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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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