I don't love the code style in terms of indentation, line length, alignment and bracket placing but IMO at least it doesn't look childish / reckless.

I think there could be bug in he_internal_send_auth_userpass when it copies the strings because when calculating the string lengths it uses the size of he_conn->username and he_conn->password which are "HE_CONFIG_TEXT_FIELD_LENGTH +1" whereas the sizes of the destination fields in he_msg_auth_t are "HE_CONFIG_TEXT_FIELD_LENGTH" so .

Take it with a grain of salt, I just took a very quick look mostly to see if I liked the coding style and it's far too early for my brain to be functional but it seemed that way to me. Other than that I didn't hate the code which is cool!

Thanks for opening it!