Is it possible to use PFS with a PGP message?
$ gpg2 --encrypt --armor --something? --something? -r [email protected]
PFS with GPG probably isn't possible -- otherwise everyone would be using it already, right? But I thought I'd ask anyway.
Forward secrecy occurs when you delete all copies of a key that exist anywhere in the world, thereby converting all past communications encrypted with that key into opaque garbage (until such a time when algorithms/computers are able to crack the key). Forward secrecy requires you to set up temporary keys that are impossible for anyone (including yourself) to find later on.
If you want to do forward secrecy with GPG, here are a couple of options:
In each case you set up a finite time window (can be very short in the latter case) during which time you can read the message. Afterwards, you discard the key and your forward secrecy begins.
It is essential that you make the private key unrecoverable. This requires some care. Ideally your temporary key only stays in your computer's RAM or on a flammable piece of paper, because once it hits harddrive, it's very difficult to ensure all copies are wiped out. Unfortunately, GPG software typically requires secret keys to be stored on a secring file. Either A) create a temporary GPG keyring on a ramfs, or B) save on your drive but use a very strong randomized passphrase that you will discard. (don't store this phrase in a text editor; don't use tmpfs -- their memory may be swapped to harddrive)
If the message is very sensitive, you should take similar precautions to wipe out all copies of the cleartext or remove incriminating parts, after you decrypt it. If you want deniable authentication, that is a separate topic and requires some care.
If neither of the options above sounds like perfect forward secrecy, too bad --- that's as good as it gets in any protocol! Forward secrecy requires you to have full trust in your hardware and software for the entire time when your temporary key is live. It only guarantees that you are protected if you delete your key before your setup is compromised.