cosmian_kyber 0.1.0

use crate::params::KYBER_N;
use crate::poly::Poly;

// Name:        load32_littleendian
//
// Description: load 4 bytes into a 32-bit integer
//              in little-endian order
//
// Arguments:   - const [u8] x: input byte array
//
// Returns 32-bit unsigned integer loaded from x
fn load32_littleendian(x: &[u8]) -> u32 {
    let mut r = x[0] as u32;
    r |= (x[1] as u32) << 8;
    r |= (x[2] as u32) << 16;
    r |= (x[3] as u32) << 24;
    r
}

// Name:        load32_littleendian
//
// Description: load 3 bytes into a 32-bit integer
//              in little-endian order
//              This function is only needed for Kyber-512
//
// Arguments:   - const [u8] x: input byte array
//
// Returns 32-bit unsigned integer loaded from x
fn load24_littleendian(x: &[u8]) -> u32 {
    let mut r = x[0] as u32;
    r |= (x[1] as u32) << 8;
    r |= (x[2] as u32) << 16;
    r
}

// Name:        cbd2
//
// Description: Given an array of uniformly random bytes, compute
//              polynomial with coefficients distributed according to
//              a centered binomial distribution with parameter eta=2
//
// Arguments:   - poly *r:                  output polynomial
//              - const [u8] buf: input byte array
pub fn cbd2(r: &mut Poly, buf: &[u8]) {
    let (mut d, mut t, mut a, mut b);
    for i in 0..(KYBER_N / 8) {
        t = load32_littleendian(&buf[4 * i..]);
        d = t & 0x55555555;
        d += (t >> 1) & 0x55555555;
        for j in 0..8 {
            a = ((d >> (4 * j)) & 0x3) as i16;
            b = ((d >> (4 * j + 2)) & 0x3) as i16;
            r.coeffs[8 * i + j] = a - b;
        }
    }
}

// Name:        cbd3
//
// Description: Given an array of uniformly random bytes, compute
//              polynomial with coefficients distributed according to
//              a centered binomial distribution with parameter eta=3
//              This function is only needed for Kyber-512
// Arguments:   - poly *r:                  output polynomial
//              - const [u8] buf: input byte array
pub fn cbd3(r: &mut Poly, buf: &[u8]) {
    let (mut d, mut t, mut a, mut b);
    for i in 0..(KYBER_N / 4) {
        t = load24_littleendian(&buf[3 * i..]);
        d = t & 0x00249249;
        d += (t >> 1) & 0x00249249;
        d += (t >> 2) & 0x00249249;
        for j in 0..4 {
            a = ((d >> (6 * j)) & 0x7) as i16;
            b = ((d >> (6 * j + 3)) & 0x7) as i16;
            r.coeffs[4 * i + j] = a - b;
        }
    }
}

pub fn poly_cbd_eta1(r: &mut Poly, buf: &[u8]) {
    if cfg!(feature = "kyber512") {
        cbd3(r, buf)
    } else {
        cbd2(r, buf)
    }
}

pub fn poly_cbd_eta2(r: &mut Poly, buf: &[u8]) {
    cbd2(r, buf)
}