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>.