Yes cool and dry but not so dry that they die. I had to mist mine last February. Do a search for instant feedback to your question.

I store mine bone-dry in open-topped paper bags or cardboard boxes in a cool attic (probably 50's during the coldest part of the winter). I have had excellent results with the cannas that produce big fat rhizomes, less success with the ones that produce thin ropey rhizomes.

My advice comes by way of Marek Busiakiewicz's excellent site. Link is here, and should be bookmarked on our favorites list. On his site he refers to overwintering as resting.

Here is a link that might be useful: http://www.canna.pl/