[cryptography] urandom vs random

James A. Donald jamesd at echeque.com
Fri Aug 16 22:01:27 EDT 2013


At startup, likely to be short of entropy.

Actual behavior, and even existence, of /dev/random and /dev/urandom 
varies substantially from one implementation to another.

If /dev/random blocks when short of entropy, then likely to block at 
startup, which is good.  Services that need entropy do not need to start 
immediately.  If they take a while to come up, no big deal.

If /dev/urandom seeded at startup, and then seeded no further, bad, but 
not very bad.

If /dev/urandom seeded at startup from /dev/random, then should block at 
startup.

If /dev/urandom never blocks, bad.  Should block at startup waiting to 
receive 160 bits from /dev/random, and never block again.

Ron Peterson reports /dev/random not very random 
<http://bytes.com/topic/c/answers/219952-dev-urandom-vs-dev-random>








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