The only novel thing is its use of symmetric encryption keys which are used to encrypt the data and are included in the backup repository, encrypted by your regular private gpg key. This allows giving additional gpg keys access to the backup after it has been made. http://liw.fi/obnam/encryption/

(Which is useful for eg, backing up a server. You can make a dedicated gpg key for that server, but give your personal gpg key access to the backups to restore later.)

Anyway, the generation of the symmetric encryption key is what needs an entropy source. AFAIK this is done once per repository.