[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 

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 

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.randombit.net/pipermail/cryptography/attachments/20130817/345e1b45/attachment.html>

More information about the cryptography mailing list