Convert GPG keys to subkeys

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Revision as of 23:31, 21 March 2011 by MacFreek (Talk | contribs) (Create New Subkeys)

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This is a short log (how to) I converted my PGP key to PGP subkeys.

First of all, make yourself familiar with your key.

Start and Desired Situation

I had the following key:

pub  1024D/B804CF07 2006-02-05 Freek Dijkstra <> [usage: SCA]
uid                            Freek Dijkstra <>
uat                            [jpeg image of size 5225]
sub  2048g/2C33CF18 2006-02-05 [usage: E]

Desired end situation:

pub   1024D/B804CF07 2006-02-05 [usage: SCA]
uid                  Freek Dijkstra (born 1975-07-03 in Hilversum, the Netherlands)
uid                  Freek Dijkstra <>
uid                  Freek Dijkstra <>
uat                  [jpeg image of size 5225]
uid                  Freek Dijkstra <>
sub   2048g/2C33CF18 2006-02-05 [expires: 2010-03-18] [usage: E]
sub   1024D/8B02309E 2008-02-26 [expires: 2010-03-18] [usage: S]

(Note: the usage information can be retrieved using gpg --edit-key 0xB804CF07 list)

My desired changes were:

  • I wanted me as my identity, rather than my e-mail address. After all, that is what is checked in PGP keysigning
  • I wanted a master key, with temporary subkeys, so that if my laptop is compromised, I do not have to invalidate my PGP key, and start all over again.
  • I did not want to get a new key, and start key signing from scratch.


The implementation was in fact more straightforward than I thought.

The most important decision was if I should get a stronger key or not. My current master key is a 1024 bit DSA key. Some security experts move to 4096 bit RSA (DSA and RSA are roughly the same strength, but the DSA specs says that only 1024 bit is allowed).

I decided not to follow this trend. I think it is far more likely that someone cracks my secret passphrase than it is to crack my 1024-bit DSA key, so increasing the key size would not gain me anything. Also, it would force me to either get a new key, or go through a complex process of changing my main key into a subkey.

Use a masterkey

A very important concept is that a PGP key contains mulitple subkeys. This is my configuration:

  • A master(sub)key for certification (signing of other keys)
  • An encryption (sub)key
  • A signing (sub)key

The mastersubkey should be locked aways safely, and off-line. The encryption and signing keys are temporary keys, which can easily be revoked if they are compromised. The private part of the mastersubkey must never be present on an online machines. The private part of the encryption and signing keys are present on on-line computers, as they are in active use.

Create a signing only subkey:

gpg --edit-key 0xB804CF07
choose 2 (DSA, sign only) or 5 (RSA, sign only)

Set an expiration date for the existing encryption only subkey:

gpg --edit-key 0xB804CF07
key 2

Remove private part of masterkey

First, and most important: make a backup of your key. Really. Do that now.

gpg --export 0xB804CF07 > publickey.gpg
gpg --export-secret-keys > secretkeys.gpg
gpg --export-secret-subkeys > secretsubkeys.gpg

To remove the private (secret) part of the subkey:

gpg --delete-secret-key 0xB804CF07
gpg --import publickey.gpg
gpg --import secretsubkeys.gpg

The only thing that has been removed after this procedure is the private part of the master key. The private parts of the subkeys are still there. That is exactly what you want.

Store publickey.gpg and secretkeys.gpg in a safe location. That is, off-line.

Change Primary Identity

To add a new identity, first add it:

gpg --edit-key 0xB804CF07
Enter yhe UID of your choice.
key 3

At the "key 3" step, make sure to choose the new UID. In this case, I added an UID with my name and date of birth, and without an e-mail address.

Replace Temporary Subkeys

In my desired situation, I have a static certification key (the master key), and temporary encryption and signing subkeys. I let these keys expire in 25 months, meaning that I must create new subkey after two years.

gpg --edit-key 0xB804CF07
[choose 2 (DSA, sign only) or 5 (RSA, sign only)]

gpg --edit-key 0xB804CF07
[choose 4 (Elgamal, encrypt only) or 6 (RSA, encrypt only)]

Note that I do not remove the old keys. I still need (the private part of) the old encryption subkey and (the public part of) the old signing subkey to respectively decrypt old e-mails and to verify signatures of old e-mails.

If your key has multiple signing (usage: s) or encryption (usage: e) subkeys, GPG will pick the one with the latest creation date. That is what you want.

Off-line Actions

Since the key on my computer no longer has a secret (master) key, I can no longer use it for signing (neither can a culprit who steals my laptop).

For all administrative actions, I use a USB stick. It would be more secure to dedicate a specific machine for that purpose, but I regard a USB key safe enough. Just be sure to store the USB key at home and never take it with you on the road.

My USB stick contains the following layout:


The README is basically the following text (since I can't remember these things by head). The 2009-02-22 directory contains a backup of my key (both public and secret keys). The gnupg directory contains all keys, but with secret key.

Before I run any administrative PGP task, I insert my USB key and set $GNUPGHOME variable.

export GNUPGHOME=/mnt/keys/gnupg

After I am finished, I unset $GNUPGHOME and eject the USB key from my computer.

Sign Other Keys

Singing another key is simple:

export GNUPGHOME=/mnt/keys/gnupg
gpg --recv-key 0x12341234
gpg --sign-key --ask-cert-level 0x12341234
gpg --edit-key 0x12341234 trust
gpg --send-key 0x12341234

To load it in my online keyring:

gpg --recv-key <key>

Create New Subkeys

This works the same as described above.

First backup the key (if you haven't done so already). This is just a temporary backup in case you screw up.

export GNUPGHOME=/mnt/keys/gnupg
gpg --export 0xB804CF07     > 2009-02-22/B804CF07-publickey.gpg
gpg --export-secret-keys    > 2009-02-22/B804CF07-secretkey.gpg
gpg --export-secret-subkeys > 2009-02-22/B804CF07-secretsubkey.gpg

Of course, you should not store this backup on your online computer, but on the USB stick. Otherwise it would still be online after you remove the USB stick.

Then create the new subkeys. In my case, one sign-only subkey, and one encrypt-only subkey.

gpg --edit-key 0xB804CF07

My current choices (for no specific reason) are DSA (sign only) and Elgamal (encrypt only), both 2048 bits and valid for 750 days.

Third, you should revoke the old keys:

gpg --edit-key 0xB804CF07
key 1      # select the subkeys to revoke (watch the * in front of the subkeys)
key 2 

Remember not to delete the keys: you will need the old decryption key to decrypt old files.

If you made a mistake during this process, simply delete /mnt/keys/gnupg, and import your backups (you did make those backups, did you?)

If you are happy and confident that everything works as you want (you may want to analyze your key if you feel unsure), you can commit your changes. Step four starts by making the same export as we did for the backup.

export GNUPGHOME=/mnt/keys/gnupg
gpg --export 0xB804CF07     > 2009-02-22/B804CF07-publickey.gpg
gpg --export-secret-keys    > 2009-02-22/B804CF07-secretkey.gpg
gpg --export-secret-subkeys > 2009-02-22/B804CF07-secretsubkey.gpg

This export can be used to import the changes to your regular keychain. (Be sure not to accidentally import your master secret.)

gpg --import 2009-02-22/B804CF07-publickey.gpg
gpg --import 2009-02-22/B804CF07-secretsubkey.gpg

Finally, let the rest of the world know:

gpg --send-key 0xB804CF07