Creating a new CSR

rick edited this page Jun 27, 2018 · 4 revisions

Creating a CSR with CFSSL

CFSSL is CloudFlare's PKI toolkit, and, among other things, it's useful for generating Certificate Signature Requests, or CSRs. A CSR is what you give to a Certificate Authority; they'll sign it and give you back a certificate that you install on your webserver. CFSSL can generate both a private key and certificate request.

In order to generate a CSR, you must provide a JSON file containing the relevant details of your request. This JSON file looks something like:

{
    "hosts": [
        "example.com",
        "www.example.com"
    ],
    "CN": "www.example.com",
    "key": {
        "algo": "rsa",
        "size": 2048
    },
    "names": [{
        "C": "US",
        "L": "San Francisco",
        "O": "Example Company, LLC",
        "OU": "Operations",
        "ST": "California"
    }]
}

The "hosts" value is a list of the domain names which the certificate should be valid for. The "CN" value is used by some CAs to determine which domain the certificate is to be generated for instead; these CAs will most often provide a certificate for both the "www" (e.g. www.example.net) and "bare" (e.g. example.net) domain names if the "www" domain name is provided. The "key" value in the example is the default that most CAs support. (It may even be omitted in this case; it is shown here for completeness.)

The "names" value is actually a list of name objects. Each name object should contain at least one "C", "L", "O", "OU", or "ST" value (or any combination of these). These values are:

  • "C": country
  • "L": locality or municipality (such as city or town name)
  • "O": organisation
  • "OU": organisational unit, such as the department responsible for owning the key; it can also be used for a "Doing Business As" (DBS) name
  • "ST": the state or province

With our JSON request ready, a CSR and private key can be generated either through the API or through the command line interface. Both return a JSON response, but the cfssljson tool can be used to convert the response to files. With the command line interface, assuming the above certificate request is saved in "csr.json", the command line tool should be called with

cfssl genkey csr.json | cfssljson -bare certificate

This will produce a "certificate.csr" and "certificate-key.pem" file; the latter is the private key: it should be stored securely. The CSR should be sent to the CA (most often by copying and pasting it into a form on their site).

The API can also be used by making a POST request to "/api/v1/cfssl/newkey". For example, a cURL request to a locally-running CFSSL would look like:

curl -X POST -H "Content-Type: application/json" -d @csr.json \
    http://127.0.0.1:8888/api/v1/cfssl/newkey | cfssljson certificate

This will produce the same pair of files.

Creating a self-signed cert

CFSSL can also create a self-signed certificate. You should be aware that self-signed certificates present some security issues: they cannot be revoked, and their authenticity is suspect. However, they can be useful as a stopgap. CFSSL generates self-signed certificates which are valid for only three months; this is just long enough for them to serve as a temporary measure.

CFSSL's self-signer takes the same certificate request JSON as before; using the example JSON above, saved in csr.json, issue the command:

cfssl selfsign www.example.net csr.json | cfssljson -bare selfsigned

This creates three files: "selfsigned.pem", which is the self-signed certificate; "selfsigned.pem", which is the private key; and "selfsigned.csr", which is a certificate request. Once an acceptable CA is found, the CSR can be used to obtain a certificate signed by that CA.

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