add hmac tester to repo (#87)

Author: AaronWDS

src/ConnectWebhookHMACValidation.php

@@ -0,0 +1,32 @@
+<?php
+
+class DocuSign_HMAC
+{
+    /*
+     * Useful reference: https://www.php.net/manual/en/function.hash-hmac.php
+     * NOTE: Currently DocuSign only supports SHA256.
+     */
+    private static function ComputeHash($secret,$payload)
+    {
+        $hexHash = hash_hmac('sha256',$payload,utf8_encode($secret));
+        $base64Hash = base64_encode(hex2bin($hexHash));
+        return $base64Hash;
+    }
+    public static function HashIsValid($secret,$payload,$verify)
+    {
+        return hash_equals($verify, self::ComputeHash($secret,$payload));
+    }
+}
+
+
+$hmac = new DocuSign_HMAC();
+
+if ($hmac::HashIsValid("{DocuSign HMAC private key}",file_get_contents("payload.txt"), "{JSON response Signature}")){
+    echo "Signature matches the HMAC key provided";
+}
+
+else
+{
+
+    echo "Signature does not match the HMAC key provided";
+}