pqcrypto/crypto_kem/ntruhps4096821/clean/sample_iid.c

#include "sample.h"

static uint16_t mod3(uint16_t a) {
    uint16_t r;
    int16_t t, c;

    r = (a >> 8) + (a & 0xff); // r mod 255 == a mod 255
    r = (r >> 4) + (r & 0xf); // r' mod 15 == r mod 15
    r = (r >> 2) + (r & 0x3); // r' mod 3 == r mod 3
    r = (r >> 2) + (r & 0x3); // r' mod 3 == r mod 3

    t = r - 3;
    c = t >> 15;

    return (c & r) ^ (~c & t);
}

void PQCLEAN_NTRUHPS4096821_CLEAN_sample_iid(poly *r, const unsigned char uniformbytes[NTRU_SAMPLE_IID_BYTES]) {
    int i;
    /* {0,1,...,255} -> {0,1,2}; Pr[0] = 86/256, Pr[1] = Pr[-1] = 85/256 */
    for (i = 0; i < NTRU_N - 1; i++) {
        r->coeffs[i] = mod3(uniformbytes[i]);
    }

    r->coeffs[NTRU_N - 1] = 0;
}