TLSv1_2_method, TLSv1_2_server_method, TLSv1_2_client_method, SSL_CTX_new,
  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
 #include <openssl/ssl.h>
 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
SSL_CTX_new() creates a new SSL_CTX object as framework to
  establish TLS/SSL or DTLS enabled connections. An SSL_CTX object is
  reference counted. Creating an SSL_CTX object for the first time
  increments the reference count. Freeing it (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.
The SSL_CTX object uses method as connection method. The methods exist in
  a generic type (for client and server use), a server only type, and a client
  only type. method can be of the following types:
  - 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.
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. 
SSL_CTX_set_options(3), SSL_CTX_free(3), SSL_accept(3),
  SSL_CTX_set_min_proto_version(3), ssl(7),
  SSL_set_connect_state(3)
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.
Copyright 2000-2019 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the OpenSSL license (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>.