x/crypto/openpgp: remove support for unauthenticated messages #25388
Description
Messages without authentication tags (called MDC by OpenPGP) can be modified by an attacker. Malleable plaintexts lead to all sorts of attacks, the latest example being EFail.
The spec is hopelessly broken in therms of downgrade protection, so a message originating with a MDC can be stripped to look like a normal message without MDC. So the only fix is to disable support for MDC-less messages entirely.
Also, MDC were added in 2001, and it's 2018.