diff --git a/acme4j-client/src/main/java/module-info.java b/acme4j-client/src/main/java/module-info.java
index 3dfe4708..761b249f 100644
--- a/acme4j-client/src/main/java/module-info.java
+++ b/acme4j-client/src/main/java/module-info.java
@@ -25,6 +25,7 @@ module org.shredzone.acme4j {
     exports org.shredzone.acme4j.toolbox;
 
     uses org.shredzone.acme4j.provider.AcmeProvider;
+    uses org.shredzone.acme4j.provider.ChallengeProvider;
 
     provides org.shredzone.acme4j.provider.AcmeProvider
             with org.shredzone.acme4j.provider.GenericAcmeProvider,
diff --git a/acme4j-client/src/main/java/org/shredzone/acme4j/challenge/TokenChallenge.java b/acme4j-client/src/main/java/org/shredzone/acme4j/challenge/TokenChallenge.java
index 0dd74092..d94a64bb 100644
--- a/acme4j-client/src/main/java/org/shredzone/acme4j/challenge/TokenChallenge.java
+++ b/acme4j-client/src/main/java/org/shredzone/acme4j/challenge/TokenChallenge.java
@@ -56,14 +56,30 @@ public class TokenChallenge extends Challenge {
     }
 
     /**
-     * Returns the authorization string.
+     * Computes the key authorization for the given token.
      * <p>
      * The default is {@code token + '.' + base64url(jwkThumbprint)}. Subclasses may
      * override this method if a different algorithm is used.
+     *
+     * @param token
+     *         Token to be used
+     * @return Key Authorization string for that token
+     * @since 2.12
+     */
+    protected String keyAuthorizationFor(String token) {
+        PublicKey pk = getLogin().getKeyPair().getPublic();
+        return token + '.' + base64UrlEncode(JoseUtils.thumbprint(pk));
+    }
+
+    /**
+     * Returns the authorization string.
+     * <p>
+     * The default uses {@link #keyAuthorizationFor(String)} to compute the key
+     * authorization of {@link #getToken()}. Subclasses may override this method if a
+     * different algorithm is used.
      */
     public String getAuthorization() {
-        PublicKey pk = getLogin().getKeyPair().getPublic();
-        return getToken() + '.' + base64UrlEncode(JoseUtils.thumbprint(pk));
+        return keyAuthorizationFor(getToken());
     }
 
 }
diff --git a/acme4j-client/src/main/java/org/shredzone/acme4j/provider/AbstractAcmeProvider.java b/acme4j-client/src/main/java/org/shredzone/acme4j/provider/AbstractAcmeProvider.java
index 0ee2053c..3721b2dc 100644
--- a/acme4j-client/src/main/java/org/shredzone/acme4j/provider/AbstractAcmeProvider.java
+++ b/acme4j-client/src/main/java/org/shredzone/acme4j/provider/AbstractAcmeProvider.java
@@ -20,7 +20,7 @@ import java.util.Collections;
 import java.util.HashMap;
 import java.util.Map;
 import java.util.Objects;
-import java.util.function.BiFunction;
+import java.util.ServiceLoader;
 
 import org.shredzone.acme4j.Login;
 import org.shredzone.acme4j.Session;
@@ -44,7 +44,7 @@ import org.shredzone.acme4j.toolbox.JSON;
  */
 public abstract class AbstractAcmeProvider implements AcmeProvider {
 
-    private static final Map<String, BiFunction<Login, JSON, Challenge>> CHALLENGES = challengeMap();
+    private static final Map<String, ChallengeProvider> CHALLENGES = challengeMap();
 
     @Override
     public Connection connect(URI serverUri) {
@@ -81,13 +81,35 @@ public abstract class AbstractAcmeProvider implements AcmeProvider {
         }
     }
 
-    private static Map<String, BiFunction<Login, JSON, Challenge>> challengeMap() {
-        Map<String, BiFunction<Login, JSON, Challenge>> map = new HashMap<>();
+    private static Map<String, ChallengeProvider> challengeMap() {
+        Map<String, ChallengeProvider> map = new HashMap<>();
 
         map.put(Dns01Challenge.TYPE, Dns01Challenge::new);
         map.put(Http01Challenge.TYPE, Http01Challenge::new);
         map.put(TlsAlpn01Challenge.TYPE, TlsAlpn01Challenge::new);
 
+        for (ChallengeProvider provider : ServiceLoader.load(ChallengeProvider.class)) {
+            ChallengeType typeAnno = provider.getClass().getAnnotation(ChallengeType.class);
+            if (typeAnno == null) {
+                throw new IllegalStateException("ChallengeProvider "
+                        + provider.getClass().getName()
+                        + " has no @ChallengeType annotation");
+            }
+            String type = typeAnno.value();
+            if (type == null || type.trim().isEmpty()) {
+                throw new IllegalStateException("ChallengeProvider "
+                        + provider.getClass().getName()
+                        + ": type must not be null or empty");
+            }
+            if (map.containsKey(type)) {
+                throw new IllegalStateException("ChallengeProvider "
+                        + provider.getClass().getName()
+                        + ": there is already a provider for challenge type "
+                        + type);
+            }
+            map.put(type, provider);
+        }
+
         return Collections.unmodifiableMap(map);
     }
 
@@ -107,9 +129,9 @@ public abstract class AbstractAcmeProvider implements AcmeProvider {
 
         String type = data.get("type").asString();
 
-        BiFunction<Login, JSON, Challenge> constructor = CHALLENGES.get(type);
+        ChallengeProvider constructor = CHALLENGES.get(type);
         if (constructor != null) {
-            return constructor.apply(login, data);
+            return constructor.create(login, data);
         }
 
         if (data.contains("token")) {
diff --git a/acme4j-client/src/main/java/org/shredzone/acme4j/provider/ChallengeProvider.java b/acme4j-client/src/main/java/org/shredzone/acme4j/provider/ChallengeProvider.java
new file mode 100644
index 00000000..5769f349
--- /dev/null
+++ b/acme4j-client/src/main/java/org/shredzone/acme4j/provider/ChallengeProvider.java
@@ -0,0 +1,39 @@
+/*
+ * acme4j - Java ACME client
+ *
+ * Copyright (C) 2021 Richard "Shred" Körber
+ *   http://acme4j.shredzone.org
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ */
+package org.shredzone.acme4j.provider;
+
+import org.shredzone.acme4j.Login;
+import org.shredzone.acme4j.challenge.Challenge;
+import org.shredzone.acme4j.toolbox.JSON;
+
+/**
+ * A provider that creates a Challenge from a matching JSON.
+ *
+ * @since 2.12
+ */
+@FunctionalInterface
+public interface ChallengeProvider {
+
+    /**
+     * Creates a Challenge.
+     *
+     * @param login
+     *         {@link Login} of the user's account
+     * @param data
+     *         {@link JSON} of the challenge as sent by the CA
+     * @return Created and initialized {@link Challenge}. It must match the JSON type.
+     */
+    Challenge create(Login login, JSON data);
+
+}
diff --git a/acme4j-client/src/main/java/org/shredzone/acme4j/provider/ChallengeType.java b/acme4j-client/src/main/java/org/shredzone/acme4j/provider/ChallengeType.java
new file mode 100644
index 00000000..7f8b69c1
--- /dev/null
+++ b/acme4j-client/src/main/java/org/shredzone/acme4j/provider/ChallengeType.java
@@ -0,0 +1,37 @@
+/*
+ * acme4j - Java ACME client
+ *
+ * Copyright (C) 2021 Richard "Shred" Körber
+ *   http://acme4j.shredzone.org
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ */
+package org.shredzone.acme4j.provider;
+
+import java.lang.annotation.Documented;
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+/**
+ * Annotates the challenge type that is generated by the {@link ChallengeProvider}.
+ *
+ * @since 2.12
+ */
+@Documented
+@Retention(RetentionPolicy.RUNTIME)
+@Target(ElementType.TYPE)
+public @interface ChallengeType {
+
+    /**
+     * Challenge type.
+     */
+    String value();
+
+}