path: root/src/lib/tls/tls_server.cpp

/*
* TLS Server
* (C) 2004-2011,2012,2016 Jack Lloyd
*     2016 Matthias Gierlings
*
* Botan is released under the Simplified BSD License (see license.txt)
*/

#include <botan/tls_server.h>
#include <botan/tls_messages.h>
#include <botan/internal/tls_handshake_state.h>
#include <botan/internal/stl_util.h>
#include <botan/tls_magic.h>

namespace Botan {

namespace TLS {

class Server_Handshake_State final : public Handshake_State
   {
   public:
      Server_Handshake_State(Handshake_IO* io, Callbacks& cb)
         : Handshake_State(io, cb) {}

      Private_Key* server_rsa_kex_key() { return m_server_rsa_kex_key; }
      void set_server_rsa_kex_key(Private_Key* key)
         { m_server_rsa_kex_key = key; }

      bool allow_session_resumption() const
         { return m_allow_session_resumption; }
      void set_allow_session_resumption(bool allow_session_resumption)
         { m_allow_session_resumption = allow_session_resumption; }


   private:
      // Used by the server only, in case of RSA key exchange. Not owned
      Private_Key* m_server_rsa_kex_key = nullptr;

      /*
      * Used by the server to know if resumption should be allowed on
      * a server-initiated renegotiation
      */
      bool m_allow_session_resumption = true;
   };

namespace {

bool check_for_resume(Session& session_info,
                      Session_Manager& session_manager,
                      Credentials_Manager& credentials,
                      const Client_Hello* client_hello,
                      std::chrono::seconds session_ticket_lifetime)
   {
   const std::vector<uint8_t>& client_session_id = client_hello->session_id();
   const std::vector<uint8_t>& session_ticket = client_hello->session_ticket();

   if(session_ticket.empty())
      {
      if(client_session_id.empty()) // not resuming
         return false;

      // not found
      if(!session_manager.load_from_session_id(client_session_id, session_info))
         return false;
      }
   else
      {
      // If a session ticket was sent, ignore client session ID
      try
         {
         session_info = Session::decrypt(
            session_ticket,
            credentials.psk("tls-server", "session-ticket", ""));

         if(session_ticket_lifetime != std::chrono::seconds(0) &&
            session_info.session_age() > session_ticket_lifetime)
            return false; // ticket has expired
         }
      catch(...)
         {
         return false;
         }
      }

   // wrong version
   if(client_hello->version() != session_info.version())
      return false;

   // client didn't send original ciphersuite
   if(!value_exists(client_hello->ciphersuites(),
                    session_info.ciphersuite_code()))
      return false;

   // client didn't send original compression method
   if(!value_exists(client_hello->compression_methods(),
                    session_info.compression_method()))
      return false;

#if defined(BOTAN_HAS_SRP6)
   // client sent a different SRP identity
   if(client_hello->srp_identifier() != "")
      {
      if(client_hello->srp_identifier() != session_info.srp_identifier())
         return false;
      }
#endif

   // client sent a different SNI hostname
   if(client_hello->sni_hostname() != "")
      {
      if(client_hello->sni_hostname() != session_info.server_info().hostname())
         return false;
      }

   // Checking extended_master_secret on resume (RFC 7627 section 5.3)
   if(client_hello->supports_extended_master_secret() != session_info.supports_extended_master_secret())
      {
      if(!session_info.supports_extended_master_secret())
         {
         return false; // force new handshake with extended master secret
         }
      else
         {
         /*
         Client previously negotiated session with extended master secret,
         but has now attempted to resume without the extension: abort
         */
         throw TLS_Exception(Alert::HANDSHAKE_FAILURE,
                             "Client resumed extended ms session without sending extension");
         }
      }

   // Checking encrypt_then_mac on resume (RFC 7366 section 3.1)
   if(!client_hello->supports_encrypt_then_mac() && session_info.supports_encrypt_then_mac())
      {
      /*
      Client previously negotiated session with Encrypt-then-MAC,
      but has now attempted to resume without the extension: abort
      */
      throw TLS_Exception(Alert::HANDSHAKE_FAILURE,
                             "Client resumed Encrypt-then-MAC session without sending extension");
      }

   return true;
   }

/*
* Choose which ciphersuite to use
*/
uint16_t choose_ciphersuite(
   const Policy& policy,
   Protocol_Version version,
   Credentials_Manager& creds,
   const std::map<std::string, std::vector<X509_Certificate> >& cert_chains,
   const Client_Hello& client_hello)
   {
   const bool our_choice = policy.server_uses_own_ciphersuite_preferences();
   const bool have_srp = creds.attempt_srp("tls-server", client_hello.sni_hostname());
   const std::vector<uint16_t> client_suites = client_hello.ciphersuites();
   const std::vector<uint16_t> server_suites = policy.ciphersuite_list(version, have_srp);

   if(server_suites.empty())
      throw TLS_Exception(Alert::HANDSHAKE_FAILURE,
                          "Policy forbids us from negotiating any ciphersuite");

   const bool have_shared_ecc_curve =
      (policy.choose_curve(client_hello.supported_ecc_curves()) != "");

   /*
   Walk down one list in preference order
   */
   std::vector<uint16_t> pref_list = server_suites;
   std::vector<uint16_t> other_list = client_suites;

   if(!our_choice)
      std::swap(pref_list, other_list);

   const std::set<std::string> client_sig_algos = client_hello.supported_sig_algos();

   for(auto suite_id : pref_list)
      {
      if(!value_exists(other_list, suite_id))
         continue;

      const Ciphersuite suite = Ciphersuite::by_id(suite_id);

      if(suite.valid() == false)
         continue;

      if(suite.ecc_ciphersuite() && have_shared_ecc_curve == false)
         continue;

      // For non-anon ciphersuites
      if(suite.sig_algo() != "")
         {
         // Do we have any certificates for this sig?
         if(cert_chains.count(suite.sig_algo()) == 0)
            continue;

         // Client reques
         if(!client_sig_algos.empty() && client_sig_algos.count(suite.sig_algo()) == 0)
            continue;
         }

      if(version.supports_negotiable_signature_algorithms() && suite.sig_algo() != "")
         {
         const std::vector<std::pair<std::string, std::string>> client_sig_hash_pairs =
            client_hello.supported_algos();

         if(client_hello.supported_algos().empty() == false)
            {
            bool we_support_some_hash_by_client = false;

            for(auto&& hash_and_sig : client_hello.supported_algos())
               {
               if(hash_and_sig.second == suite.sig_algo() &&
                  policy.allowed_signature_hash(hash_and_sig.first))
                  {
                  we_support_some_hash_by_client = true;
                  break;
                  }
               }

            if(we_support_some_hash_by_client == false)
               {
               throw TLS_Exception(Alert::HANDSHAKE_FAILURE,
                                   "Policy does not accept any hash function supported by client");
               }
            }
         else
            {
            if(policy.allowed_signature_hash("SHA-1") == false)
               throw TLS_Exception(Alert::HANDSHAKE_FAILURE,
                                   "Client did not send signature_algorithms extension "
                                   "and policy prohibits SHA-1 fallback");
            }
         }

#if defined(BOTAN_HAS_SRP6)
      /*
      The client may offer SRP cipher suites in the hello message but
      omit the SRP extension.  If the server would like to select an
      SRP cipher suite in this case, the server SHOULD return a fatal
      "unknown_psk_identity" alert immediately after processing the
      client hello message.
       - RFC 5054 section 2.5.1.2
      */
      if(suite.kex_algo() == "SRP_SHA" && client_hello.srp_identifier() == "")
         throw TLS_Exception(Alert::UNKNOWN_PSK_IDENTITY,
                             "Client wanted SRP but did not send username");
#endif

      return suite_id;
      }

   throw TLS_Exception(Alert::HANDSHAKE_FAILURE,
                       "Can't agree on a ciphersuite with client");
   }


/*
* Choose which compression algorithm to use
*/
uint8_t choose_compression(const Policy& policy,
                        const std::vector<uint8_t>& c_comp)
   {
   std::vector<uint8_t> s_comp = policy.compression();

   for(size_t i = 0; i != s_comp.size(); ++i)
      for(size_t j = 0; j != c_comp.size(); ++j)
         if(s_comp[i] == c_comp[j])
            return s_comp[i];

   return NO_COMPRESSION;
   }

std::map<std::string, std::vector<X509_Certificate> >
get_server_certs(const std::string& hostname,
                 Credentials_Manager& creds)
   {
   const char* cert_types[] = { "RSA", "DSA", "ECDSA", nullptr };

   std::map<std::string, std::vector<X509_Certificate> > cert_chains;

   for(size_t i = 0; cert_types[i]; ++i)
      {
      std::vector<X509_Certificate> certs =
         creds.cert_chain_single_type(cert_types[i], "tls-server", hostname);

      if(!certs.empty())
         cert_chains[cert_types[i]] = certs;
      }

   return cert_chains;
   }

}

/*
* TLS Server Constructor
*/
Server::Server(Callbacks& callbacks,
               Session_Manager& session_manager,
               Credentials_Manager& creds,
               const Policy& policy,
               RandomNumberGenerator& rng,
               bool is_datagram,
               size_t io_buf_sz) :
   Channel(callbacks, session_manager, rng, policy,
           is_datagram, io_buf_sz),
   m_creds(creds)
   {
   }

Server::Server(output_fn output,
               data_cb data_cb,
               alert_cb alert_cb,
               handshake_cb handshake_cb,
               Session_Manager& session_manager,
               Credentials_Manager& creds,
               const Policy& policy,
               RandomNumberGenerator& rng,
               next_protocol_fn next_proto,
               bool is_datagram,
               size_t io_buf_sz) :
   Channel(output, data_cb, alert_cb, handshake_cb,
           Channel::handshake_msg_cb(), session_manager,
           rng, policy, is_datagram, io_buf_sz),
   m_creds(creds),
   m_choose_next_protocol(next_proto)
   {
   }


Server::Server(output_fn output,
               data_cb data_cb,
               alert_cb alert_cb,
               handshake_cb handshake_cb,
               handshake_msg_cb hs_msg_cb,
               Session_Manager& session_manager,
               Credentials_Manager& creds,
               const Policy& policy,
               RandomNumberGenerator& rng,
               next_protocol_fn next_proto,
               bool is_datagram) :
   Channel(output, data_cb, alert_cb, handshake_cb, hs_msg_cb,
           session_manager, rng, policy, is_datagram),
   m_creds(creds),
   m_choose_next_protocol(next_proto)
   {
   }

Handshake_State* Server::new_handshake_state(Handshake_IO* io)
   {
   std::unique_ptr<Handshake_State> state(new Server_Handshake_State(io, callbacks()));

   state->set_expected_next(CLIENT_HELLO);
   return state.release();
   }

std::vector<X509_Certificate>
Server::get_peer_cert_chain(const Handshake_State& state) const
   {
   if(state.client_certs())
      return state.client_certs()->cert_chain();
   return std::vector<X509_Certificate>();
   }

/*
* Send a hello request to the client
*/
void Server::initiate_handshake(Handshake_State& state,
                                bool force_full_renegotiation)
   {
   dynamic_cast<Server_Handshake_State&>(state).
       set_allow_session_resumption(!force_full_renegotiation);

   Hello_Request hello_req(state.handshake_io());
   }

/*
* Process a CLIENT HELLO Message
*/
void Server::process_client_hello_msg(const Handshake_State* active_state,
                                      Server_Handshake_State& pending_state,
                                      const std::vector<uint8_t>& contents)
   {
   const bool initial_handshake = !active_state;

   if(initial_handshake == false && policy().allow_client_initiated_renegotiation() == false)
      {
      send_warning_alert(Alert::NO_RENEGOTIATION);
      return;
      }

   if(!policy().allow_insecure_renegotiation() &&
      !(initial_handshake || secure_renegotiation_supported()))
      {
      send_warning_alert(Alert::NO_RENEGOTIATION);
      return;
      }

   pending_state.client_hello(new Client_Hello(contents));
   const Protocol_Version client_version = pending_state.client_hello()->version();

   if(client_version.major_version() < 3)
      throw TLS_Exception(Alert::PROTOCOL_VERSION, "Client offered version with major version under 3");
   if(client_version.major_version() == 3 && client_version.minor_version() == 0)
      throw TLS_Exception(Alert::PROTOCOL_VERSION, "SSLv3 is not supported");

   Protocol_Version negotiated_version;

   const Protocol_Version latest_supported =
      policy().latest_supported_version(client_version.is_datagram_protocol());

   if((initial_handshake && client_version.known_version()) ||
      (!initial_handshake && client_version == active_state->version()))
      {
      /*
      Common cases: new client hello with some known version, or a
      renegotiation using the same version as previously
      negotiated.
      */

      negotiated_version = client_version;
      }
   else if(!initial_handshake && (client_version != active_state->version()))
      {
      /*
      * If this is a renegotiation, and the client has offered a
      * later version than what it initially negotiated, negotiate
      * the old version. This matches OpenSSL's behavior. If the
      * client is offering a version earlier than what it initially
      * negotiated, reject as a probable attack.
      */
      if(active_state->version() > client_version)
         {
         throw TLS_Exception(Alert::PROTOCOL_VERSION,
                              "Client negotiated " +
                              active_state->version().to_string() +
                              " then renegotiated with " +
                              client_version.to_string());
         }
      else
         negotiated_version = active_state->version();
      }
   else
      {
      /*
      New negotiation using a version we don't know. Offer them the
      best we currently know and support
      */
      negotiated_version = latest_supported;
      }

   if(!policy().acceptable_protocol_version(negotiated_version))
      {
      throw TLS_Exception(Alert::PROTOCOL_VERSION,
                           "Client version " + negotiated_version.to_string() +
                           " is unacceptable by policy");
      }

   if(pending_state.client_hello()->sent_fallback_scsv())
      {
      if(latest_supported > client_version)
         throw TLS_Exception(Alert::INAPPROPRIATE_FALLBACK,
                              "Client signalled fallback SCSV, possible attack");
      }

   secure_renegotiation_check(pending_state.client_hello());

   pending_state.set_version(negotiated_version);

   Session session_info;
   const bool resuming =
      pending_state.allow_session_resumption() &&
      check_for_resume(session_info,
                       session_manager(),
                       m_creds,
                       pending_state.client_hello(),
                       std::chrono::seconds(policy().session_ticket_lifetime()));

   bool have_session_ticket_key = false;

   try
      {
      have_session_ticket_key =
         m_creds.psk("tls-server", "session-ticket", "").length() > 0;
      }
   catch(...) {}

   m_next_protocol = "";
   if(pending_state.client_hello()->supports_alpn())
      {
      m_next_protocol = callbacks().tls_server_choose_app_protocol(pending_state.client_hello()->next_protocols());

      // if the callback return was empty, fall back to the (deprecated) std::function
      if(m_next_protocol.empty() && m_choose_next_protocol)
         {
         m_next_protocol = m_choose_next_protocol(pending_state.client_hello()->next_protocols());
         }
      }

   if(resuming)
      {
      this->session_resume(pending_state, have_session_ticket_key, session_info);
      }
   else // new session
      {
      this->session_create(pending_state, have_session_ticket_key);
      }
   }

void Server::process_certificate_msg(Server_Handshake_State& pending_state,
                                     const std::vector<uint8_t>& contents)
   {
   pending_state.client_certs(new Certificate(contents, policy()));
   pending_state.set_expected_next(CLIENT_KEX);
   }

void Server::process_client_key_exchange_msg(Server_Handshake_State& pending_state,
                                             const std::vector<uint8_t>& contents)
   {
   if(pending_state.received_handshake_msg(CERTIFICATE) && !pending_state.client_certs()->empty())
      pending_state.set_expected_next(CERTIFICATE_VERIFY);
   else
      pending_state.set_expected_next(HANDSHAKE_CCS);

   pending_state.client_kex(new Client_Key_Exchange(contents, pending_state,
                                                    pending_state.server_rsa_kex_key(),
                                                    m_creds, policy(), rng()));

   pending_state.compute_session_keys();
   }

void Server::process_change_cipher_spec_msg(Server_Handshake_State& pending_state)
   {
   pending_state.set_expected_next(FINISHED);
   change_cipher_spec_reader(SERVER);
   }

void Server::process_certificate_verify_msg(Server_Handshake_State& pending_state,
                                            Handshake_Type type,
                                            const std::vector<uint8_t>& contents)
   {
   pending_state.client_verify(new Certificate_Verify(contents, pending_state.version()));

   const std::vector<X509_Certificate>& client_certs =
      pending_state.client_certs()->cert_chain();

   const bool sig_valid =
      pending_state.client_verify()->verify(client_certs[0], pending_state, policy());

   pending_state.hash().update(pending_state.handshake_io().format(contents, type));

   /*
   * Using DECRYPT_ERROR looks weird here, but per RFC 4346 is for
   * "A handshake cryptographic operation failed, including being
   * unable to correctly verify a signature, ..."
   */
   if(!sig_valid)
      throw TLS_Exception(Alert::DECRYPT_ERROR, "Client cert verify failed");

   try
      {
      const std::string sni_hostname = pending_state.client_hello()->sni_hostname();
      auto trusted_CAs = m_creds.trusted_certificate_authorities("tls-server", sni_hostname);

      callbacks().tls_verify_cert_chain(client_certs,
                                        {}, // ocsp
                                        trusted_CAs,
                                        Usage_Type::TLS_CLIENT_AUTH,
                                        sni_hostname,
                                        policy());
      }
   catch(std::exception& e)
      {
      throw TLS_Exception(Alert::BAD_CERTIFICATE, e.what());
      }

   pending_state.set_expected_next(HANDSHAKE_CCS);
   }

void Server::process_finished_msg(Server_Handshake_State& pending_state,
                                  Handshake_Type type,
                                  const std::vector<uint8_t>& contents)
   {
   pending_state.set_expected_next(HANDSHAKE_NONE);

   pending_state.client_finished(new Finished(contents));

   if(!pending_state.client_finished()->verify(pending_state, CLIENT))
      throw TLS_Exception(Alert::DECRYPT_ERROR,
                          "Finished message didn't verify");

   if(!pending_state.server_finished())
      {
      // already sent finished if resuming, so this is a new session

      pending_state.hash().update(pending_state.handshake_io().format(contents, type));

      Session session_info(
         pending_state.server_hello()->session_id(),
         pending_state.session_keys().master_secret(),
         pending_state.server_hello()->version(),
         pending_state.server_hello()->ciphersuite(),
         pending_state.server_hello()->compression_method(),
         SERVER,
         pending_state.server_hello()->supports_extended_master_secret(),
         pending_state.server_hello()->supports_encrypt_then_mac(),
         get_peer_cert_chain(pending_state),
         std::vector<uint8_t>(),
         Server_Information(pending_state.client_hello()->sni_hostname()),
         pending_state.srp_identifier(),
         pending_state.server_hello()->srtp_profile());

      if(save_session(session_info))
         {
         if(pending_state.server_hello()->supports_session_ticket())
            {
            try
               {
               const SymmetricKey ticket_key = m_creds.psk("tls-server", "session-ticket", "");

               pending_state.new_session_ticket(
                  new New_Session_Ticket(pending_state.handshake_io(),
                                         pending_state.hash(),
                                         session_info.encrypt(ticket_key, rng()),
                                         policy().session_ticket_lifetime()));
               }
            catch(...) {}
            }
         else
            session_manager().save(session_info);
         }

      if(!pending_state.new_session_ticket() &&
         pending_state.server_hello()->supports_session_ticket())
         {
         pending_state.new_session_ticket(
            new New_Session_Ticket(pending_state.handshake_io(), pending_state.hash()));
         }

      pending_state.handshake_io().send(Change_Cipher_Spec());

      change_cipher_spec_writer(SERVER);

      pending_state.server_finished(new Finished(pending_state.handshake_io(), pending_state, SERVER));
      }

   activate_session();
   }

/*
* Process a handshake message
*/
void Server::process_handshake_msg(const Handshake_State* active_state,
                                   Handshake_State& state_base,
                                   Handshake_Type type,
                                   const std::vector<uint8_t>& contents)
   {
   Server_Handshake_State& state = dynamic_cast<Server_Handshake_State&>(state_base);
   state.confirm_transition_to(type);

   /*
   * The change cipher spec message isn't technically a handshake
   * message so it's not included in the hash. The finished and
   * certificate verify messages are verified based on the current
   * state of the hash *before* this message so we delay adding them
   * to the hash computation until we've processed them below.
   */
   if(type != HANDSHAKE_CCS && type != FINISHED && type != CERTIFICATE_VERIFY)
      {
      state.hash().update(state.handshake_io().format(contents, type));
      }

   switch(type)
      {
      case CLIENT_HELLO:
         return this->process_client_hello_msg(active_state, state, contents);

      case CERTIFICATE:
         return this->process_certificate_msg(state, contents);

      case CLIENT_KEX:
         return this->process_client_key_exchange_msg(state, contents);

      case CERTIFICATE_VERIFY:
         return this->process_certificate_verify_msg(state, type, contents);

      case HANDSHAKE_CCS:
         return this->process_change_cipher_spec_msg(state);

      case FINISHED:
         return this->process_finished_msg(state, type, contents);

      default:
         throw Unexpected_Message("Unknown handshake message received");
      }
   }

void Server::session_resume(Server_Handshake_State& pending_state,
                            bool have_session_ticket_key,
                            Session& session_info)
   {
   // Only offer a resuming client a new ticket if they didn't send one this time,
   // ie, resumed via server-side resumption. TODO: also send one if expiring soon?

   const bool offer_new_session_ticket =
      (pending_state.client_hello()->supports_session_ticket() &&
       pending_state.client_hello()->session_ticket().empty() &&
       have_session_ticket_key);

   pending_state.server_hello(new Server_Hello(
                                 pending_state.handshake_io(),
                                 pending_state.hash(),
                                 policy(),
                                 rng(),
                                 secure_renegotiation_data_for_server_hello(),
                                 *pending_state.client_hello(),
                                 session_info,
                                 offer_new_session_ticket,
                                 m_next_protocol));

   secure_renegotiation_check(pending_state.server_hello());

   pending_state.compute_session_keys(session_info.master_secret());

   if(!save_session(session_info))
      {
      session_manager().remove_entry(session_info.session_id());

      if(pending_state.server_hello()->supports_session_ticket()) // send an empty ticket
         {
         pending_state.new_session_ticket(
            new New_Session_Ticket(pending_state.handshake_io(),
                                   pending_state.hash()));
         }
      }

   if(pending_state.server_hello()->supports_session_ticket() && !pending_state.new_session_ticket())
      {
      try
         {
         const SymmetricKey ticket_key = m_creds.psk("tls-server", "session-ticket", "");

         pending_state.new_session_ticket(
            new New_Session_Ticket(pending_state.handshake_io(),
                                   pending_state.hash(),
                                   session_info.encrypt(ticket_key, rng()),
                                   policy().session_ticket_lifetime()));
         }
      catch(...) {}

      if(!pending_state.new_session_ticket())
         {
         pending_state.new_session_ticket(
            new New_Session_Ticket(pending_state.handshake_io(), pending_state.hash()));
         }
      }

   pending_state.handshake_io().send(Change_Cipher_Spec());

   change_cipher_spec_writer(SERVER);

   pending_state.server_finished(new Finished(pending_state.handshake_io(), pending_state, SERVER));
   pending_state.set_expected_next(HANDSHAKE_CCS);
   }

void Server::session_create(Server_Handshake_State& pending_state,
                            bool have_session_ticket_key)
   {
   std::map<std::string, std::vector<X509_Certificate> > cert_chains;

   const std::string sni_hostname = pending_state.client_hello()->sni_hostname();

   cert_chains = get_server_certs(sni_hostname, m_creds);

   if(sni_hostname != "" && cert_chains.empty())
      {
      cert_chains = get_server_certs("", m_creds);

      /*
      * Only send the unrecognized_name alert if we couldn't
      * find any certs for the requested name but did find at
      * least one cert to use in general. That avoids sending an
      * unrecognized_name when a server is configured for purely
      * anonymous operation.
      */
      if(!cert_chains.empty())
         send_alert(Alert(Alert::UNRECOGNIZED_NAME));
      }

   const uint16_t ciphersuite = choose_ciphersuite(policy(), pending_state.version(),
                                                   m_creds, cert_chains,
                                                   *pending_state.client_hello());

   Server_Hello::Settings srv_settings(
      make_hello_random(rng(), policy()), // new session ID
      pending_state.version(),
      ciphersuite,
      choose_compression(policy(), pending_state.client_hello()->compression_methods()),
      have_session_ticket_key);

   pending_state.server_hello(new Server_Hello(
                                 pending_state.handshake_io(),
                                 pending_state.hash(),
                                 policy(),
                                 rng(),
                                 secure_renegotiation_data_for_server_hello(),
                                 *pending_state.client_hello(),
                                 srv_settings,
                                 m_next_protocol));

   secure_renegotiation_check(pending_state.server_hello());

   const std::string sig_algo = pending_state.ciphersuite().sig_algo();
   const std::string kex_algo = pending_state.ciphersuite().kex_algo();

   if(sig_algo != "")
      {
      BOTAN_ASSERT(!cert_chains[sig_algo].empty(),
                   "Attempting to send empty certificate chain");

      pending_state.server_certs(new Certificate(pending_state.handshake_io(),
                                                 pending_state.hash(),
                                                 cert_chains[sig_algo]));
      }

   Private_Key* private_key = nullptr;

   if(kex_algo == "RSA" || sig_algo != "")
      {
      private_key = m_creds.private_key_for(
         pending_state.server_certs()->cert_chain()[0],
         "tls-server",
         sni_hostname);

      if(!private_key)
         throw Internal_Error("No private key located for associated server cert");
      }

   if(kex_algo == "RSA")
      {
      pending_state.set_server_rsa_kex_key(private_key);
      }
   else
      {
      pending_state.server_kex(new Server_Key_Exchange(pending_state.handshake_io(),
                                                       pending_state, policy(),
                                                       m_creds, rng(), private_key));
      }

   auto trusted_CAs = m_creds.trusted_certificate_authorities("tls-server", sni_hostname);

   std::vector<X509_DN> client_auth_CAs;

   for(auto store : trusted_CAs)
      {
      auto subjects = store->all_subjects();
      client_auth_CAs.insert(client_auth_CAs.end(), subjects.begin(), subjects.end());
      }

   if(!client_auth_CAs.empty() && pending_state.ciphersuite().sig_algo() != "")
      {
      pending_state.cert_req(
         new Certificate_Req(pending_state.handshake_io(),
                             pending_state.hash(),
                             policy(),
                             client_auth_CAs,
                             pending_state.version()));

      /*
      SSLv3 allowed clients to skip the Certificate message entirely
      if they wanted. In TLS v1.0 and later clients must send a
      (possibly empty) Certificate message
      */
      pending_state.set_expected_next(CERTIFICATE);
      }
   else
      {
      pending_state.set_expected_next(CLIENT_KEX);
      }

   pending_state.server_hello_done(new Server_Hello_Done(pending_state.handshake_io(), pending_state.hash()));
   }
}

}