msg_encrypted_extensions.cpp

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/*
* TLS Hello Request and Client Hello Messages
* (C) 2022 Jack Lloyd
*     2022 René Meusel, Hannes Rantzsch - neXenio GmbH
*
* Botan is released under the Simplified BSD License (see license.txt)
*/
 
#include <botan/tls_messages_13.h>
 
#include <botan/tls_callbacks.h>
#include <botan/tls_exceptn.h>
#include <botan/tls_policy.h>
#include <botan/internal/stl_util.h>
#include <botan/internal/tls_reader.h>
 
namespace Botan::TLS {
 
Encrypted_Extensions::Encrypted_Extensions(const Client_Hello_13& client_hello,
                                           const Policy& policy,
                                           Callbacks& cb,
                                           bool is_resumption,
                                           bool requesting_client_auth) {
   const auto& exts = client_hello.extensions();
 
   // NOLINTBEGIN(*-owning-memory)
 
   // RFC 8446 4.2.7
   //    As of TLS 1.3, servers are permitted to send the "supported_groups"
   //    extension to the client.  Clients [...] MAY use the information
   //    learned from a successfully completed handshake to change what groups
   //    they use in their "key_share" extension in subsequent connections.
   if(exts.has<Supported_Groups>()) {
      m_extensions.add(new Supported_Groups(policy.key_exchange_groups()));
   }
 
   const auto record_size_limit = policy.record_size_limit();
   const auto max_record_size = MAX_PLAINTEXT_SIZE + 1 /* encrypted content type byte */;
   if(exts.has<Record_Size_Limit>()) {
      // RFC 8449 4
      //    Endpoints SHOULD advertise the "record_size_limit" extension, even
      //    if they have no need to limit the size of records. [...]  For
      //    servers, this allows clients to know that their limit will be
      //    respected.
      m_extensions.add(new Record_Size_Limit(record_size_limit.value_or(max_record_size)));
   } else if(record_size_limit.has_value() && record_size_limit.value() < max_record_size) {
      // RFC 8449 4
      //    Endpoints SHOULD advertise the "record_size_limit" extension, even if
      //    they have no need to limit the size of records. For clients, this
      //    allows servers to advertise a limit at their discretion.
      throw TLS_Exception(Alert::MissingExtension,
                          "Server cannot enforce record size limit without the client supporting it");
   }
 
   // RFC 7250 4.2
   //    If the TLS server wants to request a certificate from the client
   //    (via the certificate_request message), it MUST include the
   //    client_certificate_type extension in the server hello.
   //    [...]
   //    If the server does not send a certificate_request payload [...],
   //    then the client_certificate_type payload in the server hello MUST be
   //    omitted.
   //
   // Note: requesting_client_auth tracks whether the caller will actually
   // emit a CertificateRequest. The server-side decision in
   // Certificate_Request_13::maybe_create depends on both the policy flag
   // *and* the credentials manager's CA list, so re-checking just the
   // policy flag here would miss the trusted-CAs-only configuration.
   if(auto* ch_client_cert_types = exts.get<Client_Certificate_Type>();
      ch_client_cert_types != nullptr && requesting_client_auth) {
      m_extensions.add(new Client_Certificate_Type(*ch_client_cert_types, policy));
   }
 
   // RFC 7250 4.2
   //    The server_certificate_type extension in the client hello indicates the
   //    types of certificates the client is able to process when provided by
   //    the server in a subsequent certificate payload. [...] With the
   //    server_certificate_type extension in the server hello, the TLS server
   //    indicates the certificate type carried in the Certificate payload.
   if(auto* ch_server_cert_types = exts.get<Server_Certificate_Type>()) {
      m_extensions.add(new Server_Certificate_Type(*ch_server_cert_types, policy));
   }
 
   // RFC 6066 3
   //    A server that receives a client hello containing the "server_name"
   //    extension [...] SHALL include an extension of type "server_name" in the
   //    (extended) server hello. The "extension_data" field of this extension
   //    SHALL be empty.
   //
   //    When resuming a session, the server MUST NOT include a server_name
   //    extension in the server hello.
   if(exts.has<Server_Name_Indicator>() && !is_resumption) {
      m_extensions.add(new Server_Name_Indicator(""));
   }
 
   if(auto* alpn_ext = exts.get<Application_Layer_Protocol_Notification>()) {
      const auto& offered = alpn_ext->protocols();
      const auto next_protocol = cb.tls_server_choose_app_protocol(offered);
      if(!next_protocol.empty()) {
         // RFC 7301 3.2: if a protocol is selected, the server MUST select
         // one of the protocols advertised by the client.
         if(!value_exists(offered, next_protocol)) {
            throw TLS_Exception(Alert::InternalError,
                                "Application chose an ALPN protocol that the client did not offer");
         }
         m_extensions.add(new Application_Layer_Protocol_Notification(next_protocol));
      }
   }
 
   // NOLINTEND(*-owning-memory)
 
   // TODO: Implement handling for (at least)
   //       * SRTP
 
   cb.tls_modify_extensions(m_extensions, Connection_Side::Server, type());
 
   // After the application's tls_modify_extensions callback runs, re-check the
   // RFC-MUST invariants we just established above. The application can add or
   // reorder extensions, but shouldn't remove ones required direct response to
   // ClientHello extensions would put us out of spec.
   if(exts.has<Server_Certificate_Type>() && !m_extensions.has<Server_Certificate_Type>()) {
      throw TLS_Exception(
         Alert::InternalError,
         "Application tls_modify_extensions callback removed Server_Certificate_Type from EncryptedExtensions");
   }
 
   if(requesting_client_auth && exts.has<Client_Certificate_Type>() && !m_extensions.has<Client_Certificate_Type>()) {
      throw TLS_Exception(
         Alert::InternalError,
         "Application tls_modify_extensions callback removed Client_Certificate_Type from EncryptedExtensions");
   }
}
 
Encrypted_Extensions::Encrypted_Extensions(const std::vector<uint8_t>& buf) {
   TLS_Data_Reader reader("encrypted extensions reader", buf);
 
   // Encrypted Extensions contains a list of extensions. This list may legally
   // be empty. However, in that case we should at least see a two-byte length
   // field that reads 0x00 0x00.
   if(buf.size() < 2) {
      throw TLS_Exception(Alert::DecodeError, "Server sent an empty Encrypted Extensions message");
   }
 
   m_extensions.deserialize(reader, Connection_Side::Server, type());
 
   // RFC 8446 4.2
   //    If an implementation receives an extension which it recognizes and
   //    which is not specified for the message in which it appears, it MUST
   //    abort the handshake with an "illegal_parameter" alert.
   //
   // Note that we cannot encounter any extensions that we don't recognize here,
   // since only extensions we previously offered are allowed in EE.
   const auto allowed_exts = std::set<Extension_Code>{
      // Allowed extensions listed in RFC 8446 and implemented in Botan
      Extension_Code::ServerNameIndication,
      // MAX_FRAGMENT_LENGTH
      Extension_Code::SupportedGroups,
      Extension_Code::UseSrtp,
      // HEARTBEAT
      Extension_Code::ApplicationLayerProtocolNegotiation,
      // RFC 7250
      Extension_Code::ClientCertificateType,
      Extension_Code::ServerCertificateType,
      // EARLY_DATA
 
      // Allowed extensions not listed in RFC 8446 but acceptable as Botan implements them
      Extension_Code::RecordSizeLimit,
   };
   if(m_extensions.contains_implemented_extensions_other_than(allowed_exts)) {
      throw TLS_Exception(Alert::IllegalParameter, "Encrypted Extensions contained an extension that is not allowed");
   }
 
   reader.assert_done();
}
 
std::vector<uint8_t> Encrypted_Extensions::serialize() const {
   // RFC 8446 4.3.1: EncryptedExtensions carries Extension extensions<0..2^16-1>;
   // an empty list still requires a 2-byte length-prefix on the wire.
   // Extensions::serialize collapses empty to {} to suit other contexts, so
   // emit the explicit length here. Mirrors the same fallback in
   // New_Session_Ticket_13::serialize.
   if(m_extensions.empty()) {
      return {0x00, 0x00};
   }
   return m_extensions.serialize(Connection_Side::Server);
}
 
}  // namespace Botan::TLS