Use a CAM
Content Addressable Memory (CAM) is an object store where objects are addressed by their hash. According to https://doc.obnam.org/obnam.html, an obnam client gets a chunk id from the server (i.e., chunk ids are server assigned). This means that the client has to wait for the server before it can fill in a pointer to the chunk. Using a CAM means the client wouldn't have to wait on the server, which offers a huge performance benefit when the connection has high latency. In fact, it would potentially allow the client to create the whole backup offline, and then transfer it to the server at a later point!