Posted on Sep 6, 2020 • Edited on Sep 9, 2020

Using your Yubikey to get started with GPG

Why store your RSA key on a Yubikey?

Digital security is generally much more difficult than physical security. If I have a secret written on a piece of paper, I could hide that paper somewhere, I could keep it on my person, I could put it in safety deposit box. All of these are pretty good ways to make sure nobody reads whatever is on the paper, and all are simple to reason about. If I have that same secret on a file on my laptop, however, there is much more to think about. Every piece of software I run has permission to read that file.

My laptop has a network connection, so an attacker does not need to ever meet me to steal my secret. A Yubikey almost turns a digital security problem into a physical security one.

If your SSH key is stored on a Yubikey it cannot be copied, it cannot be stolen remotely, and the software on whatever machine you stick it into cannot read it. You can even PIN protect the key, which means that if somebody physically steals your Yubikey they would have to guess the PIN correctly to be able to use it, and even then, they would be unable to extract the key.

PGP can use a number of encryption algorithms to generate its keys. One of those is Rivest–Shamir–Adleman (RSA). RSA is named for its developers, Ron Rivest, Adi Shamir, and Leonard Adleman, who developed the algorithm in 1978.

RSA was one of the first asymmetrical encryption algorithms published. Both its private key and public key can be used to encrypt and decrypt data. Whichever is used to encrypt, the other is used to decrypt.

Set Yubikey PINs

Install gpg using your operating system’s package manager so that you can interact with your Yubikey.

Plug in your Yubikey, and run gpg --change-pin to change the PIN from the default of 123456. This is the PIN you’ll be asked to enter whenever you need to access the private key, for example when doing git push.

$ gpg --change-pin 1 - change PIN 2 - unblock PIN 3 - change Admin PIN 4 - set the Reset Code Q - quit Your selection? 1 Please enter the PIN New PIN New PIN PIN changed.

If you enter your newly-set PIN incorrectly three times then the Yubikey will be locked, and you must unlock it using the admin PIN, which is 12345678 by default.

You should change the admin PIN too, by running the same command again, this time selecting option 3:

$ gpg --change-pin 1 - change PIN 2 - unblock PIN 3 - change Admin PIN 4 - set the Reset Code Q - quit Your selection? 3 gpg: 3 Admin PIN attempts remaining before card is permanently locked Please enter the Admin PIN New Admin PIN New Admin PIN Admin PIN changed.

If you later enter the admin PIN incorrectly three times you will need to factory-reset the Yubikey.

Configure Machines to use GPG Agent

Firstly, you need to configure ssh on the machines you’ll be working on to use gpg-agent to handle authentication, which will in turn load an RSA key from your Yubikey - provided that you enter the correct PIN.

The below snippet appends the appropriate config to your .bashrc (if you use zsh you will need to append these lines to ~/.zshrc instead).

$ cat <<EOF >> ~/.bashrc export GPG_TTY=$(tty) gpg-connect-agent updatestartuptty /bye unset SSH_AGENT_PID export SSH_AUTH_SOCK=$(gpgconf --list-dirs agent-ssh-socket) EOF $ . ~/.bashrc

Generate an RSA key on your Yubikey for sign GPG

I recommend generating your RSA key on the Yubikey itself, rather than generating the key on your computer and then copying it to the Yubikey. This way you know that it has never been on the filesystem, where it could be snooped upon.

$ gpg --card-edit Reader ...........: Yubico YubiKey OTP FIDO CCID 00 00 Application ID ...: D****************************0 Application type .: OpenPGP Version ..........: 2.1 Manufacturer .....: Yubico Serial number ....: 10300768 Name of cardholder: Paul Micheli Language prefs ...: en Salutation .......: URL of public key : [not set] Login data .......: [not set] Signature PIN ....: not forced Key attributes ...: rsa2048 rsa2048 rsa4096 Max. PIN lengths .: 127 127 127 PIN retry counter : 3 0 3 Signature counter : 0 Signature key ....: [none] Encryption key....: [none] Authentication key: [none] General key info..: [none]

Input the admin argument to enable admin commands

gpg/card> admin Admin commands are allowed

Run the Generate command to generate the RSA key.

gpg/card> generate

You will be asked if you want to backup the encryption key. If you are only using this key for SSH authentication I would recommend you do not do this. It is fairly easy to go though this process again if you lose your Yubikey, and it means your key will never leave your Yubikey.

Make off-card backup of encryption key? (Y/n) n Please note that the factory settings of the PINs are PIN = `123456' Admin PIN = `12345678' You should change them using the command --change-pin gpg: gpg-agent is not available in this session Please enter the PIN What keysize do you want for the Signature key? (2048) What keysize do you want for the Encryption key? (2048) What keysize do you want for the Authentication key? (2048) 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

By default, your key will not expire. I would recommend setting an expiry date.

Key is valid for? (0) 1y Key expires at Thu 11 Aug 2021 14:18:02 GMT Is this correct? (y/N) y You need a user ID to identify your key; the software constructs the user ID from the Real Name, Comment and Email Address in this form: "Heinrich Heine (Der Dichter) <heinrichh@duesseldorf.de>" Real name: Paul Micheli Email address: paul@*********.com> Comment: You selected this USER-ID: "Paul Micheli <paul@*********.com>" Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? O gpg: generating new key gpg: 3 Admin PIN attempts remaining before card is permanently locked Please enter the Admin PIN gpg: please wait while key is being generated ... gpg: key generation completed (4 seconds) gpg: signatures created so far: 0 gpg: generating new key gpg: please wait while key is being generated ... gpg: key generation completed (6 seconds) gpg: signatures created so far: 1 gpg: signatures created so far: 2 gpg: generating new key gpg: please wait while key is being generated ... gpg: key generation completed (9 seconds) gpg: signatures created so far: 3 gpg: signatures created so far: 4 gpg: key A4D581D2 marked as ultimately trusted public and secret key created and signed. gpg: checking the trustdb gpg: public key of ultimately trusted key 53157188 not found gpg: 3 marginal(s) needed, 1 complete(s) needed, PGP trust model gpg: depth: 0 valid: 2 signed: 0 trust: 0-, 0q, 0n, 0m, 0f, 2u gpg: next trustdb check due at 2018-11-14 pub 2**/********2 2017-11-14 [expires: 2018-11-14] Key fingerprint = 3***************************2 uid Paul Micheli <paul@*********.com>> sub 2***/*********A 2020-08-11 [expires: 2021-08-11] sub 2***/*********A 2020-08-11 [expires: 2021-08-11]

Quit

gpg/card> quit

Your RSA key is now generated, and the public key is stored on your GPG keychain.

Viewing the public key

With your Yubikey inserted and unlocked, find the ID of your GPG key:

$ gpg --list-secret-keys --keyid-format LONG /home/paulmicheli/.gnupg/pubring.kbx ------------------------------------ sec r***/*********E 2020-07-30 [SC] [expires: 2022-07-30] 6**************************************E uid [ultimate] Paul Micheli <paul@********.com> ssb r***/*********0 2020-07-30 [E] [expires: 2022-07-30] sec> r***/*********4 2020-07-30 [SC] 7**************************************H Card serial no. = 0006 10300768 uid [ultimate] Paul Micheli <paul@*********.com> ssb r***/********8 2020-07-30 [E]

Then, get your public key so that you can tell GitHub about it. The argument here is the long ID from the above command:

$ gpg --armor --export A97FDF705EF51C50 -----BEGIN PGP PUBLIC KEY BLOCK----- mQG********************kyd [NOPE NOT HAVING THE MIDDLE] I**********************== =ZDdO -----END PGP PUBLIC KEY BLOCK-----

I have a whole series of YubiKey tips & Tricks if anyone is interested let me know