Feature suggestion: options to require MDC or trusted signature on decryption

[Patrick Brunschwig]

On 31.05.18 20:44, Werner Koch wrote:
> On Thu, 31 May 2018 16:51, patrick at enigmail.net said:
> 
>> May I suggest that for GnuPG 2.3 you implement some more rules? For example:
>> * refuse encrypting emails if MDC is not enabled in the key prefs
> 
> RFC-4880 can be read to allow using MDC even without the feature flag.
> For RFC-4880bis non-MDC will be deprected:
> 
>   This packet is obsolete.  An implementation MUST not create this
>   packet.  An implementation MAY process such a packet but it MUST
>   return a clear diagnostic that a non-integrity protected packet has
>   been processed.  The implementation SHOULD also return an error in
>   this case and stop processing.
> 
>> * remove options like --ignore-mdc-error, --ignore-mdc-warning and
>>   --allow-multiple-messages, or at least require them to be combined
>>   with something like --dangerous-options
> 
> Already done.  The MDC options in 2.3 and 2.2 are now NOPs.  The
> allow-multiple options and the --pgpg6 options are NOPs in 2.3.  For
> testing --rfc2440 can be used which has always had the effect not to
> create an MDC.

But then you contradict yourself. You wrote that gpg prints the
following messages, but ignore-mdc-error is now a NOP:

  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.**
  [GNUPG:] ERROR nomdc_with_legacy_cipher 152
  gpg: decryption forced to fail!
  [GNUPG:] DECRYPTION_FAILED
  [GNUPG:] END_DECRYPTION


-Patrick