On 06/24/2014 06:01 PM, Martin Buchholz wrote:
On Tue, Jun 24, 2014 at 7:03 AM, Peter Levart <peter.lev...@gmail.com
<mailto:peter.lev...@gmail.com>> wrote:
I would rather use SecureRandom.generateSeed() instance method
instead of SecureRandom.nextBytes(). Why? Because every
SecureRandom instance has to initialize it's seed 1st before
getBytes() can provide the next random bytes from the PRNG. Since
we only need the 1st 8 bytes from the SecureRandom instance to
initialize TLR's seeder, we might as well directly call the
SecureRandom.generateSeed() method.
If I strace this program on Linux using strace -ff -q java SecureRandoms:
public class SecureRandoms {
public static void main(String[] args) throws Throwable {
byte[] bytes = new byte[8];
new java.security.SecureRandom().nextBytes(bytes);
}
}
I see a read from /dev/urandom, but not from /dev/random, so I
conclude your intuitive understanding of how the seeding works must be
wrong. It makes sense that NativePRNG doesn't need to do any special
seeding of its own, since it reuses the operating system's.
You're right. I checked again. The NativePRNG is actually using
/dev/urandom (by default unless java.security.egd or securerandom.source
is defined). It's mixing the /dev/urandom stream with the stream
obtained from SHA1 generator which is seeded by 20 bytes from
/dev/urandom too. So by default yes, plain NativePRNG (the default on
UNIX-es) is using /dev/urandom for nextBytes(), but this can be changed
by defining java.security.egd or securerandom.source system property. I
still think that for configuration-independent PRNG seed on UNIX-es it's
better to invoke generateSeed() on NativePRNG$NonBlocking, which
hard-codes /dev/urandom and doesn't mix it with SHA1 stream.
On Windows, there's a different story, since the default SecureRandom
algorithm is SHA1, seeded by SeedGenerator.getSystemEntropy() and
SeedGenerator.generateSeed(). The former call includes invoking
networking code and resolving local host name. Which we would like to
avoid. So I think we need a nicer story on windows then just: new
SecureRandom().nextBytes(). I propose requesting explicit algorithm /
provider on each particular platform that we know does best what we
want, rather than using default which can still be used as a fall-back.
Regards, Peter