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SSL_COMP_add_compression_method(3) OpenSSL SSL_COMP_add_compression_method(3)
SSL_COMP_add_compression_method - handle SSL/TLS integrated compression
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 applica-
tion. It cannot be set for specific SSL_CTX or SSL objects.
The TLS standard (or SSLv3) allows the integration of compression meth-
ods into the communication. The TLS RFC does however not specify com-
pression methods or their corresponding identifiers, so there is cur-
rently 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 com-
pression 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
An OpenSSL server will match the identifiers listed by a client against
its own compression methods and will unconditionally activate compres-
sion 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.
It is also not recommended to use compression if data transfered con-
tain 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.
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.
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