ACME Provider

Basically, it is possible to connect to any kind of ACME server just by connecting to the URL of its directory resource:

Session session = new Session("https://acme-v02.api.letsencrypt.org/directory");

ACME providers are “plug-ins” to acme4j that are specialized on a single CA. For example, the Let’s Encrypt provider offers URIs that are much easier to remember. The example above would look like this:

Session session = new Session("acme://letsencrypt.org");

Writing your own Provider

Every CA that provides an ACME server should also have an own AcmeProvider, and if it is just for the sake of a pretty acme: URI.

However, it is also possible to adapt the behavior of wide parts of acme4j to special characteristics of the CA, just by overriding methods and extending classes.

A client provider implements the AcmeProvider interface, but usually it is easier to extend AbstractAcmeProvider and implement only these two methods:

  • accepts(URI) checks if the client provider is accepting the provided URI. Usually it would be an URI like acme://example.com. Note that the http and https schemes are reserved for the generic provider and cannot be used by other providers.
  • resolve(URI) parses the URI and returns the corresponding URL of the directory service.

The AcmeProvider implementation needs to be registered with Java’s ServiceLoader. In the META-INF/services path of your project, create a file org.shredzone.acme4j.provider.AcmeProvider and write the fully qualified class name of your implementation into that file.

When acme4j tries to connect to an acme URI, it first invokes the accepts(URI) method of all registered AcmeProviders. Only one of the providers must return true for a successful connection. acme4j then invokes the resolve(URI) method of that provider, and connects to the directory URL that is returned.

The connection fails if no or more than one AcmeProvider implementation accepts the acme URI.

Certificate Pinning

The standard Java mechanisms are used to verify the HTTPS certificate provided by the ACME server. To pin the certificate, or use a self-signed certificate, override the createHttpConnector() method of AbstractAcmeProvider and return a subclassed HttpConnector class that modifies the HttpURLConnection as necessary.

Individual Challenges

If your ACME server provides challenges that are not specified in the ACME protocol, there should be an own Challenge implementation for each of your challenge, by extending the Challenge class.

In your AcmeProvider implementation, override the createChallenge(Login, JSON) method so it returns a new instance of your Challenge implementation when your individual challenge type is requested. All other types should be delegated to the super method.

Amended Directory Service

To override single entries of an ACME server’s directory, or to use a static directory, override the directory(Session, URI) method, and return a JSON of all available resources and their respective URL.

Adding your Provider to acme4j

After you completed your provider code, you can send in a pull request and apply for inclusion in the acme4j code base.

These preconditions must be met:

  • Your provider’s source code must be published under the terms of Apache License 2.0.
  • The source code of your ACME server must be publicly available under an OSI compliant license.
  • To avoid name conflicts, the acme: URI used must have the official domain name of your service as domain part.
  • You have the permission of all trademark holders involved, to use their trademarks in the source codes, package names, and the acme URI.

The acme4j development team reserves the right to reject your pull request, without giving any reason.

If you cannot meet these preconditions, you can always publish a JAR package of your acme4j provider yourself. Due to the plug-in nature of acme4j providers, it is sufficient to have that package in the Java classpath at runtime. There is no need to publish the source code.