RNP reports that it cannot decrypt if no Key ID of the Public-Key Encrypted Session Key Packets matches a known private key.

It would be great if it instead could check for "hidden" recipients. In case the Key ID 0x00000000 is present, RNP should try to decrypt with all known private keys. See https://tools.ietf.org/html/rfc4880#section-5.1 (last paragraph of that section).

Background: Currently Thunderbird 78 an later fails to decrypt messages where the user is a hidden recipient. Such ciphertexts could e.g. be created by Enigmail, or in general with GnuPG's hidden-encrypt-to config option. Downstream issue: https://thunderbird.topicbox.com/groups/e2ee/T7e31aaa38399d4b3-Mefc2734288f0dfa17d01b823/fails-to-decrypt-with-recipient-key-id-0x00000000-hidden-recipient.