/**
* @brief Perform a single Barrett reduction.
* @param ctx [in] The bigint session context.
* @param bi [in] A bigint.
* @return The result of the Barrett reduction.
*/
bigint * ICACHE_FLASH_ATTR bi_barrett(BI_CTX *ctx, bigint *bi)
{
bigint *q1, *q2, *q3, *r1, *r2, *r;
uint8_t mod_offset = ctx->mod_offset;
bigint *bim = ctx->bi_mod[mod_offset];
int k = bim->size;
check(bi);
check(bim);
/* use Classical method instead - Barrett cannot help here */
if (bi->size > k*2)
{
return bi_mod(ctx, bi);
}
q1 = comp_right_shift(bi_clone(ctx, bi), k-1);
/* do outer partial multiply */
q2 = regular_multiply(ctx, q1, ctx->bi_mu[mod_offset], 0, k-1);
q3 = comp_right_shift(q2, k+1);
r1 = comp_mod(bi, k+1);
/* do inner partial multiply */
r2 = comp_mod(regular_multiply(ctx, q3, bim, k+1, 0), k+1);
r = bi_subtract(ctx, r1, r2, NULL);
/* if (r >= m) r = r - m; */
if (bi_compare(r, bim) >= 0)
{
r = bi_subtract(ctx, r, bim, NULL);
}
return r;
}
/**
* @brief Perform a multiplication operation between two bigints.
* @param ctx [in] The bigint session context.
* @param bia [in] A bigint.
* @param bib [in] Another bigint.
* @return The result of the multiplication.
*/
bigint *ICACHE_FLASH_ATTR bi_multiply(BI_CTX *ctx, bigint *bia, bigint *bib) {
check(bia);
check(bib);
#ifdef CONFIG_BIGINT_KARATSUBA
if (min(bia->size, bib->size) < MUL_KARATSUBA_THRESH) {
return regular_multiply(ctx, bia, bib, 0, 0);
}
return karatsuba(ctx, bia, bib, 0);
#else
return regular_multiply(ctx, bia, bib, 0, 0);
#endif
}