Re: [openpgp] Put Signature in an Email's Header

[Kai Engert <[e-mail]>]

On 11.12.20 11:14, Neal H. Walfield wrote:
> On Fri, 11 Dec 2020 10:03:44 +0100,
> Wiktor Kwapisiewicz wrote:
>> On 11.12.2020 09:58, Hanno Böck wrote:
>>> FWIW I have heard lots of complains about the signature attachment,
>>> which is why I stopped sending signed emails by default a long time ago.
>>
>> Too bad signatures cannot be put into headers (just like DKIM
>> signatures). That would not confuse people (because they generally
>> don't inspect headers) while providing necessary info for clients that
>> understand them.
> 
> When you say cannot, I think you mean there is no standard to so.
> Technically, I think, it is possible, and it is a neat idea.


Thanks Wiktor and Neal for this inspiration.

Your suggestion didn't solve the requirement I had in the past, but 
after all, I like the suggestion to put a digital signature into an 
email header, because it could allow digital signing emails with OpenPGP 
to become more ubiquitous - solving the problem that Hanno had reported.

(I filed a tracker bug at [1] to consider adding that ability to 
Thunderbird.)

I agree that it would be good to standardize it.
How would it be done?

Instead of ...

===[ start of multipart signed message ]===
   Content-Type: multipart/signed; micalg=pgp-sha256;
    protocol="application/pgp-signature";
    boundary="------------EJEW0mgGSuTOKqQ8iZH7hKyL"

   --------------EJEW0mgGSuTOKqQ8iZH7hKyL
   Content-Type: text/plain; charset=UTF-8; format=flowed

   The inner message

   --------------EJEW0mgGSuTOKqQ8iZH7hKyL
   Content-Type: application/pgp-signature; name="OpenPGP_signature.asc"
   Content-Description: OpenPGP digital signature
   Content-Disposition: attachment; filename="OpenPGP_signature.asc"

   -----BEGIN PGP SIGNATURE-----

   wsF5BAABCAAjFiEEIdFuZ...
   =9HqM
   -----END PGP SIGNATURE-----

   --------------EJEW0mgGSuTOKqQ8iZH7hKyL--
===[ end of multipart signed message ]===


... it could be done as in the following example below?

===[ end of message with signature in header ]===
   Header-Signature: micalg=pgp-sha256;
    protocol="application/pgp-signature"; sigdata=
    wsF5BAABCAAjFiEEIdFuZ...
    =9HqM
   Content-Type: text/plain; charset=UTF-8; format=flowed

   The inner message
===[ start of message with signature in header ]===


Would this have to be suggested in a new draft document that intends to 
update PGP/MIME, RFC 3156 ?

The document should probably say how to handle a message that contains 
both this header and another signature at the topmost MIME layer. Should 
one of the layers be ignored?

Thanks,
Kai


[1] https://bugzilla.mozilla.org/show_bug.cgi?id=1847695