Transport Layer Security (TLS) ======================================== .. versionadded:: 1.11.0 Botan supports both client and server implementations of the SSL/TLS protocols, including SSL v3, TLS v1.0, TLS v1.1, and TLS v1.2 (the insecure and obsolete SSL v2 protocol is not supported, beyond processing SSL v2 client hellos which some clients still send for backwards compatability with ancient servers). There is also some initial support for DTLS (v1.0 and v1.2), a variant of TLS adapted for operation on datagram sockets such as UDP. However currently many important DTLS features (including timeouts and retransmission during handshaking) are not yet implemented. 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, and whenever the application receives some data from the counterparty it passes that information to TLS using :cpp:func:`TLS::Channel::received_data`. If the data passed in results in a handshake completing, then the user provided ``handshake_complete`` is called, and if some application data being received, or a TLS :ref:`alert ` is received, the another user provided callback, hereafter ``proc_fn``, is called. (If the reader is familiar with OpenSSL's BIO layer, it might be analagous to saying the only way of interacting with the SSL protocol stack is via a `BIO_mem` I/O abstraction.) The callbacks that TLS calls have the signatures .. cpp:function:: void output_fn(const byte data[], size_t data_len) TLS requests that all bytes of *data* be queued up to send. After this function returns, *data* will be overwritten, so a copy should be made if the callback cannot send the data immediately. .. cpp:function:: void proc_fn(const byte data[], size_t data_len, \ const TLS::Alert& alert) Called whenever application data or an alert is received from the other side of the connection, in which case *data* and *data_len* specify the data received. This array will be overwritten sometime after the callback returns, so again a copy should be made if need be. If :cpp:func:`TLS::Alert::is_valid` is true then *alert* specifies a valid alert that was received. Note that if received before the handshake has completed, an attacker can easily insert false alert values. .. note:: Currently, if an alert was passed in then no other data was provided. This allows an appliction to easily distinguish between data sent before and after the alert was received. .. cpp:function:: bool handshake_complete(const TLS::Session& session) Called whenever a negotiation completes. This can happen more than once on any connection. The *session* parameter provides information about the session which was 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. You can of course use tools like ``std::bind`` to bind additional parameters to your callback functions. 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) 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) If the connection has completed the initial handshake process, the data provided is sent to the counterparty as TLS traffic. Otherwise, an exception is thrown. .. cpp:function:: void close() A close notification is sent to the counterparty, and 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:: 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_complete* 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:: TLS::Client( \ std::function output_fn, \ std::function proc_fn, \ std::function handshake_complete, \ TLS::Session_Manager& session_manager, \ Credentials_Manager& credendials_manager, \ const TLS::Policy& policy, \ RandomNumberGenerator& rng, \ const Server_Information& server_info, \ const Protocol_Version offer_version, \ std::function > next_protocol) Initialize a new TLS client. The constructor will immediately initiate a new session. The *output_fn* callback will be called with output that should be sent to the counterparty. For instance this will be called immediately from the constructor after the client hello message is constructed. An implementation of *output_fn* is allowed to defer the write (for instance if writing when the callback occurs would block), but should eventually write the data to the counterparty *in order*. The *proc_fn* will be called with data sent by the counterparty after it has been processed. The byte array and size_t represent the plaintext; the :cpp:class:`TLS::Alert` value provides notification if the counterparty sent an alert via the TLS alert system. Possible values of alert data are included in the Alert_Type enum. Particularly relevant is the CLOSE_NOTIFY value. The *handshake_complete* function is called when a handshake (either initial or renegotiation) is completed. The return value of the callback specifies if the session should be cached for later resumption. If the function for some reason desires to prevent the connection from completing, it should throw an exception (preferably a TLS::Exception, which can provide more specific alert information to the counterparty). The :cpp:class:`TLS::Session` provides information about the session that was just established. 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 intformation; see :doc:`credentials_manager` for more information. Use *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 *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 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. Setting *offer_version* is also used to offer DTLS instead of TLS; use :cpp:func:`TLS::Protocol_Version::latest_dtls_version`. .. warning:: Implementing such a backdown strategy allows an attacker to downgrade your connection to the weakest protocol that both you and the server support. The optional *next_protocol* callback is called if the server indicates it supports the next protocol notification extension. The callback wlil be called with a list of protocol names that the server advertises, and the client can select from them or return an unadvertised protocol. A simple TLS client example: .. literalinclude:: examples/tls_client.cpp TLS Servers ---------------------------------------- .. cpp:class:: TLS::Server .. cpp:function:: TLS::Server( \ std::function output_fn, \ std::function proc_fn, \ std::function handshake_complete, \ TLS::Session_Manager& session_manager, \ Credentials_Manager& creds, \ const TLS::Policy& policy, \ RandomNumberGenerator& rng, \ const std::vector& protocols) The first 7 arguments are treated similiarly to the :ref:`client `. The final (optional) argument, protocols, specifies the protocols the server is willing to advertise it supports. .. cpp:function:: std::string TLS::Server::next_protocol() const If a handshake has completed, and if the client indicated a next protocol (ie, the protocol that it intends to run over this TLS session) this return value will specify it. The next protocol extension is somewhat unusual in that it applies to the connection rather than the session. The next protocol can not change during a renegotiation, but might change across different connections using that session. A TLS server that can handle concurrent connections using asio: .. literalinclude:: examples/asio_tls_server.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. There are also functions for serialization and deserializing sessions: .. cpp:class:: TLS::Session .. 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 uses AES-256 in CBC mode with a SHA-256 HMAC. The keys for these are derived from *key* using KDF2(SHA-256). .. 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 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. .. cpp:class:: TLS::Session_Manager_In_Memory .. cpp:function:: Session_Manager_In_Memory(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. .. cpp:class:: TLS::Policy .. 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: True for all known protocol versions .. cpp:function:: std::vector allowed_ciphers() const Returns the list of ciphers we are willing to negotiate, in order of preference. Default: "AES-256", "AES-128", "ARC4", "3DES" Also allowed: "Camellia-256", "Camellia-128", "SEED" .. note:: ARC4 will never be negotiated in DTLS due to protocol limitations .. 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: "SHA-384", "SHA-256", "SHA-1" Also allowed: "MD5" .. 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: "ECDH", "DH", "RSA" Also allowed: "SRP_SHA", "ECDHE_PSK", "DHE_PSK", "PSK" .. cpp:function:: std::vector allowed_signature_hashes() const Returns the list of algorithms we are willing to use for public key signatures, in order of preference. Default: "SHA-512", "SHA-384", "SHA-256", "SHA-224", "SHA-1" Also allowed (although **not recommended**): "MD5" .. 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", "DSA" Also allowed: "" (meaning anonymous) .. cpp:function:: std::vector allowed_ecc_curves() const Return a list of ECC curves we are willing to use, in order of preference. Default: "secp521r1", "secp384r1", "secp256r1", "secp256k1", "secp224r1", "secp224k1" Also allowed: "secp192r1", "secp192k1", "secp160r2", "secp160r1", "secp160k1" .. cpp:function:: std::vector compression() const Return the list of compression methods we are willing to use, in order of preference. .. note:: TLS compression is not currently supported. .. 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. .. 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: true .. 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:: DL_Group dh_group() const For ephemeral Diffie-Hellman key exchange, the server sends a group parameter. Return 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:: 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 name of ciphersuite .. 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 *proc_fn*. Most of the time, no alert was sent and so the alert will be of type ``NULL_ALERT``. .. 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:type:: enum Version_Code ``SSL_V3``, ``TLS_V10``, ``TLS_V11``, ``TLS_V12``, ``DTLS_V10``, ``DTLS_V12`` .. cpp:function:: static Protocol_Version latest_tls_version() Returns the latest version of TLS supported by this implementation (currently TLS v1.2) .. cpp:function:: static Protocol_Version latest_dtls_version() Returns the latest version of DTLS supported by this implementation (currently DTLS v1.2) .. 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 "SSL v3", "TLS v1.1", or "DTLS v1.0". .. cpp:function:: static Protocol_Version latest_tls_version() Returns the latest version of the TLS protocol known the the library (currently TLS v1.2) .. cpp:function:: static Protocol_Version latest_dtls_version() Returns the latest version of the DTLS protocol known the the library (currently DTLS v1.2)