/* @brief Perform a modular exponentiation.
* This function requires bi_set_mod() to have been called previously. This is
* one of the optimisations used for performance.
* @param ctx [in] The bigint session context.
* @param bi [in] The bigint on which to perform the mod power operation.
* @param biexp [in] The bigint exponent.
* @return The result of the mod exponentiation operation
* @see bi_set_mod(). */
internal bigint bi_mod_power(bigint bi, bigint biexp)
{
int i = biexp.find_max_exp_index();
int window_size = 1;
bigint biR = this.int_to_bi(1);
this.g = new bigint[] { this.bi_clone(bi) };
this.window = 1;
this.g[0].bi_permanent();
/* if sliding-window is off, then only one bit will be done at a time and
* will reduce to standard left-to-right exponentiation */
do
{
if (biexp.exp_bit_is_one(i))
{
int l = i - window_size + 1;
int part_exp = 0;
if (l < 0)
{
/* LSB of exponent will always be 1 */
l = 0;
}
else
{
while (!biexp.exp_bit_is_one(l))
{
l++; /* go back up */
}
}
/* build up the section of the exponent */
for (int j = i; j >= l; j--)
{
biR = this.bi_Modulo(this.bi_square(biR));
if (biexp.exp_bit_is_one(j))
{
part_exp++;
}
if (j != l)
{
part_exp <<= 1;
}
}
part_exp = (part_exp - 1) / 2; /* adjust for array */
biR = this.bi_Modulo(this.bi_multiply(biR, this.g[part_exp]));
i = l - 1;
}
else /* square it */
{
biR = this.bi_Modulo(this.bi_square(biR));
i--;
}
} while (i >= 0);
/* cleanup */
for (i = 0; i < this.window; i++)
{
bi_depermanent(this.g[i]);
this.bi_free(this.g[i]);
}
this.g = null;
this.bi_free(bi);
this.bi_free(biexp);
return(biR);
}
