CFSSL: CloudFlare's PKI toolkit

go get



CloudFlare's SSL tool

CF-SSL is CloudFlare's SSL swiss army knife. It is both a command line tool and an HTTP API server for signing, verifying, and bundling SSL certificates. It requires Go 1.3 to build.

Using the Command Line Tool

The command line tool takes a command to specify what operation it should carry out:

   sign             signs a certificate
   bundle           build a certificate bundle
   genkey           generate a private key and a certificate request
   gencert          generate a private key and a certificate
   serve            start the API server
   version          prints out the current version

Use "cfssl [command] -help" to find out more about a command. The version command takes no arguments.


cfssl sign [-ca cert] [-ca-key key] hostname clientcert

The hostname and clientcert are the client's host name and client certificate. The -ca and -ca-key flags are the CA's certificate and private key, respectively. By default, they are "ca.pem" and "ca_key.pem". For example, assuming the CA's private key is in /etc/ssl/private/cfssl_key.pem and the CA's certificate is in /etc/ssl/certs/cfssl.pem, to sign the cloudflare.pem certificate for

cfssl sign -ca /etc/ssl/certs/cfssl.pem \
           -ca-key /etc/ssl/private/cfssl_key.pem \

It is also possible to specify hostname and clientcert through '-hostname' and '-cert' flags. By doing so, flag values take precedence and will overwrite the arguments.


cfssl bundle [-ca-bundle bundle] [-int-bundle bundle] \
             cert [key] [intermediates]

The bundles are used for the root and intermediate certificate pools. The certificate and key parameters are paths to the PEM-encoded client certificate to be bundled. If key is specified, the bundle will be built and verified with the key. Otherwise the bundle will be built without a private key.

It is also possible to specify cert, key and intermediates through '-cert', '-key' and '-intermediates' respectively. And like other commands, flag values will take precedence and overwrite the arguments.

Generating certificate signing request and private key

cfssl genkey csrjson

To generate a private key and corresponding certificate request, specify the key request as a JSON file. This file should follow the form

    "hosts": [
    "key": {
        "algo": "rsa",
        "size": 2048
    "names": [
            "C": "US",
            "L": "San Francisco",
            "O": "Internet Widgets, Inc.",
            "OU": "WWW",
            "ST": "California"

Generating self-signed root CA certificate and private key

cfssl gencert -initca csrjson

To generate a self-signed root CA certificate, specify the key request as the JSON file in the same format as in 'genkey'. Three PEM-encoded entities will appear in the output: the private key, the csr, and the self-signed certificate.

Generating a remote-issued certificate and private key.

cfssl gencert -remote=remote_server hostname csrjson

This is calls genkey, but has a remote CFSSL server sign and issue a certificate.

Generating a local-issued certificate and private key.

cfssl gencert -ca cert -ca-key key hostname csrjson

This is generates and issues a certificate and private key from a local CA via a JSON request.

Starting the API Server

CF-SSL comes with an HTTP-based API server; the endpoints are documented in doc/api.txt. The server is started with the "serve" command:

cfssl serve [-address address] [-ca cert] [-ca-bundle bundle] \
            [-ca-key key] [-int-bundle bundle] [-port port]

Address and port default to "". The -ca and -ca-key arguments should be the PEM-encoded certificate and private key to use for signing; by default, they are "ca.pem" and "ca_key.pem". The -ca-bundle and -int-bundle should be the certificate bundles used for the root and intermediate certificate pools, respectively. These default to "ca-bundle.crt" and "int-bundle."

The amount of logging can be controlled with the -loglevel option. This comes before the serve command:

cfssl -loglevel 2 serve

The levels are:

  • 0. DEBUG
  • 1. INFO (this is the default level)
  • 2. WARNING
  • 3. ERROR

The mkbundle Utility

mkbundle is used to build the root and intermediate bundles used in verifying certificates. It can be installed with

go get

It takes a collection of certificates, checks for CRL revocation (OCSP support is planned for the next release) and expired certificates, and bundles them into one file. It takes directories of certificates and certificate files (which may contain multiple certificates). For example, if the directory intermediates contains a number of intermediate certificates,

mkbundle -f int-bundle.crt intermediates

will check those certificates and combine valid ones into a single int-bundle.crt file.

The -f flag specifies an output name; -loglevel specifies the verbosity of the logging (using the same loglevels above), and -nw controls the number of revocation-checking workers.

The cfssljson Utility

Most of the output from cfssl is in JSON. The cfssljson will take this output and split it out into separate key, certificate, CSR, and bundle files as appropriate. The tool takes a single flag, -f, that specifies the input file, and an argument that specifies the base name for the files produced. If the input filename is "-" (which is the default), cfssljson reads from standard input. It maps keys in the JSON file to filenames in the following way:

  • if there is a "cert" (or if not, if there's a "certificate") field, the file "basename.pem" will be produced.
  • if there is a "key" (or if not, if there's a "private_key") field, the file "basename-key.pem" will be produced.
  • if there is a "csr" (or if not, if there's a "certificate_request") field, the file "basename.csr" will be produced.
  • if there is a "bundle" field, the file "basename-bundle.pem" will be producd.

Additional Documentation

Additional documentation can be found in the "doc/" directory:

  • api.txt: documents the API endpoints
  • bootstrap.txt: a walkthrough from building the package to getting up and running