Add option to verify attestations from local image (#1174)

This is very similar to a previous commit that added
support for verifying signatures from a local image.

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

cmd/cosign/cli/options/verify.go

@@ -70,7 +70,7 @@ func (o *VerifyOptions) AddFlags(cmd *cobra.Command) {
 		"signature content or path or remote URL")
 
 	cmd.Flags().BoolVar(&o.LocalImage, "local-image", false,
-		"whether the path to the image is a local directory")
+		"whether the specified image is a path to an image saved locally via 'cosign save'")
 }
 
 // VerifyAttestationOptions is the top level wrapper for the `verify attestation` command.
@@ -85,6 +85,7 @@ type VerifyAttestationOptions struct {
 	Registry    RegistryOptions
 	Predicate   PredicateRemoteOptions
 	Policies    []string
+	LocalImage  bool
 }
 
 var _ Interface = (*VerifyAttestationOptions)(nil)
@@ -108,6 +109,9 @@ func (o *VerifyAttestationOptions) AddFlags(cmd *cobra.Command) {
 
 	cmd.Flags().StringVarP(&o.Output, "output", "o", "json",
 		"output format for the signing image information (json|text)")
+
+	cmd.Flags().BoolVar(&o.LocalImage, "local-image", false,
+		"whether the specified image is a path to an image saved locally via 'cosign save'")
 }
 
 // VerifyBlobOptions is the top level wrapper for the `verify blob` command.

cmd/cosign/cli/verify.go

@@ -136,6 +136,9 @@ against the transparency log.`,
   # verify image with public key
   cosign verify-attestation --key cosign.pub <IMAGE>
 
+  # verify image attestations with an on-disk signed image from 'cosign save'
+  cosign verify-attestation --key cosign.pub --local-image <PATH>
+
   # verify image with public key provided by URL
   cosign verify-attestation --key https://host.for/<FILE> <IMAGE>
 
@@ -164,6 +167,7 @@ against the transparency log.`,
 				FulcioURL:       o.Fulcio.URL,
 				PredicateType:   o.Predicate.Type,
 				Policies:        o.Policies,
+				LocalImage:      o.LocalImage,
 			}
 			return v.Exec(cmd.Context(), args)
 		},

cmd/cosign/cli/verify/verify_attestation.go

@@ -29,6 +29,7 @@ import (
 	"github.com/pkg/errors"
 	"github.com/sigstore/cosign/pkg/cosign/pkcs11key"
 	"github.com/sigstore/cosign/pkg/cosign/rego"
+	"github.com/sigstore/cosign/pkg/oci"
 
 	"github.com/sigstore/cosign/cmd/cosign/cli/fulcio"
 	"github.com/sigstore/cosign/cmd/cosign/cli/options"
@@ -52,6 +53,7 @@ type VerifyAttestationCommand struct {
 	RekorURL      string
 	PredicateType string
 	Policies      []string
+	LocalImage    bool
 }
 
 // Exec runs the verification command
@@ -109,14 +111,24 @@ func (c *VerifyAttestationCommand) Exec(ctx context.Context, images []string) (e
 	}
 
 	for _, imageRef := range images {
-		ref, err := name.ParseReference(imageRef)
-		if err != nil {
-			return err
-		}
+		var verified []oci.Signature
+		var bundleVerified bool
 
-		verified, bundleVerified, err := cosign.VerifyImageAttestations(ctx, ref, co)
-		if err != nil {
-			return err
+		if c.LocalImage {
+			verified, bundleVerified, err = cosign.VerifyLocalImageAttestations(ctx, imageRef, co)
+			if err != nil {
+				return err
+			}
+		} else {
+			ref, err := name.ParseReference(imageRef)
+			if err != nil {
+				return err
+			}
+
+			verified, bundleVerified, err = cosign.VerifyImageAttestations(ctx, ref, co)
+			if err != nil {
+				return err
+			}
 		}
 
 		var cuePolicies, regoPolicies []string

pkg/cosign/verify.go

@@ -407,6 +407,57 @@ func VerifyImageAttestations(ctx context.Context, signedImgRef name.Reference, c
 	if err != nil {
 		return nil, false, err
 	}
+
+	return verifyImageAttestations(ctx, atts, h, co)
+}
+
+// VerifyLocalImageAttestations verifies attestations from a saved, local image, without any network calls,
+// returning the verified attestations.
+// If there were no valid signatures, we return an error.
+func VerifyLocalImageAttestations(ctx context.Context, path string, co *CheckOpts) (checkedAttestations []oci.Signature, bundleVerified bool, err error) {
+	// Enforce this up front.
+	if co.RootCerts == nil && co.SigVerifier == nil {
+		return nil, false, errors.New("one of verifier or root certs is required")
+	}
+
+	se, err := layout.SignedImageIndex(path)
+	if err != nil {
+		return nil, false, err
+	}
+
+	var h v1.Hash
+	// Verify either an image index or image.
+	ii, err := se.SignedImageIndex(v1.Hash{})
+	if err != nil {
+		return nil, false, err
+	}
+	i, err := se.SignedImage(v1.Hash{})
+	if err != nil {
+		return nil, false, err
+	}
+	switch {
+	case ii != nil:
+		h, err = ii.Digest()
+		if err != nil {
+			return nil, false, err
+		}
+	case i != nil:
+		h, err = i.Digest()
+		if err != nil {
+			return nil, false, err
+		}
+	default:
+		return nil, false, errors.New("must verify either an image index or image")
+	}
+
+	atts, err := se.Attestations()
+	if err != nil {
+		return nil, false, err
+	}
+	return verifyImageAttestations(ctx, atts, h, co)
+}
+
+func verifyImageAttestations(ctx context.Context, atts oci.Signatures, h v1.Hash, co *CheckOpts) (checkedAttestations []oci.Signature, bundleVerified bool, err error) {
 	sl, err := atts.Get()
 	if err != nil {
 		return nil, false, err

test/e2e_test.go

@@ -720,12 +720,15 @@ func TestSaveLoadAttestation(t *testing.T) {
 	}
 	verifyAttestation.PredicateType = "slsaprovenance"
 	verifyAttestation.Policies = []string{policyPath}
-	// Success case
+	// Success case (remote)
 	cuePolicy := `builder: id: "2"`
 	if err := os.WriteFile(policyPath, []byte(cuePolicy), 0600); err != nil {
 		t.Fatal(err)
 	}
 	must(verifyAttestation.Exec(ctx, []string{imgName2}), t)
+	// Success case (local)
+	verifyAttestation.LocalImage = true
+	must(verifyAttestation.Exec(ctx, []string{imageDir}), t)
 }
 
 func TestAttachSBOM(t *testing.T) {