Keybase: PGP encryption made easy

Posted on Sat 21 October 2017 in HowTo • Tagged with GnuPG, PGP, Security, Encryption, Keybase

Using PGP can be quite hard, even if you have a lot of experience with computers. By the way encryption is what gives us privacy and permits us to safely transmit information and for this reason it should be easy to use, for everyone.

Keybase really makes encryption easy to use.

PGP identity

When Keybase was launched it was mainly a wrapper for PGP commands to encrypt and decrypt a message for a certain user, but it also introduced a very nice chain of trust.

In Keybase it's possible to either generate a new PGP key or import an existing one but the most important thing is being able to verify our own identity using multiple proofs.

Many of us have a personal blog, a Twitter or Facebook accounts, a GitHub account etc... All these accounts combined together make our online identity.

Every Keybase account can be verified by other online identities. In Keybase you don't just say "I'm Andrea Grandi, this is my PGP key...". In Keybase you can link your existing online accounts to your Keybase account and show additional proofs of your identity.

Unless an attacker controls all your social accounts, they cannot impersonate and verify themselves as if they were you.

keybase identity

Once you are on Keybase, other users can look for you even using your GitHub or Twitter username without having to know your email address or Keybase username. This concept can be very useful in some situations, we will see it later.

Encrypted Filesystem

One of the first features launched by Keybase was their encrypted filesystem. There is a virtual folder located at /keybase (on OSX/Linux or k:\keybase on Windows) where you will find at least three other folders: public, private, team.

Public folders

Anything you place inside the /public folder can be accessed by any Keybase user and it's automatically signed. Every user public folder/file can be accessed using their Keybase username, like for example /keybase/public/andreagrandi/hello.txt but you can also use any other identity like /keybase/public/[email protected]b/hello.txt or /keybase/public/[email protected]/hello.txt

Note: This is very useful if you only know a person on Twitter (or GitHub etc...) and you want to share a file with them (or send a message, as we will see later) but you don't follow each other and you can't reach them privately.

This is a public folder example of one of the Keybase developers:

keybase chris folder

You can put whatever you want in these folders: your public PGP key, your official avatar, your Signal fingerprint etc... the other users will access these files with the assurance they haven't been changed by anyone else in the middle.

Note: please keep in mind that Keybase doesn't work like Dropbox or similar. Files are not synced between your devices and Keybase servers. Files are streamed on demand, so you won't be able to access these files without a working Internet connection.

Private folders

Hey but... where is the encryption here?! Whatever you put inside your private folder can only be read by you and only you. Not even Keybase employees can access the content of your files, because they are encrypted before leaving your devices and decrypted on demand when you want to access them.

Do you want to share files with anotheruser? No problem. Just create a file inside /keybase/private/andreagrandi,anotheruser (the folder andreagrandi,anotheruser will implicitely exist already) and that file will only be readable by you and anotheruser.

Security and other information

Keybase employes only have access to: 1) your top level folder names (like: "andreagrandi,anotheruser"), 2) when and for how long you are reading/writing, 3) how much space you are using.

They won't be able to access the content of your files and not even the files or folders names.

Every user initially had 10GB quota available, but a few hints (including one of their recent screenshots) say that now users have 250GB available to store their files.

You can find more technical information about Keybase encrypted folders in this article: https://keybase.io/docs/kbfs

Encrypted Chat

A few months ago Keybase introduced the encrypted chat. Messages between users are end to end encrypted and cannot be read by anyone else, not even having access to Keybase servers.

kybase chat

A better address book

When we use services like WhatsApp or Signal, we are forced to share our telephone number if we want the other person to be able to contact us.

On Keybase I don't need to share my telephone number. Anyone can reach me using one of my online identities: [email protected], [email protected] etc...

You can even send a message to a person who is not on Keybase yet: if you send a message to [email protected], when randomuser joins Keybase and verify their Twitter account, the message will be encrypted for them and will be safely delivered.

Security

Keybase doesn't use PGP to encrypt chat or files. Transmitting the key across all devices wouldn't be safe so each message is encrypted using the public key of every device connected to the account.

Command line

Keybase works from the command line too. There is no need to use the graphic client to send a message to another user, you can do something like this:

keybase chat send andreagrandi "Hello mate!"

You can integrate messages in any script and it's even available a JSON API:

keybase chat help api

For more details you can have a look a this blog post on their website: https://keybase.io/blog/keybase-chat

Teams

Keybase has recently introduced Teams feature. The Chat becomes more similar to Slack, but with the difference that only team members can read the content of messages and files: the server only knows about team names and users, nobody else can access the content.

teams announcement

It's important to mention that in Keybase there aren't private channels like there are in Slack: if a team wants to have channels accessible only from a restricted group of users, the admin needs to create a sub team. For example if you have a team called keybaselovers you can create a sub team for admins only called keybaselovers.admins

Teams have a dedicated encrypter folder that you will find under /keybase/team/keybaselovers

At the moment the features available from the UI are quite limited and are only available from the command line. In the next weeks these features will be available from the UI too. In the mean time you can have a look at the commandline help:

keybase team --help # for admin'ing teams
keybase chat --help # for admin'ing chat channels

Create a Team

keybase team create keybaselovers

Add a user to a Team

keybase team add-member keybaselovers --user=alice --role=writer

For more information you can have a look at the official announcement page: https://keybase.io/blog/introducing-keybase-teams

Git

Sometimes we have the need to store private information in a safe way and we want to be sure that nobody else is able to access these information.

Latest feature that has been added to Keybase is encrypted Git repositories. They are like normal GitHub repositories, but their content is stored in a safer way.

keybase git

Privacy and Security

What is the difference with GitHub private repositories? In GitHub a private repository is used to store information that only our account can access, but the files are accessible in plain text by GitHub employees. With encrypted Git repositories instead, the information are encrypted before they leave our device and they are stored encrypted. Nobody, without having our private key can read them, not even Keybase employees.

Teams and Quota

Encrypted Git repositories are of course available for teams too. Creating a team repository, it will be available to all the members of the team.

Both teams and single users have 100GB of space available (which is separate from Folders quota).

Usage

If I create my personal repository called documents all I have to do to clone it and use it is:

git clone keybase://private/andreagrandi/documents

and I can use it as a normal git repository. Every time I commit and push something, the content will be signed and encrypted and only available to the repository owner (which is me) or to the whole team if it's a team repository.

For more information, please have a look at the official announcement here: https://keybase.io/blog/encrypted-git-for-everyone

Conclusion

Keybase is still in continuous development but it already offers a few interesting features which can help people in their every day life. I strongly advise anyone to get an account, play with the available features and report any bug so the developers will be able to fix them and build an even better product. I can't wait to see the features they will announce in the next months!


Configuring an offline GnuPG master key and subkeys on YubiKey

Posted on Sat 30 September 2017 in HowTo • Tagged with GnuPG, PGP, Security, YubiKey, Encryption

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.