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/*
* cifra - embedded cryptography library
* Written in 2014 by Joseph Birr-Pixton <jpixton@gmail.com>
*
* To the extent possible under law, the author(s) have dedicated all
* copyright and related and neighboring rights to this software to the
* public domain worldwide. This software is distributed without any
* warranty.
*
* You should have received a copy of the CC0 Public Domain Dedication
* along with this software. If not, see
* <http://creativecommons.org/publicdomain/zero/1.0/>.
*/
#include "poly1305.h"
#include "bitops.h"
#include "handy.h"
#include "blockwise.h"
#include <string.h>
#include <stdio.h>
void cf_poly1305_init(cf_poly1305 *ctx,
const uint8_t r[16],
const uint8_t s[16])
{
memset(ctx, 0, sizeof *ctx);
ctx->r[0] = r[0];
ctx->r[1] = r[1];
ctx->r[2] = r[2];
ctx->r[3] = r[3] & 0x0f;
ctx->r[4] = r[4] & 0xfc;
ctx->r[5] = r[5];
ctx->r[6] = r[6];
ctx->r[7] = r[7] & 0x0f;
ctx->r[8] = r[8] & 0xfc;
ctx->r[9] = r[9];
ctx->r[10] = r[10];
ctx->r[11] = r[11] & 0x0f;
ctx->r[12] = r[12] & 0xfc;
ctx->r[13] = r[13];
ctx->r[14] = r[14];
ctx->r[15] = r[15] & 0x0f;
ctx->r[16] = 0;
memcpy(ctx->s, s, 16);
}
static void poly1305_add(uint32_t h[17],
const uint32_t x[17])
{
uint32_t carry = 0;
int i;
for (i = 0; i < 17; i++)
{
carry += h[i] + x[i];
h[i] = carry & 0xff;
carry >>= 8;
}
}
/* Minimal reduction/carry chain. */
static void poly1305_min_reduce(uint32_t x[17])
{
uint32_t carry = 0;
int i;
for (i = 0; i < 16; i++)
{
carry += x[i];
x[i] = carry & 0xff;
carry >>= 8;
}
/* 2 ** 130 - 5 = 0x3fffffffffffffffffffffffffffffffb
* ^
* So 2 bits of carry are put into top word.
* Remaining bits get multiplied by 5 and carried back
* into bottom */
carry += x[16];
x[16] = carry & 0x03;
carry = 5 * (carry >> 2);
for (i = 0; i < 16; i++)
{
carry += x[i];
x[i] = carry & 0xff;
carry >>= 8;
}
x[16] += carry;
}
/* This is - 2 ** 130 - 5 in twos complement. */
static const uint32_t negative_1305[17] = {
0x05, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0xfc
};
static void poly1305_full_reduce(uint32_t x[17])
{
uint32_t xsub[17];
size_t i;
for (i = 0; i < 17; i++)
xsub[i] = x[i];
poly1305_add(xsub, negative_1305);
/* If x - (2 ** 130 - 5) is negative, then
* x didn't need reduction: we discard the results.
* Do this in a side-channel silent way. */
uint32_t negative_mask = mask_u32(xsub[16] & 0x80, 0x80);
uint32_t positive_mask = negative_mask ^ 0xffffffff;
for (i = 0; i < 17; i++)
x[i] = (x[i] & negative_mask) | (xsub[i] & positive_mask);
}
static void poly1305_mul(uint32_t x[17],
const uint32_t y[17])
{
uint32_t r[17];
int i;
for (i = 0; i < 17; i++)
{
uint32_t accum = 0;
int j;
for (j = 0; j <= i; j++)
accum += x[j] * y[i - j];
/* Add in carries. These get shifted 130 bits
* to the right, with a combination of byte indexing
* and shifting (136 bits right, then 6 bits left).
*
* nb. 5 << 6 is made up of two parts:
* 5: reduction of 2 ** 130 leaves a multiple 5
* shift 6 places left
* 17 * 8: byte indexing shift (136 bits)
* 130: desired shift
*/
for (j = i + 1; j < 17; j++)
accum += (5 << 6) * x[j] * y[i + 17 - j];
r[i] = accum;
}
poly1305_min_reduce(r);
for (i = 0; i < 17; i++)
x[i] = r[i];
}
static void poly1305_block(cf_poly1305 *ctx,
const uint32_t c[17])
{
poly1305_add(ctx->h, c);
poly1305_mul(ctx->h, ctx->r);
}
static void poly1305_whole_block(void *vctx,
const uint8_t *buf)
{
cf_poly1305 *ctx = vctx;
uint32_t c[17];
int i;
for (i = 0; i < 16; i++)
c[i] = buf[i];
c[16] = 1;
poly1305_block(ctx, c);
}
static void poly1305_last_block(cf_poly1305 *ctx)
{
uint32_t c[17] = { 0 };
size_t i;
for (i = 0; i < ctx->npartial; i++)
c[i] = ctx->partial[i];
c[ctx->npartial] = 1;
poly1305_block(ctx, c);
}
void cf_poly1305_update(cf_poly1305 *ctx,
const uint8_t *buf,
size_t nbytes)
{
cf_blockwise_accumulate(ctx->partial, &ctx->npartial,
sizeof ctx->partial,
buf, nbytes,
poly1305_whole_block,
ctx);
}
void cf_poly1305_finish(cf_poly1305 *ctx,
uint8_t out[16])
{
if (ctx->npartial)
poly1305_last_block(ctx);
uint32_t s[17];
size_t i;
for (i = 0; i < 16; i++)
s[i] = ctx->s[i];
s[16] = 0;
poly1305_full_reduce(ctx->h);
poly1305_add(ctx->h, s);
for (i = 0; i < 16; i++)
out[i] = ctx->h[i];
mem_clean(ctx, sizeof *ctx);
}
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