diff --git a/src/main/java/io/kamax/mxisd/crypto/SignatureManager.java b/src/main/java/io/kamax/mxisd/crypto/SignatureManager.java
index bcfdcfa..99d0470 100644
--- a/src/main/java/io/kamax/mxisd/crypto/SignatureManager.java
+++ b/src/main/java/io/kamax/mxisd/crypto/SignatureManager.java
@@ -20,12 +20,45 @@
 
 package io.kamax.mxisd.crypto;
 
+import com.google.gson.JsonElement;
 import com.google.gson.JsonObject;
+import io.kamax.matrix.event.EventKey;
+import io.kamax.matrix.json.MatrixJson;
 
 import java.nio.charset.StandardCharsets;
+import java.util.Objects;
 
 public interface SignatureManager {
 
+    /**
+     * Sign the message and add the signature to the <code>signatures</code> key.
+     * <p>
+     * If the key does not exist yet, it is created. If the key exist, the produced signature will be merged with any
+     * existing ones.
+     *
+     * @param domain  The domain under which the signature should be added
+     * @param message The message to sign and add the produced signature to
+     * @return The provided message with the new signature
+     * @throws IllegalArgumentException If the <code>signatures</code> value is not a JSON object
+     */
+    default JsonObject signMessageGson(String domain, JsonObject message) throws IllegalArgumentException {
+        JsonElement signEl = message.remove(EventKey.Signatures.get());
+        JsonObject oldSigns = new JsonObject();
+        if (!Objects.isNull(signEl)) {
+            if (!signEl.isJsonObject()) {
+                throw new IllegalArgumentException("Message contains a signatures key that is not a JSON object value");
+            }
+
+            oldSigns = signEl.getAsJsonObject();
+        }
+
+        JsonObject newSigns = signMessageGson(domain, MatrixJson.encodeCanonical(message));
+        oldSigns.entrySet().forEach(entry -> newSigns.add(entry.getKey(), entry.getValue()));
+        message.add(EventKey.Signatures.get(), newSigns);
+
+        return message;
+    }
+
     /**
      * Sign the message and produce a <code>signatures</code> object that can directly be added to the object being signed.
      *
diff --git a/src/main/java/io/kamax/mxisd/http/undertow/handler/identity/v1/SingleLookupHandler.java b/src/main/java/io/kamax/mxisd/http/undertow/handler/identity/v1/SingleLookupHandler.java
index d81d38c..87cf6f1 100644
--- a/src/main/java/io/kamax/mxisd/http/undertow/handler/identity/v1/SingleLookupHandler.java
+++ b/src/main/java/io/kamax/mxisd/http/undertow/handler/identity/v1/SingleLookupHandler.java
@@ -21,9 +21,7 @@
 package io.kamax.mxisd.http.undertow.handler.identity.v1;
 
 import com.google.gson.JsonObject;
-import io.kamax.matrix.event.EventKey;
 import io.kamax.matrix.json.GsonUtil;
-import io.kamax.matrix.json.MatrixJson;
 import io.kamax.mxisd.config.MxisdConfig;
 import io.kamax.mxisd.config.ServerConfig;
 import io.kamax.mxisd.crypto.SignatureManager;
@@ -73,11 +71,8 @@ public class SingleLookupHandler extends LookupHandler {
             respondJson(exchange, "{}");
         } else {
             SingleLookupReply lookup = lookupOpt.get();
-
-            // FIXME signing should be done in the business model, not in the controller
             JsonObject obj = GsonUtil.makeObj(new SingeLookupReplyJson(lookup));
-            obj.add(EventKey.Signatures.get(), signMgr.signMessageGson(cfg.getName(), MatrixJson.encodeCanonical(obj)));
-
+            signMgr.signMessageGson(cfg.getName(), obj);
             respondJson(exchange, obj);
         }
     }
diff --git a/src/test/java/io/kamax/mxisd/test/crypto/SignatureManagerTest.java b/src/test/java/io/kamax/mxisd/test/crypto/SignatureManagerTest.java
index 06d2fed..b48a723 100644
--- a/src/test/java/io/kamax/mxisd/test/crypto/SignatureManagerTest.java
+++ b/src/test/java/io/kamax/mxisd/test/crypto/SignatureManagerTest.java
@@ -21,6 +21,7 @@
 package io.kamax.mxisd.test.crypto;
 
 import com.google.gson.JsonObject;
+import io.kamax.matrix.event.EventKey;
 import io.kamax.matrix.json.GsonUtil;
 import io.kamax.matrix.json.MatrixJson;
 import io.kamax.mxisd.crypto.Signature;
@@ -36,10 +37,14 @@ import org.junit.Test;
 
 import static org.hamcrest.core.Is.is;
 import static org.hamcrest.core.IsEqual.equalTo;
+import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertThat;
 
 public class SignatureManagerTest {
 
+    private static final String lookupData = "{\n" + "  \"not_before\": 0,\n" + "  \"address\": \"mxisd-federation-test@kamax.io\",\n"
+            + "  \"medium\": \"email\",\n" + "  \"mxid\": \"@mxisd-lookup-test:kamax.io\",\n"
+            + "  \"not_after\": 253402300799000,\n" + "  \"ts\": 1523482030147\n" + "}";
     private static SignatureManager signMgr;
 
     private static SignatureManager build(String keySeed) {
@@ -98,12 +103,19 @@ public class SignatureManagerTest {
 
     @Test
     public void onIdentityLookup() {
-        String value = MatrixJson.encodeCanonical("{\n" + "  \"address\": \"mxisd-federation-test@kamax.io\",\n"
-                + "  \"medium\": \"email\",\n" + "  \"mxid\": \"@mxisd-lookup-test:kamax.io\",\n"
-                + "  \"not_after\": 253402300799000,\n" + "  \"not_before\": 0,\n" + "  \"ts\": 1523482030147\n" + "}");
-
+        String value = MatrixJson.encodeCanonical(lookupData);
         String sign = "ObKA4PNQh2g6c7Yo2QcTcuDgIwhknG7ZfqmNYzbhrbLBOqZomU22xX9raufN2Y3ke1FXsDqsGs7WBDodmzZJCg";
         testSign(value, sign);
     }
 
+    @Test
+    public void onIdentityLookupFull() {
+        JsonObject data = GsonUtil.parseObj(lookupData);
+        signMgr.signMessageGson("localhost", data);
+        JsonObject signatures = EventKey.Signatures.getObj(data);
+        JsonObject domainSign = GsonUtil.getObj(signatures, "localhost");
+        String sign = GsonUtil.getStringOrThrow(domainSign, "ed25519:0");
+        assertEquals(sign, "ObKA4PNQh2g6c7Yo2QcTcuDgIwhknG7ZfqmNYzbhrbLBOqZomU22xX9raufN2Y3ke1FXsDqsGs7WBDodmzZJCg");
+    }
+
 }