Simplify SCEP provisioner context handling

Instead of relying on an intermediate `scep.Service` struct,
initialize the `scep.Authority` directly. This removes one redundant
layer of indirection.

api/api.go

@@ -244,11 +244,24 @@ func (p ProvisionersResponse) MarshalJSON() ([]byte, error) {
 			continue
 		}
 
-		old := scepProv.ChallengePassword
+		type old struct {
+			challengePassword    string
+			decrypterCertificate []byte
+			decrypterKey         string
+			decrypterKeyPassword string
+		}
+		o := old{scepProv.ChallengePassword, scepProv.DecrypterCertificate, scepProv.DecrypterKey, scepProv.DecrypterKeyPassword}
 		scepProv.ChallengePassword = "*** REDACTED ***"
-		defer func(p string) { //nolint:gocritic // defer in loop required to restore initial state of provisioners
-			scepProv.ChallengePassword = p
-		}(old)
+		scepProv.DecrypterCertificate = []byte("*** REDACTED ***")
+		scepProv.DecrypterKey = "*** REDACTED ***"
+		scepProv.DecrypterKeyPassword = "*** REDACTED ***"
+
+		defer func(o old) { //nolint:gocritic // defer in loop required to restore initial state of provisioners
+			scepProv.ChallengePassword = o.challengePassword
+			scepProv.DecrypterCertificate = o.decrypterCertificate
+			scepProv.DecrypterKey = o.decrypterKey
+			scepProv.DecrypterKeyPassword = o.decrypterKeyPassword
+		}(o)
 	}
 
 	var list = struct {

authority/authority.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"context"
 	"crypto"
+	"crypto/rsa"
 	"crypto/sha256"
 	"crypto/x509"
 	"encoding/hex"
@@ -61,7 +62,7 @@ type Authority struct {
 	x509Enforcers         []provisioner.CertificateEnforcer
 
 	// SCEP CA
-	scepService *scep.Service
+	scepAuthority *scep.Authority
 
 	// SSH CA
 	sshHostPassword         []byte
@@ -261,6 +262,15 @@ func (a *Authority) ReloadAdminResources(ctx context.Context) error {
 	a.config.AuthorityConfig.Admins = adminList
 	a.admins = adminClxn
 
+	// update the SCEP service with the currently active SCEP
+	// provisioner names and revalidate the configuration.
+	if a.scepAuthority != nil {
+		a.scepAuthority.UpdateProvisioners(a.getSCEPProvisionerNames())
+		if err := a.scepAuthority.Validate(); err != nil {
+			log.Printf("failed validating SCEP authority: %v\n", err)
+		}
+	}
+
 	return nil
 }
 
@@ -640,48 +650,63 @@ func (a *Authority) init() error {
 		return err
 	}
 
-	// Check if a KMS with decryption capability is required and available
-	if a.requiresDecrypter() {
-		if _, ok := a.keyManager.(kmsapi.Decrypter); !ok {
-			return errors.New("keymanager doesn't provide crypto.Decrypter")
-		}
-	}
-
-	// TODO: decide if this is a good approach for providing the SCEP functionality
-	// It currently mirrors the logic for the x509CAService
-	if a.requiresSCEPService() && a.scepService == nil {
+	// The SCEP functionality is provided through an instance of
+	// scep.Service. It is initialized once when the CA is started.
+	// TODO(hs): should the SCEP service support reloading? For example,
+	// when the admin resources are reloaded, specifically the provisioners,
+	// it can happen that the SCEP service is no longer required and can
+	// be destroyed, or that it needs to be instantiated. It may also need
+	// to be revalidated, because not all SCEP provisioner may have a
+	// valid decrypter available.
+	if a.requiresSCEP() && a.GetSCEP() == nil {
 		var options scep.Options
-
-		// Read intermediate and create X509 signer and decrypter for default CAS.
-		options.CertificateChain, err = pemutil.ReadCertificateBundle(a.config.IntermediateCert)
+		options.Roots = a.rootX509Certs
+		options.Intermediates, err = pemutil.ReadCertificateBundle(a.config.IntermediateCert)
 		if err != nil {
 			return err
 		}
-		options.CertificateChain = append(options.CertificateChain, a.rootX509Certs...)
-		options.Signer, err = a.keyManager.CreateSigner(&kmsapi.CreateSignerRequest{
+		options.SignerCert = options.Intermediates[0]
+		if options.Signer, err = a.keyManager.CreateSigner(&kmsapi.CreateSignerRequest{
 			SigningKey: a.config.IntermediateKey,
 			Password:   a.password,
-		})
-		if err != nil {
+		}); err != nil {
 			return err
 		}
 
-		if km, ok := a.keyManager.(kmsapi.Decrypter); ok {
-			options.Decrypter, err = km.CreateDecrypter(&kmsapi.CreateDecrypterRequest{
+		// TODO(hs): instead of creating the decrypter here, pass the
+		// intermediate key + chain down to the SCEP service / authority,
+		// and only instantiate it when required there. Is that possible?
+		// Also with entering passwords?
+		// TODO(hs): if moving the logic, try improving the logic for the
+		// decrypter password too? Right now it needs to be entered multiple
+		// times; I've observed it to be three times maximum, every time
+		// the intermediate key is read.
+		_, isRSA := options.Signer.Public().(*rsa.PublicKey)
+		if km, ok := a.keyManager.(kmsapi.Decrypter); ok && isRSA {
+			if decrypter, err := km.CreateDecrypter(&kmsapi.CreateDecrypterRequest{
 				DecryptionKey: a.config.IntermediateKey,
 				Password:      a.password,
-			})
-			if err != nil {
-				return err
+			}); err == nil {
+				// only pass the decrypter down when it was successfully created,
+				// meaning it's an RSA key, and `CreateDecrypter` did not fail.
+				options.Decrypter = decrypter
 			}
 		}
 
-		a.scepService, err = scep.NewService(ctx, options)
+		// provide the current SCEP provisioner names, so that the provisioners
+		// can be validated when the CA is started.
+		options.SCEPProvisionerNames = a.getSCEPProvisionerNames()
+
+		// create a new SCEP authority
+		a.scepAuthority, err = scep.New(a, options)
 		if err != nil {
 			return err
 		}
 
-		// TODO: mimick the x509CAService GetCertificateAuthority here too?
+		// validate the SCEP authority
+		if err := a.scepAuthority.Validate(); err != nil {
+			a.initLogf("failed validating SCEP authority: %v", err)
+		}
 	}
 
 	// Load X509 constraints engine.
@@ -833,17 +858,9 @@ func (a *Authority) IsRevoked(sn string) (bool, error) {
 	return a.db.IsRevoked(sn)
 }
 
-// requiresDecrypter returns whether the Authority
-// requires a KMS that provides a crypto.Decrypter
-// Currently this is only required when SCEP is
-// enabled.
-func (a *Authority) requiresDecrypter() bool {
-	return a.requiresSCEPService()
-}
-
-// requiresSCEPService iterates over the configured provisioners
-// and determines if one of them is a SCEP provisioner.
-func (a *Authority) requiresSCEPService() bool {
+// requiresSCEP iterates over the configured provisioners
+// and determines if at least one of them is a SCEP provisioner.
+func (a *Authority) requiresSCEP() bool {
 	for _, p := range a.config.AuthorityConfig.Provisioners {
 		if p.GetType() == provisioner.TypeSCEP {
 			return true
@@ -852,13 +869,18 @@ func (a *Authority) requiresSCEPService() bool {
 	return false
 }
 
-// GetSCEPService returns the configured SCEP Service.
-//
-// TODO: this function is intended to exist temporarily in order to make SCEP
-// work more easily. It can be made more correct by using the right
-// interfaces/abstractions after it works as expected.
-func (a *Authority) GetSCEPService() *scep.Service {
-	return a.scepService
+func (a *Authority) getSCEPProvisionerNames() (names []string) {
+	for _, p := range a.config.AuthorityConfig.Provisioners {
+		if p.GetType() == provisioner.TypeSCEP {
+			names = append(names, p.GetName())
+		}
+	}
+	return
+}
+
+// GetSCEP returns the configured SCEP Authority
+func (a *Authority) GetSCEP() *scep.Authority {
+	return a.scepAuthority
 }
 
 func (a *Authority) startCRLGenerator() error {

authority/authority_test.go

@@ -478,7 +478,7 @@ func testScepAuthority(t *testing.T, opts ...Option) *Authority {
 	return a
 }
 
-func TestAuthority_GetSCEPService(t *testing.T) {
+func TestAuthority_GetSCEP(t *testing.T) {
 	_ = testScepAuthority(t)
 	p := provisioner.List{
 		&provisioner.SCEP{
@@ -542,7 +542,7 @@ func TestAuthority_GetSCEPService(t *testing.T) {
 				return
 			}
 			if tt.wantService {
-				if got := a.GetSCEPService(); (got != nil) != tt.wantService {
+				if got := a.GetSCEP(); (got != nil) != tt.wantService {
 					t.Errorf("Authority.GetSCEPService() = %v, wantService %v", got, tt.wantService)
 				}
 			}

authority/provisioner/scep.go

@@ -2,13 +2,19 @@ package provisioner
 
 import (
 	"context"
+	"crypto"
+	"crypto/rsa"
 	"crypto/subtle"
+	"crypto/x509"
+	"encoding/pem"
 	"fmt"
 	"net/http"
 	"time"
 
 	"github.com/pkg/errors"
 
+	"go.step.sm/crypto/kms"
+	kmsapi "go.step.sm/crypto/kms/apiv1"
 	"go.step.sm/linkedca"
 
 	"github.com/smallstep/certificates/webhook"
@@ -32,6 +38,12 @@ type SCEP struct {
 	// MinimumPublicKeyLength is the minimum length for public keys in CSRs
 	MinimumPublicKeyLength int `json:"minimumPublicKeyLength,omitempty"`
 
+	// TODO
+	KMS                  *kms.Options `json:"kms,omitempty"`
+	DecrypterCertificate []byte       `json:"decrypterCertificate"`
+	DecrypterKey         string       `json:"decrypterKey"`
+	DecrypterKeyPassword string       `json:"decrypterKeyPassword"`
+
 	// Numerical identifier for the ContentEncryptionAlgorithm as defined in github.com/mozilla-services/pkcs7
 	// at https://github.com/mozilla-services/pkcs7/blob/33d05740a3526e382af6395d3513e73d4e66d1cb/encrypt.go#L63
 	// Defaults to 0, being DES-CBC
@@ -41,6 +53,9 @@ type SCEP struct {
 	ctl                           *Controller
 	encryptionAlgorithm           int
 	challengeValidationController *challengeValidationController
+	keyManager                    kmsapi.KeyManager
+	decrypter                     crypto.Decrypter
+	decrypterCertificate          *x509.Certificate
 }
 
 // GetID returns the provisioner unique identifier.
@@ -177,6 +192,45 @@ func (s *SCEP) Init(config Config) (err error) {
 		s.GetOptions().GetWebhooks(),
 	)
 
+	if s.KMS != nil {
+		if s.keyManager, err = kms.New(context.Background(), *s.KMS); err != nil {
+			return fmt.Errorf("failed initializing kms: %w", err)
+		}
+		km, ok := s.keyManager.(kmsapi.Decrypter)
+		if !ok {
+			return fmt.Errorf("%q is not a kmsapi.Decrypter", s.KMS.Type)
+		}
+		if s.DecrypterKey != "" || len(s.DecrypterCertificate) > 0 {
+			if s.decrypter, err = km.CreateDecrypter(&kmsapi.CreateDecrypterRequest{
+				DecryptionKey: s.DecrypterKey,
+				Password:      []byte(s.DecrypterKeyPassword),
+			}); err != nil {
+				return fmt.Errorf("failed creating decrypter: %w", err)
+			}
+
+			// parse the decrypter certificate
+			block, rest := pem.Decode(s.DecrypterCertificate)
+			if len(rest) > 0 {
+				return errors.New("failed parsing decrypter certificate: trailing data")
+			}
+			if block == nil {
+				return errors.New("failed parsing decrypter certificate: no PEM block found")
+			}
+			if s.decrypterCertificate, err = x509.ParseCertificate(block.Bytes); err != nil {
+				return fmt.Errorf("failed parsing decrypter certificate: %w", err)
+			}
+
+			// validate the decrypter key
+			decrypterPublicKey, ok := s.decrypter.Public().(*rsa.PublicKey)
+			if !ok {
+				return fmt.Errorf("only RSA keys are supported")
+			}
+			if !decrypterPublicKey.Equal(s.decrypterCertificate.PublicKey) {
+				return errors.New("mismatch between decryption certificate and decrypter public keys")
+			}
+		}
+	}
+
 	// TODO: add other, SCEP specific, options?
 
 	s.ctl, err = NewController(s, s.Claims, config, s.Options)
@@ -259,3 +313,7 @@ func (s *SCEP) selectValidationMethod() validationMethod {
 	}
 	return validationMethodNone
 }
+
+func (s *SCEP) GetDecrypter() (*x509.Certificate, crypto.Decrypter) {
+	return s.decrypterCertificate, s.decrypter
+}

authority/provisioners.go

@@ -8,6 +8,7 @@ import (
 	"encoding/pem"
 	"fmt"
 	"os"
+	"strings"
 
 	"github.com/pkg/errors"
 	"gopkg.in/square/go-jose.v2/jwt"
@@ -15,6 +16,7 @@ import (
 	"go.step.sm/cli-utils/step"
 	"go.step.sm/cli-utils/ui"
 	"go.step.sm/crypto/jose"
+	"go.step.sm/crypto/kms"
 	"go.step.sm/linkedca"
 
 	"github.com/smallstep/certificates/authority/admin"
@@ -235,7 +237,7 @@ func (a *Authority) StoreProvisioner(ctx context.Context, prov *linkedca.Provisi
 	}
 
 	if err := certProv.Init(provisionerConfig); err != nil {
-		return admin.WrapError(admin.ErrorBadRequestType, err, "error validating configuration for provisioner %s", prov.Name)
+		return admin.WrapError(admin.ErrorBadRequestType, err, "error validating configuration for provisioner %q", prov.Name)
 	}
 
 	// Store to database -- this will set the ID.
@@ -960,7 +962,7 @@ func ProvisionerToCertificates(p *linkedca.Provisioner) (provisioner.Interface,
 		}, nil
 	case *linkedca.ProvisionerDetails_SCEP:
 		cfg := d.SCEP
-		return &provisioner.SCEP{
+		s := &provisioner.SCEP{
 			ID:                            p.Id,
 			Type:                          p.Type.String(),
 			Name:                          p.Name,
@@ -972,7 +974,19 @@ func ProvisionerToCertificates(p *linkedca.Provisioner) (provisioner.Interface,
 			EncryptionAlgorithmIdentifier: int(cfg.EncryptionAlgorithmIdentifier),
 			Claims:                        claims,
 			Options:                       options,
-		}, nil
+		}
+		if decrypter := cfg.GetDecrypter(); decrypter != nil {
+			if dkms := decrypter.GetKms(); dkms != nil {
+				s.KMS = &kms.Options{
+					Type:            kms.Type(strings.ToLower(linkedca.KMS_Type_name[int32(dkms.Type)])),
+					CredentialsFile: dkms.CredentialsFile,
+				}
+			}
+			s.DecrypterCertificate = decrypter.DecrypterCertificate
+			s.DecrypterKey = decrypter.DecrypterKey
+			s.DecrypterKeyPassword = decrypter.DecrypterKeyPassword
+		}
+		return s, nil
 	case *linkedca.ProvisionerDetails_Nebula:
 		var roots []byte
 		for i, root := range d.Nebula.GetRoots() {

ca/ca.go

@@ -250,19 +250,24 @@ func (ca *CA) Init(cfg *config.Config) (*CA, error) {
 
 	var scepAuthority *scep.Authority
 	if ca.shouldServeSCEPEndpoints() {
-		scepPrefix := "scep"
-		scepAuthority, err = scep.New(auth, scep.AuthorityOptions{
-			Service: auth.GetSCEPService(),
-			DNS:     dns,
-			Prefix:  scepPrefix,
-		})
-		if err != nil {
-			return nil, errors.Wrap(err, "error creating SCEP authority")
+		// validate the SCEP authority configuration. Currently this
+		// will not result in a failure to start if one or more SCEP
+		// provisioners are not correctly configured. Only a log will
+		// be emitted.
+		scepAuthority = auth.GetSCEP()
+		if err := scepAuthority.Validate(); err != nil {
+			err = errors.Wrap(err, "failed validating SCEP authority")
+			shouldFail := false
+			if shouldFail {
+				return nil, err
+			}
+			log.Println(err)
 		}
 
 		// According to the RFC (https://tools.ietf.org/html/rfc8894#section-7.10),
 		// SCEP operations are performed using HTTP, so that's why the API is mounted
 		// to the insecure mux.
+		scepPrefix := "scep"
 		insecureMux.Route("/"+scepPrefix, func(r chi.Router) {
 			scepAPI.Route(r)
 		})
@@ -584,10 +589,10 @@ func (ca *CA) getTLSConfig(auth *authority.Authority) (*tls.Config, *tls.Config,
 
 // shouldServeSCEPEndpoints returns if the CA should be
 // configured with endpoints for SCEP. This is assumed to be
-// true if a SCEPService exists, which is true in case a
-// SCEP provisioner was configured.
+// true if a SCEPService exists, which is true in case at
+// least one SCEP provisioner was configured.
 func (ca *CA) shouldServeSCEPEndpoints() bool {
-	return ca.auth.GetSCEPService() != nil
+	return ca.auth.GetSCEP() != nil
 }
 
 //nolint:unused // useful for debugging

go.mod

@@ -146,3 +146,5 @@ require (
 
 // use github.com/smallstep/pkcs7 fork with patches applied
 replace go.mozilla.org/pkcs7 => github.com/smallstep/pkcs7 v0.0.0-20230302202335-4c094085c948
+
+replace go.step.sm/linkedca => ./../linkedca

go.sum

@@ -1066,8 +1066,6 @@ go.step.sm/cli-utils v0.7.6 h1:YkpLVrepmy2c5+eaz/wduiGxlgrRx3YdAStE37if25g=
 go.step.sm/cli-utils v0.7.6/go.mod h1:j+FxFZ2gbWkAJl0eded/rksuxmNqWpmyxbkXcukGJaY=
 go.step.sm/crypto v0.31.0 h1:8ZG/BxC+0+LzPpk/764h5yubpG3GfxcRVR4E+Aye72g=
 go.step.sm/crypto v0.31.0/go.mod h1:Dv4lpkijKiZVkoc6zp+Xaw1xmy+voia1mykvbpQIvuc=
-go.step.sm/linkedca v0.19.1 h1:uY0ByT/uB3FCQ8zIo9mU7MWG7HKf5sDXNEBeN94MuP8=
-go.step.sm/linkedca v0.19.1/go.mod h1:vPV2ad3LFQJmV7XWt87VlnJSs6UOqgsbVGVWe3veEmI=
 go.uber.org/atomic v1.3.2/go.mod h1:gD2HeocX3+yG+ygLZcrzQJaqmWj9AIm7n08wl/qW/PE=
 go.uber.org/atomic v1.4.0/go.mod h1:gD2HeocX3+yG+ygLZcrzQJaqmWj9AIm7n08wl/qW/PE=
 go.uber.org/atomic v1.5.0/go.mod h1:sABNBOSYdrvTF6hTgEIbc7YasKWGhgEQZyfxyTvoXHQ=

scep/api/api.go

@@ -221,7 +221,7 @@ func lookupProvisioner(next http.HandlerFunc) http.HandlerFunc {
 			return
 		}
 
-		ctx = context.WithValue(ctx, scep.ProvisionerContextKey, scep.Provisioner(prov))
+		ctx = scep.NewProvisionerContext(ctx, scep.Provisioner(prov))
 		next(w, r.WithContext(ctx))
 	}
 }

scep/authority.go

@@ -2,10 +2,10 @@ package scep
 
 import (
 	"context"
+	"crypto"
 	"crypto/x509"
 	"errors"
 	"fmt"
-	"net/url"
 
 	microx509util "github.com/micromdm/scep/v2/cryptoutil/x509util"
 	microscep "github.com/micromdm/scep/v2/scep"
@@ -18,12 +18,13 @@ import (
 
 // Authority is the layer that handles all SCEP interactions.
 type Authority struct {
-	prefix                  string
-	dns                     string
-	intermediateCertificate *x509.Certificate
-	caCerts                 []*x509.Certificate // TODO(hs): change to use these instead of root and intermediate
-	service                 *Service
 	signAuth             SignAuthority
+	roots                []*x509.Certificate
+	intermediates        []*x509.Certificate
+	signerCertificate    *x509.Certificate
+	signer               crypto.Signer
+	defaultDecrypter     crypto.Decrypter
+	scepProvisionerNames []string
 }
 
 type authorityKey struct{}
@@ -49,19 +50,6 @@ func MustFromContext(ctx context.Context) *Authority {
 	}
 }
 
-// AuthorityOptions required to create a new SCEP Authority.
-type AuthorityOptions struct {
-	// Service provides the certificate chain, the signer and the decrypter to the Authority
-	Service *Service
-	// DNS is the host used to generate accurate SCEP links. By default the authority
-	// will use the Host from the request, so this value will only be used if
-	// request.Host is empty.
-	DNS string
-	// Prefix is a URL path prefix under which the SCEP api is served. This
-	// prefix is required to generate accurate SCEP links.
-	Prefix string
-}
-
 // SignAuthority is the interface for a signing authority
 type SignAuthority interface {
 	Sign(cr *x509.CertificateRequest, opts provisioner.SignOptions, signOpts ...provisioner.SignOption) ([]*x509.Certificate, error)
@@ -69,26 +57,54 @@ type SignAuthority interface {
 }
 
 // New returns a new Authority that implements the SCEP interface.
-func New(signAuth SignAuthority, ops AuthorityOptions) (*Authority, error) {
+func New(signAuth SignAuthority, opts Options) (*Authority, error) {
+	if err := opts.Validate(); err != nil {
+		return nil, err
+	}
 	authority := &Authority{
-		prefix:   ops.Prefix,
-		dns:      ops.DNS,
-		signAuth: signAuth,
+		signAuth:             signAuth, // TODO: provide signAuth through context instead?
+		roots:                opts.Roots,
+		intermediates:        opts.Intermediates,
+		signerCertificate:    opts.SignerCert,
+		signer:               opts.Signer,
+		defaultDecrypter:     opts.Decrypter,
+		scepProvisionerNames: opts.SCEPProvisionerNames,
 	}
-
-	// TODO: this is not really nice to do; the Service should be removed
-	// in its entirety to make this more interoperable with the rest of
-	// step-ca, I think.
-	if ops.Service != nil {
-		authority.caCerts = ops.Service.certificateChain
-		// TODO(hs): look into refactoring SCEP into using just caCerts everywhere, if it makes sense for more elaborate SCEP configuration. Keeping it like this for clarity (for now).
-		authority.intermediateCertificate = ops.Service.certificateChain[0]
-		authority.service = ops.Service
-	}
-
 	return authority, nil
 }
 
+// Validate validates if the SCEP Authority has a valid configuration.
+// The validation includes a check if a decrypter is available, either
+// an authority wide decrypter, or a provisioner specific decrypter.
+func (a *Authority) Validate() error {
+	noDefaultDecrypterAvailable := a.defaultDecrypter == nil
+	for _, name := range a.scepProvisionerNames {
+		p, err := a.LoadProvisionerByName(name)
+		if err != nil {
+			return fmt.Errorf("failed loading provisioner %q: %w", name, err)
+		}
+		if scepProv, ok := p.(*provisioner.SCEP); ok {
+			cert, decrypter := scepProv.GetDecrypter()
+			// TODO(hs): return sentinel/typed error, to be able to ignore/log these cases during init?
+			if cert == nil && noDefaultDecrypterAvailable {
+				return fmt.Errorf("SCEP provisioner %q does not have a decrypter certificate", name)
+			}
+			if decrypter == nil && noDefaultDecrypterAvailable {
+				return fmt.Errorf("SCEP provisioner %q does not have decrypter", name)
+			}
+		}
+	}
+
+	return nil
+}
+
+// UpdateProvisioners updates the SCEP Authority with the new, and hopefully
+// current SCEP provisioners configured. This allows the Authority to be
+// validated with the latest data.
+func (a *Authority) UpdateProvisioners(scepProvisionerNames []string) {
+	a.scepProvisionerNames = scepProvisionerNames
+}
+
 var (
 	// TODO: check the default capabilities; https://tools.ietf.org/html/rfc8894#section-3.5.2
 	defaultCapabilities = []string{
@@ -108,87 +124,52 @@ func (a *Authority) LoadProvisionerByName(name string) (provisioner.Interface, e
 	return a.signAuth.LoadProvisionerByName(name)
 }
 
-// GetLinkExplicit returns the requested link from the directory.
-func (a *Authority) GetLinkExplicit(provName string, abs bool, baseURL *url.URL, inputs ...string) string {
-	return a.getLinkExplicit(provName, abs, baseURL, inputs...)
-}
-
-// getLinkExplicit returns an absolute or partial path to the given resource and a base
-// URL dynamically obtained from the request for which the link is being calculated.
-func (a *Authority) getLinkExplicit(provisionerName string, abs bool, baseURL *url.URL, _ ...string) string {
-	link := "/" + provisionerName
-	if abs {
-		// Copy the baseURL value from the pointer. https://github.com/golang/go/issues/38351
-		u := url.URL{}
-		if baseURL != nil {
-			u = *baseURL
-		}
-
-		// If no Scheme is set, then default to http (in case of SCEP)
-		if u.Scheme == "" {
-			u.Scheme = "http"
-		}
-
-		// If no Host is set, then use the default (first DNS attr in the ca.json).
-		if u.Host == "" {
-			u.Host = a.dns
-		}
-
-		u.Path = a.prefix + link
-		return u.String()
-	}
-
-	return link
-}
-
-// GetCACertificates returns the certificate (chain) for the CA
-func (a *Authority) GetCACertificates(ctx context.Context) ([]*x509.Certificate, error) {
-	// TODO: this should return: the "SCEP Server (RA)" certificate, the issuing CA up to and excl. the root
+// GetCACertificates returns the certificate (chain) for the CA.
+//
+// This methods returns the "SCEP Server (RA)" certificate, the issuing CA up to and excl. the root.
 // Some clients do need the root certificate however; also see: https://github.com/openxpki/openxpki/issues/73
 //
-	// This means we might need to think about if we should use the current intermediate CA
-	// certificate as the "SCEP Server (RA)" certificate. It might be better to have a distinct
-	// RA certificate, with a corresponding rsa.PrivateKey, just for SCEP usage, which is signed by
-	// the intermediate CA. Will need to look how we can provide this nicely within step-ca.
-	//
-	// This might also mean that we might want to use a distinct instance of KMS for doing the key operations,
-	// so that we can use RSA just for SCEP.
+// In case a provisioner specific decrypter is available, this is used as the "SCEP Server (RA)" certificate
+// instead of the CA intermediate directly. This uses a distinct instance of a KMS for doing the SCEp key
+// operations, so that RSA can be used for just SCEP.
 //
 // Using an RA does not seem to exist in https://tools.ietf.org/html/rfc8894, but is mentioned in
-	// https://tools.ietf.org/id/draft-nourse-scep-21.html. Will continue using the CA directly for now.
-	//
-	// The certificate to use should probably depend on the (configured) provisioner and may
-	// use a distinct certificate, apart from the intermediate.
+// https://tools.ietf.org/id/draft-nourse-scep-21.html.
+func (a *Authority) GetCACertificates(ctx context.Context) (certs []*x509.Certificate, err error) {
+	p := provisionerFromContext(ctx)
 
-	p, err := provisionerFromContext(ctx)
-	if err != nil {
-		return nil, err
+	// if a provisioner specific RSA decrypter is available, it is returned as
+	// the first certificate.
+	if decrypterCertificate, _ := p.GetDecrypter(); decrypterCertificate != nil {
+		certs = append(certs, decrypterCertificate)
 	}
 
-	if len(a.caCerts) == 0 {
-		return nil, errors.New("no intermediate certificate available in SCEP authority")
-	}
+	// TODO(hs): ensure logic is in place that checks the signer is the first
+	// intermediate and that there are no double certificates.
+	certs = append(certs, a.intermediates...)
 
-	certs := []*x509.Certificate{}
-	certs = append(certs, a.caCerts[0])
-
-	// NOTE: we're adding the CA roots here, but they are (highly likely) different than what the RFC means.
-	// Clients are responsible to select the right cert(s) to use, though.
-	if p.ShouldIncludeRootInChain() && len(a.caCerts) > 1 {
-		certs = append(certs, a.caCerts[1])
+	// the CA roots are added for completeness when configured to do so. Clients
+	// are responsible to select the right cert(s) to store and use.
+	if p.ShouldIncludeRootInChain() {
+		certs = append(certs, a.roots...)
 	}
 
 	return certs, nil
 }
 
 // DecryptPKIEnvelope decrypts an enveloped message
-func (a *Authority) DecryptPKIEnvelope(_ context.Context, msg *PKIMessage) error {
+func (a *Authority) DecryptPKIEnvelope(ctx context.Context, msg *PKIMessage) error {
 	p7c, err := pkcs7.Parse(msg.P7.Content)
 	if err != nil {
 		return fmt.Errorf("error parsing pkcs7 content: %w", err)
 	}
 
-	envelope, err := p7c.Decrypt(a.intermediateCertificate, a.service.decrypter)
+	cert, pkey, err := a.selectDecrypter(ctx)
+	if err != nil {
+		return fmt.Errorf("failed selecting decrypter: %w", err)
+	}
+
+	envelope, err := p7c.Decrypt(cert, pkey)
 	if err != nil {
 		return fmt.Errorf("error decrypting encrypted pkcs7 content: %w", err)
 	}
@@ -208,6 +189,9 @@ func (a *Authority) DecryptPKIEnvelope(_ context.Context, msg *PKIMessage) error
 		if err != nil {
 			return fmt.Errorf("parse CSR from pkiEnvelope: %w", err)
 		}
+		if err := csr.CheckSignature(); err != nil {
+			return fmt.Errorf("invalid CSR signature; %w", err)
+		}
 		// check for challengePassword
 		cp, err := microx509util.ParseChallengePassword(msg.pkiEnvelope)
 		if err != nil {
@@ -226,6 +210,21 @@ func (a *Authority) DecryptPKIEnvelope(_ context.Context, msg *PKIMessage) error
 	return nil
 }
 
+func (a *Authority) selectDecrypter(ctx context.Context) (cert *x509.Certificate, pkey crypto.PrivateKey, err error) {
+	p := provisionerFromContext(ctx)
+
+	// return provisioner specific decrypter, if available
+	if cert, pkey = p.GetDecrypter(); cert != nil && pkey != nil {
+		return
+	}
+
+	// fallback to the CA wide decrypter
+	cert = a.signerCertificate
+	pkey = a.defaultDecrypter
+
+	return
+}
+
 // SignCSR creates an x509.Certificate based on a CSR template and Cert Authority credentials
 // returns a new PKIMessage with CertRep data
 func (a *Authority) SignCSR(ctx context.Context, csr *x509.CertificateRequest, msg *PKIMessage) (*PKIMessage, error) {
@@ -234,10 +233,7 @@ func (a *Authority) SignCSR(ctx context.Context, csr *x509.CertificateRequest, m
 	// poll for the status. It seems to be similar as what can happen in ACME, so might want to model
 	// the implementation after the one in the ACME authority. Requires storage, etc.
 
-	p, err := provisionerFromContext(ctx)
-	if err != nil {
-		return nil, err
-	}
+	p := provisionerFromContext(ctx)
 
 	// check if CSRReqMessage has already been decrypted
 	if msg.CSRReqMessage.CSR == nil {
@@ -358,10 +354,11 @@ func (a *Authority) SignCSR(ctx context.Context, csr *x509.CertificateRequest, m
 	// as the first certificate in the array
 	signedData.AddCertificate(cert)
 
-	authCert := a.intermediateCertificate
+	authCert := a.signerCertificate
+	signer := a.signer
 
 	// sign the attributes
-	if err := signedData.AddSigner(authCert, a.service.signer, config); err != nil {
+	if err := signedData.AddSigner(authCert, signer, config); err != nil {
 		return nil, err
 	}
 
@@ -429,7 +426,7 @@ func (a *Authority) CreateFailureResponse(_ context.Context, _ *x509.Certificate
 	}
 
 	// sign the attributes
-	if err := signedData.AddSigner(a.intermediateCertificate, a.service.signer, config); err != nil {
+	if err := signedData.AddSigner(a.signerCertificate, a.signer, config); err != nil {
 		return nil, err
 	}
 
@@ -457,10 +454,7 @@ func (a *Authority) CreateFailureResponse(_ context.Context, _ *x509.Certificate
 
 // GetCACaps returns the CA capabilities
 func (a *Authority) GetCACaps(ctx context.Context) []string {
-	p, err := provisionerFromContext(ctx)
-	if err != nil {
-		return defaultCapabilities
-	}
+	p := provisionerFromContext(ctx)
 
 	caps := p.GetCapabilities()
 	if len(caps) == 0 {
@@ -477,9 +471,6 @@ func (a *Authority) GetCACaps(ctx context.Context) []string {
 }
 
 func (a *Authority) ValidateChallenge(ctx context.Context, challenge, transactionID string) error {
-	p, err := provisionerFromContext(ctx)
-	if err != nil {
-		return err
-	}
+	p := provisionerFromContext(ctx)
 	return p.ValidateChallenge(ctx, challenge, transactionID)
 }

scep/common.go

@@ -1,29 +0,0 @@
-package scep
-
-import (
-	"context"
-	"errors"
-)
-
-// ContextKey is the key type for storing and searching for SCEP request
-// essentials in the context of a request.
-type ContextKey string
-
-const (
-	// ProvisionerContextKey provisioner key
-	ProvisionerContextKey = ContextKey("provisioner")
-)
-
-// provisionerFromContext searches the context for a SCEP provisioner.
-// Returns the provisioner or an error.
-func provisionerFromContext(ctx context.Context) (Provisioner, error) {
-	val := ctx.Value(ProvisionerContextKey)
-	if val == nil {
-		return nil, errors.New("provisioner expected in request context")
-	}
-	p, ok := val.(Provisioner)
-	if !ok || p == nil {
-		return nil, errors.New("provisioner in context is not a SCEP provisioner")
-	}
-	return p, nil
-}

scep/database.go

@@ -1,7 +0,0 @@
-package scep
-
-import "crypto/x509"
-
-type DB interface {
-	StoreCertificate(crt *x509.Certificate) error
-}

scep/options.go

@@ -4,65 +4,76 @@ import (
 	"crypto"
 	"crypto/rsa"
 	"crypto/x509"
-
-	"github.com/pkg/errors"
+	"errors"
 )
 
 type Options struct {
-	// CertificateChain is the issuer certificate, along with any other bundled certificates
-	// to be returned in the chain for consumers. Configured in the ca.json crt property.
-	CertificateChain []*x509.Certificate
+	// Roots contains the (federated) CA roots certificate(s)
+	Roots []*x509.Certificate `json:"-"`
+	// Intermediates points issuer certificate, along with any other bundled certificates
+	// to be returned in the chain for consumers.
+	Intermediates []*x509.Certificate `json:"-"`
+	// SignerCert points to the certificate of the CA signer. It usually is the same as the
+	// first certificate in the CertificateChain.
+	SignerCert *x509.Certificate `json:"-"`
 	// Signer signs CSRs in SCEP. Configured in the ca.json key property.
 	Signer crypto.Signer `json:"-"`
 	// Decrypter decrypts encrypted SCEP messages. Configured in the ca.json key property.
 	Decrypter crypto.Decrypter `json:"-"`
+	// SCEPProvisionerNames contains the currently configured SCEP provioner names. These
+	// are used to be able to load the provisioners when the SCEP authority is being
+	// validated.
+	SCEPProvisionerNames []string
+}
+
+type comparablePublicKey interface {
+	Equal(crypto.PublicKey) bool
 }
 
 // Validate checks the fields in Options.
 func (o *Options) Validate() error {
-	if o.CertificateChain == nil {
-		return errors.New("certificate chain not configured correctly")
+	switch {
+	case len(o.Intermediates) == 0:
+		return errors.New("no intermediate certificate available for SCEP authority")
+	case o.Signer == nil:
+		return errors.New("no signer available for SCEP authority")
+	case o.SignerCert == nil:
+		return errors.New("no signer certificate available for SCEP authority")
 	}
 
-	if len(o.CertificateChain) < 1 {
-		return errors.New("certificate chain should at least have one certificate")
+	// check if the signer (intermediate CA) certificate has the same public key as
+	// the signer. According to the RFC it seems valid to have different keys for
+	// the intermediate and the CA signing new certificates, so this might change
+	// in the future.
+	signerPublicKey := o.Signer.Public().(comparablePublicKey)
+	if !signerPublicKey.Equal(o.SignerCert.PublicKey) {
+		return errors.New("mismatch between signer certificate and public key")
 	}
 
-	// According to the RFC: https://tools.ietf.org/html/rfc8894#section-3.1, SCEP
-	// can be used with something different than RSA, but requires the encryption
-	// to be performed using the challenge password. An older version of specification
-	// states that only RSA is supported: https://tools.ietf.org/html/draft-nourse-scep-23#section-2.1.1
-	// Other algorithms than RSA do not seem to be supported in certnanny/sscep, but it might work
+	// decrypter can be nil in case a signing only key is used; validation complete.
+	if o.Decrypter == nil {
+		return nil
+	}
+
+	// If a decrypter is available, check that it's backed by an RSA key. According to the
+	// RFC: https://tools.ietf.org/html/rfc8894#section-3.1, SCEP can be used with something
+	// different than RSA, but requires the encryption to be performed using the challenge
+	// password in that case. An older version of specification states that only RSA is
+	// supported: https://tools.ietf.org/html/draft-nourse-scep-23#section-2.1.1. Other
+	// algorithms do not seem to be supported in certnanny/sscep, but it might work
 	// in micromdm/scep. Currently only RSA is allowed, but it might be an option
 	// to try other algorithms in the future.
-	intermediate := o.CertificateChain[0]
-	if intermediate.PublicKeyAlgorithm != x509.RSA {
-		return errors.New("only the RSA algorithm is (currently) supported")
-	}
-
-	// TODO: add checks for key usage?
-
-	signerPublicKey, ok := o.Signer.Public().(*rsa.PublicKey)
-	if !ok {
-		return errors.New("only RSA public keys are (currently) supported as signers")
-	}
-
-	// check if the intermediate ca certificate has the same public key as the signer.
-	// According to the RFC it seems valid to have different keys for the intermediate
-	// and the CA signing new certificates, so this might change in the future.
-	if !signerPublicKey.Equal(intermediate.PublicKey) {
-		return errors.New("mismatch between certificate chain and signer public keys")
-	}
-
 	decrypterPublicKey, ok := o.Decrypter.Public().(*rsa.PublicKey)
 	if !ok {
-		return errors.New("only RSA public keys are (currently) supported as decrypters")
+		return errors.New("only RSA keys are (currently) supported as decrypters")
 	}
 
 	// check if intermediate public key is the same as the decrypter public key.
 	// In certnanny/sscep it's mentioned that the signing key can be different
-	// from the decrypting (and encrypting) key. Currently that's not supported.
-	if !decrypterPublicKey.Equal(intermediate.PublicKey) {
+	// from the decrypting (and encrypting) key. These options are only used and
+	// validated when the intermediate CA is also used as the decrypter, though,
+	// so they should match.
+	if !decrypterPublicKey.Equal(o.SignerCert.PublicKey) {
 		return errors.New("mismatch between certificate chain and decrypter public keys")
 	}
 

scep/provisioner.go

@@ -2,6 +2,8 @@ package scep
 
 import (
 	"context"
+	"crypto"
+	"crypto/x509"
 	"time"
 
 	"github.com/smallstep/certificates/authority/provisioner"
@@ -16,6 +18,25 @@ type Provisioner interface {
 	GetOptions() *provisioner.Options
 	GetCapabilities() []string
 	ShouldIncludeRootInChain() bool
+	GetDecrypter() (*x509.Certificate, crypto.Decrypter)
 	GetContentEncryptionAlgorithm() int
 	ValidateChallenge(ctx context.Context, challenge, transactionID string) error
 }
+
+// provisionerKey is the key type for storing and searching a
+// SCEP provisioner in the context.
+type provisionerKey struct{}
+
+// provisionerFromContext searches the context for a SCEP provisioner.
+// Returns the provisioner or panics if no SCEP provisioner is found.
+func provisionerFromContext(ctx context.Context) Provisioner {
+	p, ok := ctx.Value(provisionerKey{}).(Provisioner)
+	if !ok {
+		panic("SCEP provisioner expected in request context")
+	}
+	return p
+}
+
+func NewProvisionerContext(ctx context.Context, p Provisioner) context.Context {
+	return context.WithValue(ctx, provisionerKey{}, p)
+}

scep/service.go

@@ -1,28 +0,0 @@
-package scep
-
-import (
-	"context"
-	"crypto"
-	"crypto/x509"
-)
-
-// Service is a wrapper for crypto.Signer and crypto.Decrypter
-type Service struct {
-	certificateChain []*x509.Certificate
-	signer           crypto.Signer
-	decrypter        crypto.Decrypter
-}
-
-// NewService returns a new Service type.
-func NewService(_ context.Context, opts Options) (*Service, error) {
-	if err := opts.Validate(); err != nil {
-		return nil, err
-	}
-
-	// TODO: should this become similar to the New CertificateAuthorityService as in x509CAService?
-	return &Service{
-		certificateChain: opts.CertificateChain,
-		signer:           opts.Signer,
-		decrypter:        opts.Decrypter,
-	}, nil
-}