diff --git a/internal/repository/repository.go b/internal/repository/repository.go
index 3b26b1f90..9abdaeec2 100644
--- a/internal/repository/repository.go
+++ b/internal/repository/repository.go
@@ -787,6 +787,22 @@ func (r *Repository) createIndexFromPacks(ctx context.Context, packsize map[rest
 	return invalid, nil
 }
 
+func (r *Repository) NewAssociatedBlobSet() restic.AssociatedBlobSet {
+	return &associatedBlobSet{*index.NewAssociatedSet[struct{}](r.idx)}
+}
+
+// associatedBlobSet is a wrapper around index.AssociatedSet to implement the restic.AssociatedBlobSet interface.
+type associatedBlobSet struct {
+	index.AssociatedSet[struct{}]
+}
+
+func (s *associatedBlobSet) Intersect(other restic.AssociatedBlobSet) restic.AssociatedBlobSet {
+	return &associatedBlobSet{*s.AssociatedSet.Intersect(other)}
+}
+func (s *associatedBlobSet) Sub(other restic.AssociatedBlobSet) restic.AssociatedBlobSet {
+	return &associatedBlobSet{*s.AssociatedSet.Sub(other)}
+}
+
 // prepareCache initializes the local cache. indexIDs is the list of IDs of
 // index files still present in the repo.
 func (r *Repository) prepareCache() error {
diff --git a/internal/restic/repository.go b/internal/restic/repository.go
index 2f1373641..ed0c64cf0 100644
--- a/internal/restic/repository.go
+++ b/internal/restic/repository.go
@@ -2,6 +2,7 @@ package restic
 
 import (
 	"context"
+	"iter"
 
 	"github.com/restic/restic/internal/backend"
 	"github.com/restic/restic/internal/crypto"
@@ -26,6 +27,7 @@ type Repository interface {
 	LookupBlob(t BlobType, id ID) []PackedBlob
 	LookupBlobSize(t BlobType, id ID) (size uint, exists bool)
 
+	NewAssociatedBlobSet() AssociatedBlobSet
 	// ListBlobs runs fn on all blobs known to the index. When the context is cancelled,
 	// the index iteration returns immediately with ctx.Err(). This blocks any modification of the index.
 	ListBlobs(ctx context.Context, fn func(PackedBlob)) error
@@ -186,3 +188,13 @@ type FindBlobSet interface {
 	Has(bh BlobHandle) bool
 	Insert(bh BlobHandle)
 }
+
+type AssociatedBlobSet interface {
+	Has(bh BlobHandle) bool
+	Insert(bh BlobHandle)
+	Delete(bh BlobHandle)
+	Len() int
+	Keys() iter.Seq[BlobHandle]
+	Intersect(other AssociatedBlobSet) AssociatedBlobSet
+	Sub(other AssociatedBlobSet) AssociatedBlobSet
+}