Re: Collecting entropy from device_attach() times.

On Wed, Sep 19, 2012 at 7:30 PM, Jonathan Anderson
<jonathan.anderson@cl.cam.ac.uk> wrote:
> On Tuesday, 18 September 2012 at 22:14, Pawel Jakub Dawidek wrote:
>> [=85] we have more
>> than 19 bits of entropy from this one call, but I reduced if to four
>> bits only, because there are devices that are much faster to attach.
>>
>
> Another reason for doing the above characterization is that, if a particu=
lar device_attach() really does provide 12 bits of uncertainty, it's a sham=
e to drop eight of them on the floor.

Estimating at 4 bits does not drop any entropy on the floor, it just
means that if you are going to unblock the PRNG once a certain amount
of entropy is present, then this input counts for 4 bits against that
certain amount. The amount of entropy harvested is unchanged.

The reason to work out how much entropy there is is to:

a) Unblock as early as possible

b) Not unblock too early

Erring on the side of underestimation is wise here.
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