CFSSL

CloudFlare’s PKI/TLS toolkit

CFSSL is CloudFlare’s PKI/TLS swiss army knife. It is both a command line tool and an HTTP API server for signing, verifying, and bundling TLS certificates. It requires Go 1.4 to build.

Note that certain linux distributions have certain algorithms removed (RHEL-based distributions in particular), so the golang from the official repositories will not work. Users of these distributions should install go manually to install CFSSL.

CFSSL consists of:

• a set of packages useful for building custom TLS PKI tools
• the cfssl program, which is the canonical command line utility using the CFSSL packages.
• the multirootca program, which is a certificate authority server that can use multiple signing keys.
• the mkbundle program is used to build certificate pool bundles.
• the cfssljson program, which takes the JSON output from the cfssl and multirootca programs and writes certificates, keys, CSRs, and bundles to disk.

Building

See BUILDING

Installation

Installation requires a working Go installation and a properly set GOPATH. The default behaviour is to build with PKCS #11, which requires the gcc compiler and the libtool development library and header files. On Ubuntu, this is libltdl-dev. On Centos/RHEL, this is ‘libtool’ and ‘libtool-ltdl’. If these are not installed, you can pass -tags nopkcs11 to the below go get commands.

$ go get -u github.com/cloudflare/cfssl/cmd/cfssl

will download and build the CFSSL tool, installing it in $GOPATH/bin/cfssl. To install the other utility programs that are in this repo:

$ go get -u github.com/cloudflare/cfssl/cmd/...

This will download, build, and install cfssl, cfssljson, and mkbundle into $GOPATH/bin/.

Using the Command Line Tool

The cfssl 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
   selfsign         generates a self-signed certificate
   print-defaults    print default configurations

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

Signing

cfssl sign [-ca cert] [-ca-key key] [-hostname comma,separated,hostnames] csr [subject]

The csr is the client’s certificate request. 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”. The -hostname is a comma separated hostname list that overrides the DNS names and IP address in the certificate SAN extension. 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 cloudflare.com:

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

It is also possible to specify csr through ‘-csr’ flag. By doing so, flag values take precedence and will overwrite the argument.

The subject is an optional file that contains subject information that should be used in place of the information from the CSR. It should be a JSON file with the type:

{
    "CN": "example.com",
    "names": [
        {
            "C": "US",
            "L": "San Francisco",
            "O": "Internet Widgets, Inc.",
            "OU": "WWW",
            "ST": "California"
        }
    ]
}

Bundling

cfssl bundle [-ca-bundle bundle] [-int-bundle bundle] \
             [-metadata metadata_file] [-flavor bundle_flavor] \
             -cert certificate_file [-key key_file]

The bundles are used for the root and intermediate certificate pools. In addition, platform metadata is specified through ‘-metadata’ The bundle files, metadata file (and auxiliary files) can be found at cfssl_trust

Specify PEM-encoded client certificate and key through ‘-cert’ and ‘-key’ respectively. If key is specified, the bundle will be built and verified with the key. Otherwise the bundle will be built without a private key. Instead of file path, use ‘-‘ for reading certificate PEM from stdin. It is also acceptable the certificate file contains a (partial) certificate bundle.

Specify bundling flavor through ‘-flavor’. There are three flavors: ‘optimal’ to generate a bundle of shortest chain and most advanced cryptographic algorithms, ‘ubiquitous’ to generate a bundle of most widely acceptance across different browsers and OS platforms, and ‘force’ to find an acceptable bundle which is identical to the content of the input certificate file.

Alternatively, the client certificate can be pulled directly from a domain. It is also possible to connect to the remote address through ‘-ip’.

cfssl bundle [-ca-bundle bundle] [-int-bundle bundle] \
             [-metadata metadata_file] [-flavor bundle_flavor] \
             -domain domain_name [-ip ip_address]

The bundle output form should follow the example

{
    "bundle": "CERT_BUNDLE_IN_PEM",
    "crt": "LEAF_CERT_IN_PEM",
    "crl_support": true,
    "expires": "2015-12-31T23:59:59Z",
    "hostnames": ["example.com"],
    "issuer": "ISSUER CERT SUBJECT",
    "key": "KEY_IN_PEM",
    "key_size": 2048,
    "key_type": "2048-bit RSA",
    "ocsp": ["http://ocsp.example-ca.com"],
    "ocsp_support": true,
    "root": "ROOT_CA_CERT_IN_PEM",
    "signature": "SHA1WithRSA",
    "subject": "LEAF CERT SUBJECT",
    "status": {
        "rebundled": false,
        "expiring_SKIs": [],
        "untrusted_root_stores": [],
        "messages": [],
        "code": 0
    }
}

Generating certificate signing request and private key

cfssl genkey csr.json

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

{
    "hosts": [
        "example.com",
        "www.example.com"
    ],
    "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 genkey -initca csr.json | cfssljson -bare ca

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=comma,separated,hostnames] csr.json

This is calls genkey, but has a remote CFSSL server sign and issue a certificate. You may use -hostname to override certificate SANs.

Generating a local-issued certificate and private key.

cfssl gencert -ca cert -ca-key key [-hostname=comma,separated,hostnames] csr.json

This is generates and issues a certificate and private key from a local CA via a JSON request. You may use -hostname to override certificate SANs.

Updating a OCSP responses file with a newly issued certificate

cfssl ocspsign -ca cert -responder key -responder-key key -cert cert \
 | cfssljson -bare -stdout >> responses

This will generate a OCSP response for the cert and add it to the responses file. You can then pass responses to ocspserve to start a OCSP server.

Starting the API Server

CFSSL 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]   \
            [-remote remote_server]

Address and port default to “127.0.0.1:8888”. 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.” If the “remote” option is provided, all signature operations will be forwarded to the remote CFSSL.

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
• 4. CRITICAL

The multirootca

The cfssl program can act as an online certificate authority, but it only uses a single key. If multiple signing keys are needed, the multirootca program can be used. It only provides the sign, authsign, and info endpoints. The documentation contains instructions for configuring and running the CA.

The mkbundle Utility

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

go get -u github.com/cloudflare/cfssl/cmd/mkbundle

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 produced.
• if there is a “ocspResponse” field, the file “basename-response.der” will be produced.

Instead of saving to a file, you can pass -stdout to output the encoded contents.

Static Builds

By default, the web assets are accessed from disk, based on their relative locations. If you’re wishing to distribute a single, statically-linked, cfssl binary, you’ll want to embed these resources before building. This can by done with the go.rice tool.

pushd cli/serve && rice embed-go && popd

Then building with go build will use the embedded resources.

Using a PKCS#11 hardware token / HSM

For better security, you may want to store your private key in an HSM or smartcard. The interface to both of these categories of device is described by the PKCS#11 spec. If you need to do approximately one signing operation per second or fewer, the Yubikey NEO and NEO-n are inexpensive smartcard options: https://www.yubico.com/products/yubikey-hardware/yubikey-neo/. In general you are looking for a product that supports PIV (personal identity verification). If your signing needs are in the hundreds of signatures per second, you will need to purchase an expensive HSM (in the thousands to many thousands of USD).

If you want to try out the PKCS#11 signing modes without a hardware token, you can use the SoftHSM implementation. Please note that using SoftHSM simply stores your private key in a file on disk and does not increase security.

To get started with your PKCS#11 token you will need to initialize it with a private key, PIN, and token label. The instructions to do this will be specific to each hardware device, and you should follow the instructions provided by your vendor. You will also need to find the path to your ‘module’, a shared object file (.so). Having initialized your device, you can query it to check your token label with:

pkcs11-tool --module <module path> --list-token-slots

You’ll also want to check the label of the private key you imported (or generated). Run the following command and look for a ‘Private Key Object’:

pkcs11-tool --module <module path> --pin <pin> \
  --list-token-slots --login --list-objects

You now have all the information you need to use your PKCS#11 token with CFSSL. CFSSL supports PKCS#11 for certificate signing and OCSP signing. To create a Signer (for certificate signing), import signer/universal and call NewSigner with a Root object containing the module, pin, token label and private label from above, plus a path to your certificate. The structure of the Root object is documented in universal.go.

The setup for an OCSP signer is slightly different. Import ocsp/pkcs11 and call NewPKCS11Signer with the appropriate configuration structure defined in ocsp/config.

Alternately, you can construct a pkcs11key.Key or pkcs11key.Pool yourself, and pass it to ocsp.NewSigner (for OCSP) or local.NewSigner (for certificate signing). This will be necessary, for example, if you are using a single-session token like the Yubikey and need both OCSP signing and certificate signing at the same time.

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