Section: OpenSSL (3)
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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 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 connection basis.
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.
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 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:
The operation succeeded.
The operation failed. Check the error queue to find out the reason.
- RETURN VALUES
- SEE ALSO
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