diff --git a/README.md b/README.md
index 15ff13a..b5426fd 100644
--- a/README.md
+++ b/README.md
@@ -14,6 +14,8 @@
 
 > Note: RFC8188 relies heavily on base64 URL encoding. 
 
+### Simple callback function for encryption key lookup
+
 ```php
 require_once "vendor/autoload.php";
 
@@ -35,8 +37,41 @@ $decoded = RFC8188::rfc8188_decode(
 $this->assertEquals($message, $decoded);
 ```
 
+### Invocable class for key lookup
+In this example we use a simple incovable class to provide key lookup. This may be more useful in complex framework integrations such as providing middleware that looks up keys from a database. This sample does not cover service injection to the key lookup class.
+
+```php
+use DevJack\EncryptedContentEncoding\RFC8188;
+use DevJack\EncryptedContentEncoding\Exception\EncryptionKeyNotFound;
+use Base64Url\Base64Url as b64;
+
+class MockKeyLookupProvider {
+
+    protected $keys = [];
+
+    public function addKey($key, $keyid='') {
+        $this->keys[$keyid] = $key;
+    }
+    public function __invoke($keyid) {
+        if (in_array($keyid, array_keys($this->keys))) {
+            return $this->keys[$keyid];
+        }
+        throw new EncryptionKeyNotFound("Encryption key not found.");
+    }
+}
 
 
+$encoded = b64::decode("uNCkWiNYzKTnBN9ji3-qWAAAABkCYTHOG8chz_gnvgOqdGYovxyjuqRyJFjEDyoF1Fvkj6hQPdPHI51OEUKEpgz3SsLWIqS_uA");
+
+$keyProvider = new MockKeyLookupProvider();
+$keyProvider->addKey(b64::decode("BO3ZVPxUlnLORbVGMpbT1Q"), 'a1');
+
+$decoded = RFC8188::rfc8188_decode(
+    $encoded, // data to decode
+    $keyProvider
+);
+```
+
 ## Installation
 
 ```
diff --git a/src/Exception/EncryptionKeyNotFound.php b/src/Exception/EncryptionKeyNotFound.php
new file mode 100644
index 0000000..33dee93
--- /dev/null
+++ b/src/Exception/EncryptionKeyNotFound.php
@@ -0,0 +1,7 @@
+<?php
+
+namespace DevJack\EncryptedContentEncoding\Exception;
+
+class EncryptionKeyNotFound extends \Exception {
+
+}
\ No newline at end of file
diff --git a/src/RFC8188.php b/src/RFC8188.php
index 14c40aa..ba1f703 100644
--- a/src/RFC8188.php
+++ b/src/RFC8188.php
@@ -25,7 +25,7 @@ protected static function hkdf($salt, $ikm, $sequence=0)
     public static function rfc8188_decode($payload, $key_lookup)
     {       
         if (!is_callable($key_lookup)) {
-            throw new Exception(sprintf(
+            throw new \Exception(sprintf(
                 '$key_lookup must be invocable'
             ));
         }
@@ -124,7 +124,7 @@ public static function rfc8188_encode($payload, $key, $keyid=null, $rs=25)
                 $encrypted = openssl_encrypt($plaintext_record, "aes-128-gcm", $hkdf['cek'], OPENSSL_RAW_DATA, $hkdf['nonce'], $tag);
                 $block = $encrypted.$tag;
                 if (false === $encrypted) {
-                    throw new Exception(sprintf(
+                    throw new \Exception(sprintf(
                         "OpenSSL error: %s", openssl_error_string()
                     ));
                 }
diff --git a/test/Mock/MockKeyLookupProvider.php b/test/Mock/MockKeyLookupProvider.php
new file mode 100644
index 0000000..f108bd6
--- /dev/null
+++ b/test/Mock/MockKeyLookupProvider.php
@@ -0,0 +1,19 @@
+<?php
+namespace DevJack\EncryptedContentEncoding\Test\Mock;
+
+use DevJack\EncryptedContentEncoding\Exception\EncryptionKeyNotFound;
+
+class MockKeyLookupProvider {
+
+    protected $keys = [];
+
+    public function addKey($key, $keyid='') {
+        $this->keys[$keyid] = $key;
+    }
+    public function __invoke($keyid) {
+        if (in_array($keyid, array_keys($this->keys))) {
+            return $this->keys[$keyid];
+        }
+        throw new EncryptionKeyNotFound("Encryption key not found.");
+    }
+}
\ No newline at end of file
diff --git a/test/RFC8188Test.php b/test/RFC8188Test.php
index b1de58d..d632f73 100644
--- a/test/RFC8188Test.php
+++ b/test/RFC8188Test.php
@@ -4,6 +4,8 @@
 use PHPUnit\Framework\TestCase;
 
 use DevJack\EncryptedContentEncoding\RFC8188;
+use DevJack\EncryptedContentEncoding\Test\Mock\MockKeyLookupProvider;
+use DevJack\EncryptedContentEncoding\Exception\EncryptionKeyNotFound;
 use Base64Url\Base64Url as b64;
 
 final class RFC8188Test extends TestCase
@@ -76,6 +78,32 @@ function($keyid ) use ($key) { return $key; }
         $this->assertEquals($message, $decoded);
     }
 
+    public function testCanInvokeCallableClassAsKeyProvider() {
+        $encoded = b64::decode("uNCkWiNYzKTnBN9ji3-qWAAAABkCYTHOG8chz_gnvgOqdGYovxyjuqRyJFjEDyoF1Fvkj6hQPdPHI51OEUKEpgz3SsLWIqS_uA");
+        $keyProvider = new MockKeyLookupProvider();
+        $keyProvider->addKey(b64::decode("BO3ZVPxUlnLORbVGMpbT1Q"), 'a1');
+        $decoded = RFC8188::rfc8188_decode(
+            $encoded, // data to decode 
+            $keyProvider
+        );
+
+        $this->assertEquals("I am the walrus", $decoded);
+    }
+
+    public function testMockLookupProviderThrowsKeyNotFoundException() {
+        $encoded = b64::decode("uNCkWiNYzKTnBN9ji3-qWAAAABkCYTHOG8chz_gnvgOqdGYovxyjuqRyJFjEDyoF1Fvkj6hQPdPHI51OEUKEpgz3SsLWIqS_uA");
+        
+        $keyProvider = new MockKeyLookupProvider();
+        $keyProvider->addKey(b64::decode("BO3ZVPxUlnLORbVGMpbT1Q"), ''); // intentionally set keyid NOT to 'a1'
+        
+        $this->expectException(EncryptionKeyNotFound::class);
+        
+        $decoded = RFC8188::rfc8188_decode(
+            $encoded, // data to decode 
+            $keyProvider
+        );
+    }
+
     /**
      * @requires PHP 5.6
      */