The Lattice of Machine Invariant Sets and Subword Complexity
  We investigate the lattice of machine invariant classes. This is an infinite completely distributive lattice but it is not a Boolean lattice. We show the subword complexity and the growth function create machine invariant classes. So the lattice would serve as a measure of words cryptographic quality if we like to identify new stream ciphers suitable for widespread adoption. 
Janis Buls