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 a 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, etc.
The callbacks for TLS have the signatures
- void
output_fn
(const byte data[], size_t data_len)¶TLS requests that all bytes of data be queued up to send to the counterparty. After this function returns, data will be overwritten, so a copy of the input must be made if the callback cannot send the data immediately.
- void
data_cb
(const byte data[], size_t data_len)¶Called whenever application data 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.
- void
alert_cb
(Alert alert, const byte data[], size_t data_len)¶Called when an alert is received. Normally, data is null and data_len is 0, as most alerts have no associated data. However, if TLS heartbeats (see RFC 6520) were negotiated, and we initiated a heartbeat, then if/when the other party responds,
alert_cb
will be called with whatever data was included in the heartbeat response (if any) along with a psuedo-alert value ofHEARTBEAT_PAYLOAD
.
- bool
handshake_cb
(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:
-
class
TLS::
Channel
¶ -
type std::function<void(const byte[], size_t)>
output_fn
¶
-
type std::function<void(const byte[], size_t)>
data_cb
¶
-
type std::function<bool(const Session&)>
handshake_cb
¶ Typedefs used in the code for the functions described above
-
size_t
received_data
(const byte buf[], size_t buf_size)¶
-
size_t
received_data
(const std::vector<byte> &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 byte buf[], size_t buf_size)¶
-
void
send
(const std::string &str)¶
-
void
send
(const std::vector<byte> &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 beforesend
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.
-
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.
-
type std::function<void(const byte[], size_t)>
TLS Clients¶
-
class
TLS::
Client
¶ -
TLS::
Client
(output_fn output, data_cb data, alert_cb alert, handshake_cb 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, next_protocol_fn npn, size_t reserved_io_buffer_size)¶
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 data_cb will be called with data sent by the counterparty after it has been processed. The byte array and size_t represent the plaintext value and size.
The alert_cb will be called when a protocol alert is received, commonly with a close alert during connection teardown.
The handshake_cb 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
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
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.
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
.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.
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/cmd/tls_client.cpp
TLS Servers¶
-
class
TLS::
Server
¶ -
TLS::
Server
(std::function<void, const byte *, size_t> output_fn, std::function<void, const byte *, size_t> data_cb, std::function<TLS::Alert, const byte *, size_t> alert_cb, TLS::Session_Manager &session_manager, Credentials_Manager &creds, const TLS::Policy &policy, RandomNumberGenerator &rng, const std::vector<std::string> &protocols, bool is_datagram = false, bool reserved_io_buffer_size)¶
-
The first 7 arguments as well as the final argument reserved_io_buffer_size, are treated similiarly to the client. The (optional) argument, protocols, specifies the protocols the server is willing to advertise it supports. The 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 receive a hello without this parameter.
-
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 connection) 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.
Code for a TLS server using asio is in src/cmd/tls_proxy.cpp.
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:
-
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.
-
Protocol_Version
There are also functions for serialization and deserializing sessions:
-
class
TLS::
Session
¶ -
std::vector<byte>
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.
-
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.
-
secure_vector<byte>
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.
-
std::vector<byte>
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<byte> &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<byte> &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 andfalse
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.
-
void
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.
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.
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 compatability) 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.
Default value: “ChaCha20Poly1305”, “AES-256/GCM”, “AES-128/GCM”, “AES-256/CCM”, “AES-128/CCM”, “AES-256/CCM-8”, “AES-128/CCM-8”, “AES-256”, “AES-128”
Also allowed: “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”, “RC4”
Note
RC4 will never be negotiated in DTLS due to protocol limitations
-
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-384”, “SHA-256”, “SHA-1”
Also allowed (although not recommended): “MD5”
-
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: “ECDH”, “DH”, “RSA”
Also allowed: “SRP_SHA”, “ECDHE_PSK”, “DHE_PSK”, “PSK”
-
std::vector<std::string>
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”
Also allowed (although not recommended): “MD5”, “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”, “DSA”
Also allowed: “” (meaning anonymous)
-
std::vector<std::string>
allowed_ecc_curves
() const¶ Return a list of ECC curves we are willing to use, in order of preference.
Default: “brainpool512r1”, “secp521r1”, “brainpool384r1”, “secp384r1”, “brainpool256r1”, “secp256r1”
Also allowed (disabled by default): “secp256k1”, “secp224r1”, “secp224k1”, “secp192r1”, “secp192k1”, “secp160r2”, “secp160r1”, “secp160k1”
-
std::vector<byte>
compression
() const¶ Return the list of compression methods we are willing to use, in order of preference. Default is null compression only.
Note
TLS compression is not currently supported.
-
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.
-
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
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
-
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
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
-
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”)
-
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
-
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)
-
std::vector<std::string>
TLS Ciphersuites¶
-
class
TLS::
Ciphersuite
¶ -
u16bit
ciphersuite_code
() const¶ Return the numerical code for this ciphersuite
-
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”)
-
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
-
u16bit
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
-
static Protocol_Version
latest_tls_version
()¶ Returns the latest version of TLS supported by this implementation (currently TLS v1.2)
-
static Protocol_Version
latest_dtls_version
()¶ Returns the latest version of DTLS supported by this implementation (currently DTLS v1.2)
-
Protocol_Version
(Version_Code named_version)¶ Create a specific version
-
byte
major_version
() const¶ Returns major number of the protocol version
-
byte
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)
-