Re: [PATCH] CPU Jitter RNG: inclusion into kernel crypto API and /dev/random

[Stephan Mueller]

Am Mittwoch, 6. November 2013, 14:26:35 schrieb Pavel Machek:

Hi Pavel,

>Hi!
>
>> >I plugged that idea into my current Jitter RNG processing and
>> >disabled
>> >the other jitter measurements to get a clear, isolated picture.
>> >
>> >The result is also a white noise! And it is even quite fast.
>> 
>> After doing some more research on this approach, I have to admit that
>> the output not good (i.e. white noise) in all situations. Therefore,
>> I
>> dropped that (for now).
>
>Is there chance to extract at least some entropy from it? (Can you
>post the code you used for testing?) Because in this case we know
>where the entropy comes from, which is important for Ted.

The code is as follows -- it hooks into the framework of the RNG I 
already have, so the code folds the obtained data into one bit (use the 
following function as a drop-in replacement to my RNG code.

static __u64 jent_measure_jitter(struct rand_data *entropy_collector)
{
        __u64 starttime = 0;
        __u64 currtime = 0;
        __u64 counter = 0;
        __u64 data = 0;

        jent_get_ustime(&starttime);
        jent_get_ustime(&currtime);
        while(starttime == currtime)
        {
                jent_get_ustime(&currtime);
                counter++;
        }
        jent_fold_time(counter, &data, 1);
        return data;
}

Consider the following in addition:

static inline void jent_get_ustime(__u64 *out)
{
        __u64 tmp = 0;
        struct timeval time;
        if(gettimeofday(&time, NULL) == 0)
                tmp = time.tv_usec;
        *out = tmp;
}

For the kernel land, I implemented jent_get_ustime to be identical to 
do_gettimeofday().

The result is the following on my i7 2nd gen without using the Von-
Neumann unbias operation:

- user space: looks like good white noise based on the results of ent 
(Chi square, etc). When I print out the counter variable above and 
calculate the Shannon Entropy, I get about 1.5 bits, so we have 
variations. But when you look at the data manually, you see quite some 
streaks that alternate between two values. Here is an example:

4
6
10
2
3
2
3
4
4
4
4
4
5
3
4
5
4
4
4
5
4
4
5
4
4
5
4
4
5
4
4
5
4
4
4
5
4
4


- kernel space: the resulting binary string is not very good: the chi 
square is very bad. Moreover, the resulting data string is slightly 
skewed. The reason is simple by looking at the counter value which I 
obtained with another debugfs file: there are very very long streaks of 
the same or alternating values.

So, I guess you may get some entropy, but I am not sure how much.

Also, when I enlarge the timer value to look something like that:

        if(gettimeofday(&time, NULL) == 0)
                tmp = time.tv_usec>>3;

the counter value is not getting really better, it is still alternating 
between two or three values.


>
>Thanks,
>							Pavel


Ciao
Stephan
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