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/*
* Montgomery Reduction
* (C) 1999-2008 Jack Lloyd
* 2006 Luca Piccarreta
*
* Distributed under the terms of the Botan license
*/
#include <botan/internal/mp_core.h>
#include <botan/internal/mp_asm.h>
#include <botan/internal/mp_asmi.h>
namespace Botan {
extern "C" {
/*
* Montgomery Reduction Algorithm
*/
void bigint_monty_redc(word z[], u32bit z_size,
const word x[], u32bit x_size, word u)
{
const u32bit blocks_of_8 = x_size - (x_size % 8);
for(u32bit i = 0; i != x_size; ++i)
{
word* z_i = z + i;
const word y = z_i[0] * u;
word carry = 0;
for(u32bit j = 0; j != blocks_of_8; j += 8)
carry = word8_madd3(z_i + j, x + j, y, carry);
for(u32bit j = blocks_of_8; j != x_size; ++j)
z_i[j] = word_madd3(x[j], y, z_i[j], &carry);
word z_sum = z_i[x_size] + carry;
carry = (z_sum < z_i[x_size]);
z_i[x_size] = z_sum;
for(u32bit j = x_size + 1; carry && j != z_size - i; ++j)
{
++z_i[j];
carry = !z_i[j];
}
}
// Check if z[x_size...x_size+1] >= x[0...x_size] using bigint_cmp (inlined)
if(!z[x_size + x_size])
{
for(u32bit i = x_size; i > 0; --i)
{
if(z[x_size + i - 1] > x[i-1])
break;
if(z[x_size + i - 1] < x[i-1])
return;
}
}
// If the compare above is true, subtract using bigint_sub2 (inlined)
word carry = 0;
for(u32bit i = 0; i != blocks_of_8; i += 8)
carry = word8_sub2(z + x_size + i, x + i, carry);
for(u32bit i = blocks_of_8; i != x_size; ++i)
z[x_size + i] = word_sub(z[x_size + i], x[i], &carry);
if(carry)
--z[x_size+x_size];
}
}
}
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