Configuring an offline GnuPG master key and subkeys on YubiKey

I've recently bought a YubiKey 4 and decided to use it for GnuPG too, other than using it as hardware 2FA.

I've also decided to make my GnuPG configuration much more safe, generating the master key on an offline computer (in my case a simple RaspberryPi not connected to Internet) and generating a subkey that will be moved to my YubiKey.

Disclaimer

Always think about what your threat model is before deciding something is 100% safe for you. I'm not claiming this setup/configuration is bullet proof. If you want to protect your GnuPG key from most of the hackers, keyloggers and if you want to use it on different computers without ever compromising your secret key, this setup can be what you are looking for. If you think you may be victim of a targeted state sponsored attack, I'm not sure this setup could be enough.

Why keeping offline the master key?

If you only use your master key on a computer that never connects to Internet (I reckon you will want to update/patch it from time to time, that's why we are going to keep the master key on an external USB key) you are at least safe from remote attacks.

Why using subkeys?

Your GnuPG master key is also your "identity" among every PGP user. If you loose your master key or if your key is compromised you need to rebuild your identity and reputation from scratch. Instead, if a subkey is compromised, you can revoke the subkey (using your master key) and generate a new subkey.

How a YubiKey makes things safer?

If you always use your subkey from a YubiKey, it's very unlikely that your private key can be stolen: it's impossible to read it from the YubiKey and if you loose your YubiKey or if it's physically stolen, the attacker will still need your passphrase and your YubiKey PIN.

Requirements

  • 1 YubiKey 4
  • 2 USB keys (in theory you only need one, but I strongly suggest you have another one as backup)
  • 1 offline computer (a simple RaspberryPi with no Internet connection will be fine)

Initial setup

From now on, I will assume that you have prepared a computer for offline use (in my case I'm using a RaspberryPi 2 with Raspbian) and you will type the next commands there and only there.

Plug one of the USB key (you can format it with VFAT for simplicity) in the offline computer and wait for the system to mount it. At this point it should be mounted in a path like this: /media/AABB-BAAC

Now set the GnuPG working directory and create it:

[email protected]:~$ export GNUPGHOME=/media/AABB-BAAC/gnupghome
[email protected]:~$ mkdir $GNUPGHOME

Second disclaimer

If you think your threat model doesn't include someone can hack your computer from remote, you can ignore my advice and type these commands on your main laptop (at your own risk).

Note

For my own convenience, to write this tutorial I reproduced all these steps on my MacBook because it was easier to copy/paste commands and outputs but I've tested it with the exact setup I'm describing, and it should be compatible with OSX and Linux. When you see something has been masked it's just to hide (from spam) things like my email or to protect the serial number of my YubiKey. Last but not least, the output shown here could not match exactly the one you get on your own PC and this also depends on the GnuPG version you are using.

Generating the master key

The master key must be generated using the advanced mode, because by default when a new master key is generated, also a new subkey is created with all the capabilities (Authentication + Signing + Encryption), while we want something different.

Note: PGP keys up to 4096 bits are only supported in YubiKey 4 models. If you have a YubiKey NEO you must use a 2048 bits key because it's the maximum size supported. Here you will create a PGP key with only the Authentication capability. If your GnuPG version doesn't allow this, choose "sign only", just don't create the encryption capability at this time.

[email protected]:~$ gpg --expert --gen-key
gpg (GnuPG) 2.0.30; Copyright (C) 2015 Free Software Foundation, Inc.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

gpg: directory `/media/AABB-BAAC/gnupghome' created
gpg: new configuration file `/media/AABB-BAAC/gnupghome/gpg.conf' created
gpg: WARNING: options in `/media/AABB-BAAC/gnupghome/gpg.conf' are not yet active during this run
gpg: keyring `/media/AABB-BAAC/gnupghome/secring.gpg' created
gpg: keyring `/media/AABB-BAAC/gnupghome/pubring.gpg' created
Please select what kind of key you want:
(1) RSA and RSA (default)
(2) DSA and Elgamal
(3) DSA (sign only)
(4) RSA (sign only)
(7) DSA (set your own capabilities)
(8) RSA (set your own capabilities)
Your selection? 8

Possible actions for a RSA key: Sign Certify Encrypt Authenticate
Current allowed actions: Sign Certify Encrypt

(S) Toggle the sign capability
(E) Toggle the encrypt capability
(A) Toggle the authenticate capability
(Q) Finished

Your selection? s

Possible actions for a RSA key: Sign Certify Encrypt Authenticate
Current allowed actions: Certify Encrypt

(S) Toggle the sign capability
(E) Toggle the encrypt capability
(A) Toggle the authenticate capability
(Q) Finished

Your selection? e

Possible actions for a RSA key: Sign Certify Encrypt Authenticate
Current allowed actions: Certify

(S) Toggle the sign capability
(E) Toggle the encrypt capability
(A) Toggle the authenticate capability
(Q) Finished

Your selection? q
RSA keys may be between 1024 and 4096 bits long.
What keysize do you want? (2048) 4096
Requested keysize is 4096 bits
Please specify how long the key should be valid.
        0 = key does not expire
    <n>  = key expires in n days
    <n>w = key expires in n weeks
    <n>m = key expires in n months
    <n>y = key expires in n years
Key is valid for? (0) 2y
Key expires at Wed 25 Sep 18:39:49 2019 BST
Is this correct? (y/N) y

GnuPG needs to construct a user ID to identify your key.

Real name: Andrea Grandi
Email address: [email protected]
Comment:
You selected this USER-ID:
    "Andrea Grandi <[email protected]>"

Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? o
You need a Passphrase to protect your secret key.

We need to generate a lot of random bytes. It is a good idea to perform
some other action (type on the keyboard, move the mouse, utilize the
disks) during the prime generation; this gives the random number
generator a better chance to gain enough entropy.
gpg: /media/AABB-BAAC/gnupghome/trustdb.gpg: trustdb created
gpg: key 2240402E marked as ultimately trusted
public and secret key created and signed.

gpg: checking the trustdb
gpg: 3 marginal(s) needed, 1 complete(s) needed, PGP trust model
gpg: depth: 0  valid:   1  signed:   0  trust: 0-, 0q, 0n, 0m, 0f, 1u
gpg: next trustdb check due at 2019-09-25
pub   4096R/2240402E 2017-09-25 [expires: 2019-09-25]
    Key fingerprint = 7D4C 4090 DB50 1693 4614  F6FC 6206 9DE9 2240 402E
uid       [ultimate] Andrea Grandi <[email protected]>

Note: please remember to save your passphrase in a safe place. Choose something you can remember because you will need it every time you need to sign, encrypt or decrypt something.

Creating a revocation certificate

It's very important to create a revocation certificate to be used if and when in the future you want to change your master key and revoke the existing one:

[email protected]:~$ gpg --gen-revoke 2240402E > 2240402E-revocation-certificate.asc

sec  4096R/2240402E 2017-09-25 Andrea Grandi <[email protected]>

Create a revocation certificate for this key? (y/N) y
Please select the reason for the revocation:
0 = No reason specified
1 = Key has been compromised
2 = Key is superseded
3 = Key is no longer used
Q = Cancel
(Probably you want to select 1 here)
Your decision? 3
Enter an optional description; end it with an empty line:
>
Reason for revocation: Key is no longer used
(No description given)
Is this okay? (y/N) y

You need a passphrase to unlock the secret key for
user: "Andrea Grandi <[email protected]>"
4096-bit RSA key, ID 2240402E, created 2017-09-25

ASCII armored output forced.
Revocation certificate created.

Please move it to a medium which you can hide away; if Mallory gets
access to this certificate he can use it to make your key unusable.
It is smart to print this certificate and store it away, just in case
your media become unreadable.  But have some caution:  The print system of
your machine might store the data and make it available to others!

Creating Encryption subkey

To create a subkey we need to edit the existing key (please note that 2240402E is the last 8 chars from the fingerprint of the previously generated master key) and specify we want to create an Encryption only key.

[email protected]:~$ gpg --edit-key 2240402E
gpg (GnuPG) 2.0.30; Copyright (C) 2015 Free Software Foundation, Inc.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Secret key is available.

pub  4096R/2240402E  created: 2017-09-25  expires: 2019-09-25  usage: C
                    trust: ultimate      validity: ultimate
[ultimate] (1). Andrea Grandi <[email protected]>

gpg> addkey
Key is protected.

You need a passphrase to unlock the secret key for
user: "Andrea Grandi <[email protected]>"
4096-bit RSA key, ID 2240402E, created 2017-09-25

Please select what kind of key you want:
(3) DSA (sign only)
(4) RSA (sign only)
(5) Elgamal (encrypt only)
(6) RSA (encrypt only)
Your selection? 6
RSA keys may be between 1024 and 4096 bits long.
What keysize do you want? (2048) 4096
Requested keysize is 4096 bits
Please specify how long the key should be valid.
        0 = key does not expire
    <n>  = key expires in n days
    <n>w = key expires in n weeks
    <n>m = key expires in n months
    <n>y = key expires in n years
Key is valid for? (0) 2y
Key expires at Wed 25 Sep 18:47:21 2019 BST
Is this correct? (y/N) y
Really create? (y/N) y
We need to generate a lot of random bytes. It is a good idea to perform
some other action (type on the keyboard, move the mouse, utilize the
disks) during the prime generation; this gives the random number
generator a better chance to gain enough entropy.

pub  4096R/2240402E  created: 2017-09-25  expires: 2019-09-25  usage: C
                    trust: ultimate      validity: ultimate
sub  4096R/01731555  created: 2017-09-25  expires: 2019-09-25  usage: E
[ultimate] (1). Andrea Grandi <[email protected]>

gpg> save

Export a backup of the secret keys

It's very important to export a backup of the secret keys at this point. Writing the secret subkey to the YubiKey is a destructive process: keys are moved to the YubiKey, they are not copied.

[email protected]:~$ gpg --export-secret-key 2240402E > 2240402E-secret.pgp

Note: this backup includes both the secret master key and the secret subkey. Please remember to save a backup of this key on a couple of separate USB keys: you will need this keys to generate future subkeys and/or to revoke the existing ones.

Programming the YubiKey with all GnuPG keys

We have previously created the master key and the encryption subkey. Now we will create the authentication and signing keys directly on the YubiKey (we don't need to have a copy of these keys) and we will move the secret encryption key to the YubiKey.

[email protected]:~$ gpg --edit-key 2240402E
gpg (GnuPG) 2.0.30; Copyright (C) 2015 Free Software Foundation, Inc.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Secret key is available.

pub  4096R/2240402E  created: 2017-09-25  expires: 2019-09-25  usage: C
                    trust: ultimate      validity: ultimate
sub  4096R/01731555  created: 2017-09-25  expires: 2019-09-25  usage: E
[ultimate] (1). Andrea Grandi <[email protected]>

gpg> addcardkey
Signature key ....: [none]
Encryption key....: [none]
Authentication key: [none]

Please select the type of key to generate:
(1) Signature key
(2) Encryption key
(3) Authentication key
Your selection? 1

What keysize do you want for the Signature key? (4096)
Key is protected.

You need a passphrase to unlock the secret key for
user: "Andrea Grandi <[email protected]>"
4096-bit RSA key, ID 2240402E, created 2017-09-25

Please specify how long the key should be valid.
        0 = key does not expire
    <n>  = key expires in n days
    <n>w = key expires in n weeks
    <n>m = key expires in n months
    <n>y = key expires in n years
Key is valid for? (0) 2y
Key expires at Wed 25 Sep 18:50:42 2019 BST
Is this correct? (y/N) y
Really create? (y/N) y

pub  4096R/2240402E  created: 2017-09-25  expires: 2019-09-25  usage: C
                    trust: ultimate      validity: ultimate
sub  4096R/01731555  created: 2017-09-25  expires: 2019-09-25  usage: E
sub  4096R/771B0554  created: 2017-09-25  expires: 2019-09-25  usage: S
[ultimate] (1). Andrea Grandi <[email protected]>

gpg> addcardkey
Signature key ....: 6FAB DC46 1847 3550 3769  2D32 0DE1 36B4 771B 0554
Encryption key....: [none]
Authentication key: [none]

Please select the type of key to generate:
(1) Signature key
(2) Encryption key
(3) Authentication key
Your selection? 3

What keysize do you want for the Authentication key? (4096)
Key is protected.

You need a passphrase to unlock the secret key for
user: "Andrea Grandi <[email protected]>"
4096-bit RSA key, ID 2240402E, created 2017-09-25

Please specify how long the key should be valid.
        0 = key does not expire
    <n>  = key expires in n days
    <n>w = key expires in n weeks
    <n>m = key expires in n months
    <n>y = key expires in n years
Key is valid for? (0) 2y
Key expires at Wed 25 Sep 18:54:51 2019 BST
Is this correct? (y/N) y
Really create? (y/N) y

pub  4096R/2240402E  created: 2017-09-25  expires: 2019-09-25  usage: C
                    trust: ultimate      validity: ultimate
sub  4096R/01731555  created: 2017-09-25  expires: 2019-09-25  usage: E
sub  4096R/771B0554  created: 2017-09-25  expires: 2019-09-25  usage: S
sub  4096R/A9B5334C  created: 2017-09-25  expires: 2019-09-25  usage: A
[ultimate] (1). Andrea Grandi <[email protected]>

gpg> toggle

sec  4096R/2240402E  created: 2017-09-25  expires: 2019-09-25
ssb  4096R/01731555  created: 2017-09-25  expires: never
ssb  4096R/771B0554  created: 2017-09-25  expires: 2019-09-25
                    card-no: 0006 05672181
ssb  4096R/A9B5334C  created: 2017-09-25  expires: 2019-09-25
                    card-no: 0006 05672181
(1)  Andrea Grandi <[email protected]>

gpg> key 1

sec  4096R/2240402E  created: 2017-09-25  expires: 2019-09-25
ssb* 4096R/01731555  created: 2017-09-25  expires: never
ssb  4096R/771B0554  created: 2017-09-25  expires: 2019-09-25
                    card-no: 0006 05672181
ssb  4096R/A9B5334C  created: 2017-09-25  expires: 2019-09-25
                    card-no: 0006 05672181
(1)  Andrea Grandi <[email protected]>

gpg> keytocard
Signature key ....: 6FAB DC46 1847 3550 3769  2D32 0DE1 36B4 771B 0554
Encryption key....: [none]
Authentication key: BD26 3AD8 985E CAB0 9F32  7307 DF7C F7C0 A9B5 334C

Please select where to store the key:
(2) Encryption key
Your selection? 2

You need a passphrase to unlock the secret key for
user: "Andrea Grandi <[email protected]>"
4096-bit RSA key, ID 01731555, created 2017-09-25


sec  4096R/2240402E  created: 2017-09-25  expires: 2019-09-25
ssb* 4096R/01731555  created: 2017-09-25  expires: never
                    card-no: 0006 05672181
ssb  4096R/771B0554  created: 2017-09-25  expires: 2019-09-25
                    card-no: 0006 05672181
ssb  4096R/A9B5334C  created: 2017-09-25  expires: 2019-09-25
                    card-no: 0006 05672181
(1)  Andrea Grandi <[email protected]>

gpg> save

Check public keys

Just to verify everything has been created correctly, we check the public keys. We should see one pub key and three sub:

[email protected]:~$ gpg -k
/media/AABB-BAAC/gnupghome/pubring.gpg
--------------------------------
pub   4096R/2240402E 2017-09-25 [expires: 2019-09-25]
uid       [ultimate] Andrea Grandi <[email protected]>
sub   4096R/01731555 2017-09-25 [expires: 2019-09-25]
sub   4096R/771B0554 2017-09-25 [expires: 2019-09-25]
sub   4096R/A9B5334C 2017-09-25 [expires: 2019-09-25]

Check private keys

When we check the private keys we should see that one key is still local, marked as sec (it's the private key of the master key), while three other keys are marked as ssb> which means they have been moved to the YubiKey:

[email protected]:~$ gpg -K
/media/AABB-BAAC/gnupghome/secring.gpg
--------------------------------
sec   4096R/2240402E 2017-09-25 [expires: 2019-09-25]
uid                  Andrea Grandi <[email protected]>
ssb>  4096R/01731555 2017-09-25
ssb>  4096R/771B0554 2017-09-25
ssb>  4096R/A9B5334C 2017-09-25

Import back secret keys from backup (only for multiple YubiKeys)

As previously said, when we write the encryption subkey to the YubiKey, the key is moved and not just copied, so we need to import back the secret key into the keyring. It's important to have a backup of the subkey too, not because we need it in case the key is compromised etc... but because we need it in case we want to write multiple YubiKeys with the same encryption key, so that we have a backup key to use.

[email protected]:~$ gpg --import < 2240402E-secret.pgp

Completely remove secret keys from laptop

Once you have programmed the YubiKey and you are sure the secret keys are backed up on a couple of USB keys, you are ready to remove the secret keys from your laptop.

Note: you don't need to remove anything if you have conducted the whole setup on a spare offline PC (or on a RaspberryPi) because that's not your every day computer.

[email protected]:~$ gpg --delete-secret-key 2240402E

Exporting the public PGP key

As you know, PGP keys are composed by a secret part and a public one. The public one must be distributed publicly and it's the one people will use to encrypt messages directed to you.

[email protected]:~$ gpg --armor --export 2240402E > 2240402E.asc

If you have a personal blog/website I suggest to upload it there (for example mine can be found here https://www.andreagrandi.it/2240402E.asc)

Change YubiKey PINs and complete configuration

Every YubiKey is sold with a certain default configuration: there is a user PIN that is required every time we need to use the key to sign/decrypt something (in addition to our passphrase) and there is an admin PIN that is required every time we change certain settings on the YubiKey.

The default values are:

  • user PIN: 123456
  • admin PIN: 12345678

I strongly recommend you to change them following this example:

[email protected]:~$ gpg --card-edit

Reader ...........: Yubico Yubikey 4 OTP U2F CCID
Application ID ...: D000000000000000000000000000000000
Version ..........: 2.1
Manufacturer .....: Yubico
Serial number ....: 012345678
Name of cardholder: [not set]
Language prefs ...: [not set]
Sex ..............: unspecified
URL of public key : [not set]
Login data .......: [not set]
Signature PIN ....: not forced
Key attributes ...: rsa4096 rsa4096 rsa4096
Max. PIN lengths .: 127 127 127
PIN retry counter : 3 0 3
Signature counter : 3
Signature key ....: 6FAB DC46 1847 3550 3769  2D32 0DE1 36B4 771B 0554
    created ....: 2017-09-25 17:50:37
Encryption key....: FC6F 40BC 4173 8D13 2D7C  E958 BCDC EA84 0173 1555
    created ....: 2017-09-25 17:47:09
Authentication key: BD26 3AD8 985E CAB0 9F32  7307 DF7C F7C0 A9B5 334C
    created ....: 2017-09-25 17:54:49
General key info..: sub  rsa4096/0DE136B4771B0554 2017-09-25 Andrea Grandi <[email protected]>
sec#  rsa4096/62069DE92240402E  created: 2017-09-25  expires: 2019-09-25
ssb>  rsa4096/BCDCEA8401731555  created: 2017-09-25  expires: 2019-09-25
                                card-no: 0006 05672181
ssb>  rsa4096/0DE136B4771B0554  created: 2017-09-25  expires: 2019-09-25
                                card-no: 0006 05672181
ssb>  rsa4096/DF7CF7C0A9B5334C  created: 2017-09-25  expires: 2019-09-25
                                card-no: 0006 05672181

gpg/card> admin
Admin commands are allowed

# Change the PIN and Admin PINs
gpg/card> passwd
gpg: OpenPGP card no. D000000000000000000000000000000000 detected

1 - change PIN
2 - unblock PIN
3 - change Admin PIN
4 - set the Reset Code
Q - quit

Your selection? 1
PIN changed.

1 - change PIN
2 - unblock PIN
3 - change Admin PIN
4 - set the Reset Code
Q - quit

Your selection? 3
PIN changed.

1 - change PIN
2 - unblock PIN
3 - change Admin PIN
4 - set the Reset Code
Q - quit

Your selection? q

# Make sure the PIN is entered before signing
gpg/card> forcesig

# Set the URL where the OpenPGP public key can be found.
gpg/card> url
URL to retrieve public key: https://www.andreagrandi.it/2240402E.asc

# Fetch the public key into the local keyring
gpg/card> fetch

gpg/card> quit

Note: when you want to use your YubiKey on any computer (for example your work laptop) you need to at least import your public PGP key into the keyring. If the key is not read automatically, you may need to give it a refresh using this command:

[email protected]:~$ gpg --card-status

Careful with PINs

Please remember that you can only digit a wrong user PIN for a maximum of three times. After three time you will need to edit the YubiKey (with gpg --card-edit) become admin and use the unblock PIN option. If you digit the wrong admin PIN for three time, you will have to follow a quite complicated procedure (explained at this address: https://developers.yubico.com/ykneo-openpgp/ResetApplet.html) and your YubiKey will be reset with factory settings, deleting your PGP keys from it.

References

To write this tutorial I originally followed other articles online. The main ones are:

Amazon Association disclaimer

I'm trying a little experiment with the Amazon Association program. Basically, if you click on any of the YubiKey links and decide to buy it, I will get a little commission from it. I've never tried this before and I've no idea if it works or not. I'm writing this here just for the sake of transparency.

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