[openpgp] Remove session key checksum and padding for v6 ECDH

[Daniel Kahn Gillmor <[e-mail]>]

Hi OpenPGP folks--

in https://gitlab.com/openpgp-wg/rfc4880bis/-/merge_requests/223 we have
a proposal to remove the checksum and padding steps for ECDH when used
with v6 PKESK:

   https://mailarchive.ietf.org/arch/msg/openpgp/em4s5PL9GO7EUrigDbQdc8TOP9c

(the MR currently reads "v5", but Aron Wussler has already volunteered
to update it to v6 assuming that we land !238)

Given that we've added the new algorithm IDs for X25519 and X448,
implementations adopting those algorithm IDs will *not* be using the
ECDH mechanism in v6 at all, so this change would apply only to keys
using the other ECDH mechanisms (e.g., NIST or Brainpool curves).

As i understand it, the rationale for dropping these pieces in v6 ECDH
is that they are unnecessary:

 - ECDH already uses a standard keywrapping function
 
 - the padding doesn't hide anything (as the SEIPDv2 packet exposes the
   choice of symmetric algorithm directly)

 - the padding also doesn't fill any needed space, since the algorithms
   used are all multiples of 8 octets anyway.

The downside, expressed by Daniel Huigens on gitlab, is that this
creates a bit of additional complexity for implementers who want to
support both v4 and v6 ECDH, and it isn't necessary for the CFRG curves
anyway now that they have their own dedicated algorithm IDs and PKESK
wrapping formats.

Please use this thread to endorse or object to this proposal to change
the value keywrapped in v6 ECDH PKESK packets!.

       --dkg