@deftypefun {int} {gnutls_x509_crt_get_key_id} (gnutls_x509_crt_t @var{crt}, unsigned int @var{flags}, unsigned char * @var{output_data}, size_t * @var{output_data_size})
@var{crt}: Holds the certificate

@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 private key.

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

@strong{Returns:} In case of failure a negative error code will be
returned, and 0 on success.
@end deftypefun