// Copyright 2018 Developers of the Rand project.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.
//
// Based on jitterentropy-library, http://www.chronox.de/jent.html.
// Copyright Stephan Mueller <smueller@chronox.de>, 2014 - 2017.
//
// With permission from Stephan Mueller to relicense the Rust translation under
// the MIT license.
//! Non-physical true random number generator based on timing jitter.
//!
//! Note that this RNG is not suited for use cases where cryptographic security is
//! required (also see this [discussion]).
//!
//! This is a true random number generator, as opposed to pseudo-random
//! generators. Random numbers generated by `JitterRng` can be seen as fresh
//! entropy. A consequence is that it is orders of magnitude slower than `OsRng`
//! and PRNGs (about 10<sup>3</sup>..10<sup>6</sup> slower).
//!
//! There are very few situations where using this RNG is appropriate. Only very
//! few applications require true entropy. A normal PRNG can be statistically
//! indistinguishable, and a cryptographic PRNG should also be as impossible to
//! predict.
//!
//! `JitterRng` can be used without the standard library, but not conveniently,
//! you must provide a high-precision timer and carefully have to follow the
//! instructions of [`JitterRng::new_with_timer`].
//!
//! This implementation is based on [Jitterentropy] version 2.1.0.
//!
//! Note: There is no accurate timer available on WASM platforms, to help
//! prevent fingerprinting or timing side-channel attacks. Therefore
//! [`JitterRng::new()`] is not available on WASM. It is also unavailable
//! with disabled `std` feature.
//!
//! [Jitterentropy]: http://www.chronox.de/jent.html
//! [discussion]: https://github.com/rust-random/rand/issues/699
#![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk.png",
html_favicon_url = "https://www.rust-lang.org/favicon.ico",
html_root_url = "https://rust-random.github.io/rand/")]
#![deny(missing_docs)]
#![deny(missing_debug_implementations)]
#![doc(test(attr(allow(unused_variables), deny(warnings))))]
// Note: the C implementation of `Jitterentropy` relies on being compiled
// without optimizations. This implementation goes through lengths to make the
// compiler not optimize out code which does influence timing jitter, but is
// technically dead code.
#![no_std]
#[cfg(feature = "std")]
extern crate std;
pub use rand_core;
// Coming from https://crates.io/crates/doc-comment
#[cfg(test)]
macro_rules! doc_comment {
($x:expr) => {
#[doc = $x]
extern {}
};
}
#[cfg(test)]
doc_comment!(include_str!("../README.md"));
#[allow(unused)]
macro_rules! trace { ($($x:tt)*) => (
#[cfg(feature = "log")] {
log::trace!($($x)*)
}
) }
#[allow(unused)]
macro_rules! debug { ($($x:tt)*) => (
#[cfg(feature = "log")] {
log::debug!($($x)*)
}
) }
#[allow(unused)]
macro_rules! info { ($($x:tt)*) => (
#[cfg(feature = "log")] {
log::info!($($x)*)
}
) }
#[allow(unused)]
macro_rules! warn { ($($x:tt)*) => (
#[cfg(feature = "log")] {
log::warn!($($x)*)
}
) }
#[allow(unused)]
macro_rules! error { ($($x:tt)*) => (
#[cfg(feature = "log")] {
log::error!($($x)*)
}
) }
#[cfg(feature = "std")]
mod platform;
mod error;
use rand_core::{RngCore, Error, impls};
pub use crate::error::TimerError;
use core::{fmt, mem, ptr};
#[cfg(feature = "std")]
use std::sync::atomic::{AtomicUsize, Ordering};
const MEMORY_BLOCKS: usize = 64;
const MEMORY_BLOCKSIZE: usize = 32;
const MEMORY_SIZE: usize = MEMORY_BLOCKS * MEMORY_BLOCKSIZE;
/// A true random number generator based on jitter in the CPU execution time,
/// and jitter in memory access time.
///
/// Note that this RNG is not suitable for use cases where cryptographic
/// security is required.
pub struct JitterRng {
data: u64, // Actual random number
// Number of rounds to run the entropy collector per 64 bits
rounds: u8,
// Timer used by `measure_jitter`
timer: fn() -> u64,
// Memory for the Memory Access noise source
mem_prev_index: u16,
// Make `next_u32` not waste 32 bits
data_half_used: bool,
}
// Note: `JitterRng` maintains a small 64-bit entropy pool. With every
// `generate` 64 new bits should be integrated in the pool. If a round of
// `generate` were to collect less than the expected 64 bit, then the returned
// value, and the new state of the entropy pool, would be in some way related to
// the initial state. It is therefore better if the initial state of the entropy
// pool is different on each call to `generate`. This has a few implications:
// - `generate` should be called once before using `JitterRng` to produce the
// first usable value (this is done by default in `new`);
// - We do not zero the entropy pool after generating a result. The reference
// implementation also does not support zeroing, but recommends generating a
// new value without using it if you want to protect a previously generated
// 'secret' value from someone inspecting the memory;
// - Implementing `Clone` seems acceptable, as it would not cause the systematic
// bias a constant might cause. Only instead of one value that could be
// potentially related to the same initial state, there are now two.
// Entropy collector state.
// These values are not necessary to preserve across runs.
struct EcState {
// Previous time stamp to determine the timer delta
prev_time: u64,
// Deltas used for the stuck test
last_delta: i32,
last_delta2: i32,
// Memory for the Memory Access noise source
mem: [u8; MEMORY_SIZE],
}
impl EcState {
// 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.
// This test is a heuristic to see whether the last measurement holds
// entropy.
fn stuck(&mut self, current_delta: i32) -> bool {
let delta2 = self.last_delta - current_delta;
let delta3 = delta2 - self.last_delta2;
self.last_delta = current_delta;
self.last_delta2 = delta2;
current_delta == 0 || delta2 == 0 || delta3 == 0
}
}
// Custom Debug implementation that does not expose the internal state
impl fmt::Debug for JitterRng {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
write!(f, "JitterRng {{}}")
}
}
impl Clone for JitterRng {
fn clone(&self) -> JitterRng {
JitterRng {
data: self.data,
rounds: self.rounds,
timer: self.timer,
mem_prev_index: self.mem_prev_index,
// The 32 bits that may still be unused from the previous round are
// for the original to use, not for the clone.
data_half_used: false,
}
}
}
// Initialise to zero; must be positive
#[cfg(all(feature = "std", not(target_arch = "wasm32")))]
static JITTER_ROUNDS: AtomicUsize = AtomicUsize::new(0);
impl JitterRng {
/// Create a new `JitterRng`. Makes use of `std::time` for a timer, or a
/// platform-specific function with higher accuracy if necessary and
/// available.
///
/// During initialization CPU execution timing jitter is measured a few
/// hundred times. If this does not pass basic quality tests, an error is
/// returned. The test result is cached to make subsequent calls faster.
#[cfg(all(feature = "std", not(target_arch = "wasm32")))]
pub fn new() -> Result<JitterRng, TimerError> {
if cfg!(target_arch = "wasm32") {
return Err(TimerError::NoTimer);
}
let mut state = JitterRng::new_with_timer(platform::get_nstime);
let mut rounds = JITTER_ROUNDS.load(Ordering::Relaxed) as u8;
if rounds == 0 {
// No result yet: run test.
// This allows the timer test to run multiple times; we don't care.
rounds = state.test_timer()?;
JITTER_ROUNDS.store(rounds as usize, Ordering::Relaxed);
info!("JitterRng: using {} rounds per u64 output", rounds);
}
state.set_rounds(rounds);
// Fill `data` with a non-zero value.
state.gen_entropy();
Ok(state)
}
/// Create a new `JitterRng`.
/// A custom timer can be supplied, making it possible to use `JitterRng` in
/// `no_std` environments.
///
/// The timer must have nanosecond precision.
///
/// This method is more low-level than `new()`. It is the responsibility of
/// the caller to run [`test_timer`] before using any numbers generated with
/// `JitterRng`, and optionally call [`set_rounds`]. Also it is important to
/// consume at least one `u64` before using the first result to initialize
/// the entropy collection pool.
///
/// # Example
///
/// ```
/// # use rand_jitter::rand_core::{RngCore, Error};
/// use rand_jitter::JitterRng;
///
/// # fn try_inner() -> Result<(), Error> {
/// fn get_nstime() -> u64 {
/// use std::time::{SystemTime, UNIX_EPOCH};
///
/// let dur = SystemTime::now().duration_since(UNIX_EPOCH).unwrap();
/// // The correct way to calculate the current time is
/// // `dur.as_secs() * 1_000_000_000 + dur.subsec_nanos() as u64`
/// // But this is faster, and the difference in terms of entropy is
/// // negligible (log2(10^9) == 29.9).
/// dur.as_secs() << 30 | dur.subsec_nanos() as u64
/// }
///
/// let mut rng = JitterRng::new_with_timer(get_nstime);
/// let rounds = rng.test_timer()?;
/// rng.set_rounds(rounds); // optional
/// let _ = rng.next_u64();
///
/// // Ready for use
/// let v: u64 = rng.next_u64();
/// # Ok(())
/// # }
///
/// # let _ = try_inner();
/// ```
///
/// [`test_timer`]: JitterRng::test_timer
/// [`set_rounds`]: JitterRng::set_rounds
pub fn new_with_timer(timer: fn() -> u64) -> JitterRng {
JitterRng {
data: 0,
rounds: 64,
timer,
mem_prev_index: 0,
data_half_used: false,
}
}
/// Configures how many rounds are used to generate each 64-bit value.
/// This must be greater than zero, and has a big impact on performance
/// and output quality.
///
/// [`new_with_timer`] conservatively uses 64 rounds, but often less rounds
/// can be used. The `test_timer()` function returns the minimum number of
/// rounds required for full strength (platform dependent), so one may use
/// `rng.set_rounds(rng.test_timer()?);` or cache the value.
///
/// [`new_with_timer`]: JitterRng::new_with_timer
pub fn set_rounds(&mut self, rounds: u8) {
assert!(rounds > 0);
self.rounds = rounds;
}
// Calculate a random loop count used for the next round of an entropy
// collection, based on bits from a fresh value from the timer.
//
// The timer is folded to produce a number that contains at most `n_bits`
// bits.
//
// Note: A constant should be added to the resulting random loop count to
// prevent loops that run 0 times.
#[inline(never)]
fn random_loop_cnt(&mut self, n_bits: u32) -> u32 {
let mut rounds = 0;
let mut time = (self.timer)();
// Mix with the current state of the random number balance the random
// loop counter a bit more.
time ^= self.data;
// We fold the time value as much as possible to ensure that as many
// bits of the time stamp are included as possible.
let folds = (64 + n_bits - 1) / n_bits;
let mask = (1 << n_bits) - 1;
for _ in 0..folds {
rounds ^= time & mask;
time >>= n_bits;
}
rounds as u32
}
// CPU jitter noise source
// 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.
// 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.
#[inline(never)]
fn lfsr_time(&mut self, time: u64, var_rounds: bool) {
fn lfsr(mut data: u64, time: u64) -> u64{
for i in 1..65 {
let mut tmp = time << (64 - i);
tmp >>= 64 - 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.
data ^= tmp;
data ^= (data >> 63) & 1;
data ^= (data >> 60) & 1;
data ^= (data >> 55) & 1;
data ^= (data >> 30) & 1;
data ^= (data >> 27) & 1;
data ^= (data >> 22) & 1;
data = data.rotate_left(1);
}
data
}
// Note: in the reference implementation only the last round effects
// `self.data`, all the other results are ignored. To make sure the
// other rounds are not optimised out, we first run all but the last
// round on a throw-away value instead of the real `self.data`.
let mut lfsr_loop_cnt = 0;
if var_rounds { lfsr_loop_cnt = self.random_loop_cnt(4) };
let mut throw_away: u64 = 0;
for _ in 0..lfsr_loop_cnt {
throw_away = lfsr(throw_away, time);
}
black_box(throw_away);
self.data = lfsr(self.data, time);
}
// 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 `self.mem` memory must be quite large which is usually not desirable.
#[inline(never)]
fn memaccess(&mut self, mem: &mut [u8; MEMORY_SIZE], var_rounds: bool) {
let mut acc_loop_cnt = 128;
if var_rounds { acc_loop_cnt += self.random_loop_cnt(4) };
let mut index = self.mem_prev_index as usize;
for _ in 0..acc_loop_cnt {
// Addition of memblocksize - 1 to index with wrap around logic to
// ensure that every memory location is hit evenly.
// The modulus also allows the compiler to remove the indexing
// bounds check.
index = (index + MEMORY_BLOCKSIZE - 1) % MEMORY_SIZE;
// memory access: just add 1 to one byte
// memory access implies read from and write to memory location
mem[index] = mem[index].wrapping_add(1);
}
self.mem_prev_index = index as u16;
}
// 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.
//
// Ensure that `ec.prev_time` is primed before using the output of this
// function. This can be done by calling this function and not using its
// result.
fn measure_jitter(&mut self, ec: &mut EcState) -> Option<()> {
// Invoke one noise source before time measurement to add variations
self.memaccess(&mut ec.mem, true);
// Get time stamp and calculate time delta to previous
// invocation to measure the timing variations
let time = (self.timer)();
// Note: wrapping_sub combined with a cast to `i64` generates a correct
// delta, even in the unlikely case this is a timer that is not strictly
// monotonic.
let current_delta = time.wrapping_sub(ec.prev_time) as i64 as i32;
ec.prev_time = time;
// Call the next noise source which also injects the data
self.lfsr_time(current_delta as u64, true);
// Check whether we have a stuck measurement (i.e. does the last
// measurement holds entropy?).
if ec.stuck(current_delta) { return None };
// Rotate the data buffer by a prime number (any odd number would
// do) to ensure that every bit position of the input time stamp
// has an even chance of being merged with a bit position in the
// entropy pool. We do not use one here as the adjacent bits in
// successive time deltas may have some form of dependency. The
// chosen value of 7 implies that the low 7 bits of the next
// time delta value is concatenated with the current time delta.
self.data = self.data.rotate_left(7);
Some(())
}
// 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.
#[inline(never)]
fn stir_pool(&mut self) {
// 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
// The order does not really 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.
const CONSTANT: u64 = 0x67452301efcdab89;
// 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
let mut mixer = 0x98badcfe10325476;
// This is a constant time function to prevent leaking timing
// information about the random number.
// The normal code is:
// ```
// for i in 0..64 {
// if ((self.data >> i) & 1) == 1 { mixer ^= CONSTANT; }
// }
// ```
// This is a bit fragile, as LLVM really wants to use branches here, and
// we rely on it to not recognise the opportunity.
for i in 0..64 {
let apply = (self.data >> i) & 1;
let mask = !apply.wrapping_sub(1);
mixer ^= CONSTANT & mask;
mixer = mixer.rotate_left(1);
}
self.data ^= mixer;
}
fn gen_entropy(&mut self) -> u64 {
trace!("JitterRng: collecting entropy");
// Prime `ec.prev_time`, and run the noice sources to make sure the
// first loop round collects the expected entropy.
let mut ec = EcState {
prev_time: (self.timer)(),
last_delta: 0,
last_delta2: 0,
mem: [0; MEMORY_SIZE],
};
let _ = self.measure_jitter(&mut ec);
for _ in 0..self.rounds {
// If a stuck measurement is received, repeat measurement
// Note: we do not guard against an infinite loop, that would mean
// the timer suddenly became broken.
while self.measure_jitter(&mut ec).is_none() {}
}
// Do a single read from `self.mem` to make sure the Memory Access noise
// source is not optimised out.
black_box(ec.mem[0]);
self.stir_pool();
self.data
}
/// Basic quality tests on the timer, by measuring CPU timing jitter a few
/// hundred times.
///
/// If successful, this will return the estimated number of rounds necessary
/// to collect 64 bits of entropy. Otherwise a [`TimerError`] with the cause
/// of the failure will be returned.
pub fn test_timer(&mut self) -> Result<u8, TimerError> {
debug!("JitterRng: testing timer ...");
// 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.
let mut delta_sum = 0;
let mut old_delta = 0;
let mut time_backwards = 0;
let mut count_mod = 0;
let mut count_stuck = 0;
let mut ec = EcState {
prev_time: (self.timer)(),
last_delta: 0,
last_delta2: 0,
mem: [0; MEMORY_SIZE],
};
// TESTLOOPCOUNT needs some loops to identify edge systems.
// 100 is definitely too little.
const TESTLOOPCOUNT: u64 = 300;
const CLEARCACHE: u64 = 100;
for i in 0..(CLEARCACHE + TESTLOOPCOUNT) {
// Measure time delta of core entropy collection logic
let time = (self.timer)();
self.memaccess(&mut ec.mem, true);
self.lfsr_time(time, true);
let time2 = (self.timer)();
// Test whether timer works
if time == 0 || time2 == 0 {
return Err(TimerError::NoTimer);
}
let delta = time2.wrapping_sub(time) as i64 as i32;
// 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 == 0 {
return Err(TimerError::CoarseTimer);
}
// 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 i < CLEARCACHE { continue; }
if ec.stuck(delta) { count_stuck += 1; }
// Test whether we have an increasing timer.
if !(time2 > time) { time_backwards += 1; }
// Count the number of times the counter increases in steps of 100ns
// or greater.
if (delta % 100) == 0 { count_mod += 1; }
// 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_delta value
delta_sum += (delta - old_delta).abs() as u64;
old_delta = delta;
}
// Do a single read from `self.mem` to make sure the Memory Access noise
// source is not optimised out.
black_box(ec.mem[0]);
// We allow the time to run backwards for up to three times.
// This can happen if the clock is being adjusted by NTP operations.
// 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 time_backwards > 3 {
return Err(TimerError::NotMonotonic);
}
// Test that the available amount of entropy per round does not get to
// low. We expect 1 bit of entropy per round as a reasonable minimum
// (although less is possible, it means the collector loop has to run
// much more often).
// `assert!(delta_average >= log2(1))`
// `assert!(delta_sum / TESTLOOPCOUNT >= 1)`
// `assert!(delta_sum >= TESTLOOPCOUNT)`
if delta_sum < TESTLOOPCOUNT {
return Err(TimerError::TinyVariantions);
}
// Ensure that we have variations in the time stamp below 100 for at
// least 10% of all checks -- on some platforms, the counter increments
// in multiples of 100, but not always
if count_mod > (TESTLOOPCOUNT * 9 / 10) {
return Err(TimerError::CoarseTimer);
}
// If we have more than 90% stuck results, then this Jitter RNG is
// likely to not work well.
if count_stuck > (TESTLOOPCOUNT * 9 / 10) {
return Err(TimerError::TooManyStuck);
}
// Estimate the number of `measure_jitter` rounds necessary for 64 bits
// of entropy.
//
// We don't try very hard to come up with a good estimate of the
// available bits of entropy per round here for two reasons:
// 1. Simple estimates of the available bits (like Shannon entropy) are
// too optimistic.
// 2. Unless we want to waste a lot of time during intialization, there
// only a small number of samples are available.
//
// Therefore we use a very simple and conservative estimate:
// `let bits_of_entropy = log2(delta_average) / 2`.
//
// The number of rounds `measure_jitter` should run to collect 64 bits
// of entropy is `64 / bits_of_entropy`.
let delta_average = delta_sum / TESTLOOPCOUNT;
if delta_average >= 16 {
let log2 = 64 - delta_average.leading_zeros();
// Do something similar to roundup(64/(log2/2)):
Ok( ((64u32 * 2 + log2 - 1) / log2) as u8)
} else {
// For values < 16 the rounding error becomes too large, use a
// lookup table.
// Values 0 and 1 are invalid, and filtered out by the
// `delta_sum < TESTLOOPCOUNT` test above.
let log2_lookup = [0, 0, 128, 81, 64, 56, 50, 46,
43, 41, 39, 38, 36, 35, 34, 33];
Ok(log2_lookup[delta_average as usize])
}
}
/// Statistical test: return the timer delta of one normal run of the
/// `JitterRng` entropy collector.
///
/// Setting `var_rounds` to `true` will execute the memory access and the
/// CPU jitter noice sources a variable amount of times (just like a real
/// `JitterRng` round).
///
/// Setting `var_rounds` to `false` will execute the noice sources the
/// minimal number of times. This can be used to measure the minimum amount
/// of entropy one round of the entropy collector can collect in the worst
/// case.
///
/// See this crate's README on how to use `timer_stats` to test the quality
/// of `JitterRng`.
pub fn timer_stats(&mut self, var_rounds: bool) -> i64 {
let mut mem = [0; MEMORY_SIZE];
let time = (self.timer)();
self.memaccess(&mut mem, var_rounds);
self.lfsr_time(time, var_rounds);
let time2 = (self.timer)();
time2.wrapping_sub(time) as i64
}
}
// A function that is opaque to the optimizer to assist in avoiding dead-code
// elimination. Taken from `bencher`.
fn black_box<T>(dummy: T) -> T {
unsafe {
let ret = ptr::read_volatile(&dummy);
mem::forget(dummy);
ret
}
}
impl RngCore for JitterRng {
fn next_u32(&mut self) -> u32 {
// We want to use both parts of the generated entropy
if self.data_half_used {
self.data_half_used = false;
(self.data >> 32) as u32
} else {
self.data = self.next_u64();
self.data_half_used = true;
self.data as u32
}
}
fn next_u64(&mut self) -> u64 {
self.data_half_used = false;
self.gen_entropy()
}
fn fill_bytes(&mut self, dest: &mut [u8]) {
// Fill using `next_u32`. This is faster for filling small slices (four
// bytes or less), while the overhead is negligible.
//
// This is done especially for wrappers that implement `next_u32`
// themselves via `fill_bytes`.
impls::fill_bytes_via_next(self, dest)
}
fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
Ok(self.fill_bytes(dest))
}
}