Add methods for pickling and unpickling sessions

include/axolotl/crypto.hh

@@ -25,6 +25,7 @@ struct Curve25519PublicKey {
 
 
 struct Curve25519KeyPair : public Curve25519PublicKey {
+    static const int LENGTH = 64;
     std::uint8_t private_key[32];
 };
 

include/axolotl/ratchet.hh

@@ -84,7 +84,7 @@ struct Session {
     );
 
     /** A some strings identifing the application to feed into the KDF. */
-    KdfInfo kdf_info;
+    const KdfInfo &kdf_info;
 
     /** The last error that happened encypting or decrypting a message. */
     ErrorCode last_error;
@@ -121,6 +121,24 @@ struct Session {
         Curve25519KeyPair const & our_ratchet_key
     );
 
+    /** The number of bytes needed to persist the current session. */
+    std::size_t pickle_max_output_length();
+
+    /** Persists a session as a sequence of bytes, encrypting using a key
+     * Returns the number of output bytes used. */
+    std::size_t pickle(
+        std::uint8_t const * key, std::size_t key_length,
+        std::uint8_t * output, std::size_t max_output_length
+    );
+
+    /** Loads a session from a sequence of bytes, decrypting using a key.
+     * Returns 0 on success, or std::size_t(-1) on failure. The last_error
+     * will be BAD_SESSION_KEY if the supplied key is incorrect. */
+    std::size_t unpickle(
+        std::uint8_t const * key, std::size_t key_length,
+        std::uint8_t * input, std::size_t input_length
+    );
+
     /** The maximum number of bytes of output the encrypt method will write for
      * a given message length. */
     std::size_t encrypt_max_output_length(

src/ratchet.cpp

@@ -223,6 +223,143 @@ void axolotl::Session::initialise_as_alice(
 }
 
 
+std::size_t axolotl::Session::pickle_max_output_length() {
+    std::size_t counter_length = 4;
+    std::size_t send_chain_length = counter_length + 64 + 32;
+    std::size_t recv_chain_length = counter_length + 32 + 32;
+    std::size_t skip_key_length = counter_length + 32 + 32 + 32 + 16;
+    std::size_t pickle_length = 3 * counter_length + 32;
+    pickle_length += sender_chain.size() * send_chain_length;
+    pickle_length += receiver_chains.size() * recv_chain_length;
+    pickle_length += skipped_message_keys.size() * skip_key_length;
+    return axolotl::aes_encrypt_cbc_length(pickle_length) + MAC_LENGTH;
+}
+
+namespace {
+
+std::uint8_t * pickle_counter(
+    std::uint8_t * output, std::uint32_t value
+) {
+    unsigned i = 4;
+    output += 4;
+    while (i--) { *(--output) = value; value >>= 8; }
+    return output + 4;
+}
+
+std::uint8_t * unpickle_counter(
+    std::uint8_t *input, std::uint32_t &value
+) {
+    unsigned i = 4;
+    value = 0;
+    while (i--) { value <<= 8; value |= *(input++); }
+    return input;
+}
+
+std::uint8_t * pickle_bytes(
+    std::uint8_t * output, std::size_t count, std::uint8_t const * bytes
+) {
+    std::memcpy(output, bytes, count);
+    return output + count;
+}
+
+std::uint8_t * unpickle_bytes(
+    std::uint8_t * input, std::size_t count, std::uint8_t * bytes
+) {
+    std::memcpy(bytes, input, count);
+    return input + count;
+}
+
+} // namespace
+
+
+std::size_t axolotl::Session::pickle(
+    std::uint8_t const * key, std::size_t key_length,
+    std::uint8_t * output, std::size_t max_output_length
+) {
+    std::uint8_t * pos = output;
+    if (max_output_length < pickle_max_output_length()) {
+        last_error = axolotl::ErrorCode::OUTPUT_BUFFER_TOO_SMALL;
+        return std::size_t(-1);
+    }
+
+    pos = pickle_counter(pos, sender_chain.size());
+    pos = pickle_counter(pos, receiver_chains.size());
+    pos = pickle_counter(pos, skipped_message_keys.size());
+    pos = pickle_bytes(pos, 32, root_key);
+    for (const axolotl::SenderChain &chain : sender_chain) {
+        pos = pickle_counter(pos, chain.chain_key.index);
+        pos = pickle_bytes(pos, 32, chain.chain_key.key);
+        pos = pickle_bytes(pos, 32, chain.ratchet_key.public_key);
+        pos = pickle_bytes(pos, 32, chain.ratchet_key.private_key);
+    }
+    for (const axolotl::ReceiverChain &chain : receiver_chains) {
+        pos = pickle_counter(pos, chain.chain_key.index);
+        pos = pickle_bytes(pos, 32, chain.chain_key.key);
+        pos = pickle_bytes(pos, 32, chain.ratchet_key.public_key);
+    }
+    for (const axolotl::SkippedMessageKey &key : skipped_message_keys) {
+        pos = pickle_counter(pos, key.message_key.index);
+        pos = pickle_bytes(pos, 32, key.message_key.cipher_key.key);
+        pos = pickle_bytes(pos, 32, key.message_key.mac_key);
+        pos = pickle_bytes(pos, 16, key.message_key.iv.iv);
+        pos = pickle_bytes(pos, 32, key.ratchet_key.public_key);
+    }
+}
+
+std::size_t axolotl::Session::unpickle(
+    std::uint8_t const * key, std::size_t key_length,
+    std::uint8_t * input, std::size_t input_length
+) {
+
+    std::uint8_t * pos = input;
+    std::uint8_t * end = input + input_length;
+    std::uint32_t send_chain_num, recv_chain_num, skipped_num;
+
+    if (end - pos < 4 * 3 + 32) {} // input too small.
+
+    pos = unpickle_counter(pos, send_chain_num);
+    pos = unpickle_counter(pos, recv_chain_num);
+    pos = unpickle_counter(pos, skipped_num);
+    pos = unpickle_bytes(pos, 32, root_key);
+
+    if (end - pos < send_chain_num * (32 * 3 + 4)) {} // input too small.
+
+    while (send_chain_num--) {
+        axolotl::SenderChain & chain = *sender_chain.insert(
+            sender_chain.end()
+        );
+        pos = unpickle_counter(pos, chain.chain_key.index);
+        pos = unpickle_bytes(pos, 32, chain.chain_key.key);
+        pos = unpickle_bytes(pos, 32, chain.ratchet_key.public_key);
+        pos = unpickle_bytes(pos, 32, chain.ratchet_key.private_key);
+    }
+
+    if (end - pos < recv_chain_num * (32 * 2 + 4)) {} // input too small.
+
+    while (recv_chain_num--) {
+        axolotl::ReceiverChain & chain = *receiver_chains.insert(
+            receiver_chains.end()
+        );
+        pos = unpickle_counter(pos, chain.chain_key.index);
+        pos = unpickle_bytes(pos, 32, chain.chain_key.key);
+        pos = unpickle_bytes(pos, 32, chain.ratchet_key.public_key);
+    }
+
+    if (end - pos < skipped_num * (32 * 3 + 16 + 4)) {} // input too small.
+
+    while (skipped_num--) {
+        axolotl::SkippedMessageKey &key = *skipped_message_keys.insert(
+            skipped_message_keys.end()
+        );
+        pos = unpickle_counter(pos, key.message_key.index);
+        pos = unpickle_bytes(pos, 32, key.message_key.cipher_key.key);
+        pos = unpickle_bytes(pos, 32, key.message_key.mac_key);
+        pos = unpickle_bytes(pos, 16, key.message_key.iv.iv);
+        pos = unpickle_bytes(pos, 32, key.ratchet_key.public_key);
+    }
+}
+
+
 std::size_t axolotl::Session::encrypt_max_output_length(
     std::size_t plaintext_length
 ) {

test.py

@@ -26,7 +26,11 @@ source_files = glob.glob("src/*.cpp")
 compile_args = "g++ -g -O0 -Itests/include -Iinclude -Ilib --std=c++11".split()
 compile_args += source_files
 
+def run(args):
+    print " ".join(args)
+    subprocess.check_call(args)
+
 for test_file in test_files:
     exe_file = "build/" + test_file[5:-4]
-    subprocess.check_call(compile_args + [test_file, "-o", exe_file])
+    run(compile_args + [test_file, "-o", exe_file])
-    subprocess.check_call([exe_file])
+    run([exe_file])