From EmacsWiki#AutoEncryption

;; crypt++ – i use this in conjunction with emacs-wiki (require 'crypt++)
(setq crypt-encryption-type 'mcrypt
crypt-encryption-file-extension "\\(Secure\\)$\\|\\(\\.enc\\)$")
(setq emacs-wiki-ignored-extensions-regexp "\\.\\(bz2\\|gz\\|[Zz]\\|enc\\)\\'")