These tests are pulled from the OpenPGP Interoperability Test Suite,
in particular from those tests tagged "forward-compat":

   https://tests.sequoia-pgp.org/?q=forward-compat

These failures make it difficult to use PGPy in an OpenPGP ecosystem
that can evolve, because PGPy will complain about OpenPGP objects that
contain something it doesn't understand, even though the rest of the
message is otherwise comprehensible and usable.

See Justus Winter's message to openpgp@ietf.org describing his
concerns:

https://mailarchive.ietf.org/arch/msg/openpgp/QUiEKx3PQeJOXnkcvvnuHpv739M

I've selected specific examples that PGPy is known to currently fail with.

a PGPMessage object can contain more than one signature.  Detached signatures should
also be able to handle having more than one signature.

https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-09.html#name-detached-signatures says:

> These detached signatures are simply one or more Signature packets
> stored separately from the data for which they are a signature.

A PGPSignatures object makes the most sense to represent such a thing.

Closes: SecurityInnovation#197

…nknown

…en discovered

The cryptography module supports encoding and decoding these signatures
directly, so no need for pyasn1 or the custom ASN1 decoder to translate
between the OpenPGP format and the RFC 3279 format.

OpenSSL 1.0.2 is ancient at this point -- Brainpool is part of the standard
distribution.  At any rate, we need 1.1.0 for X25519 and 1.1.1 for Ed25519.

And python's cryptography module has supported Brainpool since version 2.2
(also ancient).

Registering subclasses with the cryptography module is complicated across
versions (see pyca/cryptography#7234 which removed
register_interface), but we don't need any of that functionality as long as
we depend on non-ancient modules.

At the same time, we don't need pyasn1 any longer if we just treat the OID
as a bytestring label.

As this also drops all the shenanigans around
cryptography.utils.register_interface, we can also say it
Closes: SecurityInnovation#402