/*

   * Non-physical true random number generator based on timing jitter --
   * Jitter RNG standalone code.

   *

   * Copyright Stephan Mueller <smueller@chronox.de>, 2015 - 2020

   *
   * 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 2.2.0 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;
  #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 osr;	/* Oversample rate */

  #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 */

  
  	/* Repetition Count Test */
  	int rct_count;			/* Number of stuck values */
  
  	/* Adaptive Proportion Test for a significance level of 2^-30 */
  #define JENT_APT_CUTOFF		325	/* Taken from SP800-90B sec 4.4.2 */
  #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 apt_base_set:1;	/* APT base reference set? */
  
  	unsigned int health_failure:1;	/* Permanent health failure */

  };
  
  /* 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 */
  
  #include "jitterentropy.h"

  
  /***************************************************************************

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

   ***************************************************************************/

  /**
   * 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++;
  
  		if (ec->apt_count >= JENT_APT_CUTOFF)
  			ec->health_failure = 1;
  	}
  
  	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 we have a count less than zero, a previous RCT round identified
  	 * a failure. We will not overwrite it.
  	 */
  	if (ec->rct_count < 0)
  		return;
  
  	if (stuck) {
  		ec->rct_count++;
  
  		/*
  		 * The cutoff value is based on the following consideration:
  		 * alpha = 2^-30 as recommended in FIPS 140-2 IG 9.8.
  		 * 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 64 * OSR deltas for inserting them into
  		 * the entropy pool which should then have (close to) 64 bits
  		 * of entropy.
  		 *
  		 * 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.
  		 */
  		if ((unsigned int)ec->rct_count >= (31 * ec->osr)) {
  			ec->rct_count = -1;
  			ec->health_failure = 1;
  		}
  	} else {
  		ec->rct_count = 0;
  	}
  }
  
  /**
   * Is there an RCT health test failure?
   *
   * @ec [in] Reference to entropy collector
   *
   * @return
   * 	0 No health test failure
   * 	1 Permanent health test failure
   */
  static int jent_rct_failure(struct rand_data *ec)
  {
  	if (ec->rct_count < 0)
  		return 1;
  	return 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);
  	unsigned int delta_masked = current_delta & JENT_APT_WORD_MASK;
  
  	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, delta_masked);
  
  	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
   * 	0 No health test failure
   * 	1 Permanent health test failure
   */
  static int jent_health_failure(struct rand_data *ec)
  {
  	/* Test is only enabled in FIPS mode */
  	if (!jent_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:
   * @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 - 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 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.

   *

   * @ec [in] entropy collector struct
   * @time [in] time stamp to be injected
   * @loop_cnt [in] if a value not equal to 0 is set, use the given value as
   *		  number of loops to perform the folding
   * @stuck [in] Is the time stamp identified as stuck?

   *
   * Output:

   * updated ec->data

   *
   * @return Number of loops the folding operation is performed
   */

  static void jent_lfsr_time(struct rand_data *ec, __u64 time, __u64 loop_cnt,
  			   int stuck)

  {
  	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 = ec->data;

  		for (i = 1; (DATA_SIZE_BITS) >= i; i++) {
  			__u64 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.
  			*/
  			tmp ^= ((new >> 63) & 1);
  			tmp ^= ((new >> 60) & 1);
  			tmp ^= ((new >> 55) & 1);
  			tmp ^= ((new >> 30) & 1);
  			tmp ^= ((new >> 27) & 1);
  			tmp ^= ((new >> 22) & 1);
  			new <<= 1;

  			new ^= tmp;
  		}
  	}

  	/*
  	 * If the time stamp is stuck, do not finally insert the value into
  	 * the entropy pool. Although this operation should not do any harm
  	 * even when the time stamp has no entropy, SP800-90B requires that
  	 * any conditioning operation (SP800-90B considers the LFSR to be a
  	 * conditioning operation) to have an identical amount of input
  	 * data according to section 3.1.5.
  	 */
  	if (!stuck)
  		ec->data = new;

  }
  
  /**
   * 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(ec, 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 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 */

  	jent_lfsr_time(ec, current_delta, 0, stuck);

  	return stuck;

  }
  
  /**
   * Generator of one 64 bit random number
   * Function fills rand_data->data
   *

   * @ec [in] 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;
  	}

  }
  
  /**

   * 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

   *	-2	RCT failed
   *	-3	APT test failed

   */

  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);

  
  		if (jent_health_failure(ec)) {
  			int ret;
  
  			if (jent_rct_failure(ec))
  				ret = -2;
  			else
  				ret = -3;
  
  			/*
  			 * Re-initialize the noise source
  			 *
  			 * If the health test fails, the Jitter RNG remains
  			 * in failure state and will return a health failure
  			 * during next invocation.
  			 */
  			if (jent_entropy_init())
  				return ret;
  
  			/* Set APT to initial state */
  			jent_apt_reset(ec, 0);
  			ec->apt_base_set = 0;
  
  			/* Set RCT to initial state */
  			ec->rct_count = 0;
  
  			/* Re-enable Jitter RNG */
  			ec->health_failure = 0;
  
  			/*
  			 * Return the health test failure status to the
  			 * caller as the generated value is not appropriate.
  			 */
  			return ret;
  		}

  		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;

  	/* 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);

  }

  int jent_entropy_init(void)

  {
  	int i;
  	__u64 delta_sum = 0;
  	__u64 old_delta = 0;

  	unsigned int nonstuck = 0;

  	int time_backwards = 0;

  	int count_mod = 0;

  	int count_stuck = 0;
  	struct rand_data ec = { 0 };

  	/* Required for RCT */
  	ec.osr = 1;

  	/* 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 time = 0;
  		__u64 time2 = 0;

  		__u64 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, 0);

  		jent_get_nstime(&time2);
  
  		/* test whether timer works */
  		if (!time || !time2)
  			return JENT_ENOTIME;

  		delta = jent_delta(time, time2);

  		/*
  		 * 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;

  		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++;

  		else {
  			nonstuck++;
  
  			/*
  			 * Ensure that the APT succeeded.
  			 *
  			 * With the check below that count_stuck must be less
  			 * than 10% of the overall generated raw entropy values
  			 * it is guaranteed that the APT is invoked at
  			 * floor((TESTLOOPCOUNT * 0.9) / 64) == 14 times.
  			 */
  			if ((nonstuck % JENT_APT_WINDOW_SIZE) == 0) {
  				jent_apt_reset(&ec,
  					       delta & JENT_APT_WORD_MASK);
  				if (jent_health_failure(&ec))
  					return JENT_EHEALTH;
  			}
  		}
  
  		/* Validate RCT */
  		if (jent_rct_failure(&ec))
  			return JENT_ERCT;

  		/* 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 JENT_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 JENT_EVARVAR;

  
  	/*
  	 * 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;

  	/*
  	 * If we have more than 90% stuck results, then this Jitter RNG is
  	 * likely to not work well.
  	 */
  	if ((TESTLOOPCOUNT/10 * 9) < count_stuck)
  		return JENT_ESTUCK;

  	return 0;
  }