browser-csr-generation/handlers/signing.go

package handlers

import (
	"bytes"
	"crypto/x509"
	"encoding/json"
	"encoding/pem"
	"errors"
	"fmt"
	"io/ioutil"
	"net/http"
	"os"
	"os/exec"

	log "github.com/sirupsen/logrus"
)

type CertificateSigningHandler struct {
	caCertificates []*x509.Certificate
	caChainMap     map[string][]string
}

func NewCertificateSigningHandler(caCertificates []*x509.Certificate) *CertificateSigningHandler {
	return &CertificateSigningHandler{caCertificates: caCertificates, caChainMap: make(map[string][]string)}
}

func (h *CertificateSigningHandler) sign(csrPem string, commonName string) (certPem string, caChain []string, err error) {
	log.Printf("received CSR for %s:\n\n%s", commonName, csrPem)
	subjectDN := fmt.Sprintf("/CN=%s", commonName)
	var csrFile *os.File
	if csrFile, err = ioutil.TempFile("", "*.csr.pem"); err != nil {
		log.Errorf("could not open temporary file: %s", err)
		return
	}
	if _, err = csrFile.Write([]byte(csrPem)); err != nil {
		log.Errorf("could not write CSR to file: %s", err)
		return
	}
	if err = csrFile.Close(); err != nil {
		log.Errorf("could not close CSR file: %s", err)
		return
	}
	defer func(file *os.File) {
		err = os.Remove(file.Name())
		if err != nil {
			log.Errorf("could not remove temporary file: %s", err)
		}
	}(csrFile)

	opensslCommand := exec.Command(
		"openssl", "ca", "-config", "ca.cnf",
		"-policy", "policy_match", "-extensions", "client_ext",
		"-batch", "-subj", subjectDN, "-utf8", "-rand_serial", "-in", "in.pem")
	var out, cmdErr bytes.Buffer
	opensslCommand.Stdout = &out
	opensslCommand.Stderr = &cmdErr
	err = opensslCommand.Run()
	if err != nil {
		log.Print(err)
		log.Print(cmdErr.String())
		return
	}

	var block *pem.Block
	if block, _ = pem.Decode(out.Bytes()); block == nil {
		err = fmt.Errorf("could not decode pem")
		return
	}
	var certificate *x509.Certificate
	if certificate, err = x509.ParseCertificate(block.Bytes); err != nil {
		return
	}

	certPem = string(pem.EncodeToMemory(&pem.Block{
		Type:  "CERTIFICATE",
		Bytes: certificate.Raw,
	}))

	caChain, err = h.getCAChain(certificate)
	return
}

func (h *CertificateSigningHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	if r.Method != "POST" {
		http.Error(w, "Only POST requests support", http.StatusMethodNotAllowed)
		return
	}
	if r.Header.Get("content-type") != "application/json" {
		http.Error(w, "Only JSON content is accepted", http.StatusNotAcceptable)
		return
	}
	var err error
	var requestBody requestData
	var responseData responseData

	if err = json.NewDecoder(r.Body).Decode(&requestBody); err != nil {
		log.Print(err)
		http.Error(w, err.Error(), http.StatusBadRequest)
		return
	}

	responseData.Certificate, responseData.CAChain, err = h.sign(requestBody.Csr, requestBody.CommonName)
	if err != nil {
		http.Error(w, "Could not sign certificate", http.StatusInternalServerError)
		return
	}

	var jsonBytes []byte
	if jsonBytes, err = json.Marshal(&responseData); err != nil {
		log.Print(err)
	}

	if _, err = w.Write(jsonBytes); err != nil {
		log.Print(err)
	}
}

type requestData struct {
	Csr        string `json:"csr"`
	CommonName string `json:"commonName"`
}

type responseData struct {
	Certificate string   `json:"certificate"`
	CAChain     []string `json:"ca_chain"`
}

func (h *CertificateSigningHandler) getCAChain(certificate *x509.Certificate) ([]string, error) {
	issuerString := string(certificate.RawIssuer)

	if value, exists := h.caChainMap[issuerString]; exists {
		return value, nil
	}

	result := make([]string, 0)

	appendCert := func(cert *x509.Certificate) {
		result = append(
			result,
			string(pem.EncodeToMemory(&pem.Block{Bytes: cert.Raw, Type: "CERTIFICATE"})))
		log.Debugf("added %s to cachain", result[len(result)-1])
	}

	var previous *x509.Certificate
	var count = 0
	for {
		if len(h.caCertificates) == 0 {
			return nil, errors.New("no CA certificates loaded")
		}
		if count > len(h.caCertificates) {
			return nil, errors.New("could not construct certificate chain")
		}
		for _, caCert := range h.caCertificates {
			if previous == nil {
				if bytes.Equal(caCert.RawSubject, certificate.RawIssuer) {
					previous = caCert
					appendCert(caCert)
				}
			} else if bytes.Equal(previous.RawSubject, previous.RawIssuer) {
				h.caChainMap[issuerString] = result
				return result, nil
			} else if bytes.Equal(caCert.RawSubject, previous.RawIssuer) {
				previous = caCert
				appendCert(caCert)
			} else {
				log.Debugf("skipped certificate %s", caCert.Subject)
			}
		}
		count++
	}
}