Different models will work differently, the ones this guy worked on are apparently always open, despite the manufacturer's intent. The simplest solution that d

Different models will work differently, the ones this guy worked on are apparently always open, despite the manufacturer's intent. The simplest solution that doesn't make ridiculous assumptions about the time and place is to just do HMAC + replay prevention, as has already been mentioned. Don't do infosec half-assed, they'll just get in through the other cheek.