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

/*
* Client Key Exchange Message
* (C) 2004-2010 Jack Lloyd
*
* Released under the terms of the Botan license
*/

#include <botan/internal/tls_messages.h>
#include <botan/internal/tls_reader.h>
#include <botan/internal/tls_extensions.h>
#include <botan/internal/tls_handshake_io.h>
#include <botan/credentials_manager.h>
#include <botan/pubkey.h>
#include <botan/dh.h>
#include <botan/ecdh.h>
#include <botan/rsa.h>
#include <botan/srp6.h>
#include <botan/rng.h>
#include <botan/loadstor.h>

namespace Botan {

namespace TLS {

namespace {

secure_vector<byte> strip_leading_zeros(const secure_vector<byte>& input)
   {
   size_t leading_zeros = 0;

   for(size_t i = 0; i != input.size(); ++i)
      {
      if(input[i] != 0)
         break;
      ++leading_zeros;
      }

   secure_vector<byte> output(&input[leading_zeros],
                              &input[input.size()]);
   return output;
   }

}

/*
* Create a new Client Key Exchange message
*/
Client_Key_Exchange::Client_Key_Exchange(Handshake_IO& io,
                                         Handshake_State& state,
                                         const Policy& policy,
                                         Credentials_Manager& creds,
                                         const Public_Key* server_public_key,
                                         const std::string& hostname,
                                         RandomNumberGenerator& rng)
   {
   const std::string kex_algo = state.ciphersuite().kex_algo();

   if(kex_algo == "PSK")
      {
      std::string identity_hint = "";

      if(state.server_kex())
         {
         TLS_Data_Reader reader(state.server_kex()->params());
         identity_hint = reader.get_string(2, 0, 65535);
         }

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

      const std::string psk_identity = creds.psk_identity("tls-client",
                                                          hostname,
                                                          identity_hint);

      append_tls_length_value(m_key_material, psk_identity, 2);

      SymmetricKey psk = creds.psk("tls-client", hostname, psk_identity);

      std::vector<byte> zeros(psk.length());

      append_tls_length_value(m_pre_master, zeros, 2);
      append_tls_length_value(m_pre_master, psk.bits_of(), 2);
      }
   else if(state.server_kex())
      {
      TLS_Data_Reader reader(state.server_kex()->params());

      SymmetricKey psk;

      if(kex_algo == "DHE_PSK" || kex_algo == "ECDHE_PSK")
         {
         std::string identity_hint = reader.get_string(2, 0, 65535);

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

         const std::string psk_identity = creds.psk_identity("tls-client",
                                                             hostname,
                                                             identity_hint);

         append_tls_length_value(m_key_material, psk_identity, 2);

         psk = creds.psk("tls-client", hostname, psk_identity);
         }

      if(kex_algo == "DH" || kex_algo == "DHE_PSK")
         {
         BigInt p = BigInt::decode(reader.get_range<byte>(2, 1, 65535));
         BigInt g = BigInt::decode(reader.get_range<byte>(2, 1, 65535));
         BigInt Y = BigInt::decode(reader.get_range<byte>(2, 1, 65535));

         if(reader.remaining_bytes())
            throw Decoding_Error("Bad params size for DH key exchange");

         if(p.bits() < policy.minimum_dh_group_size())
            throw TLS_Exception(Alert::INSUFFICIENT_SECURITY,
                                "Server sent DH group of " +
                                std::to_string(p.bits()) +
                                " bits, policy requires at least " +
                                std::to_string(policy.minimum_dh_group_size()));

         /*
         * A basic check for key validity. As we do not know q here we
         * cannot check that Y is in the right subgroup. However since
         * our key is ephemeral there does not seem to be any
         * advantage to bogus keys anyway.
         */
         if(Y <= 1 || Y >= p - 1)
            throw TLS_Exception(Alert::INSUFFICIENT_SECURITY,
                                "Server sent bad DH key for DHE exchange");

         DL_Group group(p, g);

         if(!group.verify_group(rng, true))
            throw Internal_Error("DH group failed validation, possible attack");

         DH_PublicKey counterparty_key(group, Y);

         DH_PrivateKey priv_key(rng, group);

         PK_Key_Agreement ka(priv_key, "Raw");

         secure_vector<byte> dh_secret = strip_leading_zeros(
            ka.derive_key(0, counterparty_key.public_value()).bits_of());

         if(kex_algo == "DH")
            m_pre_master = dh_secret;
         else
            {
            append_tls_length_value(m_pre_master, dh_secret, 2);
            append_tls_length_value(m_pre_master, psk.bits_of(), 2);
            }

         append_tls_length_value(m_key_material, priv_key.public_value(), 2);
         }
      else if(kex_algo == "ECDH" || kex_algo == "ECDHE_PSK")
         {
         const byte curve_type = reader.get_byte();

         if(curve_type != 3)
            throw Decoding_Error("Server sent non-named ECC curve");

         const u16bit curve_id = reader.get_u16bit();

         const std::string name = Supported_Elliptic_Curves::curve_id_to_name(curve_id);

         if(name == "")
            throw Decoding_Error("Server sent unknown named curve " + std::to_string(curve_id));

         EC_Group group(name);

         std::vector<byte> ecdh_key = reader.get_range<byte>(1, 1, 255);

         ECDH_PublicKey counterparty_key(group, OS2ECP(ecdh_key, group.get_curve()));

         ECDH_PrivateKey priv_key(rng, group);

         PK_Key_Agreement ka(priv_key, "Raw");

         secure_vector<byte> ecdh_secret =
            ka.derive_key(0, counterparty_key.public_value()).bits_of();

         if(kex_algo == "ECDH")
            m_pre_master = ecdh_secret;
         else
            {
            append_tls_length_value(m_pre_master, ecdh_secret, 2);
            append_tls_length_value(m_pre_master, psk.bits_of(), 2);
            }

         append_tls_length_value(m_key_material, priv_key.public_value(), 1);
         }
      else if(kex_algo == "SRP_SHA")
         {
         const BigInt N = BigInt::decode(reader.get_range<byte>(2, 1, 65535));
         const BigInt g = BigInt::decode(reader.get_range<byte>(2, 1, 65535));
         std::vector<byte> salt = reader.get_range<byte>(1, 1, 255);
         const BigInt B = BigInt::decode(reader.get_range<byte>(2, 1, 65535));

         const std::string srp_group = srp6_group_identifier(N, g);

         const std::string srp_identifier =
            creds.srp_identifier("tls-client", hostname);

         const std::string srp_password =
            creds.srp_password("tls-client", hostname, srp_identifier);

         std::pair<BigInt, SymmetricKey> srp_vals =
            srp6_client_agree(srp_identifier,
                              srp_password,
                              srp_group,
                              "SHA-1",
                              salt,
                              B,
                              rng);

         append_tls_length_value(m_key_material, BigInt::encode(srp_vals.first), 2);
         m_pre_master = srp_vals.second.bits_of();
         }
      else
         {
         throw Internal_Error("Client_Key_Exchange: Unknown kex " +
                              kex_algo);
         }

      reader.assert_done();
      }
   else
      {
      // No server key exchange msg better mean RSA kex + RSA key in cert

      if(kex_algo != "RSA")
         throw Unexpected_Message("No server kex but negotiated kex " + kex_algo);

      if(!server_public_key)
         throw Internal_Error("No server public key for RSA exchange");

      if(auto rsa_pub = dynamic_cast<const RSA_PublicKey*>(server_public_key))
         {
         const Protocol_Version offered_version = state.client_hello()->version();

         m_pre_master = rng.random_vec(48);
         m_pre_master[0] = offered_version.major_version();
         m_pre_master[1] = offered_version.minor_version();

         PK_Encryptor_EME encryptor(*rsa_pub, "PKCS1v15");

         std::vector<byte> encrypted_key = encryptor.encrypt(m_pre_master, rng);

         if(state.version() == Protocol_Version::SSL_V3)
            m_key_material = encrypted_key; // no length field
         else
            append_tls_length_value(m_key_material, encrypted_key, 2);
         }
      else
         throw TLS_Exception(Alert::HANDSHAKE_FAILURE,
                             "Expected a RSA key in server cert but got " +
                             server_public_key->algo_name());
      }

   state.hash().update(io.send(*this));
   }

/*
* Read a Client Key Exchange message
*/
Client_Key_Exchange::Client_Key_Exchange(const std::vector<byte>& contents,
                                         const Handshake_State& state,
                                         const Private_Key* server_rsa_kex_key,
                                         Credentials_Manager& creds,
                                         const Policy& policy,
                                         RandomNumberGenerator& rng)
   {
   const std::string kex_algo = state.ciphersuite().kex_algo();

   if(kex_algo == "RSA")
      {
      BOTAN_ASSERT(state.server_certs() && !state.server_certs()->cert_chain().empty(),
                   "RSA key exchange negotiated so server sent a certificate");

      if(!server_rsa_kex_key)
         throw Internal_Error("Expected RSA kex but no server kex key set");

      if(!dynamic_cast<const RSA_PrivateKey*>(server_rsa_kex_key))
         throw Internal_Error("Expected RSA key but got " + server_rsa_kex_key->algo_name());

      PK_Decryptor_EME decryptor(*server_rsa_kex_key, "PKCS1v15");

      Protocol_Version client_version = state.client_hello()->version();

      /*
      * This is used as the pre-master if RSA decryption fails.
      * Otherwise we can be used as an oracle. See Bleichenbacher
      * "Chosen Ciphertext Attacks against Protocols Based on RSA
      * Encryption Standard PKCS #1", Crypto 98
      *
      * Create it here instead if in the catch clause as otherwise we
      * expose a timing channel WRT the generation of the fake value.
      * Some timing channel likely remains due to exception handling
      * and the like.
      */
      secure_vector<byte> fake_pre_master = rng.random_vec(48);
      fake_pre_master[0] = client_version.major_version();
      fake_pre_master[1] = client_version.minor_version();

      try
         {
         if(state.version() == Protocol_Version::SSL_V3)
            {
            m_pre_master = decryptor.decrypt(contents);
            }
         else
            {
            TLS_Data_Reader reader(contents);
            m_pre_master = decryptor.decrypt(reader.get_range<byte>(2, 0, 65535));
            }

         if(m_pre_master.size() != 48 ||
            client_version.major_version() != m_pre_master[0] ||
            client_version.minor_version() != m_pre_master[1])
            {
            throw Decoding_Error("Client_Key_Exchange: Secret corrupted");
            }
         }
      catch(...)
         {
         m_pre_master = fake_pre_master;
         }
      }
   else
      {
      TLS_Data_Reader reader(contents);

      SymmetricKey psk;

      if(kex_algo == "PSK" || kex_algo == "DHE_PSK" || kex_algo == "ECDHE_PSK")
         {
         const std::string psk_identity = reader.get_string(2, 0, 65535);

         psk = creds.psk("tls-server",
                         state.client_hello()->sni_hostname(),
                         psk_identity);

         if(psk.length() == 0)
            {
            if(policy.hide_unknown_users())
               psk = SymmetricKey(rng, 16);
            else
               throw TLS_Exception(Alert::UNKNOWN_PSK_IDENTITY,
                                   "No PSK for identifier " + psk_identity);
            }
         }

      if(kex_algo == "PSK")
         {
         std::vector<byte> zeros(psk.length());
         append_tls_length_value(m_pre_master, zeros, 2);
         append_tls_length_value(m_pre_master, psk.bits_of(), 2);
         }
      else if(kex_algo == "SRP_SHA")
         {
         SRP6_Server_Session& srp = state.server_kex()->server_srp_params();

         m_pre_master = srp.step2(BigInt::decode(reader.get_range<byte>(2, 0, 65535))).bits_of();
         }
      else if(kex_algo == "DH" || kex_algo == "DHE_PSK" ||
              kex_algo == "ECDH" || kex_algo == "ECDHE_PSK")
         {
         const Private_Key& private_key = state.server_kex()->server_kex_key();

         const PK_Key_Agreement_Key* ka_key =
            dynamic_cast<const PK_Key_Agreement_Key*>(&private_key);

         if(!ka_key)
            throw Internal_Error("Expected key agreement key type but got " +
                                 private_key.algo_name());

         try
            {
            PK_Key_Agreement ka(*ka_key, "Raw");

            std::vector<byte> client_pubkey;

            if(ka_key->algo_name() == "DH")
               client_pubkey = reader.get_range<byte>(2, 0, 65535);
            else
               client_pubkey = reader.get_range<byte>(1, 0, 255);

            secure_vector<byte> shared_secret = ka.derive_key(0, client_pubkey).bits_of();

            if(ka_key->algo_name() == "DH")
               shared_secret = strip_leading_zeros(shared_secret);

            if(kex_algo == "DHE_PSK" || kex_algo == "ECDHE_PSK")
               {
               append_tls_length_value(m_pre_master, shared_secret, 2);
               append_tls_length_value(m_pre_master, psk.bits_of(), 2);
               }
            else
               m_pre_master = shared_secret;
            }
         catch(std::exception &e)
            {
            /*
            * Something failed in the DH computation. To avoid possible
            * timing attacks, randomize the pre-master output and carry
            * on, allowing the protocol to fail later in the finished
            * checks.
            */
            m_pre_master = rng.random_vec(ka_key->public_value().size());
            }
         }
      else
         throw Internal_Error("Client_Key_Exchange: Unknown kex type " + kex_algo);
      }
   }

}

}