[ TechnoCage | Caskey | gnupg | pgp 2 gpg ]

# Note: THIS INFORMATION ONLY APPLIES TO PRE-1.0 VERSIONS OF GNUPG

# Moving from PGP to GnuPG

Arghhh! How do I switch to GnuPG when I (and my friends) already use PGP?

I recently had my old PGP 5.0 key brought back from the dead when a colleague of mine wanted to send me some private info and all he had available was PGP 5.5. Luckily I had created a DSS key under 5.0 and so I figured I may be able to make use of my old keys. I regret not setting an expiration date on my old key as it would have been very useful in this case. I now have four keys that I must deal with. Two RSA (PGP 2.6), one DSS/Diffie-Hellman (PGP 5.0) and one DSA/ElGamal (GnuPG 3.6).

Always set an expiration date on your keys.

That said, there are several issues at hand. First, I have a pair of DSS/Diffie-Hellman keys that my friend is using PGP 5.5 to communicate with me. Second, I have a DSS/Diffie-Hellman public key that my friend is using. Thirdly, I have a copy of gnupg 0.4.0.

This document has two parts. The first describes how to prepare to encrypt messages to a user of PGP5.x. The second details how to take a PGP5.x KEY and install it on your GnuPG keyring so you may decrypt messages from a PGP5.x user.

If these instructions do or do not work for you I would like to hear about it. Thanks to all the people on the GnuPG mailing list who have unwittingly provided most of the information I used in creating this.

## Encrypting TO a user of PGP 5.0+

### Fetch user's key from remote key server

In order to encrypt mail to my friend, I must be able to use GnuPG to encrypt against a DSS/Diffie-Hellman key. I used PGP to retrieve his public key from the keys.pgp.com public key server.

> `$ pgpk -a hkp://keys.pgp.com/friend@sample.com
> `

>     Looking up host keys.pgp.com
>     Establishing connection
>     Sending request
>     Receiving data
>     Cleaning up
>     Complete.

>     Adding keys:

>     Key ring: 'hkp://keys.pgp.com/taylor@technocage.com'
>     Type Bits KeyID      Created    Expires    Algorithm       Use
>     pub  1024 0x01234567 1998-10-10 ---------- DSS             Sign & Encrypt
>     sub  2048 0x89ABCDEF 1998-10-10 ---------- Diffie-Hellman
>     uid  Friend <friend@sample.com>

>     1 matching key found

>     Add these keys to your keyring? [Y/n] y

>     Keys added successfully.

### Verify key actually belongs to friend

First, I printed out the key ID and fingerprint of each of the keys so I can ask my friend if they match.

> `$ pgpk -ll friend@sample.com

>     Type Bits KeyID      Created    Expires    Algorithm       Use
>     pub  1024 0x01234567 1998-10-10 ---------- DSS             Sign & Encrypt
>     f20    Fingerprint20 = 0123 4567 89AB CDEF FEDC  BA98 7654 3210 DEAD BEEF
>     sub  2048 0x89ABCDEF 1998-10-10 ---------- Diffie-Hellman
>     f20    Fingerprint20 = FFFF DDDD 8888 5555 3333  2222 1111 0000 BAEF FADE
>     uid  Friend <friend@sample.com>
>     sig       0x01234567 1998-10-10 Friend <friend@sample.com>

> `

I then phoned my friend and asked him to verify the key fingerprint and ID over the telephone. As luck would have it, the information matched up.

### Export key to ascii file

After verifying that the key was his, I exported it like so:

> `$ pgpk -xa friend > friend.key

> `

### Import key to GnuPG

I then used gnupg to import that key.

> `$ gpg --import < friend.key`

>     gpg (GNUPG) 0.4.0; Copyright (C) 1998 Free Software Foundation, Inc.
>     This program comes with ABSOLUTELY NO WARRANTY.
>     This is free software, and you are welcome to redistribute it
>     under certain conditions. See the file COPYING for details.

>     gpg:[stdin]: key ABCDEFGH: public key imported

I could have used the following command as well.

> `$ pgpk -xa friend | gpg --import`

>     gpg (GNUPG) 0.4.0; Copyright (C) 1998 Free Software Foundation, Inc.
>     This program comes with ABSOLUTELY NO WARRANTY.
>     This is free software, and you are welcome to redistribute it
>     under certain conditions. See the file COPYING for details.

>     gpg:[stdin]: key ABCDEFGH: public key imported

### Create test message for friend

Now that I have loaded my friends key onto my GnuPG keyring, I can make a little test message for him to try decrypting.

> `$ echo 'Hello Friend!' | gpg -ear friend`

>     -----BEGIN PGP MESSAGE-----
>     Version: GNUPG v0.4.0 (GNU/Linux)
>     Comment: Get GNUPG from ftp://ftp.guug.de/pub/gcrypt/

>     ibxH/34oeT+0yFF0qQYqSp1AXTOfKV1Khw6GXgyxrnAbUozYFHuNaSU1uI2VlezTosb+ZFpu
>     03+PUoiyGbq/YPgcsoZOv65TSd8q3b2JbbMDsp9SAMq3WyIRffw2YBHwCu3Jxw/lQjGEkw8r
>     Q5Nea1Ph2cFby6f7abIauEKWcdBxb7Nk5VOBN7yVPcp4dNBPIOijB8v/EeMz8a16YXQVYxk5
>     FmXuh4PzLJ09woaJRbYAJPVtLXiAHANYrqonfJ8tvM3ziP6mC20CRQebV9YWguMXJTYEC6An
>     hQIOAx1s/MTwZKdhEAf/WZnNy7RILDVxKJperR3k2xOPy5rNxRXxSVFvboXEQf/B01zvE8Fn
>     udEgGwqE808X6sbxVZ7JWiOG7k0uk+M1Dxsi7ujzZdL6rWYq830DnC/LmtWs/XM2V9HwnoRo
>     CUlEsXi7zPykSxhAm5TAMhn2Al5iFrF/y3ROLf2DK+xUflZxs+uvnLLl2vRRKYmy0DXdE0z1
>     EKB2KNMPFVruuphzspH1wrNGxKs3WU0THEp/GvVj6WR0fCjQ9n5qBaKuT8kknbXbgm5zJCTC
>     J+KNOw2VwOCI7lKeKvzrr/p7tg8gasRnca3a9u3cTXXWk3cWYKcw/mdghwUUwmOUCvTeZI2k
>     MwgA5520XYbuDi2HX0LeOKf7DEVXOPixRrrnydJcnjoAYLYO3/xpQqh0IzJ1P1HwZbVnJVNl
>     z+yXNvfzAVedlkQ+98G/yraLtzASWNwbpaQZ
>     =xYcK
>     -----END PGP MESSAGE-----

I take the resulting cyphertext and paste it into an email addressed to my friend. Now we wait....

We wait some more (my friend is notoriously slow to respond to messages).

Finally after a long (long) time, my friend responds that he was able to read my message. Of course, he is kind enough to use my PGP key to send this. Thus we segue into the next section on using GnuPG to decrypt messages sent by users of PGP5.0 to DSS/DH keys generated by PGP5.0.

One important note is that my friend was using PGP5.5, not 5.0. In later tests with pgp 5.0i I discovered that you must add two more parameters to the encryption command. Specifically `--no-armor --no-comment`

> `$ echo 'Hello Friend!' | gpg -ear --no-armor --no-comment friend > mesg`

Remember that `gpg --no-armor` will produce 'binary' output and so you are wise to stick it in a file and then attach that file to an email.

## Decrypting with a PGP DSS/Diffie-Hellman key

Now the problem is, how do I make it so that my friend can use my GPG key to send me messages? Is this even possible?

I must now import my old DSS/Diffie-Hellman key onto my GnuPG key ring. This is not as easy as one may think. It is complicated by the fact that the PGP key utility `pgpk` does not have a parameter for exporting a private key. As luck would have it, superior software packages such as GnuPG are capable of solving this problem for us.

It is important to note that this procedure puts your private key at risk for a short period of time and therefore should not be done on a multi-user or public system

### Step one: import your public key

This is the easy part. We use pgpk to extract your public key from your keyring and import it into GnuPG.

> `$ pgpk -x 7BBD08DC | gpg --import

>     gpg (GNUPG) 0.4.0; Copyright (C) 1998 Free Software Foundation, Inc.
>     This program comes with ABSOLUTELY NO WARRANTY.
>     This is free software, and you are welcome to redistribute it
>     under certain conditions. See the file COPYING for details.

>     gpg:[stdin]: key 7BBD08DC: not changed
>     gpg:[stdin]: key 00000000: no valid user ids
>     gpg: this may be caused by a missing self-signature
>     gpg:[stdin]: key 0809AD24: no valid user ids
>     gpg: this may be caused by a missing self-signature

> `

A quick check of the gnupg keyring shows that my key has, in fact been imported properly. I don't entirely understand the output of the import however nothing in it seems troublesome. Perhaps someone who knows more than I do can explain it to me.

> `$ gpg --list-keys

>     gpg (GNUPG) 0.4.0; Copyright (C) 1998 Free Software Foundation, Inc.
>     This program comes with ABSOLUTELY NO WARRANTY.
>     This is free software, and you are welcome to redistribute it
>     under certain conditions. See the file COPYING for details.

>     pub  1024D/7BBD08DC 1997-07-23 Caskey L. Dickson
>     uid                            Caskey L. Dickson
>     uid                            Caskey L. Dickson
>     sub  4096g/2B65D18B 1997-07-23

> `

That certainly does look like my key.

### Remove passphrase from private PGP key

This is the dangerous part. Depending upon your paranoia level you can do everything from a simple `w` in order to see who else is on your machine to unplugging your network card and modem from the wall. It all depends upon the environment you operate in and how much you trust the sytem you are using.

> `$ pgpk -e 0x7BBD08DC

>     sec+ 1024 0x7BBD08DC 1997-07-23 ---------- DSS             Sign & Encrypt
>     sub  4096 0x2B65D18B 1997-07-23 ---------- Diffie-Hellman
>     uid  Caskey L. Dickson <caskey@one.sample.com>
>     uid  Caskey L. Dickson <caskey@two.sample.com>
>     uid  Caskey L. Dickson <caskey@three.sample.com>

>       1024 bits, Key ID 0x7BBD08DC, created 1997-07-23
>        "Caskey L. Dickson <caskey@one.sample.com>"
>        "Caskey L. Dickson <caskey@two.sample.com>"
>        "Caskey L. Dickson <caskey@three.sample.com>"

>     Do you want to unset this key as axiomatic [y/N]? N

>     Do you want to unset this key as axiomatic [y/N]? N

>     Do you want to add a new user ID [y/N]? N

>     Do you want to change your pass phrase (y/N)? Y
>     Need old passphrase. Enter pass phrase: passphrase
>     Need new passphrase. Enter pass phrase: *nothing*
>     Enter it a second time. Enter pass phrase: *nothing*
>     Changing master key passphrase...
>     Changing subkey passphrase...

>     Do want to set this as your default key [y/N]? N
>     Keyrings updated.

> `

### Export private key into GnuPG

Now that we have removed the passphrase from the key we can export it using GnuPG.

> `$ mkdir ~/private
> $ chmod 700 ~/private
> $ chdir ~/private
> $ gpg --armor --export-secret-keys --secret-keyring ~/.pgp/secring.skr 0x7BBD08DC > mykey.sec

>     gpg (GNUPG) 0.4.0; Copyright (C) 1998 Free Software Foundation, Inc.
>     This program comes with ABSOLUTELY NO WARRANTY.
>     This is free software, and you are welcome to redistribute it
>     under certain conditions. See the file COPYING for details.

>     gpg: key 456260DC: secret key without public key
>     gpg: failed to initialize the TrustDB: Public key not found

> `

The messages about not having a TrustDB entry and a missing public key are more or less normal as we did not specify the matching public key ring on the GnuPG command line.

### Import secret key file to GnuPG

We have a file named mykey.sec which contains an ascii armored private key sans passphrase. Now we must quickly load it into our keyring.

> `$ gpg --import < mykey.sec

> `

I don't have the output of this command (yet).

### Set passphrase on both keys IMMEDIATELY

We must quickly put the passphrases back onto *both* keyrings now.

Some of you have asked 'what the heck does the third instruction do?' It's actually a rather simple trick. The `mykey.sec` file contains an un-protected secret key. Obviously we want to get rid of it ASAP. If we were to just `rm` the file, yes that would eliminate the file, however the now unused blocks would be floating around on the disk somewhere with your secret key bytes still in it. Not a terrible thing, but if you're going to jump off the building from the 10th floor, why not make it the 100th and enjoy the view on the way down. Seriously though, what we've done is take the GnuPG binary and overwritten the secret key file. This (mostly) ensures that the data is gone. It isn't as strong as the techniques used by the military (xor, 3 writes) but I'm assuming that you aren't worried about someone disassembling your hard drive to find that file.

> `$ pgpk -e 0x7BBD08DC
> $ gpg --edit-key 0x7BBD08DC
> $ cat `which gpg` > mykey.sec
> $ rm mykey.sec

> `

### Test key.

Now we have our public and private DSS/Diffie-Hellman key on our GnuPG key ring. We shall employ pgp to create a test message for us to decrypt.

> `$ echo 'hello world' | pgpe -r 0x7BBD08DC | gpg --decrypt

>     No files specified.  Using stdin.

>       1024 bits, Key ID gpg (GNUPG) 0.4.0; Copyright (C) 1998 Free Software Foundation, Inc.
>     This program comes with ABSOLUTELY NO WARRANTY.
>     This is free software, and you are welcome to redistribute it
>     under certain conditions. See the file COPYING for details.

>     7BBD08DC, Created 1997-07-23
>        "Caskey L. Dickson <caskey@one.sample.com>"
>        "Caskey L. Dickson <caskey@two.sample.com>"
>        "Caskey L. Dickson <caskey@three.sample.com>"

>     You need a passphrase to unlock the secret key for
>     user: "Caskey L. Dickson "
>     (4096-bit ELG-E key, ID 2B65D18B, created 1997-07-23)

>     hello world

> `

And thus, we have used gpg to decrypt a message encrypted with PGP5 and a PGP5 key.

Comments welcome.

Copyright © 1998
Last modified: 1998-10-12