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+;;; org-crypt.el --- Public Key Encryption for Org Entries -*- lexical-binding: t; -*-
+;;
+;; Copyright (C) 2007-2021 Free Software Foundation, Inc.
+
+;; Author: John Wiegley <johnw@gnu.org>
+
+;; This file is part of GNU Emacs.
+;;
+;; GNU Emacs is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs. If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; Right now this is just a set of functions to play with. It depends
+;; on the epg library. Here's how you would use it:
+;;
+;; 1. To mark an entry for encryption, tag the heading with "crypt".
+;; You can change the tag to any complex tag matching string by
+;; setting the `org-crypt-tag-matcher' variable.
+;;
+;; 2. Set the encryption key to use in the `org-crypt-key' variable,
+;; or use `M-x org-set-property' to set the property CRYPTKEY to
+;; any address in your public keyring. The text of the entry (but
+;; not its properties or headline) will be encrypted for this user.
+;; For them to read it, the corresponding secret key must be
+;; located in the secret key ring of the account where you try to
+;; decrypt it. This makes it possible to leave secure notes that
+;; only the intended recipient can read in a shared-org-mode-files
+;; scenario.
+;; If the key is not set, org-crypt will default to symmetric encryption.
+;;
+;; 3. To later decrypt an entry, use `org-decrypt-entries' or
+;; `org-decrypt-entry'. It might be useful to bind this to a key,
+;; like C-c C-/.
+;;
+;; 4. To automatically encrypt all necessary entries when saving a
+;; file, call `org-crypt-use-before-save-magic' after loading
+;; org-crypt.el.
+
+;;; Thanks:
+
+;; - Carsten Dominik
+;; - Vitaly Ostanin
+
+;;; Code:
+
+(require 'org-macs)
+(require 'org-compat)
+
+(declare-function epg-decrypt-string "epg" (context cipher))
+(declare-function epg-list-keys "epg" (context &optional name mode))
+(declare-function epg-make-context "epg"
+ (&optional protocol armor textmode include-certs
+ cipher-algorithm digest-algorithm
+ compress-algorithm))
+(declare-function epg-encrypt-string "epg"
+ (context plain recipients &optional sign always-trust))
+(defvar epg-context)
+
+(declare-function org-back-over-empty-lines "org" ())
+(declare-function org-back-to-heading "org" (&optional invisible-ok))
+(declare-function org-before-first-heading-p "org" ())
+(declare-function org-end-of-meta-data "org" (&optional full))
+(declare-function org-end-of-subtree "org" (&optional invisible-ok to-heading))
+(declare-function org-entry-get "org" (pom property &optional inherit literal-nil))
+(declare-function org-flag-subtree "org" (flag))
+(declare-function org-make-tags-matcher "org" (match))
+(declare-function org-previous-visible-heading "org" (arg))
+(declare-function org-scan-tags "org" (action matcher todo-only &optional start-level))
+(declare-function org-set-property "org" (property value))
+
+(defgroup org-crypt nil
+ "Org Crypt."
+ :tag "Org Crypt"
+ :group 'org)
+
+(defcustom org-crypt-tag-matcher "crypt"
+ "The tag matcher used to find headings whose contents should be encrypted.
+
+See the \"Match syntax\" section of the org manual for more details."
+ :type 'string
+ :group 'org-crypt)
+
+(defcustom org-crypt-key ""
+ "The default key to use when encrypting the contents of a heading.
+
+If this variable is nil, always use symmetric encryption, unconditionally.
+
+Otherwise, The string is matched against all keys in the key ring.
+In particular, the empty string matches no key. If no key is found,
+look for the `epa-file-encrypt-to' local variable. Ultimately fall back
+to symmetric encryption.
+
+This setting can be overridden in the CRYPTKEY property."
+ :group 'org-crypt
+ :type '(choice
+ (string :tag "Public key(s) matching")
+ (const :tag "Symmetric encryption" nil)))
+
+(defcustom org-crypt-disable-auto-save 'ask
+ "What org-decrypt should do if `auto-save-mode' is enabled.
+
+t : Disable auto-save-mode for the current buffer
+ prior to decrypting an entry.
+
+nil : Leave auto-save-mode enabled.
+ This may cause data to be written to disk unencrypted!
+
+`ask' : Ask user whether or not to disable auto-save-mode
+ for the current buffer.
+
+`encrypt': Leave auto-save-mode enabled for the current buffer,
+ but automatically re-encrypt all decrypted entries
+ *before* auto-saving.
+ NOTE: This only works for entries which have a tag
+ that matches `org-crypt-tag-matcher'."
+ :group 'org-crypt
+ :version "24.1"
+ :type '(choice (const :tag "Always" t)
+ (const :tag "Never" nil)
+ (const :tag "Ask" ask)
+ (const :tag "Encrypt" encrypt)))
+
+(defun org-crypt--encrypted-text (beg end)
+ "Return encrypted text in between BEG and END."
+ ;; Ignore indentation.
+ (replace-regexp-in-string
+ "^[ \t]*" ""
+ (buffer-substring-no-properties beg end)))
+
+(defun org-at-encrypted-entry-p ()
+ "Is the current entry encrypted?
+When the entry is encrypted, return a pair (BEG . END) where BEG
+and END are buffer positions delimiting the encrypted area."
+ (org-with-wide-buffer
+ (unless (org-before-first-heading-p)
+ (org-back-to-heading t)
+ (org-end-of-meta-data 'standard)
+ (let ((case-fold-search nil)
+ (banner-start (rx (seq bol
+ (zero-or-more (any "\t "))
+ "-----BEGIN PGP MESSAGE-----"
+ eol))))
+ (when (looking-at banner-start)
+ (let ((start (point))
+ (banner-end (rx (seq bol
+ (or (group (zero-or-more (any "\t "))
+ "-----END PGP MESSAGE-----"
+ eol)
+ (seq (one-or-more "*") " "))))))
+ (when (and (re-search-forward banner-end nil t) (match-string 1))
+ (cons start (line-beginning-position 2)))))))))
+
+(defun org-crypt-check-auto-save ()
+ "Check whether auto-save-mode is enabled for the current buffer.
+
+`auto-save-mode' may cause leakage when decrypting entries, so
+check whether it's enabled, and decide what to do about it.
+
+See `org-crypt-disable-auto-save'."
+ (when buffer-auto-save-file-name
+ (cond
+ ((or
+ (eq org-crypt-disable-auto-save t)
+ (and
+ (eq org-crypt-disable-auto-save 'ask)
+ (y-or-n-p "org-decrypt: auto-save-mode may cause leakage. Disable it for current buffer? ")))
+ (message "org-decrypt: Disabling auto-save-mode for %s"
+ (or (buffer-file-name) (current-buffer)))
+ ;; The argument to auto-save-mode has to be "-1", since
+ ;; giving a "nil" argument toggles instead of disabling.
+ (auto-save-mode -1))
+ ((eq org-crypt-disable-auto-save nil)
+ (message "org-decrypt: Decrypting entry with auto-save-mode enabled. This may cause leakage."))
+ ((eq org-crypt-disable-auto-save 'encrypt)
+ (message "org-decrypt: Enabling re-encryption on auto-save.")
+ (add-hook 'auto-save-hook
+ (lambda ()
+ (message "org-crypt: Re-encrypting all decrypted entries due to auto-save.")
+ (org-encrypt-entries))
+ nil t))
+ (t nil))))
+
+(defun org-crypt-key-for-heading ()
+ "Return the encryption key(s) for the current heading.
+Assume `epg-context' is set."
+ (and org-crypt-key
+ (or (epg-list-keys epg-context
+ (or (org-entry-get nil "CRYPTKEY" 'selective)
+ org-crypt-key))
+ (bound-and-true-p epa-file-encrypt-to)
+ (progn
+ (message "No crypt key set, using symmetric encryption.")
+ nil))))
+
+;;;###autoload
+(defun org-encrypt-entry ()
+ "Encrypt the content of the current headline."
+ (interactive)
+ (unless (org-at-encrypted-entry-p)
+ (require 'epg)
+ (setq-local epg-context (epg-make-context nil t t))
+ (org-with-wide-buffer
+ (org-back-to-heading t)
+ (let ((start-heading (point))
+ (crypt-key (org-crypt-key-for-heading))
+ (folded? (org-invisible-p (line-beginning-position))))
+ (org-end-of-meta-data 'standard)
+ (let ((beg (point))
+ (folded-heading
+ (and folded?
+ (save-excursion
+ (org-previous-visible-heading 1)
+ (point)))))
+ (goto-char start-heading)
+ (org-end-of-subtree t t)
+ (org-back-over-empty-lines)
+ (let* ((contents (delete-and-extract-region beg (point)))
+ (key (get-text-property 0 'org-crypt-key contents))
+ (checksum (get-text-property 0 'org-crypt-checksum contents)))
+ (condition-case err
+ (insert
+ ;; Text and key have to be identical, otherwise we
+ ;; re-crypt.
+ (if (and (equal crypt-key key)
+ (string= checksum (sha1 contents)))
+ (get-text-property 0 'org-crypt-text contents)
+ (epg-encrypt-string epg-context contents crypt-key)))
+ ;; If encryption failed, make sure to insert back entry
+ ;; contents in the buffer.
+ (error
+ (insert contents)
+ (error (error-message-string err)))))
+ (when folded-heading
+ (goto-char folded-heading)
+ (org-flag-subtree t))
+ nil)))))
+
+;;;###autoload
+(defun org-decrypt-entry ()
+ "Decrypt the content of the current headline."
+ (interactive)
+ (pcase (org-at-encrypted-entry-p)
+ (`(,beg . ,end)
+ (require 'epg)
+ (setq-local epg-context (epg-make-context nil t t))
+ (org-with-point-at beg
+ (org-crypt-check-auto-save)
+ (let* ((folded-heading
+ (and (org-invisible-p)
+ (save-excursion
+ (org-previous-visible-heading 1)
+ (point))))
+ (encrypted-text (org-crypt--encrypted-text beg end))
+ (decrypted-text
+ (decode-coding-string
+ (epg-decrypt-string epg-context encrypted-text)
+ 'utf-8)))
+ ;; Delete region starting just before point, because the
+ ;; outline property starts at the \n of the heading.
+ (delete-region (1- (point)) end)
+ ;; Store a checksum of the decrypted and the encrypted text
+ ;; value. This allows reusing the same encrypted text if the
+ ;; text does not change, and therefore avoid a re-encryption
+ ;; process.
+ (insert "\n"
+ (propertize decrypted-text
+ 'org-crypt-checksum (sha1 decrypted-text)
+ 'org-crypt-key (org-crypt-key-for-heading)
+ 'org-crypt-text encrypted-text))
+ (when folded-heading
+ (goto-char folded-heading)
+ (org-flag-subtree t))
+ nil)))
+ (_ nil)))
+
+(defvar org--matcher-tags-todo-only)
+
+;;;###autoload
+(defun org-encrypt-entries ()
+ "Encrypt all top-level entries in the current buffer."
+ (interactive)
+ (let ((org--matcher-tags-todo-only nil))
+ (org-scan-tags
+ 'org-encrypt-entry
+ (cdr (org-make-tags-matcher org-crypt-tag-matcher))
+ org--matcher-tags-todo-only)))
+
+;;;###autoload
+(defun org-decrypt-entries ()
+ "Decrypt all entries in the current buffer."
+ (interactive)
+ (let ((org--matcher-tags-todo-only nil))
+ (org-scan-tags
+ 'org-decrypt-entry
+ (cdr (org-make-tags-matcher org-crypt-tag-matcher))
+ org--matcher-tags-todo-only)))
+
+;;;###autoload
+(defun org-crypt-use-before-save-magic ()
+ "Add a hook to automatically encrypt entries before a file is saved to disk."
+ (add-hook
+ 'org-mode-hook
+ (lambda () (add-hook 'before-save-hook 'org-encrypt-entries nil t))))
+
+(add-hook 'org-reveal-start-hook 'org-decrypt-entry)
+
+(provide 'org-crypt)
+
+;;; org-crypt.el ends here