certificates/docs/kms.md

# Key Management Services

This document describes how to use a key management service or KMS to store the
private keys and sign certificates.

Support for multiple KMS are planned, but currently the only supported one is
Google's Cloud KMS.

## Google's Cloud KMS

[Cloud KMS](https://cloud.google.com/kms) is the Google's cloud-hosted KMS that
allows you to store the cryptographic keys, and sign certificates using their
infrastructure. Cloud KMS supports two different protection levels, SOFTWARE and
HSM.

To configure Cloud KMS in your CA you need add the `"kms"` property to you
`ca.json`, and replace the property`"key"` with the Cloud KMS key name of your
intermediate key:

```json
{
    ...
    "key": "projects/<project-id>/locations/global/keyRings/<ring-id>/cryptoKeys/<key-id>/cryptoKeyVersions/<version-number>",
    ...
    "kms": {
        "type": "cloudkms",
        "credentialsFile": "path/to/credentials.json"
    }
}
```

In a similar way, for SSH certificate, the SSH keys must be Cloud KMS names:

```json
{
    ...
    "ssh": {
        "hostKey": "projects/<project-id>/locations/global/keyRings/<ring-id>/cryptoKeys/<key-id>/cryptoKeyVersions/<version-number>",
        "userKey": "projects/<project-id>/locations/global/keyRings/<ring-id>/cryptoKeys/<key-id>/cryptoKeyVersions/<version-number>"
    },
}
```

Currently [step](https://github.com/smallstep/cli) does not provide an automatic
way to initialize the public key infrastructure (PKI) using Cloud KMS, but an
experimental tool named `step-cloudkms-init` is available for this use case. At
some point this tool will be integrated into `step` and it will be deleted.

To use `step-cloudkms-init` just enable Cloud KMS in your project and run:

```sh
$ export GOOGLE_APPLICATION_CREDENTIALS=/path/to/credentials.json
$ step-cloudkms-init --project your-project-id --ssh
Creating PKI ...
✔ Root Key: projects/your-project-id/locations/global/keyRings/pki/cryptoKeys/root/cryptoKeyVersions/1
✔ Root Certificate: root_ca.crt
✔ Intermediate Key: projects/your-project-id/locations/global/keyRings/pki/cryptoKeys/intermediate/cryptoKeyVersions/1
✔ Intermediate Certificate: intermediate_ca.crt

Creating SSH Keys ...
✔ SSH User Public Key: ssh_user_ca_key.pub
✔ SSH User Private Key: projects/your-project-id/locations/global/keyRings/pki/cryptoKeys/ssh-user-key/cryptoKeyVersions/1
✔ SSH Host Public Key: ssh_host_ca_key.pub
✔ SSH Host Private Key: projects/your-project-id/locations/global/keyRings/pki/cryptoKeys/ssh-host-key/cryptoKeyVersions/1
```

See `step-cloudkms-init --help` for more options.

## YubiKey

And incomplete and experimental support for [YubiKeys](https://www.yubico.com)
is also available. Support for YubiKeys is not enabled by default and only TLS
signing can be configured.

The YubiKey implementation requires cgo, and our build system does not produce
binaries with it. To enable YubiKey download the source code and run:

```sh
make build GOFLAGS=""
```

The implementation uses [piv-go](https://github.com/go-piv/piv-go), and it
requires PCSC support, this is available by default on macOS and Windows
operating systems, but on Linux piv-go requires PCSC lite.

To install on Debian-based distributions, run:

```sh
sudo apt-get install libpcsclite-dev
```

On Fedora:

```sh
sudo yum install pcsc-lite-devel
```

On CentOS:

```sh
sudo yum install 'dnf-command(config-manager)'
sudo yum config-manager --set-enabled PowerTools
sudo yum install pcsc-lite-devel
```

The initialization of the public key infrastructure (PKI) for YubiKeys, is not
currently integrated into [step](https://github.com/smallstep/cli), but an
experimental tool named `step-yubikey-init` is available for this use case. At
some point this tool will be integrated into `step` and it will be deleted.

To configure your YubiKey just run:

```sh
$ bin/step-yubikey-init
What is the YubiKey PIN?:
Creating PKI ...
✔ Root Key: yubikey:slot-id=9a
✔ Root Certificate: root_ca.crt
✔ Intermediate Key: yubikey:slot-id=9c
✔ Intermediate Certificate: intermediate_ca.crt
```

See `step-yubikey-init --help` for more options.

Finally to enable it in the ca.json, point the `root` and `crt` to the generated
certificates, set the `key` with the yubikey URI generated in the previous step
and configure the `kms` property with the `type` and your `pin` in it.

```json
{
    "root": "/path/to/root_ca.crt",
    "crt": "/path/to/intermediate_ca.crt",
    "key": "yubikey:slot-id=9c",
    "kms": {
        "type": "yubikey",
        "pin": "123456"
    },
    ...
}
```