pqc/crypto_kem/mceliece460896/clean/bm.c

/*
  This file is for the Berlekamp-Massey algorithm
  see http://crypto.stanford.edu/~mironov/cs359/massey.pdf
*/
#include "bm.h"

#include "params.h"

#define min(a, b) (((a) < (b)) ? (a) : (b))

/* the Berlekamp-Massey algorithm */
/* input: s, sequence of field elements */
/* output: out, minimal polynomial of s */
void PQCLEAN_MCELIECE460896_CLEAN_bm(gf *out, gf *s) {
    int i;

    uint16_t N = 0;
    uint16_t L = 0;
    uint16_t mle;
    uint16_t mne;

    gf T[ SYS_T + 1  ];
    gf C[ SYS_T + 1 ];
    gf B[ SYS_T + 1 ];

    gf b = 1, d, f;

    //

    for (i = 0; i < SYS_T + 1; i++) {
        C[i] = B[i] = 0;
    }

    B[1] = C[0] = 1;

    //

    for (N = 0; N < 2 * SYS_T; N++) {
        d = 0;

        for (i = 0; i <= min(N, SYS_T); i++) {
            d ^= PQCLEAN_MCELIECE460896_CLEAN_gf_mul(C[i], s[ N - i]);
        }

        mne = d;
        mne -= 1;
        mne >>= 15;
        mne -= 1;
        mle = N;
        mle -= 2 * L;
        mle >>= 15;
        mle -= 1;
        mle &= mne;

        for (i = 0; i <= SYS_T; i++) {
            T[i] = C[i];
        }

        f = PQCLEAN_MCELIECE460896_CLEAN_gf_frac(b, d);

        for (i = 0; i <= SYS_T; i++) {
            C[i] ^= PQCLEAN_MCELIECE460896_CLEAN_gf_mul(f, B[i]) & mne;
        }

        L = (L & ~mle) | ((N + 1 - L) & mle);

        for (i = 0; i <= SYS_T; i++) {
            B[i] = (B[i] & ~mle) | (T[i] & mle);
        }

        b = (b & ~mle) | (d & mle);

        for (i = SYS_T; i >= 1; i--) {
            B[i] = B[i - 1];
        }
        B[0] = 0;
    }

    for (i = 0; i <= SYS_T; i++) {
        out[i] = C[ SYS_T - i ];
    }
}