Allow using `server-auth-key' to set a permanent shared key

* server.el (server-auth-key): New variable.
(server-generate-key): New function.
(server-get-auth-key): New function.
(server-start): Use the new variable and functions to allow
setting a permanent server key.

Fixes: debbugs:9423

etc/NEWS

@@ -53,8 +53,11 @@ character when doing minibuffer filename prompts.
 ** which-function-mode now applies to all applicable major modes by default.
 
 ** erc will look up server/channel names via auth-source and use the
-   channel keys found, if any.
+channel keys found, if any.
 
+** The `server-auth-key' variable can be used to set a permanent
+shared key for Emacs Server.
+   
 ** Obsolete packages:
 
 *** mailpost.el

lisp/ChangeLog

+2012-04-14  Michal Nazarewicz  <mina86@mina86.com>
+
+	* server.el (server-auth-key): New variable.
+	(server-generate-key): New function.
+	(server-get-auth-key): New function.
+	(server-start): Use the new variable and functions to allow
+	setting a permanent server key (bug#9423).
+
 2012-04-14  Leo Liu  <sdl.web@gmail.com>
 
 	* vc/diff-mode.el (diff-file-prev/next): Fix typo.

lisp/server.el

@@ -139,6 +139,33 @@ directory residing in a NTFS partition instead."
 ;;;###autoload
 (put 'server-auth-dir 'risky-local-variable t)
 
+(defcustom server-auth-key nil
+  "Server authentication key.
+
+Normally, authentication key is generated on random when server
+starts, which guarantees some level of security.  It is
+recommended to leave it that way.  Using a long-lived shared key
+may decrease security (especially since the key is transmitted as
+plain text).
+
+In some situations however, it can be difficult to share randomly
+generated password with remote hosts (eg. no shared directory),
+so you can set the key with this variable and then copy server
+file to remote host (with possible changes to IP address and/or
+port if that applies).
+
+The key must consist of 64 US-ASCII printable characters except
+for space (this means characters from ! to ~; or from code 33
+to 126).
+
+You can use \\[server-generate-key] to get a random authentication
+key."
+  :group 'server
+  :type '(choice
+	  (const :tag "Random" nil)
+	  (string :tag "Password"))
+  :version "24.2")
+
 (defcustom server-raise-frame t
   "If non-nil, raise frame when switching to a buffer."
   :group 'server
@@ -522,6 +549,32 @@ See variable `server-auth-dir' for details."
       (unless safe
 	(error "The directory `%s' is unsafe" dir)))))
 
+(defun server-generate-key ()
+  "Generates and returns a random 64-byte strings of random chars
+in the range `!'..`~'. If called interactively, also inserts it
+into current buffer."
+  (interactive)
+  (let ((auth-key
+	 (loop repeat 64
+	       collect (+ 33 (random 94)) into auth
+	       finally return (concat auth))))
+    (if (called-interactively-p)
+	(insert auth-key))
+    auth-key))
+
+(defun server-get-auth-key ()
+  "Returns server's authentication key.
+
+If `server-auth-key' is nil this function will just call
+`server-generate-key'.  Otherwise, if `server-auth-key' is
+a valid authentication it will return it.  Otherwise, it will
+signal an error."
+  (if server-auth-key
+    (if (string-match "^[!-~]\\{64\\}$" server-auth-key)
+        server-auth-key
+      (error "The key '%s' is invalid" server-auth-key))
+    (server-generate-key)))
+
 ;;;###autoload
 (defun server-start (&optional leave-dead inhibit-prompt)
   "Allow this Emacs process to be a server for client processes.
@@ -615,13 +668,7 @@ server or call `M-x server-force-delete' to forcibly disconnect it.")
 	  (unless server-process (error "Could not start server process"))
 	  (process-put server-process :server-file server-file)
 	  (when server-use-tcp
-	    (let ((auth-key
-		   (loop
-		    ;; The auth key is a 64-byte string of random chars in the
-		    ;; range `!'..`~'.
-		    repeat 64
-		    collect (+ 33 (random 94)) into auth
-		    finally return (concat auth))))
+	    (let ((auth-key (server-get-auth-key)))
 	      (process-put server-process :auth-key auth-key)
 	      (with-temp-file server-file
 		(set-buffer-multibyte nil)