Substitution-permutation network visualizer I'm doing differential analysis of a toy block cipher in which I've to extract the differential characteristic of the first n-1 rounds. 

However, I can't do that very well as I can't see the network itself. Basically, I wonder if there is a software which generates a SPN network automatically given round permutations&amp;S boxes and presents it visually. This would make the analysis much simpler.
I know of no software that lets you visualize an arbitrary SPN network; however, many of us on the semiconductor side of things have their verification tools output visualizations based on the bit states.  We used these to compare the SPICE output to the software simulated.  My Simon Cipher and AES verification tools print out bit grids.  My AES tools are async, but the simontool program gives synchronous outputs and will give you an example of how to generate these types of bit grids if you dig into the code.  As a tractable example using the SIMON32/64 test vector:

simontool -e -b 32 -k 64 -s 1918111009080100 -t 65656877 -l log.e.32.64.txt -x simon-e-32-64


That dynamically creates a LaTeX file that contains a bit grid based on the input vector.