=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);

Deprecated:

 #if OPENSSL_API_COMPAT < 0x10100000L
 void SSL_COMP_free_compression_methods(void)
 #endif

=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>.

In versions of OpenSSL prior to 1.1.0 SSL_COMP_free_compression_methods() freed
the internal table of compression methods that were built internally, and
possibly augmented by adding SSL_COMP_add_compression_method(). However this is
now unnecessary from version 1.1.0.  No explicit initialisation or
de-initialisation is necessary. See L<OPENSSL_init_crypto(3)> and
L<OPENSSL_init_ssl(3)>. From OpenSSL 1.1.0 calling this function does nothing.

=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
COMP_zlib() compression method available.

=head1 WARNINGS

Once the identities of the compression methods for the TLS protocol have
been standardized, the compression API will most likely be changed. Using
it in the current state is not recommended.

=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

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

=head1 COPYRIGHT

Copyright 2001-2016 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
L<https://www.openssl.org/source/license.html>.

=cut
