tls_extensions.h

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/*
* TLS Extensions
* (C) 2011,2012,2016,2018,2019 Jack Lloyd
* (C) 2016 Juraj Somorovsky
* (C) 2016 Matthias Gierlings
* (C) 2021 Elektrobit Automotive GmbH
* (C) 2022 René Meusel, Hannes Rantzsch - neXenio GmbH
*
* Botan is released under the Simplified BSD License (see license.txt)
*/
 
#ifndef BOTAN_TLS_EXTENSIONS_H_
#define BOTAN_TLS_EXTENSIONS_H_
 
#include <botan/tls_algos.h>
#include <botan/tls_magic.h>
#include <botan/tls_version.h>
#include <botan/secmem.h>
#include <botan/pkix_types.h>
#include <botan/tls_signature_scheme.h>
#include <botan/tls_session.h>
 
#include <algorithm>
#include <optional>
#include <variant>
#include <vector>
#include <string>
#include <set>
#include <memory>
 
namespace Botan {
 
class RandomNumberGenerator;
 
namespace TLS {
 
#if defined(BOTAN_HAS_TLS_13)
class Callbacks;
class Session_Manager;
class Cipher_State;
class Ciphersuite;
class Transcript_Hash_State;
 
enum class PSK_Key_Exchange_Mode : uint8_t {
   PSK_KE = 0,
   PSK_DHE_KE = 1
};
 
#endif
class Policy;
class TLS_Data_Reader;
 
enum class Extension_Code : uint16_t {
   ServerNameIndication                = 0,
   CertificateStatusRequest            = 5,
 
   SupportedGroups                     = 10,
   EcPointFormats                      = 11,
   SignatureAlgorithms                 = 13,
   CertSignatureAlgorithms             = 50,
   UseSrtp                             = 14,
   ApplicationLayerProtocolNegotiation = 16,
 
   // SignedCertificateTimestamp          = 18,  // NYI
 
   EncryptThenMac                      = 22,
   ExtendedMasterSecret                = 23,
 
   RecordSizeLimit                     = 28,
 
   SessionTicket                       = 35,
 
   SupportedVersions                   = 43,
#if defined(BOTAN_HAS_TLS_13)
   PresharedKey                        = 41,
   EarlyData                           = 42,
   Cookie                              = 44,
 
   PskKeyExchangeModes                 = 45,
   CertificateAuthorities              = 47,
   // OidFilters                          = 48,  // NYI
 
   KeyShare                            = 51,
#endif
 
   SafeRenegotiation                   = 65281,
};
 
/**
* Base class representing a TLS extension of some kind
*/
class BOTAN_UNSTABLE_API Extension
   {
   public:
      /**
      * @return code number of the extension
      */
      virtual Extension_Code type() const = 0;
 
      /**
      * @return serialized binary for the extension
      */
      virtual std::vector<uint8_t> serialize(Connection_Side whoami) const = 0;
 
      /**
      * @return if we should encode this extension or not
      */
      virtual bool empty() const = 0;
 
      /**
       * @return true if this extension is known and implemented by Botan
       */
      virtual bool is_implemented() const { return true; }
 
      virtual ~Extension() = default;
   };
 
/**
* Server Name Indicator extension (RFC 3546)
*/
class BOTAN_UNSTABLE_API Server_Name_Indicator final : public Extension
   {
   public:
      static Extension_Code static_type()
         { return Extension_Code::ServerNameIndication; }
 
      Extension_Code type() const override { return static_type(); }
 
      explicit Server_Name_Indicator(std::string_view host_name) :
         m_sni_host_name(host_name) {}
 
      Server_Name_Indicator(TLS_Data_Reader& reader,
                            uint16_t extension_size);
 
      std::string host_name() const { return m_sni_host_name; }
 
      std::vector<uint8_t> serialize(Connection_Side whoami) const override;
 
      bool empty() const override { return false; }
 
   private:
      std::string m_sni_host_name;
   };
 
/**
* Renegotiation Indication Extension (RFC 5746)
*/
class BOTAN_UNSTABLE_API Renegotiation_Extension final : public Extension
   {
   public:
      static Extension_Code static_type()
         { return Extension_Code::SafeRenegotiation; }
 
      Extension_Code type() const override { return static_type(); }
 
      Renegotiation_Extension() = default;
 
      explicit Renegotiation_Extension(const std::vector<uint8_t>& bits) :
         m_reneg_data(bits) {}
 
      Renegotiation_Extension(TLS_Data_Reader& reader,
                             uint16_t extension_size);
 
      const std::vector<uint8_t>& renegotiation_info() const
         { return m_reneg_data; }
 
      std::vector<uint8_t> serialize(Connection_Side whoami) const override;
 
      bool empty() const override { return false; } // always send this
   private:
      std::vector<uint8_t> m_reneg_data;
   };
 
/**
* ALPN (RFC 7301)
*/
class BOTAN_UNSTABLE_API Application_Layer_Protocol_Notification final : public Extension
   {
   public:
      static Extension_Code static_type()
         { return Extension_Code::ApplicationLayerProtocolNegotiation; }
 
      Extension_Code type() const override { return static_type(); }
 
      const std::vector<std::string>& protocols() const { return m_protocols; }
 
      std::string single_protocol() const;
 
      /**
      * Single protocol, used by server
      */
      explicit Application_Layer_Protocol_Notification(std::string_view protocol) :
         m_protocols(1, std::string(protocol)) {}
 
      /**
      * List of protocols, used by client
      */
      explicit Application_Layer_Protocol_Notification(const std::vector<std::string>& protocols) :
         m_protocols(protocols) {}
 
      Application_Layer_Protocol_Notification(TLS_Data_Reader& reader,
                                              uint16_t extension_size,
                                              Connection_Side from);
 
      std::vector<uint8_t> serialize(Connection_Side whoami) const override;
 
      bool empty() const override { return m_protocols.empty(); }
   private:
      std::vector<std::string> m_protocols;
   };
 
/**
* Session Ticket Extension (RFC 5077)
*/
class BOTAN_UNSTABLE_API Session_Ticket_Extension final : public Extension
   {
   public:
      static Extension_Code static_type()
         { return Extension_Code::SessionTicket; }
 
      Extension_Code type() const override { return static_type(); }
 
      /**
      * @return contents of the session ticket
      */
      const Session_Ticket& contents() const { return m_ticket; }
 
      /**
      * Create empty extension, used by both client and server
      */
      Session_Ticket_Extension() = default;
 
      /**
      * Extension with ticket, used by client
      */
      explicit Session_Ticket_Extension(Session_Ticket session_ticket) :
         m_ticket(std::move(session_ticket)) {}
 
      /**
      * Deserialize a session ticket
      */
      Session_Ticket_Extension(TLS_Data_Reader& reader, uint16_t extension_size);
 
      std::vector<uint8_t> serialize(Connection_Side) const override { return m_ticket.get(); }
 
      bool empty() const override { return false; }
   private:
      Session_Ticket m_ticket;
   };
 
 
/**
* Supported Groups Extension (RFC 7919)
*/
class BOTAN_UNSTABLE_API Supported_Groups final : public Extension
   {
   public:
      static Extension_Code static_type()
         { return Extension_Code::SupportedGroups; }
 
      Extension_Code type() const override { return static_type(); }
 
      const std::vector<Group_Params>& groups() const;
      std::vector<Group_Params> ec_groups() const;
      std::vector<Group_Params> dh_groups() const;
 
      std::vector<uint8_t> serialize(Connection_Side whoami) const override;
 
      explicit Supported_Groups(const std::vector<Group_Params>& groups);
 
      Supported_Groups(TLS_Data_Reader& reader,
                       uint16_t extension_size);
 
      bool empty() const override { return m_groups.empty(); }
   private:
      std::vector<Group_Params> m_groups;
   };
 
// previously Supported Elliptic Curves Extension (RFC 4492)
//using Supported_Elliptic_Curves = Supported_Groups;
 
/**
* Supported Point Formats Extension (RFC 4492)
*/
class BOTAN_UNSTABLE_API Supported_Point_Formats final : public Extension
   {
   public:
      enum ECPointFormat : uint8_t {
         UNCOMPRESSED = 0,
         ANSIX962_COMPRESSED_PRIME = 1,
         ANSIX962_COMPRESSED_CHAR2 = 2, // don't support these curves
      };
 
      static Extension_Code static_type()
         { return Extension_Code::EcPointFormats; }
 
      Extension_Code type() const override { return static_type(); }
 
      std::vector<uint8_t> serialize(Connection_Side whoami) const override;
 
      explicit Supported_Point_Formats(bool prefer_compressed) :
         m_prefers_compressed(prefer_compressed) {}
 
      Supported_Point_Formats(TLS_Data_Reader& reader,
                              uint16_t extension_size);
 
      bool empty() const override { return false; }
 
      bool prefers_compressed() { return m_prefers_compressed; }
 
   private:
      bool m_prefers_compressed = false;
   };
 
/**
* Signature Algorithms Extension for TLS 1.2 (RFC 5246)
*/
class BOTAN_UNSTABLE_API Signature_Algorithms final : public Extension
   {
   public:
      static Extension_Code static_type()
         { return Extension_Code::SignatureAlgorithms; }
 
      Extension_Code type() const override { return static_type(); }
 
      const std::vector<Signature_Scheme>& supported_schemes() const { return m_schemes; }
 
      std::vector<uint8_t> serialize(Connection_Side whoami) const override;
 
      bool empty() const override { return m_schemes.empty(); }
 
      explicit Signature_Algorithms(std::vector<Signature_Scheme> schemes) :
         m_schemes(std::move(schemes)) {}
 
      Signature_Algorithms(TLS_Data_Reader& reader,
                           uint16_t extension_size);
   private:
      std::vector<Signature_Scheme> m_schemes;
   };
 
/**
* Signature_Algorithms_Cert for TLS 1.3 (RFC 8446)
*
* RFC 8446 4.2.3
*    TLS 1.3 provides two extensions for indicating which signature algorithms
*    may be used in digital signatures.  The "signature_algorithms_cert"
*    extension applies to signatures in certificates, and the
*    "signature_algorithms" extension, which originally appeared in TLS 1.2,
*    applies to signatures in CertificateVerify messages.
*
* RFC 8446 4.2.3
*    TLS 1.2 implementations SHOULD also process this extension.
*/
class BOTAN_UNSTABLE_API Signature_Algorithms_Cert final : public Extension
   {
   public:
      static Extension_Code static_type()
         { return Extension_Code::CertSignatureAlgorithms; }
 
      Extension_Code type() const override { return static_type(); }
 
      const std::vector<Signature_Scheme>& supported_schemes() const { return m_schemes; }
 
      std::vector<uint8_t> serialize(Connection_Side whoami) const override;
 
      bool empty() const override { return m_schemes.empty(); }
 
      explicit Signature_Algorithms_Cert(std::vector<Signature_Scheme> schemes)
         : m_schemes(std::move(schemes)) {}
 
      Signature_Algorithms_Cert(TLS_Data_Reader& reader, uint16_t extension_size);
 
   private:
      std::vector<Signature_Scheme> m_schemes;
   };
 
/**
* Used to indicate SRTP algorithms for DTLS (RFC 5764)
*/
class BOTAN_UNSTABLE_API SRTP_Protection_Profiles final : public Extension
   {
   public:
      static Extension_Code static_type()
         { return Extension_Code::UseSrtp; }
 
      Extension_Code type() const override { return static_type(); }
 
      const std::vector<uint16_t>& profiles() const { return m_pp; }
 
      std::vector<uint8_t> serialize(Connection_Side whoami) const override;
 
      bool empty() const override { return m_pp.empty(); }
 
      explicit SRTP_Protection_Profiles(const std::vector<uint16_t>& pp) : m_pp(pp) {}
 
      explicit SRTP_Protection_Profiles(uint16_t pp) : m_pp(1, pp) {}
 
      SRTP_Protection_Profiles(TLS_Data_Reader& reader, uint16_t extension_size);
   private:
      std::vector<uint16_t> m_pp;
   };
 
/**
* Extended Master Secret Extension (RFC 7627)
*/
class BOTAN_UNSTABLE_API Extended_Master_Secret final : public Extension
   {
   public:
      static Extension_Code static_type()
         { return Extension_Code::ExtendedMasterSecret; }
 
      Extension_Code type() const override { return static_type(); }
 
      std::vector<uint8_t> serialize(Connection_Side whoami) const override;
 
      bool empty() const override { return false; }
 
      Extended_Master_Secret() = default;
 
      Extended_Master_Secret(TLS_Data_Reader& reader, uint16_t extension_size);
   };
 
/**
* Encrypt-then-MAC Extension (RFC 7366)
*/
class BOTAN_UNSTABLE_API Encrypt_then_MAC final : public Extension
   {
   public:
      static Extension_Code static_type()
         { return Extension_Code::EncryptThenMac; }
 
      Extension_Code type() const override { return static_type(); }
 
      std::vector<uint8_t> serialize(Connection_Side whoami) const override;
 
      bool empty() const override { return false; }
 
      Encrypt_then_MAC() = default;
 
      Encrypt_then_MAC(TLS_Data_Reader& reader, uint16_t extension_size);
   };
 
class Certificate_Status_Request_Internal;
 
/**
* Certificate Status Request (RFC 6066)
*/
class BOTAN_UNSTABLE_API Certificate_Status_Request final : public Extension
   {
   public:
      static Extension_Code static_type()
         { return Extension_Code::CertificateStatusRequest; }
 
      Extension_Code type() const override { return static_type(); }
 
      std::vector<uint8_t> serialize(Connection_Side whoami) const override;
 
      bool empty() const override { return false; }
 
      const std::vector<uint8_t>& get_responder_id_list() const;
      const std::vector<uint8_t>& get_request_extensions() const;
      const std::vector<uint8_t>& get_ocsp_response() const;
 
      // TLS 1.2 Server generated version: empty
      Certificate_Status_Request();
 
      // TLS 1.2 Client version, both lists can be empty
      Certificate_Status_Request(std::vector<uint8_t> ocsp_responder_ids,
                                 std::vector<std::vector<uint8_t>> ocsp_key_ids);
 
      // TLS 1.3 version
      Certificate_Status_Request(std::vector<uint8_t> response);
 
      Certificate_Status_Request(TLS_Data_Reader& reader,
                                 uint16_t extension_size,
                                 Handshake_Type message_type,
                                 Connection_Side from);
 
      ~Certificate_Status_Request() override;
 
   private:
      std::unique_ptr<Certificate_Status_Request_Internal> m_impl;
   };
 
/**
* Supported Versions from RFC 8446
*/
class BOTAN_UNSTABLE_API Supported_Versions final : public Extension
   {
   public:
      static Extension_Code static_type()
         { return Extension_Code::SupportedVersions; }
 
      Extension_Code type() const override { return static_type(); }
 
      std::vector<uint8_t> serialize(Connection_Side whoami) const override;
 
      bool empty() const override { return m_versions.empty(); }
 
      Supported_Versions(Protocol_Version version, const Policy& policy);
 
      Supported_Versions(Protocol_Version version)
         {
         m_versions.push_back(version);
         }
 
      Supported_Versions(TLS_Data_Reader& reader,
                         uint16_t extension_size,
                         Connection_Side from);
 
      bool supports(Protocol_Version version) const;
 
      const std::vector<Protocol_Version>& versions() const { return m_versions; }
   private:
      std::vector<Protocol_Version> m_versions;
   };
 
using Named_Group = Group_Params;
 
/**
* Record Size Limit (RFC 8449)
*
* TODO: the record size limit is currently not honored by the TLS 1.2 stack
*/
class BOTAN_UNSTABLE_API Record_Size_Limit final : public Extension
   {
   public:
      static Extension_Code static_type()
         { return Extension_Code::RecordSizeLimit; }
 
      Extension_Code type() const override { return static_type(); }
 
      explicit Record_Size_Limit(const uint16_t limit);
 
      Record_Size_Limit(TLS_Data_Reader& reader, uint16_t extension_size, Connection_Side from);
 
      uint16_t limit() const { return m_limit; }
 
      std::vector<uint8_t> serialize(Connection_Side whoami) const override;
 
      bool empty() const override { return m_limit == 0; }
 
   private:
      uint16_t m_limit;
   };
 
using Named_Group = Group_Params;
 
#if defined(BOTAN_HAS_TLS_13)
/**
* Cookie from RFC 8446 4.2.2
*/
class BOTAN_UNSTABLE_API Cookie final : public Extension
   {
   public:
      static Extension_Code static_type()
         { return Extension_Code::Cookie; }
 
      Extension_Code type() const override { return static_type(); }
 
      std::vector<uint8_t> serialize(Connection_Side whoami) const override;
 
      bool empty() const override { return m_cookie.empty(); }
 
      const std::vector<uint8_t>& get_cookie() const { return m_cookie; }
 
      explicit Cookie(const std::vector<uint8_t>& cookie);
 
      explicit Cookie(TLS_Data_Reader& reader,
                      uint16_t extension_size);
 
   private:
      std::vector<uint8_t> m_cookie;
   };
 
/**
* Pre-Shared Key Exchange Modes from RFC 8446 4.2.9
*/
class BOTAN_UNSTABLE_API PSK_Key_Exchange_Modes final : public Extension
   {
   public:
      static Extension_Code static_type()
         { return Extension_Code::PskKeyExchangeModes; }
 
      Extension_Code type() const override { return static_type(); }
 
      std::vector<uint8_t> serialize(Connection_Side whoami) const override;
 
      bool empty() const override { return m_modes.empty(); }
 
      const std::vector<PSK_Key_Exchange_Mode>& modes() const { return m_modes; }
 
      explicit PSK_Key_Exchange_Modes(std::vector<PSK_Key_Exchange_Mode> modes)
         : m_modes(std::move(modes)) {}
 
      explicit PSK_Key_Exchange_Modes(TLS_Data_Reader& reader, uint16_t extension_size);
 
   private:
      std::vector<PSK_Key_Exchange_Mode> m_modes;
   };
 
 
/**
 * Certificate Authorities Extension from RFC 8446 4.2.4
 */
class BOTAN_UNSTABLE_API Certificate_Authorities final : public Extension
   {
   public:
      static Extension_Code static_type()
         { return Extension_Code::CertificateAuthorities; }
 
      Extension_Code type() const override { return static_type(); }
 
      std::vector<uint8_t> serialize(Connection_Side whoami) const override;
 
      bool empty() const override { return m_distinguished_names.empty(); }
 
      const std::vector<X509_DN>& distinguished_names() const
         { return m_distinguished_names; }
 
      Certificate_Authorities(TLS_Data_Reader& reader, uint16_t extension_size);
      explicit Certificate_Authorities(std::vector<X509_DN> acceptable_DNs);
 
   private:
      std::vector<X509_DN> m_distinguished_names;
   };
 
/**
 * Pre-Shared Key extension from RFC 8446 4.2.11
 */
class BOTAN_UNSTABLE_API PSK final : public Extension
   {
   public:
      static Extension_Code static_type() { return Extension_Code::PresharedKey; }
      Extension_Code type() const override { return static_type(); }
 
      std::vector<uint8_t> serialize(Connection_Side side) const override;
 
      /**
       * Returns the cipher state representing the PSK selected by the server.
       * Note that this destructs the list of offered PSKs and its cipher states
       * and must therefore not be called more than once.
       */
      std::unique_ptr<Cipher_State> select_cipher_state(const PSK& server_psk,
                                                        const Ciphersuite& cipher);
 
      /**
       * Selects one of the offered PSKs that is compatible with \p cipher.
       * @retval PSK extension object that can be added to the Server Hello response
       * @retval std::nullptr if no PSK offered by the client is convenient
       */
      std::unique_ptr<PSK> select_offered_psk(const Ciphersuite& cipher,
                                              Session_Manager& session_mgr,
                                              Callbacks& callbacks,
                                              const Policy& policy);
 
      /**
       * Remove PSK identities from the list in \p m_psk that are not compatible
       * with the passed in \p cipher suite.
       * This is useful to react to Hello Retry Requests. See RFC 8446 4.1.4.
       */
      void filter(const Ciphersuite& cipher);
 
      /**
       * Pulls the Session to resume from a PSK extension in Server Hello.
       */
      Session take_session_to_resume();
 
      bool empty() const override;
 
      PSK(TLS_Data_Reader& reader, uint16_t extension_size, Handshake_Type message_type);
 
      /**
       * Creates a PSK extension with a TLS 1.3 session object containing a
       * master_secret. Note that it will extract that secret from the session,
       * and won't create a copy of it.
       *
       * @param session_to_resume  the session to be resumed; note that the
       *                           master secret will be taken away from the
       *                           session object.
       * @param callbacks          the application's callbacks
       */
      PSK(Session_with_Handle& session_to_resume, Callbacks& callbacks);
 
      ~PSK();
 
      void calculate_binders(const Transcript_Hash_State& truncated_transcript_hash);
      bool validate_binder(const PSK& server_psk, const std::vector<uint8_t>& binder) const;
 
      // TODO: Implement pure PSK negotiation that is not used for session
      //       resumption.
 
   private:
      /**
       * Creates a PSK extension that specifies the server's selection of an
       * offered client PSK. The @p session_to_resume is kept internally
       * and used later for the initialization of the Cipher_State object.
       *
       * Note: This constructor is called internally in PSK::select_offered_psk().
       */
      PSK(Session session_to_resume, const uint16_t psk_index);
 
   private:
      class PSK_Internal;
      std::unique_ptr<PSK_Internal> m_impl;
   };
 
/**
* Key_Share from RFC 8446 4.2.8
*/
class BOTAN_UNSTABLE_API Key_Share final : public Extension
   {
   public:
      static Extension_Code static_type()
         { return Extension_Code::KeyShare; }
 
      Extension_Code type() const override { return static_type(); }
 
      std::vector<uint8_t> serialize(Connection_Side whoami) const override;
 
      bool empty() const override;
 
      /**
       * Perform key exchange with the peer's key share.
       * This method can be called on a ClientHello's Key_Share with a ServerHello's Key_Share or vice versa.
       */
      secure_vector<uint8_t> exchange(const Key_Share& peer_keyshare, const Policy& policy, Callbacks& cb, RandomNumberGenerator& rng) const;
 
      /**
       * Update a ClientHello's Key_Share to comply with a HelloRetryRequest.
       *
       * This will create new Key_Share_Entries and should only be called on a ClientHello Key_Share with a HelloRetryRequest Key_Share.
       */
      void retry_offer(const Key_Share& retry_request_keyshare, const std::vector<Named_Group>& supported_groups, Callbacks& cb, RandomNumberGenerator& rng);
 
      /**
       * @return key exchange groups the peer offered key share entries for
       */
      std::vector<Named_Group> offered_groups() const;
 
      /**
       * @return key exchange group that was selected by a Hello Retry Request
       */
      Named_Group selected_group() const;
 
      /**
       * Delete all private keys that might be contained in Key_Share_Entries in this extension.
       */
      void erase();
 
      Key_Share(TLS_Data_Reader& reader,
                uint16_t extension_size,
                Handshake_Type message_type);
 
      // constructor used for ClientHello msg
      Key_Share(const Policy& policy, Callbacks& cb, RandomNumberGenerator& rng);
 
      // constructor used for ServerHello msg
      Key_Share(Named_Group group, Callbacks& cb, RandomNumberGenerator& rng);
 
      // constructor used for HelloRetryRequest msg
      explicit Key_Share(Named_Group selected_group);
 
      // destructor implemented in .cpp to hide Key_Share_Impl
      ~Key_Share();
 
   private:
      class Key_Share_Impl;
      std::unique_ptr<Key_Share_Impl> m_impl;
   };
 
/**
 * Indicates usage or support of early data as described in RFC 8446 4.2.10.
 */
class BOTAN_UNSTABLE_API EarlyDataIndication final : public Extension
   {
   public:
      static Extension_Code static_type()
         { return Extension_Code::EarlyData; }
 
      Extension_Code type() const override { return static_type(); }
      std::vector<uint8_t> serialize(Connection_Side whoami) const override;
 
      bool empty() const override;
 
      std::optional<uint32_t> max_early_data_size() const
         { return m_max_early_data_size; }
 
      EarlyDataIndication(TLS_Data_Reader& reader,
                          uint16_t extension_size,
                          Handshake_Type message_type);
 
      /**
       * The max_early_data_size is exclusively provided by servers when using
       * this extension in the NewSessionTicket message! Otherwise it stays
       * std::nullopt and results in an empty extension. (RFC 8446 4.2.10).
       */
      EarlyDataIndication(std::optional<uint32_t> max_early_data_size = std::nullopt)
         : m_max_early_data_size(std::move(max_early_data_size)) {}
 
   private:
      std::optional<uint32_t> m_max_early_data_size;
   };
 
#endif
 
/**
* Unknown extensions are deserialized as this type
*/
class BOTAN_UNSTABLE_API Unknown_Extension final : public Extension
   {
   public:
      Unknown_Extension(Extension_Code type,
                        TLS_Data_Reader& reader,
                        uint16_t extension_size);
 
      std::vector<uint8_t> serialize(Connection_Side whoami) const override;
 
      const std::vector<uint8_t>& value() { return m_value; }
 
      bool empty() const override { return false; }
 
      Extension_Code type() const override { return m_type; }
 
      bool is_implemented() const override { return false; }
 
   private:
      Extension_Code m_type;
      std::vector<uint8_t> m_value;
   };
 
/**
* Represents a block of extensions in a hello message
*/
class BOTAN_UNSTABLE_API Extensions final
   {
   public:
      std::set<Extension_Code> extension_types() const;
 
      const std::vector<std::unique_ptr<Extension>>& all() const
         {
         return m_extensions;
         }
 
      template<typename T>
      T* get() const
         {
         return dynamic_cast<T*>(get(T::static_type()));
         }
 
      template<typename T>
      bool has() const
         {
         return get<T>() != nullptr;
         }
 
      bool has(Extension_Code type) const
         {
         return get(type) != nullptr;
         }
 
      size_t size() const
         {
         return m_extensions.size();
         }
 
      void add(std::unique_ptr<Extension> extn);
 
      void add(Extension* extn)
         {
         add(std::unique_ptr<Extension>(extn));
         }
 
      Extension* get(Extension_Code type) const
         {
         const auto i = std::find_if(m_extensions.cbegin(), m_extensions.cend(),
                                     [type](const auto &ext) {
                                        return ext->type() == type;
                                     });
 
         return (i != m_extensions.end()) ? i->get() : nullptr;
         }
 
      std::vector<uint8_t> serialize(Connection_Side whoami) const;
 
      void deserialize(TLS_Data_Reader& reader,
                       const Connection_Side from,
                       const Handshake_Type message_type);
 
      /**
       * @param allowed_extensions        extension types that are allowed
       * @param allow_unknown_extensions  if true, ignores unrecognized extensions
       * @returns true if this contains any extensions that are not contained in @p allowed_extensions.
       */
      bool contains_other_than(const std::set<Extension_Code>& allowed_extensions,
                               const bool allow_unknown_extensions = false) const;
 
      /**
       * @param allowed_extensions  extension types that are allowed
       * @returns true if this contains any extensions implemented by Botan that
       *          are not contained in @p allowed_extensions.
       */
      bool contains_implemented_extensions_other_than(const std::set<Extension_Code>& allowed_extensions) const
         {
         return contains_other_than(allowed_extensions, true);
         }
 
      /**
       * Take the extension with the given type out of the extensions list.
       * Returns a nullptr if the extension didn't exist.
       */
      template<typename T>
      decltype(auto) take()
         {
         std::unique_ptr<T> out_ptr;
 
         auto ext = take(T::static_type());
         if (ext != nullptr) {
            out_ptr.reset(dynamic_cast<T*>(ext.get()));
            BOTAN_ASSERT_NOMSG(out_ptr != nullptr);
            ext.release();
         }
 
         return out_ptr;
         }
 
      /**
       * Take the extension with the given type out of the extensions list.
       * Returns a nullptr if the extension didn't exist.
       */
      std::unique_ptr<Extension> take(Extension_Code type);
 
      /**
      * Remove an extension from this extensions object, if it exists.
      * Returns true if the extension existed (and thus is now removed),
      * otherwise false (the extension wasn't set in the first place).
      *
      * Note: not used internally, might be used in Callbacks::tls_modify_extensions()
      */
      bool remove_extension(Extension_Code type)
         {
         return take(type) != nullptr;
         }
 
      Extensions() = default;
      Extensions(Extensions&&) = default;
      Extensions& operator=(Extensions&&) = default;
 
      Extensions(TLS_Data_Reader& reader, Connection_Side side, Handshake_Type message_type)
         {
         deserialize(reader, side, message_type);
         }
 
   private:
      Extensions(const Extensions&) = delete;
      Extensions& operator=(const Extensions&) = delete;
 
      std::vector<std::unique_ptr<Extension>> m_extensions;
   };
 
}
 
}
 
#endif