Securely Storing Optional Entropy While Using DPAPI

Question

So I am trying to store the symmetric key using DPAPI. All is well and great, but what to do with the entropy? This answered question here really doesn\'t provide enough ins

Answer 1

First, let me address the original post question. It boils down to the fact that the entropy must be stored under the authority of the user and/or the authority of the application if it is going to be used for persisted storage. I suppose you could use a key stored with the application to encrypt the information in the persisted store but again a malicious application would be able to access this encryption key. So, I do not feel there is a means to protect against the scenario you mention in comments. However, given what you have said is the intended use of the entropy, I do not feel it helps in solving your problem.

It sounds as if the actual problem is establishing a secure channel of communication between your client application and the server. In your design, you are exchanging keys that will be used to encrypt communication. I think that trying to use custom code to solve this issue will lead to additional security vulnerabilities.

Given all of that, I would suggest creating a WCF (Windows Communication Foundation) service that is used to retrieve sensitive information. It could obviously be used to retrieve all information, but the least amount of change would be to confine the service to sensitive information.

With WCF, you can configure both the client and the server to use a secure channel. WCF has plenty of options for establishing a secure channel of communication to the server.

Once you have a secure channel, many of the other problems are simpler such as access to the CC data. If that data is sent down a secure channel, it becomes an issue of authorization instead of channel security.

See How to: Create a Secure Session for more.