Commit 09d8e84c authored by Mark Haines's avatar Mark Haines
Browse files

Implement the axlotl ratchet

parent 186df912
#include "axololt/crypto.hh"
#include "axololt/list.hh"
#include "axolotl/crypto.hh"
#include "axolotl/list.hh"
namespace axolotl {
......@@ -52,7 +52,10 @@ enum struct ErrorCode {
static std::size_t const MAX_RECEIVER_CHAINS = 5;
static std::size_t const MAX_SKIPPED_MESSAGE_KEYS = 40;
struct KdfInfo {
std::uint8_t const * root_info;
std::size_t root_info_length;
std::uint8_t const * ratchet_info;
std::size_t ratchet_info_length;
std::uint8_t const * message_info;
......@@ -61,15 +64,30 @@ struct KdfInfo {
struct Session {
Session(
KdfInfo const & kdf_info
);
/** A pair of string to feed into the KDF identifing the application */
KdfInfo kdf_info;
/** The last error that happened encypting or decrypting a message */
ErrorCode last_error;
SharedKey root_key;
List<SenderChain, 1> sender_chain;
List<ReceiverChain, MAX_RECEIVER_CHAINS> reciever_chains;
List<ReceiverChain, MAX_RECEIVER_CHAINS> receiver_chains;
List<SkippedMessageKey, MAX_SKIPPED_MESSAGE_KEYS> skipped_message_keys;
void initialise_as_bob(
std::uint8_t const * shared_secret, std::size_t shared_secret_length,
Curve25519PublicKey const & their_ratchet_key
);
void initialise_as_alice(
std::uint8_t const * shared_secret, std::size_t shared_secret_length,
Curve25519KeyPair const & our_ratchet_key
);
std::size_t encrypt_max_output_length(
std::size_t plaintext_length
);
......
......@@ -15,8 +15,9 @@ struct Curve25519KeyPair : public Curve25519PublicKey {
};
Curve25519KeyPair generate_key(
std::uint8_t const * random_32_bytes
void generate_key(
std::uint8_t const * random_32_bytes,
Curve25519KeyPair & key_pair
);
......
......@@ -60,6 +60,11 @@ public:
return pos;
}
/**
* Make space for an item in the list at the start of the list
*/
T * insert() { return insert(begin()); }
/**
* Insert an item into the list at a given position.
* If inserting the item makes the list longer than max_size then
......
#include "axolotl/axolotl.hh"
#include "axolotl/message.hh"
#include <cstring>
namespace {
std::uint8_t PROTOCOL_VERSION = 3;
std::size_t MAC_LENGTH = 8;
std::size_t KEY_LENGTH = Curve25519PublicKey::Length;
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;
template<typename T>
void unset(
T & value
) {
std::memset(&value, 0, sizeof(T));
}
void create_chain_key(
axolotl::SharedKey const & root_key,
Curve25519KeyPair const & our_key,
Curve25519PublicKey const & their_key,
std::uint8_t const * info, std::size_t info_length,
SharedSecret & new_root_key,
ChainKey & new_chain_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::SharedSecret secret;
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, info_length,
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;
std::memset(derived_secrets, 0, sizeof(derived_secrets);
std::memset(secret, 0, sizeof(secret));
unset(derived_secrets);
unset(secret);
}
void advance_chain_key(
ChainKey const & chain_key,
ChainKey & new_chain_key,
axolotl::ChainKey const & chain_key,
axolotl::ChainKey & new_chain_key
) {
axolotl::hmac_sha256(
chain_key.key, sizeof(chain_key.key),
......@@ -49,11 +59,11 @@ void advance_chain_key(
void create_message_keys(
ChainKey const & chain_key,
std::uint8_t const * info, std::size_t info_length,
MessageKey & message_key
axolotl::ChainKey const & chain_key,
axolotl::KdfInfo const & info,
axolotl::MessageKey & message_key
) {
axolotl::SharedSecret secret;
axolotl::SharedKey secret;
axolotl::hmac_sha256(
chain_key.key, sizeof(chain_key.key),
MESSAGE_KEY_SEED, sizeof(MESSAGE_KEY_SEED),
......@@ -62,45 +72,43 @@ void create_message_keys(
std::uint8_t derived_secrets[80];
axolotl::hkdf_sha256(
secret, sizeof(secret),
root_key, sizeof(root_key),
info, info_length,
NULL, 0,
info.message_info, info.message_info_length,
derived_secrets, sizeof(derived_secrets)
);
std::memcpy(message_key.cipher_key, derived_secrets, 32);
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, derived_secrets + 64, 16);
std::memcpy(message_key.iv.iv, derived_secrets + 64, 16);
message_key.index = chain_key.index;
std::memset(derived_secrets, 0, sizeof(derived_secrets);
std::memset(secret, 0, sizeof(secret));
unset(derived_secrets);
unset(secret);
}
bool verify_mac(
MessageKey const & message_key,
axolotl::MessageKey const & message_key,
std::uint8_t const * input,
axolotl::MessageReader const & reader
) {
std::uint8_t mac[HMAC_SHA256_OUTPUT_LENGTH];
std::uint8_t mac[axolotl::HMAC_SHA256_OUTPUT_LENGTH];
axolotl::hmac_sha256(
keys.mac_key, sizeof(keys.mac_key),
ciphertext, reader.body_length,
message_key.mac_key, sizeof(message_key.mac_key),
input, reader.body_length,
mac
);
bool result = std::memcmp(mac, reader.mac, MAC_LENGTH) == 0;
std::memset(&mac, 0, HMAC_SHA256_OUTPUT_LENGTH);
unset(mac);
return result;
}
bool verify_mac_for_existing_chain(
axolotl::Session const & session,
axolotl::ReceiverChain const & chain,
axolotl::ChainKey const & chain,
std::uint8_t const * input,
axolotl::MessageReader const & reader
) {
ReceiverChain new_chain = chain;
if (reader.counter < chain.index) {
return false;
}
......@@ -110,18 +118,17 @@ bool verify_mac_for_existing_chain(
return false;
}
axolotl::ChainKey new_chain = chain;
while (new_chain.index < reader.counter) {
advance_chain_key(new_chain, new_chain);
}
MessageKey message_key;
create_message_keys(
new_chain_key, sender.message_info, sender.message_info_length,
message_key
);
axolotl::MessageKey message_key;
create_message_keys(new_chain, session.kdf_info, message_key);
bool result = verify_mac(message_key, input, reader);
std::memset(&new_chain, 0, sizeof(new_chain.ratchet_key);
unset(new_chain);
return result;
}
......@@ -131,8 +138,8 @@ bool verify_mac_for_new_chain(
std::uint8_t const * input,
axolotl::MessageReader const & reader
) {
SharedSecret new_root_key;
ReceiverChain new_chain;
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 */
......@@ -144,30 +151,78 @@ bool verify_mac_for_new_chain(
if (reader.counter > MAX_MESSAGE_GAP) {
return false;
}
std::memcpy(new_chain.ratchet_key, reader.ratchet_key, KEY_LENGTH);
std::memcpy(
new_chain.ratchet_key.public_key, reader.ratchet_key, KEY_LENGTH
);
create_chain_key(
root_key, sender_chain[0].ratchet_key, new_chain.ratchet_key,
session.kdf_info.ratchet_info, session.kdf_info.ratchet_info_length,
new_root_key, new_chain
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, input, reader
session, new_chain.chain_key, input, reader
);
std::memset(&new_root_key, 0, sizeof(new_root_key));
std::memset(&new_chain, 0, sizeof(new_chain.ratchet_key);
unset(new_root_key);
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;
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;
unset(derived_secrets);
}
std::size_t axolotl::Session::encrypt_max_output_length(
std::size_t plaintext_length
) {
std::size_t key_length = 1 + varstring_length(Curve25519PublicKey::Length);
std::size_t counter = sender_chain.empty() ? 0 : sender_chain[0].index;
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
......@@ -176,7 +231,7 @@ std::size_t axolotl::Session::encrypt_max_output_length(
std::size_t axolotl::Session::encrypt_random_length() {
return sender_chain.size() ? Curve25519PublicKey::Length : 0;
return sender_chain.empty() ? KEY_LENGTH : 0;
}
......@@ -189,29 +244,36 @@ std::size_t axolotl::Session::encrypt(
last_error = axolotl::ErrorCode::NOT_ENOUGH_RANDOM;
return std::size_t(-1);
}
if (max_output_length < encrypt_max_output_length()) {
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()) {
/** create sender chain */
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 and advance chain */
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::size_t key_length = Curve25519PublicKey::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, cipher_text
PROTOCOL_VERSION, counter, KEY_LENGTH, padded, output
));
std::memcpy(writer.ratchet_key, ratchet_key.public_key, key_length);
std::memcpy(writer.ratchet_key, ratchet_key.public_key, KEY_LENGTH);
axolotl::aes_encrypt_cbc(
keys.cipher_key, keys.iv,
......@@ -219,19 +281,20 @@ std::size_t axolotl::Session::encrypt(
writer.ciphertext
);
std::uint8_t mac[HMAC_SHA256_OUTPUT_LENGTH];
std::uint8_t mac[axolotl::HMAC_SHA256_OUTPUT_LENGTH];
axolotl::hmac_sha256(
keys.mac_key, sizeof(keys.mac_key),
ciphertext, writer.body_length,
output, writer.body_length,
mac
);
std::memcpy(writer.mac, mac, MAC_LENGTH);
unset(keys);
return writer.body_length + MAC_LENGTH;
}
std::size_t decrypt_max_plaintext_length(
std::size_t axolotl::Session::decrypt_max_plaintext_length(
std::size_t input_length
) {
return input_length;
......@@ -256,8 +319,7 @@ std::size_t axolotl::Session::decrypt(
return std::size_t(-1);
}
if (reader.body_length == 0
|| reader.ratchet_key_length != Curve25519PublicKey::Length) {
if (reader.body_length == 0 || reader.ratchet_key_length != KEY_LENGTH) {
last_error = axolotl::ErrorCode::BAD_MESSAGE_FORMAT;
return std::size_t(-1);
}
......@@ -265,7 +327,8 @@ std::size_t axolotl::Session::decrypt(
ReceiverChain * chain = NULL;
for (axolotl::ReceiverChain & receiver_chain : receiver_chains) {
if (0 == std::memcmp(
receiver_chain.ratchet_key, reader.ratchet_key, KEY_LENGTH
receiver_chain.ratchet_key.public_key, reader.ratchet_key,
KEY_LENGTH
)) {
chain = &receiver_chain;
break;
......@@ -278,15 +341,16 @@ std::size_t axolotl::Session::decrypt(
return std::size_t(-1);
}
} else {
if (chain->index > reader.counter) {
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 (const axolotl::SkippedMessageKey & skipped
: skipped_message_keys) {
for (axolotl::SkippedMessageKey & skipped : skipped_message_keys) {
if (reader.counter == skipped.message_key.index
&& 0 == std::memcmp(
skipped.ratchet_key, reader.ratchet_key, KEY_LENGTH
)) {
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;
......@@ -307,6 +371,7 @@ std::size_t axolotl::Session::decrypt(
/* Remove the key from the skipped keys now that we've
* decoded the message it corresponds to. */
unset(skipped);
skipped_message_keys.erase(&skipped);
return result;
}
......@@ -314,18 +379,54 @@ std::size_t axolotl::Session::decrypt(
/* 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(*chain, input, reader)) {
} 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
);
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
);
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;
}
}
......@@ -23,6 +23,7 @@ static const std::size_t SHA256_HASH_LENGTH = 32;
static const std::size_t SHA256_BLOCK_LENGTH = 64;
static const std::uint8_t HKDF_DEFAULT_SALT[32] = {};
template<std::size_t block_size>
inline static void xor_block(
std::uint8_t * block,
......@@ -86,15 +87,14 @@ inline void hmac_sha256_final(
} // namespace
axolotl::Curve25519KeyPair axolotl::generate_key(
std::uint8_t const * random_32_bytes
void axolotl::generate_key(
std::uint8_t const * random_32_bytes,
axolotl::Curve25519KeyPair & key_pair
) {
axolotl::Curve25519KeyPair key_pair;
std::memcpy(key_pair.private_key, random_32_bytes, 32);
::curve25519_donna(
key_pair.public_key, key_pair.private_key, CURVE25519_BASEPOINT
);
return key_pair;
}
......
import subprocess
import glob
import os
if not os.path.exists("build"):
os.mkdir("build")
test_files = glob.glob("tests/test_*.cpp")
source_files = glob.glob("src/*.cpp")
compile_args = "g++ -Itests/include -Iinclude -Ilib --std=c++11".split()
compile_args += source_files
for test_file in test_files:
exe_file = "build/" + test_file[:4]
subprocess.check_call(compile_args + [test_file, "-o", exe_file])
subprocess.check_call([exe_file])
......@@ -44,12 +44,14 @@ std::uint8_t expected_agreement[32] = {
0x76, 0xF0, 0x9B, 0x3C, 0x1E, 0x16, 0x17, 0x42
};
axolotl::Curve25519KeyPair alice_pair = axolotl::generate_key(alice_private);
axolotl::Curve25519KeyPair alice_pair;
axolotl::generate_key(alice_private, alice_pair);
assert_equals(alice_private, alice_pair.private_key, 32);
assert_equals(alice_public, alice_pair.public_key, 32);
axolotl::Curve25519KeyPair bob_pair = axolotl::generate_key(bob_private);
axolotl::Curve25519KeyPair bob_pair;
axolotl::generate_key(bob_private, bob_pair);
assert_equals(bob_private, bob_pair.private_key, 32);
assert_equals(bob_public, bob_pair.public_key, 32);
......
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