Add a message encoder/decoder

include/axolotl/message.hh

+#include <cstddef>
+#include <cstdint>
+
+
+namespace axolotl {
+
+/**
+ * The length of the buffer needed to hold a message.
+ */
+std::size_t encode_message_length(
+    std::uint32_t counter,
+    std::size_t ratchet_key_length,
+    std::size_t ciphertext_length,
+    std::size_t mac_length
+);
+
+
+struct MessageWriter {
+    std::size_t body_length;
+    std::uint8_t * ratchet_key;
+    std::uint8_t * ciphertext;
+    std::uint8_t * mac;
+};
+
+
+struct MessageReader {
+    std::size_t body_length;
+    std::uint8_t version;
+    std::uint32_t counter;
+    std::size_t ratchet_key_length;
+    std::size_t ciphertext_length;
+    std::size_t mac_length;
+    std::uint8_t const * ratchet_key;
+    std::uint8_t const * ciphertext;
+    std::uint8_t const * mac;
+};
+
+
+/**
+ * Writes the message headers into the output buffer.
+ * Returns a writer struct populated with pointers into the output buffer.
+ */
+MessageWriter encode_message(
+    std::uint8_t version,
+    std::uint32_t counter,
+    std::size_t ratchet_key_length,
+    std::size_t ciphertext_length,
+    std::uint8_t * output
+);
+
+
+/**
+ * Reads the message headers from the input buffer.
+ * Returns a reader struct populated with pointers into the input buffer.
+ * On failure the returned body_length will be 0.
+ */
+MessageReader decode_message(
+    std::uint8_t const * input, std::size_t input_length,
+    std::size_t mac_length
+);
+
+
+} // namespace axolotl

src/message.cpp

+#include "axolotl/message.hh"
+
+namespace {
+
+template<typename T>
+std::size_t varint_length(
+    T value
+) {
+    std::size_t result = 1;
+    while (value > 128U) {
+        ++result;
+        value >>= 7;
+    }
+    return result;
+}
+
+
+template<typename T>
+std::uint8_t * varint_encode(
+    std::uint8_t * output,
+    T value
+) {
+    while (value > 128U) {
+        *(output++) = (0x7F & value) | 0x80;
+    }
+    (*output++) = value;
+    return output;
+}
+
+
+template<typename T>
+T varint_decode(
+    std::uint8_t const * varint_start,
+    std::uint8_t const * varint_end
+) {
+    T value = 0;
+    do {
+        value <<= 7;
+        value |= 0x7F & *(--varint_end);
+    } while (varint_end != varint_start);
+    return value;
+}
+
+
+std::uint8_t const * varint_skip(
+    std::uint8_t const * input,
+    std::uint8_t const * input_end
+) {
+    while (input != input_end) {
+        std::uint8_t tmp = *(input++);
+        if ((tmp & 0x80) == 0) {
+            return input;
+        }
+    }
+    return input;
+}
+
+
+std::size_t varstring_length(
+    std::size_t string_length
+) {
+    return varint_length(string_length) + string_length;
+}
+
+static std::size_t const VERSION_LENGTH = 1;
+static std::uint8_t const RATCHET_KEY_TAG = 012;
+static std::uint8_t const COUNTER_TAG = 020;
+static std::uint8_t const CIPHERTEXT_TAG = 042;
+
+} // namespace
+
+
+std::size_t axolotl::encode_message_length(
+    std::uint32_t counter,
+    std::size_t ratchet_key_length,
+    std::size_t ciphertext_length,
+    std::size_t mac_length
+) {
+    std::size_t length = VERSION_LENGTH;
+    length += 1 + varstring_length(ratchet_key_length);
+    length += 1 + varint_length(counter);
+    length += 1 + varstring_length(ciphertext_length);
+    return length + mac_length;
+}
+
+
+axolotl::MessageWriter axolotl::encode_message(
+    std::uint8_t version,
+    std::uint32_t counter,
+    std::size_t ratchet_key_length,
+    std::size_t ciphertext_length,
+    std::uint8_t * output
+) {
+    axolotl::MessageWriter result;
+    std::uint8_t * pos = output;
+    *(pos++) = version;
+    *(pos++) = COUNTER_TAG;
+    pos = varint_encode(pos, counter);
+    *(pos++) = RATCHET_KEY_TAG;
+    pos = varint_encode(pos, ratchet_key_length);
+    result.ratchet_key = pos;
+    pos += ratchet_key_length;
+    *(pos++) = CIPHERTEXT_TAG;
+    pos = varint_encode(pos, ciphertext_length);
+    result.ciphertext = pos;
+    pos += ciphertext_length;
+    result.body_length = pos - output;
+    result.mac = pos;
+    return result;
+}
+
+
+axolotl::MessageReader axolotl::decode_message(
+    std::uint8_t const * input, std::size_t input_length,
+    std::size_t mac_length
+) {
+    axolotl::MessageReader result;
+    result.body_length = 0;
+    std::uint8_t const * pos = input;
+    std::uint8_t const * end = input + input_length - mac_length;
+    std::uint8_t flags = 0;
+    result.mac = end;
+    result.mac_length = mac_length;
+    if (pos == end) return result;
+    result.version = *(pos++);
+    while (pos != end) {
+        uint8_t tag = *(pos);
+        if (tag == COUNTER_TAG) {
+            ++pos;
+            std::uint8_t const * counter_start = pos;
+            pos = varint_skip(pos, end);
+            result.counter = varint_decode<std::uint32_t>(counter_start, pos);
+            flags |= 1;
+        } else if (tag == RATCHET_KEY_TAG) {
+            ++pos;
+            std::uint8_t const * len_start = pos;
+            pos = varint_skip(pos, end);
+            std::size_t len = varint_decode<std::size_t>(len_start, pos);
+            if (len > end - pos) return result;
+            result.ratchet_key_length = len;
+            result.ratchet_key = pos;
+            pos += len;
+            flags |= 2;
+        } else if (tag == CIPHERTEXT_TAG) {
+            ++pos;
+            std::uint8_t const * len_start = pos;
+            pos = varint_skip(pos, end);
+            std::size_t len = varint_decode<std::size_t>(len_start, pos);
+            if (len > end - pos) return result;
+            result.ciphertext_length = len;
+            result.ciphertext = pos;
+            pos += len;
+            flags |= 4;
+        } else if (tag & 0x7 == 0) {
+            pos = varint_skip(pos, end);
+            pos = varint_skip(pos, end);
+        } else if (tag & 0x7 == 2) {
+            std::uint8_t const * len_start = pos;
+            pos = varint_skip(pos, end);
+            std::size_t len = varint_decode<std::size_t>(len_start, pos);
+            if (len > end - pos) return result;
+            pos += len;
+        } else {
+            return result;
+        }
+    }
+    if (flags == 0x7) {
+        result.body_length = end - input;
+    }
+    return result;
+}

tests/test_message.cpp

+#include "axolotl/message.hh"
+#include "unittest.hh"
+
+int main() {
+
+std::uint8_t message1[36] = "\x03\n\nratchetkey\x10\x01\"\nciphertexthmacsha2";
+std::uint8_t message2[36] = "\x03\x10\x01\n\nratchetkey\"\nciphertexthmacsha2";
+std::uint8_t ratchetkey[11] = "ratchetkey";
+std::uint8_t ciphertext[11] = "ciphertext";
+std::uint8_t hmacsha2[9] = "hmacsha2";
+
+{ /* Message decode test */
+
+TestCase test_case("Message decode test");
+
+axolotl::MessageReader reader(axolotl::decode_message(message1, 35, 8));
+
+assert_equals(std::size_t(27), reader.body_length);
+assert_equals(std::uint8_t(3), reader.version);
+assert_equals(std::uint32_t(1), reader.counter);
+assert_equals(std::size_t(10), reader.ratchet_key_length);
+assert_equals(std::size_t(10), reader.ciphertext_length);
+assert_equals(std::size_t(8), reader.mac_length);
+
+assert_equals(ratchetkey, reader.ratchet_key, 10);
+assert_equals(ciphertext, reader.ciphertext, 10);
+assert_equals(hmacsha2, reader.mac, 8);
+
+
+} /* Message decode test */
+
+{ /* Message encode test */
+
+TestCase test_case("Message encode test");
+
+std::size_t length = axolotl::encode_message_length(1, 10, 10, 8);
+assert_equals(std::size_t(35), length);
+
+std::uint8_t output[length];
+
+axolotl::MessageWriter writer(axolotl::encode_message(3, 1, 10, 10, output));
+assert_equals(std::size_t(27), writer.body_length);
+
+std::memcpy(writer.ratchet_key, ratchetkey, 10);
+std::memcpy(writer.ciphertext, ciphertext, 10);
+std::memcpy(writer.mac, hmacsha2, 8);
+
+assert_equals(message2, output, 35);
+
+
+} /* Message encode test */
+
+}