Configuring passwordFormat="Encrypted" in ASP.NET

When setting passwordFortmat="Encrypted" (compared to "Hashed" by default) in web.config, I got an exception after trying to create a user.

Solution:
A tag is necessary in this case. I think it is used to encrypt password. MSDN says about machineKey:
Configures keys to use for encryption and decryption of forms authentication cookie data and view-state data, and for verification of out-of-process session state identification.
A typical tag looks like this:


The following is a generator:
http://www.aspnetresources.com/tools/keycreator.aspx