do not ignore mdc errors on a permanent basis (was: org-crypt broken on Ubuntu 18.04)

[Gregor Zattler <telegraph@gmx.net>]

Hi Óscar,
* Óscar Fuentes <ofv@wanadoo.es> [2018-07-26; 13:57]:
> For the record: executing gpg2 from the command line is revealing:
>
> gpg: WARNING: message was not integrity protected
> gpg: Hint: If this message was created before the year 2003 it is
>      likely that this message is legitimate.  This is because back
>      then integrity protection was not widely used.
> gpg: Use the option '--ignore-mdc-error' to decrypt anyway.
> gpg: decryption forced to fail!
>
> The solution is to add `ignore-mdc-error' to ~/.gnupg/gpg.conf.

I hope you'll do this only as a temporary meassure.  Your could
decrypt and re-encrypt the org-crypt parts in question iff you
are sure, they were encrypted years ago and their contents is ok.

But having this option in  ~/.gnupg/gpg.conf  otherwise weakens
the security of GnuPG usage considerably.

From the gpg man page:

     --ignore-mdc-error
          This option changes a  MDC integrity protection failure
          into a  warning.  This  can be useful  if a  message is
          partially corrupt, but  it is necessary to  get as much
          data as possible out  of the corrupt message.  However,
          be aware  that a MDC  protection failure may  also mean
          that the message was  tampered with intentionally by an
          attacker.

The usage scenario described in the first sentence is clearly a
one time thing.  Putting this option in gpg.conf ignores these
kind of errors for all future usage, for risks and side effects
see the second sentence.

Ciao; Gregor