I saw that there are encryption chips like the Atmel CryptoAuthentication. You can integrate them on your Arduino project without letting the Arduino do the job which cost resources but I think that you'll need to write a library to integrate those in you project. I haven't seen any project that utilize them but if so then I hope someone does. Here is a link so that you can ask around or find someone with the same interest as you:
Edit: I found a pretty good informative discussion about implementing security with IC .
As this discussion explains the XMEGA has an engine to support AES encryption
There is a fork of Arduino here
But at the moment of this writing it doesn't support the ethernet library. So you need to stay in touch for any changes for the support of AES, Ethernet or other libraries you may need. Maybe in a year or two everything will be set and then you will need not to worry about implementing encryption with a separated IC or about consuming too much resources, that could mean an upgrade to your project!