xref: /openbmc/linux/crypto/jitterentropy.c (revision a06c488d)

/*
 * Non-physical true random number generator based on timing jitter --
 * Jitter RNG standalone code.
 *
 * Copyright Stephan Mueller <smueller@chronox.de>, 2015
 *
 * Design
 * ======
 *
 * See http://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 1.1.0 provided at http://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;
#define NULL    ((void *) 0)

/* The entropy pool */
struct rand_data {
	/* 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. */
	__u64 data;		/* SENSITIVE Actual random number */
	__u64 old_data;		/* SENSITIVE Previous random number */
	__u64 prev_time;	/* SENSITIVE Previous time stamp */
#define DATA_SIZE_BITS ((sizeof(__u64)) * 8)
	__u64 last_delta;	/* SENSITIVE stuck test */
	__s64 last_delta2;	/* SENSITIVE stuck test */
	unsigned int stuck:1;	/* Time measurement stuck */
	unsigned int osr;	/* Oversample rate */
	unsigned int stir:1;		/* Post-processing stirring */
	unsigned int disable_unbias:1;	/* Deactivate Von-Neuman unbias */
#define JENT_MEMORY_BLOCKS 64
#define JENT_MEMORY_BLOCKSIZE 32
#define JENT_MEMORY_ACCESSLOOPS 128
#define JENT_MEMORY_SIZE (JENT_MEMORY_BLOCKS*JENT_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 */
};

/* Flags that can be used to initialize the RNG */
#define JENT_DISABLE_STIR (1<<0) /* Disable stirring the entropy pool */
#define JENT_DISABLE_UNBIAS (1<<1) /* Disable the Von-Neuman Unbiaser */
#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_EMINVARIATION	4 /* Timer variations too small for RNG */
#define JENT_EVARVAR		5 /* Timer does not produce variations of
				   * variations (2nd derivation of time is
				   * zero). */
#define JENT_EMINVARVAR		6 /* Timer variations of variations is tooi
				   * small. */

/***************************************************************************
 * Helper functions
 ***************************************************************************/

void jent_get_nstime(__u64 *out);
__u64 jent_rol64(__u64 word, unsigned int shift);
void *jent_zalloc(unsigned int len);
void jent_zfree(void *ptr);
int jent_fips_enabled(void);
void jent_panic(char *s);
void jent_memcpy(void *dest, const void *src, unsigned int n);

/**
 * 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 __u64 jent_loop_shuffle(struct rand_data *ec,
			       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);
	/*
	 * 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 folds the time into one bit units by iterating
 * through the DATA_SIZE_BITS bit time value as follows: assume our time value
 * is 0xabcd
 * 1st loop, 1st shift generates 0xd000
 * 1st loop, 2nd shift generates 0x000d
 * 2nd loop, 1st shift generates 0xcd00
 * 2nd loop, 2nd shift generates 0x000c
 * 3rd loop, 1st shift generates 0xbcd0
 * 3rd loop, 2nd shift generates 0x000b
 * 4th loop, 1st shift generates 0xabcd
 * 4th loop, 2nd shift generates 0x000a
 * Now, the values at the end of the 2nd shifts are XORed together.
 *
 * The code is deliberately inefficient 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 folded
 * @loop_cnt if a value not equal to 0 is set, use the given value as number of
 *	     loops to perform the folding
 *
 * Output:
 * @folded result of folding operation
 *
 * @return Number of loops the folding operation is performed
 */
static __u64 jent_fold_time(struct rand_data *ec, __u64 time,
			    __u64 *folded, __u64 loop_cnt)
{
	unsigned int i;
	__u64 j = 0;
	__u64 new = 0;
#define MAX_FOLD_LOOP_BIT 4
#define MIN_FOLD_LOOP_BIT 0
	__u64 fold_loop_cnt =
		jent_loop_shuffle(ec, MAX_FOLD_LOOP_BIT, MIN_FOLD_LOOP_BIT);

	/*
	 * testing purposes -- allow test app to set the counter, not
	 * needed during runtime
	 */
	if (loop_cnt)
		fold_loop_cnt = loop_cnt;
	for (j = 0; j < fold_loop_cnt; j++) {
		new = 0;
		for (i = 1; (DATA_SIZE_BITS) >= i; i++) {
			__u64 tmp = time << (DATA_SIZE_BITS - i);

			tmp = tmp >> (DATA_SIZE_BITS - 1);
			new ^= tmp;
		}
	}
	*folded = 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, __u64 loop_cnt)
{
	unsigned char *tmpval = NULL;
	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(ec, MAX_ACC_LOOP_BIT, MIN_ACC_LOOP_BIT);

	if (NULL == ec || NULL == ec->mem)
		return 0;
	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++) {
		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 void jent_stuck(struct rand_data *ec, __u64 current_delta)
{
	__s64 delta2 = ec->last_delta - current_delta;
	__s64 delta3 = delta2 - ec->last_delta2;

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

	if (!current_delta || !delta2 || !delta3)
		ec->stuck = 1;
}

/**
 * This is the heart of the entropy generation: calculate time deltas and
 * use the CPU jitter in the time deltas. The jitter is folded into one
 * bit. You can call this function the "random bit generator" as it
 * produces one random bit per invocation.
 *
 * 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 One random bit
 */
static __u64 jent_measure_jitter(struct rand_data *ec)
{
	__u64 time = 0;
	__u64 data = 0;
	__u64 current_delta = 0;

	/* 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 folds the data */
	jent_fold_time(ec, current_delta, &data, 0);

	/*
	 * Check whether we have a stuck measurement. The enforcement
	 * is performed after the stuck value has been mixed into the
	 * entropy pool.
	 */
	jent_stuck(ec, current_delta);

	return data;
}

/**
 * Von Neuman unbias as explained in RFC 4086 section 4.2. As shown in the
 * documentation of that RNG, the bits from jent_measure_jitter are considered
 * independent which implies that the Von Neuman unbias operation is applicable.
 * A proof of the Von-Neumann unbias operation to remove skews is given in the
 * document "A proposal for: Functionality classes for random number
 * generators", version 2.0 by Werner Schindler, section 5.4.1.
 *
 * Input:
 * @entropy_collector Reference to entropy collector
 *
 * @return One random bit
 */
static __u64 jent_unbiased_bit(struct rand_data *entropy_collector)
{
	do {
		__u64 a = jent_measure_jitter(entropy_collector);
		__u64 b = jent_measure_jitter(entropy_collector);

		if (a == b)
			continue;
		if (1 == a)
			return 1;
		else
			return 0;
	} while (1);
}

/**
 * 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 {
		__u64 u64;
		__u32 u32[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;

	/*
	 * 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.u32[1] = 0x67452301;
	constant.u32[0] = 0xefcdab89;
	mixer.u32[1] = 0x98badcfe;
	mixer.u32[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.u64 ^= constant.u64;
		mixer.u64 = jent_rol64(mixer.u64, 1);
	}
	entropy_collector->data ^= mixer.u64;
}

/**
 * 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) {
		__u64 data = 0;

		if (ec->disable_unbias == 1)
			data = jent_measure_jitter(ec);
		else
			data = jent_unbiased_bit(ec);

		/* enforcement of the jent_stuck test */
		if (ec->stuck) {
			/*
			 * We only mix in the bit considered not appropriate
			 * without the LSFR. The reason is that if we apply
			 * the LSFR and we do not rotate, the 2nd bit with LSFR
			 * will cancel out the first LSFR application on the
			 * bad bit.
			 *
			 * And we do not rotate as we apply the next bit to the
			 * current bit location again.
			 */
			ec->data ^= data;
			ec->stuck = 0;
			continue;
		}

		/*
		 * Fibonacci LSFR with polynom 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 polynom values minus one
		 * due to counting bits from 0 to 63). As the current
		 * position is always the LSB, the polynom only needs
		 * to shift data in from the left without wrap.
		 */
		ec->data ^= data;
		ec->data ^= ((ec->data >> 63) & 1);
		ec->data ^= ((ec->data >> 60) & 1);
		ec->data ^= ((ec->data >> 55) & 1);
		ec->data ^= ((ec->data >> 30) & 1);
		ec->data ^= ((ec->data >> 27) & 1);
		ec->data ^= ((ec->data >> 22) & 1);
		ec->data = jent_rol64(ec->data, 1);

		/*
		 * 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 void jent_fips_test(struct rand_data *ec)
{
	if (!jent_fips_enabled())
		return;

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

	if (ec->data == ec->old_data)
		jent_panic("jitterentropy: Duplicate output detected\n");

	ec->old_data = ec->data;
}

/**
 * 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 0 when request is fulfilled or an error
 *
 * The following error codes can occur:
 *	-1	entropy_collector is NULL
 */
int jent_read_entropy(struct rand_data *ec, unsigned char *data,
		      unsigned int len)
{
	unsigned char *p = data;

	if (!ec)
		return -1;

	while (0 < len) {
		unsigned int tocopy;

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

		len -= tocopy;
		p += tocopy;
	}

	return 0;
}

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

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 (!entropy_collector)
		return NULL;

	if (!(flags & JENT_DISABLE_MEMORY_ACCESS)) {
		/* Allocate memory for adding variations based on memory
		 * access
		 */
		entropy_collector->mem = jent_zalloc(JENT_MEMORY_SIZE);
		if (!entropy_collector->mem) {
			jent_zfree(entropy_collector);
			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;
}

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

int jent_entropy_init(void)
{
	int i;
	__u64 delta_sum = 0;
	__u64 old_delta = 0;
	int time_backwards = 0;
	int count_var = 0;
	int count_mod = 0;

	/* 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++) {
		__u64 time = 0;
		__u64 time2 = 0;
		__u64 folded = 0;
		__u64 delta = 0;
		unsigned int lowdelta = 0;

		jent_get_nstime(&time);
		jent_fold_time(NULL, time, &folded, 1<<MIN_FOLD_LOOP_BIT);
		jent_get_nstime(&time2);

		/* test whether timer works */
		if (!time || !time2)
			return JENT_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 JENT_ECOARSETIME;

		/*
		 * 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;

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

		/*
		 * Avoid modulo of 64 bit integer to allow code to compile
		 * on 32 bit architectures.
		 */
		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 (i) {
			if (delta != old_delta)
				count_var++;
			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 JENT_ENOMONOTONIC;
	/* Error if the time variances are always identical */
	if (!delta_sum)
		return JENT_EVARVAR;

	/*
	 * 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 JENT_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 JENT_ECOARSETIME;

	return 0;
}