download attestation: support --platform flag

cmd/cosign/cli/download/attestation.go

@@ -21,8 +21,11 @@ import (
 	"fmt"
 
 	"github.com/google/go-containerregistry/pkg/name"
+	v1 "github.com/google/go-containerregistry/pkg/v1"
 	"github.com/sigstore/cosign/v2/cmd/cosign/cli/options"
 	"github.com/sigstore/cosign/v2/pkg/cosign"
+	"github.com/sigstore/cosign/v2/pkg/oci"
+	ociremote "github.com/sigstore/cosign/v2/pkg/oci/remote"
 )
 
 func AttestationCmd(ctx context.Context, regOpts options.RegistryOptions, attOptions options.AttestationDownloadOptions, imageRef string) error {
@@ -43,7 +46,50 @@ func AttestationCmd(ctx context.Context, regOpts options.RegistryOptions, attOpt
 		}
 	}
 
-	attestations, err := cosign.FetchAttestationsForReference(ctx, ref, predicateType, ociremoteOpts...)
+	se, err := ociremote.SignedEntity(ref, ociremoteOpts...)
+	if err != nil {
+		return err
+	}
+
+	idx, isIndex := se.(oci.SignedImageIndex)
+
+	// We only allow --platform on multiarch indexes
+	if attOptions.Platform != "" && !isIndex {
+		return fmt.Errorf("specified reference is not a multiarch image")
+	}
+
+	if attOptions.Platform != "" && isIndex {
+		targetPlatform, err := v1.ParsePlatform(attOptions.Platform)
+		if err != nil {
+			return fmt.Errorf("parsing platform: %w", err)
+		}
+		platforms, err := getIndexPlatforms(idx)
+		if err != nil {
+			return fmt.Errorf("getting available platforms: %w", err)
+		}
+
+		platforms = matchPlatform(targetPlatform, platforms)
+		if len(platforms) == 0 {
+			return fmt.Errorf("unable to find an attestation for %s", targetPlatform.String())
+		}
+		if len(platforms) > 1 {
+			return fmt.Errorf(
+				"platform spec matches more than one image architecture: %s",
+				platforms.String(),
+			)
+		}
+
+		nse, err := idx.SignedImage(platforms[0].hash)
+		if err != nil {
+			return fmt.Errorf("searching for %s image: %w", platforms[0].hash.String(), err)
+		}
+		if nse == nil {
+			return fmt.Errorf("unable to find image %s", platforms[0].hash.String())
+		}
+		se = nse
+	}
+
+	attestations, err := cosign.FetchAttestations(se, predicateType)
 	if err != nil {
 		return err
 	}

cmd/cosign/cli/options/download.go

@@ -24,6 +24,7 @@ type SBOMDownloadOptions struct {
 
 type AttestationDownloadOptions struct {
 	PredicateType string // Predicate type of attestation to retrieve
+	Platform      string // Platform to download attestations
 }
 
 var _ Interface = (*SBOMDownloadOptions)(nil)
@@ -40,4 +41,6 @@ func (o *SBOMDownloadOptions) AddFlags(cmd *cobra.Command) {
 func (o *AttestationDownloadOptions) AddFlags(cmd *cobra.Command) {
 	cmd.Flags().StringVar(&o.PredicateType, "predicate-type", "",
 		"download attestation with matching predicateType annotation")
+	cmd.Flags().StringVar(&o.Platform, "platform", "",
+		"download attestation for a specific platform image")
 }

pkg/cosign/fetch.go

@@ -19,13 +19,15 @@ import (
 	"context"
 	"crypto/x509"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"os"
 	"runtime"
 	"sync"
 
 	"github.com/google/go-containerregistry/pkg/name"
 	"github.com/sigstore/cosign/v2/pkg/cosign/bundle"
+	"github.com/sigstore/cosign/v2/pkg/oci"
 	ociremote "github.com/sigstore/cosign/v2/pkg/oci/remote"
 	"golang.org/x/sync/errgroup"
 )
@@ -121,12 +123,15 @@ func FetchSignaturesForReference(_ context.Context, ref name.Reference, opts ...
 }
 
 func FetchAttestationsForReference(_ context.Context, ref name.Reference, predicateType string, opts ...ociremote.Option) ([]AttestationPayload, error) {
-	simg, err := ociremote.SignedEntity(ref, opts...)
+	se, err := ociremote.SignedEntity(ref, opts...)
 	if err != nil {
 		return nil, err
 	}
+	return FetchAttestations(se, predicateType)
+}
 
-	atts, err := simg.Attestations()
+func FetchAttestations(se oci.SignedEntity, predicateType string) ([]AttestationPayload, error) {
+	atts, err := se.Attestations()
 	if err != nil {
 		return nil, fmt.Errorf("remote image: %w", err)
 	}
@@ -135,7 +140,7 @@ func FetchAttestationsForReference(_ context.Context, ref name.Reference, predic
 		return nil, fmt.Errorf("fetching attestations: %w", err)
 	}
 	if len(l) == 0 {
-		return nil, fmt.Errorf("no attestations associated with %s", ref)
+		return nil, errors.New("found no attestations")
 	}
 
 	attestations := make([]AttestationPayload, 0, len(l))