diff --git a/src/HttpSignatures/GuzzleHttpSignatures.php b/src/HttpSignatures/GuzzleHttpSignatures.php
index 8b774f7..0d060ff 100644
--- a/src/HttpSignatures/GuzzleHttpSignatures.php
+++ b/src/HttpSignatures/GuzzleHttpSignatures.php
@@ -25,13 +25,14 @@ class GuzzleHttpSignatures
      */
     public static function middlewareFromContext(Context $context)
     {
-        return function (callable $handler) use ($context)
-        {
+        return function (callable $handler) use ($context) {
             return function (
                 Request $request,
                 array $options
-            ) use ($handler, $context)
-            {
+            ) use ($handler, $context) {
+
+                $contentsRequest = (string) $request->getBody();
+                $request = $request->withAddedHeader("digest",  base64_encode(hash('sha512', $contentsRequest, true)));
                 $request = $context->signer()->sign($request);
                 return $handler($request, $options);
             };