It has been pointed out that the Linux random driver will block if it computes there is no entropy available, and this is correct. However, last August there were bugs discovered in entropy accounting that caused it to overestimate current entropy, and the Red Hat 2.4.20-19.9 kernel still doesn't seem to have the fix. If one has a lot of faith in md5, this isn't a problem on an up-and-running machine with ~some~ source of entropy, but at very early points in the entropy pool's life, the amount does matter when something very important is being generating early on, such as ssh keys. (Ironically, would adopting the random fix, lock up a Red Hat first boot?) Also, not everyone is comfortable with disk or network timings as entropy sources. Again, on a running machine, having too many entropy sources, some of which might not actually have much entropy in them, isn't a problem, but a fresh entropy pool is different. Solution? One approach is to delay creation of ssh keys. (Or, as I did, manually create new keys after one is certain there is enough entropy available.) Even if disk jitter is the only available entropy source, waiting for more of it to accumulate would help. And don't forget embedded Linux cases, such as routers, wireless access points, etc., where there might not be a mechanical disk. It is not hard to imagine really getting hard up for entropy in a factory-fresh box. One idea I had recently is to hash the power up state of a reasonable portion of RAM. By no means are the initial contents of RAM completely random, but RAM *is* volatile, I am pretty sure it has some entropy in it. Even if there is little entropy from one power up to another power up of a given DRAM chip, there must be some entropy between different physical chips. A warm boot might have extremely little to no entropy in RAM contents, but by that point some entropy could have been stored from previously. It is the really, really cold boot that is the problem. -kb, the Kent who wiggles the mouse on his basement server whenever he walks by, a bit like a stroking rabbit's foot. P.S. A few months ago I wrote a RAM entropy grabber for embedded PPC, but it is too groady in how it passes it to the kernel.
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