Hi Colin,

You will have to implement a custom assertion to do something like that.
In the sample below I created a custom assertion using the turn-key
assertion UsernameForCertificate. (You have to configure this class as an
extension in the policy file)

public class MyCustomAssertion : UsernameForCertificateAssertion
   {
       public override SoapFilter
CreateClientOutputFilter(FilterCreationContext context)
       {
           return new MyClientOutputFilter(this);
       }

       protected class MyClientOutputFilter : ClientOutputFilter
       {
           public MyClientOutputFilter(UsernameForCertificateAssertion
assertion)
               : base(assertion)
           {
           }

           public override void SecureMessage(SoapEnvelope envelope,
Security security, MessageProtectionRequirements request)
           {
               base.SecureMessage(envelope, security, request);

               foreach (ISecurityElement element in security.Elements)
               {
                   if (element is MessageSignature)
                   {
                       MessageSignature signature =
(MessageSignature)element;

                       //XmlSignature.AlgorithmURI.InclusiveC14NTransform =
http://www.w3.org/TR/2001/REC-xml-c14n-20010315
                       signature.Signature.SignedInfo.CanonicalizationMethod
= XmlSignature.AlgorithmURI.InclusiveC14NTransform;
                   }
               }
           }
       }
   }

Regards,
Pablo Cibraro
http://weblogs.asp.net/cibrax