fix: layout attestation resolver

pkg/oci/layout.go

@@ -115,6 +115,9 @@ func attestationManifestFromOCILayout(path string, platform *v1.Platform) (*atte
 			}
 		}
 	}
+	if subjectDescriptor == nil {
+		return nil, fmt.Errorf("platform not found in index")
+	}
 	for i := range mfs2.Manifests {
 		mf := &mfs2.Manifests[i]
 		if mf.Annotations[att.DockerReferenceType] != attestation.AttestationManifestType {

pkg/oci/referrers.go

@@ -3,6 +3,7 @@ package oci
 import (
 	"context"
 	"fmt"
+	"strings"
 
 	"github.com/docker/attest/pkg/attestation"
 	att "github.com/docker/attest/pkg/attestation"
@@ -16,14 +17,6 @@ type ReferrersResolver struct {
 }
 
 func NewReferrersAttestationResolver(src ImageDetailsResolver, options ...func(*ReferrersResolver) error) (*ReferrersResolver, error) {
-	// currently only supports RegistryImageDetailsResolver
-	switch src.(type) {
-	case *RegistryImageDetailsResolver:
-	case *MockRegistryResolver:
-	default:
-		return nil, fmt.Errorf("unsupported referrers image details resolver type: %T", src)
-	}
-
 	res := &ReferrersResolver{
 		ImageDetailsResolver: src,
 	}
@@ -66,7 +59,7 @@ func (r *ReferrersResolver) resolveAttestations(ctx context.Context, predicateTy
 	}
 	var referrersSubjectRef name.Digest
 	if r.referrersRepo != "" {
-		referrersSubjectRef, err = name.NewDigest(fmt.Sprintf("%s@%s", r.referrersRepo, subjectDigest))
+		referrersSubjectRef, err = name.NewDigest(fmt.Sprintf("%s@%s", strings.TrimPrefix(r.referrersRepo, RegistryPrefix), subjectDigest))
 		if err != nil {
 			return nil, fmt.Errorf("failed to create referrers reference: %w", err)
 		}