@deftypefun {int} {gnutls_hkdf_extract} (gnutls_mac_algorithm_t @var{mac}, const gnutls_datum_t * @var{key}, const gnutls_datum_t * @var{salt}, void * @var{output})
@var{mac}: the mac algorithm used internally

@var{key}: the initial keying material

@var{salt}: the optional salt

@var{output}: the output value of the extract operation

This function will derive a fixed-size key using the HKDF-Extract
function as defined in RFC 5869.

@strong{Returns:} Zero or a negative error code on error.

@strong{Since:} 3.6.13
@end deftypefun