@deftypefun {int} {gnutls_hkdf_expand} (gnutls_mac_algorithm_t @var{mac}, const gnutls_datum_t * @var{key}, const gnutls_datum_t * @var{info}, void * @var{output}, size_t @var{length})
@var{mac}: the mac algorithm used internally

@var{key}: the pseudorandom key created with HKDF-Extract

@var{info}: the optional informational data

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

@var{length}: the desired length of the output key

This function will derive a variable length keying material from
the pseudorandom key using the HKDF-Expand function as defined in
RFC 5869.

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

@strong{Since:} 3.6.13
@end deftypefun