Refactor: encapsulate code that will be shared with the sync version.

lib/secure-random-string.js

@@ -12,15 +12,19 @@ function srs(options, cb) {
 	crypto.randomBytes(length, function(ex, buf) {
 		if (ex) throw ex;
 
+		cb(_finish(buf));
+	});
+
+  function _finish (buf) {
 		var string = buf.toString('base64');
 		if (options.urlsafe) {
 			string = string.replace(/\//g,'_').replace(/\+/g,'-');
 		}
-
-		cb(string.substr(0, length));
-	});
+    return string.substr(0, length);
+  }
 
 };
 
 
+
 module.exports = srs;