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.\"
.\" $OpenBSD: SSL_COMP_add_compression_method.3,v 1.2 2014/12/02 14:11:01 jmc Exp $
.\"
.Dd $Mdocdate: December 2 2014 $
.Dt SSL_COMP_ADD_COMPRESSION_METHOD 3
.Os
.Sh NAME
.Nm SSL_COMP_add_compression_method
.Nd handle SSL/TLS integrated compression methods
.Sh SYNOPSIS
.In openssl/ssl.h
.Ft int
.Fn SSL_COMP_add_compression_method "int id" "COMP_METHOD *cm"
.Sh DESCRIPTION
.Fn SSL_COMP_add_compression_method
adds the compression method
.Fa cm
with the identifier
.Fa 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.
.Sh 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.
.Pp
An OpenSSL client speaking a protocol that allows compression (SSLv3, TLSv1)
will unconditionally send the list of all compression methods enabled with
.Fn 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.
.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
The OpenSSL library has the compression methods
.Fn COMP_rle
and (when especially enabled during compilation)
.Fn COMP_zlib
available.
.Sh 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.
.Sh RETURN VALUES
.Fn SSL_COMP_add_compression_method
may return the following values:
.Bl -tag -width Ds
.It 0
The operation succeeded.
.It 1
The operation failed.
Check the error queue to find out the reason.
.El
.Sh SEE ALSO
.Xr ssl 3
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