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Fix wNAF scalar multiplication for big-endian scalar repr #1689

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Fix wNAF scalar multiplication for big-endian scalar repr #1689

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1 change: 1 addition & 0 deletions p256/tests/projective.proptest-regressions
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Original file line number Diff line number Diff line change
Expand Up @@ -5,3 +5,4 @@
# It is recommended to check this file in to source control so that
# everyone who runs the test benefits from these saved cases.
cc e19ee42c127b7289fbe7e42df47abf141eb644afcbd13ac141e39b9960362174 # shrinks to point = ProjectivePoint { x: FieldElement(0x823CD15F6DD3C71933565064513A6B2BD183E554C6A08622F713EBBBFACE98BE), y: FieldElement(0x55DF5D5850F47BAD82149139979369FE498A9022A412B5E0BEDD2CFC21C3ED91), z: FieldElement(0x4FE342E2FE1A7F9B8EE7EB4A7C0F9E162BCE33576B315ECECBB6406837BF51F5) }, scalar = Scalar(0x0000000000000000000000000000000000000000000000000000000000000001)
cc 67d76546dee30db7f75f666ed335f84d90da4ce8775d612dcdb88a3058ef7071 # shrinks to point = ProjectivePoint { x: FieldElement(0x823CD15F6DD3C71933565064513A6B2BD183E554C6A08622F713EBBBFACE98BE), y: FieldElement(0x55DF5D5850F47BAD82149139979369FE498A9022A412B5E0BEDD2CFC21C3ED91), z: FieldElement(0x4FE342E2FE1A7F9B8EE7EB4A7C0F9E162BCE33576B315ECECBB6406837BF51F5) }, scalar = Scalar(0xAE74000000000000000000000000000000000000000000000000000000000000)
20 changes: 19 additions & 1 deletion primeorder/src/projective.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -605,7 +605,25 @@ where
FieldBytes<C>: Copy,
{
fn recommended_wnaf_for_num_scalars(num_scalars: usize) -> usize {
todo!()
// Empirical heuristic from the zcash/bellman implementation.
if num_scalars >= 32 {
3
} else if num_scalars >= 1 {
2
} else {
4
}
}

fn scalar_repr_to_le_bytes(
repr: &<Scalar<C> as PrimeField>::Repr,
) -> Vec<u8> {
// SEC1/NIST curves use big-endian scalar representations;
// reverse to get little-endian for wNAF decomposition.
let mut le: Vec<u8> =
AsRef::<[u8]>::as_ref(repr).to_vec();
le.reverse();
le
}
}

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