Am Samstag, 9. November 2013, 23:04:07 schrieb Clemens Ladisch: Hi Clemens, > Stephan Mueller wrote: > > Am Donnerstag, 7. November 2013, 02:03:57 schrieb Nicholas Mc Guire: > >> On Wed, 06 Nov 2013, Stephan Mueller wrote: > >>> Besides, how on earth shall an attacker even gain knowledge about the > >>> state of the CPU or disable CPU mechanisms? Oh, I forgot, your NSA > >>> guy. But if he is able to do that, all discussions are moot because > >>> he simply disables any noise sources by flipping a bit, reads the > >>> memory that is used to hold the state of the RNG or just overwrites > >>> the memory locations where data is collected, because the general > >>> protection mechanisms offered by the kernel and the underlying > >>> hardware are broken. > >> > >> No need to gain knowledge of the internal CPU state itt would be > >> sufficient to be able to put the CPU in a sub-state-space in which > >> the distribution is shifted. it may be enough to reduce the truely > >> random bits of some key only by a few bits to make it suceptible to > >> brute force attacks. > > > > Note, the proposed RNG contains an unbias operation (the Von-Neumann > > unbiaser) which is proven to remove any bias when it is established that > > the individual observations are independent. And the way the > > observations are generated ensures that they are independent. > > "Independent" does not mean that your own code avoids reusing data from > the previous loop iteration; it means that the _entire_ process that > generates the bits is not affected by any memory of the past. In the other email, I explained the different types of tests I performed. All of these tests show proper statistical results. Now, I also performed these tests without the Von-Neumann unbiaser. All of the statistical tests results still showed a white noise (note, in the next code release, I will have an allocation flag added that you can use to very simply deactivate the Von-Neumann unbiaser for testing). So, the Von-Neumann unbiaser is to be considered a line of defence against (not yet observed, but potential) skews. Similarly, the optional whitening (non-cryptographic) function of jent_stir_pool is yet another line of defence. So, bottom line: I fully concur that using two separate measurements may not imply that they are independent. But testing shows that it does not matter. > > The observations are derived from the internal CPU state, which is *not* > reset between measurements. > > > Regards, > Clemens Ciao Stephan -- | Cui bono? | -- 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
