Add FromUniformBytes<64> to {k,p}256::Scalar (#1379)

Author: kayabaNerve

k256/src/arithmetic/scalar.rs

@@ -17,7 +17,7 @@ use core::{
 use elliptic_curve::{
     Curve, Error, ScalarPrimitive,
     bigint::{Limb, U256, U512, Word, prelude::*},
-    ff::{self, Field, PrimeField},
+    ff::{self, Field, FromUniformBytes, PrimeField},
     ops::{Invert, Reduce, ReduceNonZero},
     rand_core::{CryptoRng, TryCryptoRng, TryRngCore},
     scalar::{FromUintUnchecked, IsHigh},
@@ -380,6 +380,12 @@ impl From<u128> for Scalar {
     }
 }
 
+impl FromUniformBytes<64> for Scalar {
+    fn from_uniform_bytes(bytes: &[u8; 64]) -> Self {
+        WideScalar::from_bytes(bytes).reduce()
+    }
+}
+
 impl From<NonZeroScalar> for Scalar {
     fn from(scalar: NonZeroScalar) -> Self {
         *scalar.as_ref()

p256/src/arithmetic/scalar.rs

@@ -14,7 +14,7 @@ use core::{
 use elliptic_curve::{
     Curve,
     bigint::{Limb, NonZero, U256, prelude::*},
-    group::ff::{self, Field, PrimeField},
+    group::ff::{self, Field, FromUniformBytes, PrimeField},
     ops::{Invert, Reduce, ReduceNonZero},
     rand_core::TryRngCore,
     scalar::{FromUintUnchecked, IsHigh},
@@ -489,6 +489,15 @@ impl From<&Scalar> for U256 {
     }
 }
 
+impl FromUniformBytes<64> for Scalar {
+    fn from_uniform_bytes(bytes: &[u8; 64]) -> Self {
+        Self(barrett_reduce(
+            U256::from_be_slice(&bytes[32..]),
+            U256::from_be_slice(&bytes[..32]),
+        ))
+    }
+}
+
 #[cfg(feature = "bits")]
 impl From<&Scalar> for ScalarBits {
     fn from(scalar: &Scalar) -> ScalarBits {