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When I'm doing watch cat /proc/sys/kernel/random/entropy_avail, the amount of entropy steadily decreases, until I get about 150-200. Initially I get over 2000. When I stop reading that value and wait a minute or two, and then read it again, I am at about 2000 again. Why is reading that value reducing the entropy?

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It was answered on Stackoverflow. Main point is that creating a process does consume entropy. cat is a process, thus running watch cat consumes entropy. –  Nikolay Jan 6 at 1:36
    
If you compose an answer out of that, I will accept it. –  Sibbo Jan 6 at 2:09
    
OK. I did. Only because you asked. I feel like I just only retyped data from one place to another... Meanwhile I did learn this as well, because I did not know it before. –  Nikolay Jan 6 at 2:26
    
Relevant: unix.stackexchange.com/q/96847/22222 –  terdon Jan 6 at 10:35

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up vote 2 down vote accepted

Main point here is that creating a process will consume some amount of entropy.

As stated in comments in the blog post Entropy Broken entropy is needed at least for randomization of the address space layout.

cat is a process, thus running watch cat ... will be consuming entropy constantly. Obviously, when you kill watch your system is restoring entropy to the usual level.

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