[BUG] org-crypt: CRYPTKEY property does not work as expected [9.5.1 (9.5.1-g36086a @ /home/ignacio/.emacs.d/elpa/org-9.5.1/)]

[Ignacio Casso <ignaciocasso@hotmail.com>]

Hello,

I think I have found a bug in org-crypt, or that org-crypt documentation
is not clear enough. The following configuration snippet in the
org-crypt section of the org manual, as well as the docstring for
org-crypt-key, suggest that a key value of nil can be used to specify
symmetric encryption:

  (setq org-crypt-key nil)
  ;; GPG key to use for encryption
  ;; Either the Key ID or set to nil to use symmetric encryption.

The same section of the manual, and again the docstring for
org-crypt-key, suggest that the value of org-crypt-key can be overridden
with the CRYPTKEY property.

However, if I set in my configuration org-crypt-key to nil, to use
symmetric encryption by default, and try to override it in a heading
setting the CRYPTKEY property to one of my keys, it still uses symmetric
encryption and prompts me for a password.

And if I set in my configuration org-crypt-key to one of my keys, to use
it by default, and try to override it in a heading setting the CRYPTKEY
property to nil to use symmetric encryption, it still uses my key and
does not prompt me for a password.

In both cases, if I use as key value a string that does not match any
key instead of nil (e.g, the empty string ""), which also means
symmetric encryption if epa-file-encrypt-to is not set according to the
docstring of org-crypt-key, everything works as expected.

So maybe that is the only value that should be used to specify symmetric
encryption when you want to combine it with asymmetric encryption
depending of the file or entry, but if that is the case I think the
documentation does not insist on that fact enough. It's true that the
docstring for org-crypt-key does say "If this variable is nil, always
use symmetric encryption, unconditionally.", and that "unconditionally"
could be interpreted as even if a CRYPTKEY property has been set, but
for me that is not clear enough and I only considered that
interpretation after having this issue and debugging it for a while.

To exhibit the behaviour exposed, the following simple org and init
files and playing around with the value of org-crypt-key suffice:

Org file:

  * Encrypted heading 1                                                   :crypt:
    :PROPERTIES:
    :CRYPTKEY: <some-key-id>
    :END:
    This text should be encrypted using <some-key>, regardless of the
    value of org-crypt-key
* Encrypted heading 2                                                   :crypt:
    :PROPERTIES:
    :CRYPTKEY: nil
    :END:
    This text should be encrypted using symmetric encryption,
    regardless of the value of org-crypt-key

Init file:

  (require 'package)

  (package-initialize)

  (require 'org)

  (require 'org-crypt)

  (org-crypt-use-before-save-magic)
  (add-to-list 'org-tags-exclude-from-inheritance "crypt")
  (customize-set-variable org-crypt-disable-auto-save t)

  (setq org-crypt-key nil)
  ;; (setq org-crypt-key "<some-key-id>")


Emacs  : GNU Emacs 26.3 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.14)
 of 2020-03-26, modified by Debian
Package: Org mode version 9.5.1 (9.5.1-g36086a @ /home/ignacio/.emacs.d/elpa/org-9.5.1/)