Which digest module to use to create the checksum portion of the session key. By default this is Digest::MD5, but you could use any other module with the same interface, such as Digest::SHA1.

Normally the form state is stored in the user's session session using a key like:

form_state_storage_name_84eb13cfed01764d9c401219faa56d53

If you want to name keys differently, you can provide a subroutine to generate the name. The subroutine is passed the name of the form state storage.