IPsec is complex, so complex that it doesn't work properly. Go in shop, by 10 different firewalls, and then try to cross connect those using IPsec. I'm sure you

IPsec is complex, so complex that it doesn't work properly. Go in shop, by 10 different firewalls, and then try to cross connect those using IPsec. I'm sure you're going to have fun time. After you manage to get the SAs connected, you'll find out that those tunnels work unreliably, connecting, disconnecting, state machine & key renegotiation totally broken etc. If it's not crap on paper, at least it is in reality. I've been using IPsec with over 50 different devices and I find it to be real pain point. Some devices do not offer all options in UI, but still have hidden values for those built in, which you don't know and need to figure out by trian and error. Devices like ZyWALL (Zyxel) and WatchGuard, StoneGate (Stonesoft) etc, have constant probelms with IPsec. If you want real challenge, things get much worse if you're using aggressive mode and dynamic IPs with DDNS etc. Then it's total disaster, even many firewalls from same manufacturer won't work properly. I just now have two ZyWALL USG 1000 boxes, that can't maintain reliable IPsec main mode tunnel between those, even if there's no network issues. There's simply something wrong with the software. Old whines: http://www.dslreports.com/forum/r25350958-Zywall-35-vs-USG-100-IPsec-issues http://www.dslreports.com/forum/r25350958-Zywall-35-vs-USG-1... About null cipher downgrade attacks, simply don't allow "multiple proposals", then what's specified has to be exact match. (Or in some cases, there' list of options, which means that any option like null sipher isn't allowed.)