From 09d8e84c7cbbf21195f3fd2eabbcff44042d5a4e Mon Sep 17 00:00:00 2001
From: Mark Haines <mjark@negativecurvature.net>
Date: Thu, 26 Feb 2015 16:30:19 +0000
Subject: Implement the axlotl ratchet
---
include/axolotl/axolotl.hh | 24 ++++-
include/axolotl/crypto.hh | 5 +-
include/axolotl/list.hh | 5 +
src/axolotl.cpp | 239 ++++++++++++++++++++++++++++++++-------------
src/crypto.cpp | 8 +-
test.py | 17 ++++
tests/test_crypto.cpp | 6 +-
7 files changed, 224 insertions(+), 80 deletions(-)
create mode 100644 test.py
diff --git a/include/axolotl/axolotl.hh b/include/axolotl/axolotl.hh
index 34280d4..ead52fc 100644
--- a/include/axolotl/axolotl.hh
+++ b/include/axolotl/axolotl.hh
@@ -1,6 +1,6 @@
-#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
);
diff --git a/include/axolotl/crypto.hh b/include/axolotl/crypto.hh
index 42c154b..f1e81ac 100644
--- a/include/axolotl/crypto.hh
+++ b/include/axolotl/crypto.hh
@@ -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
);
diff --git a/include/axolotl/list.hh b/include/axolotl/list.hh
index a3c3d01..4c87630 100644
--- a/include/axolotl/list.hh
+++ b/include/axolotl/list.hh
@@ -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
diff --git a/src/axolotl.cpp b/src/axolotl.cpp
index a1ea03f..7384d79 100644
--- a/src/axolotl.cpp
+++ b/src/axolotl.cpp
@@ -1,43 +1,53 @@
#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;
+ }
}
diff --git a/src/crypto.cpp b/src/crypto.cpp
index e93a1e9..2397d7c 100644
--- a/src/crypto.cpp
+++ b/src/crypto.cpp
@@ -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;
}
diff --git a/test.py b/test.py
new file mode 100644
index 0000000..250f43e
--- /dev/null
+++ b/test.py
@@ -0,0 +1,17 @@
+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])
diff --git a/tests/test_crypto.cpp b/tests/test_crypto.cpp
index 1838132..1b9947b 100644
--- a/tests/test_crypto.cpp
+++ b/tests/test_crypto.cpp
@@ -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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