[snassar]

I don't know where people would run three versions at the same time. The information from the GnuPG project is clear: You can run GnuPG 1.x and GnuPG 2.x at the same time, but you should not have GnuPG 2.0 and GnuPG 2.1 installed at the same time, as bad things might happen.

What 3rd party software is using the keystorage mechanisms directly? Do you mean how information is output from GnuPG?

It sounds like the situation you are describing is the keystore, which has changed formats. GnuPG 2.1, as far as I can remember, will oll use the older versions keystore, but you are correct, once you have a 2.1 keystore it can't be used by GnuPG 2.0 and 1.x.

It's a tough call for the GnuPG developers and something distributions should help with. On one hand there is immense pressure to improve GnuPG, on the other hand, you have many actors who kick GnuPG around when it makes any deviation.

I would say defaulting to GnuPG 1.x is a bug and new releases of Linux, Homebrew, etc., should use GnuPG 2.0 at the very least, but better yet, use GnuPG 2.1 which has many of the things that people complain about fixed or in process of being fixed.

[okket]

> I don't know where people would run three versions at the same time.

Not on the same machine, but server/automatic -> GnuPG 1, desktop 2.0 or 2.1. Also different people may run different versions, even GnuPG 1 on desktop because they are used to. This compatibility mess that seems to persist for while was what I meant why people prefer to use the lowest common denominator in their sigs/cards/slides -> evil32.

> What 3rd party software is using the keystorage mechanisms directly?

Aren't there any? Good, then I misunderstood that argument in the Homebrew debates I read, sorry. That leaves only the fear of automatic upgrading and the inability to downgrade again as a blocker.

[snassar]

In practice you can use all three versions of GnuPG on three different devices without a particular difference. One problem you might see is if you are using the newer experimental curve-based algorithms on a computer running GnuPG 1.4 and you get blocked, but you really ought not to do that anyway.

As for the downgrading issue:

It used to be you could just copy your .gnupg directory from computer to computer to computer and that's what constituted migrating your PGP keys.

This was also true for moving frmo GnuPG 1.4 to GnuPG 2.x. If you are starting with a new GnuPG keystore from 2.1 you can't just copy .gnupg and use it in a GnuPG 1.4 system, you have to export your public keys, your private keys, and your trustdb (although I am iffy on what this does) and import them on the systems where you are running GnuPG 1.4 or 2.0

I am unaware of any 3rd party software directly accessing the GnuPG keystore, but that doesn't say much.