MachineKey Generator

Run the following tool to create a valid random machine key to place in your web.config or machine.config file. If you plan to deploy your application in a Web farm, you must ensure that the configuration files on each server share the same value for validationKey and decryptionKey, which are used for hashing and decryption. Since you cannot guarantee which web server will handle web requests, it is required to have the machine key shared across web servers in the farm.

It is also beneficial on a single box to keep the machine key consistent between IIS recycles and server reboots.

Lastly, each time you upgrade to a new version of ASP.NET, the key material used to automatically generate keys changes. Therefore, even if your application runs on a single Web server, if the keys need to remain stable over time, you should explicitly set the values for both validation and decryption.
For more information about MachineKey, click on the links below:

• How To: Configure MachineKey in ASP.NET 2.0

MachineKey

MachineKey in web.config