, the challenge must be completed over HTTP, not
   HTTPS.

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

   token (required, string):  A random value that uniquely identifies
      the challenge.  This value MUST have at least 128 bits of entropy,
      in order to prevent an attacker from guessing it.  It MUST NOT
      contain any characters outside the URL-safe Base64 alphabet and
      MUST NOT contain any padding characters ("=").

   {
     "type": "http-01",
     "token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA",
   }

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

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

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

   The client's response to this challenge indicates its agreement to
   this challenge by sending the server the key authorization covering
   the challenge's token and the client's account key.  In addition, the
   client MAY advise the server at which IP the challenge is
   provisioned.

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

   address (optional, string):  An IPv4 or IPv6 address, in dotted
      decimal form or [RFC4291] form, respectively.  If given, this
      address MUST be included in the set of IP addresses to which the
      domain name resolves when the server attempts validation.  If
      given, the server SHOULD connect to that specific IP address



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      instead of arbitrarily choosing an IP from the set of A and AAAA
      records to which the domain name resolves.

   {
     "keyAuthorization": "evaGxfADs...62jcerQ"
   }
   /* Signed as JWS */

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

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

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

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

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

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

   3.  If the client has supplied an address to use, verify that the
       address is included in the A or AAAA records to which the domain
       name resolves.  If the address is not included in the result, the
       validation fails.

   4.  Dereference the URI using an HTTP GET request.  If an address was
       supplied by the client, use that address to establish the HTTP
       connection.

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

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

   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.





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7.3.  TLS with Server Name Indication (TLS SNI)

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

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

   token (required, string):  A random value that uniquely identifies
      the challenge.  This value MUST have at least 128 bits of entropy,
      in order to prevent an attacker from guessing it.  It MUST NOT
      contain any characters outside the URL-safe Base64 alphabet and
      MUST NOT contain any padding characters ("=").

   {
     "type": "tls-sni-02",
     "token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA"
   }

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

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

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

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

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

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



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

   {
     "keyAuthorization": "evaGxfADs...62jcerQ",
   }
   /* Signed as JWS */

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

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

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

   2.  Open a TLS connection to the domain name being validated on the
       requested port, presenting SAN A in the SNI field.  In the
       ClientHello initiating the TLS handshake, the server MUST include
       a server_name extension (i.e., SNI) containing SAN A.  The server
       SHOULD ensure that it does not reveal SAN B in any way when
       making the TLS connection, such that the presentation of SAN B in
       the returned certificate proves association with the client.

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

   It is RECOMMENDED that the 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.

7.4.  DNS

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



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

   token (required, string):  A random value that uniquely identifies
      the challenge.  This value MUST have at least 128 bits of entropy,
      in order to prevent an attacker from guessing it.  It MUST NOT
      contain any characters outside the URL-safe Base64 alphabet and
      MUST NOT contain any padding characters ("=").

   {
     "type": "dns-01",
     "token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA"
   }

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

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

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

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

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

   {
     "keyAuthorization": "evaGxfADs...62jcerQ",
   }
   /* Signed as JWS */

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

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



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

   2.  Query for TXT records under the validation domain name

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

   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

   [[ Editor's Note: Should we create a registry for tokens that go into
   the various JSON objects used by this protocol, i.e., the field names
   in the JSON objects? ]]

9.  Well-Known URI for the HTTP Challenge

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

   URI suffix: acme-challenge

   Change controller: IETF

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

   Related information: N/A

9.1.  Replay-Nonce HTTP Header

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

   | Header Field Name | Protocol | Status | Reference |
   +:------------+:------+:------+:-----------+ | Replay-Nonce | http |
   standard | Section 5.4.1 |

9.2.  "nonce" JWS Header Parameter

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

   o  Header Parameter Name: "nonce"

   o  Header Parameter Description: Nonce




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

   o  Change Controller: IESG

   o  Specification Document(s): Section 5.4.2 of RFC XXXX

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

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

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

   Registry name:  acme

   Specification:  RFC XXXX

   Repository:  URL-TBD

   Index value:  No transformation needed.  The

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

9.4.  New Registries

   This document requests that IANA create three new registries:

   1.  ACME Error Codes

   2.  ACME Identifier Types

   3.  ACME Challenge Types

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

9.4.1.  Error Codes

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

   Template:





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

   o  Description: A human-readable description of the error

   o  Reference: Where the error is defined

   Initial contents: The codes and descriptions in the table in
   Section 5.5 above, with the Reference field set to point to this
   specification.

9.4.2.  Identifier Types

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

   Template:

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

   o  Reference: Where the identifier type is defined

   Initial contents:

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

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

9.4.3.  Challenge Types

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

   Template:

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

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



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   o  Reference: Where the challenge type is defined

   Initial Contents

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

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

10.  Security Considerations

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

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

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

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

10.1.  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





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   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
   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).

10.2.  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



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   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 only the
   account private key holder can successfully respond to the validation
   query:

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

   o  TLS SNI: 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.

   The association of challenges to identifiers is typically done by
   requiring the client to perform some action that only someone who



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   effectively controls the identifier can perform.  For the challenges
   in this document, the actions are:

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

   o  TLS SNI: Configure a TLS server for the domain

   o  DNS: Provision DNS resource records for the domain

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

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

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

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

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

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

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




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   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 public key associated with the
   domain this attack can be prevented by requiring the client to prove
   control of the corresponding private key.

10.3.  Denial-of-Service Considerations

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

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

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

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







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10.4.  CA Policy Considerations

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

   o  Has the client agreed to a subscriber agreement?

   o  Is the claimed identifier syntactically valid?

   o  For domain names:

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

      *  Is the name on the Public Suffix List?

      *  Is the name a high-value name?

      *  Is the name a known phishing domain?

   o  Is the key in the CSR sufficiently strong?

   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.

11.  Operational Considerations

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

   o  It is advisable to perform DNS queries via TCP to mitigate DNS
      forgery attacks over UDP

   [[ TODO: Other operational considerations ]]

11.1.  Default Virtual Hosts

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



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

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

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

11.2.  Use of DNSSEC Resolvers

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

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

12.  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



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   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.

13.  References

13.1.  Normative References

   [I-D.ietf-appsawg-http-problem]
              mnot, m. and E. Wilde, "Problem Details for HTTP APIs",
              draft-ietf-appsawg-http-problem-03 (work in progress),
              January 2016.

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

   [RFC2314]  Kaliski, B., "PKCS #10: Certification Request Syntax
              Version 1.5", RFC 2314, DOI 10.17487/RFC2314, March 1998,
              <http://www.rfc-editor.org/info/rfc2314>.

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

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

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

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






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

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <http://www.rfc-editor.org/info/rfc4291>.

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

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

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

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

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

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

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

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




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

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

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

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

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

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

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

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

13.2.  Informative References

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

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







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   [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


   Jacob Hoffman-Andrews
   EFF

   Email: jsha@eff.org


   James Kasten
   University of Michigan

   Email: jdkasten@umich.edu






















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