package main

import (
	"bytes"
	"crypto/rand"
	"crypto/rsa"
	"crypto/sha1"
	"crypto/x509"
	"crypto/x509/pkix"
	"fmt"
	"log"
	"math/big"
	"time"
)

var MaxSerialNumber = big.NewInt(0).SetBytes(bytes.Repeat([]byte{255}, 20))

func main() {
	name := "Test name"
	organization := "Test Org"
	validity := time.Duration(24 * time.Hour)

	priv, err := rsa.GenerateKey(rand.Reader, 2048)
	if err != nil {
		log.Fatal(err)
	}
	pub := priv.Public()

	// Subject Key Identifier support for end entity certificate.
	// https://www.ietf.org/rfc/rfc3280.txt (section 4.2.1.2)
	pkixpub, err := x509.MarshalPKIXPublicKey(pub)
	if err != nil {
		log.Fatal(err)
	}
	h := sha1.New()
	h.Write(pkixpub)
	keyID := h.Sum(nil)

	serial := MaxSerialNumber

	tmpl := &x509.Certificate{
		SerialNumber: serial,
		Subject: pkix.Name{
			CommonName:   name,
			Organization: []string{organization},
		},
		SubjectKeyId:          keyID,
		KeyUsage:              x509.KeyUsageKeyEncipherment | x509.KeyUsageDigitalSignature | x509.KeyUsageCertSign,
		ExtKeyUsage:           []x509.ExtKeyUsage{x509.ExtKeyUsageServerAuth},
		BasicConstraintsValid: true,
		NotBefore:             time.Now().Add(-validity),
		NotAfter:              time.Now().Add(validity),
		DNSNames:              []string{name},
		IsCA:                  true,
	}

	fmt.Println(MaxSerialNumber)
	fmt.Println(serial)

	raw, err := x509.CreateCertificate(rand.Reader, tmpl, tmpl, pub, priv)
	fmt.Println(err, len(raw), raw)
}