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GPG Process Notes

I followed the TAILS setup guide to get a secure offline environment running to perform this generation task. The steps I took are documented in the tails document.

Initial Key Creation

For simplicity I wanted to clear out the GnuPG configuration that starts out in place. Makes things a lot nicer later on.

rm -rf ~/.gnupg/*

I pulled in the .gnupg/gpg.conf from my dotfiles by hand.

And begin the key generation process

gpg --expert --gen-key

Choose ‘8’ which is RSA (set your own capabilities). Disable all the capabilities except for Certify and press ‘q’ to continue. Use a 4096 bit key. Set the expiration to 2 years. The certificate can be resigned and republished with a later expiration date.

Set the personal attributes appropriately, a passphrase and let the key generation happen. Once done, sub-keys need to be generated. We need to edit the existing key to create more keys.

gpg --expert --edit-key <email used in key>

Add any additional email addresses you might need to the key.

gpg> adduid
gpg> uid 1
gpg> primary

Generate the sub keys:

gpg> addkey

The above will require the master key’s private key. Create another ‘8’ type key with only the ‘Sign’ capability and 2048 bits valid for 6 months Do this again for the encryption and authentication keys. A higher bit usage may be useful, some smartcards only support 2048, if higher bits are supported 3072 or 4096 may be better.

I cropped and scale an image of myself down to 120x144 (Quality at 50% still accurately reflected my likeness quite well and came in at 2.8Kb). The image has to be under 4Kb but the smaller the better. I ended by ensuring the file was stripped of metadata and minimized as much as I could with jpegoptim -s. It has to be transferred to the secure TAILS environment and then added to the certificate while in the editkey mode we’re currently in at this point in the tutorial.

Turns out the size can be up to 240x288…

gpg> addphoto

And follow the prompts. When everything is good save the changes made:

gpg> save

Now I need to setup a backup for all the contents in case they’re lost or displaced. We need to backup the keyrings, a raw copy of the master private key, a revocation certificate just in case, then the public key. Backup and export ~/.gnupg/secring.gpg and ~/.gnupg/pubring.gpg

mkdir ~/gpg_backups
cp ~/.gnupg/{sec,pub}ring.gpg ~/gpg_backups/
gpg -a --export-secret-key sstelfox@bedroomprogrammers.net > ~/gpg_backups/secret_key.gpg
gpg -a --export sstelfox@bedroomprogrammers.net > ~/gpg_backups/publickey.gpg
gpg -a --gen-revoke sstelfox@bedroomprogrammers.net > ~/gpg_backups/revocation_cert.gpg

For the revocation key choose ‘Key has been compromised’ and an empty description, as that is the intended usage for this particular CRL.

We also want a paperkey backup file of the private key.

gpg --export-secret-key sstelfox@bedroomprogrammers.net | paperkey > ~/gpg_backups/paperkey.bak

It is expected that the paperkey.bak file is printed out on a piece of acid-free paper and put someplace very very safe. In the worst possible scenario, this needs to be hand typed back into a text file. Once done restoration can be done by pulling in your public key and the typed file like the following:

paperkey --pubring ~/gpg_backups/publickey.gpg \
  --secrets ~/gpg_backups/paperkey.bak --output ~/recovered_secret.gpg

We need to then export just the subkeys for day to day usage. This is a non-obvious practice.

Take out the subkeys:

gpg -a --export-secret-subkeys sstelfox@bedroomprogrammers.net \
  > ~/gpg_backups/subkey_secrets.gpg

Delete the secret keys:

gpg --delete-secret-keys sstelfox@bedroomprogrammers.net

You’ll need to double confirm the deletion. We now have all the public keys we want and no secret keys… We need to now import back in just the subkeys.

gpg --import ~/gpg_backups/subkey_secrets.gpg

You should see just the subkeys in the the secret ring:

gpg -K

We can then export just the ‘laptop’ keys.

gpg -a --export-secret-keys sstelfox@bedroomprogrammers.net > ~/gpg_backup/laptop_keys_secret.gpg
gpg -a --export sstelfox@bedroomprogrammers.net > ~/gpg_backup/laptop_keys_public.gpg

These two files need are what will be transferred to the laptop and other machines that they are needed. To import the two files:

gpg --import laptop_keys_public.gpg
gpg --import laptop_keys_secret.gpg

Laptop keyrings aren’t needed with the smartcard assuming the public key portions are published at the URL set on the smartcard.

Normal Smartcards

The secret keys are best suited to be stored on a smartcard (such as a YubiKey, though that needs extra configuration). This assumes that pcscd and libccid are installed on the system being used. TAILS is already setup for it.

The recommended reader & tokens for this kind of use are OpenPGP Smartcard V2 (with breakout) combined with a Gemalto USB Shell Token V2.

By default these keys have a user & admin key set respectively to 123456 and 12345678. If the user pin is mistyped 3 times the card is blocked until the admin pin is provided. If the admin pin is provided incorrectly three times the card will be destroyed. We need to change these from the default like so:

gpg --card-edit
> admin
> passwd

Change the pin then the admin pin (option 1, then 3 respectively). While in we should set the url metadata field to provide the location where your public key can be downloaded. In my case https://stelfox.net/publickey.gpg this can be set using the following command:

> url

Also a few other bits of metadata:

> name
> lang

One thing that may be worth considering is the ‘Signature PIN’ value being set to ‘not forced’. As far as I can tell this is only used to tell the gpg-agent whether it’s allowed to cache the PIN for performing signatures or not.

It will reduce the security of your card if a PIN isn’t required for every signature performed but it make it quite a bit easier to use as part of a normal workflow. Hostiles won’t be able to get the key material but they will be able to sign data on your behalf that will be very difficult to prove didn’t come from you.

The recommendation is to always force the requirement of a PIN. I believe with a five minute timeout the risk is acceptable and can make lots of fast changes inside a git repository managable. The pin will be required again regardless of time if the card is removed and readded.

Exit out and open up the gpg –edit-key view again. We need to add the subkeys. First we need to switch to the private key view:

> toggle

For the three keys (numbered 1-3) you want to transfer them using the following commands:

> key 1
> keytocard
> key 1

This selects the keys individually, copies them, then deselects them. It needs to be done for each of them. End this with a ‘save’ command and you should be left with just stubs of the keys in the secret keyring.

On new machines that need to have the stubs added we can perform the following status:

gpg --card-edit
> fetch
> quit

You should be able to view the stubs and their presence on the card with:

gpg -K
gpg --card-status

Should be able to test that the card is working by encrypting a message and then decrypting it with smartcard.

cat << EOF > message.txt
Just a secret test message...
EOF

gpg -esar sstelfox@bedroomprogrammers.net message.txt

It should ask you for your pin before continuing. Decrypting can be done using the following:

gpg -d message.txt.asc

That confirms that the signing key & encryption key are both working. The authentication key is for using the GPG agent as an SSH agent. To test this one we need to run the gpg-agent with it’s SSH compatibility layer like so:

gpg-agent --enable-ssh-support
source ~/.gpg-agent-info

Test to make sure a card is showing up:

ssh-add -l

To get it in an appropriate format for authorized_keys file:

ssh-add -L

For use on other linux systems the scdaemon binary is required which may be in a different package…

We need to check the GPG configuration (gpg.conf) against that link as we may need to sign and encrypt with an additional alternate key (hidden-encrypt-to and default-recipient entries).

On gentoo I needed to change the default use flags for app-crypt/gnupg with the following:

# /etc/portage/package.use/gnupg

app-crypt/gnupg smartcard usb

Final Tasks

Get the key ID of the primary key from gpg -k and push it to the common public key server:

gpg --send-keys 0xBEBEF280BCE92620

Also export the file for uploading to my website:

gpg --armor --export 0xBEBEF280BCE92620 > publickey.gpg

The only time the master key should be required to come out to play:

  • You need your main key (e.g. to sign another PGP key)
  • You have to replace your card and want to reuse the subkeys
  • Revoking subkeys that have been compromised
  • Your card was lost or stolen and you need to revoke the subkeys

TAILS should be booted backup, if a new key needs to get signed it needs to be imported from a file. Then signed with gpg --sign-key <Key ID>.

Key Signing Party

Get a fingerprint summary file and print out copies for people present using the following command:

gpg --fingerprint sstelfox@bedroomprogrammers.net > key_for_partying.txt

Pull keys by their IDs (0x12345678 for the example), compare the contents of the key to what you expect, sign the key, push it to the keyserver, and email the owner a copy of their signature.

gpg --recv-keys 0x12345678
gpg --list-keys 0x12345678
gpg --sign-key 0x12345678

gpg --send-keys --keyserver keyserver.ubuntu.com 0x12345678
gpg --send-keys --keyserver hkps.pool.sks-keyservers.net 0x12345678

gpg --armor --export 0x12345678 --output 0x12345678.signed-by.your-id.asc

Email the 0x12345678.signed-by.your-id.asc file to one of the emails listed in the key. When someone sends you a signature just import it like so:

gpg --import your-id.signed-by.0x12345678.asc

You can see your signatures with:

gpg --list-sigs <your-id>

More note URLs:

Git Notes

Git needs to be configured with which key to use for signatures. First you need to find your key ID, this can be done using the following command:

gpg --list-secret-keys

The key ID will look something like: 8EE30EAB. Configure the global signing key using the following command, replacing the sample key ID with yours:

git config --global user.signingkey 8EE30EAB

Ensure that they are doing this automatically:

git config --global commit.gpgsign true

This will ask you for your pin or password on your GPG key everytime you commit which can be remedied by using a GPG agent. This will reduce your security generally though.

Key Transitions

For key transitions where your key hasn’t been compromised, a transition statement needs to be published by both the old and new key. I have a sample transition statement that can be filled in with your respective information. Variables in message are embedded with {var} commands are {{cmd}}.

The variables in the template are firstName, fullName, email, oldKeyId, newKeyId, pubKeyUrl, and pubStmntUrl. The pubKeyUrl and pubStmntUrl variables should be valid paths to files on an HTTPS protected webserver.

Once you’ve filled out the transition statement you need to ensure both keys are cross signed and the signatures are published.

gpg --local-user $oldKeyId --sign-key $newKeyId
gpg --local-user $newKeyId --sign-key $oldKeyId
gpg --send-keys $oldKeyId $newKeyId

The file itself still needs to be signed by both keys which can be done with the following command:

gpg --local-user $oldKeyId --local-user $newKeyId --clearsign $keyTransitionFile

If both keys don’t exist on the same machine you’ll need to refer to the section titled ‘Multiple Clearsigned Signatures’ for how to handle the situation.

I recommend you publish the key transition with a name that includes the date, like key-transition-2017-08-17.txt.

Expiration / Renewal of Subkeys

TODO: I need to cover certificate key transitions and signed key transition statements.

Option 1: Generate a new signing / encryption key

Pros:

  • Most secure
  • Some level of forward secrecy (over large time scales)
  • Helps protect against unknown key compromises

Cons:

  • Only one keypair stored per smartcard (decrypting old files means restoring a backup of the old key)
  • More complicated
  • Requires users to refresh their keys about you

Steps:

  1. Generate new keys
  2. Generate CRLs for old keys
  3. Load keys on to smartcard
  4. Push new keys and CRLs to keyservers

Option 2: Extend Expiration

Pros:

  • Simple
  • Fast
gpg --edit-key 0x12345678
gpg> key 1
gpg> expire
...
gpg> key 1
gpg> key 2
gpg> expire
...
gpg> save

gpg --send-keys 0x12345678

Maintenance

To ensure we get updated keys and revocation announcements the keys in your keyring should be periodically refreshed with the public key server. This can be done all at once using the following command:

gpg2 --refresh-keys

Likewise after modifying your own key it should be pushed to the public key servers for consumption and availability of others.

PRIMARY_KEY_EMAIL=sstelfox@bedroomprogrammers.net
gpg2 --send-key $(gpg2 -k $PRIMARY_EMAIL | grep pub | awk '{ print $2 }' | cut -d '/' -f 2)

There is a social network disclosure that occurs when all keys are refreshed at once. The description of this disclosure is:

We assume there probably exists at least one subset of public keys in this keyring that identifies it, i.e. no other individual’s keyring contain the same subset of public keys.

A personal thought potentially making this attack easier, it is very likely that one of the keys being refreshed is owner of the keyring itself.

Trusted SSL communications (hkps) eliminates this threat for passive snoopers, the analysis could still be done on the keyserver itself.

To avoid this individual keys would have to be refreshed independently over time. The requests could still be tied to the keyring machine’s IP address to correlate all the refreshed keys to build back up the contents of the keyring. Avoiding this would require use of individual Tor circuits to mask and distribute key refreshes.

A project has been built up to handle this issue. The following are relevant:

Diagnostics

No Usuable Subkey

One one of my devices I was persistently getting an odd error message whenever I tried to sign something. The error message was:

$ gpg2 -esar sam@stelfox.net sample-file
gpg: no default secret key: Unusable secret key
gpg: tor-setup-script.sh: sign+encrypt failed: Unusable secret key

Turns out I hadn’t updated my public keys on that device and gpg2 --card-edit followed by a fetch didn’t actually update my certificate. I needed to refresh the key from the key server to get my new signatures and public key and retrust it.

gpg2 --refresh-keys 0x30856D4EA0FFBA8F
gpg2 --edit-key 0x30856D4EA0FFBA8F
trust

Card Not Visible to User (May be seen by root)

I installed pcsc-tools to get pcsc_scan but I don’t believe that contributed to the fix. Ultimately I needed to start up the pcscd service and replug my yubikey.

Unable to connect to dirmngr (IPC connect call failed)

While trying to refresh keys I was getting the following error:

gpg: connecting dirmngr at '/run/user/1000/gnupg/S.dirmngr' failed: IPC connect call failed

I rebooted the machine, and that didn’t solve it. Ultimately I believe it was permissions / ownership issues on the ~/.gnupg/crls.d directory but I can’t be entirely sure.

I solved this issue by deleting everything in my .gnupg directory and checking out the version from my dotfiles again.

Remote Usage of Smartcard

Basically relies on the extra-socket option for gpg-agent. May be able to accomplish this with something like the following command (path may be incorrect):

ssh -R /run/user/1000/gnupg/S.gpg-agent:/home/sstelfox/.gnupg/S.gpg-agent.remote  -o "StreamLocalBindUnlink=yes" remote-host

This could also live in an ssh/config parameter like so:

Host remote
  RemoteForward /run/user/1000/gnupg/S.gpg-agent:/home/sstelfox/.gnupg/S.gpg-agent.remote
  StreamLocalBindUnlink yes

The following command should work but will warn about the agent being in restricted mode.

gpg-connect-agent /bye

Multiple Clearsigned Signatures

If multiple keys on different machines need to perform a clearsign on a document the normal method doesn’t support combining these signatures to be verified in one pass. This is valuable for allowing multiple independent people to attest to the validity of a document and make it easy for people to validate the correctness of it.

Notes

If the private keys are on the same machine the --local-user flag can simply be specified multiple times. This is likely the most applicable for key-transition statements.

gpg2 --local-user pers1 --local-user pers2 --clearsign content

ONLY PARTIALLY IDEAL: Detached signatures can be combined in the same way and don’t require a lot of the sed magic used here. A detached signature can be generated with:

gpg2 --armor --detach-sign content

If this is done only the dearmor, split, combine, enarmor steps need to be performed.

Process

First we need content to sign:

echo 'A very important statement about a very important topic.' > content

Each signatory should receive or generate a copy of the content and verify it. This transport is beyond the scope of this document but could easily be done with a standard gpg2 -esa -r pers1 -r pers2 content and emailed then decrypted.

Once each user has a copy and verified it. They need to perform a normal clearsign on the content and send the signed message back to an individual to perform the combinatory process.

gpg2 --clearsign content

This assumes you now have the signed contents file from pers1 and pers2 in the files content.pers1.asc and content.pers2.asc. Verify both signatures are valid:

gpg2 --verify content.pers1.asc
gpg2 --verify content.pers2.asc

We need to verify the content is still identical in both files (one of them could have changed the content before signing to be tricky). This is kind of a formality since the signature won’t check out for at least one of them if the content differs between the two in any way.

sed -n '1,/SIGNATURE/ p' content.pers1.asc | sha1sum
sed -n '1,/SIGNATURE/ p' content.pers2.asc | sha1sum

We then need to extract just the signature blocks from each message and turn them into the raw gpg2 packets:

sed -n '/SIGNATURE/,$ p' content.pers1.asc | gpg2 --dearmor | gpgsplit --no-split > pers1.sig
sed -n '/SIGNATURE/,$ p' content.pers2.asc | gpg2 --dearmor | gpgsplit --no-split > pers2.sig

We then combine them into one enarmored signature (order doesn’t matter):

cat pers1.sig pers2.sig | gpg2 --enarmor | sed -n '5,$ p' | grep -v -- ----- > combo.sig

And append the signatures back onto the content:

(sed -n '1,/SIGNATURE/ p' content.pers1.asc; echo; cat combo.sig; \
  echo '-----END PGP SIGNATURE-----') > content.combo.asc

Verify the signatures are still good:

gpg2 --verify content.combo.asc