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ff: Move pow_vartime back into Field trait

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The only places we don't use constant u64 limbs, we use PrimeField::char
instead (except in a single test where we use a field element).
This commit is contained in:
Jack Grigg
2020-05-02 17:57:19 +12:00
parent fb31d09218
commit 15e229509a
13 changed files with 36 additions and 49 deletions
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33
ff/src/lib.rs
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@@ -76,36 +76,21 @@ pub trait Field:
/// Returns the square root of the field element, if it is
/// quadratic residue.
fn sqrt(&self) -> CtOption<Self>;
}
pub trait PowVartime<L>: Field
where
L: Copy + PartialEq + PartialOrd + AddAssign,
L: BitAnd<Output = L>,
L: Shr<Output = L>,
L: Sub<Output = L>,
{
const ZERO: L;
const ONE: L;
const LIMB_SIZE: L;
/// Exponentiates `self` by `exp`, where `exp` is a little-endian order
/// integer exponent.
///
/// **This operation is variable time with respect to the exponent.** If the
/// exponent is fixed, this operation is effectively constant time.
fn pow_vartime<S: AsRef<[L]>>(&self, exp: S) -> Self {
fn pow_vartime<S: AsRef<[u64]>>(&self, exp: S) -> Self {
let mut res = Self::one();
for e in exp.as_ref().iter().rev() {
let mut i = Self::ZERO;
while i < Self::LIMB_SIZE {
for i in (0..64).rev() {
res = res.square();
if ((*e >> (Self::LIMB_SIZE - Self::ONE - i)) & Self::ONE) == Self::ONE {
if ((*e >> i) & 1) == 1 {
res.mul_assign(self);
}
i += Self::ONE;
}
}
@@ -113,18 +98,6 @@ where
}
}
impl<T: Field> PowVartime<u8> for T {
const ZERO: u8 = 0;
const ONE: u8 = 1;
const LIMB_SIZE: u8 = 8;
}
impl<T: Field> PowVartime<u64> for T {
const ZERO: u64 = 0;
const ONE: u64 = 1;
const LIMB_SIZE: u64 = 64;
}
/// Helper trait for converting the binary representation of a prime field element into a
/// specific endianness. This is useful when you need to act on the bit representation
/// of an element generically, as the native binary representation of a prime field is