rsa/pkg/ca/create.go

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2023-03-01 21:28:48 +00:00
/*
Package ca implements a disk file-backed certificate authority with a built-in
CRL signer. The certificate authority can issue new certificates (including the
creation of intermediate CAs) and revoke existing certificates, resulting in a
fresh CRL.
*/
package ca
import (
"crypto/rand"
"crypto/rsa"
"crypto/x509"
"errors"
"fmt"
"io/fs"
"math/big"
"os"
"path/filepath"
"time"
"src.lwithers.me.uk/go/rsa/pkg/pemfile"
"src.lwithers.me.uk/go/rsa/pkg/serial"
)
const (
crlSignerBits = 3072
)
func Create(dir string, template *x509.Certificate, key *rsa.PrivateKey) (*CA, error) {
return createCA(dir, template, template, key, key)
}
func (ca *CA) CreateIntermediate(dir string, template *x509.Certificate, key *rsa.PrivateKey) (*CA, error) {
return createCA(dir, template, ca.root, key, ca.key)
}
// createCA holds the shared code between Create and CreateIntermediate. All
// that really differs between those operations is the signing key and whether
// or not there is a parent certificate.
func createCA(dir string, template, parent *x509.Certificate, newRootKey, signingKey *rsa.PrivateKey) (*CA, error) {
// template must have at least the common name set
if template.Subject.CommonName == "" {
return nil, errors.New("template CA certificate must have common name set")
}
// directory must not exist
if _, err := os.Lstat(dir); !errors.Is(err, fs.ErrNotExist) {
return nil, &fs.PathError{
Op: "mkdir",
Path: dir,
Err: fs.ErrExist,
}
}
// create the directory
if err := os.Mkdir(dir, 0700); err != nil {
return nil, err
}
// write out the private key
der, err := x509.MarshalPKCS8PrivateKey(newRootKey)
if err != nil {
return nil, fmt.Errorf("failed to marshal private key: %w", err)
}
if err = pemfile.Write(filepath.Join(dir, rootKeyFilename), pemfile.TypePKCS8PrivateKey, der); err != nil {
return nil, err
}
// ensure the template is valid for a root CA
template.BasicConstraintsValid = true
template.IsCA = true
template.KeyUsage |= x509.KeyUsageCertSign
if template.SerialNumber == nil {
template.SerialNumber = serial.Rand()
}
if template.NotBefore.IsZero() {
template.NotBefore = time.Now().Add(-2 * time.Hour)
}
if template.NotAfter.IsZero() {
template.NotAfter = time.Now().Add(24 * 365 * 40 * time.Hour)
}
// create the self-signed certificate; ensure we can parse it, and write
// it to disk
der, err = x509.CreateCertificate(rand.Reader, template, parent, &newRootKey.PublicKey, signingKey)
if err != nil {
return nil, fmt.Errorf("failed to create/sign new root CA certificate: %w", err)
}
root, err := x509.ParseCertificate(der)
if err != nil {
return nil, fmt.Errorf("failed to parse just-signed root CA certificate: %w", err)
}
if err = pemfile.Write(filepath.Join(dir, rootCertFilename), pemfile.TypeX509Certificate, der); err != nil {
return nil, err
}
// now create a CRL signer key and write it to disk
crlKey, err := rsa.GenerateKey(rand.Reader, crlSignerBits)
if err != nil {
return nil, fmt.Errorf("failed to generate new CRL signer key: %w", err)
}
der, err = x509.MarshalPKCS8PrivateKey(crlKey)
if err != nil {
return nil, fmt.Errorf("failed to marshal CRL signer private key: %w", err)
}
if err = pemfile.Write(filepath.Join(dir, crlKeyFilename), pemfile.TypePKCS8PrivateKey, der); err != nil {
return nil, err
}
// create a template for a CRL signing certificate
crlTemplate := &x509.Certificate{
SerialNumber: serial.Rand(),
Subject: template.Subject,
SubjectKeyId: ComputeSubjectKeyId(&crlKey.PublicKey),
NotBefore: template.NotBefore,
NotAfter: template.NotAfter,
KeyUsage: x509.KeyUsageCRLSign,
BasicConstraintsValid: true,
}
crlTemplate.Subject.CommonName += " CRL signer"
// create the CRL signing certificate, parse it, and write it to disk
der, err = x509.CreateCertificate(rand.Reader, crlTemplate, root, &crlKey.PublicKey, newRootKey)
if err != nil {
return nil, fmt.Errorf("failed to sign CRL signer certificate: %w", err)
}
crlCert, err := x509.ParseCertificate(der)
if err != nil {
return nil, fmt.Errorf("failed to parse just-signed CRL signer certificate: %w", err)
}
if err = pemfile.Write(filepath.Join(dir, crlCertFilename), pemfile.TypeX509Certificate, der); err != nil {
return nil, err
}
// finally, create an empty CRL
crl := &x509.RevocationList{
Number: big.NewInt(1),
ThisUpdate: template.NotBefore,
NextUpdate: template.NotAfter,
}
der, err = x509.CreateRevocationList(rand.Reader, crl, crlCert, crlKey)
if err != nil {
return nil, fmt.Errorf("failed to create empty CRL: %w", err)
}
crl, err = x509.ParseRevocationList(der)
if err != nil {
return nil, fmt.Errorf("failed to parse just-created CRL: %w", err)
}
if err = pemfile.Write(filepath.Join(dir, crlFilename), pemfile.TypeX509CRL, der); err != nil {
return nil, err
}
// all done
return &CA{
dir: dir,
key: newRootKey,
root: root,
crl: crl,
crlKey: crlKey,
crlCert: crlCert,
}, nil
}