Configuring step-ca Provisioners

Provisioners are methods of using the CA to get certificates for humans or machines. They offer different modes of authorization for the CA. In this section we'll discuss the different provisioners, their target use cases, and how to add, remove, and configure them.

Overview

Authorization Scope by Provisioner

Every provisioner has a slightly different scope of authorization. Below is a table detailing the authorization capabilities of each provisioner.

For an example of how to interpret this table, let's take the JWK provisioner. The JWK provisioner is capable of signing, renewing, and revoking X.509 certificates, as well signing user and host SSH certificates. A JWK provisioner cannot renew or rekey SSH certificates.

An SSHPOP provisioner can revoke and rekey SSH certificates and renew SSH host certificates. An SSHPOP provisioner cannot sign, renew, or revoke X.509 certificates, and it cannot sign SSH user and host certificates or renew SSH user certificates.

It's important to understand the capabilities and limitations when selecting a provisioner for a given workload.

Provisioner Management

Use the step ca provisioner command group to add, remove, or modify provisioner configurations. Run these commands directly on your CA machine. They need to modify the $(step path)/config/ca.json configuration file.

Some provisioner options override global defaults for your CA. For a list of global options, see the configuration guide section for the authority configuration block.

A remote provisioner management API can be enabled in step-ca. It is disabled by default. With remote provisioner management, the CA's provisioner configuration is stored in the database instead of ca.json. (The global CA configuration remains in ca.json.)

This feature can be useful if you have multiple CA administrators, run several load-balanced step-ca instances, or if you want to manage your provisioners remotely (eg. with Infrastructure as Code (IaC) tools; see below).

See Remote Provisioner Management for more.

Common Provisioner Operations

Common provisioner operations include:

Unless you are using remote provisioner management, you must send a SIGHUP signal, or restart the step-ca process, for changes to your provisioner configuration to take effect.

Add a provisioner

Use step ca provisioner add to add a provisioner:

See the command reference for complete documentation and examples.

Remove a provisioner

Use step ca provisioner remove to remove provisioners:

You can also edit the ca.json configuration file and remove the entire block containing the provisioner you'd like to remove.

List all provisioners

To get a list of all of your current provisioners, use step ca provisioner list or the /provisioners endpoint on your CA.

Modify a provisioner

Use step ca provisioner update to update provisioner configurations:

In this example we've modified the minumum, maximum, and default durations for TLS certificates generated by this provisioner.

Provisioner configuration can be used to affect X.509 and SSH certificate lifetimes, extensions, and templates. There are some provisioner-specific options, which are covered by the documentation for each provisioner type, below.

Remote Provisioner Management

This feature is disabled by default.

When remote provisioner management is enabled, your provisioner configuration is stored in the database, rather than in ca.json. Provisioner configuration is managed by running step ca provisioner commands locally or remotely. These commands require you to sign in as an Admin user.

Enable Remote Provisioner Management

To enable remote provisioner management, update your "authority" block in ca.json to include the following:

The "provisioners" list must be empty, and "enableAdmin" must be set to true.

Bootstrap Remote Provisioner Management

During this step:

"Admins" can remotely manage provisioners. "Super Admin" are Admins that can also manage the list of Admins for the CA.

Create An Admin User

To perform CA administrative operations, you sign into that provisioner as an Admin.

As a Super Admin, lets create an Admin user linked to the Admin JWK provisioner:

Output:

An Admin (or Super Admin) user is a combination of a username (aka a subject) and a provisioner.

The Admin and Super Admin privileges are not scoped to a provisioner: Admins can modify any provisioner configuration. Super Admins are Admins that can also modify Admins, regardless of provisioner.

Use Remote Provisioner Management

You're all set. 🎉

You can use the step ca provisioner commands, from any client, to modify your CA's provisioner configuration.

You will be asked for a username and provisioner password to sign in. The default username is step, and the default provisioner is called Admin JWK.

Unattended Remote Provisioner Management

If you want to manage provisioners remotely using a script or an Infrastructure as Code (IaC) tool, you will need an administrative certificate and key.

An administrative certificate must meet the following criteria:

To create an admin certificate, run:

Output:

With that certificate, you can use it as follows:

If the admin.crt represents a Super Admin, the same flags can also be used to manage Admins:

Provisioner Types

JWK

JWK is the default provisioner type. It uses public-key cryptography to sign and validate a JSON Web Token (JWT).

The step CLI tool will create a JWK provisioner when step ca init is used.

Example

Add a JWK provisioner:

In the ca.json configuration file, a complete JWK provisioner example looks like:

Decrypting the private key

We can use step crypto jwe decrypt to see the private key encrypted with the password asdf:

Changing a JWK provisioner password

Rekeying a JWK provisioner

Removing the encrypted private key from a JWK provisioner

The encrypted private key stored in the JWK provisioner configuration and published to the public /provisioners endpoint is provided for client convenience. It is not required for step-ca to operate. To remove this key:

OAuth/OIDC Single Sign-on

Sometimes it's useful to issue certificates to people. So step-ca supports single sign-on with identity providers (IdPs) like Google, Okta, Azure Active Directory, Keycloak, or any other provider that supports OAuth's OpenID Connect extension..

OpenID Connect is an extension to OAuth 2.0 that adds an identity layer. Providers that support OIDC can issue identity tokens ("ID tokens") to OAuth clients. These are JSON Web Tokens (JWTs) containing user identity information (eg. full name, username, email address). Like certificates, OIDC tokens have a validity period and are cryptographically signed by a trust authority (the OAuth provider).

Here's an example OIDC identity token issued by Google:

The OIDC provisioner in step-ca can be configured to trust and accept an OAuth provider's ID tokens for authentication. By default, the issued certificate will use the subject (sub) claim from the identity token as its subject. The value of the token's email claim is also included as an email SAN in the certificate.

Fig. 3: Diagram of how step works with individuals using a single sign-on provisioner

From the user's perspective, when requesting a certificate, step detects the OIDC provisioner and initiates the OAuth login flow automatically:

Configuring your identity provider (IdP)

When creating an OAuth app, there isn't much to configure on the IdP. Most providers will ask you to specify a Redirect URI, where the ID token will be delivered at the end of the OAuth flow. Since step starts its own local web server to receive the token, use http://127.0.0.1 as the Redirect URI.

Example: Google Identity

One of the most common providers, and the one used in the following example, is Google Identity.

Add a Google provisioner:

Example ca.json provisioner configuration for a Google provisioner:

Notes

Further reading

X5C - X.509 Certificate

An X5C provisioner allows a client to get an x509 or SSH certificate using an existing X509 certificate that is trusted by the provisioner.

An X5C provisioner is configured with a root certificate, supplied by the user, at the time the provisioner is created. The X5C provisioner can authenticate X5C tokens.

What's an X5C token? It's a JWT, signed by the certificate private key, with an x5c header that contains the chain.

Example

If you would like any certificate signed by step-ca to become a provisioner (have the ability to request new certificates with any name), you can create an X5C provisioner using the root certificate used by step-ca, like so:

Or, you can configure the X5C provisioner with an outside root, granting provisioner capabilities to a completely separate PKI.

Below is an example of an X5C provisioner in the ca.json:

Nebula

This is an experimental feature.

If you have a Nebula overlay network, you can create a Nebula provisioner and configure it with your Nebula root CA certificate. Clients can then use their Nebula client certificate and private key to request an X.509 or SSH host certificate from step-ca. The Nebula certificate they use for authentication must be issued by the Nebula root CA configured in the provisioner.

The Nebula client certificate is used for authorization, too: The client is allowed to request an X.509 or SSH host certificate with the name or any of the ips appearing on the Nebula client certificate.

To be clear, Nebula certificates can contain a single name and a list of ips. The name field is often a DNS hostname, but it could be an email, IP, or URI. And ips contains a list of CIDR blocks. In step-ca, the Nebula provisioner will authorize certificate subjects or SANs that include the name, plus IPs in any of the CIDR blocks in ips on the Nebula certificate.

To get started, create a Nebula provisioner:

Now you can get an X509 certificate with the Nebula provisioner, using a Nebula client certificate. Here's an example using the DNS name host3.example.com, and two IPs:

SSHPOP - SSH Certificate

An SSHPOP provisioner allows a client to renew, revoke, or rekey an SSH certificate using that certificate for authentication with the CA. The renew and rekey methods can only be used on SSH host certificates.

An SSHPOP provisioner is configured with the user and host root ssh certificates from the ca.json. The SSHPOP provisioner can only authenticate SSHPOP tokens generated using SSH certificates created by step-ca.

An SSHPOP token is a JWT, signed by the certificate private key, with an sshpop header that contains the SSH certificate.

When configured with the --ssh option (step ca init --ssh), the CA will contain a default SSHPOP provisioner named sshpop.

Example

Add an SSHPOP provisioner:

An example SSHPOP provisioner in the ca.json:

ACME

An ACME provisioner allows a client to request a certificate from the server using the ACME Protocol. This provisioner supports issuing X.509 certificates with IP or hostname identifiers. It supports the http-01, dns-01, and tls-alpn-01 ACME challenge types, All authentication of the CSR is managed by the ACME protocol.

Example

Add an ACME provisioner:

An example of an ACME provisioner in the ca.json:

See ACME Basics for more guidance on configuring and using the ACME protocol with step-ca.

SCEP

The SCEP provisioner can sign and renew certificates using the SCEP protocol (RFC8894). SCEP is very popular for use in network equipment and mobile device management (MDM). It runs over HTTP using POSTed binary data or base64-encoded GET parameters, using CMS (PKCS#7) and CSR (PKCS#10) data formats. A shared secret authenticates clients to the CA.

Requirements

Your CA must use an RSA intermediate CA, even if your client supports ECDSA. The RSA intermediate is used to decrypt the contents of the SCEP pkcsPKIEnvelope containing the certificate request. This operation cannot be performed using an ECDSA key.

Because step ca init creates an ECDSA chain by default, you will need to convert your CA to use an RSA CA chain before using the SCEP provisioner.

Configure the Provisioner

In this example, we will add a SCEP provisioner using challenge secret secret1234 and AES-256-CBC as the encryption algorithm:

The shared challenge should be a value that you will distribute to your SCEP clients.

Here's an example of a SCEP provisioner in $(step path)/config/ca.json:

Enable the HTTP Server

By default SCEP will only be served via HTTPS. Most SCEP clients use HTTP, so you will most likely need your CA to listen using HTTP too, which it does not do by default. Enable this by filling in the "insecureAddress" property to your top-level CA configuration:

Finally, restart step-ca. Your SCEP provisioner is now available at the endpoint http://ca.example.com:8080/scep/scepca.

K8sSA - Kubernetes Service Account

A K8sSA provisioner allows a client to request a certificate from the server using a Kubernetes Service Account Token.

As of the time when this provisioner was coded, the Kubernetes Service Account API for retrieving the token from a running instance was still in beta. Therefore, our K8sSA provisioner must be configured with the public key that will be used to validate K8sSA tokens.

K8sSA tokens are very minimal. There is no place for SANs, or other details that a user may want validated in a CSR. It is essentially a bearer token. Therefore, at this time a K8sSA token can be used to sign a CSR with any SANs. Said differently, the K8sSA provisioner does little to no validation on the CSR before signing it. You should only configure and use this provisioner if you know what you are doing. If a malicious user obtains the private key they will be able to create certificates with any SANs and Subject.

Example

Add a K8sSA provsioner:

An example of a K8sSA provisioner in the ca.json:

Cloud Provisioners

step-ca can be configured to use instance identity documents (IIDs) to authorize certificate signing requests from cloud VMs running on AWS, GCP, or Azure. IIDs are signed JSON tokens, created when the instance is launched, and made available via the instance metadata API.

Here's the contents of an example IID from AWS:

Fig. 2: Diagram of how step works with a cloud service as the provisioner

Once you’ve configured step-ca to accept IIDs for authentication, step will detect the provisioner type, obtain an IID token from your cloud provider's metadata API, and use it to obtain a certificate from step-ca.

From the CA's perspective, an IID is a single-use token. A host can only get one certificate, ever, per IID. A host can then renew its certificate using step ca renew. If the certificate ever expires, the host will need to use a different provisioner to issue a new one.

Security Risks and Limitations

While IIDs simplify the integration of step-ca, the approach comes with risks.

Unfortunately, an IID usually doesn't contain enough information to authenticate a host certificate. For example, while an IID may contain the private or public IP for the host, it will not contain all of the DNS names and IP addresses that you may want on the certificate.

Because of this, step-ca's cloud provisioners use the Trust on First Use (TOFU) security model, allowing any instance to get a certificate with any names (SANs) on it.

This allows for a "cryptographic perimeter": If a host presents a certificate that was signed by your CA using a cloud provisioner, and the CA is configured to verify the instance's account, project, or tenant ID in the IID, you can have some confidence that the request came from your infrastructure (not someone else's), but you cannot assume that the names on the certificate are authentic.

Because of this, every host in your infrastructure must be trusted.

Mitigating the risk of IIDs

Here are some things you can do to mitigate risk when using IIDs:

There are a lot of details to get right to make this model secure, many of which are environment-dependent and beyond the scope of this document.

AWS

The AWS provisioner allows granting a certificate to an Amazon EC2 instance using the Instance Identity Documents

The step CLI will generate a custom JWT token containing the instance identity document and its signature and the CA will grant a certificate after validating it.

Example

Find your AWS account ID to restrict access to our VMs:

On the host running step-ca add an AWS provisioner to your configuration by running:

In the ca.json, an AWS provisioner looks like:

GCP

The GCP provisioner grants certificates to Google Compute Engine instance using its identity token. The CA will validate the JWT and grant a certificate.

Example

On the host running step-ca, add an GCP provisioner to your configuration by running:

In the ca.json, a GCP provisioner looks like:

Azure

The Azure provisioner grants certificates to Microsoft Azure instances using the managed identities tokens. The CA will validate the JWT and grant a certificate.

Example

On the host running step-ca, add an GCP provisioner to your configuration by running:

In the ca.json, an Azure provisioner looks like: