ust is a timestamp. usg is probably some kind of HMAC (q || ust, secret) so that only google can generate the redirecting urls. If you change the usg, the redir

ust is a timestamp. usg is probably some kind of HMAC (q || ust, secret) so that only google can generate the redirecting urls. If you change the usg, the redirection fails. The same happens if you change ust or q.