Re: Why does reading from /dev/urandom deplete entropy so much?

Theodore Tso wrote:

Note that even paranoid applicatons should not be using /dev/random
for session keys; again, /dev/random isn't magic, and entropy isn't
unlimited. Instead, such an application should pull 16 bytes or so,
and then use it to seed a cryptographic random number generator.

What good does using multiple levels of RNG do? Why seed one RNG from another? Wouldn't it be better to have just one RNG that everybody uses? Doesn't the act of reading from the RNG add entropy to it, since no one reader has any idea how often and at what times other readers are stirring the pool?

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