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Implement JitterRng, based on jitterentropy-library. #28

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merged 7 commits into from
Nov 17, 2017
Merged

Implement JitterRng, based on jitterentropy-library. #28

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07e1d58
Implement `JitterRng`, based on `jitterentropy-library`.
pitdicker Oct 30, 2017
6e40910
Relicensing permission
pitdicker Nov 2, 2017
49e9bb0
Provide statistics function
pitdicker Nov 4, 2017
ed358de
Make the number of entropy colelction rounds variable
pitdicker Nov 8, 2017
c57aa1a
Address review comments
pitdicker Nov 8, 2017
08dc010
Merge remote-tracking branch 'upstream/master' into jitter_rng
pitdicker Nov 11, 2017
14136e7
Fix `no_std` errors
pitdicker Nov 11, 2017
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9 changes: 6 additions & 3 deletions src/jitter_rng.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -109,7 +109,7 @@ impl ErrorKind {
ErrorKind::CoarseTimer => "coarse timer",
ErrorKind::NotMonotonic => "timer not monotonic",
ErrorKind::TinyVariantions => "time delta variations too small",
ErrorKind::ToManyStuck => "to many stuck results",
ErrorKind::ToManyStuck => "too many stuck results",
ErrorKind::__Nonexhaustive => unreachable!(),
}
}
Expand All @@ -127,7 +127,7 @@ impl fmt::Display for TimerError {
ErrorKind::TinyVariantions =>
write!(f, "Variations of deltas of time too small."),
ErrorKind::ToManyStuck =>
write!(f, "To many stuck results (indicating no added entropy)."),
write!(f, "Too many stuck results (indicating no added entropy)."),
ErrorKind::__Nonexhaustive => unreachable!(),
}
}
Expand Down Expand Up @@ -361,6 +361,9 @@ impl JitterRng {
// 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 than is not strictly
// monotonic.
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a timer that is not strictly ... ?

let current_delta = time.wrapping_sub(self.prev_time) as i64;
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It might be worth adding a note that wrapping arithmetic on unsigned integers followed by cast to i64 generates correct deltas in most cases (including where both times have the highest bit set and where one has wrapped). [The exception is where the actual delta is not representable as an i64, but this shouldn't be the case.]

self.prev_time = time;

Expand Down Expand Up @@ -422,7 +425,7 @@ impl JitterRng {
// we rely on it to not recognise the opportunity.
for i in 0..64 {
let apply = (self.data >> i) & 1;
let mask = !((apply as i64) - 1) as u64;
let mask = !apply.wrapping_sub(1);
mixer ^= CONSTANT & mask;
mixer = mixer.rotate_left(1);
}
Expand Down