docs/concepts/kernel/jitterentropy/config-basic.md - fuchsia - Git at Google

 # Jitterentropy: basic configuration

 The jitterentropy library is written by Stephan Mueller, is available at
 <https://github.com/smuellerDD/jitterentropy-library>, and is documented at
 <http://www.chronox.de/jent.html>. In Zircon, it's used as a simple entropy
 source to seed the system CPRNG.

 This document describes and analyzes two (independent) configuration options of
 jitterentropy:

 1. Whether to use a variable, pseudorandom number of iterations in the noise
    generating functions.
 2. Whether to post-process the raw noise samples with jitterentropy's internal
    processing routines.

 I consider these basic configuration options because the affect the basic
 process that jitterentropy uses. I'm contrasting them to tunable parameters
 (like the precise value used for loop counts if they are not chosen
 pseudorandomly, or the size of the scratch memory used internal by
 jitterentropy), since the tunable parameters don't greatly affect the means by
 which jitterentropy collects entropy, just the amount it collects and the time
 it takes.

 My full conclusions are at the end of this document, but in summary I think that
 we should avoid both choosing pseudorandom iteration numbers and using the
 jitterentropy post-processed data.

 [TOC]

 ## Brief explanation of jitterentropy

 The author's documentation is available in HTML form at
 <http://www.chronox.de/jent/doc/CPU-Jitter-NPTRNG.html>, or in PDF form at
 <http://www.chronox.de/jent/doc/CPU-Jitter-NPTRNG.pdf>. In brief, the library
 collects random bits from variations in CPU instruction timing.

 Jitterentropy maintains a random state, in the form of a 64-bit number that is
 affected by many of the jitterentropy functions, and ultimately is used as the
 output randomness.

 There are two noise sources, both of which are blocks of relatively slow-running
 code whose precise runtime is measured (using a system clock, requiring roughly
 nanosecond resolution). The precise time to complete these blocks of code will
 vary. We test these times to ensure that they are unpredictable; while we can't
 be perfectly certain that they are, the test results (including the results
 below) are encouraging. Note however that the purpose of this document is not to
 justify our estimates for the min-entropy in jitterentropy samples, but rather
 to discuss the two configuration options listed above.

 The first of the code blocks used as a noise source is a CPU-intensive LFSR
 loop, implemented in
 [the `jent_lfsr_time` function](/zircon/third_party/lib/jitterentropy/jitterentropy-base.c#185).
 The number of times the LFSR logic is repeated is controlled by the
 `kernel.jitterentropy.ll` cmdline ("`ll`" stands for "LFSR loops"). If `ll = 0`,
 a pseudorandom count is used, and otherwise the value of `ll` is used.
 Looking at the source code, the outer loop repeats according to the `ll`
 parameter.  The inner loop advances an LFSR by 64 steps, each time XOR-ing in
 one bit from the most recent time sample. Passing the time sample through the
 LFSR this way serves as a processing step, generally tending to whiten the
 random timesteps. As described in the
 [entropy quality testing doc](/docs/concepts/kernel/entropy_quality_tests.md), it's important to
 skip this processing when testing the entropic content of the CPU time
 variations.  It's also the case that enabling the processing increases the
 entropy estimates by a suspicious amount in some cases (see
 [the "Effects of processing the raw samples" section](#effects-of-processing-the-raw-samples)).

 The second noise source is a memory access loop, in
 [the `jent_memaccess` function](/zircon/third_party/lib/jitterentropy/jitterentropy-base.c#261).
 The memory access loop is repeated according to the `kernel.jitterentropy.ml`
 cmdline ("`ml`" for "memory loops"), where again a value of 0 activates the
 pseudorandom loop count, and any non-zero value overrides the pseudorandom
 count. Each iteration of the actual memory access loop both reads and writes a
 relatively large chunk of memory, divided into `kernel.jitterentropy.bc`-many
 blocks of size `kernel.jitterentropy.bs` bytes each. The default values when I
 wrote the current document are `bc = 1024` and `bs = 64`; up-to-date defaults
 should be documented in
 [the cmdline document](/docs/reference/kernel/kernel_cmdline.md). For comparison, the defaults in
 the jitterentropy source code are `bc = 64` and `bs = 32`,
 [defined here](/zircon/third_party/lib/jitterentropy/include/lib/jitterentropy/jitterentropy.h#79).
 Per the comment above the `jent_memaccess` function, the total memory size
 should be larger than the L1 cache size of the target machine. Confusingly,
 `bc = 64` and `bs = 32` produce a memory size of 2048 bytes, which is much
 smaller than even most L1 caches (I couldn't find any CPU with more than 0 bytes
 but less than 4KB of L1). Using `bs = 64` and `bc = 1024` result in 64KB of
 memory, which is usually enough to overflow L1 data caches.

 ### Option 1: Pseudorandom loop counts

 Jitterentropy was originally designed so that the two noise generating functions
 run a pseudorandom number of times. Specifically,
 [the `jent_loop_shuffle` function](/zircon/third_party/lib/jitterentropy/jitterentropy-base.c#125)
 mixes together (1) the time read from the high-resolution clock and (2)
 jitterentropy's internal random state in order to decide how many times to run
 the noise sources.

 We added the ability to override these pseudorandom loop counts, and tested
 jitterentropy's performance both with and without the override. The results are
 discussed in more depth in
 [the "Effects of pseudorandom loop counts" section](#effects-of-pseudorandom-loop-counts),
 but in summary: the statistical tests suggested that the pseudorandom loop
 counts increased the entropy far more than expected.  This makes me mistrust
 these higher entropy counts, so I recommend using the lower estimates and
 preferring deterministic loop counts to pseudorandom.

 ### Jitterentropy's random data processing

 As mentioned above, jitterentropy can process its random data, which makes the
 data look "more random".  Specifically, the processing should decrease (and
 ideally remove) the deviation of the random data from the uniform distribution,
 and reduce (ideally, remove) any intercorrelations between random bytes.

 The main function of interest for generating processed samples is
 [`jent_gen_entropy`](/zircon/third_party/lib/jitterentropy/jitterentropy-base.c#462),
 which is called in a loop by
 [`jent_read_entropy`](/zircon/third_party/lib/jitterentropy/jitterentropy-base.c#544)
 to produce an arbitrarily large number of random bytes.
 In essence, `jent_gen_entropy` calls the noise functions in a loop 64 times.
 Each of the 64 invocations of `jent_lfsr_time` mixes the noisy time measurement
 into the jitterentropy random state.

 After these 64 iterations, the random state is optionally "stirred" in
 [`jent_stir_pool`](/zircon/third_party/lib/jitterentropy/jitterentropy-base.c#403)
 by XOR-ing with a "mixer" value, itself dependent on the jitterentropy random
 state. As noted in the source code, this operation cannot increase or decrease
 the entropy in the pool (since XOR is bijective), but it can potentially improve
 the statistical appearance of the random state.

 In principle, invoking the noise source functions 64 times should produce 64
 times as much entropy, up to the maximum 64 bits that the random state can hold.
 This assumes that the mixing operation in `jent_lfsr_time` is cryptographically
 sound. I'm not an expert in cryptanalysis, but a LFSR itself is not a
 cryptographically secure RNG, since 64 successive bits reveal the entire state
 of a 64-bit LFSR, after which all past and future values can be easily
 computed. I am not sure that the jitterentropy scheme &mdash; XOR-ing the time
 measurement into the "bottom" of the LFSR as the LFSR is shifted &mdash; is more
 secure. Without careful cryptographic examination of this scheme (which for all
 I know may exist, but the I did not see it mentioned in the jitterentropy
 documentation), I would lean towards using unprocessed samples, and mixing them
 into our system entropy pool in a known-good way (e.g. SHA-2, as we do now).

 That said, I did run the NIST test suite against processed data samples. My
 results are in
 [the "Effects of processing the raw samples" section](#effects-of-processing-the-raw-samples))
 below.

 ## Testing process

 The procedure for running entropy source quality tests is documented in
 [the entropy quality tests document](/docs/concepts/kernel/entropy_quality_tests.md).

 These preliminary results were gathered on a Zircon debug build on Raspberry Pi
 3, built from commit 18358de5e90a012cb1e042efae83f5ea264d1502 in the now-obsolete project: https://fuchsia.googlesource.com/zircon/+/a1a80a6a7d
 "\[virtio]\[entropy] Basic virtio-rng driver". The following flags were set in
 my `local.mk` file when building:

 ```
 ENABLE_ENTROPY_COLLECTOR_TEST=1
 ENTROPY_COLLECTOR_TEST_MAXLEN=1048576
 ```

 I ran the boot-time tests after netbooting the debug kernel on the Pi with the
 following kernel cmdline, varying the values of `$ML`, `$LL`, and `$RAW`:

 ```
 kernel.entropy-test.src=jitterentropy
 kernel.jitterentropy.bs=64
 kernel.jitterentropy.bc=1024
 kernel.jitterentropy.ml=$ML
 kernel.jitterentropy.ll=$LL
 kernel.jitterentropy.raw=$RAW
 ```

 ## Test results and analysis

 ### Effects of pseudorandom loop counts

 #### Raw Data

 Following the logic in the jitterentropy source code (search for
 [`MAX_FOLD_LOOP_BIT`](/zircon/third_party/lib/jitterentropy/jitterentropy-base.c#191)
 and
 [`MAX_ACC_LOOP_BIT`](/zircon/third_party/lib/jitterentropy/jitterentropy-base.c#265))
 the pseudorandom loop counts vary within these ranges:

 ```
 ml: 1 .. 128 (inclusive)
 ll: 1 .. 16 (inclusive)
 ```

 I have included the overall min-entropy estimate from the NIST suite in this
 table, as well as two contributing estimates: the compression estimate and the
 Markov estimate. The NIST min-entropy estimate is the minimum of 10 different
 estimates, including these two. The compression estimate is generally the
 smallest for jitterentropy raw samples with deterministic loop counts, and the
 Markov estimate is generally smallest for jitterentropy with other
 configurations.

 | `ml`              | `ll`             | min-entropy (bits / byte) | Compression estimate | Markov estimate |
 |:-----------------:|:----------------:|:-------------------------:|:--------------------:|:---------------:|
 | random (1 .. 128) | random (1 .. 16) | 5.77                      | 6.84                 | 5.77            |
 | 128               | 16               | 1.62                      | 1.62                 | 3.60            |
 | 1                 | 1                | 0.20                      | 0.20                 | 0.84            |


 In other words, varying the loop counts pseudorandomly increased the min-entropy
 estimate for raw samples by 4.15 bits (or 250%), compared to the deterministic
 version that always used the maximum values from the pseudorandom ranges.

 #### Analysis

 The pseudorandom loop count values are determined by adding one extra time
 sample per noise function. First, these time samples are not independent of the
 noise function time measurements, since the gaps between the loop count time
 samples correspond predictably to the noise function time measurements. As a
 result it would be highly questionable to assume that they increase the
 min-entropy of the output data at all.  Second, it is absurd to imagine that the
 loop count time samples were somehow about 250% as random as the noise function
 time measurements, since both rely on the same noise source, except that the
 very first loop count time samples maybe get a small boost from the random
 amount of time needed to boot the system enough to run the test.

 Consequently, I suspect that what happened is that the pseudorandom loop counts
 were enough to "fool" the particular suite of statistical tests and
 predictor-based tests in the NIST suite, but that a predictor test written with
 specific knowledge of how the jitterentropy pseudorandom loop counts are derived
 could in fact predict the output with far better accuracy. I think the "true"
 min-entropy in the pseudorandom loop count test, against an adversary that's
 specifically targeting our code, is within the bounds of the two deterministic
 tests, i.e. between about 0.20 and 1.62 bits per byte.

 Using pseudorandom counts forces us to make an additional decision: do we
 conservatively estimate the actual entropy content at 0.20 bits per byte (as if
 the pseudorandom count function always chose `ml = 1` and `ll = 1`)? Or do we
 chose an average entropy content (there is probably a more intelligent averaging
 technique than to compute (1.62 + 0.20) / 2 = 0.91 bits / byte, but that will
 serve for the purpose of this discussion) and risk the pseudorandom loop counts
 occasionally causing us to undershoot this average entropy content? If we are
 too conservative, we will spend more time collecting entropy than is needed; if
 we are too optimistic, we might have a security vulnerability. Ultimately, this
 forces a trade-off between security (which prefers conservative entropy
 estimates) and efficiency (which prefers optimistic entropy estimates).

 ### Effects of processing the raw samples

 #### Raw Data

 I repeated the three tests reported above, but with jitterentropy's internal
 processing turned on (with `kernel.jitterentropy.raw = false` instead of the
 default value `true`). For convenience, the tables below include both the raw
 sample results (copied from above) in the top three rows, and the processed
 results (newly added) in the bottom three rows.

 | `ml`              | `ll`             | raw   | min-entropy (bits / byte) | Compression estimate | Markov estimate |
 |:-----------------:|:----------------:|:-----:|:-------------------------:|:--------------------:|:---------------:|
 | random (1 .. 128) | random (1 .. 16) | true  | 5.77                      | 6.84                 | 5.77            |
 | 128               | 16               | true  | 1.62                      | 1.62                 | 3.60            |
 | 1                 | 1                | true  | 0.20                      | 0.20                 | 0.84            |

 | `ml`              | `ll`             | raw   | min-entropy (bits / byte) | Compression estimate | Markov estimate |
 |:-----------------:|:----------------:|:-----:|:-------------------------:|:--------------------:|:---------------:|
 | random (1 .. 128) | random (1 .. 16) | false | 5.79                      | 6.59                 | 5.79            |
 | 128               | 16               | false | 5.78                      | 6.97                 | 5.78            |
 | 1                 | 1                | false | 5.77                      | 6.71                 | 5.77            |

 #### Analysis

 The post-processing min-entropy estimates are all essentially equal (up to
 slight variations easily explained by randomness), and also equal to the
 min-entropy estimate for raw samples with pseudorandom loop counts.

 Recall that jitterentropy's processed entropy is formed from 64 separate random
 data samples, mixed together in a 64-bit internal state buffer. Each of the raw
 samples corresponds to a sample in the `raw = true` table. In particular, it's
 absurd to think that combining 64 samples with `ml = 1` and `ll = 1` then
 processing these could produce (5.77 \* 8) = 46.2 bits of entropy per 8 bytes of
 processed output, since that would imply (46.2 / 64) = 0.72 bits of entropy per
 unprocessed sample as opposed to the measured value of 0.20 bits.

 This argument applies against the `ml = 1`, `ll = 1`, `raw = false` measurement,
 but does *not* apply to `ml = 128`, `ll = 16`, `raw = false`. In particular,
 combining 64 raw samples with `ml = 128` and `ll = 16` could in principle
 collect (1.64 \* 64 / 8) = 13.1 bits of entropy per processed byte, except that
 of course there is a hard limit at 8 bits per byte.

 Interestingly, the minimal entropy estimator switches from the compression
 estimate to the Markov estimator. My theory is that the additional "confusion"
 from post-processing is enough to "fool" the compression estimate. If there is a
 cryptographic vulnerability in the jitterentropy processing routine, it may be
 possible to write a similar estimator that reveals a significantly smaller
 min-entropy. If we use the general-purpose tests to decide how many raw samples
 to collect in order to have 256 of min-entropy, but an adversary uses a targeted
 attack, then (relative to this targeted attack) our system may have less entropy
 in its entropy pool than we expect. This is a security vulnerability.

 If there is a very bad weakness in the jitterentropy processing routine, it may
 in fact be reducing the "true" entropy in jitterentropy's internal pool. The
 arithmetical argument regarding `ml = 1` and `ll = 1` shows that we can't trust
 the NIST test suite to accurately measure the actual min-entropy in the
 processed data, so it is possible that the processing actually reduces
 min-entropy and our tools just can't detect the loss. This would exacerbate the
 vulnerability described in the previous paragraph.

 ## Conclusions

 Jitterentropy's pseudorandom loop counts are of questionable benefit at best,
 and if used they force us to make a security/efficiency trade-off. Unless we can
 show convincing evidence that the pseudorandom times really do drastically
 increase entropy estimates rather than just defeating the NIST test suite, we
 should use deterministic loop counts, ideally tuned for performance on a
 per-target basis.

 Jitterentropy's processing is also questionable, since (to my knowledge) it
 hasn't been subjected to enough cryptographic analysis and testing to be
 trusted. Furthermore, we can't directly measure the min-entropy in the
 post-processed data via the NIST test suite, so if there is a cryptographic
 vulnerability we can't easily detect it. I think we should instead rely on the
 entropy mixing code in the Zircon CPRNG (based on SHA-2), and leave
 jitterentropy's processing disabled.

 ## TODOs

 1. Repeat the tests reported above against different versions of Zircon, and
    ensure that the entropy estimates remain consistent.
 2. Repeat the tests on different platforms and targets (note: x86 targets don't
    currently have access to a system clock during early boot, so the early boot
    entropy tests and early boot CPRNG seeding don't yet support jitterentropy on
    x86).
 3. Automate the process of running the tests and generating the reports in this
    document. Specifically, the tests should compare:

    - pseudorandom loop counts versus various deterministic loop count values
    - raw samples versus processed data
	# Jitterentropy: basic configuration

	The jitterentropy library is written by Stephan Mueller, is available at
	<https://github.com/smuellerDD/jitterentropy-library>, and is documented at
	<http://www.chronox.de/jent.html>. In Zircon, it's used as a simple entropy
	source to seed the system CPRNG.

	This document describes and analyzes two (independent) configuration options of
	jitterentropy:

	1. Whether to use a variable, pseudorandom number of iterations in the noise
	generating functions.
	2. Whether to post-process the raw noise samples with jitterentropy's internal
	processing routines.

	I consider these basic configuration options because the affect the basic
	process that jitterentropy uses. I'm contrasting them to tunable parameters
	(like the precise value used for loop counts if they are not chosen
	pseudorandomly, or the size of the scratch memory used internal by
	jitterentropy), since the tunable parameters don't greatly affect the means by
	which jitterentropy collects entropy, just the amount it collects and the time
	it takes.

	My full conclusions are at the end of this document, but in summary I think that
	we should avoid both choosing pseudorandom iteration numbers and using the
	jitterentropy post-processed data.

	[TOC]

	## Brief explanation of jitterentropy

	The author's documentation is available in HTML form at
	<http://www.chronox.de/jent/doc/CPU-Jitter-NPTRNG.html>, or in PDF form at
	<http://www.chronox.de/jent/doc/CPU-Jitter-NPTRNG.pdf>. In brief, the library
	collects random bits from variations in CPU instruction timing.

	Jitterentropy maintains a random state, in the form of a 64-bit number that is
	affected by many of the jitterentropy functions, and ultimately is used as the
	output randomness.

	There are two noise sources, both of which are blocks of relatively slow-running
	code whose precise runtime is measured (using a system clock, requiring roughly
	nanosecond resolution). The precise time to complete these blocks of code will
	vary. We test these times to ensure that they are unpredictable; while we can't
	be perfectly certain that they are, the test results (including the results
	below) are encouraging. Note however that the purpose of this document is not to
	justify our estimates for the min-entropy in jitterentropy samples, but rather
	to discuss the two configuration options listed above.

	The first of the code blocks used as a noise source is a CPU-intensive LFSR
	loop, implemented in
	[the `jent_lfsr_time` function](/zircon/third_party/lib/jitterentropy/jitterentropy-base.c#185).
	The number of times the LFSR logic is repeated is controlled by the
	`kernel.jitterentropy.ll` cmdline ("`ll`" stands for "LFSR loops"). If `ll = 0`,
	a pseudorandom count is used, and otherwise the value of `ll` is used.
	Looking at the source code, the outer loop repeats according to the `ll`
	parameter. The inner loop advances an LFSR by 64 steps, each time XOR-ing in
	one bit from the most recent time sample. Passing the time sample through the
	LFSR this way serves as a processing step, generally tending to whiten the
	random timesteps. As described in the
	[entropy quality testing doc](/docs/concepts/kernel/entropy_quality_tests.md), it's important to
	skip this processing when testing the entropic content of the CPU time
	variations. It's also the case that enabling the processing increases the
	entropy estimates by a suspicious amount in some cases (see
	[the "Effects of processing the raw samples" section](#effects-of-processing-the-raw-samples)).

	The second noise source is a memory access loop, in
	[the `jent_memaccess` function](/zircon/third_party/lib/jitterentropy/jitterentropy-base.c#261).
	The memory access loop is repeated according to the `kernel.jitterentropy.ml`
	cmdline ("`ml`" for "memory loops"), where again a value of 0 activates the
	pseudorandom loop count, and any non-zero value overrides the pseudorandom
	count. Each iteration of the actual memory access loop both reads and writes a
	relatively large chunk of memory, divided into `kernel.jitterentropy.bc`-many
	blocks of size `kernel.jitterentropy.bs` bytes each. The default values when I
	wrote the current document are `bc = 1024` and `bs = 64`; up-to-date defaults
	should be documented in
	[the cmdline document](/docs/reference/kernel/kernel_cmdline.md). For comparison, the defaults in
	the jitterentropy source code are `bc = 64` and `bs = 32`,
	[defined here](/zircon/third_party/lib/jitterentropy/include/lib/jitterentropy/jitterentropy.h#79).
	Per the comment above the `jent_memaccess` function, the total memory size
	should be larger than the L1 cache size of the target machine. Confusingly,
	`bc = 64` and `bs = 32` produce a memory size of 2048 bytes, which is much
	smaller than even most L1 caches (I couldn't find any CPU with more than 0 bytes
	but less than 4KB of L1). Using `bs = 64` and `bc = 1024` result in 64KB of
	memory, which is usually enough to overflow L1 data caches.

	### Option 1: Pseudorandom loop counts

	Jitterentropy was originally designed so that the two noise generating functions
	run a pseudorandom number of times. Specifically,
	[the `jent_loop_shuffle` function](/zircon/third_party/lib/jitterentropy/jitterentropy-base.c#125)
	mixes together (1) the time read from the high-resolution clock and (2)
	jitterentropy's internal random state in order to decide how many times to run
	the noise sources.

	We added the ability to override these pseudorandom loop counts, and tested
	jitterentropy's performance both with and without the override. The results are
	discussed in more depth in
	[the "Effects of pseudorandom loop counts" section](#effects-of-pseudorandom-loop-counts),
	but in summary: the statistical tests suggested that the pseudorandom loop
	counts increased the entropy far more than expected. This makes me mistrust
	these higher entropy counts, so I recommend using the lower estimates and
	preferring deterministic loop counts to pseudorandom.

	### Jitterentropy's random data processing

	As mentioned above, jitterentropy can process its random data, which makes the
	data look "more random". Specifically, the processing should decrease (and
	ideally remove) the deviation of the random data from the uniform distribution,
	and reduce (ideally, remove) any intercorrelations between random bytes.

	The main function of interest for generating processed samples is
	[`jent_gen_entropy`](/zircon/third_party/lib/jitterentropy/jitterentropy-base.c#462),
	which is called in a loop by
	[`jent_read_entropy`](/zircon/third_party/lib/jitterentropy/jitterentropy-base.c#544)
	to produce an arbitrarily large number of random bytes.
	In essence, `jent_gen_entropy` calls the noise functions in a loop 64 times.
	Each of the 64 invocations of `jent_lfsr_time` mixes the noisy time measurement
	into the jitterentropy random state.

	After these 64 iterations, the random state is optionally "stirred" in
	[`jent_stir_pool`](/zircon/third_party/lib/jitterentropy/jitterentropy-base.c#403)
	by XOR-ing with a "mixer" value, itself dependent on the jitterentropy random
	state. As noted in the source code, this operation cannot increase or decrease
	the entropy in the pool (since XOR is bijective), but it can potentially improve
	the statistical appearance of the random state.

	In principle, invoking the noise source functions 64 times should produce 64
	times as much entropy, up to the maximum 64 bits that the random state can hold.
	This assumes that the mixing operation in `jent_lfsr_time` is cryptographically
	sound. I'm not an expert in cryptanalysis, but a LFSR itself is not a
	cryptographically secure RNG, since 64 successive bits reveal the entire state
	of a 64-bit LFSR, after which all past and future values can be easily
	computed. I am not sure that the jitterentropy scheme — XOR-ing the time
	measurement into the "bottom" of the LFSR as the LFSR is shifted — is more
	secure. Without careful cryptographic examination of this scheme (which for all
	I know may exist, but the I did not see it mentioned in the jitterentropy
	documentation), I would lean towards using unprocessed samples, and mixing them
	into our system entropy pool in a known-good way (e.g. SHA-2, as we do now).

	That said, I did run the NIST test suite against processed data samples. My
	results are in
	[the "Effects of processing the raw samples" section](#effects-of-processing-the-raw-samples))
	below.

	## Testing process

	The procedure for running entropy source quality tests is documented in
	[the entropy quality tests document](/docs/concepts/kernel/entropy_quality_tests.md).

	These preliminary results were gathered on a Zircon debug build on Raspberry Pi
	3, built from commit 18358de5e90a012cb1e042efae83f5ea264d1502 in the now-obsolete project: https://fuchsia.googlesource.com/zircon/+/a1a80a6a7d
	"\[virtio]\[entropy] Basic virtio-rng driver". The following flags were set in
	my `local.mk` file when building:

	```
	ENABLE_ENTROPY_COLLECTOR_TEST=1
	ENTROPY_COLLECTOR_TEST_MAXLEN=1048576
	```

	I ran the boot-time tests after netbooting the debug kernel on the Pi with the
	following kernel cmdline, varying the values of `$ML`, `$LL`, and `$RAW`:

	```
	kernel.entropy-test.src=jitterentropy
	kernel.jitterentropy.bs=64
	kernel.jitterentropy.bc=1024
	kernel.jitterentropy.ml=$ML
	kernel.jitterentropy.ll=$LL
	kernel.jitterentropy.raw=$RAW
	```

	## Test results and analysis

	### Effects of pseudorandom loop counts

	#### Raw Data

	Following the logic in the jitterentropy source code (search for
	[`MAX_FOLD_LOOP_BIT`](/zircon/third_party/lib/jitterentropy/jitterentropy-base.c#191)
	and
	[`MAX_ACC_LOOP_BIT`](/zircon/third_party/lib/jitterentropy/jitterentropy-base.c#265))
	the pseudorandom loop counts vary within these ranges:

	```
	ml: 1 .. 128 (inclusive)
	ll: 1 .. 16 (inclusive)
	```

	I have included the overall min-entropy estimate from the NIST suite in this
	table, as well as two contributing estimates: the compression estimate and the
	Markov estimate. The NIST min-entropy estimate is the minimum of 10 different
	estimates, including these two. The compression estimate is generally the
	smallest for jitterentropy raw samples with deterministic loop counts, and the
	Markov estimate is generally smallest for jitterentropy with other
	configurations.

	\| `ml` \| `ll` \| min-entropy (bits / byte) \| Compression estimate \| Markov estimate \|
	\|:-----------------:\|:----------------:\|:-------------------------:\|:--------------------:\|:---------------:\|
	\| random (1 .. 128) \| random (1 .. 16) \| 5.77 \| 6.84 \| 5.77 \|
	\| 128 \| 16 \| 1.62 \| 1.62 \| 3.60 \|
	\| 1 \| 1 \| 0.20 \| 0.20 \| 0.84 \|


	In other words, varying the loop counts pseudorandomly increased the min-entropy
	estimate for raw samples by 4.15 bits (or 250%), compared to the deterministic
	version that always used the maximum values from the pseudorandom ranges.

	#### Analysis

	The pseudorandom loop count values are determined by adding one extra time
	sample per noise function. First, these time samples are not independent of the
	noise function time measurements, since the gaps between the loop count time
	samples correspond predictably to the noise function time measurements. As a
	result it would be highly questionable to assume that they increase the
	min-entropy of the output data at all. Second, it is absurd to imagine that the
	loop count time samples were somehow about 250% as random as the noise function
	time measurements, since both rely on the same noise source, except that the
	very first loop count time samples maybe get a small boost from the random
	amount of time needed to boot the system enough to run the test.

	Consequently, I suspect that what happened is that the pseudorandom loop counts
	were enough to "fool" the particular suite of statistical tests and
	predictor-based tests in the NIST suite, but that a predictor test written with
	specific knowledge of how the jitterentropy pseudorandom loop counts are derived
	could in fact predict the output with far better accuracy. I think the "true"
	min-entropy in the pseudorandom loop count test, against an adversary that's
	specifically targeting our code, is within the bounds of the two deterministic
	tests, i.e. between about 0.20 and 1.62 bits per byte.

	Using pseudorandom counts forces us to make an additional decision: do we
	conservatively estimate the actual entropy content at 0.20 bits per byte (as if
	the pseudorandom count function always chose `ml = 1` and `ll = 1`)? Or do we
	chose an average entropy content (there is probably a more intelligent averaging
	technique than to compute (1.62 + 0.20) / 2 = 0.91 bits / byte, but that will
	serve for the purpose of this discussion) and risk the pseudorandom loop counts
	occasionally causing us to undershoot this average entropy content? If we are
	too conservative, we will spend more time collecting entropy than is needed; if
	we are too optimistic, we might have a security vulnerability. Ultimately, this
	forces a trade-off between security (which prefers conservative entropy
	estimates) and efficiency (which prefers optimistic entropy estimates).

	### Effects of processing the raw samples

	#### Raw Data

	I repeated the three tests reported above, but with jitterentropy's internal
	processing turned on (with `kernel.jitterentropy.raw = false` instead of the
	default value `true`). For convenience, the tables below include both the raw
	sample results (copied from above) in the top three rows, and the processed
	results (newly added) in the bottom three rows.

	\| `ml` \| `ll` \| raw \| min-entropy (bits / byte) \| Compression estimate \| Markov estimate \|
	\|:-----------------:\|:----------------:\|:-----:\|:-------------------------:\|:--------------------:\|:---------------:\|
	\| random (1 .. 128) \| random (1 .. 16) \| true \| 5.77 \| 6.84 \| 5.77 \|
	\| 128 \| 16 \| true \| 1.62 \| 1.62 \| 3.60 \|
	\| 1 \| 1 \| true \| 0.20 \| 0.20 \| 0.84 \|

	\| `ml` \| `ll` \| raw \| min-entropy (bits / byte) \| Compression estimate \| Markov estimate \|
	\|:-----------------:\|:----------------:\|:-----:\|:-------------------------:\|:--------------------:\|:---------------:\|
	\| random (1 .. 128) \| random (1 .. 16) \| false \| 5.79 \| 6.59 \| 5.79 \|
	\| 128 \| 16 \| false \| 5.78 \| 6.97 \| 5.78 \|
	\| 1 \| 1 \| false \| 5.77 \| 6.71 \| 5.77 \|

	#### Analysis

	The post-processing min-entropy estimates are all essentially equal (up to
	slight variations easily explained by randomness), and also equal to the
	min-entropy estimate for raw samples with pseudorandom loop counts.

	Recall that jitterentropy's processed entropy is formed from 64 separate random
	data samples, mixed together in a 64-bit internal state buffer. Each of the raw
	samples corresponds to a sample in the `raw = true` table. In particular, it's
	absurd to think that combining 64 samples with `ml = 1` and `ll = 1` then
	processing these could produce (5.77 \* 8) = 46.2 bits of entropy per 8 bytes of
	processed output, since that would imply (46.2 / 64) = 0.72 bits of entropy per
	unprocessed sample as opposed to the measured value of 0.20 bits.

	This argument applies against the `ml = 1`, `ll = 1`, `raw = false` measurement,
	but does not apply to `ml = 128`, `ll = 16`, `raw = false`. In particular,
	combining 64 raw samples with `ml = 128` and `ll = 16` could in principle
	collect (1.64 \* 64 / 8) = 13.1 bits of entropy per processed byte, except that
	of course there is a hard limit at 8 bits per byte.

	Interestingly, the minimal entropy estimator switches from the compression
	estimate to the Markov estimator. My theory is that the additional "confusion"
	from post-processing is enough to "fool" the compression estimate. If there is a
	cryptographic vulnerability in the jitterentropy processing routine, it may be
	possible to write a similar estimator that reveals a significantly smaller
	min-entropy. If we use the general-purpose tests to decide how many raw samples
	to collect in order to have 256 of min-entropy, but an adversary uses a targeted
	attack, then (relative to this targeted attack) our system may have less entropy
	in its entropy pool than we expect. This is a security vulnerability.

	If there is a very bad weakness in the jitterentropy processing routine, it may
	in fact be reducing the "true" entropy in jitterentropy's internal pool. The
	arithmetical argument regarding `ml = 1` and `ll = 1` shows that we can't trust
	the NIST test suite to accurately measure the actual min-entropy in the
	processed data, so it is possible that the processing actually reduces
	min-entropy and our tools just can't detect the loss. This would exacerbate the
	vulnerability described in the previous paragraph.

	## Conclusions

	Jitterentropy's pseudorandom loop counts are of questionable benefit at best,
	and if used they force us to make a security/efficiency trade-off. Unless we can
	show convincing evidence that the pseudorandom times really do drastically
	increase entropy estimates rather than just defeating the NIST test suite, we
	should use deterministic loop counts, ideally tuned for performance on a
	per-target basis.

	Jitterentropy's processing is also questionable, since (to my knowledge) it
	hasn't been subjected to enough cryptographic analysis and testing to be
	trusted. Furthermore, we can't directly measure the min-entropy in the
	post-processed data via the NIST test suite, so if there is a cryptographic
	vulnerability we can't easily detect it. I think we should instead rely on the
	entropy mixing code in the Zircon CPRNG (based on SHA-2), and leave
	jitterentropy's processing disabled.

	## TODOs

	1. Repeat the tests reported above against different versions of Zircon, and
	ensure that the entropy estimates remain consistent.
	2. Repeat the tests on different platforms and targets (note: x86 targets don't
	currently have access to a system clock during early boot, so the early boot
	entropy tests and early boot CPRNG seeding don't yet support jitterentropy on
	x86).
	3. Automate the process of running the tests and generating the reports in this
	document. Specifically, the tests should compare:

	- pseudorandom loop counts versus various deterministic loop count values
	- raw samples versus processed data