SSL(3) | Library Functions Manual | SSL(3)

# NAME

`ssl` — OpenSSL TLS library

# DESCRIPTION

The `ssl` library implements the Transport Layer Security (TLS) protocol, the successor to the Secure Sockets Layer (SSL) protocol.

An SSL_CTX object is created as a framework to establish TLS/SSL enabled connections (see SSL_CTX_new(3)). Various options regarding certificates, algorithms, etc., can be set in this object.

When a network connection has been created, it can be assigned to an SSL object. After the SSL object has been created using SSL_new(3), SSL_set_fd(3) or SSL_set_bio(3) can be used to associate the network connection with the object.

Then the TLS/SSL handshake is performed using SSL_accept(3) or SSL_connect(3) respectively. SSL_read(3) and SSL_write(3) are used to read and write data on the TLS/SSL connection. SSL_shutdown(3) can be used to shut down the TLS/SSL connection.

# DATA STRUCTURES

Currently the `ssl` library functions deal with the following data structures:

SSL_METHOD (SSL Method)
  That's a dispatch structure describing the internal `ssl` library methods/functions which implement the various protocol versions. It's needed to create an SSL_CTX. See TLS_method(3) for constructors.
SSL_CIPHER (SSL Cipher)
  This structure holds the algorithm information for a particular cipher which is a core part of the SSL/TLS protocol. The available ciphers are configured on an SSL_CTX basis and the actually used ones are then part of the SSL_SESSION.
SSL_CTX (SSL Context)
  That's the global context structure which is created by a server or client once per program lifetime and which holds mainly default values for the SSL structures which are later created for the connections.
SSL_SESSION (SSL Session)
  This is a structure containing the current TLS/SSL session details for a connection: SSL_CIPHERs, client and server certificates, keys, etc.
SSL (SSL Connection)
  That's the main SSL/TLS structure which is created by a server or client per established connection. This actually is the core structure in the SSL API. At run-time the application usually deals with this structure which has links to mostly all other structures.

# HEADER FILES

Currently the `ssl` library provides the following C header files containing the prototypes for the data structures and functions:

ssl.h
  That's the common header file for the SSL/TLS API. Include it into your program to make the API of the `ssl` library available. It internally includes both more private SSL headers and headers from the crypto library. Whenever you need hardcore details on the internals of the SSL API, look inside this header file.
ssl3.h
  That's the sub header file dealing with the SSLv3 protocol only.
  Usually you don't have to include it explicitly because it's already included by ssl.h.
tls1.h
  That's the sub header file dealing with the TLSv1 protocol only.
  Usually you don't have to include it explicitly because it's already included by ssl.h.

# API FUNCTIONS

## Ciphers

The following pages describe functions acting on SSL_CIPHER objects: SSL_get_ciphers(3), SSL_get_current_cipher(3), SSL_CIPHER_get_name(3)

## Protocol contexts

The following pages describe functions acting on SSL_CTX objects.

Constructors and destructors: SSL_CTX_new(3), SSL_CTX_set_ssl_version(3), SSL_CTX_free(3)

Certificate configuration: SSL_CTX_add_extra_chain_cert(3), SSL_CTX_get0_certificate(3), SSL_CTX_load_verify_locations(3), SSL_CTX_set_cert_store(3), SSL_CTX_set_cert_verify_callback(3), SSL_CTX_set_client_cert_cb(3), SSL_CTX_set_default_passwd_cb(3), SSL_CTX_set_tlsext_status_cb(3)

Session configuration: SSL_CTX_add_session(3), SSL_CTX_flush_sessions(3), SSL_CTX_sess_number(3), SSL_CTX_sess_set_cache_size(3), SSL_CTX_sess_set_get_cb(3), SSL_CTX_sessions(3), SSL_CTX_set_session_cache_mode(3), SSL_CTX_set_timeout(3), SSL_CTX_set_tlsext_ticket_key_cb(3)

Various configuration: SSL_CTX_get_ex_new_index(3), SSL_CTX_set_tlsext_servername_callback(3)

## Common configuration of contexts and connections

The functions on the following pages each come in two variants: one to directly configure a single SSL connection and another to be called on an SSL_CTX object, to set up defaults for all future SSL connections created from that context.

Protocol and algorithm configuration: SSL_CTX_set_alpn_select_cb(3), SSL_CTX_set_cipher_list(3), SSL_CTX_set_min_proto_version(3), SSL_CTX_set_options(3), SSL_CTX_set_security_level(3), SSL_CTX_set_tlsext_use_srtp(3), SSL_CTX_set_tmp_dh_callback(3), SSL_CTX_set1_groups(3)

Certificate configuration: SSL_CTX_add1_chain_cert(3), SSL_CTX_get_verify_mode(3), SSL_CTX_set_client_CA_list(3), SSL_CTX_set_max_cert_list(3), SSL_CTX_set_verify(3), SSL_CTX_use_certificate(3), SSL_get_client_CA_list(3) SSL_set1_param(3)

Session configuration: SSL_CTX_set_generate_session_id(3), SSL_CTX_set_session_id_context(3)

Various configuration: SSL_CTX_ctrl(3), SSL_CTX_set_info_callback(3), SSL_CTX_set_mode(3), SSL_CTX_set_msg_callback(3), SSL_CTX_set_quiet_shutdown(3), SSL_CTX_set_read_ahead(3), SSL_set_max_send_fragment(3)

## Sessions

The following pages describe functions acting on SSL_SESSION objects.

Constructors and destructors: SSL_SESSION_new(3), SSL_SESSION_free(3)

Accessors: SSL_SESSION_get_compress_id(3), SSL_SESSION_get_ex_new_index(3), SSL_SESSION_get_id(3), SSL_SESSION_get_protocol_version(3), SSL_SESSION_get_time(3), SSL_SESSION_get0_peer(3), SSL_SESSION_has_ticket(3), SSL_SESSION_set1_id_context(3)

Encoding and decoding: d2i_SSL_SESSION(3), PEM_read_SSL_SESSION(3), SSL_SESSION_print(3)

## Connections

The following pages describe functions acting on SSL connection objects:

Constructors and destructors: SSL_new(3), SSL_dup(3), SSL_free(3), BIO_f_ssl(3)

To change the configuration: SSL_clear(3), SSL_set_SSL_CTX(3), SSL_copy_session_id(3), SSL_set_bio(3), SSL_set_connect_state(3), SSL_set_fd(3), SSL_set_session(3), SSL_set1_host(3), SSL_set_verify_result(3)

To inspect the configuration: SSL_get_certificate(3), SSL_get_default_timeout(3), SSL_get_ex_new_index(3), SSL_get_fd(3), SSL_get_rbio(3), SSL_get_SSL_CTX(3)

To transmit data: DTLSv1_listen(3), SSL_accept(3), SSL_connect(3), SSL_do_handshake(3), SSL_read(3), SSL_read_early_data(3), SSL_renegotiate(3), SSL_shutdown(3), SSL_write(3)

To inspect the state after a connection is established: SSL_export_keying_material(3), SSL_get_client_random(3), SSL_get_ex_data_X509_STORE_CTX_idx(3), SSL_get_peer_cert_chain(3), SSL_get_peer_certificate(3), SSL_get_server_tmp_key(3), SSL_get_servername(3), SSL_get_session(3), SSL_get_shared_ciphers(3), SSL_get_verify_result(3), SSL_get_version(3), SSL_session_reused(3)

To inspect the state during ongoing communication: SSL_get_error(3), SSL_get_shutdown(3), SSL_get_state(3), SSL_num_renegotiations(3), SSL_pending(3), SSL_rstate_string(3), SSL_state_string(3), SSL_want(3)

## Utility functions

SSL_alert_type_string(3), SSL_dup_CA_list(3), SSL_load_client_CA_file(3)

## Obsolete functions

OPENSSL_init_ssl(3), SSL_COMP_get_compression_methods(3), SSL_CTX_set_tmp_rsa_callback(3), SSL_library_init(3), SSL_set_tmp_ecdh(3)

# SEE ALSO

openssl(1), crypto(3), tls_init(3)

# HISTORY

The `ssl` document appeared in OpenSSL 0.9.2.

August 31, 2024 | Debian