yubikey-pki

This script uses your YubiKey as a CA to sign TLS certificates.

The benefits:

• Quickly and easily mint certificates
• A physical touch is required to mint any certificate
• No "certificate is not standards-compliant" error on iOS/macOS - the generated certs adhere to Apple's requirements

The downsides:

• There is no intermediate CA
• No support for rotation
• No support for signing CSRs that aren't generated by this tool

0. Prerequisites

The following dependencies are required:

• openssl@1.1
• opensc
• libp11
• ykman
• yubico-piv-tool

Using Nix

Nix Package Manager is strongly recommended. If you're using Nix, dependencies will be autoloaded just for this project in a manner that does not pollute your environment.

Install Nix (recommended)

Using Brew

brew install openssl@1.1 homebrew/cask/opensc libp11 ykman yubico-piv-tool

1. Set up your YubiKey

Set up the YubiKey PIV application on your YubiKey. First, change the default PIN, PUK, and Management Key, and save the values somewhere:

nix-shell # (if you're using Nix)
ykman piv access change-management-key
ykman piv access change-pin
ykman piv access change-puk

2. Create a root CA

This will overwrite slot 9c on your YubiKey's PIV application.

Generate an X.509 Root CA keypair for the specified domain and load it into your YubiKey.
Usage: ./root.sh [--(no-)keep-private-key] [--ttl <arg>] [-h|--help] <root_domain>
	<root_domain>: domain for the root CA
	--keep-private-key, --no-keep-private-key: keep the private key around after importing it to the YubiKey (off by default)
	--ttl: root CA ttl in days (default: '7300')
	-h, --help: Prints help

To create a root CA for example.com and load it into your YubiKey:

# Using Nix:
nix-shell --run "./root.sh example.com"

# Otherwise:
./root.sh example.com

This script will create a CA name-restricted to example.com, with a lifetime of 20 years, import it to your YubiKey, delete the private key, and give you a command to add the certificate to macOS Keychain for compatibility with Safari.

To add the cert to Firefox, go to Settings -> Security -> Certificates -> View Certificates, and add example.com/crt.pem to the Authorities section.

3. Generate endpoint subdomain certificates

Generate an X.509 cert pair for the specified endpoint, and sign the CSR with your YubiKey.
Usage: ./endpoint.sh [--ttl <arg>] [-h|--help] <endpoint_domain> <root_domain>
	<endpoint_domain>: subdomain to generate the certs for (e.g. 'foo' for 'foo.example.com')
	<root_domain>: domain of the root CA (e.g. 'example.com' for 'foo.example.com')
	--ttl: endpoint CA ttl in days (default: '820')
	-h, --help: Prints help

To generate a public/private key pair for foo.example.com:

# Using Nix:
nix-shell --run "./endpoint.sh foo example.com"

# Otherwise:
# (you might have to adjust the lib paths)
export SO_PATH="/opt/homebrew/Cellar/libp11/0.4.11/lib/engines-1.1/pkcs11.dylib"
export MODULE_PATH="Library/OpenSC/lib/opensc-pkcs11.so"
./endpoint.sh foo example.com

1. Input your YubiKey PIV PIN when prompted. Touch the YubiKey when it starts flashing.
2. Provide example.com/foo/crt.pem and example.com/foo/key.pem to your endpoint.
3. Delete example.com/foo/key.pem.

This creates a certificate for foo.example.com with a lifetime of 820 days in example.com/foo.