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/* Jitter RNG: GCD health test
*
* Copyright (C) 2021, Joshua E. Hill <josh@keypair.us>
* Copyright (C) 2021, Stephan Mueller <smueller@chronox.de>
*
* License: see LICENSE file in root directory
*
* THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED
* WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
* OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, ALL OF
* WHICH ARE HEREBY DISCLAIMED. IN NO EVENT SHALL THE AUTHOR BE
* LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT
* OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
* BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
* LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE
* USE OF THIS SOFTWARE, EVEN IF NOT ADVISED OF THE POSSIBILITY OF SUCH
* DAMAGE.
*/
#include "jitterentropy.h"
#include "jitterentropy-gcd.h"
/* The common divisor for all timestamp deltas */
static uint64_t jent_common_timer_gcd = 0;
static inline int jent_gcd_tested(void)
{
return (jent_common_timer_gcd != 0);
}
/* A straight forward implementation of the Euclidean algorithm for GCD. */
static inline uint64_t jent_gcd64(uint64_t a, uint64_t b)
{
/* Make a greater a than or equal b. */
if (a < b) {
uint64_t c = a;
a = b;
b = c;
}
/* Now perform the standard inner-loop for this algorithm.*/
while (b != 0) {
uint64_t r;
r = a % b;
a = b;
b = r;
}
return a;
}
static int jent_gcd_analyze_internal(uint64_t *delta_history, size_t nelem,
uint64_t *running_gcd_out,
uint64_t *delta_sum_out)
{
uint64_t running_gcd, delta_sum = 0;
size_t i;
if (!delta_history)
return -EAGAIN;
running_gcd = delta_history[0];
/* Now perform the analysis on the accumulated delta data. */
for (i = 1; i < nelem; i++) {
/*
* ensure that we have a varying delta timer which is necessary
* for the calculation of entropy -- perform this check
* only after the first loop is executed as we need to prime
* the old_data value
*/
if (delta_history[i] >= delta_history[i - 1])
delta_sum += delta_history[i] - delta_history[i - 1];
else
delta_sum += delta_history[i - 1] - delta_history[i];
/*
* This calculates the gcd of all the delta values. that is
* gcd(delta_1, delta_2, ..., delta_nelem)
* Some timers increment by a fixed (non-1) amount each step.
* This code checks for such increments, and allows the library
* to output the number of such changes have occurred.
*/
running_gcd = jent_gcd64(delta_history[i], running_gcd);
}
*running_gcd_out = running_gcd;
*delta_sum_out = delta_sum;
return 0;
}
int jent_gcd_analyze(uint64_t *delta_history, size_t nelem)
{
uint64_t running_gcd, delta_sum;
int ret = jent_gcd_analyze_internal(delta_history, nelem, &running_gcd,
&delta_sum);
if (ret == -EAGAIN)
return 0;
/*
* Variations of deltas of time must on average be larger than 1 to
* ensure the entropy estimation implied with 1 is preserved.
*/
if (delta_sum <= nelem - 1) {
ret = EMINVARVAR;
goto out;
}
/*
* Ensure that we have variations in the time stamp below 100 for at
* least 10% of all checks -- on some platforms, the counter increments
* in multiples of 100, but not always
*/
if (running_gcd >= 100) {
ret = ECOARSETIME;
goto out;
}
/* Adjust all deltas by the observed (small) common factor. */
if (!jent_gcd_tested())
jent_common_timer_gcd = running_gcd;
out:
return ret;
}
uint64_t *jent_gcd_init(size_t nelem)
{
uint64_t *delta_history;
delta_history = jent_zalloc(nelem * sizeof(uint64_t));
if (!delta_history)
return NULL;
return delta_history;
}
void jent_gcd_fini(uint64_t *delta_history, size_t nelem)
{
if (delta_history)
jent_zfree(delta_history,
(unsigned int)(nelem * sizeof(uint64_t)));
}
int jent_gcd_get(uint64_t *value)
{
if (!jent_gcd_tested())
return 1;
*value = jent_common_timer_gcd;
return 0;
}
int jent_gcd_selftest(void)
{
#define JENT_GCD_SELFTEST_ELEM 10
#define JENT_GCD_SELFTEST_EXP 3ULL
uint64_t *gcd = jent_gcd_init(JENT_GCD_SELFTEST_ELEM);
uint64_t running_gcd, delta_sum;
unsigned int i;
int ret = EGCD;
if (!gcd)
return EMEM;
for (i = 0; i < JENT_GCD_SELFTEST_ELEM; i++)
jent_gcd_add_value(gcd, i * JENT_GCD_SELFTEST_EXP, i);
if (jent_gcd_analyze_internal(gcd, JENT_GCD_SELFTEST_ELEM,
&running_gcd, &delta_sum))
goto out;
if (running_gcd != JENT_GCD_SELFTEST_EXP)
goto out;
ret = 0;
out:
jent_gcd_fini(gcd, JENT_GCD_SELFTEST_ELEM);
return ret;
}
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