char *BN_bn2dec(const BIGNUM *a)
{
    int i = 0, num, ok = 0;
    char *buf = NULL;
    char *p;
    BIGNUM *t = NULL;
    BN_ULONG *bn_data = NULL, *lp;
    int bn_data_num;

    /*-
     * get an upper bound for the length of the decimal integer
     * num <= (BN_num_bits(a) + 1) * log(2)
     *     <= 3 * BN_num_bits(a) * 0.101 + log(2) + 1     (rounding error)
     *     <= 3 * BN_num_bits(a) / 10 + 3 * BN_num_bits / 1000 + 1 + 1
     */
    i = BN_num_bits(a) * 3;
    num = (i / 10 + i / 1000 + 1) + 1;
    bn_data_num = num / BN_DEC_NUM + 1;
    bn_data = OPENSSL_malloc(bn_data_num * sizeof(BN_ULONG));
    buf = OPENSSL_malloc(num + 3);
    if ((buf == NULL) || (bn_data == NULL)) {
        BNerr(BN_F_BN_BN2DEC, ERR_R_MALLOC_FAILURE);
        goto err;
    }
    if ((t = BN_dup(a)) == NULL)
        goto err;

    p = buf;
    lp = bn_data;
    if (BN_is_zero(t)) {
        *(p++) = '0';
        *(p++) = '\0';
    } else {
        if (BN_is_negative(t))
            *p++ = '-';

        while (!BN_is_zero(t)) {
            if (lp - bn_data >= bn_data_num)
                goto err;
            *lp = BN_div_word(t, BN_DEC_CONV);
            if (*lp == (BN_ULONG)-1)
                goto err;
            lp++;
        }
        lp--;
        /*
         * We now have a series of blocks, BN_DEC_NUM chars in length, where
         * the last one needs truncation. The blocks need to be reversed in
         * order.
         */
        sprintf(p, BN_DEC_FMT1, *lp);
        while (*p)
            p++;
        while (lp != bn_data) {
            lp--;
            sprintf(p, BN_DEC_FMT2, *lp);
            while (*p)
                p++;
        }
    }
    ok = 1;
 err:
    OPENSSL_free(bn_data);
    BN_free(t);
    if (ok)
        return buf;
    OPENSSL_free(buf);
    return NULL;
}