The master page should be automatically removed form authentication what you really need to worry about are the images and the external css that are referenced in the master page. you will need to add the following in your web.config file right before the </configuration>
<location path="images">
<system.web>
<authorization>
<allow users="*"/>
</authorization>
</system.web>
</location>
<location path="css">
<system.web>
<authorization>
<allow users="*"/>
</authorization>
</system.web>
</location>