Am Montag, 14. Oktober 2013, 10:14:00 schrieb Sandy Harris: Hi Sandy, >On Mon, Oct 14, 2013 at 9:38 AM, Sandy Harris <sandyinchina@xxxxxxxxx> wrote: >> Stephan Mueller <smueller@xxxxxxxxxx> wrote: >>> Can you please help me understand why you think that a whitening >>> function (cryptographic or not) is needed in the case of the CPU >>> Jitter RNG, provided that I can show that each individual bit >>> coming from the folding operation has one bit of entropy? >> >> Basically, sheer paranoia. I'd mix and whiten just on general >> principles. Since efficiency is not a large concern, there is little >> reason not to. >> >> On the other hand, most RNGs use a hash because they need >> to distill some large amount of low-entropy input into a smaller >> high-entropy output. With high input entropy, you do not need >> the hash and can choose some cheaper mixer. > >You could use strong mixing/whitening: > >Feed into random(4) and let it do the mixing. That is exactly the goal with the patch found in patches/linux-3.9- random.patch in the code distribution. And that approach is exactly what I do in the linking code / patches for other crypto libs: - kernel crypto API - OpenSSL (implementation as an Engine that uses the internal DRNGs or a hook into RAND_poll that implements the seeding for the DRNGs) - libgcrypt (hook into the seeding of the DRNGs) > >Use some early outputs from your RNG to key an AES >instance. Then encrypt later outputs; this gives a 64 in 64 >out mixer that is cryptographically strong but perhaps a bit >slow in the context. That is exactly what the SP800-90A CTR DRBG or the X9.31 does. As these DRNGs are available for different crypto libs, I am simply reusing them with the crypto lib linking code. > >Alternately, quite a few plausible components for fast cheap >mixing are readily available. Thank you for the references. I have seen that in your maxwell(8) documentation. But again, I do not re-invent the wheel with the CPU Jitter RNG and therefore skipped the whitening step based on the reasons above. Another thing: when you start adding whitening functions, other people are starting (and did -- thus I added section 4.3 to my documentation) to complain that you hide your weaknesses behind the whiteners. I simply want to counter that argument and show that RNG produces white noise without a whitener. Ciao Stephan -- To unsubscribe from this list: send the line "unsubscribe linux-crypto" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
