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
cfsslprogram, which is the canonical command line utility using the CFSSL packages. - the
multirootcaprogram, which is a certificate authority server that can use multiple signing keys. - the
mkbundleprogram is used to build certificate pool bundles. - the
cfssljsonprogram, which takes the JSON output from thecfsslandmultirootcaprograms and writes certificates, keys, CSRs, and bundles to disk.
See BUILDING
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/.
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.
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"
}
]
}
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
}
}
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"
}
]
}
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.
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.
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.
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.
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] [-int-dir dir] [-port port] \
[-metadata file] [-remote remote_host] [-config config] \
[-responder cert] [-responder-key key]
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.
'-int-dir' specifies intermediates directory. '-metadata' is a file for root certificate presence. The content of the file is a json dictionary (k,v): each key k is SHA-1 digest of a root certificate while value v is a list of key store filenames. '-config' specifies path to configuration file. '-responder' and '-responder-key' are Certificate for OCSP responder and private key for OCSP responder certificate, respectively.
The amount of logging can be controlled with the -loglevel option. This
comes before the serve command:
cfssl -loglevel 2 serve
The levels are:
-
- DEBUG
-
- INFO (this is the default level)
-
- WARNING
-
- ERROR
-
- CRITICAL
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.
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.
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.
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.
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 can be found in the "doc/" directory:
api.txt: documents the API endpointsbootstrap.txt: a walkthrough from building the package to getting up and running