diff --git a/src/EncryptionKeyProviderInterface.php b/src/EncryptionKeyProviderInterface.php
new file mode 100644
index 0000000..7ce3f39
--- /dev/null
+++ b/src/EncryptionKeyProviderInterface.php
@@ -0,0 +1,10 @@
+<?php
+
+namespace DevJack\EncryptedContentEncoding;
+
+interface EncryptionKeyProviderInterface {
+    /*
+     * EncryptionKeyProvider's must be invokable.
+     */
+    public function __invoke($keyid);
+}
\ No newline at end of file
diff --git a/test/Mock/MockKeyLookupProvider.php b/test/Mock/MockKeyLookupProvider.php
index f108bd6..0b3f7b3 100644
--- a/test/Mock/MockKeyLookupProvider.php
+++ b/test/Mock/MockKeyLookupProvider.php
@@ -2,8 +2,9 @@
 namespace DevJack\EncryptedContentEncoding\Test\Mock;
 
 use DevJack\EncryptedContentEncoding\Exception\EncryptionKeyNotFound;
+use DevJack\EncryptedContentEncoding\EncryptionKeyProviderInterface;
 
-class MockKeyLookupProvider {
+class MockKeyLookupProvider implements EncryptionKeyProviderInterface {
 
     protected $keys = [];