Quelle jitterentropy.c Sprache: C

/*
* Non-physical true random number generator based on timing jitter --
* Jitter RNG standalone code.
*
* Copyright Stephan Mueller <smueller@chronox.de>, 2015 - 2023
*
* Design
* ======
*
* See https://www.chronox.de/jent.html
*
* License
* =======
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
*    notice, and the entire permission notice in its entirety,
*    including the disclaimer of warranties.
* 2. Redistributions in binary form must reproduce the above copyright
*    notice, this list of conditions and the following disclaimer in the
*    documentation and/or other materials provided with the distribution.
* 3. The name of the author may not be used to endorse or promote
*    products derived from this software without specific prior
*    written permission.
*
* ALTERNATIVELY, this product may be distributed under the terms of
* the GNU General Public License, in which case the provisions of the GPL2 are
* required INSTEAD OF the above restrictions.  (This clause is
* necessary due to a potential bad interaction between the GPL and
* the restrictions contained in a BSD-style copyright.)
*
* 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.
*/

/*
* This Jitterentropy RNG is based on the jitterentropy library
* version 3.4.0 provided at https://www.chronox.de/jent.html
*/

#ifdef __OPTIMIZE__
#error "The CPU Jitter random number generator must not be compiled with optimizations. See documentation. Use the compiler switch -O0 for compiling jitterentropy.c."
#endif

typedef unsigned long long __u64;
typedef long long  __s64;
typedef unsigned int  __u32;
typedef unsigned char  u8;
#define NULL    ((void *) 0)

/* The entropy pool */
struct rand_data {
/* SHA3-256 is used as conditioner */
#define DATA_SIZE_BITS 256
/* all data values that are vital to maintain the security
* of the RNG are marked as SENSITIVE. A user must not
* access that information while the RNG executes its loops to
* calculate the next random value. */
void *hash_state;  /* SENSITIVE hash state entropy pool */
__u64 prev_time;  /* SENSITIVE Previous time stamp */
__u64 last_delta;  /* SENSITIVE stuck test */
__s64 last_delta2;  /* SENSITIVE stuck test */

unsigned int flags;  /* Flags used to initialize */
unsigned int osr;  /* Oversample rate */
#define JENT_MEMORY_ACCESSLOOPS 128
#define JENT_MEMORY_SIZE      \
(CONFIG_CRYPTO_JITTERENTROPY_MEMORY_BLOCKS *   \
  CONFIG_CRYPTO_JITTERENTROPY_MEMORY_BLOCKSIZE)
unsigned char *mem; /* Memory access location with size of
* memblocks * memblocksize */
unsigned int memlocation; /* Pointer to byte in *mem */
unsigned int memblocks; /* Number of memory blocks in *mem */
unsigned int memblocksize; /* Size of one memory block in bytes */
unsigned int memaccessloops; /* Number of memory accesses per random
      * bit generation */

/* Repetition Count Test */
unsigned int rct_count;   /* Number of stuck values */

/* Adaptive Proportion Test cutoff values */
unsigned int apt_cutoff; /* Intermittent health test failure */
unsigned int apt_cutoff_permanent; /* Permanent health test failure */
#define JENT_APT_WINDOW_SIZE 512 /* Data window size */
/* LSB of time stamp to process */
#define JENT_APT_LSB  16
#define JENT_APT_WORD_MASK (JENT_APT_LSB - 1)
unsigned int apt_observations; /* Number of collected observations */
unsigned int apt_count;  /* APT counter */
unsigned int apt_base;  /* APT base reference */
unsigned int health_failure; /* Record health failure */

unsigned int apt_base_set:1; /* APT base reference set? */
};

/* Flags that can be used to initialize the RNG */
#define JENT_DISABLE_MEMORY_ACCESS (1<<2) /* Disable memory access for more
   * entropy, saves MEMORY_SIZE RAM for
   * entropy collector */

/* -- error codes for init function -- */
#define JENT_ENOTIME  1 /* Timer service not available */
#define JENT_ECOARSETIME 2 /* Timer too coarse for RNG */
#define JENT_ENOMONOTONIC 3 /* Timer is not monotonic increasing */
#define JENT_EVARVAR  5 /* Timer does not produce variations of
   * variations (2nd derivation of time is
   * zero). */
#define JENT_ESTUCK  8 /* Too many stuck results during init. */
#define JENT_EHEALTH  9 /* Health test failed during initialization */
#define JENT_ERCT        10 /* RCT failed during initialization */
#define JENT_EHASH        11 /* Hash self test failed */
#define JENT_EMEM        12 /* Can't allocate memory for initialization */

#define JENT_RCT_FAILURE 1 /* Failure in RCT health test. */
#define JENT_APT_FAILURE 2 /* Failure in APT health test. */
#define JENT_PERMANENT_FAILURE_SHIFT 16
#define JENT_PERMANENT_FAILURE(x) (x << JENT_PERMANENT_FAILURE_SHIFT)
#define JENT_RCT_FAILURE_PERMANENT JENT_PERMANENT_FAILURE(JENT_RCT_FAILURE)
#define JENT_APT_FAILURE_PERMANENT JENT_PERMANENT_FAILURE(JENT_APT_FAILURE)

/*
* The output n bits can receive more than n bits of min entropy, of course,
* but the fixed output of the conditioning function can only asymptotically
* approach the output size bits of min entropy, not attain that bound. Random
* maps will tend to have output collisions, which reduces the creditable
* output entropy (that is what SP 800-90B Section 3.1.5.1.2 attempts to bound).
*
* The value "64" is justified in Appendix A.4 of the current 90C draft,
* and aligns with NIST's in "epsilon" definition in this document, which is
* that a string can be considered "full entropy" if you can bound the min
* entropy in each bit of output to at least 1-epsilon, where epsilon is
* required to be <= 2^(-32).
*/
#define JENT_ENTROPY_SAFETY_FACTOR 64

#include <linux/array_size.h>
#include <linux/fips.h>
#include <linux/minmax.h>
#include "jitterentropy.h"

/***************************************************************************
* Adaptive Proportion Test
*
* This test complies with SP800-90B section 4.4.2.
***************************************************************************/

/*
* See the SP 800-90B comment #10b for the corrected cutoff for the SP 800-90B
* APT.
* https://www.untruth.org/~josh/sp80090b/UL%20SP800-90B-final%20comments%20v1.9%2020191212.pdf
* In the syntax of R, this is C = 2 + qbinom(1 − 2^(−30), 511, 2^(-1/osr)).
* (The original formula wasn't correct because the first symbol must
* necessarily have been observed, so there is no chance of observing 0 of these
* symbols.)
*
* For the alpha < 2^-53, R cannot be used as it uses a float data type without
* arbitrary precision. A SageMath script is used to calculate those cutoff
* values.
*
* For any value above 14, this yields the maximal allowable value of 512
* (by FIPS 140-2 IG 7.19 Resolution # 16, we cannot choose a cutoff value that
* renders the test unable to fail).
*/
static const unsigned int jent_apt_cutoff_lookup[15] = {
325, 422, 459, 477, 488, 494, 499, 502,
505, 507, 508, 509, 510, 511, 512 };
static const unsigned int jent_apt_cutoff_permanent_lookup[15] = {
355, 447, 479, 494, 502, 507, 510, 512,
512, 512, 512, 512, 512, 512, 512 };

static void jent_apt_init(struct rand_data *ec, unsigned int osr)
{
/*
* Establish the apt_cutoff based on the presumed entropy rate of
* 1/osr.
*/
if (osr >= ARRAY_SIZE(jent_apt_cutoff_lookup)) {
  ec->apt_cutoff = jent_apt_cutoff_lookup[
   ARRAY_SIZE(jent_apt_cutoff_lookup) - 1];
  ec->apt_cutoff_permanent = jent_apt_cutoff_permanent_lookup[
   ARRAY_SIZE(jent_apt_cutoff_permanent_lookup) - 1];
} else {
  ec->apt_cutoff = jent_apt_cutoff_lookup[osr - 1];
  ec->apt_cutoff_permanent =
    jent_apt_cutoff_permanent_lookup[osr - 1];
}
}
/*
* Reset the APT counter
*
* @ec [in] Reference to entropy collector
*/
static void jent_apt_reset(struct rand_data *ec, unsigned int delta_masked)
{
/* Reset APT counter */
ec->apt_count = 0;
ec->apt_base = delta_masked;
ec->apt_observations = 0;
}

/*
* Insert a new entropy event into APT
*
* @ec [in] Reference to entropy collector
* @delta_masked [in] Masked time delta to process
*/
static void jent_apt_insert(struct rand_data *ec, unsigned int delta_masked)
{
/* Initialize the base reference */
if (!ec->apt_base_set) {
  ec->apt_base = delta_masked;
  ec->apt_base_set = 1;
  return;
}

if (delta_masked == ec->apt_base) {
  ec->apt_count++;

  /* Note, ec->apt_count starts with one. */
  if (ec->apt_count >= ec->apt_cutoff_permanent)
   ec->health_failure |= JENT_APT_FAILURE_PERMANENT;
  else if (ec->apt_count >= ec->apt_cutoff)
   ec->health_failure |= JENT_APT_FAILURE;
}

ec->apt_observations++;

if (ec->apt_observations >= JENT_APT_WINDOW_SIZE)
  jent_apt_reset(ec, delta_masked);
}

/***************************************************************************
* Stuck Test and its use as Repetition Count Test
*
* The Jitter RNG uses an enhanced version of the Repetition Count Test
* (RCT) specified in SP800-90B section 4.4.1. Instead of counting identical
* back-to-back values, the input to the RCT is the counting of the stuck
* values during the generation of one Jitter RNG output block.
*
* The RCT is applied with an alpha of 2^{-30} compliant to FIPS 140-2 IG 9.8.
*
* During the counting operation, the Jitter RNG always calculates the RCT
* cut-off value of C. If that value exceeds the allowed cut-off value,
* the Jitter RNG output block will be calculated completely but discarded at
* the end. The caller of the Jitter RNG is informed with an error code.
***************************************************************************/

/*
* Repetition Count Test as defined in SP800-90B section 4.4.1
*
* @ec [in] Reference to entropy collector
* @stuck [in] Indicator whether the value is stuck
*/
static void jent_rct_insert(struct rand_data *ec, int stuck)
{
if (stuck) {
  ec->rct_count++;

  /*
* The cutoff value is based on the following consideration:
* alpha = 2^-30 or 2^-60 as recommended in SP800-90B.
* In addition, we require an entropy value H of 1/osr as this
* is the minimum entropy required to provide full entropy.
* Note, we collect (DATA_SIZE_BITS + ENTROPY_SAFETY_FACTOR)*osr
* deltas for inserting them into the entropy pool which should
* then have (close to) DATA_SIZE_BITS bits of entropy in the
* conditioned output.
*
* Note, ec->rct_count (which equals to value B in the pseudo
* code of SP800-90B section 4.4.1) starts with zero. Hence
* we need to subtract one from the cutoff value as calculated
* following SP800-90B. Thus C = ceil(-log_2(alpha)/H) = 30*osr
* or 60*osr.
*/
  if ((unsigned int)ec->rct_count >= (60 * ec->osr)) {
   ec->rct_count = -1;
   ec->health_failure |= JENT_RCT_FAILURE_PERMANENT;
  } else if ((unsigned int)ec->rct_count >= (30 * ec->osr)) {
   ec->rct_count = -1;
   ec->health_failure |= JENT_RCT_FAILURE;
  }
} else {
  /* Reset RCT */
  ec->rct_count = 0;
}
}

static inline __u64 jent_delta(__u64 prev, __u64 next)
{
#define JENT_UINT64_MAX  (__u64)(~((__u64) 0))
return (prev < next) ? (next - prev) :
          (JENT_UINT64_MAX - prev + 1 + next);
}

/*
* Stuck test by checking the:
* 1st derivative of the jitter measurement (time delta)
* 2nd derivative of the jitter measurement (delta of time deltas)
* 3rd derivative of the jitter measurement (delta of delta of time deltas)
*
* All values must always be non-zero.
*
* @ec [in] Reference to entropy collector
* @current_delta [in] Jitter time delta
*
* @return
* 0 jitter measurement not stuck (good bit)
* 1 jitter measurement stuck (reject bit)
*/
static int jent_stuck(struct rand_data *ec, __u64 current_delta)
{
__u64 delta2 = jent_delta(ec->last_delta, current_delta);
__u64 delta3 = jent_delta(ec->last_delta2, delta2);

ec->last_delta = current_delta;
ec->last_delta2 = delta2;

/*
* Insert the result of the comparison of two back-to-back time
* deltas.
*/
jent_apt_insert(ec, current_delta);

if (!current_delta || !delta2 || !delta3) {
  /* RCT with a stuck bit */
  jent_rct_insert(ec, 1);
  return 1;
}

/* RCT with a non-stuck bit */
jent_rct_insert(ec, 0);

return 0;
}

/*
* Report any health test failures
*
* @ec [in] Reference to entropy collector
*
* @return a bitmask indicating which tests failed
* 0 No health test failure
* 1 RCT failure
* 2 APT failure
* 1<<JENT_PERMANENT_FAILURE_SHIFT RCT permanent failure
* 2<<JENT_PERMANENT_FAILURE_SHIFT APT permanent failure
*/
static unsigned int jent_health_failure(struct rand_data *ec)
{
/* Test is only enabled in FIPS mode */
if (!fips_enabled)
  return 0;

return ec->health_failure;
}

/***************************************************************************
* Noise sources
***************************************************************************/

/*
* Update of the loop count used for the next round of
* an entropy collection.
*
* Input:
* @bits is the number of low bits of the timer to consider
* @min is the number of bits we shift the timer value to the right at
* the end to make sure we have a guaranteed minimum value
*
* @return Newly calculated loop counter
*/
static __u64 jent_loop_shuffle(unsigned int bits, unsigned int min)
{
__u64 time = 0;
__u64 shuffle = 0;
unsigned int i = 0;
unsigned int mask = (1<<bits) - 1;

jent_get_nstime(&time);

/*
* We fold the time value as much as possible to ensure that as many
* bits of the time stamp are included as possible.
*/
for (i = 0; ((DATA_SIZE_BITS + bits - 1) / bits) > i; i++) {
  shuffle ^= time & mask;
  time = time >> bits;
}

/*
* We add a lower boundary value to ensure we have a minimum
* RNG loop count.
*/
return (shuffle + (1<<min));
}

/*
* CPU Jitter noise source -- this is the noise source based on the CPU
*       execution time jitter
*
* This function injects the individual bits of the time value into the
* entropy pool using a hash.
*
* ec [in] entropy collector
* time [in] time stamp to be injected
* stuck [in] Is the time stamp identified as stuck?
*
* Output:
* updated hash context in the entropy collector or error code
*/
static int jent_condition_data(struct rand_data *ec, __u64 time, int stuck)
{
#define SHA3_HASH_LOOP (1<<3)
struct {
  int rct_count;
  unsigned int apt_observations;
  unsigned int apt_count;
  unsigned int apt_base;
} addtl = {
  ec->rct_count,
  ec->apt_observations,
  ec->apt_count,
  ec->apt_base
};

return jent_hash_time(ec->hash_state, time, (u8 *)&addtl, sizeof(addtl),
         SHA3_HASH_LOOP, stuck);
}

/*
* Memory Access noise source -- this is a noise source based on variations in
* memory access times
*
* This function performs memory accesses which will add to the timing
* variations due to an unknown amount of CPU wait states that need to be
* added when accessing memory. The memory size should be larger than the L1
* caches as outlined in the documentation and the associated testing.
*
* The L1 cache has a very high bandwidth, albeit its access rate is  usually
* slower than accessing CPU registers. Therefore, L1 accesses only add minimal
* variations as the CPU has hardly to wait. Starting with L2, significant
* variations are added because L2 typically does not belong to the CPU any more
* and therefore a wider range of CPU wait states is necessary for accesses.
* L3 and real memory accesses have even a wider range of wait states. However,
* to reliably access either L3 or memory, the ec->mem memory must be quite
* large which is usually not desirable.
*
* @ec [in] Reference to the entropy collector with the memory access data -- if
*     the reference to the memory block to be accessed is NULL, this noise
*     source is disabled
* @loop_cnt [in] if a value not equal to 0 is set, use the given value
*   number of loops to perform the LFSR
*/
static void jent_memaccess(struct rand_data *ec, __u64 loop_cnt)
{
unsigned int wrap = 0;
__u64 i = 0;
#define MAX_ACC_LOOP_BIT 7
#define MIN_ACC_LOOP_BIT 0
__u64 acc_loop_cnt =
  jent_loop_shuffle(MAX_ACC_LOOP_BIT, MIN_ACC_LOOP_BIT);

if (NULL == ec || NULL == ec->mem)
  return;
wrap = ec->memblocksize * ec->memblocks;

/*
* testing purposes -- allow test app to set the counter, not
* needed during runtime
*/
if (loop_cnt)
  acc_loop_cnt = loop_cnt;

for (i = 0; i < (ec->memaccessloops + acc_loop_cnt); i++) {
  unsigned char *tmpval = ec->mem + ec->memlocation;
  /*
* memory access: just add 1 to one byte,
* wrap at 255 -- memory access implies read
* from and write to memory location
*/
  *tmpval = (*tmpval + 1) & 0xff;
  /*
* Addition of memblocksize - 1 to pointer
* with wrap around logic to ensure that every
* memory location is hit evenly
*/
  ec->memlocation = ec->memlocation + ec->memblocksize - 1;
  ec->memlocation = ec->memlocation % wrap;
}
}

/***************************************************************************
* Start of entropy processing logic
***************************************************************************/
/*
* This is the heart of the entropy generation: calculate time deltas and
* use the CPU jitter in the time deltas. The jitter is injected into the
* entropy pool.
*
* WARNING: ensure that ->prev_time is primed before using the output
*     of this function! This can be done by calling this function
*     and not using its result.
*
* @ec [in] Reference to entropy collector
*
* @return result of stuck test
*/
static int jent_measure_jitter(struct rand_data *ec, __u64 *ret_current_delta)
{
__u64 time = 0;
__u64 current_delta = 0;
int stuck;

/* Invoke one noise source before time measurement to add variations */
jent_memaccess(ec, 0);

/*
* Get time stamp and calculate time delta to previous
* invocation to measure the timing variations
*/
jent_get_nstime(&time);
current_delta = jent_delta(ec->prev_time, time);
ec->prev_time = time;

/* Check whether we have a stuck measurement. */
stuck = jent_stuck(ec, current_delta);

/* Now call the next noise sources which also injects the data */
if (jent_condition_data(ec, current_delta, stuck))
  stuck = 1;

/* return the raw entropy value */
if (ret_current_delta)
  *ret_current_delta = current_delta;

return stuck;
}

/*
* Generator of one 64 bit random number
* Function fills rand_data->hash_state
*
* @ec [in] Reference to entropy collector
*/
static void jent_gen_entropy(struct rand_data *ec)
{
unsigned int k = 0, safety_factor = 0;

if (fips_enabled)
  safety_factor = JENT_ENTROPY_SAFETY_FACTOR;

/* priming of the ->prev_time value */
jent_measure_jitter(ec, NULL);

while (!jent_health_failure(ec)) {
  /* If a stuck measurement is received, repeat measurement */
  if (jent_measure_jitter(ec, NULL))
   continue;

  /*
* We multiply the loop value with ->osr to obtain the
* oversampling rate requested by the caller
*/
  if (++k >= ((DATA_SIZE_BITS + safety_factor) * ec->osr))
   break;
}
}

/*
* Entry function: Obtain entropy for the caller.
*
* This function invokes the entropy gathering logic as often to generate
* as many bytes as requested by the caller. The entropy gathering logic
* creates 64 bit per invocation.
*
* This function truncates the last 64 bit entropy value output to the exact
* size specified by the caller.
*
* @ec [in] Reference to entropy collector
* @data [in] pointer to buffer for storing random data -- buffer must already
*       exist
* @len [in] size of the buffer, specifying also the requested number of random
*      in bytes
*
* @return 0 when request is fulfilled or an error
*
* The following error codes can occur:
* -1 entropy_collector is NULL or the generation failed
* -2 Intermittent health failure
* -3 Permanent health failure
*/
int jent_read_entropy(struct rand_data *ec, unsigned char *data,
        unsigned int len)
{
unsigned char *p = data;

if (!ec)
  return -1;

while (len > 0) {
  unsigned int tocopy, health_test_result;

  jent_gen_entropy(ec);

  health_test_result = jent_health_failure(ec);
  if (health_test_result > JENT_PERMANENT_FAILURE_SHIFT) {
   /*
* At this point, the Jitter RNG instance is considered
* as a failed instance. There is no rerun of the
* startup test any more, because the caller
* is assumed to not further use this instance.
*/
   return -3;
  } else if (health_test_result) {
   /*
* Perform startup health tests and return permanent
* error if it fails.
*/
   if (jent_entropy_init(0, 0, NULL, ec)) {
    /* Mark the permanent error */
    ec->health_failure &=
     JENT_RCT_FAILURE_PERMANENT |
     JENT_APT_FAILURE_PERMANENT;
    return -3;
   }

   return -2;
  }

  tocopy = min(DATA_SIZE_BITS / 8, len);
  if (jent_read_random_block(ec->hash_state, p, tocopy))
   return -1;

  len -= tocopy;
  p += tocopy;
}

return 0;
}

/***************************************************************************
* Initialization logic
***************************************************************************/

struct rand_data *jent_entropy_collector_alloc(unsigned int osr,
            unsigned int flags,
            void *hash_state)
{
struct rand_data *entropy_collector;

entropy_collector = jent_zalloc(sizeof(struct rand_data));
if (!entropy_collector)
  return NULL;

if (!(flags & JENT_DISABLE_MEMORY_ACCESS)) {
  /* Allocate memory for adding variations based on memory
* access
*/
  entropy_collector->mem = jent_kvzalloc(JENT_MEMORY_SIZE);
  if (!entropy_collector->mem) {
   jent_zfree(entropy_collector);
   return NULL;
  }
  entropy_collector->memblocksize =
   CONFIG_CRYPTO_JITTERENTROPY_MEMORY_BLOCKSIZE;
  entropy_collector->memblocks =
   CONFIG_CRYPTO_JITTERENTROPY_MEMORY_BLOCKS;
  entropy_collector->memaccessloops = JENT_MEMORY_ACCESSLOOPS;
}

/* verify and set the oversampling rate */
if (osr == 0)
  osr = 1; /* H_submitter = 1 / osr */
entropy_collector->osr = osr;
entropy_collector->flags = flags;

entropy_collector->hash_state = hash_state;

/* Initialize the APT */
jent_apt_init(entropy_collector, osr);

/* fill the data pad with non-zero values */
jent_gen_entropy(entropy_collector);

return entropy_collector;
}

void jent_entropy_collector_free(struct rand_data *entropy_collector)
{
jent_kvzfree(entropy_collector->mem, JENT_MEMORY_SIZE);
entropy_collector->mem = NULL;
jent_zfree(entropy_collector);
}

int jent_entropy_init(unsigned int osr, unsigned int flags, void *hash_state,
        struct rand_data *p_ec)
{
/*
* If caller provides an allocated ec, reuse it which implies that the
* health test entropy data is used to further still the available
* entropy pool.
*/
struct rand_data *ec = p_ec;
int i, time_backwards = 0, ret = 0, ec_free = 0;
unsigned int health_test_result;

if (!ec) {
  ec = jent_entropy_collector_alloc(osr, flags, hash_state);
  if (!ec)
   return JENT_EMEM;
  ec_free = 1;
} else {
  /* Reset the APT */
  jent_apt_reset(ec, 0);
  /* Ensure that a new APT base is obtained */
  ec->apt_base_set = 0;
  /* Reset the RCT */
  ec->rct_count = 0;
  /* Reset intermittent, leave permanent health test result */
  ec->health_failure &= (~JENT_RCT_FAILURE);
  ec->health_failure &= (~JENT_APT_FAILURE);
}

/* We could perform statistical tests here, but the problem is
* that we only have a few loop counts to do testing. These
* loop counts may show some slight skew and we produce
* false positives.
*
* Moreover, only old systems show potentially problematic
* jitter entropy that could potentially be caught here. But
* the RNG is intended for hardware that is available or widely
* used, but not old systems that are long out of favor. Thus,
* no statistical tests.
*/

/*
* We could add a check for system capabilities such as clock_getres or
* check for CONFIG_X86_TSC, but it does not make much sense as the
* following sanity checks verify that we have a high-resolution
* timer.
*/
/*
* TESTLOOPCOUNT needs some loops to identify edge systems. 100 is
* definitely too little.
*
* SP800-90B requires at least 1024 initial test cycles.
*/
#define TESTLOOPCOUNT 1024
#define CLEARCACHE 100
for (i = 0; (TESTLOOPCOUNT + CLEARCACHE) > i; i++) {
  __u64 start_time = 0, end_time = 0, delta = 0;

  /* Invoke core entropy collection logic */
  jent_measure_jitter(ec, &delta);
  end_time = ec->prev_time;
  start_time = ec->prev_time - delta;

  /* test whether timer works */
  if (!start_time || !end_time) {
   ret = JENT_ENOTIME;
   goto out;
  }

  /*
* test whether timer is fine grained enough to provide
* delta even when called shortly after each other -- this
* implies that we also have a high resolution timer
*/
  if (!delta || (end_time == start_time)) {
   ret = JENT_ECOARSETIME;
   goto out;
  }

  /*
* up to here we did not modify any variable that will be
* evaluated later, but we already performed some work. Thus we
* already have had an impact on the caches, branch prediction,
* etc. with the goal to clear it to get the worst case
* measurements.
*/
  if (i < CLEARCACHE)
   continue;

  /* test whether we have an increasing timer */
  if (!(end_time > start_time))
   time_backwards++;
}

/*
* we allow up to three times the time running backwards.
* CLOCK_REALTIME is affected by adjtime and NTP operations. Thus,
* if such an operation just happens to interfere with our test, it
* should not fail. The value of 3 should cover the NTP case being
* performed during our test run.
*/
if (time_backwards > 3) {
  ret = JENT_ENOMONOTONIC;
  goto out;
}

/* Did we encounter a health test failure? */
health_test_result = jent_health_failure(ec);
if (health_test_result) {
  ret = (health_test_result & JENT_RCT_FAILURE) ? JENT_ERCT :
        JENT_EHEALTH;
  goto out;
}

out:
if (ec_free)
  jent_entropy_collector_free(ec);

return ret;
}

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Beweissystem der NASA

Beweissystem Isabelle

NIST Cobol Testsuite

Cephes Mathematical Library

Wiener Entwicklungsmethode

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