I’m often asked how to stand up a device trust layer that scales from homelab to enterprise without the additional complexity of Hardware security modules. In this post, I’ll show you how to wire up step-ca (Smallstep’s open-source CA), Google Cloud Certificate Authority Service (CAS) as a managed signing backend, and SCEP for mass device enrollment. The result is a modern, automatable PKI that issues device certificates for clientAuth, mTLS, Wi-Fi, VPN, Access Gateways, and beyond.

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Why this stack?

• step-ca: A lightweight CA with batteries included—ACME, OIDC, SCEP, SSH CA, templates, audit logs, and more. It’s ideal as your “front-door” RA/CA service and policy brain.
• Google Cloud CAS: A managed, audited CA that signs certificates on your behalf. Offload availability and compliance to Google while keeping your issuance logic under your control.
• SCEP: A well-supported protocol for bulk device enrollment—especially useful for legacy or embedded systems, printers, network gear, and certain MDM agents.

This combo gives you:

• Cloud scalability + low ops: Managed CA with step-ca’s simple deployment.
• Security: Keep the signing CAs inside Google CAS; expose only step-ca to your fleet.
• Automation: Scriptable bootstraps, ephemeral certs, and policy controls.

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Architecture at a glance

Flow summary

1. Devices talk SCEP to step-ca.
2. step-ca acts as a Registration Authority, relaying/signing requests via CAS.
3. SCEP returns a signed device certificate.
4. Devices use certs for clientAuth challenges from the IdP/SSO provider or an access gateway before reaching protected apps

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Prerequisites

• A Google Cloud project with CAS enabled and an active CA in an appropriate CA Pool.
• A Linux host/VM/container to run step-ca (front-door RA/CA and SCEP endpoint).
• DNS for your step-ca endpoint (public or private, as needed).
• Firewall rules allowing inbound 443 to step-ca.
• gcloud CLI and step CLI.

Installation docs for step-ca: https://smallstep.com/docs/step-ca/installation/

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Step 1 — Create a service account for step-ca → CAS access

Create a service account that step-ca will use to interact with CAS:

gcloud iam service-accounts create step-cas-sa \
    --description "Step-CA Service Account" \
    --display-name "Step-CA Service Account"

Grant this SA appropriate CAS roles (for certificate issuance). In many deployments that’s at least roles/privateca.certificateRequester on the CA Pool. (Use the principle of least privilege.)

Tip: You can utilize a certificate template within GCP CAS to further restrict the usage and options available to the service account when an certificate is requested.

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Step 2 — Install step-ca and initialize with Cloud CAS as the RA

Create a working directory and initialize step-ca’s config to use Cloud CAS as the Registration Authority:

mkdir /etc/step-ca
export STEPPATH=/etc/step-ca
step ca init --name="CasPoC" --deployment-type standalone --remote-management --provisioner="admin@example.com" --ra=CloudCAS --issuer=projects/<project>/locations/<us-central1>/caPools/<Ca Pool>/certificateAuthorities/<ca ID> --dns="<FQDN>" --address=":443"

What this does:

• Creates /etc/step-ca/config/ca.json and related directories.
• Sets CloudCAS as the RA, pointing at your CA (–issuer=projects/…/certificateAuthorities/<ca ID>).
• Binds HTTPS on :443 (we’ll allow a non-root user to bind shortly).
• Registers a default OIDC or local provisioner (here, admin@example.com) for management.

Tip: Authenticate gcloud as the service account, or run step-ca compute with that service account identity, so Cloud CAS requests succeed.

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Step 3 — Create a restricted system user and grant low-port bind

Run step-ca as a non-root user:

useradd step
passwd -l step
chown -R step:step /etc/step-ca

Allow binding to port 443 without root:

setcap CAP_NET_BIND_SERVICE=+eip /usr/bin/step-ca

This capability approach is safer than running as root.

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Step 4 — (Optional) Prepare an intermediate CA template

 (CSR → CAS-signed)

We’ll generate an intermediate key and CSR locally, then have CAS sign it to establish our operational intermediate for issuance:

cat <<EOF >  /etc/step-ca/templates/rsa_intermediate_ca.tpl
{
  "subject": {{ toJson .Subject }},
  "issuer": {{ toJson .Subject }},
  "keyUsage": ["certSign", "crlSign"],
  "basicConstraints": {
    "isCA": true,
    "maxPathLen": 0
  }
  {{- if typeIs "*rsa.PublicKey" .Insecure.CR.PublicKey }}
    , "signatureAlgorithm": "SHA256-RSAPSS"
  {{- end }}
}
EOF

Generate the CSR and key:

step certificate create "SCEP Intermediate CA" \
    /etc/step-ca/certs/intermediate_ca.csr \
    /etc/step-ca/secrets/intermediate_ca_key \
    --template /etc/step-ca/templates/rsa_intermediate_ca.tpl \
    --kty RSA \
    --size 3072 --csr

Ask Cloud CAS to sign the CSR:

gcloud privateca certificates create CERT_ID \
    --issuer-pool <Pool> \
    --issuer-location <Location> \
    --csr /etc/step-ca/certs/intermediate_ca.csr \
    --cert-output-file /etc/step-ca/certs/intermediate_ca.crt \
    --validity "P1Y"

Update your ca.json so step-ca uses the newly minted intermediate:

        "root": "/etc/step-ca/certs/root_ca.crt",
        "federatedRoots": null,
        "crt": "/etc/step-ca/certs/intermediate_ca.crt",
        "key": "/etc/step-ca/secrets/intermediate_ca_key",

Notes
• Store /etc/step-ca/secrets on encrypted disk or a secrets-managed volume.
• Rotate the intermediate on a schedule (e.g., annually) and keep a CRL/OCSP strategy in place.

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Step 5 — Dry run the CA

Before opening the floodgates, do a dry run:

sudo -u step step-ca /etc/step-ca/config/ca.json

If it boots cleanly, you should see the HTTP listener and provisioners registered in logs.

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Step 6 — Enable SCEP  for device enrollment

Add a SCEP provisioner to your existing CA service. We’ll set challenge credentials and certificate lifetimes:

step ca provisioner add poc_devicetrust \
  --type SCEP --challenge "<redacted>" \
   --x509-min-dur=24h \
   --x509-max-dur=8760h \
   --x509-default-dur=1080h \
  --encryption-algorithm-identifier 2 --admin-name step

A few practical notes:

• Challenge: Keep it secret (vault, KMS, or MDM payloads). Consider migrating to SCEP with client-side RA or EST for stronger auth if your device ecosystem supports it.
• Durations: Default shown is 45 days (1080h). Short-lived certs reduce revocation surface.
• Algorithm Identifier 2: This sets the SCEP Encryption Algorithm Identifier (commonly RSA/3DES/AES variations). Keep this consistent with your device agents.

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Step 7 — (Workaround) Clean up ca.json authorities section if needed

Some versions/paths require removing the cloudcas entry from the authorities section. If you see startup errors or odd RA behavior, adjust and restart:

sudo vi /etc/step-ca/config/ca.json

Then relaunch:

sudo -u step step-ca /etc/step-ca/config/ca.json

Note: SCEP can operate with just the local Intermediate CA being used to sign device cert requests. We don’t need direct access to the Root CA or subordinate CAs, so they can be isolated within CAS.

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Step 8 — Bootstrap devices and request a cert via SCEP

Once your root CA is trusted on the device (via MDM, config management, or manual import), request a certificate with a SCEP client. Example:

scepclient -private-key client.key -server-url=https://<Domain/IP>/scep/poc_devicetrust -challenge=<Redacted> -dnsname "Lab-PC.local" -cn "Lab-PC" -country "US" -organization "Lab" -ou "Device Trust"

This will:

• Pull the CA chain from GetCACert
• Submit a PKCSReq containing your CSR and challenge
• Receive a signed certificate in CertRep

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SCEP Enrollment Sequence

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Hardening, Operations, and Best Practices

1) TLS and network posture

• Put step-ca behind a reverse proxy or L7 load balancer for WAF/DoS controls.
• Consider utilizing a URL Map and/or HTTP targets to restrict acces only to your provisioners
• Use mTLS for internal admin APIs and restrict management endpoints by IP/VPN.

2) Identity for step-ca

• Run as the dedicated step user (as above) and consider systemd hardening (ProtectSystem, NoNewPrivileges, PrivateTmp, AmbientCapabilities=CAP_NET_BIND_SERVICE).
• Keep /etc/step-ca on a read-only or append-only partition where possible; separate secrets onto encrypted volumes.

3) Secrets management

• Store SCEP challenge in a secret manager and template it into device configs at enrollment time.
• If using multiple SCEP realms (e.g., printers vs. laptops), separate provisioners with distinct challenges and policies.

4) Short-lived certs + automation

• Favor short lifetimes (7–45 days) and auto-renew via SCEP or ACME where supported.
• For services (ingress gateways, sidecars), consider ACME provisioners in step-ca instead of SCEP.

5) Revocation and status

• Enable OCSP and/or regularly published CRLs. Some gear only understands CRLs; others can do OCSP.
• Document how to revoke by CN/serial and how MDM/CM tooling redistributes CRL/OCSP endpoints.

6) Names and OIDs

• Standardize Subject and SANs. For devices, prefer DNS SANs and URNs (e.g., urn:device:asset:1234) over stuffing identifiers in CN.
• Use policy OIDs or Extended Key Usages (EKUs) that match your relying parties (ClientAuth, ServerAuth, Wi-Fi EAP-TLS, IPsec, etc.).

7) Auditing

• Step-ca logs each issuance; forward logs to a SIEM with context (device inventory ID, enrollment workflow ID).
• Cloud CAS has control plane logs—monitor for volume spikes or unusual issuers.

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Validating the build (quick checks)

• Health: curl -ik https://<FQDN>/health (if you expose a health endpoint or use LB health checks).
• SCEP reachability: curl -I https://<FQDN>/scep/poc_devicetrust should not 404.
• CAS connectivity: Attempt a test cert request; if it fails, check service account auth and CAS IAM.
• Chain trust: On a device, ensure the root (and intermediate, if needed) are in the appropriate trust store. Many SCEP clients install the chain automatically after GetCACert.

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Troubleshooting tips

• Challenge mismatch: 401/failed enrollment—verify SCEP challenge and transport (hidden in MDM payloads).
• CAS quota or IAM: CAS may rate-limit or block by IAM; review Google Cloud audit logs.
• Template oddities: If you need custom subject or SAN logic per device group, use step templates and multiple SCEP provisioners.

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Where to go from here

• Add ACME for servers and gateways while keeping SCEP for legacy endpoints.
• Introduce device attestation checks before issuance (e.g., callbacks/webhooks from step-ca to your SCEP/ACME services).
• Build up resilience by added device posture checks with existing VPN, EDR/XDR. and other security agents
• Automate intermediate rotation with change windows and controlled CRL/OCSP updates.