Re: [PATCH] crypto: jitter - add cmdline oversampling overrides

[Stephan Mueller <smueller@chronox.de>]

Am Montag, 27. Januar 2025, 17:02:36 CET schrieb Markus Theil:

Hi Markus,

> As already mentioned in the comments, using a cryptographic
> hash function, like SHA3-256, decreases the expected entropy
> due to properties of random mappings (collisions and unused values).
> 
> When mapping 256 bit of entropy to 256 output bits, this results
> in roughly 6 bit entropy loss (depending on the estimate formula
> for mapping 256 bit to 256 bit via a random mapping):
> 
> NIST approximation (count all input bits as input): 255.0
> NIST approximation (count only entropy bits as input): 251.69 Bit
> BSI approximation (count only entropy bits as input): 250.11 Bit
> 
> Therefore add a cmdline override for the 64 bit oversampling safety margin,
> This results in an expected entropy of nearly 256 bit also after hashing,
> when desired.
> 
> Only enable this, when you are aware of the increased runtime per
> iteration.
> 
> This override is only possible, when not in FIPS mode (as FIPS mandates
> this to be true for a full entropy claim).
> 
> Signed-off-by: Markus Theil <theil.markus@gmail.com>

Reviewed-by: Stephan Mueller <smueller@chronox.de>

Ciao
Stephan