Create a new Certificate Authority or an intermediate CA from your existing PKI (BYOR)

🔎 Looking for the open-source step-ca version?

This tutorial is designed specifically for Smallstep's online Certificate Manager. If you are looking for the open-source version, you can find it here.

With Smallstep Certificate Manager, you can create an entirely new private PKI setup or an intermediate CA from your existing PKI. Intermediate CAs (also called subordinate CAs) are used to sign and issue leaf certificates to subscribers. Intermediates aren't generally included in trust stores, making them easier to revoke and rotate, so certificate issuance from an intermediate typically is online and automated.

This tutorial will walk you through creating a new PKI or an intermediate CA.

• Create a new Authority of the type, Default Authority below ACME and Managed Device Attestation

⚠️ Be sure to change the default type from DevOps to Advanced.

⚠️ Your root certificate needs to be in PEM format. You can convert to PEM using:

step certificate format YourRootCert.der

# command reference for `step certificate format`

Now, you will need to transfer the CSR (intermediate.csr) file to your existing root CA and get it signed. Below we have examples of how to do this using step, Active Directory Certificate Services, AWS Certificate Manager Private CA, OpenSSL, and CFSSL.

Use step to sign your intermediate CSR

step certificate sign --profile intermediate-ca intermediate.csr root.crt root.key

Active Directory Certificate Services

certreq -submit -attrib "CertificateTemplate:SubCA" intermediate.csr intermediate.crt

AWS Certificate Manager Private CA

You can now use the following Python script that uses issue-certificate to process the CSR:

import boto3
import sys

AWS_CA_ARN = '[YOUR_PRIVATE_CA_ARN]'

csr = ''.join(sys.stdin.readlines())

client = boto3.client('acm-pca')
response = client.issue_certificate(
    CertificateAuthorityArn=AWS_CA_ARN,
    Csr=csr,
    SigningAlgorithm='SHA256WITHRSA',
    TemplateArn='arn:aws:acm-pca:::template/SubordinateCACertificate_PathLen1/V1',
    Validity={
        'Value': 5,
        'Type': 'YEARS'
    }
)
print(f"Creating certificate with ARN {response['CertificateArn']}...", file=sys.stderr, end='')
waiter = client.get_waiter('certificate_issued')
waiter.wait(
    CertificateAuthorityArn=AWS_CA_ARN,
    CertificateArn=response['CertificateArn']
)
print('done.', file=sys.stderr)
response = client.get_certificate(
   CertificateArn=response['CertificateArn'],
   CertificateAuthorityArn=AWS_CA_ARN
)
print(response['Certificate'])

To run it, fill in the ARN of your CA and run:

$ python issue_certificate.py < intermediate.csr > intermediate.crt

OpenSSL

openssl ca -config [ROOT_CA_CONFIG_FILE] \
  -extensions v3_intermediate_ca \
  -days 3650 -notext -md sha512 \
  -in intermediate.csr \
  -out intermediate.crt

CFSSL

For CFSSL you'll need a signing profile that specifies a 10-year expiry:

$ cat > ca-smallstep-config.json <<EOF
{
  "signing": {
    "profiles": {
      "smallstep": {
        "expiry": "87660h",
        "usages": ["signing"]
      }
    }
  }
}
EOF

Now use that config to sign the intermediate certificate:

$ cfssl sign -ca ca.pem \
    -ca-key ca-key.pem \
    -config ca-smallstep-config.json \
    -profile smallstep
    -csr intermediate.csr | cfssljson -bare