function pointer to Opaque PRF extension server callback.
void * userdata
hook passed to callback function for passing application state.
Request that the server should attempt to accept the Opaque PRF
Input TLS extension. If the client requests the extension, the
provided callback cb will be invoked. The callback must have the
The callback can inspect the client-provided data in the input
parameters, and specify its own opaque prf input data in the output
variable. The function must return 0 on success, otherwise the
handshake will be aborted.