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olm/src/ratchet.cpp

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/* Copyright 2015, 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/ratchet.hh"
#include "olm/message.hh"
#include "olm/memory.hh"
#include "olm/cipher.h"
#include "olm/pickle.hh"
#include <cstring>
namespace {
static const std::uint8_t PROTOCOL_VERSION = 3;
static const std::uint8_t MESSAGE_KEY_SEED[1] = {0x01};
static const std::uint8_t CHAIN_KEY_SEED[1] = {0x02};
static const std::size_t MAX_MESSAGE_GAP = 2000;
/**
* Advance the root key, creating a new message chain.
*
* @param root_key previous root key R(n-1)
* @param our_key our new ratchet key T(n)
* @param their_key their most recent ratchet key T(n-1)
* @param info table of constants for the ratchet function
* @param new_root_key[out] returns the new root key R(n)
* @param new_chain_key[out] returns the first chain key in the new chain
* C(n,0)
*/
static void create_chain_key(
olm::SharedKey const & root_key,
_olm_curve25519_key_pair const & our_key,
_olm_curve25519_public_key const & their_key,
olm::KdfInfo const & info,
olm::SharedKey & new_root_key,
olm::ChainKey & new_chain_key
) {
olm::SharedKey secret;
_olm_crypto_curve25519_shared_secret(&our_key, &their_key, secret);
std::uint8_t derived_secrets[2 * olm::OLM_SHARED_KEY_LENGTH];
_olm_crypto_hkdf_sha256(
secret, sizeof(secret),
root_key, sizeof(root_key),
info.ratchet_info, info.ratchet_info_length,
derived_secrets, sizeof(derived_secrets)
);
std::uint8_t const * pos = derived_secrets;
pos = olm::load_array(new_root_key, pos);
pos = olm::load_array(new_chain_key.key, pos);
new_chain_key.index = 0;
olm::unset(derived_secrets);
olm::unset(secret);
}
static void advance_chain_key(
olm::ChainKey const & chain_key,
olm::ChainKey & new_chain_key
) {
_olm_crypto_hmac_sha256(
chain_key.key, sizeof(chain_key.key),
CHAIN_KEY_SEED, sizeof(CHAIN_KEY_SEED),
new_chain_key.key
);
new_chain_key.index = chain_key.index + 1;
}
static void create_message_keys(
olm::ChainKey const & chain_key,
olm::KdfInfo const & info,
olm::MessageKey & message_key) {
_olm_crypto_hmac_sha256(
chain_key.key, sizeof(chain_key.key),
MESSAGE_KEY_SEED, sizeof(MESSAGE_KEY_SEED),
message_key.key
);
message_key.index = chain_key.index;
}
static std::size_t verify_mac_and_decrypt(
_olm_cipher const *cipher,
olm::MessageKey const & message_key,
olm::MessageReader const & reader,
std::uint8_t * plaintext, std::size_t max_plaintext_length
) {
return cipher->ops->decrypt(
cipher,
message_key.key, sizeof(message_key.key),
reader.input, reader.input_length,
reader.ciphertext, reader.ciphertext_length,
plaintext, max_plaintext_length
);
}
static std::size_t verify_mac_and_decrypt_for_existing_chain(
olm::Ratchet const & session,
olm::ChainKey const & chain,
olm::MessageReader const & reader,
std::uint8_t * plaintext, std::size_t max_plaintext_length
) {
if (reader.counter < chain.index) {
return std::size_t(-1);
}
/* Limit the number of hashes we're prepared to compute */
if (reader.counter - chain.index > MAX_MESSAGE_GAP) {
return std::size_t(-1);
}
olm::ChainKey new_chain = chain;
while (new_chain.index < reader.counter) {
advance_chain_key(new_chain, new_chain);
}
olm::MessageKey message_key;
create_message_keys(new_chain, session.kdf_info, message_key);
std::size_t result = verify_mac_and_decrypt(
session.ratchet_cipher, message_key, reader,
plaintext, max_plaintext_length
);
olm::unset(new_chain);
return result;
}
static std::size_t verify_mac_and_decrypt_for_new_chain(
olm::Ratchet const & session,
olm::MessageReader const & reader,
std::uint8_t * plaintext, std::size_t max_plaintext_length
) {
olm::SharedKey new_root_key;
olm::ReceiverChain new_chain;
/* They shouldn't move to a new chain until we've sent them a message
* acknowledging the last one */
if (session.sender_chain.empty()) {
return std::size_t(-1);
}
/* Limit the number of hashes we're prepared to compute */
if (reader.counter > MAX_MESSAGE_GAP) {
return std::size_t(-1);
}
olm::load_array(new_chain.ratchet_key.public_key, reader.ratchet_key);
create_chain_key(
session.root_key, session.sender_chain[0].ratchet_key,
new_chain.ratchet_key, session.kdf_info,
new_root_key, new_chain.chain_key
);
std::size_t result = verify_mac_and_decrypt_for_existing_chain(
session, new_chain.chain_key, reader,
plaintext, max_plaintext_length
);
olm::unset(new_root_key);
olm::unset(new_chain);
return result;
}
} // namespace
olm::Ratchet::Ratchet(
olm::KdfInfo const & kdf_info,
_olm_cipher const * ratchet_cipher
) : kdf_info(kdf_info),
ratchet_cipher(ratchet_cipher),
last_error(OlmErrorCode::OLM_SUCCESS) {
}
void olm::Ratchet::initialise_as_bob(
std::uint8_t const * shared_secret, std::size_t shared_secret_length,
_olm_curve25519_public_key const & their_ratchet_key
) {
std::uint8_t derived_secrets[2 * olm::OLM_SHARED_KEY_LENGTH];
_olm_crypto_hkdf_sha256(
shared_secret, shared_secret_length,
nullptr, 0,
kdf_info.root_info, kdf_info.root_info_length,
derived_secrets, sizeof(derived_secrets)
);
receiver_chains.insert();
receiver_chains[0].chain_key.index = 0;
std::uint8_t const * pos = derived_secrets;
pos = olm::load_array(root_key, pos);
pos = olm::load_array(receiver_chains[0].chain_key.key, pos);
receiver_chains[0].ratchet_key = their_ratchet_key;
olm::unset(derived_secrets);
}
void olm::Ratchet::initialise_as_alice(
std::uint8_t const * shared_secret, std::size_t shared_secret_length,
_olm_curve25519_key_pair const & our_ratchet_key
) {
std::uint8_t derived_secrets[2 * olm::OLM_SHARED_KEY_LENGTH];
_olm_crypto_hkdf_sha256(
shared_secret, shared_secret_length,
nullptr, 0,
kdf_info.root_info, kdf_info.root_info_length,
derived_secrets, sizeof(derived_secrets)
);
sender_chain.insert();
sender_chain[0].chain_key.index = 0;
std::uint8_t const * pos = derived_secrets;
pos = olm::load_array(root_key, pos);
pos = olm::load_array(sender_chain[0].chain_key.key, pos);
sender_chain[0].ratchet_key = our_ratchet_key;
olm::unset(derived_secrets);
}
namespace olm {
static std::size_t pickle_length(
const olm::SharedKey & value
) {
return olm::OLM_SHARED_KEY_LENGTH;
}
static std::uint8_t * pickle(
std::uint8_t * pos,
const olm::SharedKey & value
) {
return olm::pickle_bytes(pos, value, olm::OLM_SHARED_KEY_LENGTH);
}
static std::uint8_t const * unpickle(
std::uint8_t const * pos, std::uint8_t const * end,
olm::SharedKey & value
) {
return olm::unpickle_bytes(pos, end, value, olm::OLM_SHARED_KEY_LENGTH);
}
static std::size_t pickle_length(
const olm::SenderChain & value
) {
std::size_t length = 0;
length += olm::pickle_length(value.ratchet_key);
length += olm::pickle_length(value.chain_key.key);
length += olm::pickle_length(value.chain_key.index);
return length;
}
static std::uint8_t * pickle(
std::uint8_t * pos,
const olm::SenderChain & value
) {
pos = olm::pickle(pos, value.ratchet_key);
pos = olm::pickle(pos, value.chain_key.key);
pos = olm::pickle(pos, value.chain_key.index);
return pos;
}
static std::uint8_t const * unpickle(
std::uint8_t const * pos, std::uint8_t const * end,
olm::SenderChain & value
) {
pos = olm::unpickle(pos, end, value.ratchet_key); UNPICKLE_OK(pos);
pos = olm::unpickle(pos, end, value.chain_key.key); UNPICKLE_OK(pos);
pos = olm::unpickle(pos, end, value.chain_key.index); UNPICKLE_OK(pos);
return pos;
}
static std::size_t pickle_length(
const olm::ReceiverChain & value
) {
std::size_t length = 0;
length += olm::pickle_length(value.ratchet_key);
length += olm::pickle_length(value.chain_key.key);
length += olm::pickle_length(value.chain_key.index);
return length;
}
static std::uint8_t * pickle(
std::uint8_t * pos,
const olm::ReceiverChain & value
) {
pos = olm::pickle(pos, value.ratchet_key);
pos = olm::pickle(pos, value.chain_key.key);
pos = olm::pickle(pos, value.chain_key.index);
return pos;
}
static std::uint8_t const * unpickle(
std::uint8_t const * pos, std::uint8_t const * end,
olm::ReceiverChain & value
) {
pos = olm::unpickle(pos, end, value.ratchet_key); UNPICKLE_OK(pos);
pos = olm::unpickle(pos, end, value.chain_key.key); UNPICKLE_OK(pos);
pos = olm::unpickle(pos, end, value.chain_key.index); UNPICKLE_OK(pos);
return pos;
}
static std::size_t pickle_length(
const olm::SkippedMessageKey & value
) {
std::size_t length = 0;
length += olm::pickle_length(value.ratchet_key);
length += olm::pickle_length(value.message_key.key);
length += olm::pickle_length(value.message_key.index);
return length;
}
static std::uint8_t * pickle(
std::uint8_t * pos,
const olm::SkippedMessageKey & value
) {
pos = olm::pickle(pos, value.ratchet_key);
pos = olm::pickle(pos, value.message_key.key);
pos = olm::pickle(pos, value.message_key.index);
return pos;
}
static std::uint8_t const * unpickle(
std::uint8_t const * pos, std::uint8_t const * end,
olm::SkippedMessageKey & value
) {
pos = olm::unpickle(pos, end, value.ratchet_key); UNPICKLE_OK(pos);
pos = olm::unpickle(pos, end, value.message_key.key); UNPICKLE_OK(pos);
pos = olm::unpickle(pos, end, value.message_key.index); UNPICKLE_OK(pos);
return pos;
}
} // namespace olm
std::size_t olm::pickle_length(
olm::Ratchet const & value
) {
std::size_t length = 0;
length += olm::OLM_SHARED_KEY_LENGTH;
length += olm::pickle_length(value.sender_chain);
length += olm::pickle_length(value.receiver_chains);
length += olm::pickle_length(value.skipped_message_keys);
return length;
}
std::uint8_t * olm::pickle(
std::uint8_t * pos,
olm::Ratchet const & value
) {
pos = pickle(pos, value.root_key);
pos = pickle(pos, value.sender_chain);
pos = pickle(pos, value.receiver_chains);
pos = pickle(pos, value.skipped_message_keys);
return pos;
}
std::uint8_t const * olm::unpickle(
std::uint8_t const * pos, std::uint8_t const * end,
olm::Ratchet & value,
bool includes_chain_index
) {
pos = unpickle(pos, end, value.root_key); UNPICKLE_OK(pos);
pos = unpickle(pos, end, value.sender_chain); UNPICKLE_OK(pos);
pos = unpickle(pos, end, value.receiver_chains); UNPICKLE_OK(pos);
pos = unpickle(pos, end, value.skipped_message_keys); UNPICKLE_OK(pos);
// pickle v 0x80000001 includes a chain index; pickle v1 does not.
if (includes_chain_index) {
std::uint32_t dummy;
pos = unpickle(pos, end, dummy); UNPICKLE_OK(pos);
}
return pos;
}
std::size_t olm::Ratchet::encrypt_output_length(
std::size_t plaintext_length
) const {
std::size_t counter = 0;
if (!sender_chain.empty()) {
counter = sender_chain[0].chain_key.index;
}
std::size_t padded = ratchet_cipher->ops->encrypt_ciphertext_length(
ratchet_cipher,
plaintext_length
);
return olm::encode_message_length(
counter, CURVE25519_KEY_LENGTH, padded, ratchet_cipher->ops->mac_length(ratchet_cipher)
);
}
std::size_t olm::Ratchet::encrypt_random_length() const {
return sender_chain.empty() ? CURVE25519_RANDOM_LENGTH : 0;
}
std::size_t olm::Ratchet::encrypt(
std::uint8_t const * plaintext, std::size_t plaintext_length,
std::uint8_t const * random, std::size_t random_length,
std::uint8_t * output, std::size_t max_output_length
) {
std::size_t output_length = encrypt_output_length(plaintext_length);
if (random_length < encrypt_random_length()) {
last_error = OlmErrorCode::OLM_NOT_ENOUGH_RANDOM;
return std::size_t(-1);
}
if (max_output_length < output_length) {
last_error = OlmErrorCode::OLM_OUTPUT_BUFFER_TOO_SMALL;
return std::size_t(-1);
}
if (sender_chain.empty()) {
sender_chain.insert();
_olm_crypto_curve25519_generate_key(random, &sender_chain[0].ratchet_key);
create_chain_key(
root_key,
sender_chain[0].ratchet_key,
receiver_chains[0].ratchet_key,
kdf_info,
root_key, sender_chain[0].chain_key
);
}
MessageKey keys;
create_message_keys(sender_chain[0].chain_key, kdf_info, keys);
advance_chain_key(sender_chain[0].chain_key, sender_chain[0].chain_key);
std::size_t ciphertext_length = ratchet_cipher->ops->encrypt_ciphertext_length(
ratchet_cipher,
plaintext_length
);
std::uint32_t counter = keys.index;
_olm_curve25519_public_key const & ratchet_key =
sender_chain[0].ratchet_key.public_key;
olm::MessageWriter writer;
olm::encode_message(
writer, PROTOCOL_VERSION, counter, CURVE25519_KEY_LENGTH,
ciphertext_length,
output
);
olm::store_array(writer.ratchet_key, ratchet_key.public_key);
ratchet_cipher->ops->encrypt(
ratchet_cipher,
keys.key, sizeof(keys.key),
plaintext, plaintext_length,
writer.ciphertext, ciphertext_length,
output, output_length
);
olm::unset(keys);
return output_length;
}
std::size_t olm::Ratchet::decrypt_max_plaintext_length(
std::uint8_t const * input, std::size_t input_length
) {
olm::MessageReader reader;
olm::decode_message(
reader, input, input_length,
ratchet_cipher->ops->mac_length(ratchet_cipher)
);
if (!reader.ciphertext) {
last_error = OlmErrorCode::OLM_BAD_MESSAGE_FORMAT;
return std::size_t(-1);
}
return ratchet_cipher->ops->decrypt_max_plaintext_length(
ratchet_cipher, reader.ciphertext_length);
}
std::size_t olm::Ratchet::decrypt(
std::uint8_t const * input, std::size_t input_length,
std::uint8_t * plaintext, std::size_t max_plaintext_length
) {
olm::MessageReader reader;
olm::decode_message(
reader, input, input_length,
ratchet_cipher->ops->mac_length(ratchet_cipher)
);
if (reader.version != PROTOCOL_VERSION) {
last_error = OlmErrorCode::OLM_BAD_MESSAGE_VERSION;
return std::size_t(-1);
}
if (!reader.has_counter || !reader.ratchet_key || !reader.ciphertext) {
last_error = OlmErrorCode::OLM_BAD_MESSAGE_FORMAT;
return std::size_t(-1);
}
std::size_t max_length = ratchet_cipher->ops->decrypt_max_plaintext_length(
ratchet_cipher,
reader.ciphertext_length
);
if (max_plaintext_length < max_length) {
last_error = OlmErrorCode::OLM_OUTPUT_BUFFER_TOO_SMALL;
return std::size_t(-1);
}
if (reader.ratchet_key_length != CURVE25519_KEY_LENGTH) {
last_error = OlmErrorCode::OLM_BAD_MESSAGE_FORMAT;
return std::size_t(-1);
}
ReceiverChain * chain = nullptr;
for (olm::ReceiverChain & receiver_chain : receiver_chains) {
if (0 == std::memcmp(
receiver_chain.ratchet_key.public_key, reader.ratchet_key,
CURVE25519_KEY_LENGTH
)) {
chain = &receiver_chain;
break;
}
}
std::size_t result = std::size_t(-1);
if (!chain) {
result = verify_mac_and_decrypt_for_new_chain(
*this, reader, plaintext, max_plaintext_length
);
} else if (chain->chain_key.index > reader.counter) {
/* Chain already advanced beyond the key for this message
* Check if the message keys are in the skipped key list. */
for (olm::SkippedMessageKey & skipped : skipped_message_keys) {
if (reader.counter == skipped.message_key.index
&& 0 == std::memcmp(
skipped.ratchet_key.public_key, reader.ratchet_key,
CURVE25519_KEY_LENGTH
)
) {
/* Found the key for this message. Check the MAC. */
result = verify_mac_and_decrypt(
ratchet_cipher, skipped.message_key, reader,
plaintext, max_plaintext_length
);
if (result != std::size_t(-1)) {
/* Remove the key from the skipped keys now that we've
* decoded the message it corresponds to. */
olm::unset(skipped);
skipped_message_keys.erase(&skipped);
return result;
}
}
}
} else {
result = verify_mac_and_decrypt_for_existing_chain(
*this, chain->chain_key,
reader, plaintext, max_plaintext_length
);
}
if (result == std::size_t(-1)) {
last_error = OlmErrorCode::OLM_BAD_MESSAGE_MAC;
return std::size_t(-1);
}
if (!chain) {
/* They have started using a new ephemeral ratchet key.
* We need to derive a new set of chain keys.
* We can discard our previous ephemeral ratchet key.
* We will generate a new key when we send the next message. */
chain = receiver_chains.insert();
olm::load_array(chain->ratchet_key.public_key, reader.ratchet_key);
// TODO: we've already done this once, in
// verify_mac_and_decrypt_for_new_chain(). we could reuse the result.
create_chain_key(
root_key, sender_chain[0].ratchet_key, chain->ratchet_key,
kdf_info, root_key, chain->chain_key
);
olm::unset(sender_chain[0]);
sender_chain.erase(sender_chain.begin());
}
while (chain->chain_key.index < reader.counter) {
olm::SkippedMessageKey & key = *skipped_message_keys.insert();
create_message_keys(chain->chain_key, kdf_info, key.message_key);
key.ratchet_key = chain->ratchet_key;
advance_chain_key(chain->chain_key, chain->chain_key);
}
advance_chain_key(chain->chain_key, chain->chain_key);
return result;
}
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