diff --git a/repository/index.go b/repository/index.go
index 2ffac231c..1f9d65ad1 100644
--- a/repository/index.go
+++ b/repository/index.go
@@ -132,6 +132,26 @@ func (idx *Index) Lookup(id backend.ID) (pb PackedBlob, err error) {
 	return PackedBlob{}, fmt.Errorf("id %v not found in index", id)
 }
 
+// ListPack returns a list of blobs contained in a pack.
+func (idx *Index) ListPack(id backend.ID) (list []PackedBlob) {
+	idx.m.Lock()
+	defer idx.m.Unlock()
+
+	for blobID, entry := range idx.pack {
+		if entry.packID == id {
+			list = append(list, PackedBlob{
+				ID:     blobID,
+				Type:   entry.tpe,
+				Length: entry.length,
+				Offset: entry.offset,
+				PackID: entry.packID,
+			})
+		}
+	}
+
+	return list
+}
+
 // Has returns true iff the id is listed in the index.
 func (idx *Index) Has(id backend.ID) bool {
 	_, err := idx.Lookup(id)
diff --git a/repository/index_test.go b/repository/index_test.go
index 480619fa6..85674de77 100644
--- a/repository/index_test.go
+++ b/repository/index_test.go
@@ -240,6 +240,18 @@ var exampleTests = []struct {
 	},
 }
 
+var exampleLookupTest = struct {
+	packID backend.ID
+	blobs  backend.IDSet
+}{
+	ParseID("73d04e6125cf3c28a299cc2f3cca3b78ceac396e4fcf9575e34536b26782413c"),
+	backend.IDSet{
+		ParseID("3ec79977ef0cf5de7b08cd12b874cd0f62bbaf7f07f3497a5b1bbcc8cb39b1ce"): struct{}{},
+		ParseID("9ccb846e60d90d4eb915848add7aa7ea1e4bbabfc60e573db9f7bfb2789afbae"): struct{}{},
+		ParseID("d3dc577b4ffd38cc4b32122cabf8655a0223ed22edfd93b353dc0c3f2b0fdf66"): struct{}{},
+	},
+}
+
 func TestIndexUnserialize(t *testing.T) {
 	oldIdx := backend.IDs{ParseID("ed54ae36197f4745ebc4b54d10e0f623eaaaedd03013eb7ae90df881b7781452")}
 
@@ -257,6 +269,17 @@ func TestIndexUnserialize(t *testing.T) {
 	}
 
 	Equals(t, oldIdx, idx.Supersedes())
+
+	blobs := idx.ListPack(exampleLookupTest.packID)
+	if len(blobs) != len(exampleLookupTest.blobs) {
+		t.Fatalf("expected %d blobs in pack, got %d", len(exampleLookupTest.blobs), len(blobs))
+	}
+
+	for _, blob := range blobs {
+		if !exampleLookupTest.blobs.Has(blob.ID) {
+			t.Errorf("unexpected blob %v found", blob.ID.Str())
+		}
+	}
 }
 
 func TestIndexUnserializeOld(t *testing.T) {
diff --git a/repository/master_index.go b/repository/master_index.go
index 5a5e499e6..bc3dea768 100644
--- a/repository/master_index.go
+++ b/repository/master_index.go
@@ -67,6 +67,22 @@ func (mi *MasterIndex) LookupSize(id backend.ID) (uint, error) {
 	return 0, fmt.Errorf("id %v not found in any index", id)
 }
 
+// ListPack returns the list of blobs in a pack. The first matching index is
+// returned, or nil if no index contains information about the pack id.
+func (mi *MasterIndex) ListPack(id backend.ID) (list []PackedBlob) {
+	mi.idxMutex.RLock()
+	defer mi.idxMutex.RUnlock()
+
+	for _, idx := range mi.idx {
+		list := idx.ListPack(id)
+		if len(list) > 0 {
+			return list
+		}
+	}
+
+	return nil
+}
+
 // Has queries all known Indexes for the ID and returns the first match.
 func (mi *MasterIndex) Has(id backend.ID) bool {
 	mi.idxMutex.RLock()