This article is primarily an introduction on protecting message data since the topic overall seems to cause some confusion. The source of confusion is what it means for a service to define a contract for protecting data. Data protection flows from two different directions and at a variety of different scopes.
The service can define a minimum standard of protection at various contract scopes.
This minimum standard appears as the ProtectionLevel on a contract and comes in three flavors: no protection, protected by digital signing, and protected by encryption (the use of encryption implies a signature as well). Signing prevents people from tampering with the messages while encryption prevents people from reading the messages.
These contracts define the application messages that are exchanged. Depending on the protocols you make use of, any number of infrastructure headers or messages may need to be inserted into the data stream to facilitate the exchange of your application messages. This infrastructure data can have its own independent rules for protection.
The ProtectionLevel truly is a minimum standard. It is ok for your application to receive messages that are protected better than the minimum standard defines. You may even send messages that are protected better than the minimum standard due to the other direction for flowing data protection. Data can be protected at the wire level by using security integrated with the network transport. For example, if you send data using HTTPS, then there is a level of protection that HTTPS must apply to your messages to transport them even if your contract specified a lower standard.
The net protection level is the maximum of that provided by the service and provided by the transport. As long as for every piece of data at every scope that maximum is greater than your minimum standard, then everything works. If that maximum fails to meet your minimum standard, then the service will refuse to send or receive those messages.
Next time: Embedding Arbitrary XML in Faults