Start implementing the ratchet

include/axolotl/axolotl.hh

+
+#include "axololt/crypto.hh"
+#include "axololt/list.hh"
+
+namespace axolotl {
+
+typedef std::uint8_t SharedKey[32];
+
+
+struct ChainKey {
+    std::uint32_t index;
+    SharedKey key;
+};
+
+
+struct MessageKey {
+    std::uint32_t index;
+    Aes256Key cipher_key;
+    SharedKey mac_key;
+    Aes256Iv iv;
+};
+
+
+struct SenderChain {
+    Curve25519KeyPair ratchet_key;
+    ChainKey chain_key;
+};
+
+
+struct ReceiverChain {
+    Curve25519PublicKey ratchet_key;
+    ChainKey chain_key;
+};
+
+
+struct SkippedMessageKey {
+    Curve25519PublicKey ratchet_key;
+    MessageKey message_key;
+};
+
+
+enum struct ErrorCode {
+    SUCCESS = 0, /*!< There wasn't an error */
+    NOT_ENOUGH_RANDOM = 1,  /*!< Not enough entropy was supplied */
+    OUTPUT_BUFFER_TOO_SMALL = 2, /*!< Supplied output buffer is too small */
+    BAD_MESSAGE_VERSION = 3,  /*!< The message version is unsupported */
+    BAD_MESSAGE_FORMAT = 4, /*!< The message couldn't be decoded */
+    BAD_MESSAGE_MAC = 5, /*!< The message couldn't be decrypted */
+};
+
+
+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 * ratchet_info;
+    std::size_t ratchet_info_length;
+    std::uint8_t const * message_info;
+    std::size_t message_info_length;
+};
+
+
+struct Session {
+    /** 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<SkippedMessageKey, MAX_SKIPPED_MESSAGE_KEYS> skipped_message_keys;
+
+    std::size_t encrypt_max_output_length(
+        std::size_t plaintext_length
+    );
+
+    std::size_t encrypt_random_length();
+
+    std::size_t 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 decrypt_max_plaintext_length(
+        std::size_t input_length
+    );
+
+    std::size_t decrypt(
+        std::uint8_t const * input, std::size_t input_length,
+        std::uint8_t * plaintext, std::size_t max_plaintext_length
+    );
+};
+
+
+} // namespace axololt

src/axolotl.cpp

+#include "axolotl/axolotl.hh"
+
+
+namespace {
+
+std::uint8_t PROTOCOL_VERSION = 3;
+std::size_t MAC_LENGTH = 8;
+std::size_t KEY_LENGTH = 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,
+    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::SharedSecret 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,
+        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));
+}
+
+
+void advance_chain_key(
+    ChainKey const & chain_key,
+    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(
+    ChainKey const & chain_key,
+    std::uint8_t const * info, std::size_t info_length,
+    MessageKey & message_key
+) {
+    axolotl::SharedSecret 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),
+        root_key, sizeof(root_key),
+        info, info_length,
+        derived_secrets, sizeof(derived_secrets)
+    );
+    std::memcpy(message_key.cipher_key, derived_secrets, 32);
+    std::memcpy(message_key.mac_key, derived_secrets + 32, 32);
+    std::memcpy(message_key.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));
+}
+
+
+bool verify_mac(
+    MessageKey const & message_key,
+    std::uint8_t const * input,
+    axolotl::MessageReader const & reader
+) {
+    std::uint8_t mac[HMAC_SHA256_OUTPUT_LENGTH];
+    axolotl::hmac_sha256(
+        keys.mac_key, sizeof(keys.mac_key),
+        ciphertext, reader.body_length,
+        mac
+    );
+
+    bool result = std::memcmp(mac, reader.mac, MAC_LENGTH) == 0;
+    std::memset(&mac, 0, HMAC_SHA256_OUTPUT_LENGTH);
+    return result;
+}
+
+
+bool verify_mac_for_existing_chain(
+    axolotl::Session const & session,
+    axolotl::ReceiverChain const & chain,
+    std::uint8_t const * input,
+    axolotl::MessageReader const & reader
+) {
+    ReceiverChain new_chain = chain;
+
+    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;
+    }
+
+    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
+    );
+
+    bool result = verify_mac(message_key, input, reader);
+    std::memset(&new_chain, 0, sizeof(new_chain.ratchet_key);
+    return result;
+}
+
+
+bool verify_mac_for_new_chain(
+    axolotl::Session const & session,
+    std::uint8_t const * input,
+    axolotl::MessageReader const & reader
+) {
+    SharedSecret new_root_key;
+    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, 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
+    );
+
+    bool result = verify_mac_for_existing_chain(
+        session, new_chain, input, reader
+    );
+    std::memset(&new_root_key, 0, sizeof(new_root_key));
+    std::memset(&new_chain, 0, sizeof(new_chain.ratchet_key);
+    return result;
+}
+
+} // namespace
+
+
+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 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.size() ? Curve25519PublicKey::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()) {
+        last_error = axolotl::ErrorCode::OUTPUT_BUFFER_TOO_SMALL;
+        return std::size_t(-1);
+    }
+
+    if (sender_chain.empty()) {
+        /** create sender chain */
+    }
+
+    MessageKey keys;
+
+    /** create message keys and advance chain */
+
+    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
+    ));
+
+    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[HMAC_SHA256_OUTPUT_LENGTH];
+    axolotl::hmac_sha256(
+        keys.mac_key, sizeof(keys.mac_key),
+        ciphertext, writer.body_length,
+        mac
+    );
+    std::memcpy(writer.mac, mac, MAC_LENGTH);
+
+    return writer.body_length + MAC_LENGTH;
+}
+
+
+std::size_t 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 != Curve25519PublicKey::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, 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->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) {
+                if (reader.counter == skipped.message_key.index
+                        && 0 == std::memcmp(
+                            skipped.ratchet_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. */
+                    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(*chain, input, reader)) {
+            last_error = axolotl::ErrorCode::BAD_MESSAGE_MAC;
+            return std::size_t(-1);
+        }
+    }
+
+    if (!chain) {
+        
+    }
+
+
+
+
+
+}