Automatic Certificate Management Environment (ACME)Ciscorlb@ipv.sxEFFjsha@eff.orgUniversity of Michiganjdkasten@umich.eduACME Working GroupCertificates in PKI using X.509 (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 certification 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).Certificates in the Web PKI are most commonly used to authenticate
domain names. Thus, certificate authorities in the Web PKI are trusted to
verify that an applicant for a certificate legitimately represents the domain
name(s) in the certificate.Different types of certificates reflect different kinds of CA verification of
information about the certificate subject. “Domain Validation” (DV)
certificates are by far the most common type. For DV validation, the CA merely
verifies that the requester has effective control of the web server and/or DNS
server for the domain, but does not explicitly attempt to verify their
real-world identity. (This is as opposed to “Organization Validation” (OV) and
“Extended Validation” (EV) certificates, where the process is intended to also
verify the real-world identity of the requester.)Existing Web PKI certificate authorities tend to run on a set of ad hoc
protocols for certificate issuance and identity verification. In the case of DV
certificates, a typical user experience is something like:Generate a PKCS#10 Certificate Signing Request (CSR).Cut-and-paste the CSR into a CA web page.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 email
address corresponding to the domain and then respond to it on the CA’s web
page.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 Transport Layer Security (TLS)
.It should be noted that while the focus of this document is on validating
domain names for purposes of issuing certificates in the Web PKI, ACME supports
extensions for uses with other identifiers in other PKI contexts. For example,
as of this writing, there is ongoing work to use ACME for issuance of WebPKI
certificates attesting to IP addresses and STIR
certificates attesting to telephone numbers .The guiding use case for ACME is obtaining certificates for websites
(HTTPS ). In this case, the user’s web server is intended to speak
for one or more domains, and the process of certificate issuance is intended to
verify that this web server actually speaks for the domain(s).DV certificate validation commonly checks claims about properties related to
control of a domain name – properties that can be observed by the certificate
issuer 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.Prior to ACME, when deploying an HTTPS server, an operator typically gets a
prompt to generate a self-signed certificate. If the operator were instead
deploying an HTTPS server using ACME, the experience would be something like this:The ACME client prompts the operator for the intended domain name(s) that the
web server is to stand for.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.The operator selects a CA.In the background, the ACME client contacts the CA and requests that it
issue a certificate for the intended domain name(s).The CA verifies that the client controls the requested domain name(s).Once the CA is satisfied, the certificate is issued and the ACME client
automatically downloads and installs it, potentially notifying the operator
via email, SMS, etc.The ACME client periodically contacts the CA to get updated certificates,
stapled OCSP responses, or whatever else would be required to keep the web
server functional and its credentials up-to-date.In this way, it would be nearly as easy to deploy with a CA-issued certificate
as with a self-signed certificate. Furthermore, the maintenance of that
CA-issued certificate would require minimal manual intervention. Such close
integration of ACME with HTTPS servers allows the immediate and automated
deployment of certificates as they are issued, sparing the human administrator
from much of the time-consuming work described in the previous section.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 .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 may run on a web server, mail server, or some other
server system which requires valid TLS certificates. Or, it may run on a separate
server that does not consume the certificate, but is authorized to respond to a
CA-provided challenge. The ACME server runs at a certification 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.ACME allows a client to request certificate management actions using a set of
JavaScript Object Notation (JSON) messages carried over HTTPS. In many ways,
ACME functions much like a traditional CA, in which a user creates an account,
requests a certificate, and proves control of the domains in that certificate in
order for the CA to sign the requested certificate.The first phase of ACME is for the client to request an account with the
ACME server. The client generates an asymmetric key pair and requests a
new account, optionally providing contact information, agreeing to terms
of service, and/or associating the account with an existing account
in another system. The creation request is signed with the generated
private key to prove that the client controls it.Once an account is registered, there are three major steps the client needs to take to
get a certificate:Submit an order for a certificate to be issuedProve control of any identifiers requested in the certificateAwait issuance and download the issued certificateThe client’s order for a certificate describes the desired certificate using a
PKCS#10 Certificate Signing Request (CSR) plus a few additional fields that
capture semantics that are not supported in the CSR format. If the server is
willing to consider issuing such a certificate, it responds with a list of
requirements that the client must satisfy before the certificate will be issued.For example, in most cases, the server will require the client to demonstrate
that it controls the identifiers in the requested certificate. Because there
are many different ways to validate possession of different types of
identifiers, the server will choose from an extensible set of challenges that
are appropriate for the identifier being claimed. The client responds with a
set of responses that tell the server which challenges the client has completed.
The server then validates the challenges to check that the client has
accomplished them.Once the validation process is complete and the server is satisfied that the
client has met its requirements, the server will issue the requested certificate
and make it available to the client.To revoke a certificate, the client sends a signed revocation request indicating
the certificate to be revoked: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
below address validation of domain names.
The use of ACME for other identifiers will require further specification in order
to describe how these identifiers are encoded in the protocol and what types of
validation challenges the server might require.All requests and responses sent via HTTP by ACME clients, ACME servers, and
validation servers as well as any inputs for digest computations MUST be encoded
using the UTF-8 character set.Communications between an ACME client and an ACME server are done over HTTPS,
using JSON Web Signature (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 URL.Each ACME function is accomplished by the client sending a sequence of HTTPS
requests to the server, carrying JSON messages . Use of
HTTPS is REQUIRED. Each subsection of
below describes the message formats used by the
function and the order in which messages are sent.In most HTTPS transactions used by ACME, the ACME client is the HTTPS client
and the ACME server is the HTTPS server. The ACME server acts as an HTTP and
HTTPS client when validating challenges via HTTP.ACME servers SHOULD follow the recommendations of when configuring
their TLS implementations. ACME servers that support TLS 1.3 MAY allow clients
to send early data (0xRTT). This is safe because the ACME protocol itself
includes anti-replay protections.ACME clients SHOULD send a User-Agent header in accordance with
, including the name and version of the ACME software in
addition to the name and version of the underlying HTTP client software.ACME clients SHOULD send an Accept-Language header in accordance with
to enable localization of error messages.ACME servers that are intended to be generally accessible need to use
Cross-Origin Resource Sharing (CORS) in order to be accessible from
browser-based clients . 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 Section 5, according to the profile specified
in JSON Web Signature Section 2. This encoding uses a URL safe
character set. Trailing ‘=’ characters MUST be stripped.All ACME requests with a non-empty body MUST encapsulate their payload
in a JSON Web Signature (JWS) object, signed using the account’s
private key unless otherwise specified. The server MUST verify the JWS before
processing the request. Encapsulating request bodies in JWS provides
authentication of requests.JWS objects sent in ACME requests MUST meet the following additional criteria:The JWS MUST NOT have the value “none” in its “alg” fieldThe JWS MUST NOT have a Message Authentication Code (MAC)-based algorithm in its “alg” fieldThe JWS Protected Header MUST include the following fields:
“alg” (Algorithm)“jwk” (JSON Web Key, only for requests to new-account and revoke-cert resources)“kid” (Key ID, for all other requests)“nonce” (defined in below)“url” (defined in below)The “jwk” and “kid” fields are mutually exclusive. Servers MUST reject requests
that contain both.For new-account requests, and for revoke-cert requests authenticated by certificate
key, there MUST be a “jwk” field.For all other requests, there MUST be a “kid” field. This field must
contain the account URL received by POSTing to the new-account resource.Note that authentication via signed JWS request bodies implies that GET requests
are not authenticated. Servers MUST NOT respond to GET requests for resources
that might be considered sensitive. Account resources are the only sensitive
resources defined in this specification.If the client sends a JWS signed with an algorithm that the server does not
support, then the server MUST return an error with status code 400 (Bad Request)
and type “urn:ietf:params:acme:error:badSignatureAlgorithm”. The problem
document returned with the error MUST include an “algorithms” field with an
array of supported “alg” values.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.It is common in deployment for the entity terminating TLS for HTTPS to be different
from the entity operating the logical HTTPS server, with a “request routing”
layer in the middle. For example, an ACME CA might have a content delivery
network terminate TLS connections from clients so that it can inspect client
requests for denial-of-service protection.These intermediaries can also change values in the request that are not signed
in the HTTPS request, e.g., the request URL and headers. ACME uses JWS to
provide an integrity mechanism, which protects against an intermediary
changing the request URL to another ACME URL.As noted in above, all ACME request objects carry a
“url” header 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” header parameter to
the request URL. If the two do not match, then the server MUST reject the
request as unauthorized.Except for the directory resource, all ACME resources are addressed with URLs
provided to the client by the server. For these resources, the client MUST set
the “url” header parameter to the exact string provided by the server (rather
than performing any re-encoding on the URL). The server SHOULD perform the
corresponding string equality check, configuring each resource with the URL
string provided to clients and having the resource check that requests have the
same string in their “url” header parameter.The “url” header parameter specifies the URL to which this JWS
object is directed. The “url” header parameter MUST be carried in the protected
header of the JWS. The value of the “url” header parameter MUST be a string
representing the URL.In order to protect ACME resources from any possible replay attacks, ACME
requests have a mandatory anti-replay mechanism. This mechanism is based on the
server maintaining a list of nonces that it has issued to clients, and requiring
any signed request from the client to carry such a nonce.An ACME server provides nonces to clients using the Replay-Nonce header field,
as specified in below. The server MUST include a Replay-Nonce
header field in every successful response to a POST request and SHOULD provide
it in error responses as well.Every JWS sent by an ACME client MUST include, in its protected header, the
“nonce” header parameter, with contents as defined in
below. As part of JWS verification, the
ACME server MUST verify that the value of the “nonce” header is a value that the
server previously provided in a Replay-Nonce header field. Once a nonce value
has appeared in an ACME request, the server MUST consider it invalid, in the same
way as a value it had never issued.When a server rejects a request because its nonce value was unacceptable (or not
present), it MUST provide HTTP status code 400 (Bad Request), and indicate the
ACME error type “urn:ietf:params:acme:error:badNonce”. An error response with
the “badNonce” error type MUST include a Replay-Nonce header with a fresh nonce.
On receiving such a response, a client SHOULD retry the request using the new
nonce.The precise method used to generate and track nonces is up to the server. For
example, the server could generate a random 128-bit value for each response,
keep a list of issued nonces, and strike nonces from this list as they are used.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 MUST generate the value provided in Replay-Nonce in such a way that
they are unique to each message, with high probability. For instance, it is
acceptable to generate Replay-Nonces randomly.The value of the Replay-Nonce field MUST be an octet string encoded according to
the base64url encoding described in Section 2 of . Clients MUST
ignore invalid Replay-Nonce values.The Replay-Nonce header field SHOULD NOT be included in HTTP request messages.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 . If
the value of a “nonce” header parameter is not valid according to this encoding,
then the verifier MUST reject the JWS as malformed.Creation of resources can be rate limited to ensure fair usage and
prevent abuse. Once the rate limit is exceeded, the server MUST respond
with an error with the type “urn:ietf:params:acme:error:rateLimited”.
Additionally, the server SHOULD send a “Retry-After” header indicating
when the current request may succeed again. If multiple rate limits are
in place, that is the time where all rate limits allow access again for
the current request with exactly the same parameters.In addition to the human-readable “detail” field of the error response, the
server MAY send one or multiple tokens in the “Link” header pointing to
documentation about the specific hit rate limits using the
“urn:ietf:params:acme:documentation” relation.Errors can be reported in ACME both at the HTTP layer and within challenge
objects as defined in . 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 a problem document . 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):TypeDescriptionbadCSRThe CSR is unacceptable (e.g., due to a short key)badNonceThe client sent an unacceptable anti-replay noncebadSignatureAlgorithmThe JWS was signed with an algorithm the server does not supportinvalidContactA contact URL for an account was invalidunsupportedContactA contact URL for an account used an unsupported protocol schemeaccountDoesNotExistThe request specified an account that does not existmalformedThe request message was malformedrateLimitedThe request exceeds a rate limitrejectedIdentifierThe server will not issue for the identifierserverInternalThe server experienced an internal errorunauthorizedThe client lacks sufficient authorizationunsupportedIdentifierIdentifier is not supported, but may be in futureuserActionRequiredVisit the “instance” URL and take actions specified therebadRevocationReasonThe revocation reason provided is not allowed by the servercaaCertification Authority Authorization (CAA) records forbid the CA from issuingdnsThere was a problem with a DNS queryconnectionThe server could not connect to validation targettlsThe server received a TLS error during validationincorrectResponseResponse received didn’t match the challenge’s requirementsThis 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 all errors.In this section, we describe the certificate management functions that ACME
enables:Account CreationOrdering a CertificateIdentifier AuthorizationCertificate IssuanceCertificate RevocationACME is structured as a REST application with the following types of resources:Account resources, representing information about an account
(, )Order resources, representing an account’s requests to issue certificates
()Authorization resources, representing an account’s authorization to act for an
identifier ()Challenge resources, representing a challenge to prove control of an
identifier (, )Certificate resources, representing issued certificates
()A “directory” resource ()A “new-nonce” resource ()A “new-account” resource ()A “new-order” resource ()A “revoke-cert” resource ()A “key-change” resource ()The server MUST provide “directory” and “new-nonce” resources.ACME uses different URLs for different management functions. Each function is
listed in a directory along with its corresponding URL, so clients only need to
be configured with the directory URL. These URLs are connected by a few
different link relations .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 “index” link relation is present on all resources other than the
directory and indicates the URL of the directory.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.The following table illustrates a typical sequence of requests required to
establish a new account with the server, prove control of an identifier, issue a
certificate, and fetch an updated certificate some time after issuance. The
“->” is a mnemonic for a Location header pointing to a created resource.ActionRequestResponseGet a nonceHEAD new-nonce204Create accountPOST new-account201 -> accountSubmit an orderPOST new-order201 -> orderFetch challengesGET authz200Respond to challengePOST challenge200Poll for statusGET authz200Check for new certGET cert200The remainder of this section provides the details of how these resources are
structured and how the ACME protocol makes use of them.In order to help clients configure themselves with the right URLs for each ACME
operation, ACME servers provide a directory object. This should be the only URL
needed to configure clients. It is a JSON object, whose fields names are drawn from
the following table and whose values are the corresponding URLs.FieldURL in valuenew-nonceNew noncenew-accountNew accountnew-orderNew ordernew-authzNew authorizationrevoke-certRevoke certificatekey-changeKey changeThere is no constraint on the actual URL of the directory except that it
should be different from the other ACME server resources’ URLs, and that it
should not clash with other services. For instance:a host which functions as both an ACME and a Web server may want to keep
the root path “/” for an HTML “front page”, and place the ACME
directory under the path “/acme”.a host which only functions as an ACME server could place the directory
under the path “/”.The object MAY additionally contain a field “meta”. If present, it MUST be a
JSON object; each field in the object 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:
A URL identifying the current terms of service.
An HTTP or HTTPS URL locating a website providing more
information about the ACME server.
Each string MUST be a lowercase hostname which the ACME server recognizes as
referring to itself for the purposes of CAA record validation as defined in
. This allows clients to determine the correct issuer domain name to
use when configuring CAA records.Clients access the directory by sending a GET request to the directory URL.An ACME account resource represents a set of metadata associated with an account.
Account resources have the following structure:
The status of this account. Possible values are: “valid”, “deactivated”, and
“revoked”. The value “deactivated” should be used to indicate client-initiated
deactivation whereas “revoked” should be used to indicate server-initiated
deactivation.
An array of URLs that the server can use to contact the client for issues
related to this account. For example, the server may wish to notify the
client about server-initiated revocation or certificate expiration.
Including this field in a new-account request, with a value of true, indicates
the client’s agreement with the terms of service. This field is not updateable
by the client.
A URL from which a list of orders submitted by this account can be fetched via
a GET request, as described in .Each account object includes an “orders” URL from which a list of orders created
by the account can be fetched via GET request. The result of the GET request
MUST be a JSON object whose “orders” field is an array of URLs, each identifying
an order belonging to the account. The server SHOULD include pending orders,
and SHOULD NOT include orders that are invalid in the array of URLs. The server
MAY return an incomplete list, along with a Link header with a “next” link
relation indicating where further entries can be acquired.An ACME order object represents a client’s request for a certificate and is
used to track the progress of that order through to issuance. Thus, the object
contains information about the requested certificate, the authorizations that
the server requires the client to complete, and any certificates that have
resulted from this order.
The status of this order. Possible values are: “pending”, “processing”,
“valid”, and “invalid”.
The timestamp after which the server will consider this order invalid, encoded
in the format specified in RFC 3339 . This field is REQUIRED for
objects with “pending” or “valid” in the status field.
A CSR encoding the parameters for the certificate being requested .
The CSR is sent in the base64url-encoded version of the DER format. (Note:
Because this field uses base64url, and does not include headers, it is different
from PEM.)
The requested value of the notBefore field in the certificate, in the date
format defined in .
The requested value of the notAfter field in the certificate, in the date
format defined in .
The error that occurred while processing the order, if any.
This field is structured as a problem document .
For pending orders, the authorizations that the client needs to complete
before the requested certificate can be issued (see
). For final orders, the authorizations that
were completed. Each entry is a URL from which an authorization can be fetched
with a GET request.
A URL for the certificate that has been issued in response to this order.The elements of the “authorizations” array are immutable once set. The server
MUST NOT change the contents of the “authorizations” array after it is created.
If a client observes a change in the contents of the “authorizations” array,
then it SHOULD consider the order invalid.The “authorizations” array in the challenge SHOULD reflect all authorizations
that the CA takes into account in deciding to issue, even if some authorizations
were fulfilled in earlier orders or in pre-authorization transactions. For
example, if a CA allows multiple orders to be fulfilled based on a single
authorization transaction, then it SHOULD reflect that authorization in all of
the orders.An ACME authorization object represents a server’s authorization for an account
to represent an identifier. In addition to the identifier, an authorization
includes several metadata fields, such as the status of the authorization (e.g.,
“pending”, “valid”, or “revoked”) and which challenges were used to validate
possession of the identifier.The structure of an ACME authorization resource is as follows:
The identifier that the account is authorized to represent
The type of identifier.
The identifier itself.
The status of this authorization. Possible values are: “pending”, “processing”,
“valid”, “invalid” and “revoked”.
The timestamp after which the server will consider this authorization invalid,
encoded in the format specified in RFC 3339 . This field is REQUIRED
for objects with “valid” in the “status” field.
If this field is present, then it MUST contain a URL for an order resource,
such that this authorization is only valid for that resource. If this field is
absent, then the CA MUST consider this authorization valid for all orders until
the authorization expires.
For pending authorizations, the challenges that the client can fulfill in
order to prove possession of the identifier. For final authorizations, the
challenges that were used. Each array entry is an object with parameters
required to validate the challenge. A client should attempt to fulfill
one of these challenges, and a server should consider any one of the challenges
sufficient to make the authorization valid. For final authorizations, it contains
the challenges that were successfully completed.The only type of identifier defined by this specification is a fully-qualified
domain name (type: “dns”). If a domain name contains non-ASCII Unicode characters
it MUST be encoded using the rules defined in . Servers MUST verify
any identifier values that begin with the ASCII Compatible Encoding prefix
“xn–” as defined in are properly encoded. Wildcard domain names
(with “*” as the first label) MUST NOT be included in authorization objects. describes a set of challenges for domain
name validation.Before sending a POST request to the server, an ACME client needs to have a
fresh anti-replay nonce to put in the “nonce” header of the JWS. In most cases,
the client will have gotten a nonce from a previous request. However, the
client might sometimes need to get a new nonce, e.g., on its first request to
the server or if an existing nonce is no longer valid.To get a fresh nonce, the client sends a HEAD request to the new-nonce resource
on the server. The server’s response MUST include a Replay-Nonce header field
containing a fresh nonce, and SHOULD have status code 204 (No Content). The
server SHOULD also respond to GET requests for this resource, returning an empty
body (while still providing a Replay-Nonce header) with a 204 (No Content) status.Proxy caching of responses from the new-nonce resource can cause
clients receive the same nonce repeatedly, leading to badNonce errors.
The server MUST include a Cache-Control header field with the “no-store”
directive in responses for the new-nonce resource, in order to prevent
caching of this resource.A client creates a new account with the server by sending a POST request to the
server’s new-account URL. The body of the request is a stub account object
containing the “contact” field and optionally the “terms-of-service-agreed”
field.
Same meaning as the corresponding server field defined in
Same meaning as the corresponding server field defined in
If this field is present with the value “true”, then the server MUST NOT
create a new account if one does not already exist. This allows a client to
look up an account URL based on an account key (see
).The server MUST ignore any values provided in the “orders”
fields in account bodies sent by the client, as well as any other fields
that it does not recognize. If new fields are specified in the future, the
specification of those fields MUST describe whether they can be provided by the
client.In general, the server MUST ignore any fields in the request object that it does
not recognize. In particular, it MUST NOT reflect unrecognized fields in the
resulting account object. This allows clients to detect when servers do not
support an extension field.The server SHOULD validate that the contact URLs in the “contact” field are
valid and supported by the server. If the server validates contact URLs it MUST
support the “mailto” scheme. Clients MUST NOT provide a “mailto” URL in the
“contact” field that contains hfields, or more than one
addr-spec in the to component. If a server encounters a “mailto” contact
URL that does not meet these criteria, then it SHOULD reject it as invalid.If the server rejects a contact URL for using an
unsupported scheme it MUST return an error of type “unsupportedContact”, with
a description describing the error and what types of contact URLs the server
considers acceptable. If the server rejects a contact URL for using a supported
scheme but an invalid value then the server MUST return an error of type
“invalidContact”.If the server wishes to present the client with terms under which the ACME
service is to be used, it MUST indicate the URL where such terms can be accessed
in the “terms-of-service” subfield of the “meta” field in the directory object,
and the server MUST reject new-account requests that do not have the
“terms-of-service-agreed” set to “true”. Clients SHOULD NOT automatically agree
to terms by default. Rather, they SHOULD require some user interaction for
agreement to terms.The server creates an account and stores the public key used to verify the
JWS (i.e., the “jwk” element of the JWS header) to authenticate future requests
from the account. The server returns this account object in a 201 (Created)
response, with the account URL in a Location header field.If the server already has an account registered with the provided account key,
then it MUST return a response with a 200 (OK) status code and provide the URL of
that account in the Location header field. This allows a client that has
an account key but not the corresponding account URL to recover the account URL.If a client wishes to find the URL for an existing account and does not want an
account to be created if one does not already exist, then it SHOULD do so by
sending a POST request to the new-account URL with a JWS whose payload has an
“only-return-existing” field set to “true” ({“only-return-existing”: true}).
If a client sends such a request and an account does not exist, then the server
MUST return an error response with status code 400 (Bad Request) and type
“urn:ietf:params:acme:error:accountDoesNotExist”.If the client wishes to update this information in the future, it sends a POST
request with updated information to the account URL. The server MUST ignore any
updates to “order” fields or any other fields it does not recognize. If the server
accepts the update, it MUST return a response with a 200 (OK) status code and the
resulting account object.For example, to update the contact information in the above account, the client
could send the following request:Servers SHOULD NOT respond to GET requests for account resources as these
requests are not authenticated. If a client wishes to query the server for
information about its account (e.g., to examine the “contact” or “certificates”
fields), then it SHOULD do so by sending a POST request with an empty update.
That is, it should send a JWS whose payload is an empty object ({}).As described above, a client can indicate its agreement with the CA’s terms of
service by setting the “terms-of-service-agreed” field in its account object to
“true”.If the server has changed its terms of service since a client initially agreed,
and the server is unwilling to process a request without explicit agreement to
the new terms, then it MUST return an error response with status code 403
(Forbidden) and type “urn:ietf:params:acme:error:userActionRequired”. This
response MUST include a Link header with link relation “terms-of-service” and
the latest terms-of-service URL.The problem document returned with the error MUST also include an “instance”
field, indicating a URL that the client should direct a human user to visit in
order for instructions on how to agree to the terms.The server MAY require a value to be present for the “external-account-binding”
field. This can be used to an ACME account with an existing account in a
non-ACME system, such as a CA customer database.To enable ACME account binding, a CA needs to provision the ACME client with a
MAC key and a key identifier. The key identifier MUST be an ASCII string. The
MAC key SHOULD be provided in base64url-encoded form, to maximize compatibility
between provisioning systems and ACME clients.The ACME client then computes a binding JWS to indicate the external account’s
approval of the ACME account key. The payload of this JWS is the account key
being registered, in JWK form. The protected header of the JWS MUST meet the
following criteria:The “alg” field MUST indicate a MAC-based algorithmThe “kid” field MUST contain the key identifier provided by the CAThe “nonce” field MUST NOT be presentThe “url” field MUST be set to the same value as the outer JWSThe “signature” field of the JWS will contain the MAC value computed with the
MAC key provided by the CA.When a CA receives a new-account request containing an
“external-account-binding” field, it decides whether or not to verify the
binding. If the CA does not verify the binding, then it MUST NOT reflect the
“external-account-binding” field in the resulting account object (if any). To
verify the account binding, the CA MUST take the following steps:Verify that the value of the field is a well-formed JWSVerify that the JWS protected meets the above criteriaRetrieve the MAC key corresponding to the key identifier in the “kid” fieldVerify that the MAC on the JWS verifies using that MAC keyVerify that the payload of the JWS represents the same key as was used to
verify the outer JWS (i.e., the “jwk” field of the outer JWS)If all of these checks pass and the CA creates a new account, then the CA may
consider the new account associated with the external account corresponding to
the MAC key and MUST reflect value of the “external-account-binding” field in
the resulting account object. If any of these checks fail, then the CA MUST
reject the new-account request.A client may wish to change the public key that is associated with an account in
order to recover from a key compromise or proactively mitigate the impact of an
unnoticed key compromise.To change the key associated with an account, the client first constructs a
key-change object describing the change that it would like the server to make:
The URL for account being modified. The content of this field MUST be the
exact string provided in the Location header field in response to the
new-account request that created the account.
The JWK representation of the new keyThe client then encapsulates the key-change object in a JWS, signed with the
requested new account key (i.e., the key matching the “newKey” value).The outer JWS MUST meet the normal requirements for an ACME JWS (see
). The inner JWS MUST meet the normal requirements,
with the following exceptions:The inner JWS MUST have the same “url” header parameter as the outer JWS.The inner JWS is NOT REQUIRED to have a “nonce” header parameter. The server
MUST ignore any value provided for the “nonce” header parameter.This transaction has signatures from both the old and new keys so that the
server can verify that the holders of the two keys both agree to the change.
The signatures are nested to preserve the property that all signatures on POST
messages are signed by exactly one key.On receiving key-change request, the server MUST perform the following steps in
addition to the typical JWS validation:Validate the POST request belongs to a currently active account, as described
in .Check that the payload of the JWS is a well-formed JWS object (the “inner
JWS”).Check that the JWS protected header of the inner JWS has a “jwk” field.Check that the inner JWS verifies using the key in its “jwk” field.Check that the payload of the inner JWS is a well-formed key-change object
(as described above).Check that the “url” parameters of the inner and outer JWSs are the same.Check that the “account” field of the key-change object contains the URL for
the account matching the old keyCheck that the “newKey” field of the key-change object also verifies the
inner JWS.Check that no account exists whose account key is the same as the key in the
“newKey” field.If all of these checks pass, then the server updates the corresponding account
by replacing the old account key with the new public key and returns status
code 200 (OK). Otherwise, the server responds with an error status code and a
problem document describing the error. If there is an existing account with
the new key provided, then the server SHOULD use status code 409 (Conflict) and
provide the URL of that account in the Location header field.Note that changing the account key for an account SHOULD NOT have any other
impact on the account. For example, the server MUST NOT invalidate pending
orders or authorization transactions based on a change of account key.A client can 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 or decommissioned.The server MUST verify that the request is signed by the account key. If the
server accepts the deactivation request, it replies with a 200 (OK) status code
and the current contents of the account object.Once an account is deactivated, the server MUST NOT accept further requests
authorized by that account’s key. The server SHOULD cancel any pending operations authorized
by the account’s key, such as certificate orders. A server may take a variety of actions in
response to an account deactivation, e.g., deleting data related to that account
or sending mail to the account’s contacts. Servers SHOULD NOT revoke
certificates issued by the deactivated account, since this could cause
operational disruption for servers using these certificates. ACME does not
provide a way to reactivate a deactivated account.The client requests certificate issuance by sending a POST request to the server’s
new-order resource. The body of the POST is a JWS object whose JSON payload is
a subset of the order object defined in , containing the fields
that describe the certificate to be issued:
A CSR encoding the parameters for the certificate being requested .
The CSR is sent in the base64url-encoded version of the DER format. (Note:
Because this field uses base64url, and does not include headers, it is different
from PEM.)
The requested value of the notBefore field in the certificate, in the date
format defined in
The requested value of the notAfter field in the certificate, in the date
format defined in 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 requesting a subjectAltName extension.The server MUST return an error if it cannot fulfill the request as specified,
and MUST NOT issue a certificate with contents other than 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 type and description.If the server is willing to issue the requested certificate, it responds with a
201 (Created) response. The body of this response is an order object reflecting
the client’s request and any authorizations the client must complete before the
certificate will be issued.The order object returned by the server represents a promise that if the
client fulfills the server’s requirements before the “expires” time, then the
server will issue the requested certificate. In the order object, any
authorization referenced in the “authorizations” array whose status is “pending”
represents an authorization transaction that the client must complete before the
server will issue the certificate (see ). If the
client fails to complete the required actions before the “expires” time, then
the server SHOULD change the status of the order to “invalid” and MAY
delete the order resource.The server MUST begin the issuance process for the requested certificate and
update the order resource with a URL for the certificate once the client has
fulfilled the server’s requirements. If the client has already satisfied the
server’s requirements at the time of this request (e.g., by obtaining
authorization for all of the identifiers in the certificate in previous
transactions), then the server MUST proactively issue the requested certificate
and provide a URL for it in the “certificate” field of the order. The server
MUST, however, still list the completed authorizations in the “authorizations”
array.Once the client believes it has fulfilled the server’s requirements, it should
send a GET request to the order resource to obtain its current state. The
status of the order will indicate what action the client should take:“invalid”: The certificate will not be issued. Consider this order process
abandoned.“pending”: The server does not believe that the client has fulfilled the
requirements. Check the “authorizations” array for entries that are still
pending.“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.“valid”: The server has issued the certificate and provisioned its URL to the
“certificate” field of the order.The order process described above presumes that authorization objects are
created reactively, in response to a certificate order. Some servers
may also wish to enable clients to obtain authorization for an identifier
proactively, outside of the context of a specific issuance. For example, a
client hosting virtual servers for a collection of names might wish to obtain
authorization before any virtual servers are created and only create a certificate when
a virtual server starts up.In some cases, a CA running an ACME server might have a completely external,
non-ACME process for authorizing a client to issue for an identifier. In these
case, the CA should provision its ACME server with authorization objects
corresponding to these authorizations and reflect them as already valid in any
orders submitted by the client.If a CA wishes to allow pre-authorization within ACME, it can offer a “new
authorization” resource in its directory by adding the field “new-authz” with a
URL for the new authorization resource.To request authorization for an identifier, the client sends a POST request to
the new-authorization resource specifying the identifier for which authorization
is being requested and how the server should behave with respect to existing
authorizations for this identifier.
The identifier that the account is authorized to represent:
The type of identifier.
The identifier itself.Before processing the authorization request, the server SHOULD determine whether
it is willing to issue certificates for the identifier. For example, the server
should check that the identifier is of a supported type. Servers might also
check names against a blacklist of known high-value identifiers. If the server
is unwilling to issue for the identifier, it SHOULD return a 403 (Forbidden)
error, with a problem document describing the reason for the rejection.If the server is willing to proceed, it builds a pending authorization object
from the inputs submitted by the client.“identifier” the identifier submitted by the client“status” MUST be “pending” unless the server has out-of-band information
about the client’s authorization status“challenges” as selected by the server’s policy for this identifierThe server allocates a new URL for this authorization, and returns a 201
(Created) response, with the authorization URL in the Location header field, and
the JSON authorization object in the body. The client then follows the process
described in to complete the authorization process.To download the issued certificate, the client simply sends a GET request to the
certificate URL.The default format of the certificate is application/pem-certificate-chain (see IANA Considerations).The server MAY provide one or more link relation header fields with
relation “alternate”. Each such field SHOULD express an alternative certificate
chain starting with the same end-entity certificate. This can be used to express
paths to various trust anchors. Clients can fetch these alternates and use their
own heuristics to decide which is optimal.A certificate resource represents a single, immutable certificate. If the client
wishes to obtain a renewed certificate, the client initiates a new order process
to request one.Because certificate resources are immutable once issuance is complete, the
server MAY enable the caching of the resource by adding Expires and
Cache-Control headers specifying a point in time in the distant future. These
headers have no relation to the certificate’s period of validity.The ACME client MAY request other formats by including an Accept
header in its request. For example, the client could use the media type
application/pkix-cert to request the end-entity certificate
in DER format. Server support for alternate formats is OPTIONAL. For
formats that can only express a single certificate, the server SHOULD
provide one or more Link: rel="up" headers pointing to an issuer or
issuers so that ACME clients can build a certificate chain as defined
in TLS.The identifier authorization process establishes the authorization of an account
to manage certificates for a given identifier. This process assures the
server of two things:That the client controls the private key of the account key pair, andThat the client controls the identifier in question.This process may be repeated to associate multiple identifiers to a key pair
(e.g., to request certificates with multiple identifiers), or to associate
multiple accounts with an identifier (e.g., to allow multiple entities to manage
certificates). The server may declare that an authorization is only valid for a
specific order by setting the “scope” field of the authorization to the
URL for that order.Authorization resources are created by the server in response to certificate
orders or authorization requests submitted by an account key holder; their
URLs are provided to the client in the responses to these requests. The
authorization object is implicitly tied to the account key used to sign the
request.When a client receives an order from the server it downloads the authorization
resources by sending GET requests to the indicated URLs. If the client
initiates authorization using a request to the new authorization resource, it
will have already received the pending authorization object in the response
to that request.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.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 URL (not authorization URL) once it is ready for
the server to attempt validation.For example, if the client were to respond to the “http-01” challenge in the
above authorization, it would send the following request: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 any reason or does not provide the
server with appropriate information to validate the challenge, then the server
MUST return an HTTP error. On receiving such an error, the client SHOULD undo
any actions that have been taken to fulfill the challenge, e.g., removing files
that have been provisioned to a web server.The server is said to “finalize” the authorization when it has completed
one of the validations, by assigning the authorization a status of “valid”
or “invalid”, corresponding to whether it considers the account authorized
for the identifier. If the final state is “valid”, then the server MUST include
an “expires” field. When finalizing an authorization, the server MAY remove
challenges other than the one that was completed, and may modify the “expires”
field. The server SHOULD NOT remove challenges with status “invalid”.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 URL, 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 200 (OK) response and MAY include a
Retry-After header field to suggest a polling interval to the client.If a client wishes to relinquish its authorization to issue certificates for an
identifier, then it may request that the server deactivates each authorization
associated with it by sending POST requests with the static object
{“status”: “deactivated”} to each authorization URL.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 updated
contents of the authorization object.The server MUST NOT treat deactivated authorization objects as sufficient for
issuing certificates.To request that a certificate be revoked, the client sends a POST request to
the ACME server’s revoke-cert URL. The body of the POST is a JWS object whose
JSON payload contains the certificate to be revoked:
The certificate to be revoked, in the base64url-encoded version of the DER
format. (Note: Because this field uses base64url, and does not include headers,
it is different from PEM.)
One of the revocation reasonCodes defined in Section 5.3.1 of
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. If a request contains a disallowed reasonCode the server MUST
reject it with the error type “urn:ietf:params:acme:error:badRevocationReason”.
The problem document detail SHOULD indicate which reasonCodes are allowed.Revocation requests are different from other ACME requests 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 MUST consider
at least the following accounts authorized for a given certificate:the account that issued the certificate.an account that holds authorizations for all of the identifiers in the
certificate.The server MUST also consider a revocation request valid if it is signed with
the private key corresponding to the public key in the certificate.If the revocation succeeds, the server responds with status code 200 (OK). If
the revocation fails, the server returns an error.There are few types of identifiers in the world for which there is a standardized
mechanism to prove possession of a given identifier. In all practical cases,
CAs rely on a variety of means to test whether an entity applying for a
certificate with a given identifier actually controls that identifier.Challenges provide the server with assurance that an account holder is also
the entity that controls an identifier. For each type of challenge, it must be
the case that in order for an entity to successfully complete the challenge the
entity must both:Hold the private key of the account key pair used to respond to the challengeControl the identifier in question 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 URL.This section describes an initial set of challenge types. Each challenge must
describe:Content of challenge objectsContent of response objectsHow the server uses the challenge and response to verify control of an
identifierChallenge objects all contain the following basic fields:
The type of challenge encoded in the object.
The URL to which a response can be posted.
The status of this authorization. Possible values are: “pending”, “valid”,
and “invalid”.
The time at which this challenge was completed by the server, encoded in the
format specified in RFC 3339 . This field is REQUIRED if the
“status” field is “valid”.
Errors that occurred while the server was validating the challenge, if any,
structured as problem documents . The server MUST NOT modify the
array except by appending entries onto the end. The server can limit the size
of this object by limiting the number of times it will retry a challenge.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 the challenge failed.Different challenges allow the server to obtain proof of different aspects of
control over an identifier. In some challenges, like HTTP, TLS SNI, and DNS, the
client directly proves its ability to do certain things related to the
identifier. The choice of which challenges to offer to a client under which
circumstances is a matter of server policy.The identifier validation challenges described in this section all relate to
validation of domain names. If ACME is extended in the future to support other
types of identifiers, there will need to be new challenge types, and they will
need to specify which types of identifier they apply to.Several of the challenges in this document make 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:The “JWK_Thumbprint” step indicates the computation specified in ,
using the SHA-256 digest . As noted in JWA any prepended
zero octets in the JWK object MUST be stripped before doing the computation.As specified in the individual challenges below, the token for a challenge is a
string comprised entirely of characters in the URL-safe base64 alphabet.
The “||” operator indicates concatenation of strings.ACME challenges typically require the client to set up some network-accessible
resource that the server can query in order to validate that the client
controls an identifier. In practice it is not uncommon for the server’s
queries to fail while a resource is being set up, e.g., due to information
propagating across a cluster or firewall rules not being in place.Clients SHOULD NOT respond to challenges until they believe that the server’s
queries will succeed. If a server’s initial validation query fails, the server
SHOULD retry the query after some time. While the server is still trying, the
status of the challenge remains “pending”; it is only marked “invalid” once the
server has given up.The server MUST provide information about its retry state to the client via the
“errors” field in the challenge and the Retry-After HTTP header field in
response to requests to the challenge resource. The server MUST add an entry to
the “errors” field in the challenge after each failed validation query. The
server SHOULD set the Retry-After header field to a time after the server’s
next validation query, since the status of the challenge will not change until
that time.Clients can explicitly request a retry by re-sending their response to a
challenge in a new POST request (with a new nonce, etc.). This allows clients
to request a retry when the state has changed (e.g., after firewall rules have been
updated). Servers SHOULD retry a request immediately on receiving such a POST
request. In order to avoid denial-of-service attacks via client-initiated
retries, servers SHOULD rate-limit such requests.With HTTP validation, the client in an ACME transaction proves its control over
a domain name by proving that for that domain name it can provision resources
to be returned by an HTTP server. 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 the DNS A and AAAA records, at its
discretion. Because many web servers allocate a default HTTPS virtual host to a
particular low-privilege tenant user in a subtle and non-intuitive manner, the
challenge must be completed over HTTP, not HTTPS.
The string “http-01”
A random value that uniquely identifies the challenge. This value MUST have
at least 128 bits of entropy.
It MUST NOT contain any characters outside the base64url alphabet, including
padding characters (“=”).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.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.
The key authorization for this challenge. This value MUST match the token
from the challenge and the client’s account key.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.Construct a URL by populating the URL template
“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.Verify that the resulting URL is well-formed.Dereference the URL using an HTTP GET request. This request MUST be sent to
TCP port 80 on the HTTP server.Verify that the body of the response is well-formed key authorization. The
server SHOULD ignore whitespace characters at the end of the body.Verify that key authorization provided by the HTTP server matches the key
authorization provided by the client in its response to the challenge.The server SHOULD follow redirects when dereferencing the URL.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.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 the DNS A and AAAA resource records for the domain name to respond to
specific connection attempts utilizing the Server Name Indication extension
. The server verifies the client’s challenge by accessing the
TLS server and verifying a particular certificate is presented.
The string “tls-sni-02”
A random value that uniquely identifies the challenge. This value MUST have
at least 128 bits of entropy. It MUST NOT contain any characters outside the
base64url alphabet, including padding characters (“=”).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 challenge token and encode it in lowercase hexadecimal form.
The dNSName is “x.y.token.acme.invalid”, where x is the first half of the
hexadecimal representation and y is the second half.SAN B MUST be constructed as follows: compute the SHA-256 digest of
the 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.
The key authorization for this challenge. This value MUST match the token
from the challenge and the client’s account key.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:Compute SAN A and SAN B in the same way as the client.Open a TLS connection to the domain name being validated, presenting SAN A in
the SNI field. This connection MUST be sent to TCP port 443 on the TLS server. 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.Verify that the certificate contains a subjectAltName extension containing
dNSName entries of SAN A and SAN B and no other entries.
The comparison MUST be insensitive to case and ordering of names.If all of the above verifications succeed, then the validation is successful.
Otherwise, the validation fails.When the identifier being validated is a domain name, the client can prove
control of that domain by provisioning a TXT resource record containing a designated
value for a specific validation domain name.
The string “dns-01”
A random value that uniquely identifies the challenge. This value MUST have
at least 128 bits of entropy. It MUST NOT contain any characters outside the
base64url alphabet, including padding characters (“=”).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.org”, then the client would provision the following
DNS record:The response to the DNS challenge provides the computed key authorization to
acknowledge that the client is ready to fulfill this challenge.
The key authorization for this challenge. This value MUST match the token
from the challenge and the client’s account key.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:Compute the SHA-256 digest of the key authorizationQuery for TXT records for the validation domain nameVerify that the contents of one of the TXT records match the digest valueIf 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.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.
The string “oob-01”
The URL to be visited. The scheme of this URL MUST be “http” or “https”.
Note that this field is distinct from the “url” field of the challenge, which
identifies the challenge itself.A client responds to this challenge by presenting the indicated URL for a human
user to navigate to. If the user chooses to complete this challenge (by visiting
the website and completing its instructions), the client indicates this by
sending a simple acknowledgement response to the server.
The string “oob-01”On receiving a response, the server MUST verify that the value of the “type”
field is “oob-01”. Otherwise, the steps the server takes to validate
identifier possession are determined by the server’s local policy.The “Media Types” registry should be updated with the following additional
value:MIME media type name: applicationMIME subtype name: pem-certificate-chainRequired parameters: NoneOptional parameters: NoneEncoding considerations: NoneSecurity considerations: Carries a cryptographic certificate and its associated certificate chainInteroperability considerations: NonePublished specification: draft-ietf-acme-acme
[[ RFC EDITOR: Please replace draft-ietf-acme-acme above with the RFC number assigned to this ]]Applications which use this media type: Any MIME-compliant transportAdditional information:File should contain one or more certificates encoded as PEM according to
RFC 7468 . In order to provide easy interoperation with TLS, the first
certificate MUST be an end-entity certificate. Each following certificate
SHOULD directly certify one preceding it. Because certificate validation
requires that trust anchors be distributed independently, a certificate
that specifies a trust anchor MAY be omitted from the chain, provided
that supported peers are known to possess any omitted certificates.The “Well-Known URIs” registry should be updated with the following additional
value (using the template from ):URI suffix: acme-challengeChange controller: IETFSpecification document(s): This document, Section Related information: N/AThe “Message Headers” registry should be updated with the following additional
value:Header Field NameProtocolStatusReferenceReplay-NoncehttpstandardThe “JSON Web Signature and Encryption Header Parameters” registry should be
updated with the following additional value:Header Parameter Name: “url”Header Parameter Description: URLHeader Parameter Usage Location(s): JWE, JWSChange Controller: IESGSpecification Document(s): of
RFC XXXX[[ RFC EDITOR: Please replace XXXX above with the RFC number assigned to this
document ]]The “JSON Web Signature and Encryption Header Parameters” registry should be
updated with the following additional value:Header Parameter Name: “nonce”Header Parameter Description: NonceHeader Parameter Usage Location(s): JWE, JWSChange Controller: IESGSpecification Document(s): of
RFC XXXX[[ RFC EDITOR: Please replace XXXX above with the RFC number assigned to this
document ]]The “IETF URN Sub-namespace for Registered Protocol Parameter Identifiers”
registry should be updated with the following additional value, following the
template in :
acme
RFC XXXX
URL-TBD
No transformation needed.[[ RFC EDITOR: Please replace XXXX above with the RFC number assigned to this
document, and replace URL-TBD with the URL assigned by IANA for registries of
ACME parameters. ]]This document requests that IANA create the following new registries:ACME Account Object Fields ()ACME Order Object Fields ()ACME Error Types ()ACME Resource Types ()ACME Identifier Types ()ACME Validation Methods ()All of these registries are under a heading of “Automated Certificate Management
Environment (ACME) Protocol” and are administered under a Specification
Required policy .This registry lists field names that are defined for use in ACME account
objects. Fields marked as “configurable” may be included in a
new-account request.Template:Field name: The string to be used as a field name in the JSON objectField type: The type of value to be provided, e.g., string, boolean, array of
stringClient configurable: Boolean indicating whether the server should accept
values provided by the clientReference: Where this field is definedInitial contents: The fields and descriptions defined in .Field NameField TypeConfigurableReferencestatusstringfalseRFC XXXXcontactarray of stringtrueRFC XXXXexternal-account-bindingobjecttrueRFC XXXXterms-of-service-agreedbooleantrueRFC XXXXordersarray of stringfalseRFC XXXXThis registry lists field names that are defined for use in ACME order
objects. Fields marked as “configurable” may be included in a
new-order request.Template:Field name: The string to be used as a field name in the JSON objectField type: The type of value to be provided, e.g., string, boolean, array of
stringClient configurable: Boolean indicating whether the server should accept
values provided by the clientReference: Where this field is definedInitial contents: The fields and descriptions defined in .Field NameField TypeConfigurableReferencestatusstringfalseRFC XXXXexpiresstringfalseRFC XXXXcsrstringtrueRFC XXXXnotBeforestringtrueRFC XXXXnotAfterstringtrueRFC XXXXauthorizationsarray of stringfalseRFC XXXXcertificatestringfalseRFC XXXXThis registry lists values that are used within URN values that are provided in
the “type” field of problem documents in ACME.Template:Type: The label to be included in the URN for this error, following
“urn:ietf:params:acme:error:”Description: A human-readable description of the errorReference: Where the error is definedInitial contents: The types and descriptions in the table in above,
with the Reference field set to point to this specification.This registry lists the types of resources that ACME servers may list in their
directory objects.Template:Field name: The value to be used as a field name in the directory objectResource type: The type of resource labeled by the fieldReference: Where the resource type is definedInitial contents:Field NameResource TypeReferencenew-accountNew accountRFC XXXXnew-orderNew orderRFC XXXXrevoke-certRevoke certificateRFC XXXXkey-changeKey changeRFC XXXX[[ RFC EDITOR: Please replace XXXX above with the RFC number assigned to this
document ]]This registry lists the types of identifiers that can be present in ACME
authorization objects.Template:Label: The value to be put in the “type” field of the identifier objectReference: Where the identifier type is definedInitial contents:LabelReferencednsRFC XXXX[[ RFC EDITOR: Please replace XXXX above with the RFC number assigned to this
document ]]This registry lists identifiers for the ways that CAs can validate control of
identifiers. Each method’s entry must specify whether it corresponds to an
ACME challenge type. The “Identifier Type” field must be contained in the
Label column of the ACME Identifier Types registry.Template:Label: The identifier for this validation methodIdentifier Type: The type of identifier that this method applies toACME: “Y” if the validation method corresponds to an ACME challenge type;
“N” otherwise.Reference: Where the validation method is definedInitial ContentsLabelIdentifier TypeACMEReferencehttp-01dnsYRFC XXXXtls-sni-02dnsYRFC XXXXdns-01dnsYRFC XXXXoob-01dnsYRFC XXXXWhen evaluating a request for an assignment in this registry, the designated
expert should ensure that the method being registered has a clear,
interoperable definition and does not overlap with existing validation methods.
That is, it should not be possible for a client and server to follow take the
same set of actions to fulfill two different validation mechanisms.Validation methods do not have to be compatible with ACME in order to be
registered. For example, a CA might wish to register a validation method in
order to support its use with the ACME extensions to CAA
.[[ RFC EDITOR: Please replace XXXX above with the RFC number assigned to this
document ]]ACME is a protocol for managing certificates that attest to identifier/key
bindings. Thus the foremost security goal of ACME is to ensure the integrity of
this process, i.e., to ensure that the bindings attested by certificates are
correct and that only authorized entities can manage certificates. ACME
identifies clients by their account keys, so this overall goal breaks down into
two more precise goals:Only an entity that controls an identifier can get an authorization for that
identifierOnce authorized, an account key’s authorizations cannot be improperly
used by another accountIn 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.As a service on the Internet, ACME broadly exists within the Internet threat
model . In analyzing ACME, it is useful to think of an ACME server
interacting with other Internet hosts along two “channels”:An ACME channel, over which the ACME HTTPS requests are exchangedA validation channel, over which the ACME server performs additional requests
to validate a client’s control of an identifierIn practice, the risks to these channels are not entirely separate, but they are
different in most cases. Each channel, for example, uses a
different communications pattern: the ACME channel will comprise inbound HTTPS
connections to the ACME server and the validation channel outbound HTTP or DNS
requests.Broadly speaking, ACME aims to be secure against active and passive attackers on
any individual channel. Some vulnerabilities arise (noted below) when an
attacker can exploit both the ACME channel and one of the others.On the ACME channel, in addition to network layer attackers, we also need to
account for man-in-the-middle (MitM) attacks at the application layer, and for
abusive use of the protocol itself. Protection against application layer MitM
addresses potential attackers such as Content Distribution Networks (CDNs) and
middleboxes with a TLS MitM function. Preventing abusive use of ACME means
ensuring that an attacker with access to the validation channel can’t obtain
illegitimate authorization by acting as an ACME client (legitimately, in terms
of the protocol).ACME allows anyone to request challenges for an identifier by registering an
account key and sending a new-order request using 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.:Legitimate domain holder registers account key pair AMitM registers account key pair BLegitimate domain holder sends a new-order request signed using account key AMitM suppresses the legitimate request but sends the same request signed
using account key BACME server issues challenges and MitM forwards them to the legitimate domain
holderLegitimate domain holder provisions the validation responseACME server performs validation query and sees the response provisioned by the
legitimate domain holderBecause 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 reflects the account public key, is provided to the server in the
validation response over the validation channel and signed afterwards by the
corresponding private key in the challenge response over the ACME channel.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:HTTP: Provision files under .well-known on a web server for the domainTLS SNI: Configure a TLS server for the domainDNS: Provision DNS resource records for the domainThere are several ways that these assumptions can be violated, both by
misconfiguration and by attacks. For example, on a web server that allows
non-administrative users to write to .well-known, any user can claim to own the
web server’s hostname by responding to an HTTP challenge, and likewise for TLS
configuration and TLS SNI. Similarly, if a server that can be used for ACME
validation is compromised by a malicious actor, then that malicious actor can
use that access to obtain certificates via ACME.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 website
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
website operator provisions a catch-all redirect rule that redirects requests
for unknown resources to a different domain. Then the target of the redirect
could use that to get a certificate through HTTP validation since the
validation path will not be known to the primary server.The DNS is a common point of vulnerability for all of these challenges. An
entity that can provision false DNS records for a domain can attack the DNS
challenge directly and can provision false A/AAAA records to direct the ACME
server to send its TLS SNI or HTTP validation query to a remote server of the
attacker’s choosing. There are a few different mitigations that ACME servers
can apply:Always querying the DNS using a DNSSEC-validating resolver (enhancing
security for zones that are DNSSEC-enabled)Querying the DNS from multiple vantage points to address local attackersApplying mitigations against DNS off-path attackers, e.g., adding entropy to
requests or only using TCPGiven 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.It is RECOMMENDED that the server perform DNS queries and make HTTP and TLS
connections from various network perspectives, in order to make MitM attacks
harder.As a protocol run over HTTPS, standard considerations for TCP-based and
HTTP-based DoS mitigation also apply to ACME.At the application layer, ACME requires the server to perform a few potentially
expensive operations. Identifier validation transactions require the ACME
server to make outbound connections to potentially attacker-controlled servers,
and certificate issuance can require interactions with cryptographic hardware.In addition, an attacker can also cause the ACME server to send validation
requests to a domain of its choosing by submitting authorization requests for
the victim domain.All of these attacks can be mitigated by the application of appropriate rate
limits. Issues closer to the front end, like POST body validation, can be
addressed using HTTP request limiting. For validation and certificate requests,
there are other identifiers on which rate limits can be keyed. For example, the
server might limit the rate at which any individual account key can issue
certificates or the rate at which validation can be requested within a given
subtree of the DNS. And in order to prevent attackers from circumventing these
limits simply by minting new accounts, servers would need to limit the rate at
which accounts can be registered.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 URL.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.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:Has the client agreed to a subscriber agreement?Is the claimed identifier syntactically valid?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?Is the key in the CSR sufficiently strong?Is the CSR signed with an acceptable algorithm?Has issuance been authorized or forbidden by a Certificate Authority
Authorization (CAA) record? CAs that use ACME to automate issuance will need to ensure that their servers
perform all necessary checks before issuing.CAs using ACME to allow clients to agree to terms of service should keep in mind
that ACME clients can automate this agreement, possibly not involving a human
user. If a CA wishes to have stronger evidence of user consent, it may present
an out-of-band requirement or challenge to require human involvement.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: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 perform DNS queries over TCP, which provides better
resistance to some forgery attacks than DNS over UDP.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.).In many cases, TLS-based services are deployed on hosted platforms that use
the Server Name Indication (SNI) TLS extension to distinguish between
different hosted services or “virtual hosts”. When a client initiates a
TLS connection with an SNI value indicating a provisioned host, the hosting
platform routes the connection to that host.When a connection comes in with an unknown SNI value, one might expect the
hosting platform to terminate the TLS connection. However, some hosting
platforms will choose a virtual host to be the “default”, and route connections
with unknown SNI values to that host.In such cases, the owner of the default virtual host can complete a TLS-based
challenge (e.g., “tls-sni-02”) for any domain with an A record that points to
the hosting platform. This could result in mis-issuance in cases where there
are multiple hosts with different owners resident on the hosting platform.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.
Typically, systems with default virtual hosts do not allow the holder of the
default virtual host to control what certificates are presented on a
request-by-request basis. Rather, the default virtual host can configure which
certificate is presented in TLS on a fairly static basis, so that the
certificate presented should be stable over small intervals.A CA can detect such a bounded default vhost 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”). If it receives the
same certificate on two different connections, then it is very likely that the
server is in a default virtual host configuration. Conversely, if the TLS
server returns an unrecognized_name alert, then this is an indication that the
server is not in a default virtual host configuration.The http-01, tls-sni-02 and dns-01 validation methods mandate the usage of
a random token value to uniquely identify the challenge. The value of the token
is required to contain at least 128 bits of entropy for the following security
properties. First, the ACME client should not be able to influence the ACME
server’s choice of token as this may allow an attacker to reuse a domain owner’s
previous challenge responses for a new validation request. Secondly, the entropy
requirement prevents ACME clients from implementing a “naive” validation server
that automatically replies to challenges without participating in the creation
of the initial authorization request.ACME provides certificate chains in the widely-used format known colloquially
as PEM (though it may diverge from the actual Privacy Enhanced Mail
specifications , as noted in ). Some current software
will allow the configuration of a private key and a certificate in one PEM
file, by concatenating the textual encodings of the two objects. In the context
of ACME, such software might be vulnerable to “key replacement” attacks. A
malicious ACME server could cause a client to use a private key of its choosing
by including the key in the PEM file returned in response to a query for a
certificate URL.When processing an file of type “application/pem-certificate-chain”, a client
SHOULD verify that the file contains only encoded certificates. If anything
other than a certificate is found (i.e., if the string “—–BEGIN” is ever
followed by anything other than “CERTIFICATE”), then the client MUST reject the
file as invalid.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.Peter Eckersley, EFFEric Rescorla, MozillaSeth Schoen, EFFAlex Halderman, University of MichiganMartin Thomson, MozillaJakub Warmuz, University of OxfordThis 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.NIST FIPS 180-4, Secure Hash StandardInternet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) ProfileThis memo profiles the X.509 v3 certificate and X.509 v2 certificate revocation list (CRL) for use in the Internet. An overview of this approach and model is provided as an introduction. The X.509 v3 certificate format is described in detail, with additional information regarding the format and semantics of Internet name forms. Standard certificate extensions are described and two Internet-specific extensions are defined. A set of required certificate extensions is specified. The X.509 v2 CRL format is described in detail along with standard and Internet-specific extensions. An algorithm for X.509 certification path validation is described. An ASN.1 module and examples are provided in the appendices. [STANDARDS-TRACK]PKCS #10: Certification Request Syntax Specification Version 1.7This memo represents a republication of PKCS #10 v1.7 from RSA Laboratories' Public-Key Cryptography Standards (PKCS) series, and change control is retained within the PKCS process. The body of this document, except for the security considerations section, is taken directly from the PKCS #9 v2.0 or the PKCS #10 v1.7 document. This memo provides information for the Internet community.The Transport Layer Security (TLS) Protocol Version 1.2This document specifies Version 1.2 of the Transport Layer Security (TLS) protocol. The TLS protocol provides communications security over the Internet. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. [STANDARDS-TRACK]HTTP Over TLSThis memo describes how to use Transport Layer Security (TLS) to secure Hypertext Transfer Protocol (HTTP) connections over the Internet. This memo provides information for the Internet community.Key words for use in RFCs to Indicate Requirement LevelsIn many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.UTF-8, a transformation format of ISO 10646ISO/IEC 10646-1 defines a large character set called the Universal Character Set (UCS) which encompasses most of the world's writing systems. The originally proposed encodings of the UCS, however, were not compatible with many current applications and protocols, and this has led to the development of UTF-8, the object of this memo. UTF-8 has the characteristic of preserving the full US-ASCII range, providing compatibility with file systems, parsers and other software that rely on US-ASCII values but are transparent to other values. This memo obsoletes and replaces RFC 2279.JSON Web Signature (JWS)JSON Web Signature (JWS) represents content secured with digital signatures or Message Authentication Codes (MACs) using JSON-based data structures. Cryptographic algorithms and identifiers for use with this specification are described in the separate JSON Web Algorithms (JWA) specification and an IANA registry defined by that specification. Related encryption capabilities are described in the separate JSON Web Encryption (JWE) specification.The JavaScript Object Notation (JSON) Data Interchange FormatJavaScript Object Notation (JSON) is a lightweight, text-based, language-independent data interchange format. It was derived from the ECMAScript Programming Language Standard. JSON defines a small set of formatting rules for the portable representation of structured data.This document removes inconsistencies with other specifications of JSON, repairs specification errors, and offers experience-based interoperability guidance.Hypertext Transfer Protocol (HTTP/1.1): Semantics and ContentThe Hypertext Transfer Protocol (HTTP) is a stateless \%application- level protocol for distributed, collaborative, hypertext information systems. This document defines the semantics of HTTP/1.1 messages, as expressed by request methods, request header fields, response status codes, and response header fields, along with the payload of messages (metadata and body content) and mechanisms for content negotiation.The Base16, Base32, and Base64 Data EncodingsThis document describes the commonly used base 64, base 32, and base 16 encoding schemes. It also discusses the use of line-feeds in encoded data, use of padding in encoded data, use of non-alphabet characters in encoded data, use of different encoding alphabets, and canonical encodings. [STANDARDS-TRACK]Uniform Resource Identifier (URI): Generic SyntaxA Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource. This specification defines the generic URI syntax and a process for resolving URI references that might be in relative form, along with guidelines and security considerations for the use of URIs on the Internet. The URI syntax defines a grammar that is a superset of all valid URIs, allowing an implementation to parse the common components of a URI reference without knowing the scheme-specific requirements of every possible identifier. This specification does not define a generative grammar for URIs; that task is performed by the individual specifications of each URI scheme. [STANDARDS-TRACK]Problem Details for HTTP APIsThis document defines a "problem detail" as a way to carry machine- readable details of errors in a HTTP response to avoid the need to define new error response formats for HTTP APIs.Web LinkingThis document specifies relation types for Web links, and defines a registry for them. It also defines the use of such links in HTTP headers with the Link header field. [STANDARDS-TRACK]DNS Certification Authority Authorization (CAA) Resource RecordThe Certification Authority Authorization (CAA) DNS Resource Record allows a DNS domain name holder to specify one or more Certification Authorities (CAs) authorized to issue certificates for that domain. CAA Resource Records allow a public Certification Authority to implement additional controls to reduce the risk of unintended certificate mis-issue. This document defines the syntax of the CAA record and rules for processing CAA records by certificate issuers. [STANDARDS-TRACK]Date and Time on the Internet: TimestampsPunycode: A Bootstring encoding of Unicode for Internationalized Domain Names in Applications (IDNA)Punycode is a simple and efficient transfer encoding syntax designed for use with Internationalized Domain Names in Applications (IDNA). It uniquely and reversibly transforms a Unicode string into an ASCII string. ASCII characters in the Unicode string are represented literally, and non-ASCII characters are represented by ASCII characters that are allowed in host name labels (letters, digits, and hyphens). This document defines a general algorithm called Bootstring that allows a string of basic code points to uniquely represent any string of code points drawn from a larger set. Punycode is an instance of Bootstring that uses particular parameter values specified by this document, appropriate for IDNA. [STANDARDS-TRACK]Internationalized Domain Names for Applications (IDNA): Definitions and Document FrameworkThis document is one of a collection that, together, describe the protocol and usage context for a revision of Internationalized Domain Names for Applications (IDNA), superseding the earlier version. It describes the document collection and provides definitions and other material that are common to the set. [STANDARDS-TRACK]The 'mailto' URI SchemeThis document defines the format of Uniform Resource Identifiers (URIs) to identify resources that are reached using Internet mail. It adds better internationalization and compatibility with Internationalized Resource Identifiers (IRIs; RFC 3987) to the previous syntax of 'mailto' URIs (RFC 2368). [STANDARDS-TRACK]PKCS #9: Selected Object Classes and Attribute Types Version 2.0This memo represents a republication of PKCS #9 v2.0 from RSA Laboratories' Public-Key Cryptography Standards (PKCS) series, and change control is retained within the PKCS process. The body of this document, except for the security considerations section, is taken directly from that specification. This memo provides information for the Internet community.Internet X.509 Public Key Infrastructure Operational Protocols: FTP and HTTPThe protocol conventions described in this document satisfy some of the operational requirements of the Internet Public Key Infrastructure (PKI). This document specifies the conventions for using the File Transfer Protocol (FTP) and the Hypertext Transfer Protocol (HTTP) to obtain certificates and certificate revocation lists (CRLs) from PKI repositories. [STANDARDS-TRACK]JSON Web Key (JWK) ThumbprintThis specification defines a method for computing a hash value over a JSON Web Key (JWK). It defines which fields in a JWK are used in the hash computation, the method of creating a canonical form for those fields, and how to convert the resulting Unicode string into a byte sequence to be hashed. The resulting hash value can be used for identifying or selecting the key represented by the JWK that is the subject of the thumbprint.JSON Web Algorithms (JWA)This specification registers cryptographic algorithms and identifiers to be used with the JSON Web Signature (JWS), JSON Web Encryption (JWE), and JSON Web Key (JWK) specifications. It defines several IANA registries for these identifiers.URI TemplateA URI Template is a compact sequence of characters for describing a range of Uniform Resource Identifiers through variable expansion. This specification defines the URI Template syntax and the process for expanding a URI Template into a URI reference, along with guidelines for the use of URI Templates on the Internet. [STANDARDS-TRACK]Transport Layer Security (TLS) Extensions: Extension DefinitionsThis document provides specifications for existing TLS extensions. It is a companion document for RFC 5246, "The Transport Layer Security (TLS) Protocol Version 1.2". The extensions specified are server_name, max_fragment_length, client_certificate_url, trusted_ca_keys, truncated_hmac, and status_request. [STANDARDS-TRACK]Textual Encodings of PKIX, PKCS, and CMS StructuresThis document describes and discusses the textual encodings of the Public-Key Infrastructure X.509 (PKIX), Public-Key Cryptography Standards (PKCS), and Cryptographic Message Syntax (CMS). The textual encodings are well-known, are implemented by several applications and libraries, and are widely deployed. This document articulates the de facto rules by which existing implementations operate and defines them so that future implementations can interoperate.Guidelines for Writing an IANA Considerations Section in RFCsMany protocols make use of points of extensibility that use constants to identify various protocol parameters. To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations are often coordinated by a central record keeper. For IETF protocols, that role is filled by the Internet Assigned Numbers Authority (IANA).To make assignments in a given registry prudently, guidance describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed. This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry.This is the third edition of this document; it obsoletes RFC 5226.ACME IP Identifier Validation ExtensionThis document specifies identifiers and challenges required to enable the Automated Certificate Management Environment (ACME) to issue certificates for IP addresses.ACME Identifiers and Challenges for Telephone NumbersThis document specifies identifiers and challenges required to enable the Automated Certificate Management Environment (ACME) to issue certificate for telephonoe numbers.Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) are widely used to protect data exchanged over application protocols such as HTTP, SMTP, IMAP, POP, SIP, and XMPP. Over the last few years, several serious attacks on TLS have emerged, including attacks on its most commonly used cipher suites and their modes of operation. This document provides recommendations for improving the security of deployed services that use TLS and DTLS. The recommendations are applicable to the majority of use cases.Cross-Origin Resource SharingAn IETF URN Sub-namespace for Registered Protocol ParametersThis document describes a new sub-delegation for the 'ietf' URN namespace for registered protocol items. The 'ietf' URN namespace is defined in RFC 2648 as a root for persistent URIs that refer to IETF- defined resources. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.Defining Well-Known Uniform Resource Identifiers (URIs)This memo defines a path prefix for "well-known locations", "/.well-known/", in selected Uniform Resource Identifier (URI) schemes. [STANDARDS-TRACK]CAA Record Extensions for Account URI and ACME Method BindingThe CAA DNS record allows a domain to communicate issuance policy to CAs, but only allows a domain to define policy with CA-level granularity. However, the CAA specification also provides facilities for extension to admit more granular, CA-specific policy. This specification defines two such parameters, one allowing specific accounts of a CA to be identified by URI and one allowing specific methods of domain control validation as defined by the ACME protocol to be required.Guidelines for Writing RFC Text on Security ConsiderationsAll RFCs are required to have a Security Considerations section. Historically, such sections have been relatively weak. This document provides guidelines to RFC authors on how to write a good Security Considerations section. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.Use of Bit 0x20 in DNS Labels to Improve Transaction IdentityThe small (16-bit) size of the DNS transaction ID has made it a frequent target for forgery, with the unhappy result of many cache pollution vulnerabilities demonstrated throughout Internet history. Even with perfectly and unpredictably random transaction ID's, random and birthday attacks are still theoretically feasible. This document describes a method by which an initiator can improve transaction identity using the 0x20 bit in DNS labels.Privacy Enhancement for Internet Electronic Mail: Part I: Message Encryption and Authentication ProceduresThis document defines message encryption and authentication procedures, in order to provide privacy-enhanced mail (PEM) services for electronic mail transfer in the Internet. [STANDARDS-TRACK]