tls_callbacks.h

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/*
* TLS Callbacks
* (C) 2016 Matthias Gierlings
*     2016 Jack Lloyd
*     2017 Harry Reimann, Rohde & Schwarz Cybersecurity
*     2022 René Meusel, Rohde & Schwarz Cybersecurity
*     2025 Frederik Dornemann, CARIAD SE
*
* Botan is released under the Simplified BSD License (see license.txt)
*/
 
#ifndef BOTAN_TLS_CALLBACKS_H_
#define BOTAN_TLS_CALLBACKS_H_
 
#include <botan/ec_point_format.h>
#include <botan/kdf.h>
#include <botan/pubkey.h>
#include <botan/tls_alert.h>
#include <botan/tls_algos.h>
#include <botan/tls_magic.h>
#include <chrono>
#include <memory>
#include <optional>
#include <variant>
 
namespace Botan {
 
enum class Signature_Format : uint8_t;
enum class Usage_Type : uint8_t;
class DL_Group;
class PK_Key_Agreement_Key;
class Certificate_Store;
class X509_Certificate;
class Public_Key;
class Private_Key;
class RandomNumberGenerator;
 
namespace OCSP {
 
class Response;
 
}
 
namespace TLS {
 
class Handshake_Message;
class Policy;
class Extensions;
class Certificate_Status_Request;
class Session_Summary;
class Session;
 
/**
* Encapsulates the callbacks that a TLS channel will make which are due to
* channel specific operations.
*/
class BOTAN_PUBLIC_API(2, 0) Callbacks /* NOLINT(*-special-member-functions) */ {
   public:
      virtual ~Callbacks() = default;
 
      /**
       * @name Mandatory
       *
       * Those callbacks must be implemented by all applications that use TLS.
       * @{
       */
 
      /**
      * Mandatory callback: output function
      *
      * The channel will call this with data which needs to be sent to the peer
      * (eg, over a socket or some other form of IPC). The array will be overwritten
      * when the function returns so a copy must be made if the data cannot be
      * sent immediately.
      *
      * As an example you could use the syscall ``send`` to perform a blocking
      * write on a socket, or append the data to a queue managed by your
      * application and initiate an asynchronous write.
      *
      * For TLS all writes must occur *in the order requested*. For DTLS this
      * ordering is not strictly required, but is still recommended.
      *
      * @param data a contiguous data buffer to send
      */
      virtual void tls_emit_data(std::span<const uint8_t> data) = 0;
 
      /**
      * Mandatory callback: process application data
      *
      * Called when application data record is received from the peer. The
      * array is overwritten immediately after the function returns.
      *
      * Currently empty records are ignored and do not instigate a callback,
      * but this may change in a future release.
      *
      * For TLS the record number will always increase. For DTLS, it is
      * possible to receive records with the @p seq_no field out of order, or
      * with gaps, corresponding to reordered or lost datagrams.
      *
      * @param seq_no the underlying TLS/DTLS record sequence number
      *
      * @param data a contiguous data buffer containing the received record
      */
      virtual void tls_record_received(uint64_t seq_no, std::span<const uint8_t> data) = 0;
 
      /**
      * Mandatory callback: alert received
      *
      * Called when an alert is received from the peer. If fatal, the
      * connection is closing. If not fatal, the connection may still be
      * closing (depending on the error and the peer).
      *
      * Note that alerts received before the handshake is complete are not
      * authenticated and could have been inserted by a MITM attacker.
      */
      virtual void tls_alert(Alert alert) = 0;
 
      /// @}
      // End of mandatory callbacks
 
      /**
      * @name Informational
      *
      * Override these to obtain deeper insights into the TLS connection.
      * Throwing from any of these callbacks will result in the termination of
      * the TLS connection.
      * @{
      */
 
      /**
      * Optional callback: session established
      *
      * Called whenever a negotiation completes. This can happen more than once
      * on TLS 1.2 connections, if renegotiation occurs. The @p session
      * parameter provides information about the session which was just
      * established.
      *
      * If this function wishes to cancel the handshake, it can throw an
      * exception which will send a close message to the counterparty and reset
      * the connection state.
      *
      * @param session the session descriptor
      */
      virtual void tls_session_established(const Session_Summary& session);
 
      /**
       * Optional callback: session activated
       *
       * By default does nothing. This is called when the session is activated,
       * that is once it is possible to send or receive data on the channel.  In
       * particular it is possible for an implementation of this function to
       * perform an initial write on the channel.
       */
      virtual void tls_session_activated() {}
 
      /**
       * Optional callback: peer closed connection (sent a "close_notify" alert)
       *
       * The peer signaled that it wishes to shut down the connection. The
       * application should not expect to receive any more data from the peer
       * and may tear down the underlying transport socket.
       *
       * Prior to TLS 1.3 it was required that peers discard pending writes
       * and immediately respond with their own "close_notify". With TLS 1.3,
       * applications can continue to send data despite the peer having already
       * signaled their wish to shut down.
       *
       * Returning `true` will cause the TLS 1.3 implementation to write all
       * pending data and then also signal a connection shut down. Otherwise
       * the application is responsible to call the `Channel::close()` method.
       *
       * For TLS 1.2 the return value has no effect.
       *
       * @return true causes the implementation to respond with a "close_notify"
       */
      virtual bool tls_peer_closed_connection() { return true; }
 
      /**
       * Optional callback: Resumption information was received/established
       *
       * TLS 1.3 calls this when we sent or received a TLS 1.3 session ticket at
       * any point after the initial handshake has finished.
       *
       * TLS 1.2 calls this when a session was established successfully and
       * its resumption information may be stored for later usage.
       *
       * Note that for servers this is called as soon as resumption information
       * is available and _could_ be sent to the client. If this callback
       * returns 'false', the information will neither be cached nor sent.
       *
       * @param session the session descriptor
       *
       * @return false to prevent the resumption information from being cached,
       *         and true to cache it in the configured Session_Manager
       */
      virtual bool tls_should_persist_resumption_information(const Session& session);
 
      /**
       * Optional callback with default impl: verify cert chain
       *
       * Default implementation performs a standard PKIX validation
       * and initiates network OCSP request for end-entity cert.
       * Override to provide different behavior.
       *
       * Check the certificate chain is valid up to a trusted root, and
       * optionally (if hostname != "") that the hostname given is
       * consistent with the leaf certificate.
       *
       * This function should throw an exception derived from
       * std::exception with an informative what() result if the
       * certificate chain cannot be verified.
       *
       * @param cert_chain specifies a certificate chain leading to a
       *        trusted root CA certificate.
       * @param ocsp_responses the server may have provided some
       * @param trusted_roots the list of trusted certificates
       * @param usage what this cert chain is being used for
       *        Usage_Type::TLS_SERVER_AUTH for server chains,
       *        Usage_Type::TLS_CLIENT_AUTH for client chains,
       *        Usage_Type::UNSPECIFIED for other uses
       * @param hostname when authenticating a server, this is the hostname
       *        the client requested (eg via SNI). When authenticating a client,
       *        this is the server name the client is authenticating *to*.
       *        Empty in other cases or if no hostname was used.
       * @param policy the TLS policy associated with the session being authenticated
       *        using the certificate chain
       */
      virtual void tls_verify_cert_chain(const std::vector<X509_Certificate>& cert_chain,
                                         const std::vector<std::optional<OCSP::Response>>& ocsp_responses,
                                         const std::vector<Certificate_Store*>& trusted_roots,
                                         Usage_Type usage,
                                         std::string_view hostname,
                                         const TLS::Policy& policy);
 
      /**
       * Optional callback. Default impl always rejects.
       *
       * This allows using raw public keys for authentication of peers as
       * described in RFC 7250 and RFC 8446 4.2.2. Applications that wish
       * to use raw public keys MUST override this callback to verify the
       * authenticity of the received public keys.
       *
       * Default implementation always rejects the raw public key.
       *
       * This function should throw an exception derived from
       * std::exception with an informative what() result if the
       * raw public key cannot be verified.
       *
       * @param raw_public_key specifies the raw public key to be used
       *        for peer authentication
       * @param usage what this cert chain is being used for
       *        Usage_Type::TLS_SERVER_AUTH for server chains,
       *        Usage_Type::TLS_CLIENT_AUTH for client chains,
       * @param hostname when authenticating a server, this is the hostname
       *        the client requested (eg via SNI). When authenticating a client,
       *        this is the server name the client is authenticating *to*.
       *        Empty in other cases or if no hostname was used.
       * @param policy the TLS policy associated with the session being authenticated
       *        using the raw public key
       */
      virtual void tls_verify_raw_public_key(const Public_Key& raw_public_key,
                                             Usage_Type usage,
                                             std::string_view hostname,
                                             const TLS::Policy& policy);
 
      /**
       * Called by default `tls_verify_cert_chain` to get the timeout to use for OCSP
       * requests. Return 0 to disable online OCSP checks.
       *
       * This function should not be "const" since the implementation might need
       * to perform some side effecting operation to compute the result.
       *
       * TODO(Botan4) change return type to uint64_t
       */
      virtual std::chrono::milliseconds tls_verify_cert_chain_ocsp_timeout() const {
         return std::chrono::milliseconds(0);
      }
 
      /**
       * Called by the TLS server whenever the client included the
       * status_request extension (see RFC 6066, a.k.a OCSP stapling)
       * in the ClientHello.
       *
       * @return the encoded OCSP response to be sent to the client which
       * indicates the revocation status of the server certificate. Return an
       * empty vector to indicate that no response is available, and thus
       * suppress the Certificate_Status message.
       *
       * Default implementation returns an empty vector, disabling certificate status
       */
      virtual std::vector<uint8_t> tls_provide_cert_status(const std::vector<X509_Certificate>& chain,
                                                           const Certificate_Status_Request& csr);
 
      /**
       * Called by TLS 1.3 client or server whenever the peer indicated that
       * OCSP stapling is supported. In contrast to `tls_provide_cert_status`,
       * this allows providing OCSP responses for each certificate in the chain.
       *
       * The default implementation invokes `tls_provide_cert_status` assuming
       * that no OCSP responses for intermediate certificates are available.
       *
       * @return a vector of OCSP response buffers. An empty buffer indicates
       *         that no OCSP response should be provided for the respective
       *         certificate (at the same list index). The returned vector
       *         MUST be exactly the same length as the incoming \p chain.
       */
      virtual std::vector<std::vector<uint8_t>> tls_provide_cert_chain_status(
         const std::vector<X509_Certificate>& chain, const Certificate_Status_Request& csr);
 
      /**
       * Optional callback with default impl: sign a message
       *
       * Default implementation uses PK_Signer::sign_message().
       * Override to provide a different approach, e.g. using an external device.
       *
       * @param key the private key of the signer
       * @param rng a random number generator
       * @param padding the encoding method to be applied to the message
       * @param format the signature format
       * @param msg the input data for the signature
       *
       * @return the signature
       */
      virtual std::vector<uint8_t> tls_sign_message(const Private_Key& key,
                                                    RandomNumberGenerator& rng,
                                                    std::string_view padding,
                                                    Signature_Format format,
                                                    const std::vector<uint8_t>& msg);
 
      /**
       * Optional callback with default impl: verify a message signature
       *
       * Default implementation uses PK_Verifier::verify_message().
       * Override to provide a different approach, e.g. using an external device.
       *
       * @param key the public key of the signer
       * @param padding the encoding method to be applied to the message
       * @param format the signature format
       * @param msg the input data for the signature
       * @param sig the signature to be checked
       *
       * @return true if the signature is valid, false otherwise
       */
      virtual bool tls_verify_message(const Public_Key& key,
                                      std::string_view padding,
                                      Signature_Format format,
                                      const std::vector<uint8_t>& msg,
                                      const std::vector<uint8_t>& sig);
 
      /**
       * Optional callback: deserialize a public key received from the peer
       *
       * Default implementation simply parses the public key using Botan's
       * public keys. Override to provide a different approach, e.g. using an
       * external device.
       *
       * If deserialization fails, the default implementation throws a
       * Botan::Decoding_Error exception that will be translated into a
       * TLS_Exception with an Alert::IllegalParameter.
       *
       * @param group the group identifier or (in case of TLS 1.2) an explicit
       *              discrete-log group of the public key
       * @param key_bits the serialized public key
       *
       * @return the deserialized and ready-to-use public key
       */
      virtual std::unique_ptr<Public_Key> tls_deserialize_peer_public_key(
         const std::variant<TLS::Group_Params, DL_Group>& group, std::span<const uint8_t> key_bits);
 
      /**
       * Generate an ephemeral KEM key for a TLS 1.3 handshake
       *
       * Applications may use this to add custom KEM algorithms or entirely
       * different key exchange schemes to the TLS 1.3 handshake. For instance,
       * this could provide an entry point to implement a hybrid key exchange
       * with both a traditional algorithm like ECDH and a quantum-secure KEM.
       * Typical use cases of the library don't need to do that and serious
       * security risks are associated with customizing TLS's key encapsulation
       * mechanism.
       *
       * Note that the KEM interface is usable for TLS 1.3 handshakes, only.
       *
       * The default implementation simply delegates this to the
       * tls_generate_ephemeral_key() call when appropriate.
       *
       * @param group the group identifier to generate an ephemeral keypair for
       * @param rng   a random number generator
       *
       * @returns a keypair whose public key will be provided to the peer and
       *          the private key will be provided to tls_kem_decapsulate later
       *          in the handshake.
       */
      virtual std::unique_ptr<Private_Key> tls_kem_generate_key(TLS::Group_Params group, RandomNumberGenerator& rng);
 
      /**
       * Performs a key encapsulation operation (used for TLS 1.3 servers)
       *
       * Applications may use this to add custom KEM algorithms or entirely
       * different key exchange schemes to the TLS 1.3 handshake. For instance,
       * this could provide an entry point to implement a hybrid key exchange
       * with both a traditional algorithm like ECDH and a quantum-secure KEM.
       * Typical use cases of the library don't need to do that and serious
       * security risks are associated with customizing TLS's key encapsulation
       * mechanism.
       *
       * Note that the KEM interface is usable for TLS 1.3 handshakes, only.
       *
       * The default implementation implements this key encapsulation as a
       * combination of tls_generate_ephemeral_key() followed by
       * tls_ephemeral_key_agreement() with the provided @p encoded_public_key.
       * The just-generated ephemeral private key is destroyed immediately.
       *
       * @param group the group identifier of the KEM/KEX algorithm
       * @param encoded_public_key the public key used for encapsulation/KEX
       * @param rng a random number generator
       * @param policy a TLS policy object
       *
       * @returns the shared secret both in plaintext and encapsulated with
       *          @p encoded_public_key.
       *
       * TODO(Botan4) change this return type to something else so the pubkey.h
       * dependency is removed
       * TODO(Botan4) change encoded_public_key to a span
       */
      virtual KEM_Encapsulation tls_kem_encapsulate(TLS::Group_Params group,
                                                    const std::vector<uint8_t>& encoded_public_key,
                                                    RandomNumberGenerator& rng,
                                                    const Policy& policy);
 
      /**
       * Performs a key decapsulation operation (used for TLS 1.3 clients).
       *
       * Applications may use this to add custom KEM algorithms or entirely
       * different key exchange schemes to the TLS 1.3 handshake. For instance,
       * this could provide an entry point to implement a hybrid key exchange
       * with both a traditional algorithm like ECDH and a quantum-secure KEM.
       * Typical use cases of the library don't need to do that and serious
       * security risks are associated with customizing TLS's key encapsulation
       * mechanism.
       *
       * Note that the KEM interface is usable for TLS 1.3 handshakes, only.
       *
       * The default implementation simply delegates this to the
       * tls_ephemeral_key_agreement() callback to obtain the shared secret.
       *
       * @param group the group identifier of the KEM/KEX algorithm
       * @param private_key the private key used for decapsulation/KEX
       * @param encapsulated_bytes the content to decapsulate (or the public key share)
       * @param rng a random number generator
       * @param policy a TLS policy object
       *
       * @returns the plaintext shared secret from @p encapsulated_bytes after
       *          decapsulation with @p private_key.
       *
       * TODO(Botan4) change encapsulated_bytes to a std::span
       */
      virtual secure_vector<uint8_t> tls_kem_decapsulate(TLS::Group_Params group,
                                                         const Private_Key& private_key,
                                                         const std::vector<uint8_t>& encapsulated_bytes,
                                                         RandomNumberGenerator& rng,
                                                         const Policy& policy);
 
      /**
       * Generate an ephemeral key pair for the TLS handshake.
       *
       * Applications may use this to add custom groups, curves or entirely
       * different ephemeral key agreement mechanisms to the TLS handshake.
       * Note that this callback must be used in conjunction with
       * Callbacks::tls_ephemeral_key_agreement.
       *
       * Typical use cases of the library don't need to do that and serious
       * security risks are associated with customizing TLS's key exchange
       * mechanism.
       *
       * @throws TLS_Exception(Alert::DecodeError) if the @p group is not known.
       *
       * @param group the group identifier to generate an ephemeral keypair for
       *              TLS 1.2 allows for specifying custom discrete logarithm
       *              parameters as part of the protocol. Hence the variant<>.
       * @param rng a random number generator
       *
       * @return a private key of an algorithm usable for key agreement
       */
      virtual std::unique_ptr<PK_Key_Agreement_Key> tls_generate_ephemeral_key(
         const std::variant<TLS::Group_Params, DL_Group>& group, RandomNumberGenerator& rng);
 
      /**
       * Generate an ECDH key pair for the TLS 1.2 handshake.
       *
       * Note that this callback is called exclusively by TLS 1.2 to handle the
       * ECDH public key serialization format explicitly. TLS 1.3 fixes this
       * format to 'uncompressed' and does not allow negotiating anything else.
       * X25519 and X448 feature a defined and fixed public key encoding and are
       * therefore not explicitly handled by this callback either.
       *
       * Users may override this if they want to provide a custom keypair type
       * to offload TLS 1.2's ECDH handling to custom hardware, for instance. It
       * is worth noting that support for compressed points in Botan is
       * deprecated and this callback will disappear when it is removed in a
       * future release.
       *
       * Typical use cases of the library don't need to do that and serious
       * security risks are associated with customizing TLS's key exchange
       * mechanism.
       *
       * @throws TLS_Exception(Alert::DecodeError) if the @p group is not known.
       *
       * @param group ECDH group identifier to generate an ephemeral keypair for
       * @param rng a random number generator
       * @param tls12_ecc_pubkey_encoding_format the key's serialization format
       *
       * @return an ECDH private key of an algorithm usable for key agreement
       */
      virtual std::unique_ptr<PK_Key_Agreement_Key> tls12_generate_ephemeral_ecdh_key(
         TLS::Group_Params group, RandomNumberGenerator& rng, EC_Point_Format tls12_ecc_pubkey_encoding_format);
 
      /**
       * Agree on a shared secret with the peer's ephemeral public key for
       * the TLS handshake.
       *
       * Applications may use this to add custom groups, curves or entirely
       * different ephemeral key agreement mechanisms to the TLS handshake.
       * Note that this callback must be used in conjunction with
       * Callbacks::tls_generate_ephemeral_key.
       *
       * Typical use cases of the library don't need to do that and serious
       * security risks are associated with customizing TLS's key exchange
       * mechanism.
       *
       * @param group         the TLS group identifier to be used
       *                      TLS 1.2 allows for specifying custom discrete
       *                      logarithm parameters as part of the protocol.
       *                      Hence the variant<>.
       * @param private_key   the private key (generated ahead in tls_generate_ephemeral_key)
       * @param public_value  the public key exchange information received by the peer
       * @param rng           a random number generator
       * @param policy        a TLS policy object
       *
       * @return the shared secret derived from public_value and private_key
       *
       * TODO(Botan4) change public_value to a std::span
       */
      virtual secure_vector<uint8_t> tls_ephemeral_key_agreement(const std::variant<TLS::Group_Params, DL_Group>& group,
                                                                 const PK_Key_Agreement_Key& private_key,
                                                                 const std::vector<uint8_t>& public_value,
                                                                 RandomNumberGenerator& rng,
                                                                 const Policy& policy);
 
      /**
       * Optional callback: inspect handshake message
       * Throw an exception to abort the handshake.
       * Default simply ignores the message.
       *
       * Note: On connections that negotiated TLS 1.3 this callback is also
       *       invoked for post-handshake messages.
       *
       * @param message the handshake message
       */
      virtual void tls_inspect_handshake_msg(const Handshake_Message& message);
 
      /**
       * Optional callback for server: choose ALPN protocol
       *
       * ALPN (RFC 7301) works by the client sending a list of application
       * protocols it is willing to negotiate. The server then selects which
       * protocol to use. RFC 7301 requires that if the server does not support
       * any protocols offered by the client, then it should close the connection
       * with an alert of no_application_protocol. Within this callback this would
       * be done by throwing a TLS_Exception(Alert::NoApplicationProtocol)
       *
       * @param client_protos the vector of protocols the client is willing to negotiate
       *
       * @return the protocol selected by the server; if the empty string is
       * returned, the server does not reply to the client ALPN extension.
       *
       * The default implementation returns the empty string, causing client
       * ALPN to be ignored.
       *
       * It is highly recommended to support ALPN whenever possible to avoid
       * cross-protocol attacks.
       */
      virtual std::string tls_server_choose_app_protocol(const std::vector<std::string>& client_protos);
 
      /**
       * Optional callback: examine/modify Extensions before sending.
       *
       * Both client and server will call this callback on the Extensions object
       * before serializing it in the specific handshake message. This allows an
       * application to modify which extensions are sent during the handshake.
       *
       * Default implementation does nothing.
       *
       * @param extn the extensions
       * @param which_side will be Connection_Side::Client or Connection_Side::Server which is the current
       *                   applications role in the exchange.
       * @param which_message will state the handshake message type containing the extensions
       */
      virtual void tls_modify_extensions(Extensions& extn, Connection_Side which_side, Handshake_Type which_message);
 
      /**
       * Optional callback: examine peer extensions.
       *
       * Both client and server will call this callback with the Extensions
       * object after receiving it from the peer. This allows examining the
       * Extensions, for example to implement a custom extension. It also allows
       * an application to require that a particular extension be implemented;
       * throw an exception from this function to abort the handshake.
       *
       * Default implementation does nothing.
       *
       * @param extn the extensions
       * @param which_side will be Connection_Side::Client if these are are the clients extensions (ie we are
       *        the server) or Connection_Side::Server if these are the server extensions (we are the client).
       * @param which_message will state the handshake message type containing the extensions
       */
      virtual void tls_examine_extensions(const Extensions& extn,
                                          Connection_Side which_side,
                                          Handshake_Type which_message);
 
      /**
       * Optional callback: parse a single OCSP Response
       *
       * Note: Typically a user of the library would not want to override this
       *       callback. We provide this callback to be able to support OCSP
       *       related tests from BoringSSL's BoGo tests that provide unparsable
       *       responses.
       *
       * Default implementation tries to parse the provided raw OCSP response.
       *
       * This function should not throw an exception but return a std::nullopt
       * if the OCSP response cannot be parsed.
       *
       * @param raw_response raw OCSP response buffer
       * @returns the parsed OCSP response or std::nullopt on error
       *
       * TODO(Botan4) change raw_response to a std::span
       */
      virtual std::optional<OCSP::Response> tls_parse_ocsp_response(const std::vector<uint8_t>& raw_response);
 
      /**
       * Optional callback: return peer network identity
       *
       * There is no expected or specified format. The only expectation is this
       * function will return a unique value. For example returning the peer
       * host IP and port.
       *
       * This is used to bind the DTLS cookie to a particular network identity.
       * It is only called if the dtls-cookie-secret PSK is also defined.
       */
      virtual std::string tls_peer_network_identity();
 
      /**
       * Optional callback: return a custom time stamp value
       *
       * This allows the library user to specify a custom "now" timestamp when
       * needed. By default it will use the current system clock time.
       *
       * Note that typical usages will not need to override this callback but it
       * is useful for testing purposes to allow for deterministic test outcomes.
       *
       * TODO(Botan4) change return type to uint64_t
       */
      virtual std::chrono::system_clock::time_point tls_current_timestamp();
 
      /**
       * Optional callback: error logging. (not currently called)
       * @param err An error message related to this connection.
       *
       * TODO(Botan4) remove this
       */
      virtual void tls_log_error(const char* err);
 
      /**
       * Optional callback: debug logging. (not currently called)
       * @param what Some hopefully informative string
       *
       * TODO(Botan4) remove this
       */
      virtual void tls_log_debug(const char* what);
 
      /**
       * Optional callback: debug logging taking a buffer. (not currently called)
       * @param descr What this buffer is
       * @param val the bytes
       * @param val_len length of val
       *
       * TODO(Botan4) remove this
       */
      virtual void tls_log_debug_bin(const char* descr, const uint8_t val[], size_t val_len);
 
      /**
       * Optional callback: Allows access to a connection's secret data
       *
       * Useful to implement the SSLKEYLOGFILE for connection debugging as
       * specified in ietf.org/archive/id/draft-thomson-tls-keylogfile-00.html
       *
       * Invoked if Policy::allow_ssl_key_log_file returns true.
       *
       * Default implementation simply ignores the inputs.
       *
       * @param label  Identifies the reported secret type
       *               See draft-thomson-tls-keylogfile-00 Section 3.1 and 3.2
       * @param client_random  random value from ClientHello message acting as
       *                       an identifier of the TLS sessions
       * @param secret         the actual secret value
       */
      virtual void tls_ssl_key_log_data(std::string_view label,
                                        std::span<const uint8_t> client_random,
                                        std::span<const uint8_t> secret) const;
 
      /**
       * Returns the key derivation function to be used for TLS 1.2
       *
       * The default implementation can be overridden to provide a user-defined
       * key derivation function, for example to delegate key derivation to a
       * hardware-protected environment when a pre-shared key must remain
       * inaccessible to the non-secure world.
       *
       * @param prf_algo  name of the hash function (e.g. "SHA-256")
       *
       * @return  TLS 1.2 KDF implementation
       */
      virtual std::unique_ptr<KDF> tls12_protocol_specific_kdf(std::string_view prf_algo) const;
};
 
}  // namespace TLS
 
}  // namespace Botan
 
#endif