Load in intermediate cert pool from TUF (sigstore#1804)

With the v3 TUF root, the intermediate CA certificate will be included,
so that if the intermediate signing key was compromised, the
intermediate certificate could be revoked by removing it from the TUF
targets and replacing it with a trusted certificate.

This change loads the intermediate certificate from TUF. However, we
don't want to force all users to follow this structure - They may choose
to use CRLs to detect revoked intermediates. Also, I don't want to
enforce TUF usage in the Verify package. Therefore, for TUF, we lazily create
a certificate pool only if an intermediate certificate is found, and if
it's not found, then VerifyImageSignature will create a pool using the
chain provided in the annotation.

Signed-off-by: Hayden Blauzvern <[email protected]>

cmd/cosign/cli/fulcio/fulcioroots/fulcioroots.go

@@ -83,8 +83,8 @@ func GetIntermediates() *x509.CertPool {
 }
 
 func initRoots() (*x509.CertPool, *x509.CertPool, error) {
-	rootPool := x509.NewCertPool()
-	intermediatePool := x509.NewCertPool()
+	var rootPool *x509.CertPool
+	var intermediatePool *x509.CertPool
 
 	rootEnv := os.Getenv(altRoot)
 	if rootEnv != "" {
@@ -99,8 +99,14 @@ func initRoots() (*x509.CertPool, *x509.CertPool, error) {
 		for _, cert := range certs {
 			// root certificates are self-signed
 			if bytes.Equal(cert.RawSubject, cert.RawIssuer) {
+				if rootPool == nil {
+					rootPool = x509.NewCertPool()
+				}
 				rootPool.AddCert(cert)
 			} else {
+				if intermediatePool == nil {
+					intermediatePool = x509.NewCertPool()
+				}
 				intermediatePool.AddCert(cert)
 			}
 		}
@@ -127,12 +133,21 @@ func initRoots() (*x509.CertPool, *x509.CertPool, error) {
 			for _, cert := range certs {
 				// root certificates are self-signed
 				if bytes.Equal(cert.RawSubject, cert.RawIssuer) {
+					if rootPool == nil {
+						rootPool = x509.NewCertPool()
+					}
 					rootPool.AddCert(cert)
 				} else {
+					if intermediatePool == nil {
+						intermediatePool = x509.NewCertPool()
+					}
 					intermediatePool.AddCert(cert)
 				}
 			}
 		}
+		if intermediatePool == nil {
+			intermediatePool = x509.NewCertPool()
+		}
 		intermediatePool.AppendCertsFromPEM([]byte(fulcioIntermediateV1))
 	}
 	return rootPool, intermediatePool, nil

cmd/cosign/cli/verify/verify.go

@@ -111,6 +111,7 @@ func (c *VerifyCommand) Exec(ctx context.Context, images []string) (err error) {
 			co.RekorClient = rekorClient
 		}
 		co.RootCerts = fulcio.GetRoots()
+		co.IntermediateCerts = fulcio.GetIntermediates()
 	}
 	keyRef := c.KeyRef
 	certRef := c.CertRef

cmd/cosign/cli/verify/verify_attestation.go

@@ -92,6 +92,7 @@ func (c *VerifyAttestationCommand) Exec(ctx context.Context, images []string) (e
 			co.RekorClient = rekorClient
 		}
 		co.RootCerts = fulcio.GetRoots()
+		co.IntermediateCerts = fulcio.GetIntermediates()
 	}
 	keyRef := c.KeyRef
 

pkg/cosign/verify.go

@@ -475,7 +475,8 @@ func VerifyImageSignature(ctx context.Context, sig oci.Signature, h v1.Hash, co
 		// If the chain annotation is not present or there is only a root
 		if chain == nil || len(chain) <= 1 {
 			co.IntermediateCerts = nil
-		} else {
+		} else if co.IntermediateCerts == nil {
+			// If the intermediate certs have not been loaded in by TUF
 			pool := x509.NewCertPool()
 			for _, cert := range chain[:len(chain)-1] {
 				pool.AddCert(cert)
@@ -653,7 +654,8 @@ func verifyImageAttestations(ctx context.Context, atts oci.Signatures, h v1.Hash
 				// If the chain annotation is not present or there is only a root
 				if chain == nil || len(chain) <= 1 {
 					co.IntermediateCerts = nil
-				} else {
+				} else if co.IntermediateCerts == nil {
+					// If the intermediate certs have not been loaded in by TUF
 					pool := x509.NewCertPool()
 					for _, cert := range chain[:len(chain)-1] {
 						pool.AddCert(cert)

pkg/cosign/verify_test.go

@@ -309,6 +309,42 @@ func TestVerifyImageSignatureWithMissingSub(t *testing.T) {
 	}
 }
 
+func TestVerifyImageSignatureWithExistingSub(t *testing.T) {
+	rootCert, rootKey, _ := test.GenerateRootCa()
+	subCert, subKey, _ := test.GenerateSubordinateCa(rootCert, rootKey)
+	leafCert, privKey, _ := test.GenerateLeafCert("subject", "oidc-issuer", subCert, subKey)
+	pemRoot := pem.EncodeToMemory(&pem.Block{Type: "CERTIFICATE", Bytes: rootCert.Raw})
+	pemSub := pem.EncodeToMemory(&pem.Block{Type: "CERTIFICATE", Bytes: subCert.Raw})
+	pemLeaf := pem.EncodeToMemory(&pem.Block{Type: "CERTIFICATE", Bytes: leafCert.Raw})
+
+	otherSubCert, _, _ := test.GenerateSubordinateCa(rootCert, rootKey)
+
+	rootPool := x509.NewCertPool()
+	rootPool.AddCert(rootCert)
+	subPool := x509.NewCertPool()
+	// Load in different sub cert so the chain doesn't verify
+	rootPool.AddCert(otherSubCert)
+
+	payload := []byte{1, 2, 3, 4}
+	h := sha256.Sum256(payload)
+	signature, _ := privKey.Sign(rand.Reader, h[:], crypto.SHA256)
+
+	ociSig, _ := static.NewSignature(payload,
+		base64.StdEncoding.EncodeToString(signature),
+		static.WithCertChain(pemLeaf, appendSlices([][]byte{pemSub, pemRoot})))
+	verified, err := VerifyImageSignature(context.TODO(), ociSig, v1.Hash{}, &CheckOpts{RootCerts: rootPool, IntermediateCerts: subPool})
+	if err == nil {
+		t.Fatal("expected error while verifying signature")
+	}
+	if !strings.Contains(err.Error(), "certificate signed by unknown authority") {
+		t.Fatal("expected error while verifying signature")
+	}
+	// TODO: Create fake bundle and test verification
+	if verified == true {
+		t.Fatalf("expected verified=false, got verified=true")
+	}
+}
+
 func TestValidateAndUnpackCertSuccess(t *testing.T) {
 	subject := "email@email"
 	oidcIssuer := "https://accounts.google.com"

pkg/sget/sget.go

@@ -91,6 +91,7 @@ func (sg *SecureGet) Do(ctx context.Context) error {
 		fulcioVerified := (co.SigVerifier == nil)
 
 		co.RootCerts = fulcio.GetRoots()
+		co.IntermediateCerts = fulcio.GetIntermediates()
 
 		sp, bundleVerified, err := cosign.VerifyImageSignatures(ctx, ref, co)
 		if err != nil {