For a long time I have wanted to switch from using PGP 2.x to using GPG
(because it's Free).  But I didn't want to give up my trusty PGP key,
which has a long history and has gathered a lot of signatures that
I would have trouble getting again.

Now that the RSA patent has expired, GPG comes with RSA support
by default.  This means that it's actually possible to convert
your PGP key to a GPG key, and use it normally with GPG.  You
will not need gpg-idea or any other non-free component.

The steps are quite easy.  I wrote them down here, because I expect
that a number of Debian maintainers are in the same situation
and can benefit from this advice.

Note that the GPG FAQ contains a section about the same thing,
but it's aimed at maintaining compatibility with PGP 2.x.
The procedure I describe here is designed solely to switch
to GPG without giving up your PGP key.  It will not let you
exchange encrypted mail with someone who still uses (only) PGP 2.x.
But if you ever need to do that, you can pull out your old copy
of PGP for that -- it's the same key!

Here are the steps:

  0. Make sure your version of GPG supports RSA.  You should be
     okay if you have GPG 1.0.3 or newer.  I used GPG 1.0.4 myself.

  1. Back up your .pgp directory, and your .gnupg directory (if any).
     Note: I started with an empty .gnupg directory for this.
     If you already have a GPG ring with a set of keys, you'll
     end up having two keys with probably the same userids.
     That won't break anything, but it can be confusing and it
     is easy to accidentally use the wrong key.

  2. Remove the passphrase from your PGP secret key.  This is necessary
     because the secret key is protected using the IDEA algorithm, and
     IDEA is still patented so GPG does not support it.
     Command:
     pgp -ke userid
     where userid is what you use to access your key, usually your name
     or email address.  Just hit Enter when PGP asks for the new
     passphrase.

  3. Import your secret and public keys into GPG.
     Command:
     gpg --import $HOME/.pgp/secring.pgp $HOME/.pgp/pubring.pgp
     There doesn't seem to be a way to tell GPG to import only selected
     keys from those files, so you may want to first use PGP to export
     your keys to a small keyring.  (Do not use ASCII armor for that.)
     I simply imported all the keys, because I wanted to convert to using
     GPG for everything.

  4. Re-protect your secret key with a passphrase.
     You removed the passphrase in step 2, you can now put it back.
     Command:
     gpg --edit-key userid
     GPG will tell you what key you accessed and prompt you to do
     something with it.  Tell it "passwd" to change the passphrase.

  5. Check if everything worked.  Sign and encrypt a small file,
     mail it to a friend, see if you can decrypt something mailed
     to you, upload a Debian package, that sort of thing.

  6. Clean up the backups.  Remember, you un-protected your PGP key.
     It's still not protected, and it's the same key you are now using
     with GPG.  Make sure no one can get at it.  You can restore
     the backup, or run pgp -ke again, or -- if you don't plan to ever
     use PGP again -- delete the key.  There is a program in fileutils
     called "shred" which may help with this.

Well, I hope this helps someone.  I wrote this down after exactly one
experience with converting keys -- I'd love to have feedback from
someone else who tried it.

Richard Braakman
with tips from Adam Rogoyski, Marco d'Itri, and Andrew Pimlott.