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GnuPG Notes

temporarily change keystore location

Useful for testing and for offline keys. Use Live CD for offline keys! Tails brings all you need in a live CD.

export GNUPGHOME=/save/location

some default preferences

cat >>~/.gnupg/gpg.conf <<EOF
no-version
personal-digest-preferences SHA512
cert-digest-algo SHA512
default-preference-list SHA512 SHA384 SHA256 SHA224 AES256 AES192 AES CAST5 ZLIB BZIP2 ZIP Uncompressed
ask-cert-level
cert-policy-url http://www.headstrong.de/keysigning-policy
EOF
  • hashing algorithms
  • disable version display
  • set default policy url for signing

key generation

My recommendation:

gpg --expert --gen-key # for master key
KEYID=ABCDABCD
gpg --expert --edit-key $KEYID
gpg> addkey # repeat for all subkeys
gpg> quit
gpg --output $KEYID-revocation-cert.gpg --gen-revoke $KEYID
gpg --quiet --batch --yes --output $KEYID-secret-subkeys.gpg --export-secret-subkeys $KEYID
gpg --quiet --batch --yes --output $KEYID-secret-master-key.gpg --export-secret-keys $KEYID
gpg --quiet --batch --yes --output $KEYID-public.gpg --export $KEYID
  • master key: capability certify ( = for signing keys)
  • separate subkeys for each other capability
  • set an expiration date on the master key and the subkeys. remind yourself to rotate subkeys and move the master key expiration date before it expires. i find 6 months to 1 year a reasonable span.

regular keyring: import only subkeys

gpg --import $KEYID-public.gpg $KEYID-secret-subkeys.gpg

when signing, specify signing policy

gpg --ask-cert-level --cert-policy-url http://www.headstrong.de/keysigning-policy --sign-key ABCDABCD

key transition statement

example: http://gagravarr.org/key-transition-2009-05-06.txt

RSA 8192 bit keys

You don't really ever need a larger key than 4096. If there's serious advances in breaking 2048+ bit RSA keys, they will go against all keysizes.

GnuPG no longer allows you to create larger keys. In previous versions, the batch mode allowed keys up to 8192 bit:

gpg --batch --gen-key <<EOF
Key-Type: RSA
Key-Length: 8192
Key-Usage: cert
Name-Real: ME
Name-Email: EMAIL
Passphrase: PASSWORD
EOF

Offline Key Storage

This section is for paranoid people only :)

You can split your secret master key into several parts to store them in different locations. This example uses Shamir's Secret Sharing. We are creating 3 parts of which you only need 2 in order to recover your key. You can change the parameters for your own needs. (ssss-split and gfsplit are already installed in Tails)

Splitting the Passphrase

$ ssss-split -t 2 -n 3 -w name_me

Type your password and hit enter. The output should look like the following:

name_me-1-ef3e98de26ce8400
name_me-2-808d8e0fc8779375
name_me-3-a5e37c40921f61a4

Recovering the Passphrase

$ ssss-combine -t 2

Enter 2 shares separated by newlines:

Share [1/2]: name_me-1-ef3e98de26ce8400
Share [2/2]: name_me-2-808d8e0fc8779375

The output should look like the following:

Resulting secret: P@ssw0rd

Splitting the Key

$ gfsplit -n 2 -m 3 secretKeyFile 
$ ls

The output should look like the following:

secretKeyFile secretKeyFile.050 secretKeyFile.179 secretKeyFile.193

Recovering the Key

$ gfcombine secretKeyFile.050 secretKeyFile.179 	

done. Your keyfile should be recovered.

Smartcard

Install

sudo apt-get install pcscd gpgsm

Init

gpg --card-status
gpg --change-pin
gpg --card-edit

Generate Subkeys on Smartcard

You don't actually want to do that because you will not have backup, except maybe for authentication subkeys.

gpg --edit-key $KEYID
gpg> addcardkey

Move Subkeys to Smartcard

Make a local copy first, as subkey will be transferred to the card and the local copy rendered unusable.

gpg -a --export-secret-keys $KEYID > $KEYID.key.asc

gpg --edit-key $KEYID
gpg> toggle
gpg> key 1 # select encryption subkey
gpg> keytocard
gpg> key 2 # select signature subkey
gpg> keytocard
...
gpg> save

Authenticate to a SSH server with GPG Smartcard

Make a Authentication Subkey on the Smartcard

gpg --edit-key $KEYID
gpg> addcardkey

Then choose option 3. Enter passphrase and key when queried.

Put the following in a text file, make sure it is run everytime your Desktop environment or window manager starts up:

#!/bin/sh
gpg-agent --daemon --enable-ssh-support > ~/.gnupg/gpg-agent.env
source ~/.gpg-agent.env

Log out of and into your Desktop environment.

Check if it is working

ssh-add -l

Copy the public key over to the server

gpgkey2ssh $AUTHSUBKEY > authorized_keys
scp authorized_keys user@server:~/.ssh/authorized_keys

Or, alternatively, use ssh-copyid

ssh-copyid user@server

Now when you ssh into this box ssh should ask for your PIN instead of your passphrase.

References

http://gagravarr.livejournal.com/137173.html

https://www.apache.org/dev/openpgp.html

http://www.eharning.us/gpg/

http://www.narf.ssji.net/~shtrom/wiki/tips/openpgpsmartcard