An advice that enables url.el to connect a server with the HTTPS protocol via a HTTP proxy.

url-with-https-via-proxy.el

;;
;; If the OpenSSL executable is compiled with the patch in
;; http://rt.openssl.org/Ticket/Display.html?id=2651&user=guest&pass=guest ,
;; the executable can connect a server via a HTTP proxy.
;; With the below code, the url.el library may connect a server with
;; the HTTPS protocol via a HTTP proxy.
;;

;;
;; Register a proxy for the "https" scheme from the environment
;; variable "https_proxy".
;;
(require 'url-methods nil t)

;; You can also specify the environment variable here as
;; (setenv "https_proxy" "http://proxy-host:proxy-port/") .
;; This `setenv' must precede the below call of `url-scheme-register-proxy'.
(url-scheme-register-proxy "https")

;;
;; Configure `tls-program' to use the proxy registered above.
;;
(let* ((proxy (cdr (assoc "https" url-proxy-services))))
  (when (and proxy
             (string-match "^\\([^:]+\\):\\([0-9]+\\)$" proxy))
    (setq tls-program
          `(,(concat "openssl s_client -connect %h:%p -no_ssl2 -ign_eof"
                     " -http_proxy " proxy)
            ,@tls-program))))

(defadvice url-proxy (around https-proxy activate)
  (cond
   ((string= (url-type url) "https")
    (let ((url-using-proxy nil))
      (url-https url callback cbargs)))
   (t
    ad-do-it)))