zircon/third_party/lib/jitterentropy/jitterentropy-base.c

/*
 * Non-physical true random number generator based on timing jitter.
 *
 * Copyright Stephan Mueller <smueller@chronox.de>, 2014 - 2017
 *
 * Design
 * ======
 *
 * See documentation in doc/ folder.
 *
 * Interface
 * =========
 *
 * See documentation in doc/ folder.
 *
 * 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.
 *
 * Modifications by the Fuchsia Authors, 2017
 * =======
 *
 * - Add #include lines for stdlib.h, string.h, and internal.h.
 * - Change #include line for Zircon file conventions.
 * - Remove CONFIG_CRYPTO_CPU_JITTERENTROPY_STAT flag.
 * - Add jent_entropy_collector_init definition.
 * - Remove '#pragma GCC optimize ("O0")' (not recognized by clang)
 * - Replace 'min' parameter by 'lfsr_loops_override' and 'mem_loops_override'
 *   in jent_lfsr_var_stat, and moved comment for jent_lfsr_var_stat to
 *   jitterentropy.h.
 * - Add jent_have_clock check to jent_entropy_init.
 */

#undef _FORTIFY_SOURCE

#include <assert.h>
#include <lib/jitterentropy/jitterentropy.h>
#include "internal.h"
#include <stdlib.h>
#include <string.h>

 /* only check optimization in a compilation for real work */
 #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-base.c."
 #endif

#define MAJVERSION 2 /* API / ABI incompatible changes, functional changes that
		      * require consumer to be updated (as long as this number
		      * is zero, the API is not considered stable and can
		      * change without a bump of the major version) */
#define MINVERSION 1 /* API compatible, ABI may change, functional
		      * enhancements only, consumer can be left unchanged if
		      * enhancements are not considered */
#define PATCHLEVEL 0 /* API / ABI compatible, no functional changes, no
		      * enhancements, bug fixes only */

/**
 * jent_version() - Return machine-usable version number of jent library
 *
 * The function returns a version number that is monotonic increasing
 * for newer versions. The version numbers are multiples of 100. For example,
 * version 1.2.3 is converted to 1020300 -- the last two digits are reserved
 * for future use.
 *
 * The result of this function can be used in comparing the version number
 * in a calling program if version-specific calls need to be make.
 *
 * Return: Version number of kcapi library
 */
JENT_PRIVATE_STATIC
unsigned int jent_version(void)
{
	unsigned int version = 0;

	version =  MAJVERSION * 1000000;
	version += MINVERSION * 10000;
	version += PATCHLEVEL * 100;

	return version;
}

/**
 * Update of the loop count used for the next round of
 * an entropy collection.
 *
 * Input:
 * @ec entropy collector struct -- may be NULL
 * @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 uint64_t jent_loop_shuffle(struct rand_data *ec,
				  unsigned int bits, unsigned int min)
{
	uint64_t time = 0;
	uint64_t shuffle = 0;
	unsigned int i = 0;
	unsigned int mask = (1<<bits) - 1;

	jent_get_nstime(&time);
	/*
	 * Mix the current state of the random number into the shuffle
	 * calculation to balance that shuffle a bit more.
	 */
	if (ec)
		time ^= ec->data;
	/*
	 * 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) > 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));
}

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

/**
 * 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 an LFSR.
 *
 * The code is deliberately inefficient with respect to the bit shifting
 * and shall stay that way. This function is the root cause why the code
 * shall be compiled without optimization. This function not only acts as
 * folding operation, but this function's execution is used to measure
 * the CPU execution time jitter. Any change to the loop in this function
 * implies that careful retesting must be done.
 *
 * Input:
 * @ec entropy collector struct -- may be NULL
 * @time time stamp to be injected
 * @loop_cnt if a value not equal to 0 is set, use the given value as number of
 *	     loops to perform the folding
 *
 * Output:
 * updated ec->data
 *
 * @return Number of loops the folding operation is performed
 */
static uint64_t jent_lfsr_time(struct rand_data *ec, uint64_t time,
			       uint64_t loop_cnt)
{
	unsigned int i;
	uint64_t j = 0;
	uint64_t new = 0;
#define MAX_FOLD_LOOP_BIT 4
#define MIN_FOLD_LOOP_BIT 0

	// loop_cnt is normally passed as 0, where a loop_cnt is decided by a function
	// based on current time. This may trick the entropy estimate program to
	// overestimate entropy. So it is not used in Fuchsia. Instead loop_cnt
	// is set by caller.
	uint64_t fold_loop_cnt = loop_cnt;
	if (fold_loop_cnt == 0) {
		fold_loop_cnt = jent_loop_shuffle(ec, MAX_FOLD_LOOP_BIT, MIN_FOLD_LOOP_BIT);
	}

	for (j = 0; j < fold_loop_cnt; j++) {
		new = ec->data;
		for (i = 1; (DATA_SIZE_BITS) >= i; i++) {
			uint64_t tmp = time << (DATA_SIZE_BITS - i);

			tmp = tmp >> (DATA_SIZE_BITS - 1);

			/*
			* Fibonacci LSFR with polynomial of
			*  x^64 + x^61 + x^56 + x^31 + x^28 + x^23 + 1 which is
			*  primitive according to
			*   http://poincare.matf.bg.ac.rs/~ezivkovm/publications/primpol1.pdf
			* (the shift values are the polynomial values minus one
			* due to counting bits from 0 to 63). As the current
			* position is always the LSB, the polynomial only needs
			* to shift data in from the left without wrap.
			*/
			new ^= tmp;
			new ^= ((new >> 63) & 1);
			new ^= ((new >> 60) & 1);
			new ^= ((new >> 55) & 1);
			new ^= ((new >> 30) & 1);
			new ^= ((new >> 27) & 1);
			new ^= ((new >> 22) & 1);
			new = rol64(new, 1);
		}
	}
	ec->data = new;

	return fold_loop_cnt;
}

/**
 * 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.
 *
 * Input:
 * @ec 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 if a value not equal to 0 is set, use the given value as number of
 *	     loops to perform the folding
 *
 * @return Number of memory access operations
 */
static unsigned int jent_memaccess(struct rand_data *ec, uint64_t loop_cnt)
{
	unsigned int wrap = 0;
	uint64_t i = 0;
#define MAX_ACC_LOOP_BIT 7
#define MIN_ACC_LOOP_BIT 0

	// loop_cnt is normally passed as 0, where a loop_cnt is decided by a function
	// based on current time. This may trick the entropy estimate program to
	// overestimate entropy. So it is not used in Fuchsia. Instead loop_cnt
	// is set by caller.
	uint64_t acc_loop_cnt = loop_cnt;
	if (acc_loop_cnt == 0) {
		acc_loop_cnt = jent_loop_shuffle(ec, MAX_ACC_LOOP_BIT, MIN_ACC_LOOP_BIT);
	}

	if (NULL == ec || NULL == ec->mem)
		return 0;
	wrap = ec->memblocksize * ec->memblocks;

	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;
	}
	return i;
}

/***************************************************************************
 * Start of entropy processing logic
 ***************************************************************************/

/**
 * Stuck test by checking the:
 * 	1st derivation of the jitter measurement (time delta)
 * 	2nd derivation of the jitter measurement (delta of time deltas)
 * 	3rd derivation of the jitter measurement (delta of delta of time deltas)
 *
 * All values must always be non-zero.
 *
 * Input:
 * @ec Reference to entropy collector
 * @current_delta 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, uint64_t current_delta)
{
	int64_t delta2 = ec->last_delta - current_delta;
	int64_t delta3 = delta2 - ec->last_delta2;

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

	if (!current_delta || !delta2 || !delta3)
		return 1;

	return 0;
}

/**
 * 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.
 *
 * Input:
 * @entropy_collector Reference to entropy collector
 *
 * @return: result of stuck test
 */
static int jent_measure_jitter(struct rand_data *ec)
{
	uint64_t time = 0;
	uint64_t 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 = time - ec->prev_time;
	ec->prev_time = time;

	/* Now call the next noise sources which also injects the data */
	jent_lfsr_time(ec, current_delta, 0);

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

	/*
	 * Rotate the data buffer by a prime number (any odd number would
	 * do) to ensure that every bit position of the input time stamp
	 * has an even chance of being merged with a bit position in the
	 * entropy pool. We do not use one here as the adjacent bits in
	 * successive time deltas may have some form of dependency. The
	 * chosen value of 7 implies that the low 7 bits of the next
	 * time delta value is concatenated with the current time delta.
	 */
	if (!stuck)
		ec->data = rol64(ec->data, 7);

	return stuck;
}

/**
 * Shuffle the pool a bit by mixing some value with a bijective function (XOR)
 * into the pool.
 *
 * The function generates a mixer value that depends on the bits set and the
 * location of the set bits in the random number generated by the entropy
 * source. Therefore, based on the generated random number, this mixer value
 * can have 2**64 different values. That mixer value is initialized with the
 * first two SHA-1 constants. After obtaining the mixer value, it is XORed into
 * the random number.
 *
 * The mixer value is not assumed to contain any entropy. But due to the XOR
 * operation, it can also not destroy any entropy present in the entropy pool.
 *
 * Input:
 * @entropy_collector Reference to entropy collector
 */
static void jent_stir_pool(struct rand_data *entropy_collector)
{
	/*
	 * to shut up GCC on 32 bit, we have to initialize the 64 variable
	 * with two 32 bit variables
	 */
	union c {
		uint64_t uint64;
		uint32_t uint32[2];
	};
	/*
	 * This constant is derived from the first two 32 bit initialization
	 * vectors of SHA-1 as defined in FIPS 180-4 section 5.3.1
	 */
	union c constant;
	/*
	 * The start value of the mixer variable is derived from the third
	 * and fourth 32 bit initialization vector of SHA-1 as defined in
	 * FIPS 180-4 section 5.3.1
	 */
	union c mixer;
	unsigned int i = 0;

	/* Ensure that the function implements a constant time operation. */
	union c throw_away;

	/*
	 * Store the SHA-1 constants in reverse order to make up the 64 bit
	 * value -- this applies to a little endian system, on a big endian
	 * system, it reverses as expected. But this really does not matter
	 * as we do not rely on the specific numbers. We just pick the SHA-1
	 * constants as they have a good mix of bit set and unset.
	 */
	constant.uint32[1] = 0x67452301;
	constant.uint32[0] = 0xefcdab89;
	mixer.uint32[1] = 0x98badcfe;
	mixer.uint32[0] = 0x10325476;

	for (i = 0; i < DATA_SIZE_BITS; i++) {
		/*
		 * get the i-th bit of the input random number and only XOR
		 * the constant into the mixer value when that bit is set
		 */
		if ((entropy_collector->data >> i) & 1)
			mixer.uint64 ^= constant.uint64;
		else
			throw_away.uint64 ^= constant.uint64;
		mixer.uint64 = rol64(mixer.uint64, 1);
	}
	entropy_collector->data ^= mixer.uint64;
}

/**
 * Generator of one 64 bit random number
 * Function fills rand_data->data
 *
 * Input:
 * @ec Reference to entropy collector
 */
static void jent_gen_entropy(struct rand_data *ec)
{
	unsigned int k = 0;

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

	while (1) {
		/* If a stuck measurement is received, repeat measurement */
		if (jent_measure_jitter(ec))
			continue;

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

/**
 * The continuous test required by FIPS 140-2 -- the function automatically
 * primes the test if needed.
 *
 * Return:
 * 0 if FIPS test passed
 * < 0 if FIPS test failed
 */
static int jent_fips_test(struct rand_data *ec)
{
	if (ec->fips_enabled == -1)
		return 0;

	if (ec->fips_enabled == 0) {
		if (!jent_fips_enabled()) {
			ec->fips_enabled = -1;
			return 0;
		} else
			ec->fips_enabled = 1;
	}

	/* prime the FIPS test */
	if (!ec->old_data) {
		ec->old_data = ec->data;
		jent_gen_entropy(ec);
	}

	if (ec->data == ec->old_data)
		return -1;

	ec->old_data = ec->data;

	return 0;
}

/**
 * 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.
 *
 * Input:
 * @ec Reference to entropy collector
 * @data pointer to buffer for storing random data -- buffer must already
 *        exist
 * @len size of the buffer, specifying also the requested number of random
 *       in bytes
 *
 * @return number of bytes returned when request is fulfilled or an error
 *
 * The following error codes can occur:
 *	-1	entropy_collector is NULL
 *	-2	FIPS test failed
 */
JENT_PRIVATE_STATIC
ssize_t jent_read_entropy(struct rand_data *ec, char *data, size_t len)
{
	char *p = data;
	size_t orig_len = len;

	if (NULL == ec)
		return -1;

	while (0 < len) {
		size_t tocopy;

		jent_gen_entropy(ec);
		if (jent_fips_test(ec))
			return -2;

		if ((DATA_SIZE_BITS / 8) < len)
			tocopy = (DATA_SIZE_BITS / 8);
		else
			tocopy = len;
		memcpy(p, &ec->data, tocopy);

		len -= tocopy;
		p += tocopy;
	}

	/*
	 * To be on the safe side, we generate one more round of entropy
	 * which we do not give out to the caller. That round shall ensure
	 * that in case the calling application crashes, memory dumps, pages
	 * out, or due to the CPU Jitter RNG lingering in memory for long
	 * time without being moved and an attacker cracks the application,
	 * all he reads in the entropy pool is a value that is NEVER EVER
	 * being used for anything. Thus, he does NOT see the previous value
	 * that was returned to the caller for cryptographic purposes.
	 */
	/*
	 * If we use secured memory, do not use that precaution as the secure
	 * memory protects the entropy pool. Moreover, note that using this
	 * call reduces the speed of the RNG by up to half
	 */
#ifndef CONFIG_CRYPTO_CPU_JITTERENTROPY_SECURE_MEMORY
	jent_gen_entropy(ec);
#endif
	return orig_len;
}

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

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

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

	if (!(flags & JENT_DISABLE_MEMORY_ACCESS)) {
		/* Allocate memory for adding variations based on memory
		 * access
		 */
		entropy_collector->mem = 
			(unsigned char *)jent_zalloc(JENT_MEMORY_SIZE);
		if (NULL == entropy_collector->mem) {
			jent_zfree(entropy_collector, sizeof(struct rand_data));
			return NULL;
		}
		entropy_collector->memblocksize = JENT_MEMORY_BLOCKSIZE;
		entropy_collector->memblocks = JENT_MEMORY_BLOCKS;
		entropy_collector->memaccessloops = JENT_MEMORY_ACCESSLOOPS;
	}

	/* verify and set the oversampling rate */
	if (0 == osr)
		osr = 1; /* minimum sampling rate is 1 */
	entropy_collector->osr = osr;

	entropy_collector->stir = 1;
	if (flags & JENT_DISABLE_STIR)
		entropy_collector->stir = 0;
	if (flags & JENT_DISABLE_UNBIAS)
		entropy_collector->disable_unbias = 1;

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

	return entropy_collector;
}

JENT_PRIVATE_STATIC
void jent_entropy_collector_free(struct rand_data *entropy_collector)
{
	if (NULL != entropy_collector) {
		if (NULL != entropy_collector->mem) {
			jent_zfree(entropy_collector->mem, JENT_MEMORY_SIZE);
			entropy_collector->mem = NULL;
		}
		jent_zfree(entropy_collector, sizeof(struct rand_data));
	}
}

JENT_PRIVATE_STATIC
int jent_entropy_init(void)
{
	int i;
	uint64_t delta_sum = 0;
	uint64_t old_delta = 0;
	int time_backwards = 0;
	int count_mod = 0;
	int count_stuck = 0;
	struct rand_data ec;

	if (!jent_have_clock()) {
		return ENOTIME;
	}

	/* 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.
	 */
#define TESTLOOPCOUNT 300
#define CLEARCACHE 100
	for (i = 0; (TESTLOOPCOUNT + CLEARCACHE) > i; i++) {
		uint64_t time = 0;
		uint64_t time2 = 0;
		uint64_t delta = 0;
		unsigned int lowdelta = 0;
		int stuck;

		/* Invoke core entropy collection logic */
		jent_get_nstime(&time);
		ec.prev_time = time;
		jent_lfsr_time(&ec, time, 0);
		jent_get_nstime(&time2);

		/* test whether timer works */
		if (!time || !time2)
			return ENOTIME;
		delta = time2 - time;
		/*
		 * 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)
			return ECOARSETIME;

		stuck = jent_stuck(&ec, delta);

		/*
		 * 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 (CLEARCACHE > i)
			continue;

		if (stuck)
			count_stuck++;

		/* test whether we have an increasing timer */
		if (!(time2 > time))
			time_backwards++;

		/* use 32 bit value to ensure compilation on 32 bit arches */
		lowdelta = time2 - time;
		if (!(lowdelta % 100))
			count_mod++;

		/*
		 * 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 > old_delta)
			delta_sum += (delta - old_delta);
		else
			delta_sum += (old_delta - delta);
		old_delta = delta;
	}

	/*
	 * 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 (3 < time_backwards)
		return ENOMONOTONIC;

	/*
	 * 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) <= 1)
		return EMINVARVAR;

	/*
	 * Ensure that we have variations in the time stamp below 10 for at least
	 * 10% of all checks -- on some platforms, the counter increments in
	 * multiples of 100, but not always
	 */
	if ((TESTLOOPCOUNT/10 * 9) < count_mod)
		return ECOARSETIME;

	/*
	 * If we have more than 90% stuck results, then this Jitter RNG is
	 * likely to not work well.
	 */
	if (JENT_STUCK_INIT_THRES(TESTLOOPCOUNT) < count_stuck)
		return ESTUCK;

	return 0;
}

/***************************************************************************
 * Statistical test logic not compiled for regular operation
 ***************************************************************************/

JENT_PRIVATE_STATIC
uint64_t jent_lfsr_var_stat(struct rand_data *ec,
                            unsigned int lfsr_loops_override,
                            unsigned int mem_loops_override)
{
	uint64_t time = 0;
	uint64_t time2 = 0;

	jent_get_nstime(&time);
	jent_memaccess(ec, mem_loops_override);
	jent_lfsr_time(ec, time, lfsr_loops_override);
	jent_get_nstime(&time2);
	return ((time2 - time));
}

/***************************************************************************
 * Zircon interface
 ***************************************************************************/

void jent_entropy_collector_init(
        struct rand_data* ec, uint8_t* mem, size_t mem_size,
        unsigned int mem_block_size, unsigned int mem_block_count,
        unsigned int mem_loops, bool stir) {
    DEBUG_ASSERT(((size_t)mem_block_size) * mem_block_count <= mem_size);
    memset(ec, 0, sizeof(*ec));
    /* Oversample rate. The jitterentropy man page (not included with Zircon)
     * suggests a value of 1. Higher values cause jitterentropy to discount its
     * entropy estimates by a factor of osr, so that more random bytes are
     * collected than would be with osr == 1. */
    ec->osr = 1;
    /* For now, we don't enable the FIPS 140-2 test mode built into
     * jitterentropy. Zircon should handle entropy source health tests itself,
     * to ensure uniform testing of all entropy sources. */
    ec->fips_enabled = 0;
    ec->stir = stir;
    /* von Neumann unbiasing is never performed, and the disable_unbias flag is
     * never even checked. To avoid confusion, always set the flag to true. */
    ec->disable_unbias = true;
    ec->mem = mem;
    ec->memlocation = 0;
    ec->memblocks = mem_block_count;
    ec->memblocksize = mem_block_size;
    ec->memaccessloops = mem_loops;
}