On Fri, Nov 24, 2017 at 2:05 PM, Stephan Müller <smueller@xxxxxxxxxx> wrote: > Am Freitag, 24. November 2017, 13:09:06 CET schrieb Krzysztof Kozlowski: > > Hi Krzysztof, >> >> >> >> 1. I was rather thinking about extending existing exynos-rng.c [1] so >> >> it would be using TRNG as seed for PRNG as this gives you much more >> >> random data. Instead you developed totally separate driver which has >> >> its own benefits - one can choose which interface he wants. Although >> >> it is a little bit duplication. >> > >> > As far as I can tell, these are two different devices. However, PRNG >> > shares hardware with the hash engine. Indeed there is a hardware to >> > connect TRNG and PRNG, but, IMHO, it might be hard to model that >> > dependency in kernel. >> >> It should be as simple as setting few more registers in SSS module >> (actually maybe just enabling TRNG_SEED_START in PRNG). You do not >> have to model it in a kernel like connecting some hw_rng entity to >> cryptoai's rng_alg. See the jitterentropy-kcapi.c. I understand that >> in that case existing exynos-rng.c could expose two different RNG >> devices - one PRNG based on user's seed and second TRNG (actually >> TRNG+PRNG). >> >> It does not seem difficult to model but the question is whether that >> makes sense. > > The usage strategy for the PRNGs registered at the kernel crypto API is as > follows: > > 1. crypto_rng_alloc > > 2. crypto_rng_reset > > 3. crypto_rng_generate > > If in step 2 you provide NULL as input, the kernel takes get_random_bytes as > seed source. Step 2 is the mandatory. > > The Linux-RNG can be fed internally from the hw_random framework by the > function hwrng_fillfn. This function is only used if the current_quality or > default_quality values in the hw_random framework is set. > > For the TRNG, it seems to be not set per default, but could be set as either a > boot argument or at runtime via /sys. > > If that variable is set and the TRNG is registered, it feeds random data into > the Linux-RNG which in turn is used per default to seed a PRNG. In this case, > no detour via user space is needed to push data from TRNG to the PRNG. Using > that mechanism allows you to benefit from additional entropy the Linux-RNG > collects elsewhere. >> >> > To me it seems easier to read TRNG (or >> > /dev/random) and and write the result to PRNG manually (in software). >> >> Indeed this gives more flexibility to the user (choice of engine) but >> first, it is slower, and second it reduces the quality of random >> numbers (PRNG reseeds itself... but in this model cannot reseed from >> TRNG). > > Given the reasons above, I would think that keeping the PRMG and TRNG separate > as offered by the current patch seems reasonable. If configured correctly, the > TRNG can seed the PRNG at any time (including boot time) without the need of > user space. Hi Stephan, Thanks for explaining the details. This convinces me so I do not have any objections against current approach of this second RNG driver for Exynos. Best regards, Krzysztof
