NAME

SSL_COMP_add_compression_method, SSL_COMP_free_compression_methods - handle SSL/TLS integrated compression methods

SYNOPSIS

 #include <openssl/ssl.h>
 int SSL_COMP_add_compression_method(int id, COMP_METHOD *cm);
 +void SSL_COMP_free_compression_methods(void);

DESCRIPTION

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().

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.

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.

If enabled during compilation, the OpenSSL library will have the COMP_zlib() compression method available.

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.

It is also not recommended to use compression if data transfered contain untrusted parts that can be manipulated by an attacker as he could then get information about the encrypted data. See the CRIME attack. For that reason the default loading of the zlib compression method is disabled and enabled only if the environment variable OPENSSL_DEFAULT_ZLIB is present during the library initialization.

RETURN VALUES

SSL_COMP_add_compression_method() may return the following values: