@deftypefun {int} {gnutls_x509_privkey_get_key_id} (gnutls_x509_privkey_t @var{key}, unsigned int @var{flags}, unsigned char * @var{output_data}, size_t * @var{output_data_size})
@var{key}: a key

@var{flags}: should be one of the flags from @code{gnutls_keyid_flags_t} 

@var{output_data}: will contain the key ID

@var{output_data_size}: holds the size of output_data (and will be
replaced by the actual size of parameters)

This function will return a unique ID that depends on the public key
parameters. This ID can be used in checking whether a certificate
corresponds to the given key.

If the buffer provided is not long enough to hold the output, then
* @code{output_data_size} is updated and @code{GNUTLS_E_SHORT_MEMORY_BUFFER}  will
be returned.  The output will normally be a SHA-1 hash output,
which is 20 bytes.

@strong{Returns:} On success, @code{GNUTLS_E_SUCCESS}  (0) is returned, otherwise a
negative error value.
@end deftypefun