# Transport Layer Security (TLS)

New in version 1.11.0.

Botan has client and server implementations of various versions of the TLS protocol, including TLS v1.0, TLS v1.1, and TLS v1.2. As of version 1.11.13, support for the insecure SSLv3 protocol has been removed.

There is also support for DTLS (v1.0 and v1.2), a variant of TLS adapted for operation on datagram transports such as UDP and SCTP. DTLS support should be considered as beta quality and further testing is invited.

The TLS implementation does not know anything about sockets or the network layer. Instead, it calls a user provided callback (hereafter `output_fn`) whenever it has data that it would want to send to the other party (for instance, by writing it to a network socket), and whenever the application receives some data from the counterparty (for instance, by reading from a network socket) it passes that information to TLS using `TLS::Channel::received_data`. If the data passed in results in some change in the state, such as a handshake completing, or some data or an alert being received from the other side, then the appropriate user provided callback will be invoked.

If the reader is familiar with OpenSSL’s BIO layer, it might be analogous to saying the only way of interacting with Botan’s TLS is via a BIO_mem I/O abstraction. This makes the library completely agnostic to how you write your network layer, be it blocking sockets, libevent, asio, a message queue, lwIP on RTOS, some carrier pidgeons, etc.

Starting in 1.11.31, the application callbacks are encapsulated as the class `TLS::Callbacks` with the following members. The first four (`tls_emit_data`, `tls_record_received`, `tls_alert`, and `tls_session_established`) are mandatory for using TLS, all others are optional and provide additional information about the connection.

> void `tls_emit_data`(const uint8_t data[], size_t data_len)

>   Mandatory. The TLS stack requests that all bytes of data be queued up to send to the counterparty. After this function returns, the buffer containing data will be overwritten, so a copy of the input must be made if the callback cannot send the data immediately.

>   As an example you could `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.

> void `tls_record_received`(uint64_t rec_no, const uint8_t data[], size_t data_len)

>   Mandatory. Called once for each application_data record which is received, with the matching (TLS level) record sequence number.

>   Currently empty records are ignored and do not instigate a callback, but this may change in a future release.

>   > As with `tls_emit_data`, the array will be overwritten sometime after the callback returns, so a copy should be made if needed.

>   > For TLS the record number will always increase.

>   > For DTLS, it is possible to receive records with the rec_no field out of order, or with gaps, corresponding to reordered or lost datagrams.

> void `tls_alert`(Alert alert)

>   Mandatory. Called when an alert is received from the peer. Note that alerts received before the handshake is complete are not authenticated and could have been inserted by a MITM attacker.

> bool `tls_session_established`(const TLS::Session &session)

>   Mandatory. Called whenever a negotiation completes. This can happen more than once on any connection, if renegotiation occurs. The session parameter provides information about the session which was just established.

>   If this function returns false, the session will not be cached for later resumption.

>   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.

> void `tls_verify_cert_chain`(const std::vector<X509_Certificate> &cert_chain, const std::vector<std::shared_ptr<const OCSP::Response>> &ocsp_responses, const std::vector<Certificate_Store *> &trusted_roots, Usage_Type usage, const std::string &hostname, const Policy &policy)

>   Optional - default implementation should work for many users. It can be overridden for implementing extra validation routines such as public key pinning.

>   Verifies the certificate chain in cert_chain, assuming the leaf certificate is the first element. Throws an exception if any error makes this certificate chain unacceptable.

>   If usage is Usage_Type::TLS_SERVER_AUTH, then hostname should match the information in the server certificate. If usage is TLS_CLIENT_AUTH, then hostname specifies the host the client is authenticating against (from SNI); the callback can use this for any special site specific auth logic.

>   The ocsp_responses is a possibly empty list of OCSP responses provided by the server. In the current implementation of TLS OCSP stapling, only a single OCSP response can be returned. A existing TLS extension allows the server to send multiple OCSP responses, this extension may be supported in the future in which case more than one OCSP response may be given during this callback.

>   The trusted_roots parameter was returned by a call from the associated Credentials_Manager.

>   The policy provided is the policy for the TLS session which is being authenticated using this certificate chain. It can be consulted for values such as allowable signature methods and key sizes.

> std::chrono::milliseconds `tls_verify_cert_chain_ocsp_timeout`() const

>   Called by default tls_verify_cert_chain to set timeout for online OCSP requests on the certificate chain. Return 0 to disable OCSP. Current default is 0.

> std::string `tls_server_choose_app_protocol`(const std::vector<std::string> &client_protos)

>   Optional. Called by the server when a client includes a list of protocols in the ALPN extension. The server then choose which protocol to use, or “” to disable sending any ALPN response. The default implementation returns the empty string all of the time, effectively disabling ALPN responses.

> void `tls_session_activated`()

>   Optional. 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.

> std::vector<uint8_t> `tls_provide_cert_status`(const std::vector<X509_Certificate> &chain, const Certificate_Status_Request &csr)

>   Optional. This can return a cached OCSP response. This is only used on the server side, and only if the client requests OCSP stapling.

> std::string `tls_peer_network_identity`()

>   Optional. Return a string that identifies the peer in some unique way (for example, by formatting the remote IP and port into a string). This is currently used to bind DTLS cookies to the network identity.

> void `tls_inspect_handshake_msg`(const Handshake_Message&)

>   This callback is optional, and can be used to inspect all handshake messages while the session establishment occurs.

> void `tls_modify_extensions`(Extensions &extn, Connection_Side which_side)

>   This callback is optional, and can be used to modify extensions before they are sent to the peer. For example this enables adding a custom extension, or replacing or removing an extension set by the library.

> void `tls_examine_extensions`(const Extensions &extn, Connection_Side which_side)

>   This callback is optional, and can be used to examine extensions sent by the peer.

> void `tls_log_error`(const char *msg)

>   Optional logging for an error message. (Not currently used)

> void `tls_log_debug`(const char *msg)

>   Optional logging for an debug message. (Not currently used)

> void `tls_log_debug_bin`(const char *descr, const uint8_t val[], size_t len)

>   Optional logging for an debug value. (Not currently used)

> std::string `tls_decode_group_param`(TLS::Group_Params group_param)

>   Optional. Called by the server when a client hello includes a list of supported groups in the supported_groups extension and by the client when decoding the server key exchange including the selected curve identifier. The function should return the name of the DH group or elliptic curve the passed TLS group identifier should be mapped to. Therefore this callback enables the use of custom elliptic curves or DH groups in TLS, if both client and server map the custom identifiers correctly. Please note that it is required to allow the group TLS identifier in in the used `TLS::Policy`.

Versions from 1.11.0 to 1.11.30 did not have `TLS::Callbacks` and instead used independent std::functions to pass the various callback functions. This interface is currently still included but is deprecated and will be removed in a future release. For the documentation for this interface, please check the docs for 1.11.30. This version of the manual only documents the new interface added in 1.11.31.

## TLS Channels

TLS servers and clients share an interface called TLS::Channel. A TLS channel (either client or server object) has these methods available:

class `TLS`::```Channel`
  size_t `received_data`(const uint8_t buf[], size_t buf_size)
  size_t `received_data`(const std::vector<uint8_t> &buf)

    This function is used to provide data sent by the counterparty (eg data that you read off the socket layer). Depending on the current protocol state and the amount of data provided this may result in one or more callback functions that were provided to the constructor being called.

    The return value of `received_data` specifies how many more bytes of input are needed to make any progress, unless the end of the data fell exactly on a message boundary, in which case it will return 0 instead.

  void `send`(const uint8_t buf[], size_t buf_size)
  void `send`(const std::string &str)
  void `send`(const std::vector<uint8_t> &vec)

    Create one or more new TLS application records containing the provided data and send them. This will eventually result in at least one call to the `output_fn` callback before `send` returns.

    If the current TLS connection state is unable to transmit new application records (for example because a handshake has not yet completed or the connection has already ended due to an error) an exception will be thrown.

  void `close`()

    A close notification is sent to the counterparty, and the internal state is cleared.

  void `send_alert`(const Alert &alert)

    Some other alert is sent to the counterparty. If the alert is fatal, the internal state is cleared.

  bool `is_active`()

    Returns true if and only if a handshake has been completed on this connection and the connection has not been subsequently closed.

  bool `is_closed`()

    Returns true if and only if either a close notification or a fatal alert message have been either sent or received.

  bool `timeout_check`()

    This function does nothing unless the channel represents a DTLS connection and a handshake is actively in progress. In this case it will check the current timeout state and potentially initiate retransmission of handshake packets. Returns true if a timeout condition occurred.

  void `renegotiate`(bool force_full_renegotiation = false)

    Initiates a renegotiation. The counterparty is allowed by the protocol to ignore this request. If a successful renegotiation occurs, the handshake_cb callback will be called again.

    If force_full_renegotiation is false, then the client will attempt to simply renew the current session - this will refresh the symmetric keys but will not change the session master secret. Otherwise it will initiate a completely new session.

    For a server, if force_full_renegotiation is false, then a session resumption will be allowed if the client attempts it. Otherwise the server will prevent resumption and force the creation of a new session.

  std::vector<X509_Certificate> `peer_cert_chain`()

    Returns the certificate chain of the counterparty. When acting as a client, this value will be non-empty unless the client’s policy allowed anonymous connections and the server then chose an anonymous ciphersuite. Acting as a server, this value will ordinarily be empty, unless the server requested a certificate and the client responded with one.

  SymmetricKey `key_material_export`(const std::string &label, const std::string &context, size_t length)

    Returns an exported key of length bytes derived from label, context, and the session’s master secret and client and server random values. This key will be unique to this connection, and as long as the session master secret remains secure an attacker should not be able to guess the key.

    Per RFC 5705, label should begin with “EXPERIMENTAL” unless the label has been standardized in an RFC.

## TLS Clients

class `TLS`::```Client`
  `Client`(Callbacks &callbacks, Session_Manager &session_manager, Credentials_Manager &creds, const Policy &policy, RandomNumberGenerator &rng, const Server_Information &server_info = Server_Information(), const Protocol_Version offer_version = Protocol_Version::latest_tls_version(), const std::vector<std::string> &next_protocols = std::vector<std::string>(), size_t reserved_io_buffer_size = 16 * 1024)

  Initialize a new TLS client. The constructor will immediately initiate a new session.

  The callbacks parameter specifies the various application callbacks which pertain to this particular client connection.

  The session_manager is an interface for storing TLS sessions, which allows for session resumption upon reconnecting to a server. In the absence of a need for persistent sessions, use `TLS::Session_Manager_In_Memory` which caches connections for the lifetime of a single process. See TLS Session Managers for more about session managers.

  The credentials_manager is an interface that will be called to retrieve any certificates, secret keys, pre-shared keys, or SRP information; see Credentials Manager for more information.

  Use the optional server_info to specify the DNS name of the server you are attempting to connect to, if you know it. This helps the server select what certificate to use and helps the client validate the connection.

  Note that the server name indicator name must be a FQDN. IP addresses are not allowed by RFC 6066 and may lead to interoperability problems.

  Use the optional offer_version to control the version of TLS you wish the client to offer. Normally, you’ll want to offer the most recent version of (D)TLS that is available, however some broken servers are intolerant of certain versions being offered, and for classes of applications that have to deal with such servers (typically web browsers) it may be necessary to implement a version backdown strategy if the initial attempt fails.

  Warning

  Implementing such a backdown strategy allows an attacker to downgrade your connection to the weakest protocol that both you and the server support.

  Setting offer_version is also used to offer DTLS instead of TLS; use `TLS::Protocol_Version::latest_dtls_version`.

  Optionally, the client will advertise app_protocols to the server using the ALPN extension.

  The optional reserved_io_buffer_size specifies how many bytes to pre-allocate in the I/O buffers. Use this if you want to control how much memory the channel uses initially (the buffers will be resized as needed to process inputs). Otherwise some reasonable default is used.

### Code Example

A minimal example of a TLS client is provided below. The full code for a TLS client using BSD sockets is in src/cli/tls_client.cpp

    #include <botan/tls_client.h>
    #include <botan/tls_callbacks.h>
    #include <botan/tls_session_manager.h>
    #include <botan/tls_policy.h>
    #include <botan/auto_rng.h>
    #include <botan/certstor.h>

    /**
     * @brief Callbacks invoked by TLS::Channel.
     *
     * Botan::TLS::Callbacks is an abstract class.
     * For improved readability, only the functions that are mandatory
     * to implement are listed here. See src/lib/tls/tls_callbacks.h.
     */
    class Callbacks : public Botan::TLS::Callbacks
    {
       public:
          void tls_emit_data(const uint8_t data[], size_t size) override
             {
             // send data to tls server, e.g., using BSD sockets or boost asio
             }

          void tls_record_received(uint64_t seq_no, const uint8_t data[], size_t size) override
             {
             // process full TLS record received by tls server, e.g.,
             // by passing it to the application
             }

          void tls_alert(Botan::TLS::Alert alert) override
             {
             // handle a tls alert received from the tls server
             }

          bool tls_session_established(const Botan::TLS::Session& session) override
             {
             // the session with the tls server was established
             // return false to prevent the session from being cached, true to
             // cache the session in the configured session manager
             return false;
             }
    };

    /**
     * @brief Credentials storage for the tls client.
     *
     * It returns a list of trusted CA certificates from a local directory.
     * TLS client authentication is disabled. See src/lib/tls/credentials_manager.h.
     */
    class Client_Credentials : public Botan::Credentials_Manager
       {
       public:
          Client_Credentials()
             {
             // Here we base trust on the system managed trusted CA list
             m_stores.push_back(new Botan::System_Certificate_Store);
             }

          std::vector<Botan::Certificate_Store*> trusted_certificate_authorities(
             const std::string& type,
             const std::string& context) override
             {
             // return a list of certificates of CAs we trust for tls server certificates
             // ownership of the pointers remains with Credentials_Manager
             return m_stores;
             }

          std::vector<Botan::X509_Certificate> cert_chain(
             const std::vector<std::string>& cert_key_types,
             const std::string& type,
             const std::string& context) override
             {
             // when using tls client authentication (optional), return
             // a certificate chain being sent to the tls server,
             // else an empty list
             return std::vector<Botan::X509_Certificate>();
             }

          Botan::Private_Key* private_key_for(const Botan::X509_Certificate& cert,
             const std::string& type,
             const std::string& context) override
             {
             // when returning a chain in cert_chain(), return the private key
             // associated with the leaf certificate here
             return nullptr;
             }

       private:
           std::vector<Botan::Certificate_Store*> m_stores;
    };

    int main()
       {
       // prepare all the parameters
       Callbacks callbacks;
       Botan::AutoSeeded_RNG rng;
       Botan::TLS::Session_Manager_In_Memory session_mgr(rng);
       Client_Credentials creds;
       Botan::TLS::Strict_Policy policy;

       // open the tls connection
       Botan::TLS::Client client(callbacks,
                                 session_mgr,
                                 creds,
                                 policy,
                                 rng,
                                 Botan::TLS::Server_Information("botan.randombit.net", 443),
                                 Botan::TLS::Protocol_Version::TLS_V12);

       while(!client.is_closed())
          {
          // read data received from the tls server, e.g., using BSD sockets or boost asio
          // ...

          // send data to the tls server using client.send_data()
          }
       }

## TLS Servers

class `TLS`::```Server`
  `Server`(Callbacks &callbacks, Session_Manager &session_manager, Credentials_Manager &creds, const Policy &policy, RandomNumberGenerator &rng, bool is_datagram = false, size_t reserved_io_buffer_size = 16 * 1024)

The first 5 arguments as well as the final argument reserved_io_buffer_size, are treated similarly to the client.

If a client sends the ALPN extension, the `callbacks` function `tls_server_choose_app_protocol` will be called and the result sent back to the client. If the empty string is returned, the server will not send an ALPN response. The function can also throw an exception to abort the handshake entirely, the ALPN specification says that if this occurs the alert should be of type NO_APPLICATION_PROTOCOL.

The optional argument is_datagram specifies if this is a TLS or DTLS server; unlike clients, which know what type of protocol (TLS vs DTLS) they are negotiating from the start via the offer_version, servers would not until they actually received a client hello.

### Code Example

A minimal example of a TLS server is provided below. The full code for a TLS server using asio is in src/cli/tls_proxy.cpp.

    #include <botan/tls_client.h>
    #include <botan/tls_callbacks.h>
    #include <botan/tls_session_manager.h>
    #include <botan/tls_policy.h>
    #include <botan/auto_rng.h>
    #include <botan/certstor.h>
    #include <botan/pk_keys.h>

    #include <memory>

    /**
     * @brief Callbacks invoked by TLS::Channel.
     *
     * Botan::TLS::Callbacks is an abstract class.
     * For improved readability, only the functions that are mandatory
     * to implement are listed here. See src/lib/tls/tls_callbacks.h.
     */
    class Callbacks : public Botan::TLS::Callbacks
    {
       public:
          void tls_emit_data(const uint8_t data[], size_t size) override
             {
             // send data to tls client, e.g., using BSD sockets or boost asio
             }

          void tls_record_received(uint64_t seq_no, const uint8_t data[], size_t size) override
             {
             // process full TLS record received by tls client, e.g.,
             // by passing it to the application
             }

          void tls_alert(Botan::TLS::Alert alert) override
             {
             // handle a tls alert received from the tls server
             }

          bool tls_session_established(const Botan::TLS::Session& session) override
             {
             // the session with the tls client was established
             // return false to prevent the session from being cached, true to
             // cache the session in the configured session manager
             return false;
             }
    };

    /**
     * @brief Credentials storage for the tls server.
     *
     * It returns a certificate and the associated private key to
     * authenticate the tls server to the client.
     * TLS client authentication is not requested.
     * See src/lib/tls/credentials_manager.h.
     */
    class Server_Credentials : public Botan::Credentials_Manager
    {
       public:
          Server_Credentials() : m_key(Botan::PKCS8::load_key("botan.randombit.net.key"))
             {
             }

          std::vector<Botan::Certificate_Store*> trusted_certificate_authorities(
             const std::string& type,
             const std::string& context) override
             {
             // if client authentication is required, this function
             // shall return a list of certificates of CAs we trust
             // for tls client certificates, otherwise return an empty list
             return std::vector<Certificate_Store*>();
             }

          std::vector<Botan::X509_Certificate> cert_chain(
             const std::vector<std::string>& cert_key_types,
             const std::string& type,
             const std::string& context) override
             {
             // return the certificate chain being sent to the tls client
             // e.g., the certificate file "botan.randombit.net.crt"
             return { Botan::X509_Certificate("botan.randombit.net.crt") };
             }

          Botan::Private_Key* private_key_for(const Botan::X509_Certificate& cert,
             const std::string& type,
             const std::string& context) override
             {
             // return the private key associated with the leaf certificate,
             // in this case the one associated with "botan.randombit.net.crt"
             return &m_key;
             }

          private:
             std::unique_ptr<Botan::Private_Key> m_key;
    };

    int main()
       {
       // prepare all the parameters
       Callbacks callbacks;
       Botan::AutoSeeded_RNG rng;
       Botan::TLS::Session_Manager_In_Memory session_mgr(rng);
       Server_Credentials creds;
       Botan::TLS::Strict_Policy policy;

       // accept tls connection from client
       Botan::TLS::Server server(callbacks,
                                 session_mgr,
                                 creds,
                                 policy,
                                 rng);

       // read data received from the tls client, e.g., using BSD sockets or boost asio
       // and pass it to server.received_data().
       // ...

       // send data to the tls client using server.send_data()
       // ...
       }

## TLS Sessions

TLS allows clients and servers to support session resumption, where the end point retains some information about an established session and then reuse that information to bootstrap a new session in way that is much cheaper computationally than a full handshake.

Every time your handshake callback is called, a new session has been established, and a `TLS::Session` is included that provides information about that session:

Note

The serialization format of Session is not considered stable and is allowed to change even across minor releases. In the event of such a change, old sessions will no longer be able to be resumed.

class `TLS`::```Session`
  Protocol_Version `version`() const

    Returns the `protocol version` that was negotiated

  Ciphersuite `ciphersite`() const

    Returns the `ciphersuite` that was negotiated.

  Server_Information `server_info`() const

    Returns information that identifies the server side of the connection. This is useful for the client in that it identifies what was originally passed to the constructor. For the server, it includes the name the client specified in the server name indicator extension.

  std::vector<X509_Certificate> `peer_certs`() const

    Returns the certificate chain of the peer

  std::string `srp_identifier`() const

    If an SRP ciphersuite was used, then this is the identifier that was used for authentication.

  bool `secure_renegotiation`() const

    Returns `true` if the connection was negotiated with the correct extensions to prevent the renegotiation attack.

  std::vector<uint8_t> `encrypt`(const SymmetricKey &key, RandomNumberGenerator &rng)

    Encrypts a session using a symmetric key key and returns a raw binary value that can later be passed to `decrypt`. The key may be of any length. The format is described in TLS Session Encryption.

  static Session `decrypt`(const uint8_t ciphertext[], size_t length, const SymmetricKey &key)

    Decrypts a session that was encrypted previously with `encrypt` and `key`, or throws an exception if decryption fails.

  secure_vector<uint8_t> `DER_encode`() const

    Returns a serialized version of the session.

    Warning

    The return value of `DER_encode` contains the master secret for the session, and an attacker who recovers it could recover plaintext of previous sessions or impersonate one side to the other.

## TLS Session Managers

You may want sessions stored in a specific format or storage type. To do so, implement the `TLS::Session_Manager` interface and pass your implementation to the `TLS::Client` or `TLS::Server` constructor.

class `TLS`::```Session_Mananger`
  void `save`(const Session &session)

    Save a new session. It is possible that this sessions session ID will replicate a session ID already stored, in which case the new session information should overwrite the previous information.

  void `remove_entry`(const std::vector<uint8_t> &session_id)

    Remove the session identified by session_id. Future attempts at resumption should fail for this session.

  bool `load_from_session_id`(const std::vector<uint8_t> &session_id, Session &session)

    Attempt to resume a session identified by session_id. If located, session is set to the session data previously passed to save, and `true` is returned. Otherwise session is not modified and `false` is returned.

  bool `load_from_server_info`(const Server_Information &server, Session &session)

    Attempt to resume a session with a known server.

  std::chrono::seconds `session_lifetime`() const

    Returns the expected maximum lifetime of a session when using this session manager. Will return 0 if the lifetime is unknown or has no explicit expiration policy.

### In Memory Session Manager

The `TLS::Session_Manager_In_Memory` implementation saves sessions in memory, with an upper bound on the maximum number of sessions and the lifetime of a session.

It is safe to share a single object across many threads as it uses a lock internally.

class `TLS`::```Session_Managers_In_Memory`
  `Session_Manager_In_Memory`(RandomNumberGenerator &rng, size_t max_sessions = 1000, std::chrono::seconds session_lifetime = 7200)

    Limits the maximum number of saved sessions to max_sessions, and expires all sessions older than session_lifetime.

### Noop Session Mananger

The `TLS::Session_Manager_Noop` implementation does not save sessions at all, and thus session resumption always fails. Its constructor has no arguments.

### SQLite3 Session Manager

This session manager is only available if support for SQLite3 was enabled at build time. If the macro `BOTAN_HAS_TLS_SQLITE3_SESSION_MANAGER` is defined, then `botan/tls_session_manager_sqlite.h` contains `TLS::Session_Manager_SQLite` which stores sessions persistently to a sqlite3 database. The session data is encrypted using a passphrase, and stored in two tables, named `tls_sessions` (which holds the actual session information) and `tls_sessions_metadata` (which holds the PBKDF information).

Warning

The hostnames associated with the saved sessions are stored in the database in plaintext. This may be a serious privacy risk in some applications.

class `TLS`::```Session_Manager_SQLite`
  `Session_Manager_SQLite`(const std::string &passphrase, RandomNumberGenerator &rng, const std::string &db_filename, size_t max_sessions = 1000, std::chrono::seconds session_lifetime = 7200)

    Uses the sqlite3 database named by db_filename.

## TLS Policies

`TLS::Policy` is how an application can control details of what will be negotiated during a handshake. The base class acts as the default policy. There is also a `Strict_Policy` (which forces only secure options, reducing compatibility) and `Text_Policy` which reads policy settings from a file.

class `TLS`::```Policy`
  std::vector<std::string> `allowed_ciphers`() const

    Returns the list of ciphers we are willing to negotiate, in order of preference.

    Clients send a list of ciphersuites in order of preference, servers are free to choose any of them. Some servers will use the clients preferences, others choose from the clients list prioritizing based on its preferences.

    No export key exchange mechanisms or ciphersuites are supported by botan. The null encryption ciphersuites (which provide only authentication, sending data in cleartext) are also not supported by the implementation and cannot be negotiated.

    Cipher names without an explicit mode refers to CBC+HMAC ciphersuites.

    Default value: “ChaCha20Poly1305”, “AES-256/GCM”, “AES-128/GCM”

    Also allowed: “AES-256”, “AES-128”, “AES-256/CCM”, “AES-128/CCM”, “AES-256/CCM(8)”, “AES-128/CCM(8)”, “Camellia-256/GCM”, “Camellia-128/GCM”, “ARIA-256/GCM”, “ARIA-128/GCM”, “Camellia-256”, “Camellia-128”

    Also allowed (though currently experimental): “AES-128/OCB(12)”, “AES-256/OCB(12)”

    In versions up to 2.8.0, the CBC and CCM ciphersuites “AES-256”, “AES-128”, “AES-256/CCM” and “AES-128/CCM” were enabled by default.

    Also allowed (although not recommended): “SEED”, “3DES”

    Note

    Before 1.11.30 only the non-standard ChaCha20Poly1305 ciphersuite was implemented. The RFC 7905 ciphersuites are supported in 1.11.30 onwards.

    Note

    Support for the broken RC4 cipher was removed in 1.11.17

    Note

    SEED and 3DES are deprecated and will be removed in a future release.

  std::vector<std::string> `allowed_macs`() const

    Returns the list of algorithms we are willing to use for message authentication, in order of preference.

    Default: “AEAD”, “SHA-256”, “SHA-384”, “SHA-1”

    A plain hash function indicates HMAC

    Note

    SHA-256 is preferred over SHA-384 in CBC mode because the protections against the Lucky13 attack are somewhat more effective for SHA-256 than SHA-384.

  std::vector<std::string> `allowed_key_exchange_methods`() const

    Returns the list of key exchange methods we are willing to use, in order of preference.

    Default: “CECPQ1”, “ECDH”, “DH”

    Note

    CECPQ1 key exchange provides post-quantum security to the key exchange by combining NewHope with a standard x25519 ECDH exchange. This prevents an attacker, even one with a quantum computer, from later decrypting the contents of a recorded TLS transcript. The NewHope algorithm is very fast, but adds roughly 4 KiB of additional data transfer to every TLS handshake. And even if NewHope ends up completely broken, the ‘extra’ x25519 exchange secures the handshake.

    For applications where the additional data transfer size is unacceptable, simply allow only ECDH key exchange in the application policy. DH exchange also often involves transferring several additional Kb (without the benefit of post quantum security) so if CECPQ1 is being disabled for traffic overhead reasons, DH should also be avoid.

    Also allowed: “RSA”, “SRP_SHA”, “ECDHE_PSK”, “DHE_PSK”, “PSK”

    Note

    Static RSA ciphersuites are disabled by default since 1.11.34. In addition to not providing forward security, any server which is willing to negotiate these ciphersuites exposes themselves to a variety of chosen ciphertext oracle attacks which are all easily avoided by signing (as in PFS) instead of decrypting.

    Note

    In order to enable RSA, SRP, or PSK ciphersuites one must also enable authentication method “IMPLICIT”, see `allowed_signature_methods`.

  std::vector<std::string> `allowed_signature_hashes`() const

    Returns the list of hash algorithms we are willing to use for public key signatures, in order of preference.

    Default: “SHA-512”, “SHA-384”, “SHA-256”

    Also allowed (although not recommended): “SHA-1”

    Note

    This is only used with TLS v1.2. In earlier versions of the protocol, signatures are fixed to using only SHA-1 (for DSA/ECDSA) or a MD5/SHA-1 pair (for RSA).

  std::vector<std::string> `allowed_signature_methods`() const

    Default: “ECDSA”, “RSA”

    Also allowed (disabled by default): “DSA”, “IMPLICIT”, “ANONYMOUS”

    “IMPLICIT” enables ciphersuites which are authenticated not by a signature but through a side-effect of the key exchange. In particular this setting is required to enable PSK, SRP, and static RSA ciphersuites.

    “ANONYMOUS” allows purely anonymous DH/ECDH key exchanges. Enabling this is not recommended

    Note

    Both DSA authentication and anonymous DH ciphersuites are deprecated, and will be removed in a future release.

  std::vector<Group_Params> `key_exchange_groups`() const

    Return a list of ECC curve and DH group TLS identifiers we are willing to use, in order of preference. The default ordering puts the best performing ECC first.

    Default: Group_Params::X25519, Group_Params::SECP256R1, Group_Params::BRAINPOOL256R1, Group_Params::SECP384R1, Group_Params::BRAINPOOL384R1, Group_Params::SECP521R1, Group_Params::BRAINPOOL512R1, Group_Params::FFDHE_2048, Group_Params::FFDHE_3072, Group_Params::FFDHE_4096, Group_Params::FFDHE_6144, Group_Params::FFDHE_8192

    No other values are currently defined.

  bool `use_ecc_point_compression`() const

    Prefer ECC point compression.

    Signals that we prefer ECC points to be compressed when transmitted to us. The other party may not support ECC point compression and therefore may still send points uncompressed.

    Note that the certificate used during authentication must also follow the other party’s preference.

    Default: false

    Note

    Support for EC point compression is deprecated and will be removed in a future major release.

  bool `acceptable_protocol_version`(Protocol_Version version)

    Return true if this version of the protocol is one that we are willing to negotiate.

    Default: Accepts TLS v1.2 and DTLS v1.2, and rejects all older versions.

  bool `server_uses_own_ciphersuite_preferences`() const

    If this returns true, a server will pick the cipher it prefers the most out of the client’s list. Otherwise, it will negotiate the first cipher in the client’s ciphersuite list that it supports.

  bool `allow_client_initiated_renegotiation`() const

    If this function returns true, a server will accept a client-initiated renegotiation attempt. Otherwise it will send the client a non-fatal `no_renegotiation` alert.

    Default: true

  bool `allow_server_initiated_renegotiation`() const

    If this function returns true, a client will accept a server-initiated renegotiation attempt. Otherwise it will send the server a non-fatal `no_renegotiation` alert.

    Default: false

  bool `abort_connection_on_undesired_renegotiation`() const

    If a renegotiation attempt is being rejected due to the configuration of `TLS::Policy::allow_client_initiated_renegotiation` or `TLS::Policy::allow_server_initiated_renegotiation`, and this function returns true then the connection is closed with a fatal alert instead of the default warning alert.

    Default: false

  bool `allow_insecure_renegotiation`() const

    If this function returns true, we will allow renegotiation attempts even if the counterparty does not support the RFC 5746 extensions.

    Warning

    Returning true here could expose you to attacks

    Default: false

  size_t `minimum_signature_strength`() const

    Return the minimum strength (as `n`, representing `2**n` work) we will accept for a signature algorithm on any certificate.

    Use 80 to enable RSA-1024 (not recommended), or 128 to require either ECC or large (~3000 bit) RSA keys.

    Default: 110 (allowing 2048 bit RSA)

  bool `require_cert_revocation_info`() const

    If this function returns true, and a ciphersuite using certificates was negotiated, then we must have access to a valid CRL or OCSP response in order to trust the certificate.

    Warning

    Returning false here could expose you to attacks

    Default: true

  Group_Params `default_dh_group`() const

    For ephemeral Diffie-Hellman key exchange, the server sends a group parameter. Return the 2 Byte TLS group identifier specifying the group parameter a server should use.

    Default: 2048 bit IETF IPsec group (“modp/ietf/2048”)

  size_t `minimum_dh_group_size`() const

    Return the minimum size in bits for a Diffie-Hellman group that a client will accept. Due to the design of the protocol the client has only two options - accept the group, or reject it with a fatal alert then attempt to reconnect after disabling ephemeral Diffie-Hellman.

    Default: 2048 bits

  bool `allow_tls10`() const

    Return true from here to allow TLS v1.0. Since 2.8.0, returns `false` by default.

  bool `allow_tls11`() const

    Return true from here to allow TLS v1.1. Since 2.8.0, returns `false` by default.

  bool `allow_tls12`() const

    Return true from here to allow TLS v1.2. Returns `true` by default.

  size_t `minimum_rsa_bits`() const

    Minimum accepted RSA key size. Default 2048 bits.

  size_t `minimum_dsa_group_size`() const

    Minimum accepted DSA key size. Default 2048 bits.

  size_t `minimum_ecdsa_group_size`() const

    Minimum size for ECDSA keys (256 bits).

  size_t `minimum_ecdh_group_size`() const

    Minimum size for ECDH keys (255 bits).

  void `check_peer_key_acceptable`(const Public_Key &public_key) const

    Allows the policy to examine peer public keys. Throw an exception if the key should be rejected. Default implementation checks against policy values minimum_dh_group_size, minimum_rsa_bits, minimum_ecdsa_group_size, and minimum_ecdh_group_size.

  bool `hide_unknown_users`() const

    The SRP and PSK suites work using an identifier along with a shared secret. If this function returns true, when an identifier that the server does not recognize is provided by a client, a random shared secret will be generated in such a way that a client should not be able to tell the difference between the identifier not being known and the secret being wrong. This can help protect against some username probing attacks. If it returns false, the server will instead send an `unknown_psk_identity` alert when an unknown identifier is used.

    Default: false

  u32bit `session_ticket_lifetime`() const

    Return the lifetime of session tickets. Each session includes the start time. Sessions resumptions using tickets older than `session_ticket_lifetime` seconds will fail, forcing a full renegotiation.

    Default: 86400 seconds (1 day)

## TLS Ciphersuites

class `TLS`::```Ciphersuite`
  uint16_t `ciphersuite_code`() const

    Return the numerical code for this ciphersuite

  std::string `to_string`() const

    Return the full name of ciphersuite (for example “RSA_WITH_RC4_128_SHA” or “ECDHE_RSA_WITH_AES_128_GCM_SHA256”)

  std::string `kex_algo`() const

    Return the key exchange algorithm of this ciphersuite

  std::string `sig_algo`() const

    Return the signature algorithm of this ciphersuite

  std::string `cipher_algo`() const

    Return the cipher algorithm of this ciphersuite

  std::string `mac_algo`() const

    Return the authentication algorithm of this ciphersuite

  bool `acceptable_ciphersuite`(const Ciphersuite &suite) const

    Return true if ciphersuite is accepted by the policy.

    Allows an application to reject any ciphersuites, which are undesirable for whatever reason without having to reimplement `TLS::Ciphersuite::ciphersuite_list`

  std::vector<uint16_t> `ciphersuite_list`(Protocol_Version version, bool have_srp) const

    Return allowed ciphersuites in order of preference

    Allows an application to have full control over ciphersuites by returning desired ciphersuites in preference order.

## TLS Alerts

A `TLS::Alert` is passed to every invocation of a channel’s alert_cb.

class `TLS`::```Alert`
  `is_valid`() const

    Return true if this alert is not a null alert

  `is_fatal`() const

    Return true if this alert is fatal. A fatal alert causes the connection to be immediately disconnected. Otherwise, the alert is a warning and the connection remains valid.

  Type `type`() const

    Returns the type of the alert as an enum

  std::string `type_string`()

    Returns the type of the alert as a string

## TLS Protocol Version

TLS has several different versions with slightly different behaviors. The `TLS::Protocol_Version` class represents a specific version:

class `TLS`::```Protocol_Version`
  enum `Version_Code`

    `TLS_V10`, `TLS_V11`, `TLS_V12`, `DTLS_V10`, `DTLS_V12`

  `Protocol_Version`(Version_Code named_version)

    Create a specific version

  uint8_t `major_version`() const

    Returns major number of the protocol version

  uint8_t `minor_version`() const

    Returns minor number of the protocol version

  std::string `to_string`() const

    Returns string description of the version, for instance “TLS v1.1” or “DTLS v1.0”.

  static Protocol_Version `latest_tls_version`()

    Returns the latest version of the TLS protocol known to the library (currently TLS v1.2)

  static Protocol_Version `latest_dtls_version`()

    Returns the latest version of the DTLS protocol known to the library (currently DTLS v1.2)

## TLS Custom Curves

The supported_groups TLS extension is used in the client hello to advertise a list of supported elliptic curves and DH groups. The server subsequently selects one of the groups, which is supported by both endpoints. The groups are represented by their TLS identifier. This 2 Byte identifier is standardized for commonly used groups and curves. In addition, the standard reserves the identifiers 0xFE00 to 0xFEFF for custom groups or curves.

Using non standardized custom curves is however not recommended and can be a serious risk if an insecure curve is used. Still, it might be desired in some scenarios to use custom curves or groups in the TLS handshake.

To use custom curves with the Botan `TLS::Client` or `TLS::Server` the following additional adjustments have to be implemented as shown in the following code examples.

1. Registration of the custom curve

2. Implementation TLS callback `tls_decode_group_param`

3. Adjustment of the TLS policy by allowing the custom curve

### Client Code Example

    #include <botan/tls_client.h>
    #include <botan/tls_callbacks.h>
    #include <botan/tls_session_manager.h>
    #include <botan/tls_policy.h>
    #include <botan/auto_rng.h>
    #include <botan/certstor.h>

    #include <botan/ec_group.h>
    #include <botan/oids.h>

    /**
     * @brief Callbacks invoked by TLS::Channel.
     *
     * Botan::TLS::Callbacks is an abstract class.
     * For improved readability, only the functions that are mandatory
     * to implement are listed here. See src/lib/tls/tls_callbacks.h.
     */
    class Callbacks : public Botan::TLS::Callbacks
    {
       public:
          void tls_emit_data(const uint8_t data[], size_t size) override
             {
             // send data to tls server, e.g., using BSD sockets or boost asio
             }

          void tls_record_received(uint64_t seq_no, const uint8_t data[], size_t size) override
             {
             // process full TLS record received by tls server, e.g.,
             // by passing it to the application
             }

          void tls_alert(Botan::TLS::Alert alert) override
             {
             // handle a tls alert received from the tls server
             }

          bool tls_session_established(const Botan::TLS::Session& session) override
             {
             // the session with the tls server was established
             // return false to prevent the session from being cached, true to
             // cache the session in the configured session manager
             return false;
             }
          std::string tls_decode_group_param(Botan::TLS::Group_Params group_param) override
              {
              // handle TLS group identifier decoding and return name as string
              // return empty string to indicate decoding failure

              switch(static_cast<uint16_t>(group_param))
                 {
                 case 0xFE00:
                    return "testcurve1102";
                 default:
                    //decode non-custom groups
                    return Botan::TLS::Callbacks::tls_decode_group_param(group_param);
                 }
              }
    };

    /**
     * @brief Credentials storage for the tls client.
     *
     * It returns a list of trusted CA certificates from a local directory.
     * TLS client authentication is disabled. See src/lib/tls/credentials_manager.h.
     */
    class Client_Credentials : public Botan::Credentials_Manager
    {
       public:
          std::vector<Botan::Certificate_Store*> trusted_certificate_authorities(
             const std::string& type,
             const std::string& context) override
             {
             // return a list of certificates of CAs we trust for tls server certificates,
             // e.g., all the certificates in the local directory "cas"
             return { new Botan::Certificate_Store_In_Memory("cas") };
             }

          std::vector<Botan::X509_Certificate> cert_chain(
             const std::vector<std::string>& cert_key_types,
             const std::string& type,
             const std::string& context) override
             {
             // when using tls client authentication (optional), return
             // a certificate chain being sent to the tls server,
             // else an empty list
             return std::vector<Botan::X509_Certificate>();
             }

          Botan::Private_Key* private_key_for(const Botan::X509_Certificate& cert,
             const std::string& type,
             const std::string& context) override
             {
             // when returning a chain in cert_chain(), return the private key
             // associated with the leaf certificate here
             return nullptr;
             }
    };

    class Client_Policy : public Botan::TLS::Strict_Policy
    {
       public:
          std::vector<Botan::TLS::Group_Params> key_exchange_groups() const override
             {
             // modified strict policy to allow our custom curves
             return
                {
                static_cast<Botan::TLS::Group_Params>(0xFE00)
                };
             }
    };

    int main()
       {
       // prepare rng
       Botan::AutoSeeded_RNG rng;

       // prepare custom curve

       // prepare curve parameters
       const Botan::BigInt p("0x92309a3e88b94312f36891a2055725bb35ab51af96b3a651d39321b7bbb8c51575a76768c9b6b323");
       const Botan::BigInt a("0x4f30b8e311f6b2dce62078d70b35dacb96aa84b758ab5a8dff0c9f7a2a1ff466c19988aa0acdde69");
       const Botan::BigInt b("0x9045A513CFFF9AE1F1CC84039D852D240344A1D5C9DB203C844089F855C387823EB6FCDDF49C909C");

       const Botan::BigInt x("0x9120f3779a31296cefcb5a5a08831f1a6d438ad5a3f2ce60585ac19c74eebdc65cadb96bb92622c7");
       const Botan::BigInt y("0x836db8251c152dfee071b72c6b06c5387d82f1b5c30c5a5b65ee9429aa2687e8426d5d61276a4ede");
       const Botan::BigInt order("0x248c268fa22e50c4bcda24688155c96ecd6ad46be5c82d7a6be6e7068cb5d1ca72b2e07e8b90d853");

       const Botan::BigInt cofactor(4);

       const Botan::OID oid("1.2.3.1");

       // create EC_Group object to register the curve
       Botan::EC_Group testcurve1102(p, a, b, x, y, order, cofactor, oid);

       if(!testcurve1102.verify_group(rng))
          {
          // Warning: if verify_group returns false the curve parameters are insecure
          }

       // register name to specified oid
       Botan::OIDS::add_oid(oid, "testcurve1102");

       // prepare all the parameters
       Callbacks callbacks;
       Botan::TLS::Session_Manager_In_Memory session_mgr(rng);
       Client_Credentials creds;
       Client_Policy policy;

       // open the tls connection
       Botan::TLS::Client client(callbacks,
                                 session_mgr,
                                 creds,
                                 policy,
                                 rng,
                                 Botan::TLS::Server_Information("botan.randombit.net", 443),
                                 Botan::TLS::Protocol_Version::TLS_V12);

       while(!client.is_closed())
          {
          // read data received from the tls server, e.g., using BSD sockets or boost asio
          // ...

          // send data to the tls server using client.send_data()

           }
       }

### Server Code Example

    #include <botan/tls_server.h>
    #include <botan/tls_callbacks.h>
    #include <botan/tls_session_manager.h>
    #include <botan/tls_policy.h>
    #include <botan/auto_rng.h>
    #include <botan/certstor.h>
    #include <botan/pk_keys.h>
    #include <botan/pkcs8.h>

    #include <botan/ec_group.h>
    #include <botan/oids.h>

    #include <memory>

    /**
     * @brief Callbacks invoked by TLS::Channel.
     *
     * Botan::TLS::Callbacks is an abstract class.
     * For improved readability, only the functions that are mandatory
     * to implement are listed here. See src/lib/tls/tls_callbacks.h.
     */
    class Callbacks : public Botan::TLS::Callbacks
    {
       public:
          void tls_emit_data(const uint8_t data[], size_t size) override
             {
             // send data to tls client, e.g., using BSD sockets or boost asio
             }

          void tls_record_received(uint64_t seq_no, const uint8_t data[], size_t size) override
             {
             // process full TLS record received by tls client, e.g.,
             // by passing it to the application
             }

          void tls_alert(Botan::TLS::Alert alert) override
             {
             // handle a tls alert received from the tls server
             }

          bool tls_session_established(const Botan::TLS::Session& session) override
             {
             // the session with the tls client was established
             // return false to prevent the session from being cached, true to
             // cache the session in the configured session manager
             return false;
             }

          std::string tls_decode_group_param(Botan::TLS::Group_Params group_param) override
             {
             // handle TLS group identifier decoding and return name as string
             // return empty string to indicate decoding failure

             switch(static_cast<uint16_t>(group_param))
                {
                case 0xFE00:
                   return "testcurve1102";
                default:
                   //decode non-custom groups
                   return Botan::TLS::Callbacks::tls_decode_group_param(group_param);
                }
             }
    };

    /**
     * @brief Credentials storage for the tls server.
     *
     * It returns a certificate and the associated private key to
     * authenticate the tls server to the client.
     * TLS client authentication is not requested.
     * See src/lib/tls/credentials_manager.h.
     */
    class Server_Credentials : public Botan::Credentials_Manager
    {
       public:
          Server_Credentials() : m_key(Botan::PKCS8::load_key("botan.randombit.net.key")
             {
             }

          std::vector<Botan::Certificate_Store*> trusted_certificate_authorities(
             const std::string& type,
             const std::string& context) override
             {
             // if client authentication is required, this function
             // shall return a list of certificates of CAs we trust
             // for tls client certificates, otherwise return an empty list
             return std::vector<Botan::Certificate_Store*>();
             }

          std::vector<Botan::X509_Certificate> cert_chain(
             const std::vector<std::string>& cert_key_types,
             const std::string& type,
             const std::string& context) override
             {
             // return the certificate chain being sent to the tls client
             // e.g., the certificate file "botan.randombit.net.crt"
             return { Botan::X509_Certificate("botan.randombit.net.crt") };
             }

          Botan::Private_Key* private_key_for(const Botan::X509_Certificate& cert,
             const std::string& type,
             const std::string& context) override
             {
             // return the private key associated with the leaf certificate,
             // in this case the one associated with "botan.randombit.net.crt"
             return m_key.get();
             }

          private:
             std::unique_ptr<Botan::Private_Key> m_key;
    };

    class Server_Policy : public Botan::TLS::Strict_Policy
    {
       public:
          std::vector<Botan::TLS::Group_Params> key_exchange_groups() const override
             {
             // modified strict policy to allow our custom curves
             return
                {
                static_cast<Botan::TLS::Group_Params>(0xFE00)
                };
             }
    };

    int main()
       {

       // prepare rng
       Botan::AutoSeeded_RNG rng;

       // prepare custom curve

       // prepare curve parameters
       const Botan::BigInt p("0x92309a3e88b94312f36891a2055725bb35ab51af96b3a651d39321b7bbb8c51575a76768c9b6b323");
       const Botan::BigInt a("0x4f30b8e311f6b2dce62078d70b35dacb96aa84b758ab5a8dff0c9f7a2a1ff466c19988aa0acdde69");
       const Botan::BigInt b("0x9045A513CFFF9AE1F1CC84039D852D240344A1D5C9DB203C844089F855C387823EB6FCDDF49C909C");

       const Botan::BigInt x("0x9120f3779a31296cefcb5a5a08831f1a6d438ad5a3f2ce60585ac19c74eebdc65cadb96bb92622c7");
       const Botan::BigInt y("0x836db8251c152dfee071b72c6b06c5387d82f1b5c30c5a5b65ee9429aa2687e8426d5d61276a4ede");
       const Botan::BigInt order("0x248c268fa22e50c4bcda24688155c96ecd6ad46be5c82d7a6be6e7068cb5d1ca72b2e07e8b90d853");

       const Botan::BigInt cofactor(4);

       const Botan::OID oid("1.2.3.1");

       // create EC_Group object to register the curve
       Botan::EC_Group testcurve1102(p, a, b, x, y, order, cofactor, oid);

       if(!testcurve1102.verify_group(rng))
          {
          // Warning: if verify_group returns false the curve parameters are insecure
          }

       // register name to specified oid
       Botan::OIDS::add_oid(oid, "testcurve1102");

       // prepare all the parameters
       Callbacks callbacks;
       Botan::TLS::Session_Manager_In_Memory session_mgr(rng);
       Server_Credentials creds;
       Server_Policy policy;

       // accept tls connection from client
       Botan::TLS::Server server(callbacks,
                                 session_mgr,
                                 creds,
                                 policy,
                                 rng);

       // read data received from the tls client, e.g., using BSD sockets or boost asio
       // and pass it to server.received_data().
       // ...

       // send data to the tls client using server.send_data()
       // ...
       }

## TLS Stream

`TLS::Stream` offers a Boost.Asio compatible wrapper around `TLS::Client`. It can be used as an alternative to Boost.Asio’s ssl::stream with minor adjustments to the using code. It offers the following interface:

template<class `StreamLayer`, class `ChannelT`>
class `TLS`::```Stream`

  StreamLayer specifies the type of the stream’s next layer, for example a Boost.Asio TCP socket. ChannelT is the type of the stream’s native handle; it defaults to `TLS::Channel` and should not be specified manually.

  template<typename ...`Args`>
  explicit `Stream`(Context &context, Args&&... args)

  Construct a new TLS stream. The context parameter will be used to set up the underlying native handle, i.e. the TLS::Client, when `handshake` is called. Using code must ensure the context is kept alive for the lifetime of the stream. The further args will be forwarded to the next layer’s constructor.

  template<typename ...`Args`>
  explicit `Stream`(Arg &&arg, Context &context)

  Convenience constructor for `boost::asio::ssl::stream` compatibility. The parameters have the same meaning as for the first constructor, but their order is changed and only one argument can be passed to the next layer constructor.

  void `handshake`(Connection_Side side, boost::system::error_code &ec)

  Set up the native handle and perform the TLS handshake. As only the client side of the stream is currently implemented, side should be `Connection_Side::CLIENT`.

  void `handshake`(Connection_Side side)

  Overload of `handshake` that throws an exception if an error occurs.

  template<typename `HandshakeHandler`>
  DEDUCED `async_handshake`(Connection_Side side, HandshakeHandler &&handler)

  Asynchronous variant of `handshake`. The function returns immediately and calls the handler callback function after performing asynchronous I/O to complete the TLS handshake. The return type is an automatically deduced specialization of `boost::asio::async_result`, depending on the HandshakeHandler type.

  void `shutdown`(boost::system::error_code &ec)

  Calls `TLS::Channel::close` on the native handle and writes the TLS alert to the next layer.

  void `shutdown`()

  Overload of `shutdown` that throws an exception if an error occurs.

  template<typename `MutableBufferSequence`>
  std::size_t `read_some`(const MutableBufferSequence &buffers, boost::system::error_code &ec)

  Reads encrypted data from the next layer, decrypts it, and writes it into the provided buffers. If an error occurs, error_code is set. Returns the number of bytes read.

  template<typename `MutableBufferSequence`>
  std::size_t `read_some`(const MutableBufferSequence &buffers)

  Overload of `read_some` that throws an exception if an error occurs.

  template<typename `MutableBufferSequence`, typename `ReadHandler`>
  DEDUCED `async_read_some`(const MutableBufferSequence &buffers, ReadHandler &&handler)

  Asynchronous variant of `read_some`. The function returns immediately and calls the handler callback function after writing the decrypted data into the provided buffers. The return type is an automatically deduced specialization of `boost::asio::async_result`, depending on the ReadHandler type. ReadHandler should suffice the requirements to a Boost.Asio read handler.

  template<typename `ConstBufferSequence`>
  std::size_t `write_some`(const ConstBufferSequence &buffers, boost::system::error_code &ec)

  Encrypts data from the provided buffers and writes it to the next layer. If an error occurs, error_code is set. Returns the number of bytes written.

  template<typename `ConstBufferSequence`>
  std::size_t `write_some`(const ConstBufferSequence &buffers)

  Overload of `write_some` that throws an exception rather than setting an error code.

  template<typename `ConstBufferSequence`, typename `WriteHandler`>
  DEDUCED `async_write_some`(const ConstBufferSequence &buffers, WriteHandler &&handler)

  Asynchronous variant of `write_some`. The function returns immediately and calls the handler callback function after writing the encrypted data to the next layer. The return type is an automatically deduced specialization of `boost::asio::async_result`, depending on the WriteHandler type. WriteHandler should suffice the requirements to a Boost.Asio write handler.

class `TLS`::```Context`

  A helper class to initialize and configure the Stream’s underlying native handle (see `TLS::Client`).

  `Context`(Credentials_Manager &credentialsManager, RandomNumberGenerator &randomNumberGenerator, Session_Manager &sessionManager, Policy &policy, Server_Information serverInfo = Server_Information())

  Constructor for TLS::Context.

  void `set_verify_callback`(Verify_Callback_T callback)

  Set a user-defined callback function for certificate chain verification. This will cause the stream to override the default implementation of the `tls_verify_cert_chain` callback.

### Stream Code Example

    #include <iostream>

    #include <botan/asio_stream.h>
    #include <botan/auto_rng.h>
    #include <botan/certstor_system.h>

    #include <boost/asio.hpp>
    #include <boost/beast.hpp>
    #include <boost/bind.hpp>

    namespace http = boost::beast::http;
    namespace _ = boost::asio::placeholders;

    // very basic credentials manager
    class Credentials_Manager : public Botan::Credentials_Manager
       {
       public:
          Credentials_Manager() {}

          std::vector<Botan::Certificate_Store*>
          trusted_certificate_authorities(const std::string&, const std::string&) override
             {
             return {&cert_store_};
             }

       private:
          Botan::System_Certificate_Store cert_store_;
       };

    // a simple https client based on TLS::Stream
    class client
       {
       public:
          client(boost::asio::io_context&                 io_context,
                 boost::asio::ip::tcp::resolver::iterator endpoint_iterator,
                 http::request<http::string_body>         req)
             : request_(req)
             , ctx_(credentials_mgr_,
                    rng_,
                    session_mgr_,
                    policy_,
                    Botan::TLS::Server_Information())
             , stream_(io_context, ctx_)
             {
             boost::asio::async_connect(stream_.lowest_layer(), endpoint_iterator,
                                        boost::bind(&client::handle_connect, this, _::error));
             }

          void handle_connect(const boost::system::error_code& error)
             {
             if(error)
                {
                std::cout << "Connect failed: " << error.message() << "\n";
                return;
                }
             stream_.async_handshake(Botan::TLS::Connection_Side::CLIENT,
                                     boost::bind(&client::handle_handshake, this, _::error));
             }

          void handle_handshake(const boost::system::error_code& error)
             {
             if(error)
                {
                std::cout << "Handshake failed: " << error.message() << "\n";
                return;
                }
             http::async_write(stream_, request_,
                               boost::bind(&client::handle_write, this, _::error, _::bytes_transferred));
             }

          void handle_write(const boost::system::error_code& error, size_t)
             {
             if(error)
                {
                std::cout << "Write failed: " << error.message() << "\n";
                return;
                }
             http::async_read(stream_, reply_, response_,
                              boost::bind(&client::handle_read, this, _::error, _::bytes_transferred));
             }

          void handle_read(const boost::system::error_code& error, size_t)
             {
             if(!error)
                {
                std::cout << "Reply: ";
                std::cout << response_.body() << "\n";
                }
             else
                {
                std::cout << "Read failed: " << error.message() << "\n";
                }
             }

       private:
          http::request<http::dynamic_body> request_;
          http::response<http::string_body> response_;
          boost::beast::flat_buffer         reply_;

          Botan::TLS::Session_Manager_Noop session_mgr_;
          Botan::AutoSeeded_RNG            rng_;
          Credentials_Manager              credentials_mgr_;
          Botan::TLS::Policy               policy_;

          Botan::TLS::Context                              ctx_;
          Botan::TLS::Stream<boost::asio::ip::tcp::socket> stream_;
       };

    int main()
       {
       boost::asio::io_context io_context;

       boost::asio::ip::tcp::resolver           resolver(io_context);
       boost::asio::ip::tcp::resolver::query    query("botan.randombit.net", "443");
       boost::asio::ip::tcp::resolver::iterator iterator = resolver.resolve(query);

       http::request<http::string_body> req;
       req.version(11);
       req.method(http::verb::get);
       req.target("/news.html");
       req.set(http::field::host, "botan.randombit.net");

       client c(io_context, iterator, req);

       io_context.run();
       }

## TLS Session Encryption

A unified format is used for encrypting TLS sessions either for durable storage (on client or server) or when creating TLS session tickets. This format is not stable even across the same major version.

The current session encryption scheme was introduced in 2.13.0, replacing the format previously used since 1.11.13.

Session encryption accepts a key of any length, though for best security a key of 256 bits should be used. This master key is used to key an instance of HMAC using the SHA-512/256 hash.

First a “key name” or identifier is created, by HMAC’ing the fixed string “BOTAN TLS SESSION KEY NAME” and truncating to 4 bytes. This is the initial prefix of the encrypted session, and will remain fixed as long as the same ticket key is used. This allows quickly rejecting sessions which are encrypted using an unknown or incorrect key.

Then a key used for AES-256 in GCM mode is created by first choosing a 128 bit random seed, and HMAC’ing it to produce a 256-bit value. This means for any one master key as many as 2128 GCM keys can be created. This is done because NIST recommends that when using random nonces no one GCM key be used to encrypt more than 232 messages (to avoid the possiblity of nonce reuse).

A random 96-bit nonce is created and included in the header.

AES in GCM is used to encrypt and authenticate the serialized session. The key name, key seed, and AEAD nonce are all included as additional data.