Implemented LFSR generator (with tests and CLI)

[AdrienCos]

This PR introduces a new LFSR-based generator. An LFSR (Linear Feedback Shift Register) will generate a non-repeating pseudo-random sequence of integers, that is bounded between 1 and a power of 2.

The changes add a new generator, that takes a size (pixel index) as an argument, and from here will return a sequence of 2^n-1 (where n is the largest power of 2 below size) integers, seemingly randomly arranged. If a shift is introduced, it prevents an attacker from easily extracting the data, as its start will be hidden in the file.

To use the generator from the CLI:

stegano-lsb-set hide -g LFSR -s $SHIFT -i $INPUT_PATH -o $OUTPUT_PATH -f $SECRET_PATH

The stegano-lsb-set will compute the size of the image, and pass it to the generator. The generator will then compute the value of n and select the optimal parameters it needs to run.

As an example of the usefulness of the generator, I have hidden 520KB of random data in the same picture with two different generators: identity and LFSR. The base image was :

The result of running the steganalysis-parity tool on the image created with the identity generator is:

Notice the very visible vertical stripes caused by the embedding of the payload.

The result of running the same tool on the image created with the LFSR generator is:

While the results are not perfect, I believe that using the LFSR generator provides an extra layer of discretion.

For more information on LFSR: Link

[coveralls]

Coverage increased (+0.5%) to 84.389% when pulling 0976929 on AdrienCos:master into bf094c0 on cedricbonhomme:master.

[cedricbonhomme]

I didn't knew this. Really interesting generator.
Thanks for these contributions!