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
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.
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.
The OpenSSL library has the compression methods
COMP_rle()
and (when especially enabled during compilation)
COMP_zlib()
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.
RETURN VALUES
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.