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+//! Traits for arithmetic operations on elliptic curve field elements.
+
+pub use core::ops::{Add, AddAssign, Mul, Neg, Shr, ShrAssign, Sub, SubAssign};
+
+use crypto_bigint::Integer;
+use group::Group;
+
+/// Perform an inversion on a field element (i.e. base field element or scalar)
+pub trait Invert {
+ /// Field element type
+ type Output;
+
+ /// Invert a field element.
+ fn invert(&self) -> Self::Output;
+
+ /// Invert a field element in variable time.
+ ///
+ /// ⚠️ WARNING!
+ ///
+ /// This method should not be used with secret values, as its variable-time
+ /// operation can potentially leak secrets through sidechannels.
+ fn invert_vartime(&self) -> Self::Output {
+ // Fall back on constant-time implementation by default.
+ self.invert()
+ }
+}
+
+/// Linear combination.
+///
+/// This trait enables crates to provide an optimized implementation of
+/// linear combinations (e.g. Shamir's Trick), or otherwise provides a default
+/// non-optimized implementation.
+// TODO(tarcieri): replace this with a trait from the `group` crate? (see zkcrypto/group#25)
+pub trait LinearCombination: Group {
+ /// Calculates `x * k + y * l`.
+ fn lincomb(x: &Self, k: &Self::Scalar, y: &Self, l: &Self::Scalar) -> Self {
+ (*x * k) + (*y * l)
+ }
+}
+
+/// Multiplication by the generator.
+///
+/// May use optimizations (e.g. precomputed tables) when available.
+// TODO(tarcieri): replace this with `Group::mul_by_generator``? (see zkcrypto/group#44)
+pub trait MulByGenerator: Group {
+ /// Multiply by the generator of the prime-order subgroup.
+ #[must_use]
+ fn mul_by_generator(scalar: &Self::Scalar) -> Self {
+ Self::generator() * scalar
+ }
+}
+
+/// Modular reduction.
+pub trait Reduce<Uint: Integer>: Sized {
+ /// Bytes used as input to [`Reduce::reduce_bytes`].
+ type Bytes: AsRef<[u8]>;
+
+ /// Perform a modular reduction, returning a field element.
+ fn reduce(n: Uint) -> Self;
+
+ /// Interpret the given bytes as an integer and perform a modular reduction.
+ fn reduce_bytes(bytes: &Self::Bytes) -> Self;
+}
+
+/// Modular reduction to a non-zero output.
+///
+/// This trait is primarily intended for use by curve implementations such
+/// as the `k256` and `p256` crates.
+///
+/// End users should use the [`Reduce`] impl on
+/// [`NonZeroScalar`][`crate::NonZeroScalar`] instead.
+pub trait ReduceNonZero<Uint: Integer>: Reduce<Uint> + Sized {
+ /// Perform a modular reduction, returning a field element.
+ fn reduce_nonzero(n: Uint) -> Self;
+
+ /// Interpret the given bytes as an integer and perform a modular reduction
+ /// to a non-zero output.
+ fn reduce_nonzero_bytes(bytes: &Self::Bytes) -> Self;
+}