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SSL_COMP_add_compression_method(3) OpenSSL SSL_COMP_add_compression_method(3)
SSL_COMP_add_compression_method - 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.
The TLS standard (or SSLv3) allows the integration of compression methods into the commu-
nication. 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 applica-
tions. 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 uncondi-
tionally send the list of all compression methods enabled with SSL_COMP_add_compres-
sion_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 compres-
sion 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 con-
The OpenSSL library has the compression methods COMP_rle() and (when especially enabled
during compilation) COMP_zlib() available.
Once the identities of the compression methods for the TLS protocol have been standard-
ized, the compression API will most likely be changed. Using it in the current state is
SSL_COMP_add_compression_method() may return the following values:
1 The operation succeeded.
0 The operation failed. Check the error queue to find out the reason.
0.9.7a 2001-08-23 SSL_COMP_add_compression_method(3)
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