diff --git a/TODO b/TODO
index 7ac88b7bf..de8369523 100644
--- a/TODO
+++ b/TODO
@@ -4,6 +4,16 @@ There are two TODO lists. This file (good for airplanes) and the online bug trac
 
 Offline list:
 
+-- change representation of *blobref.BlobRef to a value type with smaller
+   memory usage and no internal string cache, so it can be used as a map
+   key:
+       type BlobRef struct {
+           hash crypto.Hash
+           sum  interface{} // [hash.Size()]byte
+       }
+   The string cache, if needed, can in the blobref package, keyed by
+   blobref.
+
 -- verify that the HTTP blob upload interface caps uploads at MaxBlobSize
    before giving it to storage impls