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Implementation of the megolm ratchet

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Richard van der Hoff 2016-04-27 17:16:55 +01:00
parent 42a300fc62
commit 68d3c7bfa9
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72
include/olm/megolm.h Normal file
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/* Copyright 2016 OpenMarket Ltd
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#ifndef OLM_MEGOLM_H_
#define OLM_MEGOLM_H_
/**
* implementation of the Megolm multi-part ratchet used in group chats.
*/
#include <stdint.h>
#include <stdlib.h>
#ifdef __cplusplus
extern "C" {
#endif
/**
* number of bytes in each part of the ratchet; this should be the same as
* the length of the hash function used in the HMAC (32 bytes for us, as we
* use HMAC-SHA-256)
*/
#define MEGOLM_RATCHET_PART_LENGTH 32 /* SHA256_OUTPUT_LENGTH */
/**
* number of parts in the ratchet; the advance() implementations rely on
* this being 4.
*/
#define MEGOLM_RATCHET_PARTS 4
#define MEGOLM_RATCHET_LENGTH (MEGOLM_RATCHET_PARTS * MEGOLM_RATCHET_PART_LENGTH)
typedef struct Megolm {
uint8_t data[MEGOLM_RATCHET_PARTS][MEGOLM_RATCHET_PART_LENGTH];
uint32_t counter;
} Megolm;
/**
* initialize the megolm ratchet. random_data should be at least
* MEGOLM_RATCHET_LENGTH bytes of randomness.
*/
void megolm_init(Megolm *megolm, uint8_t const *random_data, uint32_t counter);
/** advance the ratchet by one step */
void megolm_advance(Megolm *megolm);
/**
* get the key data in the ratchet. The returned data is
* MEGOLM_RATCHET_LENGTH bytes long.
*/
#define megolm_get_data(megolm) ((const uint8_t *)((megolm)->data))
/** advance the ratchet to a given count */
void megolm_advance_to(Megolm *megolm, uint32_t advance_to);
#ifdef __cplusplus
} // extern "C"
#endif
#endif /* OLM_MEGOLM_H_ */

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src/megolm.c Normal file
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/* Copyright 2016 OpenMarket Ltd
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#include "olm/megolm.h"
#include <string.h>
#include "olm/crypto.h"
/* the seeds used in the HMAC-SHA-256 functions for each part of the ratchet.
*/
#define HASH_KEY_SEED_LENGTH 1
static uint8_t HASH_KEY_SEEDS[MEGOLM_RATCHET_PARTS][HASH_KEY_SEED_LENGTH] = {
{0x00},
{0x01},
{0x02},
{0x03}
};
static void rehash_part(
uint8_t data[MEGOLM_RATCHET_PARTS][MEGOLM_RATCHET_PART_LENGTH],
int rehash_from_part, int rehash_to_part,
uint32_t old_counter, uint32_t new_counter
) {
_olm_crypto_hmac_sha256(
data[rehash_from_part],
MEGOLM_RATCHET_PART_LENGTH,
HASH_KEY_SEEDS[rehash_to_part], HASH_KEY_SEED_LENGTH,
data[rehash_to_part]
);
}
void megolm_init(Megolm *megolm, uint8_t const *random_data, uint32_t counter)
{
megolm->counter = counter;
memcpy(megolm->data, random_data, MEGOLM_RATCHET_LENGTH);
}
/* simplistic implementation for a single step */
void megolm_advance(Megolm *megolm) {
uint32_t mask = 0x00FFFFFF;
int h = 0;
int i;
megolm->counter++;
/* figure out how much we need to rekey */
while (h < (int)MEGOLM_RATCHET_PARTS) {
if (!(megolm->counter & mask))
break;
h++;
mask >>= 8;
}
/* now update R(h)...R(3) based on R(h) */
for (i = MEGOLM_RATCHET_PARTS-1; i >= h; i--) {
rehash_part(megolm->data, h, i, megolm->counter-1, megolm->counter);
}
}
void megolm_advance_to(Megolm *megolm, uint32_t advance_to) {
int j;
/* starting with R0, see if we need to update each part of the hash */
for (j = 0; j < (int)MEGOLM_RATCHET_PARTS; j++) {
int shift = (MEGOLM_RATCHET_PARTS-j-1) * 8;
uint32_t increment = 1 << shift;
uint32_t next_counter;
/* how many times to we need to rehash this part? */
int steps = (advance_to >> shift) - (megolm->counter >> shift);
if (steps == 0) {
continue;
}
megolm->counter = megolm->counter & ~(increment - 1);
next_counter = megolm->counter + increment;
/* for all but the last step, we can just bump R(j) without regard
* to R(j+1)...R(3).
*/
while (steps > 1) {
rehash_part(megolm->data, j, j, megolm->counter, next_counter);
megolm->counter = next_counter;
steps --;
next_counter = megolm->counter + increment;
}
/* on the last step (except for j=3), we need to bump at least R(j+1);
* depending on the target count, we may also need to bump R(j+2) and
* R(j+3).
*/
int k;
switch(j) {
case 0:
if (!(advance_to & 0xFFFF00)) { k = 3; }
else if (!(advance_to & 0xFF00)) { k = 2; }
else { k = 1; }
break;
case 1:
if (!(advance_to & 0xFF00)) { k = 3; }
else { k = 2; }
break;
case 2:
case 3:
k = 3;
break;
}
while (k >= j) {
rehash_part(megolm->data, j, k, megolm->counter, next_counter);
k--;
}
megolm->counter = next_counter;
}
}

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/* Copyright 2016 OpenMarket Ltd
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#include "olm/megolm.h"
#include "olm/memory.hh"
#include "unittest.hh"
int main() {
std::uint8_t random_bytes[] =
"0123456789ABCDEF0123456789ABCDEF"
"0123456789ABCDEF0123456789ABCDEF"
"0123456789ABCDEF0123456789ABCDEF"
"0123456789ABCDEF0123456789ABCDEF";
{
TestCase test_case("Megolm::advance");
Megolm mr;
megolm_init(&mr, random_bytes, 0);
// single-step advance
megolm_advance(&mr);
const std::uint8_t expected1[] = {
0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39, 0x41, 0x42, 0x43, 0x44, 0x45, 0x46,
0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39, 0x41, 0x42, 0x43, 0x44, 0x45, 0x46,
0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39, 0x41, 0x42, 0x43, 0x44, 0x45, 0x46,
0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39, 0x41, 0x42, 0x43, 0x44, 0x45, 0x46,
0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39, 0x41, 0x42, 0x43, 0x44, 0x45, 0x46,
0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39, 0x41, 0x42, 0x43, 0x44, 0x45, 0x46,
0xba, 0x9c, 0xd9, 0x55, 0x74, 0x1d, 0x1c, 0x16, 0x23, 0x23, 0xec, 0x82, 0x5e, 0x7c, 0x5c, 0xe8,
0x89, 0xbb, 0xb4, 0x23, 0xa1, 0x8f, 0x23, 0x82, 0x8f, 0xb2, 0x09, 0x0d, 0x6e, 0x2a, 0xf8, 0x6a
};
assert_equals(1U, mr.counter);
assert_equals(expected1, megolm_get_data(&mr), MEGOLM_RATCHET_LENGTH);
// repeat with complex advance
megolm_init(&mr, random_bytes, 0);
megolm_advance_to(&mr, 1);
assert_equals(1U, mr.counter);
assert_equals(expected1, megolm_get_data(&mr), MEGOLM_RATCHET_LENGTH);
megolm_advance_to(&mr, 0x1000000);
const std::uint8_t expected2[] = {
0x54, 0x02, 0x2d, 0x7d, 0xc0, 0x29, 0x8e, 0x16, 0x37, 0xe2, 0x1c, 0x97, 0x15, 0x30, 0x92, 0xf9,
0x33, 0xc0, 0x56, 0xff, 0x74, 0xfe, 0x1b, 0x92, 0x2d, 0x97, 0x1f, 0x24, 0x82, 0xc2, 0x85, 0x9c,
0x70, 0x04, 0xc0, 0x1e, 0xe4, 0x9b, 0xd6, 0xef, 0xe0, 0x07, 0x35, 0x25, 0xaf, 0x9b, 0x16, 0x32,
0xc5, 0xbe, 0x72, 0x6d, 0x12, 0x34, 0x9c, 0xc5, 0xbd, 0x47, 0x2b, 0xdc, 0x2d, 0xf6, 0x54, 0x0f,
0x31, 0x12, 0x59, 0x11, 0x94, 0xfd, 0xa6, 0x17, 0xe5, 0x68, 0xc6, 0x83, 0x10, 0x1e, 0xae, 0xcd,
0x7e, 0xdd, 0xd6, 0xde, 0x1f, 0xbc, 0x07, 0x67, 0xae, 0x34, 0xda, 0x1a, 0x09, 0xa5, 0x4e, 0xab,
0xba, 0x9c, 0xd9, 0x55, 0x74, 0x1d, 0x1c, 0x16, 0x23, 0x23, 0xec, 0x82, 0x5e, 0x7c, 0x5c, 0xe8,
0x89, 0xbb, 0xb4, 0x23, 0xa1, 0x8f, 0x23, 0x82, 0x8f, 0xb2, 0x09, 0x0d, 0x6e, 0x2a, 0xf8, 0x6a,
};
assert_equals(0x1000000U, mr.counter);
assert_equals(expected2, megolm_get_data(&mr), MEGOLM_RATCHET_LENGTH);
megolm_advance_to(&mr, 0x1041506);
const std::uint8_t expected3[] = {
0x54, 0x02, 0x2d, 0x7d, 0xc0, 0x29, 0x8e, 0x16, 0x37, 0xe2, 0x1c, 0x97, 0x15, 0x30, 0x92, 0xf9,
0x33, 0xc0, 0x56, 0xff, 0x74, 0xfe, 0x1b, 0x92, 0x2d, 0x97, 0x1f, 0x24, 0x82, 0xc2, 0x85, 0x9c,
0x55, 0x58, 0x8d, 0xf5, 0xb7, 0xa4, 0x88, 0x78, 0x42, 0x89, 0x27, 0x86, 0x81, 0x64, 0x58, 0x9f,
0x36, 0x63, 0x44, 0x7b, 0x51, 0xed, 0xc3, 0x59, 0x5b, 0x03, 0x6c, 0xa6, 0x04, 0xc4, 0x6d, 0xcd,
0x5c, 0x54, 0x85, 0x0b, 0xfa, 0x98, 0xa1, 0xfd, 0x79, 0xa9, 0xdf, 0x1c, 0xbe, 0x8f, 0xc5, 0x68,
0x19, 0x37, 0xd3, 0x0c, 0x85, 0xc8, 0xc3, 0x1f, 0x7b, 0xb8, 0x28, 0x81, 0x6c, 0xf9, 0xff, 0x3b,
0x95, 0x6c, 0xbf, 0x80, 0x7e, 0x65, 0x12, 0x6a, 0x49, 0x55, 0x8d, 0x45, 0xc8, 0x4a, 0x2e, 0x4c,
0xd5, 0x6f, 0x03, 0xe2, 0x44, 0x16, 0xb9, 0x8e, 0x1c, 0xfd, 0x97, 0xc2, 0x06, 0xaa, 0x90, 0x7a
};
assert_equals(0x1041506U, mr.counter);
assert_equals(expected3, megolm_get_data(&mr), MEGOLM_RATCHET_LENGTH);
}
}