k256+p256+p384: impl Invert trait for Scalar types#741
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tarcieri
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Feb 3, 2023
| /// variable-time operation can potentially leak secrets through | ||
| /// sidechannels. | ||
| #[allow(non_snake_case)] | ||
| fn invert_vartime(&self) -> CtOption<Self> { |
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@fjarri FYI, there's now a dedicated method for variable-time inversions which is used by the generic ECDSA implementation.
This can probably be adapted to work with k256? I had previously tried to make it fully generic, but due to the use of Self(MODULUS) below it's relying on support in the underlying field implementation.
I tried porting it to k256 and it seemed to work, although ECDSA verification performance was only up by ~3%.
The `Invert` trait now provides an `invert_vartime` method which can use a faster implementation than the constant time version. Leveraging this requires an explicit impl of the trait however, whereas before it was provided by a blanket impl for types which impl `ff::Field`. This includes an implementation of Stein's algorithm for `p256::Scalar` originally provided by @nickray.
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The
Inverttrait now provides aninvert_vartimemethod which can use a faster implementation than the constant time version.Leveraging this requires an explicit impl of the trait however, whereas before it was provided by a blanket impl for types which impl
ff::Field.