Move FieldExt::get_lower_32 to SqrtRatio trait

It is only used internally by the table-based square root impl, and we
should probably refactor this further, but for now it can live in the
sqrt extension trait.

CHANGELOG.md

@@ -15,8 +15,8 @@ and this project adheres to Rust's notion of
 - `pasta_curves::arithmetic`:
   - `Field` re-export (`pasta_curves::group::ff::Field` is equivalent).
   - `FieldExt::ROOT_OF_UNITY` (use `ff::PrimeField::root_of_unity` instead).
-  - `FieldExt::{T_MINUS1_OVER2, pow_by_t_minus1_over2, sqrt_alt, sqrt_ratio}`
-    (moved to `SqrtRatio` trait).
+  - `FieldExt::{T_MINUS1_OVER2, pow_by_t_minus1_over2, get_lower_32, sqrt_alt,`
+    `sqrt_ratio}` (moved to `SqrtRatio` trait).
   - `FieldExt::{RESCUE_ALPHA, RESCUE_INVALPHA}`
   - `FieldExt::from_u64` (use `From<u64> for ff::PrimeField` instead).
   - `FieldExt::{from_bytes, read, to_bytes, write}`

src/arithmetic/fields.rs

@@ -31,6 +31,10 @@ pub trait SqrtRatio: ff::PrimeField {
         ff::Field::pow_vartime(&self, &Self::T_MINUS1_OVER2)
     }
 
+    /// Gets the lower 32 bits of this field element when expressed
+    /// canonically.
+    fn get_lower_32(&self) -> u32;
+
     /// Computes:
     ///
     /// - $(\textsf{true}, \sqrt{\textsf{num}/\textsf{div}})$, if $\textsf{num}$ and
@@ -103,10 +107,6 @@ pub trait FieldExt: SqrtRatio + From<bool> + Ord + Group<Scalar = Self> {
     /// Gets the lower 128 bits of this field element when expressed
     /// canonically.
     fn get_lower_128(&self) -> u128;
-
-    /// Gets the lower 32 bits of this field element when expressed
-    /// canonically.
-    fn get_lower_32(&self) -> u32;
 }
 
 /// Tonelli–Shanks' square-root algorithm for `p mod 16 = 1`.

src/fields/fp.rs

@@ -704,6 +704,13 @@ impl SqrtRatio for Fp {
         rs.square() // rt
     }
 
+    fn get_lower_32(&self) -> u32 {
+        // TODO: don't reduce, just hash the Montgomery form. (Requires rebuilding perfect hash table.)
+        let tmp = Fp::montgomery_reduce(self.0[0], self.0[1], self.0[2], self.0[3], 0, 0, 0, 0);
+
+        tmp.0[0] as u32
+    }
+
     fn sqrt_ratio(num: &Self, div: &Self) -> (Choice, Self) {
         FP_TABLES.sqrt_ratio(num, div)
     }
@@ -761,13 +768,6 @@ impl FieldExt for Fp {
 
         u128::from(tmp.0[0]) | (u128::from(tmp.0[1]) << 64)
     }
-
-    fn get_lower_32(&self) -> u32 {
-        // TODO: don't reduce, just hash the Montgomery form. (Requires rebuilding perfect hash table.)
-        let tmp = Fp::montgomery_reduce(self.0[0], self.0[1], self.0[2], self.0[3], 0, 0, 0, 0);
-
-        tmp.0[0] as u32
-    }
 }
 
 #[cfg(all(test, feature = "std"))]

src/fields/fq.rs

@@ -704,6 +704,13 @@ impl SqrtRatio for Fq {
         sqr(ss, 4) // st
     }
 
+    fn get_lower_32(&self) -> u32 {
+        // TODO: don't reduce, just hash the Montgomery form. (Requires rebuilding perfect hash table.)
+        let tmp = Fq::montgomery_reduce(self.0[0], self.0[1], self.0[2], self.0[3], 0, 0, 0, 0);
+
+        tmp.0[0] as u32
+    }
+
     fn sqrt_ratio(num: &Self, div: &Self) -> (Choice, Self) {
         FQ_TABLES.sqrt_ratio(num, div)
     }
@@ -761,13 +768,6 @@ impl FieldExt for Fq {
 
         u128::from(tmp.0[0]) | (u128::from(tmp.0[1]) << 64)
     }
-
-    fn get_lower_32(&self) -> u32 {
-        // TODO: don't reduce, just hash the Montgomery form. (Requires rebuilding perfect hash table.)
-        let tmp = Fq::montgomery_reduce(self.0[0], self.0[1], self.0[2], self.0[3], 0, 0, 0, 0);
-
-        tmp.0[0] as u32
-    }
 }
 
 #[cfg(all(test, feature = "std"))]

src/hashtocurve.rs

@@ -171,11 +171,7 @@ pub fn map_to_curve_simple_swu<F: FieldExt, C: CurveExt<Base = F>, I: CurveExt<B
     let y = F::conditional_select(&y2, &y1, gx1_square);
 
     // 9. If sgn0(u) != sgn0(y), set y = -y
-    let y = F::conditional_select(
-        &(-y),
-        &y,
-        (u.get_lower_32() % 2).ct_eq(&(y.get_lower_32() % 2)),
-    );
+    let y = F::conditional_select(&(-y), &y, u.is_odd().ct_eq(&y.is_odd()));
 
     I::new_jacobian(num_x * div, y * div3, div).unwrap()
 }