Md at debian.org

tales of a debian maintainer

New PGP key

Since my current PGP key is a 1024 bits DSA key generated in 1998, I decided that it is time to replace it with a stronger one: there are legitimate concerns that breaking 1024 bits DSA is well within the reach of major governments.

I have been holding out for the last year waiting for GnuPG 2.1, which will support elliptic curves cryptography, but I recently concluded that adopting ECC now would not be a good idea: Red Hat still does not fully support it due to unspecified patent concerns and there is no consensus in the cryptanalists community about the continued strength of (some?) ECC algorithms.

So I created three fancy keys: a 4096 bits main key for offline storage, which hopefully will be strong enough for a long time, and two 3072 bits subkeys for everyday use.

I have published a formal key transition statement and I will appreciate if people who have signed my old key will also sign the new one.

What follows are the instructions that I used to generate these PGP keys. They follow the current best practices and only reference modern software.

While the GnuPG defaults are usually appropriate, I think that it is a good idea to use a stronger hash for the key signatures of very long-lived keys. I could not find a simple way to "upgrade" the algorithm of key self signatures.

echo 'cert-digest-algo SHA256' >> ~/.gnupg/gpg.conf

First, generate a RSA/4096 sign only key, which will be your master key and may be stored offline. Then add to it two RSA/3072 subkeys (one sign only and one encrypt only):

# generate a RSA/4096 sign only key
gpg2 --gen-key
# add two RSA/3072 subkeys (sign only and encrypt only)
gpg2 --edit-key 8DC968B0

Since GnuPG lacks a command to remove the master secret key while keeping its secret subkeys, you need to delete the complete secret keys and then re-import only the subkeys:

gpg2 --export-secret-keys 8DC968B0 > backup.secret
gpg2 --export-secret-subkeys 8DC968B0 > backup.subkeys
gpg2 --delete-secret-key 8DC968B0
gpg2 --import backup.subkeys

Then you can import again the complete keys in a different secret keyring, which can be stored offline:

mkdir ~/.gnupg/master/
gpg2 --no-default-keyring \
  --keyring ~/.gnupg/pubring.gpg \
  --secret-keyring ~/.gnupg/master/secring.gpg \
  --import backup.secret

Now you can move ~/.gnupg/master/ to a USB stick. You are supposed to protect the master secret key with a strong passphrase, so there is no point in using block level encryption on the removable media.

Since you are only using the master key to sign other keys, it only needs to be configured as the second keyring in ~/.caffrc:

$CONFIG{'secret-keyring'} = $ENV{HOME} . '/.gnupg/master/secring.gpg';

It is also a good idea to have an hard copy backup of your keys, since the lifetime of USB sticks should not be trusted too much:

paperkey -v --output printable.txt --secret-key backup.secret
a2ps -2 --no-header -o printable.ps printable.txt

Some references that I used:

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Sono l'Italia che non vuole bene.

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