make Monty::new_params() take an Odd-wrapped modulus (#488)

Author: fjarri

src/modular/boxed_monty_form.rs

@@ -12,8 +12,7 @@ use super::{
     reduction::{montgomery_reduction_boxed, montgomery_reduction_boxed_mut},
     Retrieve,
 };
-use crate::{BoxedUint, Integer, Limb, Monty, Odd, Word};
-use subtle::CtOption;
+use crate::{BoxedUint, Limb, Monty, Odd, Word};
 
 #[cfg(feature = "std")]
 use std::sync::Arc;
@@ -233,12 +232,8 @@ impl Monty for BoxedMontyForm {
     type Integer = BoxedUint;
     type Params = BoxedMontyParams;
 
-    fn new_params(modulus: Self::Integer) -> CtOption<Self::Params> {
-        let is_odd = modulus.is_odd();
-
-        // Note: instantiates a potentially invalid `Odd`, but guards with `CtOption`.
-        let params = BoxedMontyParams::new(Odd(modulus));
-        CtOption::new(params, is_odd)
+    fn new_params(modulus: Odd<Self::Integer>) -> Self::Params {
+        BoxedMontyParams::new(modulus)
     }
 
     fn new(value: Self::Integer, params: Self::Params) -> Self {

src/modular/monty_form.rs

@@ -14,7 +14,7 @@ use super::{
     Retrieve,
 };
 use crate::{Limb, Monty, Odd, Uint, Word};
-use subtle::{Choice, ConditionallySelectable, ConstantTimeEq, CtOption};
+use subtle::{Choice, ConditionallySelectable, ConstantTimeEq};
 
 /// Parameters to efficiently go to/from the Montgomery form for an odd modulus provided at runtime.
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
@@ -204,8 +204,8 @@ impl<const LIMBS: usize> Monty for MontyForm<LIMBS> {
     type Integer = Uint<LIMBS>;
     type Params = MontyParams<LIMBS>;
 
-    fn new_params(modulus: Self::Integer) -> CtOption<Self::Params> {
-        Odd::new(modulus).map(MontyParams::new)
+    fn new_params(modulus: Odd<Self::Integer>) -> Self::Params {
+        MontyParams::new(modulus)
     }
 
     fn new(value: Self::Integer, params: Self::Params) -> Self {

src/traits.rs

@@ -8,7 +8,7 @@ pub use num_traits::{
 
 pub(crate) use sealed::PrecomputeInverterWithAdjuster;
 
-use crate::{Limb, NonZero};
+use crate::{Limb, NonZero, Odd};
 use core::fmt::Debug;
 use core::ops::{
     Add, AddAssign, BitAnd, BitAndAssign, BitOr, BitOrAssign, BitXor, BitXorAssign, Div, DivAssign,
@@ -553,9 +553,7 @@ pub trait Monty:
     type Params: Clone;
 
     /// Create the precomputed data for Montgomery representation of integers modulo `modulus`.
-    ///
-    /// `modulus` must be odd, otherwise returns `None`.
-    fn new_params(modulus: Self::Integer) -> CtOption<Self::Params>;
+    fn new_params(modulus: Odd<Self::Integer>) -> Self::Params;
 
     /// Convert the value into the representation using precomputed data.
     fn new(value: Self::Integer, params: Self::Params) -> Self;