mmm, you should add the signature on the client-side and verify it on the
server-side (Optional). The sample below shows how to create an assertion to
add a signature (The signature doens't contain any reference to other
element):

public class MyAssertion :
Microsoft.Web.Services3.Design.SecurityPolicyAssertion
{
 public class MyClientSendSecurityFilter :
Microsoft.Web.Services3.Security.SendSecurityFilter
 {
   public MyClientSendSecurityFilter()
    : base("")
   {
   }

   public override void SecureMessage(Microsoft.Web.Services3.SoapEnvelope
envelope, Microsoft.Web.Services3.Security.Security security)
   {
     CredentialSet cs =
envelope.Context.Credentials[this.GetActor(envelope.CurrentSoap)];
     UsernameToken token = cs.GetClientToken<UsernameToken>();
     MessageSignature signature = new MessageSignature(token);

     security.Elements.Add(signature);
   }
 }

 public override Microsoft.Web.Services3.SoapFilter
CreateClientInputFilter(Microsoft.Web.Services3.Design.FilterCreationContext
context)
 {
   return null;
 }

 public override Microsoft.Web.Services3.SoapFilter
CreateClientOutputFilter(Microsoft.Web.Services3.Design.FilterCreationContext
context)
 {
   return new MyClientSendSecurityFilter();
 }

 public override Microsoft.Web.Services3.SoapFilter
CreateServiceInputFilter(Microsoft.Web.Services3.Design.FilterCreationContext
context)
 {
   return null;
 }

 public override Microsoft.Web.Services3.SoapFilter
CreateServiceOutputFilter(Microsoft.Web.Services3.Design.FilterCreationContext
context)
 {
   return null;
 }
}

Regards,
Pablo Cibraro
www.lagashsystems.com