I had the EXACT same problem. Look in the msdn help file index under "machineKey". It specifies what to do. In short,
in the web.config file, you have to specify some parameters for encryption to work. In the < system.web > node, add the following:
< machineKey validationKey="
[big long honkin key],IsolateApps"
decryptionKey
="[other big honkin key],IsolateApps"
validation="3DES" / >
These big long honkin keys can be generated. There is an msdn article that has a download regarding adding custom providers. You can download and install it from here: (
http://download.microsoft.com/downlo...AMProvider.msi)
You can use the key gen exe to generate the correct size key you need based on the type of encryption you choose. For the validation key, i used 64 bytes. For the decryption key, i used 24 bytes (because I chose TripleDES).
Good luck!;)