[Cryptography] cheap sources of entropy

[James A. Donald]

We only need 128 bits of entropy, every now and then.  We are not going 
the be cpu bound all the time, and we are certainly not going to be cpu 
bound when the system is first turned on.

And if we are cpu bound all the time, there will still be timing 
randomness because all the other vms are dealing with things in the real 
world that are random and unpredictable.