I've been working with a application which is a WSE 2.0 client - which involves signing the message.
When attempting to send the request - an exception of type System.Security.Cryptography.CryptographicException is thrown, where the message is Cryptography_CSP_NoPrivateKey.
This is permissions issue - where the identity of the AppPool (the application I had the issue with was a web service), does not have the required permissions to access the private key of the certificate.
Management of the permissions can be done through the MMC Certificates snap-in - right click on the certificate, select All Tasks, then Manage private keys.
