NAME

       SSL_COMP_add_compression_method, SSL_COMP_get_compression_methods, SSL_COMP_get0_name, SSL_COMP_get_id,
       SSL_COMP_free_compression_methods - handle SSL/TLS integrated compression methods

SYNOPSIS

        #include <openssl/ssl.h>

        int SSL_COMP_add_compression_method(int id, COMP_METHOD *cm);
        STACK_OF(SSL_COMP) *SSL_COMP_get_compression_methods(void);
        const char *SSL_COMP_get0_name(const SSL_COMP *comp);
        int SSL_COMP_get_id(const SSL_COMP *comp);

       The following function has been deprecated since OpenSSL 1.1.0, and can be hidden entirely by defining
       OPENSSL_API_COMPAT with a suitable version value, see openssl_user_macros(7):

        void SSL_COMP_free_compression_methods(void);

DESCRIPTION

       SSL_COMP_add_compression_method() adds the compression method cm with the identifier id to the list of
       available compression methods. This list is globally maintained for all SSL operations within this
       application.  It cannot be set for specific SSL_CTX or SSL objects.

       SSL_COMP_get_compression_methods() returns a stack of all of the available compression methods or NULL on
       error.

       SSL_COMP_get0_name() returns the name of the compression method comp.

       SSL_COMP_get_id() returns the id of the compression method comp.

       SSL_COMP_free_compression_methods() releases any resources acquired to maintain the internal table of
       compression methods.

NOTES

       The TLS standard (or SSLv3) allows the integration of compression methods into the communication. The TLS
       RFC does however not specify compression methods or their corresponding identifiers, so there is
       currently no compatible way to integrate compression with unknown peers. It is therefore currently not
       recommended to integrate compression into applications. Applications for non-public use may agree on
       certain compression methods. Using different compression methods with the same identifier will lead to
       connection failure.

       An OpenSSL client speaking a protocol that allows compression (SSLv3, TLSv1) will unconditionally send
       the list of all compression methods enabled with SSL_COMP_add_compression_method() to the server during
       the handshake.  Unlike the mechanisms to set a cipher list, there is no method available to restrict the
       list of compression method on a per connection basis.

       An OpenSSL server will match the identifiers listed by a client against its own compression methods and
       will unconditionally activate compression when a matching identifier is found. There is no way to
       restrict the list of compression methods supported on a per connection basis.

       If enabled during compilation, the OpenSSL library will have the COMP_zlib() compression method
       available.

RETURN VALUES

       SSL_COMP_add_compression_method() may return the following values:

       0   The operation succeeded.

       1   The operation failed. Check the error queue to find out the reason.

       SSL_COMP_get_compression_methods() returns the stack of compressions methods or NULL on error.

       SSL_COMP_get0_name() returns the name of the compression method or NULL on error.

       SSL_COMP_get_id() returns the name of the compression method or -1 on error.

SEE ALSO

       ssl(7)

HISTORY

       The    SSL_COMP_free_compression_methods()    function    was   deprecated   in   OpenSSL   1.1.0.    The
       SSL_COMP_get0_name() and SSL_comp_get_id() functions were added in OpenSSL 1.1.0d.

COPYRIGHT

       Copyright 2001-2021 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>.

3.0.13                                             2025-09-18              SSL_COMP_ADD_COMPRESSION_METHOD(3SSL)