DOC: Hint on creating a random secret

[reneknuvers]

In the GitHub README a hint on how to create 72 bytes of random data could be added. The following command works well on macOS and linux:

head -c 72 /dev/urandom > JammerNetz-secret.bin

For easy distribution of a secret to users: how about a feature that calculates some sort of checksum on a file, for example an image. So if a jpg is addressed with the -k option, it reverts to that. Providing someone with a key is then as simple as sharing a picture with them. Small picture files generally don't get recompressed by WhatsApp or Facebook, and can be easily uploaded to and downloaded from forums, web pages and other sources. The entropy would suffice and people could use logo's and other human recognizable content as the 'key' to the server!

[christofmuc]

@reneknuvers Neat idea with the file hash! Not sure how random that is, but certainly it would be fairly easy to share. I also was thinking about allowing a copy and paste of the key directly into the software instead of relying on a file, so you could at least send the key via a simple chat or email message instead of trying to transport 72 bytes without having to create a zip file or similar.

Paper on randomness from images:
https://www.researchgate.net/publication/283021854_A_True_Random_Number_Generator_algorithm_from_digital_camera_image_noise_for_varying_lighting_conditions

https://borntoleave.github.io/resource/RzL_TRNG_final.pdf

Creating a secret key from an image:
https://www.ripublication.com/ijaer17/ijaerv12n16_110.pdf