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

[Markus Theil <theil.markus@gmail.com>]

Hi (& sorry for replying in HTML the last time :(),

On 09.02.25 10:18, Herbert Xu wrote:
> On Mon, Jan 27, 2025 at 05:02:36PM +0100, Markus Theil wrote:
>> 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>
>> ---
>>   crypto/jitterentropy.c | 33 +++++++++++++++++++++++++++------
>>   1 file changed, 27 insertions(+), 6 deletions(-)
> 
> Why does this need to be a toggle?

It slightly increases the runtime of the code per 256 Bit random number 
generated, which possibly doesn't suit embedded guys using this RNG but 
demanding short system bootup.
So users who can live with this penalty can enable it, others probably 
won't tolerate it. With this being a toggle, it will also be easier to 
enable this in default distribution kernels running e.g. in cloud instances.

> 
> Why can't you just make this conditional on fips_enabled?

This was also my first thought, just enable fips mode. Our workloads 
don't have to run in FIPS mode and I don't know which software may 
reacts to the kernel announcing to be fips enabled in an unexpected way.

So basically, this seems to be useful, even when not in FIPS mode.

BR
Markus