1=pod
2
3=head1 NAME
4
5SSL_COMP_add_compression_method, SSL_COMP_get_compression_methods,
6SSL_COMP_get0_name, SSL_COMP_get_id, SSL_COMP_free_compression_methods
7- handle SSL/TLS integrated compression methods
8
9=head1 SYNOPSIS
10
11 #include <openssl/ssl.h>
12
13 int SSL_COMP_add_compression_method(int id, COMP_METHOD *cm);
14 STACK_OF(SSL_COMP) *SSL_COMP_get_compression_methods(void);
15 const char *SSL_COMP_get0_name(const SSL_COMP *comp);
16 int SSL_COMP_get_id(const SSL_COMP *comp);
17
18The following function has been deprecated since OpenSSL 1.1.0, and can be
19hidden entirely by defining B<OPENSSL_API_COMPAT> with a suitable version value,
20see L<openssl_user_macros(7)>:
21
22 void SSL_COMP_free_compression_methods(void);
23
24=head1 DESCRIPTION
25
26SSL_COMP_add_compression_method() adds the compression method B<cm> with
27the identifier B<id> to the list of available compression methods. This
28list is globally maintained for all SSL operations within this application.
29It cannot be set for specific SSL_CTX or SSL objects.
30
31SSL_COMP_get_compression_methods() returns a stack of all of the available
32compression methods or NULL on error.
33
34SSL_COMP_get0_name() returns the name of the compression method B<comp>.
35
36SSL_COMP_get_id() returns the id of the compression method B<comp>.
37
38SSL_COMP_free_compression_methods() releases any resources acquired to
39maintain the internal table of compression methods.
40
41=head1 NOTES
42
43The TLS standard (or SSLv3) allows the integration of compression methods
44into the communication. The TLS RFC does however not specify compression
45methods or their corresponding identifiers, so there is currently no compatible
46way to integrate compression with unknown peers. It is therefore currently not
47recommended to integrate compression into applications. Applications for
48non-public use may agree on certain compression methods. Using different
49compression methods with the same identifier will lead to connection failure.
50
51An OpenSSL client speaking a protocol that allows compression (SSLv3, TLSv1)
52will unconditionally send the list of all compression methods enabled with
53SSL_COMP_add_compression_method() to the server during the handshake.
54Unlike the mechanisms to set a cipher list, there is no method available to
55restrict the list of compression method on a per connection basis.
56
57An OpenSSL server will match the identifiers listed by a client against
58its own compression methods and will unconditionally activate compression
59when a matching identifier is found. There is no way to restrict the list
60of compression methods supported on a per connection basis.
61
62If enabled during compilation, the OpenSSL library will have the
63following compression methods available:
64
65=over 4
66
67=item COMP_zlib()
68
69=item COMP_brotli()
70
71=item COMP_brotli_oneshot()
72
73=item COMP_zstd()
74
75=item COMP_zstd_oneshot()
76
77=back
78
79=head1 RETURN VALUES
80
81SSL_COMP_add_compression_method() may return the following values:
82
83=over 4
84
85=item Z<>0
86
87The operation succeeded.
88
89=item Z<>1
90
91The operation failed. Check the error queue to find out the reason.
92
93=back
94
95SSL_COMP_get_compression_methods() returns the stack of compressions methods or
96NULL on error.
97
98SSL_COMP_get0_name() returns the name of the compression method or NULL on error.
99
100SSL_COMP_get_id() returns the name of the compression method or -1 on error.
101
102=head1 SEE ALSO
103
104L<ssl(7)>
105
106=head1 HISTORY
107
108The SSL_COMP_free_compression_methods() function was deprecated in OpenSSL 1.1.0.
109The SSL_COMP_get0_name() and SSL_comp_get_id() functions were added in OpenSSL 1.1.0d.
110
111=head1 COPYRIGHT
112
113Copyright 2001-2020 The OpenSSL Project Authors. All Rights Reserved.
114
115Licensed under the Apache License 2.0 (the "License").  You may not use
116this file except in compliance with the License.  You can obtain a copy
117in the file LICENSE in the source distribution or at
118L<https://www.openssl.org/source/license.html>.
119
120=cut
121