I remember watching Adam Back's talk about this concept when he was in Israel at a meeting I think sponsored by Meni Rosenfeld.

Abstract
Sumcoin is peer-to-peer electronic cash where transaction amounts are
only visible to the sender and receiver. In each transaction, the output
amounts are encrypted with the public keys of the respective receivers.
Only the transaction fee is publicly revealed, to allow miners to prioritise
transactions. A homomorphic commitment for each transaction proves
that the sum of the transaction inputs matches the sum of its outputs.
A short Non-Interactive Zero-Knowledge Proof (NIZKP) for each out-
put also convinces all veriers that the sum does not over ow. Address
construction includes an additional public view key to allow senders to
encrypt output values. This approach practically resolves a core privacy
issue in Bitcoin[1], but without overwhelming implementation complexity.
The required commitments are an order of magnitude smaller than those
initially proposed for Condential Transactions[2].


In addition to standing on its own, Sumcoin can be implemented as a sidechain[42]
or integrated into the Monero (or Bitcoin) protocol as a hard fork with a new
transaction version. Unspent transaction pruning[43] is possible in Sumcoin