diskless-workstation/serving/enrol-broker/signer.go

// signer.go — turning an APPROVED CSR into a signed cert.
//
// Production path (stepSigner) shells out to the bundled `step` CLI exactly as
// the bench's tpm-provision.sh does: mint a one-time provisioner token, then
// `step ca sign` the CSR. The broker — not the host — holds the provisioner
// password, so a host can never self-sign; signing happens ONLY after an admin
// approves (the human OOB gate).
//
// Dev path (mockSigner) signs with an ephemeral in-process CA so the whole
// request -> approve -> cert flow is testable without a running step-ca. It is
// LAB-ONLY and announces itself loudly; never select it in production.
package main

import (
	"context"
	"crypto/ecdsa"
	"crypto/elliptic"
	"crypto/rand"
	"crypto/x509"
	"crypto/x509/pkix"
	"encoding/pem"
	"errors"
	"fmt"
	"log"
	"math/big"
	"os"
	"os/exec"
	"path/filepath"
	"strings"
	"time"
)

// Signer issues a cert for an approved CSR. Returns the cert PEM and its serial
// (lowercase hex, the value the hostnqn is derived from).
type Signer interface {
	Sign(ctx context.Context, csrPEM, cn string) (certPEM, serial string, err error)
	Kind() string
}

// --- production: step-ca via the `step` CLI ---------------------------------

type stepSigner struct {
	bin          string
	caURL        string
	root         string
	provisioner  string
	passwordFile string
	notAfter     string
}

func (s *stepSigner) Kind() string { return "step" }

func (s *stepSigner) Sign(ctx context.Context, csrPEM, cn string) (string, string, error) {
	if cn == "" {
		return "", "", fmt.Errorf("CSR has no CommonName; cannot mint a token subject")
	}
	dir, err := os.MkdirTemp("", "enrol-*")
	if err != nil {
		return "", "", err
	}
	defer os.RemoveAll(dir)
	csrPath := filepath.Join(dir, "req.csr")
	outPath := filepath.Join(dir, "out.pem")
	if err := os.WriteFile(csrPath, []byte(csrPEM), 0o600); err != nil {
		return "", "", err
	}

	// 1. one-time token (subject MUST equal the CSR CN for the JWK provisioner).
	tokArgs := []string{
		"ca", "token", cn,
		"--provisioner", s.provisioner,
		"--ca-url", s.caURL,
		"--root", s.root,
		"--provisioner-password-file", s.passwordFile,
	}
	if s.notAfter != "" {
		tokArgs = append(tokArgs, "--not-after", s.notAfter)
	}
	token, err := s.run(ctx, tokArgs...)
	if err != nil {
		return "", "", fmt.Errorf("step ca token: %w", err)
	}
	token = strings.TrimSpace(token)

	// 2. sign the CSR with that token.
	if _, err := s.run(ctx,
		"ca", "sign", csrPath, outPath,
		"--token", token,
		"--ca-url", s.caURL,
		"--root", s.root,
		"--force",
	); err != nil {
		return "", "", fmt.Errorf("step ca sign: %w", err)
	}
	certPEM, err := os.ReadFile(outPath)
	if err != nil {
		return "", "", err
	}
	serial, err := serialFromPEM(certPEM)
	if err != nil {
		return "", "", err
	}
	return string(certPEM), serial, nil
}

// run executes `step ...`, returning stdout. The token is sensitive but is passed
// as an argv element to the trusted local `step`; we never log argv or the
// password-file contents.
func (s *stepSigner) run(ctx context.Context, args ...string) (string, error) {
	cmd := exec.CommandContext(ctx, s.bin, args...)
	out, err := cmd.Output()
	if err != nil {
		var ee *exec.ExitError
		if errors.As(err, &ee) {
			return "", fmt.Errorf("%w: %s", err, strings.TrimSpace(string(ee.Stderr)))
		}
		return "", err
	}
	return string(out), nil
}

// --- dev only: ephemeral in-process CA -------------------------------------

type mockSigner struct {
	caCert *x509.Certificate
	caKey  *ecdsa.PrivateKey
}

func newMockSigner() (*mockSigner, error) {
	key, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
	if err != nil {
		return nil, err
	}
	tmpl := &x509.Certificate{
		SerialNumber:          big.NewInt(1),
		Subject:               pkix.Name{CommonName: "fleet-enrol-broker DEV mock CA"},
		NotBefore:             time.Now().Add(-time.Hour),
		NotAfter:              time.Now().Add(24 * time.Hour),
		KeyUsage:              x509.KeyUsageCertSign | x509.KeyUsageDigitalSignature,
		BasicConstraintsValid: true,
		IsCA:                  true,
	}
	der, err := x509.CreateCertificate(rand.Reader, tmpl, tmpl, &key.PublicKey, key)
	if err != nil {
		return nil, err
	}
	caCert, err := x509.ParseCertificate(der)
	if err != nil {
		return nil, err
	}
	log.Printf("WARNING: BROKER_SIGNER=mock — issuing certs from an EPHEMERAL in-process CA. LAB ONLY; these certs are NOT trusted by RADIUS/NVMe. Set BROKER_SIGNER=step for real enrolment.")
	return &mockSigner{caCert: caCert, caKey: key}, nil
}

func (m *mockSigner) Kind() string { return "mock" }

func (m *mockSigner) Sign(_ context.Context, csrPEM, cn string) (string, string, error) {
	csr, err := parseCSR(csrPEM)
	if err != nil {
		return "", "", err
	}
	serial, err := rand.Int(rand.Reader, new(big.Int).Lsh(big.NewInt(1), 128))
	if err != nil {
		return "", "", err
	}
	tmpl := &x509.Certificate{
		SerialNumber: serial,
		Subject:      csr.Subject,
		NotBefore:    time.Now().Add(-time.Minute),
		NotAfter:     time.Now().Add(720 * time.Hour),
		KeyUsage:     x509.KeyUsageDigitalSignature,
		ExtKeyUsage:  []x509.ExtKeyUsage{x509.ExtKeyUsageClientAuth},
	}
	der, err := x509.CreateCertificate(rand.Reader, tmpl, m.caCert, csr.PublicKey, m.caKey)
	if err != nil {
		return "", "", err
	}
	certPEM := pem.EncodeToMemory(&pem.Block{Type: "CERTIFICATE", Bytes: der})
	return string(certPEM), strings.ToLower(fmt.Sprintf("%x", serial)), nil
}

// --- shared helpers --------------------------------------------------------

func parseCSR(csrPEM string) (*x509.CertificateRequest, error) {
	blk, _ := pem.Decode([]byte(csrPEM))
	if blk == nil || blk.Type != "CERTIFICATE REQUEST" {
		return nil, fmt.Errorf("not a PEM CERTIFICATE REQUEST")
	}
	csr, err := x509.ParseCertificateRequest(blk.Bytes)
	if err != nil {
		return nil, err
	}
	if err := csr.CheckSignature(); err != nil {
		return nil, fmt.Errorf("CSR self-signature invalid: %w", err)
	}
	return csr, nil
}

func serialFromPEM(certPEM []byte) (string, error) {
	blk, _ := pem.Decode(certPEM)
	if blk == nil || blk.Type != "CERTIFICATE" {
		return "", fmt.Errorf("issued cert is not PEM")
	}
	cert, err := x509.ParseCertificate(blk.Bytes)
	if err != nil {
		return "", err
	}
	return strings.ToLower(fmt.Sprintf("%x", cert.SerialNumber)), nil
}