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@deftypefun {int} {gnutls_prf_early} (gnutls_session_t @var{session}, size_t @var{label_size}, const char * @var{label}, size_t @var{context_size}, const char * @var{context}, size_t @var{outsize}, char * @var{out})
@var{session}: is a @code{gnutls_session_t} type.
@var{label_size}: length of the @code{label} variable.
@var{label}: label used in PRF computation, typically a short string.
@var{context_size}: length of the @code{extra} variable.
@var{context}: optional extra data to seed the PRF with.
@var{outsize}: size of pre-allocated output buffer to hold the output.
@var{out}: pre-allocated buffer to hold the generated data.
This function is similar to @code{gnutls_prf_rfc5705()} , but only works in
TLS 1.3 or later to export early keying material.
Note that the keying material is only available after the
ClientHello message is processed and before the application traffic
keys are established. Therefore this function shall be called in a
handshake hook function for @code{GNUTLS_HANDSHAKE_CLIENT_HELLO} .
The @code{label} variable usually contains a string denoting the purpose
for the generated data.
The @code{context} variable can be used to add more data to the seed, after
the random variables. It can be used to make sure the
generated output is strongly connected to some additional data
(e.g., a string used in user authentication).
The output is placed in @code{out} , which must be pre-allocated.
Note that, to provide the RFC5705 context, the @code{context} variable
must be non-null.
@strong{Returns:} @code{GNUTLS_E_SUCCESS} on success, or an error code.
@strong{Since:} 3.6.8
@end deftypefun
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