Add a custom enforcer that can be used to modify a cert.

authority/authority.go

@@ -50,6 +50,7 @@ type Authority struct {
 	rootX509CertPool   *x509.CertPool
 	federatedX509Certs []*x509.Certificate
 	certificates       *sync.Map
+	x509Enforcers      []provisioner.CertificateEnforcer
 
 	// SCEP CA
 	scepService *scep.Service

authority/options.go

@@ -241,6 +241,15 @@ func WithLinkedCAToken(token string) Option {
 	}
 }
 
+// WithX509Enforcers is an option that allows to define custom certificate
+// modifiers that will be processed just before the signing of the certificate.
+func WithX509Enforcers(ces ...provisioner.CertificateEnforcer) Option {
+	return func(a *Authority) error {
+		a.x509Enforcers = ces
+		return nil
+	}
+}
+
 func readCertificateBundle(pemCerts []byte) ([]*x509.Certificate, error) {
 	var block *pem.Block
 	var certs []*x509.Certificate

authority/tls.go

@@ -180,6 +180,17 @@ func (a *Authority) Sign(csr *x509.CertificateRequest, signOpts provisioner.Sign
 		}
 	}
 
+	// Process injected modifiers after validation
+	for _, m := range a.x509Enforcers {
+		if err := m.Enforce(leaf); err != nil {
+			return nil, errs.ApplyOptions(
+				errs.ForbiddenErr(err, "error creating certificate"),
+				opts...,
+			)
+		}
+	}
+
+	// Sign certificate
 	lifetime := leaf.NotAfter.Sub(leaf.NotBefore.Add(signOpts.Backdate))
 	resp, err := a.x509CAService.CreateCertificate(&casapi.CreateCertificateRequest{
 		Template: leaf,

authority/tls_test.go

@@ -205,6 +205,17 @@ type basicConstraints struct {
 	MaxPathLen int  `asn1:"optional,default:-1"`
 }
 
+type testEnforcer struct {
+	enforcer func(*x509.Certificate) error
+}
+
+func (e *testEnforcer) Enforce(cert *x509.Certificate) error {
+	if e.enforcer != nil {
+		return e.enforcer(cert)
+	}
+	return nil
+}
+
 func TestAuthority_Sign(t *testing.T) {
 	pub, priv, err := keyutil.GenerateDefaultKeyPair()
 	assert.FatalError(t, err)
@@ -244,6 +255,7 @@ func TestAuthority_Sign(t *testing.T) {
 		extraOpts       []provisioner.SignOption
 		notBefore       time.Time
 		notAfter        time.Time
+		extensionsCount int
 		err             error
 		code            int
 	}
@@ -454,6 +466,49 @@ ZYtQ9Ot36qc=
 				code:      http.StatusInternalServerError,
 			}
 		},
+		"fail with provisioner enforcer": func(t *testing.T) *signTest {
+			csr := getCSR(t, priv)
+			aa := testAuthority(t)
+			aa.db = &db.MockAuthDB{
+				MStoreCertificate: func(crt *x509.Certificate) error {
+					assert.Equals(t, crt.Subject.CommonName, "smallstep test")
+					return nil
+				},
+			}
+
+			return &signTest{
+				auth: aa,
+				csr:  csr,
+				extraOpts: append(extraOpts, &testEnforcer{
+					enforcer: func(crt *x509.Certificate) error { return fmt.Errorf("an error") },
+				}),
+				signOpts: signOpts,
+				err:      errors.New("error creating certificate"),
+				code:     http.StatusForbidden,
+			}
+		},
+		"fail with custom enforcer": func(t *testing.T) *signTest {
+			csr := getCSR(t, priv)
+			aa := testAuthority(t, WithX509Enforcers(&testEnforcer{
+				enforcer: func(cert *x509.Certificate) error {
+					return fmt.Errorf("an error")
+				},
+			}))
+			aa.db = &db.MockAuthDB{
+				MStoreCertificate: func(crt *x509.Certificate) error {
+					assert.Equals(t, crt.Subject.CommonName, "smallstep test")
+					return nil
+				},
+			}
+			return &signTest{
+				auth:      aa,
+				csr:       csr,
+				extraOpts: extraOpts,
+				signOpts:  signOpts,
+				err:       errors.New("error creating certificate"),
+				code:      http.StatusForbidden,
+			}
+		},
 		"ok": func(t *testing.T) *signTest {
 			csr := getCSR(t, priv)
 			_a := testAuthority(t)
@@ -470,6 +525,7 @@ ZYtQ9Ot36qc=
 				signOpts:        signOpts,
 				notBefore:       signOpts.NotBefore.Time().Truncate(time.Second),
 				notAfter:        signOpts.NotAfter.Time().Truncate(time.Second),
+				extensionsCount: 6,
 			}
 		},
 		"ok with enforced modifier": func(t *testing.T) *signTest {
@@ -503,6 +559,7 @@ ZYtQ9Ot36qc=
 				signOpts:        signOpts,
 				notBefore:       now.Truncate(time.Second),
 				notAfter:        now.Add(365 * 24 * time.Hour).Truncate(time.Second),
+				extensionsCount: 6,
 			}
 		},
 		"ok with custom template": func(t *testing.T) *signTest {
@@ -536,6 +593,7 @@ ZYtQ9Ot36qc=
 				signOpts:        signOpts,
 				notBefore:       signOpts.NotBefore.Time().Truncate(time.Second),
 				notAfter:        signOpts.NotAfter.Time().Truncate(time.Second),
+				extensionsCount: 6,
 			}
 		},
 		"ok/csr with no template critical SAN extension": func(t *testing.T) *signTest {
@@ -564,6 +622,33 @@ ZYtQ9Ot36qc=
 				signOpts:        provisioner.SignOptions{},
 				notBefore:       now.Truncate(time.Second),
 				notAfter:        now.Add(365 * 24 * time.Hour).Truncate(time.Second),
+				extensionsCount: 5,
+			}
+		},
+		"ok with custom enforcer": func(t *testing.T) *signTest {
+			csr := getCSR(t, priv)
+			aa := testAuthority(t, WithX509Enforcers(&testEnforcer{
+				enforcer: func(cert *x509.Certificate) error {
+					cert.CRLDistributionPoints = []string{"http://ca.example.org/leaf.crl"}
+					return nil
+				},
+			}))
+			aa.config.AuthorityConfig.Template = a.config.AuthorityConfig.Template
+			aa.db = &db.MockAuthDB{
+				MStoreCertificate: func(crt *x509.Certificate) error {
+					assert.Equals(t, crt.Subject.CommonName, "smallstep test")
+					assert.Equals(t, crt.CRLDistributionPoints, []string{"http://ca.example.org/leaf.crl"})
+					return nil
+				},
+			}
+			return &signTest{
+				auth:            aa,
+				csr:             csr,
+				extraOpts:       extraOpts,
+				signOpts:        signOpts,
+				notBefore:       signOpts.NotBefore.Time().Truncate(time.Second),
+				notAfter:        signOpts.NotAfter.Time().Truncate(time.Second),
+				extensionsCount: 7,
 			}
 		},
 	}
@@ -645,9 +730,6 @@ ZYtQ9Ot36qc=
 								// Empty CSR subject test does not use any provisioner extensions.
 								// So provisioner ID ext will be missing.
 								found = 1
-								assert.Len(t, 5, leaf.Extensions)
-							} else {
-								assert.Len(t, 6, leaf.Extensions)
 							}
 						}
 					}
@@ -655,6 +737,7 @@ ZYtQ9Ot36qc=
 					realIntermediate, err := x509.ParseCertificate(issuer.Raw)
 					assert.FatalError(t, err)
 					assert.Equals(t, intermediate, realIntermediate)
+					assert.Len(t, tc.extensionsCount, leaf.Extensions)
 				}
 			}
 		})