mu4e: compose: auto-crypt/sign crypto replies

When replying to an encrypted message, sign/encrypt by default; see
  mu4e-compose-crypto-reply-policy.

(Issue #865)

mu4e/mu4e-compose.el

@@ -143,6 +143,23 @@ Also see `mu4e-context-policy'."
 	   :safe 'symbolp
 	   :group 'mu4e-compose))
 
+
+(defcustom mu4e-compose-crypto-reply-policy 'sign-and-encrypt
+  "Policy for signing/encrypting replies to encrypted messages.
+We have the following choices:
+
+- `sign': sign the reply
+- `sign-and-encrypt': sign and encrypt the repy
+- `encrypt': encrypt the reply, but don't sign it.
+-  anything else: do nothing."
+  :type '(choice
+	   (const :tag "Sign the reply" 'sign)
+	   (const :tag "Sign and encrypt the reply" 'sign-and-encrypt)
+	   (const :tag "Encrypt the reply" 'encrypt)
+	   (const :tag "Don't do anything" nil)
+	   :safe 'symbolp
+	   :group 'mu4e-compose))
+
 (defcustom mu4e-compose-format-flowed nil
   "Whether to compose messages to be sent as format=flowed (or
    with long lines if `use-hard-newlines' is set to nil).  The
@@ -165,6 +182,7 @@ it has been constructed, `mu4e-compose-mode-hook' would be the
 place to do that."
   :type 'hook
   :group 'mu4e-compose)
+
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 
@@ -480,6 +498,21 @@ buffers; lets remap its faces so it uses the ones for mu4e."
 		       mu4e~compose-buffer-max-name-length
 		       nil nil t)))))
 
+
+(defun mu4e~compose-crypto-reply (parent compose-type)
+  "When composing a reply to an encrypted message, we can
+automatically encrypt that reply."
+  (message "%S %S" parent compose-type)
+  (when (and  (eq compose-type 'reply)
+	  (and parent (member 'encrypted (mu4e-message-field parent :flags))))
+	(case mu4e-compose-crypto-reply-policy
+	  (sign (mml-secure-message-sign))
+	  (encrypt (mml-secure-message-encrypt))
+	  (sign-and-encrypt (mml-secure-message-sign-encrypt))
+	  (message "Do nothing"))))
+
+
+
 (defun* mu4e~compose-handler (compose-type &optional original-msg includes)
   "Create a new draft message, or open an existing one.
 
@@ -518,6 +551,8 @@ tempfile)."
   ;; insert mail-header-separator, which is needed by message mode to separate
   ;; headers and body. will be removed before saving to disk
   (mu4e~draft-insert-mail-header-separator)
+  ;; maybe encrypt/sign replies
+  (mu4e~compose-crypto-reply original-msg compose-type)
   ;; include files -- e.g. when forwarding a message with attachments,
   ;; we take those from the original.
   (save-excursion