NAME

SSL_COMP_add_compression_method − Handle SSL/TLS integrated compression methods

SYNOPSIS

#include <openssl/ssl.h>
int SSL_COMP_add_compression_method(
        int id,
        COMP_METHOD ∗cm );

DESCRIPTION

The SSL_COMP_add_compression_method() function 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. 

NOTES

The TLS standard (or SSLv3) allows the integration of compression methods into the communication. However, the TLS RFC does not specify compression methods or their corresponding identifiers, so there is no  compatible way to integrate compression with unknown peers. We do not recommend integrating 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. We do not recommend using it in its current state. 

RETURN VALUES

The following return values can occur:

0The operation failed. Check the error queue to learn the reason. 

1The operation succeeded. 

SEE ALSO

Functions: ssl(3)