How can I add encrypted comments.

[Stefan Claas]

On Thu, Jan 14, 2021 at 8:16 PM Stefan Claas
<spam.trap.mailing.lists at gmail.com> wrote:
>
> On Thu, Jan 14, 2021 at 10:46 AM Ayoub Misherghi via Gnupg-users
> <gnupg-users at gnupg.org> wrote:
> >
> >
> > I am encrypting and signing documents with myself as the receiver. Nobody else will want to look inside them. Is it possible to add encrypted comments or other information to a separated signature file; and later retrieve this additional information? I want to be able to decrypt the signature file alone and retrieve all the information I put inside it.
>
> You can add Comments: to a detached signature, yes, but beware that these
> encrypted content must be seperated for each comment line.
>
> I have not tested this yet, but you could with a shell script use some format
> or lenght preserving encryption software, like Google's Adiantum with a base64
> encoder and then would have the smallest possible symmetrically encrypted
> output for a message as Comment: line. You can do this also manually
> of course as much as you wish because it does not invalidate the signature.
>
> Hope this helps a bit.

Here is a quick manually inline sig.

First message with GnuPG symmetric content in Comment lines
and second same message with Google's Adiantum+base64

You see the difference, what I mean with format preserving.

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA256

Hello World! :-)

Regards
Stefan
-----BEGIN PGP SIGNATURE-----
Comment: -----BEGIN PGP MESSAGE-----
Comment:
Comment: jA0EBwMCMx3mMIiLwjPH0mgBh3We4k31HkKJ7W8c9oju++X96uaNVB5mMEDJhhr6
Comment: Ao5wibzeivfsfFL9Si2cCc/X9kUG2maKHSwb+51nwtcFSRNT2h99SQlbMPzRkoku
Comment: EkyCpYpeq+d8gyMeJ+uNgEvtAwHF35RYVQ==
Comment: =Vain
Comment: -----END PGP MESSAGE-----

iHUEARYIAB0WIQR61Pk5PUF7u6Rs+mem3tVibXmEGgUCYACeDgAKCRCm3tVibXmE
Gpk6AP98iXZb8gd0NDvOllByTHkrcQvQluXd/db1c5u+skm90gEAj5c991XdP5s5
clB9wwK9G8XoCDJnhfMLWljuvjCM8Ac=
=XJXL
-----END PGP SIGNATURE-----

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA256

Hello World! :-)

Regards
Stefan
-----BEGIN PGP SIGNATURE-----
Comment: vHgPAUzXglLiVFelwf0jjUzXCNIqSrinvNhjF+JRkd8K

iHUEARYIAB0WIQR61Pk5PUF7u6Rs+mem3tVibXmEGgUCYACeDgAKCRCm3tVibXmE
Gpk6AP98iXZb8gd0NDvOllByTHkrcQvQluXd/db1c5u+skm90gEAj5c991XdP5s5
clB9wwK9G8XoCDJnhfMLWljuvjCM8Ac=
=XJXL
-----END PGP SIGNATURE-----

Regards
Stefan