New answers tagged gnupg
Symmetric encryption does not rely on public/private keys (so there is no need and possibility to select a key). It does not use a key in your key ring at all. The symmetric session key (which would be encrypted with a public key without the --symmetric flag) is derived from a passphrase instead.
Synchronization through exporting and importing is safe, but be aware GnuPG cannot merge secret subkeys but starting with GnuPG 2.1 -- so if you change anything with the subkeys in GnuPG 2.1, you'd have to delete the whole key in GnuPG 1 before importing. The other way round should be safe, though. I'm not sure if you have to export/import ownertrust for ...
From my experience gpg4win's Outlook integration is not very realiable. To have a check if the key material on Windows is correct, you might try to install gpg4o. It also includes a decent keymanagement. Unfortunately it is not freeware, but it's trial mode should be sufficient to test the keys.
GnuPG does not have an option to use "unusable" keys anyway (ie., those that are revoked or expired). You'll have to fiddle with the system time, as already proposed. Faking Time with GnuPG Debug Options GnuPG even has an option for this (reading unix epoche timestamps or ISO date values like --faked-system-time 2010-12-24T180000), but this is only enabled ...
I just played it a little on GnuPG's command line but I found no option to persuade GnuPG to use an expired key. You can tweak your own computers clock but this might create other problem. So the only hope is that your friend creates a new key or extends the expiration date of the existing key.
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