Transport Layer Security (TLS) ======================================== .. versionadded:: 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 :cpp:func:`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 analagous 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. .. cpp:function:: void tls_emit_data(const byte 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 asyncronous write. For TLS all writes must occur *in the order requested*. For DTLS this ordering is not strictly required, but is still recommended. .. cpp:function:: void tls_record_received(uint64_t rec_no, const byte 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, cooresponding to reordered or lost datagrams. .. cpp:function:: 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. .. cpp:function:: 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. .. cpp:function:: std::string tls_server_choose_app_protocol(const std::vector& 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. .. cpp:function:: 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. .. cpp:function:: void tls_log_error(const char* msg) Optional logging for an error message. (Not currently used) .. cpp:function:: void tls_log_debug(const char* msg) Optional logging for an debug message. (Not currently used) .. cpp:function:: void tls_log_debug_bin(const char* descr, const uint8_t val[], size_t len) Optional logging for an debug value. (Not currently used) 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: .. cpp:class:: TLS::Channel .. cpp:function:: size_t received_data(const byte buf[], size_t buf_size) .. cpp:function:: size_t received_data(const std::vector& 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. .. cpp:function:: void send(const byte buf[], size_t buf_size) .. cpp:function:: void send(const std::string& str) .. cpp:function:: void send(const std::vector& 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 connnection has already ended due to an error) an exception will be thrown. .. cpp:function:: void close() A close notification is sent to the counterparty, and the internal state is cleared. .. cpp:function:: void send_alert(const Alert& alert) Some other alert is sent to the counterparty. If the alert is fatal, the internal state is cleared. .. cpp:function:: 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. .. cpp:function:: bool is_closed() Returns true if and only if either a close notification or a fatal alert message have been either sent or received. .. cpp:function:: 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. .. cpp:function:: 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. .. cpp:function:: std::vector 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. .. cpp:function:: 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_client: TLS Clients ---------------------------------------- .. cpp:class:: TLS::Client .. cpp:function:: 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& next_protocols = {}, \ 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 :cpp:class:`TLS::Session_Manager_In_Memory` which caches connections for the lifetime of a single process. See :ref:`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 :doc:`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. 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 :cpp:func:`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 for a TLS client using BSD sockets is in `src/cli/tls_client.cpp` TLS Servers ---------------------------------------- .. cpp:class:: TLS::Server .. cpp:function:: 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 similiarly to the :ref:`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 for a TLS server using asio is in `src/cli/tls_proxy.cpp`. .. _tls_sessions: 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: .. cpp:class:: TLS::Session .. cpp:function:: Protocol_Version version() const Returns the :cpp:class:`protocol version ` that was negotiated .. cpp:function:: Ciphersuite ciphersite() const Returns the :cpp:class:`ciphersuite ` that was negotiated. .. cpp:function:: 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. .. cpp:function:: std::vector peer_certs() const Returns the certificate chain of the peer .. cpp:function:: std::string srp_identifier() const If an SRP ciphersuite was used, then this is the identifier that was used for authentication. .. cpp:function:: bool secure_renegotiation() const Returns ``true`` if the connection was negotiated with the correct extensions to prevent the renegotiation attack. .. cpp:function:: std::vector 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. Currently the implementation encrypts the session using AES-256 in GCM mode with a random nonce. .. cpp:function:: static Session decrypt(const byte 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. .. cpp:function:: secure_vector 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: 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. .. cpp:class:: TLS::Session_Mananger .. cpp:function:: 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. .. cpp:function:: void remove_entry(const std::vector& session_id) Remove the session identified by *session_id*. Future attempts at resumption should fail for this session. .. cpp:function:: bool load_from_session_id(const std::vector& 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. .. cpp:function:: bool load_from_server_info(const Server_Information& server, \ Session& session) Attempt to resume a session with a known server. .. cpp:function:: 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. .. _tls_session_manager_inmem: 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. .. cpp:class:: TLS::Session_Managers_In_Memory .. cpp:function:: 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. .. cpp:class:: TLS::Session_Manager_SQLite .. cpp:function:: 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. .. cpp:class:: TLS::Policy .. cpp:function:: std::vector 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", "AES-256/CCM", "AES-128/CCM", "AES-256", "AES-128" Also allowed: "AES-256/CCM(8)", "AES-128/CCM(8)", "Camellia-256/GCM", "Camellia-128/GCM", "Camellia-256", "Camellia-128" Also allowed (though currently experimental): "AES-128/OCB(12)", "AES-256/OCB(12)" 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. .. cpp:function:: std::vector 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. .. cpp:function:: std::vector 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" 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. .. cpp:function:: std::vector 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). .. cpp:function:: std::vector allowed_signature_methods() const Default: "ECDSA", "RSA" Also allowed (disabled by default): "DSA", "" (empty string meaning anonymous) .. note:: DSA authentication is deprecated and will be removed in a future release. .. cpp:function:: std::vector allowed_ecc_curves() const Return a list of ECC curves we are willing to use, in order of preference. The default ordering puts the best performing ECC first. Default: "x25519", "secp256r1", "secp521r1", "secp384r1", "brainpool256r1", "brainpool384r1", "brainpool512r1" No other values are currently defined. .. cpp:function:: 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 .. cpp:function:: std::vector compression() const Return the list of compression methods we are willing to use, in order of preference. Default is null compression only. .. note:: TLS data compression is not currently supported. .. cpp:function:: 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 v1.0 or higher and DTLS v1.2 or higher. .. cpp:function:: 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. .. cpp:function:: bool negotiate_heartbeat_support() const If this function returns true, clients will offer the heartbeat support extension, and servers will respond to clients offering the extension. Otherwise, clients will not offer heartbeat support and servers will ignore clients offering heartbeat support. If this returns true, callers should expect to handle heartbeat data in their ``alert_cb``. Default: false .. cpp:function:: 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 .. cpp:function:: 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 .. cpp:function:: std::string dh_group() const For ephemeral Diffie-Hellman key exchange, the server sends a group parameter. Return a string specifying the group parameter a server should use. Default: 2048 bit IETF IPsec group ("modp/ietf/2048") .. cpp:function:: 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: 1024 bits .. cpp:function:: size_t minimum_rsa_bits() const Minimum accepted RSA key size. Default 2048 bits. .. cpp:function:: size_t minimum_dsa_group_size() const Minimum accepted DSA key size. Default 2048 bits. .. cpp:function:: size_t minimum_ecdsa_group_size() const Minimum size for ECDSA keys (256 bits). .. cpp:function:: size_t minimum_ecdh_group_size() const Minimum size for ECDH keys (255 bits). .. cpp:function:: 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`. .. cpp:function:: 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 .. cpp:function:: 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 ---------------------------------------- .. cpp:class:: TLS::Ciphersuite .. cpp:function:: u16bit ciphersuite_code() const Return the numerical code for this ciphersuite .. cpp:function:: std::string to_string() const Return the ful name of ciphersuite (for example "RSA_WITH_RC4_128_SHA" or "ECDHE_RSA_WITH_AES_128_GCM_SHA256") .. cpp:function:: std::string kex_algo() const Return the key exchange algorithm of this ciphersuite .. cpp:function:: std::string sig_algo() const Return the signature algorithm of this ciphersuite .. cpp:function:: std::string cipher_algo() const Return the cipher algorithm of this ciphersuite .. cpp:function:: std::string mac_algo() const Return the authentication algorithm of this ciphersuite .. _tls_alerts: TLS Alerts ---------------------------------------- A ``TLS::Alert`` is passed to every invocation of a channel's *alert_cb*. .. cpp:class:: TLS::Alert .. cpp:function:: is_valid() const Return true if this alert is not a null alert .. cpp:function:: 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. .. cpp:function:: Type type() const Returns the type of the alert as an enum .. cpp:function:: 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: .. cpp:class:: TLS::Protocol_Version .. cpp:enum:: Version_Code ``TLS_V10``, ``TLS_V11``, ``TLS_V12``, ``DTLS_V10``, ``DTLS_V12`` .. cpp:function:: Protocol_Version(Version_Code named_version) Create a specific version .. cpp:function:: byte major_version() const Returns major number of the protocol version .. cpp:function:: byte minor_version() const Returns minor number of the protocol version .. cpp:function:: std::string to_string() const Returns string description of the version, for instance "TLS v1.1" or "DTLS v1.0". .. cpp:function:: static Protocol_Version latest_tls_version() Returns the latest version of the TLS protocol known to the library (currently TLS v1.2) .. cpp:function:: static Protocol_Version latest_dtls_version() Returns the latest version of the DTLS protocol known to the library (currently DTLS v1.2)