Is there any work done on developing difference-calculus of Turing Machines (or simpler Formal Languages)
As it turns out, there is some fascinating work done in this direction. 

In particular, in 2003, Michael Howard, Jon Pincus, and Jeannette M. Wing's Measuring Relative Attack Surfaces in proceedings of Workshop on Advanced Developments in Software and Systems Security, Taipei, December 2003.  

Further work by the same authors over the years is quite interesting ... For anyone who found my question of interest you can check their work out at http://www.cs.cmu.edu/~pratyus/as.html ... And if you find them interesting, i hope that you will find my work interesting too :)