@deftypefun {int} {gnutls_x509_privkey_sign_data} (gnutls_x509_privkey_t @var{key}, gnutls_digest_algorithm_t @var{digest}, unsigned int @var{flags}, const gnutls_datum_t * @var{data}, void * @var{signature}, size_t * @var{signature_size})
@var{key}: a key

@var{digest}: should be a digest algorithm

@var{flags}: should be 0 for now

@var{data}: holds the data to be signed

@var{signature}: will contain the signature

@var{signature_size}: holds the size of signature (and will be replaced
by the new size)

This function will sign the given data using a signature algorithm
supported by the private key. Signature algorithms are always used
together with a hash functions.  Different hash functions may be
used for the RSA algorithm, but only SHA-1 for the DSA keys.

If the buffer provided is not long enough to hold the output, then
* @code{signature_size} is updated and @code{GNUTLS_E_SHORT_MEMORY_BUFFER}  will
be returned.

Use @code{gnutls_x509_crt_get_preferred_hash_algorithm()}  to determine
the hash algorithm.

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