Securing WCF service using basicHttpBinding which supports streaming

My question is in regards to the best (aka "least painful") way to secure access to a WCF service that is only exposed to our company's internal users. The goal is to ensure that the service is only accessed via a single Windows forms application that each of our users has installed. When the service is called, I want the service to be able to validate that it was called from the permitted application.

The service to be secured uses basicHttpBinding, which supports streaming, so I believe I am limited to Transport level security.

Below are simplified versions of the <bindings> and <services> sections from my service's config file.

<bindings>
  <basicHttpBinding>
    <binding name="Service1Binding" transferMode="Streamed"/>    
  </basicHttpBinding>
</bindings>

<services>
    <service name="WCFServiceSecurity.Service1" 
        behaviorConfiguration="WCFServiceSecurity.Service1Behavior">
        <endpoint address=""
            binding="basicHttpBinding"
            contract="WCFServiceSecurity.IService1"
            bindingConfiguration="Service1Binding"/>
        <endpoint address="mex" binding="mexHttpBinding" contract="IMetadataExchange"/>
    </service>
</services>

Can anyone offer some details as to what actions I would need to take in order to implement security on this service?

Note: I'm new to WCF and am not familiar with security at all, so let me know if I haven't provided enough detail.

UPDATE:

As suggested by marc_s, I'd like to secure the WCF service using some sort of username/password mechanism. This gives a little more direction towards an answer, but I'm still somewhat blurry on how to actually do this.