Automatic Certificate Management Environment (ACME)
draft-ietf-acme-acme-03
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 8555.
|
|
|---|---|---|---|
| Authors | Richard Barnes , Jacob Hoffman-Andrews , James Kasten | ||
| Last updated | 2016-07-08 | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews |
GENART Last Call review
(of
-11)
by Dale Worley
Ready w/issues
GENART Telechat review
(of
-07)
by Dale Worley
On the Right Track
|
||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Associated WG milestone |
|
||
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 8555 (Proposed Standard) | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-ietf-acme-acme-03
Network Working Group R. Barnes
Internet-Draft Mozilla
Intended status: Standards Track J. Hoffman-Andrews
Expires: January 9, 2017 EFF
J. Kasten
University of Michigan
July 08, 2016
Automatic Certificate Management Environment (ACME)
draft-ietf-acme-acme-03
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 January 9, 2017 [Page 1]
Internet-Draft ACME July 2016
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 9, 2017.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Deployment Model and Operator Experience . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 6
5. Message Transport . . . . . . . . . . . . . . . . . . . . . . 8
5.1. HTTPS Requests . . . . . . . . . . . . . . . . . . . . . 9
5.2. Request Authentication . . . . . . . . . . . . . . . . . 9
5.3. Request URI Integrity . . . . . . . . . . . . . . . . . . 10
5.3.1. "url" (URL) JWS header parameter . . . . . . . . . . 10
5.4. Replay protection . . . . . . . . . . . . . . . . . . . . 11
5.4.1. Replay-Nonce . . . . . . . . . . . . . . . . . . . . 11
5.4.2. "nonce" (Nonce) JWS header parameter . . . . . . . . 12
5.5. Rate limits . . . . . . . . . . . . . . . . . . . . . . . 12
5.6. Errors . . . . . . . . . . . . . . . . . . . . . . . . . 12
6. Certificate Management . . . . . . . . . . . . . . . . . . . 14
6.1. Resources . . . . . . . . . . . . . . . . . . . . . . . . 14
6.1.1. Directory . . . . . . . . . . . . . . . . . . . . . . 16
6.1.2. Registration Objects . . . . . . . . . . . . . . . . 17
6.1.3. Application Objects . . . . . . . . . . . . . . . . . 19
6.1.4. Authorization Objects . . . . . . . . . . . . . . . . 21
6.2. Registration . . . . . . . . . . . . . . . . . . . . . . 23
6.2.1. Account Key Roll-over . . . . . . . . . . . . . . . . 25
6.2.2. Account deactivation . . . . . . . . . . . . . . . . 27
6.3. Applying for Certificate Issuance . . . . . . . . . . . . 28
6.3.1. Downloading the Certificate . . . . . . . . . . . . . 30
6.4. Identifier Authorization . . . . . . . . . . . . . . . . 31
Barnes, et al. Expires January 9, 2017 [Page 2]
Internet-Draft ACME July 2016
6.4.1. Responding to Challenges . . . . . . . . . . . . . . 33
6.4.2. Deactivating an Authorization . . . . . . . . . . . . 35
6.5. Certificate Revocation . . . . . . . . . . . . . . . . . 36
7. Identifier Validation Challenges . . . . . . . . . . . . . . 38
7.1. Key Authorizations . . . . . . . . . . . . . . . . . . . 39
7.2. HTTP . . . . . . . . . . . . . . . . . . . . . . . . . . 40
7.3. TLS with Server Name Indication (TLS SNI) . . . . . . . . 42
7.4. DNS . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
7.5. Out-of-Band . . . . . . . . . . . . . . . . . . . . . . . 45
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 46
8.1. Well-Known URI for the HTTP Challenge . . . . . . . . . . 46
8.2. Replay-Nonce HTTP Header . . . . . . . . . . . . . . . . 47
8.3. "url" JWS Header Parameter . . . . . . . . . . . . . . . 47
8.4. "nonce" JWS Header Parameter . . . . . . . . . . . . . . 47
8.5. URN Sub-namespace for ACME (urn:ietf:params:acme) . . . . 48
8.6. New Registries . . . . . . . . . . . . . . . . . . . . . 48
8.6.1. Error Codes . . . . . . . . . . . . . . . . . . . . . 48
8.6.2. Resource Types . . . . . . . . . . . . . . . . . . . 49
8.6.3. Identifier Types . . . . . . . . . . . . . . . . . . 49
8.6.4. Challenge Types . . . . . . . . . . . . . . . . . . . 50
9. Security Considerations . . . . . . . . . . . . . . . . . . . 50
9.1. Threat model . . . . . . . . . . . . . . . . . . . . . . 51
9.2. Integrity of Authorizations . . . . . . . . . . . . . . . 52
9.3. Denial-of-Service Considerations . . . . . . . . . . . . 54
9.4. Server-Side Request Forgery . . . . . . . . . . . . . . . 55
9.5. CA Policy Considerations . . . . . . . . . . . . . . . . 55
10. Operational Considerations . . . . . . . . . . . . . . . . . 56
10.1. DNS over TCP . . . . . . . . . . . . . . . . . . . . . . 56
10.2. Default Virtual Hosts . . . . . . . . . . . . . . . . . 56
10.3. Use of DNSSEC Resolvers . . . . . . . . . . . . . . . . 57
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 57
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 58
12.1. Normative References . . . . . . . . . . . . . . . . . . 58
12.2. Informative References . . . . . . . . . . . . . . . . . 60
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 61
1. Introduction
Certificates in the Web PKI [RFC5280] are most commonly used to
authenticate domain names. Thus, certificate authorities in the Web
PKI are trusted to verify that an applicant for a certificate
legitimately represents the domain name(s) in the certificate.
Existing Web PKI certificate authorities tend to run on a set of ad
hoc protocols for certificate issuance and identity verification. A
typical user experience is something like:
o Generate a PKCS#10 [RFC2986] Certificate Signing Request (CSR).
Barnes, et al. Expires January 9, 2017 [Page 3]
Internet-Draft ACME July 2016
o Cut-and-paste the CSR into a CA web page.
o Prove ownership of the domain by one of the following methods:
* Put a CA-provided challenge at a specific place on the web
server.
* Put a CA-provided challenge at a DNS location corresponding to
the target domain.
* Receive CA challenge at a (hopefully) administrator-controlled
e-mail address corresponding to the domain and then respond to
it on the CA's web page.
o Download the issued certificate and install it on their Web
Server.
With the exception of the CSR itself and the certificates that are
issued, these are all completely ad hoc procedures and are
accomplished by getting the human user to follow interactive natural-
language instructions from the CA rather than by machine-implemented
published protocols. In many cases, the instructions are difficult
to follow and cause significant confusion. Informal usability tests
by the authors indicate that webmasters often need 1-3 hours to
obtain and install a certificate for a domain. Even in the best
case, the lack of published, standardized mechanisms presents an
obstacle to the wide deployment of HTTPS and other PKIX-dependent
systems because it inhibits mechanization of tasks related to
certificate issuance, deployment, and revocation.
This document describes an extensible framework for automating the
issuance and domain validation procedure, thereby allowing servers
and infrastructural software to obtain certificates without user
interaction. Use of this protocol should radically simplify the
deployment of HTTPS and the practicality of PKIX authentication for
other protocols based on TLS [RFC5246].
2. Deployment Model and Operator Experience
The major guiding use case for ACME is obtaining certificates for Web
sites (HTTPS [RFC2818]). In that case, the server is intended to
speak for one or more domains, and the process of certificate
issuance is intended to verify that the server actually speaks for
the domain(s).
Different types of certificates reflect different kinds of CA
verification of information about the certificate subject. "Domain
Validation" (DV) certificates are by far the most common type. For
Barnes, et al. Expires January 9, 2017 [Page 4]
Internet-Draft ACME July 2016
DV validation, the CA merely verifies that the requester has
effective control of the web server and/or DNS server for the domain,
but does not explicitly attempt to verify their real-world identity.
(This is as opposed to "Organization Validation" (OV) and "Extended
Validation" (EV) certificates, where the process is intended to also
verify the real-world identity of the requester.)
DV certificate validation commonly checks claims about properties
related to control of a domain name - properties that can be observed
by the issuing authority in an interactive process that can be
conducted purely online. That means that under typical
circumstances, all steps in the request, verification, and issuance
process can be represented and performed by Internet protocols with
no out-of-band human intervention.
When deploying a current HTTPS server, an operator generally gets a
prompt to generate a self-signed certificate. When an operator
deploys an ACME-compatible web server, the experience would be
something like this:
o The ACME client prompts the operator for the intended domain
name(s) that the web server is to stand for.
o The ACME client presents the operator with a list of CAs from
which it could get a certificate. (This list will change over
time based on the capabilities of CAs and updates to ACME
configuration.) The ACME client might prompt the operator for
payment information at this point.
o The operator selects a CA.
o In the background, the ACME client contacts the CA and requests
that a certificate be issued for the intended domain name(s).
o Once the CA is satisfied, the certificate is issued and the ACME
client automatically downloads and installs it, potentially
notifying the operator via e-mail, SMS, etc.
o The ACME client periodically contacts the CA to get updated
certificates, stapled OCSP responses, or whatever else would be
required to keep the server functional and its credentials up-to-
date.
The overall idea is that it's nearly as easy to deploy with a CA-
issued certificate as a self-signed certificate, and that once the
operator has done so, the process is self-sustaining with minimal
manual intervention. Close integration of ACME with HTTPS servers,
for example, can allow the immediate and automated deployment of
Barnes, et al. Expires January 9, 2017 [Page 5]
Internet-Draft ACME July 2016
certificates as they are issued, optionally sparing the human
administrator from additional configuration work.
3. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
The two main roles in ACME are "client" and "server". The ACME
client uses the protocol to request certificate management actions,
such as issuance or revocation. An ACME client therefore typically
runs on a web server, mail server, or some other server system which
requires valid TLS certificates. The ACME server runs at a
certificate authority, and responds to client requests, performing
the requested actions if the client is authorized.
An ACME client is represented by an "account key pair". The client
uses the private key of this key pair to sign all messages sent to
the server. The server uses the public key to verify the
authenticity and integrity of messages from the client.
4. Protocol Overview
ACME allows a client to request certificate management actions using
a set of JSON messages carried over HTTPS. In some ways, ACME
functions much like a traditional CA, in which a user creates an
account, adds identifiers to that account (proving control of the
domains), and requests certificate issuance for those domains while
logged in to the account.
In ACME, the account is represented by an account key pair. The "add
a domain" function is accomplished by authorizing the key pair for a
given domain. Certificate issuance and revocation are authorized by
a signature with the key pair.
The first phase of ACME is for the client to register with the ACME
server. The client generates an asymmetric key pair and associates
this key pair with a set of contact information by signing the
contact information. The server acknowledges the registration by
replying with a registration object echoing the client's input. The
server can also provide terms of service at this stage, which the
client can present to a human user.
Barnes, et al. Expires January 9, 2017 [Page 6]
Internet-Draft ACME July 2016
Client Server
Contact Information
Signature ------->
<------- Registration
Terms of Service
Once the client is registered, there are three major steps it needs
to take to get a certificate:
1. Apply for a certificate to be issued
2. Fulfill the server's requirements for issuance
3. Finalize the application and request issuance
The client's application 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 can either
proactively issue the requested certificate or wait for the client to
request that the application be "finalized", at which point the
certificate will be issued and provided to the client.
Barnes, et al. Expires January 9, 2017 [Page 7]
Internet-Draft ACME July 2016
Application
Signature ------->
<------- Requirements
(e.g., Challenges)
Responses
Signature ------->
<~~~~~~~~Validation~~~~~~~~>
Finalize application
Signature ------->
<------- Certificate
To revoke a certificate, the client simply sends a revocation request
indicating the certificate to be revoked, signed with an authorized
key pair. The server indicates whether the request has succeeded.
Client Server
Revocation request
Signature -------->
<-------- Result
Note that while ACME is defined with enough flexibility to handle
different types of identifiers in principle, the primary use case
addressed by this document is the case where domain names are used as
identifiers. For example, all of the identifier validation
challenges described in Section 7 below address validation of domain
names. The use of ACME for other protocols will require further
specification, in order to describe how these identifiers are encoded
in the protocol, and what types of validation challenges the server
might require.
5. Message Transport
Communications between an ACME client and an ACME server are done
over HTTPS, using JWS 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.
Barnes, et al. Expires January 9, 2017 [Page 8]
Internet-Draft ACME July 2016
5.1. HTTPS Requests
Each ACME function is accomplished by the client sending a sequence
of HTTPS requests to the server, carrying JSON messages
[RFC2818][RFC7159]. Use of HTTPS is REQUIRED. Clients SHOULD
support HTTP public key pinning [RFC7469], and servers SHOULD emit
pinning headers. Each subsection of Section 6 below describes the
message formats used by the function, and the order in which messages
are sent.
In all HTTPS transactions used by ACME, the ACME client is the HTTPS
client and the ACME server is the HTTPS server.
ACME servers that are intended to be generally accessible need to use
Cross-Origin Resource Sharing (CORS) in order to be accessible from
browser-based clients [W3C.CR-cors-20130129]. Such servers SHOULD
set the Access-Control-Allow-Origin header field to the value "*".
Binary fields in the JSON objects used by ACME are encoded using
base64url encoding described in [RFC4648] Section 5, according to the
profile specified in JSON Web Signature [RFC7515] Section 2. This
encoding uses a URL safe character set. Trailing '=' characters MUST
be stripped.
5.2. Request Authentication
All ACME requests with a non-empty body MUST encapsulate the body in
a JWS object, signed using the account key pair. The server MUST
verify the JWS before processing the request. (For readability,
however, the examples below omit this encapsulation.) Encapsulating
request bodies in JWS provides a simple authentication of requests by
way of key continuity.
JWS objects sent in ACME requests MUST meet the following additional
criteria:
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 Protected Header MUST include the following fields:
* "alg"
* "jwk"
* "nonce" (defined below)
Barnes, et al. Expires January 9, 2017 [Page 9]
Internet-Draft ACME July 2016
* "url" (defined below)
Note that this implies that GET requests are not authenticated.
Servers MUST NOT respond to GET requests for resources that might be
considered sensitive.
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 "...".
5.3. Request URI Integrity
It is common in deployment the entity terminating TLS for HTTPS to be
different from the entity operating the logical HTTPS server, with a
"request routing" layer in the middle. For example, an ACME CA might
have a content delivery network terminate TLS connections from
clients so that it can inspect client requests for denial-of-service
protection.
These intermediaries can also change values in the request that are
not signed in the HTTPS request, e.g., the request URI and headers.
ACME uses JWS to provide a limited integrity mechanism, which
protects against an intermediary changing the request URI to another
ACME URI of a different type. (It does not protect against changing
between URIs of the same type, e.g., from one authorization URI to
another).
As noted above, all ACME request object 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. In such cases, the
client MUST set the "url" field to the exact string provided by the
server (rather than performing any re-encoding on the URL).
5.3.1. "url" (URL) JWS header parameter
The "url" header parameter specifies the URL to which this JWS object
is directed [RFC3986]. The "url" parameter MUST be carried in the
protected header of the JWS. The value of the "nonce" header MUST be
a JSON string representing the URL.
Barnes, et al. Expires January 9, 2017 [Page 10]
Internet-Draft ACME July 2016
5.4. Replay protection
In order to protect ACME resources from any possible replay attacks,
ACME requests have a mandatory anti-replay mechanism. This mechanism
is based on the server maintaining a list of nonces that it has
issued to clients, and requiring any signed request from the client
to carry such a nonce.
An ACME server MUST include a Replay-Nonce header field in each
successful response it provides to a client, with contents as
specified below. In particular, the ACME server MUST provide a
Replay-Nonce header field in response to a HEAD request for any valid
resource. (This allows clients to easily obtain a fresh nonce.) It
MAY also provide nonces in error responses.
Every JWS sent by an ACME client MUST include, in its protected
header, the "nonce" header parameter, with contents as defined below.
As part of JWS verification, the ACME server MUST verify that the
value of the "nonce" header is a value that the server previously
provided in a Replay-Nonce header field. Once a nonce value has
appeared in an ACME request, the server MUST consider it invalid, in
the same way as a value it had never issued.
When a server rejects a request because its nonce value was
unacceptable (or not present), it SHOULD provide HTTP status code 400
(Bad Request), and indicate the ACME error code
"urn:ietf:params:acme:error:badNonce".
The precise method used to generate and track nonces is up to the
server. For example, the server could generate a random 128-bit
value for each response, keep a list of issued nonces, and strike
nonces from this list as they are used.
5.4.1. Replay-Nonce
The "Replay-Nonce" header field includes a server-generated value
that the server can use to detect unauthorized replay in future
client requests. The server should generate the value provided in
Replay-Nonce in such a way that they are unique to each message, with
high probability.
The value of the Replay-Nonce field MUST be an octet string encoded
according to the base64url encoding described in Section 2 of
[RFC7515]. Clients MUST ignore invalid Replay-Nonce values.
base64url = [A-Z] / [a-z] / [0-9] / "-" / "_"
Replay-Nonce = *base64url
Barnes, et al. Expires January 9, 2017 [Page 11]
Internet-Draft ACME July 2016
The Replay-Nonce header field SHOULD NOT be included in HTTP request
messages.
5.4.2. "nonce" (Nonce) JWS header parameter
The "nonce" header parameter provides a unique value that enables the
verifier of a JWS to recognize when replay has occurred. The "nonce"
header parameter MUST be carried in the protected header of the JWS.
The value of the "nonce" header parameter MUST be an octet string,
encoded according to the base64url encoding described in Section 2 of
[RFC7515]. If the value of a "nonce" header parameter is not valid
according to this encoding, then the verifier MUST reject the JWS as
malformed.
5.5. 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 code "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.
5.6. 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 January 9, 2017 [Page 12]
Internet-Draft ACME July 2016
+-----------------------+-------------------------------------------+
| Code | Description |
+-----------------------+-------------------------------------------+
| badCSR | The CSR is unacceptable (e.g., due to a |
| | short key) |
| | |
| badNonce | The client sent an unacceptable anti- |
| | replay nonce |
| | |
| connection | The server could not connect to |
| | validation target |
| | |
| dnssec | DNSSEC validation failed |
| | |
| caa | CAA records forbid the CA from issuing |
| | |
| malformed | The request message was malformed |
| | |
| 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 |
| | |
| rateLimited | The request exceeds a rate limit |
| | |
| invalidContact | The contact URI for a registration was |
| | invalid |
| | |
| rejectedIdentifier | The server will not issue for the |
| | identifier |
| | |
| unsupportedIdentifier | Identifier is not supported, but may be |
| | in future |
+-----------------------+-------------------------------------------+
This list is not exhaustive. The server MAY return errors whose
"type" field is set to a URI other than those defined above. Servers
MUST NOT use the ACME URN namespace for errors other than the
standard types. Clients SHOULD display the "detail" field of such
errors.
Barnes, et al. Expires January 9, 2017 [Page 13]
Internet-Draft ACME July 2016
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 Key Registration
o Application for a Certificate
o Account Key Authorization
o Certificate Issuance
o Certificate Revocation
6.1. Resources
ACME is structured as a REST application with a few types of
resources:
o Registration resources, representing information about an account
o Application resources, represnting 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-registration" resource
o A "new-application" resource
o A "revoke-certificate" resource
o A "key-change" resource
Barnes, et al. Expires January 9, 2017 [Page 14]
Internet-Draft ACME July 2016
For the "new-X" resources above, the server MUST have exactly one
resource for each function. This resource may be addressed by
multiple URIs, but all must provide equivalent functionality.
ACME uses different URIs for different management functions. Each
function is listed in a directory along with its corresponding URI,
so clients only need to be configured with the directory URI. These
URIs are connected by a few different link relations [RFC5988].
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
|
|
----------------------------------------------------
| | |
| | |
V V V
new-reg new-app revoke-cert
| | ^
| | | "revoke"
V V |
reg -------------> app -------------> cert ---------+
| ^ |
| | "up" | "up"
V | V
authz cert-chain
| ^
| | "up"
V |
challenge
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
Barnes, et al. Expires January 9, 2017 [Page 15]
Internet-Draft ACME July 2016
some time after issuance. The "->" is a mnemonic for a Location
header pointing to a created resource.
+--------------------+----------------+------------+
| Action | Request | Response |
+--------------------+----------------+------------+
| Register | POST new-reg | 201 -> reg |
| | | |
| Apply for a cert | POST new-app | 201 -> app |
| | | |
| Fetch challenges | GET authz | 200 |
| | | |
| Answer challenges | POST challenge | 200 |
| | | |
| Poll for status | GET authz | 200 |
| | | |
| Request issuance | POST app | 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.
+-------------+--------------------+
| Key | URL in value |
+-------------+--------------------+
| new-reg | New registration |
| | |
| new-app | New application |
| | |
| 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:
Barnes, et al. Expires January 9, 2017 [Page 16]
Internet-Draft ACME July 2016
o a host which function as both an ACME and Web server may want to
keep the root path "/" for an HTML "front page", and and place the
ACME directory under path "/acme".
o a host which only functions as an ACME server could place the
directory under path "/".
The dictionary MAY additionally contain a key "meta". If present, it
MUST be a JSON dictionary; each item in the dictionary is an item of
metadata relating to the service provided by the ACME server.
The following metadata items are defined, all of which are OPTIONAL:
"terms-of-service" (optional, string): A URI identifying the current
terms of service.
"website" (optional, string)): An HTTP or HTTPS URL locating a
website providing more information about the ACME server.
"caa-identities" (optional, array of string): Each string MUST be a
lowercase hostname which the ACME server recognises as referring
to itself for the purposes of CAA record validation as defined in
[RFC6844]. This allows clients to determine the correct issuer
domain name to use when configuring CAA record.
Clients access the directory by sending a GET request to the
directory URI.
HTTP/1.1 200 OK
Content-Type: application/json
{
"new-reg": "https://example.com/acme/new-reg",
"new-app": "https://example.com/acme/new-app",
"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. Registration Objects
An ACME registration resource represents a set of metadata associated
to an account key pair. Registration resources have the following
structure:
Barnes, et al. Expires January 9, 2017 [Page 17]
Internet-Draft ACME July 2016
key (required, dictionary): The public key of the account key pair,
encoded as a JSON Web Key object [RFC7517].
status (required, string): "good" or "deactivated"
contact (optional, array of string): An array of URIs that the
server can use to contact the client for issues related to this
authorization. For example, the server may wish to notify the
client about server-initiated revocation.
agreement (optional, string): A URI referring to a subscriber
agreement or terms of service provided by the server (see below).
Including this field indicates the client's agreement with the
referenced terms.
applications (required, string): A URI from which a list of
authorizations submitted by this account can be fetched via a GET
request. The result of the GET request MUST be a JSON object
whose "applications" field is an array of strings, where each
string is the URI of an authorization belonging to this
registration. The server SHOULD include pending applications, and
SHOULD NOT include applications that are invalid. The server MAY
return an incomplete list, along with a Link header with link
relation "next" indicating a URL to retrieve further entries.
certificates (required, string): A URI from which a list of
certificates issued for this account can be fetched via a GET
request. The result of the GET request MUST be a JSON object
whose "certificates" field is an array of strings, where each
string is the URI of a certificate. The server SHOULD NOT include
expired or revoked certificates. The server MAY return an
incomplete list, along with a Link header with link relation
"next" indicating a URL to retrieve further entries.
{
"contact": [
"mailto:cert-admin@example.com",
"tel:+12025551212"
],
"agreement": "https://example.com/acme/terms",
"authorizations": "https://example.com/acme/reg/1/authz",
"certificates": "https://example.com/acme/reg/1/cert"
}
Barnes, et al. Expires January 9, 2017 [Page 18]
Internet-Draft ACME July 2016
6.1.3. Application Objects
An ACME registration resource represents a client's request for a
certificate, and is used to track the progress of that application
through to issuance. Thus, the object contains information about the
requested certificate, the server's requirements, and any
certificates that have resulted from this application.
status (required, string): The status of this authorization.
Possible values are: "unknown", "pending", "processing", "valid",
and "invalid".
expires (optional, string): The timestamp after which the server
will consider this application invalid, encoded in the format
specified in RFC 3339 [RFC3339]. This field is REQUIRED for
objects with "pending" or "valid" in the status field.
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: This field
uses the same modified Base64 encoding rules used elsewhere in
this document, so it is different from PEM.)
notBefore (optional, string): The requested value of the notBefore
field in the certificate, in the date format defined in [RFC3339]
notAfter (optional, string): The requested value of the notAfter
field in the certificate, in the date format defined in [RFC3339]
requirements (required, array): The requirements that the client
needs to fulfill before the requested certificate can be granted
(for pending applications). For final applications, the
requirements that were met. Each entry is a dictionary with
parameters describing the requirement (see below).
certificate (optional, string): A URL for the certificate that has
been issued in response to this application.
Barnes, et al. Expires January 9, 2017 [Page 19]
Internet-Draft ACME July 2016
{
"status": "pending",
"expires": "2015-03-01T14:09:00Z",
"csr": "jcRf4uXra7FGYW5ZMewvV...rhlnznwy8YbpMGqwidEXfE",
"notBefore": "2016-01-01T00:00:00Z",
"notAfter": "2016-01-08T00:00:00Z",
"requirements": [
{
"type": "authorization",
"status": "valid",
"url": "https://example.com/acme/authz/1234"
},
{
"type": "out-of-band",
"status": "pending",
"url": "https://example.com/acme/payment/1234"
}
]
"certificate": "https://example.com/acme/cert/1234"
}
[[ Open issue: There are two possible behaviors for the CA here.
Either (a) the CA automatically issues once all the requirements are
fulfilled, or (b) the CA waits for confirmation from the client that
it should issue. If we allow both, we will need a signal in the
application object of whether confirmation is required. I would
prefer that auto-issue be the default, which would imply a syntax
like "confirm": true ]]
[[ Open issue: Should this syntax allow multiple certificates? That
would support reissuance / renewal in a straightforward way,
especially if the CSR / notBefore / notAfter could be updated. ]]
The elements of the "requirements" array are immutable once set,
except for their "status" fields. If any other part of the object
changes after the object is created, the client MUST consider the
application invalid.
The "requirements" array in the challenge SHOULD reflect everything
that the CA required the client to do before issuance, even if some
requirements were fulfilled in earlier applications. For example, if
a CA allows multiple applications to be fufilled based on a single
authorization transaction, then it must reflect that authorization in
all of the applications.
Barnes, et al. Expires January 9, 2017 [Page 20]
Internet-Draft ACME July 2016
Each entry in the "requirements" array expresses a requirement from
the CA for the client to takek a particular action. All requirements
objects have the following basic fields:
type (required, string): The type of requirement (see below for
defined types)
status (required, string): The status of this requirement. Possible
values are: "pending", "valid", and "invalid".
All additional fields are specified by the requirement type.
6.1.3.1. Authorization Requirement
A requirement with type "authorization" requests that the ACME client
complete an authorization transaction. The server specifies the
authorization by pre-provisioning a pending authorization resource
and providing the URI for this resource in the requirement.
url (required, string): The URL for the authorization resource
To fulfill this requirement, the ACME client should fetch the
authorization object from the indicated URL, then follow the process
for obtaining authorization as specified in Section 6.4.
6.1.3.2. Out-of-Band Requirement
A requirement with type "out-of-band" requests that the ACME client
have a human user visit a web page in order to receive further
instructions for how to fulfill the requirement. The requirement
object provides a URI for the web page to be visited.
url (required, string): The URL to be visited. The scheme of this
URL MUST be "http" or "https"
To fulfill this requirement, the ACME client should direct the user
to the indicated web page.
6.1.4. Authorization Objects
An ACME authorization object represents server's authorization for an
account to represent an identifier. In addition to the identifier,
an authorization includes several metadata fields, such as the status
of the authorization (e.g., "pending", "valid", or "revoked") and
which challenges were used to validate possession of the identifier.
The structure of an ACME authorization resource is as follows:
Barnes, et al. Expires January 9, 2017 [Page 21]
Internet-Draft ACME July 2016
identifier (required, dictionary of string): The identifier that the
account is authorized to represent
type (required, string): The type of identifier.
value (required, string): The identifier itself.
status (required, string): The status of this authorization.
Possible values are: "unknown", "pending", "processing", "valid",
"invalid" and "revoked". If this field is missing, then the
default value is "pending".
expires (optional, string): The timestamp after which the server
will consider this authorization invalid, encoded in the format
specified in RFC 3339 [RFC3339]. This field is REQUIRED for
objects with "valid" in the "status field.
scope (optional, string): If this field is present, then it MUST
contain a URI for an application 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
applications until the authorization expires. [[ Open issue: More
flexible scoping? ]]
challenges (required, array): The challenges that the client needs
to fulfill in order to prove possession of the identifier (for
pending authorizations). For final authorizations, the challenges
that were used. Each array entry is a dictionary with parameters
required to validate the challenge, as specified in Section 7.
combinations (optional, array of arrays of integers): A collection
of sets of challenges, each of which would be sufficient to prove
possession of the identifier. Clients complete a set of
challenges that covers at least one set in this array. Challenges
are identified by their indices in the challenges array. If no
"combinations" element is included in an authorization object, the
client completes all challenges.
The only type of identifier defined by this specification is a fully-
qualified domain name (type: "dns"). The value of the identifier
MUST be the ASCII representation of the domain name. Wildcard domain
names (with "*" as the first label) MUST NOT be included in
authorization requests.
Barnes, et al. Expires January 9, 2017 [Page 22]
Internet-Draft ACME July 2016
{
"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. Registration
A client creates a new account with the server by sending a POST
request to the server's new-registration URI. The body of the
request is a stub registration object containing only the "contact"
field.
POST /acme/new-reg HTTP/1.1
Host: example.com
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"jwk": {...},
"nonce": "6S8IqOGY7eL2lsGoTZYifg",
"url": "https://example.com/acme/new-reg"
})
"payload": base64url({
"contact": [
"mailto:cert-admin@example.com",
"tel:+12025551212"
]
}),
"signature": "RZPOnYoPs1PhjszF...-nh6X1qtOFPB519I"
}
The server MUST ignore any values provided in the "key",
"authorizations", and "certificates" fields in registration bodies
Barnes, et al. Expires January 9, 2017 [Page 23]
Internet-Draft ACME July 2016
sent by the client, as well as any other fields that it does not
recognize. If new fields are specified in the future, the
specification of those fields MUST describe whether they may be
provided by the client.
The server creates a registration object with the included contact
information. The "key" element of the registration is set to the
public key used to verify the JWS (i.e., the "jwk" element of the JWS
header). The server returns this registration object in a 201
(Created) response, with the registration URI in a Location header
field.
If the server already has a registration object with the provided
account key, then it MUST return a 409 (Conflict) response and
provide the URI of that registration in a Location header field.
This allows a client that has an account key but not the
corresponding registration URI to recover the registration URI.
If the server wishes to present the client with terms under which the
ACME service is to be used, it MUST indicate the URI where such terms
can be accessed in a Link header with link relation "terms-of-
service". As noted above, the client may indicate its agreement with
these terms by updating its registration to include the "agreement"
field, with the terms URI as its value. When these terms change in a
way that requires an agreement update, the server MUST use a
different URI in the Link header.
HTTP/1.1 201 Created
Content-Type: application/json
Location: https://example.com/acme/reg/asdf
Link: <https://example.com/acme/terms>;rel="terms-of-service"
Link: <https://example.com/acme/some-directory>;rel="directory"
{
"key": { /* JWK from JWS header */ },
"status": "good",
"contact": [
"mailto:cert-admin@example.com",
"tel:+12025551212"
]
}
If the client wishes to update this information in the future, it
sends a POST request with updated information to the registration
URI. The server MUST ignore any updates to the "key",
"authorizations, or "certificates" fields, and MUST verify that the
Barnes, et al. Expires January 9, 2017 [Page 24]
Internet-Draft ACME July 2016
request is signed with the private key corresponding to the "key"
field of the request before updating the registration.
For example, to update the contact information in the above
registration, the client could send the following request:
POST /acme/reg/asdf HTTP/1.1
Host: example.com
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"jwk": {...},
"nonce": "ax5RnthDqp_Yf4_HZnFLmA",
"url": "https://example.com/acme/reg/asdf"
})
"payload": base64url({
"contact": [
"mailto:certificates@example.com",
"tel:+12125551212"
]
}),
"signature": "hDXzvcj8T6fbFbmn...rDzXzzvzpRy64N0o"
}
Servers SHOULD NOT respond to GET requests for registration resources
as these requests are not authenticated. If a client wishes to query
the server for information about its account (e.g., to examine the
"contact" or "certificates" fields), then it SHOULD do so by sending
a POST request with an empty update. That is, it should send a JWS
whose payload is trivial ({}).
6.2.1. Account Key Roll-over
A client may wish to change the public key that is associated with a
registration in order to recover from a key compromise or proactively
mitigate the impact of an unnoticed key compromise.
To change the key associate with an account, the client sends a POST
request containing a key-change object with the following fields:
oldKey (required, JWK): The JWK representation of the original key
(i.e., the client's current account key)
newKey (requrired, JWK): The JWK representation of the new key
Barnes, et al. Expires January 9, 2017 [Page 25]
Internet-Draft ACME July 2016
The JWS of this POST must have two signatures: one signature from the
existing key on the account, and one signature from the new key that
the client proposes to use. This demonstrates that the client
actually has control of the private key corresponding to the new
public key. The protected header must contain a JWK field containing
the current account key.
POST /acme/key-change HTTP/1.1
Host: example.com
Content-Type: application/jose+json
{
"payload": base64url({
"oldKey": /* Old key in JWK form */
"newKey": /* New key in JWK form */
}),
"signatures": [{
"protected": base64url({
"alg": "ES256",
"jwk": /* old key */,
"nonce": "pq00v-D1KB0sReG4jFfqVg",
"url": "https://example.com/acme/key-change"
}),
"signature": "XFvVbo9diBlIBvhE...UI62sNT6MZsCJpQo"
}, {
"protected": base64url({
"alg": "ES256",
"jwk": /* new key */,
"nonce": "vYjyueEYhMjpVQHe_unw4g",
"url": "https://example.com/acme/key-change"
}),
"signature": "q20gG1f1r9cD6tBM...a48h0CkP11tl5Doo"
}]
}
On receiving key-change request, the server MUST perform the
following steps in addition to the typical JWS validation:
1. Check that the JWS protected header container a "jwk" field
containing a key that matches a currently active account.
2. Check that there are exactly two signatures on the JWS.
3. Check that one of the signatures validates using the account key
from (1).
4. Check that the "key" field contains a well-formed JWK that meets
key strength requirements.
Barnes, et al. Expires January 9, 2017 [Page 26]
Internet-Draft ACME July 2016
5. Check that the "key" field is not equivalent to the current
account key or any other currently active account key.
6. Check that one of the two signatures on the JWS validates using
the JWK from the "key" field.
If all of these checks pass, then the server updates the
corresponding registration 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.
6.2.2. 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/reg/asdf HTTP/1.1
Host: example.com
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"jwk": {...},
"nonce": "ntuJWWSic4WVNSqeUmshgg",
"url": "https://example.com/acme/reg/asdf"
})
"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 registration
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 January 9, 2017 [Page 27]
Internet-Draft ACME July 2016
6.3. Applying for Certificate Issuance
The holder of an account key pair may use ACME to submit an
application for a certificate to be issued. The client makes this
request by sending a POST request to the server's new-application
resource. The body of the POST is a JWS object whose JSON payload is
a subset of the application 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: This field
uses the same modified Base64 encoding rules used elsewhere in
this document, so it is different from PEM.)
notBefore (optional, string): The requested value of the notBefore
field in the certificate, in the date format defined in [RFC3339]
notAfter (optional, string): The requested value of the notAfter
field in the certificate, in the date format defined in [RFC3339]
POST /acme/new-app HTTP/1.1
Host: example.com
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"jwk": {...},
"nonce": "5XJ1L3lEkMG7tR6pA00clA",
"url": "https://example.com/acme/new-app"
})
"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 January 9, 2017 [Page 28]
Internet-Draft ACME July 2016
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 application object reflecting the client's request and any
requirements the client must fulfill before the certificate will be
issued.
HTTP/1.1 201 Created
Location: https://example.com/acme/app/asdf
{
"status": "pending",
"expires": "2015-03-01T14:09:00Z",
"csr": "jcRf4uXra7FGYW5ZMewvV...rhlnznwy8YbpMGqwidEXfE",
"notBefore": "2016-01-01T00:00:00Z",
"notAfter": "2016-01-08T00:00:00Z",
"requirements": [
{
"type": "authorization",
"status": "valid",
"url": "https://example.com/acme/authz/1234"
},
{
"type": "out-of-band",
"status": "pending",
"url": "https://example.com/acme/payment/1234"
}
]
}
The application 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 application object, any object in the "requirements" array
whose status is "pending" represents an action that the client must
perform before the server will issue the certificate. If the client
fails to complete the required actions before the "expires" time,
then the server SHOULD change the status of the application to
"invalid" and MAY delete the application resource.
The server SHOULD issue the requested certificate and update the
application resource with a URL for the certificate as soon as the
client has fulfilled the server's requirements. If the client has
Barnes, et al. Expires January 9, 2017 [Page 29]
Internet-Draft ACME July 2016
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 MAY
proactively issue the requested certificate and provide a URL for it
in the "certificate" field of the application. The server MUST,
however, still list the satisfied requirements in the "requirements"
array, with the state "valid".
Once the client believes it has fulfilled the server's requirements,
it should send a GET request to the application resource to obtain
its current state. The status of the application will indicate what
action the client should take:
o "invalid": The certificate will not be issued. Consider this
application process abandoned.
o "pending": The server does not believe that the client has
fulfilled the requirements. Check the "requirements" array for
requirements 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 application. Download the
certificate.
6.3.1. 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 DER (application/pkix-cert).
The client may request other formats by including an Accept header in
its request. For example, the client may use the media type
application/x-pem-file to request the certificate in PEM format.
The server provides metadata about the certificate in HTTP headers.
In particular, the server MUST send one or more link relation header
fields [RFC5988] with relation "up", each indicating a single
certificate resource for the issuer of this certificate. The server
MAY also include the "up" links from these resources to enable the
client to build a full certificate chain.
Barnes, et al. Expires January 9, 2017 [Page 30]
Internet-Draft ACME July 2016
The server MUST also provide a link relation header field with
relation "author" to indicate the application 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/app/asdf>;rel="author"
Link: <https://example.com/acme/sct/asdf>;rel="ct-sct"
Link: <https://example.com/acme/some-directory>;rel="directory"
[DER-encoded certificate]
A certificate resource represents a single, immutable certificate.
If the client wishes to obtain a renewed certificate, the client
initiates a new application 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.
6.4. Identifier Authorization
The identifier authorization process establishes the authorization of
an account to manage certificates for a given identifier. This
process must assure the server of two things: First, that the client
controls the private key of the account key pair, and second, that
the client holds the identifier in question. This process may be
repeated to associate multiple identifiers to a key pair (e.g., to
request certificates with multiple identifiers), or to associate
multiple accounts with an identifier (e.g., to allow multiple
entities to manage certificates). The server may declare that an
Barnes, et al. Expires January 9, 2017 [Page 31]
Internet-Draft ACME July 2016
authorization is only valid for a specific application by setting the
"scope" field of the authorization to the URI for that application.
Authorization resources are created by the server in response to
certificate applications submitted by an account key holder; their
URLs are provided to the client in "authorization" requirement
objects. The authorization object is implicitly tied to the account
key used to sign the new-application request.
When a client receives an application from the server with an
"authorization" requirement, it downloads the authorization resource
by sending a GET request to the indicated URL.
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",
"identifier": {
"type": "dns",
"value": "example.org"
},
"challenges": [
{
"type": "http-01",
"uri": "https://example.com/authz/asdf/0",
"token": "IlirfxKKXAsHtmzK29Pj8A"
},
{
"type": "dns-01",
"uri": "https://example.com/authz/asdf/1",
"token": "DGyRejmCefe7v4NfDGDKfA"
}
],
"combinations": [[0], [1]]
}
Barnes, et al. Expires January 9, 2017 [Page 32]
Internet-Draft ACME July 2016
6.4.1. 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
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",
"jwk": {...},
"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
Barnes, et al. Expires January 9, 2017 [Page 33]
Internet-Draft ACME July 2016
taken to fulfill the challenge, e.g., removing files that have been
provisioned to a web server.
Presumably, the client's responses provide the server with enough
information to validate one or more challenges. The server is said
to "finalize" the authorization when it has completed all the
validations it is going to complete, and assigns the authorization a
status of "valid" or "invalid", corresponding to whether it considers
the account authorized for the identifier. If the final state is
"valid", the server MUST add an "expires" field to the authorization.
When finalizing an authorization, the server MAY remove the
"combinations" field (if present) or remove any challenges still
pending. The server SHOULD NOT remove challenges with status
"invalid".
Usually, the validation process will take some time, so the client
will need to poll the authorization resource to see when it is
finalized. For challenges where the client can tell when the server
has validated the challenge (e.g., by seeing an HTTP or DNS request
from the server), the client SHOULD NOT begin polling until it has
seen the validation request from the server.
To check on the status of an authorization, the client sends a GET
request to the authorization URI, and the server responds with the
current authorization object. In responding to poll requests while
the validation is still in progress, the server MUST return a 202
(Accepted) response, and MAY include a Retry-After header field to
suggest a polling interval to the client.
Barnes, et al. Expires January 9, 2017 [Page 34]
Internet-Draft ACME July 2016
GET /acme/authz/asdf HTTP/1.1
Host: example.com
HTTP/1.1 200 OK
{
"status": "valid",
"expires": "2015-03-01T14:09:00Z",
"identifier": {
"type": "dns",
"value": "example.org"
},
"challenges": [
{
"type": "http-01"
"status": "valid",
"validated": "2014-12-01T12:05:00Z",
"token": "IlirfxKKXAsHtmzK29Pj8A",
"keyAuthorization": "IlirfxKKXA...vb29HhjjLPSggwiE"
}
]
}
6.4.2. 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 January 9, 2017 [Page 35]
Internet-Draft ACME July 2016
POST /acme/authz/asdf HTTP/1.1
Host: example.com
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"jwk": {...},
"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 registration object.
The server MUST NOT treat deactivated authorization objects as
sufficient for issuing certificates.
6.5. Certificate Revocation
To request that a certificate be revoked, the client sends a POST
request to the ACME server's revoke-cert URI. The body of the POST
is a JWS object whose JSON payload contains the certificate to be
revoked:
certificate (required, string): The certificate to be revoked, in
the base64url-encoded version of the DER format. (Note: This
field uses the same modified Base64 encoding rules used elsewhere
in this document, so it is different from PEM.)
reason (optional, int): One of the revocation reasonCodes defined in
RFC 5280 [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 January 9, 2017 [Page 36]
Internet-Draft ACME July 2016
POST /acme/revoke-cert HTTP/1.1
Host: example.com
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"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
keys authorized for a given certificate:
o the public key in the certificate.
o an account key that is authorized to act for all of the
identifier(s) in the certificate.
If the revocation succeeds, the server responds with status code 200
(OK). If the revocation fails, the server returns an error.
HTTP/1.1 200 OK
Content-Length: 0
--- or ---
HTTP/1.1 403 Forbidden
Content-Type: application/problem+json
Content-Language: en
{
"type": "urn:ietf:params:acme:error:unauthorized"
"detail": "No authorization provided for name example.net"
"instance": "http://example.com/doc/unauthorized"
}
Barnes, et al. Expires January 9, 2017 [Page 37]
Internet-Draft ACME July 2016
7. Identifier Validation Challenges
There are few types of identifiers in the world for which there is a
standardized mechanism to prove possession of a given identifier. In
all practical cases, CAs rely on a variety of means to test whether
an entity applying for a certificate with a given identifier actually
controls that identifier.
Challenges provide the server with assurance that an account key
holder is also the entity that controls an identifier. For each type
of challenge, it must be the case that in order for an entity to
successfully complete the challenge the entity must both:
o Hold the private key of the account key pair used to respond to
the challenge
o Control the identifier in question
Section 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:
1. Content of challenge objects
2. Content of response objects
3. How the server uses the challenge and response to verify control
of an identifier
Challenge objects all contain the following basic fields:
type (required, string): The type of challenge encoded in the
object.
uri (required, string): The URI to which a response can be posted.
status (required, string): The status of this authorization.
Possible values are: "pending", "valid", and "invalid".
Barnes, et al. Expires January 9, 2017 [Page 38]
Internet-Draft ACME July 2016
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
and TLS SNI, the client directly proves its ability to do certain
things related to the identifier. The choice of which challenges to
offer to a client under which circumstances is a matter of server
policy.
The identifier validation challenges described in this section all
relate to validation of domain names. If ACME is extended in the
future to support other types of identifier, there will need to be
new challenge types, and they will need to specify which types of
identifier they apply to.
[[ Editor's Note: In pre-RFC versions of this specification,
challenges are labeled by type, and with the version of the draft in
which they were introduced. For example, if an HTTP challenge were
introduced in version -03 and a breaking change made in version -05,
then there would be a challenge labeled "http-03" and one labeled
"http-05" - but not one labeled "http-04", since challenge in version
-04 was compatible with one in version -04. ]]
[[ Editor's Note: Operators SHOULD NOT issue "combinations" arrays in
authorization objects that require the client to perform multiple
challenges over the same type, e.g., ["http-03", "http-05"].
Challenges within a type are testing the same capability of the
domain owner, and it may not be possible to satisfy both at once. ]]
7.1. Key Authorizations
Several of the challenges in this document makes use of a key
authorization string. A key authorization is a string that expresses
a domain holder's authorization for a specified key to satisfy a
specified challenge, by concatenating the token for the challenge
with a key fingerprint, separated by a "." character:
Barnes, et al. Expires January 9, 2017 [Page 39]
Internet-Draft ACME July 2016
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.
type (required, string): The string "http-01"
token (required, string): A random value that uniquely identifies
the challenge. This value MUST have at least 128 bits of entropy,
in order to prevent an attacker from guessing it. It MUST NOT
contain any characters outside the URL-safe Base64 alphabet and
MUST NOT contain any padding characters ("=").
{
"type": "http-01",
"token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA"
}
Barnes, et al. Expires January 9, 2017 [Page 40]
Internet-Draft ACME July 2016
A client responds to this challenge by constructing a key
authorization from the "token" value provided in the challenge and
the client's account key. The client then provisions the key
authorization as a resource on the HTTP server for the domain in
question.
The path at which the resource is provisioned is comprised of the
fixed prefix ".well-known/acme-challenge/", followed by the "token"
value in the challenge. The value of the resource MUST be the ASCII
representation of the key authorization.
.well-known/acme-challenge/evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA
The client's response to this challenge indicates its agreement to
this challenge by sending the server the key authorization covering
the challenge's token and the client's account key. In addition, the
client MAY advise the server at which IP the challenge is
provisioned.
keyAuthorization (required, string): The key authorization for this
challenge. This value MUST match the token from the challenge and
the client's account key.
/* BEGIN JWS-signed content */
{
"keyAuthorization": "evaGxfADs...62jcerQ"
}
/* END JWS-signed content */
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.
Barnes, et al. Expires January 9, 2017 [Page 41]
Internet-Draft ACME July 2016
3. Dereference the URI using an HTTP GET request.
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.
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 URL-safe Base64 alphabet and
MUST NOT contain any padding characters ("=").
{
"type": "tls-sni-02",
"token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA"
}
A client responds to this challenge by constructing a self-signed
certificate which the client MUST provision at the domain name
concerned in order to pass the challenge.
The certificate may be constructed arbitrarily, except that each
certificate MUST have exactly two subjectAlternativeNames, SAN A and
SAN B. Both MUST be dNSNames.
SAN A MUST be constructed as follows: compute the SHA-256 digest of
the UTF-8-encoded challenge token and encode it in lowercase
hexadecimal form. The dNSName is "x.y.token.acme.invalid", where x
Barnes, et al. Expires January 9, 2017 [Page 42]
Internet-Draft ACME July 2016
is the first half of the hexadecimal representation and y is the
second half.
SAN B MUST be constructed as follows: compute the SHA-256 digest of
the UTF-8 encoded key authorization and encode it in lowercase
hexadecimal form. The dNSName is "x.y.ka.acme.invalid" where x is
the first half of the hexadecimal representation and y is the second
half.
The client MUST ensure that the certificate is served to TLS
connections specifying a Server Name Indication (SNI) value of SAN A.
The response to the TLS-SNI challenge simply acknowledges that the
client is ready to fulfill this challenge.
keyAuthorization (required, string): The key authorization for this
challenge. This value MUST match the token from the challenge and
the client's account key.
/* BEGIN JWS-signed content */
{
"keyAuthorization": "evaGxfADs...62jcerQ"
}
/* END JWS-signed content */
On receiving a response, the server MUST verify that the key
authorization in the response matches the "token" value in the
challenge and the client's account key. If they do not match, then
the server MUST return an HTTP error in response to the POST request
in which the client sent the challenge.
Given a challenge/response pair, the ACME server verifies the
client's control of the domain by verifying that the TLS server was
configured appropriately, using these steps:
1. Compute SAN A and SAN B in the same way as the client.
2. Open a TLS connection to the domain name being validated on the
requested port, presenting SAN A in the SNI field. In the
ClientHello initiating the TLS handshake, the server MUST include
a server_name extension (i.e., SNI) containing SAN A. The server
SHOULD ensure that it does not reveal SAN B in any way when
making the TLS connection, such that the presentation of SAN B in
the returned certificate proves association with the client.
3. Verify that the certificate contains a subjectAltName extension
containing dNSName entries of SAN A and SAN B and no other
Barnes, et al. Expires January 9, 2017 [Page 43]
Internet-Draft ACME July 2016
entries. The comparison MUST be insensitive to case and ordering
of names.
It is RECOMMENDED that the ACME server validation TLS connections
from multiple vantage points to reduce the risk of DNS hijacking
attacks.
If all of the above verifications succeed, then the validation is
successful. Otherwise, the validation fails.
7.4. DNS
When the identifier being validated is a domain name, the client can
prove control of that domain by provisioning a resource record under
it. The DNS challenge requires the client to provision a TXT record
containing a designated value under a specific validation domain
name.
type (required, string): The string "dns-01"
token (required, string): A random value that uniquely identifies
the challenge. This value MUST have at least 128 bits of entropy,
in order to prevent an attacker from guessing it. It MUST NOT
contain any characters outside the URL-safe Base64 alphabet and
MUST NOT contain any padding characters ("=").
{
"type": "dns-01",
"token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA"
}
A client responds to this challenge by constructing a key
authorization from the "token" value provided in the challenge and
the client's account key. The client then computes the SHA-256
digest of the key authorization.
The record provisioned to the DNS is the base64url encoding of this
digest. The client constructs the validation domain name by
prepending the label "_acme-challenge" to the domain name being
validated, then provisions a TXT record with the digest value under
that name. For example, if the domain name being validated is
"example.com", then the client would provision the following DNS
record:
_acme-challenge.example.com. 300 IN TXT "gfj9Xq...Rg85nM"
Barnes, et al. Expires January 9, 2017 [Page 44]
Internet-Draft ACME July 2016
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.
/* BEGIN JWS-signed content */
{
"keyAuthorization": "evaGxfADs...62jcerQ"
}
/* END JWS-signed content */
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
3. Verify that the contents of one of the TXT records matches the
digest value
If all of the above verifications succeed, then the validation is
successful. If no DNS record is found, or DNS record and response
payload do not pass these checks, then the validation fails.
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"
Barnes, et al. Expires January 9, 2017 [Page 45]
Internet-Draft ACME July 2016
url (required, string): The URL to be visited. The scheme of this
URL MUST be "http" or "https"
{
"type": "oob-01",
"url": "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"
/* BEGIN JWS-signed content */
{
"type": "oob-01"
}
/* END JWS-signed content */
On receiving a response, the server MUST verify that the value of the
"type" field is as required. Otherwise, the steps the server takes
to validate identifier possession are determined by the server's
local policy.
8. IANA Considerations
[[ Editor's Note: Should we create a registry for tokens that go into
the various JSON objects used by this protocol, i.e., the field names
in the JSON objects? ]]
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
Barnes, et al. Expires January 9, 2017 [Page 46]
Internet-Draft ACME July 2016
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 5.4.1 |
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 5.3.1 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 5.4.2 of RFC XXXX
[[ RFC EDITOR: Please replace XXXX above with the RFC number assigned
to this document ]]
Barnes, et al. Expires January 9, 2017 [Page 47]
Internet-Draft ACME July 2016
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
Index value: No transformation needed. The
[[ RFC EDITOR: Please replace XXXX above with the RFC number assigned
to this document, and replace URL-TBD with the URL assigned by IANA
for registries of ACME parameters. ]]
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. 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
Barnes, et al. Expires January 9, 2017 [Page 48]
Internet-Draft ACME July 2016
Initial contents: The codes and descriptions in the table in
Section 5.6 above, with the Reference field set to point to this
specification.
8.6.2. 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
o Reference: Where the identifier type is defined
Initial contents:
+-------------+--------------------+-----------+
| Key | Resource type | Reference |
+-------------+--------------------+-----------+
| new-reg | New registration | RFC XXXX |
| | | |
| new-app | New application | 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 ]]
8.6.3. 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:
Barnes, et al. Expires January 9, 2017 [Page 49]
Internet-Draft ACME July 2016
+-------+-----------+
| Label | Reference |
+-------+-----------+
| dns | RFC XXXX |
+-------+-----------+
[[ RFC EDITOR: Please replace XXXX above with the RFC number assigned
to this document ]]
8.6.4. Challenge Types
This registry lists the ways that ACME servers can offer to validate
control of an identifier. The "Identifier Type" field in template
MUST be contained in the Label column of the ACME Identifier Types
registry.
Template:
o Label: The value to be put in the "type" field of challenge
objects using this validation mechanism
o Identifier Type: The type of identifier that this mechanism
applies to
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
Barnes, et al. Expires January 9, 2017 [Page 50]
Internet-Draft ACME July 2016
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.
9.1. Threat model
As a service on the Internet, ACME broadly exists within the Internet
threat model [RFC3552]. In analyzing ACME, it is useful to think of
an ACME server interacting with other Internet hosts along three
"channels":
o An ACME channel, over which the ACME HTTPS requests are exchanged
o A validation channel, over which the ACME server performs
additional requests to validate a client's control of an
identifier
o A contact channel, over which the ACME server sends messages to
the registered contacts for ACME clients
+------------+
| ACME | ACME Channel
| Client |--------------------+
+------------+ |
^ V
| Contact Channel +------------+
+--------------------| ACME |
| Server |
+------------+
+------------+ |
| Validation |<-------------------+
| Server | Validation Channel
+------------+
In practice, the risks to these channels are not entirely separate,
but they are different in most cases. Each of the three channels,
for example, uses a different communications pattern: the ACME
channel will comprise inbound HTTPS connections to the ACME server,
Barnes, et al. Expires January 9, 2017 [Page 51]
Internet-Draft ACME July 2016
the validation channel outbound HTTP or DNS requests, and the contact
channel will use channels such as email and PSTN.
Broadly speaking, ACME aims to be secure against active and passive
attackers on any individual channel. Some vulnerabilities arise
(noted below), when an attacker can exploit both the ACME channel and
one of the others.
On the ACME channel, in addition to network-layer attackers, we also
need to account for application-layer man in the middle attacks, and
for abusive use of the protocol itself. Protection against
application-layer MitM addresses potential attackers such as Content
Distribution Networks (CDNs) and middleboxes with a TLS MitM
function. Preventing abusive use of ACME means ensuring that an
attacker with access to the validation or contact channels can't
obtain illegitimate authorization by acting as an ACME client
(legitimately, in terms of the protocol).
9.2. Integrity of Authorizations
ACME allows anyone to request challenges for an identifier by
registering an account key and sending a new-application 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-application 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
Barnes, et al. Expires January 9, 2017 [Page 52]
Internet-Draft ACME July 2016
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:
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.
Barnes, et al. Expires January 9, 2017 [Page 53]
Internet-Draft ACME July 2016
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
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.
Barnes, et al. Expires January 9, 2017 [Page 54]
Internet-Draft ACME July 2016
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
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:
Barnes, et al. Expires January 9, 2017 [Page 55]
Internet-Draft ACME July 2016
o Has the client agreed to a subscriber agreement?
o Is the claimed identifier syntactically valid?
o For domain names:
* If the leftmost label is a '*', then have the appropriate
checks been applied?
* Is the name on the Public Suffix List?
* Is the name a high-value name?
* Is the name a known phishing domain?
o Is the key in the CSR sufficiently strong?
o Is the CSR signed with an acceptable algorithm?
CAs that use ACME to automate issuance will need to ensure that their
servers perform all necessary checks before issuing.
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.
Barnes, et al. Expires January 9, 2017 [Page 56]
Internet-Draft ACME July 2016
However, some hosting platforms will choose a virtual host to be the
"default", and route connections with unknown SNI values to that
host.
In such cases, the owner of the default virtual host can complete a
TLS-based challenge (e.g., "tls-sni-02") for any domain with an A
record that points to the hosting platform. This could result in
mis-issuance in cases where there are multiple hosts with different
owners resident on the hosting platform.
A CA that accepts TLS-based proof of domain control should attempt to
check whether a domain is hosted on a domain with a default virtual
host before allowing an authorization request for this host to use a
TLS-based challenge. A default virtual host can be detected by
initiating TLS connections to the host with random SNI values within
the namespace used for the TLS-based challenge (the "acme.invalid"
namespace for "tls-sni-02").
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
Barnes, et al. Expires January 9, 2017 [Page 57]
Internet-Draft ACME July 2016
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.
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>.
[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>.
Barnes, et al. Expires January 9, 2017 [Page 58]
Internet-Draft ACME July 2016
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<http://www.rfc-editor.org/info/rfc5280>.
[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>.
[RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
2015, <http://www.rfc-editor.org/info/rfc7515>.
[RFC7517] Jones, M., "JSON Web Key (JWK)", RFC 7517,
DOI 10.17487/RFC7517, May 2015,
<http://www.rfc-editor.org/info/rfc7517>.
[RFC7518] Jones, M., "JSON Web Algorithms (JWA)", RFC 7518,
DOI 10.17487/RFC7518, May 2015,
<http://www.rfc-editor.org/info/rfc7518>.
Barnes, et al. Expires January 9, 2017 [Page 59]
Internet-Draft ACME July 2016
[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>.
[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>.
Barnes, et al. Expires January 9, 2017 [Page 60]
Internet-Draft ACME July 2016
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 January 9, 2017 [Page 61]