Add a message encoder/decoder
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63
include/axolotl/message.hh
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63
include/axolotl/message.hh
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#include <cstddef>
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#include <cstdint>
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namespace axolotl {
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/**
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* The length of the buffer needed to hold a message.
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*/
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std::size_t encode_message_length(
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std::uint32_t counter,
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std::size_t ratchet_key_length,
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std::size_t ciphertext_length,
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std::size_t mac_length
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);
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struct MessageWriter {
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std::size_t body_length;
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std::uint8_t * ratchet_key;
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std::uint8_t * ciphertext;
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std::uint8_t * mac;
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};
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struct MessageReader {
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std::size_t body_length;
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std::uint8_t version;
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std::uint32_t counter;
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std::size_t ratchet_key_length;
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std::size_t ciphertext_length;
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std::size_t mac_length;
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std::uint8_t const * ratchet_key;
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std::uint8_t const * ciphertext;
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std::uint8_t const * mac;
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};
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/**
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* Writes the message headers into the output buffer.
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* Returns a writer struct populated with pointers into the output buffer.
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*/
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MessageWriter encode_message(
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std::uint8_t version,
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std::uint32_t counter,
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std::size_t ratchet_key_length,
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std::size_t ciphertext_length,
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std::uint8_t * output
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);
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/**
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* Reads the message headers from the input buffer.
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* Returns a reader struct populated with pointers into the input buffer.
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* On failure the returned body_length will be 0.
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*/
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MessageReader decode_message(
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std::uint8_t const * input, std::size_t input_length,
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std::size_t mac_length
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);
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} // namespace axolotl
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171
src/message.cpp
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171
src/message.cpp
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#include "axolotl/message.hh"
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namespace {
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template<typename T>
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std::size_t varint_length(
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T value
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) {
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std::size_t result = 1;
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while (value > 128U) {
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++result;
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value >>= 7;
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}
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return result;
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}
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template<typename T>
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std::uint8_t * varint_encode(
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std::uint8_t * output,
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T value
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) {
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while (value > 128U) {
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*(output++) = (0x7F & value) | 0x80;
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}
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(*output++) = value;
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return output;
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}
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template<typename T>
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T varint_decode(
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std::uint8_t const * varint_start,
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std::uint8_t const * varint_end
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) {
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T value = 0;
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do {
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value <<= 7;
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value |= 0x7F & *(--varint_end);
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} while (varint_end != varint_start);
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return value;
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}
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std::uint8_t const * varint_skip(
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std::uint8_t const * input,
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std::uint8_t const * input_end
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) {
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while (input != input_end) {
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std::uint8_t tmp = *(input++);
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if ((tmp & 0x80) == 0) {
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return input;
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}
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}
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return input;
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}
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std::size_t varstring_length(
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std::size_t string_length
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) {
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return varint_length(string_length) + string_length;
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}
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static std::size_t const VERSION_LENGTH = 1;
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static std::uint8_t const RATCHET_KEY_TAG = 012;
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static std::uint8_t const COUNTER_TAG = 020;
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static std::uint8_t const CIPHERTEXT_TAG = 042;
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} // namespace
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std::size_t axolotl::encode_message_length(
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std::uint32_t counter,
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std::size_t ratchet_key_length,
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std::size_t ciphertext_length,
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std::size_t mac_length
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) {
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std::size_t length = VERSION_LENGTH;
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length += 1 + varstring_length(ratchet_key_length);
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length += 1 + varint_length(counter);
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length += 1 + varstring_length(ciphertext_length);
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return length + mac_length;
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}
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axolotl::MessageWriter axolotl::encode_message(
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std::uint8_t version,
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std::uint32_t counter,
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std::size_t ratchet_key_length,
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std::size_t ciphertext_length,
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std::uint8_t * output
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) {
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axolotl::MessageWriter result;
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std::uint8_t * pos = output;
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*(pos++) = version;
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*(pos++) = COUNTER_TAG;
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pos = varint_encode(pos, counter);
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*(pos++) = RATCHET_KEY_TAG;
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pos = varint_encode(pos, ratchet_key_length);
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result.ratchet_key = pos;
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pos += ratchet_key_length;
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*(pos++) = CIPHERTEXT_TAG;
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pos = varint_encode(pos, ciphertext_length);
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result.ciphertext = pos;
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pos += ciphertext_length;
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result.body_length = pos - output;
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result.mac = pos;
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return result;
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}
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axolotl::MessageReader axolotl::decode_message(
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std::uint8_t const * input, std::size_t input_length,
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std::size_t mac_length
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) {
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axolotl::MessageReader result;
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result.body_length = 0;
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std::uint8_t const * pos = input;
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std::uint8_t const * end = input + input_length - mac_length;
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std::uint8_t flags = 0;
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result.mac = end;
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result.mac_length = mac_length;
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if (pos == end) return result;
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result.version = *(pos++);
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while (pos != end) {
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uint8_t tag = *(pos);
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if (tag == COUNTER_TAG) {
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++pos;
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std::uint8_t const * counter_start = pos;
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pos = varint_skip(pos, end);
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result.counter = varint_decode<std::uint32_t>(counter_start, pos);
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flags |= 1;
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} else if (tag == RATCHET_KEY_TAG) {
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++pos;
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std::uint8_t const * len_start = pos;
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pos = varint_skip(pos, end);
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std::size_t len = varint_decode<std::size_t>(len_start, pos);
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if (len > end - pos) return result;
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result.ratchet_key_length = len;
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result.ratchet_key = pos;
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pos += len;
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flags |= 2;
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} else if (tag == CIPHERTEXT_TAG) {
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++pos;
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std::uint8_t const * len_start = pos;
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pos = varint_skip(pos, end);
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std::size_t len = varint_decode<std::size_t>(len_start, pos);
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if (len > end - pos) return result;
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result.ciphertext_length = len;
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result.ciphertext = pos;
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pos += len;
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flags |= 4;
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} else if (tag & 0x7 == 0) {
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pos = varint_skip(pos, end);
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pos = varint_skip(pos, end);
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} else if (tag & 0x7 == 2) {
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std::uint8_t const * len_start = pos;
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pos = varint_skip(pos, end);
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std::size_t len = varint_decode<std::size_t>(len_start, pos);
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if (len > end - pos) return result;
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pos += len;
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} else {
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return result;
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}
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}
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if (flags == 0x7) {
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result.body_length = end - input;
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}
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return result;
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}
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53
tests/test_message.cpp
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53
tests/test_message.cpp
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#include "axolotl/message.hh"
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#include "unittest.hh"
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int main() {
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std::uint8_t message1[36] = "\x03\n\nratchetkey\x10\x01\"\nciphertexthmacsha2";
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std::uint8_t message2[36] = "\x03\x10\x01\n\nratchetkey\"\nciphertexthmacsha2";
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std::uint8_t ratchetkey[11] = "ratchetkey";
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std::uint8_t ciphertext[11] = "ciphertext";
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std::uint8_t hmacsha2[9] = "hmacsha2";
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{ /* Message decode test */
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TestCase test_case("Message decode test");
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axolotl::MessageReader reader(axolotl::decode_message(message1, 35, 8));
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assert_equals(std::size_t(27), reader.body_length);
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assert_equals(std::uint8_t(3), reader.version);
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assert_equals(std::uint32_t(1), reader.counter);
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assert_equals(std::size_t(10), reader.ratchet_key_length);
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assert_equals(std::size_t(10), reader.ciphertext_length);
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assert_equals(std::size_t(8), reader.mac_length);
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assert_equals(ratchetkey, reader.ratchet_key, 10);
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assert_equals(ciphertext, reader.ciphertext, 10);
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assert_equals(hmacsha2, reader.mac, 8);
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} /* Message decode test */
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{ /* Message encode test */
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TestCase test_case("Message encode test");
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std::size_t length = axolotl::encode_message_length(1, 10, 10, 8);
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assert_equals(std::size_t(35), length);
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std::uint8_t output[length];
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axolotl::MessageWriter writer(axolotl::encode_message(3, 1, 10, 10, output));
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assert_equals(std::size_t(27), writer.body_length);
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std::memcpy(writer.ratchet_key, ratchetkey, 10);
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std::memcpy(writer.ciphertext, ciphertext, 10);
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std::memcpy(writer.mac, hmacsha2, 8);
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assert_equals(message2, output, 35);
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} /* Message encode test */
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}
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