Set up a private CA with `cfssl`

Why?#

• Got a home lab and want to secure connections through https
• I hate openssl CLI, they’re old and hard to use.
• I want a more modern solution.
• Just for fun :)

Installing cfssl#

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git clone --depth=1 git@github.com:cloudflare/cfssl.git
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cd cfssl
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make
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make install
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# or
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go install github.com/cloudflare/cfssl/cmd/...@latest

Make sure the binaries are in your PATH and let’s get started.

Generate root CA#

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mkdir pki/{ca,ocsp,crl,issued}
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cd pki
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cfssl print-defaults config > ca-config.json

and we get:

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{
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  "signing": {
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    "default": {
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      "expiry": "168h"
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    },
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    "profiles": {
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      "www": {
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        "expiry": "8760h",
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        "usages": [
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          "signing",
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          "key encipherment",
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          "server auth"
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        ]
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      },
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      "client": {
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        "expiry": "8760h",
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        "usages": [
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          "signing",
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          "key encipherment",
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          "client auth"
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        ]
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      }
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    }
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  }
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}

The default value are pretty good, so we’re not gonna change it. However, the default value for CSR isn’t what we desired, so let’s customise it.

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{
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  "CN": "",
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  "hosts": [
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    "example.net",
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    "www.example.net"
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  ],
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  "key": {
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    "algo": "ecdsa",
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    "size": 256
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  },
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  "names": [
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    {
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      "C": "US",
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      "ST": "CA",
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      "L": "San Francisco"
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    }
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  ]
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}

The default CSR uses ECDSA, which is required by CABForum to distribute keys with OSes or browsers:

For RSA key pairs the CA SHALL:

• Ensure that the modulus size, when encoded, is at least 2048 bits, and;
• Ensure that the modulus size, in bits, is evenly divisible by 8.

For ECDSA key pairs, the CA SHALL:

• Ensure that the key represents a valid point on the NIST P-256, NIST P-384 or NIST P-521 elliptic curve.

No other algorithms or key sizes are permitted.

Since no one will include our self-signed CA in their product, we’ll just use Ed25519.

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echo > ca-csr.json <EOF
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{
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    "CN": "Alkimia Root CA",
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    "key": {
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        "algo": "ed25519"
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    },
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    "names": [
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        {
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            "C": "CN",
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            "ST": "GD",
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            "L": "End City",
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            "O": "Alkimia Org",
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            "OU": "PKI"
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        }
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    ],
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    "ca": {
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        "expiry": "87600h"
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    }
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}
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EOF

Now, generate the root CA.

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cfssl gencert -initca ca-csr.json |cfssljson -bare ca

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{
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  "cert": "-----BEGIN CERTIFICATE----....----END CERTIFICATE-----\n",
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  "csr": "-----BEGIN CERTIFICATE REQUEST-----....-----END CERTIFICATE REQUEST-----\n",
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  "key": "-----BEGIN Ed25519 PRIVATE KEY-----....-----END Ed25519 PRIVATE KEY-----\n"
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}

cfssljson is responsible for saving it to files, in this case ca-key.pem, ca.csrandca.pem. ca-key.pem is our private key.

Signing server certificates#

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{
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  "CN": "alkimia.lan",
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  "hosts": [
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    "alkimia.lan",
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    "*.alkimia.lan"
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  ],
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  "key": {
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    "algo": "ed25519"
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  },
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  "names": [
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    {
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      "C": "CN",
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      "ST": "GD",
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      "L": "End City",
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      "O": "Alkimia Org"
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    }
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  ]
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}

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cfssl gencert -ca ca.pem -ca-key ca-key.pem -config ca-config.json \
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  -profile www server-csr.json \
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 |cfssljson -bare issued/alkimia.lan

Why don’t we try CRL and OCSP?#

Our CA is distributed by hand on every machine. CRL and OCSP are meaningless.