restrict embedding to embeddable signature types - #325

I proposed something with similar effect here: https://datatracker.ietf.org/doc/html/draft-gallagher-openpgp-signatures#name-recursive-embedding-inside-

It differs from this patch by classifying signature types as "embeddable" and "non-embeddable" and then only permits embeddable signature types to be embedded in non-embeddable types, thus preventing infinite recursion without counting nesting.

good catch, thanks for the link. reworked it to follow that model - an embedded signature must be an embeddable type, and embeddable types can't themselves carry an embedded signature, so the recursion gets rejected while parsing the inner sig without counting depth. pushed an update

Hi 👋 Thanks for the PR! However, this is now somewhat conflating adding (partial) support for the Third-Party Confirmation signature type, and fixing the original issue you pointed out. Since we don't support Third-Party Confirmation signatures for now, we can just check that the embedding signature is not a Primary Key Binding signature and call it a day, IMHO.

To be fully effective, it needs two checks:

1. a primary key binding signature cannot contain an embedded signature
2. an embedded signature must be a primary key binding signature

Yeah, but the second one we already have:

Aha, I didn't understand what you meant by "embedding signature". Yes, agreed. 🫣

hmm, makes sense. give me a few minutes, i'll try cutting it down to just that check

pushed. just the primary key binding check now, dropped the third-party confirmation bits and left the existing embedded-type check alone