2 * Non-physical true random number generator based on timing jitter --
3 * Jitter RNG standalone code.
5 * Copyright Stephan Mueller <smueller@chronox.de>, 2015 - 2020
10 * See https://www.chronox.de/jent.html
15 * Redistribution and use in source and binary forms, with or without
16 * modification, are permitted provided that the following conditions
18 * 1. Redistributions of source code must retain the above copyright
19 * notice, and the entire permission notice in its entirety,
20 * including the disclaimer of warranties.
21 * 2. Redistributions in binary form must reproduce the above copyright
22 * notice, this list of conditions and the following disclaimer in the
23 * documentation and/or other materials provided with the distribution.
24 * 3. The name of the author may not be used to endorse or promote
25 * products derived from this software without specific prior
28 * ALTERNATIVELY, this product may be distributed under the terms of
29 * the GNU General Public License, in which case the provisions of the GPL2 are
30 * required INSTEAD OF the above restrictions. (This clause is
31 * necessary due to a potential bad interaction between the GPL and
32 * the restrictions contained in a BSD-style copyright.)
34 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED
35 * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
36 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, ALL OF
37 * WHICH ARE HEREBY DISCLAIMED. IN NO EVENT SHALL THE AUTHOR BE
38 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
39 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT
40 * OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
41 * BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
42 * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
43 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE
44 * USE OF THIS SOFTWARE, EVEN IF NOT ADVISED OF THE POSSIBILITY OF SUCH
49 * This Jitterentropy RNG is based on the jitterentropy library
50 * version 2.2.0 provided at https://www.chronox.de/jent.html
54 #error "The CPU Jitter random number generator must not be compiled with optimizations. See documentation. Use the compiler switch -O0 for compiling jitterentropy.c."
57 typedef unsigned long long __u64;
58 typedef long long __s64;
59 typedef unsigned int __u32;
60 #define NULL ((void *) 0)
62 /* The entropy pool */
64 /* all data values that are vital to maintain the security
65 * of the RNG are marked as SENSITIVE. A user must not
66 * access that information while the RNG executes its loops to
67 * calculate the next random value. */
68 __u64 data; /* SENSITIVE Actual random number */
69 __u64 old_data; /* SENSITIVE Previous random number */
70 __u64 prev_time; /* SENSITIVE Previous time stamp */
71 #define DATA_SIZE_BITS ((sizeof(__u64)) * 8)
72 __u64 last_delta; /* SENSITIVE stuck test */
73 __s64 last_delta2; /* SENSITIVE stuck test */
74 unsigned int osr; /* Oversample rate */
75 #define JENT_MEMORY_BLOCKS 64
76 #define JENT_MEMORY_BLOCKSIZE 32
77 #define JENT_MEMORY_ACCESSLOOPS 128
78 #define JENT_MEMORY_SIZE (JENT_MEMORY_BLOCKS*JENT_MEMORY_BLOCKSIZE)
79 unsigned char *mem; /* Memory access location with size of
80 * memblocks * memblocksize */
81 unsigned int memlocation; /* Pointer to byte in *mem */
82 unsigned int memblocks; /* Number of memory blocks in *mem */
83 unsigned int memblocksize; /* Size of one memory block in bytes */
84 unsigned int memaccessloops; /* Number of memory accesses per random
87 /* Repetition Count Test */
88 unsigned int rct_count; /* Number of stuck values */
90 /* Intermittent health test failure threshold of 2^-30 */
91 #define JENT_RCT_CUTOFF 30 /* Taken from SP800-90B sec 4.4.1 */
92 #define JENT_APT_CUTOFF 325 /* Taken from SP800-90B sec 4.4.2 */
93 /* Permanent health test failure threshold of 2^-60 */
94 #define JENT_RCT_CUTOFF_PERMANENT 60
95 #define JENT_APT_CUTOFF_PERMANENT 355
96 #define JENT_APT_WINDOW_SIZE 512 /* Data window size */
97 /* LSB of time stamp to process */
98 #define JENT_APT_LSB 16
99 #define JENT_APT_WORD_MASK (JENT_APT_LSB - 1)
100 unsigned int apt_observations; /* Number of collected observations */
101 unsigned int apt_count; /* APT counter */
102 unsigned int apt_base; /* APT base reference */
103 unsigned int apt_base_set:1; /* APT base reference set? */
106 /* Flags that can be used to initialize the RNG */
107 #define JENT_DISABLE_MEMORY_ACCESS (1<<2) /* Disable memory access for more
108 * entropy, saves MEMORY_SIZE RAM for
109 * entropy collector */
111 /* -- error codes for init function -- */
112 #define JENT_ENOTIME 1 /* Timer service not available */
113 #define JENT_ECOARSETIME 2 /* Timer too coarse for RNG */
114 #define JENT_ENOMONOTONIC 3 /* Timer is not monotonic increasing */
115 #define JENT_EVARVAR 5 /* Timer does not produce variations of
116 * variations (2nd derivation of time is
118 #define JENT_ESTUCK 8 /* Too many stuck results during init. */
119 #define JENT_EHEALTH 9 /* Health test failed during initialization */
120 #define JENT_ERCT 10 /* RCT failed during initialization */
123 * The output n bits can receive more than n bits of min entropy, of course,
124 * but the fixed output of the conditioning function can only asymptotically
125 * approach the output size bits of min entropy, not attain that bound. Random
126 * maps will tend to have output collisions, which reduces the creditable
127 * output entropy (that is what SP 800-90B Section 3.1.5.1.2 attempts to bound).
129 * The value "64" is justified in Appendix A.4 of the current 90C draft,
130 * and aligns with NIST's in "epsilon" definition in this document, which is
131 * that a string can be considered "full entropy" if you can bound the min
132 * entropy in each bit of output to at least 1-epsilon, where epsilon is
133 * required to be <= 2^(-32).
135 #define JENT_ENTROPY_SAFETY_FACTOR 64
137 #include <linux/fips.h>
138 #include "jitterentropy.h"
140 /***************************************************************************
141 * Adaptive Proportion Test
143 * This test complies with SP800-90B section 4.4.2.
144 ***************************************************************************/
147 * Reset the APT counter
149 * @ec [in] Reference to entropy collector
151 static void jent_apt_reset(struct rand_data *ec, unsigned int delta_masked)
153 /* Reset APT counter */
155 ec->apt_base = delta_masked;
156 ec->apt_observations = 0;
160 * Insert a new entropy event into APT
162 * @ec [in] Reference to entropy collector
163 * @delta_masked [in] Masked time delta to process
165 static void jent_apt_insert(struct rand_data *ec, unsigned int delta_masked)
167 /* Initialize the base reference */
168 if (!ec->apt_base_set) {
169 ec->apt_base = delta_masked;
170 ec->apt_base_set = 1;
174 if (delta_masked == ec->apt_base)
177 ec->apt_observations++;
179 if (ec->apt_observations >= JENT_APT_WINDOW_SIZE)
180 jent_apt_reset(ec, delta_masked);
183 /* APT health test failure detection */
184 static int jent_apt_permanent_failure(struct rand_data *ec)
186 return (ec->apt_count >= JENT_APT_CUTOFF_PERMANENT) ? 1 : 0;
189 static int jent_apt_failure(struct rand_data *ec)
191 return (ec->apt_count >= JENT_APT_CUTOFF) ? 1 : 0;
194 /***************************************************************************
195 * Stuck Test and its use as Repetition Count Test
197 * The Jitter RNG uses an enhanced version of the Repetition Count Test
198 * (RCT) specified in SP800-90B section 4.4.1. Instead of counting identical
199 * back-to-back values, the input to the RCT is the counting of the stuck
200 * values during the generation of one Jitter RNG output block.
202 * The RCT is applied with an alpha of 2^{-30} compliant to FIPS 140-2 IG 9.8.
204 * During the counting operation, the Jitter RNG always calculates the RCT
205 * cut-off value of C. If that value exceeds the allowed cut-off value,
206 * the Jitter RNG output block will be calculated completely but discarded at
207 * the end. The caller of the Jitter RNG is informed with an error code.
208 ***************************************************************************/
211 * Repetition Count Test as defined in SP800-90B section 4.4.1
213 * @ec [in] Reference to entropy collector
214 * @stuck [in] Indicator whether the value is stuck
216 static void jent_rct_insert(struct rand_data *ec, int stuck)
226 static inline __u64 jent_delta(__u64 prev, __u64 next)
228 #define JENT_UINT64_MAX (__u64)(~((__u64) 0))
229 return (prev < next) ? (next - prev) :
230 (JENT_UINT64_MAX - prev + 1 + next);
234 * Stuck test by checking the:
235 * 1st derivative of the jitter measurement (time delta)
236 * 2nd derivative of the jitter measurement (delta of time deltas)
237 * 3rd derivative of the jitter measurement (delta of delta of time deltas)
239 * All values must always be non-zero.
241 * @ec [in] Reference to entropy collector
242 * @current_delta [in] Jitter time delta
245 * 0 jitter measurement not stuck (good bit)
246 * 1 jitter measurement stuck (reject bit)
248 static int jent_stuck(struct rand_data *ec, __u64 current_delta)
250 __u64 delta2 = jent_delta(ec->last_delta, current_delta);
251 __u64 delta3 = jent_delta(ec->last_delta2, delta2);
253 ec->last_delta = current_delta;
254 ec->last_delta2 = delta2;
257 * Insert the result of the comparison of two back-to-back time
260 jent_apt_insert(ec, current_delta);
262 if (!current_delta || !delta2 || !delta3) {
263 /* RCT with a stuck bit */
264 jent_rct_insert(ec, 1);
268 /* RCT with a non-stuck bit */
269 jent_rct_insert(ec, 0);
274 /* RCT health test failure detection */
275 static int jent_rct_permanent_failure(struct rand_data *ec)
277 return (ec->rct_count >= JENT_RCT_CUTOFF_PERMANENT) ? 1 : 0;
280 static int jent_rct_failure(struct rand_data *ec)
282 return (ec->rct_count >= JENT_RCT_CUTOFF) ? 1 : 0;
285 /* Report of health test failures */
286 static int jent_health_failure(struct rand_data *ec)
288 return jent_rct_failure(ec) | jent_apt_failure(ec);
291 static int jent_permanent_health_failure(struct rand_data *ec)
293 return jent_rct_permanent_failure(ec) | jent_apt_permanent_failure(ec);
296 /***************************************************************************
298 ***************************************************************************/
301 * Update of the loop count used for the next round of
302 * an entropy collection.
305 * @ec entropy collector struct -- may be NULL
306 * @bits is the number of low bits of the timer to consider
307 * @min is the number of bits we shift the timer value to the right at
308 * the end to make sure we have a guaranteed minimum value
310 * @return Newly calculated loop counter
312 static __u64 jent_loop_shuffle(struct rand_data *ec,
313 unsigned int bits, unsigned int min)
318 unsigned int mask = (1<<bits) - 1;
320 jent_get_nstime(&time);
322 * Mix the current state of the random number into the shuffle
323 * calculation to balance that shuffle a bit more.
328 * We fold the time value as much as possible to ensure that as many
329 * bits of the time stamp are included as possible.
331 for (i = 0; ((DATA_SIZE_BITS + bits - 1) / bits) > i; i++) {
332 shuffle ^= time & mask;
337 * We add a lower boundary value to ensure we have a minimum
340 return (shuffle + (1<<min));
344 * CPU Jitter noise source -- this is the noise source based on the CPU
345 * execution time jitter
347 * This function injects the individual bits of the time value into the
348 * entropy pool using an LFSR.
350 * The code is deliberately inefficient with respect to the bit shifting
351 * and shall stay that way. This function is the root cause why the code
352 * shall be compiled without optimization. This function not only acts as
353 * folding operation, but this function's execution is used to measure
354 * the CPU execution time jitter. Any change to the loop in this function
355 * implies that careful retesting must be done.
357 * @ec [in] entropy collector struct
358 * @time [in] time stamp to be injected
359 * @loop_cnt [in] if a value not equal to 0 is set, use the given value as
360 * number of loops to perform the folding
361 * @stuck [in] Is the time stamp identified as stuck?
366 * @return Number of loops the folding operation is performed
368 static void jent_lfsr_time(struct rand_data *ec, __u64 time, __u64 loop_cnt,
374 #define MAX_FOLD_LOOP_BIT 4
375 #define MIN_FOLD_LOOP_BIT 0
376 __u64 fold_loop_cnt =
377 jent_loop_shuffle(ec, MAX_FOLD_LOOP_BIT, MIN_FOLD_LOOP_BIT);
380 * testing purposes -- allow test app to set the counter, not
381 * needed during runtime
384 fold_loop_cnt = loop_cnt;
385 for (j = 0; j < fold_loop_cnt; j++) {
387 for (i = 1; (DATA_SIZE_BITS) >= i; i++) {
388 __u64 tmp = time << (DATA_SIZE_BITS - i);
390 tmp = tmp >> (DATA_SIZE_BITS - 1);
393 * Fibonacci LSFR with polynomial of
394 * x^64 + x^61 + x^56 + x^31 + x^28 + x^23 + 1 which is
395 * primitive according to
396 * http://poincare.matf.bg.ac.rs/~ezivkovm/publications/primpol1.pdf
397 * (the shift values are the polynomial values minus one
398 * due to counting bits from 0 to 63). As the current
399 * position is always the LSB, the polynomial only needs
400 * to shift data in from the left without wrap.
402 tmp ^= ((new >> 63) & 1);
403 tmp ^= ((new >> 60) & 1);
404 tmp ^= ((new >> 55) & 1);
405 tmp ^= ((new >> 30) & 1);
406 tmp ^= ((new >> 27) & 1);
407 tmp ^= ((new >> 22) & 1);
414 * If the time stamp is stuck, do not finally insert the value into
415 * the entropy pool. Although this operation should not do any harm
416 * even when the time stamp has no entropy, SP800-90B requires that
417 * any conditioning operation (SP800-90B considers the LFSR to be a
418 * conditioning operation) to have an identical amount of input
419 * data according to section 3.1.5.
426 * Memory Access noise source -- this is a noise source based on variations in
427 * memory access times
429 * This function performs memory accesses which will add to the timing
430 * variations due to an unknown amount of CPU wait states that need to be
431 * added when accessing memory. The memory size should be larger than the L1
432 * caches as outlined in the documentation and the associated testing.
434 * The L1 cache has a very high bandwidth, albeit its access rate is usually
435 * slower than accessing CPU registers. Therefore, L1 accesses only add minimal
436 * variations as the CPU has hardly to wait. Starting with L2, significant
437 * variations are added because L2 typically does not belong to the CPU any more
438 * and therefore a wider range of CPU wait states is necessary for accesses.
439 * L3 and real memory accesses have even a wider range of wait states. However,
440 * to reliably access either L3 or memory, the ec->mem memory must be quite
441 * large which is usually not desirable.
443 * @ec [in] Reference to the entropy collector with the memory access data -- if
444 * the reference to the memory block to be accessed is NULL, this noise
446 * @loop_cnt [in] if a value not equal to 0 is set, use the given value
447 * number of loops to perform the LFSR
449 static void jent_memaccess(struct rand_data *ec, __u64 loop_cnt)
451 unsigned int wrap = 0;
453 #define MAX_ACC_LOOP_BIT 7
454 #define MIN_ACC_LOOP_BIT 0
456 jent_loop_shuffle(ec, MAX_ACC_LOOP_BIT, MIN_ACC_LOOP_BIT);
458 if (NULL == ec || NULL == ec->mem)
460 wrap = ec->memblocksize * ec->memblocks;
463 * testing purposes -- allow test app to set the counter, not
464 * needed during runtime
467 acc_loop_cnt = loop_cnt;
469 for (i = 0; i < (ec->memaccessloops + acc_loop_cnt); i++) {
470 unsigned char *tmpval = ec->mem + ec->memlocation;
472 * memory access: just add 1 to one byte,
473 * wrap at 255 -- memory access implies read
474 * from and write to memory location
476 *tmpval = (*tmpval + 1) & 0xff;
478 * Addition of memblocksize - 1 to pointer
479 * with wrap around logic to ensure that every
480 * memory location is hit evenly
482 ec->memlocation = ec->memlocation + ec->memblocksize - 1;
483 ec->memlocation = ec->memlocation % wrap;
487 /***************************************************************************
488 * Start of entropy processing logic
489 ***************************************************************************/
491 * This is the heart of the entropy generation: calculate time deltas and
492 * use the CPU jitter in the time deltas. The jitter is injected into the
495 * WARNING: ensure that ->prev_time is primed before using the output
496 * of this function! This can be done by calling this function
497 * and not using its result.
499 * @ec [in] Reference to entropy collector
501 * @return result of stuck test
503 static int jent_measure_jitter(struct rand_data *ec)
506 __u64 current_delta = 0;
509 /* Invoke one noise source before time measurement to add variations */
510 jent_memaccess(ec, 0);
513 * Get time stamp and calculate time delta to previous
514 * invocation to measure the timing variations
516 jent_get_nstime(&time);
517 current_delta = jent_delta(ec->prev_time, time);
518 ec->prev_time = time;
520 /* Check whether we have a stuck measurement. */
521 stuck = jent_stuck(ec, current_delta);
523 /* Now call the next noise sources which also injects the data */
524 jent_lfsr_time(ec, current_delta, 0, stuck);
530 * Generator of one 64 bit random number
531 * Function fills rand_data->data
533 * @ec [in] Reference to entropy collector
535 static void jent_gen_entropy(struct rand_data *ec)
537 unsigned int k = 0, safety_factor = 0;
540 safety_factor = JENT_ENTROPY_SAFETY_FACTOR;
542 /* priming of the ->prev_time value */
543 jent_measure_jitter(ec);
545 while (!jent_health_failure(ec)) {
546 /* If a stuck measurement is received, repeat measurement */
547 if (jent_measure_jitter(ec))
551 * We multiply the loop value with ->osr to obtain the
552 * oversampling rate requested by the caller
554 if (++k >= ((DATA_SIZE_BITS + safety_factor) * ec->osr))
560 * Entry function: Obtain entropy for the caller.
562 * This function invokes the entropy gathering logic as often to generate
563 * as many bytes as requested by the caller. The entropy gathering logic
564 * creates 64 bit per invocation.
566 * This function truncates the last 64 bit entropy value output to the exact
567 * size specified by the caller.
569 * @ec [in] Reference to entropy collector
570 * @data [in] pointer to buffer for storing random data -- buffer must already
572 * @len [in] size of the buffer, specifying also the requested number of random
575 * @return 0 when request is fulfilled or an error
577 * The following error codes can occur:
578 * -1 entropy_collector is NULL
579 * -2 Intermittent health failure
580 * -3 Permanent health failure
582 int jent_read_entropy(struct rand_data *ec, unsigned char *data,
585 unsigned char *p = data;
593 jent_gen_entropy(ec);
595 if (jent_permanent_health_failure(ec)) {
597 * At this point, the Jitter RNG instance is considered
598 * as a failed instance. There is no rerun of the
599 * startup test any more, because the caller
600 * is assumed to not further use this instance.
603 } else if (jent_health_failure(ec)) {
605 * Perform startup health tests and return permanent
608 if (jent_entropy_init())
614 if ((DATA_SIZE_BITS / 8) < len)
615 tocopy = (DATA_SIZE_BITS / 8);
618 jent_memcpy(p, &ec->data, tocopy);
627 /***************************************************************************
628 * Initialization logic
629 ***************************************************************************/
631 struct rand_data *jent_entropy_collector_alloc(unsigned int osr,
634 struct rand_data *entropy_collector;
636 entropy_collector = jent_zalloc(sizeof(struct rand_data));
637 if (!entropy_collector)
640 if (!(flags & JENT_DISABLE_MEMORY_ACCESS)) {
641 /* Allocate memory for adding variations based on memory
644 entropy_collector->mem = jent_zalloc(JENT_MEMORY_SIZE);
645 if (!entropy_collector->mem) {
646 jent_zfree(entropy_collector);
649 entropy_collector->memblocksize = JENT_MEMORY_BLOCKSIZE;
650 entropy_collector->memblocks = JENT_MEMORY_BLOCKS;
651 entropy_collector->memaccessloops = JENT_MEMORY_ACCESSLOOPS;
654 /* verify and set the oversampling rate */
656 osr = 1; /* minimum sampling rate is 1 */
657 entropy_collector->osr = osr;
659 /* fill the data pad with non-zero values */
660 jent_gen_entropy(entropy_collector);
662 return entropy_collector;
665 void jent_entropy_collector_free(struct rand_data *entropy_collector)
667 jent_zfree(entropy_collector->mem);
668 entropy_collector->mem = NULL;
669 jent_zfree(entropy_collector);
672 int jent_entropy_init(void)
677 unsigned int nonstuck = 0;
678 int time_backwards = 0;
681 struct rand_data ec = { 0 };
683 /* Required for RCT */
686 /* We could perform statistical tests here, but the problem is
687 * that we only have a few loop counts to do testing. These
688 * loop counts may show some slight skew and we produce
691 * Moreover, only old systems show potentially problematic
692 * jitter entropy that could potentially be caught here. But
693 * the RNG is intended for hardware that is available or widely
694 * used, but not old systems that are long out of favor. Thus,
695 * no statistical tests.
699 * We could add a check for system capabilities such as clock_getres or
700 * check for CONFIG_X86_TSC, but it does not make much sense as the
701 * following sanity checks verify that we have a high-resolution
705 * TESTLOOPCOUNT needs some loops to identify edge systems. 100 is
706 * definitely too little.
708 * SP800-90B requires at least 1024 initial test cycles.
710 #define TESTLOOPCOUNT 1024
711 #define CLEARCACHE 100
712 for (i = 0; (TESTLOOPCOUNT + CLEARCACHE) > i; i++) {
716 unsigned int lowdelta = 0;
719 /* Invoke core entropy collection logic */
720 jent_get_nstime(&time);
722 jent_lfsr_time(&ec, time, 0, 0);
723 jent_get_nstime(&time2);
725 /* test whether timer works */
728 delta = jent_delta(time, time2);
730 * test whether timer is fine grained enough to provide
731 * delta even when called shortly after each other -- this
732 * implies that we also have a high resolution timer
735 return JENT_ECOARSETIME;
737 stuck = jent_stuck(&ec, delta);
740 * up to here we did not modify any variable that will be
741 * evaluated later, but we already performed some work. Thus we
742 * already have had an impact on the caches, branch prediction,
743 * etc. with the goal to clear it to get the worst case
755 * Ensure that the APT succeeded.
757 * With the check below that count_stuck must be less
758 * than 10% of the overall generated raw entropy values
759 * it is guaranteed that the APT is invoked at
760 * floor((TESTLOOPCOUNT * 0.9) / 64) == 14 times.
762 if ((nonstuck % JENT_APT_WINDOW_SIZE) == 0) {
764 delta & JENT_APT_WORD_MASK);
765 if (jent_health_failure(&ec))
771 if (jent_rct_failure(&ec))
774 /* test whether we have an increasing timer */
778 /* use 32 bit value to ensure compilation on 32 bit arches */
779 lowdelta = time2 - time;
780 if (!(lowdelta % 100))
784 * ensure that we have a varying delta timer which is necessary
785 * for the calculation of entropy -- perform this check
786 * only after the first loop is executed as we need to prime
789 if (delta > old_delta)
790 delta_sum += (delta - old_delta);
792 delta_sum += (old_delta - delta);
797 * we allow up to three times the time running backwards.
798 * CLOCK_REALTIME is affected by adjtime and NTP operations. Thus,
799 * if such an operation just happens to interfere with our test, it
800 * should not fail. The value of 3 should cover the NTP case being
801 * performed during our test run.
803 if (time_backwards > 3)
804 return JENT_ENOMONOTONIC;
807 * Variations of deltas of time must on average be larger
808 * than 1 to ensure the entropy estimation
809 * implied with 1 is preserved
811 if ((delta_sum) <= 1)
815 * Ensure that we have variations in the time stamp below 10 for at
816 * least 10% of all checks -- on some platforms, the counter increments
817 * in multiples of 100, but not always
819 if ((TESTLOOPCOUNT/10 * 9) < count_mod)
820 return JENT_ECOARSETIME;
823 * If we have more than 90% stuck results, then this Jitter RNG is
824 * likely to not work well.
826 if ((TESTLOOPCOUNT/10 * 9) < count_stuck)