pqcrypto/crypto_kem/mceliece460896/clean/controlbits.c

/*
  This file is for functions required for generating the control bits of the Benes network w.r.t. a random permutation
  see the Lev-Pippenger-Valiant paper https://www.computer.org/csdl/trans/tc/1981/02/06312171.pdf
*/

#include "controlbits.h"

#include "params.h"

#include <stdint.h>

typedef uint8_t bit;

#define N (1 << GFBITS)

static bit is_smaller(uint32_t a, uint32_t b) {
    uint32_t ret = 0;

    ret = a - b;
    ret >>= 31;

    return (bit)ret;
}

static bit is_smaller_63b(uint64_t a, uint64_t b) {
    uint64_t ret = 0;

    ret = a - b;
    ret >>= 63;

    return (bit)ret;
}

static void cswap(uint32_t *x, uint32_t *y, bit swap) {
    uint32_t m;
    uint32_t d;

    m = swap;
    m = 0 - m;

    d = (*x ^ *y);
    d &= m;
    *x ^= d;
    *y ^= d;
}

static void cswap_63b(uint64_t *x, uint64_t *y, bit swap) {
    uint64_t m;
    uint64_t d;

    m = swap;
    m = 0 - m;

    d = (*x ^ *y);
    d &= m;
    *x ^= d;
    *y ^= d;
}

/* output x = min(input x,input y) */
/* output y = max(input x,input y) */

static void minmax(uint32_t *x, uint32_t *y) {
    bit m;

    m = is_smaller(*y, *x);
    cswap(x, y, m);
}

static void minmax_63b(uint64_t *x, uint64_t *y) {
    bit m;

    m = is_smaller_63b(*y, *x);
    cswap_63b(x, y, m);
}

/* merge first half of x[0],x[step],...,x[(2*n-1)*step] with second half */
/* requires n to be a power of 2 */

static void merge(int n, uint32_t *x, int step) {
    int i;
    if (n == 1) {
        minmax(&x[0], &x[step]);
    } else {
        merge(n / 2, x, step * 2);
        merge(n / 2, x + step, step * 2);
        for (i = 1; i < 2 * n - 1; i += 2) {
            minmax(&x[i * step], &x[(i + 1) * step]);
        }
    }
}

static void merge_63b(int n, uint64_t *x, int step) {
    int i;
    if (n == 1) {
        minmax_63b(&x[0], &x[step]);
    } else {
        merge_63b(n / 2, x, step * 2);
        merge_63b(n / 2, x + step, step * 2);
        for (i = 1; i < 2 * n - 1; i += 2) {
            minmax_63b(&x[i * step], &x[(i + 1) * step]);
        }
    }
}

/* sort x[0],x[1],...,x[n-1] in place */
/* requires n to be a power of 2 */

static void sort(int n, uint32_t *x) {
    if (n <= 1) {
        return;
    }
    sort(n / 2, x);
    sort(n / 2, x + n / 2);
    merge(n / 2, x, 1);
}

void PQCLEAN_MCELIECE460896_CLEAN_sort_63b(int n, uint64_t *x) {
    if (n <= 1) {
        return;
    }
    PQCLEAN_MCELIECE460896_CLEAN_sort_63b(n / 2, x);
    PQCLEAN_MCELIECE460896_CLEAN_sort_63b(n / 2, x + n / 2);
    merge_63b(n / 2, x, 1);
}

/* y[pi[i]] = x[i] */
/* requires n = 2^w */
/* requires pi to be a permutation */
static void composeinv(int n, uint32_t *y, const uint32_t *x, const uint32_t *pi) { // NC
    int i;
    uint32_t t[2 * N];

    for (i = 0; i < n; ++i) {
        t[i] = x[i] | (pi[i] << 16);
    }

    sort(n, t);

    for (i = 0; i < n; ++i) {
        y[i] = t[i] & 0xFFFF;
    }
}

/* ip[i] = j iff pi[i] = j */
/* requires n = 2^w */
/* requires pi to be a permutation */
static void invert(int n, uint32_t *ip, const uint32_t *pi) {
    int i;

    for (i = 0; i < n; i++) {
        ip[i] = i;
    }

    composeinv(n, ip, ip, pi);
}


static void flow(int w, uint32_t *x, const uint32_t *y, int t) {
    bit m0;
    bit m1;

    uint32_t b;
    uint32_t y_copy = *y;

    m0 = is_smaller(*y & ((1 << w) - 1), *x & ((1 << w) - 1));
    m1 = is_smaller(0, t);

    cswap(x, &y_copy, m0);
    b = m0 & m1;
    *x ^= b << w;
}

/* input: permutation pi */
/* output: (2w-1)n/2 (or 0 if n==1) control bits c[0],c[step],c[2*step],... */
/* requires n = 2^w */
static void controlbitsfrompermutation(int w, int n, int step, int off, unsigned char *c, const uint32_t *pi) {
    int i;
    int j;
    int k;
    int t;
    uint32_t ip[N] = {0};
    uint32_t I[2 * N] = {0};
    uint32_t P[2 * N] = {0};
    uint32_t PI[2 * N] = {0};
    uint32_t T[2 * N] = {0};
    uint32_t piflip[N] = {0};
    uint32_t subpi[2][N / 2] = {{0}};

    if (w == 1) {
        c[ off / 8 ] |= (pi[0] & 1) << (off % 8);
    }
    if (w <= 1) {
        return;
    }

    invert(n, ip, pi);

    for (i = 0; i < n; ++i) {
        I[i] = ip[i] | (1 << w);
        I[n + i] = pi[i];
    }

    for (i = 0; i < 2 * n; ++i) {
        P[i] = (i >> w) + (i & ((1 << w) - 2)) + ((i & 1) << w);
    }

    for (t = 0; t < w; ++t) {
        composeinv(2 * n, PI, P, I);

        for (i = 0; i < 2 * n; ++i) {
            flow(w, &P[i], &PI[i], t);
        }

        for (i = 0; i < 2 * n; ++i) {
            T[i] = I[i ^ 1];
        }

        composeinv(2 * n, I, I, T);

        for (i = 0; i < 2 * n; ++i) {
            T[i] = P[i ^ 1];
        }

        for (i = 0; i < 2 * n; ++i) {
            flow(w, &P[i], &T[i], 1);
        }
    }

    for (i = 0; i < n; ++i) {
        for (j = 0; j < w; ++j) {
            piflip[i] = pi[i];
        }
    }

    for (i = 0; i < n / 2; ++i) {
        c[ (off + i * step) / 8 ] |= ((P[i * 2] >> w) & 1) << ((off + i * step) % 8);
    }
    for (i = 0; i < n / 2; ++i) {
        c[ (off + ((w - 1)*n + i) * step) / 8 ] |= ((P[n + i * 2] >> w) & 1) << ((off + ((w - 1) * n + i) * step) % 8);
    }

    for (i = 0; i < n / 2; ++i) {
        cswap(&piflip[i * 2], &piflip[i * 2 + 1], (P[n + i * 2] >> w) & 1);
    }

    for (k = 0; k < 2; ++k) {
        for (i = 0; i < n / 2; ++i) {
            subpi[k][i] = piflip[i * 2 + k] >> 1;
        }
    }

    for (k = 0; k < 2; ++k) {
        controlbitsfrompermutation(w - 1, n / 2, step * 2, off + step * (n / 2 + k), c, subpi[k]);
    }
}

/* input: pi, a permutation*/
/* output: out, control bits w.r.t. pi */
void PQCLEAN_MCELIECE460896_CLEAN_controlbits(unsigned char *out, const uint32_t *pi) {
    unsigned int i;
    unsigned char c[ (2 * GFBITS - 1) * (1 << GFBITS) / 16 ];

    for (i = 0; i < sizeof(c); i++) {
        c[i] = 0;
    }

    controlbitsfrompermutation(GFBITS, (1 << GFBITS), 1, 0, c, pi);

    for (i = 0; i < sizeof(c); i++) {
        out[i] = c[i];
    }
}