Assuming 80 possible characters (A-Z,a-z,0-9 + 18 punctuation symbols), log2(80^8) ~= 50. So a correct horse password would be about the same as a random 8 char

Assuming 80 possible characters (A-Z,a-z,0-9 + 18 punctuation symbols), log2(80^8) ~= 50. So a correct horse password would be about the same as a random 8 character alpha-numeric entropy wise. The XKCD argument is that correct hose style is way easier to remember. It's possible Schneier thought Munroe was arguing that a correct horse was as much entropy as a 20 character random password, even though he isn't. This is the first I've heard of this argument though so I'm not even sure how much of a controversy it is.