ABSTRACT
The keystream of a stream cipher should ideally be random, i.e., just by observing the keystream, one should not be able to differentiate it from any random bitstream.
The best method of generating a random bitstream is to toss an unbiased coin or to record thermal noise in an environment or to probe any such stochastic process in nature. These random number generators are called True Random Number Generators (TRNG).
