Secure netTCP Binding

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WCF

I have publisher-subscriber kind of service architecture with netTCPbinding over the internet.

How can I make communication secure between service & subscribed client. currently I have security mode = "message" in development environment I am validating username & password using custom validation.

<netTcpBinding>
       <binding name="netTcpBindingConfiguration" receiveTimeout="00:10:00">
         <reliableSession enabled="true" ordered="true" inactivityTimeout="00:10:00" />
         <security mode="Message">
           <transport clientCredentialType="None" protectionLevel="None" />
           <message clientCredentialType="UserName"/>
         </security>
       </binding>
     </netTcpBinding>

Here is the sample username validator class

class UserNamePassValidator : System.IdentityModel.Selectors.UserNamePasswordValidator
   {
       public override void Validate(string userName, string password)
       {
           if(  userName==null ||  password==null)
           {
               throw new ArgumentNullException();
           }

           if (!(userName == "test" && password == "test") )
           {
               throw new FaultException("Incorrect Username or Password");
           }
       }
   }

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Do I need some kind of encryption to encrypt the message communication. Does netTcpBinding is right choice over the internet? Actually I need to get some data back from wcf client using callback contact so I only have limited choice of bindings.