diff --git a/acme4j-client/src/main/java/org/shredzone/acme4j/provider/AbstractAcmeClientProvider.java b/acme4j-client/src/main/java/org/shredzone/acme4j/provider/AbstractAcmeClientProvider.java
index 51485cc2..616c0490 100644
--- a/acme4j-client/src/main/java/org/shredzone/acme4j/provider/AbstractAcmeClientProvider.java
+++ b/acme4j-client/src/main/java/org/shredzone/acme4j/provider/AbstractAcmeClientProvider.java
@@ -16,19 +16,26 @@ package org.shredzone.acme4j.provider;
 import java.io.IOException;
 import java.net.HttpURLConnection;
 import java.net.URI;
+import java.util.Collection;
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.Map;
 
+import org.shredzone.acme4j.AcmeClient;
 import org.shredzone.acme4j.challenge.Challenge;
 import org.shredzone.acme4j.challenge.DnsChallenge;
 import org.shredzone.acme4j.challenge.GenericChallenge;
 import org.shredzone.acme4j.challenge.HttpChallenge;
 import org.shredzone.acme4j.challenge.ProofOfPossessionChallenge;
 import org.shredzone.acme4j.challenge.TlsSniChallenge;
+import org.shredzone.acme4j.impl.GenericAcmeClient;
 
 /**
  * Abstract implementation of {@link AcmeClientProvider}. It consists of a challenge
  * registry and a standard {@link #openConnection(URI)} implementation.
+ * <p>
+ * Implementing classes must implement at least {@link AcmeClientProvider#accepts(URI)}
+ * and {@link AbstractAcmeClientProvider#resolve(URI)}.
  *
  * @author Richard "Shred" Körber
  */
@@ -42,6 +49,26 @@ public abstract class AbstractAcmeClientProvider implements AcmeClientProvider {
         registerBaseChallenges();
     }
 
+    /**
+     * Resolves the server URI and returns the matching directory URI.
+     *
+     * @param serverUri
+     *            Server {@link URI}
+     * @return Resolved directory {@link URI}
+     * @throws IllegalArgumentException
+     *             if the server {@link URI} is not accepted
+     */
+    protected abstract URI resolve(URI serverUri);
+
+    @Override
+    public AcmeClient connect(URI serverUri) {
+        if (!accepts(serverUri)) {
+            throw new IllegalArgumentException("This provider does not accept " + serverUri);
+        }
+
+        return createAcmeClient(resolve(serverUri));
+    }
+
     @Override
     @SuppressWarnings("unchecked")
     public <T extends Challenge> T createChallenge(String type) {
@@ -79,9 +106,26 @@ public abstract class AbstractAcmeClientProvider implements AcmeClientProvider {
      *            constructor.
      */
     protected void registerChallenge(String type, Class<? extends Challenge> clazz) {
+        if (type == null) {
+            throw new NullPointerException("type must not be null");
+        }
+        if (clazz == null) {
+            throw new NullPointerException("Challenge class must not be null");
+        }
+        if (type.trim().isEmpty()) {
+            throw new IllegalArgumentException("type must not be empty");
+        }
+
         challenges.put(type, clazz);
     }
 
+    /**
+     * Returns all registered challenge types.
+     */
+    protected Collection<String> getRegisteredChallengeTypes() {
+        return Collections.unmodifiableCollection(challenges.keySet());
+    }
+
     /**
      * Registers all standard challenges as specified in the ACME specifications.
      * <p>
@@ -95,4 +139,15 @@ public abstract class AbstractAcmeClientProvider implements AcmeClientProvider {
         registerChallenge(HttpChallenge.TYPE, HttpChallenge.class);
     }
 
+    /**
+     * Creates an {@link AcmeClient} for the given directory URI.
+     *
+     * @param directoryUri
+     *            Directory {@link URI}
+     * @return {@link AcmeClient}
+     */
+    protected AcmeClient createAcmeClient(URI directoryUri) {
+        return new GenericAcmeClient(this, directoryUri);
+    }
+
 }
diff --git a/acme4j-client/src/main/java/org/shredzone/acme4j/provider/GenericAcmeClientProvider.java b/acme4j-client/src/main/java/org/shredzone/acme4j/provider/GenericAcmeClientProvider.java
index 5fea0872..0e97663b 100644
--- a/acme4j-client/src/main/java/org/shredzone/acme4j/provider/GenericAcmeClientProvider.java
+++ b/acme4j-client/src/main/java/org/shredzone/acme4j/provider/GenericAcmeClientProvider.java
@@ -15,9 +15,6 @@ package org.shredzone.acme4j.provider;
 
 import java.net.URI;
 
-import org.shredzone.acme4j.AcmeClient;
-import org.shredzone.acme4j.impl.GenericAcmeClient;
-
 /**
  * A generic {@link AcmeClientProvider}. It should be working for all ACME servers
  * complying to the ACME specifications.
@@ -35,12 +32,8 @@ public class GenericAcmeClientProvider extends AbstractAcmeClientProvider {
     }
 
     @Override
-    public AcmeClient connect(URI serverUri) {
-        if (!accepts(serverUri)) {
-            throw new IllegalArgumentException("This provider does not accept " + serverUri);
-        }
-
-        return new GenericAcmeClient(this, serverUri);
+    protected URI resolve(URI serverUri) {
+        return serverUri;
     }
 
 }
diff --git a/acme4j-letsencrypt/src/main/java/org/shredzone/acme4j/provider/LetsEncryptAcmeClientProvider.java b/acme4j-letsencrypt/src/main/java/org/shredzone/acme4j/provider/LetsEncryptAcmeClientProvider.java
index 1c0cb905..ac543600 100644
--- a/acme4j-letsencrypt/src/main/java/org/shredzone/acme4j/provider/LetsEncryptAcmeClientProvider.java
+++ b/acme4j-letsencrypt/src/main/java/org/shredzone/acme4j/provider/LetsEncryptAcmeClientProvider.java
@@ -28,9 +28,6 @@ import javax.net.ssl.SSLContext;
 import javax.net.ssl.SSLSocketFactory;
 import javax.net.ssl.TrustManagerFactory;
 
-import org.shredzone.acme4j.AcmeClient;
-import org.shredzone.acme4j.impl.GenericAcmeClient;
-
 /**
  * An {@link AcmeClientProvider} for <em>Let's Encrypt</em>.
  * <p>
@@ -57,44 +54,7 @@ public class LetsEncryptAcmeClientProvider extends AbstractAcmeClientProvider {
     }
 
     @Override
-    public AcmeClient connect(URI serverUri) {
-        return createAcmeClient(resolve(serverUri));
-    }
-
-    @Override
-    public HttpURLConnection openConnection(URI uri) throws IOException {
-        HttpURLConnection conn = super.openConnection(uri);
-        if (conn instanceof HttpsURLConnection) {
-            ((HttpsURLConnection) conn).setSSLSocketFactory(createSocketFactory());
-        }
-        return conn;
-    }
-
-    /**
-     * Creates an {@link AcmeClient} for the given directory URI.
-     *
-     * @param directoryUri
-     *            Directory {@link URI}
-     * @return {@link AcmeClient}
-     */
-    protected AcmeClient createAcmeClient(URI directoryUri) {
-        return new GenericAcmeClient(this, directoryUri);
-    }
-
-    /**
-     * Resolves the server URI and returns the matching directory URI.
-     *
-     * @param serverUri
-     *            Server {@link URI} to resolve
-     * @return Directory {@link URI}
-     * @throws IllegalArgumentException
-     *             if the server URI cannot be resolved
-     */
     protected URI resolve(URI serverUri) {
-        if (!accepts(serverUri)) {
-            throw new IllegalArgumentException("Unknown URI " + serverUri);
-        }
-
         String path = serverUri.getPath();
         String directoryUri;
         if (path == null || "".equals(path) || "/".equals(path) || "/v01".equals(path)) {
@@ -112,6 +72,15 @@ public class LetsEncryptAcmeClientProvider extends AbstractAcmeClientProvider {
         }
     }
 
+    @Override
+    public HttpURLConnection openConnection(URI uri) throws IOException {
+        HttpURLConnection conn = super.openConnection(uri);
+        if (conn instanceof HttpsURLConnection) {
+            ((HttpsURLConnection) conn).setSSLSocketFactory(createSocketFactory());
+        }
+        return conn;
+    }
+
     /**
      * Lazily creates an {@link SSLSocketFactory} that exclusively accepts the Let's
      * Encrypt certificate.