Cryptography and SSL/TLS Toolkit



SSL_COMP_add_compression_method, SSL_COMP_free_compression_methods - handle SSL/TLS integrated compression methods


 #include <openssl/ssl.h>

 int SSL_COMP_add_compression_method(int id, COMP_METHOD *cm);


 #if OPENSSL_API_COMPAT < 0x10100000L
 void SSL_COMP_free_compression_methods(void)


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.

In versions of OpenSSL prior to 1.1.0 SSL_COMP_free_compression_methods() freed the internal table of compression methods that were built internally, and possibly augmented by adding SSL_COMP_add_compression_method(). However this is now unnecessary from version 1.1.0. No explicit initialisation or de-initialisation is necessary. See OPENSSL_init_crypto and OPENSSL_init_ssl. From OpenSSL 1.1.0 calling this function does nothing.


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.


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.




SSL_COMP_free_compression_methods() was deprecated in OpenSSL 1.1.0.

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