cbd.c

#include <stdint.h>
#include "params.h"
#include "cbd.h"

/*************************************************
* Name:        load32_littleendian
*
* Description: load bytes into a 32-bit integer
*              in little-endian order
*
* Arguments:   - const uint8_t *x: pointer to input byte array
*
* Returns 32-bit unsigned integer loaded from x
**************************************************/
static uint32_t load32_littleendian(const uint8_t x[4])
{
  uint32_t r;
  r  = (uint32_t)x[0];
  r |= (uint32_t)x[1] << 8;
  r |= (uint32_t)x[2] << 16;
  r |= (uint32_t)x[3] << 24;
  return r;
}

/*************************************************
* Name:        cbd
*
* Description: Given an array of uniformly random bytes, compute
*              polynomial with coefficients distributed according to
*              a centered binomial distribution with parameter KYBER_ETA
*
* Arguments:   - poly *r:            pointer to output polynomial
*              - const uint8_t *buf: pointer to input byte array
**************************************************/
void cbd(poly *r, const uint8_t buf[KYBER_ETA*KYBER_N/4])
{
#if KYBER_ETA != 2
#error "poly_getnoise in poly.c only supports eta=2"
#endif
  unsigned int i,j;
  uint32_t t,d;
  int16_t a,b;

  for(i=0;i<KYBER_N/8;i++) {
    t  = load32_littleendian(buf+4*i);
    d  = t & 0x55555555;
    d += (t>>1) & 0x55555555;

    for(j=0;j<8;j++) {
      a = (d >> (4*j+0)) & 0x3;
      b = (d >> (4*j+2)) & 0x3;
      r->coeffs[8*i+j] = a - b;
    }
  }
}