you can add your machine key in system.web element.
for e.g.
<system.web>
<machineKey validationKey="1234567890123456789012345678901234567890AAAAAAAAAA"
decryptionKey="123456789012345678901234567890123456789012345678"
validation="SHA1" decryption="Auto" />
</system.web>
If you don't want to put a machinekey in the web.config for each application, you need to put it in machine level web.config for each .Net Framework version.