SSL_CTX_new(3)

NAME
TLSv1_2_method, TLSv1_2_server_method, TLSv1_2_client_method, SSL_CTX_new, SSL_CTX_new_ex, SSL_CTX_up_ref, SSLv3_method, SSLv3_server_method, SSLv3_client_method, TLSv1_method, TLSv1_server_method, TLSv1_client_method, TLSv1_1_method, TLSv1_1_server_method, TLSv1_1_client_method, TLS_method, TLS_server_method, TLS_client_method, SSLv23_method, SSLv23_server_method, SSLv23_client_method, DTLS_method, DTLS_server_method, DTLS_client_method, DTLSv1_method, DTLSv1_server_method, DTLSv1_client_method, DTLSv1_2_method, DTLSv1_2_server_method, DTLSv1_2_client_method - create a new SSL_CTX object as framework for TLS/SSL or DTLS enabled functions

SYNOPSIS
 #include <openssl/ssl.h>

 SSL_CTX *SSL_CTX_new_ex(OSSL_LIB_CTX *libctx, const char *propq,
                         const SSL_METHOD *method);
 SSL_CTX *SSL_CTX_new(const SSL_METHOD *method);
 int SSL_CTX_up_ref(SSL_CTX *ctx);

 const SSL_METHOD *TLS_method(void);
 const SSL_METHOD *TLS_server_method(void);
 const SSL_METHOD *TLS_client_method(void);

 const SSL_METHOD *SSLv23_method(void);
 const SSL_METHOD *SSLv23_server_method(void);
 const SSL_METHOD *SSLv23_client_method(void);

 #ifndef OPENSSL_NO_SSL3_METHOD
 const SSL_METHOD *SSLv3_method(void);
 const SSL_METHOD *SSLv3_server_method(void);
 const SSL_METHOD *SSLv3_client_method(void);
 #endif

 #ifndef OPENSSL_NO_TLS1_METHOD
 const SSL_METHOD *TLSv1_method(void);
 const SSL_METHOD *TLSv1_server_method(void);
 const SSL_METHOD *TLSv1_client_method(void);
 #endif

 #ifndef OPENSSL_NO_TLS1_1_METHOD
 const SSL_METHOD *TLSv1_1_method(void);
 const SSL_METHOD *TLSv1_1_server_method(void);
 const SSL_METHOD *TLSv1_1_client_method(void);
 #endif

 #ifndef OPENSSL_NO_TLS1_2_METHOD
 const SSL_METHOD *TLSv1_2_method(void);
 const SSL_METHOD *TLSv1_2_server_method(void);
 const SSL_METHOD *TLSv1_2_client_method(void);
 #endif

 const SSL_METHOD *DTLS_method(void);
 const SSL_METHOD *DTLS_server_method(void);
 const SSL_METHOD *DTLS_client_method(void);

 #ifndef OPENSSL_NO_DTLS1_METHOD
 const SSL_METHOD *DTLSv1_method(void);
 const SSL_METHOD *DTLSv1_server_method(void);
 const SSL_METHOD *DTLSv1_client_method(void);
 #endif

 #ifndef OPENSSL_NO_DTLS1_2_METHOD
 const SSL_METHOD *DTLSv1_2_method(void);
 const SSL_METHOD *DTLSv1_2_server_method(void);
 const SSL_METHOD *DTLSv1_2_client_method(void);
 #endif

DESCRIPTION
SSL_CTX_new_ex() creates a new SSL_CTX object, which holds various configuration and data relevant to SSL/TLS or DTLS session establishment. These are later inherited by the SSL object representing an active session. The method parameter specifies whether the context will be used for the client or server side or both - for details see the "NOTES" below. The library context libctx (see OSSL_LIB_CTX(3)) is used to provide the cryptographic algorithms needed for the session. Any cryptographic algorithms that are used by any SSL objects created from this SSL_CTX will be fetched from the libctx using the property query string propq (see "ALGORITHM FETCHING" in crypto(7). Either or both the libctx or propq parameters may be NULL.

SSL_CTX_new() does the same as SSL_CTX_new_ex() except that the default library context is used and no property query string is specified.

An SSL_CTX object is reference counted. Creating an SSL_CTX object for the first time increments the reference count. Freeing the SSL_CTX (using SSL_CTX_free) decrements it. When the reference count drops to zero, any memory or resources allocated to the SSL_CTX object are freed. SSL_CTX_up_ref() increments the reference count for an existing SSL_CTX structure.

An SSL_CTX object should not be changed after it is used to create any SSL objects or from multiple threads concurrently, since the implementation does not provide serialization of access for these cases.

NOTES
On session establishment, by default, no peer credentials verification is done. This must be explicitly requested, typically using SSL_CTX_set_verify(3). For verifying peer certificates many options can be set using various functions such as SSL_CTX_load_verify_locations(3) and SSL_CTX_set1_param(3).

The SSL/(D)TLS implementation uses the X509_STORE_CTX_set_default(3) function to prepare checks for X509_PURPOSE_SSL_SERVER on the client side and X509_PURPOSE_SSL_CLIENT on the server side. The X509_VERIFY_PARAM_set_purpose(3) function can be used, also in conjunction with SSL_CTX_get0_param(3), to override the default purpose of the session.

The SSL_CTX object uses method as the connection method. Three method variants are available: a generic method (for either client or server use), a server-only method, and a client-only method.

The method parameter of SSL_CTX_new_ex() and SSL_CTX_new() can be one of the following:

TLS_method(), TLS_server_method(), TLS_client_method()
    These are the general-purpose version-flexible SSL/TLS methods. The actual protocol version used will be negotiated to the highest version mutually supported by the client and the server. The supported protocols are SSLv3, TLSv1, TLSv1.1, TLSv1.2 and TLSv1.3. Applications should use these methods, and avoid the version-specific methods described below, which are deprecated.

SSLv23_method(), SSLv23_server_method(), SSLv23_client_method()
    These functions do not exist anymore, they have been renamed to TLS_method(), TLS_server_method() and TLS_client_method() respectively. Currently, the old function calls are renamed to the corresponding new ones by preprocessor macros, to ensure that existing code which uses the old function names still compiles. However, using the old function names is deprecated and new code should call the new functions instead.

TLSv1_2_method(), TLSv1_2_server_method(), TLSv1_2_client_method()
    A TLS/SSL connection established with these methods will only understand the TLSv1.2 protocol. These methods are deprecated.

TLSv1_1_method(), TLSv1_1_server_method(), TLSv1_1_client_method()
    A TLS/SSL connection established with these methods will only understand the TLSv1.1 protocol. These methods are deprecated.

TLSv1_method(), TLSv1_server_method(), TLSv1_client_method()
    A TLS/SSL connection established with these methods will only understand the TLSv1 protocol. These methods are deprecated.

SSLv3_method(), SSLv3_server_method(), SSLv3_client_method()
    A TLS/SSL connection established with these methods will only understand the SSLv3 protocol. The SSLv3 protocol is deprecated and should not be used.

DTLS_method(), DTLS_server_method(), DTLS_client_method()
    These are the version-flexible DTLS methods. Currently supported protocols are DTLS 1.0 and DTLS 1.2.

DTLSv1_2_method(), DTLSv1_2_server_method(), DTLSv1_2_client_method()
    These are the version-specific methods for DTLSv1.2. These methods are deprecated.

DTLSv1_method(), DTLSv1_server_method(), DTLSv1_client_method()
    These are the version-specific methods for DTLSv1. These methods are deprecated.

SSL_CTX_new() initializes the list of ciphers, the session cache setting, the callbacks, the keys and certificates and the options to their default values.

TLS_method(), TLS_server_method(), TLS_client_method(), DTLS_method(), DTLS_server_method() and DTLS_client_method() are the version-flexible methods. All other methods only support one specific protocol version. Use the version-flexible methods instead of the version specific methods.

If you want to limit the supported protocols for the version flexible methods you can use SSL_CTX_set_min_proto_version(3), SSL_set_min_proto_version(3), SSL_CTX_set_max_proto_version(3) and SSL_set_max_proto_version(3) functions. Using these functions it is possible to choose e.g. TLS_server_method() and be able to negotiate with all possible clients, but to only allow newer protocols like TLS 1.0, TLS 1.1, TLS 1.2 or TLS 1.3.

The list of protocols available can also be limited using the SSL_OP_NO_SSLv3, SSL_OP_NO_TLSv1, SSL_OP_NO_TLSv1_1, SSL_OP_NO_TLSv1_3, SSL_OP_NO_TLSv1_2 and SSL_OP_NO_TLSv1_3 options of the SSL_CTX_set_options(3) or SSL_set_options(3) functions, but this approach is not recommended. Clients should avoid creating "holes" in the set of protocols they support. When disabling a protocol, make sure that you also disable either all previous or all subsequent protocol versions. In clients, when a protocol version is disabled without disabling all previous protocol versions, the effect is to also disable all subsequent protocol versions.

The SSLv3 protocol is deprecated and should generally not be used. Applications should typically use SSL_CTX_set_min_proto_version(3) to set the minimum protocol to at least TLS1_VERSION.

RETURN VALUES
The following return values can occur:

NULL
    The creation of a new SSL_CTX object failed. Check the error stack to find out the reason.

Pointer to an SSL_CTX object
    The return value points to an allocated SSL_CTX object.

    SSL_CTX_up_ref() returns 1 for success and 0 for failure.

SEE ALSO
SSL_CTX_set_options(3), SSL_CTX_free(3), X509_STORE_CTX_set_default(3), SSL_CTX_set_verify(3), SSL_CTX_set1_param(3), SSL_CTX_get0_param(3), SSL_connect(3), SSL_accept(3), SSL_CTX_set_min_proto_version(3), ssl(7), SSL_set_connect_state(3)

HISTORY
Support for SSLv2 and the corresponding SSLv2_method(), SSLv2_server_method() and SSLv2_client_method() functions where removed in OpenSSL 1.1.0.

SSLv23_method(), SSLv23_server_method() and SSLv23_client_method() were deprecated and the preferred TLS_method(), TLS_server_method() and TLS_client_method() functions were added in OpenSSL 1.1.0.

All version-specific methods were deprecated in OpenSSL 1.1.0.

SSL_CTX_new_ex() was added in OpenSSL 3.0.

COPYRIGHT
Copyright 2000-2023 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at <https://www.openssl.org/source/license.html>.