Fix macOS SCEP client issues

Fixes #746

authority/authority.go

@@ -437,13 +437,6 @@ func (a *Authority) init() error {
 		}
 	}
 
-	// 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 {
@@ -454,6 +447,7 @@ func (a *Authority) init() error {
 		if err != nil {
 			return err
 		}
+		options.CertificateChain = append(options.CertificateChain, a.rootX509Certs...)
 		options.Signer, err = a.keyManager.CreateSigner(&kmsapi.CreateSignerRequest{
 			SigningKey: a.config.IntermediateKey,
 			Password:   []byte(a.password),
@@ -601,6 +595,12 @@ func (a *Authority) IsRevoked(sn string) (bool, error) {
 	return a.db.IsRevoked(sn)
 }
 
+// GetIntermediateCertificate returns the x509 CA intermediate
+// certificate.
+func (a *Authority) GetIntermediateCertificate() (*x509.Certificate, error) {
+	return pemutil.ReadCertificate(a.config.IntermediateCert)
+}
+
 // requiresDecrypter returns whether the Authority
 // requires a KMS that provides a crypto.Decrypter
 // Currently this is only required when SCEP is

authority/provisioner/scep.go

@@ -18,13 +18,20 @@ type SCEP struct {
 	ForceCN           bool     `json:"forceCN,omitempty"`
 	ChallengePassword string   `json:"challenge,omitempty"`
 	Capabilities      []string `json:"capabilities,omitempty"`
+	// IncludeRoots makes the provisioner return the CA root(s) in the GetCACerts response
+	IncludeRoots bool `json:"includeRoots,omitempty"`
 	// MinimumPublicKeyLength is the minimum length for public keys in CSRs
-	MinimumPublicKeyLength int      `json:"minimumPublicKeyLength,omitempty"`
-	Options                *Options `json:"options,omitempty"`
-	Claims                 *Claims  `json:"claims,omitempty"`
-	claimer                *Claimer
+	MinimumPublicKeyLength int `json:"minimumPublicKeyLength,omitempty"`
+	// 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 2, being AES-256-CBC
+	EncryptionAlgorithmIdentifier *int     `json:"encryptionAlgorithmIdentifier,omitempty"`
+	Options                       *Options `json:"options,omitempty"`
+	Claims                        *Claims  `json:"claims,omitempty"`
+	claimer                       *Claimer
 
 	secretChallengePassword string
+	encryptionAlgorithm     int
 }
 
 // GetID returns the provisioner unique identifier.
@@ -100,6 +107,15 @@ func (s *SCEP) Init(config Config) (err error) {
 		return errors.Errorf("only minimum public keys exactly divisible by 8 are supported; %d is not exactly divisible by 8", s.MinimumPublicKeyLength)
 	}
 
+	s.encryptionAlgorithm = 2 // default to AES-256-CBC
+	if s.EncryptionAlgorithmIdentifier != nil {
+		value := *s.EncryptionAlgorithmIdentifier
+		if value < 0 || value > 4 {
+			return errors.Errorf("only encryption algorithm identifiers from 0 to 4 are valid")
+		}
+		s.encryptionAlgorithm = value
+	}
+
 	// TODO: add other, SCEP specific, options?
 
 	return err
@@ -129,3 +145,17 @@ func (s *SCEP) GetChallengePassword() string {
 func (s *SCEP) GetCapabilities() []string {
 	return s.Capabilities
 }
+
+// ShouldIncludeRootsInChain indicates if the CA should
+// return its intermediate, which is currently used for
+// both signing and decryption, as well as the other certs
+// in its chain (usually a single root certificate).
+func (s *SCEP) ShouldIncludeRootsInChain() bool {
+	return s.IncludeRoots
+}
+
+// GetContentEncryptionAlgorithm returns the numeric identifier
+// for the pkcs7 package encryption algorithm to use.
+func (s *SCEP) GetContentEncryptionAlgorithm() int {
+	return s.encryptionAlgorithm
+}

ca/ca.go

@@ -417,11 +417,6 @@ func (ca *CA) getTLSConfig(auth *authority.Authority) (*tls.Config, error) {
 		}
 	}
 
-	certPool := x509.NewCertPool()
-	for _, crt := range auth.GetRootCertificates() {
-		certPool.AddCert(crt)
-	}
-
 	// GetCertificate will only be called if the client supplies SNI
 	// information or if tlsConfig.Certificates is empty.
 	// When client requests are made using an IP address (as opposed to a domain
@@ -432,6 +427,23 @@ func (ca *CA) getTLSConfig(auth *authority.Authority) (*tls.Config, error) {
 	tlsConfig.Certificates = []tls.Certificate{}
 	tlsConfig.GetCertificate = ca.renewer.GetCertificateForCA
 
+	certPool := x509.NewCertPool()
+	for _, crt := range auth.GetRootCertificates() {
+		certPool.AddCert(crt)
+	}
+
+	// adding the intermediate CA to the pool will allow clients that
+	// fail to send the intermediate for chain building to connect to the CA
+	// successfully.
+	shouldAddIntermediateToClientCAPool := true // TODO(hs): make this into a configuration
+	if shouldAddIntermediateToClientCAPool {
+		cert, err := auth.GetIntermediateCertificate()
+		if err != nil {
+			return nil, err
+		}
+		certPool.AddCert(cert)
+	}
+
 	// Add support for mutual tls to renew certificates
 	tlsConfig.ClientAuth = tls.VerifyClientCertIfGiven
 	tlsConfig.ClientCAs = certPool

scep/api/api.go

@@ -212,7 +212,7 @@ func (h *Handler) lookupProvisioner(next nextHTTP) nextHTTP {
 // GetCACert returns the CA certificates in a SCEP response
 func (h *Handler) GetCACert(ctx context.Context) (SCEPResponse, error) {
 
-	certs, err := h.Auth.GetCACertificates()
+	certs, err := h.Auth.GetCACertificates(ctx)
 	if err != nil {
 		return SCEPResponse{}, err
 	}
@@ -289,20 +289,29 @@ func (h *Handler) PKIOperation(ctx context.Context, request SCEPRequest) (SCEPRe
 	// NOTE: at this point we have sufficient information for returning nicely signed CertReps
 	csr := msg.CSRReqMessage.CSR
 
-	if msg.MessageType == microscep.PKCSReq {
-
+	// NOTE: we're blocking the RenewalReq if the challenge does not match, because otherwise we don't have any authentication.
+	// The macOS SCEP client performs renewals using PKCSreq. The CertNanny SCEP client will use PKCSreq with challenge too, it seems,
+	// even if using the renewal flow as described in the README.md. MicroMDM SCEP client also only does PKCSreq by default, unless
+	// a certificate exists; then it will use RenewalReq. Adding the challenge check here may be a small breaking change for clients.
+	// We'll have to see how it works out.
+	if msg.MessageType == microscep.PKCSReq || msg.MessageType == microscep.RenewalReq {
 		challengeMatches, err := h.Auth.MatchChallengePassword(ctx, msg.CSRReqMessage.ChallengePassword)
 		if err != nil {
 			return h.createFailureResponse(ctx, csr, msg, microscep.BadRequest, errors.New("error when checking password"))
 		}
-
 		if !challengeMatches {
 			// TODO: can this be returned safely to the client? In the end, if the password was correct, that gains a bit of info too.
 			return h.createFailureResponse(ctx, csr, msg, microscep.BadRequest, errors.New("wrong password provided"))
 		}
 	}
 
-	// TODO: check if CN already exists, if renewal is allowed and if existing should be revoked; fail if not
+	// TODO: authorize renewal: we can authorize renewals with the challenge password (if reusable secrets are used).
+	// Renewals OPTIONALLY include the challenge if the existing cert is used as authentication, but client SHOULD omit the challenge.
+	// This means that for renewal requests we should check the certificate provided to be signed before by the CA. We could
+	// enforce use of the challenge if we want too. That way we could be more flexible in terms of authentication scheme (i.e. reusing
+	// tokens from other provisioners, calling a webhook, storing multiple secrets, allowing them to be multi-use, etc).
+	// Authentication by the (self-signed) certificate with an optional challenge is required; supporting renewals incl. verification
+	// of the client cert is not.
 
 	certRep, err := h.Auth.SignCSR(ctx, csr, msg)
 	if err != nil {

scep/authority.go

@@ -23,7 +23,7 @@ type Interface interface {
 	LoadProvisionerByID(string) (provisioner.Interface, error)
 	GetLinkExplicit(provName string, absoluteLink bool, baseURL *url.URL, inputs ...string) string
 
-	GetCACertificates() ([]*x509.Certificate, error)
+	GetCACertificates(ctx context.Context) ([]*x509.Certificate, error)
 	DecryptPKIEnvelope(ctx context.Context, msg *PKIMessage) error
 	SignCSR(ctx context.Context, csr *x509.CertificateRequest, msg *PKIMessage) (*PKIMessage, error)
 	CreateFailureResponse(ctx context.Context, csr *x509.CertificateRequest, msg *PKIMessage, info FailInfoName, infoText string) (*PKIMessage, error)
@@ -36,6 +36,7 @@ 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
 }
@@ -72,6 +73,8 @@ func New(signAuth SignAuthority, ops AuthorityOptions) (*Authority, error) {
 	// 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
 	}
@@ -82,7 +85,7 @@ func New(signAuth SignAuthority, ops AuthorityOptions) (*Authority, error) {
 var (
 	// TODO: check the default capabilities; https://tools.ietf.org/html/rfc8894#section-3.5.2
 	defaultCapabilities = []string{
-		"Renewal",
+		"Renewal", // NOTE: removing this will result in macOS SCEP client stating the server doesn't support renewal, but it uses PKCSreq to do so.
 		"SHA-1",
 		"SHA-256",
 		"AES",
@@ -100,18 +103,13 @@ func (a *Authority) LoadProvisionerByID(id string) (provisioner.Interface, error
 
 // GetLinkExplicit returns the requested link from the directory.
 func (a *Authority) GetLinkExplicit(provName string, abs bool, baseURL *url.URL, inputs ...string) string {
-	// TODO: taken from ACME; move it to directory (if we need a directory in SCEP)?
 	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, inputs ...string) string {
-
-	// TODO: do we need to provide a way to provide a different suffix?
-	// Like "/cgi-bin/pkiclient.exe"? Or would it be enough to have that as the name?
 	link := "/" + provisionerName
-
 	if abs {
 		// Copy the baseURL value from the pointer. https://github.com/golang/go/issues/38351
 		u := url.URL{}
@@ -137,7 +135,7 @@ func (a *Authority) getLinkExplicit(provisionerName string, abs bool, baseURL *u
 }
 
 // GetCACertificates returns the certificate (chain) for the CA
-func (a *Authority) GetCACertificates() ([]*x509.Certificate, error) {
+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
 	// Some clients do need the root certificate however; also see: https://github.com/openxpki/openxpki/issues/73
@@ -153,14 +151,27 @@ func (a *Authority) GetCACertificates() ([]*x509.Certificate, error) {
 	// 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
+	// The certificate to use should probably depend on the (configured) provisioner and may
 	// use a distinct certificate, apart from the intermediate.
 
-	if a.intermediateCertificate == nil {
+	p, err := provisionerFromContext(ctx)
+	if err != nil {
+		return nil, err
+	}
+
+	if len(a.caCerts) == 0 {
 		return nil, errors.New("no intermediate certificate available in SCEP authority")
 	}
 
-	return []*x509.Certificate{a.intermediateCertificate}, nil
+	certs := []*x509.Certificate{}
+	certs = append(certs, a.caCerts[0])
+
+	// TODO(hs): we're adding the roots here, but they may be something different than what the RFC means. Clients are responsible to select the right cert(s) to use, though.
+	if p.ShouldIncludeRootsInChain() && len(a.caCerts) >= 2 {
+		certs = append(certs, a.caCerts[1:]...)
+	}
+
+	return certs, nil
 }
 
 // DecryptPKIEnvelope decrypts an enveloped message
@@ -211,8 +222,6 @@ func (a *Authority) DecryptPKIEnvelope(ctx context.Context, msg *PKIMessage) err
 
 // SignCSR creates an x509.Certificate based on a CSR template and Cert Authority credentials
 // returns a new PKIMessage with CertRep data
-//func (msg *PKIMessage) SignCSR(crtAuth *x509.Certificate, keyAuth *rsa.PrivateKey, template *x509.Certificate) (*PKIMessage, error) {
-//func (a *Authority) SignCSR(ctx context.Context, msg *PKIMessage, template *x509.Certificate) (*PKIMessage, error) {
 func (a *Authority) SignCSR(ctx context.Context, csr *x509.CertificateRequest, msg *PKIMessage) (*PKIMessage, error) {
 
 	// TODO: intermediate storage of the request? In SCEP it's possible to request a csr/certificate
@@ -220,7 +229,7 @@ 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)
+	p, err := provisionerFromContext(ctx)
 	if err != nil {
 		return nil, err
 	}
@@ -292,10 +301,17 @@ func (a *Authority) SignCSR(ctx context.Context, csr *x509.CertificateRequest, m
 		return nil, err
 	}
 
+	// apparently the pkcs7 library uses a global default setting for the content encryption
+	// algorithm to use when en- or decrypting data. We need to restore the current setting after
+	// the cryptographic operation, so that other usages of the library are not influenced by
+	// this call to Encrypt().
+	encryptionAlgorithmToRestore := pkcs7.ContentEncryptionAlgorithm
+	pkcs7.ContentEncryptionAlgorithm = p.GetContentEncryptionAlgorithm()
 	e7, err := pkcs7.Encrypt(deg, msg.P7.Certificates)
 	if err != nil {
 		return nil, err
 	}
+	pkcs7.ContentEncryptionAlgorithm = encryptionAlgorithmToRestore
 
 	// PKIMessageAttributes to be signed
 	config := pkcs7.SignerInfoConfig{
@@ -434,7 +450,7 @@ func (a *Authority) CreateFailureResponse(ctx context.Context, csr *x509.Certifi
 // MatchChallengePassword verifies a SCEP challenge password
 func (a *Authority) MatchChallengePassword(ctx context.Context, password string) (bool, error) {
 
-	p, err := ProvisionerFromContext(ctx)
+	p, err := provisionerFromContext(ctx)
 	if err != nil {
 		return false, err
 	}
@@ -453,7 +469,7 @@ func (a *Authority) MatchChallengePassword(ctx context.Context, password string)
 // GetCACaps returns the CA capabilities
 func (a *Authority) GetCACaps(ctx context.Context) []string {
 
-	p, err := ProvisionerFromContext(ctx)
+	p, err := provisionerFromContext(ctx)
 	if err != nil {
 		return defaultCapabilities
 	}

scep/common.go

@@ -14,9 +14,9 @@ const (
 	ProvisionerContextKey = ContextKey("provisioner")
 )
 
-// ProvisionerFromContext searches the context for a SCEP provisioner.
+// provisionerFromContext searches the context for a SCEP provisioner.
 // Returns the provisioner or an error.
-func ProvisionerFromContext(ctx context.Context) (Provisioner, error) {
+func provisionerFromContext(ctx context.Context) (Provisioner, error) {
 	val := ctx.Value(ProvisionerContextKey)
 	if val == nil {
 		return nil, errors.New("provisioner expected in request context")

scep/provisioner.go

@@ -16,4 +16,6 @@ type Provisioner interface {
 	GetOptions() *provisioner.Options
 	GetChallengePassword() string
 	GetCapabilities() []string
+	ShouldIncludeRootsInChain() bool
+	GetContentEncryptionAlgorithm() int
 }