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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