Add prime-field crate

prime-field introduces a macro to generate a prime field, in its entitrety,
de-duplicating code across minimal-ed448, embedwards25519, and secq256k1.

crypto/evrf/secq256k1/src/lib.rs

@@ -10,20 +10,39 @@ use std_shims::io::{self, Read};
 use k256::elliptic_curve::{
   zeroize::Zeroize,
   generic_array::typenum::{Sum, Diff, Quot, U, U1, U2},
-  group::{ff::PrimeField, Group},
+  group::{
+    ff::{PrimeField, FromUniformBytes},
+    Group,
+  },
 };
 
-#[macro_use]
-mod backend;
-
-mod scalar;
-pub use scalar::Scalar;
+prime_field::odd_prime_field!(
+  Scalar,
+  "fffffffffffffffffffffffffffffffffffffffffffffffffffffffefffffc2f",
+  "03",
+  true
+);
 
 pub use k256::Scalar as FieldElement;
 
 mod point;
 pub use point::Point;
 
+pub(crate) fn u8_from_bool(bit_ref: &mut bool) -> u8 {
+  use core::hint::black_box;
+  use prime_field::zeroize::Zeroize;
+
+  let bit_ref = black_box(bit_ref);
+
+  let mut bit = black_box(*bit_ref);
+  let res = black_box(u8::from(bit));
+  bit.zeroize();
+  debug_assert!((res | 1) == 1);
+
+  bit_ref.zeroize();
+  res
+}
+
 /// Ciphersuite for Secq256k1.
 ///
 /// hash_to_F is implemented with a naive concatenation of the dst and data, allowing transposition
@@ -47,7 +66,9 @@ impl ciphersuite::Ciphersuite for Secq256k1 {
 
   fn hash_to_F(dst: &[u8], data: &[u8]) -> Self::F {
     use blake2::Digest;
-    Scalar::wide_reduce(Self::H::digest([dst, data].concat()).as_slice().try_into().unwrap())
+    <Scalar as FromUniformBytes<64>>::from_uniform_bytes(
+      &Self::H::digest([dst, data].concat()).into(),
+    )
   }
 
   // We override the provided impl, which compares against the reserialization, because