Add Scalar::halve()
#804
Merged
Add Scalar::halve()
#804
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It seems like you could implement a much more efficient divide-by-two operation using a right shift over the limbs of the unsaturated 52-bit representation used by |
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I originally did exactly that, but that doesn't have the same output and doesn't seem to work; the test will fail. |
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@daxpedda I think it might need the same trick that the |
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#805 should be close, I think |
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I see! Seems to work great! |
tarcieri
reviewed
Aug 14, 2025
| @@ -1867,6 +1867,35 @@ mod test { | |||
| } | |||
| } | |||
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| #[test] | |||
| #[cfg(all(feature = "alloc", feature = "rand_core", feature = "group"))] | |||
| fn multiply_double_and_compress_1024_random_points() { | |||
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We should really get proptest set up, this is fine for now though.
cc @rozbb
tarcieri
approved these changes
Aug 14, 2025
tarcieri
merged commit 25b424a
into
dalek-cryptography:scalar-div-by-2
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This adds a new method
Scalar::halve()which halves the givenScalar.This can facilitate batch encoding the result of scalar-point multiplications by halving the
Scalarand then usingRistrettoPoint::double_and_compress_batch(). I've added a test for this.