tls_transcript_hash_13.cpp

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/*
* TLS transcript hash implementation for TLS 1.3
* (C) 2022 Jack Lloyd
*     2022 Hannes Rantzsch, René Meusel - neXenio GmbH
*
* Botan is released under the Simplified BSD License (see license.txt)
*/
 
#include <botan/internal/tls_transcript_hash_13.h>
 
#include <botan/tls_exceptn.h>
#include <botan/tls_messages.h>
#include <botan/internal/tls_reader.h>
 
#include <utility>
 
namespace Botan::TLS {
 
Transcript_Hash_State::Transcript_Hash_State(std::string_view algo_spec) {
   set_algorithm(algo_spec);
}
 
Transcript_Hash_State::Transcript_Hash_State(const Transcript_Hash_State& other) :
      m_hash((other.m_hash != nullptr) ? other.m_hash->copy_state() : nullptr),
      m_unprocessed_transcript(other.m_unprocessed_transcript),
      m_current(other.m_current),
      m_previous(other.m_previous),
      m_truncated(other.m_truncated) {}
 
Transcript_Hash_State Transcript_Hash_State::recreate_after_hello_retry_request(
   std::string_view algo_spec, const Transcript_Hash_State& prev_transcript_hash_state) {
   // make sure that we have seen exactly 'client_hello' and 'hello_retry_request'
   // before re-creating the transcript hash state
   BOTAN_STATE_CHECK(prev_transcript_hash_state.m_hash == nullptr);
   BOTAN_STATE_CHECK(prev_transcript_hash_state.m_unprocessed_transcript.size() == 2);
 
   Transcript_Hash_State ths(algo_spec);
 
   const auto& client_hello_1 = prev_transcript_hash_state.m_unprocessed_transcript.front();
   const auto& hello_retry_request = prev_transcript_hash_state.m_unprocessed_transcript.back();
 
   const size_t hash_length = ths.m_hash->output_length();
   BOTAN_ASSERT_NOMSG(hash_length < 256);
 
   // RFC 8446 4.4.1
   //    [...], when the server responds to a ClientHello with a HelloRetryRequest,
   //    the value of ClientHello1 is replaced with a special synthetic handshake
   //    message of handshake type "message_hash" [(0xFE)] containing:
   std::vector<uint8_t> message_hash;
   message_hash.reserve(4 + hash_length);
   message_hash.push_back(0xFE /* message type 'message_hash' RFC 8446 4. */);
   message_hash.push_back(0x00);
   message_hash.push_back(0x00);
   message_hash.push_back(static_cast<uint8_t>(hash_length));
   message_hash += ths.m_hash->process(client_hello_1);
 
   ths.update(message_hash);
   ths.update(hello_retry_request);
 
   return ths;
}
 
namespace {
 
// TODO: This is a massive code duplication of the client hello parsing code,
//       as well as basic parsing of extensions. We should resolve this.
//
// Ad-hoc idea: When parsing the production objects, we could keep markers into
//              the original buffer. E.g. the PSK extensions would keep its off-
//              set into the entire client hello buffer. Using that offset we
//              could quickly identify the offset of the binders list slice the
//              buffer without re-parsing it.
//
// Finds the truncation offset in a serialization of Client Hello as defined in
// RFC 8446 4.2.11.2 used for the calculation of PSK binder MACs.
size_t find_client_hello_truncation_mark(std::span<const uint8_t> client_hello) {
   TLS_Data_Reader reader("Client Hello Truncation", client_hello);
 
   // handshake message type
   BOTAN_ASSERT_NOMSG(reader.get_byte() == static_cast<uint8_t>(Handshake_Type::ClientHello));
 
   // message length
   reader.discard_next(3);
 
   // legacy version
   reader.discard_next(2);
 
   // random
   reader.discard_next(32);
 
   // session ID
   const auto session_id_length = reader.get_byte();
   reader.discard_next(session_id_length);
 
   // TODO: DTLS contains a hello_cookie in this location
   //       Currently we don't support DTLS 1.3
 
   // cipher suites
   const auto ciphersuites_length = reader.get_uint16_t();
   reader.discard_next(ciphersuites_length);
 
   // compression methods
   const auto compression_methods_length = reader.get_byte();
   reader.discard_next(compression_methods_length);
 
   // extensions
   const auto extensions_length = reader.get_uint16_t();
   const auto extensions_offset = reader.read_so_far();
   while(reader.has_remaining() && reader.read_so_far() - extensions_offset < extensions_length) {
      const auto ext_type = static_cast<Extension_Code>(reader.get_uint16_t());
      const auto ext_length = reader.get_uint16_t();
 
      // skip over all extensions, finding the PSK extension to be truncated
      if(ext_type != Extension_Code::PresharedKey) {
         reader.discard_next(ext_length);
         continue;
      }
 
      // PSK identities list
      const auto identities_length = reader.get_uint16_t();
      reader.discard_next(identities_length);
 
      // check that only the binders are left in the buffer...
      const auto binders_length = reader.peek_uint16_t();
      if(binders_length != reader.remaining_bytes() - 2 /* binders_length */) {
         throw TLS_Exception(Alert::IllegalParameter,
                             "Failed to truncate Client Hello that doesn't end on the PSK binders list");
      }
 
      // the reader now points to the truncation point
      break;
   }
 
   // if no PSK extension was found, this will point to the end of the buffer
   return reader.read_so_far();
}
 
std::vector<uint8_t> read_hash_state(std::unique_ptr<HashFunction>& hash) {
   // Botan does not support finalizing a HashFunction without resetting
   // the internal state of the hash. Hence we first copy the internal
   // state and then finalize the transient HashFunction.
   return hash->copy_state()->final_stdvec();
}
 
}  // namespace
 
void Transcript_Hash_State::update(std::span<const uint8_t> serialized_message_s) {
   auto serialized_message = serialized_message_s.data();
   auto serialized_message_length = serialized_message_s.size();
   if(m_hash != nullptr) {
      auto truncation_mark = serialized_message_length;
 
      // Check whether we should generate a truncated hash for supporting PSK
      // binder calculation or verification. See RFC 8446 4.2.11.2.
      if(serialized_message_length > 0 && *serialized_message == static_cast<uint8_t>(Handshake_Type::ClientHello)) {
         truncation_mark = find_client_hello_truncation_mark(serialized_message_s);
      }
 
      if(truncation_mark < serialized_message_length) {
         m_hash->update(serialized_message, truncation_mark);
         m_truncated = read_hash_state(m_hash);
         m_hash->update(serialized_message + truncation_mark, serialized_message_length - truncation_mark);
      } else {
         m_truncated.clear();
         m_hash->update(serialized_message, serialized_message_length);
      }
 
      m_previous = std::exchange(m_current, read_hash_state(m_hash));
   } else {
      m_unprocessed_transcript.push_back(
         std::vector(serialized_message, serialized_message + serialized_message_length));
   }
}
 
const Transcript_Hash& Transcript_Hash_State::current() const {
   BOTAN_STATE_CHECK(!m_current.empty());
   return m_current;
}
 
const Transcript_Hash& Transcript_Hash_State::previous() const {
   BOTAN_STATE_CHECK(!m_previous.empty());
   return m_previous;
}
 
const Transcript_Hash& Transcript_Hash_State::truncated() const {
   BOTAN_STATE_CHECK(!m_truncated.empty());
   return m_truncated;
}
 
void Transcript_Hash_State::set_algorithm(std::string_view algo_spec) {
   BOTAN_STATE_CHECK(m_hash == nullptr || m_hash->name() == algo_spec);
   if(m_hash != nullptr) {
      return;
   }
 
   m_hash = HashFunction::create_or_throw(algo_spec);
   for(const auto& msg : m_unprocessed_transcript) {
      update(msg);
   }
   m_unprocessed_transcript.clear();
}
 
Transcript_Hash_State Transcript_Hash_State::clone() const {
   return *this;
}
 
}  // namespace Botan::TLS