我应该将公钥放在独立的应用程序中的什么位置

Question

I wrote an C# WPF application that signs a license xml file using the standard .Net SignedXml class. I am able to extract the public and private key as xml strings. I can safely tuck away my private key locally for the signing application, but what about the public key needed in the remote sign check application (library)? Options considered:

• KeyContainer: no good, because signing and checking happens in 2 separate environments
• Hardcoded: hardcode the public key as xml string in my checking library. I know the public key is not secret, but how can I prevent hackers from replacing the key with their own? I can sign the library, but then they could tamper with the application using the library....

Answer 1

Put the public key in a standard digital certificate that you distribute with your app. The integrity of the certificate will then be guaranteed by Windows, and you can tell if it's been changed.

Of course both the hardware and Windows itself is under the control of any potential attacker, so you can't really prevent a compromise of a specific machine.

Here's an example from a program that I used to distribute. When the program was activated from a licence perspective, it sent a hardware hash to a web service. This returned a self-signed certificate containing the hardware hash, which my program then checked whenever it was started. If the certificate had been changed in any way, the program would stop.

Answer 2

I put the public key in a directory outside of the inetpub directory, and if you move the private key off of the computer, then the worst someone could do is to replace the public key and it no longer can decrypt, but you could be informed when it can't decrypt to know something happened.

But, if someone was able to change your file you will have bigger problems that just having this one file changed.

UPDATE:

Oops, I missed that this is a WPF program. Unfortunately the best you can do is to have the private key separate from the public key, so, you can decrypt, but if the hacker changes the public key the application won't function properly.

That is one of the advantages of using the public/private key, to verify that only you can do the encryption.

The other option is to fetch the public key from a webserver, but then you have the same problem in that it is possible for someone to trick the application to go to the wrong server, so it isn't full-proof, and it will require that the user had an Internet connection, and for you to uniquely identify them.