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pub(crate) fn divide_scalar_bytes_by_cofactor(scalar: &mut [u8; 32]) {
let mut low = 0u8;
for i in scalar.iter_mut().rev() {
let r = *i & 0b00000111;
*i >>= 3;
*i += low;
low = r << 5;
}
}
pub(crate) fn multiply_scalar_bytes_by_cofactor(scalar: &mut [u8; 32]) {
let mut high = 0u8;
for i in scalar.iter_mut() {
let r = *i & 0b11100000;
*i <<= 3;
*i += high;
high = r >> 5;
}
}
#[cfg(test)]
mod tests {
use super::*;
use rand::{thread_rng, Rng};
#[test]
fn cofactor_adjustment() {
let mut x: [u8; 32] = thread_rng().gen();
x[31] &= 0b00011111;
let mut y = x.clone();
multiply_scalar_bytes_by_cofactor(&mut y);
divide_scalar_bytes_by_cofactor(&mut y);
assert_eq!(x, y);
let mut x: [u8; 32] = thread_rng().gen();
x[0] &= 0b11111000;
let mut y = x.clone();
divide_scalar_bytes_by_cofactor(&mut y);
multiply_scalar_bytes_by_cofactor(&mut y);
assert_eq!(x, y);
}
}