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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"
+#include "axolotl/message.hh"
+#include "axolotl/memory.hh"
+
+#include <cstring>
+
+namespace {
+
+std::uint8_t PROTOCOL_VERSION = 3;
+std::size_t MAC_LENGTH = 8;
+std::size_t KEY_LENGTH = axolotl::Curve25519PublicKey::LENGTH;
+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,
+ axolotl::Curve25519KeyPair const & our_key,
+ axolotl::Curve25519PublicKey const & their_key,
+ axolotl::KdfInfo const & info,
+ axolotl::SharedKey & new_root_key,
+ axolotl::ChainKey & new_chain_key
+) {
+ axolotl::SharedKey secret;
+ 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),
+ info.ratchet_info, info.ratchet_info_length,
+ 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);
+}
+
+
+void advance_chain_key(
+ axolotl::ChainKey const & chain_key,
+ axolotl::ChainKey & new_chain_key
+) {
+ 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(
+ axolotl::ChainKey const & chain_key,
+ axolotl::KdfInfo const & info,
+ axolotl::MessageKey & message_key
+) {
+ axolotl::SharedKey secret;
+ 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),
+ NULL, 0,
+ info.message_info, info.message_info_length,
+ derived_secrets, sizeof(derived_secrets)
+ );
+ std::memcpy(message_key.cipher_key.key, derived_secrets, 32);
+ std::memcpy(message_key.mac_key, derived_secrets + 32, 32);
+ std::memcpy(message_key.iv.iv, derived_secrets + 64, 16);
+ message_key.index = chain_key.index;
+ axolotl::unset(derived_secrets);
+ axolotl::unset(secret);
+}
+
+
+bool verify_mac(
+ axolotl::MessageKey const & message_key,
+ std::uint8_t const * input,
+ axolotl::MessageReader const & reader
+) {
+ std::uint8_t mac[axolotl::HMAC_SHA256_OUTPUT_LENGTH];
+ axolotl::hmac_sha256(
+ message_key.mac_key, sizeof(message_key.mac_key),
+ input, reader.body_length,
+ mac
+ );
+
+ bool result = std::memcmp(mac, reader.mac, MAC_LENGTH) == 0;
+ axolotl::unset(mac);
+ return result;
+}
+
+
+bool verify_mac_for_existing_chain(
+ axolotl::Session const & session,
+ axolotl::ChainKey const & chain,
+ 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;
+ }
+
+ axolotl::ChainKey new_chain = chain;
+
+ while (new_chain.index < reader.counter) {
+ advance_chain_key(new_chain, new_chain);
+ }
+
+ axolotl::MessageKey message_key;
+ create_message_keys(new_chain, session.kdf_info, message_key);
+
+ bool result = verify_mac(message_key, input, reader);
+ axolotl::unset(new_chain);
+ return result;
+}
+
+
+bool verify_mac_for_new_chain(
+ axolotl::Session const & session,
+ std::uint8_t const * input,
+ axolotl::MessageReader const & reader
+) {
+ axolotl::SharedKey new_root_key;
+ axolotl::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 false;
+ }
+
+ /* Limit the number of hashes we're prepared to compute */
+ if (reader.counter > MAX_MESSAGE_GAP) {
+ return false;
+ }
+ std::memcpy(
+ new_chain.ratchet_key.public_key, reader.ratchet_key, KEY_LENGTH
+ );
+
+ 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
+ );
+
+ bool result = verify_mac_for_existing_chain(
+ session, new_chain.chain_key, input, reader
+ );
+ axolotl::unset(new_root_key);
+ axolotl::unset(new_chain);
+ return result;
+}
+
+} // namespace
+
+
+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);
+}
+
+
+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);
+}
+
+
+std::size_t axolotl::Session::encrypt_max_output_length(
+ std::size_t plaintext_length
+) {
+ std::size_t counter = 0;
+ if (!sender_chain.empty()) {
+ counter = sender_chain[0].chain_key.index;
+ }
+ 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() {
+ return sender_chain.empty() ? KEY_LENGTH : 0;
+}
+
+
+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);
+ }
+ if (max_output_length < encrypt_max_output_length(plaintext_length)) {
+ last_error = axolotl::ErrorCode::OUTPUT_BUFFER_TOO_SMALL;
+ return std::size_t(-1);
+ }
+
+ if (sender_chain.empty()) {
+ 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
+ );
+ }
+
+ 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 padded = axolotl::aes_encrypt_cbc_length(plaintext_length);
+ std::uint32_t counter = keys.index;
+ const Curve25519PublicKey &ratchet_key = sender_chain[0].ratchet_key;
+
+ axolotl::MessageWriter writer(axolotl::encode_message(
+ PROTOCOL_VERSION, counter, KEY_LENGTH, padded, output
+ ));
+
+ std::memcpy(writer.ratchet_key, ratchet_key.public_key, KEY_LENGTH);
+
+ axolotl::aes_encrypt_cbc(
+ keys.cipher_key, keys.iv,
+ plaintext, plaintext_length,
+ writer.ciphertext
+ );
+
+ std::uint8_t mac[axolotl::HMAC_SHA256_OUTPUT_LENGTH];
+ axolotl::hmac_sha256(
+ keys.mac_key, sizeof(keys.mac_key),
+ output, writer.body_length,
+ mac
+ );
+ std::memcpy(writer.mac, mac, MAC_LENGTH);
+
+ axolotl::unset(keys);
+ return writer.body_length + MAC_LENGTH;
+}
+
+
+std::size_t axolotl::Session::decrypt_max_plaintext_length(
+ 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);
+ }
+
+ if (reader.body_length == 0 || reader.ratchet_key_length != KEY_LENGTH) {
+ 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(
+ receiver_chain.ratchet_key.public_key, reader.ratchet_key,
+ KEY_LENGTH
+ )) {
+ 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 {
+ 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 (axolotl::SkippedMessageKey & skipped : skipped_message_keys) {
+ if (reader.counter == skipped.message_key.index
+ && 0 == std::memcmp(
+ skipped.ratchet_key.public_key, reader.ratchet_key,
+ KEY_LENGTH
+ )
+ ) {
+ /* 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);
+ 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);
+ } else if (!verify_mac_for_existing_chain(
+ *this, chain->chain_key, input, reader
+ )) {
+ last_error = axolotl::ErrorCode::BAD_MESSAGE_MAC;
+ return std::size_t(-1);
+ }
+ }
+
+ if (!chain) {
+ /* 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]);
+ sender_chain.erase(sender_chain.begin());
+ }
+
+ 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);
+ }
+
+ 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);
+
+ advance_chain_key(chain->chain_key, chain->chain_key);
+
+ if (result == std::size_t(-1)) {
+ last_error = axolotl::ErrorCode::BAD_MESSAGE_MAC;
+ return std::size_t(-1);
+ } else {
+ return result;
+ }
+}