I want to create new WCF service and client. The 2 parties will communicate using 2-way SSL.
I want to use the SSL only for the authentication phase. After this authentication, the encryption is not necessary. Can I configure my service (and client) to use SSL only for authentication and leave the connection unencrypted (performance issue)?
Why would you want to leave the connection unencrypted after authentication?
The username and password will be passed via the network one way or another so you should consider encrypting the connection all the time to avoid leaking that information.
Edit:
If you are using a certificate it has to be encrypted all the time, since I know you can’t do authentication with a certificate and then get back to simple HTTP.
WCF need to authenticate the client for every request (if it wasn’t like that client could get a certificate for a moment then delete it and use your WCF service like nothing happened because authentication will not be needed anymore which is abnormal in your situation).