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SSL_COMP_add_compression_method(3) OpenSSL SSL_COMP_add_compression_method(3)
SSL_COMP_add_compression_method, SSL_COMP_free_compression_methods -
handle SSL/TLS integrated compression methods
int SSL_COMP_add_compression_method(int id, COMP_METHOD *cm);
SSL_COMP_add_compression_method() adds the compression method cm with
the identifier 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_free_compression_methods() frees the internal table of
compression methods that were built internally, and possibly augmented
by adding SSL_COMP_add_compression_method().
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
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
If enabled during compilation, the OpenSSL library will have the
COMP_zlib() compression method available.
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.
SSL_COMP_add_compression_method() may return the following values:
0 The operation succeeded.
1 The operation failed. Check the error queue to find out the reason.