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.\" ========================================================================
.\"
.IX Title "SSL_COMP_add_compression_method 3"
.TH SSL_COMP_add_compression_method 3 "2022-10-06" "1.0.2u" "OpenSSL"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
SSL_COMP_add_compression_method, SSL_COMP_free_compression_methods \- handle SSL/TLS integrated compression methods
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #include <openssl/ssl.h>
\&
\& int SSL_COMP_add_compression_method(int id, COMP_METHOD *cm);
\&
\& +void SSL_COMP_free_compression_methods(void);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\fBSSL_COMP_add_compression_method()\fR adds the compression method \fBcm\fR with
the identifier \fBid\fR to the list of available compression methods. This
list is globally maintained for all \s-1SSL\s0 operations within this application.
It cannot be set for specific \s-1SSL_CTX\s0 or \s-1SSL\s0 objects.
.PP
\&\fBSSL_COMP_free_compression_methods()\fR frees the internal table of
compression methods that were built internally, and possibly
augmented by adding \fBSSL_COMP_add_compression_method()\fR.
.SH "NOTES"
.IX Header "NOTES"
The \s-1TLS\s0 standard (or SSLv3) allows the integration of compression methods
into the communication. The \s-1TLS RFC\s0 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.
.PP
An OpenSSL client speaking a protocol that allows compression (SSLv3, TLSv1)
will unconditionally send the list of all compression methods enabled with
\&\fBSSL_COMP_add_compression_method()\fR 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.
.PP
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.
.PP
If enabled during compilation, the OpenSSL library will have the
\&\fBCOMP_zlib()\fR compression method available.
.SH "WARNINGS"
.IX Header "WARNINGS"
Once the identities of the compression methods for the \s-1TLS\s0 protocol have
been standardized, the compression \s-1API\s0 will most likely be changed. Using
it in the current state is not recommended.
.PP
It is also not recommended to use compression if data transfered contain
untrusted parts that can be manipulated by an attacker as he could then
get information about the encrypted data. See the \s-1CRIME\s0 attack. For
that reason the default loading of the zlib compression method is
disabled and enabled only if the environment variable \fB\s-1OPENSSL_DEFAULT_ZLIB\s0\fR
is present during the library initialization.
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
\&\fBSSL_COMP_add_compression_method()\fR may return the following values:
.IP "0" 4
The operation succeeded.
.IP "1" 4
.IX Item "1"
The operation failed. Check the error queue to find out the reason.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBssl\fR\|(3)
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