@deftypefun {int} {gnutls_x509_crt_get_authority_key_id} (gnutls_x509_crt_t @var{cert}, void * @var{id}, size_t * @var{id_size}, unsigned int * @var{critical})
@var{cert}: should contain a @code{gnutls_x509_crt_t}  type

@var{id}: The place where the identifier will be copied

@var{id_size}: Holds the size of the id field.

@var{critical}: will be non-zero if the extension is marked as critical (may be null)

This function will return the X.509v3 certificate authority's key
identifier.  This is obtained by the X.509 Authority Key
identifier extension field (2.5.29.35). Note that this function
only returns the keyIdentifier field of the extension and
@code{GNUTLS_E_X509_UNSUPPORTED_EXTENSION} , if the extension contains
the name and serial number of the certificate. In that case
@code{gnutls_x509_crt_get_authority_key_gn_serial()}  may be used.

@strong{Returns:} On success, @code{GNUTLS_E_SUCCESS}  (0) is returned, @code{GNUTLS_E_REQUESTED_DATA_NOT_AVAILABLE} 
if the extension is not present, otherwise a negative error value.
@end deftypefun