Comment by tptacek
1 year ago
It's not about AES, it's about the WireGuard protocol. AES is fine. It's possible that, if Jason had the decisions to do over again today, he might use XAES instead of ChaPoly (he didn't have an especially good AES construction to use at the time). The big thing with WireGuard is not doing ciphersuite negotiation, which is an extremely good decision that is definitely worth paying some cycles/byte for (if you must).
Maybe I'm missing something, but why would he have needed XAES rather than vanilla AES-GCM, which was certainly available at the time WireGuard was created? XAES gives you large nonces which is cool, but that's not something WireGuard needs AFAIK and it's not something regular ChaPoly gives you anyways.
Now I admit ChaPoly has some pretty nice advantages if you're implementing it in software. But with the trend of AES-GCM hardware support and the long-lived nature of WireGuard's crypto choices given the lack of ciphersuite negotiation (which I agree was a good decision!), I'm not sure AES-GCM wouldn't have been the best (albeit less cool) choice.
Although maybe on the other hand, ChaPoly can still be made to run pretty fast even just in software and it gives WireGuard the advantage of being more practical on very low-end devices that might lack AES-GCM hardware. Avoiding ciphersuite negotiation means a tradeoff needs to be made somewhere, at least with current algorithms, and I'd bet line-rate hardware encryption is probably the least likely place to see WireGuard for a while at least, so maybe WireGuard did make the best tradeoff at the time.
WireGuard is an instantiation of Noise, which slightly disfavors AES-GCM (see: the spec). I don't think it's a huge big deal, but at the time WireGuard was being designed it was pretty normal to tack away from GCM.
I agree in advance, Noise already uses counter-based nonces, the extended nonce wouldn't matter to vanilla Noise.
This has been nagging at me for a day, so just to clarify real quick:
I wanted to push back a little on the notion that Chapoly was "cool" and GCM was "lame" back in 2015-2016. At the time, GCM was coming off a pretty rough run of implementation bugs. It was the tail end of a period of time where a concern was that some mainstream architectures wouldn't be able to run performant constant-time GCM at all; like, the fast software GCMs had a table-driven multiplication? I forget the details.
But you could have done a secure WireGuard instantiated on AES-GCM. It's true that GCM was out of fashion and Chapoly was in fashion. I just want to say, that fashion had (has?) some real technical roots. That's all.