mpi_rand.c

#include "mpi_pvt.h"
#include <cry/prng.h>
#include <stdlib.h>

/*
 * Generate a random integer with a given number of bits.
 */
int cry_mpi_rand(cry_mpi *r, size_t bits)
{
    int res;
    size_t digs, msb, siz;
    unsigned char *bin;

    if (bits == 0) {
        cry_mpi_zero(r);
        return 0;
    }
    siz = (bits + 7) >> 3;
    if ((bin = malloc(siz)) == NULL)
        return CRY_ERROR_OUT_OF_MEMORY;
    if ((res = cry_prng_aes_rand(bin, siz)) != 0)
        goto e;
    if ((res = cry_mpi_load_bin(r, bin, siz)) != 0)
        goto e;

    digs = CRY_MPI_BITS_TO_DIGS(bits);
    msb = bits - CRY_MPI_DIGIT_BITS * (digs - 1);
    if (msb < CRY_MPI_DIGIT_BITS)
        r->data[digs-1] &= ((cry_mpi_digit)1 << msb) - 1; /* Clear extra bits */
    r->data[digs-1] |= ((cry_mpi_digit)1 << (msb - 1));   /* Set the msb */

e:  free(bin);
    return res;
}

int cry_mpi_rand_range(cry_mpi *a, const cry_mpi *max)
{
    int res;
    size_t n;

    n = cry_mpi_count_bits(max);
    if ((res = cry_mpi_rand(a, n)) == 0) {
        if (cry_mpi_cmp_abs(a, max) >= 0)
            res = cry_mpi_mod(a, a, max);
    }
    return res;
}