scep

scep is a Simple Certificate Enrollment Protocol server and client

Installation

Binary releases are available on the releases page.

Compiling from source

To compile the SCEP client and server you will need a Go compiler as well as standard tools like git, make, etc.

1. Clone the repository and get into the source directory: git clone https://github.com/micromdm/scep.git && cd scep
2. Compile the client and server binaries: make

The binaries will be compiled in the current directory and named after the architecture. I.e. scepclient-linux-amd64 and scepserver-linux-amd64.

Docker

See Docker documentation below.

Example setup

Minimal example for both server and client.

# SERVER:
# create a new CA
./scepserver-linux-amd64 ca -init
# start server
./scepserver-linux-amd64 -depot depot -port 2016 -challenge=secret

# SCEP request:
# in a separate terminal window, run a client
# note, if the client.key doesn't exist, the client will create a new rsa private key. Must be in PEM format.
./scepclient-linux-amd64 -private-key client.key -server-url=http://127.0.0.1:2016/scep -challenge=secret

# NDES request:
# note, this should point to an NDES server, scepserver does not provide NDES.
./scepclient-linux-amd64 -private-key client.key -server-url=https://scep.example.com:4321/certsrv/mscep/ -ca-fingerprint="e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855"

Server Usage

The default flags configure and run the scep server.

-depot must be the path to a folder with ca.pem and ca.key files. If you don't already have a CA to use, you can create one using the ca subcommand.

The scepserver provides one HTTP endpoint, /scep, that facilitates the normal PKIOperation/Message parameters.

Server usage:

$ ./scepserver-linux-amd64 -help
  -allowrenew string
    	do not allow renewal until n days before expiry, set to 0 to always allow (default "14")
  -capass string
    	passwd for the ca.key
  -challenge string
    	enforce a challenge password
  -crtvalid string
    	validity for new client certificates in days (default "365")
  -csrverifierexec string
    	will be passed the CSRs for verification
  -debug
    	enable debug logging
  -depot string
    	path to ca folder (default "depot")
  -log-json
    	output JSON logs
  -port string
    	port to listen on (default "8080")
  -version
    	prints version information
usage: scep [<command>] [<args>]
 ca <args> create/manage a CA
type <command> --help to see usage for each subcommand

Use the ca -init subcommand to create a new CA and private key.

CA sub-command usage:

$ ./scepserver-linux-amd64 ca -help
Usage of ca:
  -country string
    	country for CA cert (default "US")
  -depot string
    	path to ca folder (default "depot")
  -init
    	create a new CA
  -key-password string
    	password to store rsa key
  -keySize int
    	rsa key size (default 4096)
  -common_name string
        common name (CN) for CA cert (default "MICROMDM SCEP CA")
  -organization string
    	organization for CA cert (default "scep-ca")
  -organizational_unit string
    	organizational unit (OU) for CA cert (default "SCEP CA")
  -years int
    	default CA years (default 10)

CSR verifier

The -csrverifierexec switch to the SCEP server allows for executing a command before a certificate is issued to verify the submitted CSR. Scripts exiting without errors (zero exit status) will proceed to certificate issuance, otherwise a SCEP error is generated to the client. For example if you wanted to just save the CSR this is a valid CSR verifier shell script:

#!/bin/sh

cat - > /tmp/scep.csr

Client Usage

$ ./scepclient-linux-amd64 -help
Usage of ./scepclient-linux-amd64:
  -ca-fingerprint string
    	SHA-256 digest of CA certificate for NDES server. Note: Changed from MD5.
  -certificate string
    	certificate path, if there is no key, scepclient will create one
  -challenge string
    	enforce a challenge password
  -cn string
    	common name for certificate (default "scepclient")
  -country string
    	country code in certificate (default "US")
  -debug
    	enable debug logging
  -keySize int
    	rsa key size (default 2048)
  -locality string
    	locality for certificate
  -log-json
    	use JSON for log output
  -organization string
    	organization for cert (default "scep-client")
  -ou string
    	organizational unit for certificate (default "MDM")
  -private-key string
    	private key path, if there is no key, scepclient will create one
  -province string
    	province for certificate
  -server-url string
    	SCEP server url
  -version
    	prints version information

Note: Make sure to specify the desired endpoint in your -server-url value (e.g. 'http://scep.groob.io:2016/scep')

To obtain a certificate through Network Device Enrollment Service (NDES), set -server-url to a server that provides NDES. This most likely uses the /certsrv/mscep path. You will need to add the -ca-fingerprint client argument during this request to specify which CA to use.

If you're not sure which SHA-256 hash (for a specific CA) to use, you can use the -debug flag to print them out for the CAs returned from the SCEP server.

Docker

# first compile the Docker binaries
make docker

# build the image
docker build -t micromdm/scep:latest .

# create CA
docker run -it --rm -v /path/to/ca/folder:/depot micromdm/scep:latest ca -init

# run
docker run -it --rm -v /path/to/ca/folder:/depot -p 8080:8080 micromdm/scep:latest

SCEP library

The core scep library can be used for both client and server operations.

go get github.com/micromdm/scep/scep

For detailed usage, see the Go Reference.

Example (server):

// read a request body containing SCEP message
body, err := ioutil.ReadAll(r.Body)
if err != nil {
    // handle err
}

// parse the SCEP message
msg, err := scep.ParsePKIMessage(body)
if err != nil {
    // handle err
}

// do something with msg
fmt.Println(msg.MessageType)

// extract encrypted pkiEnvelope
err := msg.DecryptPKIEnvelope(CAcert, CAkey)
if err != nil {
    // handle err
}

// use the CSR from decrypted PKCS request and sign
// MyCSRSigner returns an *x509.Certificate here
crt, err := MyCSRSigner(msg.CSRReqMessage.CSR)
if err != nil {
    // handle err
}

// create a CertRep message from the original
certRep, err := msg.Success(CAcert, CAkey, crt)
if err != nil {
    // handle err
}

// send response back
// w is a http.ResponseWriter
w.Write(certRep.Raw)

Server library

You can import the scep endpoint into another Go project. For an example take a look at scepserver.go.

The SCEP server includes a built-in CA/certificate store. This is facilitated by the Depot and CSRSigner Go interfaces. This certificate storage to happen however you want. It also allows for swapping out the entire CA signer altogether or even using SCEP as a proxy for certificates.