Network Working Group R. Barnes
Internet-Draft Mozilla
Intended status: Standards Track J. Hoffman-Andrews
Expires: August 7, 2017 EFF
J. Kasten
University of Michigan
February 03, 2017
Automatic Certificate Management Environment (ACME)
draft-ietf-acme-acme-05
Abstract
Certificates in the Web's X.509 PKI (PKIX) are used for a number of
purposes, the most significant of which is the authentication of
domain names. Thus, certificate authorities in the Web PKI are
trusted to verify that an applicant for a certificate legitimately
represents the domain name(s) in the certificate. Today, this
verification is done through a collection of ad hoc mechanisms. This
document describes a protocol that a certificate authority (CA) and
an applicant can use to automate the process of verification and
certificate issuance. The protocol also provides facilities for
other certificate management functions, such as certificate
revocation.
DISCLAIMER: This is a work in progress draft of ACME and has not yet
had a thorough security analysis.
RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH: The source for
this draft is maintained in GitHub. Suggested changes should be
submitted as pull requests at https://github.com/ietf-wg-acme/acme .
Instructions are on that page as well. Editorial changes can be
managed in GitHub, but any substantive change should be discussed on
the ACME mailing list (acme@ietf.org).
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
Barnes, et al. Expires August 7, 2017 [Page 1]
Internet-Draft ACME February 2017
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 7, 2017.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Deployment Model and Operator Experience . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 6
5. Message Transport . . . . . . . . . . . . . . . . . . . . . . 8
5.1. HTTPS Requests . . . . . . . . . . . . . . . . . . . . . 8
5.2. Request Authentication . . . . . . . . . . . . . . . . . 9
5.3. Equivalence of JWKs . . . . . . . . . . . . . . . . . . . 11
5.4. Request URI Integrity . . . . . . . . . . . . . . . . . . 11
5.4.1. "url" (URL) JWS header parameter . . . . . . . . . . 12
5.5. Replay protection . . . . . . . . . . . . . . . . . . . . 12
5.5.1. Replay-Nonce . . . . . . . . . . . . . . . . . . . . 13
5.5.2. "nonce" (Nonce) JWS header parameter . . . . . . . . 13
5.6. Rate limits . . . . . . . . . . . . . . . . . . . . . . . 13
5.7. Errors . . . . . . . . . . . . . . . . . . . . . . . . . 14
6. Certificate Management . . . . . . . . . . . . . . . . . . . 16
6.1. Resources . . . . . . . . . . . . . . . . . . . . . . . . 16
6.1.1. Directory . . . . . . . . . . . . . . . . . . . . . . 18
6.1.2. Account Objects . . . . . . . . . . . . . . . . . . . 20
6.1.3. Order Objects . . . . . . . . . . . . . . . . . . . . 21
6.1.4. Authorization Objects . . . . . . . . . . . . . . . . 23
6.2. Getting a Nonce . . . . . . . . . . . . . . . . . . . . . 24
6.3. Account Creation . . . . . . . . . . . . . . . . . . . . 25
6.3.1. Changes of Terms of Service . . . . . . . . . . . . . 27
6.3.2. External Account Binding . . . . . . . . . . . . . . 28
6.3.3. Account Key Roll-over . . . . . . . . . . . . . . . . 30
Barnes, et al. Expires August 7, 2017 [Page 2]
Internet-Draft ACME February 2017
6.3.4. Account deactivation . . . . . . . . . . . . . . . . 32
6.4. Applying for Certificate Issuance . . . . . . . . . . . . 33
6.4.1. Pre-Authorization . . . . . . . . . . . . . . . . . . 35
6.4.2. Downloading the Certificate . . . . . . . . . . . . . 37
6.5. Identifier Authorization . . . . . . . . . . . . . . . . 39
6.5.1. Responding to Challenges . . . . . . . . . . . . . . 40
6.5.2. Deactivating an Authorization . . . . . . . . . . . . 42
6.6. Certificate Revocation . . . . . . . . . . . . . . . . . 43
7. Identifier Validation Challenges . . . . . . . . . . . . . . 45
7.1. Key Authorizations . . . . . . . . . . . . . . . . . . . 47
7.2. HTTP . . . . . . . . . . . . . . . . . . . . . . . . . . 47
7.3. TLS with Server Name Indication (TLS SNI) . . . . . . . . 50
7.4. DNS . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
7.5. Out-of-Band . . . . . . . . . . . . . . . . . . . . . . . 54
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 55
8.1. Well-Known URI for the HTTP Challenge . . . . . . . . . . 55
8.2. Replay-Nonce HTTP Header . . . . . . . . . . . . . . . . 55
8.3. "url" JWS Header Parameter . . . . . . . . . . . . . . . 56
8.4. "nonce" JWS Header Parameter . . . . . . . . . . . . . . 56
8.5. URN Sub-namespace for ACME (urn:ietf:params:acme) . . . . 56
8.6. New Registries . . . . . . . . . . . . . . . . . . . . . 57
8.6.1. Fields in Account Objects . . . . . . . . . . . . . . 57
8.6.2. Fields in Order Objects . . . . . . . . . . . . . . . 58
8.6.3. Error Codes . . . . . . . . . . . . . . . . . . . . . 59
8.6.4. Resource Types . . . . . . . . . . . . . . . . . . . 59
8.6.5. Identifier Types . . . . . . . . . . . . . . . . . . 60
8.6.6. Challenge Types . . . . . . . . . . . . . . . . . . . 60
9. Security Considerations . . . . . . . . . . . . . . . . . . . 61
9.1. Threat model . . . . . . . . . . . . . . . . . . . . . . 62
9.2. Integrity of Authorizations . . . . . . . . . . . . . . . 63
9.3. Denial-of-Service Considerations . . . . . . . . . . . . 65
9.4. Server-Side Request Forgery . . . . . . . . . . . . . . . 65
9.5. CA Policy Considerations . . . . . . . . . . . . . . . . 66
10. Operational Considerations . . . . . . . . . . . . . . . . . 67
10.1. DNS over TCP . . . . . . . . . . . . . . . . . . . . . . 67
10.2. Default Virtual Hosts . . . . . . . . . . . . . . . . . 67
10.3. Use of DNSSEC Resolvers . . . . . . . . . . . . . . . . 68
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 68
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 69
12.1. Normative References . . . . . . . . . . . . . . . . . . 69
12.2. Informative References . . . . . . . . . . . . . . . . . 71
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 72
1. Introduction
Certificates in the Web PKI [RFC5280] are most commonly used to
authenticate domain names. Thus, certificate authorities in the Web
Barnes, et al. Expires August 7, 2017 [Page 3]
Internet-Draft ACME February 2017
PKI are trusted to verify that an applicant for a certificate
legitimately represents the domain name(s) in the certificate.
Different types of certificates reflect different kinds of CA
verification of information about the certificate subject. "Domain
Validation" (DV) certificates are by far the most common type. For
DV validation, the CA merely verifies that the requester has
effective control of the web server and/or DNS server for the domain,
but does not explicitly attempt to verify their real-world identity.
(This is as opposed to "Organization Validation" (OV) and "Extended
Validation" (EV) certificates, where the process is intended to also
verify the real-world identity of the requester.)
Existing Web PKI certificate authorities tend to run on a set of ad
hoc protocols for certificate issuance and identity verification. In
the case of DV certificates, a typical user experience is something
like:
o Generate a PKCS#10 [RFC2986] Certificate Signing Request (CSR).
o Cut-and-paste the CSR into a CA web page.
o Prove ownership of the domain by one of the following methods:
* Put a CA-provided challenge at a specific place on the web
server.
* Put a CA-provided challenge at a DNS location corresponding to
the target domain.
* Receive CA challenge at a (hopefully) administrator-controlled
e-mail address corresponding to the domain and then respond to
it on the CA's web page.
o Download the issued certificate and install it on their Web
Server.
With the exception of the CSR itself and the certificates that are
issued, these are all completely ad hoc procedures and are
accomplished by getting the human user to follow interactive natural-
language instructions from the CA rather than by machine-implemented
published protocols. In many cases, the instructions are difficult
to follow and cause significant confusion. Informal usability tests
by the authors indicate that webmasters often need 1-3 hours to
obtain and install a certificate for a domain. Even in the best
case, the lack of published, standardized mechanisms presents an
obstacle to the wide deployment of HTTPS and other PKIX-dependent
Barnes, et al. Expires August 7, 2017 [Page 4]
Internet-Draft ACME February 2017
systems because it inhibits mechanization of tasks related to
certificate issuance, deployment, and revocation.
This document describes an extensible framework for automating the
issuance and domain validation procedure, thereby allowing servers
and infrastructural software to obtain certificates without user
interaction. Use of this protocol should radically simplify the
deployment of HTTPS and the practicality of PKIX authentication for
other protocols based on TLS [RFC5246].
2. Deployment Model and Operator Experience
The guiding use case for ACME is obtaining certificates for Web sites
(HTTPS [RFC2818]). In this case, the user's web server is intended
to speak for one or more domains, and the process of certificate
issuance is intended to verify that this server actually speaks for
the domain(s).
DV certificate validation commonly checks claims about properties
related to control of a domain name - properties that can be observed
by the issuing authority in an interactive process that can be
conducted purely online. That means that under typical
circumstances, all steps in the request, verification, and issuance
process can be represented and performed by Internet protocols with
no out-of-band human intervention.
At time of writing, when deploying an HTTPS server, an operator
typically gets a prompt to generate a self-signed certificate. If
the operator were instead deploying an ACME-compatible web server,
the experience would be something like this:
o The ACME client prompts the operator for the intended domain
name(s) that the web server is to stand for.
o The ACME client presents the operator with a list of CAs from
which it could get a certificate. (This list will change over
time based on the capabilities of CAs and updates to ACME
configuration.) The ACME client might prompt the operator for
payment information at this point.
o The operator selects a CA.
o In the background, the ACME client contacts the CA and requests
that a certificate be issued for the intended domain name(s).
o Once the CA is satisfied, the certificate is issued and the ACME
client automatically downloads and installs it, potentially
notifying the operator via e-mail, SMS, etc.
Barnes, et al. Expires August 7, 2017 [Page 5]
Internet-Draft ACME February 2017
o The ACME client periodically contacts the CA to get updated
certificates, stapled OCSP responses, or whatever else would be
required to keep the server functional and its credentials up-to-
date.
In this way, it would be nearly as easy to deploy with a CA-issued
certificate as with a self-signed certificate. Furthermore, the
maintenance of that CA-issued certificate would require minimal
manual intervention. Such close integration of ACME with HTTPS
servers would allow the immediate and automated deployment of
certificates as they are issued, sparing the human administrator from
much of the time-consuming work described in the previous section.
3. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
The two main roles in ACME are "client" and "server". The ACME
client uses the protocol to request certificate management actions,
such as issuance or revocation. An ACME client therefore typically
runs on a web server, mail server, or some other server system which
requires valid TLS certificates. The ACME server runs at a
certificate authority, and responds to client requests, performing
the requested actions if the client is authorized.
An ACME client is represented by an "account key pair". The client
uses the private key of this key pair to sign all messages sent to
the server. The server uses the public key to verify the
authenticity and integrity of messages from the client.
4. Protocol Overview
ACME allows a client to request certificate management actions using
a set of JSON messages carried over HTTPS. In many ways, ACME
functions much like a traditional CA, in which a user creates an
account, requests a certificate, and proves control of the domains in
that certificate in order for the CA to sign the requested
certificate.
The first phase of ACME is for the client to request an account with
the ACME server. The client generates an asymmetric key pair and
requests a new account, optionally providing contact information,
agreeing to terms of service, and/or associating the account with an
existing account in another system. The creation request is signed
with the generated private key to prove that the client controls it.
Barnes, et al. Expires August 7, 2017 [Page 6]
Internet-Draft ACME February 2017
Client Server
Contact Information
ToS Agreement
Additional Data
Signature ------->
<------- Account
Once an account is registered, there are three major steps the client
needs to take to get a certificate:
1. Submit an order for a certificate to be issued
2. Prove control of any identifiers requested in the certificate
3. Await issuance and download the issued certificate
The client's order for a certificate describes the desired
certificate using a PKCS#10 Certificate Signing Request (CSR) plus a
few additional fields that capture semantics that are not supported
in the CSR format. If the server is willing to consider issuing such
a certificate, it responds with a list of requirements that the
client must satisfy before the certificate will be issued.
For example, in most cases, the server will require the client to
demonstrate that it controls the identifiers in the requested
certificate. Because there are many different ways to validate
possession of different types of identifiers, the server will choose
from an extensible set of challenges that are appropriate for the
identifier being claimed. The client responds with a set of
responses that tell the server which challenges the client has
completed. The server then validates the challenges to check that
the client has accomplished the challenge.
Once the validation process is complete and the server is satisfied
that the client has met its requirements, the server will issue the
requested certificate and make it available to the client.
Barnes, et al. Expires August 7, 2017 [Page 7]
Internet-Draft ACME February 2017
Order
Signature ------->
Required
<------- Authorizations
Responses
Signature ------->
<~~~~~~~~Validation~~~~~~~~>
<------- Certificate
To revoke a certificate, the client sends a signed revocation request
indicating the certificate to be revoked:
Client Server
Revocation request
Signature -------->
<-------- Result
Note that while ACME is defined with enough flexibility to handle
different types of identifiers in principle, the primary use case
addressed by this document is the case where domain names are used as
identifiers. For example, all of the identifier validation
challenges described in Section 7 below address validation of domain
names. The use of ACME for other identifiers will require further
specification, in order to describe how these identifiers are encoded
in the protocol, and what types of validation challenges the server
might require.
5. Message Transport
Communications between an ACME client and an ACME server are done
over HTTPS, using JSON Web Signature (JWS) [RFC7515] to provide some
additional security properties for messages sent from the client to
the server. HTTPS provides server authentication and
confidentiality. With some ACME-specific extensions, JWS provides
authentication of the client's request payloads, anti-replay
protection, and integrity for the HTTPS request URI.
5.1. HTTPS Requests
Each ACME function is accomplished by the client sending a sequence
of HTTPS requests to the server, carrying JSON messages
[RFC2818][RFC7159]. Use of HTTPS is REQUIRED. Clients SHOULD
support HTTP public key pinning [RFC7469], and servers SHOULD emit
Barnes, et al. Expires August 7, 2017 [Page 8]
Internet-Draft ACME February 2017
pinning headers. Each subsection of Section 6 below describes the
message formats used by the function, and the order in which messages
are sent.
In most HTTPS transactions used by ACME, the ACME client is the HTTPS
client and the ACME server is the HTTPS server. The ACME server acts
as an HTTP and HTTPS client when validating challenges via HTTP.
ACME clients SHOULD send a User-Agent header in accordance with
[RFC7231], including the name and version of the ACME software in
addition to the name and version of the underlying HTTP client
software.
ACME clients SHOULD send an Accept-Language header in accordance with
[RFC7231] to enable localization of error messages.
ACME servers that are intended to be generally accessible need to use
Cross-Origin Resource Sharing (CORS) in order to be accessible from
browser-based clients [W3C.CR-cors-20130129]. Such servers SHOULD
set the Access-Control-Allow-Origin header field to the value "*".
Binary fields in the JSON objects used by ACME are encoded using
base64url encoding described in [RFC4648] Section 5, according to the
profile specified in JSON Web Signature [RFC7515] Section 2. This
encoding uses a URL safe character set. Trailing '=' characters MUST
be stripped.
5.2. Request Authentication
All ACME requests with a non-empty body MUST encapsulate their
payload in a JWS object, signed (in most cases) using the account's
private key. The server MUST verify the JWS before processing the
request. Encapsulating request bodies in JWS provides a simple
authentication of requests.
JWS objects sent in ACME requests MUST meet the following additional
criteria:
o The JWS MUST be encoded using UTF-8
o The JWS MUST NOT have the value "none" in its "alg" field
o The JWS MUST NOT have a MAC-based algorithm in its "alg" field
o The JWS Protected Header MUST include the following fields:
* "alg"
Barnes, et al. Expires August 7, 2017 [Page 9]
Internet-Draft ACME February 2017
* "jwk" (only for requests to new-reg and revoke-cert resources)
* "kid" (for all other requests).
* "nonce" (defined below)
* "url" (defined below)
The "jwk" and "kid" fields are mutually exclusive. Servers MUST
reject requests that contain both.
For new-reg requests, and for revoke-cert requests authenticated by
certificate key, there MUST be a "jwk" field.
For all other requests, there MUST be a "kid" field. This field must
contain the account URI received by POSTing to the new-reg resource.
Note that authentication via signed JWS request bodies implies that
GET requests are not authenticated. Servers MUST NOT respond to GET
requests for resources that might be considered sensitive.
If the client sends a JWS signed with an algorithm that the server
does not support, then the server MUST return an error with status
code 400 (Bad Request) and type
"urn:ietf:params:acme:error:badSignatureAlgorithm". The problem
document returned with the error MUST include an "algorithms" field
with an array of supported "alg" values.
HTTP/1.1 400 Bad Request
Replay-Nonce: IXVHDyxIRGcTE0VSblhPzw
Content-Type: application/problem+json
Content-Language: en
{
"type": "urn:ietf:params:acme:error:badSignatureAlgorithm",
"detail": "Algorithm 'ES384' is not supported",
"algorithms": ["RS256", "RS384", "ES256"]
}
In the examples below, JWS objects are shown in the JSON or flattened
JSON serialization, with the protected header and payload expressed
as base64url(content) instead of the actual base64-encoded value, so
that the content is readable. Some fields are omitted for brevity,
marked with "...".
Barnes, et al. Expires August 7, 2017 [Page 10]
Internet-Draft ACME February 2017
5.3. Equivalence of JWKs
At some points in the protocol, it is necessary for the server to
determine whether two JSON Web Key (JWK) [RFC7517] objects represent
the same key. In performing these checks, the server MUST consider
two JWKs to match if and only if they have the identical values in
all fields included in the computation of a JWK thumbprint for that
key. That is, the keys must have the same "kty" value and contain
identical values in the fields used in the computation of a JWK
thumbprint for that key type:
o "RSA": "n", "e"
o "EC": "crv", "x", "y"
Note that this comparison is equivalent to computing the JWK
thumbprints of the two keys and comparing thumbprints. The only
difference is that there is no requirement for a hash computation
(and thus it is independent of the choice of hash function) and no
risk of hash collision.
5.4. Request URI Integrity
It is common in deployment for the entity terminating TLS for HTTPS
to be different from the entity operating the logical HTTPS server,
with a "request routing" layer in the middle. For example, an ACME
CA might have a content delivery network terminate TLS connections
from clients so that it can inspect client requests for denial-of-
service protection.
These intermediaries can also change values in the request that are
not signed in the HTTPS request, e.g., the request URI and headers.
ACME uses JWS to provide an integrity mechanism, which protects
against an intermediary changing the request URI to another ACME URI.
As noted above, all ACME request objects carry a "url" parameter in
their protected header. This header parameter encodes the URL to
which the client is directing the request. On receiving such an
object in an HTTP request, the server MUST compare the "url"
parameter to the request URI. If the two do not match, then the
server MUST reject the request as unauthorized.
Except for the directory resource, all ACME resources are addressed
with URLs provided to the client by the server. For these resources,
the client MUST set the "url" field to the exact string provided by
the server (rather than performing any re-encoding on the URL). The
server SHOULD perform the corresponding string equality check,
configuring each resource with the URL string provided to clients and
Barnes, et al. Expires August 7, 2017 [Page 11]
Internet-Draft ACME February 2017
having the resource check that requests have the same string in their
"url" fields.
5.4.1. "url" (URL) JWS header parameter
The "url" header parameter specifies the URL [RFC3986] to which this
JWS object is directed. The "url" parameter MUST be carried in the
protected header of the JWS. The value of the "url" header MUST be a
JSON string representing the URL.
5.5. Replay protection
In order to protect ACME resources from any possible replay attacks,
ACME requests have a mandatory anti-replay mechanism. This mechanism
is based on the server maintaining a list of nonces that it has
issued to clients, and requiring any signed request from the client
to carry such a nonce.
An ACME server provides nonces to clients using the Replay-Nonce
header field, as specified below. The server MUST include a Replay-
Nonce header field in every successful response to a POST request,
and SHOULD provide it in error responses as well.
Every JWS sent by an ACME client MUST include, in its protected
header, the "nonce" header parameter, with contents as defined below.
As part of JWS verification, the ACME server MUST verify that the
value of the "nonce" header is a value that the server previously
provided in a Replay-Nonce header field. Once a nonce value has
appeared in an ACME request, the server MUST consider it invalid, in
the same way as a value it had never issued.
When a server rejects a request because its nonce value was
unacceptable (or not present), it MUST provide HTTP status code 400
(Bad Request), and indicate the ACME error code
"urn:ietf:params:acme:error:badNonce". An error response with the
"badNonce" error code MUST include a Replay-Nonce header with a fresh
nonce. On receiving such a response, a client SHOULD retry the
request using the new nonce.
The precise method used to generate and track nonces is up to the
server. For example, the server could generate a random 128-bit
value for each response, keep a list of issued nonces, and strike
nonces from this list as they are used.
Barnes, et al. Expires August 7, 2017 [Page 12]
Internet-Draft ACME February 2017
5.5.1. Replay-Nonce
The "Replay-Nonce" header field includes a server-generated value
that the server can use to detect unauthorized replay in future
client requests. The server should generate the value provided in
Replay-Nonce in such a way that they are unique to each message, with
high probability.
The value of the Replay-Nonce field MUST be an octet string encoded
according to the base64url encoding described in Section 2 of
[RFC7515]. Clients MUST ignore invalid Replay-Nonce values.
base64url = [A-Z] / [a-z] / [0-9] / "-" / "_"
Replay-Nonce = *base64url
The Replay-Nonce header field SHOULD NOT be included in HTTP request
messages.
5.5.2. "nonce" (Nonce) JWS header parameter
The "nonce" header parameter provides a unique value that enables the
verifier of a JWS to recognize when replay has occurred. The "nonce"
header parameter MUST be carried in the protected header of the JWS.
The value of the "nonce" header parameter MUST be an octet string,
encoded according to the base64url encoding described in Section 2 of
[RFC7515]. If the value of a "nonce" header parameter is not valid
according to this encoding, then the verifier MUST reject the JWS as
malformed.
5.6. Rate limits
Creation of resources can be rate limited to ensure fair usage and
prevent abuse. Once the rate limit is exceeded, the server MUST
respond with an error with the type
"urn:ietf:params:acme:error:rateLimited". Additionally, the server
SHOULD send a "Retry-After" header indicating when the current
request may succeed again. If multiple rate limits are in place,
that is the time where all rate limits allow access again for the
current request with exactly the same parameters.
In addition to the human readable "detail" field of the error
response, the server MAY send one or multiple tokens in the "Link"
header pointing to documentation about the specific hit rate limits
using the "rate-limit" relation.
Barnes, et al. Expires August 7, 2017 [Page 13]
Internet-Draft ACME February 2017
5.7. Errors
Errors can be reported in ACME both at the HTTP layer and within ACME
payloads. ACME servers can return responses with an HTTP error
response code (4XX or 5XX). For example: If the client submits a
request using a method not allowed in this document, then the server
MAY return status code 405 (Method Not Allowed).
When the server responds with an error status, it SHOULD provide
additional information using problem document [RFC7807]. To
facilitate automatic response to errors, this document defines the
following standard tokens for use in the "type" field (within the
"urn:ietf:params:acme:error:" namespace):
Barnes, et al. Expires August 7, 2017 [Page 14]
Internet-Draft ACME February 2017
+-----------------------+-------------------------------------------+
| Code | Description |
+-----------------------+-------------------------------------------+
| badCSR | The CSR is unacceptable (e.g., due to a |
| | short key) |
| | |
| badNonce | The client sent an unacceptable anti- |
| | replay nonce |
| | |
| badSignatureAlgorithm | The JWS was signed with an algorithm the |
| | server does not support |
| | |
| caa | CAA records forbid the CA from issuing |
| | |
| connection | The server could not connect to |
| | validation target |
| | |
| dnssec | DNSSEC validation failed |
| | |
| invalidContact | The contact URI for a registration was |
| | invalid |
| | |
| malformed | The request message was malformed |
| | |
| rateLimited | The request exceeds a rate limit |
| | |
| rejectedIdentifier | The server will not issue for the |
| | identifier |
| | |
| serverInternal | The server experienced an internal error |
| | |
| tls | The server received a TLS error during |
| | validation |
| | |
| unauthorized | The client lacks sufficient authorization |
| | |
| unknownHost | The server could not resolve a domain |
| | name |
| | |
| unsupportedIdentifier | Identifier is not supported, but may be |
| | in future |
| | |
| userActionRequired | The user visit the "instance" URL and |
| | take actions specified there |
+-----------------------+-------------------------------------------+
This list is not exhaustive. The server MAY return errors whose
"type" field is set to a URI other than those defined above. Servers
Barnes, et al. Expires August 7, 2017 [Page 15]
Internet-Draft ACME February 2017
MUST NOT use the ACME URN namespace for errors other than the
standard types. Clients SHOULD display the "detail" field of all
errors.
Authorization and challenge objects can also contain error
information to indicate why the server was unable to validate
authorization.
6. Certificate Management
In this section, we describe the certificate management functions
that ACME enables:
o Account Creation
o Ordering a Certificate
o Identifier Authorization
o Certificate Issuance
o Certificate Revocation
6.1. Resources
ACME is structured as a REST application with a few types of
resources:
o Account resources, representing information about an account
o Order resources, representing an account's requests to issue
certificates
o Authorization resources, representing an account's authorization
to act for an identifier
o Challenge resources, representing a challenge to prove control of
an identifier
o Certificate resources, representing issued certificates
o A "directory" resource
o A "new-nonce" resource
o A "new-account" resource
o A "new-order" resource
Barnes, et al. Expires August 7, 2017 [Page 16]
Internet-Draft ACME February 2017
o A "revoke-certificate" resource
o A "key-change" resource
The server MUST provide "directory" and "new-nonce" resources.
ACME uses different URIs for different management functions. Each
function is listed in a directory along with its corresponding URI,
so clients only need to be configured with the directory URI. These
URIs are connected by a few different link relations [RFC5988].
The "up" link relation is used with challenge resources to indicate
the authorization resource to which a challenge belongs. It is also
used from certificate resources to indicate a resource from which the
client may fetch a chain of CA certificates that could be used to
validate the certificate in the original resource.
The "directory" link relation is present on all resources other than
the directory and indicates the directory URL.
The following diagram illustrates the relations between resources on
an ACME server. For the most part, these relations are expressed by
URLs provided as strings in the resources' JSON representations.
Lines with labels in quotes indicate HTTP link relations
directory
|
|--> new-nonce
|
--------------------------------------------------+
| | | |
| | | |
V V V V
new-account new-authz new-order revoke-cert
| | | ^
| | | "author" | "revoke"
V | V <-------- |
acct | order --------> cert ---------+
| | ^ |
| | | "up" | "up"
| V | V
+------> authz cert-chain
| ^
| | "up"
V |
challenge
Barnes, et al. Expires August 7, 2017 [Page 17]
Internet-Draft ACME February 2017
The following table illustrates a typical sequence of requests
required to establish a new account with the server, prove control of
an identifier, issue a certificate, and fetch an updated certificate
some time after issuance. The "->" is a mnemonic for a Location
header pointing to a created resource.
+----------------------+------------------+----------------+
| Action | Request | Response |
+----------------------+------------------+----------------+
| Get a nonce | HEAD new-nonce | 204 |
| | | |
| Create account | POST new-account | 201 -> account |
| | | |
| Submit an order | POST new-order | 201 -> order |
| | | |
| Fetch challenges | GET authz | 200 |
| | | |
| Respond to challenge | POST challenge | 200 |
| | | |
| Poll for status | GET authz | 200 |
| | | |
| Check for new cert | GET cert | 200 |
+----------------------+------------------+----------------+
The remainder of this section provides the details of how these
resources are structured and how the ACME protocol makes use of them.
6.1.1. Directory
In order to help clients configure themselves with the right URIs for
each ACME operation, ACME servers provide a directory object. This
should be the only URL needed to configure clients. It is a JSON
dictionary, whose keys are drawn from the following table and whose
values are the corresponding URLs.
Barnes, et al. Expires August 7, 2017 [Page 18]
Internet-Draft ACME February 2017
+-------------+--------------------+
| Key | URL in value |
+-------------+--------------------+
| new-nonce | New nonce |
| | |
| new-account | New account |
| | |
| new-order | New order |
| | |
| new-authz | New authorization |
| | |
| revoke-cert | Revoke certificate |
| | |
| key-change | Key change |
+-------------+--------------------+
There is no constraint on the actual URI of the directory except that
it should be different from the other ACME server resources' URIs,
and that it should not clash with other services. For instance:
o a host which function as both an ACME and Web server may want to
keep the root path "/" for an HTML "front page", and and place the
ACME directory under path "/acme".
o a host which only functions as an ACME server could place the
directory under path "/".
The dictionary MAY additionally contain a key "meta". If present, it
MUST be a JSON dictionary; each item in the dictionary is an item of
metadata relating to the service provided by the ACME server.
The following metadata items are defined, all of which are OPTIONAL:
"terms-of-service" (optional, string): A URI identifying the current
terms of service.
"website" (optional, string): An HTTP or HTTPS URL locating a
website providing more information about the ACME server.
"caa-identities" (optional, array of string): Each string MUST be a
lowercase hostname which the ACME server recognises as referring
to itself for the purposes of CAA record validation as defined in
[RFC6844]. This allows clients to determine the correct issuer
domain name to use when configuring CAA record.
Clients access the directory by sending a GET request to the
directory URI.
Barnes, et al. Expires August 7, 2017 [Page 19]
Internet-Draft ACME February 2017
HTTP/1.1 200 OK
Content-Type: application/json
{
"new-nonce": "https://example.com/acme/new-nonce",
"new-account": "https://example.com/acme/new-account",
"new-order": "https://example.com/acme/new-order",
"new-authz": "https://example.com/acme/new-authz",
"revoke-cert": "https://example.com/acme/revoke-cert",
"key-change": "https://example.com/acme/key-change",
"meta": {
"terms-of-service": "https://example.com/acme/terms",
"website": "https://www.example.com/",
"caa-identities": ["example.com"]
}
}
6.1.2. Account Objects
An ACME account resource represents a set of metadata associated to
an account. Account resources have the following structure:
key (required, dictionary): The public key of the account's key
pair, encoded as a JSON Web Key object [RFC7517]. The client may
not directly update this field, but must use the key-change
resource instead.
status (required, string): The status of this account. Possible
values are: "valid", "deactivated", and "revoked". The value
"deactivated" should be used to indicate user initiated
deactivation whereas "revoked" should be used to indicate
administratively initiated deactivation.
contact (optional, array of string): An array of URIs that the
server can use to contact the client for issues related to this
account. For example, the server may wish to notify the client
about server-initiated revocation or certificate expiration.
terms-of-service-agreed (optional, boolean): Including this field in
a new-account request, with a value of true, indicates the
client's agreement with the terms of service. This field is not
updateable by the client.
orders (required, string): A URI from which a list of orders
submitted by this account can be fetched via a GET request, as
described in Section 6.1.2.1
Barnes, et al. Expires August 7, 2017 [Page 20]
Internet-Draft ACME February 2017
{
"contact": [
"mailto:cert-admin@example.com",
"tel:+12025551212"
],
"terms-of-service-agreed": true,
"orders": "https://example.com/acme/acct/1/orders"
}
6.1.2.1. Orders List
Each account object includes an "orders" URI from which a list of
orders created by the account can be fetched via GET request. The
result of the GET request MUST be a JSON object whose "orders" field
is an array of URIs, each identifying an order belonging to the
account. The server SHOULD include pending orders, and SHOULD NOT
include orders that are invalid in the array of URIs. The server MAY
return an incomplete list, along with a Link header with link
relation "next" indicating a URL to retrieve further entries.
HTTP/1.1 200 OK
Content-Type: application/json
Link: href="/acme/acct/1/orders?cursor=2", rel="next"
{
"orders": [
"https://example.com/acme/acct/1/order/1",
"https://example.com/acme/acct/1/order/2",
/* 47 more URLs not shown for example brevity */
"https://example.com/acme/acct/1/order/50"
]
}
6.1.3. Order Objects
An ACME order object represents a client's request for a certificate,
and is used to track the progress of that order through to issuance.
Thus, the object contains information about the requested
certificate, the authorizations that the server requires the client
to complete, and any certificates that have resulted from this order.
status (required, string): The status of this order. Possible
values are: "pending", "processing", "valid", and "invalid".
expires (optional, string): The timestamp after which the server
will consider this order invalid, encoded in the format specified
in RFC 3339 [RFC3339]. This field is REQUIRED for objects with
"pending" or "valid" in the status field.
Barnes, et al. Expires August 7, 2017 [Page 21]
Internet-Draft ACME February 2017
csr (required, string): A CSR encoding the parameters for the
certificate being requested [RFC2986]. The CSR is sent in the
base64url-encoded version of the DER format. (Note: Because this
field uses base64url, and does not include headers, it is
different from PEM.)
notBefore (optional, string): The requested value of the notBefore
field in the certificate, in the date format defined in [RFC3339]
notAfter (optional, string): The requested value of the notAfter
field in the certificate, in the date format defined in [RFC3339]
authorizations (required, array of string): For pending orders, the
authorizations that the client needs to complete before the
requested certificate can be issued (see Section 6.5). For final
orders, the authorizations that were completed. Each entry is a
URL from which an authorization can be fetched with a GET request.
certificate (optional, string): A URL for the certificate that has
been issued in response to this order.
{
"status": "pending",
"expires": "2015-03-01T14:09:00Z",
"csr": "jcRf4uXra7FGYW5ZMewvV...rhlnznwy8YbpMGqwidEXfE",
"notBefore": "2016-01-01T00:00:00Z",
"notAfter": "2016-01-08T00:00:00Z",
"authorizations": [
"https://example.com/acme/authz/1234",
"https://example.com/acme/authz/2345"
],
"certificate": "https://example.com/acme/cert/1234"
}
The elements of the "authorizations" array are immutable once set.
The server MUST NOT change the contents of the "authorizations" array
after it is created. If a client observes a change in the contents
of the "authorizations" array, then it SHOULD consider the order
invalid.
The "authorizations" array in the challenge SHOULD reflect all
authorizations that the CA takes into account in deciding to issue,
even if some authorizations were fulfilled in earlier orders or in
pre-authorization transactions. For example, if a CA allows multiple
Barnes, et al. Expires August 7, 2017 [Page 22]
Internet-Draft ACME February 2017
orders to be fufilled based on a single authorization transaction,
then it SHOULD reflect that authorization in all of the order.
6.1.4. Authorization Objects
An ACME authorization object represents a server's authorization for
an account to represent an identifier. In addition to the
identifier, an authorization includes several metadata fields, such
as the status of the authorization (e.g., "pending", "valid", or
"revoked") and which challenges were used to validate possession of
the identifier.
The structure of an ACME authorization resource is as follows:
identifier (required, dictionary of string): The identifier that the
account is authorized to represent
type (required, string): The type of identifier.
value (required, string): The identifier itself.
status (required, string): The status of this authorization.
Possible values are: "pending", "processing", "valid", "invalid"
and "revoked". If this field is missing, then the default value
is "pending".
expires (optional, string): The timestamp after which the server
will consider this authorization invalid, encoded in the format
specified in RFC 3339 [RFC3339]. This field is REQUIRED for
objects with "valid" in the "status" field.
scope (optional, string): If this field is present, then it MUST
contain a URI for an order resource, such that this authorization
is only valid for that resource. If this field is absent, then
the CA MUST consider this authorization valid for all orders until
the authorization expires. [[ Open issue: More flexible scoping?
]]
challenges (required, array): The challenges that the client can
fulfill in order to prove possession of the identifier (for
pending authorizations). For final authorizations, the challenges
that were used. Each array entry is a dictionary with parameters
required to validate the challenge, as specified in Section 7. A
client should attempt to fulfill at most one of these challenges,
and a server should consider any one of the challenges sufficient
to make the authorization valid.
Barnes, et al. Expires August 7, 2017 [Page 23]
Internet-Draft ACME February 2017
The only type of identifier defined by this specification is a fully-
qualified domain name (type: "dns"). The value of the identifier
MUST be the ASCII representation of the domain name. If a domain
name contains Unicode characters it MUST be encoded using the rules
defined in [RFC3492]. Servers MUST verify any identifier values that
begin with the ASCII Compatible Encoding prefix "xn-" as defined in
[RFC5890] are properly encoded. Wildcard domain names (with "*" as
the first label) MUST NOT be included in authorization objects.
{
"status": "valid",
"expires": "2015-03-01T14:09:00Z",
"identifier": {
"type": "dns",
"value": "example.org"
},
"challenges": [
{
"type": "http-01",
"status": "valid",
"validated": "2014-12-01T12:05:00Z",
"keyAuthorization": "SXQe-2XODaDxNR...vb29HhjjLPSggwiE"
}
]
}
6.2. Getting a Nonce
Before sending a POST request to the server, an ACME client needs to
have a fresh anti-replay nonce to put in the "nonce" header of the
JWS. In most cases, the client will have gotten a nonce from a
previous request. However, the client might sometimes need to get a
new nonce, e.g., on its first request to the server or if an existing
nonce is no longer valid.
To get a fresh nonce, the client sends a HEAD request to the new-
nonce resource on the server. The server's response MUST include a
Replay-Nonce header field containing a fresh nonce, and SHOULD have
status code 204 (No Content). The server SHOULD also respond to GET
requests for this resource, returning an empty body (while still
providing a Replay-Nonce header).
Barnes, et al. Expires August 7, 2017 [Page 24]
Internet-Draft ACME February 2017
HEAD /acme/new-nonce HTTP/1.1
Host: example.com
HTTP/1.1 204 No Content
Replay-Nonce: oFvnlFP1wIhRlYS2jTaXbA
Cache-Control: no-store
Proxy caching of responses from the new-nonce resource can cause
clients receive the same nonce repeatedly, leading to badNonce
errors. The server MUST include a Cache-Control header field with
the "no-store" directive in responses for the new-nonce resource, in
order to prevent caching of this resource.
6.3. Account Creation
A client creates a new account with the server by sending a POST
request to the server's new-account URI. The body of the request is
a stub account object containing only the "contact" field.
POST /acme/new-account HTTP/1.1
Host: example.com
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"jwk": {...},
"nonce": "6S8IqOGY7eL2lsGoTZYifg",
"url": "https://example.com/acme/new-account"
}),
"payload": base64url({
"terms-of-service-agreed": true,
"contact": [
"mailto:cert-admin@example.com",
"tel:+12025551212"
]
}),
"signature": "RZPOnYoPs1PhjszF...-nh6X1qtOFPB519I"
}
The server MUST ignore any values provided in the "key", and "orders"
fields in account bodies sent by the client, as well as any other
fields that it does not recognize. If new fields are specified in
the future, the specification of those fields MUST describe whether
they may be provided by the client.
In general, the server MUST ignore any fields in the request object
that it does not recognize. In particular, it MUST NOT reflect
Barnes, et al. Expires August 7, 2017 [Page 25]
Internet-Draft ACME February 2017
unrecognized fields in the resulting account object. This allows
clients to detect when servers do not support an extension field.
The server SHOULD validate that the contact URLs in the "contact"
field are valid and supported by the server. If the client provides
the server with an invalid or unsupported contact URL, then the
server MUST return an error of type "invalidContact", with a
description describing the error and what types of contact URL the
server considers acceptable.
The server creates an account object with the included contact
information. The "key" element of the account is set to the public
key used to verify the JWS (i.e., the "jwk" element of the JWS
header). The server returns this account object in a 201 (Created)
response, with the account URI in a Location header field.
If the server already has an account registered with the provided
account key, then it MUST return a 200 (OK) response and provide the
URI of that account in a Content-Location header field. This allows
a client that has an account key but not the corresponding account
URI to recover the account URI.
If the server wishes to present the client with terms under which the
ACME service is to be used, it MUST indicate the URI where such terms
can be accessed in the "terms-of-service" subfield of the "meta"
field in the directory object, and the server MUST reject new-account
requests that do not have the "terms-of-service-agreed" set to
"true". Clients SHOULD NOT automatically agree to terms by default.
Rather, they SHOULD require some user interaction for agreement to
terms.
HTTP/1.1 201 Created
Content-Type: application/json
Replay-Nonce: D8s4D2mLs8Vn-goWuPQeKA
Location: https://example.com/acme/acct/1
Link: <https://example.com/acme/some-directory>;rel="directory"
{
"key": { /* JWK from JWS header */ },
"status": "valid",
"contact": [
"mailto:cert-admin@example.com",
"tel:+12025551212"
]
}
Barnes, et al. Expires August 7, 2017 [Page 26]
Internet-Draft ACME February 2017
If the client wishes to update this information in the future, it
sends a POST request with updated information to the account URI.
The server MUST ignore any updates to the "key", or "order" fields or
any other fields it does not recognize. The server MUST verify that
the request is signed with the private key corresponding to the "key"
field of the request before updating the registration.
For example, to update the contact information in the above account,
the client could send the following request:
POST /acme/acct/1 HTTP/1.1
Host: example.com
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"kid": "https://example.com/acme/acct/1",
"nonce": "ax5RnthDqp_Yf4_HZnFLmA",
"url": "https://example.com/acme/acct/1"
}),
"payload": base64url({
"contact": [
"mailto:certificates@example.com",
"tel:+12125551212"
]
}),
"signature": "hDXzvcj8T6fbFbmn...rDzXzzvzpRy64N0o"
}
Servers SHOULD NOT respond to GET requests for account resources as
these requests are not authenticated. If a client wishes to query
the server for information about its account (e.g., to examine the
"contact" or "certificates" fields), then it SHOULD do so by sending
a POST request with an empty update. That is, it should send a JWS
whose payload is trivial ({}).
6.3.1. Changes of Terms of Service
As described above, a client can indicate its agreement with the CA's
terms of service by setting the "terms-of-service-agreed" field in
its account object to "true".
If the server has changed its terms of service since a client
initially agreed, and the server is unwilling to process a request
without explicit agreement to the new terms, then it MUST return an
error response with status code 403 (Forbidden) and type
"urn:ietf:params:acme:error:userActionRequired". This response MUST
Barnes, et al. Expires August 7, 2017 [Page 27]
Internet-Draft ACME February 2017
include a Link header with link relation "terms-of-service" and the
latest terms-of-service URL.
The problem document returned with the error MUST also include an
"instance" field, indicating a URL that the client should direct a
human user to visit in order for instructions on how to agree to the
terms.
HTTP/1.1 403 Forbidden
Replay-Nonce: IXVHDyxIRGcTE0VSblhPzw
Content-Type: application/problem+json
Content-Language: en
{
"type": "urn:ietf:params:acme:error:userActionRequired",
"detail": "Terms of service have changed",
"instance": "http://example.com/agreement/?token=W8Ih3PswD-8"
}
6.3.2. External Account Binding
The server MAY require a value to be present for the "external-
account-binding" field. This can be used to an ACME account with an
existing account in a non-ACME system, such as a CA customer
database.
To enable ACME account binding, a CA needs to provision the ACME
client with a MAC key and a key identifier. The key identifier MUST
be an ASCII string. The MAC key SHOULD be provided in base64url-
encoded form, to maximize compatibility between provisioning systems
and ACME clients.
The ACME client then computes a binding JWS to indicate the external
account's approval of the ACME account key. The payload of this JWS
is the account key being registered, in JWK form. The protected
header of the JWS MUST meet the following criteria:
o The "alg" field MUST indicate a MAC-based algorithm
o The "kid" field MUST contain the key identifier provided by the CA
o The "nonce" field MUST NOT be present
o The "url" field MUST be set to the same value as the outer JWS
The "signature" field of the JWS will contain the MAC value computed
with the MAC key provided by the CA.
Barnes, et al. Expires August 7, 2017 [Page 28]
Internet-Draft ACME February 2017
POST /acme/new-reg HTTP/1.1
Host: example.com
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"jwk": /* account key */,
"nonce": "K60BWPrMQG9SDxBDS_xtSw",
"url": "https://example.com/acme/new-account"
}),
"payload": base64url({
"contact": ["mailto:example@anonymous.invalid"],
"terms-of-service-agreed": true,
"external-account-binding": {
"protected": base64url({
"alg": "HS256",
"kid": /* key identifier from CA */,
"url": "https://example.com/acme/new-account"
}),
"payload": base64url(/* same as in "jwk" above */),
"signature": /* MAC using MAC key from CA */
}
}),
"signature": "5TWiqIYQfIDfALQv...x9C2mg8JGPxl5bI4"
}
When a CA receives a new-account request containing an "external-
account-binding" field, it must decide whether or not to verify the
binding. If the CA does not verify the binding, then it MUST NOT
reflect the "external-account-binding" field in the resulting account
object (if any). To verify the account binding, the CA MUST take the
following steps:
1. Verify that the value of the field is a well-formed JWS
2. Verify that the JWS protected meets the above criteria
3. Retrieve the MAC key corresponding to the key identifier in the
"kid" field
4. Verify that the MAC on the JWS verifies using that MAC key
5. Verify that the payload of the JWS represents the same key as was
used to verify the outer JWS (i.e., the "jwk" field of the outer
JWS)
Barnes, et al. Expires August 7, 2017 [Page 29]
Internet-Draft ACME February 2017
If all of these checks pass and the CA creates a new account, then
the CA may consider the new account associated with the external
account corresponding to the MAC key, and MUST reflect value of the
"external-account-binding" field in the resulting account object. If
any of these checks fail, then the CA MUST reject the new-
registration request.
6.3.3. Account Key Roll-over
A client may wish to change the public key that is associated with a
account in order to recover from a key compromise or proactively
mitigate the impact of an unnoticed key compromise.
To change the key associated with an account, the client first
constructs a key-change object describing the change that it would
like the server to make:
account (required, string): The URL for account being modified. The
content of this field MUST be the exact string provided in the
Location header field in response to the new-account request that
created the account.
newKey (required, JWK): The JWK representation of the new key
The client then encapsulates the key-change object in a JWS, signed
with the requested new account key (i.e., the key matching the
"newKey" value).
The outer JWS MUST meet the normal requirements for an ACME JWS (see
Section 5.2). The inner JWS MUST meet the normal requirements, with
the following exceptions:
o The inner JWS MUST have the same "url" parameter as the outer JWS.
o The inner JWS is NOT REQUIRED to have a "nonce" parameter. The
server MUST ignore any value provided for the "nonce" header
parameter.
This transaction has signatures from both the old and new keys so
that the server can verify that the holders of the two keys both
agree to the change. The signatures are nested to preserve the
property that all signatures on POST messages are signed by exactly
one key.
Barnes, et al. Expires August 7, 2017 [Page 30]
Internet-Draft ACME February 2017
POST /acme/key-change HTTP/1.1
Host: example.com
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"jwk": /* old key */,
"nonce": "K60BWPrMQG9SDxBDS_xtSw",
"url": "https://example.com/acme/key-change"
}),
"payload": base64url({
"protected": base64url({
"alg": "ES256",
"jwk": /* new key */,
"url": "https://example.com/acme/key-change"
}),
"payload": base64url({
"account": "https://example.com/acme/acct/1",
"newKey": /* new key */
}),
"signature": "Xe8B94RD30Azj2ea...8BmZIRtcSKPSd8gU"
}),
"signature": "5TWiqIYQfIDfALQv...x9C2mg8JGPxl5bI4"
}
On receiving key-change request, the server MUST perform the
following steps in addition to the typical JWS validation:
1. Validate the POST request belongs to a currently active account,
as described in Message Transport.
2. Check that the payload of the JWS is a well-formed JWS object
(the "inner JWS")
3. Check that the JWS protected header of the inner JWS has a "jwk"
field.
4. Check that the inner JWS verifies using the key in its "jwk"
field
5. Check that the payload of the inner JWS is a well-formed key-
change object (as described above)
6. Check that the "url" parameters of the inner and outer JWSs are
the same
Barnes, et al. Expires August 7, 2017 [Page 31]
Internet-Draft ACME February 2017
7. Check that the "account" field of the key-change object contains
the URL for the account matching the old key
8. Check that the "newKey" field of the key-change object contains
the key used to sign the inner JWS.
If all of these checks pass, then the server updates the
corresponding account by replacing the old account key with the new
public key and returns status code 200. Otherwise, the server
responds with an error status code and a problem document describing
the error.
6.3.4. Account deactivation
A client may deactivate an account by posting a signed update to the
server with a status field of "deactivated." Clients may wish to do
this when the account key is compromised.
POST /acme/acct/1 HTTP/1.1
Host: example.com
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"kid": "https://example.com/acme/acct/1",
"nonce": "ntuJWWSic4WVNSqeUmshgg",
"url": "https://example.com/acme/acct/1"
}),
"payload": base64url({
"status": "deactivated"
}),
"signature": "earzVLd3m5M4xJzR...bVTqn7R08AKOVf3Y"
}
The server MUST verify that the request is signed by the account key.
If the server accepts the deactivation request, it should reply with
a 200 (OK) status code and the current contents of the account
object.
Once an account is deactivated, the server MUST NOT accept further
requests authorized by that account's key. It is up to server policy
how long to retain data related to that account, whether to revoke
certificates issued by that account, and whether to send email to
that account's contacts. ACME does not provide a way to reactivate a
deactivated account.
Barnes, et al. Expires August 7, 2017 [Page 32]
Internet-Draft ACME February 2017
6.4. Applying for Certificate Issuance
A client may use ACME to submit an order for a certificate to be
issued. The client makes this request by sending a POST request to
the server's new-order resource. The body of the POST is a JWS
object whose JSON payload is a subset of the order object defined in
Section 6.1.3, containing the fields that describe the certificate to
be issued:
csr (required, string): A CSR encoding the parameters for the
certificate being requested [RFC2986]. The CSR is sent in the
base64url-encoded version of the DER format. (Note: Because this
field uses base64url, and does not include headers, it is
different from PEM.)
notBefore (optional, string): The requested value of the notBefore
field in the certificate, in the date format defined in [RFC3339]
notAfter (optional, string): The requested value of the notAfter
field in the certificate, in the date format defined in [RFC3339]
POST /acme/new-order HTTP/1.1
Host: example.com
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"kid": "https://example.com/acme/acct/1",
"nonce": "5XJ1L3lEkMG7tR6pA00clA",
"url": "https://example.com/acme/new-order"
}),
"payload": base64url({
"csr": "5jNudRx6Ye4HzKEqT5...FS6aKdZeGsysoCo4H9P",
"notBefore": "2016-01-01T00:00:00Z",
"notAfter": "2016-01-08T00:00:00Z"
}),
"signature": "H6ZXtGjTZyUnPeKn...wEA4TklBdh3e454g"
}
The CSR encodes the client's requests with regard to the content of
the certificate to be issued. The CSR MUST indicate the requested
identifiers, either in the commonName portion of the requested
subject name, or in an extensionRequest attribute [RFC2985]
requesting a subjectAltName extension.
The server MUST return an error if it cannot fulfil the request as
specified, and MUST NOT issue a certificate with contents other than
Barnes, et al. Expires August 7, 2017 [Page 33]
Internet-Draft ACME February 2017
those requested. If the server requires the request to be modified
in a certain way, it should indicate the required changes using an
appropriate error code and description.
If the server is willing to issue the requested certificate, it
responds with a 201 (Created) response. The body of this response is
an order object reflecting the client's request and any
authorizations the client must complete before the certificate will
be issued.
HTTP/1.1 201 Created
Replay-Nonce: MYAuvOpaoIiywTezizk5vw
Location: https://example.com/acme/order/asdf
{
"status": "pending",
"expires": "2016-01-01T00:00:00Z",
"csr": "jcRf4uXra7FGYW5ZMewvV...rhlnznwy8YbpMGqwidEXfE",
"notBefore": "2016-01-01T00:00:00Z",
"notAfter": "2016-01-08T00:00:00Z",
"authorizations": [
"https://example.com/acme/authz/1234",
"https://example.com/acme/authz/2345"
]
}
The order object returned by the server represents a promise that if
the client fulfills the server's requirements before the "expires"
time, then the server will issue the requested certificate. In the
order object, any authorization referenced in the "authorizations"
array whose status is "pending" represents an authorization
transaction that the client must complete before the server will
issue the certificate (see Section 6.5). If the client fails to
complete the required actions before the "expires" time, then the
server SHOULD change the status of the order to "invalid" and MAY
delete the order resource.
The server MUST issue the requested certificate and update the order
resource with a URL for the certificate shortly after the client has
fulfilled the server's requirements. If the client has already
satisfied the server's requirements at the time of this request
(e.g., by obtaining authorization for all of the identifiers in the
certificate in previous transactions), then the server MUST
proactively issue the requested certificate and provide a URL for it
in the "certificate" field of the order. The server MUST, however,
Barnes, et al. Expires August 7, 2017 [Page 34]
Internet-Draft ACME February 2017
still list the completed authorizations in the "authorizations"
array.
Once the client believes it has fulfilled the server's requirements,
it should send a GET request to the order resource to obtain its
current state. The status of the order will indicate what action the
client should take:
o "invalid": The certificate will not be issued. Consider this
order process abandoned.
o "pending": The server does not believe that the client has
fulfilled the requirements. Check the "authorizations" array for
entries that are still pending.
o "processing": The server agrees that the requirements have been
fulfilled, and is in the process of generating the certificate.
Retry after the time given in the "Retry-After" header field of
the response, if any.
o "valid": The server has issued the certificate and provisioned its
URL to the "certificate" field of the order. Download the
certificate.
6.4.1. Pre-Authorization
The order process described above presumes that authorization objects
are created reactively, in response to a certificate order. Some
servers may also wish to enable clients to obtain authorization for
an identifier proactively, outside of the context of a specific
issuance. For example, a client hosting virtual servers for a
collection of names might wish to obtain authorization before any
servers are created, and only create a certificate when a server
starts up.
In some cases, a CA running an ACME server might have a completely
external, non-ACME process for authorizing a client to issue for an
identifier. In these case, the CA should provision its ACME server
with authorization objects corresponding to these authorizations and
reflect them as already valid in any orders submitted by the client.
If a CA wishes to allow pre-authorization within ACME, it can offer a
"new authorization" resource in its directory by adding the key "new-
authz" with a URL for the new authorization resource.
To request authorization for an identifier, the client sends a POST
request to the new-authorization resource specifying the identifier
Barnes, et al. Expires August 7, 2017 [Page 35]
Internet-Draft ACME February 2017
for which authorization is being requested and how the server should
behave with respect to existing authorizations for this identifier.
identifier (required, dictionary of string): The identifier that the
account is authorized to represent
type (required, string): The type of identifier.
value (required, string): The identifier itself.
existing (optional, string): How an existing authorization should be
handled. Possible values are "accept" and "require".
POST /acme/new-authz HTTP/1.1
Host: example.com
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"jwk": {...},
"nonce": "uQpSjlRb4vQVCjVYAyyUWg",
"url": "https://example.com/acme/new-authz"
}),
"payload": base64url({
"identifier": {
"type": "dns",
"value": "example.net"
},
"existing": "accept"
}),
"signature": "nuSDISbWG8mMgE7H...QyVUL68yzf3Zawps"
}
Before processing the authorization request, the server SHOULD
determine whether it is willing to issue certificates for the
identifier. For example, the server should check that the identifier
is of a supported type. Servers might also check names against a
blacklist of known high-value identifiers. If the server is
unwilling to issue for the identifier, it SHOULD return a 403
(Forbidden) error, with a problem document describing the reason for
the rejection.
If the authorization request specifies "existing" with a value of
"accept" or "require", before proceeding, the server SHOULD determine
whether there are any existing, valid authorization resources for the
account and given identifier. If one or more such authorizations
exists, a response SHOULD returned with status code 303 (See Other)
Barnes, et al. Expires August 7, 2017 [Page 36]
Internet-Draft ACME February 2017
and a Location header pointing to the existing resource URL;
processing of the request then stops. If there are multiple such
authorizations, the authorization with the latest expiry date SHOULD
be returned. If no existing authorizations were found and the value
for "existing" was "require", then the server MUST return status code
404 (Not Found); if it was "accept" or was any other value or was
absent, processing continues as follows.
If the server is willing to proceed, it builds a pending
authorization object from the inputs submitted by the client.
o "identifier" the identifier submitted by the client
o "status": MUST be "pending" unless the server has out-of-band
information about the client's authorization status
o "challenges" and "combinations": As selected by the server's
policy for this identifier
The server allocates a new URI for this authorization, and returns a
201 (Created) response, with the authorization URI in a Location
header field, and the JSON authorization object in the body. The
client then follows the process described in Section 6.5 to complete
the authorization process.
6.4.2. Downloading the Certificate
To download the issued certificate, the client simply sends a GET
request to the certificate URL.
The default format of the certificate is PEM (application/x-pem-file)
as specified by [RFC7468]. This format should contain the end-entity
certificate first, followed by any intermediate certificates that are
needed to build a path to a trusted root. Servers SHOULD NOT include
self-signed trust anchors. The client may request other formats by
including an Accept header in its request. For example, the client
may use the media type application/pkix-cert to request the end-
entity certificate in DER format.
The server MAY provide one or more link relation header fields
[RFC5988] with relation "alternate". Each such field should express
an alternative certificate chain starting with the same end-entity
certificate. This can be used to express paths to various trust
anchors. Clients can fetch these alternates and use their own
heuristics to decide which is optimal.
Barnes, et al. Expires August 7, 2017 [Page 37]
Internet-Draft ACME February 2017
The server MUST also provide a link relation header field with
relation "author" to indicate the order under which this certificate
was issued.
If the CA participates in Certificate Transparency (CT) [RFC6962],
then they may want to provide the client with a Signed Certificate
Timestamp (SCT) that can be used to prove that a certificate was
submitted to a CT log. An SCT can be included as an extension in the
certificate or as an extension to OCSP responses for the certificate.
The server can also provide the client with direct access to an SCT
for a certificate using a Link relation header field with relation
"ct-sct".
GET /acme/cert/asdf HTTP/1.1
Host: example.com
Accept: application/pkix-cert
HTTP/1.1 200 OK
Content-Type: application/pkix-cert
Link: <https://example.com/acme/ca-cert>;rel="up";title="issuer"
Link: <https://example.com/acme/revoke-cert>;rel="revoke"
Link: <https://example.com/acme/order/asdf>;rel="author"
Link: <https://example.com/acme/sct/asdf>;rel="ct-sct"
Link: <https://example.com/acme/some-directory>;rel="directory"
-----BEGIN CERTIFICATE-----
[End-entity certificate contents]
-----END CERTIFICATE-----
-----BEGIN CERTIFICATE-----
[Issuer certificate contents]
-----END CERTIFICATE-----
-----BEGIN CERTIFICATE-----
[Other certificate contents]
-----END CERTIFICATE-----
A certificate resource represents a single, immutable certificate.
If the client wishes to obtain a renewed certificate, the client
initiates a new order process to request one.
Because certificate resources are immutable once issuance is
complete, the server MAY enable the caching of the resource by adding
Expires and Cache-Control headers specifying a point in time in the
distant future. These headers have no relation to the certificate's
period of validity.
Barnes, et al. Expires August 7, 2017 [Page 38]
Internet-Draft ACME February 2017
6.5. Identifier Authorization
The identifier authorization process establishes the authorization of
an account to manage certificates for a given identifier. This
process must assure the server of two things: First, that the client
controls the private key of the account key pair, and second, that
the client controls the identifier in question. This process may be
repeated to associate multiple identifiers to a key pair (e.g., to
request certificates with multiple identifiers), or to associate
multiple accounts with an identifier (e.g., to allow multiple
entities to manage certificates). The server may declare that an
authorization is only valid for a specific order by setting the
"scope" field of the authorization to the URI for that order.
Authorization resources are created by the server in response to
certificate orders or authorization requests submitted by an account
key holder; their URLs are provided to the client in the responses to
these requests. The authorization object is implicitly tied to the
account key used to sign the request.
When a client receives an order from the server it downloads the
authorization resource by sending a GET request to the indicated URL.
If the client initiates authorization using a request to the new
authorization resource, it will have already recevied the pending
authorization object in the response to that request.
Barnes, et al. Expires August 7, 2017 [Page 39]
Internet-Draft ACME February 2017
GET /acme/authz/1234 HTTP/1.1
Host: example.com
HTTP/1.1 200 OK
Content-Type: application/json
Link: <https://example.com/acme/some-directory>;rel="directory"
{
"status": "pending",
"expires": "2018-03-03T14:09:00Z",
"identifier": {
"type": "dns",
"value": "example.org"
},
"challenges": [
{
"type": "http-01",
"url": "https://example.com/authz/1234/0",
"token": "DGyRejmCefe7v4NfDGDKfA"
},
{
"type": "tls-sni-02",
"url": "https://example.com/authz/1234/1",
"token": "DGyRejmCefe7v4NfDGDKfA"
},
{
"type": "dns-01",
"url": "https://example.com/authz/1234/2",
"token": "DGyRejmCefe7v4NfDGDKfA"
}
]
}
6.5.1. Responding to Challenges
To prove control of the identifier and receive authorization, the
client needs to respond with information to complete the challenges.
To do this, the client updates the authorization object received from
the server by filling in any required information in the elements of
the "challenges" dictionary. (This is also the stage where the
client should perform any actions required by the challenge.)
The client sends these updates back to the server in the form of a
JSON object with the response fields required by the challenge type,
carried in a POST request to the challenge URI (not authorization
Barnes, et al. Expires August 7, 2017 [Page 40]
Internet-Draft ACME February 2017
URI). This allows the client to send information only for challenges
it is responding to.
For example, if the client were to respond to the "http-01" challenge
in the above authorization, it would send the following request:
POST /acme/authz/asdf/0 HTTP/1.1
Host: example.com
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"kid": "https://example.com/acme/acct/1",
"nonce": "Q_s3MWoqT05TrdkM2MTDcw",
"url": "https://example.com/acme/authz/asdf/0"
}),
"payload": base64url({
"type": "http-01",
"keyAuthorization": "IlirfxKKXA...vb29HhjjLPSggwiE"
}),
"signature": "9cbg5JO1Gf5YLjjz...SpkUfcdPai9uVYYQ"
}
The server updates the authorization document by updating its
representation of the challenge with the response fields provided by
the client. The server MUST ignore any fields in the response object
that are not specified as response fields for this type of challenge.
The server provides a 200 (OK) response with the updated challenge
object as its body.
If the client's response is invalid for some reason, or does not
provide the server with appropriate information to validate the
challenge, then the server MUST return an HTTP error. On receiving
such an error, the client SHOULD undo any actions that have been
taken to fulfill the challenge, e.g., removing files that have been
provisioned to a web server.
The server is said to "finalize" the authorization when it has
completed one of the validations, by assigning the authorization a
status of "valid" or "invalid", corresponding to whether it considers
the account authorized for the identifier. If the final state is
"valid", then the server MUST include an "expires" field. When
finalizing an authorization, the server MAY remove challenges other
than the one that was completed, and may modify the "expires" field.
The server SHOULD NOT remove challenges with status "invalid".
Barnes, et al. Expires August 7, 2017 [Page 41]
Internet-Draft ACME February 2017
Usually, the validation process will take some time, so the client
will need to poll the authorization resource to see when it is
finalized. For challenges where the client can tell when the server
has validated the challenge (e.g., by seeing an HTTP or DNS request
from the server), the client SHOULD NOT begin polling until it has
seen the validation request from the server.
To check on the status of an authorization, the client sends a GET
request to the authorization URI, and the server responds with the
current authorization object. In responding to poll requests while
the validation is still in progress, the server MUST return a 202
(Accepted) response, and MAY include a Retry-After header field to
suggest a polling interval to the client.
GET /acme/authz/asdf HTTP/1.1
Host: example.com
HTTP/1.1 200 OK
{
"status": "valid",
"expires": "2018-09-09T14:09:00Z",
"identifier": {
"type": "dns",
"value": "example.org"
},
"challenges": [
{
"type": "http-01"
"url": "https://example.com/authz/asdf/0",
"status": "valid",
"validated": "2014-12-01T12:05:00Z",
"token": "IlirfxKKXAsHtmzK29Pj8A",
"keyAuthorization": "IlirfxKKXA...vb29HhjjLPSggwiE"
}
]
}
6.5.2. Deactivating an Authorization
If a client wishes to relinquish its authorization to issue
certificates for an identifier, then it may request that the server
deactivate each authorization associated with that identifier by
sending a POST request with the static object {"status":
"deactivated"}.
Barnes, et al. Expires August 7, 2017 [Page 42]
Internet-Draft ACME February 2017
POST /acme/authz/asdf HTTP/1.1
Host: example.com
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"kid": "https://example.com/acme/acct/1",
"nonce": "xWCM9lGbIyCgue8di6ueWQ",
"url": "https://example.com/acme/authz/asdf"
}),
"payload": base64url({
"status": "deactivated"
}),
"signature": "srX9Ji7Le9bjszhu...WTFdtujObzMtZcx4"
}
The server MUST verify that the request is signed by the account key
corresponding to the account that owns the authorization. If the
server accepts the deactivation, it should reply with a 200 (OK)
status code and the current contents of the authorization object.
The server MUST NOT treat deactivated authorization objects as
sufficient for issuing certificates.
6.6. Certificate Revocation
To request that a certificate be revoked, the client sends a POST
request to the ACME server's revoke-cert URI. The body of the POST
is a JWS object whose JSON payload contains the certificate to be
revoked:
certificate (required, string): The certificate to be revoked, in
the base64url-encoded version of the DER format. (Note: Because
this field uses base64url, and does not include headers, it is
different from PEM.)
reason (optional, int): One of the revocation reasonCodes defined in
[RFC5280] Section 5.3.1 to be used when generating OCSP responses
and CRLs. If this field is not set the server SHOULD use the
unspecified (0) reasonCode value when generating OCSP responses
and CRLs. The server MAY disallow a subset of reasonCodes from
being used by the user.
Barnes, et al. Expires August 7, 2017 [Page 43]
Internet-Draft ACME February 2017
POST /acme/revoke-cert HTTP/1.1
Host: example.com
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"kid": "https://example.com/acme/acct/1", // OR "jwk"
"nonce": "JHb54aT_KTXBWQOzGYkt9A",
"url": "https://example.com/acme/revoke-cert"
}),
"payload": base64url({
"certificate": "MIIEDTCCAvegAwIBAgIRAP8...",
"reason": 1
}),
"signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4"
}
Revocation requests are different from other ACME request in that
they can be signed either with an account key pair or the key pair in
the certificate. Before revoking a certificate, the server MUST
verify that the key used to sign the request is authorized to revoke
the certificate. The server SHOULD consider at least the following
accounts authorized for a given certificate:
o the account that issued the certificate.
o an account that holds authorizations for all of the identifiers in
the certificate.
The server SHOULD also consider a revocation request valid if it is
signed with the private key corresponding to the public key in the
certificate.
If the revocation succeeds, the server responds with status code 200
(OK). If the revocation fails, the server returns an error.
Barnes, et al. Expires August 7, 2017 [Page 44]
Internet-Draft ACME February 2017
HTTP/1.1 200 OK
Replay-Nonce: IXVHDyxIRGcTE0VSblhPzw
Content-Length: 0
--- or ---
HTTP/1.1 403 Forbidden
Replay-Nonce: IXVHDyxIRGcTE0VSblhPzw
Content-Type: application/problem+json
Content-Language: en
{
"type": "urn:ietf:params:acme:error:unauthorized",
"detail": "No authorization provided for name example.net",
"instance": "http://example.com/doc/unauthorized"
}
7. Identifier Validation Challenges
There are few types of identifiers in the world for which there is a
standardized mechanism to prove possession of a given identifier. In
all practical cases, CAs rely on a variety of means to test whether
an entity applying for a certificate with a given identifier actually
controls that identifier.
Challenges provide the server with assurance that an account holder
is also the entity that controls an identifier. For each type of
challenge, it must be the case that in order for an entity to
successfully complete the challenge the entity must both:
o Hold the private key of the account key pair used to respond to
the challenge
o Control the identifier in question
Section 9 documents how the challenges defined in this document meet
these requirements. New challenges will need to document how they
do.
ACME uses an extensible challenge/response framework for identifier
validation. The server presents a set of challenges in the
authorization object it sends to a client (as objects in the
"challenges" array), and the client responds by sending a response
object in a POST request to a challenge URI.
This section describes an initial set of challenge types. Each
challenge must describe:
Barnes, et al. Expires August 7, 2017 [Page 45]
Internet-Draft ACME February 2017
1. Content of challenge objects
2. Content of response objects
3. How the server uses the challenge and response to verify control
of an identifier
Challenge objects all contain the following basic fields:
type (required, string): The type of challenge encoded in the
object.
url (required, string): The URL to which a response can be posted.
status (required, string): The status of this authorization.
Possible values are: "pending", "valid", and "invalid".
validated (optional, string): The time at which this challenge was
completed by the server, encoded in the format specified in RFC
3339 [RFC3339]. This field is REQUIRED if the "status" field is
"valid".
error (optional, dictionary of string): The error that occurred
while the server was validating the challenge, if any. This field
is structured as a problem document [RFC7807].
All additional fields are specified by the challenge type. If the
server sets a challenge's "status" to "invalid", it SHOULD also
include the "error" field to help the client diagnose why they failed
the challenge.
Different challenges allow the server to obtain proof of different
aspects of control over an identifier. In some challenges, like
HTTP, TLS SNI, and DNS, the client directly proves its ability to do
certain things related to the identifier. The choice of which
challenges to offer to a client under which circumstances is a matter
of server policy.
The identifier validation challenges described in this section all
relate to validation of domain names. If ACME is extended in the
future to support other types of identifier, there will need to be
new challenge types, and they will need to specify which types of
identifier they apply to.
[[ Editor's Note: In pre-RFC versions of this specification,
challenges are labeled by type, and with the version of the draft in
which they were introduced. For example, if an HTTP challenge were
introduced in version -03 and a breaking change made in version -05,
Barnes, et al. Expires August 7, 2017 [Page 46]
Internet-Draft ACME February 2017
then there would be a challenge labeled "http-03" and one labeled
"http-05" - but not one labeled "http-04", since challenge in version
-04 was compatible with one in version -04. ]]
7.1. Key Authorizations
Several of the challenges in this document makes use of a key
authorization string. A key authorization is a string that expresses
a domain holder's authorization for a specified key to satisfy a
specified challenge, by concatenating the token for the challenge
with a key fingerprint, separated by a "." character:
key-authz = token || '.' || base64url(JWK\_Thumbprint(accountKey))
The "JWK_Thumbprint" step indicates the computation specified in
[RFC7638], using the SHA-256 digest. As specified in the individual
challenges below, the token for a challenge is a JSON string
comprised entirely of characters in the URL-safe base64 alphabet.
The "||" operator indicates concatenation of strings.
In computations involving key authorizations, such as the digest
computations required for the DNS and TLS SNI challenges, the key
authorization string MUST be represented in UTF-8 form (or,
equivalently, ASCII).
An example of how to compute a JWK thumbprint can be found in
Section 3.1 of [RFC7638]. Note that some cryptographic libraries
prepend a zero octet to the representation of the RSA public key
parameters N and E, in order to avoid ambiguity with regard to the
sign of the number. As noted in JWA [RFC7518], a JWK object MUST NOT
include this zero octet. That is, any initial zero octets MUST be
stripped before the values are base64url-encoded.
7.2. HTTP
With HTTP validation, the client in an ACME transaction proves its
control over a domain name by proving that it can provision resources
on an HTTP server that responds for that domain name. The ACME
server challenges the client to provision a file at a specific path,
with a specific string as its content.
As a domain may resolve to multiple IPv4 and IPv6 addresses, the
server will connect to at least one of the hosts found in A and AAAA
records, at its discretion. Because many webservers allocate a
default HTTPS virtual host to a particular low-privilege tenant user
in a subtle and non-intuitive manner, the challenge must be completed
over HTTP, not HTTPS.
Barnes, et al. Expires August 7, 2017 [Page 47]
Internet-Draft ACME February 2017
type (required, string): The string "http-01"
token (required, string): A random value that uniquely identifies
the challenge. This value MUST have at least 128 bits of entropy,
in order to prevent an attacker from guessing it. It MUST NOT
contain any characters outside the base64url alphabet and MUST NOT
contain any padding characters ("=").
GET /acme/authz/1234/0 HTTP/1.1
Host: example.com
HTTP/1.1 200 OK
{
"type": "http-01",
"url": "https://example.com/acme/authz/0",
"status": "pending",
"token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA"
}
A client responds to this challenge by constructing a key
authorization from the "token" value provided in the challenge and
the client's account key. The client then provisions the key
authorization as a resource on the HTTP server for the domain in
question.
The path at which the resource is provisioned is comprised of the
fixed prefix ".well-known/acme-challenge/", followed by the "token"
value in the challenge. The value of the resource MUST be the ASCII
representation of the key authorization.
.well-known/acme-challenge/evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA
The client's response to this challenge indicates its agreement to
this challenge by sending the server the key authorization covering
the challenge's token and the client's account key.
keyAuthorization (required, string): The key authorization for this
challenge. This value MUST match the token from the challenge and
the client's account key.
Barnes, et al. Expires August 7, 2017 [Page 48]
Internet-Draft ACME February 2017
POST /acme/authz/1234/0
Host: example.com
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"kid": "https://example.com/acme/acct/1",
"nonce": "JHb54aT_KTXBWQOzGYkt9A",
"url": "https://example.com/acme/authz/1234/0"
}),
"payload": base64url({
"keyAuthorization": "evaGxfADs...62jcerQ"
}),
"signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4"
}
On receiving a response, the server MUST verify that the key
authorization in the response matches the "token" value in the
challenge and the client's account key. If they do not match, then
the server MUST return an HTTP error in response to the POST request
in which the client sent the challenge.
Given a challenge/response pair, the server verifies the client's
control of the domain by verifying that the resource was provisioned
as expected.
1. Form a URI by populating the URI template [RFC6570]
"http://{domain}/.well-known/acme-challenge/{token}", where:
* the domain field is set to the domain name being verified; and
* the token field is set to the token in the challenge.
2. Verify that the resulting URI is well-formed.
3. Dereference the URI using an HTTP GET request. This request MUST
be sent to TCP port 80 on the server.
4. Verify that the body of the response is well-formed key
authorization. The server SHOULD ignore whitespace characters at
the end of the body.
5. Verify that key authorization provided by the server matches the
token for this challenge and the client's account key.
Barnes, et al. Expires August 7, 2017 [Page 49]
Internet-Draft ACME February 2017
If all of the above verifications succeed, then the validation is
successful. If the request fails, or the body does not pass these
checks, then it has failed.
7.3. TLS with Server Name Indication (TLS SNI)
The TLS with Server Name Indication (TLS SNI) validation method
proves control over a domain name by requiring the client to
configure a TLS server referenced by an A/AAAA record under the
domain name to respond to specific connection attempts utilizing the
Server Name Indication extension [RFC6066]. The server verifies the
client's challenge by accessing the reconfigured server and verifying
a particular challenge certificate is presented.
type (required, string): The string "tls-sni-02"
token (required, string): A random value that uniquely identifies
the challenge. This value MUST have at least 128 bits of entropy,
in order to prevent an attacker from guessing it. It MUST NOT
contain any characters outside the base64url alphabet and MUST NOT
contain any padding characters ("=").
GET /acme/authz/1234/1 HTTP/1.1
Host: example.com
HTTP/1.1 200 OK
{
"type": "tls-sni-02",
"url": "https://example.com/acme/authz/1234/1",
"status": "pending",
"token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA"
}
A client responds to this challenge by constructing a self-signed
certificate which the client MUST provision at the domain name
concerned in order to pass the challenge.
The certificate may be constructed arbitrarily, except that each
certificate MUST have exactly two subjectAlternativeNames, SAN A and
SAN B. Both MUST be dNSNames.
SAN A MUST be constructed as follows: compute the SHA-256 digest of
the UTF-8-encoded challenge token and encode it in lowercase
hexadecimal form. The dNSName is "x.y.token.acme.invalid", where x
is the first half of the hexadecimal representation and y is the
second half.
Barnes, et al. Expires August 7, 2017 [Page 50]
Internet-Draft ACME February 2017
SAN B MUST be constructed as follows: compute the SHA-256 digest of
the UTF-8 encoded key authorization and encode it in lowercase
hexadecimal form. The dNSName is "x.y.ka.acme.invalid" where x is
the first half of the hexadecimal representation and y is the second
half.
The client MUST ensure that the certificate is served to TLS
connections specifying a Server Name Indication (SNI) value of SAN A.
The response to the TLS-SNI challenge simply acknowledges that the
client is ready to fulfill this challenge.
keyAuthorization (required, string): The key authorization for this
challenge. This value MUST match the token from the challenge and
the client's account key.
POST /acme/authz/1234/1
Host: example.com
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"kid": "https://example.com/acme/acct/1",
"nonce": "JHb54aT_KTXBWQOzGYkt9A",
"url": "https://example.com/acme/authz/1234/1"
}),
"payload": base64url({
"keyAuthorization": "evaGxfADs...62jcerQ"
}),
"signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4"
}
On receiving a response, the server MUST verify that the key
authorization in the response matches the "token" value in the
challenge and the client's account key. If they do not match, then
the server MUST return an HTTP error in response to the POST request
in which the client sent the challenge.
Given a challenge/response pair, the ACME server verifies the
client's control of the domain by verifying that the TLS server was
configured appropriately, using these steps:
1. Compute SAN A and SAN B in the same way as the client.
2. Open a TLS connection to the domain name being validated,
presenting SAN A in the SNI field. This connection MUST be sent
to TCP port 443 on the server. In the ClientHello initiating the
Barnes, et al. Expires August 7, 2017 [Page 51]
Internet-Draft ACME February 2017
TLS handshake, the server MUST include a server_name extension
(i.e., SNI) containing SAN A. The server SHOULD ensure that it
does not reveal SAN B in any way when making the TLS connection,
such that the presentation of SAN B in the returned certificate
proves association with the client.
3. Verify that the certificate contains a subjectAltName extension
containing dNSName entries of SAN A and SAN B and no other
entries. The comparison MUST be insensitive to case and ordering
of names.
It is RECOMMENDED that the server open multiple TLS connections from
various network perspectives, in order to make MitM attacks harder.
If all of the above verifications succeed, then the validation is
successful. Otherwise, the validation fails.
7.4. DNS
When the identifier being validated is a domain name, the client can
prove control of that domain by provisioning a resource record under
it. The DNS challenge requires the client to provision a TXT record
containing a designated value under a specific validation domain
name.
type (required, string): The string "dns-01"
token (required, string): A random value that uniquely identifies
the challenge. This value MUST have at least 128 bits of entropy,
in order to prevent an attacker from guessing it. It MUST NOT
contain any characters outside the base64url alphabet and MUST NOT
contain any padding characters ("=").
GET /acme/authz/1234/2 HTTP/1.1
Host: example.com
HTTP/1.1 200 OK
{
"type": "dns-01",
"url": "https://example.com/acme/authz/1234/2",
"status": "pending",
"token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA"
}
A client responds to this challenge by constructing a key
authorization from the "token" value provided in the challenge and
the client's account key. The client then computes the SHA-256
digest of the key authorization.
Barnes, et al. Expires August 7, 2017 [Page 52]
Internet-Draft ACME February 2017
The record provisioned to the DNS is the base64url encoding of this
digest. The client constructs the validation domain name by
prepending the label "_acme-challenge" to the domain name being
validated, then provisions a TXT record with the digest value under
that name. For example, if the domain name being validated is
"example.com", then the client would provision the following DNS
record:
_acme-challenge.example.com. 300 IN TXT "gfj9Xq...Rg85nM"
The response to the DNS challenge provides the computed key
authorization to acknowledge that the client is ready to fulfill this
challenge.
keyAuthorization (required, string): The key authorization for this
challenge. This value MUST match the token from the challenge and
the client's account key.
POST /acme/authz/1234/2
Host: example.com
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"kid": "https://example.com/acme/acct/1",
"nonce": "JHb54aT_KTXBWQOzGYkt9A",
"url": "https://example.com/acme/authz/1234/2"
}),
"payload": base64url({
"keyAuthorization": "evaGxfADs...62jcerQ"
}),
"signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4"
}
On receiving a response, the server MUST verify that the key
authorization in the response matches the "token" value in the
challenge and the client's account key. If they do not match, then
the server MUST return an HTTP error in response to the POST request
in which the client sent the challenge.
To validate a DNS challenge, the server performs the following steps:
1. Compute the SHA-256 digest of the key authorization
2. Query for TXT records under the validation domain name
Barnes, et al. Expires August 7, 2017 [Page 53]
Internet-Draft ACME February 2017
3. Verify that the contents of one of the TXT records matches the
digest value
It is RECOMMENDED that the server perform multiple DNS queries from
various network perspectives, in order to make MitM attacks harder.
If all of the above verifications succeed, then the validation is
successful. If no DNS record is found, or DNS record and response
payload do not pass these checks, then the validation fails.
7.5. Out-of-Band
There may be cases where a server cannot perform automated validation
of an identifier, for example if validation requires some manual
steps. In such cases, the server may provide an "out of band" (OOB)
challenge to request that the client perform some action outside of
ACME in order to validate possession of the identifier.
The OOB challenge requests that the client have a human user visit a
web page to receive instructions on how to validate possession of the
identifier, by providing a URL for that web page.
type (required, string): The string "oob-01"
href (required, string): The URL to be visited. The scheme of this
URL MUST be "http" or "https". Note that this field is distinct
from the "url" field of the challenge, which identifies the
challenge itself.
GET /acme/authz/1234/3 HTTP/1.1
Host: example.com
HTTP/1.1 200 OK
{
"type": "oob-01",
"href": "https://example.com/validate/evaGxfADs6pSRb2LAv9IZ"
}
A client responds to this challenge by presenting the indicated URL
for a human user to navigate to. If the user choses to complete this
challege (by vising the website and completing its instructions), the
client indicates this by sending a simple acknowledgement response to
the server.
type (required, string): The string "oob-01"
Barnes, et al. Expires August 7, 2017 [Page 54]
Internet-Draft ACME February 2017
POST /acme/authz/1234/3
Host: example.com
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"kid": "https://example.com/acme/acct/1",
"nonce": "JHb54aT_KTXBWQOzGYkt9A",
"url": "https://example.com/acme/authz/1234/3"
}),
"payload": base64url({
"type": "oob-01"
}),
"signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4"
}
On receiving a response, the server MUST verify that the value of the
"type" field is "oob-01". Otherwise, the steps the server takes to
validate identifier possession are determined by the server's local
policy.
8. IANA Considerations
8.1. Well-Known URI for the HTTP Challenge
The "Well-Known URIs" registry should be updated with the following
additional value (using the template from [RFC5785]):
URI suffix: acme-challenge
Change controller: IETF
Specification document(s): This document, Section Section 7.2
Related information: N/A
8.2. Replay-Nonce HTTP Header
The "Message Headers" registry should be updated with the following
additional value:
| Header Field Name | Protocol | Status | Reference |
+:------------+:------+:------+:-----------+ | Replay-Nonce | http |
standard | Section 5.5.1 |
Barnes, et al. Expires August 7, 2017 [Page 55]
Internet-Draft ACME February 2017
8.3. "url" JWS Header Parameter
The "JSON Web Signature and Encryption Header Parameters" registry
should be updated with the following additional value:
o Header Parameter Name: "url"
o Header Parameter Description: URL
o Header Parameter Usage Location(s): JWE, JWS
o Change Controller: IESG
o Specification Document(s): Section 5.4.1 of RFC XXXX
[[ RFC EDITOR: Please replace XXXX above with the RFC number assigned
to this document ]]
8.4. "nonce" JWS Header Parameter
The "JSON Web Signature and Encryption Header Parameters" registry
should be updated with the following additional value:
o Header Parameter Name: "nonce"
o Header Parameter Description: Nonce
o Header Parameter Usage Location(s): JWE, JWS
o Change Controller: IESG
o Specification Document(s): Section 5.5.2 of RFC XXXX
[[ RFC EDITOR: Please replace XXXX above with the RFC number assigned
to this document ]]
8.5. URN Sub-namespace for ACME (urn:ietf:params:acme)
The "IETF URN Sub-namespace for Registered Protocol Parameter
Identifiers" registry should be updated with the following additional
value, following the template in [RFC3553]:
Registry name: acme
Specification: RFC XXXX
Repository: URL-TBD
Barnes, et al. Expires August 7, 2017 [Page 56]
Internet-Draft ACME February 2017
Index value: No transformation needed.
[[ RFC EDITOR: Please replace XXXX above with the RFC number assigned
to this document, and replace URL-TBD with the URL assigned by IANA
for registries of ACME parameters. ]]
8.6. New Registries
This document requests that IANA create the following new registries:
1. ACME Error Codes
2. ACME Resource Types
3. ACME Identifier Types
4. ACME Challenge Types
All of these registries should be administered under a Specification
Required policy [RFC5226].
8.6.1. Fields in Account Objects
This registry lists field names that are defined for use in ACME
account objects. Fields marked as "configurable" may be included in
a new-account request.
Template:
o Field name: The string to be used as a key in the JSON dictionary
o Field type: The type of value to be provided, e.g., string,
boolean, array of string
o Client configurable: Boolean indicating whether the server should
accept values provided by the client
o Reference: Where this field is defined
Initial contents: The fields and descriptions defined in
Section 6.1.2.
Barnes, et al. Expires August 7, 2017 [Page 57]
Internet-Draft ACME February 2017
+--------------------------+-------------+--------------+-----------+
| Field Name | Field Type | Configurable | Reference |
+--------------------------+-------------+--------------+-----------+
| key | dictionary | false | RFC XXXX |
| | | | |
| status | string | false | RFC XXXX |
| | | | |
| contact | array of | true | RFC XXXX |
| | string | | |
| | | | |
| external-account-binding | dictionary | true | RFC XXXX |
| | | | |
| terms-of-service-agreed | boolean | false | RFC XXXX |
| | | | |
| orders | array of | false | RFC XXXX |
| | string | | |
+--------------------------+-------------+--------------+-----------+
8.6.2. Fields in Order Objects
This registry lists field names that are defined for use in ACME
order objects. Fields marked as "configurable" may be included in a
new-order request.
Template:
o Field name: The string to be used as a key in the JSON dictionary
o Field type: The type of value to be provided, e.g., string,
boolean, array of string
o Client configurable: Boolean indicating whether the server should
accept values provided by the client
o Reference: Where this field is defined
Initial contents: The fields and descriptions defined in
Section 6.1.3.
Barnes, et al. Expires August 7, 2017 [Page 58]
Internet-Draft ACME February 2017
+----------------+-------------------+--------------+-----------+
| Field Name | Field Type | Configurable | Reference |
+----------------+-------------------+--------------+-----------+
| status | string | false | RFC XXXX |
| | | | |
| expires | string | false | RFC XXXX |
| | | | |
| csr | string | true | RFC XXXX |
| | | | |
| notBefore | string | true | RFC XXXX |
| | | | |
| notAfter | string | true | RFC XXXX |
| | | | |
| authorizations | array of string | false | RFC XXXX |
| | | | |
| certificate | string | false | RFC XXXX |
+----------------+-------------------+--------------+-----------+
8.6.3. Error Codes
This registry lists values that are used within URN values that are
provided in the "type" field of problem documents in ACME.
Template:
o Code: The label to be included in the URN for this error,
following "urn:ietf:params:acme:"
o Description: A human-readable description of the error
o Reference: Where the error is defined
Initial contents: The codes and descriptions in the table in
Section 5.7 above, with the Reference field set to point to this
specification.
8.6.4. Resource Types
This registry lists the types of resources that ACME servers may list
in their directory objects.
Template:
o Key: The value to be used as a dictionary key in the directory
object
o Resource type: The type of resource labeled by the key
Barnes, et al. Expires August 7, 2017 [Page 59]
Internet-Draft ACME February 2017
o Reference: Where the identifier type is defined
Initial contents:
+-------------+--------------------+-----------+
| Key | Resource type | Reference |
+-------------+--------------------+-----------+
| new-account | New account | RFC XXXX |
| | | |
| new-order | New order | RFC XXXX |
| | | |
| revoke-cert | Revoke certificate | RFC XXXX |
| | | |
| key-change | Key change | RFC XXXX |
+-------------+--------------------+-----------+
[[ RFC EDITOR: Please replace XXXX above with the RFC number assigned
to this document ]]
8.6.5. Identifier Types
This registry lists the types of identifiers that ACME clients may
request authorization to issue in certificates.
Template:
o Label: The value to be put in the "type" field of the identifier
object
o Reference: Where the identifier type is defined
Initial contents:
+-------+-----------+
| Label | Reference |
+-------+-----------+
| dns | RFC XXXX |
+-------+-----------+
[[ RFC EDITOR: Please replace XXXX above with the RFC number assigned
to this document ]]
8.6.6. Challenge Types
This registry lists the ways that ACME servers can offer to validate
control of an identifier. The "Identifier Type" field in template
must be contained in the Label column of the ACME Identifier Types
registry.
Barnes, et al. Expires August 7, 2017 [Page 60]
Internet-Draft ACME February 2017
Template:
o Label: The value to be put in the "type" field of challenge
objects using this validation mechanism
o Identifier Type: The type of identifier that this mechanism
applies to
o Reference: Where the challenge type is defined
Initial Contents
+---------+-----------------+-----------+
| Label | Identifier Type | Reference |
+---------+-----------------+-----------+
| http | dns | RFC XXXX |
| | | |
| tls-sni | dns | RFC XXXX |
| | | |
| dns | dns | RFC XXXX |
+---------+-----------------+-----------+
[[ RFC EDITOR: Please replace XXXX above with the RFC number assigned
to this document ]]
9. Security Considerations
ACME is a protocol for managing certificates that attest to
identifier/key bindings. Thus the foremost security goal of ACME is
to ensure the integrity of this process, i.e., to ensure that the
bindings attested by certificates are correct, and that only
authorized entities can manage certificates. ACME identifies clients
by their account keys, so this overall goal breaks down into two more
precise goals:
1. Only an entity that controls an identifier can get an account key
authorized for that identifier
2. Once authorized, an account key's authorizations cannot be
improperly transferred to another account key
In this section, we discuss the threat model that underlies ACME and
the ways that ACME achieves these security goals within that threat
model. We also discuss the denial-of-service risks that ACME servers
face, and a few other miscellaneous considerations.
Barnes, et al. Expires August 7, 2017 [Page 61]
Internet-Draft ACME February 2017
9.1. Threat model
As a service on the Internet, ACME broadly exists within the Internet
threat model [RFC3552]. In analyzing ACME, it is useful to think of
an ACME server interacting with other Internet hosts along two
"channels":
o An ACME channel, over which the ACME HTTPS requests are exchanged
o A validation channel, over which the ACME server performs
additional requests to validate a client's control of an
identifier
+------------+
| ACME | ACME Channel
| Client |--------------------+
+------------+ |
V
+------------+
| ACME |
| Server |
+------------+
+------------+ |
| Validation |<-------------------+
| Server | Validation Channel
+------------+
In practice, the risks to these channels are not entirely separate,
but they are different in most cases. Each channel, for example,
uses a different communications pattern: the ACME channel will
comprise inbound HTTPS connections to the ACME server and the
validation channel outbound HTTP or DNS requests.
Broadly speaking, ACME aims to be secure against active and passive
attackers on any individual channel. Some vulnerabilities arise
(noted below), when an attacker can exploit both the ACME channel and
one of the others.
On the ACME channel, in addition to network-layer attackers, we also
need to account for application-layer man in the middle attacks, and
for abusive use of the protocol itself. Protection against
application-layer MitM addresses potential attackers such as Content
Distribution Networks (CDNs) and middleboxes with a TLS MitM
function. Preventing abusive use of ACME means ensuring that an
attacker with access to the validation channel can't obtain
illegitimate authorization by acting as an ACME client (legitimately,
in terms of the protocol).
Barnes, et al. Expires August 7, 2017 [Page 62]
Internet-Draft ACME February 2017
9.2. Integrity of Authorizations
ACME allows anyone to request challenges for an identifier by
registering an account key and sending a new-order request under that
account key. The integrity of the authorization process thus depends
on the identifier validation challenges to ensure that the challenge
can only be completed by someone who both (1) holds the private key
of the account key pair, and (2) controls the identifier in question.
Validation responses need to be bound to an account key pair in order
to avoid situations where an ACME MitM can switch out a legitimate
domain holder's account key for one of his choosing, e.g.:
o Legitimate domain holder registers account key pair A
o MitM registers account key pair B
o Legitimate domain holder sends a new-order request signed under
account key A
o MitM suppresses the legitimate request, but sends the same request
signed under account key B
o ACME server issues challenges and MitM forwards them to the
legitimate domain holder
o Legitimate domain holder provisions the validation response
o ACME server performs validation query and sees the response
provisioned by the legitimate domain holder
o Because the challenges were issued in response to a message signed
account key B, the ACME server grants authorization to account key
B (the MitM) instead of account key A (the legitimate domain
holder)
All of the challenges above have a binding between the account
private key and the validation query made by the server, via the key
authorization. The key authorization is signed by the account
private key, reflects the corresponding public key, and is provided
to the server in the validation response.
The association of challenges to identifiers is typically done by
requiring the client to perform some action that only someone who
effectively controls the identifier can perform. For the challenges
in this document, the actions are:
Barnes, et al. Expires August 7, 2017 [Page 63]
Internet-Draft ACME February 2017
o HTTP: Provision files under .well-known on a web server for the
domain
o TLS SNI: Configure a TLS server for the domain
o DNS: Provision DNS resource records for the domain
There are several ways that these assumptions can be violated, both
by misconfiguration and by attack. For example, on a web server that
allows non-administrative users to write to .well-known, any user can
claim to own the server's hostname by responding to an HTTP
challenge, and likewise for TLS configuration and TLS SNI.
The use of hosting providers is a particular risk for ACME
validation. If the owner of the domain has outsourced operation of
DNS or web services to a hosting provider, there is nothing that can
be done against tampering by the hosting provider. As far as the
outside world is concerned, the zone or web site provided by the
hosting provider is the real thing.
More limited forms of delegation can also lead to an unintended party
gaining the ability to successfully complete a validation
transaction. For example, suppose an ACME server follows HTTP
redirects in HTTP validation and a web site operator provisions a
catch-all redirect rule that redirects requests for unknown resources
to a different domain. Then the target of the redirect could use
that to get a certificate through HTTP validation, since the
validation path will not be known to the primary server.
The DNS is a common point of vulnerability for all of these
challenges. An entity that can provision false DNS records for a
domain can attack the DNS challenge directly, and can provision false
A/AAAA records to direct the ACME server to send its TLS SNI or HTTP
validation query to a server of the attacker's choosing. There are a
few different mitigations that ACME servers can apply:
o Always querying the DNS using a DNSSEC-validating resolver
(enhancing security for zones that are DNSSEC-enabled)
o Querying the DNS from multiple vantage points to address local
attackers
o Applying mitigations against DNS off-path attackers, e.g., adding
entropy to requests [I-D.vixie-dnsext-dns0x20] or only using TCP
Given these considerations, the ACME validation process makes it
impossible for any attacker on the ACME channel, or a passive
Barnes, et al. Expires August 7, 2017 [Page 64]
Internet-Draft ACME February 2017
attacker on the validation channel to hijack the authorization
process to authorize a key of the attacker's choice.
An attacker that can only see the ACME channel would need to convince
the validation server to provide a response that would authorize the
attacker's account key, but this is prevented by binding the
validation response to the account key used to request challenges. A
passive attacker on the validation channel can observe the correct
validation response and even replay it, but that response can only be
used with the account key for which it was generated.
An active attacker on the validation channel can subvert the ACME
process, by performing normal ACME transactions and providing a
validation response for his own account key. The risks due to
hosting providers noted above are a particular case. For identifiers
where the server already has some public key associated with the
domain this attack can be prevented by requiring the client to prove
control of the corresponding private key.
9.3. Denial-of-Service Considerations
As a protocol run over HTTPS, standard considerations for TCP-based
and HTTP-based DoS mitigation also apply to ACME.
At the application layer, ACME requires the server to perform a few
potentially expensive operations. Identifier validation transactions
require the ACME server to make outbound connections to potentially
attacker-controlled servers, and certificate issuance can require
interactions with cryptographic hardware.
In addition, an attacker can also cause the ACME server to send
validation requests to a domain of its choosing by submitting
authorization requests for the victim domain.
All of these attacks can be mitigated by the application of
appropriate rate limits. Issues closer to the front end, like POST
body validation, can be addressed using HTTP request limiting. For
validation and certificate requests, there are other identifiers on
which rate limits can be keyed. For example, the server might limit
the rate at which any individual account key can issue certificates,
or the rate at which validation can be requested within a given
subtree of the DNS.
9.4. Server-Side Request Forgery
Server-Side Request Forgery (SSRF) attacks can arise when an attacker
can cause a server to perform HTTP requests to an attacker-chosen
URL. In the ACME HTTP challenge validation process, the ACME server
Barnes, et al. Expires August 7, 2017 [Page 65]
Internet-Draft ACME February 2017
performs an HTTP GET request to a URL in which the attacker can
choose the domain. This request is made before the server has
verified that the client controls the domain, so any client can cause
a query to any domain.
Some server implementations include information from the validation
server's response (in order to facilitate debugging). Such
implementations enable an attacker to extract this information from
any web server that is accessible to the ACME server, even if it is
not accessible to the ACME client.
It might seem that the risk of SSRF through this channel is limited
by the fact that the attacker can only control the domain of the URL,
not the path. However, if the attacker first sets the domain to one
they control, then they can send the server an HTTP redirect (e.g., a
302 response) which will cause the server to query an arbitrary URI.
In order to further limit the SSRF risk, ACME server operators should
ensure that validation queries can only be sent to servers on the
public Internet, and not, say, web services within the server
operator's internal network. Since the attacker could make requests
to these public servers himself, he can't gain anything extra through
an SSRF attack on ACME aside from a layer of anonymization.
9.5. CA Policy Considerations
The controls on issuance enabled by ACME are focused on validating
that a certificate applicant controls the identifier he claims.
Before issuing a certificate, however, there are many other checks
that a CA might need to perform, for example:
o Has the client agreed to a subscriber agreement?
o Is the claimed identifier syntactically valid?
o For domain names:
* If the leftmost label is a '*', then have the appropriate
checks been applied?
* Is the name on the Public Suffix List?
* Is the name a high-value name?
* Is the name a known phishing domain?
o Is the key in the CSR sufficiently strong?
Barnes, et al. Expires August 7, 2017 [Page 66]
Internet-Draft ACME February 2017
o Is the CSR signed with an acceptable algorithm?
CAs that use ACME to automate issuance will need to ensure that their
servers perform all necessary checks before issuing.
CAs using ACME to allow clients to agree to terms of service should
keep in mind that ACME clients can automate this agreement, possibly
not involving a human user. If a CA wishes to have stronger evidence
of user consent, it may present an out-of-band requirement or
challenge to require human involvement.
10. Operational Considerations
There are certain factors that arise in operational reality that
operators of ACME-based CAs will need to keep in mind when
configuring their services. For example:
10.1. DNS over TCP
As noted above, DNS forgery attacks against the ACME server can
result in the server making incorrect decisions about domain control
and thus mis-issuing certificates. Servers SHOULD verify DNSSEC when
it is available for a domain. When DNSSEC is not available, servers
SHOULD perform DNS queries over TCP, which provides better resistance
to some forgery attacks than DNS over UDP.
10.2. Default Virtual Hosts
In many cases, TLS-based services are deployed on hosted platforms
that use the Server Name Indication (SNI) TLS extension to
distinguish between different hosted services or "virtual hosts".
When a client initiates a TLS connection with an SNI value indicating
a provisioned host, the hosting platform routes the connection to
that host.
When a connection comes in with an unknown SNI value, one might
expect the hosting platform to terminate the TLS connection.
However, some hosting platforms will choose a virtual host to be the
"default", and route connections with unknown SNI values to that
host.
In such cases, the owner of the default virtual host can complete a
TLS-based challenge (e.g., "tls-sni-02") for any domain with an A
record that points to the hosting platform. This could result in
mis-issuance in cases where there are multiple hosts with different
owners resident on the hosting platform.
Barnes, et al. Expires August 7, 2017 [Page 67]
Internet-Draft ACME February 2017
A CA that accepts TLS-based proof of domain control should attempt to
check whether a domain is hosted on a domain with a default virtual
host before allowing an authorization request for this host to use a
TLS-based challenge. A default virtual host can be detected by
initiating TLS connections to the host with random SNI values within
the namespace used for the TLS-based challenge (the "acme.invalid"
namespace for "tls-sni-02").
10.3. Use of DNSSEC Resolvers
An ACME-based CA will often need to make DNS queries, e.g., to
validate control of DNS names. Because the security of such
validations ultimately depends on the authenticity of DNS data, every
possible precaution should be taken to secure DNS queries done by the
CA. It is therefore RECOMMENDED that ACME-based CAs make all DNS
queries via DNSSEC-validating stub or recursive resolvers. This
provides additional protection to domains which choose to make use of
DNSSEC.
An ACME-based CA must use only a resolver if it trusts the resolver
and every component of the network route by which it is accessed. It
is therefore RECOMMENDED that ACME-based CAs operate their own
DNSSEC-validating resolvers within their trusted network and use
these resolvers both for both CAA record lookups and all record
lookups in furtherance of a challenge scheme (A, AAAA, TXT, etc.).
11. Acknowledgements
In addition to the editors listed on the front page, this document
has benefited from contributions from a broad set of contributors,
all the way back to its inception.
o Peter Eckersley, EFF
o Eric Rescorla, Mozilla
o Seth Schoen, EFF
o Alex Halderman, University of Michigan
o Martin Thomson, Mozilla
o Jakub Warmuz, University of Oxford
This document draws on many concepts established by Eric Rescorla's
"Automated Certificate Issuance Protocol" draft. Martin Thomson
provided helpful guidance in the use of HTTP.
Barnes, et al. Expires August 7, 2017 [Page 68]
Internet-Draft ACME February 2017
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000,
<http://www.rfc-editor.org/info/rfc2818>.
[RFC2985] Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object
Classes and Attribute Types Version 2.0", RFC 2985,
DOI 10.17487/RFC2985, November 2000,
<http://www.rfc-editor.org/info/rfc2985>.
[RFC2986] Nystrom, M. and B. Kaliski, "PKCS #10: Certification
Request Syntax Specification Version 1.7", RFC 2986,
DOI 10.17487/RFC2986, November 2000,
<http://www.rfc-editor.org/info/rfc2986>.
[RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
<http://www.rfc-editor.org/info/rfc3339>.
[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
for Internationalized Domain Names in Applications
(IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003,
<http://www.rfc-editor.org/info/rfc3492>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<http://www.rfc-editor.org/info/rfc3986>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<http://www.rfc-editor.org/info/rfc4648>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
Barnes, et al. Expires August 7, 2017 [Page 69]
Internet-Draft ACME February 2017
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<http://www.rfc-editor.org/info/rfc5280>.
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, DOI 10.17487/RFC5890, August 2010,
<http://www.rfc-editor.org/info/rfc5890>.
[RFC5988] Nottingham, M., "Web Linking", RFC 5988,
DOI 10.17487/RFC5988, October 2010,
<http://www.rfc-editor.org/info/rfc5988>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
<http://www.rfc-editor.org/info/rfc6066>.
[RFC6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
and D. Orchard, "URI Template", RFC 6570,
DOI 10.17487/RFC6570, March 2012,
<http://www.rfc-editor.org/info/rfc6570>.
[RFC6844] Hallam-Baker, P. and R. Stradling, "DNS Certification
Authority Authorization (CAA) Resource Record", RFC 6844,
DOI 10.17487/RFC6844, January 2013,
<http://www.rfc-editor.org/info/rfc6844>.
[RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
2014, <http://www.rfc-editor.org/info/rfc7159>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<http://www.rfc-editor.org/info/rfc7231>.
[RFC7468] Josefsson, S. and S. Leonard, "Textual Encodings of PKIX,
PKCS, and CMS Structures", RFC 7468, DOI 10.17487/RFC7468,
April 2015, <http://www.rfc-editor.org/info/rfc7468>.
[RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
2015, <http://www.rfc-editor.org/info/rfc7515>.
Barnes, et al. Expires August 7, 2017 [Page 70]
Internet-Draft ACME February 2017
[RFC7517] Jones, M., "JSON Web Key (JWK)", RFC 7517,
DOI 10.17487/RFC7517, May 2015,
<http://www.rfc-editor.org/info/rfc7517>.
[RFC7518] Jones, M., "JSON Web Algorithms (JWA)", RFC 7518,
DOI 10.17487/RFC7518, May 2015,
<http://www.rfc-editor.org/info/rfc7518>.
[RFC7638] Jones, M. and N. Sakimura, "JSON Web Key (JWK)
Thumbprint", RFC 7638, DOI 10.17487/RFC7638, September
2015, <http://www.rfc-editor.org/info/rfc7638>.
[RFC7807] Nottingham, M. and E. Wilde, "Problem Details for HTTP
APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016,
<http://www.rfc-editor.org/info/rfc7807>.
12.2. Informative References
[I-D.vixie-dnsext-dns0x20]
Vixie, P. and D. Dagon, "Use of Bit 0x20 in DNS Labels to
Improve Transaction Identity", draft-vixie-dnsext-
dns0x20-00 (work in progress), March 2008.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552,
DOI 10.17487/RFC3552, July 2003,
<http://www.rfc-editor.org/info/rfc3552>.
[RFC3553] Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An
IETF URN Sub-namespace for Registered Protocol
Parameters", BCP 73, RFC 3553, DOI 10.17487/RFC3553, June
2003, <http://www.rfc-editor.org/info/rfc3553>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>.
[RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
Uniform Resource Identifiers (URIs)", RFC 5785,
DOI 10.17487/RFC5785, April 2010,
<http://www.rfc-editor.org/info/rfc5785>.
[RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
<http://www.rfc-editor.org/info/rfc6962>.
Barnes, et al. Expires August 7, 2017 [Page 71]
Internet-Draft ACME February 2017
[RFC7469] Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April
2015, <http://www.rfc-editor.org/info/rfc7469>.
[W3C.CR-cors-20130129]
Kesteren, A., "Cross-Origin Resource Sharing", World Wide
Web Consortium CR CR-cors-20130129, January 2013,
<http://www.w3.org/TR/2013/CR-cors-20130129>.
Authors' Addresses
Richard Barnes
Mozilla
Email: rlb@ipv.sx
Jacob Hoffman-Andrews
EFF
Email: jsha@eff.org
James Kasten
University of Michigan
Email: jdkasten@umich.edu
Barnes, et al. Expires August 7, 2017 [Page 72]
