cosmian_kyber 0.1.0

use crate::params::*;

const QINV: i32 = 62209; // q^(-1) mod 2^16

// Name:        montgomery_reduce
//
// Description: Montgomery reduction; given a 32-bit integer a, computes
//              16-bit integer congruent to a * R^-1 mod q,
//              where R=2^16
//
// Arguments:   - i32 a: input integer to be reduced; has to be in {-q2^15,...,q2^15-1}
//
// Returns:     integer in {-q+1,...,q-1} congruent to a * R^-1 modulo q.
pub fn montgomery_reduce(a: i32) -> i16 {
    let ua = a.wrapping_mul(QINV) as i16;
    let u = ua as i32;
    let mut t = u * KYBER_Q as i32;
    t = a - t;
    t >>= 16;
    t as i16
}

// Name:        barrett_reduce
//
// Description: Barrett reduction; given a 16-bit integer a, computes
//              centered representative congruent to a mod q in {-(q-1)/2,...,(q-1)/2}
//
// Arguments:   - i16 a: input integer to be reduced
//
// Returns:     i16 in {-(q-1)/2,...,(q-1)/2} congruent to a modulo q.
pub fn barrett_reduce(a: i16) -> i16 {
    let v = ((1u32 << 26) / KYBER_Q as u32 + 1) as i32;
    let mut t = v * a as i32 + (1 << 25);
    t >>= 26;
    t *= KYBER_Q as i32;
    a - t as i16
}