Am Mittwoch, 10. August 2016, 06:04:32 CEST schrieb Pan, Miaoqing: Hi Miaoqing, > Hi Stephan, > > FIPS RNG test is supposed to be run on the output of an RNG, and not on the > RNG entropy source. It is not surprising that the RNG input fails the > entropy tests from NIST. Check the following example. > > Imagine you have a perfectly random sequence, a_1, a_2, .., a_n, where each > a_i is a byte. And imagine, this sequence passes all randomness tests. > > Now, let's say I create a new sequence a_1, 0, a_2, 0, a_3, 0, ..., 0, a_n, > where each zero is a byte > > If you give this sequence (as an entropy source) to a randomness test, it > will fail most of the tests, if not all. This does not mean this sequence > is not appropriate as an entropy source, it just means we need twice more > bytes to gain the same amount of entropy. Agreed. But that is a very simplistic view. > > I can give this 2n byte sequence to an RNG as an entropy source and it > provides the same amount of security as if I give the n byte stream. Well, I am working with standards bodies like NIST and BSI on RNG assessments. They all require that the noise source (pre-whitening, of course) pass statistical tests like the AIS20 tests, SP800-22 and similar. If you fail, you better have a good argument. And the only argument that is kind of allowed is that you oversample your noise source to seed a DRNG from (i.e. have an entropy to data ratio of significantly below 1). And the argument for the oversampling rate is always a very interesting discussion. You apply 10/32. In private, I am wondering about that ratio, but this should not be discussed here as I hope you have a valid argument for that. As we are talking about the current rngd, we have to consider that it does *not* perform an oversampling (yet) as mentioned in the previous emails. Do not get me wrong on my initial patch: your RNG may provide some entropy. But there are quite some folks who want to understand and audit a noise source before using it. Your current implementation simply does not allow switching the noise source off to feed the input_pool with data that increases the entropy estimator (at runtime). 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
