Consuming of entropy When I put:

sudo watch tail /proc/sys/kernel/random/entropy_avail &amp;&amp; dd if=/dev/random of=/dev/zero


It shows that available entropy is "consumed".

What I can't understand is: 

Why does generating random consume entropy? Why does generating random bits cause that entropy to become useless?
Why generating random consumes an entropy?


For the same reason that to generate true random bits using coin throws, one needs to throw a coin for each bit generated. Any attempt to use less throws is doomed (against a computationally unbounded adversary, or if the technique relies on few simple bit combinations).


  Why generating a random bit causes that entropy to become useless?


Revealing a generated random bit is what makes it useless towards generating further secure random bits.

There's a solution: generate a sufficient number of random bits (like 256 or more) and use these, exclusively, to seed a Cryptographically Secure Pseudo Random Number Generator which will supply an endless stream of bits that have all testable properties of truly random bits. That's part of the strategy of Unix's /dev/urandom