Golang: Demonstrate creating a CA Certificate, Creating a signed Client Certificate and Private Key, and Signing a Certificate Signing Request CSR using a given CA

README.md

Demonstrate creating a CA Certificate, Creating a signed Client Certificate and Private Key, and Signing a Certificate Signing Request CSR using a given CA.

Thanks

• Golang: Demonstrate creating a CA Certificate, and Creating and Signing Certs with the CA

• Signing certificate request with certificate authority

• How to parse collection of PEM certs

License

MIT License

Copyright (c) 2023 Ahmed Mahmoud Ahmed Mahmoud Abouelkher

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.

cert_gen.go

package main

import (
	"bytes"
	"crypto/rand"
	"crypto/rsa"
	"crypto/x509"
	"crypto/x509/pkix"
	"encoding/pem"
	"errors"
	"flag"
	"math/big"
	"net"
	"os"
	"time"
)

func main() {
	var isCA, isCRT, isSignCSR bool
	var csrFile string
	flag.BoolVar(&isCA, "ca", false, "generate a CA")
	flag.BoolVar(&isCRT, "crt", false, "generate a certificate")
	flag.BoolVar(&isSignCSR, "csr", false, "sign a CSR")
	flag.StringVar(&csrFile, "csrf", "", "CSR file to sign")
	flag.Parse()

	t := time.Now()

	if isCA {
		ca, key, err := genCA()
		if err != nil {
			panic(err)
		}
		// write the CA to disk
		writeToFile("ca.pem", ca)
		// write the key to disk
		writeToFile("ca.key", key)
	} else if isCRT {
		// load the CA
		caCert, err := os.ReadFile("ca.pem")
		if err != nil {
			panic(err)
		}
		caPrivKey, err := os.ReadFile("ca.key")
		if err != nil {
			panic(err)
		}
		// generate the client certificate
		cert, key, err := genClientCrt(bytes.NewBuffer(caCert), bytes.NewBuffer(caPrivKey))
		if err != nil {
			panic(err)
		}
		writeToFile("client_cert.pem", cert)
		writeToFile("client_cert.key", key)
	} else if isSignCSR {
		if csrFile == "" {
			flag.Usage()
			println("CSR file not specified")
			os.Exit(2)
		}
		caCert, err := os.ReadFile("ca.pem")
		if err != nil {
			panic(err)
		}
		caPrivKey, err := os.ReadFile("ca.key")
		if err != nil {
			panic(err)
		}
		clientCSR, err := os.ReadFile(csrFile)
		if err != nil {
			panic(err)
		}
		clientCRT, err := signCSR(caCert, caPrivKey, clientCSR)
		if err != nil {
			panic(err)
		}
		writeToFile("signed_client_cert.pem", clientCRT)
	} else {
		flag.Usage()
		os.Exit(1)
	}

	elapsed := time.Since(t)
	println("elapsed time:", elapsed.String())
}

func genCA() (*bytes.Buffer, *bytes.Buffer, error) {
	// set up our CA certificate
	ca := &x509.Certificate{
		SerialNumber: big.NewInt(2019),
		Subject: pkix.Name{
			Organization:  []string{"Company, INC."},
			Country:       []string{"US"},
			Province:      []string{""},
			Locality:      []string{"San Francisco"},
			StreetAddress: []string{"Golden Gate Bridge"},
			PostalCode:    []string{"94016"},
		},
		NotBefore:             time.Now(),
		NotAfter:              time.Now().AddDate(10, 0, 0),
		IsCA:                  true,
		ExtKeyUsage:           []x509.ExtKeyUsage{x509.ExtKeyUsageClientAuth, x509.ExtKeyUsageServerAuth},
		KeyUsage:              x509.KeyUsageDigitalSignature | x509.KeyUsageCertSign,
		BasicConstraintsValid: true,
	}

	// create our private and public key
	caPrivKey, err := rsa.GenerateKey(rand.Reader, 4096)
	if err != nil {
		return nil, nil, err
	}

	// create the CA
	caBytes, err := x509.CreateCertificate(rand.Reader, ca, ca, &caPrivKey.PublicKey, caPrivKey)
	if err != nil {
		return nil, nil, err
	}

	// pem encode
	caPEM := new(bytes.Buffer)
	pem.Encode(caPEM, &pem.Block{
		Type:  "CERTIFICATE",
		Bytes: caBytes,
	})

	caPrivKeyPEM := new(bytes.Buffer)
	pem.Encode(caPrivKeyPEM, &pem.Block{
		Type:  "RSA PRIVATE KEY",
		Bytes: x509.MarshalPKCS1PrivateKey(caPrivKey),
	})

	return caPEM, caPrivKeyPEM, nil
}

func genClientCrt(rawCA *bytes.Buffer, caKey *bytes.Buffer) (*bytes.Buffer, *bytes.Buffer, error) {
	// set up our server certificate
	cert := &x509.Certificate{
		SerialNumber: big.NewInt(2019),
		Subject: pkix.Name{
			Organization:  []string{"Company, INC."},
			Country:       []string{"US"},
			Province:      []string{""},
			Locality:      []string{"San Francisco"},
			StreetAddress: []string{"Golden Gate Bridge"},
			PostalCode:    []string{"94016"},
		},
		IPAddresses:  []net.IP{net.IPv4(127, 0, 0, 1), net.IPv6loopback},
		NotBefore:    time.Now(),
		NotAfter:     time.Now().AddDate(10, 0, 0),
		SubjectKeyId: []byte{1, 2, 3, 4, 6},
		ExtKeyUsage:  []x509.ExtKeyUsage{x509.ExtKeyUsageClientAuth, x509.ExtKeyUsageServerAuth},
		KeyUsage:     x509.KeyUsageDigitalSignature,
	}

	certPrivKey, err := rsa.GenerateKey(rand.Reader, 4096)
	if err != nil {
		return nil, nil, err
	}

	// parse PEM encoded CA
	pemBlock, _ := pem.Decode(rawCA.Bytes())
	if pemBlock == nil {
		return nil, nil, err
	}
	caCert, err := x509.ParseCertificate(pemBlock.Bytes)
	if err != nil {
		panic(err)
	}

	// parse PEM encoded CA private key
	pemBlock, _ = pem.Decode(caKey.Bytes())
	if pemBlock == nil {
		return nil, nil, err
	}
	caPrivKey, err := x509.ParsePKCS1PrivateKey(pemBlock.Bytes)
	if err != nil {
		panic(err)
	}

	certBytes, err := x509.CreateCertificate(rand.Reader, cert, caCert, &certPrivKey.PublicKey, caPrivKey)
	if err != nil {
		return nil, nil, err
	}

	certPEM := new(bytes.Buffer)
	pem.Encode(certPEM, &pem.Block{
		Type:  "CERTIFICATE",
		Bytes: certBytes,
	})

	certPrivKeyPEM := new(bytes.Buffer)
	pem.Encode(certPrivKeyPEM, &pem.Block{
		Type:  "RSA PRIVATE KEY",
		Bytes: x509.MarshalPKCS1PrivateKey(certPrivKey),
	})

	return certPEM, certPrivKeyPEM, nil
}

func signCSR(rawCA []byte, caKey []byte, rawCSR []byte) (*bytes.Buffer, error) {
	// parse PEM encoded CA
	pemBlock, _ := pem.Decode(rawCA)
	if pemBlock == nil {
		return nil, errors.New("failed to parse CA")
	}
	caCert, err := x509.ParseCertificate(pemBlock.Bytes)
	if err != nil {
		panic(err)
	}
	// parse PEM encoded CA private key
	pemBlock, _ = pem.Decode(caKey)
	if pemBlock == nil {
		return nil, errors.New("failed to parse CA private key")
	}
	caPrivKey, err := x509.ParsePKCS1PrivateKey(pemBlock.Bytes)
	if err != nil {
		panic(err)
	}
	// parse PEM encoded CSR
	pemBlock, _ = pem.Decode(rawCSR)
	if pemBlock == nil {
		return nil, errors.New("failed to parse CSR")
	}
	clientCSR, err := x509.ParseCertificateRequest(pemBlock.Bytes)
	if err != nil {
		panic(err)
	}
	clientCRTTemplate := x509.Certificate{
		Signature:          clientCSR.Signature,
		SignatureAlgorithm: clientCSR.SignatureAlgorithm,

		PublicKeyAlgorithm: clientCSR.PublicKeyAlgorithm,
		PublicKey:          clientCSR.PublicKey,
		Subject:            clientCSR.Subject,

		SerialNumber: big.NewInt(2),
		Issuer:       caCert.Subject,
		NotBefore:    time.Now(),
		NotAfter:     time.Now().Add(24 * time.Hour),
		KeyUsage:     x509.KeyUsageDigitalSignature,
		ExtKeyUsage:  []x509.ExtKeyUsage{x509.ExtKeyUsageClientAuth},
	}
	clientCRTBytes, err := x509.CreateCertificate(rand.Reader, &clientCRTTemplate, caCert, clientCSR.PublicKey, caPrivKey)
	if err != nil {
		panic(err)
	}
	clientPEM := new(bytes.Buffer)
	pem.Encode(clientPEM, &pem.Block{
		Type:  "CERTIFICATE",
		Bytes: clientCRTBytes,
	})
	return clientPEM, nil
}

func writeToFile(filename string, data *bytes.Buffer) error {
	f, err := os.Create(filename)
	if err != nil {
		return err
	}
	defer f.Close()

	_, err = f.Write(data.Bytes())
	if err != nil {
		return err
	}

	return nil
}