[Discuss] The RSA Keying links

[blu at nedharvey.com (Edward Ned Harvey)]

> From: discuss-bounces+blu=nedharvey.com at blu.org [mailto:discuss-
> bounces+blu=nedharvey.com at blu.org] On Behalf Of Daniel Hagerty
> 
> * Linux has /proc/sys/kernel/random.

Thanks for the pointer.
To figure out how much entropy you have, do this:
	watch -n 1 cat /proc/sys/kernel/random/entropy_avail
At any given moment, it will tell you how many bits of randomness you have
available right now, but they quickly expire.  So your system will hover
around some level...  If you have a TPM or any other hardware random
generator, or even mouse & keyboard, then you'll generate a lot more and
have a higher level.  A relatively un-random system might be around 100 or
200 (which corresponds to approx bits/sec) and a good system might be around
2,000 or 4,000.

There are pages all over the internet saying to use prngd to feed bits from
urandom into random.  This sounds foolish to me.  If there is any purpose to
distinguish random from urandom at all...  Then feeding random from urandom
defeats that purpose.

To ensure you generate good ssh keys is easy:
export SSH_USE_STRONG_RNG=256
(Notice, these are bytes.  So the above is 2048 bits.)
Notice, this could take a while.  Supposing your system is generating 128
bits/sec, a lot of them get wasted, and ssh-keygen will take about a minute
or two.

That answers the question for ssh.
Now what about SSL?

Star by checking for the existence of a RANDFILE environment variable.  If
there is none...  Look for RANDFILE in your openssl.cnf file.  In my system:
	grep RANDFILE /etc/pki/tls/openssl.cnf
	RANDFILE                = $ENV::HOME/.rnd
	RANDFILE        = $dir/private/.rand    # private random number file

And sure enough, when I look in my home directory, I have a 1K file ~/.rnd

Here's how openssl works:  It seeds from the RANDFILE, and then it uses its
own internal prng stream cipher.  And every time it runs, it will overwrite
the RANDFILE with a new seed for next time.  So to ensure good random
numbers in openssl, all you need to do is any variation of the following:
	export RANDFILE=/dev/random
	or
	dd if=/dev/random of=~/.rnd bs=1k count=1
	or
	Whatever is appropriate based on your individual RANDFILE.

Personally, I'm inclined to export RANDFILE=/dev/random, because it
eliminates the possibility you accidentally looked in the wrong openssl.cnf,
and it eliminates the possibility of someone discovering your seed by
reading your ~/.rnd file.