olm/src/ratchet.cpp

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/* Copyright 2015 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 "axolotl/ratchet.hh"
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#include "axolotl/message.hh"
#include "axolotl/memory.hh"
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#include <cstring>
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namespace {
std::uint8_t PROTOCOL_VERSION = 3;
std::size_t MAC_LENGTH = 8;
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std::size_t KEY_LENGTH = axolotl::Curve25519PublicKey::LENGTH;
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std::uint8_t MESSAGE_KEY_SEED[1] = {0x01};
std::uint8_t CHAIN_KEY_SEED[1] = {0x02};
std::size_t MAX_MESSAGE_GAP = 2000;
void create_chain_key(
axolotl::SharedKey const & root_key,
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axolotl::Curve25519KeyPair const & our_key,
axolotl::Curve25519PublicKey const & their_key,
axolotl::KdfInfo const & info,
axolotl::SharedKey & new_root_key,
axolotl::ChainKey & new_chain_key
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) {
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axolotl::SharedKey secret;
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axolotl::curve25519_shared_secret(our_key, their_key, secret);
std::uint8_t derived_secrets[64];
axolotl::hkdf_sha256(
secret, sizeof(secret),
root_key, sizeof(root_key),
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info.ratchet_info, info.ratchet_info_length,
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derived_secrets, sizeof(derived_secrets)
);
std::memcpy(new_root_key, derived_secrets, 32);
std::memcpy(new_chain_key.key, derived_secrets + 32, 32);
new_chain_key.index = 0;
axolotl::unset(derived_secrets);
axolotl::unset(secret);
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}
void advance_chain_key(
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axolotl::ChainKey const & chain_key,
axolotl::ChainKey & new_chain_key
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) {
axolotl::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;
}
void create_message_keys(
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axolotl::ChainKey const & chain_key,
axolotl::KdfInfo const & info,
axolotl::MessageKey & message_key
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) {
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axolotl::SharedKey secret;
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axolotl::hmac_sha256(
chain_key.key, sizeof(chain_key.key),
MESSAGE_KEY_SEED, sizeof(MESSAGE_KEY_SEED),
secret
);
std::uint8_t derived_secrets[80];
axolotl::hkdf_sha256(
secret, sizeof(secret),
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NULL, 0,
info.message_info, info.message_info_length,
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derived_secrets, sizeof(derived_secrets)
);
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std::memcpy(message_key.cipher_key.key, derived_secrets, 32);
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std::memcpy(message_key.mac_key, derived_secrets + 32, 32);
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std::memcpy(message_key.iv.iv, derived_secrets + 64, 16);
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message_key.index = chain_key.index;
axolotl::unset(derived_secrets);
axolotl::unset(secret);
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}
bool verify_mac(
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axolotl::MessageKey const & message_key,
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std::uint8_t const * input,
axolotl::MessageReader const & reader
) {
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std::uint8_t mac[axolotl::HMAC_SHA256_OUTPUT_LENGTH];
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axolotl::hmac_sha256(
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message_key.mac_key, sizeof(message_key.mac_key),
input, reader.body_length,
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mac
);
bool result = std::memcmp(mac, reader.mac, MAC_LENGTH) == 0;
axolotl::unset(mac);
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return result;
}
bool verify_mac_for_existing_chain(
axolotl::Session const & session,
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axolotl::ChainKey const & chain,
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std::uint8_t const * input,
axolotl::MessageReader const & reader
) {
if (reader.counter < chain.index) {
return false;
}
/* Limit the number of hashes we're prepared to compute */
if (reader.counter - chain.index > MAX_MESSAGE_GAP) {
return false;
}
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axolotl::ChainKey new_chain = chain;
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while (new_chain.index < reader.counter) {
advance_chain_key(new_chain, new_chain);
}
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axolotl::MessageKey message_key;
create_message_keys(new_chain, session.kdf_info, message_key);
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bool result = verify_mac(message_key, input, reader);
axolotl::unset(new_chain);
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return result;
}
bool verify_mac_for_new_chain(
axolotl::Session const & session,
std::uint8_t const * input,
axolotl::MessageReader const & reader
) {
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axolotl::SharedKey new_root_key;
axolotl::ReceiverChain new_chain;
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/* 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 false;
}
/* Limit the number of hashes we're prepared to compute */
if (reader.counter > MAX_MESSAGE_GAP) {
return false;
}
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std::memcpy(
new_chain.ratchet_key.public_key, reader.ratchet_key, KEY_LENGTH
);
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create_chain_key(
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session.root_key, session.sender_chain[0].ratchet_key,
new_chain.ratchet_key, session.kdf_info,
new_root_key, new_chain.chain_key
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);
bool result = verify_mac_for_existing_chain(
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session, new_chain.chain_key, input, reader
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);
axolotl::unset(new_root_key);
axolotl::unset(new_chain);
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return result;
}
} // namespace
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axolotl::Session::Session(
axolotl::KdfInfo const & kdf_info
) : kdf_info(kdf_info), last_error(axolotl::ErrorCode::SUCCESS) {
}
void axolotl::Session::initialise_as_bob(
std::uint8_t const * shared_secret, std::size_t shared_secret_length,
axolotl::Curve25519PublicKey const & their_ratchet_key
) {
std::uint8_t derived_secrets[64];
axolotl::hkdf_sha256(
shared_secret, shared_secret_length,
NULL, 0,
kdf_info.root_info, kdf_info.root_info_length,
derived_secrets, sizeof(derived_secrets)
);
receiver_chains.insert();
std::memcpy(root_key, derived_secrets, 32);
std::memcpy(receiver_chains[0].chain_key.key, derived_secrets + 32, 32);
receiver_chains[0].ratchet_key = their_ratchet_key;
axolotl::unset(derived_secrets);
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}
void axolotl::Session::initialise_as_alice(
std::uint8_t const * shared_secret, std::size_t shared_secret_length,
axolotl::Curve25519KeyPair const & our_ratchet_key
) {
std::uint8_t derived_secrets[64];
axolotl::hkdf_sha256(
shared_secret, shared_secret_length,
NULL, 0,
kdf_info.root_info, kdf_info.root_info_length,
derived_secrets, sizeof(derived_secrets)
);
sender_chain.insert();
std::memcpy(root_key, derived_secrets, 32);
std::memcpy(sender_chain[0].chain_key.key, derived_secrets + 32, 32);
sender_chain[0].ratchet_key = our_ratchet_key;
axolotl::unset(derived_secrets);
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}
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std::size_t axolotl::Session::encrypt_max_output_length(
std::size_t plaintext_length
) {
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std::size_t counter = 0;
if (!sender_chain.empty()) {
counter = sender_chain[0].chain_key.index;
}
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std::size_t padded = axolotl::aes_encrypt_cbc_length(plaintext_length);
return axolotl::encode_message_length(
counter, KEY_LENGTH, padded, MAC_LENGTH
);
}
std::size_t axolotl::Session::encrypt_random_length() {
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return sender_chain.empty() ? KEY_LENGTH : 0;
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}
std::size_t axolotl::Session::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
) {
if (random_length < encrypt_random_length()) {
last_error = axolotl::ErrorCode::NOT_ENOUGH_RANDOM;
return std::size_t(-1);
}
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if (max_output_length < encrypt_max_output_length(plaintext_length)) {
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last_error = axolotl::ErrorCode::OUTPUT_BUFFER_TOO_SMALL;
return std::size_t(-1);
}
if (sender_chain.empty()) {
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sender_chain.insert();
axolotl::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
);
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}
MessageKey keys;
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create_message_keys(sender_chain[0].chain_key, kdf_info, keys);
advance_chain_key(sender_chain[0].chain_key, sender_chain[0].chain_key);
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std::size_t padded = axolotl::aes_encrypt_cbc_length(plaintext_length);
std::uint32_t counter = keys.index;
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Curve25519PublicKey const & ratchet_key = sender_chain[0].ratchet_key;
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axolotl::MessageWriter writer(axolotl::encode_message(
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PROTOCOL_VERSION, counter, KEY_LENGTH, padded, output
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));
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std::memcpy(writer.ratchet_key, ratchet_key.public_key, KEY_LENGTH);
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axolotl::aes_encrypt_cbc(
keys.cipher_key, keys.iv,
plaintext, plaintext_length,
writer.ciphertext
);
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std::uint8_t mac[axolotl::HMAC_SHA256_OUTPUT_LENGTH];
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axolotl::hmac_sha256(
keys.mac_key, sizeof(keys.mac_key),
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output, writer.body_length,
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mac
);
std::memcpy(writer.mac, mac, MAC_LENGTH);
axolotl::unset(keys);
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return writer.body_length + MAC_LENGTH;
}
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std::size_t axolotl::Session::decrypt_max_plaintext_length(
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std::size_t input_length
) {
return input_length;
}
std::size_t axolotl::Session::decrypt(
std::uint8_t const * input, std::size_t input_length,
std::uint8_t * plaintext, std::size_t max_plaintext_length
) {
if (max_plaintext_length < decrypt_max_plaintext_length(input_length)) {
last_error = axolotl::ErrorCode::OUTPUT_BUFFER_TOO_SMALL;
return std::size_t(-1);
}
axolotl::MessageReader reader(axolotl::decode_message(
input, input_length, MAC_LENGTH
));
if (reader.version != PROTOCOL_VERSION) {
last_error = axolotl::ErrorCode::BAD_MESSAGE_VERSION;
return std::size_t(-1);
}
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if (reader.body_length == 0 || reader.ratchet_key_length != KEY_LENGTH) {
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last_error = axolotl::ErrorCode::BAD_MESSAGE_FORMAT;
return std::size_t(-1);
}
ReceiverChain * chain = NULL;
for (axolotl::ReceiverChain & receiver_chain : receiver_chains) {
if (0 == std::memcmp(
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receiver_chain.ratchet_key.public_key, reader.ratchet_key,
KEY_LENGTH
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)) {
chain = &receiver_chain;
break;
}
}
if (!chain) {
if (!verify_mac_for_new_chain(*this, input, reader)) {
last_error = axolotl::ErrorCode::BAD_MESSAGE_MAC;
return std::size_t(-1);
}
} else {
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if (chain->chain_key.index > reader.counter) {
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/* Chain already advanced beyond the key for this message
* Check if the message keys are in the skipped key list. */
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for (axolotl::SkippedMessageKey & skipped : skipped_message_keys) {
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if (reader.counter == skipped.message_key.index
&& 0 == std::memcmp(
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skipped.ratchet_key.public_key, reader.ratchet_key,
KEY_LENGTH
)
) {
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/* Found the key for this message. Check the MAC. */
if (!verify_mac(skipped.message_key, input, reader)) {
last_error = axolotl::ErrorCode::BAD_MESSAGE_MAC;
return std::size_t(-1);
}
std::size_t result = axolotl::aes_decrypt_cbc(
skipped.message_key.cipher_key,
skipped.message_key.iv,
reader.ciphertext, reader.ciphertext_length,
plaintext
);
if (result == std::size_t(-1)) {
last_error = axolotl::ErrorCode::BAD_MESSAGE_MAC;
return result;
}
/* Remove the key from the skipped keys now that we've
* decoded the message it corresponds to. */
axolotl::unset(skipped);
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skipped_message_keys.erase(&skipped);
return result;
}
}
/* No matching keys for the message, fail with bad mac */
last_error = axolotl::ErrorCode::BAD_MESSAGE_MAC;
return std::size_t(-1);
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} else if (!verify_mac_for_existing_chain(
*this, chain->chain_key, input, reader
)) {
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last_error = axolotl::ErrorCode::BAD_MESSAGE_MAC;
return std::size_t(-1);
}
}
if (!chain) {
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/* They have started using a new empheral ratchet key.
* We need to derive a new set of chain keys.
* We can discard our previous empheral ratchet key.
* We will generate a new key when we send the next message. */
chain = receiver_chains.insert();
std::memcpy(
chain->ratchet_key.public_key, reader.ratchet_key, KEY_LENGTH
);
create_chain_key(
root_key, sender_chain[0].ratchet_key, chain->ratchet_key,
kdf_info, root_key, chain->chain_key
);
axolotl::unset(sender_chain[0]);
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sender_chain.erase(sender_chain.begin());
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}
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while (chain->chain_key.index < reader.counter) {
axolotl::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);
}
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axolotl::MessageKey message_key;
create_message_keys(chain->chain_key, kdf_info, message_key);
std::size_t result = axolotl::aes_decrypt_cbc(
message_key.cipher_key,
message_key.iv,
reader.ciphertext, reader.ciphertext_length,
plaintext
);
axolotl::unset(message_key);
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advance_chain_key(chain->chain_key, chain->chain_key);
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if (result == std::size_t(-1)) {
last_error = axolotl::ErrorCode::BAD_MESSAGE_MAC;
return std::size_t(-1);
} else {
return result;
}
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}