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       gnutls_prf_early - API function


       #include <gnutls/gnutls.h>

       int  gnutls_prf_early(gnutls_session_t  session,  size_t  label_size,  const char * label,
       size_t context_size, const char * context, size_t outsize, char * out);


       gnutls_session_t session
                   is a gnutls_session_t type.

       size_t label_size
                   length of the  label variable.

       const char * label
                   label used in PRF computation, typically a short string.

       size_t context_size
                   length of the  extra variable.

       const char * context
                   optional extra data to seed the PRF with.

       size_t outsize
                   size of pre-allocated output buffer to hold the output.

       char * out  pre-allocated buffer to hold the generated data.


       This function is similar to 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 GNUTLS_HANDSHAKE_CLIENT_HELLO.

       The  label variable usually contains a string denoting the purpose for the generated data.

       The   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  out , which must be pre-allocated.

       Note that, to provide the RFC5705 context, the  context variable must be non-null.


       GNUTLS_E_SUCCESS on success, or an error code.




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