http://blog.adilakhter.com/2009/08/06/wcf-security-wcf-performance-protectionlevel-part-1/

-The following table shows the various protection levels available with message security.======================================================= If you are using message security, you can configure message protection to sign but not encrypt each message. This allows you to verify the integrity of a message without the overhead of encryption in case there is no sensitive data requiring protection. With transport security, you cannot modify the level of protection because it is transport-dependent and the WCF framework does not control transport standards.