Add certificate chain flag for signing (sigstore#1656)

* Add certificate chain flag for signing

This allows users to pass their own certificate chain to include in the
OCI signature. The chain is checked for validity using the provided
certificate.

Also refactored the check for matching public keys using a method from
sigstore/sigstore, comparing the certificate's key with the provided
key. Also added this check when extracting the PKCS11 certificate.

Certificate chains must be PEM-encoded. I changed the text of the
certificate flag to also specify a preference for PEM encoding, but
didn't remove the code that handles DER encoding for backwards
compatibility.

Signed-off-by: Hayden Blauzvern <hblauzvern@google.com>

* Adding 3rd party licenses

Signed-off-by: Hayden Blauzvern <hblauzvern@google.com>

* Added check for empty chain

Signed-off-by: Hayden Blauzvern <hblauzvern@google.com>

cmd/cosign/cli/attest.go

@@ -51,6 +51,9 @@ func Attest() *cobra.Command {
   # attach an attestation to a container image with a key pair stored in Hashicorp Vault
   cosign attest --predicate <FILE> --type <TYPE> --key hashivault://[KEY] <IMAGE>
 
+  # attach an attestation to a container image with a local key pair file, including a certificate and certificate chain
+  cosign attest --predicate <FILE> --type <TYPE> --key cosign.key --cert cosign.crt --cert-chain chain.crt <IMAGE>
+
   # attach an attestation to a container image which does not fully support OCI media types
   COSIGN_DOCKER_MEDIA_TYPES=1 cosign attest --predicate <FILE> --type <TYPE> --key cosign.key legacy-registry.example.com/my/image`,
 
@@ -70,7 +73,7 @@ func Attest() *cobra.Command {
 				OIDCClientSecret:         o.OIDC.ClientSecret,
 			}
 			for _, img := range args {
-				if err := attest.AttestCmd(cmd.Context(), ko, o.Registry, img, o.Cert, o.NoUpload,
+				if err := attest.AttestCmd(cmd.Context(), ko, o.Registry, img, o.Cert, o.CertChain, o.NoUpload,
 					o.Predicate.Path, o.Force, o.Predicate.Type, o.Replace, ro.Timeout); err != nil {
 					return errors.Wrapf(err, "signing %s", img)
 				}

cmd/cosign/cli/attest/attest.go

@@ -74,7 +74,7 @@ func uploadToTlog(ctx context.Context, sv *sign.SignerVerifier, rekorURL string,
 }
 
 //nolint
-func AttestCmd(ctx context.Context, ko sign.KeyOpts, regOpts options.RegistryOptions, imageRef string, certPath string,
+func AttestCmd(ctx context.Context, ko sign.KeyOpts, regOpts options.RegistryOptions, imageRef string, certPath string, certChainPath string,
 	noUpload bool, predicatePath string, force bool, predicateType string, replace bool, timeout time.Duration) error {
 	// A key file or token is required unless we're in experimental mode!
 	if options.EnableExperimental() {
@@ -117,7 +117,7 @@ func AttestCmd(ctx context.Context, ko sign.KeyOpts, regOpts options.RegistryOpt
 	// each access.
 	ref = digest // nolint
 
-	sv, err := sign.SignerFromKeyOpts(ctx, certPath, ko)
+	sv, err := sign.SignerFromKeyOpts(ctx, certPath, certChainPath, ko)
 	if err != nil {
 		return errors.Wrap(err, "getting signer")
 	}

cmd/cosign/cli/options/attest.go

@@ -23,6 +23,7 @@ import (
 type AttestOptions struct {
 	Key       string
 	Cert      string
+	CertChain string
 	NoUpload  bool
 	Force     bool
 	Recursive bool
@@ -51,7 +52,13 @@ func (o *AttestOptions) AddFlags(cmd *cobra.Command) {
 		"path to the private key file, KMS URI or Kubernetes Secret")
 
 	cmd.Flags().StringVar(&o.Cert, "cert", "",
-		"path to the x509 certificate to include in the Signature")
+		"path to the X.509 certificate in PEM format to include in the OCI Signature")
+
+	cmd.Flags().StringVar(&o.CertChain, "cert-chain", "",
+		"path to a list of CA X.509 certificates in PEM format which will be needed "+
+			"when building the certificate chain for the signing certificate. "+
+			"Must start with the parent intermediate CA certificate of the "+
+			"signing certificate and end with the root certificate. Included in the OCI Signature")
 
 	cmd.Flags().BoolVar(&o.NoUpload, "no-upload", false,
 		"do not upload the generated attestation")

cmd/cosign/cli/options/sign.go

@@ -23,6 +23,7 @@ import (
 type SignOptions struct {
 	Key               string
 	Cert              string
+	CertChain         string
 	Upload            bool
 	Output            string // deprecated: TODO remove when the output flag is fully deprecated
 	OutputSignature   string // TODO: this should be the root output file arg.
@@ -55,7 +56,13 @@ func (o *SignOptions) AddFlags(cmd *cobra.Command) {
 		"path to the private key file, KMS URI or Kubernetes Secret")
 
 	cmd.Flags().StringVar(&o.Cert, "cert", "",
-		"path to the x509 certificate to include in the Signature")
+		"path to the X.509 certificate in PEM format to include in the OCI Signature")
+
+	cmd.Flags().StringVar(&o.CertChain, "cert-chain", "",
+		"path to a list of CA X.509 certificates in PEM format which will be needed "+
+			"when building the certificate chain for the signing certificate. "+
+			"Must start with the parent intermediate CA certificate of the "+
+			"signing certificate and end with the root certificate. Included in the OCI Signature")
 
 	cmd.Flags().BoolVar(&o.Upload, "upload", true,
 		"whether to upload the signature")

cmd/cosign/cli/policy_init.go

@@ -175,7 +175,7 @@ func signPolicy() *cobra.Command {
 			}
 
 			// Get Fulcio signer
-			sv, err := sign.SignerFromKeyOpts(ctx, "", sign.KeyOpts{
+			sv, err := sign.SignerFromKeyOpts(ctx, "", "", sign.KeyOpts{
 				FulcioURL:                o.Fulcio.URL,
 				IDToken:                  o.Fulcio.IdentityToken,
 				InsecureSkipFulcioVerify: o.Fulcio.InsecureSkipFulcioVerify,

cmd/cosign/cli/sign.go

@@ -62,6 +62,9 @@ func Sign() *cobra.Command {
   # sign a container image with a key pair stored in a Kubernetes secret
   cosign sign --key k8s://[NAMESPACE]/[KEY] <IMAGE>
 
+  # sign a container image with a key, attaching a certificate and certificate chain
+  cosign sign --key cosign.key --cert cosign.crt --cert-chain chain.crt <IMAGE>
+
   # sign a container in a registry which does not fully support OCI media types
   COSIGN_DOCKER_MEDIA_TYPES=1 cosign sign --key cosign.key legacy-registry.example.com/my/image`,
 		Args: cobra.MinimumNArgs(1),
@@ -89,7 +92,8 @@ func Sign() *cobra.Command {
 			if err != nil {
 				return err
 			}
-			if err := sign.SignCmd(ro, ko, o.Registry, annotationsMap.Annotations, args, o.Cert, o.Upload, o.OutputSignature, o.OutputCertificate, o.PayloadPath, o.Force, o.Recursive, o.Attachment); err != nil {
+			if err := sign.SignCmd(ro, ko, o.Registry, annotationsMap.Annotations, args, o.Cert, o.CertChain, o.Upload,
+				o.OutputSignature, o.OutputCertificate, o.PayloadPath, o.Force, o.Recursive, o.Attachment); err != nil {
 				if o.Attachment == "" {
 					return errors.Wrapf(err, "signing %v", args)
 				}

cmd/cosign/cli/sign/sign.go

@@ -18,8 +18,6 @@ package sign
 import (
 	"bytes"
 	"context"
-	"crypto/ecdsa"
-	"crypto/rsa"
 	"crypto/x509"
 	"encoding/pem"
 	"fmt"
@@ -95,7 +93,8 @@ func GetAttachedImageRef(ref name.Reference, attachment string, opts ...ociremot
 
 // nolint
 func SignCmd(ro *options.RootOptions, ko KeyOpts, regOpts options.RegistryOptions, annotations map[string]interface{},
-	imgs []string, certPath string, upload bool, outputSignature, outputCertificate string, payloadPath string, force bool, recursive bool, attachment string) error {
+	imgs []string, certPath string, certChainPath string, upload bool, outputSignature, outputCertificate string,
+	payloadPath string, force bool, recursive bool, attachment string) error {
 	if options.EnableExperimental() {
 		if options.NOf(ko.KeyRef, ko.Sk) > 1 {
 			return &options.KeyParseError{}
@@ -109,7 +108,7 @@ func SignCmd(ro *options.RootOptions, ko KeyOpts, regOpts options.RegistryOption
 	ctx, cancel := context.WithTimeout(context.Background(), ro.Timeout)
 	defer cancel()
 
-	sv, err := SignerFromKeyOpts(ctx, certPath, ko)
+	sv, err := SignerFromKeyOpts(ctx, certPath, certChainPath, ko)
 	if err != nil {
 		return errors.Wrap(err, "getting signer")
 	}
@@ -310,29 +309,40 @@ func signerFromSecurityKey(keySlot string) (*SignerVerifier, error) {
 	}, nil
 }
 
-func signerFromKeyRef(ctx context.Context, certPath, keyRef string, passFunc cosign.PassFunc) (*SignerVerifier, error) {
+func signerFromKeyRef(ctx context.Context, certPath, certChainPath, keyRef string, passFunc cosign.PassFunc) (*SignerVerifier, error) {
 	k, err := sigs.SignerVerifierFromKeyRef(ctx, keyRef, passFunc)
 	if err != nil {
 		return nil, errors.Wrap(err, "reading key")
 	}
 
-	// Handle the -cert flag
+	// Attempt to extract certificate from PKCS11 token
 	// With PKCS11, we assume the certificate is in the same slot on the PKCS11
 	// token as the private key. If it's not there, show a warning to the
 	// user.
 	if pkcs11Key, ok := k.(*pkcs11key.Key); ok {
 		certFromPKCS11, _ := pkcs11Key.Certificate()
-		var pemBytes []byte
 		if certFromPKCS11 == nil {
 			fmt.Fprintln(os.Stderr, "warning: no x509 certificate retrieved from the PKCS11 token")
-		} else {
-			pemBytes, err = cryptoutils.MarshalCertificateToPEM(certFromPKCS11)
-			if err != nil {
-				pkcs11Key.Close()
-				return nil, err
-			}
+			return &SignerVerifier{
+				SignerVerifier: k,
+				close:          pkcs11Key.Close,
+			}, nil
+		}
+		pemBytes, err := cryptoutils.MarshalCertificateToPEM(certFromPKCS11)
+		if err != nil {
+			pkcs11Key.Close()
+			return nil, err
+		}
+		// Check that provided public key and certificate key match
+		pubKey, err := k.PublicKey()
+		if err != nil {
+			pkcs11Key.Close()
+			return nil, err
+		}
+		if cryptoutils.EqualKeys(pubKey, certFromPKCS11.PublicKey) != nil {
+			pkcs11Key.Close()
+			return nil, errors.New("pkcs11 key and certificate do not match")
 		}
-
 		return &SignerVerifier{
 			Cert:           pemBytes,
 			SignerVerifier: k,
@@ -342,15 +352,18 @@ func signerFromKeyRef(ctx context.Context, certPath, keyRef string, passFunc cos
 	certSigner := &SignerVerifier{
 		SignerVerifier: k,
 	}
+
 	if certPath == "" {
 		return certSigner, nil
 	}
 
+	// Handle --cert flag
+	// Allow both DER and PEM encoding
 	certBytes, err := os.ReadFile(certPath)
 	if err != nil {
 		return nil, errors.Wrap(err, "read certificate")
 	}
-	// Handle PEM.
+	// Handle PEM
 	if bytes.HasPrefix(certBytes, []byte("-----")) {
 		decoded, _ := pem.Decode(certBytes)
 		if decoded.Type != "CERTIFICATE" {
@@ -366,23 +379,44 @@ func signerFromKeyRef(ctx context.Context, certPath, keyRef string, passFunc cos
 	if err != nil {
 		return nil, errors.Wrap(err, "get public key")
 	}
-	switch kt := parsedCert.PublicKey.(type) {
-	case *ecdsa.PublicKey:
-		if !kt.Equal(pk) {
-			return nil, errors.New("public key in certificate does not match that in the signing key")
-		}
-	case *rsa.PublicKey:
-		if !kt.Equal(pk) {
-			return nil, errors.New("public key in certificate does not match that in the signing key")
-		}
-	default:
-		return nil, fmt.Errorf("unsupported key type: %T", parsedCert.PublicKey)
+	if cryptoutils.EqualKeys(pk, parsedCert.PublicKey) != nil {
+		return nil, errors.New("public key in certificate does not match the provided public key")
 	}
 	pemBytes, err := cryptoutils.MarshalCertificateToPEM(parsedCert)
 	if err != nil {
 		return nil, errors.Wrap(err, "marshaling certificate to PEM")
 	}
 	certSigner.Cert = pemBytes
+
+	if certChainPath == "" {
+		return certSigner, nil
+	}
+
+	// Handle --cert-chain flag
+	// Accept only PEM encoded certificate chain
+	certChainBytes, err := os.ReadFile(certChainPath)
+	if err != nil {
+		return nil, errors.Wrap(err, "reading certificate chain from path")
+	}
+	certChain, err := cryptoutils.LoadCertificatesFromPEM(bytes.NewReader(certChainBytes))
+	if err != nil {
+		return nil, errors.Wrap(err, "loading certificate chain")
+	}
+	if len(certChain) == 0 {
+		return nil, errors.New("no certificates in certificate chain")
+	}
+	// Verify certificate chain is valid
+	rootPool := x509.NewCertPool()
+	rootPool.AddCert(certChain[len(certChain)-1])
+	subPool := x509.NewCertPool()
+	for _, c := range certChain[:len(certChain)-1] {
+		subPool.AddCert(c)
+	}
+	if err := cosign.TrustedCert(parsedCert, rootPool, subPool); err != nil {
+		return nil, errors.Wrap(err, "unable to validate certificate chain")
+	}
+	certSigner.Chain = certChainBytes
+
 	return certSigner, nil
 }
 
@@ -418,13 +452,13 @@ func keylessSigner(ctx context.Context, ko KeyOpts) (*SignerVerifier, error) {
 	}, nil
 }
 
-func SignerFromKeyOpts(ctx context.Context, certPath string, ko KeyOpts) (*SignerVerifier, error) {
+func SignerFromKeyOpts(ctx context.Context, certPath string, certChainPath string, ko KeyOpts) (*SignerVerifier, error) {
 	if ko.Sk {
 		return signerFromSecurityKey(ko.Slot)
 	}
 
 	if ko.KeyRef != "" {
-		return signerFromKeyRef(ctx, certPath, ko.KeyRef, ko.PassFunc)
+		return signerFromKeyRef(ctx, certPath, certChainPath, ko.KeyRef, ko.PassFunc)
 	}
 
 	// Default Keyless!

cmd/cosign/cli/sign/sign_blob.go

@@ -71,7 +71,7 @@ func SignBlobCmd(ro *options.RootOptions, ko KeyOpts, regOpts options.RegistryOp
 	ctx, cancel := context.WithTimeout(context.Background(), ro.Timeout)
 	defer cancel()
 
-	sv, err := SignerFromKeyOpts(ctx, "", ko)
+	sv, err := SignerFromKeyOpts(ctx, "", "", ko)
 	if err != nil {
 		return nil, err
 	}