On 25/06/2021 15:50, Stephan Müller wrote: > Am Freitag, 25. Juni 2021, 13:09:26 CEST schrieb Mickaël Salaün: > > Hi Mickaël, > > [...] >> >>> - applies an entropy_input len of 512 bits during initial seeding >>> >>> - applies a nonce of 128 bits during initial seeding >>> >>> entropy_input == <384 bits get_random_bytes> || <256 bits Jitter RNG> >> >> We think that using "<384 bits get_random_bytes> || " makes this DRBG >> non-compliant with SP800-90A rev1 because get_random_bytes doesn't use a >> vetted conditioning component (but ChaCha20 instead): >> >> SP800-90Ar1, section 8.6.5 says "A DRBG mechanism requires an approved >> randomness source during instantiation and reseeding [...]. An approved >> randomness source is an entropy source that conforms to [SP 800-90B], or >> an RBG that conforms to [SP 800-90C] − either a DRBG or an NRBG". >> The FIPS 140-2 Implementation Guidance >> (https://csrc.nist.gov/csrc/media/projects/cryptographic-module-validation-p >> rogram/documents/fips140-2/fips1402ig.pdf), section 7.19 says "As of >> November 7, 2020, all newly submitted modules requiring an entropy >> evaluation must demonstrate compliance to SP 800-90B". In resolution 3 it >> says "all processing of the raw data output from the noise sources that >> happens before it is ultimately output from the entropy source *shall* >> occur within a conditioning chain". Data from get_random_bytes may come >> from multiple noise sources, but they are hashed with ChaCha20. >> In resolution 6 it says "a vetted conditioning component may optionally >> take a finite amount of supplemental data [...] in addition to the data >> from the primary noise source", which would be OK if get_random_bytes >> used a vetted algorithm, but it is not the case for now. > > You cite the right references, I think the interpretation is too strict. > > The specifications require that > > a) The DRBG must be seeded by a 90B entropy source > > b) The DRBG must be initially seeded with 256 bits of entropy plus some 128 > bit nonce > > We cover a) with the Jitter RNG and b) by pulling 384 bits from it. > > The standard does not forbit: > > c) the entropy string may contain data from another origin or it contains a > larger buffer > > d) the actual entropy distribution in the entropy string being not an > equidistribution over the entire entropy string > > Bullet d) implies that it is perfectly fine to have entropy distribution begin > loopsided in the entropy string. > > Bullet c) implies that other data can be provided with the entropy string. > > With that, to be 90A/B compliant, you interpret that the Jitter RNG provides > all entropy you need and credit the entropy from get_random_bytes with zero > bits of entropy. > > > Note, if you look into the implementation of the DRBG seeding, the different > input strings like entropy string or data without entropy like personalization > string are simply concatenated and handed to the DRBG. As the Jitter RNG and > get_random_bytes data is also concatenated, it follows the concepts of 90A. > > If you look into the draft 90C standard, it explicitly allows concatenation of > data from an entropy source that you credit with entropy and data without > entropy - see the crediting of entropy of multiple entropy sources defined > with "Method 1" and "Method 2" in the current 90C draft. > > This ultimately allows us to have an entropy string that is concatenated from > different entropy sources. If you have an entropy source that is not 90B > compliant, you have to credit it with zero bits of entropy in the entropy > analysis. Thus, only the entropy source(s) compliant to 90B must provide the > entire entropy as mandated by 90A. Thanks for your detailed explanation Stephan. We agree that data from get_random_bytes is not accounted as entropy, but the question is: is it still in line with the specification because it uses an algorithm not compliant to SP800-90B (i.e. ChaCha20 is not a vetted conditioning component)? Cf. IG 7.19 resolution 6 from 08/28/2020 and IG 7.20 from 05/04/2021. > > After having several discussions with the Entropy Working group sponsored by > NIST that included also representatives from the NIST crypto technology group, > there was no concern regarding such approach. > > This approach you see in the current DRBG seeding code is now taken for > different FIPS validations including FIPS validations that I work on as a FIPS > tester as part of my duties working for a FIPS lab. My colleagues have > reviewed the current kernel DRBG seeding strategy and approved of it for other > FIPS validations. Good to know. We are worried that a new FIPS validation (started after November 7, 2020) could failed because of the new SP800-90B requirement. This issue was pointed out by a lab. It seems that the specification is still open to different interpretations. Regards, Mickaël
