/*
* Karatsuba improves on regular multiplication due to only 3 multiplications
* being done instead of 4. The additional additions/subtractions are O(N)
* rather than O(N^2) and so for big numbers it saves on a few operations
*/
static bigint * ICACHE_FLASH_ATTR karatsuba(BI_CTX *ctx, bigint *bia, bigint *bib, int is_square)
{
bigint *x0, *x1;
bigint *p0, *p1, *p2;
int m;
if (is_square)
{
m = (bia->size + 1)/2;
}
else
{
m = (max(bia->size, bib->size) + 1)/2;
}
x0 = bi_clone(ctx, bia);
x0->size = m;
x1 = bi_clone(ctx, bia);
comp_right_shift(x1, m);
bi_free(ctx, bia);
/* work out the 3 partial products */
if (is_square)
{
p0 = bi_square(ctx, bi_copy(x0));
p2 = bi_square(ctx, bi_copy(x1));
p1 = bi_square(ctx, bi_add(ctx, x0, x1));
}
else /* normal multiply */
{
bigint *y0, *y1;
y0 = bi_clone(ctx, bib);
y0->size = m;
y1 = bi_clone(ctx, bib);
comp_right_shift(y1, m);
bi_free(ctx, bib);
p0 = bi_multiply(ctx, bi_copy(x0), bi_copy(y0));
p2 = bi_multiply(ctx, bi_copy(x1), bi_copy(y1));
p1 = bi_multiply(ctx, bi_add(ctx, x0, x1), bi_add(ctx, y0, y1));
}
p1 = bi_subtract(ctx,
bi_subtract(ctx, p1, bi_copy(p2), NULL), bi_copy(p0), NULL);
comp_left_shift(p1, m);
comp_left_shift(p2, 2*m);
return bi_add(ctx, p1, bi_add(ctx, p0, p2));
}
/*
* Work out g1, g3, g5, g7... etc for the sliding-window algorithm
*/
static void ICACHE_FLASH_ATTR precompute_slide_window(BI_CTX *ctx, int window, bigint *g1) {
int k = 1, i;
bigint *g2;
for (i = 0; i < window - 1; i++) { /* compute 2^(window-1) */
k <<= 1;
}
ctx->g = (bigint **)os_malloc(k * sizeof(bigint *));
ctx->g[0] = bi_clone(ctx, g1);
bi_permanent(ctx->g[0]);
g2 = bi_residue(ctx, bi_square(ctx, ctx->g[0])); /* g^2 */
for (i = 1; i < k; i++) {
ctx->g[i] = bi_residue(ctx, bi_multiply(ctx, ctx->g[i - 1], bi_copy(g2)));
bi_permanent(ctx->g[i]);
}
bi_free(ctx, g2);
ctx->window = k;
}
/**
* @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().
*/
bigint * ICACHE_FLASH_ATTR bi_mod_power(BI_CTX *ctx, bigint *bi, bigint *biexp)
{
int i = find_max_exp_index(biexp), j, window_size = 1;
bigint *biR = int_to_bi(ctx, 1);
#if defined(CONFIG_BIGINT_MONTGOMERY)
uint8_t mod_offset = ctx->mod_offset;
if (!ctx->use_classical)
{
/* preconvert */
bi = bi_mont(ctx,
bi_multiply(ctx, bi, ctx->bi_RR_mod_m[mod_offset])); /* x' */
bi_free(ctx, biR);
biR = ctx->bi_R_mod_m[mod_offset]; /* A */
}
#endif
check(bi);
check(biexp);
#ifdef CONFIG_BIGINT_SLIDING_WINDOW
for (j = i; j > 32; j /= 5) /* work out an optimum size */
window_size++;
/* work out the slide constants */
precompute_slide_window(ctx, window_size, bi);
#else /* just one constant */
ctx->g = (bigint **)SSL_MALLOC(sizeof(bigint *));
ctx->g[0] = bi_clone(ctx, bi);
ctx->window = 1;
bi_permanent(ctx->g[0]);
#endif
/* 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 (exp_bit_is_one(biexp, 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 (exp_bit_is_one(biexp, l) == 0)
l++; /* go back up */
}
/* build up the section of the exponent */
for (j = i; j >= l; j--)
{
biR = bi_residue(ctx, bi_square(ctx, biR));
if (exp_bit_is_one(biexp, j))
part_exp++;
if (j != l)
part_exp <<= 1;
}
part_exp = (part_exp-1)/2; /* adjust for array */
biR = bi_residue(ctx, bi_multiply(ctx, biR, ctx->g[part_exp]));
i = l-1;
}
else /* square it */
{
biR = bi_residue(ctx, bi_square(ctx, biR));
i--;
}
} while (i >= 0);
/* cleanup */
for (i = 0; i < ctx->window; i++)
{
bi_depermanent(ctx->g[i]);
bi_free(ctx, ctx->g[i]);
}
SSL_FREE(ctx->g);
bi_free(ctx, bi);
bi_free(ctx, biexp);
#if defined CONFIG_BIGINT_MONTGOMERY
return ctx->use_classical ? biR : bi_mont(ctx, biR); /* convert back */
#else /* CONFIG_BIGINT_CLASSICAL or CONFIG_BIGINT_BARRETT */
return biR;
#endif
}
