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/*
* Copyright (c) 2017 Thomas Pornin <pornin@bolet.org>
*
* Permission is hereby granted, free of charge, to any person obtaining
* a copy of this software and associated documentation files (the
* "Software"), to deal in the Software without restriction, including
* without limitation the rights to use, copy, modify, merge, publish,
* distribute, sublicense, and/or sell copies of the Software, and to
* permit persons to whom the Software is furnished to do so, subject to
* the following conditions:
*
* The above copyright notice and this permission notice shall be
* included in all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
* NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
* BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
* ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
* CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
* SOFTWARE.
*/
#include "inner.h"
/*
* This is a "reference" implementation of Poly1305 that uses the
* generic "i15" code for big integers. It is slow, but it handles all
* big-integer operations with generic code, thereby avoiding most
* tricky situations with carry propagation and modular reduction.
*/
/*
* Modulus: 2^130-5.
*/
static const uint16_t P1305[] = {
0x008A,
0x7FFB, 0x7FFF, 0x7FFF, 0x7FFF, 0x7FFF, 0x7FFF, 0x7FFF, 0x7FFF, 0x03FF
};
/*
* -p mod 2^15.
*/
#define P0I 0x4CCD
/*
* R^2 mod p, for conversion to Montgomery representation (R = 2^135,
* since we use 9 words of 15 bits each, and 15*9 = 135).
*/
static const uint16_t R2[] = {
0x008A,
0x6400, 0x0000, 0x0000, 0x0000, 0x0000, 0x0000, 0x0000, 0x0000, 0x0000
};
/*
* Perform the inner processing of blocks for Poly1305. The "r" array
* is in Montgomery representation, while the "a" array is not.
*/
static void
poly1305_inner(uint16_t *a, const uint16_t *r, const void *data, size_t len)
{
const unsigned char *buf;
buf = data;
while (len > 0) {
unsigned char tmp[16], rev[16];
uint16_t b[10];
uint32_t ctl;
int i;
/*
* If there is a partial block, right-pad it with zeros.
*/
if (len < 16) {
memset(tmp, 0, sizeof tmp);
memcpy(tmp, buf, len);
buf = tmp;
len = 16;
}
/*
* Decode next block and apply the "high bit". Since
* decoding is little-endian, we must byte-swap the buffer.
*/
for (i = 0; i < 16; i ++) {
rev[i] = buf[15 - i];
}
br_i15_decode_mod(b, rev, sizeof rev, P1305);
b[9] |= 0x0100;
/*
* Add the accumulator to the decoded block (modular
* addition).
*/
ctl = br_i15_add(b, a, 1);
ctl |= NOT(br_i15_sub(b, P1305, 0));
br_i15_sub(b, P1305, ctl);
/*
* Multiply by r, result is the new accumulator value.
*/
br_i15_montymul(a, b, r, P1305, P0I);
buf += 16;
len -= 16;
}
}
/*
* Byteswap a 16-byte value.
*/
static void
byteswap16(unsigned char *buf)
{
int i;
for (i = 0; i < 8; i ++) {
unsigned x;
x = buf[i];
buf[i] = buf[15 - i];
buf[15 - i] = x;
}
}
/* see bearssl_block.h */
void
br_poly1305_i15_run(const void *key, const void *iv,
void *data, size_t len, const void *aad, size_t aad_len,
void *tag, br_chacha20_run ichacha, int encrypt)
{
unsigned char pkey[32], foot[16];
uint16_t t[10], r[10], acc[10];
/*
* Compute the MAC key. The 'r' value is the first 16 bytes of
* pkey[].
*/
memset(pkey, 0, sizeof pkey);
ichacha(key, iv, 0, pkey, sizeof pkey);
/*
* If encrypting, ChaCha20 must run first, followed by Poly1305.
* When decrypting, the operations are reversed.
*/
if (encrypt) {
ichacha(key, iv, 1, data, len);
}
/*
* Run Poly1305. We must process the AAD, then ciphertext, then
* the footer (with the lengths). Note that the AAD and ciphertext
* are meant to be padded with zeros up to the next multiple of 16,
* and the length of the footer is 16 bytes as well.
*/
/*
* Apply the "clamping" operation on the encoded 'r' value.
*/
pkey[ 3] &= 0x0F;
pkey[ 7] &= 0x0F;
pkey[11] &= 0x0F;
pkey[15] &= 0x0F;
pkey[ 4] &= 0xFC;
pkey[ 8] &= 0xFC;
pkey[12] &= 0xFC;
/*
* Decode the clamped 'r' value. Decoding should use little-endian
* so we must byteswap the value first.
*/
byteswap16(pkey);
br_i15_decode_mod(t, pkey, 16, P1305);
/*
* Convert 'r' to Montgomery representation.
*/
br_i15_montymul(r, t, R2, P1305, P0I);
/*
* Accumulator is 0.
*/
br_i15_zero(acc, 0x8A);
/*
* Process the additional authenticated data, ciphertext, and
* footer in due order.
*/
br_enc64le(foot, (uint64_t)aad_len);
br_enc64le(foot + 8, (uint64_t)len);
poly1305_inner(acc, r, aad, aad_len);
poly1305_inner(acc, r, data, len);
poly1305_inner(acc, r, foot, sizeof foot);
/*
* Decode the value 's'. Again, a byteswap is needed.
*/
byteswap16(pkey + 16);
br_i15_decode_mod(t, pkey + 16, 16, P1305);
/*
* Add the value 's' to the accumulator. That addition is done
* modulo 2^128, so we just ignore the carry.
*/
br_i15_add(acc, t, 1);
/*
* Encode the result (128 low bits) to the tag. Encoding should
* be little-endian.
*/
br_i15_encode(tag, 16, acc);
byteswap16(tag);
/*
* If decrypting, then ChaCha20 runs _after_ Poly1305.
*/
if (!encrypt) {
ichacha(key, iv, 1, data, len);
}
}
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