certificates/cas/vaultcas/vaultcas.go

package vaultcas

import (
	"context"
	"crypto/sha256"
	"crypto/x509"
	"encoding/json"
	"encoding/pem"
	"math/big"
	"time"

	"github.com/pkg/errors"
	"github.com/smallstep/certificates/cas/apiv1"

	vault "github.com/hashicorp/vault/api"
	auth "github.com/hashicorp/vault/api/auth/approle"
	certutil "github.com/hashicorp/vault/sdk/helper/certutil"
)

func init() {
	apiv1.Register(apiv1.VaultCAS, func(ctx context.Context, opts apiv1.Options) (apiv1.CertificateAuthorityService, error) {
		return New(ctx, opts)
	})
}

type VaultOptions struct {
	PKI             string        `json:"pki,omitempty"`
	PKIRoleDefault  string        `json:"PKIRoleDefault,omitempty"`
	PKIRoleRSA      string        `json:"pkiRoleRSA,omitempty"`
	PKIRoleEC       string        `json:"pkiRoleEC,omitempty"`
	PKIRoleEd25519  string        `json:"PKIRoleEd25519,omitempty"`
	RoleID          string        `json:"roleID,omitempty"`
	SecretID        auth.SecretID `json:"secretID,omitempty"`
	AppRole         string        `json:"appRole,omitempty"`
	IsWrappingToken bool          `json:"isWrappingToken,omitempty"`
}

// VaultCAS implements a Certificate Authority Service using Hashicorp Vault.
type VaultCAS struct {
	client      *vault.Client
	config      VaultOptions
	fingerprint string
}

type Certificate struct {
	leaf          *x509.Certificate
	intermediates []*x509.Certificate
	root          *x509.Certificate
}

func loadOptions(config json.RawMessage) (*VaultOptions, error) {
	var vc *VaultOptions

	err := json.Unmarshal(config, &vc)
	if err != nil {
		return nil, errors.Wrap(err, "error decoding vaultCAS config")
	}

	if vc.PKI == "" {
		vc.PKI = "pki" // use default pki vault name
	}

	if vc.PKIRoleDefault == "" {
		vc.PKIRoleDefault = "default" // use default pki role name
	}

	if vc.PKIRoleRSA == "" {
		vc.PKIRoleRSA = vc.PKIRoleDefault
	}
	if vc.PKIRoleEC == "" {
		vc.PKIRoleEC = vc.PKIRoleDefault
	}
	if vc.PKIRoleEd25519 == "" {
		vc.PKIRoleEd25519 = vc.PKIRoleDefault
	}

	if vc.RoleID == "" {
		return nil, errors.New("vaultCAS config options must define `roleID`")
	}

	if vc.SecretID.FromEnv == "" && vc.SecretID.FromFile == "" && vc.SecretID.FromString == "" {
		return nil, errors.New("vaultCAS config options must define `secretID` object with one of `FromEnv`, `FromFile` or `FromString`")
	}

	if vc.PKI == "" {
		vc.PKI = "pki" // use default pki vault name
	}

	if vc.AppRole == "" {
		vc.AppRole = "auth/approle"
	}

	return vc, nil
}

func certificateSort(n []*x509.Certificate) bool {
	// sort all cert using bubble sort
	isSorted := false
	s := 0
	maxSwap := len(n) * (len(n) - 1) / 2
	for s <= maxSwap && !isSorted {
		isSorted = true
		var i = 0
		for i < len(n)-1 {
			if !isSignedBy(n[i], n[i+1]) {
				// swap
				n[i], n[i+1] = n[i+1], n[i]
				isSorted = false
			}
			i++
		}
		s++
	}
	return isSorted
}

func isSignedBy(i, j *x509.Certificate) bool {
	signer := x509.NewCertPool()
	signer.AddCert(j)

	opts := x509.VerifyOptions{
		Roots:         signer,
		Intermediates: x509.NewCertPool(), // set empty to avoid using system CA
	}
	_, err := i.Verify(opts)
	return err == nil
}

func parseCertificates(pemCert string) []*x509.Certificate {
	var certs []*x509.Certificate
	rest := []byte(pemCert)
	var block *pem.Block
	for {
		block, rest = pem.Decode(rest)
		if block == nil {
			break
		}
		cert, err := x509.ParseCertificate(block.Bytes)
		if err != nil {
			break
		}
		certs = append(certs, cert)
	}
	return certs
}

func getCertificateAndChain(certb certutil.CertBundle) (*Certificate, error) {
	// certutil.CertBundle contains CAChain and Certificate.
	// Both could have a common part or different and we are not sure
	// how user define their chain inside vault.
	// We will create an array of certificate with all parsed certificates
	// then sort the array to create a consistent chain
	var root *x509.Certificate
	var leaf *x509.Certificate
	intermediates := make([]*x509.Certificate, 0)
	used := make(map[string]bool) // ensure that intermediate are uniq
	for _, chain := range append(certb.CAChain, certb.Certificate) {
		for _, cert := range parseCertificates(chain) {
			if used[cert.SerialNumber.String()] {
				continue
			}
			used[cert.SerialNumber.String()] = true
			switch {
			case isRoot(cert):
				root = cert
			case cert.BasicConstraintsValid && cert.IsCA:
				intermediates = append(intermediates, cert)
			default:
				leaf = cert
			}
		}
	}
	if ok := certificateSort(intermediates); !ok {
		return nil, errors.Errorf("failed to sort certificate, probably one of cert is not part of the chain")
	}

	certificate := &Certificate{
		root:          root,
		leaf:          leaf,
		intermediates: intermediates,
	}

	return certificate, nil
}

func parseCertificateRequest(pemCsr string) (*x509.CertificateRequest, error) {
	block, _ := pem.Decode([]byte(pemCsr))
	if block == nil {
		return nil, errors.Errorf("error decoding certificate request: not a valid PEM encoded block, please verify\r\n%v", pemCsr)
	}

	cr, err := x509.ParseCertificateRequest(block.Bytes)
	if err != nil {
		return nil, errors.Wrap(err, "error parsing certificate request")
	}
	return cr, nil
}

func (v *VaultCAS) createCertificate(cr *x509.CertificateRequest, lifetime time.Duration) (*x509.Certificate, []*x509.Certificate, error) {
	var vaultPKIRole string

	switch {
	case cr.PublicKeyAlgorithm == x509.RSA:
		vaultPKIRole = v.config.PKIRoleRSA
	case cr.PublicKeyAlgorithm == x509.ECDSA:
		vaultPKIRole = v.config.PKIRoleEC
	case cr.PublicKeyAlgorithm == x509.Ed25519:
		vaultPKIRole = v.config.PKIRoleEd25519
	default:
		return nil, nil, errors.Errorf("createCertificate: Unsupported public key algorithm '%v'", cr.PublicKeyAlgorithm)
	}

	certPemBytes := pem.EncodeToMemory(&pem.Block{Type: "CERTIFICATE REQUEST", Bytes: cr.Raw})
	if certPemBytes == nil {
		return nil, nil, errors.Errorf("createCertificate: Failed to encode pem '%v'", cr.Raw)
	}

	y := map[string]interface{}{
		"csr":    string(certPemBytes),
		"format": "pem_bundle",
		"ttl":    lifetime.Seconds(),
	}

	secret, err := v.client.Logical().Write(v.config.PKI+"/sign/"+vaultPKIRole, y)
	if err != nil {
		return nil, nil, errors.Wrapf(err, "createCertificate: unable to sign certificate %v", y)
	}
	if secret == nil {
		return nil, nil, errors.New("createCertificate: secret sign is empty")
	}

	var certBundle certutil.CertBundle
	if err := unmarshalMap(secret.Data, &certBundle); err != nil {
		return nil, nil, errors.Wrap(err, "error unmarshaling cert bundle")
	}

	cert, err := getCertificateAndChain(certBundle)
	if err != nil {
		return nil, nil, err
	}

	// Return certificate and certificate chain
	return cert.leaf, cert.intermediates, nil
}

// New creates a new CertificateAuthorityService implementation
// using Hashicorp Vault
func New(ctx context.Context, opts apiv1.Options) (*VaultCAS, error) {
	if opts.CertificateAuthority == "" {
		return nil, errors.New("vaultCAS 'certificateAuthority' cannot be empty")
	}

	if opts.CertificateAuthorityFingerprint == "" {
		return nil, errors.New("vaultCAS 'certificateAuthorityFingerprint' cannot be empty")
	}

	vc, err := loadOptions(opts.Config)
	if err != nil {
		return nil, err
	}

	config := vault.DefaultConfig()
	config.Address = opts.CertificateAuthority

	client, err := vault.NewClient(config)
	if err != nil {
		return nil, errors.Wrap(err, "unable to initialize vault client")
	}

	var appRoleAuth *auth.AppRoleAuth
	if vc.IsWrappingToken {
		appRoleAuth, err = auth.NewAppRoleAuth(
			vc.RoleID,
			&vc.SecretID,
			auth.WithWrappingToken(),
			auth.WithMountPath(vc.AppRole),
		)
	} else {
		appRoleAuth, err = auth.NewAppRoleAuth(
			vc.RoleID,
			&vc.SecretID,
			auth.WithMountPath(vc.AppRole),
		)
	}
	if err != nil {
		return nil, errors.Wrap(err, "unable to initialize AppRole auth method")
	}

	authInfo, err := client.Auth().Login(ctx, appRoleAuth)
	if err != nil {
		return nil, errors.Wrap(err, "unable to login to AppRole auth method")
	}
	if authInfo == nil {
		return nil, errors.New("no auth info was returned after login")
	}

	return &VaultCAS{
		client:      client,
		config:      *vc,
		fingerprint: opts.CertificateAuthorityFingerprint,
	}, nil
}

// CreateCertificate signs a new certificate using Hashicorp Vault.
func (v *VaultCAS) CreateCertificate(req *apiv1.CreateCertificateRequest) (*apiv1.CreateCertificateResponse, error) {
	switch {
	case req.CSR == nil:
		return nil, errors.New("CreateCertificate: `CSR` cannot be nil")
	case req.Lifetime == 0:
		return nil, errors.New("CreateCertificate: `LIFETIME` cannot be 0")
	}

	cert, chain, err := v.createCertificate(req.CSR, req.Lifetime)
	if err != nil {
		return nil, err
	}

	return &apiv1.CreateCertificateResponse{
		Certificate:      cert,
		CertificateChain: chain,
	}, nil
}

func (v *VaultCAS) GetCertificateAuthority(req *apiv1.GetCertificateAuthorityRequest) (*apiv1.GetCertificateAuthorityResponse, error) {
	secret, err := v.client.Logical().Read(v.config.PKI + "/cert/ca_chain")
	if err != nil {
		return nil, errors.Wrap(err, "unable to read root")
	}
	if secret == nil {
		return nil, errors.New("secret root is empty")
	}

	var certBundle certutil.CertBundle
	if err := unmarshalMap(secret.Data, &certBundle); err != nil {
		return nil, errors.Wrap(err, "error unmarshaling cert bundle")
	}

	cert, err := getCertificateAndChain(certBundle)
	if err != nil {
		return nil, err
	}

	sha256Sum := sha256.Sum256(cert.root.Raw)
	expectedSum := certutil.GetHexFormatted(sha256Sum[:], "")
	if expectedSum != v.fingerprint {
		return nil, errors.Errorf("Vault Root CA fingerprint `%s` doesn't match config fingerprint `%v`", expectedSum, v.fingerprint)
	}

	return &apiv1.GetCertificateAuthorityResponse{
		RootCertificate: cert.root,
	}, nil
}

// RenewCertificate will always return a non-implemented error as renewals
// are not supported yet.
func (v *VaultCAS) RenewCertificate(req *apiv1.RenewCertificateRequest) (*apiv1.RenewCertificateResponse, error) {
	return nil, apiv1.ErrNotImplemented{Message: "vaultCAS does not support renewals"}
}

func (v *VaultCAS) RevokeCertificate(req *apiv1.RevokeCertificateRequest) (*apiv1.RevokeCertificateResponse, error) {
	if req.SerialNumber == "" && req.Certificate == nil {
		return nil, errors.New("`serialNumber` or `certificate` are required")
	}

	var serialNumber []byte
	if req.SerialNumber != "" {
		// req.SerialNumber is a big.Int string representation
		n := new(big.Int)
		n, ok := n.SetString(req.SerialNumber, 10)
		if !ok {
			return nil, errors.Errorf("serialNumber `%v` can't be convert to big.Int", req.SerialNumber)
		}
		serialNumber = n.Bytes()
	} else {
		// req.Certificate.SerialNumber is a big.Int
		serialNumber = req.Certificate.SerialNumber.Bytes()
	}

	serialNumberDash := certutil.GetHexFormatted(serialNumber, "-")

	y := map[string]interface{}{
		"serial_number": serialNumberDash,
	}

	_, err := v.client.Logical().Write(v.config.PKI+"/revoke/", y)
	if err != nil {
		return nil, errors.Wrap(err, "unable to revoke certificate")
	}

	return &apiv1.RevokeCertificateResponse{
		Certificate:      req.Certificate,
		CertificateChain: nil,
	}, nil
}

func unmarshalMap(m map[string]interface{}, v interface{}) error {
	b, err := json.Marshal(m)
	if err != nil {
		return err
	}

	return json.Unmarshal(b, v)
}

// isRoot returns true if the given certificate is a root certificate.
func isRoot(cert *x509.Certificate) bool {
	if cert.BasicConstraintsValid && cert.IsCA {
		return cert.CheckSignatureFrom(cert) == nil
	}
	return false
}