=pod

=head1 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

=head1 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 B<OPENSSL_API_COMPAT> with a suitable version value,
see L<openssl_user_macros(7)>:

 void SSL_COMP_free_compression_methods(void);

=head1 DESCRIPTION

SSL_COMP_add_compression_method() adds the compression method B<cm> with
the identifier B<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 B<comp>.

SSL_COMP_get_id() returns the id of the compression method B<comp>.

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

=head1 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
following compression methods available:

=over 4

=item COMP_zlib()

=item COMP_brotli()

=item COMP_brotli_oneshot()

=item COMP_zstd()

=item COMP_zstd_oneshot()

=back

=head1 RETURN VALUES

SSL_COMP_add_compression_method() may return the following values:

=over 4

=item Z<>0

The operation succeeded.

=item Z<>1

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

=back

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.

=head1 SEE ALSO

L<ssl(7)>

=head1 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.

=head1 COPYRIGHT

Copyright 2001-2020 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
L<https://www.openssl.org/source/license.html>.

=cut
