msg_client_hello.cpp

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/*
* TLS Hello Request and Client Hello Messages
* (C) 2004-2011,2015,2016 Jack Lloyd
*     2016 Matthias Gierlings
*     2017 Harry Reimann, Rohde & Schwarz Cybersecurity
*     2021 Elektrobit Automotive GmbH
*     2022 René Meusel, Hannes Rantzsch - neXenio GmbH
*
* Botan is released under the Simplified BSD License (see license.txt)
*/
 
 
#include <botan/tls_exceptn.h>
#include <botan/tls_messages.h>
#include <botan/tls_callbacks.h>
#include <botan/rng.h>
#include <botan/hash.h>
#include <botan/credentials_manager.h>
#include <botan/tls_version.h>
 
#include <botan/internal/stl_util.h>
#include <botan/internal/tls_reader.h>
#include <botan/internal/tls_session_key.h>
#include <botan/internal/tls_handshake_io.h>
#include <botan/internal/tls_handshake_hash.h>
 
#ifdef BOTAN_HAS_TLS_13
  #include <botan/internal/tls_transcript_hash_13.h>
  #include <botan/internal/tls_handshake_layer_13.h>
#endif
 
#include <chrono>
#include <iterator>
 
namespace Botan::TLS {
 
enum
   {
   TLS_EMPTY_RENEGOTIATION_INFO_SCSV        = 0x00FF,
   };
 
std::vector<uint8_t> make_hello_random(RandomNumberGenerator& rng,
                                       Callbacks& cb,
                                       const Policy& policy)
   {
   auto buf = rng.random_vec<std::vector<uint8_t>>(32);
 
   if(policy.hash_hello_random())
      {
      auto sha256 = HashFunction::create_or_throw("SHA-256");
      sha256->update(buf);
      sha256->final(buf);
      }
 
   // TLS 1.3 does not require the insertion of a timestamp in the client hello
   // random. When offering both TLS 1.2 and 1.3 we nevertheless comply with the
   // legacy specification.
   if(policy.include_time_in_hello_random() && (policy.allow_tls12() || policy.allow_dtls12()))
      {
      const uint32_t time32 = static_cast<uint32_t>(
                                 std::chrono::system_clock::to_time_t(cb.tls_current_timestamp()));
 
      store_be(time32, buf.data());
      }
 
   return buf;
   }
 
/**
 * Version-agnostic internal client hello data container that allows
 * parsing Client_Hello messages without prior knowledge of the contained
 * protocol version.
 */
class Client_Hello_Internal
   {
   public:
      Client_Hello_Internal() : m_comp_methods({0}) {}
 
      Client_Hello_Internal(const std::vector<uint8_t>& buf)
         {
         if(buf.size() < 41)
            { throw Decoding_Error("Client_Hello: Packet corrupted"); }
 
         TLS_Data_Reader reader("ClientHello", buf);
 
         const uint8_t major_version = reader.get_byte();
         const uint8_t minor_version = reader.get_byte();
 
         m_legacy_version = Protocol_Version(major_version, minor_version);
         m_random = reader.get_fixed<uint8_t>(32);
         m_session_id = Session_ID(reader.get_range<uint8_t>(1, 0, 32));
 
         if(m_legacy_version.is_datagram_protocol())
            {
            auto sha256 = HashFunction::create_or_throw("SHA-256");
            sha256->update(reader.get_data_read_so_far());
 
            m_hello_cookie = reader.get_range<uint8_t>(1, 0, 255);
 
            sha256->update(reader.get_remaining());
            m_cookie_input_bits = sha256->final_stdvec();
            }
 
         m_suites = reader.get_range_vector<uint16_t>(2, 1, 32767);
         m_comp_methods = reader.get_range_vector<uint8_t>(1, 1, 255);
 
         m_extensions.deserialize(reader, Connection_Side::Client, Handshake_Type::ClientHello);
         }
 
      /**
       * This distinguishes between a TLS 1.3 compliant Client Hello (containing
       * the "supported_version" extension) and legacy Client Hello messages.
       *
       * @return TLS 1.3 if the Client Hello contains "supported_versions", or
       *         the content of the "legacy_version" version field if it
       *         indicates (D)TLS 1.2 or older, or
       *         (D)TLS 1.2 if the "legacy_version" was some other odd value.
       */
      Protocol_Version version() const
         {
         // RFC 8446 4.2.1
         //    If [the "supported_versions"] extension is not present, servers
         //    which are compliant with this specification and which also support
         //    TLS 1.2 MUST negotiate TLS 1.2 or prior as specified in [RFC5246],
         //    even if ClientHello.legacy_version is 0x0304 or later.
         //
         // RFC 8446 4.2.1
         //    Servers MUST be prepared to receive ClientHellos that include
         //    [the supported_versions] extension but do not include 0x0304 in
         //    the list of versions.
         //
         // RFC 8446 4.1.2
         //    TLS 1.3 ClientHellos are identified as having a legacy_version of
         //    0x0303 and a supported_versions extension present with 0x0304 as
         //    the highest version indicated therein.
         if(!extensions().has<Supported_Versions>() ||
            !extensions().get<Supported_Versions>()->supports(Protocol_Version::TLS_V13))
            {
            // The exact legacy_version is ignored we just inspect it to
            // distinguish TLS and DTLS.
            return (m_legacy_version.is_datagram_protocol())
               ? Protocol_Version::DTLS_V12
               : Protocol_Version::TLS_V12;
            }
 
         // Note: The Client_Hello_13 class will make sure that legacy_version
         //       is exactly 0x0303 (aka ossified TLS 1.2)
         return Protocol_Version::TLS_V13;
         }
 
      Protocol_Version legacy_version() const { return m_legacy_version; }
      const Session_ID& session_id() const { return m_session_id; }
      const std::vector<uint8_t>& random() const { return m_random; }
      const std::vector<uint16_t>& ciphersuites() const { return m_suites; }
      const std::vector<uint8_t>& comp_methods() const { return m_comp_methods; }
 
      const std::vector<uint8_t>& hello_cookie() const { return m_hello_cookie; }
      const std::vector<uint8_t>& hello_cookie_input_bits() const { return m_cookie_input_bits; }
 
      const Extensions& extensions() const { return m_extensions; }
      Extensions& extensions() { return m_extensions; }
 
   public:
      Protocol_Version m_legacy_version;
      Session_ID m_session_id;
      std::vector<uint8_t> m_random;
      std::vector<uint16_t> m_suites;
      std::vector<uint8_t> m_comp_methods;
      Extensions m_extensions;
 
      std::vector<uint8_t> m_hello_cookie; // DTLS only
      std::vector<uint8_t> m_cookie_input_bits; // DTLS only
   };
 
 
Client_Hello::Client_Hello(Client_Hello&&) noexcept = default;
Client_Hello& Client_Hello::operator=(Client_Hello&&) noexcept = default;
 
Client_Hello::~Client_Hello() = default;
 
Client_Hello::Client_Hello()
   : m_data(std::make_unique<Client_Hello_Internal>()) {}
 
/*
* Read a counterparty client hello
*/
Client_Hello::Client_Hello(std::unique_ptr<Client_Hello_Internal> data)
   : m_data(std::move(data))
   {
   BOTAN_ASSERT_NONNULL(m_data);
   }
 
Handshake_Type Client_Hello::type() const
   {
   return Handshake_Type::ClientHello;
   }
 
Protocol_Version Client_Hello::legacy_version() const
   {
   return m_data->legacy_version();
   }
 
const std::vector<uint8_t>& Client_Hello::random() const
   {
   return m_data->random();
   }
 
const Session_ID& Client_Hello::session_id() const
   {
   return m_data->session_id();
   }
 
const std::vector<uint8_t>& Client_Hello::compression_methods() const
   {
   return m_data->comp_methods();
   }
 
const std::vector<uint16_t>& Client_Hello::ciphersuites() const
   {
   return m_data->ciphersuites();
   }
 
std::set<Extension_Code> Client_Hello::extension_types() const
   {
   return m_data->extensions().extension_types();
   }
 
const Extensions& Client_Hello::extensions() const
   {
   return m_data->extensions();
   }
 
void Client_Hello_12::update_hello_cookie(const Hello_Verify_Request& hello_verify)
   {
   BOTAN_STATE_CHECK(m_data->legacy_version().is_datagram_protocol());
 
   m_data->m_hello_cookie = hello_verify.cookie();
   }
 
/*
* Serialize a Client Hello message
*/
std::vector<uint8_t> Client_Hello::serialize() const
   {
   std::vector<uint8_t> buf;
   buf.reserve(1024); // working around GCC warning
 
   buf.push_back(m_data->legacy_version().major_version());
   buf.push_back(m_data->legacy_version().minor_version());
   buf += m_data->random();
 
   append_tls_length_value(buf, m_data->session_id().get(), 1);
 
   if(m_data->legacy_version().is_datagram_protocol())
      { append_tls_length_value(buf, m_data->hello_cookie(), 1); }
 
   append_tls_length_value(buf, m_data->ciphersuites(), 2);
   append_tls_length_value(buf, m_data->comp_methods(), 1);
 
   /*
   * May not want to send extensions at all in some cases. If so,
   * should include SCSV value (if reneg info is empty, if not we are
   * renegotiating with a modern server)
   */
 
   buf += m_data->extensions().serialize(Connection_Side::Client);
 
   return buf;
   }
 
std::vector<uint8_t> Client_Hello::cookie_input_data() const
   {
   BOTAN_STATE_CHECK(!m_data->hello_cookie_input_bits().empty());
 
   return m_data->hello_cookie_input_bits();
   }
 
/*
* Check if we offered this ciphersuite
*/
bool Client_Hello::offered_suite(uint16_t ciphersuite) const
   {
   return std::find(m_data->ciphersuites().cbegin(), m_data->ciphersuites().cend(), ciphersuite) != m_data->ciphersuites().cend();
   }
 
std::vector<Signature_Scheme> Client_Hello::signature_schemes() const
   {
   if(Signature_Algorithms* sigs = m_data->extensions().get<Signature_Algorithms>())
      { return sigs->supported_schemes(); }
   return {};
   }
 
std::vector<Signature_Scheme> Client_Hello::certificate_signature_schemes() const
   {
   // RFC 8446 4.2.3
   //   If no "signature_algorithms_cert" extension is present, then the
   //   "signature_algorithms" extension also applies to signatures appearing
   //   in certificates.
   if(Signature_Algorithms_Cert* sigs = m_data->extensions().get<Signature_Algorithms_Cert>())
      { return sigs->supported_schemes(); }
   else
      { return signature_schemes(); }
   }
 
std::vector<Group_Params> Client_Hello::supported_ecc_curves() const
   {
   if(Supported_Groups* groups = m_data->extensions().get<Supported_Groups>())
      { return groups->ec_groups(); }
   return {};
   }
 
std::vector<Group_Params> Client_Hello::supported_dh_groups() const
   {
   if(Supported_Groups* groups = m_data->extensions().get<Supported_Groups>())
      { return groups->dh_groups(); }
   return std::vector<Group_Params>();
   }
 
bool Client_Hello_12::prefers_compressed_ec_points() const
   {
   if(Supported_Point_Formats* ecc_formats = m_data->extensions().get<Supported_Point_Formats>())
      { return ecc_formats->prefers_compressed(); }
   return false;
   }
 
std::string Client_Hello::sni_hostname() const
   {
   if(Server_Name_Indicator* sni = m_data->extensions().get<Server_Name_Indicator>())
      { return sni->host_name(); }
   return "";
   }
 
bool Client_Hello_12::secure_renegotiation() const
   {
   return m_data->extensions().has<Renegotiation_Extension>();
   }
 
std::vector<uint8_t> Client_Hello_12::renegotiation_info() const
   {
   if(Renegotiation_Extension* reneg = m_data->extensions().get<Renegotiation_Extension>())
      { return reneg->renegotiation_info(); }
   return {};
   }
 
std::vector<Protocol_Version> Client_Hello::supported_versions() const
   {
   if(Supported_Versions* versions = m_data->extensions().get<Supported_Versions>())
      { return versions->versions(); }
   return {};
   }
 
bool Client_Hello_12::supports_session_ticket() const
   {
   return m_data->extensions().has<Session_Ticket_Extension>();
   }
 
Session_Ticket Client_Hello_12::session_ticket() const
   {
   if(auto* ticket = m_data->extensions().get<Session_Ticket_Extension>())
      { return ticket->contents(); }
   return {};
   }
 
std::optional<Session_Handle> Client_Hello_12::session_handle() const
   {
   // RFC 5077 3.4
   //    If a ticket is presented by the client, the server MUST NOT attempt
   //    to use the Session ID in the ClientHello for stateful session
   //    resumption.
   if(auto ticket = session_ticket(); !ticket.empty())
      { return ticket; }
   else if(const auto& id = session_id(); !id.empty())
      { return id; }
   else
      { return std::nullopt; }
   }
 
bool Client_Hello::supports_alpn() const
   {
   return m_data->extensions().has<Application_Layer_Protocol_Notification>();
   }
 
bool Client_Hello_12::supports_extended_master_secret() const
   {
   return m_data->extensions().has<Extended_Master_Secret>();
   }
 
bool Client_Hello_12::supports_cert_status_message() const
   {
   return m_data->extensions().has<Certificate_Status_Request>();
   }
 
bool Client_Hello_12::supports_encrypt_then_mac() const
   {
   return m_data->extensions().has<Encrypt_then_MAC>();
   }
 
bool Client_Hello::sent_signature_algorithms() const
   {
   return m_data->extensions().has<Signature_Algorithms>();
   }
 
std::vector<std::string> Client_Hello::next_protocols() const
   {
   if(auto alpn = m_data->extensions().get<Application_Layer_Protocol_Notification>())
      { return alpn->protocols(); }
   return {};
   }
 
std::vector<uint16_t> Client_Hello::srtp_profiles() const
   {
   if(SRTP_Protection_Profiles* srtp = m_data->extensions().get<SRTP_Protection_Profiles>())
      { return srtp->profiles(); }
   return {};
   }
 
const std::vector<uint8_t>& Client_Hello::cookie() const
   {
   return m_data->hello_cookie();
   }
 
/*
* Create a new Hello Request message
*/
Hello_Request::Hello_Request(Handshake_IO& io)
   {
   io.send(*this);
   }
 
/*
* Deserialize a Hello Request message
*/
Hello_Request::Hello_Request(const std::vector<uint8_t>& buf)
   {
   if(!buf.empty())
      { throw Decoding_Error("Bad Hello_Request, has non-zero size"); }
   }
 
/*
* Serialize a Hello Request message
*/
std::vector<uint8_t> Hello_Request::serialize() const
   {
   return std::vector<uint8_t>();
   }
 
 
Client_Hello_12::Client_Hello_12(std::unique_ptr<Client_Hello_Internal> data)
   : Client_Hello(std::move(data))
   {
   if(offered_suite(static_cast<uint16_t>(TLS_EMPTY_RENEGOTIATION_INFO_SCSV)))
      {
      if(Renegotiation_Extension* reneg = m_data->extensions().get<Renegotiation_Extension>())
         {
         if(!reneg->renegotiation_info().empty())
            throw TLS_Exception(Alert::HandshakeFailure,
                                "Client sent renegotiation SCSV and non-empty extension");
         }
      else
         {
         // add fake extension
         m_data->extensions().add(new Renegotiation_Extension());
         }
      }
   }
 
// Note: This delegates to the Client_Hello_12 constructor to take advantage
//       of the sanity checks there.
Client_Hello_12::Client_Hello_12(const std::vector<uint8_t>& buf)
   : Client_Hello_12(std::make_unique<Client_Hello_Internal>(buf)) {}
 
/*
* Create a new Client Hello message
*/
Client_Hello_12::Client_Hello_12(Handshake_IO& io,
                                 Handshake_Hash& hash,
                                 const Policy& policy,
                                 Callbacks& cb,
                                 RandomNumberGenerator& rng,
                                 const std::vector<uint8_t>& reneg_info,
                                 const Client_Hello_12::Settings& client_settings,
                                 const std::vector<std::string>& next_protocols)
   {
   m_data->m_legacy_version = client_settings.protocol_version();
   m_data->m_random = make_hello_random(rng, cb, policy);
   m_data->m_suites = policy.ciphersuite_list(client_settings.protocol_version());
 
   if(!policy.acceptable_protocol_version(m_data->legacy_version()))
      throw Internal_Error("Offering " + m_data->legacy_version().to_string() +
                           " but our own policy does not accept it");
 
   /*
   * Place all empty extensions in front to avoid a bug in some systems
   * which reject hellos when the last extension in the list is empty.
   */
 
   // EMS must always be used with TLS 1.2, regardless of the policy used.
   m_data->extensions().add(new Extended_Master_Secret);
 
   if(policy.negotiate_encrypt_then_mac())
      { m_data->extensions().add(new Encrypt_then_MAC); }
 
   m_data->extensions().add(new Session_Ticket_Extension());
 
   m_data->extensions().add(new Renegotiation_Extension(reneg_info));
 
   m_data->extensions().add(new Supported_Versions(m_data->legacy_version(), policy));
 
   if(!client_settings.hostname().empty())
      { m_data->extensions().add(new Server_Name_Indicator(client_settings.hostname())); }
 
   if(policy.support_cert_status_message())
      m_data->extensions().add(new Certificate_Status_Request({}, {}));
 
   auto supported_groups = std::make_unique<Supported_Groups>(policy.key_exchange_groups());
   if(!supported_groups->ec_groups().empty())
      {
      m_data->extensions().add(new Supported_Point_Formats(policy.use_ecc_point_compression()));
      }
   m_data->extensions().add(supported_groups.release());
 
   m_data->extensions().add(new Signature_Algorithms(policy.acceptable_signature_schemes()));
   if(auto cert_signing_prefs = policy.acceptable_certificate_signature_schemes())
      {
      // RFC 8446 4.2.3
      //    TLS 1.2 implementations SHOULD also process this extension.
      //    Implementations which have the same policy in both cases MAY omit
      //    the "signature_algorithms_cert" extension.
      m_data->extensions().add(new Signature_Algorithms_Cert(std::move(cert_signing_prefs.value())));
      }
 
   if(reneg_info.empty() && !next_protocols.empty())
      { m_data->extensions().add(new Application_Layer_Protocol_Notification(next_protocols)); }
 
   if(m_data->legacy_version().is_datagram_protocol())
      { m_data->extensions().add(new SRTP_Protection_Profiles(policy.srtp_profiles())); }
 
   cb.tls_modify_extensions(m_data->extensions(), Connection_Side::Client, type());
 
   hash.update(io.send(*this));
   }
 
/*
* Create a new Client Hello message (session resumption case)
*/
Client_Hello_12::Client_Hello_12(Handshake_IO& io,
                                 Handshake_Hash& hash,
                                 const Policy& policy,
                                 Callbacks& cb,
                                 RandomNumberGenerator& rng,
                                 const std::vector<uint8_t>& reneg_info,
                                 const Session_with_Handle& session,
                                 const std::vector<std::string>& next_protocols)
   {
   m_data->m_legacy_version = session.session.version();
   m_data->m_random = make_hello_random(rng, cb, policy);
 
   // RFC 5077 3.4
   //    When presenting a ticket, the client MAY generate and include a
   //    Session ID in the TLS ClientHello. [...] If a ticket is presented by
   //    the client, the server MUST NOT attempt to use the Session ID in the
   //    ClientHello for stateful session resumption.
   m_data->m_session_id = session.handle.id().value_or(Session_ID(make_hello_random(rng, cb, policy)));
   m_data->m_suites = policy.ciphersuite_list(m_data->legacy_version());
 
   if(!policy.acceptable_protocol_version(session.session.version()))
      throw Internal_Error("Offering " + m_data->legacy_version().to_string() +
                           " but our own policy does not accept it");
 
   if(!value_exists(m_data->ciphersuites(), session.session.ciphersuite_code()))
      { m_data->m_suites.push_back(session.session.ciphersuite_code()); }
 
   /*
   * As EMS must always be used with TLS 1.2, add it even if it wasn't used
   * in the original session. If the server understands it and follows the
   * RFC it should reject our resume attempt and upgrade us to a new session
   * with the EMS protection.
   */
   m_data->extensions().add(new Extended_Master_Secret);
 
   if(session.session.supports_encrypt_then_mac())
      { m_data->extensions().add(new Encrypt_then_MAC); }
 
   if(session.handle.is_ticket())
      { m_data->extensions().add(new Session_Ticket_Extension(session.handle.ticket().value())); }
 
   m_data->extensions().add(new Renegotiation_Extension(reneg_info));
 
   m_data->extensions().add(new Server_Name_Indicator(session.session.server_info().hostname()));
 
   if(policy.support_cert_status_message())
      m_data->extensions().add(new Certificate_Status_Request({}, {}));
 
   auto supported_groups = std::make_unique<Supported_Groups>(policy.key_exchange_groups());
 
   if(!supported_groups->ec_groups().empty())
      {
      m_data->extensions().add(new Supported_Point_Formats(policy.use_ecc_point_compression()));
      }
 
   m_data->extensions().add(supported_groups.release());
 
   m_data->extensions().add(new Signature_Algorithms(policy.acceptable_signature_schemes()));
   if(auto cert_signing_prefs = policy.acceptable_certificate_signature_schemes())
      {
      // RFC 8446 4.2.3
      //    TLS 1.2 implementations SHOULD also process this extension.
      //    Implementations which have the same policy in both cases MAY omit
      //    the "signature_algorithms_cert" extension.
      m_data->extensions().add(new Signature_Algorithms_Cert(std::move(cert_signing_prefs.value())));
      }
 
   if(reneg_info.empty() && !next_protocols.empty())
      { m_data->extensions().add(new Application_Layer_Protocol_Notification(next_protocols)); }
 
   cb.tls_modify_extensions(m_data->extensions(), Connection_Side::Client, type());
 
   hash.update(io.send(*this));
   }
 
#if defined(BOTAN_HAS_TLS_13)
 
Client_Hello_13::Client_Hello_13(std::unique_ptr<Client_Hello_Internal> data)
   : Client_Hello(std::move(data))
   {
   const auto& exts = m_data->extensions();
 
   // RFC 8446 4.1.2
   //    TLS 1.3 ClientHellos are identified as having a legacy_version of
   //    0x0303 and a "supported_versions" extension present with 0x0304 as the
   //    highest version indicated therein.
   //
   // Note that we already checked for "supported_versions" before entering this
   // c'tor in `Client_Hello_13::parse()`. This is just to be doubly sure.
   BOTAN_ASSERT_NOMSG(exts.has<Supported_Versions>());
 
   // RFC 8446 4.2.1
   //    Servers MAY abort the handshake upon receiving a ClientHello with
   //    legacy_version 0x0304 or later.
   if(m_data->legacy_version().is_tls_13_or_later())
      {
      throw TLS_Exception(Alert::DecodeError,
                          "TLS 1.3 Client Hello has invalid legacy_version");
      }
 
   // RFC 8446 4.1.2
   //    For every TLS 1.3 ClientHello, [the compression method] MUST contain
   //    exactly one byte, set to zero, [...].  If a TLS 1.3 ClientHello is
   //    received with any other value in this field, the server MUST abort the
   //    handshake with an "illegal_parameter" alert.
   if(m_data->comp_methods().size() != 1 || m_data->comp_methods().front() != 0)
      {
      throw TLS_Exception(Alert::IllegalParameter,
                          "Client did not offer NULL compression");
      }
 
   // RFC 8446 4.2.9
   //    A client MUST provide a "psk_key_exchange_modes" extension if it
   //    offers a "pre_shared_key" extension. If clients offer "pre_shared_key"
   //    without a "psk_key_exchange_modes" extension, servers MUST abort
   //    the handshake.
   if(exts.has<PSK>())
      {
      if(!exts.has<PSK_Key_Exchange_Modes>())
         {
         throw TLS_Exception(Alert::MissingExtension,
                             "Client Hello offered a PSK without a psk_key_exchange_modes extension");
         }
 
      // RFC 8446 4.2.11
      //     The "pre_shared_key" extension MUST be the last extension in the
      //     ClientHello [...]. Servers MUST check that it is the last extension
      //     and otherwise fail the handshake with an "illegal_parameter" alert.
      if(exts.all().back()->type() != Extension_Code::PresharedKey)
         {
         throw TLS_Exception(Alert::IllegalParameter,
                             "PSK extension was not at the very end of the Client Hello");
         }
      }
 
   // RFC 8446 9.2
   //    [A TLS 1.3 ClientHello] message MUST meet the following requirements:
   //
   //     -  If not containing a "pre_shared_key" extension, it MUST contain
   //        both a "signature_algorithms" extension and a "supported_groups"
   //        extension.
   //
   //     -  If containing a "supported_groups" extension, it MUST also contain
   //        a "key_share" extension, and vice versa.  An empty
   //        KeyShare.client_shares vector is permitted.
   //
   //    Servers receiving a ClientHello which does not conform to these
   //    requirements MUST abort the handshake with a "missing_extension"
   //    alert.
   if(!exts.has<PSK>())
      {
      if(!exts.has<Supported_Groups>() || !exts.has<Signature_Algorithms>())
         {
         throw TLS_Exception(Alert::MissingExtension,
                             "Non-PSK Client Hello did not contain supported_groups and signature_algorithms extensions");
         }
 
      }
   if(exts.has<Supported_Groups>() != exts.has<Key_Share>())
      {
      throw TLS_Exception(Alert::MissingExtension,
                          "Client Hello must either contain both key_share and supported_groups extensions or neither");
      }
 
   if(exts.has<Key_Share>())
      {
      const auto supported_ext = exts.get<Supported_Groups>();
      BOTAN_ASSERT_NONNULL(supported_ext);
      const auto supports = supported_ext->groups();
      const auto offers = exts.get<Key_Share>()->offered_groups();
 
      // RFC 8446 4.2.8
      //    Each KeyShareEntry value MUST correspond to a group offered in the
      //    "supported_groups" extension and MUST appear in the same order.
      //    [...]
      //    Clients MUST NOT offer any KeyShareEntry values for groups not
      //    listed in the client's "supported_groups" extension.
      //
      // Note: We can assume that both `offers` and `supports` are unique lists
      //       as this is ensured in the parsing code of the extensions.
      auto found_in_supported_groups = [&supports,support_offset = -1](auto group) mutable
         {
         const auto i = std::find(supports.begin(), supports.end(), group);
         if(i == supports.end())
            {
            return false;
            }
 
         const auto found_at = std::distance(supports.begin(), i);
         if(found_at <= support_offset)
            {
            return false;  // The order that groups appear in "key_share" and
                           // "supported_groups" must be the same
            }
 
         support_offset = static_cast<decltype(support_offset)>(found_at);
         return true;
         };
 
      for(const auto offered : offers)
         {
         // RFC 8446 4.2.8
         //    Servers MAY check for violations of these rules and abort the
         //    handshake with an "illegal_parameter" alert if one is violated.
         if(!found_in_supported_groups(offered))
            {
            throw TLS_Exception(Alert::IllegalParameter,
                                "Offered key exchange groups do not align with claimed supported groups");
            }
         }
      }
 
   // TODO: Reject oid_filters extension if found (which is the only known extension that
   //       must not occur in the TLS 1.3 client hello.
   // RFC 8446 4.2.5
   //    [The oid_filters extension] MUST only be sent in the CertificateRequest message.
   }
 
/*
* Create a new Client Hello message
*/
Client_Hello_13::Client_Hello_13(const Policy& policy,
                                 Callbacks& cb,
                                 RandomNumberGenerator& rng,
                                 std::string_view hostname,
                                 const std::vector<std::string>& next_protocols,
                                 std::optional<Session_with_Handle>& session)
   {
   // RFC 8446 4.1.2
   //    In TLS 1.3, the client indicates its version preferences in the
   //    "supported_versions" extension (Section 4.2.1) and the
   //    legacy_version field MUST be set to 0x0303, which is the version
   //    number for TLS 1.2.
   m_data->m_legacy_version = Protocol_Version::TLS_V12;
   m_data->m_random = make_hello_random(rng, cb, policy);
   m_data->m_suites = policy.ciphersuite_list(Protocol_Version::TLS_V13);
 
   if(policy.allow_tls12())  // Note: DTLS 1.3 is NYI, hence dtls_12 is not checked
      {
      const auto legacy_suites = policy.ciphersuite_list(Protocol_Version::TLS_V12);
      m_data->m_suites.insert(m_data->m_suites.end(), legacy_suites.cbegin(), legacy_suites.cend());
      }
 
   if(policy.tls_13_middlebox_compatibility_mode())
      {
      // RFC 8446 4.1.2
      //    In compatibility mode (see Appendix D.4), this field MUST be non-empty,
      //    so a client not offering a pre-TLS 1.3 session MUST generate a new
      //    32-byte value.
      //
      // Note: we won't ever offer a TLS 1.2 session. In such a case we would
      //       have instantiated a TLS 1.2 client in the first place.
      m_data->m_session_id = Session_ID(make_hello_random(rng, cb, policy));
      }
 
   if(!hostname.empty())
      m_data->extensions().add(new Server_Name_Indicator(hostname));
 
   m_data->extensions().add(new Supported_Groups(policy.key_exchange_groups()));
 
   m_data->extensions().add(new Key_Share(policy, cb, rng));
 
   m_data->extensions().add(new Supported_Versions(Protocol_Version::TLS_V13, policy));
 
   m_data->extensions().add(new Signature_Algorithms(policy.acceptable_signature_schemes()));
   if(auto cert_signing_prefs = policy.acceptable_certificate_signature_schemes())
      {
      // RFC 8446 4.2.3
      //    Implementations which have the same policy in both cases MAY omit
      //    the "signature_algorithms_cert" extension.
      m_data->extensions().add(new Signature_Algorithms_Cert(std::move(cert_signing_prefs.value())));
      }
 
   // TODO: Support for PSK-only mode without a key exchange.
   //       This should be configurable in TLS::Policy and should allow no PSK
   //       support at all (e.g. to disable support for session resumption).
   m_data->extensions().add(new PSK_Key_Exchange_Modes({PSK_Key_Exchange_Mode::PSK_DHE_KE}));
 
   if(policy.support_cert_status_message())
      m_data->extensions().add(new Certificate_Status_Request({}, {}));
 
   // We currently support "record_size_limit" for TLS 1.3 exclusively. Hence,
   // when TLS 1.2 is advertised as a supported protocol, we must not offer this
   // extension.
   if(policy.record_size_limit().has_value() && !policy.allow_tls12())
      m_data->extensions().add(new Record_Size_Limit(policy.record_size_limit().value()));
 
   if(!next_protocols.empty())
      m_data->extensions().add(new Application_Layer_Protocol_Notification(next_protocols));
 
   if(policy.allow_tls12())
      {
      m_data->extensions().add(new Renegotiation_Extension());
      m_data->extensions().add(new Session_Ticket_Extension());
 
      // EMS must always be used with TLS 1.2, regardless of the policy
      m_data->extensions().add(new Extended_Master_Secret);
 
      if(policy.negotiate_encrypt_then_mac())
         m_data->extensions().add(new Encrypt_then_MAC);
 
      if(m_data->extensions().has<Supported_Groups>() && !m_data->extensions().get<Supported_Groups>()->ec_groups().empty())
         m_data->extensions().add(new Supported_Point_Formats(policy.use_ecc_point_compression()));
      }
 
   if(session.has_value())
      {
      m_data->extensions().add(new PSK(session.value(), cb));
      }
 
   cb.tls_modify_extensions(m_data->extensions(), Connection_Side::Client, type());
 
   if(m_data->extensions().has<PSK>())
      {
      // RFC 8446 4.2.11
      //    The "pre_shared_key" extension MUST be the last extension in the
      //    ClientHello (this facilitates implementation [...]).
      if(m_data->extensions().all().back()->type() != Extension_Code::PresharedKey)
         {
         throw TLS_Exception(Alert::InternalError,
                             "Application modified extensions of Client Hello, PSK is not last anymore");
         }
      calculate_psk_binders({});
      }
   }
 
 
std::variant<Client_Hello_13, Client_Hello_12>
Client_Hello_13::parse(const std::vector<uint8_t>& buf)
   {
   auto data = std::make_unique<Client_Hello_Internal>(buf);
   const auto version = data->version();
 
   if(version.is_pre_tls_13())
      return Client_Hello_12(std::move(data));
   else
      return Client_Hello_13(std::move(data));
   }
 
void Client_Hello_13::retry(const Hello_Retry_Request& hrr,
                            const Transcript_Hash_State& transcript_hash_state,
                            Callbacks& cb,
                            RandomNumberGenerator& rng)
   {
   BOTAN_STATE_CHECK(m_data->extensions().has<Supported_Groups>());
   BOTAN_STATE_CHECK(m_data->extensions().has<Key_Share>());
 
   auto hrr_ks = hrr.extensions().get<Key_Share>();
   const auto& supported_groups = m_data->extensions().get<Supported_Groups>()->groups();
 
   if(hrr.extensions().has<Key_Share>())
      m_data->extensions().get<Key_Share>()->retry_offer(*hrr_ks, supported_groups, cb, rng);
 
   // RFC 8446 4.2.2
   //    When sending the new ClientHello, the client MUST copy
   //    the contents of the extension received in the HelloRetryRequest into
   //    a "cookie" extension in the new ClientHello.
   //
   // RFC 8446 4.2.2
   //    Clients MUST NOT use cookies in their initial ClientHello in subsequent
   //    connections.
   if(hrr.extensions().has<Cookie>())
      {
      BOTAN_STATE_CHECK(!m_data->extensions().has<Cookie>());
      m_data->extensions().add(new Cookie(hrr.extensions().get<Cookie>()->get_cookie()));
      }
 
   // Note: the consumer of the TLS implementation won't be able to distinguish
   //       invocations to this callback due to the first Client_Hello or the
   //       retried Client_Hello after receiving a Hello_Retry_Request. We assume
   //       that the user keeps and detects this state themselves.
   cb.tls_modify_extensions(m_data->extensions(), Connection_Side::Client, type());
 
   auto psk = m_data->extensions().get<PSK>();
   if(psk)
      {
      // Cipher suite should always be a known suite as this is checked upstream
      const auto cipher = Ciphersuite::by_id(hrr.ciphersuite());
      BOTAN_ASSERT_NOMSG(cipher.has_value());
 
      // RFC 8446 4.1.4
      //    In [...] its updated ClientHello, the client SHOULD NOT offer
      //    any pre-shared keys associated with a hash other than that of the
      //    selected cipher suite.
      psk->filter(cipher.value());
 
      // RFC 8446 4.2.11.2
      //    If the server responds with a HelloRetryRequest and the client
      //    then sends ClientHello2, its binder will be computed over: [...].
      calculate_psk_binders(transcript_hash_state.clone());
      }
   }
 
void Client_Hello_13::validate_updates(const Client_Hello_13& new_ch)
   {
   // RFC 8446 4.1.2
   //    The client will also send a ClientHello when the server has responded
   //    to its ClientHello with a HelloRetryRequest. In that case, the client
   //    MUST send the same ClientHello without modification, except as follows:
 
   if(m_data->session_id() != new_ch.m_data->session_id() ||
      m_data->random() != new_ch.m_data->random() ||
      m_data->ciphersuites() != new_ch.m_data->ciphersuites() ||
      m_data->comp_methods() != new_ch.m_data->comp_methods())
      {
      throw TLS_Exception(Alert::IllegalParameter, "Client Hello core values changed after Hello Retry Request");
      }
 
   const auto oldexts = extension_types();
   const auto newexts = new_ch.extension_types();
 
   // Check that extension omissions are justified
   for(const auto oldext : oldexts)
      {
      if(!newexts.contains(oldext))
         {
         const auto ext = extensions().get(oldext);
 
         // We don't make any assumptions about unimplemented extensions.
         if(!ext->is_implemented())
            continue;
 
         // RFC 8446 4.1.2
         //    Removing the "early_data" extension (Section 4.2.10) if one was
         //    present.  Early data is not permitted after a HelloRetryRequest.
         if(oldext == EarlyDataIndication::static_type())
            continue;
 
         // RFC 8446 4.1.2
         //    Optionally adding, removing, or changing the length of the
         //    "padding" extension.
         //
         // TODO: implement the Padding extension
         // if(oldext == Padding::static_type())
         //    continue;
 
         throw TLS_Exception(Alert::IllegalParameter, "Extension removed in updated Client Hello");
         }
      }
 
   // Check that extension additions are justified
   for(const auto newext : newexts)
      {
      if(!oldexts.contains(newext))
         {
         const auto ext = new_ch.extensions().get(newext);
 
         // We don't make any assumptions about unimplemented extensions.
         if(!ext->is_implemented())
            continue;
 
         // RFC 8446 4.1.2
         //    Including a "cookie" extension if one was provided in the
         //    HelloRetryRequest.
         if(newext == Cookie::static_type())
            continue;
 
         // RFC 8446 4.1.2
         //    Optionally adding, removing, or changing the length of the
         //    "padding" extension.
         //
         // TODO: implement the Padding extension
         // if(newext == Padding::static_type())
         //    continue;
 
         throw TLS_Exception(Alert::UnsupportedExtension, "Added an extension in updated Client Hello");
         }
      }
 
   // RFC 8446 4.1.2
   //    Removing the "early_data" extension (Section 4.2.10) if one was
   //    present.  Early data is not permitted after a HelloRetryRequest.
   if(new_ch.extensions().has<EarlyDataIndication>())
      {
      throw TLS_Exception(Alert::IllegalParameter, "Updated Client Hello indicates early data");
      }
 
   // TODO: Contents of extensions are not checked for update compatibility, see:
   //
   // RFC 8446 4.1.2
   //    If a "key_share" extension was supplied in the HelloRetryRequest,
   //    replacing the list of shares with a list containing a single
   //    KeyShareEntry from the indicated group.
   //
   //    Updating the "pre_shared_key" extension if present by recomputing
   //    the "obfuscated_ticket_age" and binder values and (optionally)
   //    removing any PSKs which are incompatible with the server's
   //    indicated cipher suite.
   //
   //    Optionally adding, removing, or changing the length of the
   //    "padding" extension.
   }
 
void Client_Hello_13::calculate_psk_binders(Transcript_Hash_State ths)
   {
   auto psk = m_data->extensions().get<PSK>();
   if(!psk || psk->empty())
      return;
 
   // RFC 8446 4.2.11.2
   //    Each entry in the binders list is computed as an HMAC over a
   //    transcript hash (see Section 4.4.1) containing a partial ClientHello
   //    [...].
   //
   // Therefore we marshal the entire message prematurely to obtain the
   // (truncated) transcript hash, calculate the PSK binders with it, update
   // the Client Hello thus finalizing the message. Down the road, it will be
   // re-marshalled with the correct binders and sent over the wire.
   Handshake_Layer::prepare_message(*this, ths);
   psk->calculate_binders(ths);
   }
 
std::optional<Protocol_Version> Client_Hello_13::highest_supported_version(const Policy& policy) const
   {
   // RFC 8446 4.2.1
   //    The "supported_versions" extension is used by the client to indicate
   //    which versions of TLS it supports and by the server to indicate which
   //    version it is using. The extension contains a list of supported
   //    versions in preference order, with the most preferred version first.
   const auto supvers = m_data->extensions().get<Supported_Versions>();
   BOTAN_ASSERT_NONNULL(supvers);
 
   std::optional<Protocol_Version> result;
 
   for(const auto& v : supvers->versions())
      {
      // RFC 8446 4.2.1
      //    Servers MUST only select a version of TLS present in that extension
      //    and MUST ignore any unknown versions that are present in that
      //    extension.
      if(!v.known_version() || !policy.acceptable_protocol_version(v))
         { continue; }
 
      result = (result.has_value())
         ? std::optional(std::max(result.value(), v))
         : std::optional(v);
      }
 
   return result;
   }
 
#endif // BOTAN_HAS_TLS_13
 
}