1SSL_COMP_add_compression_method(3)  OpenSSL SSL_COMP_add_compression_method(3)
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NAME

6       SSL_COMP_add_compression_method, SSL_COMP_free_compression_methods -
7       handle SSL/TLS integrated compression methods
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SYNOPSIS

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

17       SSL_COMP_add_compression_method() adds the compression method cm with
18       the identifier id to the list of available compression methods. This
19       list is globally maintained for all SSL operations within this
20       application.  It cannot be set for specific SSL_CTX or SSL objects.
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22       SSL_COMP_free_compression_methods() frees the internal table of
23       compression methods that were built internally, and possibly augmented
24       by adding SSL_COMP_add_compression_method().
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NOTES

27       The TLS standard (or SSLv3) allows the integration of compression
28       methods into the communication. The TLS RFC does however not specify
29       compression methods or their corresponding identifiers, so there is
30       currently no compatible way to integrate compression with unknown
31       peers. It is therefore currently not recommended to integrate
32       compression into applications. Applications for non-public use may
33       agree on certain compression methods. Using different compression
34       methods with the same identifier will lead to connection failure.
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36       An OpenSSL client speaking a protocol that allows compression (SSLv3,
37       TLSv1) will unconditionally send the list of all compression methods
38       enabled with SSL_COMP_add_compression_method() to the server during the
39       handshake.  Unlike the mechanisms to set a cipher list, there is no
40       method available to restrict the list of compression method on a per
41       connection basis.
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43       An OpenSSL server will match the identifiers listed by a client against
44       its own compression methods and will unconditionally activate
45       compression when a matching identifier is found. There is no way to
46       restrict the list of compression methods supported on a per connection
47       basis.
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49       If enabled during compilation, the OpenSSL library will have the
50       COMP_zlib() compression method available.
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WARNINGS

53       Once the identities of the compression methods for the TLS protocol
54       have been standardized, the compression API will most likely be
55       changed. Using it in the current state is not recommended.
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57       It is also not recommended to use compression if data transfered
58       contain untrusted parts that can be manipulated by an attacker as he
59       could then get information about the encrypted data. See the CRIME
60       attack. For that reason the default loading of the zlib compression
61       method is disabled and enabled only if the environment variable
62       OPENSSL_DEFAULT_ZLIB is present during the library initialization.
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RETURN VALUES

65       SSL_COMP_add_compression_method() may return the following values:
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67       0   The operation succeeded.
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69       1   The operation failed. Check the error queue to find out the reason.
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SEE ALSO

72       ssl(3)
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761.0.2o                            2019-02-08SSL_COMP_add_compression_method(3)
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